内蒙古中部锡林浩特-西乌旗早三叠世A型酸性火山岩的地球化学特征及其地质意义

Early Triassic A-type felsic volcanism in the Xilinhaote-Xiwuqi,central Inner Mongolia:Age,geochemistry and tectonic implications

通过 K-Ar 年代学、元素、同位素地球化学研究,可以厘定内蒙古锡林浩特-西乌旗英安岩-流纹岩形成于早三叠世(K-Ar:245Ma);其具有富硅碱,贫钙镁,显著的负 Eu 异常,低 Ba、Sr 丰度和高 Rh/Sr 比值,以及高 Ga、Zr、Nb、Ce、Y 等元素地球化学特征;并呈现相对富集的(^(87)Sr/^(86)Sr)_i(0.7068～0.7099)和中等亏损的 Nd(ε_(Nd)(t)=+1.66～+3.34)的同位素特征。这些类似于 A 型花岗岩的地球化学特征。它们可能是内蒙中部近于同期发育的铝质 A 型花岗岩浆的喷出相,形成于幔源岩浆底侵新增生中下地壳的直接部分熔融,并伴随其后的分异作用。这些具有 A_2型花岗岩地球化学特征的酸性火山岩与同期拉斑玄武岩构成了造山后拉张环境下发育的特征性双峰式火山岩组合,这不仅为内蒙中部在早中生代已经结束块体拼合而进入造山后阶段提供了进一步的岩石学证据,而且暗示了地幔物质的加入在中亚造山带后造山时期陆壳的垂向生长过程中起最主要的作用。

In this paper, we present the combined K-Ar geochronological, elemental and isotopic geochemical data for the felsic volcanics from Xilinhaote-Xiwuqi, Inner Mongolia. The whole-rock K-Ar dating gives an early Triassic age at around 245Ma and we interpret it as representing the intrusive age for these felsic volcanics. Geochemically, these rocks are high in silica and alkalis and low in calcium and magnesium and exhibit fractionated REE patterns with remarkably negative Eu anomalies. They are also high in Ga,Zr, Nb,Ce,Y and Rb/Sr and low in Ba and Sr and display relatively enriched （^87 Sr/^86 Sr） i （0. 7068 - 0. 7099） and moderately positive εNd （t） （ ＋ 1.66 - ＋ 3.34） values. This indicates that these felsic volcanics share geochemical characteristics with aluminous A2-type granites, and probably generated by the direct partial melting of the newly-underplated lower crustal source heated by contemporaneous intrusion of mantle-derived basaltic magmas and the subsequent differentiations. They, together with the coeval tholeiite, constitute typical bimodal volcanic rock association developed within a post-orogenic or anorogenic extension setting. This not only provides further petrological evidences for establishing that the Inner Mongolia has seen the final amalgamation of terranes in the Central Asian Orogenic Belts （CAOB） during late Permian, but also has important implications for the predominant role played by contributions of mantle- derived magmas in vertical continental growth of the CAOB during post-orogenic period.