Trace elemental and PGE geochemical constraints of Mesozoic and Cenozoic peridotitic xenoliths on...

Geochimica et Cosmochimica Acta, Vol. 69, No. 13, pp. 3401–3418, 2005Copyright © 2005 Elsevier Ltd

Printed in the USA. All rights reserved

doi:10.1016/j.gca.2005.03.020

Trace elemental and PGE geochemical constraints of Mesozoic and Cenozoic peridotiticxenoliths on lithospheric evolution of the North China Craton

JIANPING ZHENG,1,2,* MIN SUN,2 MEI-FU ZHOU,1 and PAUL ROBINSON2,3

1State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences,Wuhan 430074, China

2Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China3Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H2J5, Canada

(Received November 29, 2004; accepted in revised form March 15, 2005)

Abstract—Whole-rock major, trace, and platinum-group elemental (PGE) data, and major and trace elementcompositions of diopsides are reported for peridotite xenoliths from (1) early Mesozoic volcanic breccias inXinyang, located at the southern margin of the North China Craton, and (2) Cenozoic basalts in Hebi andShanwang, both of which are situated within the craton and lie on the North-South Gravity Line and the Tanlufault zone, respectively. The early Mesozoic Xinyang xenoliths are harzburgites containing �2% Cpx withhigh Cr# and enriched in LREE but depleted in HFSE. These xenoliths have chondritic Pd/Ir (1.9–6.6) andRu/Ir (3.5–4.0) ratios and high Ni and low CaO, Al2O3, and S contents, indicating derivation from a highlyrefractory mantle that experienced carbonatitic metasomatism. Negative Ce (mean �Ce � 0.50) and low Mg/Siratios of the Xinyang peridotites record the addition of crustal components likely produced from subductedcontinental material of the Yangtze Craton in the early Mesozoic. The subduction-related modification of thelithospheric mantle was limited to the area close to the collision zone rather than being pervasive throughoutthe craton. The Cenozoic Hebi peridotite xenoliths are harzburgites with �4.5% Cpx and have low CaO andAl2O3 but high Ni contents, chondritic Ru/Ir ratios (2.5–5.4), and a wide range of CaO/Al2O3, Na2O/TiO2,Pt/Ir (0.4–2.3), and Pd/Pt (1.1–8.5) ratios. These peridotites are interpreted as the shallow relics of the cratonicmantle. In contrast, the Cenozoic Shanwang xenoliths are lherzolites (5.6%–19.5% Cpx), which have low Nicontents and low Ni/Cu and Mg/Si ratios, but high CaO, Al2O3, S, and HREE contents, and relatively highRu/Ir and Pd/Ir ratios. The Shanwang peridotites show pronounced positive Ti and Sr, negative Th, andslightly negative Y, Zr, and Hf anomalies. They are believed to represent newly accreted fertile lithosphericmantle derived from cooling of upwelling asthenosphere. The documented temporal and spatial variations inthe Mesozoic-Cenozoic mantle support the previous suggestion that the buoyant refractory continental keel inthe eastern part of the North China Craton was heterogeneously replaced by younger fertile lithospheric mantle

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in the late Cretaceous–early Tertiary. Copyright © 2005 Elsevier Ltd

1. INTRODUCTION

Ancient depleted mantle roots are often preserved beneathcratons owing to their significant buoyancy (Griffin et al.,1998a, 1999) or viscosity (Kelemen et al., 1998). However,asthenosphere-lithosphere and mantle-crust interactions canmodify these roots or replace them with more fertile materials(e.g., Menzies et al., 1993; Griffin et al., 1998b; O’Reilly et al.,2001). The North China Craton (NCC) is one of the majorArchean cratons in eastern Eurasia. Geochemical and isotopicstudies of mantle xenoliths from Paleozoic diamond-bearingkimberlites (Zheng J. and Lu, 1999; Griffin et al., 1998b) andfrom Cenozoic basalts (Xu Y. et al., 1996; Fan et al., 2001; Gaoet al., 2002) confirm the existence of a thick (�200 km)cratonic lithosphere with a cold geotherm (�40 mW/m2) in thePaleozoic (Lu and Zheng, 1996; Griffin et al., 1998b), whichwas transformed into a thin lithosphere (80–110 km) with a hotgeotherm in the Cenozoic (e.g., Xu Y. et al., 1995; Chen et al.,2001). A seismic tomography profile across the NCC alonglatitude 36°N clearly reveals the presence of isolated high-Vpzones dispersed within a region with generally low Vp atdepths of 100 km (Sun, 1992). Interpretation of this profile has

* Author to whom correspondence should be addressed([email protected]).

3401

led to a “mushroom cloud” model, which envisages thinning ofthe lithosphere beneath the NCC during the late Mesozoic as aresult of extension of the ancient mantle root and mixing withthe upwelling asthenosphere (Menzies et al., 1993; Yuan, 1996;Griffin et al., 1998b; Zheng J. et al., 1998, 2001; Lu et al., 2000;Xu Y., 2001). Abundant information is available on the Paleo-zoic and Cenozoic lithospheric mantle beneath the NCC, butthe only data available on the nature of the Mesozoic lithos-pheric mantle come from a few studies of late Mesozoic mag-matic rocks (e.g., Zhang H. et al., 2002, 2003; Xu Y. et al.,2004).

In this paper, we present new data on recently discoveredperidotitic xenoliths from early Mesozoic volcanic rocks lo-cated at the southern margin of the NCC adjacent to theQinling-Dabie-Sulu Orogenic Belt (Fig. 1). This belt wasformed by collision of the NCC and the Yangtze Craton (YC)in the early Mesozoic (Li S. et al., 1993), and the xenolithsprovide an excellent opportunity to investigate the nature of thelithospheric mantle and the extent of crust-mantle interaction atthat time. Trace elements and platinum-group elements (PGE)are ideal for such a study because they are particularly sensitiveto mantle processes such as partial melting, metamorphism, andmelt/rock interaction (Arculus and Delano, 1981; Zindler andJagoutz, 1988; O’Neill, 1991). Transfer of these elements from
the mantle into the crust occurs only in specific geodynamic

3402 J. Zheng et al.

areas characterized by extensive mantle-crust interaction (Nal-drett, 1981). Various models have been proposed to explain thefractionation of trace elements, especially PGE (Handler andBennett, 1999; Rehkamper et al., 1999; Schmidt et al., 2000),and their behavior during mantle evolution (Garuti et al., 1997).

Here, we present whole-rock major oxide, trace element, andPGE data for five peridotite xenoliths from early MesozoicXinyang volcanic breccias, and ten peridotitic xenoliths fromCenozoic Hebi and Shanwang basalts within the NCC (Fig. 1).Trace element data for diopsides from the Xinyang peridotiteswere also obtained by LAM-ICPMS and compared with exist-ing data from Hebi and Shanwang peridotites (Zheng J. et al.,1998, 2001). The principal objectives of this study are (1) todetermine whether the mantle underneath the NCC is charac-terized by chondritic PGE abundances, 2) to characterize thebehavior of trace elements and PGE during asthenosphere-lithosphere and mantle-crust interaction, and (3) to constrainthe evolution of lithospheric mantle beneath the North ChinaCraton.

