Geochronological and Geochemical study of Palaeoproterozoic gneissic granites and clinopyroxenite xenoliths from NW Fujian, SE China: Implications for the crustal evolution of the Cathaysia Block

摘要

U-Pb zircon dating of gneissic granite samples in northwestern Fujian Province, Southeast China, gave Neoarchean to Palaeoproterozoic ages (2.6-1.9 Ga) for the rounded zircon cores and Palaeoproterozoic upper intercept ages (1851 ±21 Ma, 1857 ±29 Ma, respectively) for zoned rims that have oscillatory growth zoning and yield ε_(Hf)(t) values between -8.8 and +3.7 and T_(DM) model ages between 2.1 and 2.6 Ga. These gneissic granites are peraluminous (A/CNK = 0.94-1.28), characterized by high SiO_2 (68-72 wt.％), Al_2O_3 (14-15 wt.％) and low CaO, MgO, FeO~t, TiO_2 and P_2O_5 contents. They also possess relatively low REE contents and moderate LREE/HREE fractionation and display pronounced negative anomalies in Nb, Ta, Sr, P and Ti. All these features indicate that the precursor magmas were likely formed by partial melting of sedimentary rocks. Clinopyroxenite xenoliths with gneissosity structures are found in these gneissic granites and they have high MgO, Ni and Cr, but low contents of TiO_2, and are characterized by high CaO/Al_2O_3 (4.1-5.3), low Al_2O_3/TiO_2 ratios (8.7-9.5) and HREE depletion. These features are similar to the typical Al-depleted type komatiites. Their geochemical characteristics, such as high (Gd/Yb)_N and low HREE, Y, Zr and Hf suggest partial melting of the upper mantle source with residual garnet. The negative Nb, Ta and Ti anomalies indicate a strong arc-related affinity. Our results and recently published data for granitic and mafic basement rocks in this region, reveal that 1.85 Ga was an important period for the evolution of the Cathaysia Block, possibly related to the Columbia supercontinent assembly. Large amount of granitic magma was emplaced, probably as a result of partial melting of old Neoarchean to Palaeoproterozoic materials.