青海省拉陵高里河一带中三叠世花岗闪长岩地球化学特征及其意义

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

青海省拉陵高里河一带集中分布的中酸性侵入岩的岩石组合为中细粒闪长岩、中细粒石英闪长岩、中细粒英云闪长岩、中细粒花岗闪长岩、中细粒二长花岗岩等。通过对花岗闪长岩的分析研究发现,SiO_2含量为67.01%~70.92%,Al_2O_3含量为14.13%~15.44%,K_2O含量为3.18%~4.03%,Na_2O含量为2.78%~3.74%,CaO含量为2.6%~4.15%,里特曼指数σ为1.6~1.92,过铝质高钾钙碱性系列花岗岩;稀土总量(∑REE)为106.48×10^(-6)~145.87×10^(-6),轻重稀土比(LREE/HREE)为8.967~15.136,轻稀土元素强烈富集,δEu=0.504~0.83,Eu的亏损较为明显,具较强的负Eu异常;岩石中Rb、Th、Ce富集,Ba、Nb、Sr、P、Ti不同程度的亏损。在花岗闪长岩中获得LA-ICP-MS锆石U-Pb年龄为(234.9±1.3)Ma,形成时代为中三叠世。通过构造环境分析,认为拉陵高里河一带分布的花岗闪长岩为壳幔混合源成因,属陆内俯冲阶段地壳物质部分熔融的产物,形成过程中有地幔物质的参与。

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.