The Cambrian (Pampean) and early Ordovician (Famatinian) magmatic/migmatitic belt in the Eastern Sierras Pampeanas, Argentina

Abstract

Two consecutive orogenies, named Pampean and Famatinian, delineated most of the metamorphic and igneous rocks that are currently exposed in the Sierras Pampeanas. During the Cambrian and early Ordovician periods development of a classic magmatic arc and another magmatic/migmatitic belt took place, the latter nowadays located in the easternmost sector of the Eastern Sierras Pampeanas. The Sierra Norte-Ambargasta batholith is considered the Pampean magmatic arc and shows a calc-alkaline trend, similar to those defined by magmas associated with andesitic sources. The continuation to the north of this Cambrian magmatism is represented by smaller plutons located in the Cumbres Calchaquíes, Guasayán and Cordillera Oriental ranges, emplaced in areas of medium-upper crust, mostly in schists, phyllites and slates of low- and medium metamorphic grade. The assimilation of metasedimentary material from the host rocks by magmas similar to those described in the Sierra Norte-Ambargasta would have been the process that gave rise to the geochemical characteristics of these granitoids. On the contrary, the southernmost Sierras de Córdoba exposes a deep crustal section dominated by migmatites and high-grade metamorphic rocks with a diverse variety of protoliths such as pelites-greywackes and amphibolites, and minor marbles, calc-silicate and ultramafic rocks. In this context, the diversity of magmatic rocks is due to the variety of protoliths involved in the region of origin, which gave rise to granitic and trondhjemitic melts related to leucocratic bodies formed during partial melting. On the other hand, the early Ordovician back-arc magmatism exposed in the Eastern Sierras Pampeanas is more homogeneous in the northern part compared to the southern one. It is represented by Pl-rich tonalites and granodiorites with high Na2O, CaO, Al2O3 and Sr contents, which define calcic to calc-alkaline, slight to highly peraluminous and magnesian to ferroan series. The petrogenesis of these rocks would be related to a combination of plagioclase fractionation from tonalitic-trondhjemitic primary melts and hybridization processes between two end-members (monzogranitic and tonalitic-trondhjemitic melts), which gave rise to an intermediate magmatism of hybrid composition. The source area for this Famatinian magmatism could have its origin within the same domain where the Cambrian hot orogen developed along the Pampean convergent margin. Small amounts of Grt-bearing leucogranites and pegmatitic dykes could have been generated in response to the emplacement of intermediate magmas in middle-crust levels producing the partial melting of metasediments from Puncoviscana Fm. The Famatinian back-arc was associated with a generalized extension, marine sediment deposition and anatexis in the lower crust, causing differentiation and crustal recycling in the high-grade Pampean metamorphic basement.