浙江洋滨黄玉花岗斑岩的Nd、Sr、Pb同位素研究

NEODYMIUM STRONTIUM AND LEAD ISOTOPIC STUDY OF TOPAZ GRANITE PORPHYRY IN YANGBIN ZHEJIANPROVINCE

洋滨斑岩锡矿位于我国东南沿海中生代火山活动带中部．含矿岩石主要由青黄玉的钾长花岗斑岩与二长花岗斑岩及黄英斑岩组成．这些岩石具有高硅、强过铝质、富集不相容元素、弱的铈和强的铕负异常以及高的稀土总量等特征．采用最小二乘法计算的全岩Ｒｂ－Ｓｒ等时线年龄为１０１．７±２．１Ｍａ，（８７Ｓｒ／８６Ｓｒ）ｉ＝０．７１４２±９．同时，它们具有低的εＮｄ值（－８．４～－８．０），较高的２０７Ｐｂ／２０４Ｐｂ（１５．５８８～１５．６６６）以及较古老的二阶段Ｎｄ模式年龄（１５９７～１７４３）Ｍａ．这些特征表明，洋滨花岗斑岩的母岩浆属于Ｓ型，它们可能是由本区未出露的中元古代变质沉积岩经部分熔融方式形成．初始岩浆经历了强烈的分离结晶，继之以流体岩浆／分离过程。

The porphyry tin deposit in Yangbin, Zhejing Province is located in themiddle portion .of Mesozoic volcanic active belt on the southeastern coast of China. Themineralizing rocks are composed mainly of topaz K-feldspar granite porphyry, topazmonzogranite porphyry, and topzaite porphyry. They commonly have typical porphyritictextures. The phenocrysts consist of quartz, microcline perthite, albite and protlithionite,while the groundmasses comprise topaz, quartz and protlithionite. These rocks have highsilica (SiO2> 71%), superperalumina (A / NKC = 1.574￣ 12.938), K2O >Na2O, lowNK / A ratios (<0.6), rich in incopmatible elements(Rb, Zr, W, Sn, .Nb) and depleted incompatible elements(Sr, Eu) and high F / Cl ratios (146- 303). Moreover, they also possesshigh total REE (282￣ 320ppm), low LREE/HREE ratios (0.76-0.84), weak Ce(δCe = 0.78 ￣ 0.82) and intensive Eu(δEu = 0.01-0.03) negative anomalies, etc.More detailed Nd, Sr and Ph isotopic data on these rocks haVe been reported in thispaper. Whole rock Rb-Sr isochron age and (87Sr /86Sr).ratio calculated by the least squaremethod are 101 .7 ± 2.1 Ma and 0.7142 ± 9 respectively. Topaz K-feldspar granite porphyryand topaz monzdgranite porphyry have similar εNdvalues (-8.4￣-8.0 respectively), but thedifference of εNdvalues between the dark- and light-topazite porphyries is large.(-8.6￣-6.7 respectively). Meantime, these rocks also have higher 207Pb / 204Pb ratios (15.588-15.666). The mineralizing parent rocks in Yangbin have very high Sm / Nd ratios (0.242￣0.387), which may be related to the intensive concentration of fluiorine in the parentmagma. The Nd model ages of granite porphyries and topazite porphyries calculated by thetwo-stage model are 1597- 1743Ma, indicating that they probably come from middleProterozoic crust.All above mentioned characteristics suggest that the parent magma of graniteporphyries and topazite porphyries in Yangbin belongs to S-type other than A-type orI-type. They were probably formed by. partial melting of middle Proterozoicmetasedimentary rocks which were unexposed in the study area. The initial magma experienced intensively fractional evolution and was followed by fluid / magma separation.