ORIGINAL PAPER

Yuejun Wang Æ Weiming Fan Æ Touping Peng Æ Feng Guo

Elemental and Sr–Nd isotopic systematics of the early Mesozoicvolcanic sequence in southern Jiangxi Province, South China:petrogenesis and tectonic implications

Received: 2 October 2003 / Accepted: 27 August 2004 / Published online: 14 October 2004� Springer-Verlag 2004

Abstract Elemental and Sr–Nd isotopic results are pre-sented for the early Mesozoic volcanic sequence(�172 Ma) in southern Jiangxi Province, South China.The sequence is voluminously composed of �45% sub-alkaline basaltic rocks (group 1), <5% high-mg andes-ite–dacites (group 2) and �50% rhyolites (group 3). Thegroup 1 rocks are characterized by (La/Yb)cn = 3.8–7.2,Eu/Eu* = 0.65–1.15, Nb/La = 0.64–0.99, 87Sr/86Sr(t) =0.70602 � 0.70822 and �Nd(t) = �1.63 to +0.11, similarto those of an EMII-like source. The group 2 rocks havemg=0.42 � 0.60, SiO2=60.24 � 66.71%, MgO =2.65� 5.54%, Ni=24 � 102 ppm and Cr=84 �266 ppm,classified as high-mg andesitic rocks. These rocks aremore enriched in LILEs and LREE with more signifi-cant negative Eu anomaly (0.63�0.79), are more de-pleted in HFSEs with Nb/La ratios of 0.40–0.56 andhave lower �Nd(t) (�9.44 to �7.78) and higher87Sr/86Sr(t) (0.70985 � 0.71016), in comparison with thegroup 1 rocks. They most likely originated from meta-somatised veins in the lithospheric mantle. The origi-nation of the group 1 and group 2 magma suggests thedevelopment of a peridotite-plus-vein lithospheric man-tle during early Mesozoic era beneath the interior of theCathaysian block. The group 3 rhyolites are character-ized by high SiO2 (72.75 � 77.97%), Zr (99 � 290 ppm),Hf (3.9 � 9.7 ppm) and Ga/Al (2.76 � 3.87) andsignificant Nb–Ta, Ba–Sr and P–Ti depletions. Theserhyolites exhibit Sr–Nd isotopic compositions

(87Sr/86Sr(t) = 0.70962�0.71104, �Nd(t) = �4.63 to�5.80) similar to the contemporaneous Zhaibei andPitou A-type granites in the area. Such characteristicssuggest that they might be derived from the underplating

basaltic magma contaminated by crustal materials.Therefore, an early Mesozoic rifting model in responseto intracontinental lithospheric extension is proposed toaccount for the early Mesozoic volcanism in southernJiangxi Province, South China.

Keywords Elemental and Sr–Nd isotopic systematics ÆEarly Mesozoic volcanism Æ Peridotite-plus-veinlithospheric mantle Æ Intracontinental lithosphericextension Æ South China

Introduction

The Cathaysian block, which is also termed the ‘‘Hua-nan block’’ (Hsu et al. 1990), is a major part of the SouthChina Block (SCB). The block includes the ‘‘SoutheastChina Caledonian fold belt’’ and the ‘‘Southeast Chinamaritime fold belt’’, which are separated by the Zhen-ghe-Dapu fault as illustrated in Fig. 1a (Chen and Jahn1998), and has a complex history of tectono-magmaticevolution. Over the past 20 years, the tectonic nature ofthe Cathaysian block and the early Mesozoic tectonichistory of the SCB have been debated (Hsu et al. 1990;Faure et al. 1996; Rodgers 1989). Two distinct view-points can be summarized. One suggests that the earlyMesozoic tectonic evolution of the region is related toeither westward subduction of a Mesozoic Pacific plate,or the closure of an oceanic basin in the interior of theSCB, and as such, considers the region an Andean-typeactive continental margin (Holloway 1982; Faure et al.1996), Alps-type collision belt (Hsu et al. 1990) and alithospheric subduction zone with underplating of maficmagma (Zhou and Li 2000). The other viewpoint favorsan intracontinental lithospheric extension scenario inresponse to asthenospheric upwelling (Gilder et al. 1996;Li et al. 2003, 2004; Wang et al. 2003a, b). This view-point also proposes that the Cathaysian block is an oldcontinental block, and the consolidation between theCathaysian and Yangtze blocks took place during the

