西藏冈底斯南缘中新世含矿斑岩源区组成与成因

Petrogenesis of the Miocene Ore-Bearing Granite Porphyries in the Southern Gangdese, Tibet

青藏高原南部中新世下地壳流动是当前大陆动力学研究的热点,关键科学问题是下地壳的流动方向。LA-ICP-MS定年结果表明冲江斑状黑云母二长花岗岩的形成时代为14.9~14.8 Ma,朱诺斑状花岗岩的形成时代为15.3~14.9 Ma。含矿埃达克质斑岩的特征如下:Si O2含量67.72%~74.49%,K2O含量2.85%~5.98%,Sr含量93~804μg/g,高Sr/Y(16~139)、(La/Yb)N(21~43)比值,Eu/Eu*值为0.6~0.91。冲江岩体锆石εHf(t)值为1.2~5.1,朱诺岩体锆石εHf(t)值为–6.9~–0.1,他们与徐旺春等报道的镁铁质麻粒岩的锆石Hf值(–2.5~4.8)具有很好的叠合性,暗示镁铁质麻粒岩(印度镁铁质下地壳)可以作为他们的岩浆源区。此外,Sr-Nd同位素表现出雅鲁藏布江蛇绿岩和拉萨地块两个端元混合的特征,Pb同位素表现出雅鲁藏布江蛇绿岩和喜马拉雅富集陆壳端元的特征。上述同位素地球化学特征表明,冈底斯中新世埃达克质斑岩的岩浆源区物质组成包括:拉萨陆壳、印度陆壳、雅鲁藏布江蛇绿岩(地幔成分),表现出加厚下地壳部分熔融特征,暗示青藏高原南部由南向北的下地壳流动方向。

The Miocene lower crust flow in the southern Tibetan plateau is one of the present significant topics regarding continental dynamics, but the direction of lower crust flow has not yet been well constrained. Here we report an integrated study of zircon U-Pb ages, major and trace element geochemistry, and Sr-Nd-Pb-Hf isotope compositions of the ore-bearing adakite porphyries from the Chongjiang and Zhunuo areas in the southern margin of the Gangdese, Tibet. The U-Pb ages revealed that both the Chongjiang porphyritic biotite monzogranite and the Zhunuo porphyritic granite formed at ca. 15 Ma. The zircons from the Chongjiang porphyritic biotite monzogranite haveεHf（t） values of 1.2~5.1, corresponding to two-stage model ages of 1.1~1.4 Ga, which indicate that the Chongjiang porphyritic biotite monzogranite were derived from a juvenile crust source region. The zircons from the Zhunuo porphyritic granite haveεHf（t） values of-6.9^-0.1, corresponding to two-stage model ages of 1.6~2.2 Ga, indicating that they originated from partial melting of ancient crust. The major and trace element data show that both the Chongjiang monzogranite and the Zhunuo porphyritic granite have geochemical characteristics similar to those of adakite rocks, indicating that they are both from partial melting of thickened lower crust. The Sr-Nd isotopic compositions revealed that the source of the Chongjiang and Zhunuo granites may include mixed components from both the Lhasa continental crust and the Yarlung Zangbo ophiolite. The Pb isotopic compositions show that they contain both the Yarlung Zangbo ophiolite and the Himalayan enriched continental crustal components. We conclude that the Miocene Gangdese adakite porphyry has strong indications of melting of the thickened lower crustal which mixed with the Lhasa and Indian continental crusts, the Yarlung Zangbo ophiolite, and the mantle. These findings further imply that the lower curst flowed from the southern Tibetan toward the north, which partly explains the emplacement mechanism of the adakite ore-bearing granite porphyry.