Magnetic structure of the continental crust as revealed by the Wutai-Jining crustal cross-section in the North China craton

Abstract

The Wutai-Jining crustal cross-section is located in the central North China Craton. The upper crustal cross-section is represented by the Wutai granite-greenstone terrain and overlying post-Archean sedimentary rocks, the middle crust by the comparable Henshan and Fuping amphibolite- to granulite-facies terrains, and the upper lower crust by the granulite-facies Jining terrain. Correlation of measured seismic velocities of rocks from the crustal cross-section with data on seismic refractions suggests that the section is likely to represent a ca 30 km crust column with the 5 km thick lowermost crust being not exposed (Kern, H., Gao Shan, Liu Qingsheng, 1996. Earth Planet. Sci. Lett. 139, 439–455). Forty-four samples from the cross-section, some of which were used for measurements of seismic velocities and densities ( Kern et al., 1996), are analyzed for saturation magnetization ( J s) and saturation isothermal remanent magnetization (SIRM). Rock magnetism depends primarily on metamorphic grade and thus on different depth of mineral equilibrium. Lithology plays a less important role. Rocks of greenschist-, amphibolite- and granulite-facies have average J s of 58.7, 686 and 1068 A/m, respectively. SIRM corresponds to 4.1, 77.9 and 138 A/m. Intermediate and felsic granulites from the Jining terrain show even higher magnetism than greenschist-facies metabasalts from the Wutai terrain. Variation coefficient ( V c) of J s increases from the upper (62.2%) through the middle (64.3%) to the lower (144%) crust. Similarly, SIRM increases from 70.7 to 82.9 and to 165%. This documents considerably greater magnetic heterogeneity in the lower crust compared to the upper and middle crust. For the same lithology, magnetism ( J s and SIRM) of rocks from the Jining terrain is remarkably higher than that of rocks from the Archean Taihua granulite terrain at the southern margin of the North China craton adjacent to the Qinling orogenic belt. This is attributed to lower heat flow (52–56 mWm −2) in the central North China craton compared to the southern margin of the craton (62 mWm −2). Combined with longwavelength magnetic anomalies from Magsat, it is inferred that the mafic granulite in the cross-section is responsible for lower crustal magnetization in the region. They are comparable to magnetization of lower crust from shield areas in other parts of the world. The strong magnetism of the granulites under this investigation and pyroxenite xenoliths from the nearby Hannuoba alkaline basalt suggest that the magnetic bottom of the lithosphere in the central North China craton lies at the base of the crust or the uppermost mantle.