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

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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 and felsic gneisses. Sm±Nd and zircon U±Pb isotopic dating yielded ages of 2.8 Ga and 2.5 Ga for amphibolite inclusions and tonalitic gneiss, respectively (Tian et al., 1992). Thermobarometric studies and mineral assemblage indicate metamorphic pressure between 450 and 8708C (Xu, 1986). 中国科技论文在线______www.paper.edu.cn 4 Liu Qingsheng et al. / Journal of Geodynamics 29 (2000) 1±13

3. The Fuping amphibolite to granulite facies terrain, located to the south of the Wutai terrain, consists mostly of TTGG gneiss and amphibolite and locally preserved ma®c granulite with minor marble and pelite. The amphibolite and ma®c granulite were dated at 2.8 Ga by the Sm±Nd method (Zhang et al., 1991). The metamorphic conditions were estimated to be within the range of 650±8508C and 300±700 MPa (Wu et al., 1989). The Fuping and Henshan terrains show close similarities in rock association, metamorphic conditions, age and geochemical characteristics. For example, both terrains consist of 85±90% TTGG gneiss and 7±12% amphibolite and ma®c granulite and 3% metasedimentary rock according to the exposed area. Therefore, geological and geochemical evidence suggests a correlation between the Fuping and Henshan terrains (Tian et al., 1992; Xu and Xu, 1992). It has been argued that the two terrains represent the exhumed deep root of the Wutai terrain

Fig. 1. Generalized geological map of the central North China craton (Kern et al., 1996). Inset shows tectonic locations of the study area (small block) and the Xiangshui±Mandal Geoscience Transect (dotted bars) in China. NC=North China craton; YC=Yangtze craton; SC=South China Caledonian Fold Belt; KL=Kunlun orogenic belt; QD=Qinling±Dabie orogenic belt; IM=Inner Mongolian orogenic belt; 1= Post-Archean sedimentary rocks; 2=Archean Wutai granite-greenstone terrain; 3=Archean Henshan amphibolite to granulite facies terrain; 4=Archean Fuping amphibolite to granulite facies terrain; 5=Archean Jining granulie facies terrain; 6=Archean Wulashan amphibolite to granulite facies terrain; 7=granite; 8=sample locality. 中国科技论文在线______www.paper.edu.cn Liu Qingsheng et al. / Journal of Geodynamics 29 (2000) 1±13 5

(Xu and Xu, 1992). This is supported by the similar major element composition between the whole of the Henshan/Fuping terrains and the Wutai terrain. To the north of the Henshan terrain is the Jining granulite facies terrain, which may extend to the west and correlate with the Wulashan terrain in the Baotou area (Fig. 1; Shen et al., 1989). The Jining terrain north of Daton, where our samples were collected (Fig. 1), consists of TTGG gneiss, ma®c granulite of probably intrusive origin and supracrustal rocks. The supracrustal rocks are dominated by metasandstone, quartzite and metapelite that commonly contain sillimanite, garnet and graphite (Shen et al., 1989; Condie et al., 1992; Lu et al., 1992). Unlike the Henshan and Fuping terrains, ma®c granulites and TTGG gneisses of the Jining terrain are characterized by ubiquitous orthopyroxene and clinopyroxene. The peak metamorphic conditions were estimated to be 800±9008C and 700±1000 MPa (Shen et al., 1989). A preliminary U±Pb age from a single zircon from the TTGG gneiss yields an age of 3323 Ma (Krosner et al., 1987). In summary, the metamorphic grade shows a northward progressive increase, from the subgreenschist to low amphibolite facies Wutai terrain, through the high-amphibolite facies dominated Henshan/Fuping terrain, to the granulite facies Jining terrain. This is accompanied by an apparent tilting of the lowermost crust, with Vp of 6.8±7.4 km/s, a slight increase in depth of the Moho, a prominent increase of 600 nT in the magnetic anomaly (Ma et al., 1991) and a moderate increase of 30 mgal in the gravity anomaly (Shan Chen, unpublished data 1989), in the same direction. These features characterize exposed continental crustal cross- sections and indicate an uplift of lower crustal rocks of relatively high density and high magnetism (Fountain and Salisbury, 1981; Kern and Schenk, 1985; Schlinger, 1985; Wasilewski, 1987). The result is consistent with a signi®cant increase in the amount of ma®c rock from the Henshan/Fuping to the Jining terrains. It is, therefore, proposed that the Wutai to Jining terrains represent an oblique cross-section through the middle to lower crust of the North China craton. The crustal structure and seismic velocities are similar along the entire Xiangshui±Mandal Transect (Ma et al., 1991). The petrophysical and petrological data of the present study thus provides an important basis for the interpretation of the structure and composition of the deep crust of the North China craton.

