| 关于三阶段铅演化体系：以大厂锡矿床及Lewisian片麻岩为例 |
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| 引用本文： | 韩发,李振清.关于三阶段铅演化体系：以大厂锡矿床及Lewisian片麻岩为例[J].地学前缘,2007,14(2):173-181. |
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| 作者姓名： | 韩发 李振清 |
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| 作者单位： | 中国地质科学院,矿产资源研究所,北京,100037 |
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| 基金项目： | 中国地质科学院KL05-8专项资金
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成矿作用与资源评价开放实验室项目 |
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| 摘 要： | 以大厂锡-多金属矿床、Lewisian片麻岩为例,讨论了成矿/成岩过程中铅的三阶段演化轨迹。在206Pb/204Pb对207Pb/204Pb图解上,大厂硫化物铅的数据点构成了Pb-Pb等时线,该等时线给出了1.508Ga的矿化源岩年龄,说明成矿物质主要来自中元古代基底岩石——四堡群。四堡群本身又可能来自太古宙(2.86Ga)老陆壳,故大厂硫化物中的痕量铅应该具有三阶段的演化史。用三阶段铅演化模式解释大厂硫化物铅同位素资料,结果表明,太古宙老陆壳从上地幔源区分离出来时,其μ2=11.93,大厂矿床矿化源岩的μ3=9.76。不同变质程度的Lewisian杂岩全岩铅具有不同的演化历史:北部区角闪岩相片麻岩具有两阶段的演化史,中心区麻粒岩相片麻岩具有三阶段的演化史。北部区片麻岩两阶段Pb-Pb等时线给出了2.951Ga的成岩年龄,其μ1=7.81。中心区片麻岩三阶段Pb-Pb等时线给出了2.68Ga的变质年龄,其火成源岩的μ2=3.51。在解释异常铅等时线时,对其演化阶段级次的判别具有十分重要意义,它直接涉及不同演化阶段U-Pb体系μ值的计算。资料证明,如果把三次等时线当作二次等时线加以解释,则会导致一系列错误结论的产生。在一般情况下,用三阶段铅演化模式解释某组铅同位素资料时,只是假设第二阶段U-Pb体系是处于封闭状态的,但无任何直接证据。Lewisian北部区片麻岩两阶段铅演化体系与中心区片麻岩三阶段铅演化体系共存的情况为解决上述问题提供了极好的资料。
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| 关 键 词： | 三阶段铅演化体系 硫化物铅 全岩铅 大厂锡矿床 Lewisian片麻岩 |
| 文章编号： | 1005-2321(2007)02-0173-09 |
| 修稿时间： | 2006年8月28日 |
On the three-stage evolutionary systems of lead: as examplified by the Dachang tin deposits and the Lewisian gneisses |
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| Han Fa,Li Zhenqing.On the three-stage evolutionary systems of lead: as examplified by the Dachang tin deposits and the Lewisian gneisses[J].Earth Science Frontiers,2007,14(2):173-181. |
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| Authors: | Han Fa Li Zhenqing |
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| Abstract: | As examplified by the Dachang tin-polymetallic deposits in Guangxi province, China and Lewisian gneisses of mainland north-west Scotland, this paper discusses a three-stage evolution of the lead from the sulfides of the Dachang ores and from the whole-rocks of the Lewisian gneisses.On 207Pb/204Pb versus 206Pb/204Pb diagram, the data points of the sulfide lead from the Dachang tin doposits hosted in the Middle to Upper Devonian sequence, and from Yuanbaoshan and Baotan tin deposits surrounded by the Middle Proterozoic metasedimentary rocks 100 km to the east, define a 1.51 Ga to 0.38 Ga isochron, suggesting that the ore-forming materials are mainly derived from the Middle Protorozoic basement rocks, namely the Sibao Group, which underlies the Dachang orefield. On the basis of regional geology and zircon U-Pb dates in the range 2935-2573 Ma, even older Archean crust (2.86 Ga) may have been present in the northern part of Guangxi province, suggesting that the rocks of the Sibao Group mainly composed of terrigenous detritus, were possibly derived from the Archean rocks. Therefore, the Dachang lead must have involved three-stages of evolution rather than two-stages. Our interpretation based on the three-stages of evolution indicates that the Archean crust separated from the mantle reservoir during crust-forming event has high U/Pb value(μ2=11.93), and source rocks of the Dachang ores have a medium U/Pb value (μ3=9.76) similar to that of average crust rocks.According to the data of whole rock lead of Lewisian gneisses from Chapman and Moorbath (1977) and Whitehouse and Moorbath (1986), thes especially discussed the three-stage evolution of the lead, and obtained μ2 ratio of 3.51 for the igneous protoliths of the granulite-facies gneisses during 270 Ma period and μ3 ratio of 2.25 (weighted avg.) for the gneisses during the 2680 Ma period.An evolution-stage order of the lead is very important for explaining a certain isochron of anomalous lead. On a lithologically diverse suite of the Lewisian gneisses, there exist two-stage U-Pb systems and three-stage U-Pb systems. This case provides a very typical example for discussing the relationship between two evolutionary systems in detail. |
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| Keywords: | three-stage model sulfide lead whole-rock lead Dachang tin ores Lewisian gneisses |
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