See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/262953899 Metasomatism of ferroan granites in the northern Aravalli orogen, NW India: Geochemical and isotopic constraints, and its metallogenic significance ARTICLE in INTERNATIONAL JOURNAL OF EARTH SCIENCES · FEBRUARY 2014 Impact Factor: 2.09 · DOI: 10.1007/s00531-014-1005-x CITATION READS 1 113 7 AUTHORS, INCLUDING: Parampreet Kaur Albrecht W. Hofmann 19 PUBLICATIONS 311 CITATIONS 302 PUBLICATIONS 22,123 CITATIONS Panjab University Max Planck Institute for Chemistry SEE PROFILE SEE PROFILE Martin Okrusch Juergen Koepke 148 PUBLICATIONS 2,334 CITATIONS 114 PUBLICATIONS 1,694 CITATIONS University of Wuerzburg SEE PROFILE Leibniz Universität Hannover SEE PROFILE Available from: Albrecht W. Hofmann Retrieved on: 03 February 2016 Int J Earth Sci (Geol Rundsch) DOI 10.1007/s00531-014-1005-x ORIGINAL PAPER Metasomatism of ferroan granites in the northern Aravalli orogen, NW India: geochemical and isotopic constraints, and its metallogenic significance Parampreet Kaur · Naveen Chaudhri · Albrecht W. Hofmann · Ingrid Raczek · Martin Okrusch · Susanne Skora · Jürgen Koepke Received: 8 February 2013 / Accepted: 6 February 2014 © Springer-Verlag Berlin Heidelberg 2014 Abstract The late Palaeoproterozoic (1.72–1.70 Ga) ferroan granites of the Khetri complex, northern Aravalli orogen, NW India, were extensively metasomatised ~900 Ma after their emplacement, at around 850–830 Ma by lowtemperature (ca. 400 °C) meteoric fluids that attained metamorphic character after exchanging oxygen with the surrounding metamorphic rocks. Albitisation is the dominant metasomatic process that was accompanied by Mg and Ca metasomatism. A two-stage metasomatic model is applicable to all the altered ferroan intrusives. The stage I is represented by a metasomatic reaction interface that developed as a result of transformation of the original microcline– oligoclase (An12–14) granite to microcline–albite (An1–3) P. Kaur (*) · N. Chaudhri Centre of Advanced Study in Geology, Panjab University, Chandigarh 160 014, India e-mail: param.geol@gmail.com A. W. Hofmann · I. Raczek Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany M. Okrusch Lehrstuhl für Geodynamik und Geomaterialforschung, Institut für Geographie und Geologie, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany S. Skora Institute of Mineralogy and Geochemistry, University of Lausanne, 1015 Lausanne, Switzerland S. Skora School of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol BS81JP, UK J. Koepke Institut für Mineralogie, Leibniz Universität Hannover, Callinstr. 3, 30167 Hannover, Germany granite, and this stage is rarely preserved. In contrast, the stage II metasomatic reaction front, where the microclinebearing albite granite has been transformed to microclinefree albite granite, is readily recognisable in the field and present in most of the intrusives. Some of them lack an obvious reaction interface due to the presence of stage II albite granites only. When studied in isolation, these intrusives were incorrectly classified and their tectonic setting was misinterpreted. Furthermore, our results show that the mafic mineralogy of metasomatised granites has a significant impact on the characterisation of such rocks in the magmatic classification and discrimination diagrams. Nevertheless, the stage I metasomatised granites can be appropriately characterised in these diagrams, whereas the characterisation of the stage II granites will lead to erroneous interpretations. The close spatial association of these high heat producing ferroan granites with iron oxide–copper–gold (IOCG), U and REE mineralisation in the region indicates a genetic link between the metasomatism and the mineralisation. World-class IOCG, U and REE deposits are associated with metasomatised ferroan granites, suggesting that such a relationship may act as a critical first-order exploration target for undiscovered mineral deposits. Keywords Ferroan granites · Metasomatism · Albitisation · IOCG metallogeny · Khetri complex · Aravalli orogen · NW India Introduction A-type granites, which are recommended to be more appropriately termed as ferroan granites (Frost and Frost 2011), have high metallogenic potential and are associated with many mineral deposits (Dall’Agnol et al. 2012 and 13 Int J Earth Sci (Geol Rundsch) references therein). These granites are enriched repositories of Fe, REE, U and other incompatible elements, and if suitably metasomatised, can provide important constituents for ore deposits. Metasomatised ferroan granites are, therefore, known worldwide to be closely associated, in space and time, with iron oxide–copper–gold (IOCG), U and REE mineral deposits. Some of such well-studied occurrences are located in the Gawler craton of southern Australia where metasomatism of early Mesoproterozoic (1.59– 1.55 Ga) ferroan granites was responsible for the IOCG– U–REE mineralisation (e.g. Williams et al. 2005; Pollard 2006). The temporal relationships between the intrusion of ferroan granites and formation of IOCG–U–REE deposits vary substantially, i.e. these deposits can form either immediately after the emplacement of the granites or sometimes more than a billion years later as exemplified by the Olympic Dam and the Curnamona province deposits of southern Australia, respectively (Elburg et al. 2012). This indicates that the study of metasomatised ferroan granites is of great importance in localising important ore deposits. Ferroan granites are widespread in the Khetri and Alwar complexes of the northern Aravalli orogen, NW India (Fig. 1; e.g. Biju-Sekhar et al. 2003; Kaur et al. 2006a, 2007, 2011a). These late Palaeoproterozoic (1.72–1.70 Ga) high heat producing granites in the Khetri complex, which host IOCG and uranium mineralisation (Knight et al. 2002), are extensively metasomatised (Kaur et al. 2011a, 2012). Albitisation is the main metasomatic process that affected ferroan granites of the Khetri complex. The subtle occurrence of the unaltered original granite, which has so far been observed only in one intrusive, the Dosi pluton, reflects the pervasive nature of metasomatism in the region. This enabled Kaur et al. (2012) to present a twostage metasomatic model for albitisation in the Khetri complex that resulted in the formation of two metasomatic fronts. These advanced through the original granite simultaneously, but at different rates. The leading metasomatic front converted oligoclase of the original granite to albite, producing microcline-bearing albite granite and a second, trailing front converted microcline to albite, forming microcline-free albite granite (for details see Kaur et al. 2012). This model is yet to be tested for other intrusions in the northern Aravalli orogen where either both metasomatic fronts or only the second metasomatic front is represented. Moreover, in some of the intrusions, only the completely albitised granites, without any metasomatic reaction front, are exposed. For such severely metasomatised rocks, it becomes difficult to ascertain their original protoliths. When such ferroan granites were studied in isolation, this led to their misclassification and to a misinterpretation of their tectonic setting. Furthermore, the nature and volume of mafic phases in these ferroan rocks varies, as 13 metasomatism has produced either totally new mafic minerals or the same mafic phases with different composition, or their complete disappearance in the severely albitised rocks. For example, in the granites studied by Kaur et al. (2012), biotite, although having remained annite during the first stage of metasomatism, changed its composition from XFe 0.71–0.74 to XFe 0.90–0.91 and nearly disappeared in the second stage. Alongside, the original ferropargasite amphibole was altered to hastingsite during the first stage of metasomatism, and the hastingsite was subsequently transformed to actinolite in the final stage. There are other albitised ferroan granites in the region with either biotite or clinopyroxene as the original mafic minerals that have been transformed to amphiboles of varied compositions. The significance and the impact of these mafic mineralogical transformations on the characterisation of metasomatic granites have not been studied in detail. This work complements our previous study (Kaur et al. 2012) by considering additional metasomatised intrusives from the northern Khetri complex with entirely new objectives. In this study, we aim to (1) establish the temporal relations between the ferroan granite intrusion, metasomatism and the IOCG–U mineralisation in the Khetri complex by taking into account recently available geological and geochronological information; (2) present new petrological, geochemical and isotopic data for metasomatically altered ferroan granites that show different original mafic mineralogy; and in addition, (3) we demonstrate the pitfalls in the petrological and tectonic interpretations if such intensely metasomatised rocks are not studied with extreme care. Geological setting The Precambrian geology of the Aravalli orogen (Fig. 1a) is characterised by two prominent Proterozoic cover sequences, the Aravalli fold belt and the Delhi fold belt, which unconformably rest over a Palaeo–Neoarchaean (ca. 3.3–2.5 Ga) basement (Fig. 1b; e.g. Sinha-Roy et al. 1998; Roy and Jakhar 2002) known as the Banded Gneissic Complex or BGC (Heron 1953) in the southern part and the Sandmata complex in the central part (Roy et al. 2012). The present study is confined to the Khetri complex, which is the westernmost entity of the northern Delhi fold belt (Fig. 1b) and forms an important metallogenic terrane. Here, the rocks of the Delhi Supergroup form a cover sequence over a Palaeoproterozoic basement. The Delhi rocks are divisible into two lithostratigraphic units, an arenaceous-dominated older sequence (the Alwar Group) and an argillaceous-dominated younger unit (the Ajabgarh Group) with a gradational and conformable contact between the two (e.g. Das Gupta 1968). Int J Earth Sci (Geol Rundsch) The basement, which is largely composed of 1.72– 1.70 Ga ferroan granites (Fig. 1c) and their cover sequence, records multiple phases of deformation and medium- to high-grade regional metamorphism (e.g. Sarkar and Dasgupta 1980; Lal and Ackermand 1981; Naha et al. 1988; Gupta et al. 1998). Judging from microprobe dating of monazites, this metamorphism likely took place between ca. 950–910 Ma (Kaur et al. 2006b; Pant et al. 2008). The metasomatism in the region is constrained to have occurred at around 850–830 Ma as indicated by an U–Pb SHRIMP titanite age (Knight et al. 2002) and a whole-rock–mineral (clinopyroxene and scapolite) Sm–Nd isochron age (Kaur Fig. 1 a Map showing the extent of the Aravalli orogen in NW India; b simplified geological map of the Aravalli orogen showing major Precambrian lithotectonic units (modified after Roy 1988; Roy and Jakhar 2002) and location of the study area, and c generalised geolog- ical map of the northern Khetri complex showing the distribution of the ferroan granites (compiled after Heron 1923; Geological Survey of India 1997; Gupta et