The Chronology, Geochemical Characteristics and Geological Significance of the Mesozoic Shiyaogou Hidden Granite at the East Qinling

The Shiyaogou granite is a related to the molybdenum mineralization hidden granite, which is found for the first time in the Xionger Mountain at the East Qinling, southern margin of North China landmass. The lithology of the granite is mainly medium-fine biotite monzogranite and porphyritic granite, and the LA-ICP-MS zircon U-Pb dating results indicate that it was formed in the early Cretaceous(140.45±0.75 Ma-136.64±0.55 Ma). The geochemical data show that the granite is characterized by SiO₂(70.27%-73.22%), Al₂O₃(12.71%-14.96%), MgO(0.23%-0.54%), the total alkaline(K₂O+Na₂O) ranging from 6.43% to 11.78wt%, which suggests that the granite has high silicon and rich alkali characteristics. The Rittmann Index (δ) ranges from 2.11 to 3.02, and AR ranges from 1.48 to 5.73, which shows the granite is calc-alkaline series. The A/CNK value is 0.95-1.01, and shows a Aluminum-peraluminous Ⅰ-type granite characteristic. The ∑REE of the granite ranges form 147×10^-6 to 322×10^-6, the ratio of LREE/HREE ranges from 15.2 to 25.2, and the value of La_N/Yb_N ranges from 19.10×10^-6 to 50.50×10^-6. The chonrite standardized distribution pattern is characterized by enrichment of LREE in the right-dipping type with medium-weak negative Eu anomalies(δEu=0.53-0.71, average 0.62). The trace elements are characterized by the value of Sr(133×10^-6-759×10^-6, average 371×10^-6), the low value of Y and Yb(Y=10.02×10^-6-18.80×10^-6, average 12.57×10^-6; Yb=1.16×10^-6-2.02×10^-6, average 1.40×10^-6), and the lower ratio of Sr/Y(12.77-61.66, average 30.44). The geochemical characteristics reflect the granite melt experienced feldspar fractional crystallization in the magma. The initial isotopic Sr ratio(I_sr=0.707 44-0.713 84), the initial epsilon Sr(ε_Sr(t)= 44.1-134.9) and Nd(ε_Nd(t)=-12.96 to -13.46) disclose the Shiyaogou hidden granite has a magma homology with the Mesozoic Heyu granite. The Nd model ages(t_DM2) of the granites are concentrated in 2.00-2.01 Ga. All the isotopic data suggest the granite formed by the melting of the South Qinling and Yangtze block crystalline basement and with participation of Taihua and Xionger groups. The regional geology and geochemical characteristics suggest that the formtion of the Shiyaogou granite experienced two stages: Before Jurassic, the crust of East Qinling thickened with the subduction-collision of South Qinling and Yangtze Block under the North China block; In the Jurassic and Cretaceous, when the extrusion environment changed to the extension condition, with the decompression and warming, partial melting of the middle-lower continental crust, finally formed the Shiyaogou granite in the Early Cretaceous.