Re–Os isotopes of sulfides in mantle xenoliths from eastern China: Progressive modification of lithospheric mantle

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Lithos 102 (2008) 43 – 64 www.elsevier.com/locate/lithos

Re–Os isotopes of sulfides in mantle xenoliths from eastern China: Progressive modification of lithospheric mantle Xisheng Xu a,b,⁎, William L. Griffin b , Suzanne Y. O'Reilly b , Norman J. Pearson b , Hongyan Geng a , Jianping Zheng c a

b

State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, China ARC National Key Centre for the Geochemical Evolution and Metallogeny of Continents, Macquarie University, NSW 2109, Australia c State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China Received 12 October 2006; accepted 15 June 2007 Available online 27 June 2007

Abstract In situ Re–Os isotopic data for sulfide grains in mantle-derived peridotite xenoliths from eastern China demonstrate a close temporal linkage between crustal tectonism and fluid-migration events in the subcontinental lithospheric mantle (SCLM). In the Cathaysia block, TRD and TMA ages of sulfides with 187Re/188Os b 0.11 cluster in four groups: Paleoproterozoic (∼ 1.8), Mesoproterozoic (∼ 1.3 to 1.5 Ga) and Neoproterozoic (0.9 Ga and 0.6 Ga), corresponding to known major crustal growth events. In the Sino-Korean block, the most robust TRD and TMA ages from sulfides, and some published whole-rock data, indicate that the earliest SCLM formed together with the oldest Archean crust and was modified in Paleoproterozoic time (ca 1.8 Ga), corresponding to the collision between the eastern and western parts of the block. Meso- to Neoproterozoic ages (ca 1.4 Ga, 0.9 Ga, 0.6 Ga) record younger thermal events, the latest of which also is known from zircon ages in lower-crustal xenoliths. The scarcity of Archean sulfide- and whole-rock model ages may reflect widespread Proterozoic modification of the SCLM, but may also be due to preferential sampling of young SCLM by volcanoes situated along zones of asthenospheric upwelling, controlled by a network of major shear zones. Widespread Mesozoic magmatism in the Cathaysia block may be represented by abundant mantle sulfides with mildly superchondritic 187Os/188Os and “future” model ages. This would imply that the sublithospheric mantle has developed a superchondritic Re/Os over perhaps the last 1 Ga, requiring its isolation from the convecting asthenosphere. The SCLM beneath eastern China has had a very complex history, and is now a mixture of refractory and fertile mantle domains with different ages, modified during a number of events. © 2007 Elsevier B.V. All rights reserved. Keywords: Os isotopes; Mantle sulfides; Re–Os mantle ages; Eastern China lithospheric mantle

1. Introduction ⁎ Corresponding author. State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, China. Tel.: +86 25 83592185; fax: +86 25 83686016. E-mail addresses: [email protected] (X. Xu), [email protected] (W.L. Griffin), [email protected] (S.Y. O'Reilly). 0024-4937/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2007.06.010

Eastern China is a key region for understanding the relationships between the subcontinental lithospheric mantle (SCLM) and tectonic activity in the overlying crust. Most regions of Archean crust worldwide are underlain by thick roots of cool, depleted SCLM, and

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X. Xu et al. / Lithos 102 (2008) 43–64

the persistence of these roots through time attests to the difficulty of modifying them. However, in the eastern Sino-Korean block, differences in paleo-geotherms and SCLM composition, recorded in mantle xenoliths from Paleozoic kimberlites and Cenozoic basalts, suggest that relict ancient lithosphere may persist only at shallow depths, and may be underlain by, or interleaved with, more fertile lithosphere emplaced in Phanerozoic time (Griffin et al., 1998). The removal or modification of the older lithospheric mantle has been interpreted as a Mesozoic process (Menzies et al., 1993; Griffin et al., 1998), and this interpretation is supported by the evolution of isotopic signatures in Mesozoic–Cenozoic basalts in the region (e.g. Zhang et al., 2003). Lithosphere thinning since Mesozoic time has also been proposed for other parts of eastern China, including the Cathaysia block (Wu et al., 2003; Xu et al., 2003). However, the mechanism, extent and timing of lithospheric thinning in eastern China are still unclear. A better understanding of the lithosphere-modification processes requires a detailed history of events in the SCLM beneath these different crustal blocks. The Re–Os isotopic system in mantle-derived xenoliths has been widely used to estimate the age of meltdepletion events in the SCLM (e.g. Carlson and Irving, 1994; Peslier et al., 2000; Lee et al., 2000, 2001; Schmidt and Snow, 2002). In principle, partial melting of the mantle, leading to the creation of SCLM, leaves a Re-depleted residue that retards the growth of the 187 Os/188 Os ratio, compared to the evolution of the convecting upper mantle (e.g. Walker et al., 2002). The Os budget of mantle peridotites is dominated by trace sulfide phases. The analysis of Re–Os systematics in single sulfide grains by in situ LA-MC-ICPMS methods has demonstrated that different sulfide populations in mantle-derived xenoliths can display a range of Re–Os model ages (Alard et al., 2000; Griffin et al., 2002, 2004). Only the oldest of these model ages, if any, can record the original melt-depletion event that led to stabilisation of a lithospheric volume. For the younger model ages to be meaningful, these sulfides must have been introduced into the lithosphere together with fluids or melts derived from the convecting mantle (“asthenosphere”), and carried the Os-isotope signature of that reservoir. Correlations between the model ages of such sulfide populations and the ages of major events in the overlying crust (Pearson et al., 2002; Griffin et al., 2002, 2004) suggest that this assumption is broadly valid. This approach can thus be used, with due caution, to date both melt-depletion events (residual sulfides) and metasomatic episodes (introduced sulfides), and to track the series of events that has affected individual mantle domains.

In this paper we use Re–Os isotopic data for sulfide phases in xenoliths from ten localities across eastern China continent, and from the Penghu Islands (offshore from southeastern China; Wang et al., 2003) to examine the timing of depletion and metasomatism events in the SCLM beneath this complex region. Previously published whole-rock Re–Os data (Gao et al., 2002; Wu et al., 2003; Reisberg et al., 2005; Wu et al., 2006) have been incorporated for comparison, and to demonstrate the modification of the lithospheric mantle. 2. Geological background and samples 2.1. Geological background The tectonic framework of eastern China is made up of the Cathaysia, Yangtze, Sino-Korean and XingMeng blocks, from south to north (Fig. 1). The lateMesoproterozoic Grenvillian orogeny, which led to the

Fig. 1. A sketch map of the tectonic framework of eastern China, showing localities sampled in this study.

X. Xu et al. / Lithos 102 (2008) 43–64

formation of the supercontinent Rodinia, joined the Cathaysia block with the Yangtze block (Charvet et al., 1996; Li, 1998; Li et al., 2002) (Fig. 1). The earlyMesozoic Qinling-Dabie orogenic belt is the boundary between the Yangtze and Sino-Korean blocks in the west (e.g. Li, 1998). The Sulu orogenic belt is interpreted as the surface expression of the suture in the east but the location of the deep boundary between these two blocks near Nanjing is still controversial (Li, 1994; Gao et al., 2002; Reisberg et al., 2005). The southern edge of the Central Asian Orogenic Belt is the boundary between the Sino-Korean and Xing-Meng blocks, which were joined near the end of the Jurassic by the closure of the Mongol–Okhotsk Ocean (Li, 1998). Outcrops of the Cathaysia Proterozoic basement include the Badu group, Mayuan group, Yunkai group and the Lanhe gneiss. This basement was overprinted by Caledonian, Indosinian and Yanshanian events (Xu et al., 2005 and references therein). The late Mesozoic Yanshanian volcanic-intrusive magmatism was a major regional event. The Yangtze block has a Paleoproterozoic to Mesoproterozoic basement overlain by a Neoproterozoic (Sinian) to Cenozoic cover; zircon data (Zheng et al., 2006) indicate that the deeper crust contains large volumes of Archean material. The basement of the Sino-Korean block contains the Ordos nucleus in the west and the Liaolu nucleus in the east (Wang and Mo, 1995; Wu et al., 2005); it was consolidated about 1.85 Ga ago (Zhao et al., 2000, 2005). Several Ordovician diamondiferous kimberlites are found in the eastern part. Mantle-derived magmas were injected into the lower crust in late Mesozoic time (Wu et al., 2005). The Xing-Meng block in northeast China is a Paleozoic fold belt, representing the eastern part of the Paleozoic Central Asian Orogenic Belt, located between the Siberian craton and the Sino-Korean craton (Jahn et al., 2000).

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spinel = 0.074. Olivine and pyroxenes contain fluid inclusions, showing the presence of a fluid phase. Phlogopite and amphibole in microcracks or on grain boundaries between olivine and pyroxene provide other evidence of metasomatism. The Mingxi locality (Fig. 1) contains abundant fertile (garnet 4.7–10.8%, cpx N10%) garnet ± spinel lherzolite xenoliths (Fan and Hooper, 1989; Qi et al., 1995) in Miocene to Pliocene alkali basalts (b 10 Ma Chen and Zhang, 1992). Other xenoliths include spinel lherzolite, spinel pyroxenite, garnet pyroxenite and websterite. The spinel peridotites from Mingxi may have either high (N10%) or low (b5%) clinopyroxene contents. Some peridotites from Mingxi contain rare phlogopite, reflecting modal metasomatism. Details of mineral compositions in xenoliths from Mingxi, Niutoushan and Qilin (see below) are given by Xu et al. (1998, 2000). 2.2.1.2. Niutoushan and Qilin. Niutoushan (Fig. 1) is a late Tertiary (K–Ar ages 16.6–19.2 Ma; Chen and Zhang, 1992) basalt vent 0.5 km2 in area. The spinel lherzolites are less than 10 cm in diameter with most about 5 cm; all have coarse equant (1–5 mm) microstructures. The mineral assemblage is olivine (40–54%), orthopyroxene (23–41%), clinopyroxene (6.6–21%) and spinel (ca 4.5%). The Qilin pipe (Fig. 1) is composed of mafic to ultramafic breccia, intruded into Mesozoic granitic rocks and cut by basaltic dikes. It has been assigned a Tertiary age (BGMRGP, 1990). Deep-seated xenoliths can be divided into three groups (Xu, X.S. et al., 1996): spinel peridotites, metapyroxenites including garnet websterites, garnet clinopyroxenites, spinel pyroxenites (re-equilibrated basaltic compositions), and granulites. Some spinel peridotites contain amphibole and carbonate, reflecting modal metasomatism. 2.2.2. Yangtze block

2.2. Host basalts and xenolith samples 2.2.1. Cathaysia block 2.2.1.1. Anyuan and Mingxi. The Anyuan ultramafic breccia pipe (Fig. 1) contains spinel peridotite and garnet peridotite mantle xenoliths, as well as clinopyroxene and phlogopite megacrysts. A K–Ar age of 44 Ma for phlogopite megacrysts and a Rb–Sr whole rock age of 37 Ma were reported by Ye et al. (2001, 2002). Peridotite xenoliths (b 10 cm in their long dimension, most ca 3 cm) mostly have a porphyroclastic microstructure; The rocks are relatively fertile, with Mg# (Mg/ (Mg + Fe)) of olivine = 0.90, and Cr# (Cr/(Cr + Al)) of

2.2.2.1. Nanjing area. About twenty Cenozoic basaltic volcanic cones occur near Nanjing; Panshishan, Tashan and Fangshan have provided xenoliths for this study. These three nearby cones erupted during Miocene time (K–Ar ages of 16 Ma for Tashan, 9 Ma for Fangshan;Chen and Peng, 1988). The alkali basalts contain megacrysts of aluminous augite and anorthoclase, and abundant large, fresh peridotite xenoliths. Most of these are spinel lherzolites with porphyroclastic or heteroblastic texture. Their mineral assemblage is olivine (42–70%), orthopyroxene (20–45%), clinopyroxene (5–20%) and spinel (1– 5%); a few samples are harzburgite. Chen et al. (1994) present the mineral compositions for these peridotites.

