Abstract
The south margin of Central Altyn (SMCA) has long been considered as Precambrian stratigraphic unit of the Altyn Orogen. However, their strata attribution is subjected to challenge by the new finding of Siluran-Devonian metamorphic rocks. Aiming to further understand the geological evolution of the SMCA and its relations with the adjacent tectonic units, here we investigate the metasedimentary, metamafic and metagranitic rocks and in turn to re-examine the tectonic characteristics of the region. Detrital zircons from six metasedimentary rocks hold a sharp peak of ~ 452 Ma and moderate peak of 800–1600 Ma, whose spectra characteristics are similar to those of extensional basins formed under subduction setting of active continental margins. Two metamafic rocks and one metagranitic rock obtained crystallization ages for their protolith of c.456 Ma, c.462 Ma and c.457 Ma, which fell in the range of the 462–451 Ma magmatism of South Altyn (SA). These ages are widely distributed from west to east along the SMCA, and congruously indicate its stratum should formed in the Middle- Late Ordovician sedimentary environment under the extensional background of the SA subduction–early exhumation. All rocks record three stages metamorphism by zircons of ~ 430 Ma, ~ 400 Ma and ~ 377 Ma, consistent with previously reported HP pelitic gneiss. These protolith and metamorphic studies suggest that the SMCA should be a Paleozoic subduction-collision zone, rather than Mesoproterozoic sedimentary strata, and it should be separated from the Central Altyn block. Moreover, the similar protolith and metamorphic age have been also widely reported in the North Qaidam (NQ) and Dunhuang (DH) orogen, suggesting they may be the response of the same geological event in different tectonic units.
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Introduction
Subduction-collision events usually result in the juxtaposition or mixing of several unrelated rock units1,2,3. Ancient basement or sedimentary basins are often involved in younger subduction processes and preserved as old continental fragments or incomplete stratigraphic units, challenging the reconstruction of their geological history4,5,6. Understanding the intermittently emerging young rocks or young age information in a tectonic belt is the key to determine whether a tectonic attribute of a region is a “young orogenic belt” or an “ancient block”7.
The Altyn Orogen situated in the northeastern Tibetan Plateau (Fig. 1a). The Central Altyn (CA) block, also called the Milanhe-Jinyanshan block, represents a major Precambrian stratigraphic unit of the Altyn Orogen, extends ENE for over 400 km (Fig. 1b). The CA block is composed mainly of the Bashikuergan Group of the Changcheng period (Ch), the Taxidaban Group of the Jixian period (Jxz), the Suoerkuli Group of the Qingbaikou period (Qh)8. These groups are dominated by weakly metamorphosed clastic sedimentary rocks and volcanic rocks, which were previously interpreted as the Mesoproterozoic–Neoproterozoic sedimentary assemblages9,10,11. However, this interpretation has been challenged by lastest geochronological evidence.
Recently, a set of high-pressure pelitic gneiss has been discovered in the “Bashikuergan Group”8 in the Younusisayi area of SMCA (Fig. 1c). A NE-trending regional fault separates the “Bashikuergan Group” from the South Altyn (SA) complex. The gneiss recorded a clockwise P-T-t path with an Alpine type peak metamorphism of ~ 430 Ma12, and is inferred as the product of the Early Paleozoic Altyn orogenic event. Later, various types of Siluran-Devonian metamorphic rocks were found in the area13. Therefore, the attribution of the Precambrian strata of SMCA is challenged by the new finding of field-based lithological, geochronological and metamorphism study. There are two possibilities: (1) the ancient SMCA material was involved in the Altyn Early Paleozoic orogenesis process to form the Siluran-Devonian metamorphic rocks; (2) the SMCA is a set of young metamorphic strata.
In addition, current studies on the chronology of the “Bashikuergan Group” in SMCA area are relatively weak, and its strata ages are mainly determined by stratigraphic correlation16,17, lacking accurate chronological data. What is the formation age of these “Bashikuergan Group” in SMCA? Are there other Altyn formations among them? What is the connection with the SA subduction-collision-exhumation process? All these problems restrict the understanding of the Altyn tectonic framework.
