Chronology, Geochemistry And Petrogenesis Of The Late Mesozoic Granitoids In The Pingnan-Nanping Area, Fujian

The Late Mesozonic igneous rocks are widespread in the South China tectonic block, and the interpretation for the petrogenesis and geological setting of these igneous rocks is uncertain. In this paper, the Pingnan-Nanping granitoids was selected as an example from the northeastern area of Fujian province for a detailed geochronological and geochemical study in order to discuss its petrogenesis and tectonic dynamic background. The dating results show their LA-MC-ICP-MS U-Pb zircon ages for granite porphyry and granite are 145.5±1.1Ma、146.0±1.1Ma、148.3±1.0Ma and 108.3±0.7Ma respectively in the Pingnan area, indicating that they were generated from late Jurassic to early cretaceous. What’s more, the dating results show their LA-MC-ICP-MS U-Pb zircon ages for granite and biotite granodiorite are 154.0±0.9Ma、153.6±0.9Ma respectively in the Nanping area, indicating that they were generated from late Jurassic. Zircon Hf isotopic compositions of the Pingnan pluton is highly variable with a εHf(t) variation range over 10εHf units(-18.25~-6.47). Such large variation in zircon Hf isotopic compositions suggests that mantle derived materials have contributed magma genesis. The Pingnan pluton is weakly peraluminous(A/CNK<1.1) and belongs to high contents of silica and alkalis, low abundances of calcium, magnesium and iron. They are enriched in Rb、Th、K、U、Pb and depleted in Ba、Sr、Ti、P、Eu. Their TFe O/Mg O ratios are relatively low, and 10000×Ga/Al ratios and Zr+Nb+Ce+Y contents are less than the low-limit values of the A-type granites. Zircon saturated temperatures of the Pingnan pluton are relatively high(758~774℃). Integrated geological and geochemical data suggest that the Pingnan pluton belongs to the high-K calc alkaline I-type granites. The roll-back and break-off of the subducting Paleo-Pacific Plate from late Jurassic to Cretaceous facilitated a rapid and strong upwelling of the asthenospheric mantle. During this process, magmatic underplating provided the necessary heat to cause partial melting of Paleoproterozoic crust. Meanwhile, Zhenghe-Dapu fault belt was activated again, the molten magma with a small amount of mantle derived materials rising along its cracks, and forming granitoids of the research area.