al. 1998 and our own observations) 13 Int J Earth Sci (Geol Rundsch) et al. 2013a). The north-westernmost part of the Khetri complex represents an older (1.85–1.82 Ga) allochthonous terrane of basement rocks (probably equivalent to the Aravalli Supergroup), which are now juxtaposed tectonically along a shear zone with the younger Delhi Supergroup sequences exposed in the east (Kaur et al. 2009, 2013b). These ~1.82 Ga subduction-related magnesian granitoids do not show any evidence of metasomatism, indicating that this terrane was docked to its current position after the metasomatic event in the region (for details see Kaur et al. 2009). Field characteristics Four granitoid plutons located around the villages of Tehara, Bansiyal, Gorir and Gothara in the northern Khetri complex (Figs. 1c, 2) were studied for the present work. Their petrological, geochemical and isotopic characteristics are compared with other well-studied intrusives of Dabla, Biharipur and Dosi (Chaudhri et al. 2003; Kaur et al. 2006a, 2012) that occur in the north-eastern extremity of the same region (Fig. 1c). It should be noted that, owing to the deformation during the metamorphic overprint, the Khetri granites attained Fig. 2 Detailed geological maps of the a Tehara, b Bansiyal and c Gorir intrusives; d is the generalised geological map of the Gothara intrusive (after Geological Survey of India 1997) 13 Int J Earth Sci (Geol Rundsch) a foliated structure and consequently should be designated as meta-granites. The foliation was, however, widely obliterated by the subsequent metasomatic albitisation and dissolution of mafic phases or replacement of biotite by amphibole (see below) leading to a structural appearance of undeformed granite. Therefore, for simplicity, we maintain the traditional name granites for these rocks. Furthermore, as the original oligoclase and in some cases, both oligoclase and K-feldspar have been transformed to albite, these granites are attributed to the field of “normal granites” (field 3 of QAP diagram) as per the recommendations of the IUGS (Streckeisen 1976), and the albitised granites are named according to the nature of feldspars present in these rocks, such as microcline–albite granite or albite granite. The term alkali-feldspar granite has been discontinued for these rocks (see Chaudhri et al. 2003; Kaur et al. 2006a, 2007, 2011a). Tehara The Tehara pluton, located about 2 km east of Khetri, is an elongated intrusive (ca. 2 km × 1 km), exposed in the axial depression of an anticline (Das Gupta 1968). A sedimentary breccia, similar to that in the Gothara intrusion (see below), containing granitic and quartzitic material in an iron oxide ore matrix runs almost all along the periphery of the pluton. This is followed by the Alwar quartzite sequence and other rocks of the Delhi Supergroup. Detailed mapping carried out during this study reveals that the albite granite is the most widespread facies, whereas the microcline–albite granite is present as irregular, relic-like enclave bodies within the albite granite (Fig. 2a). The microcline–albite granite is foliated in a NE–SW direction with westerly dips; it crops out as local topographic highs, separated by low-lying albite granite (Fig. 3a, b). A small outcrop of the original granite, exposed in the eastern part, occurs in contact with the microcline–albite granite. The albite granite is profusely commingled, at the margins, with mafic rocks, similar to what is observed at Biharipur (for details see Kaur et al. 2006a, 2013a). In addition, there are pegmatites, occurring mainly in the northern part, and a few NW–SE trending dolerite dykes cutting through the granites, pegmatites and the quartzite of the cover sequence. This phase of mafic magmatism postdates the regional metasomatic event because the calcic plagioclase in these rocks has preserved its magmatic composition (An57.8±8.7; Kaur et al. unpublished data) and is not albitised. The time of original emplacement of this pluton was dated by the Pb–Pb zircon evaporation method at 1,711 ± 0.5 Ma (Kaur et al. 2007). Bansiyal The Bansiyal body, located about 4 km SE of Tehara (Fig. 1c), is contiguous with the Tehara pluton. It is an oval-shaped small (ca. 1 km × 1 km) intrusive, which is surrounded by quartzites in the north, unclassified metamorphics in the west, while its eastern and southern portions are covered by alluvium (Fig. 2b). The Bansiyal body is essentially made up of albite granite, showing a small outcrop of biotite-bearing foliated albite granite in the middle that is surrounded by amphibole-bearing albite granite. The biotite-bearing albite granite has yielded a U–Pb zircon age of 1,727 ± 28 Ma (Kaur et al. 2011a), which is considered as the time of crystallisation of this intrusive. Gorir The Gorir intrusive is located about 16 km ENE of Khetri, and the granites here crop out as a number of isolated hillocks within alluvial cover trending in NE–SW direction and covering a distance of at least 6 km (Fig. 2c); the south-western extension of these rocks is much larger than the mapped area (see Dinkar et al. 2013). No previous work on these granites is available. Most of these rocks are albite granite with a small outcrop of microcline–albite granite occurring to the south of Gorir (Fig. 2c). The latter is brick red in colour (Fig. 3c) and also occurs as relict patches or veins within the grey coloured albite granite (Fig. 3d), defining a metasomatic reaction front between the two rock types. The albite granites, north-west of Gorir, also contain bands of magnetite (Fig. 3e), and these rocks show extensive commingling with mafic material, in places producing a banded structure. The microcline–albite granite has provided a U–Pb zircon age of 1,727 ± 8 Ma (Kaur et al. unpublished data). Gothara The Gothara pluton, situated about 8 km NE of Khetri, extends over a distance of ~7 km in NW–SE direction as separate hillocks. The best exposures occur to the SW of Gothara, covering a distance of about 1.5 km. The eastern margin of the pluton is covered by alluvium, whereas a sedimentary breccia (Gupta et al. 1998) occurs all along its north-western margin. The breccia is followed by an albitic quartzite, containing iron bands (Fig. 2d), and shows a sharp contact with the granite (Fig. 3f). An in situ laser ablation ICP–MS study on detrital zircons from the albitic quartzite unit indicates that the age of deposition of these sediments was <1.7 Ga (Kaur et al. 2011b). Previous workers described the Gothara intrusive as quartz-hornblende granites with layers and patches of granophyres (Das Gupta 1968), an I-type K-feldspar–albite granite of magmatic origin (Bhattacharyya and Dasgupta 1981; Gupta et al. 1998), a hydrothermally altered granite (Knight et al. 2002) or even as unaltered plagiogranite of magmatic origin, providing evidence of oceanic magmatism in the region (Kaur and Mehta 2005). SHRIMP U–Pb zircon data yield an age of 1,691 ± 4 Ma for the Gothara pluton (Knight et al. 2002). 13 Int J Earth Sci (Geol Rundsch) Fig. 3 Representative field photographs illustrating the general characteristics of the albitised ferroan granites. a Microcline–albite granite occurring as local topographic highs (marked by yellow arrows) separated by the low-lying albite granite in the Tehara intrusive; b close-up showing microcline–albite granite and the albite granite at Tehara. The dashed line marked the contact between the two; c brick red coloured microcline–albite granite at Gorir; d the same micro- cline–albite granite occurring as relict patches within the grey albite granite, illustrating the stage II metasomatic reaction interface; e iron oxide band within the weathered Gorir albite granite and f Gothara albite granite showing a sharp contact with the brecciated zone. The coin is 2.5 cm across in Fig. 3c, the length of hammer’s handle is 28 cm in Figs. 3d and e and the length of the pen is about 14 cm in Fig. 3f Analytical procedures Chemie, Mainz (JEOL Superprobe JXA-8200) and the Mineralogisches Institut, Universität Würzburg (CAMECA SX 50). In Würzburg, a CAMECA SX 50 electron microprobe was used, which was equipped with three independent WDS channels. Acceleration voltage was 15 kV, beam Electron microprobe analyses of minerals were carried out at the Institut für Mineralogie, Leibniz Universität Hannover (CAMECA SX 100), the Max-Planck-Institut für 13 Int J Earth Sci (Geol Rundsch) current 10 or 15 nA and integration time 20 s (30 s for Fe); beam size was 3–4 microns on feldspars and 1 micron on other minerals. Metal oxides and synthetic mineral standards supplied by CAMECA were used for reference and the PAP program for correction procedures (Pouchou and Pichoir 1985). The analytical details for Hannover and Mainz EMP analyses are provided in Kaur et al. (2006a). In general, the relative analytical errors were approximately 1 % for element concentrations >2 wt% and 5 % for minor elements (<2 wt%), and the detection limit was at about 0.1 wt%. Mineral abbreviations used are those given by Whitney and Evans (2010). Major and trace elements were analysed, using Philips PW 1480 X-ray fluorescence (XRF) spectrometers in the Department of Geosciences, University of Mainz (for details see Kaur et al. 2009) and in the Mineralogisches Institut, Universität Würzburg (see Kaur et al. 2006a). REE as well as Hf, Ta and U concentrations of the samples were obtained from the Activation Laboratories Ltd., Ontario, Canada, by ICP–MS, using a PerkinElmer SCIEX ELAN 6000 ICP–MS. Duplicate measurements of sample GL-6 provided a precision generally better than 1 % (SD). The accuracy of the analyses is monitored by many international reference materials, e.g. W2, WMG1, STM1, DNC1, BIR1, SY3, etc. Based on the rock standard STM1, it is usually better than 5 %, except for La, Er (<10 %) and Pr (<30 %). Sm–Nd and Rb–Sr isotopic analyses were carried out on a Finnigan MAT 261 thermal ionisation mass spectrometer in the Max-Planck-Institut für Chemie, Mainz as per the procedures outlined by Kaur et al. (2009). Twenty analyses of the La Jolla Nd standard performed during this study yielded a mean 143Nd/144Nd ratio of 0.511854 ± 7 (2σ), whereas 17 analyses of NBS 987 provided a mean 87Sr/86Sr ratio of 0.710231 ± 17 (2σ). For oxygen isotope analyses, ~1.5–2.5 mg whole-rock samples were extracted on a platinum sample holder in a F2 atmosphere, using a CO2 laser system for heating (for details see Sharp 1992). Purified O2 gas was analysed directly on a Finnigan MAT 253 mass spectrometer at the University of Lausanne, Switzerland. The Gore Mountain garnet standard (δ18O = 5.8 ‰) used for reference (Valley et al. 1995) suggests an analytical uncertainty (1σ) of 0.23 ‰. All oxygen isotope values are reported relative to VSMOW. Petrography Full petrographic description of the granitoids is given in the Appendix along with representative mineral analyses. Here, we outline only some of the main features which are particularly relevant in the context of this paper. Rock referred to as “original granite” is the only rock type that has not been affected by albitisation. It is only preserved at Tehara in a small