How mafic is the lower continental crust?

Details How mafic is the lower continental crust? How mafic is the lower continental crust?

Sep 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998e%26psl.161..101g&link_type=abstract

Earth and Planetary Science Letters, vol. 161, Issue 1-4, pp.101-117

Physics

39

China, Lower Crust, Velocity Structure, Composition, Heat Flow, Delamination

Scientific paper

Crustal structures for nine broad tectonic units in China, except the Tarim craton, are derived from seismic data for 18 geophysical refraction profiles, which include twelve geoscience transects and have a total length of 16216 km. All the tectonic units except the Qinling orogen show a four-layered crustal structure, consisting of upper, middle, upper lower, and lowermost crust. If corrections are made to room temperature 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 averages. By correlation with experimental results on velocities of typical lower crustal rocks, the upper lower crust is suggested to be intermediate, while the lowermost crust is mafic in composition. Due to dominance 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 East China as a whole is suggested to have ~57% SiO 2 . The results 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 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 (34 km) 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 37-82 km thick eclogitic lower crust is required to have been delaminated in order to explain the relative Eu, Sr and transition metal 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.

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