Paleoproterozoic tectonic transition from collision to extension in the eastern Cathaysia Block, South China: Evidence from geochemistry, zircon U-Pb geochronology and Nd-Hf isotopes of a granite-charnockite suite in southwestern Zhejiang

摘要

The Badu complex and associated Paleoproterozoic granitoids are among the oldest known rocks in the Cathaysia Block in South China. The Paleoproterozoic units' of the Badu complex are dominantly composed of metapelitic rocks and meta-greywackes. Here we report LA-SS-ICP-MS (laser ablation-split-stream inductively coupled plasma-mass spectrometry) zircon U-Pb data from a newly discovered garnet-bearing granite which show an emplacement age of 1929 ± 15 Ma and metamorphism at 1872 ± 34 Ma. We also report U-Pb ages of 1886 ±16 Ma, 1858 ± 7 Ma, 1848 ± 11 Ma from a gneissic granodiorite, and two charnockites respectively. The garnet-bearing granite is peraluminous with A/CNK range from 1.1 to 1.3. The rock shows relatively high SiO2, K2O and Rb contents, and low total REE, Sr, CaO and ferromagnesian components, typical of leucogranites. The whole rock Nd two-stage model age(T_(DM2)(Nd)) of this rock is ca. 2.7 Ga, zircon Hf crustal model ages(ToM(Hf)) peak at about 2.7 Ga, and abundant inherited zircons occur with U-Pb ages in the range of 2044 to 2803 Ma. Evidences from zircon U-Pb age and Hf isotope compositions, whole rock Nd isotopes and whole rock major and trace elements suggest a metasedimentary protolith, and that the garnet-bearing granite (leucogranite) was derived by partial melting in a thickened crust at about 1.93 Ga. The gneissic granodiorite and charnockites show indistinguishable major and trace element features, as well as zircon Hf and whole rock Nd isotope compositions, indicating that they were generated from the same source rocks. The gneissic granodiorite and charnockites are ferroan, calc-alkalic and metaluminous with A/CNK range from 0.80 to 0.98. They display relatively low SiO2 contents and Ga/Al ratios, suggesting their A-type affinity. Their zircon T_(DM)~C(Hf) age-peak is 2.9 Ga and whole rock T_(DM2)(Nd) ages range from 2.8 to 2.9 Ga. These high temperature rocks were generated possibly through the partial melting of ancient amphibolites (2.9 Ga), with heat input from astheno-sphere upwelling and magmatic underplating in an extensional setting at about 1.85-1.88 Ga. Combined with data from previous studies, we suggest three episodes of major crustal growth in this region at 1.8-1.9 Ga, -2.7 Ga and -2.9 Ga. During the 1.8-1.9 Ga event, extensive reworking also took place. From the 1.93 Ga syn-collision leucogranite (garnet-bearing granite), the 1.88-1.85 Ga post-collision A-type granites and charnockites (the granite-charnockite suite), and thel.85-1.77 Ga intraplate basaltic rocks (Li 1997 and Xiang et al. 2008) in the region, a Paleoproterozoic orogenic cycle can be deciphered in the Cathaysia Block. The rapid transition of tectonic regime from collision to post-collisional extension can be correlated to the assembly of the supercontinent Columbia.