Early Palaeozoic Barrovian Metamorphism in the Northern Central Qilian Block, NW China: Structure and Evolution of a Continental Arc in the Proto-Tethys Tectonic Domain

Abstract

Regional metamorphism within the Central Qilian Block provides critical constraints on the tectonic evolution and geodynamic processes in the Qilian orogenic system. Combined phase equilibrium modelling and multiple isotope-dating methods reveal the P–T–t evolution of the Barrovian metamorphic zones preserved in the Huangyuan Group in the northern Central Qilian Block. A metamorphic zircon U–Pb age of 466 ± 2 Ma from a paragneiss, a muscovite ⁴⁰Ar/³⁹Ar age of 434.8 ± 3.9 Ma and a biotite ⁴⁰Ar/³⁹Ar age of 365.6 ± 1.2 Ma from a micaschist and a biotite ⁴⁰Ar/³⁹Ar age of 372.4 ± 0.9 Ma from an amphibolite in the group record Palaeozoic polyphase metamorphism. Phase equilibrium modelling indicates clockwise P–T paths with peak conditions of 6.1–6.8 kbar and 575°C–625°C in the garnet zone, 6.2–7.2 kbar and 610°C–670°C in the staurolite zone and 6.0–7.3 kbar and 750°C–780°C in the sillimanite zone. The Barrovian metamorphism is inferred to result from crustal thickening and the resulting elevated geothermal gradient in the northern Central Qilian Block. A Palaeozoic thickened continental arc resulting from southward subduction of the North Qilian Ocean at ~466 Ma is proposed for the northern Central Qilian Block. Southward subduction of the North Qilian Ocean beneath the Central Qilian Block lasted until 457–428 Ma. With the waning of arc-related magmatism, a decrease in the regional geothermal gradient led to greenschist-facies to amphibolite-facies retrograde metamorphism in the Huangyuan Group at ~435 Ma. After the closure of the South Qilian Ocean, the Quanji block collided with the Hualong and Central Qilian Blocks. Post-collisional extensional collapse at 400–350 Ma affected the Central Qilian Block, resulting in hydrothermal activity in the Huangyuan Group at 372–366 Ma.