| 西藏甲玛铜多金属矿区成矿斑岩的岩浆混合作用：石英及长石斑晶新证据 |
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| 引用本文：彭惠娟,汪雄武,Müller Axel,秦志鹏,侯林,周云.2011.Magma mixing in Jiama Cu-polymetalic deposit of Tibet: Evidence from quartz and feldspar phenocrysts[J].Mineral Deposits,30(2):249~265 |
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| 基金项目:本文得到国家973项目（编号: 2011CB403103）、中央级公益性行业科研专项（编号: 20091107-02）、地质调查项目“念青唐古拉地区成矿条件研究与找矿靶区优选（编号: N0807）”、青藏专项（编号: 1212010818089）、西藏华泰龙矿业开发有限公司勘探项目、技术开发项目“西藏墨竹工卡县甲玛铜多金属矿床地质特征及找矿方向研究项目（编号: E0804)”、教育部岩石学矿床学国家重点（培育）学科建设项目（编号: SZD0407）、国土资源部西藏自治区矿产资源潜力评价项目（编号: 1212010813025）的联合资助 |
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| 中文摘要:西藏甲玛矿区斑岩内石英和长石斑晶的阴极发光（CL）特征及元素含量变化有效记录了岩浆演化、混合及补给事件。石英斑晶的显微生长结构表明，原始岩浆经历过2次铁镁质岩浆混合作用。根据石英斑晶中Ti含量的变化可知，在2次溶蚀前后，石英结晶温度分别增高了约110 ℃和80 ℃。此外，斜长石斑晶的反环带及其Ba、Sr、Fe等元素的浓度梯度、钾长石的镶边结构、黑云母的筛状结构等，都有效地证明了铁镁质岩浆与长英质岩浆混合作用的存在。根据这些研究结果，初步构建了该矿区岩浆混合作用模型，并推测了岩浆混合过程。在16 Ma左右，岩石圈地幔拆沉，软流圈物质上涌导致正常下地壳部分熔融，产生了含Mo的钾质岩浆。原始岩浆房内形成了第Ⅰ世代自形的高温石英晶体核。软流圈上涌诱发了含大量地幔组分的新生镁铁质下地壳部分熔融，产生了含Cu、富水、高f(O2)的埃达克质岩浆熔体，并与含Mo的长英质岩浆发生了第1次岩浆混合作用。在此过程中，早期石英核部溶蚀，形成了高Ti的溶蚀表面。这些高f(O2)的混合岩浆在浅部地壳形成了大型岩浆房，并排泄出含Cu、Mo岩浆流体。在岩浆房中，石英晶体形成了均匀的生长环带和第Ⅱ世代的石英斑晶核。地幔减薄和岩石圈拆沉直接引起了地壳强烈伸展，形成了垂直于碰撞带的正断层系统和裂谷，使岩浆房内部的压力急剧减小，岩浆快速侵位。岩浆房的突然腾空，诱发了地幔物质上涌，造成了第2次基性岩浆的混入，促使第Ⅰ世代和第Ⅱ世代石英斑晶的边缘发生溶蚀。正是两次基性岩浆的加入，为成矿提供了大量的Cu、S。这是形成甲玛超大型铜多金属矿床不可或缺的因素。 |
| 中文关键词:地质学 石英 长石 显微结构 岩浆混合 甲玛 西藏 |
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| Magma mixing in Jiama Cu-polymetalic deposit of Tibet: Evidence from quartz and feldspar phenocrysts |
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| Abstract:Quartz and feldspar phenocrysts of igneous rocks in the Jiama Cu-polymetalic deposits were studied by cathodoluminescence and minor and trace element profiling. The results verify the suitability of quartz and feldspar phenocrysts as recorders of magmatic evolution, magma mixing and recharge events. The microscopic structure of quartz phenocrysts indicate that primary magma has gone through two times of magma mixing events. After two times of resorption, the recrystallization temperature of quartz phenocrysts was raised by 110℃ and 80℃ respectively, according to the content changing of Ti in zoned quartz. In addition, element concentration steps in feldspar phenocrysts ( Ba, Sr, Fe), plagioclase-mantled K-feldspars etc. indicate mixing of silicic magma with a more mafic magma for several magmatic phases of the porphyry in the Jiama Cu-polymetalic deposits. Based on the result, the process of magma mixing has been established. At 16 Ma, as a result of asthenosphere up swelling and lithosphere dismantling and subsiding, the lower crust remelted and formed potassic magma containing Mo, forming a primary magmatic chamber in the first generation quartz crystals. During the post-collisional crustal extension period, shallow emplacement and fluid exsolution of the newly-born adakitic magma, resulting from the lower crust and rich in metals, water and highf(O2), and mixed with primary magma which contained Mo, formed the shallow crustal magma chamber. Owing to the first mixing with mafic magma, resorption surfaces appeared around the core of quartz phenocrysts. And then, the quartz experienced stable growth conditions, forming a steady growth zone with low CL contrasts. Gangdise orogenic belt was in a extensional condition. Under these conditions, quite a few of normal faults were formed. The pressure of the magma reservoir was reduced rapidly, and there occurred magmatic reemplacement. Owing to the sudden empty of the magma chamber, the second magma mixing occurred. |
| keywords:geology, quartz, feldspar, microstructure, magma mixing, Jiama, Tibet |
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