| 作者单位 |
1. 中国地质调查局 花岗岩成岩成矿地质研究中心, 湖北 武汉 430205;
2. 中国地质调查局 武汉地质调查中心(中南地质科技创新中心), 湖北 武汉 430205;
3. 江西省地质局 第八地质大队, 江西 上饶 334000;
4. 江西省地质局 第二地质大队, 江西 九江 332000
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| 摘要 |
为探究石英闪长玢岩成因及幔源基性岩浆对斑岩铜矿的贡献,本文选取德兴矿床石英闪长玢岩开展了锆石U-Pb定年、Hf同位素和全岩地球化学研究。获得石英闪长玢岩LA-ICP-MS锆石U-Pb年龄为169 Ma,与成矿花岗闪长斑岩侵位时间一致,岩体为中侏罗世岩浆活动的产物。石英闪长玢岩具有低的SiO2(58.41%~63.12%)和K2O(1.68%~2.94%)含量及A/CNK值(0.85~1.04),富集大离子亲石元素和轻稀土元素,亏损高场强元素Nb、Ta、Ti和重稀土元素,属于钙碱性到高钾钙碱性系列岩石。具有相对亏损的锆石Hf同位素组成,εHf(t)=2.20~7.93(最大值7.93),指示其源区为岩石圈地幔。锆石稀土元素配分模式图显示出明显的正Ce异常,岩浆氧逸度(lg fO2)为-20.05~-6.66,达到磁铁矿-赤铁矿氧逸度等级,指示石英闪长玢岩结晶自高氧逸度岩浆。全岩地球化学特征显示,德兴石英闪长玢岩与成矿花岗闪长斑岩及其暗色包体符合岩浆混合的演化趋势,说明成矿花岗闪长斑岩可能是中侏罗世幔源基性岩浆和地壳酸性岩浆大规模混合作用的产物,并且石英闪长玢岩代表了岩浆混合过程中的幔源基性端员。结合前人研究成果,认为在中侏罗世伸展构造背景下,软流圈物质上涌导致新元古代受交代的岩石圈地幔部分熔融形成幔源基性岩浆,基性岩浆的底侵作用诱发下地壳物质熔融并与之发生一定程度的岩浆混合作用,形成了花岗闪长斑岩的母岩浆。高氧逸度幔源岩浆的加入可抑制斑岩体系硫化物的过早饱和,同时为德兴矿床注入了成矿所需的部分挥发分和金属元素。
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| Abstract |
In order to explore the petrogenesis of quartz diorite porphyry and the ore-forming contribution of mantle-derived basic magma, this paper present LA-ICP-MS zircon U-Pb dating, Hf isotope and whole rock geochemical study for quartz diorite porphyries from Dexing porphyry Cu deposit. The LA-ICP-MS zircon U-Pb age of quartz diorite porphyries obtained is 169 Ma, in accordance with that of ore-forming granodiorite porphyries, indicating that they were emplaced during the middle Jurassic. The quartz diorite porphyries are characterized by low SiO2 (58.41%~63.12%), K2O (1.68%~2.94%) contents and A/CNK values(0.85~1.04), belonging to the calc-alkaline to high-potassium calc-alkaline rocks. They are enriched in large ion lithophile elements and light rare earth elements but depleted in high field strength elements Nb, Ta, Ti, and heavy rare earth elements. The Hf isotope composition of zircon with relative depletion, εHf(t)=2.20~7.93 (maximum 7.93), indicating that they were crystallized from lithospheric mantle-derived magmas. The zircon REE distribution pattern diagram shows an obvious positive Ce anomaly, and the magmatic oxygen fugacity (lg fO2) varies from-20.05 to -6.66, reaching the magnetite to hematite oxygen fugacity buffer, indicating that they were crystallized from magma with high oxygen fugacity. Geochemical results show that quartz diorite porphyries, granodiorite porphyries (and the enclosed enclaves) conform to the evolution trend of magma mixing, which indicates that the ore-forming granodiorite porphyry may be the product of large-scale mixing of mantle-derived basic magma and crust-derived acidic magma in the Middle Jurassic, and the quartz diorite porphyry represents the mantle-derived basic end member in the process of magma mixing. Combined with previous research results, it is believed that quartz diorite porphyries in the Dexing deposit were generated by melting of subduction-modified lithospheric mantle, which was triggered by upwelling of the asthenospheric mantle related to the extensional setting during the middle Jurassic in South China. Underplating of the mantle-derived basic magmas could have provided heat for melting of the lower crust and subsequently mixed with the crust-derived acidic magmas, which generated the parent magmas of ore-forming granodiorite porphyries. In addition, the magmatic system of quartz diorite porphyry was highly oxidized, which was favorable for preventing the early sulfide saturation and contributing volatiles and metals to the Dexing ore-forming systems.
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