Late Paleozoic onset of subduction and exhumation at the western margin of Gondwana (Chilenia Terrane): Counterclockwise P–T paths and timing of metamorphism of deep-seated garnet–mica schist and amphibolite of Punta Sirena, Coastal Accretionary Complex, central Chile (34° S)

Abstract

In this study, the Paleozoic albite–epidote–amphibolite occurring as meter-sized intercalations within garnet–mica schist at Punta Sirena beach (Pichilemu region, central Chile) is characterized for the first time. These rocks constitute an unusual exposure of subduction-related rocks within the Paleozoic Coastal Accretionary Complex of central Chile. Whereas high pressure (HP) greenschist and cofacial metasediments are the predominant rocks forming the regional metamorphic basement, the garnet–mica schist and amphibolite yield higher P–T conditions (albite–epidote amphibolite facies) and an older metamorphic age. Combining detailed mineral chemistry and textural information, P–T calculations and Ar–Ar ages, including previously published material from the Paleozoic Accretionary Complex of central Chile, we show that the garnet–mica schist and associated amphibolite (locally retrograded to greenschist) are vestiges of the earliest subducted material now forming exotic bodies within the younger HP units of the paleo-accretionary wedge. These rocks are interpreted as having been formed during the onset of subduction at the southwestern margin of Gondwana. However, we show that the garnet–mica schist formed at a slightly greater depth (ca. 40 km) than the amphibolite (ca. 30 km) along the same hot-subduction gradient developed during the onset of subduction. Both lithotypes reached their peak-P conditions at ca. 335–330 Ma and underwent near-isobaric cooling followed by cooling and decompression (i.e., counterclockwise P–T paths). The forced return flow of the garnet–mica schist from the subduction channel started at ca. 320 Ma and triggered the exhumation of fragments of shallower accreted oceanic crust (amphibolite). Cores of phengite (garnet–mica schist) and amphibole (amphibolite) grains have similar chemical compositions in both the S1 and S2 domains, indicating rotation of these grains during the transposition of the burial-related (prograde peak-T) foliation S1 by the non-coaxial exhumation-related foliation S2. During exhumation and retrograde D2 deformation, the garnet–mica schist and amphibolite were tectonically mingled at a depth of ca. 30 km at ca. 315 Ma. We propose that the Punta Sirena unit comprises a “pseudo”-coherent sequence formed by heterogeneous lithologies that followed non-chaotic exhumation mingling, now representing the remnants of the fossil subduction channel developed at the onset of the Late Paleozoic subduction at central Chile.