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Table 1 Re–Os analyses of sulfide inclusions in mantle xenoliths from eastern China a Sample no

Age (Ma)

Cathaysia block Anyuan A3

44

A7 A8

A13

Qilin QL26

Re/188Os

1SE

187

Os/188Os

1SE

A3-1 A3-1b A3-1c A7-1 A8(1-1) A8(1-2) A8(1-2) A13(1) A13(2) A13(4) A15(1)

0.0097 0.0207 0.0030 0.1931 0.6403 0.7018 0.7264 0.1523 0.0350 0.1672 0.6954

0.0007 0.0012 0.0003 0.0020 0.0072 0.0040 0.0130 0.0055 0.0055 0.0018 0.0570

0.1226 0.1155 0.1204 0.1261 0.1442 0.1465 0.1473 0.1392 0.1187 0.1251 0.1691

0.0022 0.0008 0.0005 0.0010 0.0012 0.0007 0.0007 0.0048 0.0014 0.0005 0.0046

MX3-1 MX3-2 MX3-2

0.0581 0.1032 0.0964

0.0039 0.0006 0.0005

0.1229 0.1128 0.1140

QL26-1 QL26-1-1 QL26-2 QL26-3 QL26-4 QL26-5 QL27-1 QL27-1-1 QL27-2 QL27-3 QL27-4 QL27-4-1 Q9332-1 Q9332-2 Q9335-1 Q9335-1-1 Q9335-3 Q9335-4 Q9335-5 Q9335-7 Q9347-1 Q9347-2

0.2406 0.5204 0.3792 1.0350 1.1800 1.3610 0.2963 0.1841 0.4317 0.1431 0.3175 0.3276 0.0576 0.0167 0.5009 0.0991 0.2740 0.7341 0.1573 0.4121 0.2744 0.1124

0.0320 0.0120 0.0056 0.0600 0.0440 0.0580 0.0016 0.0008 0.0072 0.0002 0.0022 0.0001 0.0011 0.0001 0.0140 0.0048 0.0096 0.0062 0.0093 0.0092 0.0015 0.0011

NTS-1-5

0.2705

0.0060

Initial γOs

Pt b (ppm)

Re/Os

8.19 113.39 19.84 29.19 11.55 13.41 12.52 1.56 18.09 28.11 3.03

38.88 17.56 21.54 − 2.22 5.15 6.63 8.25 1.01 18.18 6.25 − 2.14

0.002 0.004 0.001 0.041 0.136 0.149 0.154 0.032 0.007 0.036 0.148

208 147 102

6.00 18.00 4.00

5.88 5.88 11.76

0.012 0.022 0.020

−15940 −10838 39229 − 712 − 840 604 3406 2287 − 6398 2965 9449 10382 2057 2091 − 4010 668 4102 − 1860 613 − 7124 − 595 1554

7101 2036 10244 406 569 444 147 52 3346 192 421 363 240 144 1207 572 747 241 294 16808 237 88

1.66 4.95 13.52 1.78 2.39 2.20 21.10 22.58 12.75 33.83 16.00 24.60 15.96 235.74 2.00 13.00 7.74 8.79 32.74 8.17 25.00 8.60

12.65 18.30 11.19 0.45 2.17 0.73 24.47 22.31 11.63 2.46 19.61 30.00 22.77 96.80 7.84 8.52 8.64 12.11 26.76 5.78 33.33 17.65

0.051 0.111 0.081 0.220 0.251 0.289 0.063 0.039 0.092 0.030 0.067 0.070 0.012 0.004 0.106 0.021 0.058 0.156 0.033 0.088 0.058 0.024

2715

1035

7.86

1.63

0.057

TRD

1SE

TMA

1SE

−3.42 −9.00 −5.13 −0.75 13.23 14.99 15.64 9.58 −6.49 −1.52 32.82

655 1692 975 152 − 2552 − 2902 − 3031 − 1835 1229 297 − 6558

327 118 73 171 194 111 162 722 208 83 1461

670 1780 982 252 4180 3777 3645 − 3011 1341 477 8044

335 124 73 284 318 145 195 1186 227 133 1792

0.0012 0.0008 0.0005

−3.19 − 11.20 −10.22

607 2093 1912

179 110 78

708 2798 2502

0.1646 0.1074 0.1061 0.1195 0.1162 0.1367 0.1208 0.1186 0.1240 0.1139 0.1126 0.1130 0.1150 0.1134 0.1206 0.1236 0.1180 0.1169 0.1245 0.1259 0.1283 0.1194

0.0150 0.0031 0.0018 0.0042 0.0073 0.0071 0.0003 0.0002 0.0014 0.0009 0.0005 0.0005 0.0014 0.0009 0.0017 0.0029 0.0015 0.0013 0.0012 0.0025 0.0005 0.0004

29.65 −15.44 −16.50 −5.96 −8.62 7.52 −4.84 −6.63 −2.38 −10.28 − 11.32 − 11.04 −9.41 −10.71 −5.05 −2.63 −7.11 −8.03 −1.94 −0.90 1.01 −5.94

− 5892 2868 3059 1130 1622 − 1432 921 1255 460 1929 2118 2067 1768 2006 961 507 1342 1513 378 181 − 181 1126

2625 539 799 645 1099 1052 40 29 240 125 94 72 206 138 289 434 244 196 181 427 72 64

0.1209

0.0023

−4.79

912

347

Os b (ppm)

5

10

QL27

Q9332 Q9335

Q9347 Niutoushan NTS-1

187

16

X. Xu et al. / Lithos 102 (2008) 43–64

A15 Mingxi MX3

Sulfide grain no

NTS-2

NTS-6 NTS-7

NTS-8 NTS-9 NTS-11

KPH9822

KPH9826

KP0201

KP0214

KP0215

0.2589 0.3028 0.1468 0.1782 0.7766 0.3744 0.0166 0.8375 0.5794 0.3680 0.6673 0.5613 0.6805 0.4415 0.4697 0.4796

0.0069 0.0023 0.0009 0.0011 0.0022 0.0085 0.0008 0.0340 0.0082 0.0062 0.0091 0.0020 0.0022 0.0080 0.0066 0.0064

0.1203 0.1152 0.1204 0.1223 0.1147 0.1144 0.1182 0.1139 0.1098 0.1136 0.1058 0.1119 0.1138 0.1224 0.1161 0.1290

0.0046 0.0005 0.0004 0.0007 0.0032 0.0027 0.0047 0.0058 0.0018 0.0017 0.0014 0.0012 0.0036 0.0031 0.0011 0.0018

− 5.28 − 9.25 − 5.19 − 3.65 − 9.74 − 9.91 − 6.86 − 10.42 − 13.61 − 10.59 − 16.75 −11.88 − 10.47 − 3.62 − 8.59 1.60

1003 1744 987 701 1841 1866 1297 1966 2545 1990 3113 2230 1973 692 1623 −286

690 78 54 105 479 703 695 880 290 443 232 180 536 488 228 254

2749 6736 1539 1241 −2004 22664 1352 −1841 −6138 20073 −5000 −5956 −2924 −7384 − 10501 1554

1892 301 84 186 521 8540 724 824 700 4467 372 480 795 5207 1473 1378

5.28 24.30 20.60 1.20 5.00 3.80 30.83 1.60 7.40 6.00 5.20 6.80 2.60 6.00 7.20 6.00

3.49 18.43 11.18 6.86 0.78 1.94 5.00 2.75 13.43 0.31 0.00 2.94 1.57 0.18 10.39 7.25

0.055 0.064 0.031 0.038 0.165 0.080 0.004 0.178 0.123 0.078 0.142 0.119 0.145 0.094 0.100 0.102

KPH9810-s1 KPH9816/3-1-s1 KPH9816/3-1-s1 KPH9816/3-1-s4 KPH9822-s0

0.1417 0.2168 0.1395 0.2951 0.7094

0.0015 0.0024 0.0017 0.0110 0.0036

0.1228 0.1160 0.1194 0.1061 0.1187

0.0014 0.0018 0.0008 0.0015 0.0015

− 3.29 − 8.66 − 5.97 − 16.47 − 6.61

630 1629 1130 3054 1251

210 267 119 383 226

964 3469 1716 10736 −1644

320 569 181 1348 297

70.00 10.00 20.00 43.00 36.00

6.00 8.00 25.00 10.00 0.00

0.030 0.046 0.030 0.063 0.151

KPH9822-s4 KPH9822-s4-2 KPH9826-s1 KPH9826-s2 KPH9826-s4 KP0201-s2 KP0201-s4 KP0201-s5 KP0201-s7 KP0214-11-s0 KP0214-11-s3-1 KP0214-11-s6 KP0214-16-s0-1 KP0215-s1 KP0215-s4 KP0215-s5 KP0215-s6 KP0215-s7 KP0215-s8

0.7240 0.1176 0.1509 0.2931 0.5683 0.7777 0.5252 1.0776 0.8974 0.4320 0.7201 0.7004 0.1526 0.4560 0.2838 0.2111 0.3364 0.1914 0.4089

0.0069 0.0022 0.0031 0.0038 0.0390 0.0110 0.0091 0.0160 0.0140 0.0200 0.0069 0.0110 0.0029 0.0120 0.0072 0.0060 0.0051 0.0022 0.0024

0.1091 0.1182 0.1190 0.1239 0.1327 0.1202 0.1256 0.1123 0.1210 0.1212 0.1270 0.1240 0.1155 0.1249 0.1216 0.1167 0.1195 0.1172 0.1216

0.0008 0.0007 0.0011 0.0005 0.0032 0.0025 0.0023 0.0007 0.0014 0.0013 0.0018 0.0011 0.0018 0.0016 0.0010 0.0005 0.0007 0.0008 0.0006

