东天山哈密地区早二叠世花岗岩体及镁铁质包体的年代学,岩石地球化学特征及其构造意义

Geochronology and Geochemistry Characteristics of the Early Permian Monzogranite and Dioritic Enclaves of East Tianshan and Their Tectonic Implications

花岗岩中包体是区域构造与深部过程研究结合的良好窗口，对研究花岗岩的成因和壳-幔相互作用有非常重要的意义。东天山南山口黑云母二长花岗岩中广泛发育镁铁质包体。主量元素组成上，寄主花岗岩岩具有中酸性、准铝质、富碱、富钾等特征；镁铁质包体则偏基性、贫钾。微量和稀土元素组成上，富集Rb、K、Th、U，贫Sr、P、Nb、Ta、Ti，且Zr、Hf含量相对较高，具中-强的铕负异常（0.46~0.57）。镁铁质包体与寄主岩具有相似的微量元素特征，但相对富集Sr、P，贫Zr、Hf，铕负异常中等或不明显（0.80~0.93）。闪长质包体属于同源包体，为寄主花岗岩同源母岩浆经结晶分异形成的早期产物。运用LAICPMS锆石UPb定年技术，该南山口岩体暗色镁铁质微粒包体与寄主黑云母二长花岗岩的锆石UPb年龄分别为298.2±2.0Ma和294.0±2.7Ma，包体与寄主岩样品具有相似的Hf同位素组成，暗示其母岩浆来自新元古代晚期和早古生代亏损地幔中分离的新生地壳。该高钾钙碱性的花岗岩及其包体可能形成于东天山后碰撞背景下的板片断离后引起软流圈上涌，底侵体带来的热能使得新元古代晚期和早古生代的新生地壳发生部分熔融。

Collisional magmatism records many information of material cycling between crust and mantle during subduction and collision and it is of great importance to reveal their source nature and crust mantle interaction for understanding tectonic evolution of the orogenic belt. Mafic microgranular enclaves （MMEs） are widely distributed in Nanshankou biotite monzogranite of the East Tianshan. Geochemically, the host biotite monzogranites show intermedia acidic, metaluminous, alkaline and potassium rich signatures, whereas the MMEs are relatively poor in silicon and potassium. The host biotite monzogranite are enriched in Rb, K, Th, U and depleted in Sr, P, Nb, Ti and display moderately Zr, Hf peaks in the primitive mantle normalized trace element spidergram, and moderately to strongly Eu depletions （0.46-0.57） in the chondritite normalized REE distribution patterns. The MMEs share similar trace and rare earth elements characteristics to those of the host rocks, but are relatively enriched in Sr, P and more depleted in Zr, Hf, and display weakly to moderately Eu depletions （0.80-0.93）. Zircon La ICP MS U Pb dating gave ages of 298.2 ± 2.0 Ma and 294.0 ± 2.7 Ma for MMEs and biotite monzogranite, respectively, indicating the Early Permian emplacement. The dioritic enclaves and their host granites have positive εHf（t） values （7.31-11.98 for MMEs, 6.94-12.14 for host granites） and relatively low n（176Hf）/n（177Hf） with fairly similar model ages （450-656Ma）. It is suggested that the MMEs and host granites were derived from the same parental magma that resulted from partial melting of the Neoproterozoic Early Paleozoic depleted mantle sourced juvenile crust which was triggered by the heat of asthenosphere upwelling after the post collisional slab break off.