Geophysical properties of the lower crustal granulites from the Qinling Orogenic Belt, China

401

Tectonophysics, 204 (1992) 401-408

Elsevier Science Publishers B.V., Amsterdam

geophysical properties of the lower crustal granulites from the Qinling erogenic belt, China Liu Qingsheng a and Gao Shan b ’

Department of Applied Geophysics, China &rive&y of Geosciences, W&m 430074, China b Department of Geochemistry, China Vniversi@ of Geosciences, Wuhun 430074, China

(Received January 22, 1990; revised version accepted August 26, 1991)

ABSTRACT Liu Qingsheng and Gao Shan, 1992. Geophysical properties of the lower crustai granulites from the Qinling Qrogenic Belt, China. Tectonophysics, 204: 401-408. The lower continental crust plays a crucial role in continental evolution and mountain building. It is widely accepted that the lower crust is do~nated by granuhtes, mafic or felsic. In the present paper our studies are presented on the geophysical properties of granulites from two adjacent tectonic units, the Late Archean Taihau Complex along the southern margin of the North China craton in Lushan, and the Early Proterozoid Dahe Formation in the Qinling orogen in Tongbai, eastern Henan Province, China. The results indicate that the granulites from the two units were formed within the P-T regime of the lower crust, but in distinct environments.

It is well known that the lower crust remains a poorly understood region of the Earth. The lower crust is an intermediate layer between the upper crust and the upper mantle. It is a key link through which the crust and mantle differentiate and exchange materials, and it records a wealth of information on the history of crustal evolution. Therefore, synthetic geological, geophysical and geochemical studies of the nature and process of the lower crust are of great importance for revealing the evolution of the lithosphere. The continental lower crust can be studied geophysically through seismic, magnetotelluric, geomagnetic and gravity methods. Previous results have indicated that the geophysical (e.g. wave velocity, density, electric and magnetic) boundaries may be related to geological (e.g.

Corresponderrce to: Liu Qingsheng, China University of Geosciences, Department of Applied Geophysics, 430074 Wuhan, China. 0040-1951/92/$05.00

lithologic, geochemical, metamorphic and structural) boundaries within the crust. However, geophysical inversion results are frequently inconclusive. Thus, reliable synthetic interpration of geophysical data can provide important information on the nature of the deep crust. To do this, it is important to measure in situ the physical properties of deep-crustal rocks, and reconcile them with the geophysical results in the interpretation. There are two purposes for the in situ measurements. First, the results can constrain the multiplicity of geophysical inversion. For example, previous studies indicate that magnetization of granulite-facies rocks can reach l-5 A/m and is required for the magnetization of sources of long-wavelength magnetic anomalies (Wasilewski and Mayhew, 1982; Mayhew and Labrecque, 1987; Shive and Fountain, 1988; Wasilewski and Warner, 1988). The compression wave velocity V, and density of granulites also are distinctly higher than these of upper-middle-crustal rocks. These results can be used to construct prel~in~ models of geophysical inversion. Second, physical properties and geological features of rocks can be

8 1992 - Elsevier Science Publishers B.V. All rights reserved

402

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related. For instance, Schlinger (1985) indicates that the susceptibility K and the NRM (natural remanent magnetization) increase systematically with metamorphic grades in Lofoten and VesterHlen, Norway. In this paper are presented our preliminary results of measurements of P-wave velocities, densities and magnetic properties of granulites from the southern margin of the North China craton and its adjacent Qinling erogenic belt, and discussion of their tectonic settings. Geological background The Qinling Mountains are an E-W-trending mountain range which separates, both geographically and geologically, North and South China and extends westwards to the Qillan, Kunlun and Pamir mountains, and eastwards to the Dabie

Fig. 1. Location of the Qinling region in China. NC = North China craton; SC = South China Caledonian Fold Belt; YC = Yangtze (South China) craton; KL., QLS and QL denote the Kulun, the Qilian and the Qinling Mountains, respectively.

Mountains (Fig. 1). Therefore, it is one of the most important tectonic units in China. The Taihau Complex is a Late Archean high-grade ter-

Fig. 2. Geological sketch map of East Qinling and adjacent regions. 1 = Quaternary; 2 = sedimentary cover of North China craton; 3 = sedimentary cover of South Qinling Belt; 4 = sedimentary cover of Yangtze craton; 5 = Proterozoic; 6 = Archaean; 7 = ophiolites; 8 = Late Mesozoic granites; 9 = Late Palaeozoic-Early Mesozoic granites; IO = Early Palaeozoic granites; 11 = Middle-Late Proterozoic granites; 12 = basis-ultrabasic rocks; asterisks denote sampling sites.

