Pressure–temperature–time (P–T–t) evolution of schist in the Qinling Orogenic Belt, China

Abstract

The Qinling Orogenic Belt marks the amalgamation of the South and North China Cratons during a protracted but punctuated period spanning the Neoproterozoic through to Triassic. The complex evolution of the Qinling Orogen has been extensively studied through U–Pb zircon geochronology, but lacks fundamental characterisation of its thermal history. Moreover, metamorphic studies, of which there are few, focus on high-pressure rocks at the margins of major tectonic divisions within the architecture of the Qinling Orogen. Cordierite schists investigated in this study have metamorphic monazite age affinities to Late Triassic magmatism and metamorphism in the South Qinling Belt (SQB) occurring at 220–230 Ma. Calculated phase equilibrium modelling constrains metamorphism to 0.60–4.25 kbar and 540–570˚C, corresponding to steep (extreme) apparent thermal gradients between 135–900˚C/kbar. This probably represents contact metamorphism of the Liuling Group turbidite sequence by intruding magmas. Garnet–staurolite schist within the North Qinling Belt (NQB) has metamorphic monazite age data that overlaps with Late Palaeozoic events occurring at ca. 400 Ma. Calculated phase equilibrium modelling constrains peak metamorphism to ~7.1 kbar and 615˚C, corresponding to a thermal gradient of ~87˚C/kbar. This represents Barrovian-style metamorphism of forearc sedimentary units during arc-continent collision marking the closure of the Shangdan Ocean. Metamorphism of these forearc sequences has a comparable thermal gradient to Guishan Complex equivalents in the Tongbai Orogen, which comprises a continuation of the Qinling to the east. This study establishes previously undocumented contact metamorphism in the northern SQB and Barrovian-style metamorphism in the NQB providing fundamental data that is vital for better constrained tectonic models of the evolution of the Qinling Orogenic Belt.