3. Samples and analytical method

Forty-four representative samples from the Wutai-Jining section were measured for magnetic hysteresis loop using an LDJ-9500 Vibration Magnetometer (LDJ Company, USA). Some of the samples were previously measured for seismic velocities and densities by Kern et al. (1996). The samples include 15 greenschist facies rocks from the Wutai granite-greenstone terrain, 16 amphibolite- to granulite facies rocks from the Henshan and Fuping terrains and 13 granulite- facies rocks from the Jining terrain. The lithology covers major rock types of the four terrains and spans a composition range from ma®c through intermediate to felsic. An applied ®eld of 0.8 T was used in this study. Analytical precision is better than 1% with resolution 0.1 A/m. 中国科技论文在线______www.paper.edu.cn 6 Liu Qingsheng et al. / Journal of Geodynamics 29 (2000) 1±13

Table 1 Magnetic parameters of rocks from the Wutai-Jining section

Sample Lithology Js (A/m) SIRM(A/m) Hc (mT) R1

Wutai granite greenstone terrain WTH4 Ma®c volcanic 83.9 3.6 19 0.0425 WTH10 Ma®c volcanic 156 2.8 12 0.0182 WTH9 Ma®c volcanic 30.7 1.9 16 0.0630 WTH12 Metasedimentary 25.2 1.8 19 0.0694 WSZ1 Metarhyolite 75.5 4.2 19 0.0553 WGY1 Metasedimentary 35.4 2.2 18 0.0618 WGY5 Metasedimentary 29.2 2.6 19 0.0873 WSJ1 Ma®c volcanic 18.9 5.2 10 0.0277 WTB3 Ma®c volcanic 120 6.4 20 0.0531 HDQ7 Metarhyolite 50.4 13.2 59 0.2621 HDT4 Metapelite 63.8 1.1 9 0.0172 HDD3 Metasedimentary 38.6 3.8 55 0.0976 HDG1 Metasedimentary 49.3 7.2 35 0.1451 HGX1 Metasedimentary 41.5 3.4 20 0.0816 BT1 Trondhjemitic gneiss 62.6 2.6 14 0.0421 Average 58.7236.5 4.122.9 23215 0.070120.06 Henshan/Fuping amphibolite-granulite facies terrains H4 Metagraywacke 18.3 2.2 11 0.1171 H5 Amphibolite 101 1.9 10 0.0182 H8 Amphibolite 232 22.8 23 0.0982 H9 Ma®c granulite 177 10.8 30 0.0610 H10 Ma®c granulite 459 48.8 23 0.1064 H12 Ma®c granulite 441 44.7 21 0.1015 H14 TTG gneiss 666 65.5 17 0.0983 FP1 Metagraywacke 53.9 0.8 6 0.0145 FP3 Ma®c granulite 6830 848 17 0.1242 FP11 TTG gneiss 260 20.5 15 0.0788 FP12 Amphibolite 286 21.6 21 0.0756 FP13 TTG gneiss 208 23.7 19 0.1144 FP19 Amphibolite 507 77.5 19 0.1528 FP21 TTG gneiss 334 37.1 17 0.1111 FP22 Amphibolite 154 4.4 17 0.0284 FP29 Amphibolite 253 16.4 21 0.0647 Average 68621160 782200 1826 0.085320.038 Jining granulite facies terrain JN1 TTG gneiss 380 44.3 15 0.1165 JN2 TTG gneiss 1722 194 16 0.1128 JN3 Ma®c granulite 3337 471 13 0.1410 JN5 Ma®c granulite 2379 232 15 0.0975 JN6 Ma®c granulite 17.8 10.3 16 0.0581 JN8 Granite gneiss 196 22.2 18 0.1130 JN11 Ma®c granulite 5040 784 23 0.1555 JN12 Ma®c granulite 164 9.0 21 0.0552 JN13 TTG gneiss 84.0 6.1 16 0.0724 JN14 Metasedimentary 92.1 3.1 12 0.0334 JN15 Metasedimentary 88.3 3.5 10 0.0403 JN18 Metasedimentary 57.5 3.4 13 0.0589 JN24 Metasedimentary 167 17.2 30 0.1031 Average 10682154 1382229 1725 0.089120.037 中国科技论文在线______www.paper.edu.cn Liu Qingsheng et al. / Journal of Geodynamics 29 (2000) 1±13 7

4. Results and discussion

In order to study the eects of metamorphism and mineral composition on rock magnetism, comparisons are made for same rock types of varying metamorphic grades and for dierent rock types at the same metamorphic grades.

