Relict refractory mantle beneath the eastern North China block: significance for lithosphere evolution

Lithos 57 Ž2001. 43–66 www.elsevier.nlrlocaterlithos

Relict refractory mantle beneath the eastern North China block: significance for lithosphere evolution Jianping Zheng a,b, Suzanne Y. O’Reilly b,) , W.L. Griffin b,c , Fengxiang Lu a , Ming Zhang b, N.J. Pearson b a

b

Faculty of Earth Sciences, China UniÕersity of Geosciences, Wuhan, 430074, People’s Republic of China GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie UniÕersity, Sydney, NSW 2109, Australia c CSIRO Exploration and Mining, P.O. Box 136, North Ryde, NSW 1670, Australia Received 14 March 2000; accepted 2 November 2000

Abstract Xenolith-bearing Neogene basalts occur in Hebi county, at the southern end of the Taihangshan–Luliangshan paleo-rift zone in the North China block of the Sino–Korean craton. This locality lies on the North–South Gravity Lineament, which divides the craton into two geophysical zones. The spinel peridotite xenoliths hosted by the basalts can be divided into two groups based on the Mg a values of olivine. The whole-rock compositions of the low-Mg ŽFo - 91. xenoliths have high Al 2 O 3 q CaO Žaverage 3.06 wt.%. and Na 2 O Žaverage 0.19 wt.%., and low MgrSi; they are similar to xenoliths from many localities in eastern China and other Phanerozoic volcanic areas. The dominant high-Mg ŽFo G 92. group consists of harzburgites Ž66%. and depleted lherzolites Ž34%. with coarse-grained Žmainly. and porphyroclastic microstructures, and high-Cr spinels Žmean Cr a s 0.51.. The high-Mg xenoliths have low Al 2 O 3 q CaO Žaverage 1.36 wt.%. and high MgrSi, are in general strongly depleted in HREE, Ti, Zr and Y, and are compositionally similar to xenoliths in kimberlites from Archean cratons. The Archean lithospheric root beneath the eastern part of the Sino–Korean craton, which was sampled by Paleozoic kimberlites, was largely replaced by fertile Phanerozoic mantle during Mesozoic extension and subduction events. The high-Mg xenoliths are interpreted as relics of the Archean lithosphere, preserved locally at relatively shallow levels, and re-equilibrated to spinel facies in a regime of high heat flow caused by advective heat transport during extension. Their calculated mean room-temperature density Ž3.36 grcm3 . and Vp Ž8.39 kmrs. are consistent with this interpretation and with geophysical data for the Hebi area. Regional geophysical data suggest that similar material may be widespread in the uppermost mantle west of the North–South Gravity Lineament, and more locally in the eastern part of the former craton. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Archean mantle; Mantle xenoliths; Gravity lineament; Lithosphere evolution; Tanlu fault zone; Sino–Korean craton

)

Corresponding author. Fax: q61-2-9850-8943; Internet address: www.es.mq.edu.aurGEMOCr. E-mail addresses: [email protected] ŽJ. Zheng., [email protected] ŽS.Y. O’Reilly., [email protected] ŽW.L. Griffin., [email protected] ŽF. Lu., [email protected] ŽM. Zhang., [email protected] ŽN.J. Pearson.. 0024-4937r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 4 - 4 9 3 7 Ž 0 0 . 0 0 0 7 3 - 6

44

J. Zheng et al.r Lithos 57 (2001) 43–66

1. Introduction The preservation of ancient depleted cratonic roots appears to be common, due to their significant buoyancy and viscosity ŽGriffin et al., 1998a, 1999a; Kelemen et al., 1998., but lithosphere–asthenosphere interaction can modify these roots and lead to their replacement by more fertile material Že.g. Griffin et al., 1992, 1998a,b; Menzies et al., 1993; Lee et al., 1996; O’Reilly et al., 1998; Pearson, 1999.. Mantle xenoliths and xenocrysts in Paleozoic diamondiferous kimberlites ŽFig. 1. in Mengyin county ŽShandong province. and Fuxian county ŽLiaoning province. document the presence of a thick lithospheric root at least as late as middle Ordovician time beneath the North China block of the Sino– Korean craton Že.g. Griffin et al., 1998b.. Chemical data on minerals Žincluding olivine, pyroxenes, garnet and chromite. from diamond inclusions, mantle xenoliths and xenocrysts indicate that these kimberlites intruded through a typical Archean mantle section with cool geotherms Že.g. Eggler et al., 1988; Lu et al., 1991; Griffin et al., 1992, 1998b; Zhou et al., 1994; Lu and Zheng, 1996.. However, since late Mesozoic time the region has been tectonically active with widespread volcanism of both calc-alkaline and intraplate type, lithosphere thinning and the development of large sedimentary basins and high heat

flow. Tertiary to Neogene basalts in the eastern part of the Sino Korean craton contain abundant xenoliths derived from a thin, hot and fertile lithosphere, indicating replacement of the Archean lithospheric root by younger material ŽGriffin et al., 1992, 1998b; Fan and Menzies, 1992; Zheng et al., 1998; Xu et al., 1998, 2000.. Liu Ž1987. summarized available geological and geophysical data and proposed a subduction-related mantle upwelling model to erode the older lithosphere. Tomographic analysis of the lithosphere structure in this region by Yuan Ž1996a,b. refined this interpretation, revealing upwelling new hot mantle with residual buoyant old mantle regions. Based on the Sr–Nd isotopic data from clinopyroxenes in the mantle xenoliths from the Paleozoic kimberlites and Tertiary basalts, Fan and Menzies Ž1992. speculated that a depleted lithospheric mantle with an AoceanicB affinity could underlie the Archean–Proterozoic crust in the North China block. From a comparison of the Cenozoic mantle between Shanwang, lying directly above the Tancheng–Lujiang ŽTanlu. fault zone and Qixia ŽFig. 1., lying east of the fault, Zheng et al. Ž1998. concluded that the Tanlu fault had played an important part in the Mesozoic–Cenozoic replacement of the pre-existing Archean lithospheric mantle. According to this model, shallow relics of the Archean mantle root

Fig. 1. Locality map and tectonic setting. NSGL, North–South Gravity Lineament; TLFZ, Tanlu fault zone; THLLPR, Taihang–Luliang paleo-rift zone; HP, Su–Lu high pressure metamorphism zone.

J. Zheng et al.r Lithos 57 (2001) 43–66

45

might be found beneath the areas away from the Tanlu fault zone in the eastern part of the North China block. Hebi county, about 400 km west of the Tanlu fault zone, is a place where young basalts might sample the Archean mantle in the eastern part of the North China block. In this paper, detailed petrological and mineralogical data for peridotitic xenoliths from Neogene basalts, including trace element analysis of constituent minerals using laser ablation ICPMS, are used to investigate the nature of the subcontinental lithospheric mantle beneath the eastern part of the North China block. Mantle peridotites from Hebi are compared with Phanerozoic mantle sampled by young basalts wShanwang and Qixia ŽZheng et al., 1998. and Nushan ŽXu et al., 1998.x and Archean mantle sampled by kimberlites ŽMengyin. shown in Fig. 1.

megacrysts Žup to 5 cm long. of garnet and pyroxene. The kimberlites, occurring 6 km west of Hebi city, form a NE–NEE swarm intruded into Cambrian sediments, and contain rare strongly altered dunite and spinel lherzolites, and abundant fresh diorite and syenite xenoliths ŽGriffin et al., 1998b.. The diorite and syenite xenoliths from the kimberlites are dated between 121 and 45 Ma ŽK–Ar age; Zhou et al., 1994.. The Tanlu fault zone is a major wrench fault system ŽXu et al., 1987. that cuts trough the eastern part of the North China block. Occurrences of Cenozoic basalts with abundant mantle xenoliths along the entire fault zone and its NE extensions Že.g. Peng et al., 1986; Fan and Hooper, 1989; Xu et al., 1993, 1996. demonstrate that the Tanlu fault zone extends deep into the lithospheric mantle.

2. Geological background

3. Petrography of Hebi peridotites

The North China block is divided in two by a major linear gradient in the gravity field, extending from Guizhou in southwest China northwards into Russia, and running approximately parallel to and about 400 km west of the Tanlu fault zone ŽFig. 1.. This North–South Gravity Lineament coincides approximately with the Taihang–Luliang paleo-rift zone separating the Archean Liaolu and Ordos cratonic nuclei ŽFig. 1.. To the east of the North–South Gravity Lineament, the Liaolu nucleus Ž; 3800 Ma; Liu et al., 1992. is characterized by a thin crust and lithosphere, high heat flow and weak negative to positive regional Bouguer anomalies. To the west of the North–South Gravity Lineament, the Ordos nucleus is characterized by a thick crust and lithosphere, low heat flow and strong negative Bouguer gravity anomalies ŽMa, 1987; Yuan, 1996a; Griffin et al., 1998b.. The Hebi area of Henan province lies east of the southern end of the Taihang–Luliang paleo-rift zone. Both Neogene basalts and Cretaceous–Eogene barren carbonatitic kimberlites occur here. NNW-oriented pipes andror dikes of olivine nephelinite are found 10 km south of Hebi city ŽSun et al., 1996.. These erupted 4.0–4.3 Ma ago ŽK–Ar age; Liu et al., 1990., and contain abundant mantle xenoliths and

Peridotite xenoliths in the Hebi Neogene olivine nephelinites are angular or rounded and belong to the Cr-diopside suite or Type 1 ŽWilshire and Shervais, 1975; Frey and Prinz, 1978.. They range from 1 to 6.5 cm and average about 3 cm across. Modes have been determined by point-counting more than 1000 points in each thin section ŽTable 1.. The nomenclature of the ultramafic xenoliths is based on the IUGS scheme ŽLe Maitre, 1982., except that AharzburgiteB is used only to describe clinopyroxene-free rocks. Two xenolith groups, high-Mg a and low-Mg a, can be distinguished and are defined below. Spinel harzburgite and clinopyroxene ŽCpx.poor lherzolites ŽCpx - 5 vol.%. are abundant Ž66% and 34%, respectively, of the high-Mg a group.. Amphibole-bearing lherzolite Že.g. Hb12., wehrlite Že.g. Hb29., spinel dunite and spinel pyroxenite Že.g. Hb92044. are rare. Both the high-Mg a and the low-Mg a xenoliths have high modal orthopyroxene Žaverage 27 vol.%.. This value is much higher than that from the North American Proterozoic craton Ž12 vol.%, Schmidberger and Francis, 1999. and typical Phanerozoic peridotites Ž12.5 vol.%, Boyd, 1989., but similar to those from the Archean Kaapvaal Ž30 vol.%, Boyd, 1989. and Siberia craton roots Ž20 vol.%, Boyd et al., 1997..