403

GEOPHYSICAL PROPERTIES OF LOWER CRUSTAL GRANULITES

rain at the southern margin of the North China Craton. Its granulites are scattered in Lushan (Fig. 2), where the complex comprises tonalitic gneisses interlayered with amphibolites in the lower part, and graphite gneisses, garnet-sillimanite gneisses, diopside marble and BIF of khondalite nature in the upper part. Garnet-two pyroxene mafic granulites, which are the only type, occur in layers in the upper part, with the mineral assemblage hypersthene + almandine + hornblende + andesine + quartz. The Dahe Formation, about 160 km south of Lushan, is well exposed in Tongbai, in the Qinling Orogenic Belt, where it is composed of various khondalitic aluminous schists and gneisses, quartzites, marbles, amphibolites, and lenses and layers of granulites. The granulites are dominated by felsic types, with the common mineral assemblage quartz + biotite + hypersthene + hornblende + almandine. In addition, granulites in the two regions change bilaterally into high-amphibolite to amphibolite facies rocks. The granulites of the Taihau Complex equilibrated at temperatures of 825-876°C and pressures of 9-11 kbar, and the Dahe granulites at 637-714°C and 5.5-7.6 kb are on the basis of geothermobarometric studies (Liu and Gao, 1990). Sample processing and analytical methods Samples used for the thermal demagnetization of NRM are 2 cm cubes or cylinderss of 2 cm diameter and height. On heating, the temperature is raised from room temperature to lOO”C, and then to 700°C at intervals of 50°C at which point the natural remanent magnetization (NRM) drops below 0.01% of the values at room temperature. The thermal demagnetization of the NRM of samples was completed using a TDS-1 Thermal Demagnetizer, and DSM-2 and SSM-2A Spinner Magnetometers (Schonstede Company, U.S.A.). The magnetic hysteresis loops were performed on powdered samples. After crushing in a hard-steel jaw crusher, samples were ground in an agate mill in order to pass through a 180 mesh. Samples used for magnetic hysteresis loops weighed 100-200 mg, and the magnetic field increased continuously up to kO.5 T.

The compressionai wave velocities V, and densities D under ambient conditions and thermal demagnetization curves were measured, respectively, by the Seismic Laboratory, the Gravity Laboratory, and the Palaeomagnetism Laboratory of the Department of Applied Geophysics, China University of Geosciences. The magnetic hysteresis loops were measured using an LDJ-9500 Vibration Magnetometer (LDJ Company, U.S.A.) in the Magnetic Materials Laboratory of the Institute of Wuhan, the Ministry of Aeronautics and Space, China. Results and discussion P-wave velocity and density The bulk rock contrasts in velocity and density are important geophysical boundaries of the crust. Geophysical parameters of granulites in the Taihau Group and the Dahe Formation are shown in Table 1. P-wave velocities determined under ambient conditions range from 6.30 to 8.00 km/s. Densities range from 2.68 to 2.95 g/cm3. Results from deep-seismic sounding for various lowercrustal sections and measurements on the vp and density of exposed lower continental crust rocks and lower-crustal xenoliths around the world have shown that the lower crust generally has V, = 6.00-7.30 km/s and D = 2.65-3.30 g/cm3 (Carmichael, 1982; Gao, 1988). Therefore the VP and D values for the Qinling granulites fall within these general ranges. The completed deep-seismic sounding and gravity experiments in the Qinling region indicate that the VP and D values of the lower crust are 6.49-7.7 km/s and 2.88-2.96 g/cm3, respectively (Chen Chao, pers. commun., 1989). These values agree with those for the Dahe Formation (low values of VP and D) and the Taihau Group (high values of V, and D), respectively. Magnetic properties We have measured the thermal demagnetization curves of NRM and magnetic hysteresis loops for typical samples in the two sites. The magnetic parameters are listed in Table 1. The NRM ther-

404

TABLE

1

Geophysical

parameters

Site

No.

of granulites

Lithology (km/s)

Notes: ration

(A/m/g)

(mT)

(mT)

5.23 32.00

16.96 27.78

61 76

(lOmh SI/g)

6807

hyp-cpx-bi-pl

6.62

2.89

hyp-cpx-amp-pi

7.30

2.83

114.9 212.0

6809

hyp-cpx-amp-ga-pl

2.95

212.1

19.66

17.19

58

838

6810

hyp-cpx-amp-ga-pl

8.00

2.90

658.8

125.20

27.41

62

4398

6819 6820

hyp-bi-pl-qz hyp-bi-pl-qz

6.53 1.33

2.76 2.77

122.0 176.8

14.57

21.25

62

628

0.12

25.57

21.88

59

842

0.14

6821

hyp-ga-bi-pl-qz

6.67

2.83

19.09

hyp-ga-bi-pl-qz

7.00

2.77

162.0 167.5

20.00

6822

20.69

21.27

53

628

0.12

6823

hyp-ga-bi-pi-qz

2.68

141.6

15.11

18.38

58

842

0.11

6824

hyp-ga-bi-pl-qz

2.76

151.5

17.08

17.79

49

943

0.1

Geophysical

saturation

(A/m/g)