2. GEOLOGIC SETTING

The North China Craton is bounded by the Central AsianOrogenic Belt on the north and by the Qinling-Dabie-SuluOrogenic Belt (QDSOB) on the south. As recorded in rocks�3.6 Ga old (Liu D. et al. 1992; Zheng J. et al., 2004a), the

Fig. 1. Localities of peridotite xenoliths from North China Cra-ton.

craton underwent a series of tectonic events in the late Archean

and Paleoproterozoic (e.g., Jahn, 1990; Zhai et al., 2001; ZhengJ. et al., 2004b) and was stabilized in the late Paleoproterozoicfollowing collision of the Eastern and the Western Blocks(Zhao et al., 1999, 2000). Following this collision, the cratonwas magmatically and tectonically quiescent until eruption ofthe Fuxian and Mengyin kimberlites in the middle Ordovician(Lu et al., 1998; Fig. 1). Late Ordovician to middle Carbonif-erous sedimentary rocks are absent, indicating uplift and ero-sion of the craton during this period. Since late Mesozoic timethe region has been tectonically reactivated, as evidenced bywidespread calc-alkaline and intraplate volcanism, lithospherethinning, development of large sedimentary basins, and highheat flow.

The NCC is divided into two parts by a North-South GravityLine (NSGL) running approximately parallel to, and �400 kmwest of the Tanlu fault zone, a major wrench fault system thatcuts through the eastern part of the NCC and extends deep intothe lithospheric mantle (Xu J. et al., 1987; Fig. 1). East of theNSGL, the craton is characterized by a thin crust and litho-sphere, high heat flow, and weak negative-to-positive regionalBouguer anomalies. West of the NSGL, the craton is charac-terized by thick crust and lithosphere, low heat flow, and strongnegative Bouguer gravity anomalies (Yuan, 1996; Griffin et al.,1998b).

Studies of peridotitic xenoliths from Paleozoic kimberlitesand from Cenozoic basalts on the craton reveal the existence ofa thick lithosphere (�200 km) in the Paleozoic (Griffin et al.,1992, 1998b; Lu and Zheng, 1996), and thin lithosphere in theCenozoic (Xu X. et al., 1998; Chen et al., 2001). Lithosphericthinning beneath the eastern part of the NCC was accompaniedby heterogeneous replacement by fertile peridotites in Meso-zoic-Cenozoic time (Zheng J. et al., 1998). Fertile peridotites ofPhanerozoic age now make up much of the lithospheric mantlebeneath the eastern part of the NCC (Griffin et al., 1998b;Menzies and Xu, 1998; Zheng J., 1999), especially along thetranslithospheric fault zone (Zheng J. et al., 1998, 2004c).Relics of refractory Archean mantle are uncommon and arepresent mostly in the region �400 km west of the Tanlu fault(Zheng J. et al., 2001).

The Qinling-Dabie-Sulu Orogenic Belt is an ultrahigh-pres-sure metamorphic terrane containing micro-iamonds (Xu S. etal., 1992) and coesite (Okay et al., 1989; Wang et al., 1989).Subduction of the Yangtze Craton beneath the NCC com-menced in the late Permian or early Triassic and was followedby collision of the blocks during the Triassic and uplift in thelate Jurassic to early Cretaceous (Li S. et al., 1993; Jahn, 1998;Banno et al., 2000).

Nine early Mesozoic diatremes have been discovered �75km north of Xinyang city, Henan Province, along the southernmargin of the NCC (Fig. 1). These diatremes are filled withbasaltic andesite, peridotite, high-pressure mafic granulite(Zheng J. et al., 2003), and early Archean felsic granulites(Zheng J. et al., 2004a). Geologic relationships indicate that thediatremes were erupted later than the Ordovician, and K-Argeochronological data indicate that the volcanic activity oc-curred in the Jurassic (206–178 Ma; Lu et al., 2003).

Cenozoic volcanic fields on the NCC occur at Hebi andShanwang. The Hebi area of Henan Province lies at the westernmargin of the Eastern Block of the NCC. NNW-oriented pipes
and/or dikes of olivine nephelinite crop out 10 km south of

3403Trace elements and PGE of peridotite from the North China Craton

Hebi city. These nephelinites erupted at �4.0–4.3 Ma (K-Arages; Liu R. et al., 1990) and contain abundant xenoliths ofharzburgite. The Shanwang area of Shandong Province liesastride the Tanlu fault zone (Xu J. et al., 1987) that cuts throughthe lithosphere of the eastern part of the NCC. Three episodesof volcanism, including 15 lava flows (episodes 1, 2, and 3comprising 10, 2, and 3 flows, respectively) have been recog-nized in Shanwang. The mantle xenoliths described here werecollected from lavas of episode 1 (18.2–16.8 Ma, by K-Ardating; Jin, 1985).

3. PETROGRAPHY

Modal mineralogy of the peridotites was determined by pointcounting (Table 1), and the rocks were classified using theIUGS scheme (Le Maitre, 1982). Lherzolites are divided intoCpx-poor lherzolites, lherzolites, and Cpx-rich lherzolites,based on the ratio of Cpx/Opx (�1/3, 1/3–2/3, and �2/3,respectively). The textural terms used to describe the mantlexenoliths are after Harte (1977).

The Xinyang xenoliths (8–10 cm in diameter) are porphy-roclastic to sheared, Cpx-poor spinel harzburgites containing�2% fresh diopside and spinel. Olivine and enstatite in theharzburgites have been replaced by talc. However, all of theother xenoliths, such as high-pressure mafic granulites,eclogites, garnet pyroxenites, and early Archean felsic granu-lites, from these diatremes are quite fresh (Zheng J. et al., 2003,2004a). The Hebi xenoliths (6–8 cm in diameter) contain20–33 modal% enstatite and are coarse grained (Fig. 2A), anddominantly Cpx free. A few contain �4.5% Cpx (e.g., HB5).Both olivine and pyroxene grains in these rocks contain abun-dant melt and fluid inclusions, which are generally distributedalong healed fractures. Patches and small veins of siliceous,alumina-rich, and alkali-rich glasses with diopside microphe-nocrysts are also present (Fig. 2B). Inclusions of monosulfidesolid solution and pentlandite also occur in the enstatite andolivine grains (Fig. 2C). The Shanwang peridotites (5–6 cm indiameter) are mainly fine-grained, foliated (Fig. 2D), Cpx-richspinel lherzolites (8.2–19.5 modal% Cpx) with minor Cpx-poorvarieties.

4. ANALYTICAL METHODS

Major oxides of whole rocks were obtained using a Philips PW2400x-ray fluorescence (XRF) spectrometer on glass disks at the Universityof Hong Kong. Some trace elements, such as S, Cr, Ni, Cu, and Znwere determined by XRF on pressed-powder pellets. Accuracy andprecision are better than �2% for the major oxides and �5% for thetrace elements.

Other trace elements, including REE, were determined by ICP-MSafter digestion of the Li2B4O7 fused beads with mixed acid (HF �HNO3). Pure solution external standards were used for calibration, andgeologic standards (USGS standards PCC-1, SARM-39, G-2, SY-4,and W-2 and Chinese National Standards GSR-1 and GSB-3) wereanalyzed to monitor the analytical accuracy. Measurements were per-formed on a VG Elemental PQ Excell ICP-MS at the University ofHong Kong. Precision for the REE is better than 5%, and the precisionfor Rb, Sr, Ba, Nb, Ta, Zr, Hf, U, and Th is better than 10%.

