摘要
讨论了下地壳拆沉作用的地球化学示踪方法，并以近来对秦岭－大别造山带和美国西部内华达岩基地区的研究成果为例，说明了下地壳拆沉在两地区壳－幔演化方面可能起的重要作用。
It has been suggested that during collision events leading to crustal thickening and formation of eclogite, or through underplating and fractionation of basaltic magma at the base of the crust, the mafic/ultramafic root of the continental crust will become denser than the underlying upper mantle. This density instability will lead to delamination that occurs during continental collision and drives lower crust recycling into the mantle. Lower crustal delamination has been proposed to explain the dilemma that the primary mantle additions into the crust are principally basaltic, whereas the present day continental crust has an intermediate bulk composition. Eu, Sr and transition metals(Cr,Ni,Co,V,Sc and Ti) are enriched in mafic granulites and eclogites. As a result, delamination of lower crust will reduce abundances of these elements as well as Eu/Eu and ratios of Sr and transition metals to Nd in the continental crust. Correspondingly, SiO 2 increases. Therefore, Eu, Sr and transition metals can be used together to test the consistency of a geochemical model of lower crustal delamination. The seismic consequence of delamination will be evident in lower seismic velocities due to the loss of high velocity mafic root, while geologically, delamination is expected to produce crustal exhumation followed by extension and formation of basins and leads to a lack of substantial mountain roots. Our estimates of the total crust composition in central East China show a more evolved character compared to models of Rudnick and Fountain(1995) and Taylor and McLennan(1985,1995) and are characterized by a prominent negative Eu anomaly(Eu/Eu =0 80),higher SiO 2(61 8%),and lower Sr/Nd(～10) as well as lower Sr/Nd、Cr/Nd、Ni/Nd、Co/Nd、V/Nd and Ti/Nd ratios. This ,together with slower crustal velocities and remarkably thin crustal thicknesses(34km) for the paleozoic Mesozoic Qinling Dabie orogenic belt,leads to the suggestion that lower crustal delamination played an important role in modification of the East
出处
《高校地质学报》
CAS
CSCD
1998年第3期241-249,共9页
Geological Journal of China Universities
基金
国家自然科学基金
国家教委基金
地质矿产部壳幔体系组成物质交换及动力学开放研究实验室基金联合资助
关键词
拆沉作用
下地壳
壳幔变换作用
造山带
地壳演化
delamination, lower crust, crust mantle interaction, Qinling Dabie orogenic belt, Sierra Nevada batholith
