秦岭-祁连结合部位早古生代埃达克岩的发现及其造山作用意义  

The Discovery of the Early Paleozoic Adakitic Rocks in the Conjunction of the Qinling and Qilian Orogenic Belts and Its Implications on the Orogenic Processes

作　　者：孟祥舒 何艳红 陈亮[1] 务磊 

机构地区：[1]西北大学大陆动力学重点实验室,西北大学地质学系,西安710069

出　　处：《地质学报》2017年第12期2679-2696,共18页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号:41421002,41102121)资助的成果

摘　　要：阎家店闪长岩体位于北祁连造山带东段,是秦-祁构造结合部位最大的早古生代复合闪长岩体,主要岩性为石英闪长岩-闪长岩。早期研究显示阎家店闪长岩体北部样品的测年结果为~440Ma,是以红土堡基性火山岩为代表的弧间裂谷(或初始弧后盆地)在中晚奥陶世向北俯冲形成的岛弧侵入岩体。区域资料显示阎家店闪长岩自北向南岩体逐渐发育平行定向构造,具有糜棱结构,片麻状构造。南部糜棱岩化闪长岩LA-ICP-MS锆石U-Pb定年结果显示其形成于418±3.2Ma。主微量元素地球化学特征显示其与埃达克岩地球化学特征一致,具有高Si O2,Al2O3,Na2O和Sr,低Yb和Y以及高Sr/Y和(La/Yb)N比值,无或微弱的Eu异常。低Na/K,Nb/U和Ce/Pb比值以及负的εHf(t)值表明阎家店闪长岩体主要起源于壳源物质的部分熔融。较高的Y/Yb和Ho/Yb以及Nb/Ta暗示其源岩已经转变为榴辉岩相物质。高密度的榴辉岩相基性下地壳由于重力不稳定性拆沉进入地幔,受地幔橄榄岩交代导致了阎家店闪长岩体南部样品的高Mg O含量以及Mg#。因此,自~440Ma至~418Ma秦岭-祁连结合部位的地壳在不断地加厚,~418Ma的阎家店闪长岩形成于俯冲后碰撞造山阶段,其动力学背景和时限与北祁连构造带一致。Yanjiadian dioritic pluton, located in the east of the Qilian orogenic belt, is the largest Early Paleozoic composite dioritic pluton in the conjunction area of the Qinling and Qilian orogenic belts, and consists mainly of quart diorites and diorites. Previous studies suggest that the samples from northern Yanjiadian diorite has a formation age of -440Ma by U Pb zircon dating, produced by the northward subduction of the inter arc rift（or initial back arc basin） represented by Hongtubao basic volcanic rock during the Middle Late Ordovician period. Regional geology survey shows that s parallel oriented structure is well developed from north to south within the Yanjiadian dioritie with mylonitic texture and gneissic structure. LA ICP MS U Pb zircon dating of the Yanjiadian dioritie in the south indicates that the diorite formed at 418±32Ma. Geochemical analysis of major and trace elements display high SiO2, Al2O3, Na2O and Sr contents, low Yb and Y contents, high ratios of Sr/Y and （La/Yb）N, and no or weakly Eu anomalies, similar to the geochemical characteristics of the adakites. Low ratios of Na/K, Nb/U and Ce/Pb, and nagtive εHf（t） values suggest that these adakitic rocks resulted from the partial melting of the continental crust. High ratios of Y/Yb, Ho/Y, and Nb/Ta imply that the rocks in the source have been metamorphosed into the eclogite facies. High density, eclogite facies mafic lower crust may be detached from crust and sink into the mantle due to gravitational instability, and subsequent reaction with mantle peridotites results in the high MgO and Mg^# of the samples from the southern of the Yanjiadian diorite. In conclusion, the crust thickness in the conjunction area of Qinling Qilian progressively increased during the period of -440Ma to -418Ma, and -418 Ma diorite at Yanjiadian formed in the post subuduction and collision orogenic stage. Both events occurred at the same geodynamic setting and time framework as the North Qilian tectonic belt did.

关 键 词：埃达克岩 石英闪长岩 秦-祁结合部位 下地壳拆沉 晚志留世 

分 类 号：P588.122[天文地球—岩石学]