outcrop area and consists of the major phases quartz, microcline, oligoclase and biotite. The “microcline–albite granite” refers to the moderately albitised granite in which there is a complete replacement of oligoclase by albite and an incipient replacement of K-feldspar by albite. This facies is well preserved at Tehara only, with a minor outcrop at Gorir (for differences in mafic minerals, see full description in the “Appendix”). The albite granite is the completely albitised rock and is characterised by almost complete replacement of K-feldspar and oligoclase by albite, often with small amounts of mafic minerals. This rock type exclusively crops out at Gothara, covers a large tract around Gorir and also forms a large part of the Tehara intrusive. Geochemical and isotopic results The SiO2 contents in most of the intrusives are ≥70 wt%, indicating their evolved nature (Tables 1, 2, 3). Overall, the albite granites are more enriched in Si than the microcline–albite granites. All the albite granites can be distinguished from other granites by their disparity in Na and K abundances, with Na2O as high as 8 wt% and K2O as low as 0.1 wt%. The Rb, Ba and Pb abundances are in consonance with the extremely low K2O values. Using Na/K as an index of albitisation, a progressive depletion in Rb from 178 ppm in the original granite to <1 ppm in the albite granite occurred with advancing metasomatic replacement of K-feldspar by albite and biotite by amphibole (Fig. 4). The amphibole-bearing albite granites of Bansiyal and Tehara as well as the Gothara albite granites show lower Fe and higher Ca values than the biotite-bearing granites, including the original granite at Tehara. No such particular trend is seen for the concentrations of Fe, Ca or Mg in the Gorir microcline–albite granite and albite granites, where Ca tends to be lower in the latter. Th/U ratios of the original granite and the microcline–albite granite at Tehara and some of the albite granites of Gothara are close to the “normal” crustal Th/U ratio of about 4 (Tables 1, 2). By contrast, the other albitised granites mostly show relatively high Th/U ratios, especially those of Gorir (Table 3), indicating U loss. The internal heat production values of the Tehara, Bansiyal and Gothara granites are high, mostly around 5 µWm−3, but some albite granites show values as low as 4.1 µWm−3 due to an extreme loss of K and also U from these rocks. The relatively high loss of U in addition to K in granites of the Gorir pluton may have resulted in even lower heat production values of 3.8–2.6 µWm−3. Overall, the REE of all the granites are enriched and generally fractionated; however, the degree of fractionation or the degree of REE enrichment differs. All the 13 Int J Earth Sci (Geol Rundsch) Table 1 Whole-rock chemical compositions for the Tehara granitoids, Rajasthan, NW India Sample no Original granite Microcline–albite granite GL-7 GL-2 GL-5 GL-8 GL-1 GL-3 GL-4 GL-6 (Lat °N) 27°59′ 57.2″ 28°0′ 22.2″ 27°59′ 50.2″ 28°0′ 7.0″ 28°0′ 15.2″ 28°0′ 17.2″ 28°0′ 7.8″ 27°59′ 56.2″ (Long °E) 75°49′ 7.4″ 70.9 0.65 12.58 5.11 0.03 0.57 1.00 3.26 4.55 0.13 0.61 99.4 75°48′ 25.8″ 70.7 0.62 12.60 4.87 0.03 0.66 1.01 3.63 4.35 0.13 0.88 99.5 75°48′ 44.8″ 69.4 0.69 12.60 5.43 0.02 0.67 1.21 3.99 3.83 0.14 1.04 99.0 75°49′ 6.8″ 71.6 0.61 12.54 4.40 0.02 0.75 0.70 4.40 3.43 0.13 0.81 99.4 75°48′ 37.8″ 71.8 0.65 12.86 4.24 0.05 0.75 1.51 7.07 0.09 0.13 0.46 99.6 75°48′ 20.8″ 72.6 0.63 12.64 2.91 0.05 0.99 1.61 6.92 0.10 0.13 0.65 99.2 75°48′ 45.2″ 73.0 0.58 12.69 3.48 0.05 0.72 1.44 6.94 0.13 0.12 0.44 99.6 75°48′ 51.8″ 72.7 0.66 12.57 3.35 0.05 0.81 1.61 6.79 0.11 0.12 0.53 99.3 SiO2 (wt%) TiO2 Al2O3 Fe2Ot3 MnO MgO CaO Na2O K2O P2O5 LOI Sum Albite granite V (ppm) 30 31 34 38 32 34 28 30 Cr Co Ni Zn Ga Rb Sr Y Zr Nb Ba Pb Th U Hf Ta La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 7 18 3 17 19 178 28 55 326 21 551 7 36.6 8.7 11.9 1.6 59.8 108 12.4 45.6 10.3 1.45 9.2 1.6 9.0 1.8 5.6 0.83 5.0 0.74 271.3 2 27 5 15 18 142 25 53 314 21 521 8 36.6 8.1 10 2.4 69.6 123 13.8 50.0 10.9 1.58 9.1 1.6 8.8 1.7 5.4 0.78 5.1 0.72 302.1 6 23 5 12 19 113 20 55 322 23 462 7 33.8 9.1 10.5 0.7 4 21 4 18 18 89 25 48 312 20 313 7 35.2 8.1 9.7 2.9 5 19 8 11 18 2 17 43 327 22 13 6 35.4 8.6 10.1 3.4 7 17 7 12 19 1.3 20 52 341 22 12 7 37.2 6 11.9 2.9 61.6 105 12.0 44.5 9.6 1.32 8.8 1.5 8.3 1.7 5.4 0.78 5.0 0.75 226.3 3 19 4 12 18 4 16 43 306 21 18 12 43.7 6.9 11.2 0.8 5 29 3 12 18 1.3 13 56 316 21 6 8 35.5 6.5 11.2 2.5 56.1 100 11.7 42.4 9.5 1.10 8.5 1.5 8.6 1.8 5.5 0.80 4.9 0.71 253.1 ∑REE 13 Int J Earth Sci (Geol Rundsch) Table 1 continued Sample no Original granite Microcline–albite granite GL-7 GL-2 GL-5 Albite granite GL-8 GL-1 GL-3 GL-4 GL-6 (La/Yb)N 8.1 9.3 8.4 7.8 (La/Sm)N 3.6 4.0 4.0 3.7 (Gd/Yb)N Eu/Eu* Th/U HPU 1.5 1.4 1.4 1.4 0.45 4.2 5.3 0.48 4.5 5.1 3.7 5.1 4.3 4.9 4.1 4.8 0.44 6.2 4.2 6.3 4.9 0.37 5.5 4.2 ASI 1.07 1.03 1.00 1.05 0.91 0.90 0.91 0.90 Eu/Eu* = EuN/(SmN × GdN)½. HPU = D[0.0967U (ppm) + 0.0263Th (ppm) + 0.035 K (wt%)] µWm−3 is heat production units (after Beardsmore and Cull 2001) where D refers to density in gcm−3 and is taken as 2.7 for all the samples Source: Data from Kaur et al. (2007). Bold Rb value: data from TIMS (see Table 4) Tehara granites show exceptionally homogeneous REE concentrations and profiles rather similar to that of the original granite at Dosi. The patterns are fractionated with (La/Yb)N = 7.8–9.3 and (La/Sm)N = 3.6–4.0, but nearly flat heavy REE (HREE) profiles [(Gd/Yb)N = 1.4–1.5) and prominent negative Eu anomalies with Eu/Eu* = 0.37– 0.48 (Fig. 5a). The Bansiyal albite granites display similar REE patterns to those of Tehara (Fig. 5b), but are slightly more fractionated [(La/Yb)N = 10.9–13.8], and the negative Eu anomalies are more distinct in the amphibolebearing albite granites (Eu/Eu* = 0.56–0.78). The REE patterns of the Gorir granites (Fig. 5c) are also similar, although relatively less fractionated than those of Tehara [(La/Yb)N = 2.8–5.5], and are also nearly identical to those of the original granite of Dosi. In contrast, the REE patterns of the Gothara albite granites are quite variable with (La/Yb)N ratios ranging from 2 to 20 (Table 2; Fig. 5d). These patterns fall within the range of the Biharipur, Dabla and Dosi albite granites (Fig. 5d). In a primitive mantle-normalised multielement diagram (Fig. 5e), the original granite and the microcline–albite granites of all studied plutons show similar patterns, characterised by variably pronounced negative spikes in Ba, Sr, P and Ti, and variably marked enrichment in REE. The completely albitised granites additionally display negative spikes for K and Rb with more pronounced troughs for Ba, Sr, P and Ti (Fig. 5f). Figure 6 shows the Sm–Nd data of the newly analysed granites together with our previously published data (Kaur et al. 2012). Although there is substantial scatter in this “isochron” diagram, the combined dataset appears to show two separate trends, one with a slope roughly corresponding to 2.6 Ga for the granites and most of the albite granites, with 147Sm/144Nd < 0.15, the other one for four samples of albite granites, all with 147Sm/144Nd > 0.15; in other words, those samples (from the Dosi, Dabla and Gothara intrusives) showing evidence for significant LREE depletion. The slope of the latter trend corresponds to about 1.4 Ga. We, therefore, speculate that selective LREE loss occurred during an advanced substage of albitisation through the partial loss of allanite, thought to be the major host of LREE in these rocks (see also Kaur et al. 2012). The apparent 2.6 Ga “isochron”, on the other hand, would be older than the actual emplacement of the granites, but would match the Nd model ages. Our tentative interpretation that allanite-related LREE loss occurred during albitisation, rather than via magmatic fractionation crystallisation of allanite, is consistent with the REE mineralisation in the Khetri complex noted by Knight et al. (2002). A severe disturbance in the Rb–Sr system is noted for all the analysed granite samples because of the high mobility of both Rb and Sr during metasomatism (Kaur et al. 2012). In the albite granites samples, which are devoid of both biotite and K-feldspar, the Rb/Sr ratios are close to zero because of extreme loss of Rb during albitisation of K-feldspar. Such samples show high initial 87Sr/86Sr ratios (0.710–0.725), but low 87Rb/86Sr ratios (Table 4). Kaur et al. (2012) inferred that these high “initial” Sr values in these albite granites are due to the severe loss of Rb, which effectively “froze” relatively radiogenic Sr at the time of albitisation. The whole-rock δ18O values (Table 4) for the original, the microcline–albite granites of Tehara and the biotitebearing albite granites of Bansiyal are almost identical (7.9–8.1 ‰). Generally, the albite granites have higher δ18O values than the original granites or the microcline– albite granites (Fig. 7), except for the Tehara albite granites, which possess somewhat lower δ18O values. Overall, there is an increasing trend of whole-rock δ18O values with progressive albitisation and Si enrichment (Fig. 7). In particular, the whole-rock δ18O values at Gorir show a significant increase from 8.1 ‰ in the microcline–albite granite to 10.0 ‰ in the albite granite with concomitant increase in Si (Fig. 7b). A similar relationship has also been observed for the Dosi, Biharipur and Dabla counterparts. 13 Int J Earth Sci (Geol Rundsch) Table 2 Whole-rock chemical compositions for the Bansiyal and Gothara albite granites, Rajasthan, NW India Sample no BS-4a BS-7 BS-8 GO-1 GO-2 GO-3 GO-4 GO-5 (Lat °N) 27°59′ 1.9″ 27°59′ 23.6″ 27°59′ 24.4″ 28°03′ 16.8″ 28°03′ 22.2″ 28°03′ 41.3″ 28°03′ 30.1″ 28°03′ 44.6″ (Long °E) 75°51′ 41.9″ 71.2 0.60 12.93 5.33 0.07 0.88 0.88 5.69 1.78 0.13 0.71 100.2 75°51′ 46.0″ 73.1 0.63 12.75 3.51 0.05 0.61 1.09 7.16 0.24 0.11 0.46 99.7 75°52′ 7.2″ 74.1 0.51 13.35 2.18 0.04 0.42 1.19 7.39 0.34 0.14 0.38 100.0 75°48′ 18.9″ 73.8 0.63 12.21 1.17 0.03 0.34 2.27 6.96 0.10 0.11 1.80 99.4 75°48′ 27.4″ 74.6 0.62 11.91 0.93 0.02 0.21 1.77 6.82 0.08 0.14 1.53 98.6 75°48′ 31.6″ 75.4 0.69 12.19 0.83 0.01 0.15 1.55 7.01 0.12 0.15 0.60 98.7 75°48′ 40.5″ 73.7 0.63 12.79 1.16 0.03 0.75 2.60 7.27 0.13 0.12 0.59 99.8 75°48′ 34.5″ 74.7 0.69 12.40 0.97 0.02 0.49 1.86 7.03 0.13 0.14 0.67 99.1 SiO2 (wt%) TiO2 Al2O3 Fe2Ot3 MnO MgO CaO Na2O K2O P2O5 LOI Sum V (ppm) 47 47 45 44 28 52 30 50 Cr Co Ni Zn Ga Rb Sr Y Zr Nb Ba Pb Th U Hf Ta La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ∑REE <10 20 6 7 14 29 16 38 283 17 93 7 38 4.9 7.9 1.8 69.1 118 12.9 46.1 9.4 1.36 7.8 1.2 6.5 1.2 3.9 0.57 3.4 0.50 281.5 <10 34 9 6 18 9.11 19 51 288 18 21 9 42 7.4 7.9 1.9 86.6 147 17.4 65.8 14.1 3.14 10.7 1.6 8.4 1.6 5.0 0.71 4.3 0.62 367.0 13 31 6 <5 21 6 13 58 251 19 20 <5 39 9.8 7.3 2.0 71.9 132 15.3 56.9 12.6 2.13 10.7 1.8 9.6 1.8 5.5 0.77 4.5 0.61 326.1 5 20 4 8 15 2.43 19 59 322 25 9 8 37 9.6 11.4 1 116 192 20.3 69.6 13.1 1.94 10.7 1.8 9.7 2.0 6.0 0.86 5.1 0.74 449.8 7 32 5 5 16 3 21 51 344 23 0 8 38.1 6.8 12.9 2.5 57.2 105 10.8 40.2 8.7 1.84 8.5 1.6 9.1 1.7 5.2 0.79 4.7 0.67 256.0 4 30 3 6 17 3 