− 14.18 − 6.91 − 6.28 − 2.45 4.44 − 5.44 − 1.15 −11.72 − 4.83 − 4.60 − 0.08 − 2.44 − 9.04 − 1.69 − 4.26 − 8.10 − 5.92 − 7.71 − 4.28

2640 1305 1189 472 −837 1033 227 2192 920 875 25 470 1700 329 812 1527 1122 1454 816

124 104 164 70 510 381 371 116 216 613 4461 173 267 260 158 82 131 120 292

−3434 1832 1886 1692 2018 −1098 −690 −1321 −732 − 12854 −2 −608 2710 −2386 2689 3159 6519 2735 − 87817

161 146 260 252 1229 404 1127 70 172 9001 340 223 425 1885 523 170 759 225 31482

83.00 95.00 40.00 43.00 10.00 6.00 5.00 87.00 110.00 20.00 5.00 10.00 48.00 9.00 16.00 63.00 12.00 5.00 16.00

40.00 75.00 82.00 4.00 12.00 0.00 1.00 154.00 187.00 20.00 14.00 25.00 330.00 16.00 13.00 29.00 31.00 1.00 4.00

0.154 0.025 0.032 0.062 0.121 0.165 0.112 0.229 0.191 0.092 0.153 0.149 0.032 0.097 0.060 0.045 0.071 0.041 0.087

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X. Xu et al. / Lithos 102 (2008) 43–64

Penghu Kueipi peridotites KPH9810 KPH9816

NTS-1-6 NTS-2-1 NTS-2-4 NTS-2-6 NTS-2-7 NTS-2-8 NTS-6-2 NTS-7-1 NTS-7-2 NTS-7-3 NTS-7-4 NTS-8-1 NTS-8-2 NTS-9-1 NTS-11-1 NTS-11-2

(continued on next page) 47

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Table 1 (continued) Sample no

Age (Ma)

Tungchiyu peridotite TC0242

Kueipi pyroxenite KPH9801 Tungchiyu pyroxenites TC0248

Yangtze block Around Nanjing PS1-1 PSS-4 PSS-5 PSS-10 PSS-12 TS2

16

16

187

Re/188Os

1SE

187

Os/188Os

1SE

Initial γOs

TRD

1SE

TCHUR

1SE

Os (ppm)

Pt (ppm)

Re/Os

TC0242/2-s6 TC0242/2-s11 TC0242/2-s12 TC0242/2-s13

0.0632 0.2528 0.4753 0.2770

0.0005 0.0018 0.0039 0.0042

0.1139 0.1157 0.1198 0.1128

0.0011 0.0010 0.0009 0.0017

− 10.28 − 8.90 − 5.71 − 11.19

1929 1674 1082 2094

162 149 147 260

2280 4390 − 6203 6470

191 392 841 804

32.00 9.00 6.00 5.00

122.00 9.00 17.00 9.00

0.013 0.054 0.101 0.059

KPH9801/2-1-s1 KPH9801/2-1-s2

0.2150 0.2294

0.0059 0.0034

0.1204 0.1148

0.0027 0.0014

− 5.19 − 9.60

986 1804

404 210

2087 4106

855 478

11.00 34.00

10.00 141.00

0.046 0.049

TC0248-s6 TC0248-s11 TC0248-s12 TC0248-s13 TC0248-s16 TC0248-s18 TC0248-s30 TC0248-s31 TC0223-s1 TC0223-s2 TC0223-s3 TC0223-s7 TC0223-s8 TC0223-s11 TC0223-s12 TC0223-s13 TC0223-s14 TC0223-s15

0.2172 0.1909 0.2149 0.4387 0.1204 0.2590 0.1536 0.0824 0.2734 0.3746 0.0252 0.1951 0.0356 0.3363 0.0202 0.1698 0.2087 0.1871

0.0024 0.0023 0.0026 0.0110 0.0010 0.0021 0.0013 0.0011 0.0054 0.0049 0.0002 0.0030 0.0009 0.0017 0.0004 0.0023 0.0036 0.0012

0.1187 0.1223 0.1224 0.1133 0.1186 0.1186 0.1170 0.1193 0.1274 0.1241 0.1212 0.1272 0.1231 0.1247 0.1214 0.1281 0.1214 0.1197

0.0006 0.0005 0.0014 0.0023 0.0008 0.0014 0.0008 0.0020 0.0015 0.0009 0.0005 0.0016 0.0010 0.0010 0.0005 0.0020 0.0010 0.0009

− 6.53 − 3.69 − 3.61 − 10.82 − 6.59 − 6.62 − 7.86 − 6.03 0.31 − 2.30 − 4.53 0.17 −3.03 − 1.82 − 4.37 0.88 − 4.40 − 5.74

1235 706 692 2027 1247 1251 1482 1143 − 49 445 862 − 22 582 355 833 − 158 839 1088

83 68 210 694 111 209 111 297 190 159 74 186 141 154 67 289 150 134

2642 1325 1462 − 27938 1767 3433 2373 1431 − 183 6088 919 − 55 637 2079 876 − 283 1718 2009

178 127 444 9569 158 574 178 371 702 2178 79 465 155 901 70 519 308 248

23.00 27.00 20.00 8.00 44.00 15.00 16.00 23.00 5.00 3.00 36.00 3.00 10.00 9.00 46.00 4.00 4.00 7.00

37.00 50.00 20.00 13.00 15.00 27.00 1.00 8.00 8.00 4.00 30.00 3.00 1.00 20.00 6.00 7.00 5.00 18.00

0.046 0.041 0.046 0.093 0.026 0.055 0.033 0.018 0.058 0.080 0.005 0.041 0.008 0.071 0.004 0.036 0.044 0.040

PS1-1 PSS-4-1 PSS-4-4 PSS-5(2) PSS-10 PSS-12(1) TS2-1 TS2-1R TS2-2 TS2-2R TS2-3

0.0520 0.1959 0.1687 0.4820 0.0122 0.6023 0.1041 0.3537 0.0294 0.2649 0.1776

0.0020 0.0100 0.0011 0.0100 0.0013 0.0120 0.0070 0.0150 0.0055 0.0069 0.0048

0.1271 0.1229 0.1142 0.1398 0.1450 0.1456 0.1234 0.1253 0.1178 0.1226 0.1226

0.0010 0.0053 0.0037 0.0010 0.0047 0.0020 0.0017 0.0015 0.0002 0.0002 0.0005

0.09 − 3.25 − 10.04 10.00 14.19 14.55 − 2.83 − 1.36 − 7.19 − 3.49 − 3.44

−7 624 1886 − 1917 − 2741 − 2814 544 267 1358 668 659

110 798 546 282 718 347 255 250 31 49 76

− 10 1197 3203 8862 − 2830 5318 727 2081 1462 1909 1163

163 1531 927 1305 741 655 341 1948 33 141 135

18.99 1.39 0.60 9.43 3.55 2.23 13.30 9.40 62.98 27.47 33.55

3.70 2.44 4.61 0.48 42.54 0.26 − 5.50 2.03 1062.60 59.25 0.45

0.011 0.042 0.036 0.102 0.003 0.128 0.022 0.075 0.006 0.056 0.038

X. Xu et al. / Lithos 102 (2008) 43–64

TC0223

Sulfide grain No

TS3

9

Sino-Korean block Damaping DM02-4

15

DM02-5 DM02-6 DM02-11 DM02-13 DM02-15 DM02-17

DM02-19 Hebi Hb028 Hb003

0.0643 0.3950 0.1587 0.3098 0.4132 0.1834 0.6363 1.9867 0.3005 0.4555 0.2028 0.2490 0.2331 0.2665 0.5842 0.5859 1.0675

0.0002 0.0070 0.0036 0.0057 0.0038 0.0034 0.0110 0.0240 0.0009 0.0018 0.0016 0.0005 0.0042 0.0009 0.0061 0.0064 0.0630

0.1238 0.1217 0.1211 0.1196 0.1208 0.1337 0.1517 0.1139 0.1272 0.1375 0.1261 0.1278 0.1273 0.1270 0.1269 0.1272 0.1500

0.0003 0.0026 0.0004 0.0010 0.0006 0.0015 0.0025 0.0014 0.0005 0.0004 0.0003 0.0002 0.0002 0.0004 0.0019 0.0018 0.0039

− 2.47 − 4.20 − 4.63 − 5.83 − 4.93 5.29 19.34 − 10.51 0.15 8.21 − 0.72 0.63 0.22 0.02 − 0.12 0.15 18.00

477 801 882 1105 938 − 1001 − 3773 1998 − 18 − 1567 147 − 110 − 31 6 32 − 19 − 3503

40 907 55 158 325 225 422 216 42 81 44 32 25 61 475 162 682

564 34329 1440 4631 − 47693 − 1867 6001 − 497 − 118 10701 280 − 315 − 97 − 15 − 42 71 2034

48 38861 90 661 16549 420 671 54 279 551 84 91 78 168 626 587 396

88.96 3.02 76.12 11.71 14.73 6.13 2.56 1.06 56.39 19.72 64.04 45.18 38.88 6.81 15.55 15.11 1.31

4.41 0.49 0.74 0.08 1.80 − 0.629 − 3.235 0.46 0.59 − 0.06 1.49 7.55 0.80 1.12 0.34 0.70 − 2.39

0.014 0.084 0.034 0.066 0.088 0.039 0.135 0.422 0.064 0.097 0.043 0.053 0.050 0.057 0.124 0.124 0.227

DM02-4-1 DM02-4-2 DM02-4-2b DM02-4-3 DM02-5-1 DM02-5-2 DM02-6-1 DM02-11-2 DM02-11-5 DM02-13-1 DM02-13-2 DM02-15-2 DM02-15-3 DM02-17-1 DM02-17-2 DM02-17-2-1 DM02-17-3 DM02-17-3-1 DM02-17-4 DM02-17-5 DM02-17-7 DM02-19-2

0.1744 0.1285 0.1325 0.0778 0.6475 1.4729 1.1888 0.0851 0.0469 0.8763 0.2886 0.3415 0.3163 0.1320 0.1977 0.0481 0.1068 0.1184 0.0491 0.0217 0.0622 0.4490

0.0014 0.0077 0.0200 0.0008 0.0040 0.0170 0.0860 0.0014 0.0007 0.0200 0.0150 0.0053 0.0064 0.0160 0.0078 0.0007 0.0016 0.0030 0.0020 0.0019 0.0009 0.0066

0.1152 0.1140 0.1058 0.1169 0.1185 0.1119 0.1229 0.1233 0.1211 0.1307 0.1311 0.1210 0.1173 0.1252 0.1231 0.1214 0.1170 0.1208 0.1178 0.1188 0.1184 0.1248

0.0004 0.0046 0.0032 0.0006 0.0006 0.0016 0.0081 0.0004 0.0004 0.0042 0.0031 0.0005 0.0027 0.0018 0.0025 0.0010 0.0007 0.0014 0.0010 0.0046 0.0008 0.0010

