Geología

ABSTRACT New understanding of the mechanisms and rates by which magmatic arcs are constructed requires geological control from crustal cross-sections. Previously characterized arc sections are generally from island arcs, and may reflect relatively low-flux conditions potentially quite different than those associated with the large scale volcanism and rapid formation of pluton-flooded, mid-to-upper crust such as occurs in the Puna-Altiplano region today. However the deepest levels of the early Ordovician Famatinian arc crust provides a window into the 'plumbing' of a continental arc in flare-up mode and is exposed in an approximately 140 km by 40 km cross-section in the Sierras Valle Fertil-La Huerta in northwestern Argentina. The Valle Fertil portion of the arc grades continuously from regional-scale calc-alkaline metaluminous I-type granitoids (granodiorite into deeper heterogeneous tonalities) downward into a complex arrangement of large mafic intrusions (both cumulate and non-cumulate) and partially melted supracrustal packages composed largely of siliciclastic sediments. The region is characterized by granulite grade conditions formed broadly synchronously with igneous plutonic activity spanning up to 20 Ma, with a portion of the section recording 5.2-7.1 kbar and temperatures of 770-840°C. Peak metamorphic conditions produced thermal gradients of around 36° per km and there was little pressure change upon cooling producing a very flat ccw P-T loop. No resolvable differences in peak metamorphic conditions from localities along strike 70 km apart were detected, indicating that the arc is exposed at the same paleo-depth and the average crustal thickness was more than 20-25 km. It has also been proposed that this orogen formed in a margin without continent-continent collision and hence no terrane accretion. Thus the lower crustal section exposed in the Sierras Valle Fertil-La Huerta may be an analog for the Cenozoic high plateau of the Central Andes. This is an especially attractive idea as the large mafic bodies and heterogeneous tonalities might be an analog for the attenuating mid-crustal low velocity layer imaged in Puna-Altiplano region today. Our presentation will present work in progress on our efforts to refine the regional architecture of this arc with a special emphasis on applications to continental arcs in flare- up mode.

Traditional measures of mutual funds persistence are based on measures like Jensen's alpha, factor models and so on. As expected, their results on a broad data-set of European mutual funds show a small, but relevant, amount of persistent funds, depending on the method of estimation used. We go further in our analysis and, by using a multiple hypothesis testing methodology, find that those measures overstate the performance on the sample across countries, signalling persistence in situations where the positive results are due to luck.

RESUMEN. Se describe por primera vez la petrografía y geoquímica de unidades volcánicas terciarias del sur de la provincia de Mendoza. Se analizan la nomenclatura estratigráfica y sus edades, estableciéndose una secuencia de eventos magmáticos ...

This work presents the field setting, petrography, mineralogy and geochemistry of a gabbroic and peridotitic layered body that is lens-shaped and surrounded by gabbronorites, diorites, and metasedimentary migmatites. This body exposed at Jaboncillo Valley is one among ...

The whole Valle Fértil-La Huerta section appears as a calc-alkaline plutonic suite typical of a destructive plate margin. New Sr and Nd isotopic whole-rock data and published whole-rock geochemistry suggest that the less-evolved intermediate (dioritic) rocks can be derived by magmatic differentiation, mainly by hornblende + plagioclase ± Fe–Ti oxide fractional crystallization, from mafic (gabbroic) igneous precursors. Closed-system differentiation, however, cannot produce the typical

Interaction between mafic magmas and crustal metamorphic rocks at mid-crustal levels causes modification of mafic magma composition by assimilation or reaction with crustal materials. This petrologic process is studied at the Suya Taco Complex where a suite of mafic magmas interacted with metamorphic rocks at 6.6±1 kbar. The Suya Taco Complex consists of close-spaced bodies of biotite-bearing gabbronorites. These magmas intruded into a supracrustal sequence consisting of amphibolite– to granulite–facies gneisses and migmatites. The peak-metamorphic mineral assemblage Bt+Qtz+Pl(An30)+Grt+Kfs±Crd±Sil of supracrustal rocks crystallised before igneous mafic magmatism. Interaction between mafic magmas and metamorphic rocks produces two major hybrid lithologies: diorites made up of Opx+Bt+Pl (An38–50)+Ilm±Cpx±Qtz; and garnet- and orthopyroxene-bearing rocks characterised by the coexistence of Bt+Pl (An30–45)+Grt+Opx+Qtz+Ilm±Spl±Kfs. The latter lithology may locally appear as a quartz-absent and high-aluminous mineral assemblage. The two hybrid rocks occur interlayered at the contact zone between igneous mafic rocks and high-grade gneisses. Individual layers range from centimeter to meter scales; moreover, dioritic and orthopyroxene- and garnet-bearing layers present a spectrum of mineral assemblages and mineral chemistry that spread among those that appear in igneous mafic and metamorphic rocks. The work shows that the changes of mineral assemblages and variations of mineral chemistry in the hybrid diorites and orthopyroxene- and garnet-bearing rocks are the result of interaction processes between mafic magmas and regional metamorphic rocks. Field relationships, petrographic features, mineral compositional changes and qualitative analyses of phase relations suggest that rocks formed by interaction between mafic magmas and metamorphic rocks results from: (1) assimilation of metamorphic rocks by mafic magmas at variable degrees and (2) ion diffusion among mineral assemblages of contrasting composition. The implication of these processes is that Ca is transferred from mafic magmas to metamorphic rocks, which forces the breakdown of garnet and quartz, and induces the crystallisation of plagioclase and orthopyroxene in the resulting hybrid rocks.