Mesozoic-Cenozoic evolution of the North Patagonian Batholith in Aysen, southern Chile

A detailed Rb-Sr geochronological and geochemical study has been carried ou t on granitoids of the North Patagonian batholith in Aysen. The results for 25 plutonic bodies reveal a complex age pattern. The principal zones defin ed are, from west to east: Late Cretaceous (a single 76 Ma pluton), Early C retaceous (c. 135 Ma), Eocene (c. 45 Ma), and Early Miocene (25-15 Ma), rev erting to mid-Cretaceous (120-90 Ma) in the Main Cordillera. The igneous su ite is typically metaluminous and calc-alkaline, with hornblende-biotite gr anodiorite and tonalite dominant, although small bodies of Late Miocene/Pli ocene (c. 10-5 Ma) peraluminous leucogranite occur sporadically. Tertiary p lutonism extends to gabbroic compositions and is concentrated in the vicini ty of the dextral strike-slip Liquine-Ofqui fault zone. The highest initial Sr-87/Sr-86 ratios (c. 0.7050) occur in the Early Cretaceous group, with a sharp decrease to 0.7034-0.7045 that persists until a Late Miocene reversa l to higher values (0.7040-0.7048). Thane variations are reflected by Nd is otopes, but trends in the epsilon Sr-t v. epsilon Nd-t diagram show that th is is not due to contamination from the accretionary complex into which the batholith was emplaced. An origin by melting of mafic crustal underplate a nd lower crust is suggested for the main magma suite. The discrete episodes of magmatism correlate with significant changes in subduction kinematics.