19 113 339 27 7 8 37.3 8.3 10.1 2.7 253 431 41.0 133 21.1 4.19 19.4 3.7 20.8 4.0 12.3 1.79 10.4 1.35 957.0 5 30 4 9 17 1.96 20 62 326 26 6 9 40.8 9.9 9.6 3.2 10.0 20.7 3.12 15.1 5.7 1.37 7.9 1.7 9.9 2.1 6.2 0.94 5.4 0.73 90.9 2 34 3 13 15 1.76 24 67 357 27 11 10 38.7 7.3 13.4 3.6 185 299 30.0 103 18.6 3.62 15.0 2.4 12.3 2.2 6.8 1.05 6.2 0.84 686.0 (La/Yb)N 13.8 13.7 10.9 15.5 8.3 16.5 1.26 20.3 13 Int J Earth Sci (Geol Rundsch) Table 2 continued Sample no BS-4a BS-7 BS-8 GO-1 GO-2 GO-3 GO-4 GO-5 6.2 (La/Sm)N 4.6 3.8 3.6 5.5 4.1 7.5 1.1 (Gd/Yb)N Eu/Eu* Th/U HPU 1.9 2.0 1.9 1.7 1.5 1.5 1.2 2.0 0.48 7.8 4.1 0.78 5.7 4.9 0.56 4.0 5.4 0.50 3.9 5.0 0.65 5.6 4.4 0.63 4.5 4.7 0.62 4.1 5.4 0.66 5.3 4.6 ASI 1.03 0.93 0.93 0.79 0.84 0.86 0.77 0.84 Eu/Eu* = EuN/(SmN × GdN)½. HPU = D[0.0967U (ppm) + 0.0263Th (ppm) + 0.035 K (wt%)] µWm−3 is heat production units (after Beardsmore and Cull 2001) where D refers to density in gcm−3 and is taken as 2.7 for all the samples a Data from Kaur et al. (2011a). Bold Rb value: data from TIMS (see Table 4) Discussion The occurrence of two kinds of albitised granites, viz. the microcline–albite granite and the albite granite, reflects the two different stages of progressive metasomatism. The almost complete replacement of magmatic oligoclase to albite and incipient alteration of K-feldspar in the microcline–albite granite indicate that these rocks are moderately metasomatised. The more severely metasomatised albite granites show nearly complete transformation of both microcline and oligoclase to albite. The albite granites are characterised by the whitening of feldspars, lack of foliation, occurrence of chessboard albite, presence of microcline as relics and exceptionally low K and high Na abundances. It is this difference in mineralogy and chemistry between the two variably metasomatised granites, which at times allows to clearly recognise the metasomatism on outcrop scale in the form of a reaction interface developed as a result of transformation of pink K-feldspar to white albite (Putnis and Austrheim 2010). That the original granite occurs in contact with the microcline–albite granite and not with the albite granite supports the previously proposed two-stage metasomatic replacement model for the albitised granites in the northeastern part of the Khetri complex (Kaur et al. 2012). The microcline–albite granite represents the first stage of albitisation (stage I), formed by the leading replacement front where oligoclase is converted to albite, and the albite granite is an outcome of the most advanced (second) stage of albitisation (stage II) where K-feldspar is transformed to albite by the second, slow-moving, trailing front. Isocon analysis In order to assess the chemical changes that resulted from metasomatism during the two stages, we present isocon diagrams (Fig. 8), following the approach of Grant (1986, 2005) and as more specifically outlined in Kaur et al. (2012). Only at Tehara, both stages (stage I and stage II) of metasomatism are preserved, whereas at Gorir, merely stage II can be observed. In the Bansiyal and Gothara intrusives, it is not possible to apply isocon analysis, as they completely consist of albite granites that are the final product of stage II metasomatism. As Al has remained conserved during the metasomatism of the Khetri ferroan granites (Kaur et al. 2012), conforming to the common immobility of Al during a metasomatic event (Grant 1986), the isocons are drawn through the data point of this element and the origin. Two isocon diagrams are presented for the Tehara intrusive, with the first (Fig. 8a) representing the chemical changes during alteration of the original granite (sample GL-7) to the microcline–albite granite (samples GL-2) and the second (Fig. 8b) documenting the changes during the transformation of the microcline–albite granite to the albite granite (average of samples GL-3 and GL-6). Element mobility during the stage I metasomatic alteration At Tehara, the stage I microcline–albite granite shows addition of H (for H2O, determined by loss on ignition, LOI), relative to the original granite, indicating hydration at this stage of metasomatism. This may be partly related to the formation of epidote and, more abundantly, to sericitisation of plagioclase in the microcline–albite granite. The minor gain in Na is reflected by the transformation of oligoclase to albite, whereas the slight Rb loss may be related to the incipient alteration of K-feldspar, although K has remained virtually immobile. The slight gain in Mg may be related to the relatively Mg-rich biotite in the microcline–albite granite (sample GL-2; Table 7). The minor gain in La is more likely to be a result of progressive fractional crystallisation in the original granite. The other elements plot either on or close to the isocon, indicating no significant losses or gains for most of the elements. Element mobility during the stage II metasomatic alteration The gains and losses are more pronounced during the stage II, i.e. the formation of the albite granite from the 13 Int J Earth Sci (Geol Rundsch) Table 3 Whole-rock chemical compositions for the Gorir albitised granites, Rajasthan, NW India Sample no Mc–Ab granite Albite granite GR-27 GR-1 GR-4 GR-5 GR-13 GR-15 GR-16 GR-17 GR-18 GR-20 GR-22 (Lat °N) 28°0′ 43.9″ 28°2′ 3.4″ 28°01′ 34.2″ 28°01′ 37.7″ 28°0′ 48.5″ 28°0′ 11.3″ 28°0′ 11.2″ 28°0′ 1.4″ 27°59′ 56.2″ 27°59′ 36.1″ 28°0′ 14.8″ (Long oE): 75°57′ 4.9″ 69.0 0.60 13.50 3.69 0.02 0.47 2.30 5.32 4.15 0.13 0.61 99.8 75°57′ 19.7″ 72.1 0.48 13.32 3.47 0.02 0.47 1.21 6.69 1.81 0.09 0.20 99.9 75°57′ 8.6″ 72.4 0.56 13.74 1.56 0.02 0.71 2.03 8.13 0.11 0.07 0.36 99.7 75°57′ 18.5″ 72.8 0.45 13.27 3.32 0.02 0.48 1.17 7.11 1.15 0.08 0.42 100.3 75°57′ 1.5″ 68.1 0.69 13.44 5.84 0.03 0.47 1.71 7.86 0.15 0.15 0.64 99.1 75°56′ 31.5″ 67.3 0.73 13.50 6.39 0.02 0.63 2.41 7.99 0.13 0.16 0.47 99.7 75°56′ 38.8″ 73.1 0.35 13.41 1.72 0.02 0.56 0.93 7.98 0.13 0.01 0.59 98.8 75°56′ 46.4″ 75.9 0.27 12.90 1.14 0.01 0.24 0.52 7.66 0.11 0.02 0.33 99.1 75°56′ 22.5″ 74.7 0.40 13.65 1.12 0.01 0.24 0.60 7.77 0.63 0.02 0.46 99.6 75°56′ 17.4″ 72.7 0.40 12.87 4.32 0.01 0.39 0.76 7.70 0.09 0.07 0.25 99.6 75°56′ 57.8″ 69.0 0.61 13.49 5.37 0.02 0.56 1.98 7.36 0.98 0.06 0.30 99.7 SiO2 (wt%) TiO2 Al2O3 Fe2Ot3 MnO MgO CaO Na2O K2O P2O5 LOI Sum V (ppm) 20 29 30 29 38 35 10 6 10 36 36 Cr Co Ni Zn Ga Rb Sr Y Zr Nb Ba Pb Th U Hf Ta La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu <10 34 <5 <5 21 67 22 82 372 18 272 <5 22 6 9.5 1.5 33.3 84 11 46.8 11.3 1.97 12.1 2.1 13.3 2.8 7.9 1.18 8 1.24 237.0 <10 44 <5 <5 16 42 34 72 339 17 343 <5 31 <5 11 32 <5 <5 17 <5 29 67 318 19 33 <5 32 3.7 8.7 1.6 47.6 97.7 12.0 45.9 10.7 1.75 10.2 1.8 10.0 2.1 6.4 0.98 5.9 0.86 253.9 <10 30 <5 <5 17 26 32 73 300 14 208 <5 29 <5 3 31 10 7 23 1.09 23 77 371 19 7 3 22.6 3.9 12.2 1.8 51.0 111 13.7 52.1 12.3 1.95 12.1 2.1 12.2 2.5 7.9 1.20 7.2 1.06 288.3 3 25 13 7 23 0.64 37 88 373 22 20 3 23.5 4 13.1 bdl 4 24 6 6 20 0.43 20 58 249 15 12 2 26.3 3.3 6.8 2.1 28.1 64.2 8.3 33.4 8.4 0.85 8.4 1.6 9.3 2.0 6.0 0.92 5.4 0.74 177.6 3 32 2 3 21 3 15 76 244 18 9 3 33.2 5.3 6.8 2.1 1 24 4 4 20 14.21 16 68 350 18 49 3 33.4 3.4 11.9 2.5 <10 31 6 <5 22 <5 17 71 466 21 <20 <5 31 <5 <10 27 9 <5 21 14 47 68 365 17 153 <5 22 <5 ∑REE 13 Int J Earth Sci (Geol Rundsch) Table 3 continued Sample no Mc–Ab granite Albite granite GR-27 GR-1 GR-4 GR-5 GR-13 GR-15 GR-16 GR-17 GR-18 GR-20 GR-22 (La/Yb)N 2.8 5.5 4.8 3.5 (La/Sm)N 1.8 2.8 2.6 2.1 (Gd/Yb)N Eu/Eu* Th/U HPU 1.2 1.4 1.4 1.3 0.51 3.7 3.5 >6 3.4 0.51 8.6 3.2 >6 3.2 0.49 5.8 2.6 5.9 2.7 0.31 8.0 2.7 6.3 3.8 9.8 3.3 >6 3.3 >4 2.7 ASI 0.79 0.89 0.81 0.89 0.85 0.78 0.90 0.95 0.94 0.92 0.81 Eu/Eu* = EuN/(SmN × GdN)½. HPU = D[0.0967U (ppm) + 0.0263Th (ppm) + 0.035 K (wt%)] µWm−3 is heat production units (after Beardsmore and Cull 2001) where D refers to density in gcm−3 and is taken as 2.7 for all the samples Bold Rb value: data from TIMS (see Table 4) microcline–albite granite, which is seen only in the Tehara and Gorir intrusives. At Tehara, the high field strength elements (HFSE) and the REE have remained immobile during this stage, whereas there are significant gains in Na and losses in Ba, Rb and K. These gains and losses can be attributed to the almost complete replacement of K-feldspar to albite by an allochemical, Al-conservative replacement reaction (e.g. Moore and Liou 1979):     + K+ aq + Na+ aq ⇔ NaAlSi3 Osolid KAlSi3 Osolid 8 8 (1) In addition, the gains in Mg and Ca and the attendant losses in Fe and H are reflected by the transformation of annite to magnesiohornblende in the albite granite. This may also point out that the granites experienced Mg and Ca metasomatism as the formation of magnesiohornblende requires more Ca and Mg, and lesser Fe and H2O than the annite. This can be shown below by a simplified, Al-conservative, allochemical reaction considering the biotite of sample GL-2 and the amphibole of sample GL-6:   K2.0 Mg1.0 Fe2+ 3.9 AlVI 0.5 Ti0.3 (OH)4 AlIV 2.5 Si5.5 O20 + 1.4Na+ annite + 3.4Ca+2 + 3.2Mg+2 + 2.7Fe3+ + 10.9H4 SiO4 =  3+ VI  IV 2Na0.3 Ca1.7 Mg2.1 Fe2+ 1.6 Fe1.2 Al0.1 (OH)2 Al1.0 Si7 O22 magnesiohornblende + 2K+ + 0.7Fe2+ + 0.3FeTiO3 + 0.8Na[AlSi3 O8 ] + ilmenite + 12.3H2 O + 19.0H Fig. 4 Rb (ppm) versus Na/K (cationic) diagram showing the depletion of Rb with progressive albitisation of K-feldspar. Data sources Dabla, Chaudhri et al. (2003), Biharipur, Kaur et al. (2006a) and Dosi, Kaur et al. (2012) albite (2) Reaction (2) mirrors, in a schematic way, the decomposition of annite to form magnesiohornblende with minor albite and ilmenite and that a part of the K loss can also be related to the breakdown of biotite to amphibole. For the Gorir rocks, we have carried out an isocon analysis between the representative albite granite sample GR-13 and the microcline–albite granite sample GR-27 because both occur in close proximity in the same outcrop (Fig. 2c). In addition to gains and losses observed at Tehara, the isocon diagram (Fig. 8c) suggests loss of Ca and significant gains in Fe and La with minor gain in Ti and P. These compositional changes are likely related to the partial transformation of diopside to amphibole (see above) because the resultant calcic amphibole is richer in Fe, Ti and poorer in Ca than its parent diopside. The gain in LREE is attributed 13 Int J Earth Sci (Geol Rundsch) Fig. 5 Chondrite-normalised REE patterns for the a Tehara, b Bansiyal, c Gorir, and d Gothara granites; primitive mantle (PRIMA)-normalised multielement patterns for the e Tehara original granite and the microcline–albite granites of Tehara and Gorir, and f albite granites of the studied intrusives. Chondrite-normalising and the primitive mantlenormalising values are after McDonough and Sun (1995). Data sources are the same as in Fig. 4 Fig. 6 Sm–Nd isochron diagram for all analysed granites. Tehara et al. include the newly studied intrusives of Tehara– Bansiyal–Gorir–Gothara and Dosi et al. comprise granites of Dosi–Biharipur–Dabla intrusives, and their data are taken from Kaur et al. (2012) 13 Sm (ppm) Nd (ppm) 147 143 Tehara pluton Original granite GL-7e Mc–Ab granite