摘要:
提要: 卡而却卡铜多金属矿区花岗闪长岩体中广泛分布具有似斑状结构的闪长质包体, 包体主要为椭圆形, 定向排列, 具有与寄主岩石相似的矿物组合, 但角闪石含量明显比寄主岩石高, 具有明显的不平衡反应结构, 发育针状磷灰石。LA-MC-ICP-MS锆石U-Pb测试表明, 暗色微粒包体形成于(234.1±0.6) Ma, 寄主花岗闪长岩形成于(234.4±0.6) Ma, 为岩浆混合作用的存在提供了有力证据。地球化学特征显示, 暗色包体SiO2、MgO含量较寄主岩石高, 更偏基性; 均富集大离子亲石元素和轻稀土元素, 亏损高场强元素, 但寄主花岗闪长岩LaN/YbN值(11.6~19.0)明显高于暗色包体(5.0~9.7), Nb/Ta值(10.6~11.7)低于暗色包体(11.6~14.8)。二者具有相近的Sr-Nd-Hf同位素组成, 花岗闪长岩的εNd(t)值为-5.3~-4.2, (87Sr/86Sr)i 值为0.71110~0.71125, εHf(t)为-5.8~-3.4, tDM(Hf)为1012~1102 Ma; 暗色包体的εNd(t)值为-5.2~-5.0, (87Sr/86Sr)i 值为0.71114~0.71171, εHf(t)为-6.3~-1.6, tDM(Hf)为937~1129 Ma。综上推测, 在中三叠世俯冲—碰撞转换阶段, EMⅡ型富集岩石圈地幔在上涌软流圈物质的作用下发生部分熔融产生基性岩浆, 并底侵下地壳物质局部熔融形成长英质岩浆房, 进而由基性岩浆注入长英质岩浆房后, 经过演化形成微粒包体, 演化了的基性岩浆与长英质岩浆混合, 最终形成含微粒包体的均一岩浆, 这种岩浆结晶后形成含暗色包体花岗闪长岩。
Abstract:
Abstract: The porphyritoid dioritic enclaves are widely distributed in granodiorites of the Kaerqueka copper deposit, East Kunlun belt. The enclaves are mostly aligned ovals, with similar mineralogy to the host rocks. However, the modal percentage of hornblende is significantly higher than that of the host rock. It has apparent imbalanced structure with development of needle-like apatite. The granodiorite and enclaves have the same age of 234 Ma, which indicates that they might have been formed by magma mixing. As for geochemical characteristics, the enclaves contain higher values of SiO2 and MgO than the host rock and are hence more mafic. They are also enriched in large LILE and LREE, and are depleted in HFSE. The LaN/YbN ratios of host granodiorite (11.6?19.0) are significantly higher than the ratios of the enclaves (5.0?9.7). The Nb/Ta values of host granodiorite (10.6?11.7) are lower than those of the enclaves (11.6?14.8). They have similar Sr, Nd and Hf isotopic compositions. The granodiorite has εNd(t) values of ?5.3 to ?4.2, (87Sr/86Sr)i values of 0.71110?0.71125, εHf(t) values of ?5.8 to ?1.6, tDM(Hf) values of 1012?1102 Ma. The enclaves have εNd(t) values of ?5.2 to ?5.0, (87Sr/86Sr)i values of 0.71114?0.71171, εHf(t) values of ?6.3 to ?3.4, and tDM(Hf) values of 937?1129 Ma. These data indicate that, at the subduction-collision transition stage of Triassic period, the enriched EMII type lithosphere mantle underwent partial melting and produced mafic magma under the influence of upwelling asthenosphere materials. The partial melting of lower crust materials, which had been underplated by the mantle, formed the felsic magma chamber. The injection of mafic magma into the felsic magma chamber formed enclaves. After mixing of evolved mafic magma with felsic magma, enclaves-bearing homogeneous magma was formed. The crystallization of this homogeneous magma formed granodiorites containing enclaves.
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