− 9.25 − 10.19 − 16.69 − 7.94 − 6.79 − 12.13 − 3.43 − 2.87 − 4.59 2.76 3.20 − 4.75 − 7.63 − 1.41 − 3.03 − 4.41 − 7.87 − 4.87 − 7.25 − 6.43 − 6.71 − 1.78

1739 1912 3094 1497 1284 2267 657 551 874 − 515 − 599 905 1439 276 583 841 1484 926 1369 1217 1269 346

59 680 520 95 87 243 1292 61 65 593 464 114 416 274 377 149 110 209 142 681 114 164

3028 2783 4551 1848 − 2121 − 852 − 316 695 987 460 − 2191 5700 6428 403 1127 953 2006 1303 1555 1285 1496 − 2882

102 990 765 117 143 91 620 76 74 530 1698 715 1858 400 729 168 148 294 161 719 134 1365

30.00 25.69 25.77 24.00 11.10 10.47 3.57 29.08 54.31 4.10 3.62 14.69 4.49 19.48 7.08 11.99 39.41 13.16 23.67 6.24 28.00 8.98

31.37 142.91 103.60 13.73 25.16 25.55 1.03 42.38 71.87 1.21 3.38 6.63 3.75 40.31 1.68 3.77 21.17 4.28 7.88 19.62 31.37 1.42

0.037 0.027 0.028 0.017 0.138 0.313 0.253 0.018 0.010 0.186 0.061 0.073 0.067 0.028 0.042 0.010 0.023 0.025 0.010 0.005 0.013 0.095

Hb028-1 Hb003-1

0.0010 0.0059

0.0000 0.0006

0.1065 0.1102

0.0007 0.0007

− 16.10 − 13.16

2987 2454

101 108

2994 2489

101 110

372.73 27.27

144.94 255.06

0.000 0.001

X. Xu et al. / Lithos 102 (2008) 43–64

FS2

TS2-4 TS2-5 TS2-2(3) TS2-3(3) TS2-4(3) TS2-(1) TS2-(2) TS3-1 TS3-1-1 TS3(1) TS3-3 TS3-3-1 TS3-3-2 TS3-3-3 FS2-1 FS2-1-1 FS2-2

4

49

(continued on next page)

50

Table 1 (continued) Sample no Xing-Meng block Yitong YT01 YT03

YT07 YT-08

YT09

a b

Sulfide grain No

187

Re/188Os

1SE

187

Os/188Os

1SE

Initial γOs

TRD

1SE

TCHUR

1557 − 1798 −231 − 114 −407 371 763 404 642 −98 583 2681 1905 −30 − 1916 691 897 1567 545 715 949 704

207 738 107 612 478 100 315 132 187 217 76 82 61 397 267 58 359 39 225 71 345 99

2124 24677 −310 −168 −453 377 1879 − 1979 3030 −154 2376 18606 6106 −142 − 16952 1206 2412 4147 986 1562 7441 2467

1SE

Os (ppm)

Pt (ppm)

Re/Os

22.31 14.49 13.12 5.17 10.39 20.86 131.20 20.00 20.00 11.24 24.93 14.00 26.00 0.59 11.01 38.18 6.46 48.00 19.30 33.70 14.28 16.79

20.38 5.43 0.39 112.33 16.94 2.47 59.83 43.73 19.22 34.46 1.05 0.00 0.78 5.93 24.39 0.63 49.22 21.96 43.00 71.12 5.57 12.83

0.023 0.090 0.021 0.026 0.009 0.002 0.051 0.103 0.068 0.029 0.065 0.075 0.060 0.063 0.074 0.037 0.054 0.054 0.039 0.047 0.075 0.062

10 YT01-2 YT03-1 YT03-3 YT05-4-1 YT05-4-2 YT07-2 YT-08-6 YT-08-8 YT-08-8b YT09-1 YT09-1-1 YT09-1-2 YT09-2 YT09-2-1 YT09-2-2 YT09-2-3 YT09-2b YT09-3 YT09(1) YT09(2) YT09(3) YT09(4)

0.1092 0.4253 0.0994 0.1220 0.0403 0.0071 0.2413 0.4831 0.3194 0.1381 0.3059 0.3515 0.2813 0.2945 0.3503 0.1740 0.2553 0.2538 0.1825 0.2206 0.3538 0.2900

0.0017 0.0095 0.0013 0.0024 0.0014 0.0002 0.0049 0.0160 0.0062 0.0097 0.0027 0.0005 0.0027 0.0100 0.0038 0.0110 0.0170 0.0018 0.0020 0.0023 0.0170 0.0037

0.1165 0.1389 0.1286 0.1278 0.1297 0.1245 0.1219 0.1244 0.1227 0.1277 0.1231 0.1087 0.1141 0.1273 0.1397 0.1224 0.1210 0.1164 0.1234 0.1222 0.1207 0.1223

0.0014 0.0008 0.0007 0.0042 0.0032 0.0007 0.0021 0.0007 0.0012 0.0015 0.0005 0.0005 0.0003 0.0033 0.0015 0.0003 0.0023 0.0002 0.0015 0.0005 0.0005 0.0006

− 8.27 9.39 1.27 0.65 2.19 − 1.91 − 4.00 − 2.09 − 3.35 0.57 − 3.04 − 14.41 − 10.15 0.21 10.00 − 3.61 − 4.71 − 8.32 − 2.83 −3.74 − 4.99 − 3.68

The parameters used in the calculation are λRe = 1.666 × 10− 11/year, (187Re/188Os)CHUR = 0.40186 and (187Os/188Os)CHUR,0 = 0.1270. Semiquantitative: measured by comparison of signal with PGE-A.

282 10123 143 902 533 102 775 648 882 342 308 572 197 1847 267 102 965 103 407 155 2706 346

X. Xu et al. / Lithos 102 (2008) 43–64

YT05

Age (Ma)

X. Xu et al. / Lithos 102 (2008) 43–64

2.2.3. Sino-Korean block 2.2.3.1. Damaping. The Cenozoic Hannuoba basalts, including the xenolith-rich locality of Damaping (Fig. 1), form a highland near the northern margin of the Sino-Korean block and on the boundary between the eastern and western parts of that block. Most of the basalts erupted 22 to 10 Ma ago (Chen et al., 2001, references therein). Alkali basalts in the lower part of the lava sequence host abundant deep-seated xenoliths, which are dominated by peridotites, with subordinate pyroxenites and mafic granulites and rare felsic granulites. Most peridotites are four-phase spinel lherzolites; detailed mineral compositions are given by Chen et al. (2001). 2.2.3.2. Hebi. The Hebi locality lies just east of the Trans-North China Orogen that marks the boundary between the eastern and western parts of the SinoKorean block. 4.0–4.3 Ma pipes and dikes of olivine nephelinite (Liu, R. et al., 1992) contain abundant mantle xenoliths, and megacrysts of garnet and clinopyroxene. The spinel peridotites are small (1-6.5 cm), generally coarse-grained and angular. About 1/3 are lherzolites, and 2/3 are highly depleted harzburgites. The harzburgites have high orthopyroxene/olivine ratios, and mineral chemistry that in most respects resembles that of peridotite xenoliths from beneath Archean terrains worldwide (Zheng et al., 2001). 2.2.4. Xing-Meng block 2.2.4.1. Yitong. In the National Volcano Reserve Area of Yitong, there are 16 volcanoes, including the 9.9 Ma basalts of Dongjianshan and Chuandishan (Liu et al., 2001). The ultramafic xenoliths can be assigned to three suites according to their modal composition (Xu, Y. et al., 1996). The lherzolite suite includes spinel lherzolites with protogranular, porphyroclastic to equigranular textures. The pyroxenite suite is characterized by coarse-grained igneous textures and distinctive mineral compositions; they are considered to have crystallised from alkali basalt magmas in the upper mantle. The wehrlite suite (olivine–clinopyroxene) suite originated from interaction between the lherzolite mantle and an infiltrating metasomatic melt.

51

fides are very similar to those described in detail by Alard et al. (2000, 2002). In general, they may be divided into interstitial sulfides and those enclosed in primary minerals, usually olivine and pyroxenes (Lorand and Gregoire, 2006). The “enclosed sulfides” commonly are intergrowths of pyrrhotite, pentlandite and minor chalcopyrite, suggesting low-T exsolution from pre-existing monosulfide solid solutions. They typically, but not always, have higher contents of Os than the interstitial sulfides. Pt commonly is heterogeneously distributed, in the form of micronuggets that appear as “spikes” in the time-resolved analytical signals, whereas Os and Re are more homogeneous. Interstitial sulfide grains may be pyrrhotite-pentlandite intergrowths, or consist only of pyrrhotite or pentlandite. They generally, but not always, have higher Re/Os ratios than the enclosed sulfide grains in the same sample. Both types of sulfides may appear as large (N100 μm diameter) and small grains. In many samples from most localities, interstitial sulfides are very abundant but in grains too small (b 10 μm) to be analysed by laser microprobe. 3. Methods and results Re–Os isotopes were determined in single grains of sulfides using LA-MC-ICPMS. The analytical procedures are described in detail by Pearson et al. (2002) and Griffin et al. (2002) (see Appendix). Reported analytical uncertainties on 187Os/188Os include the propogated uncertainty arising from the correction of 187Re overlap, and uncertainties on TMA model ages include the uncertainties in the measured 187Re/188 Os. In the time-

2.3. Sulfide minerals The abundance of sulfide minerals varies widely between individual xenoliths, and between localities. The microstructural and chemical features of the sul-

Fig. 2. 187Re/188Os–187Os/188Os plots of sulfide samples from eastern China, showing that most are subchondritic. Reference isochrons are shown for sulfides derived from the depleted mantle at 2.5 Ga, 1 Ga and 0.5 Ga.

