Petrogenetic evolution and thermobarometry of the Late Jurassic La Hoya pluton, early stages of the North Patagonian batholith, southwestern Argentina

Abstract

The Patagonian batholith was formed through continuous subduction in the southwestern Gondwana margin, from the Early–Middle Jurassic to present times. This study presents a petrological analysis of the La Hoya pluton, an intrusive body located in the North Patagonian Andes near Esquel that is part of the Patagonian batholith. The La Hoya pluton consists of porphyritic biotite–amphibole granodiorites that grade into porphyritic biotite monzogranites, and both facies are associated with minor occurrences of gabbroic-to-tonalitic stocks and doleritic dikes. Amphibole dating yielded a new 40Ar–39Ar stepwise age of 161.5 ± 0.5 Ma for this shallow emplaced pluton. Field, petrographic, whole rock, and mineral geochemical data suggest and indicate that the petrogenetic evolution of the La Hoya pluton involved fractional crystallization, mafic and felsic magma interaction, and possibly mixing of multiple magma sources. Early formed brown resorbed amphibole and labradoritic plagioclase antecrysts crystallized in alkaline-to-transitional (alkaline-to-subalkaline) magmas with temperatures of 834–962 °C and pressures of ~ 2 kbar (up to ~ 7 km depth). Green amphiboles, plagioclases of andesine–oligoclase composition, and biotites crystallized in equilibrium with subalkaline magmas at lower temperatures and pressures (750–806 °C and of ~ 1 kbar; ~ 3.5 km depth). Actinolitic amphibole and albitic plagioclase formed during the hydrothermal alteration associated with the overprinting of post-emplacement solid-state deformation. The La Hoya pluton involved early formed alkaline-to-transitional magmas that progressively became more calc-alkaline with ongoing differentiation.