西秦岭多福屯和甘加地区晚中生代陆内火山岩Mg-Os-Pb-Sr-Nd同位素地球化学及岩石成因研究

Located in the northeastern margin of the Qinghai-Tibet Plateau, West Qinling is tectonically surrounded by the Tethys Domain, Pacific domain and Paleo-Asian Ocean domain, with a very special tectonic background. According to our preliminary study and former research work, this project mainly focuses on the late Mesozoic intracontinental sodic alkali basalts and potassic tholeiitic basalts and associated volcanic rocks in the Duofutun area and Ganjia area, West Qinling. Based on the systematic analyses in terms of mineralogy, petrology, geochronology and conventional geochemical techniques (e.g. major, trace elements and Pb-Sr-Nd isotopes), this project will utilized Mg and Re-Os isotopes as a breakthrough, aiming to dig and discover the mantle characteristic, composition, thermal state, type of metasomatism, partial melting mechanism, interaction between lithosphere and asthenosphere and interaction between crust and mantle beneath West Qinling during the late Mesozoic, as well as the origin of olivine xenocrysts within alkali basalts. This project will closely combine the late Mesozoic magmatism with the deep dynamic process. This study will not only provide an important case study to probe into the origin of the intracontinental volcanic rocks in terms of non-traditional isotopes, but also offer reliably petrological and geochemical constraints on the tectono-magmatic evolution of West Qinling before the India-Eurasia collision and the uplift of Tibet in the late Mesozoic.

西秦岭位于青藏高原东北缘，是特提斯构造域、滨太平洋构造域和古亚洲洋构造域的交接转换带，具有极为特殊的构造环境。本项目在早期预研究和工作积累之上，选择西秦岭多福屯地区和甘加地区出露的晚中生代陆内钠质碱性玄武岩、钾质拉斑玄武岩及相关火山岩为重点剖析对象，在系统的矿物学、岩石学、同位素年代学、主微量和Pb-Sr-Nd等常规地球化学分析相结合的基础上，进一步以Mg同位素和Re-Os同位素为突破口，重点剖析西秦岭晚中生代下覆地幔源区性质、物质组成、热状态、交代作用类型、熔融机制、岩石圈/软流圈相互作用和壳/幔相互作用过程中的相关细节，同时查明碱性玄武岩中特殊橄榄石“捕掳晶”的成因，并将西秦岭晚中生代陆内岩浆作用和深部动力学背景建立有机联系。该研究不仅在非常规同位素探究陆内火山岩成因方面提供了一个重要的研究实例，而且为印度－欧亚大陆碰撞及高原隆升之前西秦岭构造—岩浆演化提供可靠的岩石学和地球化学约束。

本项目选择西秦岭多福屯（麦秀）-甘加（八角城）-红墙（麻当）中生代陆内火山岩为主要研究对象，综合火山地质学与室内综合研究，运用岩相学、矿物学、元素地球化学和Sr-Nd-Pb-Os同位素，以及全岩基质玻璃激光40Ar/39Ar定年技术，查明了西秦岭晚中生代火山岩的时空分布，即三个露头产出的火山岩（以玄武岩为主，兼有少量的玄武安山岩）的喷发时限为109～99Ma，表明其形成于早晚白垩世之交。其中多福屯碱性玄武岩在元素地球化学和Sr-Nd-Pb同位素方面，具有与洋岛玄武岩相似的地球化学特征，其原始地幔标准化图解上广泛出现的K元素的亏损和部分大离子亲石元素（例如Ba、Sr等）的富集指示了源区可能存在角闪石岩脉体。多福屯碱性玄武岩的87Sr/86Sr 同位素初始比值范围为0.70326 ～0.70503，143Nd/144Nd 初始比值为0.51272 ～0.51287, (206Pb/204Pb)t 变化范围为17.876～18.253（与EM2 型地幔端元相近），(207Pb/204Pb)t 变化范围为15.421～15.518，(208Pb/204Pb)t 变化范围为37.580～38.104，上述同位素特征指示多福屯火山岩源区具有DMM 和EM2 型富集地幔端元混合的特征。多福屯玄武岩具有低的Os含量（4.6～42.02 ppt）和较大变化范围的Re/Os比值特征，初始187Os/188Os变化范围为0.2112～0.6784。通过AFC模拟，指示该套玄武岩受到了地壳混染的影响。西秦岭多福屯及周缘地区发育的晚中生代中基性火山岩暗示了区域拉张活动引发的岩石圈与软流圈的相互作用，发生深部富角闪石脉体及周围地幔橄榄岩的部分熔融，并沿着区域断裂喷发至地表，保存在一系列拉分盆地中。