The role of mafic microgranular enclaves in the generation of Early Cretaceous granitic rocks of SE China: evidence from zircon U-Pb geochronology, geochemistry, and Hf isotopic data for the Liangnong pluton, eastern Zhejiang Province

摘要

In order to constrain the timing and petrogenesis of both the hosting rocks and the inner mafic microgranular enclaves (MMEs) of the Liangnong pluton, SE China, we have performed a series of bulk-rock geochemistry, zircon U-Pb, and Hf isotopic analysis, respectively. Zircon laser ablation-inductively coupled plasma-mass spectrometry U-Pb isotopic analysis yielded ages of 106.3 +/- 1.1 Ma for the granodiorite and 103.9 +/- 1.6 to 105 +/- 1.8 Ma for monzogranite phases within the hosting pluton, as well as an age of 104.7 +/- 0.8 Ma for the associated MMEs. The host rocks are metaluminous, have A/CNK values of 0.91-1.09, contain relatively high concentrations of SiO2 and K2O, are enriched in Rb, Th, Ba, Zr, and Hf, are depleted of Sr, P, Ti, Nd, and Ta, contain high concentrations of the rare earth elements (REE) and the light REE, and have moderately negative Eu anomalies (Eu*/Eu = 0.6-0.8). In comparison, the MMEs contain high concentrations of Al2O3, FeO, MgO, and TiO2, are relatively enriched in Ba, U, and Sr, and are depleted in Th, Nd, and Zr. They have lower total REE concentrations and higher Eu*/Eu values than the hosting granites. The zircons within the hosting granites have Hf crustal model ages (T-DM(C)) that show a peak at 1.29-1.85 Ga. Zircons within the MMEs have different eHf(t) values (-3.7 to + 4.9) than the zircons within the hosting granites (-10.8 to -1.9). The results indicate that the MMEs and the hosting granites crystallized from magmas with different sources, thereby showing that the Early Cretaceous magmatism in the coastal areas of SE China was generated by the widespread injection of mantle-derived magmas caused by rollback of the subducting palaeo-Pacific Plate.