52

X. Xu et al. / Lithos 102 (2008) 43–64

Fig. 3. Re–Os systematics of the analysed sulfides, expressed in terms of TRD and TMA model ages.

resolved analysis used here, the overlap correction is performed on each individual reading (0.2 s). While Re/ Os may vary widely during a single run (and thus may have a large 1σ), the recorded 187 Os/188Os values have been corrected for the overlap at each 0.2 s step. Because of the large number of measurements in a single run, the reported uncertainties on 187Os/188 Os for a typical 20–60 second analysis therefore typically is smaller than those for 187Re/188 Os, especially where 187 Re/188 Os is large. The Os contents of the sulfides vary from b 1 to N 370 ppm (Table 1), within the range observed in sulfides enclosed in olivine macrocrysts from the Udachnaya (Siberia) kimberlite pipe (Griffin et al., 2002). 187Os/188Os ranges from 0.1058 to 0.1691, and 187Re/188 Os from 0.001 to 1.99, but most samples are subchondritic (Table 1, Fig. 2). Sulfides with γOs b 0 and Re/Os b CHUR will give positive TRD and TMA model ages (Fig. 3). Samples with γOs b 0 and Re/Os N CHUR will give future TMA model ages, and can be interpreted in terms of recent Readdition processes (Griffin et al., 2004). Some sulfides

have γOs N 0 and can be interpreted in terms of a multistage evolution involving high Re/Os at some stage. Only those analyses falling in the NE quadrant of Fig. 3 can in principle be interpreted as residual after melting (Griffin et al., 2002, 2004). Fig. 4 also shows that samples with high Re/Os will give unrealistic TRD and TMA model ages. Many of the peridotites studied here contain several generations of sulfides, spanning from Archean to “future” model ages (Table 1). In general the older ages are found in enclosed sulfides, and superchondritic 187 Os/188Os is found in interstitial sulfides, as noted by Alard et al. (2002). However, many enclosed sulfides have superchondritic 187Os/188 Os or young model ages, and Proterozoic model ages have been measured in many interstitial sulfides. The isotopic distinction between enclosed and interstitial sulfides probably has been obscured in many samples by recrystallisation and grain-boundary adjustment. Re–Os isochrons can be defined by sulfide data, where a single event has affected a suite of related xenoliths; Aulbach et al. (2004) found such an isochron for sulfides enclosed in olivine grains from the Slave Craton. However, subsequent metasomatism, which is very common in xenoliths from eastern China (e.g. Xu et al., 2003) would probably destroy such relationships. Sulfide data from some individual samples in the present study define 187Re/188Os–187Os/188Os arrays with positive slopes, which could be interpreted as “isochron ages”. However, most of these show large MSWD values and high initial 187Os/188Os relative to the apparent age, and are interpreted as reflecting the mixing of different generations of sulfide. For example, the data for all samples from Anyuan define an “errorchron” with an age of 2640 ± 930 Ma (MSWD = 9.6), but the initial 187Os/188 Os = 0.1194, which corresponds to a model age of 1130 Ma. Far more samples show arrays with negative slopes, which cannot reflect radiogenic

Fig. 4. Re/Os versus TMA and TRD model ages, showing how samples with higher Re/Os may give “future” ages, or ages older than Earth.

X. Xu et al. / Lithos 102 (2008) 43–64

ingrowth of 187 Os, and can only be mixing lines (Alard et al., 2002; Wang et al., 2003; Griffin et al., 2004). In other samples, there is no correlation between 187 Re/188 Os and 187 Os/188Os over large ranges in Re/ Os. We therefore do not regard any of the in situ data from single samples or suites as representing a single isochronous event, either melting or metasomatism; instead, they give clear evidence of the presence of several sulfide populations. 4. Discussion Osmium in mantle xenoliths is strongly partitioned into sulfide phases, and Os is unlikely to be transported by hydrous or carbonatitic fluids except under highly oxidizing conditions (Alard et al., 2000). This means that (1) any radiogenic Os generated in a sulfide grain by in-situ decay of 187Re will be retained in the grain; (2) the Os-isotope compositions of individual sulfides will be resistant to change by metasomatic processes; (3) “resetting” of sulfide ages is unlikely except through direct contact with later sulfide melts. Osmium model ages of whole-rock xenolith samples provide different types of constraints on the timing of melt depletion and metasomatic processes involving sulfide melts. TMA model ages (Walker et al., 1989; Shirey and Walker, 1998) are based on the extrapolation of the Os isotope composition of a xenolith to the chondritic evolution curve, using the measured 187Re/188Os ratio of the sample. However, TMA calculations may yield both future ages and ages older than the Earth, because Re may be added to, or removed from, a xenolith by processes in the mantle and in the host basalt. TRD (Time of Re depletion) model ages, which assume all Re was added to the sample near the time of eruption, are more robust indicators of the minimum age of melt depletion (Walker et al., 1989; Pearson et al., 1995a,b). TRD ages may approach the true age of melting for highly refractory peridotites, as Re is almost completely removed from the residue at high degrees of melting. However, TRD ages may significantly underestimate the age of a melt depletion event if the samples experienced only partial Re removal, or if Os was added to the sample at some later time. As described above, if sulfide melts are derived from the convecting mantle and infiltrate the lithospheric mantle as part of metasomatic events, their Os-isotope composition may provide constraints on the timing of such events. In a lithospheric volume that has undergone melt depletion, followed by several metasomatic events, we might expect to find sulfides with a wide range of Re–Os model ages. The interpretation of these ages

53

must consider the possibility of reaction between “old” sulfides and later sulfide melts; the presence of discrete age populations can provide further constraints on such processes. 4.1. Data screening and interpretation of in situ model ages A first screening of the data (Italic font in Table 1) eliminated samples with superchondritic 187Os/188 Os, which are interpreted as reflecting the addition of radiogenic Os from the host basalts or a precursor fluid or melt. The remaining data are presented in cumulativeprobability histograms of TRD for individual localities (Fig. 5). For many of these sulfides TMA ages are much higher than TRD ages (Table 1), raising the question of whether the measured Re/Os of the grains is primary. Griffin et al. (2002) suggested that residual sulfides in highly depleted rocks have 187Re/188Os b 0.07. Sulfides with higher 187Re/188 Os may have been affected by Re addition, perhaps from later metasomatic fluids, or may have crystallised from sulfide melts, rather than being residual. Relatively few sulfides in the present data set have 187 Re/188Os b 0.07, and this cutoff value may be too low for rocks such as the spinel peridotites studied here. We therefore have further constrained the data to those sulfides with γOs b 0 and 187 Re/188Os b 0.11 (Re/ Os b 0.023), to obtain a simplified data set with TRD ≈ TMA ages (Italic-bold font in Table 1); these data are shown in Fig. 6, and may provide the most robust ages for mantle events. The occurrence of multiple generations of sulfides requires in situ analysis to understand such complex processes. However, the in situ data also raise the question of what sort of event, if any, is represented by the model ages of the individual sulfide grains. Even moderate levels of melt extraction from mantle peridotites should remove any sulfur; this makes it unlikely that many of the observed sulfides, especially those with low Os contents, represent residual sulfides that date melt depletion events. It is more likely that sulfides have been reintroduced to the depleted SCLM by asthenosphere-derived melts/fluids; in this case their Os isotope ratios could represent the depleted-mantle source at the time of metasomatism (Gao et al., 2002). We therefore consider that the model ages calculated for many of the sulfides in these rocks may roughly date major fluid-infiltration events in the mantle. However, the occurrence of abundant sulfide inclusions with superchondritic 187 Os/188Os suggests that sources other than the asthenospheric mantle may provide Os to

54

X. Xu et al. / Lithos 102 (2008) 43–64

metasomatic fluids. The alteration of pre-existing sulfides by such fluids could produce mixed “ages”, and obscure the record of older events. 4.2. Timing of possible events in the SCLM beneath eastern China 4.2.1. Cathaysia block Samples from inland (Anyuan and Mingxi), coastal (Niutoushan and Qilin) and offshore (Penghu Islands) localities show some broad similarities, and some clear differences, in their patterns of TRD ages (Fig. 5a). Samples from Anyuan and Mingxi show three TRD peaks at 1.7–2.1 Ga, 1.0 Ga and 0.3 Ga. Samples from Niutoushan and Qilin also show broad peaks around 1.8–2.1 Ga and 0.9–1.2 Ga, as well as clear evidence for an Archean component, but show less evidence for Mesozoic events. Samples from Penghu show TRD peaks at 1.2–1.5 Ga, 0.8 Ga and ca 0.5 Ga; there is also evidence for Paleoproterozoic and Archean components. The differences in the TRD spectra, though based on limited data, suggest that the effects of different thermal events are heterogeneously distributed in the SCLM beneath the Cathaysia block. A cumulative plot of all the screened Cathaysia data (Fig. 5b) shows a range of TRD ages, with most between Paleoproterozoic (ca 2.1 Ga) and Neoproterozoic (ca 0.8 Ga). Many of the sulfides plotted in Fig. 5 have high Re/ Os ratios, suggesting that they have been affected by later metasomatism; such metasomatism also may have

Fig. 5. Cumulative probability plots of TRD model ages for the sulfides with subchondritic 187Os/188Os. Fig. 6. The TMA and TRD model ages of low-Re/Os sulfides (data shown in Italic–bold in Table 1) from eastern China.

X. Xu et al. / Lithos 102 (2008) 43–64

disturbed the 187 Os/ 188 Os ratios. The low-Re/Os sulfides, which are most likely to yield meaningful TMA model ages, show a more distinct grouping (Fig. 6). These data define clusters with TMA at ca 0.7 Ga, 0.9 Ga, 1.3–1.5 Ga, 1.7–2.3 Ga and 2.5–2.7 Ga. Nd model ages of granites (Chen and Jahn, 1998; Wang and Shen, 2003) and U–Pb and Hf isotope data on zircons from granites (Xu et al., 2005) in the Cathaysia block indicate three major periods of Precambrian crustal growth: late Archean (∼2.5 to 2.7 Ga), Paleoproterozoic (∼ 1.8 Ga) and early Mesoproterozoic (1.3– 1.5 Ga). The in situ Os isotope data suggest that these same events are recorded by different generations of sulfides in the peridotite xenoliths. The TMA cluster at ca 0.9 Ga corresponds to the timing of the collision between the Cathaysia and Yangtze blocks, while the 0.7 Ga cluster corresponds approximately to rifting related to the disruption of the Rodinian supercontinent (Powell et al., 1993; Li, 1998). In the Cathaysia block, the most important magmatism occurred during Mesozoic time, but this event is not clearly recorded in the sulfide data, except for a few analyses from the Anyuan locality. This anomaly will be discussed below. 4.2.2. Yangtze block Whole-rock Re–Os analyses of xenoliths from the Nanjing area show no correlation between 187Os/188 Os and 187Re/188Os, despite a good correlation between 87 Os/188 Os and Yb, indicating a recent perturbation of the Re–Os system (Reisberg et al., 2005). The sulfide populations in xenoliths from this area are dominated by grains that yield Neoproterozoic to Mesozoic TRD model ages, with some older ages. Archean model ages are not obvious (Fig. 5c). Therefore, the TRD model age spectrum is different from those of the Cathaysia and Sino-Korean blocks (see below). The Yangtze block has a mainly Archean to Mesoproterozoic basement overlain by a Neoproterozoic (Sinian) to Cenozoic cover. The robust TRD and TMA in situ ages for sulfides from the Yangtze block fall into the Neoproterozoic to Mesoproterozoic clusters (Fig. 6). This is consistent with some whole-rock Re–Os data (Zhi and Qin, 2004; Reisberg et al., 2005), and reflects a major period of crustal growth in the eastern Yangtze block. 4.2.3. Sino-Korean block More than 80% of the exposed basement in the eastern part of the Sino-Korean block is composed of 2.8–2.6 Ga TTG gneisses. Older basement is present as 3.8–3.5 Ga gneisses, amphibolites and metasediments

