Petrochemical Characteristics and Geological Significance of Middle Triassic Granodiorites from Lalinggaoli River Area in Qinghai

The intermediate-acid intrusive rocks are intensively developed in Lalinggaili river area, Qinghai, which is a rock combination of diorite, quartz diorite, tonalite, granodiorite and monzonitic granite with fine grain. Through the analysis of the granite diorite, the results show that SiO₂, Al₂O₃, K₂O, Na₂O, CaO values and σ indexes range from 67.01% to 70.92%, from 14.13% to 15.44%, from 3.18% to 4.03%, from 2.78% to 3.74%, from 2.6% to 4.15% and from 1.6 to1.92, respectively. The geochemical characteristics suggest that these granites belong to high potassium calc-alkaline series. REE characteristics show that, the ∑REE values are 106.48×10^-6~145.87×10^-6, LREE/HREE ratios vary from 8.967 to15.136, and the δEu value range from 0.504 to 0.83. The spider patterns are the right-incline, with low Eu positive abnormality and LREE-rich smooth curves. The trace element characteristics indicate that they are enriched in LILE (Rb, Ce, Th, LREE) and depleted in HFSE (Nb, P, Ti, Sr, Ba). The LA-ICP-MS zircon U-Pb age of granodiorite is (234.9±1.3)Ma, which was formed in Middle Triassic epoch. By constructing environmental analysis, it is believed that the granodiorite from Lalinggaili river area is came from the crust-mantle mixed source, which was served as the product of crustal melting during intracontinental subduction stage, with the participation of mantle materials.