GL-2e Mc–Ab granite GL-5e Ab granite GL-3e Ab granite GL-6e 10.79 10.94 10.29 9.78 9.63 54.83 56.67 50.37 50.21 48.69 0.1189 0.1167 0.1235 0.1177 0.1195 0.511446 ± 8 0.511456 ± 8 0.511566 ± 7 0.511496 ± 7 0.511498 ± 8 −6.3 −5.7 −5.0 −5.1 −5.5 Bansiyal pluton Ab granite BS-4f BS-7 Ab granite 9.33 13.94 50.09 70.67 0.1125 0.1192 0.511371 ± 9 0.511438 ± 8 Gothara pluton GO-1 Ab granite GO-4 Ab granite GO-5 Ab granite 12.43 5.71 17.60 70.32 16.06 100.75 0.1068 0.2151 0.1056 Gorir pluton GR-27 Mc–Ab granite GR-13 Ab granite 12.00 GR-15 Ab granite 13.14 GR-16 Ab granite 7.89 56.40 59.71 33.50 GR-18 47.67 Sample a Rock type Ab granite 10.53 Sm/144Nda Nd/144Ndb εc Nd(t) Td(Ga) DM Rb (ppm) Sr (ppm) 87 Rb/86Sr 87 Sr/86Sr δ18O WR 2.59 2.54 2.49 2.50 2.52 180.54 144.33 115.60 1.30 1.30 26.04 23.10 17.65 18.08 10.27 20.94 18.78 19.74 0.21 0.37 1.158711 ± 10 1.102403 ± 7 1.131751 ± 13 0.726102 ± 8 0.729020 ± 15 −6.4 −6.6 2.60 2.61 30.61 9.11 13.72 14.62 6.62 1.81 0.860200 ± 15 0.746105 ± 10 8.1 9.0 0.511374 ± 10 0.512555 ± 7 0.511328 ± 8 −5.1 −5.7 -5.7 2.50 2.54 2.55 2.43 1.96 1.76 16.65 17.01 22.05 0.42 0.33 0.23 0.720563 ± 12 0.722464 ± 14 0.726434 ± 11 7.9 0.1286 0.1330 0.1423 0.511704 ± 11 0.511773 ± 9 0.511897 ± 9 −3.4 −3.0 −2.6 2.36 2.34 2.31 1.09 0.64 0.43 19.86 32.75 16.45 0.16 0.06 0.08 0.727634 ± 14 0.723522 ± 14 0.727682 ± 14 0.1335 0.511760 ± 13 −3.4 2.36 14.21 12.99 3.18 0.756933 ± 15 8.0 7.9 7.5 8.1 10.0 Estimated error for 147Sm/144Nd is less than 0.5 % b143 Nd/144Nd normalised to 146Nd/144Nd = 0.7219; Within run error expressed as 2σ in the least significant digits c Initial εNd values calculated using the present-day values of (143Nd/144Nd) = 0.512638 and (147Sm/144Nd) = 0.1966 for the chondritic uniform reservoir (Jacobsen and Wasserburg 1980), and the crystallisation age of 1,700 Ma for the granites d Depleted mantle model ages calculated using the two-stage model of Liew and Hofmann (1988) e Nd data from Kaur et al. (2007) f Data from Kaur et al. (2011a); Mc microcline, Ab albite, WR whole rock Int J Earth Sci (Geol Rundsch) Table 4 Nd, Sr and O isotopic systematics for the ferroan granites, Rajasthan, NW India 13 Int J Earth Sci (Geol Rundsch) Fig. 7 Whole-rock δ18O values against a an index of albitisation, the Na/K (cationic), and b SiO2 (wt%), displaying increasing values during progressive albitisation. Data source for Dosi, Biharipur and Dabla, Kaur et al. (2012) and symbols as in Fig. 4 to fractional crystallisation as in the case of the Tehara pluton. The Bansiyal intrusion consists only of albite granites, but their contiguous occurrence with the Tehara pluton suggests that the protolith of Bansiyal albite granites should be similar to that of the Tehara. This assumption is also supported by the similar ages of both intrusives (see above, field characteristics section) as well as almost identical REE patterns (Fig. 8c). Moreover, the biotites of the Bansiyal albite granite (sample BS-4) and the Tehara microcline–albite granite (GL8) have almost identical compositions (Fig. 11a). Despite this, it would not be appropriate to consider the microcline–albite granites of Tehara as protolith of the Bansiyal albite granites for isocon analyses because the latter intrusive shows two types of albite granites with different mafic mineralogy, i.e. either containing amphibole or biotite. The occurrence of amphibole-bearing albite granite at the margins and the biotite-bearing counterpart in the core of Bansiyal intrusive may indicate that the metasomatic fluid, when infiltrated from the margins towards the core, transformed biotite to amphibole at the margins. Subsequently, the Mg/Ca ratio became high 13 Fig. 8 Isocon diagrams (Grant 2005), illustrating the whole-rock chemical changes during the a stage I metasomatic alteration in the Tehara intrusive, i.e. transformation of the original granite to the microcline–albite granite; b stage II metasomatic alteration in the Tehara intrusive, i.e. transformation of the microcline–albite granite to the albite granite; and c stage II metasomatic alteration in the Gorir intrusive. Major oxides are given in wt% and trace elements in ppm; labels for major oxides are abbreviated to the elements Int J Earth Sci (Geol Rundsch) enough not to transform the biotite to amphibole in the core of the intrusive. Thus, the different nature of the mafic phases in Bansiyal albite granites is most likely due to variation in the fluid composition on a local scale. The proximity of the Gothara to Tehara intrusive may suggest that the albite granites of Gothara might have been transformed from a Tehara–like protolith. At the same time, it is noteworthy that the breakdown of biotite at Tehara has yielded amphibole of largely magnesiohornblende composition, whereas the Gothara albite granites are dominantly composed of actinolite (Fig. 11b), like the amphiboles of the Biharipur and Dabla albite granites. The variations in amphibole chemistry, however, most likely reflect the local compositional gradients in the evolving fluid phase. This is because the cationic composition of amphibole is dictated by crystal chemical constraints, induced on the crystal structure by Cl incorporated on the anion side (Kullerud 1996). Hence, the compositional variation in the secondary amphibole is principally controlled − by the fluid activity ratio a− Cl/aOH (Kullerud and Erambert 1999). Therefore, it is difficult to ascertain the protolith of the Gothara pluton, which may have been derived from a Dosi-like protolith, and it is also equally possible that these rocks have a similar protolith which crops out at Tehara. Magmatic characterisation and magmatic misclassification Based on the arguments presented above, the original and the microcline–albite granites can be characterised in magmatic classification and discrimination diagrams. However, geochemical characterisation of severely metasomatised rocks, like the albite granites where any reaction front is absent, can cause misleading geodynamic interpretations. In the CIPW normative Ab–Or–An granitoid classification diagram of Barker (1979), the microcline–albite granites of Tehara and Gorir can be classified as granite (Fig. 9a). The original granite of Tehara also falls in the same field, but more towards the Or corner, reflecting the higher amount of biotite in this rock. The same holds true for the biotite-bearing albite granite of Bansiyal. On the other hand, amphibole-bearing Bansiyal samples, which are devoid of significant amount of biotite, cluster at the Ab corner. Due to the extreme gain in Na and the loss in K during the second stage of metasomatism, the normative composition of the albite granites from Tehara, Gorir and Gothara isshifted towards, or tightly cluster at, the Ab corner, providing an erroneous notion that these rocks are trondhjemites. The original and the microcline–albite granites of Tehara are peraluminous (Fig. 9b) with aluminium saturation indices molar Al2O3/(CaO-3.3P2O5 + Na2O + K2O) (ASI or A/CNK) ≥ 1. In contrast, the albite granites of the same as well as other plutons, including the microcline–albite of Gorir, are metaluminous, having ASI < 1. The lowering in the ASI values at Tehara can be explained by the transformation of biotite (of the microcline–albite granite) to calcic amphibole (in the albite granite). During this stage of metasomatism, the albite granites gained Ca (reaction 2) and Na (reaction 1) with the concomitant additional loss of K due to decomposition of biotite, finally leading to relatively high CNK values resulting in their lower ASI. In the other studied granites, however, the original mafic phases, such as amphibole or clinopyroxene, were not K-bearing, and therefore, K was lost only as per reaction (1) without much affecting their ASI values. For example, the amphibole-bearing original, moderately and completely albitised granites at Dosi maintained their metaluminous character (Kaur et al. 2012). The classification diagrams discussed above demonstrate the significant role of mafic mineralogy of a metasomatised granite in its characterisation. For instance, an albite granite, derived from a Tehara-like peraluminous protolith, will be misinterpreted as a metaluminous trondhjemite in the absence of any reaction fronts between the original/microcline–albite granite and the albite granite. In a Fe-number [Fe* = wt% FeOt/(FeOt + MgO)] versus SiO2 diagram (Frost et al. 2001; Frost and Frost 2008), the original granite, all the microcline–albite granites and a few albite granites are classified as ferroan granites (Fig. 9c). The majority of the amphibole-bearing Tehara and Gothara albite granites extend towards the magnesian field because of their relatively low Fe*. This is reflected by the metasomatic transformation of biotite (XFe ~ 0.80) to amphibole (XFe ~ 0.55) in Tehara. An analogous assumption seems quite reasonable for Gothara, if actinolite (XFe ~ 0.30) was derived from a mafic phase with high XFe, from a Tehara-like or Dosi-like protolith. Some of the Gorir albite granites are ferroan, while others are magnesian as reflected by the degree of replacement of clinopyroxene by amphibole. In the albite granites that plot in the magnesian field, the clinopyroxene shows minor alteration to amphibole, whereas those falling within the ferroan field show relatively more alteration. In the microcline–albite granites extending to the ferroan field, clinopyroxene is considerably altered to Fe-rich amphibole. Therefore, in the absence of a reaction interface between the original granite and the microcline–albite granite, it becomes difficult to constrain the composition of pristine clinopyroxene and classify these rocks either as ferroan or magnesian. Similarly, the biotite-bearing Bansiyal albite granite is ferroan, whereas its amphibole-bearing counterparts straddle the boundary between ferroan and magnesian. This shows how misleading it can be to classify the metasomatised rocks when their modified geochemical characteristics are viewed as primary signatures. 