Platinum-group elements were preconcentrated by Ni-sulphide fireassay and Te coprecipitation (Zhou et al., 2000, 2001). The preparedsolutions were analyzed by ICP-MS at the University of Hong Kongalong with standard reference materials PCC-1, SARM-39, WPR-1,
and TDB-1. Analytical precision is better than 10% for Ru and Rh and15% for Ir, Pd, and Pt (Table 1). Osmium data are not reported in this
study because it is difficult to control the loss of the Os oxides duringsample preparation (Sun et al., 2001).

Major oxide analyses of diopsides were carried out in the GEMOCKey Centre at Macquarie University using a Cameca SX50 electronmicroprobe (EMP). The EMP was fitted with five crystal spectrometersand used an accelerating voltage of 15 kV and a sample current of 20nA. The width of the electron beam was 5 �m. Natural minerals wereused as standards, and matrix corrections were carried out using themethod of Pouchou and Pichoir (1984). Counting time was 10 s forpeaks and 5 s for background on either side of the peak. The majorelement abundances reported in Table 2 generally represent averages ofmore than five point analyses of each grain and several grains fromdifferent parts of each sample.

Trace elements in diopside were also determined in the GEMOCKey Centre at Macquarie University using a 266 nm UV laser ablationmicroprobe coupled to an ICPMS (LAM-ICPMS) similar to the onedescribed by Jackson et al. (1992) and Jenner et al. (1994). The laser isa Continuum Surelite I-20 Q-switched and frequency-quadrupled Nd:YAG laser with a fundamental infrared (IR) wavelength at 1064 nmand a pulse width of 5–7 ns. Most analyses were done with a beam inthe range of 0.5–3 mJ per pulse. NIST 610 and 612 glasses and CaOcontents were used as external and internal standards for trace elementanalysis. Data were reduced using the in-house GLITTER on-linesoftware. Trace element abundances reported in Table 2 generallyrepresent averages of at least three analyses for each phase.

5. ANALYTICAL RESULTS

5.1. Chemistry of Xinyang Diopsides

Diopsides in the Xinyang peridotites are generally similar tothose of the Hebi peridotites but distinctly different from thoseof the fertile mantle reflected by the Shanwang samples (Fig.3). The Xinyang diopsides have Mg# and Cr# of 0.92–0.93 and0.15–0.23 respectively, and are characterized by moderatelyhigh Al2O3 (2.88–5.78 wt%) and Cr2O3 (1.15–1.47 wt%) butlow Na2O and TiO2 (Table 2).

Diopside is the main repository for REE and other incom-patible trace elements in “dry” peridotite. Chondrite-normal-ized REE patterns for the Xinyang diopsides are generallysteeply inclined from La to Ho and then concave upward for theHREE ((La/Nd)n � 1.22–2.16; (Ho/Lu)n � 0.49–1.77; Fig. 4).Sample Y97-5 is distinguished from the others in havingmarked depletion of La and Ce, leading to an upwardly convexpattern in the LREE ((La/Nd)n � 0.32, (Ho/Lu)n � 1.74; Fig.4), relatively higher total REE (84.8 ppm), and enrichment inthe MREE (�Nd). Lead and high field strength elements(HFSE) such as Nb, Ta, Zr, Hf, and Ti show strong negativeanomalies relative to REE in all of the Xinyang diopsides.Primitive mantle–normalized trace elemental patterns of theXinyang diopsides are similar and fall into the field defined bythe Hebi diopsides (Fig. 5).

5.2. Major Oxides of Peridotites

The Xinyang peridotites have low CaO (�0.30 wt%) andAl2O3 (�0.84 wt%) contents and very low Mg/Si (0.63–0.65)and CaO/Al2O3 (0.31–0.48) ratios. The high SiO2 contents(�56 wt%) of these peridotites reflect complete replacement ofolivine and orthopyroxene by talc. The Hebi peridotites havesomewhat higher CaO (0.89–2.25 wt%) and Al2O3 (1.26–1.94wt%) contents than those from Xinyang, but these values arestill lower than commonly accepted primitive mantle compo-sitions (e.g., Hart and Zindler, 1986; McDonough and Sun,
1995). The Hebi peridotites have high Mg/Si ratios (1.3–1.4)

Table 1. Modes and compsitions of major and trace element and PGE of peridotites from east China

Locality: Mesozoic Xinyang xenolith* Cenozoic Hebi xenolith Cenozoic Shanwang xenolith

Sample: Y97-0 Y97-1 Y97-2 Y97-4 Y97-5 HB1 HB2 HB3 HB4 HB5 LQ1 LQ2 LQ13 LQ20 LQ21Texture: F F S P P C C C C P F F F S S

