| 东昆仑小灶火软玉中热液锆石U-Pb年龄及Hf同位素特征:对成矿时代的制约 |
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| 引用本文： | 雷成,杨明星,钟增球.东昆仑小灶火软玉中热液锆石U-Pb年龄及Hf同位素特征:对成矿时代的制约[J].大地构造与成矿学,2018(1):108-125. |
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| 作者姓名： | 雷成 杨明星 钟增球 |
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| 作者单位： | 中国地质大学(武汉) 地球科学学院,湖北 武汉,430074
中国地质大学(武汉) 珠宝学院,湖北 武汉,430074 |
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| 基金项目： | 国家公益性科研专项基金项目 |
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| 摘 要： | 小灶火软玉矿床位于东昆仑造山带西段,同新疆和田玉处于昆仑山脉同一条成矿带上。该矿床软玉种类主要为青玉,其中透闪石含量达99%以上,为特殊的镁矽卡岩矿床。对该矿区青玉及其相关的黑云二长花岗岩进行了系统的LA-MC-ICP-MS锆石微量元素、U-Pb及Lu-Hf同位素研究。结果显示,黑云二长花岗岩中均为具典型韵律振荡环带的岩浆成因锆石,锆石U-Pb定年得到其成岩年龄为415.8±1.7 Ma;锆石ε_(Hf)(t)值为1.5~6.6,亏损地幔Hf模式年龄(tDM)为779~965 Ma,指示黑云二长花岗岩为东昆仑原特提斯造山带后碰撞阶段新生的新元古代地壳物质熔融的产物。青玉中的锆石可分为两类,其中主要锆石群——I类锆石呈补丁状环带、面状环带或无环带,且具有与黑云二长花岗岩中的岩浆锆石明显不同的稀土元素及Hf同位素特征,显示典型的热液锆石特点,其U-Pb年龄416.4±1.5 Ma代表了青玉的形成时间;II类锆石(416 Ma、471 Ma、818 Ma)呈韵律振荡环带,为热液流体从围岩中捕获的继承锆石。小灶火地区成岩-成矿作用的同时性(~416 Ma)表明岩浆岩侵入是软玉矿床形成的岩浆岩条件,软玉成矿作用方式为热液交代和充填作用。值得注意的是,用热液锆石的Hf同位素来示踪软玉成矿流体性质时应十分谨慎,因为成矿热液中的Hf元素在同时结晶的两种不同矿物(热液锆石和透闪石)间发生了重新分配。
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| 关 键 词： | 软玉 热液锆石 U-Pb年龄 Hf同位素 东昆仑造山带 nephrite hydrothermal zircon U-Pb age Hf isotopes East Kunlun Orogen |
Zircon U-Pb Ages and Hf Isotopes of the Xiaozaohuo Nephrite,Eastern Kunlun Orogenic Belt: Constraints on its Ore-forming Age |
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| LEI Cheng,YANG Mingxing,ZHONG Zengqiu.Zircon U-Pb Ages and Hf Isotopes of the Xiaozaohuo Nephrite,Eastern Kunlun Orogenic Belt: Constraints on its Ore-forming Age[J].Geotectonica et Metallogenia,2018(1):108-125. |
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| Authors: | LEI Cheng YANG Mingxing ZHONG Zengqiu |
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| Abstract: | The Xiaozaohuo nephrite deposit, occurring in the same nephrite belt along with the famous Hetian nephrite deposits, is located in the western part of the East Kunlun Orogen. The nephrites from this deposit are mainly green nephrites, which are composed of more than 99% tremolite with trace constituents of zircon and titanite. This deposit is a typical Mg-skarn deposit hosted in the lower Proterozoic dolomitic marble while the green nephrite veins occur mainly in the contact zone of a biotite adamellite pluton. In this study, U-Pb ages, trace elements, and Hf isotope compositions of zircons from the green nephrite and its related biotite adamellite have been determined to provide constraints on the timing and genesis of the nephrite deposit. Zircons from the biotite adamellite sample are characterized by oscillatory zoning in CL images and gave a weighted mean 206Pb/238U age of 415.8±1.7 Ma, representing the emplacement time of the pluton. They have positive εHf(t) values of 1.5–6.6 and tDMages of 779–965 Ma, indicating the biotite adamellite was produced by partial melting of a juvenile Neoproterozoic crust source during the post-collisional stage of the Proto-Tethys orogeny. Zircons from the green nephrite sample can be divided into two groups based on their internal structures revealed by CL images. The type-I, the main zircon group in the nephrite sample, exhibits patchy zoning, planar zoning or no obvious zoning. Although zircons of this group yielded a U-Pb age of 416.4±1.5 Ma resembling that for the biotite adamellite, they possess REE and Hf isotopic characteristics distinct from those of the latter, suggesting they are hydrothermal zircons precipitated from aqueous fluid responsible for the nephrite formation. Thus the age of 416.4±1.5 Ma is considered as the age of nephrite formation. The type-II, however, contains zircons showing oscillatory zoning. They gave U-Pb ages of 416?818 Ma and have distinct REE and Hf isotopic characteristics from those of type-I, which can be interpreted as inherited zircons picked up by hydrothermal fluids from wall rock. The similar ages for the magmatism and mineralization (ca. 416 Ma) reveal a close relationship between the intrusion and nephrite formation. The predominantly vein or lens occurrences of nephrites in the Xiaozaohuo deposit also support a hydrothermal filling origin for nephrite mineralization. Nevertheless, the nature of the ore-forming fluid is still not clear. Hafnium isotopes should be used with caution to trace the nature of the nephrite-forming fluid because Hf was active during the hydrothermal processes. |
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