55

(Liu, D.Y. et al., 1992; Song et al., 1996). In the southern part of the block, lower-crustal xenoliths are much older than the surface outcrop; zircon U–Pb and Hf-isotope data (Zheng et al., 2004a) document a history starting with extraction of protoliths from the mantle at ca 4 Ga, remelting at 3.7–3.6 Ga, and metamorphism at 2.1– 1.9 Ga. U–Pb dating and Hf-isotope data for zircons in lower crustal mafic xenoliths from the Fuxian kimberlites (Zheng et al., 2004b) suggest that they are the products of basaltic underplating, derived from a depleted mantle source in Neoarchean time. Two important tectonothermal events overprinted the lower crust at 1.8–1.9 Ga and 0.6–0.7 Ga. The contrasting petrochemistry and temperature-pressure estimates for Ordovician and Tertiary mantle xenoliths from the SinoKorean block suggest that a significant portion of the original Archean–Proterozoic SCLM was removed or strongly modified after early Paleozoic time (Menzies et al., 1993; Griffin et al., 1998; Menzies and Xu, 1998; Fan et al., 2000). Whole-rock Re–Os data on xenoliths from the Fuxian kimberlites and the Tertiary basalts (Gao et al., 2002; Wu et al., 2003; Xia et al., 2004; Wu et al., 2006) are consistent with this process. The Damaping xenoliths contain many sulfides with TRD ages b1 Ga, but a higher proportion of Mesoproterozoic TRD ages (1.2–1.8 Ga; Fig. 5d); an Archean component also appears to be present. The two analysed sulfides from Hebi yield robust Archean ages. Overall, the TRD age spectrum of Sino-Korean block suggests an Archean protolith and strong Proterozoic-Phanerozoic modification (Fig. 5d). The most robust TRD and TMA in situ ages for sulfides from the Sino-Korean block fall into the same clusters as those from the Cathaysia block (Fig. 6). 4.2.4. Xing-Meng block Whole-rock Re–Os data on peridotite xenoliths from the Xing-Meng block yield Mesoproterozoic to Phanerozoic TRD model ages (Wu et al., 2003). The sulfide populations in xenoliths from Yitong are dominated by grains that yield Neoproterozoic to Mesozoic TRD model ages, with smaller numbers of Paleoproterozoic to Mesoproterozoic ages. Archean model ages are rare (Fig. 5e). The most robust TRD and TMA in situ ages for sulfides from Yitong fall into the Paleoproterozoic and Phanerozoic clusters (Fig. 6). 4.2.5. Integration of published whole-rock ages The published whole-rock Re–Os data (Table 2 and Fig. 1) (Meisel et al., 2001; Gao et al., 2002; Wu et al., 2003; Xia et al., 2004; Reisberg et al., 2005; Wu et al., 2006) can be used to further evaluate the timing of

56

X. Xu et al. / Lithos 102 (2008) 43–64

Table 2 Re–Os analyses of whole-rock mantle xenoliths from eastern China Sample no

Age (Ma)

Xing-Meng block ABS1-5 40 ABS2-21 40 BLS1-8 40 BLS1-9 40 BL1-10 40 BL1-12 40 BBT1-2 40 BBT1-8 40 BBT1-12 40 BBT1-14 40 BBT2-12 40 WQ00-6 3 WQ00-12 3 WQ00-13 3 WQ00-17 3 WQ00-18 3 WQ00-19 3 WQ00-26 3

Re/188Os

187

0.4060 0.3950 0.1440 0.9260 0.1650 0.4260 0.2730 0.1380 0.4790 0.0550 0.1490 0.3700 0.0100 0.0460 0.3940 0.2090 0.4450 0.1620

0.1263 0.1268 0.1181 0.1252 0.1225 0.1289 0.1213 0.1188 0.1235 0.1234 0.1222 0.1271 0.1246 0.1227 0.1247 0.1180 0.1269 0.1256

− 0.76 − 0.32 − 7.02 − 1.83 − 3.56 1.31 − 4.63 − 6.53 − 2.96 − 2.79 − 3.85 0.11 − 1.84 − 3.33 − 1.78 − 7.06 − 0.06 − 1.06

0.1171 0.1230 0.1268 0.1249 0.1271 0.1136 0.1189 0.1274 0.1228 0.1269

Sino-Korean block(North China Craton) Kuandian HY1-01 0.5 0.0510 HY2-01 0.5 0.0980 HY2-02 0.5 0.0940 HY2-03 0.5 0.2140 HY2-04 0.5 0.0820 HY2-05 0.5 0.1240 HY2-06 0.5 0.0430 HY2-07 0.5 0.3300 HY2-14 0.5 0.0550 HY2-29 0.5 0.5080 Tieling FW04-350 460 0.0250 FW04-351 460 0.0110 FW04-352 460 0.0390 FW04-353 460 0.0120 FW04-354 460 0.5370 FW04-355 460 0.0450 FW04-356 460 0.0090 Longgang L-1 1.2 0.0260 L-2 1.2 0.1240 L-5 1.2 0.1010 LQL1-01 1.2 0.0240 LQL1-02 1.2 0.0920 LQL1-13 1.2 0.0430 DYS1-01 1.2 0.1340 DYS1-03 1.2 0.0580 DYS1-05 1.2 0.6250 DAL02-2 1.2 0.0360 DAL02-3 1.2 0.3220 Qixia Q1 1 0.1785 Q4 1 0.3346 Q5 1 0.0805 Q6 1 0.0734 Q8 1 0.0167

Os/188Os

Initial γOs

187

TRD

TCHUR

Os (ppb)

Re (ppb)

Re/Os

154 71 1326 357 682 −239 881 1235 569 536 735 − 11 357 638 347 1334 21 210

− 12174 1810 2032 − 204 1125 4502 2625 1850 − 2795 615 1140 − 170 367 719 15466 2744 − 167 349

4.95 3.48 5.09 0.45 1.92 3.40 3.74 2.56 3.37 3.37 4.48 1.09 1.59 1.72 1.11 1.92 1.32 0.99

0.42 0.29 0.15 0.09 0.07 0.30 0.21 0.07 0.34 0.04 0.14 0.08 0.00 0.02 0.09 0.08 0.12 0.03

0.084 0.082 0.030 0.192 0.034 0.089 0.057 0.029 0.099 0.012 0.031 0.077 0.002 0.009 0.081 0.044 0.092 0.033

− 7.74 − 3.14 − 0.08 − 1.63 0.10 − 10.50 − 6.35 0.36 − 3.24 − 0.07

1459 602 26 318 −9 1968 1202 − 58 621 23

1669 795 33 677 −11 2825 1345 − 327 719 − 85

2.53 2.78 0.88 2.19 0.69 1.75 2.22 1.06 1.52 1.98

0.03 0.06 0.02 0.10 0.01 0.05 0.02 0.07 0.02 0.21

0.011 0.020 0.019 0.044 0.017 0.026 0.009 0.068 0.011 0.105

0.1112 0.1112 0.1126 0.1156 0.1839 0.1145 0.1113

− 12.53 − 12.44 −11.51 − 9.02 41.61 − 10.06 − 12.39

2340 2323 2154 1697 −8448 1887 2315

2462 2374 2333 1734 21089 2065 2356

4.17 4.65 4.35 4.21 0.10 3.34 5.07

0.02 0.01 0.04 0.01 0.01 0.03 0.01

0.005 0.002 0.008 0.002 0.113 0.009 0.002

0.1192 0.1256 0.1268 0.1243 0.1245 0.1201 0.1289 0.1188 0.1221 0.1186 0.1229

− 6.13 − 1.05 − 0.14 − 2.10 − 1.94 − 5.38 1.54 − 6.45 − 3.83 − 6.57 − 3.19

1161 209 36 406 377 1022 −283 1220 732 1243 612

1241 302 48 432 488 1143 − 425 1423 − 1338 1364 3012

1.86 2.81 2.66 2.84 0.86 1.86 2.31 1.31 0.11 3.25 1.12

b0.010 0.07 0.06 0.01 0.02 0.02 0.06 0.02 0.02 0.02 0.08

0.026 0.021 0.005 0.020 0.009 0.028 0.012 0.134 0.007 0.067

0.1267 0.1299 0.1241 0.1200 0.1266

− 0.18 2.34 − 2.22 − 5.47 − 0.27

44 −435 430 1038 61

78 − 2655 537 1268 64

2.53 1.57 0.84 7.15 2.29

0.09 0.11 0.01 0.11 0.01

0.037 0.069 0.017 0.015 0.004

X. Xu et al. / Lithos 102 (2008) 43–64

57

Table 2 (continued) Sample no Q17 QX-07 QX-11 QX-13 QX-14

Age (Ma) 1 1 1 1 1 1

Damaping (Hannuoba) DMP04 15 15 DMP19 15 DMP23A 15 DMP25 15 DMP41 15 DMP51 15 DMP56 15 15 DMP58 15 DMP59 15 DMP60 15 15 DMP67c 15 DM1-4 15 DM1-7 15 P-1 15 15 P-2 15 P-3 15 15 P-7 15 P-9 15 P-10 15 15 P-12 15 15 P-13 15 P-14 15 P-15 15 P-16 15 15 P-17 15 FP-1 15 FP-2 15 15 Fuxian F50-9270 460 460 F50-9271 460 Mengyin SD9405 460 Yangtze block Around Nanjing Lianshan LHLS-1 16 16 16 LHLS-2 16

Re/188Os

Os/188Os

Initial γOs

187

187

Re (ppb)

Re/Os

0.3370 0.4301 0.2109 0.2547 0.2662 0.0455

0.1280 0.1286 0.1257 0.1244 0.1243 0.1261

0.86 1.32 − 0.98 − 2.01 − 2.06 − 0.70

− 153 − 241 196 389 400 142

− 961 3348 410 1055 1174 160

0.66 0.54 2.23 2.77 3.43 4.28

0.05 0.05 0.10 0.15 0.19 0.04

0.070 0.088 0.044 0.053 0.055 0.009

0.2412 0.2427 0.1189 0.1043 0.0476 0.2599 0.1903 0.2947 0.3080 0.2253 0.2387 0.3359 0.3450 0.1026 0.4600 0.2610 0.2098 0.1703 0.2043 0.0557 0.0476 0.1592 0.1889 0.3000 0.2004 0.1647 0.1551 0.5047 0.1950 0.2703 0.1902 0.1996 0.1294 0.0531 0.1007 0.0916

0.1235 0.1231 0.1203 0.1188 0.1166 0.1236 0.1236 0.1277 0.1282 0.1259 0.1238 0.1267 0.1264 0.1233 0.1263 0.1183 0.1228 0.1222 0.1213 0.1214 0.1208 0.1221 0.1234 0.1237 0.1244 0.1225 0.1218 0.1220 0.1241 0.1217 0.1232 0.1238 0.1183 0.1211 0.1193 0.1189

