Ana María Sato

The granitoids of the Sierra de San Luis

Abstract The Sierra de San Luis consists of Lower Palaeozoic igneous and metamorphic rocks. The granitoids were emplaced between Late Cambrian and Early Carboniferous times, and the most important metamorphic and deformational events occurred during this period. Older metamorphic events have also been recognized but they are poorly constrained. The most important group of granitoids are as follows: (1) Deformed tonalites and granites. The tonalites have a calc-alkaline signature with low La/Yb ratio. They are the Early Famatinian, pre-orogenic, granitoids of Late Cambrian to Early Ordovician age and form part of a magmatic arc. (2) The La Escalerilla granite is a deformed biotite monzogranite and consists of a single elongated pluton; its relation with the other granitoids is not clear. (3) Garnet-biotite-muscovite granodiorites and granites and pegmatites. The granitoids have S-type signatures, with high 87Sr/86Sr and La/Yb ratios. They are possible crustal magmas related to a thickened crust and are interpreted as early- to syn-orogenic. (4) Biotite- and biotite-hornblende granites and minor tonalites of Devonian to Early Carboniferous age. The granites form large, post-orogenic, plutons and batholiths related to crustal uplift, and represent the last igneous activity of the Famatinian cycle (and also of the Palaeozoic era) in the Sierra de San Luis.

Las Matras Block, Central Argentina (37°S-67°W): the Southernmost Cuyania Terrane and its Relationship with the Famatinian Orogeny

Abstract The Las Matras Block in Central Argentina constitutes the southernmost part of the Cuyania terrane, which was accreted to the southwestern margin of Gondwana during the Early to Mid Ordovician Famatinian orogeny. The Grenville-aged rocks of the Las Matras Block are represented by the tonalitic to trondhjemitic Las Matras pluton. A new U-Pb conventional zircon age of 1244±42 Ma confirms previous Sm-Nd and Rb-Sr isochron ages of this pluton. Mineral composition data are consistent with the tonalitic-trondhjemitic character of the pluton, and constrain its emplacement level to 1.9 to 2.6 kb. This shallow level of emplacement and the undeformed character of the pluton are distinctive features of this southernmost basement. A regional comparison indicates that the igneous-metamorphic evolution of the Grenville-aged basement rocks of the Cuyania terrane occurred over a period of more than 200 million years, with ages older than 1200 Ma up to those close to 1000 Ma. The shallowest crustal level is found in Las Matras, suggesting a southward shallowing of the exposed level of basement. The deformation and metamorphism associated with the collisional Famatinian orogeny affect both the Cuyania terrane and the adjacent western margin of Gondwana, and the Gondwana margin was also the locus of the related arc magmatism, but the compressive effects of the collision decrease in intensity toward the south. The Famatinian metamorphism and magmatism continue even further south into the Patagonia region, but the southern continuity of the Cuyania terrane into this region remains uncertain.

A view from the roof: magmatic stoping in the shallow crust, Chita pluton, Argentina

Nearly 2 km of vertical relief and exposure of continuous pluton roof-wall corners around the Chita pluton, northwest Argentina, provides constraints on the 3-D host rock displacement field attending magma emplacement in the shallow crust. The pluton is rectangular in crosssectional view and consists of weakly to non-deformed granite to granodiorite. Structures in surrounding metasedimentary host rocks are truncated at a knife-sharp contact and are only weakly deflected from their regional orientations. Final emplacement occurred by stoping together with minor doming of the earth’s surface. Structural restoration of a cross-section indicates that host-rocks above the pluton may have been domed upwards during emplacement by as much as a few hundred meters, accounting for up to 20% of the exposed pluton volume. The remaining 80% of the space for pluton emplacement was made by downward removal of host-rock from the present level of exposure. Although mechanisms such as floor-subsidence are permissive, late stoping has destroyed evidence for the early emplacement history. However, geometrical constraints provided by exposure of continuous wall-to-roof contacts indicate that early emplacement mechanisms at this level in the crust and immediately below were also dominated by the downward displacement of host rocks through a conduit that now is represented by the solidified pluton. q 2002 Elsevier Science Ltd. All rights reserved.

Proterozoico medio y Paleozoico inferior de la sierra de Umango, antepaís andino (29°S), Argentina: edades U-Pb y caracterizaciones isotópicas

