青藏高原碰撞造山带:I.主碰撞造山成矿作用

Metallogenesis in Tibetan collisional orogenic belt: Ⅰ. Mineralization in main collisional orogenic setting

大陆碰撞与成矿作用是当代成矿学研究的重要前沿。与板块构造成矿作用研究相比,大陆碰撞造山带的成矿作用研究则明显薄弱。文章以青藏高原主碰撞带为对象,研究了印度—亚洲大陆主碰撞过程与区域成矿作用的耦合关系,并初步建立了主碰撞造山成矿模型。研究表明,印度—亚洲大陆主碰撞始于65Ma,延续至41Ma,形成了以藏南前陆冲断带、冈底斯主碰撞构造_岩浆带和藏北陆内褶皱_逆冲带为特征的青藏高原碰撞造山带主体。伴随陆_陆碰撞,在冈底斯带相继发育①壳源白云母花岗岩_钾质钙碱性花岗岩组合(66～50Ma)、②+εNd花岗岩_辉长岩组合(52～47Ma)和③幔源玄武质次火山岩_辉绿岩脉组合(42Ma),以及大面积分布的巨厚(5000m)的林子宗火山岩系(65～43Ma),反映深部相继发生大陆碰撞和板片陡深俯冲(65～52Ma)→板片断离(52～42Ma)→板片低角度俯冲(<40Ma)等重要过程。在主碰撞期,初步识别出4个重要的成矿事件:①与壳源花岗岩有关的Sn、稀有金属成矿事件,在藏东滇西形成腾冲Sn、稀有金属矿集区;②与壳/幔花岗岩有关的Cu_Au_Mo成矿事件,在冈底斯南缘形成长达百余公里的Cu_Au矿化带;③与碰撞造山有关的剪切带型Au成矿事件,沿雅鲁藏布江缝合带分布,形成具有较大成矿潜力的Au矿化带;④与挤压抬升有关的Cu_Au成矿事件,形成以雄村大型铜金矿为代表的斑岩型/浅成低温复合型Cu_Au矿床。在综合研究基础上,初步建立了大陆主碰撞造山区域成矿模型。

Metallogenesis of continents is an important research frontier in the present regional metallogeny, and its theoretic framework might be built up by deepening the understanding of metallogenesis in the collisional orogenic belt. However, metallogenesis in the collisional orogenic belt is rather poorly understood in comparison with metallogenesis in the plate tectonic setting. The Himalayan-Tibetan orogenic belt, which is characterized by the existence of large-scale, intense and young mineralizations as well as various kinds of large-size and weakly reformed deposits, is regarded as an ideal field for understanding the ore-forming processes in the, collisional orogenic belt. Based on three years＇ systematical work on metallogeny in the Tibetan collisional orogenic belt, the authors have summarized the main ore-forming events and their temporal-spatial distribution, analyzed four important metallogenic belts and representative deposits as well as their plutonic processes and tectonic control factors and, on such a basis, set up a metallogenic model in the main collisional orogenic setting. The main collisional event lasted about 25 Ma （from 65 Ma to 41 Ma）, resulting in the formation of the principal part of the Tibetan plateau orogenic belt characterized by the South Tibet foreland thrust belt, the Gangdese main collisional tectonic-magmatic belt, and the North Tibet fold-thrust belt. In company with the collisional orogenic event, many important magrnatic events were developed, as evidenced by the existence of ① the crust-derived muscovite granite-potassic calc-alkaline granite assemblage （66-50 Ma）, ② the positive εNd granite-gabbro assem-blage （52-47 Ma）, and ③ the mantle-derived basaltic subvolcanics-diabase assemblage, as well as widespread and thick Linzizong volcanics （64- 43 Ma）. The development of above magrnatic activities suggests that the deep part of Tibetan plateau experienced in turn continental collision （65-52 Ma）, breakoff of continental plate （52-42 Ma） and low angle underthrust of continental plate （〈 40 Ma）. At least four important ore-forming events occurred during the main collisional orogenic epoch, which included ① the Sn and rare metal event related to crust-derived granite, forming the Tengchong Sn and rare metal ore concentration area in east Tibetan plateau, ② the copper-gold-molybdenum event related to crust/mantle-derived granite, forming a Cu-Au ore belt 100 km long in south Gangdese, ③ the shear zone gold event related to collisional orogeny, forming the potential Au belt along the Yarlung Zangbo suture zone, and ④the copper-gold event related to crust uplift, forming porphyry/epithermal transitional type Cu-Au deposits represented by the Xiongcun Cu-Au deposit.