13 Int J Earth Sci (Geol Rundsch) Fig. 9 Whole-rock chemical compositions of the Khetri ferroan granites in the a normative Ab–Or–An ternary diagram (Barker 1979); b Shand Index plot with discrimination fields after Maniar and Piccoli (1989); c FeOt/(FeOt + MgO) versus SiO2 (wt%) classification diagram (Frost et al. 2001). The Fe-number (Fe*) dividing line is after Frost and Frost (2008); d Rb (ppm) versus Y + Nb (ppm) tectonic discrimination diagram (Pearce et al. 1984); e Nb (ppm) versus Y (ppm) tectonic discrimination diagram (Pearce et al. 1984), and f Zr (ppm) versus 104 × Ga/Al classification diagram (Whalen et al. 1987). Data sources are the same as in Fig. 4 Although Rb is a highly mobile element, Pearce et al. (1984) used this element in their tectonic discrimination diagrams Rb versus SiO2 and Rb versus Nb + Y. The extreme loss of Rb in the Khetri albitised granites (see Fig. 4) gives rise to some interesting and potentially useful interpretations (Fig. 9d). The original granites of Tehara and Dosi fall in the within plate granite (WPG) field. The microcline–albite granites of Tehara and Gorir 13 Int J Earth Sci (Geol Rundsch) are slightly shifted vertically downward because of some Rb loss (see Fig. 8a) due to incipient K-feldspar alteration, but still plot well within the WPG field. The albite granites experienced substantial loss in Rb, together with K, as reflected by nearly complete replacement of K-feldspar and biotite, while Y and Nb remained immobile during the second stage of metasomatism (see isocon analysis section). Therefore, the albite granites display a vertical drop in Rb with respect to their protolith and thus plot in the ocean ridge granite (ORG) field. Some of the Gorir albite granite samples plot in the WPG field because they still retain some Rb substituting for K in minor biotite or relict K-feldspar or both. Similarly, the Bansiyal albite granites, which are biotite-bearing, plot in the WPG field, although below the original and microcline–albite granites of other intrusives, because they still retain some Rb and, with varying degree of alteration of biotite to amphibole, they range towards the ORG field. The same holds true for the transitional granites of Dosi and Dabla, which mostly plot close to or at the boundary between the WPG and ORG. The ORG affinity for the Gothara albite granites is an apparent case of “mistaken ORG identity” because Kaur and Mehta (2005), who studied these rocks in isolation, (mis)interpreted these as a plagiogranite occurrence of magmatic origin. The present study, in conjunction with the findings of Knight et al. (2002), demonstrates amply that the Gothara intrusive is an extremely metasomatised equivalent of the ferroan/A-type granites. Therefore, the existence of an oceanic magmatic activity in the Khetri complex is untenable. Finally, when the data of original, moderately albitised and completely albitised granites are plotted in an immobile trace element discrimination diagram (Fig. 9e), all the granites confirm their WPG affinity. Besides Al, the trace elements Zr and Ga behaved as immobile elements during metasomatism of these granites as shown by the isocon analysis (Fig. 8). The discrimination diagram of Whalen et al. (1987), using Zr versus Ga/ Al, classifies all the studied samples as A-type granites (Fig. 9f). Note the tight cluster shown by the Tehara and the Gothara granites. Nature of fluid The whole-rock δ18O of the studied plutons range between 7.5 and 10 ‰ (Fig. 7). It is noteworthy that during the transformation of the original granite to the microcline–albite granite, there is either virtually no increase (in Tehara) or a significant increase (in Dosi) in whole-rock δ18O values along with the increase in Si content (Fig. 7). Oxygen isotope analyses for mineral separates are not available for the studied intrusives. The conclusions presented below are therefore based on oxygen isotope patterns detailed in Kaur et al. (2012) for the Dosi, Biharipur and Dabla plutons. As compared to Dosi, there is no noticeable evidence of (re-)crystallisation of isotopically heavy new quartz at stage I in Tehara, and therefore, it does not show any appreciable increase in whole-rock δ18O values. This conclusion is based on the strong tendency of quartz to concentrate 18O at any given temperature (Zheng 1993), as well as isotopic bulk rock/quartz patterns presented in Kaur et al. (2012). The replacement of oligoclase by heavy albite has not contributed much to 18O enrichment. On the contrary, there is a significant increase in wholerock δ18O values during stage II of metasomatism, specifically in case of Gorir and Bansiyal, similar to that observed in the Dosi, Biharipur and Dabla plutons (Kaur et al. 2012). Also the Si enrichment is quite appreciable at this stage in all the above-mentioned intrusives. Hence, the increase in whole-rock δ18O values may be principally linked to the crystallisation of isotopically heavier Si-rich phases (amphiboles and albite), (re-)crystallisation of isotopically heavy quartz and to some extent to the replacement of microcline by isotopically heavy albite. Therefore, part of the whole-rock δ18O increase is the reflection of mineralogical transformation. During the stage II in Tehara, there is, however, a slight decrease in whole-rock δ18O values. This can be related, in parts, to the fact that amphibole forming from biotite was the significant mineralogical change that contributed to bulk Si enrichment. This is in contrast to the other intrusives where the breakdown of mafic phases is accompanied by albite crystallisation. The tendency of albite to concentrate 18O values is much stronger as compared to amphibole (Zheng 1993), resulting into a net increase in Si as well as 18 O in albite granites. Amphibole, however, still tends to be isotopically heavier than biotite (Zheng 1993); hence, the mineralogical change should have still resulted in a slight net increase in bulk 18O. One possibility is that the amount of biotite to amphibole transformation was too small to significantly alter the bulk rock composition. In the absence of 18O data specific for mafic phases, it can also not be excluded that slight local differences in the isotopic composition of the fluid phase, and/or alteration temperature, could have contributed to the isotopic pattern observed here. Moreover, biotite is also sometimes altered to chlorite, which could further lower the whole-rock δ18O values (especially if chlorite was formed as a response to an even later alteration event that involved an isotopically lighter fluid phase). Nevertheless, the overall enrichment in the whole-rock δ18O values with progressive albitisation and silicification (Fig. 7) and the fact that the metasomatism occurred immediately after the regional metamorphic event point to the metamorphic nature of the metasomatising fluid because meteoric or surface-derived waters are depleted in δ18O (δ18O = ~ 0 to −10 ‰, Sheppard 1986). Considering the 13 Int J Earth Sci (Geol Rundsch) pervasive nature of the regionally widespread metasomatism, Kaur et al. (2012) suggested that the required high “water–rock” ratios may ultimately point to a meteoric origin for the fluid, which acquired the metamorphic character after exchanging oxygen with the surrounding low-temperature metamorphic rocks. Metasomatism, metallogeny and ferroan granites In the northern Khetri complex Cu mineralisation, associated with minor Au and Ag, extends over a strike length of 10 km (Fig. 1c), and thus, this metallogenic province is more commonly known as the Khetri Copper Belt. In the southern domain, iron sulphide (pyrite) with minor Cu–Au constitutes an important ore deposit. Besides, U mineralisation occurs throughout the belt along with minor REE, Mo, Fe, fluorite and ilmenite–magnetite deposits (e.g. Das Gupta 1970; Ray 1987; Sarkar 2000; Singh 2000). The copper and iron sulphide mineralisation in the region was considered either to be sedimentary diagenetic (e.g. Sarkar and Dasgupta 1980; Sarkar 2000) or hydrothermal epigenetic (e.g. Roy Chowdhury and Das Gupta 1965; Das Gupta, 1970) in origin. Knight et al. (2002), for the first time, proposed that the Khetri complex is an important IOCG-mineralised province, covering a large area of 150 × 150 km. These deposits are comparable to the well-known IOCG sensu stricto deposits worldwide (Groves et al. 2010). In such deposits, Fe oxides are associated with Cu, Au, Ag, U, Ba, F and LREE and these are formed due to metasomatic activity where fluids and ore components are likely sourced from magmatic bodies (e.g. Pollard 2006). The results of our studies (Kaur et al. 2012; this study) demonstrate that during the second stage of metasomatism, a significant amount of Fe, Ti, U and REE were mobilised from the Khetri ferroan granites due to the breakdown of the mafic and accessory phases. It should be noted, however, that the REE mobility documented in the north-eastern Khetri ferroan granites (Kaur et al. 2012) is found to be negligible in the newly studied intrusives, except for Gothara. The loss of these elements is manifested by the ore formation in and around the ferroan granites. For example, a breccia with iron-rich bands occurs all along the western margin of the Gothara and almost along the entire margin of the Tehara intrusives. Earlier, these breccias were Temperature of albitisation We have used two-feldspar geothermometry to estimate the temperature of albitisation; for this, microcline–albite granites of Tehara and Gorir were considered. A number of feldspar pairs of pure albite and microcline were used to calculate equilibration temperatures (Table 5), employing the program SOLVCALC (Wen and Nekvasil 1994) and the thermometer of Fuhrman and Lindsley (1988). The other details of the method are outlined in Kaur et al. (2012). The coexisting feldspars yield exceptionally concordant temperatures for the Or, Ab and An components of all the analysed feldspar pairs, indicating that the feldspar compositions represent equilibrium (for more discussion on this aspect, see Kaur et al. 2012). The microcline–albite granites yield a discordant temperature (Tdis) range of 267–288 °C with very small errors of ±3 °C (Table 5). As lattice constants determined on an albite crystal from the Dosi granite conform to a completely ordered structural state (Kaur et al. 2012), this temperature was increased by about 100 °C to account for the approximation of the equilibrium temperature (Tord) for ordered feldspar pairs (Brown and Parsons 1989). The corresponding “ordered” temperature range of 370–390 °C conforms well to the Tord estimate of 360–395 °C obtained by Kaur et al. (2012) for the north-eastern Khetri intrusives. Moreover, quartz-feldspar oxygen thermometry yielded, within error, a nearly identical temperature range of 350–400 °C for the albitisation of the granites in the Khetri complex (Kaur et al. 2012). Table 5 Temperature (°C) estimates from the two-feldspar geothermometer for the ferroan albitised granites, Rajasthan, NW India Sample Rock type Tehara pluton GL-2 Mc-Ab granite GL-5 Mc-Ab granite GL-8 Mc-Ab granite Plagioclase K-feldspar Ab Or An Ab Or An TAb TOr TAn 95.6 97.3 96.9 0.4 0.4 0.5 4.0 2.3 2.6 5.4 5.5 6.0 94.6 94.5 94.0 0.0 0.0 0.0 280 278 288 280 278 288 280 278 288 98.4 0.5 1.1 5.1 94.9 0.0 267 267 267 n TDis TOrd 9 9 9 280 ± 3 278 ± 2 288 ± 3 380 380 390 10 267 ± 2 370 Gorir pluton GR-27 Mc–Ab granite n = Number of feldspar pairs used to calculate the temperatures; TDis = Equilibrium temperature for disordered feldspars; TOrd = Approximate equivalent temperatures for ordered feldspars, which is ~100 °C more than the TDis (Brown and Parsons 1989) Mc microcline, Ab albite 13 Int J Earth Sci (Geol Rundsch) thought to be of sedimentary origin (Gupta et al. 1998), but they are now interpreted as hydrothermal breccia, typical of many IOCG deposits (Williams et al. 2005). Small magnetite deposits also occur within some of the albitised granites (e.g. Gorir and Dhanota). Part of Fe, mobilised during this metasomatism, might have also been involved in the formation of Cu–Au ores, as most of these intrusives are spatially associated with these ore deposits. A very close spatial association of the Gothara intrusive