云南普朗复式岩体地质地球化学特征及成因

Geological and geochemical feature and petrogenesis of Pulang complex,Yannan Province,China

根据接触关系和岩石特征将普朗复式岩体划分为3个阶段的侵入岩,由早至晚分别为石英闪长玢岩、石英二长斑岩和花岗闪长斑岩,锆石U-Pb年代学研究揭示侵入岩形成于221～206Ma之间,形成于印支期。岩石地球化学特征表明,岩石富硅、高铝、低碱且相对富钠,由早至晚岩石向富硅、富碱、高钾、低铝、低钙镁的方向演化。Cs、Rb、K、Ba、Sr等大离子亲石元素(LIL)在岩石中明显富集,Y、Hf、Zr、Ti、Nb、Ta等较不活泼的高场强元素(HFS)则在岩石中相对亏损,岩石显示负的Nb异常和高的正Pb异常,表明岩浆起源于上地幔或下地壳,且地壳物质参与了岩浆作用过程。结合Nd同位素测试结果,认为不同阶段的侵入岩为同源岩浆演化和侵入作用的结果。通过构造环境图解判别,斑岩体形成于主动大陆边缘的火山弧构造环境,是洋壳沿甘孜-理塘海沟向西俯冲的产物。

Pulang complex can be divided into 3-stage intrusion based on contact relationship and lithologic features, namely quartz dioritic porphyrite, quartz monzonite-porphyry and granitic diorite porphyry. It is indicated by the zircon U-Pb chronology resulte that these intrusion formed in indo-chinese epoch, between 221 and 206Ma. According to the geochemical feature, these rocks are high in Si and A1, low in alkali but relatively rich in Na. Along with those development, they become richer in Si, alkali and K, lower in Al, Ca and Mg. Such LIL elements including Cs, Pb, K, Ba and Sr are apparently concentrated, while opposite for those inactive HFS elements including Y, Hf, Zr, Ti, Nb and Ta. Generally the rocks display negative Nb and positive Pb anomaly, which indicate that the magma originated from upper mantle or lower crust with crust substance involved in. Combines with Nd isotope test results, it is agreed that these mtflti-phase intrusions derived from comagrnatic evolution and intrusion. With the help of tectonic background, porphyry rocks formed in volcanic arc structural environment of active continental margin, which can be regarded as the outcome of westward subduction of oceanic crust along Ganzi-Litang trench.