U-PB AND RB-SR ISOTOPIC SYSTEMATICS OF FLUIDS ASSOCIATED WITH MINERALIZATION OF THE DARTMOOR GRANITE, SOUTHWEST ENGLAND
U-PB AND RB-SR ISOTOPIC SYSTEMATICS OF FLUIDS ASSOCIATED WITH MINERALIZATION OF THE DARTMOOR GRANITE, SOUTHWEST ENGLAND
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英格兰西南部达特穆尔花岗岩矿化相关流体的 U-PB 和 RB-SR 同位素系统
DOI：
10.1016/0016-7037(95)00410-6
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发表时间：
1996
影响因子：
5
通讯作者：
D. Banks
中科院分区：
文献类型：
--
作者：
D. Wayne;M. F. Miller;R. Scrivener;D. Banks
280马达特穆尔花岗岩早期热液成矿石英脉中含有丰富的流体包裹体，采用破碎浸出技术取样，分析Rb，Sr，U和Pb含量和同位素比值。现今铅同位素签名的一些crushleach流体是类似的花岗岩碱性长石从达特穆尔和其他地方在英格兰西南部，虽然一些流体样品含有更多的放射性铅成分。更多的放射性流体落入，或附近，花岗岩钾长石领域后，校正了现今的流体铅比在现场铀衰变超过约。280 Ma，但低放射性流体的铅同位素比值变得显着低于花岗质钾长石。明显的过校正可能是真实的，或者可能反映由于次生流体包裹体的沥滤、U和Pb的差异沥滤、石英内未确定的富U包含相（或矿物再填充）的存在而引起的流体混合，或者上述的组合。流体U/Pb比值的变化也表明一些复杂的选择性保留的U和Pb的石英表面，或存在亚显微富U包裹体。因此，我们建议，在现场衰变校正使用破碎浸出技术获得的铅同位素数据应非常谨慎。流体中的87 Rb/86 Sr值变化很小（1.40-1.74），其现今的锶同位素特征也是如此（87 Rb/86 Sr = 0.71814-0.71968）。当校正原位87 Rb衰变超过280 Ma，流体含有显着过量的放射性Sr（Sri= 0.7118-0.7141），相对于Dartmoor花岗岩Si（=0.7101）。如果流体中的Sr完全来自花岗岩，或一些花岗岩相关的来源，并假定Rb-Sr同位素系统的封闭系统演化，过剩87 Sr的量是一个函数之间的时间差的Sr封闭在花岗岩和Sr封闭在静脉。年龄估计的花岗岩和流体包裹体Sr演化曲线的交叉点是一致的，在Birch Tor-Vitifer的晚期脉岩和绢云母蚀变的40 Ar-39 Ar年龄。我们的研究结果进一步表明，流体有关的石英-电气石-辉长岩矿化在达特穆尔花岗岩本身有关的花岗岩衍生的，后期细晶熔体。
Quartz veins from early hydrothermal mineralization within the 280 Ma Dartmoor granite contain abundant fluid inclusions, which were sampled using crush-leach techniques and analyzed for Rb, Sr, U, and Pb content and isotopic ratios. The present-day lead isotopic signature of some of the crushleach fluids is similar to that of granitic alkali feldspars from Dartmoor and other localities in southwest England, although some fluid samples contain a more radiogenic Pb component. The more radiogenic fluids fell into, or near, the granitic K-feldspar field upon correction of the present-day fluid Pb ratios for in situ U decay over ca. 280 Ma, but the lead isotope ratios of the less-radiogenic fluids became significantly lower than that of the granitic K-feldspars. The apparent overcorrection may be real, or may reflect either fluid mixing due to leaching of secondary fluid inclusions, differential leaching of U and Pb, the presence of an undetermined U-rich included phase (or fracture-filling) within the quartz, or a combination of the above. The variation of fluid U/Pb ratios also suggests some complications regarding either the selective retention of U and Pb on quartz surfaces, or the presence of submicroscopic U-rich inclusions. Therefore, we suggest that in situ decay corrections to lead isotopic data obtained using crush-leach techniques be applied with great caution. The fluid87Rb/86Sr values vary little (1.40–1.74), as do their present-day strontium isotope signatures (87Rb/86Sr = 0.71814–0.71968). When corrected for in situ87Rb decay over 280 Ma, the fluids contain significant excess radiogenic Sr (Sri= 0.7118–0.7141), relative to the Dartmoor granite Si(=0.7101). If the Sr in the fluids is derived exclusively from the granite, or some granite-related source, and closedsystem evolution of the Rb-Sr isotopic system is assumed, the amount of excess87Sr is a function of the time difference between Sr closure in the granite and Sr closure in the vein. Age estimates from the intersection of granite and fluid inclusion Sr evolution curves are in agreement with published40Ar-39Ar ages of late dikes and sericitic alteration at Birch Tor-Vitifer. Our results further suggest that the fluids related to quartz-tourmaline-cassiterite mineralization within the Dartmoor granite are themselves related to granite-derived, late-stage aplitic melts.