Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex using the sedimentary record in the Hefei Basin, China

Abstract

Jurassic rocks in the Hefei Basin were deposited by braided-fluvial and alluvial-fan systems, characterized by a general coarsening-upward sequence. Multiproxy provenance analyses demonstrate that the sediment source areas for the Hefei Basin are composed of a variety of rocks, including ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic rocks and Yangtze basement rocks of the axial Dabie Shan metamorphic complex, the Luzhenguan complex granite, low- and medium-grade metamorphic rocks, and the Yangshan Group sandstone in the North Huaiyang fold and thrust belt. A Middle Jurassic section in the western part of the basin is characterized by relatively high e Nd values (at 176 Ma), ranging from −13.8 to −11.3, whereas a section in the middle part of the basin has higher 147 Sm/ 144 Nd ratios, from 0.1168 to 0.1266 and somewhat lower e Nd values (at 176 Ma), from −15.0 to −14.5. Sediments in a section in the eastern part of the basin have the lowest e Nd values (at 176 Ma), ranging from −22.0 to −14.6, the highest T DM values, from 1.8 to 2.4 Ga, and low 147 Sm/ 144 Nd ratios, from 0.0937 to 0.1067. Provenance analyses of detrital compositions and Nd isotopic compositions of the sediments in the Hefei Basin clearly demonstrate that the depth of exhumation in the Dabie Shan orogen increases from the west to the east; the unroofing ages of the UHP and HP metamorphic rocks change from Early Jurassic to Late Jurassic westward. The exhumation rate during the Late Triassic and Jurassic is inferred to have increased eastward from ∼1.4 mm/a to ∼2.5 mm/a on average. The sediments in the basin record the episodic thrusting events and periodic unroofing in the orogen.