In this contribution, we present detailed zircon U–Pb chronological studies on metasedimentary, metamafic and metagranitic rocks from SMCA, with aims to: (1) constrain the formation age of the SMCA; (2) constrain and compare their metamorphism; (3) explore the relationship between SMCA formation and SA subduction-exhumation by integrating with previous studies; (4) provide information about the tectonic evolution of SMCA during Early Paleozoic and enable correlations with other units adjacent to Altyn Orogen.
Geological backgrounds
The Altyn orogenic belt comprises four units with a triangular NE–SW extension, namely (from north to south): the North Altyn Tagh (I), the North Altyn subduction-accretion belt (II), the Central Altyn block (III), the SA subduction-collision belt (IV) (Fig. 1b).
Unit I was dominated by Archean-Proterozoic metamorphic rocks with an affinity to the Tarim Craton8, with major lithologies of ca. 3.7 Ga and 2.8–2.5 Ga tonalite-trondhjemite-granodiorite gneiss, 2.0 Ga felsic and mafic gneiss, 1.9 Ga paragneiss, and 1.85 Ga granitic veins and mafic dykes9,10,18,19.
Unit II was consist of supra-subduction zone (SSZ)-type ophiolitic mélanges (520–480 Ma)20,21, flysch sediments, magmatic rocks (520–400 Ma)22,23,24,25,26,27, LT–HP blueschist and eclogite (520–491 Ma)28,29. The Unit II has been considered as an early Paleozoic accretionary orogenic belt30.
Unit III was mainly composed of Meso- to Neoproterozoic clastic and volcanic rocks of Changcheng System (Ch) Bashikuergan Group, Jixian System (Jx) Taxidaban Group and Qingbaikou System (Qb) Suoerkuli Group8. Bashikuergan Group (Ch) is the metamorphic basement, consisting of Mesoproterozoic sandstones, marbles, weakly metamorphic phyllites and schists, and the overlying thick-bedded limestones (Jinyanshan Group, Jx). The Taxidaban Group (Jx) predominately consists of clasolites in lower part and thick limestones in upper. The Suoerkuli Group (Qb) overlied the former strata along an unconformity. It is composed of conglomerates, sandstones, dolomites, killas and limestones with stromatolites. Minimum ages from detrital zircons in Suoerkuli Group showing 1.3 ~ 1.2Ga, limit the sedimentary period of the protolith. These sedimentary strata were invaded by 930 ± 10 Ma rhyolites and 922 ± 6 Ma granites9, indicating their formation predates the Neoproterozoic.
Unit IV comprises two parts, the Mangya ophiolite tectonic mélange belt and the SA HP-UHP metamorphic belt (Fig. 1b), which recording the converging and rifting of the Rodina supercontinent14,15,31,32, and the opening and closure of the Proto-Tethys Ocean33,34,35. The SA HP-UHP metamorphic belt consists mainly of granitic gneiss and minor paragneisses of different metamorphic grade, together with corresponding-grade eclogite, granulite, and garnet amphibolite lenses14,31. The HP-UHP rocks record the protolith age of 950–730 Ma14,31, the peak metamorphic ages of 505–485 Ma15,36,37,38, and two stages of retrograde metamorphism of 485 − 450 Ma and ~ 420 Ma31,39,40,41. Granitic magmatism are widespread in the SA, which generated mainly in four episodes. These magmatic rocks were derived from various magma source under different tectonic setting, including the partial melting of the subducted oceanic crust (ca. 517 Ma, adakitic granitoids), the thickened continental crust due to continental subduction (501–496 Ma, adakitic granitoids), the mid-upper crust in response to slab break off at (462–451 Ma, I- and S-type granitoids), and the tectonic transition from contraction to extension at (426–385 Ma, A-type granitoids)42,43,44,45,46.
Sample location and selection
For the study, nine samples were collected along the SMCA, including arenous biotite–plagioclase (Bt–Pl) gneiss (18AM051, 18AM052, 18AM053) and argillaceous Bt–Pl gneiss (18AM071, 18AM072, 18AM073) from Munabulake, granitic gneiss (17A15) and amphibolite (17A16)from Yaolesayi, biotite–amphibole (Bt–Amp) gneiss (18AJ01) from Jianggalesayi (Fig. 1c). The sampling areas is located in the south margin of the Central Altyn block (Fig. 1c), where originally designated as “Bashikurgan Group” (Ch), here we called it SMCA. And it adjoins the SA HP–UHP metamorphic belt to the south and are bounded by regional faults (Fig. 1b, c).
