【摘要】 阿翁错地区早白垩世花岗闪长岩位于班公湖-怒江成矿带西段,广泛发育暗色微粒包体,是研究岩浆混合作用的理想对象。本文从地质学、岩相学和矿物化学等方面对花岗闪长岩开展了详细研究。包体形态多样,与寄主岩呈渐变或截然型接触关系,包体与寄主岩之间相互穿插,接触带发育冷凝边,包体从寄主岩中捕获了大量捕虏晶矿物,包体和寄主岩中均见针状磷灰石。电子探针结果显示:具正环带特征的寄主岩斜长石An值变化范围不大,在48.16～59.05之间,具致密韵律环带特征,可能代表镁铁质岩浆注入长英质岩浆房前结晶的斜长石;具反环带特征斜长石的An值变化范围较大,核部An值为29.86,往外An值陡增至51.09,整体呈逐渐增大趋势,暗示存在富钙基性岩浆的加入;具正-反环带特征的寄主岩斜长石核部至中间区域An值变化相对复杂,具稀疏韵律环带结构特征,反映斜长石结晶过程中因岩浆混合作用岩浆房环境发生突变,随着混合作用持续进行,岩浆逐渐冷却,斜长石在静态环境下继续结晶形成干净的正环带边部;包体斜长石虽然存在正环带,但其An值变化范围较大,在28.63～62.40之间,核部An值高,边部An值骤然降低,可能是镁铁质岩浆与长英质岩浆混合的结果;具反环带和正-反环带特征的包体斜长石核部遭受了弱的熔融作用,但An值变化范围相对较小,具致密韵律环带特征,核部熔蚀作用可能是镁铁质岩浆上升过程中压力快速降低引起的,形成于静态岩浆房环境。寄主岩黑云母和包体黑云母为原生岩浆黑云母,二者的Al~Ⅵ值分别为0～0.16 apfu和0～0.04 apfu,氧化系数分别为0.11～0.63和0.16～1.66,镁质率分别为0.40～0.47和0.42～0.46,表明寄主岩和包体均为钙碱性Ⅰ型花岗岩,具壳幔混源成因,其形成与岛弧的俯冲消减作用有关。结合区域构造背景,本文认为阿翁错地区早白垩世花岗闪长岩形成于狮泉河-纳木错特提斯洋盆北向俯冲向碰撞过渡的构造背景。更多还原

【Abstract】 Located at the western part of the Bangong Co-Nujiang metallogenic belt, the Early Cretaceous granodiorites from the Awengcuo area are characterized by the widely developed mafic microgranular enclaves(MMEs) and obvious magma mixing. Plagioclase and biotite, as the main rock-forming minerals, record the material composition and physicochemical conditions of the magmatic system, which are ideal objects for studying the petrogenesis, magmatic evolution and tectonic setting. This paper focuses on the granodiorites(host rock) and MMEs from the aspects of geology, petrography and mineral chemistry. The MMEs are in various shapes and have sharp or transitional contact boundaries with the host rocks. The enclaves often contain back vein. The MMEs and host rocks are commonly interpenetrated with each other, and chilled borders with width of 1–2 mm on the enclaves’ side can be observed occasionally. Many phenocrysts in the enclaves are captured from the host rocks, and acicular apatite formed by quenching is found in both the MMEs and host rocks. The above characteristics suggest that the MMEs are the products of magma mixing and mingling, and material interchanges are common. The results show that the plagioclases in the host rocks have intensive oscillatory normal zoning features with narrow An value range of 48.16 to 59.05, which may represent the plagioclase crystallized before the injection of mafic magma. The An values of the plagioclase with normal zoning features vary widely, with a core An value of 29.86 and a sudden increase to 51.09 outward, showing an overall gradual increase, indicating the addition of calcium-rich magma. Plagioclases of the host rocks show normal and reverse zoning features, with relatively complex core to mantle variations of An values and wide oscillatory zonation. Formation of the wide core to mantle oscillatory zonation of the plagioclase may due to the abrupt changes triggered by magma mixing. With the continuous mixing and gradual cooling of the magma, the plagioclase continued to crystallize in the static environment, and formed a clean edge of normal zonation. Although there is a normal zoning in the inclusion matrix plagioclase, the An values vary significantly from 62.40 in the core to 28.63 in the edge. It is obviously different from the normal zoning characteristics of the host rock plagioclase, which may be the result of the mixing of mafic magma and felsic magma. The core of plagioclases in the matrix of the MMEs with the characteristics of reverse zoning and normal-reverse zoning suffered weak melting erosion, but the range of the An values is relatively small, with the characteristics of intensive oscillatory zoning. It is suggested that the core melting erosion may be caused by the rapid decrease of pressure during the ascent of the mafic magma and formed in a static magma chamber. The host rock biotite and enclave matrix biotite are primary magmatic biotite, with Al~Ⅵ values of 0–0.16 apfu and 0–0.04 apfu, oxidation coefficients of 0.11–0.63 and 0.16–1.66, and magnesia contents of 0.40–0.47 and 0.42–0.46, respectively, indicating that both of the host rocks and MMEs are calc alkaline Ⅰ-type granites, whose formation is related to the subduction of island arc. Combined with the regional tectonic setting, it is considered that the early Cretaceous granodiorites in the Awngcuo area were formed under the tectonic setting of the transition from northward subduction to collision of the Shiquanhe-Namucuo Tethys ocean basin.更多还原