Argentina

Located in NW Argentina, the Sierras Pampeanas are a series of mountainous ranges, comprising crystalline basement intruded by diverse Paleozoic igneous rocks, that were elevated during Miocene to recent compressional (Andean) tectonics. The granites were emplaced during three main orogenic events: (a) Pampean (mid-Cambrian), (b) Famatinian (Cambro-Ordovician), and (c) Achalian (Devonian-Carboniferous). This collaborative project, with Pablo Alasino and Juan Dahlquist, CONICET, focuses on the Achalian granitoids that outcrop in the Pampean ranges. The study areas are the San Blas pluton of the Sierra de Velasco, the Santa Rosa and Sauce Guacho plutons of the Sierra de Ancasti, and the Achala batholith of the Sierra Grande de Cordoba. The project is an integrated field and laboratory study that will include geologic mapping, petrographic descriptions, major and trace element rock chemistry, mineral chemistry, and geochronology. Results of the study, as they become available, will be posted on this page. Photographs of the field area, major lithologic units, and structural features are found below.

Fault controlled Carboniferous A-type magmatism in the proto-Andean foreland (Sierras Pampeanas, Argentina): Geochemical constraints and petrogenesis

Dahlquist, J. A., Alasino, P. H., Eby, G. N., Galindo, C., Casquet, C.

The intrusion of granitoids into the Eastern Sierras Pampeanas in the Early Carboniferous took place after a long period of mainly compressional deformation that included the Famatinian (Ordovician) and Achalian (Devonian) orogenies. These granitoids occur as small scattered plutons emplaced in a dominant extensional setting, within older metamorphic and igneous rocks, and many of them are arranged along a reactivated large shear zone. A set of 46 samples from different granitic rocks: Huaco granitic complex, San Blas pluton, and the La Chinchilla stock from the Sierra de Velasco, Zapata granitic complex from Sierra de Zapata, and the Los Árboles pluton from Sierra de Fiambalá, display high and restricted SiO₂ contents between 69.2 and 76.4 wt. %. On both FeO/(FeO+MgO) vs. SiO₂ and [(Na₂O+K₂O)-CaO] vs. SiO₂ plots the samples plot in the ferroan and alkaline-calcic to calco-alkaline fields (FeO/(FeO+MgO) = 0.88-1.0%;[(Na₂O+K₂O)-CaO] = 6.3–8.3%), thus showing an A-type granitoid signature. The high concentrations for the High Field Strength Elements (HSFE), such as Y, Nb, Ga, Ta, U, Th, etc. and flat REE patterns showing significant negative Eu anomalies are also typical features of A-type granites. Our petrogenetic model supports progressive fractional crystallization with dominant fractionation of feldspar and a source mineral assemblage enriched in plagioclase. Biotites have distinctive compositions with high FeO/MgO ratios (7.8-61.5), F (360–5610 ppm), and Cl (120–1050 ppm). The FeO/MgO ratios together with the F and Cl content of igneous biotites seem to reflect the nature of their parental host magmas and may be useful in identifying A-type granitoids. The isotopic data (Rb-Sr and Sm-Nd) confirm that the A-type granites represent variable mixtures of asthenospheric mantle and continental crust and different mixtures lead to different subtypes of A-type granite (illustrating the lack of consensus about A-type magma origin). We conclude that prominent shear zones play an important role providing suitable conduits for ascending asthenospheric material and heat influx in the crust, a hypothesis that is in accord with other recent work on A-type granites.

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Canyon cut in Huaco granite. Near TIPA shear zone.	Canyon in Huaco granite. Near TIPA shear zone.	TIPA shear zone. Fractured feldspar in shear zone indicating dextral movement.	Large K-feldspar in Antinaco granite.

San Blas pluton.	Porphyritic granite. San Blas pluton.	Porphyritic granite. San Blas pluton.	Aplite dike in San Blas pluton.

Shear zone in San Blas pluton.	Huaco granite.	Shear zone. Northeastern end of the Sierra de Velasco.	Spotted hornfels. Shear zone at northeastern end of the Sierra de Velcasco.

Southern end of Sierra de Velasco.	Large feldspar phenocrysts in Antinaco granite.	TIPA shear zone at southern end of Sierra de Velasco.	Landslides on western flank of Sierra de Ancasti.

Catamarca valley and Catamarca city from Sierra de Ancasti.	Sierra de Ancasti.	Achala batholith. Sierra de Cordoba.	Large feldspars in porphyritic granite. Achala batholith. Sierra de Cordoba.

Fine-grained grantic phase of Achala batholith in coarse-grained porphyritic granite. Sierra de Cordoba.	Block of fine-grained granitic phase of Achala batholith in coarse-grained porphyritic granite. Sierra de Cordoba.	Pegmatite mine. Sierra de Cordoba.	Boudinage in metamorphic country rock. Sierra de Cordoba.