Sample 18AM051, 18AM052 and 18AM053 are taken from moderately deformed Bt–Pl gneiss (Fig. 2a-c) that is dominated by feldspar and contains a small amount of biotite (Fig. 3a-c). The protolith of the Bt–Pl gneiss may be sandy sedimentary rock. Sample 18AM071, 18AM072 and 18AM073 are sampled from strongly deformed Bt–Pl gneiss (Fig. 2d-f) that is composed of Pl + Bt + Qz, with plagioclase augens (Fig. 3d-f). The biotite content of these rocks is relatively high, and the protolith should be argillaceous sedimentary rocks. The mafic interlayer has been identified in the local outcrop of the gneiss (Fig. 2e). Sample 17A15 is composed of Kfs + Pl + Qz + Bt (Figs. 2g and 3g). Sample 17A16 is composed of Cpx + Amp + Pl + Qz + Bt + Ms, with transitional structure from Cpx to Amp and multi-stage component zoning of plagioclase (Figs. 2h and 3h). Sample 18AJ01 is composed of Amp + Bt + Pl + Qz + Ms (Fig. 3i) and strongly weathered (Fig. 2i). Ma12 reported that the amphibole transformed into biotite coronal (Fig. 4b), and recorded growth zoning from core to rim with decreasing Si (p.f.u) and Mg (p.f.u) and increasing Al (p.f.u). Mineral abbreviations here are after Whitney and Evans (2010)47.
Field photographs taken from the SMCA. (a-c) Arenous pararocks from Munabulake area; (d-f) Argillaceous pararocks from Munabulake area; Metagrantic rocks from Yaolesayi area; (h) Metabasites from Yaolesayi area; (i) Metabasites from Jianggalesayi area.
Microscopic photographs showing the mineralogy and texture of analyzed samples. (a) Sample 18AM051, plane polarized light; (b) Sample 18AM052, crossed polarized light; (c) Sample 18AM053, BSE image; (d) Sample 18AM071, plane polarized light; (e) Sample 18AM072, crossed polarized light; (f) Sample 18AM073, BSE image; (g) Sample 17A15, crossed polarized light; (h) Sample 17A16, crossed polarized light; (i) Sample 18AJ01, plane polarized light.
Cathode luminescence (CL) images of representative zircons from different samples. The meanings of the different colored circles are clarified as: Purple - protolith; Red - metamorphic ages of ~ 430 Ma; Blue - metamorphic ages of ~ 400 Ma; Green - metamorphic ages of ~ 375 Ma.
Sample analytical method
The analyses were completed at the Wuhan SampleSolution Analytical Technology Co., Ltd and the State Key Laboratory of Continental Dynamics, Northwest University, P.R.China.
Zircon grains were separated using standard heavy liquid and magnetic techniques from different samples. Intact grains were selected for target preparation comma and epoxy resin was polished to expose the center, which was followed by transmitted and reflected light photographing and cathodoluminescence (CL) imaging. CL images were obtained from Quanta 400FEG environmental scanning electron microscope equipped with an Oxford energy dispersive spectroscopy system and a Gatan CL3+ detector. Representative CL images of zircons from all samples are presented in Fig. 5. U-Pb isotopes and trace element for zircons were analysed using LA-ICP-MS equipment, consisting of an Agilent 7500c connected to a 193 nm ArF Excimer laser with an automatic positioning system. Synthetic silicate glass NIST 610 was used as the optimizing standard for instrument calibration, Harvard zircon 91,500 as the external standard, and29 Si as the internal standard for element concentrations. U-Pb isotopes and trace elements were obtained from spots with a diameter of 24 μm and a depth of 20–40 μm. Isotopic ratios were calculated using the ICPMSDataCal 8.6 program, and age calculations and concordia plot integration were performed using ISOPLOT 3.048. Detailed analysis steps and data processing methods are presented in Yuan et al.49.
Th/U ratios of representative zircons from different samples.
