Zircon U-Pb geochronology and geochemistry of two episodes of granitoids from the northwestern Zhejiang Province, SE China: Implication for magmatic evolution and tectonic transition

摘要

Granitoids (175-80 Ma) representing a prominent Yanshanian (Jurassic to Cretaceous) magmatic event in South China widely intrude the Precambrian crystalline basement and Paleozoic strata. Here we report zircon U-Pb age data, geochemical characteristics and Sr-Nd isotopes of the Late Jurassic and Early Cretaceous granitoids from the northwestern Zhejiang Province (ZXB) of southeastern China. Our results reveal two distinct episodes for the Yanshanian magmatism. The Jiemeng and Datongkeng granodiorites formed at 148.6 ± 1.1 Ma, whereas the Huangshitan, Jiuligang and Ruhong aluminous A-type granites were generated between 129.0 ± 0.6 Ma and 126.1 ±1.1 Ma. The two magmatic phases represent a tectonic transition from an active continental margin to post-orogenic setting during the Late Jurassic (ca. 150 Ma) to Early Cretaceous (ca. 128 Ma). Geochemically, these intrusions are granodioritic to granitic in composition and show an affinity of S-type and A-type granitoids, respectively. The S-type granodiorites of Jiemeng and Datongkeng are characterized by moderate SiO2 (65.0-69.6 wt%), high K2O + Na2O (5.0-7.6 wt%), K2O/Na2O (1.2-1.5), Zr (31-109 ppm), Sr (71-190 ppm) and high field strength elements, low to intermediate Mg#, and moderate Nb depletion. The A-type granites of Huangshitan, Jiuligang and Ruhong are characterized by high SiO2 (72.7-77.2 wt%), K2O + Na2O (6.9-8.8 wt%), K2O/Na2O (13-2.1), Fe_T(Fe_T + Mg),Ga(17-29 ppm,>20 ppm commonly), Zr (96-197 ppm) and Sr (8-45 ppm) with slight Nb depletion. The S-type granodiorites have higher Mg#, A/NK, Sr, Sr/Ba, Sr/Y, (La/Yb)_N. and LREE/HREE, and lower SiO2, K2O + Na2O, Ga and Zr with weak negative Eu anomalies compared to those of the A-type granites with negative Eu anomalies. All these rocks show Y/Nb ratios > 1.2, high initial ~(87)Sr/~(86)Sr (I_(Sr)) ratios and low ε_(Nd)(t), and are depleted in Nb, Ti and Sr, indicating crustal origin with subduction zone signatures. We suggested that the ZXB S-type granitic bodies might have been derived from the Mesoproterozoic metamorphic basement rocks through partial melting induced by mantle-derived magma, followed by limited fractional crystallization. The ZXB aluminous A-type granites were also derived from a similar magma source but underwent fractional crystallization at higher crustal levels. The A-type granites in the ZXB correlate with a post-orogenic tectonic setting. A geological comparison between ZXB and adjacent areas indicates that the geochemical features of the ZXB A-type granites are comparable with the Baijuhuajian A-type granites and also the adjacent areas A-type granites, but are distinct from the Late Cretaceous A-type granites (105-90 Ma) distributed along the southeastern coastal area of South China We correlate the formation of the ZXB S-type granodiorites to inland compression associated with the subduction and collision of the paleo-Pacific plate in the Late Jurassic (170-145 Ma). In contrast, the A-type granites formed under a post-orogenic setting during the Early Cretaceous period (145-120 Ma) resulting from lithospheric thinning and continent extension accompanied by slab roll-back of the paleo-Pacific plate following the subduction-collision event