Ol (vol%) 73.2 75.1 76.3 69.8 70.4 70.3 66.9 78.2 76.0 74.0 69.3 63.9 67.6 75.5 70.6Opx 26.2 24.6 22.9 29.3 28.2 28.7 32.5 20.4 23.5 21.5 16.7 14.4 20.0 16.3 18.2Cpx 0.4 0.3 0.6 0.8 1.2 0.8 free free free 4.5 11.3 19.5 8.6 5.6 8.2Sp 0.2 free 0.2 0.1 0.2 0.2 0.6 0.8 0.5 free 2.7 2.2 3.5 2.6 3.1SiO2 (wt%) 56.8 56.8 56.0 56.5 56.6 42.4 43.2 42.9 42.4 41.3 43.3 42.8 43.4 43.4 43.4TiO2 0.02 0.02 0.03 0.01 0.02 0.12 0.11 0.19 0.08 0.12 0.35 0.40 0.34 0.30 0.32Al2O3 0.62 0.62 0.84 0.40 0.62 1.52 1.94 1.26 1.38 1.50 4.40 4.51 4.34 4.33 4.34FeO 7.59 6.74 8.73 7.17 8.00 8.07 7.42 7.72 7.70 9.44 9.61 9.48 9.73 9.55 9.65MnO 0.03 0.03 0.03 0.03 0.03 0.12 0.11 0.11 0.11 0.14 0.14 0.15 0.13 0.14 0.14MgO 28.3 28.6 27.4 28.4 28.1 44.9 44.4 45.2 46.1 44.0 34.8 34.2 34.4 35.7 35.1CaO 0.23 0.30 0.25 0.12 0.19 1.29 1.03 0.99 0.89 2.25 5.04 6.00 4.81 4.26 4.54Na2O 0.49 0.48 0.52 0.46 0.49 0.21 0.29 0.30 0.14 0.13 0.22 0.25 0.21 0.19 0.21K2O 0.03 0.03 0.03 0.02 0.02 0.07 0.08 0.14 0.05 0.04 0.09 0.07 0.13 0.09 0.10P2O5 0.01 0.01 0.01 0.01 0.01 0.07 0.06 0.09 0.04 0.10 0.36 0.33 0.47 0.27 0.37LOI 5.72 5.53 6.01 5.93 5.80 0.83 0.66 0.76 0.91 1.00 1.58 1.55 1.53 1.66 1.60Total 99.9 99.1 99.8 99.1 99.9 99.6 99.3 99.7 99.8 100.0 99.9 99.8 99.5 99.9 99.7S (ppm) 35.6 40.8 25.6 40.6 33.1 82.8 89.6 101 101 39.6 106 102 136 78.6 107Cr 1249 768 1594 1386 1490 2272 2186 2084 2345 2472 2480 2487 2511 2443 2477Ni 2585 2737 2545 2473 2509 2266 2223 2245 2378 2219 1979 1992 1903 2042 1972Cu 7.15 7.98 6.61 6.88 6.74 8.80 5.04 5.86 6.50 17.8 22.3 23.0 27.3 16.6 22.0Zn 40.4 37.9 45.7 37.5 41.6 53.6 50.0 54.0 50.4 60.0 58.9 59.0 60.9 56.8 58.9Sc 4.55 3.37 4.70 5.59 5.15 4.67 2.66 3.73 4.23 8.04 7.00 6.85 8.51 7.09 9.23V 24.5 18.5 32.8 22.2 27.5 27.6 18.2 22.3 21.3 48.7 53.1 56.5 49.2 53.5 51.4Rb (ppm) 0.38 0.34 0.36 0.45 0.41 4.07 3.73 6.10 3.50 2.96 1.66 1.84 1.73 1.40 1.57Sr 5.60 6.64 6.14 4.02 5.08 77.2 65.1 106 58.3 79.0 87.4 108 42.5 112 77.1Y 1.35 1.57 1.58 0.89 1.24 1.13 0.96 1.58 0.72 1.27 2.41 2.02 3.10 2.10 2.60Zr 1.17 1.94 1.41 0.17 0.79 19.6 16.8 33.2 13.8 14.6 6.46 4.71 7.60 7.07 7.34Nb 0.09 0.14 0.07 0.05 0.06 9.22 7.93 15.5 8.38 5.13 1.22 0.97 0.96 1.74 1.35Ba 8.16 9.81 9.82 4.86 7.34 38.6 33.1 59.2 34.4 28.0 8.45 5.19 10.7 9.49 10.1La 1.12 1.53 0.86 0.67 0.79 3.18 2.75 4.54 2.10 3.34 1.66 1.72 1.23 1.72 1.58Ce 1.29 2.11 0.96 0.59 0.81 6.02 5.13 9.00 3.81 6.12 2.88 3.71 2.06 0.39 0.35Pr 0.23 0.30 0.23 0.17 0.20 0.78 0.66 1.22 0.46 0.77 0.39 0.49 0.30 1.53 1.44Nd 0.90 1.03 0.98 0.68 0.83 3.17 2.69 5.08 1.81 3.08 1.60 1.92 1.34 0.34 0.33Sm 0.23 0.22 0.26 0.18 0.21 0.62 0.52 1.00 0.35 0.61 0.35 0.38 0.39 0.11 0.13Eu 0.08 0.07 0.09 0.07 0.08 0.17 0.14 0.26 0.10 0.17 0.12 0.12 0.14 0.37 0.42Gd 0.24 0.25 0.29 0.19 0.24 0.48 0.42 0.73 0.29 0.49 0.40 0.37 0.47 0.06 0.07Tb 0.04 0.04 0.05 0.03 0.04 0.06 0.05 0.09 0.04 0.06 0.07 0.06 0.08 0.43 0.51Dy 0.27 0.27 0.34 0.20 0.27 0.31 0.26 0.46 0.19 0.32 0.47 0.40 0.58 0.09 0.11Ho 0.06 0.06 0.07 0.04 0.06 0.05 0.04 0.07 0.03 0.06 0.10 0.09 0.13 0.28 0.33Er 0.17 0.18 0.21 0.11 0.16 0.13 0.11 0.18 0.09 0.15 0.31 0.28 0.38 0.04 0.05Tm 0.03 0.03 0.03 0.02 0.03 0.02 0.01 0.02 0.01 0.02 0.05 0.04 0.06 0.31 0.36Yb 0.17 0.20 0.23 0.11 0.18 0.11 0.10 0.13 0.07 0.14 0.35 0.30 0.41 0.05 0.06Lu 0.03 0.03 0.03 0.02 0.03 0.02 0.01 0.02 0.01 0.02 0.05 0.04 0.06 0.19 0.21Hf 0.03 0.05 0.04 0.00 0.02 0.46 0.39 0.79 0.29 0.35 0.17 0.09 0.23 0.11 0.09Ta 0.01 0.01 0.01 0.01 0.01 0.53 0.47 0.89 0.41 0.33 0.08 0.05 0.07 0.03 0.04

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612

588

.686

.669

.61.

532.

24R

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500.

430.

381.

340.

550.

571.

210.

510.

240.

881.

530.

472.

893.

772.

97/Y

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4.48

5.20

2.54

4.14

2.98

19.7

18.7

23.7

20.4

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3.22

3.89

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2.25

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352.

320.

890.

790.

932.

342.

382.

172.

632.

421.

992.

361.

281.

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85/L

a)n

0.08

0.09

0.08

0.07

0.08

2.86

2.85

3.36

3.94

1.52

0.73

0.56

0.77

3.42

4.08

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534.

013.

863.

823.

533.

265.

423.

273.

612.

494.

628.

373.

452.

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1.42

1.32

1.71

0.97

1.44

1.12

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0.58

0.72

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2.42

2.23

3.39

2.06

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262.

241.

906.

622.

502.

398.

472.

131.

102.

829.

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0.36

0.37

0.18

1.66

0.36

1.28

0.87

0.37

2.31

0.80

1.95

0.93

3.13

3.40

3.75

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oars

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aine

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ined

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lan

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pxar

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mor

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ers,

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and a wide range of CaO/Al2O3 ratios (0.53–1.5). The Shan-wang samples have very high CaO (4.26–6.00 wt%) and Al2O3

(4.37–4.57 wt%) contents, with Mg/Si ratios of �1.0 andCaO/Al2O3 ratios of 0.98–1.33. High contents of P2O5 (0.27–0.47 wt%) and CaO in the Shanwang peridotites may suggestthe presence fine-grained apatite but it has not been recognisedpetrographically.

5.3. Trace Elements of Peridotites

The Xinyang peridotites have low total REE (3.08–6.32ppm) and HREE abundances (0.30–0.57 ppm). Their chon-drite-normalized REE patterns are flat from Lu to Gd andslightly enriched in Sm to La, with strong negative Ce anom-alies (mean �Ce � 0.50; �Ce defined as 2Cen/(Lan�Prn)) (Fig.6A). In contrast, the Hebi peridotites possess high total REEabundances (9.4–23 ppm) and high (Ce/Eu)n ratios (3.1–3.4).They have very low HREE abundances (0.21–0.42 ppm), andtherefore high (La/Yb)n ratios (16–23). Their chondrite-nor-malized REE patterns are flat from Lu to Er, with strongenrichment from La to Ho (Fig. 6B). The Shanwang xenolithshave similar (La/Sm)n ratios (1.9–3.1) to the Hebi peridotitesbut higher HREE abundances (0.75–1.04 ppm) and lower totalREE abundances (7.63–9.92 ppm). Their chondrite-normalizedREE patterns display slight enrichment in LREE and flat HREEfrom Lu to Eu (Fig. 6C).

On primitive mantle–normalized diagrams, all the peridotitesin this study show negative Th anomalies relative to REE. TheXinyang peridotites have low abundances of the incompatibletrace elements and show strongly negative HFSE and Sr anom-alies and slightly negative Y anomalies (Fig. 7A). The Hebiperidotites are enriched in highly incompatible trace elementsand have negative Ti and positive Nb and Ta anomalies (exceptHB5, which has a positive U anomaly) (Fig. 7B). The Shan-wang peridotites show clearly positive Ti, U, and Sr anomaliesand slightly negative Y, Nb, Ta, Zr and Hf anomalies (Fig. 7C).

