中冈底斯成矿带龙根铅锌矿床含矿斑岩年代学与地球化学特征

Geochronology and geochemical characteristics of ore-bearing porphyry in Longgen lead-zinc deposit of middle-Gangdese metallogenic belt,Tibet

龙根铅锌矿床是在西藏中冈底斯成矿带中段发现的一个矽卡岩型矿床。对该矿床含矿斑岩进行了LA-ICP-MS锆石U-Pb定年,主量、微量元素分析和Sr-Nd-Hf同位素组成研究,首次获得中冈底斯成矿带中段铅锌银矿床的矿化时代。含矿斑岩锆石U-Pb年龄明显可分为2组,206Pb/238U加权平均年龄分别为(70.5±2.0)Ma和(61.4±1.2)Ma,前者记录了印-亚板块俯冲大陆边缘弧后伸展过程中的早期岩浆-流体成矿事件,后者代表了印-亚板块主碰撞聚合中的主成矿期岩浆-流体成矿作用时代。斑岩具富硅、富钾,贫钛、贫磷等特征,铝饱和指数(A/CNK)为1.11～1.16,富集大离子亲石元素Rb、Th、U,亏损高场强元素Zr、Nd、Sm等,与冈底斯成熟大陆地壳物质相比具相对高的εNd(t)值(-6.42～-6.05)和相对低的(87Sr/86Sr)i值(0.708 026～0.709 071),并具不均一的锆石εHf(t)值(-3.34～-1.12)以及古老的锆石Hf同位素地壳模式年龄(TCDM=1.06～1.19 Ga),属于过铝质"S"型花岗岩类。笔者认为中冈底斯成矿带中段晚白垩世—古新世岩浆活动和成矿作用很可能是与雅鲁藏布江洋盆闭合之后的印-亚板块俯冲大陆边缘弧后伸展和主碰撞聚合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。

Located in the middle section of the middle-Gangdese metallogenic belt, the Longgen porphyry-skarn deposit is a recently-discovered deposit in Tibet. However, the high-precision chronologic data of porphyritic intrusion and deposit are absent. The zircon U-Pb ages are reported in this study. Based on whole rock major elements, trace elements and zircon Sr-Nd-Hf isotopic data of the ore-bearing rocks in the Longgen deposit, the authors obtained for the first time the ore-forming age of the middle section of the middle-Gangdese metallogenic belt. Zircon U-Pb ages of the granitic porphyry are divided into two groups, -70.5 Ma and -61.4 Ma, with the former probably recording an earlier magmatic-fluid activity related to the initial collision of Indian-Asian continent, and the latter representing magmatic crystallization related to the extensional environment in the back-arc of the active continent. The ore-bearing porphyry of the Longgen deposit has high SiO2, K2O, Rb, Th, and U content and low TiO2, P2O5 content as well as strong negative anomalies of Zr, Nd and Sm, variable εHf（t） （-3.34 - -1.12）and ancient TCDM （1.06-1.19 Ga）. In comparison with Gangdise continental crust, the ore-bearing porphyry of the Longgen deposit has high εNd（t）（-3.34 - -1.12）and low （^87Sr/^86Sr）i（0.708 026 - 0.709 071）. These geochemical features suggest that the samples from the Longgen deposit belong to the peraluminous S-type granite. The authors hold that the Late Cretaceous-Paleocene magmatism and mineralization in the middle section of the middle-Gangdese metallogenic belt might have been produced by partial melting of ancient crust materials of the Gangdese micro-continent accompanied by fractional crystallization, resulting probably from underplating of mantle-derived magmas in the epicontinental back-arc extensional environment and the main collision setting of the collision between Indian and Eurasia.