东昆仑香加花岗质岩体中镁铁质包体成因:岩相学及地球化学证据

东昆仑造山带以广泛发育富含镁铁质包体的早-中三叠世花岗岩为主要特征，但目前尚缺乏对不同类型镁铁质包体系统的岩相学和矿物学研究.在本文中，我们选择了极具代表性的香加花岗岩体及其中包体为研究对象，从岩相学和矿物化学角度揭示了东昆仑地区壳幔岩浆相互作用的详细过程.研究表明包体发育眼球状石英、韵律环带斜长石和针状磷灰石等不平衡结构和快速结晶现象，指示存在岩浆混合作用，而似辉绿辉长结构包体代表了岩浆混合的基性端元.此外，长石的多阶段生长证明可能存在多次的岩浆混合过程.镁铁质包体相对寄主岩（Mg#值为0.39~0.56，Fe#值为0.44~0.62）具高Mg#和低Fe#特征.包体具有两类角闪石:一类结晶源自早期深部幔源岩浆（TiO₂=2.1%~2.9%，SiO₂=41.75%~44.49%），另一类则起源于浅部壳幔混合作用（TiO₂=1.0%~1.8%，SiO₂=42.49%~48.10%）.部分黑云母具有高镁特征（MgO=9.78%~11.53%，Mg#=0.462~0.541），与幔源成因黑云母成分相当.斜长石的韵律环带及化学组成指示其岩浆混合成因.幔源基性岩浆在5×108 bar（约18 km）左右深度结晶并形成高钛角闪石，玄武质岩浆底侵上升，并发生壳幔岩浆混合作用，混合的岩浆上升至2.5×108 bar（约8 km）左右深度结晶形成低钛角闪石.以上证据指示，东昆仑地区在三叠纪时期可能经历了多期次的岩浆混合作用，地幔岩浆的注入在地壳深熔作用和地壳生长过程中扮演了重要角色.广泛的壳幔岩浆相互作用可能是三叠纪时期阿尼玛卿洋板片断离的重要响应. 

The Origin of Mafic Enclaves in Xiangjia Granitic Pluton of East Kunlun Orogenic Belt: Evidence from Petrography and Geochemistry

The East Kunlun orogenic belt is characterized by widespread Early-Middle Triassic Epoch granitic rocks with a large proportion of mafic enclaves therein, however, there is still lack of petrography and mineralogy studies on the Mafic enclaves with different textures. In this paper, a systematic petrography and mineral chemistry study has been carried out for the representative Xiangjia granite pluton as well as the enclosed Mafic enclaves, which have clarified the processes of crust-mantle interaction in East Kunlun area. Studies on petrography have shown that the mafic enclaves have some disequilibrium textures and some phenomenon point to rapid crystallization, such as needle-like apatite, needle-like amphibole and augen-shaped quartz, all of which suggest the possible of magma mixing process, and the enclave with ophitic-gabbro texture likely to be the basic end-member. In addition, multi-stage growing of feldspar indicates there might be multi-mixing process. Whole rock geochemistry shows that the mafic enclaves have lower Mg# and higher Fe# values relative to their host rocks (Mg#:0.39-0.56, Fe#:0.44-0.62). Electron microprobe investigation shows that enclaves have two kinds of amphiboles, one kind in mafic enclaves (TiO₂:2.1%-2.9%, SiO₂:41.75%-44.49%) crystallized from mantle-derived basic magma in the early stage, while the other kind (TiO₂:1.0%-1.8%, SiO₂:42.49%-48.10%) probably crystallized from the mixed magma in the shallow crust level. Some biotites (MgO:9.78%-11.53%, Mg#:0.462-0.541) are similar to biotites crystallizing in the mantle-derived magma. The rhythmic zoning and geochemistry contents of Plagioclases show that they may have suffered the process of magma mixing. Calculated results by geobarameter indicate that the mantle-derived basic magma crystallized under 5×108 bar, i.e. about 18 km depth, producing the high-Ti amphiboles, and then mixed with the acid magma. The mixed magma emplaced to upper crust, maybe at 2.5×108 bar, i.e. about 8km depth, producing the Low-Ti amphiboles. All these features above show that the East Kunlun orogenic belt might have experienced crust-mantle interaction repeatly during the Triassic Epoch, and the injection of mantle magma played an important role in the crustal growth and crust anatexis. The wide crust-mantle interaction may be the response to the slab break-off of Animaqing Ocean during the Triassic Period.