西藏中冈底斯成矿带查个勒铅锌矿床含矿斑岩年代学及其地质意义

Geochronology and Its Geological Significance of the Ore-Bearing Porphyry in Chagele Lead-Zinc Deposit in Middle-Gangdese Metallogenic Belt,Tibet

查个勒铅锌矿床是西藏中冈底斯成矿带中段新发现的一个大型斑岩-矽卡岩型矿床,但其成岩成矿时代一直缺乏年代学约束。本文对该矿床含矿斑岩进行了LA-ICP-MS锆石U-Pb定年、辉钼矿Re-Os定年、主量和微量元素分析及Sr-Nd-Hf同位素组成研究,获得中冈底斯成矿带中段铜钼铅锌银矿化时代。含矿斑岩LA-ICP-MS锆石U-Pb年龄为64.6~62.9 Ma,代表岩浆的结晶年龄;辉钼矿Re-Os等时线年龄为(62.3±1.4)Ma,代表查个勒主成矿年龄,与其构造岩浆事件一致;结合区域林子宗群大规模火山活动(65~45 Ma)以及以亚贵拉铅锌矿床(68.6~65.0 Ma)为代表的成矿作用,表明在印度与欧亚大陆主碰撞过程中均可产生不同规模的成矿作用。查个勒含矿斑岩具富硅、富钾,贫钛、贫磷特征,铝饱和指数(A/CNK)为1.12~1.60,富集大离子亲石元素Rb、Th、U,亏损高场强元素Nb、Zr等,与冈底斯成熟大陆地壳物质相比具相对高的εNd(t)值(-6.64^-5.79)和相对低的(87Sr/86Sr)i值(0.711 813~0.717 307),并具不均一的锆石εHf(t)值(-7.02^-1.27)以及古老的锆石Hf同位素地壳模式年龄(TCDM=1 093~1419 Ma),属于过铝质S型花岗岩类。本文认为中冈底斯成矿带中段古新世岩浆活动和成矿作用,很可能与雅鲁藏布江洋盆闭合之后的印-亚大陆碰撞诱发幔源岩浆底侵导致的冈底斯地体古老地壳物质部分熔融有关,岩浆在上升过程中有不同程度的分离结晶。

The newly discovered Chagele skarn‐porphyry deposit is located in the Middle‐Gangdese metallogenic belt . This study focuses on the zircon U Pb age of the ore‐bearing porphyry and molybdenite Re Os ,the whole rock major elements ,trace elements ,and the Sr Nd Hf isotopic data of the ore‐bearing rocks in Chagele deposit .The zircon U Pb age of the ore‐bearing porphyry changes from 64 .6 to 62 .9 Ma ,which represents the magmatic crystallization age .The Re Os isotopic age shows that the deposit was formed at (62 .3 ± 1 .4) Ma ,which is consistent with the magmatic age in the study area .Combing with the extensive volcanic event of Linzizong Group ,these new data suggests that the different scales of mineralization were formed at different stages in the entire collision between India and Eurasia continent .The ore‐bearing porphyry of Chagele deposit has high contents of SiO 2 ,K2 O , Rb ,Th ,and U ,coupled with low contents of TiO 2 , P2 O5 ,negative anomalies of Nb ,Zr ,variableεHf (t) ( - 7 .02 ~ - 1 .27) ,and ancient TCDM (1 093 ~ 1 419 Ma) .In comparison with the Gangdese continental crust composition ,the ore‐bearing porphyry is high in εNd (t) ( - 6 .64 ~ - 5 .79) and low in (87 Sr/86 Sr)i (0 .711 813 ~ 0 .717 307) .These geochemical features indicate that the porphyry of Chagele deposit is similar to the peraluminous feature of S‐type granite .The above discussion suggests that the Paleocene magmatism and mineralization in the middle section of the Middle‐Gangdese metallogenic belt could be formed through partial melting of the ancient crust materials of Gangdese micro‐continent accompanying with fractional crystallization ,which was probably induced by upwelling of mantle‐derived magma in the main collision between Indian and Eurasia continent .