5.4. Cu, Ni, and PGE of Peridotites

The Xinyang peridotites have variable PGE contents andratios. For example, Ir contents range from 0.29 to 1.72 ppb, Rufrom 1.73 to 10.4 ppb, Pd/Ir ratios from 2.3 to 8.0, and Pt/Irratios from 0.4 to 3.7. These rocks all have low Pd and Pt(2.32–3.96 ppb and 0.68–1.19 ppb, respectively), high Ni(2290–2930 ppm), and narrow ranges of Cu (6.61–7.98 ppm),S (25.6–40.8 ppm), and Ni/Cu ratios (342–385).

The Hebi peridotites contain 0.63–3.74 ppb Ir, 5.33–21.1 ppbRu, 0.34–1.50 ppb Rh, 4.96–12.0 ppb Pd, 1.22–19.2 ppb Pt,and 10.4–13.5 ppb Au and have variable ratios of Pd/Ir (1.3–10.2), Pt/Ir (0.8–5.1), and Pd/Pt (0.3–5.3). They have slightlyhigher S (39.6–101 ppm) and lower Ni contents (2219–2378ppm) than the Xinyang samples. The Shanwang peridotiteshave lower Au (3.40–6.70 ppb) and overall lower Ir (0.51–1.05ppb), Ru (2.72–13.7 ppb), Rh (0.30–0.67 ppb), Pd (4.16–10.3ppb), and Pt (2.71–4.58 ppb) and higher Ru/Ir (5.3–13.1), Pd/Ir(6.2–15.5), and Pt/Ir (2.1–7.5) ratios than the Hebi samples.

All of the peridotites show Ru enrichment except for samplesLQ13 and LQ21, which have flat patterns from Ru to Pt. TheXinyang samples all show negative Pt anomalies (Fig. 8A).
Most of the Hebi peridotites (HB2, HB3, and HB5) haveT
hU �

RIr

(R

uR

hPt Pd A

uM

gM

gN

l/Pd

/(L

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negative Pt anomalies, but sample HB4 has a positive anomalyand sample HB1 shows no anomaly (Fig. 8B). The Shanwangperidotites exhibit PGE patterns with slightly negative Ptanomalies (Fig. 8C). Ir correlates positively with Rh and Ru inall samples and with Pt in the Hebi samples but correlatesnegatively with Pt in the Shanwang samples. Correlations be-tween Ir and other elements such as Pd, Ni, and Cu varybetween areas (Fig. 9).

6. DISCUSSION

6.1. Refractory or Fertile Mantle?

Partial melting of relative fertile peridotite (e.g., lherzolite)will decrease modal contents of clinopyroxene and form har-zburgite, which will be relatively depleted in CaO, Al2O3, andTiO2. During partial melting, S and Cu behave as incompatibletrace elements and are extracted from the mantle. Ir and Ru(IPGE) typically behave as compatible elements and are mainlyassociated with spinel, whereas Pt and Pd (PPGE) are inferredto be incompatible (Amosse et al., 1990; Capobianco et al.,1994; Handler and Bennett, 1999).

The Cenozoic Hebi xenoliths consist mainly of Cpx-freeharzburgite with minor Cpx-rich varieties (�4.5% Cpx in HB5)and have Mg-rich olivine (Fo �92) and Cr-rich spinel (meanCr# � 0.51; Zheng J. et al., 2001). These peridotites are

Fig. 2. Textures of peridotite xenoliths. (A) HB2, cosiliceous, alumina- and alkali-rich glasses and diopside (D(D) LQ2, foliation shown by olivine. Scale bar: A, B, D

characterized by having low CaO and Al2O3, and high Ni

contents. They also have high Mg/Si and Ni/Cu ratios andsuperchondritic Ru/Ir and Pd/Ir ratios (Table 1). They arestrongly depleted in HREE (Fig. 6B), and thus have high(La/Yb)n ratios (16.2–20.4) similar to peridotitic xenoliths inkimberlites from Archean cratons (Fig. 10), reflecting depletionin basaltic components (e.g., Bernstein et al., 1998). In contrast,the Cenozoic Shanwang xenoliths are mainly Cpx-rich lherzo-lites, with less magnesian olivine (mean 89.4) and low-Crspinel (Cr# � 0.21; Zheng J. et al., 1998). They have lower Ni,Ni/Cu, and Mg/Si and higher CaO, Al2O3, S, HREE, Ru/Ir, andPd/Ir than the Hebi peridotites, indicative of a more fertilemantle (Zheng J. et al., 1998, and references therein). The earlyMesozoic Xinyang peridotitic xenoliths are Cpx-poor (�2%)spinel harzburgites containing high-Cr and high-Mg diopside(Fig. 3). They have high Ni and Ni/Cu ratios but low CaO,Al2O3, S, Cu (Fig. 11), and Pd/Ru (Fig. 12), reflecting a highlyrefractory character similar to the cratonic mantle (see Fig. 10)

6.2. Metasomatic Agent Reflected by Mantle Diopside

Mantle peridotites commonly exhibit local evidence of meta-somatism by transient melts/fluids, leading to the formation ofvolatile-bearing minerals (e.g., amphibole, mica, and apatite) orthe enrichment of strongly incompatible trace elements in di-opside. The depletion of Al O with increasing Mg# and the

ained peridotite; (B) HB2, patches or small veins withophenocrysts; (C) HB2, sulfide inclusion in olivine (Ol);mm; C � 30 �m.

arse-gr

2 3

positive correlation between Ni and Cr# in diopside reflect the

0.20 0.12 0.05


progressive depletion of the Xinyang diopsides from samplesY97-5 to Y97-1 (Fig. 13). The LREE-enriched patterns of thediopsides (Fig. 4), on the other hand, record subsequent meta-somatic enrichment.

Enrichment in LILE and LREE of mantle diopside in “dry”peridotite has been attributed to metasomatism by carbonatiticmelts (e.g., Yaxley et al., 1998), volatile-rich silicate melts(Zangana et al., 1999), or H2O-CO2 fluids (Ionov et al., 1997;Stalder et al., 1998). Trace element partitioning between diop-side and melt has been experimentally determined for carbon-ate and silicate systems under mantle conditions. The experi-mental results show that diopside-melt partition coefficients forSi, Al, HREE, Ti, and Zr in the carbonate system are higher byfactors of 5 to 200 than in the silicate system. Partition coef-ficients for Nb, LREE, and alkali metals show much lessfractionation (�3 times; Blundy and Dalton, 2000). On theother hand, carbonatitic melts can fractionate REE and HFSE

Table 2. Major (wt%) and trace eleme

Sample: Y97-0 Y97-1

SiO2 53.2 53.5TiO2 0.03 0.02Al2O3 3.30 2.88Cr2O3 1.19 1.15FeO 2.27 2.28MnO 0.08 0.06MgO 17.3 17.6CaO 19.6 21.1Na2O 1.16 0.80Total 98.2 99.4Mg* 0.93 0.93Cr* 0.20 0.21Sc 53.4 51.8Ti 150 120V 113 111Co 19.5 20.9Ni 338 361Cu 1.50 1.96Zn 7.94 8.49Ga 0.98 1.16Sr 580 175Y 1.73 0.92Zr 8.64 3.35Nb 0.57 1.05Ba 0.59 6.65La 17.5 6.79Ce 38.0 14.9Pr 4.52 1.69Nd 15.8 5.95Sm 2.23 0.77Eu 0.62 0.22Gd 1.25 0.48Tb 0.11 0.04Dy 0.48 0.22Ho 0.07 0.04Er 0.18 0.15Tm 0.03 0.03Yb 0.21 0.20Lu 0.03 0.04Hf 0.12 0.11Ta 0.07 0.09Pb 0.45 0.13Th 0.63 0.38U 0.15 0.10

more effectively than silicate melts or CO2-rich fluids (Blusz-
# #
nt concentrations (ppm) of Xinyang diopsides