− 2.74 − 3.04 − 5.29 − 6.45 − 8.12 − 2.70 − 2.69 0.53 0.92 − 0.86 − 2.55 − 0.27 − 0.46 − 2.87 − 0.58 − 6.89 − 3.32 − 3.78 − 4.47 − 4.35 − 4.83 − 3.82 − 2.80 − 2.61 − 2.07 − 3.55 − 4.10 − 3.96 − 2.27 − 4.19 − 2.96 − 2.55 − 6.80 − 4.59 − 6.05 − 6.34

526 584 1004 1221 1531 520 519 − 91 − 165 172 491 62 98 551 120 1302 636 722 851 830 919 730 538 502 400 679 783 756 439 799 569 491 1286 874 1146 1200

1286 1442 1415 1638 1732 1433 968 − 382 − 759 373 1180 300 599 734 − 717 3617 1308 1237 1704 960 1040 1195 998 1914 781 1136 1261 − 2973 836 2379 1062 957 1880 1004 1519 1545

3.95 3.94 4.35 3.47 3.26 2.92 3.49 3.75 3.64 3.95 4.65 4.19 4.10 1.88 3.86 3.49 2.81 3.13 4.30 1.92 2.19 3.12 3.31 2.55 3.28 3.77 3.93 1.24 3.29 3.43 3.06 2.94 2.83 3.50 3.02 3.32

0.20 0.20 0.11 0.08 0.03 0.16 0.14 0.23 0.23 0.19 0.23 0.29 0.29 0.04 0.37 0.19 0.12 0.11 0.18 0.02 0.02 0.10 0.13 0.16 0.16 0.13 0.13 0.13 0.13 0.19 0.12 0.12 0.08 0.04 0.06 0.06

0.050 0.050 0.025 0.022 0.010 0.054 0.040 0.061 0.064 0.047 0.050 0.070 0.072 0.021 0.096 0.054 0.044 0.035 0.043 0.011 0.010 0.033 0.039 0.062 0.048 0.034 0.032 0.105 0.040 0.056 0.040 0.042 0.027 0.011 0.021 0.019

0.0620 0.0631 0.0705

0.1101 0.1114 0.1092

− 13.64 − 12.66 − 14.41

2541 2363 2682

2913 2711 3144

1.20 1.15 1.72

0.02 0.02 0.03

0.013 0.013 0.015

0.4800

0.1222

− 6.68

1263

− 3846

2.88

0.29

0.100

0.4410 0.0210

0.1250 0.1337 0.1230 0.1273

− 1.52 5.35 − 3.18 0.31

298 − 1012 610 − 48

− 6454 − 52

1.08 3.78

0.099 0.016

0.091 0.004

TRD

TCHUR

Os (ppb)

(continued on next page)

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X. Xu et al. / Lithos 102 (2008) 43–64

Table 2 (continued) Sample no Yangtze block Around Nanjing LHLS-3 LHLS-4 LHLS-5 LHLS-6 LHLS-7 LHLS-8 LHLS-8 LHLS-9 LHLS-10 LHLS-11 LHLS-12 LHLS-13 LHLS-14 LHLS-15 LHLS-16 LHLS-17 LHLS-18 Panshishan LHPSS-1 LHPSS-2 LHPSS-3 LHPSS-4 LHPSS-8 LHPSS-9 LHPSS-11 LHPSS-12

Age (Ma)

16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16

Re/188Os

187

0.3380

0.0530

0.1261 0.1239 0.1193 0.1253 0.1255 0.1267 0.1251 0.1248 0.1254 0.1262 0.1345 0.1230 0.1172 0.1303 0.1252 0.1243 0.1223 0.1252 0.1186 0.1178

− 0.76 − 2.40 − 6.07 − 1.32 − 1.18 − 0.22 − 1.52 − 1.71 − 1.20 − 0.67 5.98 − 3.17 − 7.71 2.64 − 1.58 − 2.10 − 3.71 − 1.37 − 6.58 − 7.25

154 464 1150 259 234 53 297 334 237 138 − 1134 608 1455 −492 310 408 709 269 1245 1369

0.1880

0.1269

− 0.11

0.4420 0.3500

0.1278 0.1248 0.1259 0.1254 0.1257 0.1254 0.1239 0.1276 0.1262

0.54 − 1.76 − 0.83 − 1.18 − 1.09 − 1.31 − 2.49 0.45 − 0.73

0.0500 0.3010 0.3130 0.2070 0.1470 0.2330 0.2600

1.0090 0.0930 0.0900

0.3470 0.3980 0.2720 0.4920 0.5190

Os/188Os

Initial γOs

187

TRD

TCHUR

Os (ppb)

Re (ppb)

Re/Os

877

2.14

0.151

0.071

1309

2.45

0.026

0.011

880 182 595 516

0.301 0.110 0.080 0.060

0.204 0.065 0.043 0.031

0.074

0.049

0.067

0.054

0.038

0.210

0.038 0.027

0.019 0.019

1572

1.48 1.69 1.85 1.96 1.83 1.52 0.11 1.23 1.01 0.14 0.18 1.94 1.98 1.42 2.07 2.16

0.011

31

45

2.05

0.024 0.110 0.080

0.039

− 93 343 167 234 217 257 481 − 75 149

1096 2505

2.82 3.72 2.97 6.07 3.24 3.05 3.71 3.33 2.91

0.259 0.272

0.092 0.073

0.241 0.252 0.194 0.342 0.314

0.074 0.083 0.052 0.103 0.108

305 1677

− 179 916 342

1470 21035 1443 418 − 442

Samples of Xing-Meng block and Longgang are from Wu et al. (2003). Kuandian and Tieling are from Wu et al. (2006). Qixia, Damaping (DMP-04 to DMP-67c), Fuxian, and Mengyin samples are from Gao et al. (2002). DM1-4 and DM1-7 are from Meisel et al. (2001). P-1 to P-17 and FP-1, FP-2 are from Xia et al. (2004). Around Nanjing samples are from Reisberg et al. (2005).

mantle events, but this must be approached with caution, since the presence of multiple sulfide generations is likely to distort the whole-rock model ages. Gao et al. (2002) obtained a whole-rock Re–Os isochron with an age of 1910 Ma from Damaping, Sino-Korean block. The inference from this whole-rock isochron is that the range of 187 Re/188Os compositions could be produced by varying degrees of partial melting, and in-growth of radiogenic osmium over time produced the spread of 187 Os/188 Os. In most cases, the whole-rock data from an individual locality do not yield an isochron (e.g., the analytical data for Qixia, Sino-Korean block by Gao et al. (2002)). The scatter implies that the Re–Os isotopic systematics have been affected by multiple events. Furthermore, it is difficult to explain why the sulfide

data within a single peridotite from Damaping show almost the same range of 187Re/188 Os and 187Os/188 Os data as the entire whole-rock dataset for this locality (Tables 1 and 2). As with the sulfides, some xenoliths give negative whole-rock TRD model ages (Table 2). We have therefore applied the same screening process used for the sulfide data to the whole-rock Re–Os data, to identify the samples most likely to have robust model ages; 40 analysis pass this screen (Italic-bold font in Table 2). Some published data (Meisel et al., 2001; Gao et al., 2002; Reisberg et al., 2005) have also been recalculated to the CHUR model to allow direct comparison. Four peridotites from the Nanjing area pass the screen (Italic-bold font in Table 2); their robust TMA and

X. Xu et al. / Lithos 102 (2008) 43–64

TRD model ages range from Meso- to Neoproterozoic. The integration of sulfide and whole-rock Re–Os data (Figs. 6 and 7) suggests that the oldest part of the lithospheric mantle sampled beneath the eastern Yangtze block may be Paleoproterozoic in age, and has been modified in Mesoproterozoic and Phanerozoic time. Re–Os data (Gao et al., 2002) for two xenoliths (three analyses) from the Fuxian kimberlites yield TMA and TRD ages ranging from Neo-to Mesoarchean (Fig. 7). A few of the robust whole-rock analyses from localities in the Sino-Korean block show Paleoproterozoic model ages, but most give Mesoproterozoic to Neoproterozoic model ages, and four samples show TMA and TRD ages younger than Paleozoic (Fig. 7). Therefore, the collision of the eastern and western parts of the Sino-Korean block around 1.8 Ga and the subsequent modification of the lithospheric mantle may be reflected by both robust sulfide and wholerock data. Wu et al. (2003) reported whole-rock Re–Os data for mantle xenoliths from Shungliao and Wangqing (Fig. 1) in the Xing-Meng block. Three screened samples give TRD model ages of 1.24–1.34 Ga, corresponding closely with Nd model ages for the overlying crust (Wu et al., 2003), but the other three give model ages ranging from 0.3–0.6 Ga (Fig. 7), corresponding roughly with the timing of granite magmatism in the fold belt.

Fig. 7. The TMA and TRD model ages of screened whole-rock samples (data shown in Italic–bold in Table 2). Fields show groupings of sulfide data from Fig. 6.

59

4.3. Lithosphere replacement or lithosphere modification? The most striking feature of the available Re–Os data from the Sino-Korean block is the scarcity of model ages corresponding to the most ancient crust-forming events. This contrasts with data from other Archean areas (Griffin et al., 2002, 2004), where the oldest sulfide model ages from mantle-derived xenoliths correspond to U–Pb ages for the oldest crustal rocks. The only robust evidence for Archean SCLM is from wholerock ages on xenoliths in the Fuxian kimberlites (Gao et al., 2002) and the nearby Tieling basalts (Wu et al., 2006), and two TRD ages on sulfides in highly depleted Archean-type spinel peridotites from Hebi. One possible explanation for the apparent scarcity of Archean SCLM is that there was little introduction of sulfide into the SCLM between the original Archean depletion event, and a series of Proterozoic metasomatic events. In this case, Archean mantle could be present, but its original depletion age would be difficult to establish. However, this explanation does not account for the dominantly fertile composition of the xenoliths in the Tertiary basalts; this type of composition is very rare in Archean xenolith suites. Some form of lithospheric delamination may have occurred during Proterozoic time, as suggested by Gao et al. (2002) and others. However, typical thick, depleted Archean SCLM, of the type sampled by the Ordovician Fuxian and Mengyin kimberlites, is buoyant relative to the underlying asthenosphere. This buoyancy, and the higher viscosity imparted by extreme depletion and dehydration, contribute to its stability and make it difficult to “delaminate” (Griffin et al., 1998; Poudjom Djomani et al., 2001). Furthermore, such Proterozoic delamination cannot have affected the eastern part of the craton, where the Ordovician kimberlites sampled Archean SCLM. The events recorded in the mantle beneath Damaping therefore may reflect modification of the SCLM by asthenospheric upwelling, rather than a craton-wide delamination. Detailed seismic tomography beneath the eastern part of the Sino-Korean block (Yuan, 1996) shows a complex pattern of high and low velocities at shallow depths, defining channels of material with low seismic velocity. This pattern is most simply interpreted as reflecting a combination of thermal and compositional differences, produced by lithosphere extension and associated asthenospheric upwelling (Yuan, 1996). In this model the low-velocity channels represent upwelling asthenospheric material (fertile, hot) and the highervelocity areas are remnant SCLM, cooler and more

