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

Abstract

Twoconsecutive orogenies, named Pampean and Famatinian, delineated most of themetamorphic and igneous rocks that are currently exposed in the SierrasPampeanas. During the Cambrian and early Ordovician periods development of aclassic magmatic arc and another magmatic/migmatitic belt took place, thelatter nowadays located in the easternmost sector of the Eastern Sierras Pampeanas.TheSierra Norte-Ambargasta batholith is considered the Pampean magmatic arc and showsa calc-alkaline trend, similar to those defined by magmas associated withandesitic sources. The continuation to the north of this Cambrian magmatism isrepresented by smaller plutons located in the Cumbres Calchaquíes, Guasayán andCordillera Oriental ranges, emplaced in areas of medium-upper crust, mostly inschists, phyllites and slates of low- and medium metamorphic grade. Theassimilation of metasedimentary material from the host rocks by magmas similarto those described in the Sierra Norte-Ambargasta would have been the processthat gave rise to the geochemical characteristics of these granitoids. On thecontrary, the southernmost Sierras de Córdoba exposes a deep crustal sectiondominated by migmatites and high-grade metamorphic rocks with a diverse varietyof protoliths such as pelites-30 greywackes and amphibolites, and minormarbles, calc-silicate and ultramafic rocks. In this context, the diversity ofmagmatic rocks is due to the variety of protoliths involved in the region oforigin, which gave rise to granitic and trondhjemitic melts related toleucocratic bodies formed during partial melting. On the other hand, the early Ordovicianback-arc magmatism exposed in the Eastern Sierras Pampeanas is more homogeneousin the northern part compared to the southern one. It is represented by Pl-richtonalites and granodiorites with high Na2O, CaO, Al2O3 and Sr contents, whichdefine calcic to calc-alkaline, slight to highly peraluminous and magnesian toferroan series. The petrogenesis of these rocks would be related to acombination of plagioclase fractionation from tonalitic-trondhjemitic primary meltsand hybridization processes between two end-members (monzogranitic andtonalitic trondhjemitic melts), which gave rise to an intermediate magmatism ofhybrid composition. The sourcearea for this Famatinian magmatism could have its origin within the same domainwhere the Cambrian hot orogen developed along the Pampean convergent margin.Small amounts of Grt-bearing leucogranites and pegmatitic dykes could have beengenerated in response to the emplacement of intermediate magmas in middle-crustlevels producing the partial melting of metasediments from Puncoviscana Fm. TheFamatinian back-arc was associated with a generalized extension, marinesediment deposition and anatexis in the lower crust, causing differentiationand crustal recycling in the high-grade Pampean metamorphic basement.