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Magnetic Structure of the Continental Crust As Revealed by the Wutai-Jining Crustal Cross-Section in the North China Craton

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Magnetic Structure of the Continental Crust As Revealed by the Wutai-Jining Crustal Cross-Section in the North China Craton __________________________________________________________________________www.paper.edu.cn Journal of Geodynamics 29 (2000) 1±13 Magnetic structure of the continental crust as revealed by the Wutai-Jining crustal cross-section in the North China craton Liu Qingsheng a,*, Gao Shan b, c, Liu Yongsheng c aDepartment of Applied Geophysics, China University of Geosciences, Wuhan, 430074, China bDepartment of Geology, Northwest University, Xi'an, 710069, China cFaculty of Earth Sciences, China University of Geosciences, Wuhan, 430074, China Received 3 December 1998; received in revised form 9 July 1999; accepted 14 July 1999 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 (Js) and saturation isothermal remanent magnetization (SIRM). Rock magnetism depends primarily on metamorphic grade and thus on dierent depth of mineral equilibrium. Lithology plays a less important role. Rocks of greenschist-, amphibolite- and granulite-facies have average Js 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 coecient (Vc)ofJs 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 (Js 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 * Corresponding author. Fax: +86-27-87801763. E-mail address: [email protected] (Liu Qingsheng) 0264-3707/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0264-3707(99)00064-2 中国科技论文在线_________________________________________________________________________www.paper.edu.cn 2 Liu Qingsheng et al. / Journal of Geodynamics 29 (2000) 1±13 adjacent to the Qinling orogenic belt. This is attributed to lower heat ¯ow (52±56 mWm2) in the central North China craton compared to the southern margin of the craton (62 mWm2). Combined with longwavelength magnetic anomalies from Magsat, it is inferred that the ma®c 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. # 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Geophysical surveys (gravity, magnetic, electric and seismic) have shown that the continental lithosphere exhibits a distinct layered structure in physical properties. Their spatial distributions are related to geodynamic processes (Kay and Kay, 1981; Percival et al., 1989; Percival and West, 1994). The geological and seismic studies of the continental crust indicate that the upper continental crust is dominated by low-grade metamorphic rocks and is felsic in composition. In middle crustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content (Fountain and Salisbury, 1981; Christensen and Mooney, 1995). However, the non-uniqueness in correlation between geophysical properties and lithology and the complexity of deep geology lead to discrepancies between geophysical, petrological and geochemical boundaries within the continental lithosphere (Grin and O'Reilly, 1987, 1987). Granulite xenoliths carried by basaltic volcanics and exposed crustal sections form a window into the nature and processes of the deep crust. Combined geological, geochemical and geophysical studies of these deep crustal samples provide an important way to revealing structure and composition of the continental crust (Fountain and Salisbury, 1981; Schlinger, 1985, Schlinger et al., 1989; Wasilewski and Fountain, 1982; Wasilewski and Warner, 1988; Hahn and Roser, 1989; Liu and Gao, 1992; Liu et al., 1994, 1996a,b; Percival and West, 1994; Kern et al., 1996; Warner and Wasilewski, 1997). Rock magnetism is one important aspect of geophysical structure and is controlled by mineralogy, metamorphism, and anisotropy related to mineral fabrics. Experimental results show that thermal remanent magnetization (TRM) is the most stable at pressures of >800 MPa (Kobranova, 1989). The Curie Point (Tc) of magnetite varies with pressure with a coecient of only 208C/GPa (Samara and Giardini, 1969; Schult, 1970). Comparison of magnetic susceptibility (k ) measured on core samples with that derived from data on downhole magnetic survey shows excellent agreement (Bakhvalov et al., 1984). The Kola well investigations do not suggest the supposedly considerable in¯uence of pressure on rock k. Consequently, measurements of magnetic properties at room pressure can be applied directly to at least the upper part of the Earth's crust (Bakhvalov et al., 1984). In addition, the magnetism of amphibolite- to granulite-facies rocks from the Lofoten region, Norway, indicates that the Hopkinson eect (i.e., the relation between magnetic susceptibility and temperature) is insigni®cant (Schlinger, 1985). These results show that magnetic measurements 中国科技论文在线_________________________________________________________________________www.paper.edu.cn Liu Qingsheng et al. / Journal of Geodynamics 29 (2000) 1±13 3 of exposed deep rocks under room conditions are important for elucidating the magnetic structure of the continental crust. Studies of the magnetic structure of the continental crustal cross-sections can be used to evaluate (1) the relationship between rock magnetism and crustal apparent depths, (2) the correlations between magnetic and mineralogical and geochemical structures, (3) the relationships between magnetization of the continental lower crust and sources of magnetic anomalies of longwavelength in aeromagnetic and Magsat data, and (4) the magnitude and origin of magnetization in the lower crust. This paper presents magnetic data of representative rocks from the Wutai-Jining upper to upper lower crustal section. The results show a distinct layered structure in magnetism, which is controlled mainly by metamorphic grade. Ma®c granulites account for the majority of regional lower crust magnetization and are the source of longwavelength magnetic anomalies from Magsat data. 2. Geological background The North China craton is one of the world's oldest Archean cratons and preserves crustal remnants as old as 3.8 Ga (Liu and Gao, 1992). Although Proterozoic and Phanerozoic volcanosedimentary rocks were also developed in the area, and although this craton experienced signi®cant reactivation during Proterozoic and Mesozoic±Cenozoic rifting, studies of neodymium model ages of granite and sedimentary rocks indicate that most of the crustal growth occurred during the Archean in the North China craton. This implies that the crustal composition of the North China craton has not changed signi®cantly during the Proterozoic and Phanerozoic. The Wutai-Jining section is located in the central North China craton and oers a good continental crustal cross-section exposing rocks of the upper crust, middle crust and lower crust (Kern et al., 1996). The section consists of the three Archean terrains: 1. The Wutai granite-greenstone terrain. This terrain consists of bimodal ma®c-ultrama®c and felsic metavolcanic rocks, with minor banded-iron formations (BIF) in the lower unit and thick sequences of metagraywackes of turbidite character in the upper unit (Bai, 1986). The volcanic and sedimentary rocks were intruded by large volumes of contemporaneous tonalitic-trondhjemitic-granodioritic and granitic (TTGG) gneisses. SHRIMP zircon dating shows identical age (2550±2520 Ma) for the felsic volcanics and the felsic intrusions and suggests a co-magmatic origin for the two rock types (Wilde et al., 1998). 2. The Henshan amphibolite to granulite facies terrain, which is immediately adjacent to the Wutai terrain to the south, is predominated by TTGG gneisses with minor pelite (Wang et al., 1991). The felsic gneisses frequently contain amphibolite and ma®c granulite inclusions. Detailed geological mapping indicates that characteristic rock units of the Wutai terrain can be traced into the Henshan terrain with a northward increase in metamorphic grade (Xu and Xu, 1992). Anatexis is a common feature of the amphibolite
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