青海大黑山钨矿黑云二长花岗岩的锆石U-Pb同位素定年及岩石地球化学特征

大黑山钨矿位于祁连山加里东造山带,其形成与宝库河黑云二长花岗岩密切相关。黑云二长花岗岩锆石LA-ICPMS U-Pb测年结果显示其形成年龄为:450.2±2.8Ma,为加里东期岩浆活动的产物。地球化学数据显示,宝库河黑云二长花岗岩富硅(SiO2含量为73.03%～74.18%)、富钾(K2O/Na2O为1.13～1.94,K2O+Na2O含量为7.25%～8.51%)、铝过饱和(A/CNK为1.04～1.12),为过铝质钙碱性-高钾钙碱性花岗岩。P2O5含量低(0.03%～0.08%),且具有随SiO2含量的增长呈现负增长的趋势。稀土含量低,Eu明显负异常,LREE分异强烈,HREE分异不明显。微量元素蛛网图中Th、U、Pb、Zr、Hf呈现明显的正异常,Ba、Sr、Ta、Nb、P、Ti呈现负异常,为I型花岗岩。结合对区域动力地质背景的分析,表明宝库河黑云二长花岗岩形成于活动大陆边缘,由地壳物质熔融并结晶形成。

Zircon U-Pb isotopic dating and lithogeochemical features of the biotite monzonitic granite in Daheishan tungsten deposit, Qinghai Province, China

The Daheishan tungsten deposit related to the biotite monzonitic granite from Baokuhe plutons locates at the Caledonian orogeny of Qilian Mountains. LA-ICP-MS U-Pb dating of zircons shows that the formation age of biotite monzonitic granite is 450.2±2.8Ma, indicating that the pluton was formed at Caledonian magmatic-thermal event. Geochemical data reveal that the biotite monzonitic granite from Baokuhe plutons is rich in silicon (73.03%~74.18%), alkalis (K₂O/Na₂O=1.13~1.94, K₂O+Na₂O=7.25%~8.51%) and is peraluminous (A/CNK=1.04~1.12). The biotite monzonitic granites from Baokuhe peraluminous plutons should be high potassic calc-alkaline granite. P₂O₅ content of the granites is low, and shows a trend of decrease with the increase of SiO₂ content. The content of rare earth element is low. Chondrite-nornalized REE patterns are right-inclined and display significant negative Eu anomalies. Differentiation of LREE is strong, and that of HREE is weak. Based on spider diagram of trace elements, Th, U, Pb, Zr, Hf are obviously positive anormaly, and Ba, Sr, Ta, Nb, P, Ti are negative anormaly. It suggests that it is an I-type granite. Combined with the tectonic evolution of the North Qilian, we suggest that the biotite monzonitic granite from Baokuhe plutons formed in the active continental margin, and the magmas were derived from the melting of crustal rocks, followed by crystal fractionation.