4.1. Magnetic properties

Table 1 gives magnetic parameters for the 44 samples. Rocks of dierent metamorphic grades display distinct saturation magnetization (Js) and saturation isothermal remanent magnetization (SIRM). The greenschist facies and essentially unmetamorphosed rocks from the

Wutai granite-greenstone terrain and the Hutuo group have Js ranging from 18.9 to 156 A/m with the average 58.7 A/m. SIRM ranges between 1.2 and 13.2 A/m and averages 4.1 A/m.

The Js of the amphibolite to granulite facies Henshan and Fuping terrains is 18.3±6830 A/m with the average of 686 A/m and SIRM 0.8±840 A/m with the average of 77.9 A/m. Js of the granulite facies Jining terrain varies from 57.5 to 5040 A/m and averages 1068 A/m and SIRM from 3.1 to 784 A/m with the average of 138 A/m. The average Js and SIRM of ®ve ma®c granulites are 2220 and 302 A/m, respectively, which indicates unusually strong magnetism.

Fig. 2. Magnetic hysteresis loops of representative rocks of (a) granulite facies, (b) amphibolite to granulite facies and (c) greenschist facies. 中国科技论文在线______www.paper.edu.cn 8 Liu Qingsheng et al. / Journal of Geodynamics 29 (2000) 1±13

Amphibolite- and granulite-facies rocks from this section display signi®cantly higher magnetism than those from the Dengfeng±Lushan crustal cross-section at the southern margin of the North China craton adjacent to the Qinling orogenic belt, whose average SIRM is 14.3 A/m (amphibolite facies) and 66.9 A/m (granulite facies), respectively (Liu and Gao, 1992; Liu et al., 1994). As pressure has insigni®cant eect on rock magnetism within the crustal depth (Bakhvalov et al., 1984; Kobranova, 1989), this is attributed to present-day lower heat ¯ow (52±56 mWm2) in the central North China craton compared to the southern margin of the craton (62 mWm2) (Yuan, 1996). Another possibility is a hotter thermal history experienced by the Dengfeng±Lushan section during the continental collision related to the orogeny of the Qinling belt which may have resulted in thermal demagnetization. Such mechanism was used

Fig. 3. Magnetic hysteresis loops of granulite facies rocks: (a) TTG gneiss and (b) ma®c granulite. 中国科技论文在线______www.paper.edu.cn Liu Qingsheng et al. / Journal of Geodynamics 29 (2000) 1±13 9 to explain the relatively low susceptibilities and weak induced magnetization of rocks in continental collision zones (Toft et al., 1993). Magnetic hysteresis loops of representative samples are shown in Fig. 2. The Wutai and Hutuo lower grade rocks are predominantly paramagnetic (Fig. 2c). The Henshan and Fuping amphibolite-granulite facies rocks show mixed paramagnetic and ferrimagnetic characteristics (Fig. 2b), while both ma®c granulites and TTG gneisses from the Jining terrain exhibit clear ferrimagnetic behavior (Fig. 2a). This is well exempli®ed by both ma®c and felsic granulites in Fig. 3, except JN13.

The ratio (R1) of SIRM to Js is of interest in that it relates to the domain state of the magnetic carriers in rocks. R1 values of rocks from greenschist-, amphibolite- and granulite- facies are 0.0701, 0.0853 and 0.0891, respectively. For felsic rocks, R1 increases from 0.0875 to 0.1037 for the Henshan, Fuping and Jining terrains. For ma®c rocks, R1 also shows a systematic increase with metamorphic grade from 0.0831 to 0.1015 (Table 2). The results document that metamorphic grade plays a dominant role in R1. Wasilewski (1987) and Dunlop (1986) suggest that R1 is either less than 0.05 or higher than 0.5 for multi-domain (MD) and single domain (SD), while pseudo-single-domain (PSD) and MD+PSD have R1=0.3±0.05. Therefore, R1 shows an apparent transition from MD through PSD to SD with increasing metamorphic grade. Consequently, R1 provides a basis for studying the origin of magnetization in the continental crust.

4.2. Relationship between rock magnetism and metamorphic grade

The relationship between rock magnetism and apparent depth can be constructed according to increasing depth from greenschist- through amphibolite- to granulite-facies, which broadly corresponds to the upper, middle and lower crust (Fountain and Salisbury, 1981; Liu et al., 1994; Christensen and Mooney, 1995). As shown in Table 2 and Fig. 4, magnetism of both ma®c and intermediate-felsic rocks shows a strong dependence on metamorphic grade. Clastic sedimentary rocks exhibit a less signi®cant dependence. Ma®c and sedimentary rocks from the greenschist-facies Wutai terrain have comparable SIRM, which is 4.5 and 4.1 A/m on average, respectively. However, the Jining ma®c granulites have Js and SIRM, which are greater than the associated metasandstones by a factor of 22±50. Three ma®c granulites (FP3, FP19 and