Y97-2 Y97-4 Y97-5

53.5 53.9 53.40.07 0.07 0.183.22 3.79 5.781.40 1.30 1.472.34 2.32 2.420.06 0.07 0.10

17.1 16.9 15.719.7 19.8 18.7

1.25 1.33 2.1198.7 99.6 99.90.93 0.93 0.920.23 0.19 0.15

59.7 61.4 80.8420 442 1079182 151 20022.0 20.3 18.9

381 352 3271.64 2.51 4.669.68 8.51 7.921.71 1.95 3.96

410 412 4836.93 4.03 6.54

41.8 27.5 56.41.47 1.13 1.431.19 4.04 7.72

15.5 11.3 5.2235.9 28.5 25.44.28 4.00 5.52

16.7 17.4 31.23.46 3.39 7.121.16 1.01 2.053.08 2.34 4.550.38 0.25 0.451.94 1.15 1.970.29 0.17 0.270.63 0.41 0.530.07 0.05 0.060.36 0.30 0.430.07 0.05 0.071.48 0.93 2.000.19 0.13 0.170.33 0.26 0.150.85 0.50 0.14

Fig. 3. Plot of Mg vs. Cr of the Xinyang diopsides. Data sources:Hebi, Zheng J. et al. (2001); Shanwang, Zheng J. et al. (1998).


tajn and Shimizu, 1994) and have high contents of LILE(Meen, 1987). High HFSE depletion and low Ti/Eu ratios inmantle diopside, coupled with LREE enrichment, have beenwidely interpreted as the key signatures of carbonatite-relatedmetasomatism (Klemme et al., 1995; Coltorti et al., 1999).Recent experimental investigations on the partitioning of traceelements between minerals and H2O-rich fluids (Brenan et al.,1995; Keppler, 1996) have shown that fluids and melts havesimilar transport capabilities for LILE elements such as Sr andBa. In contrast, highly charged elements such as Nb are morestrongly partitioned into silicate melts than into hydrous fluids.

LREE-depleted patterns are common for the Shanwang di-opsides (Zheng J. et al., 1998) but are not found in the Hebi(Zheng J. et al., 2001) or Xinyang diopsides (see Fig. 4). Incontrast, the upwardly concave HREE patterns are common indiopsides of Hebi and Xinyang but not in those of Shanwang.Diopsides from Hebi and Xinyang have higher proportions ofLREE-enriched samples and generally higher LREE contentsand La/Yb ratios than those from Shanwang. On the other hand,diopsides from Shanwang show variations from LREE-de-pleted through spoon-shaped to LREE-enriched patterns in

Fig. 4. Chondrite-normalized REE patterns of the Xinyang diop-sides. Data sources: same as Figure 3.

Fig. 5. Spidergrams of the Xinyang diopsides. Data sources: same asFigure 3.

different grains of the same peridotite (e.g., sample SW36;Zheng J. et al., 1998), indicating a strong disequilibrium inREE within these mantle peridotites.

In conclusion, metasomatic signatures in the Xinyang diop-sides include (1) enrichment in LREE, Th, U and Sr and (2)fractionation of Zr, Hf, Nb, Ta, and Ti from the LREE (see Fig.5). These diopsides have high (La/Yb)n (8.1–54.4) but lowTi/Eu (242–534) ratios, suggesting carbonatitic metasomatismsimilar to that of the Hebi mantle but clearly different fromsilicate metasomatism reflected by the Shanwang diopsides(Fig. 14).

6.3. Distribution of Incompatible Trace Elements in theHebi and Shanwang Mantle

To constrain geochemical models of the lithospheric mantle,Bedini and Bodinier (1999) carried out a study on the distri-bution of incompatible trace elements between the various

Fig. 6. Chondrite-normalized REE patterns for the peridotite xeno-liths.

constituents of spinel peridotite. They concluded that whole-


rock trace element compositions are controlled by five distinctcomponents: silicate minerals, fluid inclusions, pervasive grain-boundary components, thin reaction layers, and apatite. Thefresh Hebi peridotites have a wide range of CaO/Al2O3, Na2O/TiO2, Pd/Pt, Pt/Ir, and Pd/Ir ratios (Table 1) and high Nb/Laand Ce/Sm ratios (Fig. 15). These xenoliths are enriched inLREE, Nb, Ta, Sr, and Ba but relatively depleted in Ti. Theyhave distinctly different spider patterns from those “recon-structed” by mass-balance calculations based on trace elementcompositions and modal compositions of diopside (Fig. 7B;

Fig. 7. Primitive mantle–normalized trace element spidergrams forthe peridotite xenoliths.

Zheng J. et al., 2001). This is consistent with the abundant fluid

inclusions in olivine and enstatite and abundant patches andsmall veins of glass containing minute grains of diopside inthese rocks (see Fig. 2B). The fresh Shanwang peridotites alsohave different spider patterns from their “reconstructed” ones(see Fig. 7C; Zheng J. et al., 1998). These peridotites showpronounced positive Ti, Sr, and U anomalies, negative Thanomalies, and slightly negative Y, Zr, and Hf anomalies. Theyhave high P2O5 and CaO contents and 1.5–1.6 wt% loss onignition (LOI). These features are consistent with the presenceof abundant CO2-rich fluid inclusions in olivine and enstatite(Xia et al., 1996) and the probable presence of fine-grainedapatite in these rocks. A positive Ti anomaly and slightly highK2O (up to 0.13 wt%) in the Shanwang peridotites may suggestthe presence of a very thin reaction layer composed of Ti-oxides and phlogopite coating the surfaces of spinel grains (cf.Bodinier et al., 1996; Condie et al., 2004).

In summary, the incompatible trace elements of the perido-

Fig. 8. Primitive mantle–normalized PGE spidergrams for the peri-dotite xenoliths.

(D), A


tites occur not only in diopside but also in fluid inclusions, thinreaction layers, and patches or small veins of glass in the “dry”mantle.

6.4. Crustal Components in the Early Mesozoic XinyangMantle

The Xinyang peridotites have very low LREE, Pd, and Ptand low Ce/Sm, Nb/La and Mg/Si ratios. They are also char-acterized by prominent negative Y, HFSE, and Ce anomalies(see Fig. 6A) and high HREE contents compared to the Hebisamples. These data, however, are inconsistent with the modalabundances and compositions of diopside in these rocks (Ta-bles 1 and 2), which show that the Xinyang samples havesimilar levels of depletion compared to the Hebi diopsides (seeFig. 3). Olivine and orthopyroxene in the peridotitic xenoliths

Fig. 9. Plots of Ir against Ru (A), Rh (B), Pt (C), Pd

from the Paleozoic Mengyin and Fuxian kimberlites at the

interior of the NCC (see Fig. 1) are completely altered toserpentine. High SiO2 contents (up to 56.8 wt%) and high LOI(5–6 wt%) of the Xinyang peridotites reflect replacement ofolivine and enstatite by talc, and record an obvious addition ofSiO2 and H2O. To interpret correctly the geochemistry of theXinyang peridotites, particularly the origin of the negative Ceanomaly, it is necessary to determine the conditions underwhich the talc formed.

