Structural characterization by anisotropy of magmatic fabrics and microstructures of the Nahuelbuta Batholith and its emplacement within the Metamorphic Complex of the Eastern Series, South-Central Chile

Abstract

The Nahuelbuta Batholith (~37°–38.5° S) represents the southern end of the Chilean Coastal Batholith (~33°–38.5° S) from South-Central Chile. In the Late Carboniferous intruded in the Metamorphic Complex of the Eastern Series, a turbiditic sequence representing a fossil accretionary wedge. Typical intrusive rocks are coarse-grained diorites, tonalites to monzogranites with I-type geochemical characteristics, but showing a crustal-derived geochemical component. They were syntec-tonically emplaced in a N–S striking shear zone as a result of the Late Paleozoic subduction beneath the southwestern margin of Gondwana. These intrusive rocks were exposed before the Upper Triassic. During this time a volcano-sedimentary sequence was deposited and deformed by two folding events in the mid-and Upper Cretaceous. The intrusive rocks and their hosts were affected by subsequent brittle deformation. In this work, the structures of the batholith and the host rock are analyzed to decipher its emplacement mechanism and deformation history of the plutonic rocks. At the northern end of the Nahuelbuta Batholith the magmatic structures were overprinted by high temperature solid-state fabrics. Garnet, andalusite, staurolite and/or sillimanite appear predominantly along the western contact with the metamorphic schists and migmatites of the Eastern Series. At the northern end, the Eastern Series is represented by amphibolite to granulite facies mylonites that are moderately to high-ly dipping to the East. The down-dip stretching lineation denotes a reverse sense of shear. Microgran-ular mafic enclaves, which are abundant in a few zones in the igneous host, show a preferred shape alignment, striking ENE–WSW to NE–SW. They are considered as directions of the feeder channels, which opened in high angle to the σ1-direction. The batholith is elongated in a N-S direction, parallel to the current margin of the continental crust. Based on these directions and considering the evolution of the SW margin of South America, the emplacement mode can be derived as being almost coaxial.