Zircon geochronology of the SMCA
Metasedimentary rocks
Six representative pararocks were selected for chronological analysis.
Detrital zircons from the studied samples are colorless to light yellow, and 100–300 μm in length with aspect ratios of 1.0–3.0 (Fig. 5a-f). Most zircon grains from samples 18AM051, 18AM052 and 18AM053 are rounded or ellipsoidal and generally less than 150 μm in length (Fig. 5a-c), indicating that they have experienced long-distance transportation and abrasion. Most zircon grains from sample 18AM071, 18AM072 and 18AM073 are euhedral to subhedral with a few prismatic grains (Fig. 5d-f). Despite the discrepancy in psephicity, most zircons (~ 90%) show a common morphologic feature of core–rim structures. Zircons display clear oscillatory zoning in the core (Fig. 5a-f) and have high Th/U ratios (80% grains > 0.2) (Fig. 6a-f; Tables 1, 2, 3, 4, 5 and 6), implying that they were derived from intermediate–acid igneous protoliths50,51. Zircons display gray planar or weak fir-tree or sector zoning in the rim (Fig. 5a-f) and have low Th/U ratios (most grains < 0.1–0.2) (Fig. 6a-f; Tables 1, 2, 3, 4, 5 and 6), implying that they underwent metamorphic growth as rims52,53,54,55,56,57. A few zircons underwent metamorphic growth as single grain (Fig. 5f).
Total 216 detrital zircon spots (zircon core of sample 18AM051, 18AM052, 18AM053, 18AM071, 18AM072 and 18AM073) were analysed for U-Pb dating, and 208 concordant analyses (24 points > 80%, 47 points > 90%, 137 points > 95%) were accepted (Fig. 7a-f). They define a wide range of 207Pb/206Pb (> 1.0 Ga) and 206Pb/238U (< 1.0 Ga) ages from 3063 Ma to 442 Ma (Tables 1, 2, 3, 4, 5 and 6). On the probability density distribution plots, detrital zircons show a prominent age peak of the Paleozoic zircons (the minimum age cluster, 451.8 ± 2.9 Ma) and several subordinate age peaks of the older Precambrian zircons (800–1600 Ma) (Fig. 8a). Total 91 metamorphic zircon spots (zircon rim) were analysed for metamorphic dating, and 83 concordant analyses (6 points > 80%, 17 points > 90%, 60 points > 95%) were accepted (Fig. 7a-f). In the zircon U–Pb concordia diagrams, sample 18AM052 and 18AM053 have one metamorphic age group of ~ 430 Ma (Fig. 7b, c); sample 18AM051 and 18AM072 have two age group of ~ 430 Ma and ~ 400 Ma (Fig. 7a, e); Sample 18AM071 and 18AM073 have three age group of ~ 430 Ma, ~ 400 Ma and ~ 377 Ma (Fig. 7d, f). On the probability density distribution plots, metamorphic zircons of six samples synthetically show three prominent age cluster of 432.7 ± 2.8 Ma, 400.9 ± 3.6 Ma and 376.8 ± 3.3 Ma (Fig. 8b).
Detrital (a) and metamorphic (b) zircon age spectra. Sources of data are available from sample 18AM051, 18AM052, 18AM053, 18AM071, 18AM072 and 18AM073.
Chondrite-normalized REE pattern of representative zircons from different samples. Normalization after Sun and McDonough, 198958. The trace elements are presented in Table S1–S7. The trace elements data of metamafic rocks will be published in other papers. The meanings of the different colored lines are clarified as: Purple - protolith; Red - metamorphic ages of ~ 430 Ma; Blue - metamorphic ages of ~ 400 Ma; Green - metamorphic ages of ~ 375 Ma.
Trace-elements patterns from 216 detrital zircon spots show high REE and HREE contents with strong negative Eu anomalies (Fig. 4a-f; Table S1-S6). Most metamorphic spots of ~ 430 Ma show moderate REE contents and flat HREE patterns, with moderate negative Eu anomalies (Fig. 4a-f). Spots of ~ 400 Ma show low REE contents and moderate HREE patterns, with moderate negative Eu anomalies (Fig. 4a, d-f). Spots of ~ 377 Ma show low REE and HREE contents, with strong negative Eu anomalies (Fig. 4d, f).
