HOW MAFIC IS THE LOWER CONTINENTAL-CRUST

Crustal structures for nine broad tectonic units in China, except the Tarim craton, are derived from seismic data for 18 geophysical refract ion profiles, which include twelve geoscience transects and have a tot al length of 16216 km. AU the tectonic units except the Qinling orogen show a four-layered crustal structure, consisting of upper, middle, u pper lower, and lowermost crust. If corrections are made to room tempe rature and 600 MPa, the upper lower crust has P-wave velocities of 6.7 -6.8 km s(-1) and the lowermost crust 7.0-7.2 km s(-1). Velocities of the bulk lower crust and total crust are 6.8-7.0 and 6.4-6.5 km s(-1), respectively. They are slower by 0.2-0.4 km s(-1), than the global av erages. By correlation with experimental results on velocities of typi cal lower crustal rocks, the upper lower crust is suggested to be inte rmediate, while the lowermost crust is mafic in composition. Due to do minance of the former, the bulk lower crust is still intermediate for most tectonic units. However, inter-unit compositional variations are also evident, as indicated by large variations in the thickness ratio (0.3-3.0) of the upper lower to lowermost crust. The lower crust in Ea st China as a whole is suggested to have similar to 57% SiO2. The resu lts contrast with generally accepted models of mafic lower crust. Our estimates of the total crust composition in central East China show a more evolved character compared to models of Rudnick and Fountain, and Taylor and McLennan and are characterized by a prominent negative Eu anomaly (Eu/Eu = 0.80), higher SiO2 (61.8%), and lower Sr/Nd (similar to 10) as well as lower Sr/Nd, Cr/Nd, Ni/Nd, Co/Nd, V/Nd and Ti/Nd ra tios. This, together with slower crustal velocities and remarkably thi n crustal thicknesses (34 km) for the Paleozoic-Mesozoic Qinling-Dabie orogenic belt, leads to the suggestion that lower crustal delaminatio n played an important role in modification of the East China crust. Ma ss balance modeling further suggests that eclogite from the Dabie-Sulu ultrahigh pressure metamorphic belt is the most likely candidate as t he delaminated material, and that a cumulative 37-82 km thick eclogiti c lower crust is required to have been delaminated in order to explain the relative Eu, Sr and transition metal deficits in the crust of cen tral East China. Delamination of eclogites can also explain the signif icantly higher than eclogite Poisson's ratio in the present Dabie lowe r crust and upper mantle and lack of eclogite in Cenozoic xenolith pop ulations of the lower crust and upper mantle in East China. (C) 1998 E lsevier Science B.V. All rights reserved.