闽西南地区中生代花岗闪长质岩石的特征及其构造演化

Characteristics of Mesozoic granodioritic rocks in southwestern Fujian and their tectonic evolution

闽西南地区中生代存在两期花岗闪长质岩石 ,其形成年龄分别为 183～ 16 2Ma和 10 8～10 5Ma。花岗闪长质岩石K2 O +Na2 O >6 .0 % ,晚期花岗闪长岩相对富K2 O ,均属板内高钾钙碱型岩石。岩石富LREE和LILE ,δEu =0 .80～ 0 .86 ,Nb、Ta、Ti、Y亏损 ,具有岛弧 /活动大陆边缘钙碱性岩石的特征 ,指示源区性质很可能与受到早期俯冲作用所改造的岩石圈富集地幔有关 ,或是源于软流圈的基性岩浆与地壳物质混合作用的结果。闽西南地区可能自早中生代以来存在多期次的岩石圈强烈伸展减薄作用 ,幔源岩浆与中下地壳物质部分熔融形成的花岗质岩浆的混合形成了闽西南钾质岩石。随扩张增强 。

Mesozoic granodiorites occurring in southwestern Fujian, whose ages are 183～162 Ma and 108～106 Ma respectively, belong to high_K calc_alkaline series of intraplate with K 2O+Na 2O>6.0%. Early and late granodiorites are enriched in Na 2O and K 2O respectively. The chondrite_normalized REE patterns are characterized by LREE enrichment with weak negative Eu anomalies (δEu=0.80～0.86). In the primitive mantle_normalized spidergrams, they are enriched in LILE and LREE with significant Nb, Ta, Ti, Y depletion and possess the geochemical features of calc_alkaline rocks in island arc or acive continental margin. The geochemical characteristics of the two types of Mesozoic granodiorites suggest that their source region may be an enriched lithosphere mantle affected and altered by ancient subduction materials or source mixing between asthenosphere_derived basalts and crust materials. Perhaps there have occurred many times of lithospheric extension and thinning since Early Mesozoic in southwestern Fujian. The granodiorites were mixed between partial melts of middle/lower crust_derived materials and mantle magma. With the increasing extension, the mantle components in Late Mesozoic granodiorites gradually grow.