Regional lithologic variations in the Patagonian batholith

Abstract

The Patagonian batholith, in the southern Andes of Chile, represents a major addition to the Pacific margin of the South American plate due to semi-continual subduction since the Late Jurassic. Plutonic ages range from 166 to 12 Ma, with an apparent peak around 100-70 Ma. Most of the batholith is composed of I-type, metaluminous, calc-alkaline rocks ultimately related to partial melting of mantle wedge material and/or subducted material. Tonalite (plus minor quartz diorite) is the dominant lithology (36% of 585 samples); other lithologic groups (granite, granodiorite + quartz monzodiorite, and diorite + gabbro) make up roughly equal proportions of the samples (18–25% each). Minor amounts of peraluminous rock suggest some amount of crustal recycling (contamination of mantle magmas or melting of continental crust). Most plutons are undeformed; foliated rocks crop out in Andean orogenic zones and locally along lineaments. Local, protracted, felsic to mafic progressions are probably related to intrusion geometry and erosion level rather than evolution of a single magma. Similarities with the Coastal batholith of Peru include size, the period of intrusion (mainly Cretaceous-Tertiary), and the general I-type, tonalitic character. Differences include no unidirectional migration in time, thinner and younger pre-batholithic crust, deeper level of erosion (generally mesozonal), and significant primary biotite in mafic rocks. These differences probably reflect different mechanisms of magmatic evolution and arc construction related to subduction kinematics and crustal setting.