6808

Lushan

Tongbei

(g/cm3)

parameters

isothermal

remanence;

6.30 (SI units):

VP = compressional

H, = intrinsic

coercivity;

wave velocity;

H,, = remanent

D = density;

coercivity;

314

0.04 0.15 0.09 0.1 Y

0.12

J, = saturation

K, = initial

magnetization;

susceptibility;

I J, =

SR = rectangle

(e.g. Jr/J,).

Abbreviations:

hyp, hypersthere:

epx, clinopyroxene;

bi, biotite;

pl, plagioclase;

ma1 demagnetization curves have been explored as a possible method for magnetic phase identification. As NRM thermal demagnetization is independent of a paramagnetic component, it principally reflects the distribution of the ferromag-

amp, amphiboilite;

qz, quart.

netic or ferrimagnetic carriers within samples. The thermal demagnetization curves for eight samples are shown in Fig. 3. It can be seen from the figure that all samples exhibit the Curie point CT,) of magnetite (%O”Cl, and four samples may J(A/m.)

J(A/m)

ga, garnet;

‘:

1

A/d

-P

0. IW

6610 0.0111

J(Alm)

.I (A/m) J(A/m)

Fig. 3. Thermal

demagnetization

curves

of NRNI

for granulites:

6819-6824

samples

6807-6810

from the Dahe Formation.

from

the Taihau

Group;

and

samples

GEOPHYSICAL

PROPERTIES

OF

LOWER

CRUSTAL

GRANULITES

contain magnetic phases with a Curie point below that of magnetite (7” = 320°C e.g. pyrrhotite 7”). Most samples have stable remanent magnetization, and their NRMs are almost constant be-

tween room temperature and about 540°C above which they drop sharply to zero. The convex curve shapes for most samples suggest that the magnetite is of fine grain size. These results agree

J f A/m)

J (A/m)

I

.-IO0 .-

so 6807

/

0.X

0. 6

-0.6

: 0.8

.w

H(T) I,. w

0. 6

Il. 1

0. 2

0. 2

I,. 1

0. 6

: 0. n

H(T)

J (A/m)

J (A/m)

J (A/m)

I .-200

H(T) 0. 8

0.6

0.2

0.4

0.6

: 0.8

H(T)

-200

t J (A/m)

l

J (A/m)

200

H(T) - 0. 8

- 0.6

-0.4

-0.:

Fig. 4. Magnetic hysteresis loops of granulites: samples 6807-6810 from the Taihau Group; and samples 6819-6824 from the Dahe Formation.

406

H( 7’)

H( Tl

Fig. 4 (continued).

with results for deep-crustal rocks from the Ivrea Zone, Italy by Wasilewski and Warner (1988). Embodied in the shape of the magnetic hysteresis loop and derived magnetic parameters, e.g. saturation magnetization J,, saturation remanence J,, intrinsic coercivity H,, remanent coercivity H,, paramagnetic susceptibility K, and rectangle ratio SR
the world (Haggerty and Toft, 1985; Wasilewski, 1987; Liu and Lu, 1991). Granulite-forming environment

For the Taihau granulites we obtained temperatures of 825°C and 876°C on the basis of the orthopyroxene-clinopyroxene geothermometers of Wood and Banno (1973), as well as that of Wells (1977), respectively. From the gamet-orthopyroxene-clinopyroxene-plagioclase-quartz geobrometer of Newton and Perkins (19821, we obtained pressures of 9.1 kbar (825°C) and 10.9 kbar (876°C); from the Fe-reaction geobarometer of Perkins and Chipera (1985) for the same mineral assemblage, the pressures were calculated to be 9.1 kbar (825°C) and 9.8 kbar (876°C). For Dahe felsic granulites we obtained temperatures of 637-714°C and pressures of 5.5-7.4 kbar using the biotite-garnet geobarometer of Indres and Martignole (1985) in conjunction with the above two geobarometers, and temperatures of 733741°C and pressures of 5.8-7.6 kbar using the biotite-garnet geobarometer of Hoinkes (1986) in conjunction with the same geobarometers. Therefore, we obtained average pressures of 5.5-7.6 kbar for the Dahe granulites and 9.1-10.9 kbar for the Taihau granulites using geothermobarometry. The pressures obtained for the Taihau granulites corresponds to 33-40 km (9-11 kbar) and for the Dahe granulites (5.5-7.1 kbar) 20-28 km in depth in general cases (0.275 kbar load pressure increase per kilometer in depth; Liu and Gao, 1990). Most regions in the world have a

