西藏班公湖-怒江成矿带主碰撞期成矿作用:荣嘎钼矿岩石地球化学及同位素年龄的证据  

Main collisional mineralization of Bangong-Nujiang metallogenic belt,Tibet: Geochronological,geochemical and isotopic evidence from Rongga molybdenum deposit

作　　者：彭勃 李宝龙[1] 刘海永 秦广洲 龚福志 周磊[5] PENG Bo;LI BaoLong;LIU HaiYong;QIN GuangZhou;GONG FuZhi;ZHOU Lei(MNR Key Laboratory of Melallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 ,China;Tibet Institution of Geological Survey, IJiasa 850000, China;College of Earth Sciences, Chengdu University of Technology, Chengdu 610059 , China;No. 2 Geological Team of Tibet Bureau of Geology and Mineral Exploration and Development, Lhasa 850003 , China;Geochemistry Exploration Brigade of Sichuan Bureau of Exploration and Development of Geology and Minerals Resources, Deyang 618000 , China)

机构地区：[1]中国地质科学院矿产资源研究所自然资源部成矿作用与资源评价重点实验室,北京100037 [2]西藏自治区地质调查院,拉萨850000 [3]成都理工大学地球科学学院,成都610059 [4]西藏自治区地质矿产勘查开发局第二地质大队,拉萨850003 [5]四川省地质矿产勘查开发局化探队,德阳618000

出　　处：《岩石学报》2019年第3期705-723,共19页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41802103);中国博士后科学基金(2018M641434);中国地质调查局项目(DD20160026)联合资助

摘　　要：荣嘎钼矿是新近在班公湖-怒江成矿带发现并证实的首例斑岩型钼矿床。为了查明班公湖-怒江成矿带主碰撞期的成岩成矿作用,探讨荣嘎钼矿的富Mo机制,对该矿床进行岩石地球化学以及同位素年代学的研究。本次研究获得荣嘎含矿二长花岗斑岩的锆石SHRIMP206Pb/238U加权平均年龄为99. 8±1. 9Ma (MSWD=2. 0),属晚白垩世早期。7件辉钼矿样品的模式年龄较为一致,介于98. 0±1. 6Ma～101. 8±1. 7Ma之间,加权平均年龄为99. 7±1. 1Ma。荣嘎含矿二长花岗斑岩样品主量元素表现为富硅(SiO_2=72. 83%～75. 49%)、富碱(Na_2O+K_2O=8. 12%～8. 55%)、过铝质(A/CNK=1. 08～1. 13)的高钾钙碱性系列岩石。样品明显富集Rb、K、Th、U等元素,中等亏损Nb、Ta,强烈亏损Ba、Sr、P、Ti等元素,具中等程度的负Eu异常(δEu=0. 53～0. 63),稀土元素配分曲线呈轻稀土相对较陡,重稀土较为平坦的右倾型。荣嘎含矿二长花岗斑岩样品的(^(87)Sr/^(86)Sr)i值为0. 7042～0. 7055,ε_(Nd)(t)值介于0. 03～0. 68之间,锆石ε_(Hf)(t)值介于+12. 4～+17. 9之间,两阶段模式年龄t_(DM2)较为年轻(31～314Ma)。基于以上同位素以及岩石地球化学特征研究,荣嘎含矿二长花岗斑岩很可能是幔源岩浆诱发了富Mo洋底沉积物的熔融并与壳源熔体混合形成母岩浆,再经历结晶分异作用而形成。结合本文以及区域上的研究资料,荣嘎钼矿可能形成于拉萨地块与南羌塘地块碰撞的大地构造背景,富Mo大洋沉积物熔体的加入及随后的结晶分异是荣嘎含矿岩浆中金属Mo富集的主要原因。班公湖-怒江成矿带南缘侏罗-白垩纪海相地层分布的地区,可能为勘查钼矿床的有利靶区,应重点评价晚白垩世早期高分异中酸性岩体的含矿性。Rongga deposit is the first porphyry molybdenum deposit to be newly discovered in Bangong-Nujiang metallogenic belt, Tibet. For the purpose to find out the main collisional magmatism and related mineralization of Bangong-Nujiang metallogenic belt, we do geochronological, geochemical and isotopic study on Rongga deposit. In this paper, SHRIMP zircon U-Pb dating yields a weighted mean age of 99.8±1.9Ma (MSWD=2.0), belonging to early Late Cretaceous. Seven molybdenite samples of Rongga mining area yield a weighted model age of 98.0±1.6Ma^101.8±1.7Ma with a mean of 99.7±1.1Ma. Rongga ore-bearing monzonitic granite porphyry is rich in SiO2 (72.83%~75.49%) and alkaline elements (8.12%~8.55%), belonging to the high-K calc-alkaline series. It is strongly enriched in Rb, K, Th, U, and are moderately depleted in Nb, Ta, and are strongly depleted in Ba, Sr, P, Ti, also has moderately negative Eu anomalies (δEu=0.53~0.63). Chondrite-normalized REE patterns show light REEs enrichment, heavy REEs depletion. The zircon εHf(t) values are mainly positive (+12.4^+17.9), and the model ages (tDM2) range from 31Ma to 314Ma. The ore-bearing monzonitic granite porphyry yielded whole-rock εNd(t) of +0.03^+0.68 and (87Sr/86Sr)i of 0.7042~0.7055. Based on integrated isotopic and geochemical data, we infer that the ore-bearing monzonitic granite porphyry formed mantle-derived magma mixed with the Mo-rich oceanic sedimentary melting. Based on the above data, combined with the regional geological background, Rongga molybdenum deposit formed in a main collisional tectonic regime. The porphyry molybdenum deposit is also a key type of mineral prospecting in the southern margin of the Bangong-Nujiang metallogenic belt in addition to skarn Cu-Au deposit. The exploration targeting of the molybdenum deposits should be focused on the margin of the Shiquanhe back-arc basin where the Jurassic-Cretaceous marine strata occur, the ore-bearing properties of the highly differentiated medium-acidic intrusion in the Early Cretaceous should be evaluate

关 键 词：荣嘎 拉萨地块 主碰撞 班公湖-怒江特提斯洋 富Mo机制 年代学 地球化学 

分 类 号：P588.12[天文地球—岩石学] P597.3[天文地球—地质学]