LA-ICP-MS zircon U-Pb geochronology, petrochemistry of the Late Jurassic granite porphyry in central Zhejiang Province and their geological significance

Abstract: There are a few small granite porphyry plutons scattered in Houshandian and Cao’e area of central Zhejiang Province on the northeastern margin of South China block. The LA-MC-ICP-MS zircon U-Pb dating shows that they were both generated in Late Jurassic of the Mesozoic simultaneously (149.1±1.1 Ma and 150.3±1.6 Ma respectively), indicating that they both formed at the late stage of Early Yanshanian. The compositions of these two granite porphyry plutons fall into the high-K calc-alkaline category，with the A/CNK ratios being 0.96-1.28 (mostly less than 1.1). They are of metaluminous and weakly peraluminous series. They have moderate REE and strong light-heavy REE fractionation and strong Eu depletion with a slightly right inclined “sea-gull” shaped REE pattern. The spider diagrams of trace elements show that the granite porphyry samples are enriched in LILEs (e.g., Rb, Th and U) and HFSEs (e.g., Nb, Hf and Y) , but depleted in Ba, Sr, P and Ti significantly. The Houshandian granite porphyries belong to S-type granite and Cao’e granite porphyries are of A-type granite. Zircon εHf(t) values of the granites all are negative(-9.5 to -2.9 ), and two-stage Hf model ages (tHf2) range from 1364 Ma to 1783 Ma, suggesting that these granites originated from remelting of the Mesoproterozoic upper-crustal materials. Combined with the previous results, the authors infer that the Late Jurassic granite porphyries in central Zhejiang Province on the northeastern margin of South China block were formed in a post-orogenic extension environment. The formation age may represent a late stage of conversion from the Paleo-Tethys tectonic domain to the Paleo-Pacific tectonic domain.