J. Zheng et al.r Lithos 57 (2001) 43–66

46 Table 1 Modes of Hebi peridotites Sample

Rock type

Microstructure

Mode Žvol.%. Ol

High-Mg Hb1 Hb2 Hb4 Hb5 Hb6a Hb6b Hb9 Hb10 Hb16 Hb17 Hb17-1 Hb18 Hb20 Hb22 Hb23 Hb25 Hb27 Hb28 Hb31 Hb34 Hb49 Hb92001 Hb92009 Hb92010 Hb92019 Hb92020 Hb92023 Hb92029 Hb92030 Hb92039 Hb92047 Hb92061

a

Opx

Cpx

Sp

Amp

group spinel dunite spinel harzburgite lherzolite spinel lherzolite spinel lherzolite spinel harzburgite spinel harzburgite spinel lherzolite spinel harzburgite spinel harzburgite harzburgite spinel harzburgite spinel harzburgite spinel lherzolite spinel harzburgite spinel lherzolite spinel harzburgite lherzolite spinel harzburgite harzburgite spinel harzburgite spinel harzburgite spinel harzburgite harzburgite spinel lherzolite spinel harzburgite spinel dunite lherzolite lherzolite spinel lherzolite spinel lherzolite spinel harzburgite

Low-Mg a group Hb3 spinel harzburgite Hb12 amphibole lherzolite Hb19 harzburgite Hb92006 spinel lherzolite Hb92045 spinel peridotite Hb29 wehrilite Hb92044 spinel pyroxenite

coarse coarse medium coarse coarse coarse coarse coarse medium-coarse coarse coarse coarse coarse coarse coarse coarse medium-coarse porphyroclastic medium coarse coarse porphyroclastic coarse porphyroclastic coarse coarse porphyroclastic coarse porphyroclastic coarse coarse coarse coarse

98.2 75.4 61.0 62.9 60.7 66.9 61.2 81.5 58.2 78.2 85.5 78.7 77.5 80.9 60.9 70.3 61.2 66.7 74.5 75.5 74.0 76.0 68.1 73.7 71.9 68.6 94.5 74.0 76.5 63.0 75.0 75.2

coarse coarse medium coarse porphyroclastic coarse porphyroclastic medium

65.1 71.2 67.1 64.6 83.0 88.0

Two major microstructural types, coarse-grained and porphyroclastic, are recognised in the Hebi xenoliths, based on the classification of Harte Ž1977.. The coarse-grained microstructures are especially common, and the grain size of olivine and orthopyroxene is generally from 2.5 to 6 mm, but may be up to 12

24.5 33.5 33.1 35.4 32.5 37.2 15.3 40.8 20.4 14.5 19.0 17.2 18.5 36.8 28.7 38.5 29.2 25.1 24.5 24.5 23.5 31.2 25.5 23.7 29.2 21.5 20.5 34.5 21.4 24.3

33.5 16.5 32.9 28.1

35.6

1.8 0.1 6.4 3.6 3.4

2.0

0.5 0.8

0.5 0.5 0.6 0.6 1.2 1.0 0.8 2.3 5.0 0.1 2.3 0.2 0.3

4.1 0.4

4.2

4.5 3.0 1.5 3.3

1.5 0.5 0.7 0.8 0.2 2.2 5.5

1.0 0.3 0.5

1.4 7.8 5.8 16.5 12.0 64.2

4.5 1.5 0.5 0.2

mm. In the porphyroclastic xenoliths at least 10% of the olivine occurs as coarse, commonly strained porphyroclasts, set in a matrix of small, mainly strain-free recrystallised grains. The relict porphyroclastic microstructure in Hb29 shows porphyroblasts of coarse olivine in a groundmass of fine-grained

Table 2 Representative electron microprobe analyses of minerals from Hebi peridotite xenoliths High-Mg a group Hb2 spinel harzburgite

Hb4 veinlet lherzolite

Mineral Ol Points 5

Sp 5

Opx 5

5

SiO 2 TiO 2 Al 2 O 3 Cr2 O 3 FeO MnO MgO CaO Na 2 O K 2O NiO Ý Mg a

0.0 56.6 41.5 0.11 0.04 0.01 19.98 2.89 0.02 45.37 0.76 0.02 17.52 4.84 7.63 0.00 0.09 0.12 14.46 34.27 50.7 0.01 0.89 0.06 0.02 0.08 0.01 0.00 0.00 0.00 0.16 0.06 0.36 97.6 100.6 100.5 0.60 0.93 0.92

40.8 0.01 0.02 0.03 7.64 0.10 51.2 0.02 0.00 0.01 0.37 100.1 0.92

Ol 5

Sp 5

0.1 52.3 0.21 0.81 28.63 18.08 38.59 0.00 15.02 0.37 0.00 0.02 15.79 0.39 0.00 7.89 0.01 0.65 0.01 3.34 0.19 0.00 98.5 83.8 0.65 0.65

Ol 14

Opx 4

40.1 55.2 0.01 0.05 0.02 2.53 0.05 0.76 7.87 4.87 0.12 0.10 50.6 34.26 0.07 0.92 0.02 0.18 0.01 0.00 0.37 0.08 99.2 99.0 0.92 0.93

Hb5 spinel lherzolite Cpx 7

Ol 12

Opx 4

52.7 40.2 55.0 0.13 0.01 0.00 3.77 0.02 2.88 1.66 0.04 0.71 2.48 7.35 4.59 0.08 0.12 0.11 16.84 51.2 34.22 18.10 0.07 0.88 1.75 0.01 0.05 0.01 0.01 0.02 0.04 0.35 0.08 97.5 99.4 98.5 0.92 0.93 0.93

Hb6a spinel lherzolite

Cpx 6

Sp 5

Ol 7

Opx 7

Cpx 11

52.3 0.03 3.16 1.09 2.18 0.07 17.47 20.33 0.71 0.01 0.06 97.4 0.93

0.0 40.9 56.3 0.34 0.01 0.02 23.03 0.04 2.92 41.66 0.06 0.80 14.37 7.57 4.72 0.00 0.10 0.07 15.64 51.1 34.02 0.00 0.07 0.99 0.05 0.03 0.17 0.00 0.01 0.00 0.19 0.38 0.11 95.3 100.3 100.1 0.66 0.92 0.93

Hb6b spinel harzburgite Ol Opx 6 5

Sp 5

54.1 41.1 58.4 0.02 0.00 0.01 3.45 0.02 1.12 1.29 0.02 0.26 2.35 7.07 4.39 0.09 0.11 0.13 17.13 51.3 35.71 18.88 0.03 0.41 1.62 0.01 0.11 0.01 0.01 0.00 0.09 0.40 0.09 99.0 100.1 100.6 0.93 0.93 0.94

Hb9 veinlet spinel glass harzburgite Ol 5 5

0.1 46.7 0.01 0.03 17.22 24.73 49.57 0.02 13.59 0.20 0.00 0.01 14.99 0.00 0.00 9.47 0.00 0.08 0.00 4.10 0.16 0.01 95.6 85.4 0.66 0.00

41.2 0.00 0.01 0.02 7.18 0.11 51.4 0.03 0.01 0.00 0.41 100.3 0.93

High-Mg a group Sample Rock Mineral Opx Points 5 SiO 2 TiO 2 Al 2 O 3 Cr2 O 3 FeO MnO MgO CaO Na 2 O K 2O NiO Ý Mg a

57.5 0.01 1.24 0.39 4.54 0.10 35.66 0.40 0.14 0.00 0.13 100.1 0.93

Sp 5

Hb10 veinlet spinel lherzolite Cpx Ol Opx Cpx 5 5 5 5

Sp 5

0.0 0.01 15.99 50.32 13.45 0.00 14.69 0.01 0.04 0.00 0.15 94.7 0.66

49.8 2.70 3.45 0.36 4.58 0.08 14.22 23.56 0.63 0.01 0.00 99.4 0.85

0.1 41.4 58.2 0.24 0.01 0.02 29.00 0.01 1.03 36.67 0.02 0.16 12.77 7.12 4.70 0.00 0.11 0.11 16.29 51.4 35.96 0.01 0.01 0.15 0.02 0.00 0.01 0.01 0.00 0.01 0.23 0.38 0.08 95.0 100.5 100.4 0.69 0.93 0.93

40.2 56.7 0.00 0.04 0.02 2.69 0.03 0.72 7.97 4.93 0.11 0.11 50.4 34.00 0.07 0.89 0.02 0.22 0.01 0.01 0.36 0.15 98.8 100.3 0.92 0.92

