Pressure–temperature–time (P–T–t) evolution of fore-arc and foreland schist in the Qinling Orogenic Belt, China: Implications for Late Paleozoic and Triassic subduction termination

Abstract

The Qinling Orogenic Belt (QOB) is a segment of the larger Central China Orogen and marks the amalgamation of the South and North China Cratons during a protracted period spanning the Neoproterozoic through to Triassic. The complex evolution of the QOB has been extensively studied through U–Pb zircon geochronology, but lacks fundamental characterisation of its time-integrated thermal history. Moreover, the comparatively few metamorphic studies that exist focus exclusively on high-pressure rocks at the margins of major tectonic divisions within the QOB. Samples for this study are metapelitic schists sourced from the Qinling Mountains, North Qinling Belt (NQB), deep in the QOB interior. Cordierite schist has metamorphic monazite age affinities to Late Triassic magmatism and metamorphism in the South Qinling Belt (SQB) occurring at 230–220 Ma. Calculated phase equilibria modelling constrains metamorphism to 1.0–2.0 kbar and 530–545 °C, corresponding to a steep apparent thermal gradient between ~275–530 °C/kbar. This probably represents contact metamorphism of the Liuling Group turbidite sequence by intruding magma. Garnet–staurolite schist has metamorphic monazite age data that overlaps with Late Paleozoic events occurring between 420 and 400 Ma. Calculated phase equilibria modelling constrains peak metamorphism to ~7.1 kbar and 615 °C, corresponding to a shallower thermal gradient of ~87 °C/kbar. This represents Barrovian-style metamorphism of fore-arc sedimentary units during arc–continent collision marking the closure of the Shangdan Ocean. Metamorphism of these fore-arc sequences has a comparable thermal gradient to Guishan Complex equivalents in the Tongbai Orogen, which is a continuation of the Qinling Orogen to the east. This study establishes previously undocumented contact metamorphism in the northern SQB and Barrovian-style metamorphism in the NQB, providing key constraints for tectonic models that explain the evolution of the QOB and broader Central China Orogen.