西藏当雄地区拉屋矿床二长花岗岩地球化学特征及构造背景

Geochemical signatures and tectonic setting of monzogranites in the Lawu ore deposit, Damxung, Xizang

关键词:  过铝质花岗岩  地球化学  构造环境  拉屋  西藏

Key words:  peraluminous granite  geochemistry  tectonic setting  Lawu  Xizang

摘要:

西藏拉屋矿床位于冈底斯铜铅锌多金属成矿带东段。通过对拉屋矿区出露的二长花岗岩地球化学研究表明,岩石中SiO₂、Al₂O₃和CaO等的含量均高、贫Fe和低Na₂O;w(SiO₂)含量为65.40%~74.43%, A/CNK值在1.41~2.20之间,为强过铝质花岗岩。岩石稀土总量ΣREE (不含Y)较低,为(5.08~80.38)×10^-6, La_N/Yb_N值为2.43~5.65,ΣLREE/ΣHREE为1.63~3.64,稀土元素配分型式为右倾型,具负铕异常。Nb、Ti、Zr、Ce等高场强元素和Ba明显的亏损,K、Rb、Sr等大离子亲石元素明显的富集。综合研究认为,本区花岗岩的物质来源于上部陆壳,岩浆源区岩石成分为泥质岩;该岩体为形成于同碰撞构造环境、地壳加厚阶段的过铝质花岗岩。

Abstract:

The Lawu ore deposit is located in the eastern part of the Gangdise Cu-Pb-Zn polymetallic metallogenic belt in Damxung, Xizang. The geochemical signatures have revealed that the monzogranites outcropped in the Lawu ore deposit are rich in SiO₂(65.40%-74.43%), Al₂O₃ and CaO, and poor in Fe and Na₂O. The A/CNK ratios range between 1.41 and 2.20, suggesting the highly peraluminous granites. The ΣREE values vary between 5.08×10^-6 and 80.38×10^-6. The La_N/Yb_N ratios range between 2.43 and 5.65. The ΣLREE/ΣHREE ratios range from 1.63 to 3.64. The REE distribution patterns display the right-leaning curves, and negative Eu anomalies. The high field strength elements such as Nb, Ti, Zr and Ce as well as Ba are markedly depleted while the large ion lithophile elements such as K, Rb and Sr are markedly enriched. It follows that the monzogranites in the Lawu ore deposit were derived from the upper crust, and the source rocks are represented by mudstones. The peraluminous granites cited above are interpreted to be formed in the syn-collisional tectonic settings during crustal thickening.