新疆北部的两类埃达克岩

Two types of adakites in north Xinjiang,China.

新疆北部有两类埃达克岩,一是俯冲型,形成于早、中泥盆世-早石炭世晚期(≥320Ma),包括了埃达克岩、富Nb玄武岩、高(富)Mg安山岩。第二类埃达克岩是底侵型,形成于中晚二叠世(≤280Ma)。第一类埃达克岩分布于西天山的阿拉套山、博罗科努山,中天山的骆驼沟和巴仑台,东天山的土屋-延东,阿尔泰山陆缘南富蕴-青河南,准噶尔盆地中部陆梁,克拉玛依等地。在阿尔泰陆缘南,苦橄岩与埃达克岩、富Nb玄武岩和高(富)Mg安山岩密切组合。第二类埃达克岩分布于西天山的阿吾拉勒山和东天山的三岔口,未发现富Nb玄武岩和高(富)Mg安山岩组合。俯冲型埃达克岩、富Nb玄武岩和高(富) Mg安山岩的高Sr低Y、Yb、富Eu及高ε_(Nd)(t)(+1．5～+10．0),低(^(87)Sr/^(86)Sr);(＜0．7070)的同位素组成,均一致表明其源区物质为洋壳板片,部分为地幔楔、弧前棱柱,产于岛弧环境;而底侵型埃达克岩源于底侵的幔源玄武质物质,形成于后造山环境。两类埃达克岩及其组合岩石的地质及地球化学特点,展示了中亚型造山在本区晚古生代陆壳增生作用的多样性:在增生构造过程上,有洋壳板片的斜俯冲、俯冲板片的撕裂、板片窗、俯冲剥蚀及玄武质物质的底侵作用等;在增生方向上,有洋壳板片的侧向斜俯冲,也有玄武质物质垂向上底侵于壳-幔边界;在增生物质上,有洋壳板片、地幔楔、受地幔楔混染的洋壳板片熔体,弧前棱柱、地幔楔受板片熔体交代后形成熔体及底侵的幔源玄武质物质。与两类埃达克岩有关,尤其是第一类埃达克岩及其组合岩石,在本区广泛发育了Cu、Au成矿作用,其中部分达到大型-超大型规模。因此,对埃达克岩及其组合岩石的识别及相关Cu、Au成矿作用的找矿勘探应予以足够重视。

Two types of adakites were recognized in northern Xinjiang. The first one is subducted oceanic slab-related adakite and composed of Nb-enriched basalt（NEB） and high-Mg or Mg andesite （HMA or MA）. The second one is underplating basalt-related. The first type of adakites were mainly generated during early-middle Devonian-late early Carboniferous （≥ 320Ma） and distributed in the western Tiansha （ Alataw, Boloholo）, central Tianshan （ Luotuogou）, eastern Tianshan （ Tuwu-Yandong）, south margin of Altay （Fuyun, south Qinghe）, central Junggar basin （Luliang） and souther Kalamay （Baogutu）. These adakites are characterized by depleted HFSE （ Nb, Ta and Ti ） and HREE （ Yb and Y）, enriched in Sr, high positive εNd （t） values, low TDM model ages （〈900Ma） and low （^87Sr/^86Sr）1 ratios. The HMA or MA are of high Mg^# values, high contents of Cr and Ni and lower positive εNd （t） values. High contents of Nb and relative rich in Yb and Y are the typical features of NEB. These geochemical characteristics for the adakites, HMA and NEB have displayed that the subducted oceanic slab and mantle wedge are the main source materials for these adakite and associated rocks. Particularly, picritic rocks have been found in the south margin of Ahay and associated closely spatially and temporally with adakite, HMA and NEB. The second type of adakite was mainly generated in mid-late Permian （≤280Ma） and distributed in Awulale mountain （west Tianshan） and Sanchakou （east Tianshan）. No associated NEB and HMA have been found. Based on the distribution of two types of adakites and the close association of the first type of adakites with HMA, NEB and picritic rocks, it can be deduced that the crust accretion in north Xinjiang possesses multi-fashions. For accretion direction, the vertical and horizontal accretion are both exist. For the accretion tectonic process, the oblique subduction of oceanic slab, slab tera, slab window, subduction erosion and the underplating of basaltic materials are all able to generate the adakites, NEB, HMA and picritic rocks. For the accretion materials, there are subducted oceanic slab, mantle wedge, forearc prism, mantle wedge modified by adakitic melt, adakitic melt contaminated by the manle wedge and upwelling asthenospheric thru the slab window. Relatively extensive Cu and Au mineralization associated with adakite, NEB and HMA have been foud in north Xinjiang, particular for the first type of adakites. Some of them can be achive lageosuperlarge scale and showing the high potential of adakite, NEB and HMA for the exploration of Cu and Au in north Xinjiang.