Metagranitic rocks
Sample 17A15 were selected for chronological analysis. Zircons from the studied samples are colorless, and 50–150 μm in length with aspect ratios of 1.0–3.0 (Fig. 5g). Most zircon grains are euhedral to subhedral with a few prismatic grains (Fig. 5g). CL images show a common morphologic feature of core–rim structures (Fig. 5g). Zircons display clear oscillatory zoning in the core (Fig. 5g) and have high Th/U ratios (> 0.2) (Fig. 6g; Table 7), implying that they were derived from intermediate–acid igneous protoliths. Zircons display gray planar zoning in the rim (Fig. 5g), implying that they underwent metamorphic growth as rims.
Total 24 magmatic zircon spots (zircon rim) were analysed for metamorphic dating, and 15 concordant analyses (> 95% and concentrated between 447 and 459 Ma) were accepted. The U–Pb analytical spots (Table 7) plot on or close to the concordia line (Fig. 7g), forming one age groups, and yielding a weighted mean age of 452.0 ± 2.2 Ma (MSWD = 1.40). Regretfully, the rims are too narrow to get an accurate metamorphic age.
Zircon trace-elements data show high REE contents (ΣREE = 684.37–3569.84 ppm) with strong negative Eu anomalies (Fig. 4g; Table S7).
Metamafic rocks
Sample 17A16 and 18AJ01 were selected for chronological analysis to define the lower limit of the protolith formation time13.
Sample 17A16
Zircons from Sample 17A16 are colorless, and 100–200 μm in length with aspect ratios of 1.0–2.0 (Fig. 5h). Most zircon grains are euhedral to subhedral (Fig. 5h). CL images show a common morphologic feature of core–rim structures (Fig. 5h). Zircons display dark weak oscillatory zoning in the core (Fig. 5h) and have high Th/U ratios (> 0.52) (Fig. 6h), implying that they were derived from mafic igneous protoliths. Zircons display gray planar zoning in the rim (Fig. 5h) and have low Th/U ratios (< 0.50) (Fig. 6h), implying that they underwent metamorphic growth as rims.
Total 24 zircon spots were analysed and 23 concordant analyses (> 90%) were accepted. The U–Pb analytical spots plot on or close to the concordia line (Fig. 7h), forming two age groups. Ages of 18 spots range from 468 to 456 Ma and yield a weighted mean age of 462.4 ± 1.4 Ma (MSWD = 1.02). Ages of 5 spots range from 437 to 430 Ma and yield a weighted mean age of 433.5 ± 5.4 Ma (MSWD = 0.24).
Zircon trace-elements data from 23 spots show two distinct trace-element patterns (Fig. 4h). 18 spots show high REE contents (ΣREE = 741.48–6416.20 ppm) with strong negative Eu anomalies. 5 spots have low REE contents (ΣREE = 85.76–975.16 ppm) with moderate negative Eu anomalies (Fig. 4h).
Sample 18AJ01
Zircons from Sample 18AJ01 are colorless, and 150–250 μm in length with aspect ratios of 1.0–3.0 (Fig. 5i). Most zircon grains are euhedral to subhedral with a few prismatic grains (Fig. 5i). CL images show a common morphologic feature of core–rim structures (Fig. 5i). Zircons from Sample 18AJ01 could divided into two groups by CL images. Group I zircons have core–rim structures with nebulae or homogeneous zoning and have low Th/U ratios (50% < 0.10) (Fig. 6i), indicating a continuous metamorphic growth process59,60. Group II zircons display distinct oscillatory zoning in the core of magmatic origin51,55, and show light rims of metamorphic overgrowth.
Total 42 zircon spots were analysed and 41 concordant analyses (> 80%) were accepted. The U–Pb analytical spots plot on or close to the concordia line (Fig. 7i), forming four age groups. Ages of 8 spots of magmatic ages range from 459 to 451 Ma and yield a weighted mean age of 455.9 ± 2.6 Ma (MSWD = 0.48). Ages of 13 spots range form 434 to 427 Ma and yield a weighted mean age of 431.8 ± 2.3 Ma (MSWD = 0.56). Ages of 13 spots range form 405to 396 Ma and yield a weighted mean age of 400.2 ± 2.2 Ma (MSWD = 0.39). Ages of 7 spots range from 381 to 373 Ma and yield a weighted mean age of 377.1 ± 4.1 Ma (MSWD = 0.21).