52.4 0.21 5.28 1.84 2.41 0.08 15.51 17.59 2.60 0.01 0.04 98.0 0.92

Hb16 spinel harzburgite Ol Opx Sp 6 6 6

Hb17-1 harzburgite Ol Opx 5 5

Hb17 spinel harzburgite Ol Opx Sp 5 5 6

0.0 41.2 56.4 40.4 56.1 0.03 0.00 0.02 0.01 0.10 28.43 0.01 2.90 0.02 2.14 37.00 0.01 0.78 0.03 0.70 16.00 7.44 4.72 7.76 4.89 0.00 0.13 0.11 0.12 0.12 15.06 50.9 34.27 50.6 34.16 0.00 0.07 1.02 0.07 0.91 0.01 0.01 0.07 0.01 0.13 0.00 0.01 0.00 0.00 0.01 0.10 0.42 0.11 0.37 0.10 96.6 100.2 100.4 99.4 99.4 0.63 0.92 0.93 0.92 0.93

Hb18 Sp harzburgite Ol Opx 5 4

0.1 41.0 56.1 0.46 0.01 0.02 21.43 0.02 2.85 42.36 0.05 0.75 15.16 7.55 4.95 0.00 0.10 0.08 15.15 51 34.52 0.00 0.09 0.91 0.01 0.00 0.06 0.01 0.01 0.01 0.22 0.40 0.11 94.9 100.5 100.4 0.64 0.92 0.93

Sp 5

Hb20 spinel harzburgite Ol Opx Sp 5 5 5

0.1 41.0 57.5 0.06 0.00 0.02 29.05 0.02 1.44 38.34 0.03 0.49 13.63 7.54 4.68 0.00 0.11 0.12 17.13 50.9 35.49 0.01 0.03 0.46 0.01 0.01 0.16 0.00 0.00 0.00 0.23 0.31 0.10 98.5 100.0 100.4 0.69 0.92 0.93

0.0 0.02 18.35 47.50 14.78 0.00 14.36 0.02 0.01 0.00 0.08 95.1 0.63

J. Zheng et al.r Lithos 57 (2001) 43–66

Sample Hb1 Rock spinel dunite

(continued on next page) 47

48

Table 2 Ž continued . High-Mg a group Hb22 spinel lherzolite Ol Opx Cpx 5 5 5

Sp 5

SiO 2 TiO 2 Al 2 O 3 Cr2 O 3 FeO MnO MgO CaO Na 2 O K 2O NiO Ý Mg a

40.9 57.0 0.02 0.01 0.01 2.36 0.04 0.79 7.79 4.84 0.12 0.11 50.9 34.35 0.09 1.07 0.00 0.04 0.01 0.01 0.32 0.10 99.9 100.6 0.92 0.93

0.1 41.2 57.2 0.07 0.01 0.00 23.70 0.01 1.92 42.10 0.22 0.40 15.05 7.72 5.04 0.00 0.11 0.10 15.39 51.1 35.58 0.01 0.04 0.51 0.01 0.00 0.01 0.01 0.01 0.01 0.19 0.39 0.06 96.4 100.8 100.8 0.65 0.92 0.93

53.3 0.02 2.45 1.19 2.22 0.06 17.87 21.61 0.49 0.01 0.07 99.2 0.93

Hb23 spinel harzburgite Ol Opx Sp 5 5 5

Hb25 spinel harzburgite Ol Opx Sp 11 11 4

0.0 41.1 56.6 0.02 0.02 0.08 27.14 0.02 2.66 37.25 0.04 0.67 16.99 7.06 4.45 0.00 0.11 0.11 15.64 51.1 34.22 0.01 0.06 0.87 0.01 0.01 0.20 0.01 0.01 0.01 0.21 0.41 0.12 97.3 100.0 100.0 0.62 0.93 0.93

Hb27 spinel harzburgite Ol1 Opx Sp 7 5 5

0.0 41.5 57.6 0.31 0.01 0.02 26.90 0.02 2.03 38.34 0.01 0.15 12.42 7.33 4.77 0.00 0.09 0.12 16.60 51.6 35.98 0.01 0.01 0.15 0.02 0.01 0.02 0.01 0.01 0.00 0.19 0.38 0.08 94.8 101.0 100.9 0.70 0.93 0.93

Hb28 lherzolite Ol Opx 10 5

0.0 41.3 56.7 0.03 0.01 0.02 37.21 0.00 1.82 29.29 0.02 0.29 12.99 7.30 4.91 0.00 0.11 0.10 18.30 51.7 36.32 0.01 0.03 0.21 0.00 0.02 0.16 0.01 0.00 0.05 0.20 0.34 0.14 98.1 100.9 100.7 0.72 0.93 0.93

Hb31 patch spinel harzburgite Cpx Ol Opx Sp 5 9 5 5 53.6 41.4 57.6 0.10 0.01 0.02 2.10 0.01 1.78 2.90 0.01 0.37 2.27 7.90 5.19 0.07 0.11 0.10 18.24 50.4 35.16 17.24 0.02 0.09 1.77 0.01 0.07 0.11 0.01 0.00 0.00 0.40 0.05 98.4 100.3 100.5 0.93 0.92 0.92

Hb34 veinlet harzburgite Ol 4 5

0.4 52.5 0.28 0.16 25.97 18.25 39.80 0.00 16.34 0.46 0.00 0.02 14.62 0.07 0.04 5.52 0.14 1.47 0.03 8.98 0.13 0.10 97.7 87.5 0.61 0.22

41.2 0.03 0.02 0.03 8.04 0.11 51.2 0.08 0.01 0.00 0.42 101.2 0.92

High-Mg a group Sample Rock

Hb49 veinlet spinel harzburgite

Mineral Opx Points 5

4

SiO 2 TiO 2 Al 2 O 3 Cr2 O 3 FeO MnO MgO CaO Na 2 O K 2O NiO Ý Mg a

56.5 48.1 0.02 0.00 2.52 22.35 0.78 0.00 5.00 0.00 0.06 0.00 34.68 0.00 1.03 6.51 0.07 0.81 0.01 8.73 0.09 0.00 100.8 86.6 0.93 0.00

Ol 8

Opx 5

41.0 57.0 0.02 0.01 0.02 2.08 0.02 0.23 7.46 4.66 0.09 0.11 51.6 35.83 0.02 0.13 0.00 0.10 0.00 0.00 0.33 0.09 100.5 100.3 0.92 0.93

Sp 5

Hb92001 spinel harzburgite

Hb92009 spinel harzburgite

Hb92010 harzburgite

Hb92019 veinlet spinel lherzolite

Ol 4

Ol 5

Ol 5

3

Ol 5

Cpx 5

55.4 0.74 18.02 0.02 0.59 0.03 0.00 3.98 1.44 10.66 0.01 90.9 0.00

41.6 0.01 0.01 0.01 7.92 0.10 50.3 0.05 0.00 0.00 0.37 100.3 0.92

53.6 57.5 0.08 0.01 2.45 1.79 2.31 0.62 2.42 4.95 0.06 0.13 16.42 34.66 21.29 0.69 1.12 0.16 0.10 0.00 0.04 0.10 99.9 100.6 0.92 0.93

Opx 5

0.0 41.7 56.5 0.04 0.00 0.02 39.03 0.02 3.12 28.74 0.04 0.83 11.54 7.59 4.65 0.00 0.09 0.12 18.67 51.3 34.45 0.01 0.07 1.00 0.01 0.01 0.05 0.00 0.01 0.00 0.25 0.42 0.09 98.3 101.3 100.8 0.74 0.92 0.93

Sp 5

Opx 5

0.1 40.9 57.3 0.02 0.01 0.01 32.02 0.01 1.78 36.50 0.03 0.45 11.96 7.58 4.84 0.00 0.10 0.12 17.75 51.4 35.27 0.00 0.03 0.36 0.00 0.00 0.13 0.00 0.01 0.01 0.22 0.40 0.10 98.6 100.5 100.3 0.73 0.92 0.93

Sp 5

Opx 5

0.0 41.5 56.6 0.02 0.00 0.02 20.75 0.02 2.78 46.63 0.05 0.80 15.32 7.94 4.91 0.00 0.13 0.09 14.85 50.5 34.17 0.03 0.09 1.02 0.00 0.00 0.14 0.00 0.01 0.00 0.16 0.34 0.13 97.8 100.6 100.7 0.63 0.92 0.93

Opx 5

Sp 5

Hb92020 veinlet spinel harzburgite Ol Opx 4 5 5

0.0 50.4 0.12 0.83 24.44 16.16 42.98 0.01 15.97 3.48 0.00 0.07 14.71 5.94 0.00 4.77 0.00 0.56 0.01 5.85 0.17 0.06 98.4 88.1 0.62 0.75

41.3 55.9 0.00 0.01 0.01 3.31 0.03 0.79 7.72 4.96 0.13 0.10 50.4 33.92 0.08 1.06 0.01 0.04 0.00 0.01 0.40 0.12 100.1 100.2 0.92 0.92

J. Zheng et al.r Lithos 57 (2001) 43–66

Sample Rock Mineral Points

Table 2 Ž continued . High-Mg a group Sample Rock Mineral Sp Points 5

0.1 41.2 0.02 0.01 32.43 0.02 35.16 0.06 12.61 8.04 0.00 0.08 17.26 50.2 0.00 0.06 0.01 0.02 0.00 0.00 0.19 0.41 97.8 100.2 0.71 0.92

Hb92029 lherzolite Ol Opx 6 4

Cpx 6

Hb92030 lherzolite Ol Opx 13 8

0.1 41.4 58.0 53.1 40.9 56.0 0.10 0.02 0.01 0.02 0.02 0.02 18.74 0.01 1.52 4.04 0.02 3.12 49.17 0.00 0.19 1.41 0.04 0.83 15.70 7.46 4.99 1.81 7.62 4.69 0.00 0.11 0.13 0.04 0.10 0.12 14.41 51.1 35.71 17.63 50.5 33.99 0.01 0.02 0.29 21.83 0.09 1.04 0.00 0.00 0.01 0.38 0.01 0.09 0.00 0.01 0.01 0.01 0.00 0.01 0.15 0.40 0.04 0.08 0.41 0.16 98.3 100.5 100.8 100.3 99.8 100.0 0.62 0.92 0.93 0.95 0.92 0.93