GEOPHYSICAL

PROPERTIES

OF LOWER

CRUSTAL

407

GRANULITES

lower-crustal depth ranging from 20 to 35 km (Gao, 1988). The present lower-crustal depth of the Qinling region ranges from 20 to 34 km, while the Dahe granulites were formed in the upper part of the lower crust. This is supported by the measured V, values of the granulite samples from both regions, which fall in the VP range of the regional lower crust (the Taihau granulite maximum V, value can reach 8 km/s). In addition, the depth of the source of the average long wavelength magnetic anomaly in Tongbai is about 21.6 km, and corresponds to a high-conductivity layer in the lower crust (resistivity 2-4 ohm, Zhang Shengye, pers. commun., 1989). This result is consistent with the viewpoint that the source of long-wavelength magnetic anomalies corresponds with the magnetic layer within the lower crust (Wasilewski and Mayhew, 1982). The average V, and D values of the Taihau granulites are 0.54 km/s and 0.13 g/cm3 higher than those of Dahe Formation granulites, so that the Taihau granmites may have formed at greater depth. The magnetic parameters of the Dahe Formation are generally less than those of the Taihau Group. It can be shown that the magnetism increased from the low-pressure granulite facies of the Dahe Formation to the high-pressure granulite facies of the Taihau Group. This conclusion is consistent with Schlinger’s results for the Lofoten and Vesterilen regions of Norway (Schlinger, 1985). Therefore, the synthetic results for the compression velocity, density and magnetism for the granulites of the two sites indicate that they were formed under lower-crustal conditions, with the Taihau Group at the base of the lower crust and the Dahe Formation at the top of the lower crust. However, the complex remanence and multi-magnetic phases of the Dahe granulites (Fig. 3) probably reflect their multi-episode that ranges in time from the Early Proterozoic to the Early Triassic, with peaks at 990, 452 and 152 Ma, respectively (You Zhendong et al., 1990). Conclusions

1. Granulites from the Taihau Complex and Dahe Formation were formed under lower-crustal conditions. The former were located at the base

of the lower crust or near the Moho, and the latter were supracrustal rocks subjected to granulite facies matamorphism within the upper part of the lower crust. 2. The granulites have stable remanent magnetization and their major magnetic carriers are magnetite, and some also contain significant pyrrhotite. They may be soures of long-wavelength anomalies in the region under study. 3. Magnetic contrasts of the Taihau and the Dahe granulites show that the magnetism of the granulites increased with increasing metamorphic grades. The complex remanent magnetization and multi-magnetic phases of the Dahe granulite may reflect its complicated tectonic history. Acknowledgements

This study was supported by the National Science Foundation of China (NSFC 48900026). We are grateful to Professor K.H. Wedepohl, whose kind hospitality made it possible for us to complete part of the analytical work at the Geochemical Institute of the University of Giittingen, Germany; to Professor Zhang Guwei for his considerable assistent during fieldwork; and to Professor You Zhengdong for reviews of the manuscript. References Christensen, NJ., 1982. Seismic velocities. In: R.S. Carmichael (Editor), Handbook of Physical Properties of Rocks. Vol. II. CRC Press, Boca Raton, Fla., pp. 1-128. Gao Shan, 1988. The lower crust: its present state and prospects of research. Geol. Sci. Technol. Inform., 7: 15-22 (in Chinese). Haggerty, SE. and Taft, P.B., 1985. Native iron in the continental lower crust: petrological and geophysical implications. Science, 229: 647-649. Hoinkes, G., 1986. Effect of grossular content in garnet on the partitioning of Fe and Mg between garnet and biotite. Contrib. Mineral. Petrol., 92: 393-399. Indares, A. and Martignole, J., 1985. Biotite-garnet geothermometry in the granulite facies: the influence of Ti and Al in biotite. Am. Mineral., 70: 272-278. Kern, H. and Schenk, V., 1985. Elastic wave velocities in rocks from a lower crustal section in southern Calabria (Italy). Phys. Earth Planet. Inter., 40: 147-160. Liu Qingsheng and Gao Shan, 1990. Geochemistry of the lower crustal granulites from Qinling Orogenic Belt. Earth Sci.-J. China Univ. Geosci., 15(4): 441-449 (in Chinese).

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