华北北部早中新世玄武岩成因的Mg-O同位素制约

中国东部分布有大量的新生代玄武岩，分为碱性和拉斑两类，两者具有密切的时空关系。研究表明，对于华北北缘的新生代玄武岩地幔源区，是受古亚洲洋板块俯冲的影响，还是受西太平洋板块俯冲的影响，存在很大的争议。本申请项目拟选取华北北部距大兴安岭-太行重力梯度带和中亚造山带不同距离的早中新世同时期碱性和拉斑玄武岩作为研究对象，探讨不同板块俯冲体系对这些玄武岩的地幔源区的可能影响。鉴于中国东部新生代玄武岩地幔源区普遍体现出再循环碳酸盐交代的特征，而沉积碳酸盐具有明显不同于地幔的Mg-O同位素特征，因此Mg-O同位素的结合是示踪地幔再循环碳酸盐交代作用的强有力手段。本项目将重点利用Mg-O同位素并辅以元素地球化学和Sr-Nd-Hf同位素地球化学，对华北北部不同地区同时期的早中新世碱性和拉斑玄武岩进行系统研究，确定它们的地幔源区是受古亚洲洋板块俯冲的影响，还是受西太平洋板块俯冲的影响，抑或受两者的共同影响。

Voluminous Cenozoic basalts occur in eastern China. The basalts can be grouped into alkali and tholeiitic and both are usually associated spatially and temporally. Despite of a number of studies, it is still controversial whether the mantle source for the Cenozoic basalts at the northern margin of the North China craton is affected by the Paleo-Asian slab subduction or by the western Pacific slab subduction. We will choose early Miocene alkali and tholeiitic basalts that have various distances away from the Daxinganling-Taihangshan gravity lineament and from the Central-Asian Orogenic Belt as investigation objectives to discuss the possible affection of different subduction systems. Given that a common carbonated mantle source is suggested for the Cenozoic basalts of the eastern part of China, a combination of Mg-O isotopes is a powerful tool for tracing mantle metasomatism by recycled carbonates because sedimentary carbonates have δ26Mg and δ18O values significantly different from the mantle. This project will utilize a combination of Mg-O isotopes together with elemental and Sr-Nd-Hf isotopic geochemistry to systematically investigate the coeval early Miocene alkali and tholeiitic basalts from a number of regions of the northern part of the North China. The aim is to determine whether the mantle source for the early Miocene basalts is affected by the Paleo-Asian slab subduction, or by the western Pacific slab subduction, or both.