青海阿尼玛卿蛇绿混杂岩带西段哥日卓托杂岩体年代学、地球化学及Hf同位素

Geochronology,Geochemistry and Hf Isotopes of Gerizhuotuo Complex Intrusion in West of Anyemaqen Suture Zone

布青山构造混杂岩带中的哥日卓杂岩体与区域构造线方向不一致,属于典型的＂钉合岩体＂,对限定地体拼贴或增生的时间上限具有特殊的构造年代学意义.锆石LA-ICP-MS U-Pb定年结果显示岩体形成于晚三叠世中期（似斑状花岗岩形成于227Ma,闪长岩形成于224Ma）.在地球化学上,似斑状花岗岩为高钾钙碱性系列,高Sr（479~573μg/g）,低Yb（1.03~1.20μg/g）,Mg#值为48.7~51.8,判断岩石属于埃达克岩类.岩石的εHf（t）值为-2.40~2.98,判断岩石源于加厚陆壳部分熔融.闪长岩Ti/Yb值为1 930~2 178,Nb/Ta值为15.9~17.4,属混合成因.综合前人研究成果,哥日卓托杂岩体形成于古特提斯洋闭合后碰撞环境,并且标志同碰撞到后碰撞的构造体制转换.

The Gerizhuotuo complex intrusion,a typical ‘stitching pluton’with its long axis cutting regional structural lines,is significant in tectonic geochronology for providing an upper-age for the timing of plate collision.The zircon U-Pb dating results indicate that the pluton was formed in 227-224 Ma.Geochemically,porphyritic granite falls into the high-K calc-alkaline se-ries,with high Sr（479-573μg/g）and low Yb（1.03-1.20μg/g）and Mg^#（48.7-51.8）,being of the characteristics of ada-kitic rocks.The in situ zirconεHf （t）value varies from -2.40 to 2.98,which suggests that the primary magma was derived from partial melting of lower crustal materials.Diorite also shows high-K calc-alkaline affinitie,with a Ti/Yb ratio ranging from 1 930 to 2 178,Nb/Ta ratio ranging from 15.9 to 17.4.It is suggested that a mixture,between basaltic magma derived from partial melting of metasomatized mantle and acidic magma derived from crustal material,produced the dioritic magma.It is concluded that the pluton was formed in a post-collisional tensional environment.