The ~2.4 Ga granitoids in the Quanji Massif，NW China: Petrogenesis and Implication for the early Paleoproterozoic Tectonics

Abstract:

The 2.4–2.2 Ga collision-related magmatism had once been considered to be weak or lacking, and were interpreted to represent the stagnation and deceleration of plate subduction. However, large-scale intermediate to acid intrusions at 2.4–2.2 Ga are widely reported in the Trans-North China Orogen and Tarim Craton, NW China. Here we presented geochronological and geochemical and Nd-Hf isotopic studies of the Hudesheng and Delingha granitoids in the Quanji Massif, which could shed new light on the NW China and global tectonics in the Paleoproterozoic.

The two granitoids are exposed in the eastern and central parts of the Quanji Massif, showing similar rock types and including mainly the syenogranitic, monzogranitic and adamellitic gneisses, with ~2.41 Ga low P/T-type amphibolite enclaves. They were formed at 2.37–2.39 Ga and underwent medium P/T-type amphibolite-facies metamorphism at ~1.95–1.90 Ga. Both granitoids have relatively high Si, K contents and FeO^*/Mg, 10⁴Ga/Al ratios, and are characterized by a relative enrichment in LILEs and LREEs, but depletion in Ba, Sr, Eu and Nb. These signatures are similar to those of the high-K calc-alkaline I-type granites, and with some geochemical features of A-type granites as well, suggesting that their formation might be related to a post-collisional setting with transition from compression to extension regimes at ~2.4 Ga. The whole-rock Nd and zircon Hf isotopes suggest important crustal growth occurred at ~2.5–2.8 Ga, and the precursor magmas of both granitoids were derived from partial melting of the Neo-Archean juvenile and ancient crustal components.

Collectively, the ~2.37–2.39 Ga magmatic activities in the Quanji Massif took place right after arc-related metamorphism and arc-continent collision, thus probably making up part of the arc-related ~2.4–2.2 Ga magmatic activities, including those in the Trans-North China Orogen and Tarim Craton, NW China. This implies that a prolonged strong subduction-accretion-collision process occurred in the North China, consistent with those magmatic events having been discovered recently worldwide.

This study is supported by National Natural Science Foundation of China (Grant Nos. 40972042, 41172069 and 41372075)