Zircon trace-elements of four age groups have chaotic distribution pattern (Fig. 4i).
Discussions
Redefining formation age of “bashikuergan group” in the SMCA
The “Bashikuergan Group” was created in Hongliugou-Beiketan area from North Altyn. Later, due to Chinese translation and other problems, it was renamed “Bashikuergan Group”, and has been used until now. The Bashikurgan Group was originally defined as a metamorphic volcanic-sedimentary successions that was unconformably overlain by the Jixian System. It could be subdivided into three formations, from the bottom up, named Zhasikansaihe Formation (Chz.), the Hongliuquan Formation (Chh.) and the Beiketan Formation (Chb.)8. Recently research of regional investigation and geochronological data took a new understanding on the stratigraphic division of the original “Bashikuergan Group” in the Hongliugou-Beiketan area of North Altyn. The so-called “Bashikuergan Group” is actually composed of multiple tectonic slabs or blocks. Lithologic assemblages are mainly sericoid chlorite quartz schist, HP pelitic rocks, bluesschist, eclogite, marble, quartzite, metamorphic sandstone, ultramafite, pillow tholeiitic basalt, siliceous rock, etc. In the Hongliugou-Beiketan area, this set of rock unconformably lies under the thick Ordovician limestone. Recently, tholeiitic basalt of 524–508 Ma20,21,61, blueschist and eclogite of 520–491 Ma (Ar-Ar ages of phengite and paragonite)28,29, gneissic granodiorite of 481.5 ± 5.3 Ma, quartstone of 497 ± 5 Ma (maximum depositional age) in the Hongliuquan Formation11, plagiogranite of 501 ± 3 Ma and granodiorite of 496 ± 2 Ma in the Beiketan Formation62, and A-type granite of 857–851 Ma have been successively reported in the “Bashikuergan Group” in the Northern Altyn. Hao63 obtained the minimum zircon U-Pb ages of ~ 1117 Ma, 816 Ma, 780 Ma, 777 Ma, 775 Ma, 773 Ma, 730 Ma, and 586 Ma through detailed chronology analysis of several sets of detrital samples from the Bashikuergan Group. It can be observed that a significant portion of the original “Bashikuergan Group” in the Hongliugou-Beiketan area is likely part of the North Altyn subduction-accretion belt and should be disintegrated.
The “Bashikuergan Group” in the SMCA is mainly distributed in the northern parts of Munabulake, Jianggalesayi, Yunusisayi and Yaolesayi area (Fig. 1b). The strata is mainly determined by comparing with typical profiles of Zhasikansaihe (Chz.), the Hongliuquan (Chh.) and the Beiketan (Chb.) formations in North Altyn16,17. Recent studies on stratigraphic chronology show that the age of the original “Bashikuergan Group” in the SMCA are generally quite young. Cao et al.64 discovered identified HP pelitic granulites in the Hongliuquan formations in the Munabulake area, with the protolith age of 579 Ma and metamorphic age of 486 ± 5 Ma, which was considered to be disintegrated as part of the SA HP-UHP belt. Later, Cao et al.65 discovered monzonite gneiss and quartz schist in the Hongliuquan and Zhasikansaihe Formation, respectively. Detrital zircon studies show that the formation ages of the protolith rocks are between 840 and 498 Ma and 647–500 Ma, respectively, indicating that the two protolith rocks should have formed after the Early Cambrian.
In this study, the minimum detrital zircon age cluster of 451.8 ± 2.9 Ma is obtained from six Munabulake paracetamorphic rocks; two crystallization ages of mafic rocks of 455.9 ± 2.6 Ma and 462.4 ± 1.4 Ma obtained from Jianggalesayi Bt/Amp- gneiss and Yaolesayi amphibolite; one crystallization age of granitic rock of 452.0 ± 2.2 Ma obtained from Yaolesayi granitic gneiss. These ages are distributed from west to east along the SMCA, and congruously indicate that the main body of the SMCA should formed in the Middle- Late Ordovician (460–450 Ma). Thus, the SMCA should be a young geological unit, rather than the Meso- to Neoproterozoic sedimentary strata as previously suggested8. Further, the SMCA should be separated from the proto- Bashikuergan Group in the Central Altyn block in future studies.
