摘要:
西藏阿里地区革吉县乃不扎康巴流纹斑岩是班公湖—怒江成矿带南缘新发现的一套次火山岩,本文在地质调查和资料收集的基础上,开展了流纹斑岩锆石U-Pb定年和岩石地球化学和锆石Hf同位素特征研究.流纹斑岩具有高硅(SiO2=73.93%~74.71%)、高Na2O+K2O含量(6.73%~7.30%),低TFe和MgO含量等特征,属于弱过铝质高钾钙碱性系列且具有I型花岗岩地球化学特征;轻重稀土元素分馏明显,富集轻稀土元素,具有明显的负Eu异常,其LREE/HREE值为9.71~12.38,(La/Yb)N值为10.64~14.42,δEu=0.65~0.73;流纹斑岩相对亏损Nb、Ta、Ti等高场强元素,富集Rb、Th、U等大离子亲石元素,与典型活动陆源火山岩的微量元素特征类似;LA-ICP-MS锆石U-Pb定年结果表明乃不扎康巴流纹斑岩的形成时间为(151.6±1.2)Ma(MSWD=0.9),锆石εHf(t)值整体为负值(–9.4~2.6,平均为–3.1)且变化范围较大(可达11个ε单位)呈现出混合源区特征.Hf同位素数据和全岩锆石饱和温度(750~791℃)表明幔源物质在流纹斑岩形成过程中起到了重要作用.结合区域构造演化特征,认为该套流纹斑岩主要产于Slainajap洋的南向俯冲的背景之下,由幔源岩浆提供热量诱发古老拉萨下地壳发生部分熔融并与幔源岩浆发生混合形成初始母岩浆,之后经历一定程度的分离结晶作用形成.
Abstract:
Zircon U-Pb age, geochemistry and zircon Hf isotopes of the Naibuzhakangba rhyolite porphyry in Geji area of Tibet are reported in this study. LA-ICP-MS zircon U-Pb dating indicates that the Naibuzhakangba rhyolite porphyry was emplaced at (151.6±1.2) Ma (MSWD=0.9). Geochemical data show that the rhyolite porphyry rocks are weakly peraluminous and have high content of SiO2 (73.93%~74.71%) and Na2O+K2O (6.73%~7.30%), and low content of TFe and MgO, belonging to high-K calc-alkaline series. These rhyoliteporphyry rocks are enriched in LREE and show obvious negative Eu anomalies in the chondrite-normalizeddiagram and significant negative Nb, Ta, Ti and P anomalies in the primary mantle-normalized diagram. Theserocks have LREE/HREE ratios of 9.71~12.38, (La/Yb)N values of 10.64~14.42, and δEu values of 0.65~0.73. Therhyolite porphyry rocks have geochemical features of subduction-related I-type granitoids and have negativezircon εHf(t) values (–3.1 on average). The wide range of zircon εHf(t) values (up to 11ε units) indicates that therhyolite porphyry has a mixture source region. Hf isotopes and whole-rock Zr saturation temperature of theserocks (750~791℃) indicates that the mantle magma played an important role in the formation of the parentmagma. In combination with the regional geology, the results obtained by the authors indicate that the rhyoliteporphyry was formed in the southward subduction setting of the Slainajap Ocean. Thus, the mantle-derivedmagma provided the heat and materials for the partial melting of ancient mafic lower crust and was mixed with it,and then the mixed magma moved upward and experienced the fractional crystallization process to form theNaibuzhakangba rhyolite porphyry.
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