with the main copper deposit (Madan Kudan mine) of the region is noteworthy (Fig. 1c); besides, numerous old workings of Cu occur in and around Bansiyal, Tehara and Dhanota. Although the U–REE–Mo–F mineralisation is regionally widespread, the major ore deposits are located in the southern part of the complex in association with severely albitised granites. Under oxidising conditions, the immobile U4+ is converted into highly mobile U6+, which can be easily leached out of the rocks during hydrothermal alteration. The occurrence of U deposits along with Cu–Au–REE and others exclusively in the regional-scale albite–haematite and calc silicate alteration zones in the region (Knight et al. 2002) supports that albitisation was accompanied by oxidation, which lead to the hydrothermal U loss. Knight et al. (2002) established a genetic and temporal relationship between the widespread Cu ± Au ± Ag ± Co ± Fe ± Mo ± REE ± U mineralisation and the regional metasomatic alteration (also see Yadav et al. 2000), and thus, there is an apparent synchronicity between the metasomatic activity and the metallogenic mineralisations in the Khetri complex. This metasomatic event is Early Cryogenian in age (850–830 Ma), which occurred nearly 900 Ma after the emplacement of the Khetri ferroan granites, and it closely followed the metamorphic activity (950–910 Ma) in the region (e.g. Knight et al. 2002; Kaur et al. 2013a). As this alteration event is associated with significant mobility of elements on a large scale, it requires long-term horizontal thermal gradients in the upper crust to serve as heat pumps for hydrothermal fluid circulation (for details see Kaur et al. 2012). The high U–Th contents of ferroan granites result in high internal heat production of such intrusives. For example, the original ferroan granite of Tehara has high present-day internal heat production of 5.3 µWm−3 (Table 1). The abnormally high heat flow values in the Khetri and adjoining regions, induced by the high internal heat production of the granites (Gupta et al. 1967; Sundar et al. 1990), support the existence of permanently elevated crustal temperatures in the region. Thus, the widespread occurrence of high heat producing ferroan granites in the northern Khetri complex may have provided the heat source for driving the metasomatising fluid circulation long after their emplacement. Additionally, a granitoid emplacement event at 830–820 Ma recorded in the adjoining areas (Kaur et al. 2013a and references therein) might have acted as an additional heat pump for the ore formation in the region. In this regard, the Khetri IOCG deposits are similar to the Fe–Cu–U–REE deposits of the Curnamona Province of South Australia, where the 1,557–1,579 Ma old ferroan granites were extensively metasomatised, more than 1 Ga later, at ~455 Ma and these granites provided the essential chemical constituents for the IOCG mineralisation in the province (Elburg et al. 2012). The present study in conjunction with the new results on the origin of IOCG deposits at Curnamona Province, therefore reiterates that ferroan granites, which are enriched repositories of Fe, U and REE, if suitably metasomatised can act as potential sources for important ore deposits. Conclusions Detailed mapping, petrography, whole-rock elemental and isotope geochemistry of the ferroan granites of the Khetri complex, in conjunction with previously published data, lead to the following conclusions: •฀ All the 1.72–1.70 Ga ferroan granites of the northern Khetri complex are metasomatised in two-discrete steps during a single metasomatic event at around 850–830 Ma by the infiltration of a low-temperature (~400 °C) meteoric fluid that acquired metamorphic character by exchanging oxygen with the surrounding metamorphic rocks. This produced two types of granites with different extents of albitisation: moderately albitised stage I and completely albitised stage II granites. During the stage I of metasomatism, the original (K-feldspar-oligoclase) granite was transformed into a microcline–albite granite, and during the stage II, the microcline–albite granite was finally converted to an albite granite. •฀ The rare preservation of the stage I metasomatic reaction front signifies the pervasive nature of metasomatism; the stage II is present in most of the plutons and is decipherable by the characteristic whitening of granite outcrops and by destruction of foliation. •฀ Isocon analyses reveal that most of the elements preserved their near-primary abundances during the first stage of metasomatism, except for some minor loss of Rb or a slight gain in Na. More pronounced losses in K, Rb, and Ba, and gains in Na, Mg, and Ca are recorded for the second stage of metasomatism, whereas Fe displays gain or loss depending on the original composition of the mafic phase involved. •฀ The results show that completely metasomatised granites should be studied with extreme caution, especially in the absence of any reaction interface either between original protolith and its relatively less altered deriva- 13 Int J Earth Sci (Geol Rundsch) tive or between the two albitised derivatives differing in the extent of albitisation. We demonstrate that the stage I microcline–albite granites can be correctly characterised in magmatic classification and discrimination diagrams, whereas the albite granites of the stage II, if studied in isolation, will lead to incorrect interpretations. The mafic mineralogy of the albitised granites can play a significant role in the characterisation of such granites. •฀ The close association of the high heat producing U–Thrich ferroan granites with Cu–Au, Fe-oxide and U mineralisation in the region suggests that such rocks, which are enriched repositories of Fe, REE and U, if suitably metasomatised, would act as a potential magmatichydrothermal system for driving the IOCG and U mineralisation. Acknowledgments We are grateful to Dmitry V. Kuzmin (Mainz), Uli Schuessler (Würzburg), Nora Groschopf (Mainz) and Doris Neuhäuser (Mainz) for their assistance in microprobe work, XRF analyses and clean laboratory, and Rosemarie Baur (Würzburg) for performing some of the XRF analyses. We thank two anonymous reviewers and the Editor Ingo Braun for their comments that led to improvement of the manuscript. Fruitful discussions with, and valuable suggestions by, Hartwig Frimmel (Würzburg) are warmly acknowledged. This work was supported by grants from the Department of Science and Technology, New Delhi (DST, SR/S4/ES-388/2008), and the German Academic Exchange Service, Bonn (DAAD, A/03/02882). Appendix: Detailed petrographic description Tehara At Tehara, the original granite is a medium-grained, pinkish grey coloured foliated granite and shows a subhedralgranular microstructure (Fig. 10a). It is composed of quartz, plagioclase, K-feldspar and biotite as the major minerals; titanite and apatite are dominant accessories, while zircon, tourmaline, muscovite, ilmenite and magnetite are subordinate. At places, biotite and feldspars are deformed. The plagioclase (An14.3±0.7, n = 10; Table 6) mostly shows albite twinning and is usually sericitised. K-feldspar (Or94.5±0.4, n = 10) largely displays tartan twinning typical of microcline and in places shows perthitic intergrowths. In contact with plagioclase, myrmekite intergrowths are occasionally noticed. Biotite, which imparts the foliation to the rock, is annite (Fig. 11a) and is quite uniform in composition with XFe [=Fet/(Fet + Mg)] of 0.80 (Table 7). Megascopically, the microcline–albite granite appears to be almost identical to the original granite with the same colour, microstructure and mineral content. However, except for minor relics of magmatic oligoclase (An9.7–14.0, n = 5), plagioclase in this rock is virtually 13 Fig. 10 Photomicrographs illustrating the textural and mineralogi-▸ cal aspects of the Khetri ferroan granites. a Original granite at Tehara showing a subhedral-granular microstructure and the foliation defined by biotite (crossed nicols); b Microcline (Mc) displaying its incipient alteration to untwinned albite (Ab) in Tehara microcline–albite granite (crossed nicols); c albite showing deformed twin lamellae in Tehara albite granite (crossed nicols); d chessboard–twinned albite, which typically develops due to complete replacement of K-feldspar by albite in Tehara albite granite (crossed nicols); e Bansiyal amphibole-bearing albite granite displaying evidence of transformation of biotite (Bt) to amphibole (Amp); (one nicol); f Gorir microcline– albite granite, illustrating partial replacement of clinopyroxene (Cpx) to amphibole (Amp); Zrn: zircon (one nicol); g Gorir albite granite, showing replacement of clinopyroxene (Cpx) to amphibole (Amp, bottom left) as well as that of amphibole to biotite (Bt); Ttn titanite (one nicol) and h Chessboard–type albite in Gothara albite granite, displaying graphic microstructure (crossed nicols) pure albite (An3.0±1.2, n = 29). The composition of microcline (Or94.2±0.8, n = 26) is almost identical to that of the original granite, and the mineral also shows its incipient replacement by untwinned albite (Fig. 10b). Biotite, which is still annite, shows more varied compositions (Fig. 11a), with the average XFe ranging between 0.80 (in sample GL2) and 0.73 (sample GL-8). Accessories are the same as in the original granite, while calcite and epidote occur as secondary minerals. The albite granite is megascopically medium grained, non-foliated and is distinctly recognised due to whitening of outcrop, which is an important field indicator of severe albitisation (e.g. Kaur et al. 2012; Petersson et al. 2012). K-feldspar is absent, and almost pure albite (An2.0±0.6, n = 40) is the dominant felsic phase; instead of biotite, amphibole is the main mafic mineral. Although this rock lacks foliation because of the transformation of biotite to amphibole (see below, the section on isocon analysis), the presence of bending in twin lamellae of albite that has formed from deformed plagioclase (Fig. 10c) indicates that the original rock was deformed prior to albitisation. Besides albite twinning, the albite also shows chessboard twinning (Fig. 10d), which is considered to have formed due to complete replacement of K-feldspar by Na-feldspar (e.g. Moore and Liou 1979; Slaby 1992; Kaur et al. 2012). In places, this chessboard-type albite also contains cuneiform quartz, which most likely represents earlier graphic intergrowths between K-feldspar and quartz; this feature is better developed and common at Gothara (see below). There is no compositional difference between chessboard- and albitetwinned albite. Bluish green amphibole is largely magnesiohornblende, with minor ferrohornblende and actinolite with XFe values of 0.46–0.65 (Fig. 11b; Table 8). In places, the amphibole has altered to form chlorite. The amounts of titanite and apatite are lower than in the microcline–albite granite, while the other accessories are the same. Allanite appears as an additional accessory mineral in the albite granite. Int J Earth Sci (Geol Rundsch) 13 13 Table 6 Average plagioclase and K-feldspar compositions for the ferroan