Meanwhile, the metamorphic ages of ~ 430 Ma were recorded by all samples, and metamorphic ages of ~ 400 Ma and ~ 375 Ma were recorded by metasedimentary and metamafic rocks. These chronological records of metamorphic evolution are in complete agreement with previously reported high-pressure pelitic gneiss and mafic granulite in the Younusisayi area12,13, which together indicate that SMCA underwent Siluran-Devonian metamorphism.
Possible background of SMCA formation
Detrital zircon age spectra have characteristic distribution patterns that reflect the tectonic setting of the basin where they formed66. From the dating results, it can be inferred that the SMCA was formed at ~ 450 Ma (maximum deposition age). The spectra of young detrital zircons with high deposition flux and old detrital zircons with relatively low flux and wide intervals indicate that the sedimentary basin received detrital material both from ancient crust and synsedimentary magmatic activity67. Detral zircons from six samples from SMCA together constitute a predominate peak of 451.8 ± 2.9 Ma, and subordinate peak of 800–1600 Ma (Fig. 8a), whose detrital zircon spectra characteristics are similar to those of backarc basins or extensional basins formed under subduction setting of active continental margins66.
Four episodes of magmatism occurred in the Early Paleozoic, including 517 Ma, 501–496 Ma, 462–451 Ma and 426–385 Ma, among which the 462–451 Ma magmatism are the most widespread event34,43,45,46. This period of magmatism is widely distributed in the SA and Central Altyn regions (Table 8). Combined with the first exhumation stage of the SA HP-UHP metamorphic rocks at ~ 450 Ma, the magmatism may be generated from the upwelling of mantle source materials after the break-off of deep-subducted SA continental plate, and the resulting partial melting of middle and upper crustal materials46. Among them, Munablak granodiorite, Washixia monzogranite and Ruoqiang gneiss granite have intrusive ages of 450 Ma43,68 and metamorphic ages of ~ 411 Ma69. They belong to SMCA tectonically with the Jianggalesayi Bt-Amp gneiss, Yaolesayi amphibolite and granitic gneiss in this paper, and have the same magmatic and metamorphic age records, which should be the products of primary tectonic setting of stress release during the transition from collision compression to extensional uplift.
Moreover, Ma13 reported that the phengite pelite gneiss, HP mafic granulite and marble in the SMCA show a similar geological occurrence of interbedding distribution, indicating that the protolith assemblage should be volcanic-sedimentary formation. Geochemical studies show that mafic metamorphic rocks may mainly formed by magmatism with the attribute of island arc, and the metasedimentary rock are derived from the felsic source of continental island arc.
Therefore, based on the age characteristics of detrital zircons source, the correlation of magmatic ages, and the previous geochemical results, the SMCA formation should be formed in the sedimentary environment under the extensional background of the SA subduction-early exhumation, accompanied by synchronous magmatic emplacement.
Tectonic implications
Previous metamorphic studies show that the SMCA experienced a Silurian - Devonian subduction-collision event, and vast areas were overprinted by early Palaeozoic greenschist to amphibolite facies metamorphism, and several areas reached HP granulite–amphibole eclogite-facies12,13. According to the study of representative MP-HP rocks, as Ph pelitic gneiss, HP mafic granulite and Grt–Sil pelitic gneiss, the SMCA has undergone three stages of metamorphic evolution13, including a HP peak metamorphism of < 9.2–11.8 ℃/km (dT/dP) at ~ 430 Ma, a (HP) granulite facies overprinting at ~ 400 Ma, and a amphibole facies retrograde metamorphism at ~ 375 Ma. The P–T–t paths show priority depressurization–heating followed by clockwise depressurization–cooling with an Alpine type peak metamorphism, consistent with the geothermal gradient transformation in subduction zone under modern tectonic system, suggesting the origin of subduction-collision under compression regime6.
