东准噶尔晚古生代依旧存在俯冲消减作用--来自石炭纪火山岩岩石学、地球化学及年代学证据

摘要

The volcanic rocks from the Carboniferous Batamayineishan Formation are widely distributed in eastern Junggar. Petrology, geochemistry and isotope geochemistry of the rocks indicate that:1) They have widely varied rock types including basalt-andesite-dacite-rhyolite rocks and are of high-K calc-alkaline, enriched in large ion lithophile elements (LILE) (Sr, K, Rb, Ba, Th) and LREE and depleted in high field strength elements (HFSE) (Nb, Ta, Ti)and HREE. Besides, the initial values of 87Sr/86Sr and 143Nd/144Nd are low and the εNd(t) values generally vary in range of 3.0~6.2. All these features suggest that they are immature back-arc basin volcanics related to subduction;2) The parent magma of the volcanic rocks mainly derived from the depleted mantle with involvement of minor juvenile lower crust comprised of remnants of Paleozoic oceanic crust and island arc system. The genesis of basic lavas may be attributed to a mixing process by hybridization of depleted mantle-derived magma and a small amount of juvenile crust-derived magma, which was related to partial melting of mantle wedge induced by fluid and sediments of subducted oceanic crust. Distinguished from the basic ones, the acid lavas originated from mantle-derived magma, coupled with strong fractional crystallization and crust contamination. 3) Consistent with regional geological background and discovered fossils, the Sm-Nd isochron age of 319.7±5.9 Ma can represent the time of eruption of the volcanic rocks. Based on comprehensive analysis, it is inferred that subduction of the Paleo-Asian Ocean was still underway at the time of 320 Ma and the final closure of the ancient Junggar ocean basin occurred in the period 320 to 311 Ma. Enriched in ore-forming substances, the intensified magmatism during this process made the eastern Junggar region of great prospecting potential.%东准噶尔地区石炭纪巴塔玛依内山组火山岩分布广泛,规模巨大。对其岩石学、元素地球化学及同位素地球化学研究表明：1)该套火山岩岩石类型复杂,具玄武岩-安山岩-英安岩-流纹岩组合特征,属高钾钙碱性火山岩系,并富集大离子亲石元素(Sr、K、Rb、Ba、Th)和轻稀土元素,亏损高场强元素(Nb、Ta、Ti)和重稀土元素,87Sr/86Sr和143Nd/144Nd初始值分别为0.70385~0.71312,0.152378~0.512998,εNd(t)多介于3.0~6.2之间,显示了与俯冲消减作用相关的不成熟弧后盆地火山岩地球化学特征。2)火山岩物质来源以亏损地幔源为主,并混有少量主体由古生代残余洋壳、岛弧体系组成的年轻下地壳物质。其中,基性熔岩以亏损地幔源为主,并在岩浆源区与少量新生地壳物质发生了近乎完全的壳-幔岩浆混合和Sm-Nd同位素体系均一化,其形成很可能与俯冲沉积物和(或)俯冲洋壳变质脱水产生的流体引起上覆地幔楔物质的部分熔融有关；而酸性熔岩与基性熔岩存在明显的不同,酸性熔岩是少量幔源岩浆经强烈结晶分异和经历较多壳源物质混染的结果。3)玄武岩全岩 Sm-Nd等时线年龄为319.7±5.9 Ma,与区域地质构造背景和已有化石证据基本一致,代表了火山岩的形成时代。综合研究表明,东准噶尔地区320 Ma左右依旧存在古亚洲洋的俯冲消减作用,准噶尔古洋盆最终闭合时间应介于320~311 Ma之间。在此过程中,火山岩浆作用强烈,蕴含成矿物质丰富,东准噶尔地区找矿前景值得期待。