下地壳拆沉作用及大陆地壳演化

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。

LOWER CRUSTAL DELAMINATION AND EVOLUTIONOF CONTINENTAL CRUST

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/uhramafic 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 continenta1 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 wil1 reduce abundances of these elements as wel1 as Eu/Eu and ratios of Sr and transition metals to Nd in the continental crust. Correspondingly, SiO2 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 wil1 be evident in 1ower 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 SiO2(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 crusta1 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 China crust. Mass balance modeling further suggests that eclogite from the Dabie Sulu ultrahigh pressure metamorphic belt is the most likely candidate as the delaminated material, and that a cumulative 37～82km thick eclogitic lower crust is required to have been delaminated in order to expl~n the relative Eu, Sr and transition meta1 deficits in the crust of central East China. Delamination of eclogites can also explain the significantly higher than eclogite Poisson’s ratio in the present Dabie lower crust and upper mantle and lack of eclogite in Cenozoic xenolith populations of the lower crust and upper mantle in East China. As an analogue, xenolith and geophysical evidences from the Sierra Nevada batholith indicate existence of a～ 70km mafic-ultramafic eclogite facies root in Mid-Miocene, which complements the ～ 30km granitic intrusion as cumulates/residues. Delamination of the thick eclogitic root has also been proposed to explain the very thin crust of 30～40km currently in the southern Sierra Nevada, which is mainly granitic, as indicated by Vp 6.0～ 6.3km s-1, and is underlain by a peridotitic upper mantle, as revealed by younger Late Miocene xenoliths. Therefore, delamination of eclogite is a common and important process for evolution of continental crust and crust-mantle interaction.