Geochronological studies of protolith in this study and the results of previous metamorphism studies suggested that the SMCA was a Paleozoic stratigraphic unit formed during the subduction-exhumation process of the SA, and then involved in the Siluran-Devonian subduction-collision event. Contemporaneous metamorphic events have also been widely reported in the North Qaidam (NQ) and Dunhuang Orogen (DH) adjacent to Altyn. HP-UHP rocks from NQ are mainly eclogite, garnet peridotite and various gneiss rocks, with the peak metamorphic age of 457–423 Ma and retrograde age of 420–397 Ma, which appear from west to east in Yuka, Lvliang Mountain (Shengli Kou), Xitie Mountain and Duran70,71,72,73,74,75,76,77,78,79,80,81,82,83,84. And protolith of some eclogites have the characteristics of nascent oceanic crustal with age of < 500 Ma85,86,87. In DH complex, eclogite, mafic granulite, and amphibolite are preserved as puddingstones within a matrix of metapelitic schist and/or gneiss or marble88. These MP-HP rocks fromed a tectonic mélange recording the Silurian–Devonian subduction-metamorphic process89,90,91. The protolith studies show that the DH complex could be divided into two units: (1) metasedimentary rocks with maximum sedimentary age of Neoproterozoic-Paleozoic (ca.739/565–442 Ma); (2) gneiss unit with protolith of Archean-Paleoproterozoic7.
Nine samples from SMCA record metamorphic age groups of 432.7 ± 2.8 Ma, 400.9 ± 3.6 Ma and 376.8 ± 3.3 Ma (Fig. 8b), suggest that they also experienced multistage metamorphism from Siluran to Devonian. Based on the correlation of protolith age and metamorphic evolution, the SMCA MP-HP rocks are highly consistent with partial rocks in the NQ and DH (Fig. 9), which may be the response of the same geological event in different tectonic units.
Summarizing of SHRIMP/LA-ICP-MS zircon U-Pb ages for Silurian-Devonian metamorphic events in the Altyn orogen and adjacent geological unit. Data sources are superscripted as follows: (a) this study; (b) Song et al.92; (c) Ren et al.87; (d) Zhang et al.93; (e) Zhang et al.82; (f) Song et al.94; (g) Zhang et al.95; (h) Zhang et al.96; (i) Song et al.97; (j) Mattinson et al.74; (k) Mattinson et al.98; (l) Zhang et al.80; (m) Yu et al.99; (n) Meng et al.100; (o) Zhao et al.101; (p) Wang et al.88; (q) Wang et al.91; (r) Wang et al.102; (s) Song et al.103.
Conclusion
-
(1)
The minimum detrital zircon age cluster of 451.8 ± 2.9 Ma from paracetamorphic rocks, the crystallization ages of 455.9 ± 2.6 Ma, 462.4 ± 1.4 Ma and 456.5 ± 5.2 from mafic and granitic metamorphic rocks, suggest that the main body of the SMCA formed in the Middle–Late Ordovician (460–450 Ma).
-
(2)
Nine samples also record three stages metamorphism by zircons of ~ 433 Ma, ~ 401 Ma and ~ 377 Ma. Combined with protolith and previous metamorphic studies, the SMCA should be a Paleozoic subduction-collision zone, rather than a Mesoproterozoic sedimentary stratum as previously thought, which should be separated from the “proto- Bashikuergan Group” in the Central Altyn block.
-
(3)
The similar protolith and metamorphic age are distributed in the NQ and DH orogen, suggesting that it may be the response of the same geological event in different tectonic units.
Data availability
All data generated or analysed during this study are included in this published article [and its supplementary information files].
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Acknowledgements
This work was supported by National Natural Science Foundation of China (42302056, 42030307, 41972054) and MOST Special Fund from the State Key Laboratory of Continental Dynamics.
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Tuo Ma completed the experiment, collected and processed the data and wrote the manuscript. Liang Liu and Mingguo Zhai designed the study, and participated in writing and discussion. YongSheng Gai, Shihao Zhang participated in the experimental analysis.
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Ma, T., Liu, L., Zhang, S. et al. The south margin of the central Altyn is an early paleozoic tectonic unit confirmed by Zircon dating evidence. Sci Rep 15, 6218 (2025). https://doi.org/10.1038/s41598-025-90898-0
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DOI: https://doi.org/10.1038/s41598-025-90898-0
