Geochronology, Geochemistry And Petrogenesis Of Some Mesozoic Granite And The Assessing Of The Prospecting Potential For Uranium Deposit In The Northwest Fujian Province

Three Mesozoic granite plutons, including the Luoguyan granite, the Buhujian granite and the Zhouzhai granite, which are located in the northwest of Fujian Province, have been studied for LA-ICP-MS zircon U-Pb ages, whole-rock elemental and Sr-Nd-Hf isotopic compositions to constrain their sources and tectonic settings. And the mafic microgranular enclaves, which are hosted in the Buhujian granite, have been studied for whole-rock elemental and Sr-Nd isotopic compositions to constrain their sources. This study also assesses the prospecting potential for uranium deposit of the first granite, which were formed by the Indosinian magmatism events.LA-ICP-MS U-Pb zircon ages of two samples (LGY02and LGY10) for Luoguyan pluton are227.1±2.6Ma (LGY02) and234.4±1.5Ma (LGY10), indicating that this pluton was generated in Indosinian stage. Chemically, this pluton is metaluminous (most of the A/CNK values are lower than1.0), and show high contents of silica (SiO2>74%) and alkalis (K20+Na20=8.57%～9.22%, K2O/Na2O>1.4), low abundances of calcium, magnesium and iron. The rocks display low total REE contents (EREE=116.72～236.53ppm) and low contents of Zr+Ce+Nb+Y(<350ppm) with strongly negative Eu anomalies (δEu=0.17～0.37), and enriched in Rb, Nd, Th, U, Hf, and depeted in Ba, Nb, Sr, Zr, Ti. Overall, our integrated mineralogical and geochemical features suggest that the Luoguyan granites are highly fractionated I-type granites. Isotopically, samples show high and variable initial87Sr/86Sr ratios (0.7078～0.7130), yet low and constant sNd(t)(-12.17～10.70), corresponding to two stage model age of1.87～2.00Ga. However, their zircon Hf isotopes show large variations, with εHf(t) values varying between-13.0～-6.8, corresponding to two stage model age of1.69-2.08Ga. Combining with the discovery of Indosinian YangFang syenite located in Pucheng-Wuping fault zone and the Xiamao diabase originated from the enriched mantle, we suggest that the Luoguyuan pluton was derived from a mixed magma which mingled an enriched mantle-derived component similar to the Xiamao diabase with a crustal felsic component derived from the partial melting of basement Mayuan metamorphic rocks, during which the tectonic setting was in an extension environment, and then experienced a high degree of differentiation when crystallizing.LA-ICP-MS U-Pb zircon ages for the Buhujian pluton and the Zhouzhai pluton are149.1±1.2Ma and152.9±1.1Ma respectively, indicating that these two plutons were generated in early Yanshannian stage. Chemically, these two plutons are both metaluminous (most of the A/CNK values are lower than1.0), and they show high alkalis (K2O+Na2O=7.38%-8.57%and7.65%-8.44%, K2O/Na2O>1.0, respectively), low abundances of calcium, magnesium and iron, but Buhujian pluton shows lower contents of silica (SiO2=68.76%～71.30%) than Zhouzhai pluton (SiO2=71.80%～75.88). Comparing the mafic microgranular enclave with these two granite plutons, the mafic microgranular enclave shows lowest contents of silica (SiO2=53.42%) and highest contents of aptite (P2O5=0.50%), calcium (CaO=5.61%), magnesium (MgO=3.67%) and iron (FeOT=9.20%). These three groups of rocks display low total REE contents (∑EE=146.54-256.04ppm,151.70～202.10ppm and211.75ppm, respectively), with differently negative Eu anomalies (8Eu=0.66～0.89,0.17～0.37and0.46～0.71, respectively), and enriched in Rb, Th, U, La, Nd and Sm, and depeted in Ba, Nb, Sr and Zr. These two granite putons also show low contents of Zr+Ce+Nb+Y (<350ppm). Overall, our integrated mineralogical and geochemical features suggest that the Buhujian granites and the Zhouzhai granites are also I-type granites. Isotopically, samples show high and variable initial87Sr/86Sr ratios (0.70940～0.71165,0.71083～0.71127and0.70929, respectively), yet low sNd(/) values (-13.00～-9.74,-14.41～-12.32and-8.09, respectively), corresponding to two stage model age of1.73～2.00Ga,1.94～2.11Ga and1.60Ga. However, zircon Hf isotopes for the Buhujian pluton and Zhouzhai pluton show large variations, with εHf(t) values varying between-15.51～-9.97and-15.90～-9.89respectively, corresponding to two stage model age of1.84～2.18Ga and1.83-2.21Ga. Combining with the discovery of the mafic microgranular enclaves from the Buhujian granite pluton, we suggest that these two granite plutons were derived from a mixed magma which mingled an enriched mantle-derived component similar to the Huziyan balsalt with a crustal felsic component derived from the partial melting of basement Mayuan metamorphic rocks by variable extent of crust-mantle interaction, during which the tectonic setting was in an extension environment, and then experienced a different degree of differentiation when crystallizing.