Cpx 9

Hb92039 spinel lherzolite Ol Opx Cpx 5 6 5

53.8 41.2 56.2 0.02 0.00 0.00 3.30 0.02 2.60 1.11 0.00 0.76 2.33 7.57 4.83 0.06 0.12 0.10 17.24 50.5 34.33 20.54 0.10 1.05 1.11 0.00 0.05 0.02 0.01 0.00 0.09 0.38 0.12 99.6 99.5 99.9 0.93 0.92 0.93

52.3 0.01 2.66 1.14 2.19 0.04 17.32 21.36 0.76 0.01 0.07 97.8 0.93

Sp 5

Hb92047 spinel lherzolite Ol Opx Cpx 6 6 6

0.1 40.7 55.3 0.02 0.02 0.06 25.18 0.02 4.37 40.40 0.04 0.66 13.38 7.13 4.54 0.00 0.12 0.11 16.04 51.0 33.70 0.01 0.06 0.91 0.02 0.01 0.12 0.01 0.01 0.00 0.27 0.43 0.15 95.1 99.6 100.0 0.68 0.93 0.93

52.4 0.15 6.28 1.09 2.42 0.11 15.90 19.82 1.62 0.01 0.07 99.9 0.92

Sp 6

Hb92061 spinel harzburgite Ol Opx Sp 8 8 8

0.1 41.4 56.2 0.08 0.02 0.01 47.64 0.02 3.10 18.13 0.04 0.80 10.38 7.44 4.73 0.00 0.10 0.11 20.29 50.7 34.03 0.01 0.09 1.02 0.00 0.00 0.07 0.01 0.01 0.01 0.33 0.42 0.08 96.9 100.3 100.1 0.78 0.92 0.93

0.1 0.04 31.83 35.08 12.24 0.00 17.55 0.02 0.01 0.01 0.24 97.1 0.72

Low-Mg a group Sample Hb3 Rock spinel harzburgite

Hb12 amphibole lherzolite

Mineral Ol Points 12

Ol 5

SiO 2 TiO 2 Al 2 O 3 Cr2 O 3 FeO MnO MgO CaO Na 2 O K 2O NiO Ý Mg a

Opx 8

40.5 56.2 0.02 0.06 0.02 2.48 0.03 0.68 11.02 6.19 0.16 0.15 47.9 33.44 0.09 0.75 0.02 0.14 0.01 0.01 0.37 0.12 100.1 100.2 0.89 0.91

Sp 7

0.1 40.8 0.76 0.02 25.00 0.03 36.33 0.02 18.58 9.94 0.00 0.16 14.22 49.5 0.02 0.06 0.02 0.01 0.01 0.01 0.25 0.40 95.2 100.9 0.58 0.90

Cpx 5

Opx 5

51.5 54.7 0.47 0.09 4.89 3.83 1.24 0.43 2.70 6.29 0.05 0.13 15.34 33.27 22.53 0.87 0.67 0.18 0.24 0.00 0.05 0.11 99.7 99.9 0.91 0.90

Hb19 harzburgite Amp Ol 5 8

Opx 5

Hb92006 spinel lherzolite Ol 5

Opx 5

43.3 40.5 55.6 41.1 57.2 1.70 0.00 0.05 0.01 0.02 13.90 0.02 3.40 0.01 1.85 1.38 0.01 0.69 0.02 0.26 4.13 9.46 5.23 8.67 5.70 0.04 0.14 0.14 0.18 0.22 17.31 49.4 33.60 49.6 34.81 10.00 0.07 0.92 0.01 0.26 3.25 0.03 0.16 0.00 0.01 1.64 0.01 0.00 0.01 0.00 0.09 0.31 0.10 0.39 0.11 96.7 99.9 99.9 100.0 100.5 0.88 0.90 0.92 0.91 0.92

Hb92045 spinel peridotite Ol Cpx 10 5

Cpx 7

Sp 5

52.9 0.031 3.93 0.67 2.20 0.04 18.25 21.39 0.23 0.01 0.10 99.8 0.94

0.0 41.1 0.00 0.00 42.77 0.01 23.36 0.05 14.24 8.29 0.00 0.15 16.80 49.6 0.02 0.05 0.00 0.01 0.00 0.01 0.20 0.37 97.4 99.6 0.68 0.91

Hb29 wehrilite Ol 13

52.2 40.6 0.50 0.01 4.44 0.01 2.13 0.05 2.49 11.22 0.04 0.24 15.85 47.7 19.84 0.04 1.58 0.01 0.01 0.01 0.07 0.39 99.1 100.3 0.92 0.88

Cpx 10

Hb92044 spinel pyroxenite Ol Opx core, 5 5

Cpx 5

54.7 41.1 58.0 54.4 0.24 0.01 0.01 0.11 0.44 0.00 1.46 3.03 3.46 0.09 0.46 2.11 2.92 9.36 5.05 2.35 0.01 0.22 0.13 0.05 16.70 49.16 34.90 15.85 19.15 0.14 0.72 20.47 1.86 0.02 0.19 1.89 0.02 0.00 0.00 0.01 0.04 0.40 0.11 0.03 99.5 100.5 101.0 100.3 0.91 0.90 0.92 0.92

Sp 5

J. Zheng et al.r Lithos 57 (2001) 43–66

SiO 2 TiO 2 Al 2 O 3 Cr2 O 3 FeO MnO MgO CaO Na 2 O K 2O NiO Ý Mg a

Hb92023 spinel dunite Ol Sp 5 5

0.0 0.08 19.31 48.97 13.44 0.00 15.81 0.02 0.01 0.00 0.08 97.8 0.68

49

50

J. Zheng et al.r Lithos 57 (2001) 43–66

secondary clinopyroxene" spinel, "olivine due to fluid reaction. Sheared microstructures have not been observed. Partial melting of pyroxenes is reflected in sieved textures and dark reaction rims containing abundant fine-grained olivine Žusually - 0.1 mm.. The olivine and pyroxene in the peridotite xenoliths contain abundant melt and fluid inclusions, generally distributed along healed fractures. Siliceous, aluminium- and alkali-rich glasses with clinopyroxene microphenocrysts are commonly found in patches and small veins. Inclusions of monosulfide solid solution and pentlandite occur in olivine and orthopyroxene. These sulfide inclusions co-exist with silicate ones, and thus may have been derived from immiscible sulfide melts trapped in the residual mantle during partial melting or melt–rock reaction.

4. Analytical methods Analyses of major elements of minerals were carried out at Macquarie University using a Cameca SX 50 electron microprobe ŽEMP., fitted with five crystal spectrometers, at an accelerating voltage of 15 kV and a sample current of 20 nA. The width of the electron beam was 5 mm. Standards were natural minerals and matrix corrections were done by the PAP method ŽPouchou and Pichoir, 1984.. Counting times were 10 s for peaks and 5 s for background on either side of the peak. Major element compositions of rock-forming minerals reported in Table 2 generally represent averages of five point analyses of each grain and several grains from different parts of each sample. In order to examine whether equilibrium had been attained, considerable attention was paid to determining the homogeneity of individual phases between core and rim. The minerals in all of the Hebi peridotites are homogeneous, except for olivine in the porphyroclastic sample Hb29. Trace element analyses of clinopyroxenes were carried out at Macquarie University using a UV laser ablation microprobe coupled to an ICPMS ŽLAMICPMS.. The laser ablation system and supporting software were designed by Drs. Simon Jackson and Henry Longerich ŽJackson et al., 1992; Jenner et al., 1994.. The laser is a Continuum Surelite I-20 Qswitched and frequency quadrupled Ž266 nm. Nd: YAG laser with a fundamental infrared ŽIR. wave-

length at 1064 nm and a pulse width of 5–7 ns. Most analyses were done with a beam energy of 0.5–3 mJ per pulse. The ICPMS is a Perkin-Elmer Sciex ELAN 5100, and the NIST 610 and 612 glasses were used as calibration standards. Typical operating conditions, precision, accuracy and detection limits for laser ablation analyses are described by Norman et al. Ž1996.. Trace element abundances reported in Tables 3 and 4 generally represent averages of at least five pyroxenes, amphibole and spinel point analyses of each sample and detection limits are indicated where relevant.

5. Mineral chemistry Minerals of all samples are homogeneous within analytical precision except for sample Hb29 that shows zoned olivine. 5.1. OliÕine Olivine compositions ŽTable 2. cluster in two groups: a high-Mg a group ŽG 0.92. is more common, while a low-Mg a group Ž- 0.91. is represented by six samples ŽHb3, Hb12, Hb19, Hb29,

Fig. 2. Mg a and MnO in olivine from Hebi spinel peridotites. Data for peridotites from the Mengyin Paleozoic kimberlites are from Zheng Ž1999. and for peridotites from Shanwang and Qixia from Zheng et al. Ž1998..

