Two-stage rollbacks of the paleo-Pacific plate beneath the Cathaysia block during Cretaceous: Insights from A-type granites and volcanic rocks

Abstract

Petrogenesis and tectonic implications of the Cretaceous A-type rocks in the Cathaysia block remain controversial. We address this issue by the inland early Cretaceous (137–136 Ma) A-type Tiantangshan volcanic-intrusive complex and late Cretaceous A-type (96 Ma) Shihu-Huilong syenogranites, and integrating them with Cretaceous A-type rocks in the Cathaysia block. The Tiantangshan volcanics (trachyte-trachydacite-rhyolite) have εNd(t) (−3.5 to −2.6) and zircon εHf(t) (−3.0 to −1.3), suggesting their forming by partial melting of crust with significant addition of asthenosphere mantle-derived magma. The Tiantangshan granite, genetically connected with the volcanics, having higher εNd(t) (-1.2 to −0.8) and zircon εHf(t) (avg. 0.6) with more evolved composition, was likely formed by extensive fractional crystallization of the parental magma of the volcanics, recharged by asthenosphere-derived magma. The Shihu-Huilong syenogranites have initial 87Sr/86Sr (0.707987 to 0.708939), εNd(t) (−3.7 to −4.0) and zircon εHf(t) (avg. −1.6 to −0.6), suggesting a hybrid crustal and asthenospheric magma source. The early Cretaceous A-type rocks, distributed in the inland region with northeastward younging trend, mostly have A2 chemical features and inhomogeneous isotopic composition. However, the Tiantangshan and Baijuhuajian A-type rocks are distinct by A1 chemical subgroup and more depleted isotopic composition, likely reflective of intensified extensional back-arc environment due to slab rollback with high dipping angle. We propose an asynchronous and asymmetric rollback model during early Cretaceous that southern subducted paleo-Pacific plate started to retreat earlier than its northern part, and the rollback gradually developed northeastward. The late Cretaceous A-type granites in the coastal region, and Shihu-Huilong granites, are all characterized by A1 subgroup with relatively homogeneous isotopic composition, suggesting more involvement of asthenospheric materials in magma sources, and that the coastal region was entirely under an intensified extensional back-arc environment due to slab rollback with high dipping angle. The rollback of the subducted paleo-Pacific plate during Cretaceous likely occurred as two discrete stages.