Petrology and geochemistry of the ore-forming porphyries in the Lamasu copper deposit, western Tianshan: Implications for petrogenesis
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摘要: 位于西天山别珍套-科古琴晚古生代岛弧西段的喇嘛苏铜矿床是区内最大的铜矿床,与成矿作用有着密切关系的斑岩体为英云闪长斑岩、花岗闪长斑岩,是同源岩浆分异演化的产物,且花岗闪长斑岩可能属于岩浆演化晚期的产物。本区成矿斑岩的主量、微量元素和Sr-Nd同位素地球化学特征表明,其富集大离子亲石元素,而相对亏损高场强元素,出现了较为明显的Ta、Nb负异常,初始锶同位素ISr和εNd(t=390Ma)值分别为0.7072~0.7076和-0.32~0.17,显示壳幔混合源的特征,利用Sr和Nd同位素估算其源区物质约有50%来源于地壳。岩石地球化学特征指示了其为典型钙碱性火山弧花岗岩,暗示其形成于大陆弧环境。结合区域地质背景,推测本区成矿斑岩是在洋壳俯冲作用下发生部分熔融,交代原先的地幔楔,并混合了部分下地壳的物质,经历分离结晶作用的产物,其形成可能与晚古生代准噶尔洋板块向南的俯冲作用有关。结合东西天山的成矿斑岩的地球化学特征对比研究,岩浆源区的差别可能导致不同类型斑岩型矿床的形成,斑岩型铜矿床的形成较斑岩型钼矿床可能有更少的地壳物质贡献。Abstract: The western Tianshan is located in the north part of the Yili plate, i.e., orogenic belt between Junggar plate and Yili basin. The Lamasu deposit is situated in the western section of the Late Paleozoic Bigjintao-Kokirqin island arc. The porphyries closely associated with mineralization, are predominated by tonalitic porphyry and granodioritic porphyry. The two types of porphyries are co-magmatic. Furthermore, granodioritic porphyry may be highly evolved from a relative mafic parental magma. Their major- and trace-element, Sr-Nd isotopic compositions suggest that they are characterized by relative enrichment of large ion lithophile elements and depletion of high strength elements coupled with significantly negative Nb and Ta anomalies. Their ISr and εNd(t=390Ma) ratios range from 0.7072 to 0.7076 and -0.32 to 0.17 respectively, suggesting that they were mainly derived from a mixture source of crust and mantle. Sr and Nd isotope modeling implies that the crustal materials were made up of 50% in the source region. The geochemical characteristics show that they are the typically calc-alkaline volcanic arc granites, in a continental arc setting. Combined with geological background, the formation of the porphyries may be attributed to fractional crystallization of a parental magma produced by partial melting of MORB-like mantle wedge which was modified by the addition of a melt component derived from altered subducted oceanic crust, and then contaminated by materials of lower crust. The porphyries might be related to the southward subduction of the Junggar plate during the Late Paleozoic time. Based on previous work, different types of deposits could be linked to distinct sources, i.e., the formation of copper deposit could be attributed to less crustal contamination involved in magma generation than that of porphyry molybdenum deposit.
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Key words:
- Porphyry /
- Geochemistry /
- Source region /
- Lamasu /
- Western Tianshan
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