H12) from the Henshan and Fuping terrains exhibit an unusually high magnetism with Js

Table 2 Average values of magnetic parameters for rocks from the Wutai-Jining section

All samples Metasedimentary Intermediate & felsci Ma®c

a a Js SIRM R1 Js SIRM R1 Js SIRM R1 Js SIRM R1

Wutai 58.7 4.12 0.0701 44.0 4.12 0.0937 94.7 4.5 0.0475 Fuping/Hengshan 686 77.9 0.0827 257 25.0 0.0875 944 110 0.0831 Jining 1068 138 0.0891 101 6.8 0.0589 596 66.7 0.1037 2220 301 0.1015

a Js and SIRM are reported in A/m. 中国科技论文在线______www.paper.edu.cn 10 Liu Qingsheng et al. / Journal of Geodynamics 29 (2000) 1±13

Fig. 4. Correlation between rock magnetism, lithology and metamorphic grade. being 441, 507 and 6830 A/m and SIRM 44.7, 77.5 and 848 A/m. The Wutai metabasalts show magnetism which is even lower than that of felsic rocks from the Henshan/Fuping and Jining terrains. The results clearly indicate the dominant eect of metamorphic grade over lithology for deep crustal rocks and thus infer increasing magnetism with depth.

4.3. Magnetism of granulites and magnetization in the continental lower crust

The relation between magnetization in the continental lower crust and sources of longwavelength magnetic anomalies from aeromagnetic and Magsat data is a matter of great controversy, which is important for understanding crust±mantle interactions. Longwavelength magnetic anomalies from Magsat data given by Ravat et al. (1995) show a local DZ anomaly over the central North China craton. A susceptibility anomaly is present in the same area (Arkani-Hamed and Dyment, 1996). There is a prominent increase of 600 nT in magnetic anomaly from the high-amphibolite facies dominated Henshan/Fuping terrain to the granulite facies Jining terrain (Ma et al., 1991). Therefore, ma®c granulites from the Jining terrain are likely to produce the observed regional magnetic anomalies over the central North China craton. The temperature at the base of the crust in the central North China craton is estimated to be 5308C from surface heat ¯ow (Liu et al., 1996b). This infers that the magnetic bottom of lithosphere lies at the lowermost crust. Pyroxenite xenoliths carried by the Hannuoba alkaline 中国科技论文在线______www.paper.edu.cn Liu Qingsheng et al. / Journal of Geodynamics 29 (2000) 1±13 11 basalt, which occurs 30 km east of the Jining terrain, have P-wave velocities that are 0.5±0.9 km s1 higher than the lowermost crust in the central North China craton (Gao et al., 1999). They are interpreted to have been derived from the uppermost mantle. Magnetism of the pyroxenite xenoliths is signi®cant with SIRM =15±30 A/m. Thus, another possibility is that the magnetic bottom of lithosphere lies at the uppermost mantle. Magnetic measurements of >400 lower crustal xenolith samples by Wasilewski and Mayhew (1992) show that the induced magnetization is commonly several A/m, and can readily account for longwavelength magnetic anomalies measured by satellite and aircraft. Consequently, there is no need to search for exotic sources for the ``missing'' crustal magnetization.

Variation coecients (Vc) of rock magnetism can be used to evaluate the heterogeneity of magnetism in the continental crust. Js and SIRM have similar Vc in upper and middle crustal rocks with Vc < 85%. However, Vc values of Js and SIRM for lower crustal rocks reach up to 150% and 168%, respectively. This documents a signi®cantly greater heterogeneity in lower crustal magnetism.

5. Conclusions

1. Magnetic measurements of rock samples from the Wutai-Jining crustal cross-section show a layered structure in magnetism for the continental crust. This section represents an excellent continental crustal cross-section, whose apparent depth corresponds to the upper, middle and upper lower crust of the central North China craton. 2. Metamorphic grade is the most important factor in controlling rock magnetism. Lithology plays a less important role. This is particularly true for ma®c rocks. 3. Granulites and pyroxenite xenoliths show strong magnetism. This, together with low geotherm, suggests that the magnetic bottom of the lithosphere in the central North China craton lies at the lowermost crust or the uppermost mantle. 4. Variation coecients of magnetic parameters show remarkably greater heterogeneity in the lower crust compared to the shallower crustal layers.

Acknowledgements

This study is co-supported by a special grant from the Northwest University, the National Natural Science Foundation of China, the State Ministry of Education of China and the Open Laboratory of Constitution, Interaction and Dynamics of the Crust±Mantle System. We thank two anonymous reviewers for their helpful comments.

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