Present address: Y. Wang (&) Æ W. Fan Æ T. Peng Æ F. GuoGuangzhou Institute of Geochemistry,Chinese Academy of Sciences, Wushan, Guangzhou,510640, Peoples Republic of ChinaE-mail: yjwang@gig.ac.cn; wangyj6@21cn.comTel.: +86-20-85290527Fax: +86-20-85290708

Int J Earth Sci (Geol Rundsch) (2005) 94: 53–65DOI 10.1007/s00531-004-0441-4

Jinning collisional event at �970 Ma (Li 1998; Li andMcCulloch 1996; Chen et al. 1991).

The Cathaysian block has a tectonic history andcrustal ages distinctive from the Yangtze block (JBMGR1989; Li and McCulloch 1996), suggesting that bothblocks should have different mantle nature (Wang et al.2003a). To achieve a better understanding of the earlyMesozoic tectonic evolution of the SCB, one of the keysis to constrain the petrogenesis and source characteris-tics of the early Mesozoic volcanism of the Yangtze andCathaysian blocks. Recent studies have showed that theearly Mesozoic magmatism in the interior of the Yan-gtze block has the signatures of an EMI-like sourcecontaminated by an OIB component (Wang et al. 2003a;Li et al. 2004), suggesting a major intracontinentallithospheric extension/thinning scenario during earlyMesozoic (Chen and Jahn 1998; Gilder et al. 1996;Wang et al. 2003a, b). However, little is known aboutthe systematic geochemistry of the early Mesozoic vol-canic sequence in the interior of the Cathaysian block(Chen et al. 1999; Li et al. 2003). Questions remain as to:(1) what is the source of the interior of the Cathaysianblock during early Mesozoic? (2) how was the earlyMesozoic volcanism generated? and (3) what was thedriving mechanism? This paper sets out to probe intothe nature of the mantle underneath the interior of theCathaysian block and to advance our understanding ofthe early Mesozoic tectonic evolution of the SCB, bypresenting a set of major, trace elemental and Sr–Ndisotopic data for the early Mesozoic volcanic sequencefrom southern Jiangxi Province in the interior of theCathaysian block.

Background geology and petrography

The Cathaysian block, intruded by the voluminousgranites since the Neoproterozoic age, consists of aPrecambrian basement, an overlying Paleozoic neriticmarine sedimentary cover and a Mesozoic continentalvolcanic-sedimentary sequence from upper Triassiconwards. The metamorphic basement has Paleo- toMesoproterozoic and possibly late Archaean ages(JBGMR 1989; Chen and Jahn 1998; Li 1998; Li andMcCulloch 1996). The early Mesozoic volcanic sequencein the Cathaysian block dominantly developed in theMesozoic continental volcanic-sedimentary basins witha thickness of hundreds of meters (>800 m), such as theDongkeng-Linjiang and Changpu-Baimianshan basinsin southern Jiangxi Province (Fig. 1b). The volcanicsequence uncomfortably overlies the Pre-J1 strata andcomfortably underlies J2–3 volcanic rocks (Fig. 1c). Thevolcanic sequence was previously dated at 163–179 Maby Rb–Sr whole-rock analysis (Chen et al. 1999).However, Li et al. (2003) recently reported the SHRIMPzircon ages of 172.9±4.3 and 171.6±4.6 Ma for theChebu gabbro and Zhaibei A-type granite from the re-gion (see Fig. 1b for locations), which are considered tobe the equivalents of the basalt and rhyolite within thevolcanic sequence, respectively (Chen et al. 1999;JBGMR 1989). A high-mg andesite sample (20GN-72)from the Changpu profile (Fig. 1c) yielded a K–Ar ageof 172 Ma (Wang et al. 2003a). This suggests that thevolcanic sequence is most likely the product of con-temporaneous eruption at �172 Ma.