granites, Rajasthan, NW India Pluton Tehara Rock type Or. granite (GL7) Bansiyal Gorir Gothara Tehara Gorir Mc–Ab granite Mc–Ab granite Ab granite (GL1, Ab granite (GL2, 5 & 8) (GL2 & 8) 3, 4 & 6) (BS4 & 7) Mc–Ab granite Ab granite (GR27) (GR4 & 16) Ab granite (GO1, Or. granite 2, 3 & 4) (GL-7) Mc–Ab granite Mc–Ab granite (GL2, 5 & 8) (GR27) n 10 (Pl) 29 (Pl) 5 (Pl) 40 (Pl) 35 (Pl) 10 (Pl) 15 (Pl) 35 (Pl) 10 (Kfs) 26 (Kfs) 8 (Kfs) SiO2 64.73 ± 0.31 0.01 ± 0.01 20.07 ± 0.16 0.09 ± 0.04 0.03 ± 0.03 0.00 2.98 ± 0.14 9.78 ± 0.21 0.11 ± 0.01 99.66 64.33 ± 0.99 0.01 ± 0.01 21.95 ± 0.33 0.04 ± 0.04 0.01 ± 0.01 0.00 0.63 ± 0.26 11.31 ± 0.26 0.09 ± 0.08 99.49 64.57 ± 0.45 0.04 ± 0.00 21.27 ± 0.51 0.14 ± 0.06 0.00 0.01 ± 0.01 2.44 ± 0.30 10.32 ± 0.21 0.07 ± 0.01 98.83 68.09 ± 0.60 0.01 ± 0.01 19.84 ± 0.18 0.04 ± 0.03 0.01 ± 0.02 0.01 ± 0.01 0.42 ± 0.13 11.43 ± 0.13 0.04 ± 0.02 99.87 68.21 ± 0.79 0.01 ± 0.01 19.91 ± 0.26 0.04 ± 0.02 0.01 ± 0.01 0.01 ± 0.01 0.60 ± 0.32 11.72 ± 0.20 0.07 ± 0.02 100.57 67.67 ± 0.49 67.73 ± 0.32 0.01 ± 0.01 19.92 ± 0.31 0.04 ± 0.04 0.01 ± 0.02 0.01 ± 0.02 0.45 ± 0.09 11.31 ± 0.31 0.07 ± 0.04 99.55 68.43 ± 0.13 0.01 ± 0.01 18.30 ± 0.15 0.01 ± 0.02 0.01 ± 0.01 0.00 0.01 ± 0.01 0.59 ± 0.05 15.69 ± 0.12 99.45 64.60 ± 0.37 0.01 ± 0.01 18.32 ± 0.15 0.04 ± 0.04 0.01 ± 0.01 0.00 0.00 0.63 ± 0.09 15.68 ± 0.15 99.29 63.71 ± 0.20 0.23 ± 0.15 11.43 ± 0.14 0.13 ± 0.05 99.05 68.54 ± 0.66 0.01 ± 0.02 19.27 ± 0.30 0.08 ± 0.04 0.01 ± 0.01 0.01 ± 0.01 0.22 ± 0.07 11.75 ± 0.29 0.12 ± 0.03 100.03 0.00 0.56 ± 0.05 15.88 ± 0.19 98.35 Cations per 32 oxygens Si 11.44 ± 0.03 Al 4.57 ± 0.03 0.01 ± 0.01 Fe+3 Ca 0.56 ± 0.03 Na 3.35 ± 0.06 K 0.02 ± 0.00 Sum 19.96 An 14.3 ± 0.7 Ab 85.1 ± 0.8 11.84 ± 0.07 4.16 ± 0.08 0.01 ± 0.01 0.12 ± 0.05 3.86 ± 0.09 0.02 ± 0.02 20.01 3.0 ± 1.2 96.5 ± 1.5 11.51 ± 0.05 4.47 ± 0.06 0.02 ± 0.01 0.46 ± 0.05 3.56 ± 0.10 0.02 ± 0.00 20.04 11.5 ± 1.5 88.1 ± 1.5 11.92 ± 0.04 4.09 ± 0.04 0.01 ± 0.00 0.08 ± 0.02 3.88 ± 0.04 0.01 ± 0.00 19.98 2.0 ± 0.6 97.8 ± 0.2 11.88 ± 0.07 4.09 ± 0.06 0.01 ± 0.00 0.11 ± 0.06 3.96 ± 0.07 0.02 ± 0.00 20.06 2.7 ± 1.5 96.9 ± 1.6 11.94 ± 0.04 4.06 ± 0.04 0.01 ± 0.01 0.04 ± 0.03 3.98 ± 0.04 0.03 ± 0.01 19.99 1.1 ± 0.7 98.3 ± 0.6 11.99 ± 0.03 3.97 ± 0.04 0.01 ± 0.01 0.04 ± 0.01 3.99 ± 0.12 0.03 ± 0.01 20.03 1.0 ± 0.3 98.3 ± 0.4 11.89 ± 0.04 4.12 ± 0.06 0.01 ± 0.01 0.08 ± 0.02 3.85 ± 0.10 0.02 ± 0.01 19.97 2.1 ± 0.4 97.4 ± 0.4 12.02 ± 0.02 4.00 ± 0.03 0.00 0.00 0.21 ± 0.02 3.71 ± 0.03 19.94 0.0 5.5 ± 0.4 12.00 ± 0.03 4.01 ± 0.04 0.01 ± 0.01 0.00 0.23 ± 0.03 3.72 ± 0.04 19.96 0.0 5.7 ± 0.8 11.98 ± 0.02 4.01 ± 0.03 0.02 ± 0.01 0.00 0.20 ± 0.02 3.81 ± 0.04 20.02 0.0 5.1 ± 0.5 Or 0.5 ± 0.5 0.4 ± 0.1 0.2 ± 0.1 0.4 ± 0.1 0.6 ± 0.3 0.7 ± 0.2 0.5 ± 0.2 94.5 ± 0.4 94.3 ± 0.8 94.9 ± 0.5 TiO2 Al2O3 FeO MnO MgO CaO Na2O K 2O Sum n number of analyses, Or. original, Mc–Ab Microcline–albite, Ab Albite 18.11 ± 0.16 0.10 ± 0.06 Int J Earth Sci (Geol Rundsch) 0.6 ± 0.1 19.51 ± 0.23 0.08 ± 0.04 Int J Earth Sci (Geol Rundsch) Fig. 11 Mineral compositions of the Khetri ferroan granites. a Classification of analysed biotite in the annite–siderophyllite–eastonite–phlogopite quadrilateral. Data for Dosi original granite from Kaur et al. (2012); b IMA classification of analysed amphiboles (Leake et al. 2004). Data for Dabla and Biharipur albite granites from Chaudhri et al. (2003) and Kaur et al. (2006a), respectively, and c IMA classification of analysed pyroxenes (Morimoto et al. 1988). Ab albite, Mc microcline Bansiyal At Bansiyal, the biotite-bearing albite–granite is medium grained, foliated, like the Tehara granite or microcline– albite granite, but is dark grey because of absence of pink feldspars. Accessory phases are apatite, zircon and secondary calcite along with relict K-feldspar, which shows irregular and diffused boundaries with albite (Kaur et al. 2011a). The amphibole-bearing albite granite is nonfoliated and shows whitening of feldspars. Moreover, 13 Int J Earth Sci (Geol Rundsch) Table 7 Average biotite and clinopyroxene compositions for the ferroan granites, Rajasthan, NW India Pluton Tehara Rock Or. Granite (GL-7) Mc–Ab granite (GL-2) Mc–Ab granite (GL-5) n 10 (Bt) 16 (Bt) SiO2 33.62 ± 0.32 2.98 ± 0.13 15.31 ± 0.12 29.15 ± 0.61 0.15 ± 0.04 3.93 ± 0.12 0.03 ± 0.02 0.06 ± 0.02 9.01 ± 0.22 94.23 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O F Cl Sum F=O Cl=O Sum Cations per 22 oxygens Si 5.48 ± 0.02 Ti 0.37 ± 0.02 Al 2.94 ± 0.03 3.97 ± 0.05 Fe+2 Mn 0.02 ± 0.01 Mg 0.95 ± 0.03 Ca 0.01 ± 0.00 Na 0.02 ± 0.01 K 1.87 ± 0.03 F Cl Sum 15.63 En Fs Wo XFe 0.81 ± 0.00 Bansiyal Gorir Gorir Mc–Ab granite (GL-8) Ab granite (BS-4) Mc–Ab granite (GR-27) Ab granite (GR-4 & 16) 17 (Bt) 10 (Bt) 35 (Bt) 5 (Cpx) 15 (Cpx) 33.86 ± 0.28 2.48 ± 0.12 16.10 ± 0.18 29.30 ± 0.35 0.19 ± 0.03 4.19 ± 0.14 0.01 ± 0.01 0.07 ± 0.03 9.18 ± 0.16 34.26 ± 0.60 3.28 ± 0.22 15.96 ± 0.74 28.63 ± 0.49 0.13 ± 0.05 4.57 ± 0.14 0.01 ± 0.02 0.06 ± 0.03 9.46 ± 0.15 34.46 ± 0.35 3.07 ± 0.18 15.65 ± 0.15 26.65 ± 0.42 0.12 ± 0.03 5.55 ± 0.13 0.03 ± 0.01 0.06 ± 0.03 9.27 ± 0.11 51.43 ± 0.12 <0.05 0.40 ± 0.17 12.86 ± 0.59 0.21 ± 0.03 10.27 ± 0.52 23.47 ± 0.26 0.76 ± 0.08 52.65 ± 0.35 0.05 ± 0.03 0.59 ± 0.16 12.30 ± 0.68 0.18 ± 0.06 10.74 ± 0.32 20.18 ± 1.00 2.14 ± 0.50 95.38 96.37 94.86 34.73 ± 0.43 3.30 ± 0.06 15.56 ± 0.31 26.28 ± 0.36 0.35 ± 0.03 5.27 ± 0.14 0.01 ± 0.01 0.07 ± 0.05 9.34 ± 0.12 0.12 ± 0.03 0.46 ± 0.02 95.49 0.05 ± 0.01 0.10 ± 0.01 95.34 99.40 98.83 5.45 ± 0.02 0.30 ± 0.01 3.05 ± 0.03 3.94 ± 0.05 0.03 ± 0.00 1.01 ± 0.04 0.00 0.02 ± 0.01 1.88 ± 0.03 5.44 ± 0.07 0.39 ± 0.02 2.98 ± 0.10 3.80 ± 0.08 0.02 ± 0.01 1.08 ± 0.04 0.00 0.02 ± 0.01 1.91 ± 0.03 5.50 ± 0.03 0.37 ± 0.02 2.94 ± 0.03 3.56 ± 0.07 0.02 ± 0.01 1.32 ± 0.03 0.00 0.02 ± 0.01 1.89 ± 0.02 15.68 15.65 15.61 0.80 ± 0.01 0.78 ± 0.01 0.73 ± 0.01 Cations per 6 oxygens 5.53 ± 0.05 1.98 ± 0.00 0.40 ± 0.01 2.92 ± 0.06 0.02 ± 0.01 3.50 ± 0.05 0.41 ± 0.02 0.05 ± 0.00 0.01 ± 0.00 1.25 ± 0.03 0.59 ± 0.03 0.00 0.97 ± 0.01 0.02 ± 0.01 0.06 ± 0.01 1.90 ± 0.02 0.06 ± 0.01 0.12 ± 0.01 15.75 4.04 29.8 ± 1.2 21.3 ± 1.2 48.9 ± 0.3 0.74 ± 0.01 0.41 ± 0.02 2.02 ± 0.01 0.00 0.03 ± 0.01 0.39 ± 0.02 0.01 ± 0.00 0.61 ± 0.02 0.83 ± 0.04 0.16 ± 0.04 4.05 33.2 ± 0.6 21.7 ± 1.5 45.0 ± 1.3 0.39 ± 0.02 n number of analyses, Or. original, Mc–Ab Microcline–albite, Ab albite in the latter rock, biotite occurs as a minor relict phase, and sometimes displays evidence of its transformation to amphibole (Fig. 10e); traces of apatite, titanite, allanite and secondary calcite occur as accessory minerals. In both the rocks, albite is the main felsic phase in addition to quartz, showing a compositional range of An3.4±1.3 (n = 21) to An1.6±0.7 (n = 14) in the biotite- and amphibole-bearing varieties, respectively. Biotite is sometimes deformed in the biotite-bearing albite granite; it is annite with XFe at 0.72–0.75 (Table 7) and has nearly the same composition to that of the microcline–albite granite of 13 Tehara (Fig. 11a). In the amphibole-bearing albite granite, amphibole is largely magnesiohornblende with minor actinolite (Fig. 11b). Gorir The microcline–albite granite at Gorir is brick red in colour due to haematite precipitation that accompanies albitisation (Engvik et al. 2008; Kaur et al. 2012); it is a medium-grained and non-foliated rock. The major minerals are quartz, nearly pure albite (An1.1±0.7, n = 10), Int J Earth Sci (Geol Rundsch) Table 8 Average amphibole compositions for the ferroan granites, Rajasthan, NW India Pluton Tehara Bansiyal Gorir Gothara Rock type Ab granite (GL1, 3, 4 & 6) Ab granite (BS4 & 7) Mc–Ab granite (GR27) Ab granite (GO1, 2, 3 & 4) n 48 22 10 50 SiO2 47.31 ± 2.34 0.28 ± 0.10 6.26 ± 1.89 0.01 ± 0.02 20.79 ± 1.53 0.30 ± 0.07 9.76 ± 1.26 11.16 ± 0.14 1.05 ± 0.31 0.18 ± 0.08 50.91 ± 1.08 0.32 ± 0.10 3.67 ± 0.89 0.01 ± 0.01 16.37 ± 0.80 0.45 ± 0.04 13.00 ± 0.66 10.98 ± 0.22 0.75 ± 0.20 0.23 ± 0.09 0.08 ± 0.03 0.08 ± 0.03 96.84 0.03 ± 0.01 0.02 ± 0.01 96.79 37.11 ± 1.44 0.20 ± 0.14 9.57 ± 0.74 <0.05 30.14 ± 0.59 0.21 ± 0.04 3.10 ± 0.31 10.80 ± 0.29 1.37 ± 0.16 2.62 ± 0.27 53.06 ± 1.67 0.07 ± 0.07 1.93 ± 1.24 0.02 ± 0.02 13.88 ± 1.52 0.37 ± 0.09 14.93 ± 1.23 12.58 ± 0.31 0.33 ± 0.20 0.06 ± 0.05 95.13 97.22 7.44 ± 0.13 0.56 ± 0.13 6.14 ± 0.15 1.84 ± 0.15 7.70 ± 0.18 0.30 ± 0.19 8.00 8.00 8.00 8.00 Al Ti Fe+3 Mg Fe+2 Mn 0.16 ± 0.09 0.03 ± 0.01 0.80 ± 0.31 2.17 ± 0.26 1.79 ± 0.27 0.04 ± 0.01 0.07 ± 0.04 0.04 ± 0.01 0.72 ± 0.12 2.83 ± 0.13 1.28 ± 0.13 0.05 ± 0.00 0.03 ± 0.03 0.03 ± 0.02 0.97 ± 0.16 0.76 ± 0.07 3.19 ± 0.14 0.02 ± 0.01 0.04 ± 0.04 0.01 ± 0.01 0.24 ± 0.15 3.23 ± 0.24 1.45 ± 0.18 0.04 ± 0.02 ΣC 5.00 5.00 5.00 5.00 Mg Fe+2 Mn Ca Na 0.00 0.00 0.00 1.79 ± 0.09 0.19 ± 0.05 0.00 0.00 0.00 1.72 ± 0.04 0.21 ± 0.05 0.00 0.01 0.01 1.92 ± 0.06 0.07 ± 0.06 0.00 0.00 0.01 1.96 ± 0.04 0.04 ± 0.03 ΣB 1.98 1.93 2.01 2.00 Na K 0.11 ± 0.10 0.04 ± 0.02 0.15 0.01 ± 0.10 0.04 ± 0.00 0.05 0.37 ± 0.05 0.55 ± 0.06 0.92 0.05 ± 0.04 0.01 ± 0.01 0.06 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O F Cl Sum F=O Cl=O Sum 97.10 Cations per 23 oxygens Si 7.06 ± 0.28 0.94 ± 0.28 AlIV ΣT VI ΣA Σ T ALL 15.13 14.97 15.91 15.06 XFe 0.55 ± 0.05 0.41 ± 0.02 0.85 ± 0.01 0.34 ± 0.04 n number of analyses, Or. original, Mc–Ab Microcline–albite, Ab albite K-feldspar (Or94.9±0.6, n = 8) and clinopyroxene. Secondary amphibole and calcite occur as minor phases, whereas titanite, apatite, zircon and fluorite are important accessories. K-feldspar occurs as microcline and is similar in composition to that of the Tehara counterparts. Green coloured clinopyroxene is diopside (Fig. 11c), uniform in composition with En31Fs20Wo49 to En28Fs23Wo49 and XFe at 0.39–0.44 (Table 7). The mineral generally shows its partial transformation to deep bluish green amphibole (Fig. 10f), which is quite variable in composition. Although it is largely potassic hastingsite (K > 0.50 apfu; Leake et al. 1997) to potassian Fe-edenite (with K = 0.43–0.44 apfu), 13 Int J Earth Sci (Geol Rundsch) Fe-actinolite, Fe-hornblende and potassian katophorite also occur in minor amounts (Fig. 10b). The albite granites are greyish white, non-foliated, quartz-albite rocks with clinopyroxene as the major mafic phase. K-feldspar, secondary amphibole and biotite occur in minor to accessory amounts. The other accessory minerals are same as in the microcline–albite granite. Albite is identical in composition (An1.1±0.3, n = 15) to that of the microcline–albite granite. Clinopyroxene is largely diopside (XFe 0.33–0.41), but has a higher Na/Ca ratio than diopside of the microcline–albite granite (Table 7). The mineral invariably shows its alteration to amphibole, which varies from minor to almost negligible in some samples (e.g. GR-4 and GR-16). In part of the samples, the amphibole is further replaced by biotite (Fig. 10g), and in such cases, titanite often occurs in close association with biotite. Gothara The Gothara pluton is entirely made up of medium-grained, non-foliated albite granites, which are greenish white to grey in colour. In some of the samples, albite forms porphyroblasts in a matrix consisting of quartz, amphibole and albite. 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