Table 3 Trace element concentrations in pyroxenes and amphibole Žppm. by LAM-ICPMS HB4

HB5 HB6a HB10

HB22 Hb92030 Hb92039 Hb92047 HB12

High-Mg a group Clinopyroxene 8.2 52 180 81 7460 19 334 1.2 7.9 0.9 b.d. 879 0.5 8 0.4 0.4 25.9 51.2 5.72 18.1 2.0 0.47 0.85 0.05 0.14 0.02 b.d. 0.02 0.12 0.03 0.14 0.04 0.70 0.84 0.18

13.7 55 120 145 8830 20 343 1.8 8.0 1.0 b.d. 281 2.9 9 0.7 0.7 9.2 24.8 3.32 13.6 2.5 0.77 1.66 0.18 0.82 0.11 0.24 0.03 0.30 0.04 0.09 0.09 0.20 0.43 0.13

21.3 83 1259 211 12,590 18 302 1.4 7.4 5.4 1.8 583 6.2 61 1.9 10.0 7.4 31.3 6.45 35.2 7.1 1.96 4.38 0.44 1.89 0.25 0.49 0.05 0.32 0.05 2.05 0.22 0.10 0.17 0.05

36.8 49 120 127 8140 21 370 b.d. 8.5 1.0 0.4 180 0.9 2 1.3 5.1 7.8 14.5 1.48 4.7 0.5 0.17 0.36 0.03 0.18 0.04 b.d. b.d. 0.16 b.d. 0.10 0.08 0.12 0.51 0.14

18.7 54 120 94 7600 20 353 2.0 8.4 1.3 0.3 171 1.0 5 0.8 8.2 5.8 15.3 1.90 7.3 1.0 0.28 0.59 0.05 0.25 0.05 0.17 b.d. 0.23 0.04 0.13 0.09 0.15 0.24 0.06

9.4 45 60 146 7800 23 417 1.6 9.5 1.0 b.d. 470 0.6 2 0.8 1.2 16.2 36.5 3.67 11.2 1.3 0.32 0.64 0.05 0.18 0.03 b.d. 0.02 0.11 b.d. 0.04 0.08 0.49 0.84 0.17

20.9 79 899 190 7460 20 352 7.9 8.5 2.5 1.4 383 6.9 52 0.9 5.4 3.0 19.5 4.59 27.1 7.1 2.14 4.72 0.46 2.05 0.29 0.57 0.07 0.53 0.09 1.95 0.12 0.19 0.11 0.04

Hb4

Hb5

Clinopyroxene

Amp

Orthopyroxene

17.1 59 540 246 8490 27 409 3.0 16.5 8.3 2.3 537 10.5 89 2.8 10.3 9.3 38.2 6.16 27.1 5.3 1.64 3.95 0.49 2.66 0.44 1.03 0.13 0.82 0.10 3.06 0.14 0.32 0.45 0.21

1.2 35 10,192 300 9440 38 753 1.4 18.9 29.3 3.9 924 8.9 92 32.0 234.1 9.6 37.1 5.85 25.7 4.9 1.47 3.62 0.43 2.24 0.37 0.85 0.11 0.66 0.08 3.66 2.24 0.55 0.40 0.16

n.d. 14 215 61 4180 85 1470 n.d. n.d. 3.34 n.d. 2 1.30 8.21 1.19 n.d. 0.16 0.27 - 0.09 - 0.61 - 0.59 0.13 - 0.71 n.d. - 0.44 0.09 - 0.37 n.d. 0.46 - 0.07 0.39 n.d. n.d. - 0.13 0.11

4.5 57 2998 150 14,580 16 277 b.d. 11.0 3.6 b.d. 258 9.3 152 0.8 0.6 3.4 15.6 3.09 17.5 5.0 1.60 4.19 0.52 2.67 0.41 0.82 0.10 0.54 0.06 3.66 0.13 0.50 0.06 0.02

Hb6a

Hb92029 Hb92030 Hb92047

n.d. 22 47 90 6130 65 960 n.d. n.d. 1.83 n.d. 1 0.51 1.53 - 0.18 n.d. - 0.20 0.13 - 0.18 -1.10 -1.21 - 0.34 - 0.81 n.d. - 0.46 - 0.11 - 0.60 n.d. - 0.81 - 0.16 - 0.36 n.d. n.d. - 0.18 - 0.19

n.d. 24 - 22.14 45 1600 53 595 n.d. n.d. - 2.41 n.d. - 0.84 - 0.79 -1.98 -1.25 n.d. - 0.76 - 0.82 - 0.68 - 2.80 -6.08 -1.05 - 4.35 n.d. - 4.10 - 0.50 -1.88 n.d. - 4.05 - 0.69 -1.76 n.d. n.d. - 0.71 -1.12

High-Mg a group

n.d. 21 108 45 4590 56 806 n.d. n.d. 1.59 n.d. 5 - 0.13 1.47 0.28 n.d. 0.25 0.34 - 0.11 - 0.53 - 0.90 - 0.23 0.59 n.d. - 0.53 - 0.08 - 0.27 n.d. - 0.36 - 0.06 - 0.35 n.d. n.d. - 0.17 - 0.16

n.d. 23 -8.41 61 5430 64 912 n.d. n.d. 1.85 n.d. - 0.29 - 0.20 0.82 - 0.19 n.d. - 0.24 - 0.24 - 0.20 - 0.97 -1.21 - 0.35 -1.05 n.d. - 0.75 - 0.15 - 0.54 n.d. - 0.98 - 0.12 - 0.34 n.d. n.d. - 0.28 - 0.20

n.d. 27 146 64 5090 54 817 n.d. n.d. 1.47 n.d. 1 0.53 2.14 - 0.70 n.d. - 0.65 - 0.37 - 0.36 - 2.76 - 2.56 - 0.87 - 2.68 n.d. -1.27 - 0.27 -1.41 n.d. -1.37 - 0.22 - 0.62 n.d. n.d. -1.96 - 0.47

J. Zheng et al.r Lithos 57 (2001) 43–66

Li 25.0 Sc 74 Ti 779 V 218 Cr 11,360 Co 21 Ni 345 Cu 1.7 Zn 9.8 Ga 2.4 Rb 0.1 Sr 349 Y 13.3 Zr 82 Nb 2.1 Ba 1.1 La 14.7 Ce 35.4 Pr 4.89 Nd 22.2 Sm 5.6 Eu 2.00 Gd 5.52 Tb 0.72 Dy 3.71 Ho 0.55 Er 1.12 Tm 0.11 Yb 0.61 Lu 0.07 Hf 2.91 Ta 0.29 Pb 0.16 Th 0.85 U 0.23

Hb92045 HB12

Low-Mg a group

Amp, amphibole; n.a., not analysed; b.d., below detection limit. 51

52

J. Zheng et al.r Lithos 57 (2001) 43–66

ŽG 0.93., and Cr2 O 3 , but low TiO 2 and Al 2 O 3 relative to those in the low-Mg a group ŽHb3, Hb12, Hb19 and Hb92006. and in xenoliths from Shanwang ŽZheng et al., 1998. and Nushan ŽXu et al., 2000. near the Tanlu fault zone ŽFig. 1.. Trace element abundances for orthopyroxenes from six Hebi peridotites in the high-Mg a group are given in Table 3. 5.3. Clinopyroxene Compositional data for clinopyroxenes from Hebi xenoliths are given in Tables 2 and 3.

Fig. 3. Na vs. Al Ža., and Cr Žb. in clinopyroxenes from Hebi spinel peridotites. Data for garnet peridotites from Mengyin Paleozoic kimberlites are from Zheng Ž1999.; data for spinel peridotites from Shanwang and Qixia area from Zheng et al. Ž1998.; for Nushan garnet ŽGt. peridotites from Xu et al. Ž2000..

Hb92006, and Hb92045.. NiO contents are relatively high, and MnO is low, in the high-Mg a group. Olivine in the high-Mg a group is similar to the olivine in xenoliths of Archean mantle sampled by the Paleozoic kimberlites of the Shandong area ŽZheng, 1999., while the low-Mg a group is similar to the olivines in spinel peridotite xenoliths from young basalts Že.g. Shanwang and Qixia; Fig. 2.. 5.2. Orthopyroxene The orthopyroxenes ŽEn 91 – 93 Fs 6 – 8Wo 0 – 2 . in the peridotites of the high-Mg a group have high Mg a

Fig. 4. Representative chondrite-normalised REE patterns of clinopyroxenes in Hebi peridotites.

J. Zheng et al.r Lithos 57 (2001) 43–66

5.3.1. Mg a in clinopyroxene Clinopyroxenes in the high-Mg a Hebi peridotite xenoliths have Mg a, Al and Cr a close to that of Cpx in Archean garnet peridotite xenoliths from the Mengyin kimberlites and higher than those from the Phanerozoic spinel peridotites in Cenozoic basalts from Shanwang and Qixia ŽZheng et al., 1998. and garnet peridotites from Nushan ŽXu et al., 2000. ŽFig. 3.. Cr a of the Hebi clinopyroxenes lies between these two groups except for sample Hb29 which has anomalously high Cr a. 5.3.2. Trace elements in clinopyroxene Most clinopyroxenes in the Hebi xenoliths have moderate to high concentrations of incompatible trace elements. They have similar Ga Ž0.9–8.3 ppm., low Ni Ž302–417 ppm. and Co Ž18–27 ppm., but high Sc Ž49–79 ppm., V Ž81–246 ppm., and Cr Ž7460–14580 ppm. compared to coexisting orthopyroxenes. Ni, Co and Sc contents of Hebi clinopyroxenes in the high-Mg a group are intermediate between the higher Ni and Co and lower Sc abundances of those in Archean peridotites and the lower Ni and Co and higher Sc abundances of those from younger mantle samples. Those analysed from the low-Mg a group have a wider spread. Clinopyroxene is the main repository for REE and many other incompatible trace elements in AdryB

53

peridotite. The chondrite-normalised REE patterns of the Hebi clinopyroxenes can be divided into two types ŽFig. 4.: one shows HREE depletion from Lu to Er with strong enrichment from Ho or Er to La; the other shows flat HREE Žand higher HREE abundances. with strong enrichment from Er to Nd and either flat LREE or depletion from Nd to La. All clinopyroxenes with the steep pattern belong to the high-Mg a group. Those with the flat HREE patterns and low LarNd n include the low-Mg a group ŽHb12 and Hb92045. and three samples from the high-Mg a group Že.g. Hb4, Hb10 and Hb92047., The clinopyroxenes in both the high- and lowMg a groups show similar relative abundances of the strongly incompatible trace elements ŽFig. 5., but the low-Mg a group has a narrower range, and higher relative abundances of moderately incompatible elements. Ba, Pb and high field strength elements ŽHFSE. such as Nb, Ta, Zr and Hf show strong negative anomalies relative to REE in all of the Hebi clinopyroxenes except Hb92045, which has a Zr content similar to primitive mantle. 5.4. Spinels The spinels of the Hebi peridotites have high Cr2 O 3 contents Ž23.36–50.32 wt.%. and high Cr a Ž0.20–0.69. with a broad negative correlation be-

Fig. 5. Fields of variation of trace element abundances Žnormalised to Primitive Mantle. in clinopyroxenes from the low- and high-Mg a groups of Hebi peridotites.

