冈底斯中北部晚侏罗世—早白垩世地球动力学环境:火山岩约束  

Late Jurassic-Early Cretaceous geodynamic setting in middle-northern Gangdese:New insights from volcanic rocks.

作　　者：朱弟成[1] 潘桂棠[1] 莫宣学[2] 王立全[1] 廖忠礼[1] 赵志丹[2] 董国臣[2] 周长勇[1] 

机构地区：[1]成都地质矿产研究所,成都610082 [2]中国地质大学,北京100083

出　　处：《岩石学报》2006年第3期534-546,共13页Acta Petrologica Sinica

基　　金：国家973项目(No.2002CB412609);国家自然科学基金项目(No.40503005;40473020);国土资源部百名优秀青年科技人才计划;中国地质调查局"青藏高原南部空白区基础地质综合研究"项目(No.200313000025)共同资助.

摘　　要：利用新近完成的1∶25万区域地质调查资料和笔者分析数据,初步研究了西藏冈底斯带晚侏罗世—早白垩世火山岩的分布特点及地层古生物、同位素年代学约束的火山活动时代以及冈底斯中北部火山岩的性质、构造环境、岩石成因和俯冲极性。冈底斯带晚侏罗世—早白垩世火山岩主要分布于南冈底斯、中冈底斯、北冈底斯和班公湖-怒江缝合带南缘,基本同期开始于早白垩世(大约120～130 Ma)并可能延续到晚白垩世,以冈底斯中北部地区规模最大为特征。已有数据显示中冈底斯则弄群火山岩包括部分中钾钙碱性系列的中基性岩石和占优势的高钾钙碱性系列的中酸性火山岩,不同于传统岛弧火山岩,但与中安第斯厚地壳背景下的岛弧火山岩相似,北冈底斯主要为与传统岛弧火山岩相似的中钾钙碱性系列火山岩。中冈底斯则弄群中基性火山岩很可能与来自消减沉积物和/或蚀变玄武质洋壳的含水流体引起上覆地幔楔物质的部分熔融有关,并在岩浆上升过程中经历了明显的分离结晶作用和中上部地壳物质的同化混染(即AFC过程),长英质火山岩很可能主要与地壳重熔有关,但并不能完全排除镁铁质岩浆的分离结晶作用。目前冈底斯中北部晚侏罗世—早白垩世火山岩地球化学数据揭示的成分变化、地壳厚度、与俯冲带的距离以及冈底斯带同期火山岩的分布特点暗示冈底斯中北部火山岩很可能与班公湖-怒江特提斯洋壳向南的俯冲作用有关。Distribution, volcanism-age constrained by stratigraphy, paleontology and isotopic dating for the Late Jurassic-Early Cretaceous volcanic rocks on Gangdese, and their tectonic setting, petrogenesis and subducted polarity have been preliminarily studied based on regional geological survey （ 1 ： 250000） and our data. Our study shows that the Late Jurassic-Early Cretaceous volcanic rocks are not only distributed on southern Gangdese, but also widely exposed on middle, northern Gangdese and southern margin of Bangong Co-Nujiang suture zone. They started to erupt at about 120 - 130 Ma and likely lasted to the Late Cretaceous. However, the maximum volcanic activity occurred in middle-northern rather than southern Gangdese. Present geochemical data indicate that the Zenong Group volcanic rocks on middle Gangdese, which are mainly composed of partly intermediate-basic rocks of medium-K calc-alkaline series and dominant intermediate-acid rocks of high-K calc-alkaline series, are not consistent with traditional island-arc volcanic rocks but similar to volcanic rocks erupted in a thick crust from Central Andes, whereas the Duoni Formation volcanic rocks on northern Gangdese consist of mainly medium-K calc-alkaline series, resembling traditional island-arc volcanic rocks. It should be noted that all of these volcanic rocks have affinities with island-arc volcanic rocks. The Zenong Group intermediate-basic volcanic rocks on middle Gangdese are most likely derived from the partial melting of mantle wedge materials induced by fluid from subducted sediments and/or subducted basaltic crust and then experienced assimilation-fractional crystallization （AFC process） during ascending. The Zenong Group felsic rocks are dominantly related to crustal remelting, however, fractional crystallization of mafic magma cannot be entirely ruled out. The compositional variation, crustal thickness, distance from subducted zone revealed by present geochemical data of the Late Jurassic- Early Cretaceous volcanic rocks on middle-northern Ga

关 键 词：俯冲极性 岛弧火山岩 晚休罗世-早白垩世 冈底斯中北部 西藏 

分 类 号：P588.14[天文地球—岩石学]