Early crustal evolution of the eastern Yangtze Block: Evidence from detrital zircon U-Pb ages and Hf isotopic composition of the Neoproterozoic Huashan Group in the Dahongshan area

Abstract

The Yangtze Block is separated into the eastern Yangtze Block (EYB) and western Yangtze Block (WYB) by the Chongqing–Huaying line (CHL). An integrated study of detrital zircon Lu-Hf isotopes and U-Pb ages from two sandstones in the Huashan Group together with previous studies reveals six major tectonothermal events in the EYB occurring at about 2.87Ga, 2.67Ga, 2.5Ga, 2.0Ga, 1.84Ga and 0.8Ga. Combined with paleogeographic map, this study reveals distinct detrital zircon U-Pb ages and Hf isotopic characteristics in the western EYB and eastern EYB. The former has most zircon U-Pb ages grouped at about 2.0Ga, 2.67Ga and 2.87Ga with sporadic 2.5Ga ages. The ∼2.5Ga zircons show negative to slightly positive εHf(t) values. However, the latter has significant ages grouped at ∼2.0Ga and ∼2.5Ga with negligible 2.67Ga and older zircon ages. The ∼2.5Ga zircons show negative to high positive εHf(t) values. The Early Neoarchean (2.7–2.6Ga) was a significant period for production of mature continental crust in the western EYB and the majority of crustal materials were extracted from the depleted mantle until then. Zircon Hf isotopic compositions indicate that the Late Neoarchean to Early Paleoproterozoic event (∼2.5Ga) involved both reworking of old crust and juvenile crust growth. With previous studies of sedimentary facies, lithochemical compositions, zircon morphology and zircon Hf isotopes, it is proposed that there are unexposed ∼2.5Ga basement rocks in the EYB. During the 2.2–1.8Ga, subduction processes, collisional and post-collisional events were recorded in the north and west of the EYB, which can be correlated to the assembly and breakup of the supercontinent Columbia. Two changes in εHf(t) value at ∼840Ma and ∼790Ma have been found from detrital zircons in the northern margin of the EYB, which probably represent arc–continent collision and continent–continent collision events, respectively. This long-term subduction process occurring at 1100–790Ma in the EYB appeals for a peripheral position for the Yangtze Block in the supercontinent Rodinia.