Talc occurs widely in metamorphosed mafic and ultramaficrocks (Chopin, 1981; Zhang R. et al., 1989), in subduction zoneenvironments (Schreyer, 1988), and in shallow hydrothermalsystems (Evans and Guggenheim, 1988). Talc is also know tooccur in coesite-bearing ultrahigh-pressure metamorphic as-semblages such as talc � garnet � omphacite � kyanite �zoisite (Zhang R. et al., 1995). There is abundant evidence that

u (E), Ni (F), and Cu (G) of peridotite xenoliths.

talc is stable in silica-enriched peridotites at mantle P-T con-


ditions (e.g., Schreyer, 1988; Bose and Ganguly, 1995; ZhangR. et al., 1995; Peacock and Wang, 1999). Guillot et al. (2001)reported talc-bearing serpentinites from a strongly depletedmantle wedge in the Indus suture zone, NW Himalayas. Theserocks contain a high-grade metamorphic assemblage of antig-orite � magnetite � olivine � talc, and trace element and Ndisotopic data show that fluids responsible for hydration of themantle wedge were derived from subducting clastic sediments.Based on the elastic properties of hydrated peridotite, Baileyand Holloway (2000) suggest that talc is present in a 10-km-thick layer directly above subducting slabs. Because all of theother lithologies in the Xinyang diatremes, such as basalticandesite, mafic granulite and felsic granulites are fresh, webelieve that the peridotites were altered to talc during subduc-tion of the Yangtze block rather than by shallow hydrothermalprocesses. Thus, the negative Ce anomalies in the peridotitesare believed to be due to recycling of crustal material into themantle wedge and/or mantle metasomatism (cf. Neal and Tay-lor, 1989).

Recent research shows that the Mesozoic lithospheric mantlebeneath the NCC is spatially heterogeneous (Zhang H. et al.,2002; Guo et al., 2003), with the Yangtze continental subduc-tion–related modification of the mantle being most pronouncedclose to the collision zone (Jahn et al., 1999; Zheng Y. et al.,2003). The Xinyang samples come from the southern margin ofthe NCC adjacent to the Qinling-Dabie-Sulu Orogenic Belt.Eruption of the host magmas (206–178 Ma) took place shortlyafter collision of the Yangtze Craton with the NCC (e.g.,230–210 Ma; Li S. et al., 1993). Peridotitic xenoliths from thislocality have much lower HFSE contents and show strongly Ceand HFSE negative anomalies compared to the Hebi samples,although they are also refractory as indicated by the modalabundance and composition of the diopside in the rocks. Neg-

Fig. 10. Plot of CaO vs. Al2O3 for peridotite xenoliths. Other datasources: Hebi and Shanwang, same as Figure 3; Xinyang, Lu et al.(2003); Kaapvaal low-T xenoliths, Boyd and Mertzman (1987), Cox etal. (1987), Nixon (1987), and Boyd et al. (1993); Oceanic trend adaptedfrom Boyd (1989) and Boyd et al. (1997); Primitive mantle (P.M.),McDonough and Sun (1995).

ative Ce anomaly does not exist in the diopsides, reflecting the

presence of a grain boundary phase in the Xinyang peridotites.However, none of the other components of the Xinyang dia-tremes show significant alteration or modification (Zheng J. etal., 2003, 2004a), suggesting that the peridotites may be mod-ified by metasomatic fluids/melts before eruption. Therefore,we interpret that metasomatic fluids derived from the subductedcontinental materials of the Yangtze Craton affected the litho-spheric mantle wedge along the southern margin of the NCCand that this influence is recorded in the early Mesozoic Xin-

Fig. 11. Plots of CaO vs. S (A), Ni (B), and Cu (C) for peridotitexenoliths.


yang peridotites (e.g., alteration to talc, high SiO2 contents, andhigh LOI).

6.5. Fractionation of PGE and Their Significance

Studies demonstrate that abyssal peridotites (Rehkamper etal., 1999), ultramafic rocks from orogenic lherzolite massifs(Schmidt et al., 2000; Lorand et al., 2000), and peridotitexenoliths (Rehkamper et al., 1997; Lorand et al., 2003; Schmitt,2003) do not have chondritic PGE ratios, and this feature hasbeen interpreted as indicating heterogeneous accretion or frac-tionation. There are typically two generations of sulphides inmantle peridotites, which typically host the bulk PGE in mantleperidotites: One is primary (residues after partial melting), andthe other is metasomatic, probably introduced by infiltration ofbasaltic melts (Alard et al., 2000). The two different genera-tions of sulphides have considerably different PGE concentra-tions and distribution patterns. These observations demonstratethat it is difficult to discuss the distribution of PGE in mantlexenoliths given the ubiquitous presence of the two differentpopulations of sulphides (Alard et al., 2002; Lorand et al.,2004). Because of these complexities, we studied the occur-rence of the sulphides in our samples with great care. More thanninety thin sections from the studied samples show that over95% of the sulphides occur as inclusions in olivine and enstatite(see Fig. 2C), representing residues after partial melting. Thus,the whole-rock PGE contents of these peridotites are believedto reside largely in the primary sulphides and to reflect originalmantle compositions.

The good positive correlation between Ir and Ru, and moresubtle positive correlations of Ir with Rh through Pt to Pd (seeFig. 9), reflect a decrease in compatibility from IPGE to PPGEin the peridotite xenoliths studied. This suggests that Ir and Ruare generally compatible during melt extraction from mantle

Fig. 12. Plot of Ni/Cu vs. Pd/Ru for peridotite xenoliths. Fields ofgarnet harzburgite, depleted sp. lherzolite, fertile sp. lherzolite, komat-iite, ocean basalt, and continental tholeiite are drawn after Zhou et al.(2000, 2001).

peridotites, whereas Rh, Pt, and Pd are slightly incompatible

(Brandon et al., 1996; Handler and Bennett, 1999, and refer-ences therein). This interpretation provides a possible explana-tion for the highly fractionated PGE patterns of the peridotitexenoliths observed in this study (see Fig. 8). The strong Irdepletion in samples from Eastern China (Orberger et al., 1998,and references therein) is ascribed to Ir retention in the uppermantle reflected by the peridotite xenoliths, owing to the highmetal/silicate distribution coefficient of Ir at fO2 � IW�1.

Super-chondritic Ru/Ir, Rh/Ir, and Pd/Ir ratios have nowbeen documented in a large number of pristine mantle samplesfrom various tectonic settings as well as in ophiolitic andabyssal peridotites, arguing for primary variations of highlysiderophile element (HSE) abundances in the terrestrial uppermantle (Lorand et al., 2000). Recent studies show that there isno systematic relationship between the contents of Ir and otherPGE in xenolithic and massive peridotites, in the degree offertility of the host rock (amount of Cpx) or in the geologicsetting (Schmidt et al., 2000). The PGE compositions of theserocks can be explained by melt depletion accompanying or

# #
Fig. 13. Plots of Mg vs. Al2O3 (A) and Ni vs. Co (B) and Cr (C)of the Xinyang diopsides.


followed by mixing of depleted residues with sulfides, with orwithout the addition of basaltic melt (Rehkamper et al., 1999).Good positive correlations between CaO and IPGE (e.g., Ir andRu), and more subtle positive correlations of CaO with PPGE(e.g., Rh and Pd) exist in the fertile Shanwang peridotites. Incontrast, CaO shows good positive correlations with PPGE butslightly weaker positive correlations with IPGE in the refrac-tory samples from Xinyang and Hebi (Fig. 16). This is consis-tent with the observed similarity between the Cenozoic Hebiand Mesozoic Xinyang mantle (refractory with carbonatiticmetasomatism) and the differences between the Hebi samplesand the Cenozoic Shanwang peridotites (fertile with silicatemetasomatism).