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depleted. The Archean peridotites from the Hebi locality (Zheng et al., 2001; Fig. 1) come from the edge of a high-velocity block, lending support to the seismic model. Such a model suggests that the apparent scarcity of preserved Archean SCLM is partly a sampling effect. Some of the Tertiary volcanoes that have provided xenoliths for these studies are located over, or near, the “hot channels”, while others are close to major lithospheric shear zones (Fig. 1). These shear zones may have served as loci for metasomatism in Proterozoic and later times, to produce the observed model-age patterns. The Tertiary volcanoes, whose eruption is controlled by these fertile zones, thus may give a biased picture of the craton's SCLM. The Sino-Korean block may be underlain by a patchwork of Archean remnants, dissected by fertile zones metasomatised in Proterozoic and in Mesozoic time. However, it is also possible that wholesale refertilisation of the Archean SCLM took place over much of Proterozoic and Paleozoic time, perhaps by movement of asthenosphere-derived fluids along such networks of lithospheric shear zones. As the bulk composition of the SCLM approached a fertile composition similar to typical Phanerozoic SCLM, it would have become gravitationally unstable and susceptible to delamination (Poudjom Djomani et al., 2001). Delamination could have occurred in response to external stresses (Neil and Houseman, 1999) such as the Jurrasic–Triassic collisions along the northern and southern margins of the craton, or the subduction of the Pacific plate beneath the area. The upwelling and freezing of asthenospheric mantle would produce a relatively fertile SCLM, such as that along the Tan-Lu fault zone (Menzies and Xu, 1998; Gao et al., 2002), making it difficult to distinguish from any highly metasomatised Archean remnants. An important event recorded by the sulfide and whole-rock data at 600-800 Ma in the four blocks of eastern China may correspond to incipient rifting in the Rodinia supercontinent. However, most reconstructions of Rodinia place the North China block very far from the South China block, and the link between these events across most of present-day eastern China remains to be explained. In the Sino-Korean block, major modification or even delamination of the SCLM apparently occurred during Mesozoic time (Menzies et al., 1993; Griffin et al., 1998), again accompanied by felsic magmatism (Chen et al., 2004). These events clearly have involved the movement of magmas and other fluids from the lithospheric mantle into the crust, and the screened whole-rock Os-isotope data (Fig. 7) may suggest that

some of the xenoliths were part of the SCLM during these events. 4.4. Cathaysia: the case of the missing Mesozoic The Mesozoic was a period of major significance in the evolution of the crust and upper mantle in the Cathaysia block. Bimodal volcanism was accompanied by the widespread intrusion of granitoids in this region. This large scale magmatism requires a major input of heat, and could be expected to involve the movement of magmas and other fluids from the lithospheric mantle into the crust. We therefore expected to find such fluid movements marked by spikes in the Os-isotope record, but they are notably rare (Fig. 5). However, the scarcity of sulfides with Mesozoic model ages may be simply an artefact of the model-age calculation. The calculation of depleted-mantle model ages assumes a homogeneous convecting mantle with a constant Re/Os (except for the slow decay of 187 Re), whereas the range of 187Os/188Os in modern MOR basalts (0.128–0.163; Schiano et al., 1997; Alard et al., 2005) indicates that measurable heterogeneity may be present. This heterogeneity limits the precision of Osisotope model ages in young rocks, unless constraints can be placed on the nature, degree and origin of the mantle heterogeneity. The sulfide data from Cathaysia may offer insights into this problem. Global seismic tomography suggests that the influence of continental roots extends to depths N250 km (eg, Ritsema and van Heijst, 2000; Grand, 2002). Thus the “asthenosphere” beneath large continents, or inboard from large subduction systems, may not be efficiently homogenised with the global convecting mantle reservoir, and may evolve differently in terms of Os-isotope composition. For example, a concentration of mafic rocks, derived from repeated episodes of magmatism or

Fig. 8. Cumulative probability plots of TRD model ages (from − 1.5 to 1.5 Ga) for sulfides including those with superchondritic 187Os/188Os, in xenoliths from Cathaysia block.

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5. Conclusions

Fig. 9. CHUR evolution curve and evolution curve for “asthenosphere” with higher 187Re/188Os since an event around 1 Ga, which may yield ca 150 Ma TRD model ages for samples with superchondritic 187 Os/188Os (mean values of 0.1282).

the delamination of lower-crustal rocks, could raise the mean Re/Os of the subcontinental asthenosphere. Young sulfides derived from this contaminated source would have spuriously low model ages. A plot of TRD model ages that includes the “future” ages required by sulfides with superchondritic 187Os/188Os shows a marked peak at −180 Ma for the samples from the Cathaysia block (Fig. 8). Let us assume that this peak actually represents the “missing” Mesozoic events (150 ± 50 Ma), and that the sublithospheric mantle beneath the Cathaysia block has evolved with a higher Re/Os since the collision between the Cathaysia and Yangtze blocks (ca 1 Ga). It only requires an increase in the mean 187Re/188Os of the “asthenosphere” from 0.402 (the CHUR value) to 0.560, to yield +150 Ma TRD model ages for the sulfides that give −180 Ma model ages with the CHUR model (Fig. 9). This scheme implies that all TRD model ages b 1 Ga are only minimum ages, and that the discrepancy between “real” and “model” age increases with decreasing age. Putting the increase in Re/Os further back in time would reduce required change in Re/Os, but the general coincidence between crustal events and peaks of TRD model ages up to Neoproterozoic time suggests that the CHUR model is a reasonable approximation for ages at least N1 Ga. The use of other suggested models, such as PUM, would make these ages somewhat older. We do not use the PUM model here because studies of PGE abundances in sulfides indicate that this model may bear little resemblance to a truly primitive mantle (Alard et al., 2005). Rather than being missing from the sulfide record, the dramatic Mesozoic crustal events in Cathaysia may be recorded largely by sulfides with superchondritic 187 Os/188 Os. This would require that the subcontinental “asthenosphere” beneath Cathaysia has acquired an elevated Re/Os (relative to CHUR) at some time in the relatively recent past.

1. In situ Re–Os isotopic data for sulfides in mantle xenoliths from eastern China show a wide range in Os isotopic composition, which reflects the complexity of mantle events. Robust data with TRD ≈ TMA ages, defined by sulfides and published whole-rock analyses with low Re/Os, cluster in several age groups corresponding to major crustal events. 2. In the Cathaysia block, sulfide data indicate mantle events in Paleoproterozoic (1.8 Ga), Mesoproterozoic (∼ 1.3–1.5 Ga), and Neoproterozoic (0.9 Ga and 0.6 Ga) time, corresponding to the major periods of Precambrian crustal growth revealed by Nd and Hf isotopes. Residues of Neoarchean (2.5–2.7 Ga) lithosphere and overlying crustal basement may still exist beneath the Cathaysia block. 3. In the Sino-Korean block, Archean ages are recorded only by a few sulfide grains, and by some published whole-rock data (Gao et al., 2002; Wu et al., 2006); the in situ data are dominated by Meso- to Neoproterozoic ages. The modification of the Sino-Korean SCLM may have begun already in Proterozoic time, controlled by a network of major shear zones. 4. The major Mesozoic magmatic event in the Cathaysia block is poorly recorded in the sulfide TRD spectrum. However, these young events may be represented by clusters of sulfides with superchondritic 187 Os/188 Os but subchondritic Re/Os (and hence future ages). This interpretation would require a relatively small increase in the mean Re/Os of the subcontinental “asthenosphere”, perhaps in Neoproterozoic time, and would imply that this source of magmas did not homogenise efficiently with the global convecting mantle. 5. Integration of data from seismic tomography, xenolith studies and in situ Re–Os analysis of sulfides suggests that the present SCLM beneath the Sino-Korean block is a patchwork of Archean relics, highly metasomatised Proterozoic SCLM and younger material occupying deep fault- and suture zones. Metasomatism of the Archean SCLM may have reduced its buoyancy enough to allow localised delamination in Proterozoic time, and more widespread thinning and/or delamination across the craton in Triassic–Jurassic time in response to collisions along the northern and southern boundaries. Acknowledgments This work was supported by National Major Basic Research Project (2006CB403508) and NSF of China

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Grants (No. 40221301, 40125007), an ARC Discovery and Linkage International Grants (SYO'R and WLG). Analytical data were obtained using instrumentation funded by ARC LIEF, and DEST Systemic Infrastructure Grants and Macquarie University. The manuscript was improved by thoughtful reviews from Fuyuan Wu, Michel Gregoire and Laurie Reisberg. This is contribution no. 490 from the ARC National Key Centre for the Geochemical Evolution and Metallogeny of Continents (www.es.mq.edu.au/GEMOC/). Appendix Analytical methods As described by Pearson et al. (2002) and Griffin et al. (2002), the technique uses a New Wave Research 266 nm laser microprobe with a modified ablation cell, attached to the Nu Plasma multicollector ICPMS. All ablations are carried out in He, and a “dry” Ir aerosol is introduced into the gas line between the ablation cell and the ICPMS, to provide a mass-bias correction with a precision independent of the size of the Os signal from the sample. Pearson et al. (2002) have demonstrated that the relative mass fractionations of Re, Ir and Os are identical within analytical uncertainty on the Nu Plasma instrument. Masses 188–194 are measured in Faraday cups, and masses 185 and 187 in ETP ion counters. The ion counters are initially calibrated against the Faraday cups using a two-cycle analysis of an Os solution. During ablation runs, a synthetic NiS bead (PGE-A) with 200 ppm Os and Pt is analysed between samples, to monitor any drift in the ion counters. These drift corrections are typically less than 1% over a long day. The overlap of 187Re on 187Os is corrected assuming 187Re/185Re = 1.6742. Data are collected using the Nu Plasma time-resolved software, which allows the selection of the most stable intervals of the signal for integration. The selected interval is divided into 40 replicates to calculate standard deviation and standard error. Typical ablation pits are 50–80 μm in diameter and depth. In sulfides ≥50 μm in diameter and with ≥40 ppm Os, an internal precision on 187Os/188Os of 0.1–0.3% (2se) is routinely obtainable. For smaller grains or lower Os contents, an internal precision of 1–2% is routine. The external reproducibility of 187Os/188Os for the PGE-A standard over several months is ±0.00048 (2sd), and the mean value is indistinguishable from the TIMS value. Following each analytical session, the polished blocks were ground down 200–300 μm and repolished to find new sulfide targets. This process has been repeated 2–5 times for different samples.

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