Geochronology and geochemistry of the alkaline granite porphyry and diabase dikes in Huanglongpu area of Shaanxi Province： Petrogenesis and implications for tectonic environment

Abstract：Granite porphyry and diabase dikes are outcropped sporadically in irregular shapes in Huanglongpu area, Shaanxi Province. Megacrysts of granite porphyry contain reddish orthoclase (~60%) and quartz (~40%), and the matrix comprises plagioclase (~15%) and biotite (~10%). The mineral assemblages of the diabase mainly consists of plagioclase (～45%), pyroxene (20%), amphibole (6%～7% ) and quartz (～10%). The composition of the granite porphyry exhibits geochemical signatures of high SiO2 (72.23%～74.86%) and alkali composition (K2O+Na2O=7.95%～10.25%) as well as peraluminous nature (A/CNK=1.32～1.72), and the projects of the composition in TAS shows that it belongs to alkaline granite porphyry. Its rocks are characterized by weakly inclined REE patterns with strong depleted Eu, enrichment of Th, U, Zr, Hf elements and depletion of large ion lithophile elements such as Ba, K, Sr, and high field strength element Ta. Its geochemical signatures and Pb isotopic compositions are similar to Mo-bearing granitoids in East Qingling region, implying that it was produced by partial melting of the lower crust and subjected to fractional crystallization during its evolution. The diabase contains SiO2 between 48.78% and 48.92% and has preserved flat, identical and rightly inclined REE patterns without obvious depletion of Eu. High initial Sr (0.718～0.762) and extremely low εNd values (-19.53～-19.69) of the samples are consistent with things of the mafic dikes occurring in the Jiaodong gold Peninsula, indicating an enriched mantle source. SHRIMP zircon U-Pb dating was conducted for both granite porphyry and diabase, and yielded（131±1）Ma（MSWD=1.5, n=13）and （129±2） Ma（MSWD=0.71, n=12）of weighted mean ages respectively. They are considered to be the products of lithospheric extensional environment of intraplate in Early Cretaceous age.