J. Zheng et al.r Lithos 57 (2001) 43–66

54

lower Ni, and higher abundances of Sc, Sr, Zr and Nd relative to those from Nushan ŽXu et al., 2000.. There are no obvious difference in Ni, Co or V contents between the high-Mg a group and low-Mg a group spinels. Co and Zn contents fall within the AMantle ArrayB ŽYao et al., 1998. which is controlled by temperature-dependent partitioning between chromite and mantle olivine. The lower Co and similar Zn in the Hebi spinels relative to the Mengyin spinels implies a higher equilibration temperature. Zr and Nb show a positive correlation and the Hebi spinels overlap the lower end of the range for Mengyin spinels.

5.5. Amphibole Fig. 6. Cr a vs. Mg a in spinels from Hebi peridotites. Other data sources as for Fig. 2.

tween Mg a and Cr a . Cr a lies between that for the Mengyin and ShanwangrQixia xenolith spinels ŽFig. 6.. Spinels from seven Hebi peridotites have been analysed for trace elements ŽTable 4.. They have

Amphibole in peridotites reflects modal metasomatism of the mantle. Three grains of amphibole coexisting with clinopyroxene were found in Hb12 Žin the low-Mg a group.. Major and trace element contents are given in Tables 2 and 3. The chondrite-normalised REE abundances of the amphibole are similar to those of coexisting clinopyroxene but the amphibole has strong positive Ba, Nb, Ta, and Sr anomalies, negative Th and U anomalies, and no Ti anomaly.

Table 4 Trace element concentrations in spinels from Hebi peridotites Žppm. High-Mg a group

Low-Mg a group

Sample

Hb5

Hb16

Hb25

Hb92001

Hb92047

Hb92061

Hb3

Sc Ti V Mn Co Ni Zn Ga Ge Sr Zr Nb Pb T ZnŽ8C.

2.47 434 482 n.a. 227 1444 n.a. 20 n.a. - 0.25 0.76 0.74 n.a.

3.77 103 621 1135 336 1004 1539 23 - 1.29 23.64 1.97 3.09 3.09 778

1.75 1583 639 1141 250 1554 901 81 - 2.18 - 0.14 0.75 0.48 0.98 849

2.45 156 543 1006 233 1617 737 27 - 2.53 - 0.19 0.53 0.74 - 0.57 920

1.57 478 485 829 214 2234 668 25 - 2.85 8.04 1.22 0.78 - 0.77 958

2.81 201 586 1088 256 1899 891 27 - 2.03 3.47 0.88 1.01 - 0.35 855

2.47 3488 635 1197 228 1642 1188 79 2.37 65.61 12.65 7.64 3.02 773

n.a., not analysed.

J. Zheng et al.r Lithos 57 (2001) 43–66

55

6. Whole-rock composition The whole-rock compositions of 38 Hebi peridotites have been reconstructed using their modal composition ŽTable 1. and mineral compositions ŽTable 2.. The compositional comparison between the high-Mg a group and the low-Mg a group is shown in Table 5. The peridotites of both groups display wide ranges in SiO 2 , MgO, Al 2 O 3 , and CaO. However, the high-Mg a group has lower contents of CaO and Na 2 O, and high values of MgrSi relative to those in the low-Mg a group. In terms of CaO q Al 2 O 3 vs. FeOrMgO ŽFig. 7., most of the high-Mg a peridotites plot in the field defined by low-temperature granular peridotites from the Kaapvaal craton ŽHerzberg, 1993., while most of the low-Mg a group xenoliths fall in the field occupied by high-temperature sheared garnet peridotites and by spinel lherzolites worldwide Žincluding those from eastern China, Griffin et al., 1998a, 1999a..

7. Equilibration temperatures Equilibration temperatures ŽT . for Hebi spinel peridotites have been estimated using thermobarometry protocols recommended in O’Reilly et al. Ž1997. and Xu et al. Ž1998. and given in Table 6. The

Fig. 7. CaOqAl 2 O 3 vs. FeOrMgO from Hebi spinel peridotites. Data for: Shangwang peridotites from Zheng et al. Ž1998.; Nushan peridotites from Xu et al. Ž2000.; Kaapvaal low-T xenoliths from Boyd and Mertzman Ž1987., Cox et al. Ž1987., Nixon Ž1987. and Boyd et al. Ž1993.. The line separating low-T granular peridotites and high-T sheared garnet peridotites from Kaapvaal craton and spinel lherzolites worldwide is from Herzberg Ž1993.; Primitive mantle ŽP.M.., McDonough and Sun Ž1995..

Sachtleben and Seck Ž1981. method generally gives slightly higher T estimates, while the Brey and

Table 5 Compositional comparison of peridotites of the high-Mg a group and low-Mg a group High-Mg a group Ž n s 32.

SiO 2 TiO 2 Al 2 O 3 Cr2 O 3 FeO MnO MgO CaO Na 2 O K 2O NiO Total Mg a CaO q Al 2 O 3 FeOrMgO MgrSi

Low-Mg a group Ž n s 6.

Range

Average

Range

Average

39.0–47.6 0.00–0.04 0.38–1.69 0.10–2.48 6.09–8.47 0.07–0.13 44.1–50.5 0.02–2.71 0.00–0.18 0.00–0.02 0.24–0.39 99.0–101.1 0.91–0.93 0.40–3.94 0.14–0.18 1.30–1.88

44.8 0.01 0.87 0.58 6.87 0.11 46.1 0.49 0.05 0.01 0.30 100.2 0.92 1.36 0.15 1.47

41.9–45.7 0.01–0.14 0.15–1.66 0.19–1.18 7.54–9.51 0.14–0.18 37.4–47.2 0.31–6.37 0.02–0.62 0.00–0.10 0.24–0.35 99.6–100.5 0.89–0.91 1.50–4.05 0.17–0.23 1.11–1.63

44.5 0.06 1.03 0.62 8.34 0.15 42.8 2.03 0.19 0.02 0.29 100.0 0.90 3.06 0.20 1.33

J. Zheng et al.r Lithos 57 (2001) 43–66

56 Table 6 Estimated temperatures Ž8C. for Hebi xenoliths

High-Mg Hb2 Hb4 Hb5 Hb6a Hb9 Hb10 Hb17 Hb18 Hb20 Hb22 Hb23 Hb25 Hb27 Hb28 HB31 Hb49 Hb92001 Hb92019 Hb92009 Hb92020 Hb92029 Hb92030 Hb92039 Hb92044 Hb92047 Hb92061

a

T ŽSS.

T ŽBK.

T ŽWEK, a.

T ŽWEK, b.

T ŽWells.

Sp–Opx

Ca-in-Opx

Cr–Al–Opx

Sp–Opx

Opx–Cpx

1057

1126

1044

1231

841 1033 1004 1053 891 1045 874 1013 780

820 1022 1050 1078 864 1070 899 1042 804

856 814 1092 953 887 1089

880 798 1069 906 962 1113

group

Low-Mg a group Hb3 Hb12 Hb92006

1153 1198 894 1045 1120 1079 930 1116 957 1073 821

1034 1025 1051 1100

1044

1225 911 812 1088 942 1021 1134

1123

962

794 1063 998

1015

1039

1146

1045

1079

1047 1091

1029

1082 1083

1040 1069

1155

984 886 776

1012

1089

816

771

776

1085 1070 1071

955

1031 1037 1004 914 1013

908 1078

T ŽSS., Sachtleben and Seck Ž1981., Sp–Opx thermometer; T ŽBK., Brey and Kohler Ž1990., Ca-in-Opx thermometer; T ŽWEK, a. and T ŽWEK, b., Witt-Eickschen and Seck Ž1991., Cr–Al–Opx thermometer and Sp–Opx thermometer; T ŽWells., Wells Ž1977., two-pyroxene thermometer.

Kohler Ž1990. Ca-in-Opx, Witt-Eickschen and Seck Ž1991. and Wells Ž1977. thermometers in general show good agreement for most samples. The estimated equilibrium temperatures of the high-Mg peridotites vary from 8008C to ; 12008C ŽSachtleben and Seck, 1981., indicating that the high-Mg group xenoliths occur over the whole depth range sampled ŽFig. 8.. The two T values for the low-Mg group xenoliths represent both shallow and deep levels Ž7808C and 11558C..

No garnet-bearing xenoliths have been found in Hebi, so we cannot construct a xenolith-based geotherm. However, the estimated Moho depth of 38 km ŽMa et al., 1991., combined with the lowest temperature estimates Ž- 7008C. for the Hebi spinel peridotites, suggests a geotherm significantly lower than the Cenozoic geotherm established for Nushan in the eastern part of the craton ŽXu et al., 1998.. This is consistent with the difference in modern heat flow between the Hebi area Ž32–77 mWrm2 , mean

J. Zheng et al.r Lithos 57 (2001) 43–66

Fig. 8. T Ž8C. Žfrom Sachtleben and Seck, 1981., plotted against Mg a in olivine from Hebi peridotites. Data for peridotites from Shanwang and Qixia from Zheng et al. Ž1998.. The spread of temperatures qualitatively reflects the relative depths of derivation of the xenoliths.

52 mWrm2 . and the Tanlu fault area Ž54–82 mWrm2 , mean 69 mWrm2 ; Ma et al., 1991..

