LP/HTmetamorphism as a temporal marker of change of deformation style within the Late Palaeozoic accretionary wedge of central Chile

Abstract

AbstractA Late Palaeozoic accretionary prism, formed at the southwestern margin of Gondwana from Early Carboniferous to Late Triassic, comprises the Coastal Accretionary Complex of central Chile (34–41°S). This fossil accretionary system is made up of two parallel contemporaneous metamorphic belts: a high‐pressure/low temperature belt (HP/LT– Western Series) and a low pressure/high temperature belt (LP/HT– Eastern Series). However, the timing of deformation events associated with the growth of the accretionary prism (successive frontal accretion and basal underplating) and the development of theLP/HTmetamorphism in the shallower levels of the wedge are not continuously observed along this paired metamorphic belt, suggesting the former existence of local perturbations in the subduction regime. In the Pichilemu region, a well‐preserved segment of the paired metamorphic belt allows a first order correlation between the metamorphic and deformational evolution of the deep accreted slices of oceanic crust (blueschists andHPgreenschists from the Western Series) and deformation at the shallower levels of the wedge (the Eastern Series).LP/HTmineral assemblages grew in response to arc‐related granitic intrusions, and porphyroblasts constitute time markers recording the evolution of deformation within shallow wedge material. IntegratedP–T–t–danalysis reveals that theLP/HTbelt is formed between the stages of frontal accretion (D1) and basal underplating of basic rocks (D2) forming blueschists atc. 300 Ma. A timeline evolution relating the formation of blueschists and the formation and deformation ofLP/HTmineral assemblages at shallower levels, combined with published geochronological/thermobarometric/geochemistry data suggests a cause–effect relation between the basal accretion of basic rocks and the deformation of the shallowerLP/HTbelt. The S2foliation that formed during basal accretion initiated near the base of the accretionary wedge at ~30 km depth atc. 308 Ma. Later, the S2foliation developed atc. 300 Ma and ~15 km depth shortly after the emplacement of the granitoids and formation of the (LP/HT) peak metamorphic mineral assemblages. This shallow deformation may reflect a perturbation in the long‐term subduction dynamics (e.g. entrance of a seamount), which would in turn have contributed to the coeval exhumation of the nearby blueschists atc. 300 Ma. Finally,40Ar–39Ar cooling ages reveal that foliated LP/HT rocks were already at ~350 °C atc. 292 Ma, indicating a rapid cooling for this metamorphic system.