Petrogenesis of the Puerto Eden Igneous and Metamorphic Complex,Magallanes,Chile: Late Jurassic syn-deformational anatexis of metapelites and granitoid magma genesis

摘要

A suite of schists,gneisses,migmatites,and biotite granitoids from the Puerto Eden Igneous and Metamorphic Complex (PEIMC) and biotite-hornblende granitoids of the South Patagonian batholith (southern Chile) has been studied.For that purpose,the chemistry of minerals and the bulk rock composition of major and trace elements including Rb-Sr and Sm-Nd isotopes were determined.Mineralogical observations and geothermobarometric calculations indicate high-temperature and low-pressure conditions (ca.600-700 deg C and 3 to 4.5 kbar) for an event of metamorphism and partial melting of metapelites in Late Jurassic times (previously determined by SHRIMP U-Pb zircon ages).Structures in schists,gneisses,migmatites and mylonites indicate non-coaxial deformation flow during and after peak metamorphic and anatectic conditions.Andalusite schists and sillimanite gneisses yield initial ~(87)Sr/~(86)Sr ratios of up to 0.7134 and epsilon Nd_(150) values as low as - 7.6.Contemporaneous biotite granitoids and a coarse-grained orthogneiss have initial ~(87)Sr/~(86)Sr ratios between 0.7073 and 0.7089,and epsilon Nd_(150) values in the range - 7.6 to - 4.4.This indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source.Thus,a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids.The PEIMC is considered as a segment of an evolving kilometre-sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in Late Jurassic times.A biotite-hornblende granodiorite and a muscovite-garnet leucogranite show initial ~(87)Sr/~(86)Sr ratios of 0.7048 and 0.7061,and epsilon Nd_(100) values of - 2.6 and - 1.8,respectively,and are thus probably related to Early Cretaceous magmas not involved in the anatexis of the metasedimentary rocks.