6.6. Tectonic Implications for Mantle ReplacementBeneath the North China Craton

The most significant feature of ancient mantle is the presenceof depleted harzburgites with very Cpx-poor lherzolites (Boyd,1996; Griffin et al., 1999), which clearly distinguishes it fromPhanerozoic mantle. The dominant lherzolites become progres-sively less depleted from Archean through Proterozoic to Pha-nerozoic time (Griffin et al., 1998a). Mercier and Nicolas(1975) and Harte (1977) noted that some mantle xenoliths haveundergone more than one cycle of recrystallization, passingfrom coarse-grained porphyroclastic to granoblastic textures.The diverse textures of peridotitic xenoliths provide usefulinformation about the effects of tectonic processes in the man-tle and are therefore important for understanding mantle evo-lution (O’Reilly et al., 1989).

The lithosphere beneath the eastern part of the NCC inPaleozoic time was thick with a cool geotherm (Griffin et al.,1992; Lu and Zheng, 1996) and consisted mainly of depletedlherzolite and harzburgite (Griffin et al., 1998b; Zheng J. andLu, 1999). The early Mesozoic lithospheric mantle at the south-ern margin of the NCC, sampled by the Xinyang xenoliths,

Fig. 14. Plots of Ti/Eu vs. (La/Yb)n of the Xinyang diopsides. Datasources: same as Figure 3. “Carbonatitic” and “silicate” metasomatismafter Coltorti et al. (1999).

consisted of highly depleted peridotites with high-Cr# and

high-Mg# diopsides, high whole-rock Ni/Cu ratios, and lowRh/Ir and Pd/Ir ratios relative to the primary mantle. Thesesamples have porphyroclastic, sheared, and fine-grained tex-tures. The obviously negative Ce and HFSE anomalies in theserocks indicate that the depleted cratonic mantle experiencedstrong addition of SiO2 and H2O. Subduction of the YangtzeCraton along the southern margin of the NCC in late earlyPermian–early Jurassic (e.g., Yin and Nie, 1993; Li, 1994)could have been responsible for both the deformation and theaddition of SiO2 and H2O.

The Cenozoic Hebi spinel harzburgites/lherzolites are typi-cally coarse grained with porphyroclastic textures and havechemical characteristics similar to those of garnet peridotitesfrom the Archean Kaapvaal craton and the Paleozoic Mengyinand Fuxian kimberlites (Zheng J., 1999). These peridotites havelow modal contents of Cpx and spinel, high Opx/Ol ratios, highMg# and Cr# values of diopside, high whole-rock Ni/Cu ratios,and low Rh/Ir and Pd/Ir ratios. They also show strong enrich-ment of trace elements in diopsides, indicating that the upper-

Fig. 15. Plots of (Pd/Ir)n vs. (La/Yb)n (A), (Ce/Sm)n (B), and(Nb/La)n (C) of peridotite xenoliths.

), Pt (D


most part of the Archean lithospheric mantle (Zheng J. et al.2001) escaped the deformation and asthenospheric erosion thatoccurred beneath the interior of the NCC. The Cenozoic Shan-wang peridotites with fine-grained and foliated textures arefertile, as indicated by their high CaO � Al2O3 contents, highPd/Ir ratios, and low La/Yb and Ni/Cu ratios. Such fertilematerials now make up much of the lithospheric mantle be-neath the eastern part of the NCC (Griffin et al., 1998b; Chen

Fig. 16. Plots of CaO vs. Ir (A), Ru (B), Rh (C

et al., 2001; Gao et al., 2002; Wu et al., 2003), especially along

the translithospheric Tanlu fault zone (Xu Y. et al., 1996),which acted as an important channel for the asthenosphericupwelling (Zheng J. et al., 1998, 2004c).

Late Mesozoic (e.g., early Cretaceous) basaltic rocks in theinterior of the NCC are characterized by an EM1-like signature,in contrast to the EM2-like character of the rocks adjacent tothe collision (Guo et al., 2003; Fan et al., 2004). Zhang H. et al.(2002) interpreted the basalts (125–116 Ma) to have originated

), Pd (E), and Au (F) of peridotite xenoliths.

from a Paleozoic mantle that had undergone extensive interac-


tion with a melt derived from subducted crust. Depleted peri-dotitic xenoliths are absent in the Neocene basalts (Zheng J. etal., 1998, 2004c; Xu X. et al., 1998), implying that the depletedmantle was removed before the Neocene. Therefore, we inter-pret the Shanwang peridotites as fragments of newly accretedlithospheric mantle derived from upwelling asthenospheric ma-terials during the late Mesozoic to Eocene. The asthenosphericupwelling may be relevant to the behavior of the cratonicsubcontinental mantle during episodes of major continentalbreakup for eastern North China (Wilde et al., 2003) andcoincided with the development of the large basins and strongcrust-mantle interaction recorded in Sr-Nd isotopes of the lateMesozoic–Tertiary magmatic rocks (Zhang H. et al., 2002,2003; Guo et al., 2003; Xu Y. et al., 2004).

7. CONCLUSIONS

High IPGE, low PPGE/IPGE, and good correlations of CaOwith IPGE and PPGE in the Cenozoic Hebi and MesozoicXinyang xenoliths suggest that these samples were derivedfrom cratonic mantle. The Cenozoic Shanwang peridotites dif-fer significantly from those of Hebi and Xinyang and werederived from noncratonic mantle. The differences support theprevious suggestion that the buoyant refractory continental keelwas heterogeneously replaced by younger more fertile lithos-pheric mantle in eastern China.

Early Mesozoic peridotite xenoliths from Xinyang on thesouthern margin of the North China Craton consist of harzbur-gites enriched in LREE, suggesting a highly refractory mantlemodified by carbonatitic metasomatism. The Cenozoic Hebiperidotite xenoliths are harzburgites, believed to represent shal-low relics of the cratonic mantle, whereas the Cenozoic Shan-wang xenoliths are lherzolites representing newly accreted fer-tile lithospheric mantle derived from upwelling asthenosphericmaterials during late Mesozoic to Eocene time.

Patches and small veins of glass with very fine-graineddiopside and apatite and the presence of very thin reactionlayers of Ti-oxides and phlogopite coating minerals probablycontrolled the trace element compositions of the “dry” mantlewithin the craton. However, low LREE, Pd, and Pt contents,low Ce/Sm, Nb/La, and Mg/Si ratios, high HREE contents, andprominent negative HFSE and Ce anomalies of the early Me-sozoic Xinyang mantle record addition of SiO2- and H2O-richfluids produced from the subducted continent materials of theYangtze Craton along the margin of the North China Craton.

Acknowledgments—The authors thank Ms. Xiao Fu of the Universityof Hong Kong for her valuable assistance with the geochemical anal-yses, and Professors Sue O’Reilly and Bill Griffin of Macquarie Uni-versity for their assistance in the collection and interpretation of themineral data. Two anonymous reviewers and Dr. Martin Menzies(Associate Editor) are thanked for their constructive comments. Thisstudy was supported by the Chinese Nature Science Funding(40425002 and 40273001) and “973” project (2003CB716500) and theHong Kong Research Grants Council (to MS).

Associate editor: M. Menzies

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