Two stages of granitoid intrusions and their implications on the early Paleoproterozoic tectonic evolution of the Quanji Massif

Abstract

Tectonic regimes took different forms worldwide during the early Paleoproterozoic, ranging from a slowing or halting of plate tectonics to vigorous subduction-related magmatism. A detailed study is conducted on the Delingha granitoid pluton in the Quanji Massif of northwestern China to reveal the crustal evolution of this Precambrian microcontinent and to provide potential insight on complex and heterogeneous evolutionary processes occurring in this period. The Delingha pluton was mainly composed of gneissic biotite granite and gneissic granodiorite, which formed at ca. 2.48 – 2.44 Ga and 2.38 – 2.35 Ga, respectively, as evidenced by LA-ICP-MS zircon U-Pb ages. The gneissic biotite granite exhibited high-K I-type granite affinities and variable zircon εHf(t) values (−12.9 −4.7) with a major peak of approximately 2.8 and a secondary peak of approximately 3.9, indicating that the source rock was dominated by heterogeneous mafic crustal materials. The gneissic granodiorite shows A-type granite affinities and a nearly unimodal distribution of zircon εHf(t) values (−3.1 −4.0, with a peak of approximately 1.5). These data indicate that the gneissic granodiorite was mainly sourced from Archean mafic igneous rocks. Our study suggests that the gneissic biotite granite formed in a subduction-related environment while the gneissic granodiorite formed in a post-collisional tectonic regime. Alongside previous studies on other Siderian granitoids in the Quanji Massif, we propose a tectonic evolution model, including (1) plate subduction occurring before 2.44 Ga; (2) a collision occurring from ca. 2.43 to ca. 2.40 Ga; (3) a post-collisional extension occurring from ca. 2.39 to ca. 2.32 Ga. This tectonic evolution reveals that the operation of plate tectonics continued during the Siderian and that a slowdown or pause might have occurred in the Quanji Massif after ca. 2.3 Ga.