Search

El área Los Gemelos (31° 58’ 26’’ S, 69° 36’ 7’ O) se enmarca en la Cordillera Frontal de la provincia de San Juan, Argentina, en la porción sur del segmento de subducción horizontal pampeano de los Andes Centrales. Estudios petro - mineralógicos de muestras de superficie permitieron establecer la presencia de un cuerpo subvolcánico dacítico y otro daci-andesítico, ambos hospedados en rocas volcaniclásticas de edad pérmica media. A partir del análisis de los datos de geoquímica se determinó que el pulso magmático que generó dicho intrusivos habría sido producto de un magmatismo de arco emplazado en una corteza de espesor normal (~30 km) en transición a una de espesor intermedio (~45 km), caracterizado por el elevado contenido de álcalis (en particular del Na2O) que se interpreta como una característica intrínseca de estos magmas. Por las semejanzas en cuanto al modo de yacer, textura y composición mineralógica con otras vulcanitas de edad miocena que afloran en el ámbito de la Cordillera Frontal y Principal sanjuanina y que constituyen las rocas hospedantes de depósitos mineralizados (Chita, Co.Bola, Qda. de las Minas, Pachón y Morro del Cobre), se le asigna preliminarmente una edad miocena al pulso magmático que generó los intrusivos del área de estudio. Los Gemelos area (31°58’26’’S, 69°36’7”W) is located in the Frontal Cordillera of San Juan province, Argentina, over the southern part of the Pampean flat slab segment of the Central Andes. Petro-mineralogical studies of surface samples allowed establishing the presence of a dacitic and a dacitic-andesitic subvolcanic bodies, both hosted in Middle Permian volcaniclastic rocks. By means of a comparative geochemical study, similarities could be established between the subvolcanic rocks of the study area and Miocene rocks from Frontal and Main Cordillera, and Western Precordillera. This allowed determining that the dacitic-andesitic body (Western Intrusive) resembles the Divisadero Sur and Cerro Puntas Negras units, both assigned to the Middle Miocene. These rocks would have originated in a normal thickness crust where there was no fractionation of plagioclase due to high pH2 O conditions. On the other hand, the dacitic body (Eastern Intrusive) shows geochemical characteristics similar to those of El Altar, Chita, Cerro Bola and Cerro Negro Upper Miocene units with an amphibole in transition to garnet residual mineralogy equilibrated under crustal thickening conditions. Fil: Bugueño Manrique, Mariana Soledad. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geología; Argentina Fil: Previley, Lorena Cristina. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan; Argentina Fil: Bastías Torres, María Verónica. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan; Argentina Fil: Rubinstein, Nora Alicia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentina

The Morro del Cobre porphyry (Cu-Mo-Au) deposit, genetically linked to the upper Miocene magmatism (~9–10 Ma), is located at the eastern margin of the Andean Frontal Cordillera of San Juan, Argentina, over the Pampean flat-slab segment (28–33°S) of the Central Andes. The petrogenetic analysis of these rocks reveals an arc affinity and an adakite-like signature with the source magmas evolving under thickened crust conditions which resulted in a high pressure (amphibole ± garnet) residual mineralogy. These mineralizing magmas, that evolved towards shallower magma chambers (

Textural and compositional changes affecting a quartzose metagabbro/metadiorite in a ductile shear zone which is part of the Early Palaeozoic (Famatinian) orogenic belt of the Western Sierras Pampeanas, Argentina, allow reconstruction of its tectono-metamorphic evolution as well as the metamorphic conditions achieved. On the basis of paragenetic associations and microfabrics, three overprinted deformation events are differentiated: a) a relict ductile event developed within the granulite facies, at temperatures exceeding 800 °C and pressures ≤5–5.5 kbar; b) a two-stage mylonitic deformation event under high-to medium-grade metamorphic conditions, at temperatures 500–700 °C and pressures between 6 and 7 kbar and c) a brittle deformation event developed at low greenschist facies below 400 °C. Evidence of the first event is preserved in the protolith although the subsequent deformation obliterated it partially. Slight modal and chemical changes were detected between the protolith and its mylonitic products. Deformation ages of the mylonites that vary from 441.9 ± 1.9 Ma to 438.7 ± 1.9 Ma are internally consistent and compatible with existing geological and geochronological data for the region, suggesting that during the Famatinian orogeny the western margin of Gondwana was characterized by several episodes of ductile deformation that varied in time and space.

The geodynamic evolution of the proto-Andean margin of Gondwana during the Paleozoic was characterized by repeated subduction processes associated with the docking of several terranes, including the Cuyania-Precordillera terrane and the Chilenia terrane, and the development of the calc-alkaline Famatinian continental magmatic arc. In the Sierra de La Huerta (30º56' - 31º29' S and 67º17' - 67º32' W), at the southwestern end of the Western Sierras Pampeanas, some mafic to ultramafic igneous bodies belonging to the Famatinian arc were affected by regional metamorphism of the medium-pressure granulite facies (7-7.5 kbar, ≈ 850 ºC). After this regional metamorphism, local mylonitization under amphibolite facies along discrete NW-SE to NNW-SSE striking ductile shear zones occurred. A 40Ar/39Ar plateau age of 432 ± 4 Ma was obtained on rather homogeneous hornblende porphyroclasts from a metagabbro mylonite. The textures and mineral chemistry of the mylonitized metagabbro allow its P-T-t evolution from magmatic crystallization to mylonitization to be constrained. Geochronological data obtained from mylonite provide evidence that orogenesis was active at least until the Early Silurian. This deformational event would have been related to uplift and decompression during the later stages of the orogenesis, probably associated with the accretion of the Precordillera terrane to the southwestern Gondwana margin. Fil: Castro, Brigida Marta Ester. Universidad Nacional de San Juan; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan; Argentina Fil: Arancibia, Gloria. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Geología "Dr. Emiliano Aparicio"; Argentina. Laboratorio Servicio Nacional de Geología y Minería,; Chile Fil: Morata Cespedes, Diego Antonio. Universidad Católica de Chile; Chile Fil: Belmar, Mauricio. Universidad de Chile. Facultad de Ciencias Fisicas y Matematicas; Chile Fil: Previley, Lorena Cristina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan; Argentina Fil: Pontoriero, Sandra Inés. Universidad Nacional de San Juan; Argentina