8. Discussion 8.1. Archean or Phanerozoic lithosphere? The most significant feature of Archean mantle, which clearly distinguishes it from the mantle beneath younger terrains Že.g. Phanerozoic mantle., is the presence of very depleted harzburgites with strongly subcalcic garnets Žreflecting low CarAl. and high OpxrOl ratios, giving low MgrSi ŽBoyd, 1996; Griffin et al., 1998a.. These harzburgites are interlayered with depleted lherzolites that have similarly low-MgrSi ŽBoyd, 1996; Griffin et al., 1999a.. Most of the Archean lherzolites contain - 3% Cpx, and the compositional differences between harzburgites and lherzolites are small. In general, subcalcic Žcpx-free. harzburgites are restricted to Archean mantle, and the dominant lherzolites become pro-

57

gressively less depleted from Archean through Proterozoic to Phanerozoic time ŽGriffin et al., 1998a.. Most Archean subcontinental lithospheric mantle with low MgrSi is believed to be the product of high-degree melting at depths ) 150 km, with no Cr–Al phase present in the residue ŽGriffin et al., 1999a.. Kelemen et al. Ž1998. and Smith et al. Ž1999. explained silica enrichment in the continental upper mantle as the result of melt– and water–rock interactions, while Herzberg Ž1999. suggested that the opx-rich nature of many Archean peridotites is the result of magmatic sorting of olivine and high-T pyroxenes. There is a marked difference in the processes that produced the subcontinental lithospheric mantle in Archean and Phanerozoic time, which is illustrated by the differences in major-element composition between cratonic and circum-cratonic mantle ŽBoyd and Mertzman, 1987; Boyd, 1996.. These differences are also seen in a comparison between the high-Mg a group in the Hebi peridotites and the newly accreted lithosphere from the Tanlu fault area. Most Hebi peridotites, and especially the high-Mg a group, have much lower CaO and Al 2 O 3 contents than commonly accepted primitive mantle compositions Že.g. Hart and Zindler, 1986; McDonough and Sun, 1995., reflecting a high degree of depletion in basaltic components Že.g. Bernstein et al., 1998.. These depleted peridotites have very low Ž- 5%. modal proportions of clinopyroxene. Only the peridotites of the low-Mg a group, with higher CaO and Al 2 O 3 contents, are more fertile. In contrast to many peridotites from Shanwang ŽZheng et al., 1998. and Nushan ŽXu et al., 2000. within the Tanlu fault zone, the high-Mg a group in Hebi plots in or near the area of depleted cratonic peridotite xenoliths in terms of Ca and Al contents ŽFig. 9.. As shown in Fig. 10, most of the high-Mg a peridotites plot in the field of Archean lherzolitesr harzburgites, and some of high-Mg a group plot between the Archean and Proterozoic fields. The samples of the low-Mg a group, which are similar to the peridotites from Shanwang within the Tanlu fault zone and Qixia lying east of the fault ŽZheng et al., 1998., vary widely in Mg a and modal content of olivine, but none plots within the Archean field. Similarly, most Hebi peridotites Že.g. the high-Mg a group. plot in the Archon field of Griffin et al. Ž1999a., while only a few samples Že.g. the low-Mg a

58

J. Zheng et al.r Lithos 57 (2001) 43–66

Fig. 9. CaO vs. Al 2 O 3 for Hebi spinel peridotites. Other data sources as for Fig. 7.

group. similar to those from Shanwang ŽZheng et al., 1998. and Nushan ŽXu et al., 2000. zone, plot in the Proton and Tecton areas ŽFig. 11..

Fig. 10. Modal Ž%. of olivine vs. Mg a of olivine from Hebi spinel peridotites. Data for peridotites from Shanwang and Qixia from Zheng et al. Ž1998.; oceanic trend from Boyd Ž1989.; Archean, Proterozoic and Phanerozoic fields are from Griffin et al. Ž1999a..

Fig. 11. Plots of Mg a vs. MgrSi for Hebi peridotites. Data for peridotites from Kaapvaal craton from Cox et al. Ž1987.; other data sources as for Fig. 3. ŽArchons are defined as lithosphere domains that experienced their last major tectonothermal event more than 2.5 Ga ago, Protons between 2.5 and 1.0 Ga and Tectons less than 1.0 Ga, as modified from Janse’s, 1994 classification by Griffin et al., 1998a..

The lithosphere present beneath the eastern part of the North China block in Paleozoic time was thick ŽG 200 km; Lu et al., 1991; Lu and Zheng, 1996; Griffin et al., 1992, 1998b; Zhou et al., 1994. with a relatively cool geotherm. The dominant rock types were depleted garnet lherzolite and garnet harzburgite, variably affected by phlogopite-related metasomatism at relatively shallow depths and melt-related metasomatism closer to the base of the lithosphere ŽGriffin et al., 1998b.. The Hebi spinel harzburgitesrlherzolites with high Mg a have chemical characteristics similar to garnet peridotites from the Archean Kaapvaal craton and the Paleozoic Mengyin kimberlites, with low modal contents of Cpx and spinel, high opxrol, high Cr a, and strong enrichment of trace elements in clinopyroxene. We suggest that they represent relics of the Archean lithospheric mantle. In contrast, the Hebi peridotites with low-Mg a olivine and low-Cr a spinel are similar to the xenoliths from Shanwang ŽZheng et al., 1998. and Nushan ŽXu et al., 2000.; they are typical of Phanerozoic mantle, rather than an Archean or Proterozoic lithospheric mantle.

J. Zheng et al.r Lithos 57 (2001) 43–66

59

Fig. 12. Results of modelling of partial melting fractions using clinopyroxene compositions wnormalised to Primitive Mantle ŽMcDonough and Sun, 1995.x for Hebi peridotites: Data for peridotites from Shanwang and Qixia from Zheng et al. Ž1998.; fractional melting degrees from 1% to 25%, were calculated using the method of Norman Ž1998..

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J. Zheng et al.r Lithos 57 (2001) 43–66

8.2. Partial melting and mantle metasomatism in the Hebi peridotites The incompatible trace element patterns of the clinopyroxenes reflect mantle processes such as partial melting, metasomatism and sub-solidus re-equilibration. To help constrain the conditions of melt depletion, the concentrations of incompatible elements in mantle clinopyroxene were modelled for both batch and fractional melting of a primitive mantle using the approaches of Johnson et al. Ž1990. and Norman Ž1998. ŽFigs. 12 and 13.. The degrees of batch melting Ž) 40%. required to produce the HREE abundances of most Hebi samples are too high to be realistic, but they could be produced by - 25% fractional melting. The Y–Yb plot ŽFig. 12a. and Ho–Yb plot Žnot shown. show that all high-Mg a group except one ŽHb4. have HREE patterns consistent with about 10% to 25% fractional melting, whereas the samples in low-Mg a group experienced only 5–10% melting. This is consistent with the high Cr a in most of Hebi phases, and the high Mg a of the olivine in the high-Mg a group peridotites ŽFig. 14., which also indicate that a large volume of the uppermost mantle in Hebi underwent higher degrees of melt extraction than the mantle beneath Shanwang and Qixia. The Ti–Yb plot ŽFig. 12b. and Zr–Yb plot ŽFig. 12c. show that the Ti and Zr contents of five samples in high-Mg a group can be explained simply by the partial melting process, using a D cpxrmelt s 0.35 Ti cpxrmelt and a D Zr s 0.12 ŽHart and Dunn, 1993; Jenner et al., 1994; Norman, 1998; Xu et al., 2000.. The other samples Žthree high-Mg peridotites and two low-Mg peridotites. fall to the right of the melting curves ŽFig. 12b and c., implying later Ti and Zr addition. The plots of Gd, La, Sr and Nb against Yb ŽFig. 12d–f. also indicate metasomatic addition of these elements in most of the high-Mg peridotites after partial melting. The generally higher LREE contents and LarYb ratios in the Hebi clinopyroxenes are in contrast to LREE-depleted patterns clinopyroxenes common in those from Shanwang and Qixia ŽZheng et al., 1998. and in some peridotite types from Nushan ŽXu et al., 2000.. Therefore, we infer that the subcontinental lithospheric mantle beneath Hebi has undergone stronger modification by

Fig. 13. ZrrZr ) vs. TirTi ) Ža., ŽSmrDy. n Žb. and ŽCerNd. n Žc. for Hebi clinopyroxenes. Other data and methods as for Fig. 9.

mantle metasomatism than the mantle beneath Shanwang.

J. Zheng et al.r Lithos 57 (2001) 43–66

Fig. 14. Mg a in olivine vs. Cr a in spinel Ža., and Cr a in clinopyroxene; Žb. from Hebi peridotites. Other data sources as for Fig. 3.

