用玄武岩组成反演中-新生代华北岩石圈的演化

玄武岩的化学组成与地幔源区特征、部分熔融程度、地幔温度和岩石圈厚度等多个因素有关,因此可以用来反演深部地幔的演化。文中简要地阐述了用玄武岩组成获得岩石圈厚度及其变化的方法,并总结了有关华北中—新生代岩浆演化的两个最主要特征:(a)晚中生代岩浆活动经历了由早期的源自富集地幔的岩浆向后期亏损地幔起源岩浆的转变,而两个阶段为一岩浆间隙期(~10Ma)所分隔;(b)华北东、西部新生代玄武岩具有相反的碱性强度随时间的变化趋势。这些岩浆演化特征可以用岩石圈减薄过程中地幔地温梯度的逐渐升高、岩石圈地幔中富集组分在短时间内的不可再生以及岩石圈盖效应来解释。该认识为华北岩石圈减薄的时间尺度和机制以及减薄作用的时空不均一性提供了新的制约。

Using basalt geochemistry to constrain Mesozoic-Cenozoic evolution of the lithosphere beneath North China Craton

Because the composition of basalts depends on a number of factors including source characteristics, degree of partial melting, mantle temperature and lithospheric thickness, it can be used to constrain the mantle evolution.This paper describes how to use basalt geochemistry to constrain the lithospheric thickness and summarizes two major features regarding the evolution of Mesozoic-Cenozoic magmas in the North China craton:(a) there is a transition from enriched mantle-derived magmas to depleted mantle-derived basalts during the late Mesozoic, and these two stages are separated by a magmatic hiatus (~10 Ma); (b) the temporal variations in alkali content for Cenozoic basalts from western and eastern North China are different.All of these features can be understood in terms of a gradual increase in thermal gradient of the lithosphere during lithospheric thinning, the consumption of fusible components in the lithospheric mantle that cannot be re-supplied over a short period of time, and the lithospheric lid effect.The present interpretation yields important constraints on the time scale and mechanism of lithospheric thinning in North China.It also implies a temporally and spatially heterogeneous process for the destruction of the lithospheric keel beneath the North China craton.