Early Cretaceous granitic dykes in the western Dabie Orogen, China: Geochronology, petrogenesis, and tectonic implications

摘要

Whole-rock major-trace elemental, whole-rock Sr-Nd, and zircon U-Pb-Hf isotopic analyses have been carried out on two suites of Early Cretaceous granitic dykes in the western Dabie Orogen (Central China) with the aim of addressing their petrogenesis and geodynamic significance. LA-ICPMS zircon U-Pb dating reveals that the E-W-trending granitic dykes have emplacement ages of 133.9 +/- 1.6 and 132.8 +/- 1.0 Ma, while the NW-trending granitic dykes have emplacement ages of 121.4 +/- 0.9 and 120.5 +/- 0.7 Ma. These rocks have high concentrations of SiO2 and K2O, belonging to the high-K calc-alkaline and shoshonitic series. They are metaluminous to peraluminous and are mainly fractionated I-type granite. The E-W-trending granites have moderate initial Sr-87/Sr-86 ratios (0.70614 to 0.70878) and negative epsilon(Nd)(t) values varying from -16.2 to -17.2 with two-stage Nd model ages of 2.20 to 2.43 Ga. They exhibit a narrow range of zircon epsilon(Hf)(t) values (-22.34 to -23.70) with two-stage Hf model ages of 2.31 to 2.36 Ga. These isotopic signatures suggest a homogeneous crustal source without significant magma contamination or mixing. The NW-trending granites have initial Sr-87/Sr-86 ratios ranging from 0.70774 to 0.71034 and negative epsilon(Nd)(t) values varying from -10.9 to -9.5 with two-stage Nd model ages of 1.69 to 1.80 Ga. They yield variable zircon epsilon(Hf)(t) values of -17.06 to -0.39 with two-stage Hf model ages of 1.07 to 1.99 Ga, implying heterogeneous crustal signatures with minor mantle materials involved. Combining the geological, geochemical, and isotopic evidences, the E-W-trending granitic dykes are considered to be derived from partial melting of crustal materials under normal thickness during a transitional period following the tectonic collapse, while the NW-trending granitic dykes resulted from crust melting in an extensional setting, most likely induced by the asthenospheric upwelling. Our results support that the western Dabie Orogen could have experienced S-N-directed compression (pre. 133 Ma), S-N-directed extension (ca. 133 Ma), and general E-W-directed extension (ca. 120 Ma). We propose that the Dabie Orogen has experienced tectonic collapse of the thickened lower crust and also has been affected by the subduction of the Izanagi (or Palaeo-Pacific) Plate during the Early Cretaceous.