ZrrZr ) and TirTi ) of clinopyroxene Žwhere Zr s ŽSm n q Nd n .r2 and Ti ) s ŽGd n q Eu n .r2; Rampone et al., 1991. range from 0.03–0.47 Žexcept for 1.07 in Hb92045, a sample of low-Mg a . and 0.02–0.28, respectively. Negative Ti anomalies in clinopyroxene are characteristic of Hebi peridotites. In contrast to the samples from Shanwang and Qixia ŽZheng et al., 1998., most Hebi clinopyroxenes, especially the high–Mg a peridotites, cannot be described by the melting curve in plots of ZrrZr ) vs. ŽSmrDy. n and ŽCerNd. n ŽFig. 13.. Plots of ZrrZr ) vs. TirTi ) indicate the Hebi clinopyroxenes experi)

61

enced high Žabout 5–10%. degrees of partial melting relative to those Ž- 5%. from Shanwang within the Tanlu fault zone. Trace-element studies of mantle clinopyroxenes indicate that cryptic metasomatism is a widespread phenomenon in volatile-free spinel peridotites in continental settings Že.g. Frey and Green, 1974; Yaxley et al., 1998; Zangana et al., 1999.. Secondary Cr-rich clinopyroxenes " spinels " olivine Že.g. Hb29. and amphibole Že.g. Hb12. in the low-Mg a peridotites reflect modal metasomatism. Such enrichment has been attributed to metasomatism by carbonatite melts Že.g. Meen, 1987; Yaxley et al., 1991, 1998., silicate melts with H 2 O and CO 2 Že.g. O’Reilly and Griffin, 1988; O’Reilly et al., 1991.; volatile-rich silicate melts Že.g. Zangana et al., 1999. or subduction-derived H 2 O–CO 2 liquids Že.g. Stalder et al., 1998.. Experimental studies of the peridotite– H 2 O system ŽGreen, 1973. constrained models of mantle metasomatism by hydrous silicate melts which may be very enriched in large-ion lithophile elements ŽLILE. and high-field-strength elements ŽHFSE. in the mantle environment. In contrast, experiments on amphibole-bearing peridotites with carbonatite Žcarbonate-rich. metasomatism show that such fluids may have high REE, LILE, Th, U and P, but low contents of HFSE Že.g. Green and Wallace, 1988.. Based on petrographic and geochemical data, the metasomatic fluid that affected the Hebi mantle was rich in LREE, Nb and Sr but relatively low in Ti and Zr, requiring a silicate-rich melt with carbonate and hydrous components. The metasomatic imprints characteristic of such a broad compositional spectrum represent different episodes and agents through time. In summary, petrographic and geochemical information indicate more complex and higher degrees of both mantle metasomatism and melt extraction beneath Hebi than in the inferred relatively young lithospheric mantle material ŽZheng et al., 1998; Xu et al., 2000. near the Tanlu fault zone. 8.3. Vp calculation and mantle structure An average density of 3.36 grcm3 at room temperature has been calculated for the refractory peridotites Ž66% harzburgiteq 34% lherzolite. from Hebi, using major element compositions of individ-

62

J. Zheng et al.r Lithos 57 (2001) 43–66

ual minerals, their average modal abundance and the experimentally determined densities of their endmember components ŽSmyth and McCormich, 1995.. The calculated density of Hebi peridotites is slightly higher than the calculated densities of strongly depleted Archean garnet lherzolite Ž3.30 grcm3 ., but lower than that for high-T sheared peridotite Ž3.39 grcm3 ; Boyd and McAllister, 1976.. It is also higher than the density of an average depleted lherzolite from South African and Siberia Ž3.20 grcm3, Griffin et al., 1996.. A Vp of 8.39 kmrs Žat room temperature. is calculated for the average composition of the refractory peridotites, using the modes and compositions of constituent minerals and the known Vp values for the relevant mineral compositions from the end-member components Žmineral Vp values were interpolated from the data of Anderson and Isaak, 1995.. A Vp of 8.05 kmrs at Moho temperatures of 700–8008C was derived using the dVprdT relationship of Anderson and Sammis Ž1970.. West of the North–South Gravity Lineament ŽFig. 1., the Vp immediately below Moho ranges from 8.1 to 8.2 kmrs ŽYuan, 1996a.. However, east of the North–South Gravity Lineament, where higher Moho temperatures would predict lower velocities, Vp measured by seismic refraction traverses range from 7.9–8.15 kmrs with an average of 8.0 kmrs ŽMa, 1987.. This is consistent with presence of material like the high-Mg a xenoliths immediately below the crust in this region. The lherzolitic xenoliths from the Tanlu fault area are less depleted, and although they have higher densities Ž3.29–3.36 mgrcm3 ., they give calculated Vp of 7.5–7.8 kmrs at 10 kb and 800–9008C ŽXu et al., 2000., the conditions expected at sub-Moho depths beneath these areas of the thinnest lithosphere. Cooling of these lherzolites to 400–5008C temperatures, corresponding to Moho temperatures under the cratonic areas west of the North–South Gravity Lineament, will increase their Vp to only ca. 7.9 kmrs ŽGriffin et al., 1999a.. This indicates that such fertile Phanerozoic material does not lie immediately below the Archean crust in the sampled areas of thicker lithosphere such as in Hebi. A seismic tomography profile cross the Sino– Korean craton along 368N shows the presence of isolated high-Vp regions dispersed within a background of generally low-Vp regions at depths of 100 km ŽMa et al., 1991; Sun, 1992; Yuan 1996a.. The

tomographic imaging ŽYuan, 1996b. shows a Amushroom-cloudB structure indicating that lowervelocity material in the upper mantle has welled up from depths greater than 150 km and spread out beneath the crust. 8.4. Multi-stage eÕolution of the lithospheric mantle beneath Hebi The Proterozoic Taihang–Luliang paleo-rift zones bisects the Sino–Korean craton ŽFig. 1., separating the Archean nuclei of Liaolu and Ordos. The lithospheric mantle beneath this area in Paleozoic time may have been Archean material reworked in Proterozoic time or primary Proterozoic-type mantle at depths. Garnet concentrates from the Cretaceous– Eogene carbonate-kimberlitic hosts in Hebi county indicate a fertile mantle, with a geotherm intermediate between the cratonic geotherm found in the Paleozoic kimberlites and the elevated geotherm inferred above for the Neogene volcanism ŽZhou et al., 1994; Lu and Zheng, 1996; Griffin et al., 1998b.. The appearance of Phanerozoic-type lithosphere beneath the Taihang–Luliang area during Cretaceous– Eocene time Ž121–45 Ma. suggests that the older lithosphere has been replaced by hotter, more fertile material at depths of ) 80 km. These processes coincide with the development of the large Mesozoic–Cenozoic basins in eastern China, and especially in the eastern part of the North China block. The basins have formed in two episodes during Mesozoic–Cenozoic time: the first in Jurassic–Cretaceous time and second in Eocene time ŽLi et al., 1997.. It can be suggested that at least three episodes of mantle erosion and replacement have occurred beneath the North China block. The first replacement of Archean mantle by Proterozoic material would have been related to the formation of the Proterozoic Taihang–Luliang paleo-rift zones, which bisects the North China block. The second stage, the replacement of Archean or Proterozoic mantle by Phanerozoic material, may have occurred during Jurassic–Cretaceous time, and the third stage of replacement by entirely Phanerozoic material may have occurred during Eogene time Ž; 45–57 Ma.. The last two episodes coincide with two peaks of heat flow calculated from coal reflectivity Ž R 0 . measurements in the Songliao Basin Ž99 mWrm2 at 115

J. Zheng et al.r Lithos 57 (2001) 43–66

Ma and 86.5 mWrm2 at 57 Ma; Li, 1995.. However, it should be noted that the Songliao Basin lies about 1000 km northeast of Hebi, so similarities in tectonism at this time assume the northeastern region of China was affected by similar and contemporaneous tectonic events. These episodes may have accompanied subduction of the Kula Plate in Jurassic–Cretaceous time and the Pacific Plate in the Tertiary Že.g. Ma et al., 1984; Liu, 1987; Tian et al., 1992; Griffin et al., 1998b.. The peridotite xenoliths from Cretaceous–Eogene kimberlites and Neogene basalts in Hebi represent sampling at different levels, from deeper to shallower, through time. Lithospheric thinning and mantle replacement may also have accompanied the collision between the North China block and the South China block at 220 Ma ŽLi et al., 1993. along the Qinling–Dabieshan orogenic belt during late Early Permian–Early Jurassic Že.g. Yin and Nie, 1993; Li, 1994.. This lithosphere thinning and replacement appears to be a consequence of mechanical rifting, providing an extensional regime for conduits of upwelling new material, and is consistent with the Amushroom-cloudB structure of the mantle in this region, revealed in detailed seismic tomography ŽYuan, 1996b..

9. Conclusions Ž1. The Hebi xenolith suite comprises two geochemically distinct populations: a low-Mg a group Žwith olivine of Fo less than 91. and a high-Mg a group Žwith olivine of Fo greater than 92.. Ž2. The high-Mg a peridotite xenoliths from the Hebi volcanics are similar in microstructure, their high whole-rock MgrSi and Mg a , modal OpxrOlivine, Mg a of olivine and pyroxenes, and Cr a of spinel and clinopyroxene, to mantle xenoliths from typical Archean regions such as the Kaapvaal craton. The incompatible elements and moderately compatible elements of the clinopyroxenes record subsequent mantle metasomatism. These high-Mg a xenoliths are interpreted as samples of relict Archean lithosphere preserved at shallow levels of the mantle. Ž3. Relics of Archean lithospheric mantle may be preserved in areas of relatively thick lithosphere such as Hebi, east of the North–South Gravity Lineament in northeastern China. Lithosphere thinning and re-

63

placement were heterogeneously distributed in space and time beneath the North China block. Relatively undepleted Phanerozoic mantle now makes up much of the subcontinental lithospheric mantle beneath the eastern part of the North China block and represents the bulk of most Tertiary xenolith suites sampled by young basalts. Relics of depleted Archean mantle are less common, and are mainly distributed away from the Tanlu fault, as at Hebi.

10. Uncited references Brey et al., 1990 Griffin et al., 1999b Zheng et al., 1996

Acknowledgements This work was supported by the ACILP AusAID Program ŽSYO’R and W.L.G.., ARC ŽS.Y.O’R., Macquarie University, the Chinese National Scientific Foundation ŽF.L.., the Chinese Geological Youth Foundation ŽJ.Z.., and the Open Laboratory of Constitution Interaction and Dynamics of the Crust– Mantle ŽJ.Z... We thank Ms. Carol Lawson and Ms. Ashwini Sharma for their tireless help with the EMP and ICPMS analyses. Joe Boyd and Martin Menzies provided helpful and constructive reviews. This is contribution no. 230 from the ARC National Key Centre for the Geochemical Evolution and Metallogeny of Continents Žwww.es.mq.edu.aur GEMOCr..

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