En el basamento cristalino de la sierra de Umango se han distinguido varias unidades igneo-metamorficas, formadas principalmente entre el Proterozoico medio y el Paleozoico inferior. Incluye ortogneises graniticos, una sucesion de esquistos y marmoles, anfibolitas, y tres plutones graniticos de reducido tamano. Se han realizado analisis quimicos de granitoides y anfibolitas, y determinaciones de edad por los metodos U-Pb y K-Ar. La edad U-Pb en circon del Ortogneis Juchi (1108 ± 13 Ma) ratifica la asignacion al Proterozoico medio por anterior datacion Rb-Sr (1030& ± 30 Ma). La edad U-Pb en circon del Granito El Penon (473 ± 17 Ma) es ligeramente mayor que la obtenida por el metodo Rb-Sr (469 ± 9 Ma), aunque concordante si se tiene en cuenta el error de las determinaciones. Las edades K-Ar sobre muscovitas del Granito El Penon (409 ± 14 y 381 ± 11 Ma) son relacionadas a deformacion y metamorfismo ocurridos en el Devonico. Los isotopos de Nd y Sr permitieron establecer diferencias entre un conjunto de unidades relacionadas con el Ciclo Grenvilliano y otro con los Ciclos Pampeano-Famatiniano. Los datos obtenidos en la sierra de Umango indican que en las Sierras Pampeanas Occidentales hay nucleos de un basamento del Proterozoico medio (Ciclo Grenvilliano), mas una sucesion de esquistos y marmoles con anfibolitas, y plutones graniticos de reducido tamano, formados en el lapso Proterozoico superior-Devonico (Ciclos Pampeano-Famatiniano). La estructura interna de mayor relevancia, formada en el Ordovicico-Devonico muestra clara vergencia tectonica occidental y estaria relacionada con laminas de corrimiento (nappes) de una tectonica colisional. Las mismas se proyectarian desde el interior orogenico (Sierras Pampeanas Orientales) hacia el contiguo antepais de una de las placas interactuantes. Las sedimentitas clasticas del Ordovicico superior-Devonico en Precordillera (cuenca de antepais) completan el escenario geodinamico.

Early Paleozoic Structural and Metamorphic Evolution of Western Sierra de San Luis (Argentina), in Relation to Cuyania Accretion

Abstract Structural, metamorphic and isotopic data obtained from the Nogoli Metamorphic Complex of western Sierra de San Luis indicate that the Early Paleozoic Famatinian Orogeny overprinted an already structured and metamorphosed older basement. The older geological features are relict NW trending fabric associated with high-grade (amphibolite facies) regional metamorphism preserved within thin strips of schists and paragneisses and in the core of mafic to ultramafic lenses. Arc magmatism, medium P (Barrovian type)/high T (amphibolite to granulite facies) regional metamorphism and penetrative NNE to NE trending foliation are related to the building of the Famatinian orogenic belt. The P-T conditions of the Famatinian prograde metamorphism reached a pressure peak of ca. 8 kb, with a thermal peak from -750°C up to -820°C. U-Pb conventional and chemical dating and Ar-Ar plateau ages constrain the peak of the main orogenic phase related to the Famatinian belt to 470–457 Ma (Early to Mid-Ordovician). Greenschist facies retrograde metamorphism closely associated with shear zones and secondary Ar-Ar plateau and Sm-Nd ages suggest that a late to post-orogenic phase of the Famatinian belt was active at least since -445 Ma. This phase continued during the Silurian to Late Devonian times through multiple reactivation of early shear zones. The Famatinian Orogeny reset a previous thermal history and therefore, the timing of the relict fabric could not be constrained conclusively with radiometric dates. Despite this difficulty, a range of 520 to 490 Ma suggests some inheritance from Pampean events registered by the older NW-SE fabric. The Early to Mid-Ordovician regional metamorphism and ductile deformation of the western Sierra de San Luis is interpreted as the orogenic effects of the collision of the allochthonous Cuyania terrane with the autochthonous proto-Pacific margin of Gondwana during the Famatinian Orogeny.

Middle Proterozoic and Lower Paleozoic of the Sierra de Umango, Andean Foreland (29°S), Argentina: U-Pb ages and isotopic characteristics.

En el basamento cristalino de la sierra de Umango se han distinguido varias unidades igneo-metamorficas, formadas principalmente entre el Proterozoico medio y el Paleozoico inferior. Incluye ortogneises graniticos, una sucesion de esquistos y marmoles, anfibolitas, y tres plutones graniticos de reducido tamano. Se han realizado analisis quimicos de granitoides y anfibolitas, y determinaciones de edad por los metodos U-Pb y K-Ar. La edad U-Pb en circon del Ortogneis Juchi (1108 ± 13 Ma) ratifica la asignacion al Proterozoico medio por anterior datacion Rb-Sr (1030& ± 30 Ma). La edad U-Pb en circon del Granito El Penon (473 ± 17 Ma) es ligeramente mayor que la obtenida por el metodo Rb-Sr (469 ± 9 Ma), aunque concordante si se tiene en cuenta el error de las determinaciones. Las edades K-Ar sobre muscovitas del Granito El Penon (409 ± 14 y 381 ± 11 Ma) son relacionadas a deformacion y metamorfismo ocurridos en el Devonico. Los isotopos de Nd y Sr permitieron establecer diferencias entre un conjunto de unidades relacionadas con el Ciclo Grenvilliano y otro con los Ciclos Pampeano-Famatiniano. Los datos obtenidos en la sierra de Umango indican que en las Sierras Pampeanas Occidentales hay nucleos de un basamento del Proterozoico medio (Ciclo Grenvilliano), mas una sucesion de esquistos y marmoles con anfibolitas, y plutones graniticos de reducido tamano, formados en el lapso Proterozoico superior-Devonico (Ciclos Pampeano-Famatiniano). La estructura interna de mayor relevancia, formada en el Ordovicico-Devonico muestra clara vergencia tectonica occidental y estaria relacionada con laminas de corrimiento (nappes) de una tectonica colisional. Las mismas se proyectarian desde el interior orogenico (Sierras Pampeanas Orientales) hacia el contiguo antepais de una de las placas interactuantes. Las sedimentitas clasticas del Ordovicico superior-Devonico en Precordillera (cuenca de antepais) completan el escenario geodinamico.