Petrology and geochronology of the San Martín de los Andes batholith: Insights into the Devonian magmatism of the North Patagonian Andes

Abstract

Evidence is growing that a belt of Devonian plutonism is present in the Northern Patagonian segment of the present-day Andes. This plutonism formed along the paleo-Pacific margin of the Gondwana supercontinent. New whole rock chemistry and SHRIMP U–Pb zircon data for plutonic rocks of the San Martín de los Andes batholith (southwestern Argentina) provide insights into the magmatic and tectonic evolution of the southwestern Gondwana margin during the middle Palaeozoic. The San Martín de los Andes batholith was formed between 390 and 374 Ma and intruded into a Lower Paleozoic metamorphic basement that had been previously affected by Late Ordovician-middle Silurian deformation. Most of the batholith is composed of metaluminous, calc-alkaline tonalites and granodiorites, with subordinate volume of peraluminous garnet-bearing granites. Discrete and disperse gabbroic and dioritic rocks hosted in tonalites and granodiorites occur as either microgranular enclaves or syn-magmatic dykes. Tonalitic, granodioritic and granitic rocks develop transitional contacts among them revealing coetaneous building up throughout the plutonic suite, but when interpreting their chemical variability, they are not strictly comagmatic rocks. Collectively, field, petrographic, and geochemical data of the San Martín de los Andes batholith are interpreted to reflect concurrence of crystal fractionation, crustal assimilation and magma mixing in driving petrological diversification. Yet, gabbroic and dioritic rocks in the San Martín de los Andes batholith display geochemical characteristics of calc-alkaline primitive basalts that concur with what is being hypothesized to be a magmatic arc zone for the Devonian period. This study discusses how to interpret the evolution of the San Martín de Los Andes batholith together with other geodynamic models such as the Chaitenia magmatic arc.