摘要
浙东地区晚中生代花岗岩类在岩性上分为三类:花岗岩-二长花岗岩、钾长花岗岩和A型花岗岩。对后两类花岗岩已有较多研究,但对前一类,尤其是二长花岗岩的研究还较薄弱。选择浙东具代表性的北漳和梁弄二长花岗岩体及其所含暗色岩石包体,以及共生的石英闪长岩类,通过系统的岩石学与地球化学对比研究,提出浙东二长花岗岩属准铝质、高钾钙碱性I型花岗岩类演化系列,暗色岩石包体是由花岗质岩浆在深部析离出的镁铁质微粒包体(MME),成分特征类似于石英闪长岩,说明三者具内在成因联系,均与俯冲作用关系密切。
Late Mesozoic granites in eastern Zhejiang province can be subdivided into three types according to their lithology: monzonitic granites (with an isotopic age range of 85~101 Ma), K-feldspar granites (100~110 Ma), and A-type granites (90~109 Ma). The last two types are well studied, but no enough attention was paid to the first one. In this contribution, Beizhang and Lianglong plutons, as representatives of monzonitic granites in eastern Zhejiang, dark enclaves hosted in monzolitic granites, and associated quartz diorites are selected to discuss their characteristics and genetic relations. We have systematically studied their petrological and geochemical characteristics, and have concluded that monzonitic granites in eastern Zhejiang belong to meta-aluminous, calc-alkaline series, I-type granites with a relatively higher Mg #. Both the monzonitic granites and the associated dioritic rocks show geochemical feature of volcanic arc magmas, characterized by enriched large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to the high field strength elements (HFSE). This is consistent with early Cretaceous basalts from eastern Zhejiang. Major and trace elements (or ratios) covariations suggest that the monzonitic granite and dioritic rocks were resulted from mixing of mantle-derived basic magma and crust-derived acid magma in different proprotions. With a range of isotopic ages from 85 to 101 Ma, the formation age of monzonitic granite and quartz diorites is contemporary with the late Mesozoic A-type granites in coastal area of Zhejiang and Fujian Province. This may be a hysteresis effect of volcanic arc magma. The dark enclaves in Beizhang monzonitic granite are mafic microgranular enclaves (MME). The MME and the host rocks are similar in REE patterns, but the MME have higher total REE contents and the element ratios between the MME and host rock are nearly constant. The MME, which are relatively more mafic, contrarily have higher Rb, Zr, La, Nb, Cs, Tl, Ga contents, and lower Sr, Ba, Cr contents than the host rocks. This is quite different from the element distribution produced by crystallization-differentiation of magma, and indicates that some elements were enriched during formation of the MME. The ratios of Mg/(Mg+Fe) and Na/(Na+Ca) in the MME and their host rocks are very close (0.39~0.43 and 0.66~0.77 respectively), indicating that the formation of the MME is related to the immiscible fractionation of the magma.
出处
《高校地质学报》
CAS
CSCD
北大核心
2005年第2期264-275,共12页
Geological Journal of China Universities
基金
中国地质调查局1:25万嵊县幅区调项目(编号:20001300006141)
国家自然科学基金项目(批准号:40002005)
关键词
暗色岩石包体
北漳和梁弄花岗岩体
白垩纪
浙东
dark enclave
monzonitic granite
Beizhang and Liangnong
Cretaceous
eastern Zhejiang
