柴北缘早古生代岛弧火山岩中埃达克质英安岩的发现及其地质意义

The discovery of adakitic dacite in Early Palaeozoic island arc volcanic rocks on the northern margin of Qaidam basin and its geological significance

在柴达木北缘吉绿素滩间山群火山岩中发现埃达克质 (adakitic)英安岩 ,其主要地球化学特征如下 :SiO26 3.7%～ 6 4.3%,Al2 O3 15 .0 7%～ 15 .2 7%,MgO 3.47%～ 3.72 %,K2 O/Na2 O 0 .14～ 0 .2 5 ,Y和Yb含量较低 ,Y为17.82× 10 -6～ 18.10× 10 -6,平均 17.96× 10 -6,Yb为 1.47× 10 -6～ 1.5 5× 10 -6,Sr含量较高 ,为 5 2 7× 10 -6～ 5 37× 10 -6,亏损HREE ,具弱负Eu和Sr异常。除MgO高于正常的埃达克岩外 ,与世界上典型埃达克岩极为相似。据其地球化学研究结果并结合该区埃达克质英安岩 (5 14.2± 8.5Ma)和榴辉岩 (494.6± 6 .5Ma)的年龄数据 ,推测柴北缘板片在晚寒武世开始俯冲 ,俯冲板片在 75～ 85km深处开始部分熔融形成埃达克质岩浆 ,而熔融残留部分继续俯冲 ,在早奥陶世形成含石榴石残留相的榴辉岩。柴北缘埃达克质英安岩的发现进一步证实了早古生代柴北缘发生过洋陆俯冲作用。

The dacite cropping out at Jilusu on the northern margin of Qaidam basin has lots of adakitic features: SiO 2 63.7%～64.3%, Al 2O 3 15.07%～15.27 %, MgO 3.47%～3.72 %, K 2O/Na 2O ratio is about 0.14～0.25, Y and Yb are both low: Y is 17.82×10 -6～18.10×10 -6, 17.96×10 -6 on average, and Yb is 1.47×10 -6～1.55×10 -6, Sr is high (527×10 -6～537×10 -6), HREE is depleted and Eu and Sr negative anomalies are relatively weak, but the content of MgO is higher than that of the typical adakite in the world. Generally, the presence of adakite signifies the beginning of slab subduction. The age of adakitic dacite is about 514.2± 8.5 Ma, and the age of eclogite from the northern margin of Qaidam basin is 494.6±6.5 Ma. Therefore, it is considered that the slab started its subduction in Late Cambrian, that the adakitic magma was formed at the subduction depth of about 75～85 km, and that the partial melting residues with garnet facies continued subducting and finally formed eclogite in Early Ordovician. The discovery of adakitic dacite confirms that ocean-continent subduction occurred in Early Palaeozoic on the northern margin of Qaidam basin.