西藏仁布县帕夏始新世埃达克质岩的发现及其意义

本次工作对出露于冈底斯中段南缘的仁布县帕夏石英闪长岩进行了LA-ICP-MS锆石U-Pb定年和全岩岩石化学研究。利用LA-ICP-MS同位素定年技术,测得石英闪长岩锆石U-Pb年龄为49.5±1.7Ma (n=9,MSWD=2.0)。地球化学研究表明,石英闪长岩属于准铝质高钾钙碱性系列岩石,稀土总量ΣREE为(110.14~143.36)×10^-6,LREE/HREE为7.09~10.23,轻稀土相对重稀土富集,δEu为0.93~1.14,铕异常不明显;相对富集大离子亲石元素(LILE) U、Th、K、Ba、Rb,相对亏损高场强元素(HFSE) Ta、Nb、Ti;具有高Sr低Y特征,显示出埃达克质岩的性质。研究认为,该埃达克质石英闪长岩源区为角闪岩相下地壳和地幔楔的混合源区,残留相为角闪石、金红石,无斜长石残留,在侵位过程中经分离结晶作用。该岩体代表了50Ma左右与俯冲板片断离诱发的地幔物质上涌有关的冈底斯中段南缘壳幔混源岩浆活动。

The discovery and geological significance of the Eocene adakitic rocks in Paxia,Renbu,Xizang

The emphasis in the present paper is placed on LA-ICP-MS zircon U-Pb dating and whole-rock lithogeochemical analysis of quartz diorite cropped out in the middle part of the Gangdise tectonic belt. The sampled quartz diorite yields a weighted average age of 49.5 ±1.7 Ma (n=9,MSWD=2.0) dated by using LA-ICP-MS zircon U-Pb dating method. Geochemical data suggest that the quartz diorite belongs to the metaluminous high-K cala-alkaline series rocks,with the total REE contents (ΣREE) ranging from 110.14×10-6 to 143.36×10-6,LREE/HREE ratios ranging from 7.09 to 10.23,and δEu values ranging from 0.93 to 1.14. These rocks are characteric of the enrichment of the large ion lithophile elements (LILE) U,Th,K,Ba and Rb,depletion of the high field strength elements (HFSE) Ta,Nb and Ti,and high Sr contents and low Y contents,which represent typical features of adakitic rocks. The authors in the present paper contend that the adakitic quartz diorite in the study area may originate from a mixed magmatic origin including the lower crust and mantle wedge,with the residual phases of rutile and hornblende and without the plagioclase phase,and with the fractional crystallization during the emplacement of the magmas. The quartz diorite pluton may record the mixed crust-mantle magmatic origin associated with the mantle upwelling triggered by the broken subducting plate in the middle part of the Gangdise tectonic belt at about 50 Ma.