Fig. 1 a Regional geologicalmap (revised after Chen andJahn 1998; JBGMR 1989;Wang et al. 2003a).b Distributions of the earlyMesozoic volcanic sequence insouthern Jiangxi Province.c A synthetically stratigraphicprofile of the sequence

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In previous literature, the volcanic sequence wasdeciphered as bimodal rocks that consist of basalts/gabbros and rhyolites/A-type granites (JBGMR 1989;Chen et al. 1999; Li et al. 2003). In our field investi-gation, however, we found that there are wide spec-trums of rock types in the sequence, including basalt,basaltic andesite, rhyolitic lava, rhyolitic tuff and asmall amount of andesite and dacite. In volume, thevolcanic sequence is roughly composed of 45% basaltsand basaltic andesites (named group 1 here), <5%andesite–dacites (named group 2) and 50% rhyolites(named group 3). The stratigraphic relationship ofthese groups is summarized in Fig. 1c.

The group 1 and 2 rocks are commonly subaphyric toporphyritic with clinopyroxene phenocrysts (less vol-ume) and alkaline feldspar. The matrix is mainly com-posed of fine-grained or aphanitic clinopyroxene,plagioclase and minor opaque oxides. Plagioclase phe-nocrysts are less developed in the group 1 rocks incomparison with the group 2 rocks. The mineral com-positions of the group 3 rocks include plagioclase(25–45%), feldspar (5–25%) and quartz (20–35%),biotite (3–5%), amphibole (5–10%) and minor euhedralsphene, apatite, zircon and Fe–Ti oxides.

Analytical methods

Samples were ground in an agate mill. Major elementswere determined by a wavelength X-ray fluorescencespectrometry at the Hubei Institute of Geology andMineral Resource, the Chinese Ministry of Land andResource. The relative standard derivations (RSD) arewithin 5%, and totals are all 100±1%. FeO content insamples was analyzed solely by a wet chemical meth-od. Trace elements were analyzed by ICP-MS (Perkin-Elmer ELAN 6000) in the Guangzhou Institute ofGeochemistry (GIG), the Chinese Academy of Sciences(CAS). The international standard BCR-1 was chosento calibrate the element concentrations of measuredsamples. In-run analytical precision for most elements(e.g., Rb, Th, U, REE, Nb, Ta, Y, Zr, Hf) is betterthan 5%. Detailed analytical procedures of ICP-MSwere reported by Liu et al. (1996). The analyticalresults of major and trace elements are presented inTable 1.

Sr and Nd isotopic ratios were measured by a VG354mass spectrometer at the Institute of Geology andGeophysics, CAS. The 87Sr/86Sr ratio of the NBS 987standard and 143Nd/144Nd ratio of the La Jolla standardare 0.710265±12 (2r) and 0.511862±10 (2r), respec-

tively. The measured 87Sr/86Sr and 143Nd/144Nd ratios

were normalized to 86Sr/88Sr=0.1194 and 146Nd/144Nd=

0.7219, respectively. 87Rb/86Sr and 147Sm/144Nd ratios

were calculated using the Rb, Sr, Sm and Nd abun-

dances measured by ICP-MS. The measured and age-

corrected 87Sr/86Sr and 143Nd/144Nd ratios are listed in

Table 2.

Results

Major elements

The group 1 rocks have SiO2=46.56–56.89%,MgO=3.29–8.54% and Na2O/K2O=1.00–5.83. mg-number (=Mg2+ /(Mg2+ +Fe2+) in atomic ratio) is inthe range of 0.41–0.60 and Ni of 26–132 ppm, repre-senting relatively primitive to evolved compositions.They are relatively high in Al2O3 (13.71–17.98%, aver-age of 15.80%), TiO2 (1.16–2.27%, average of 1.68%)and P2O5 (0.21–0.68%, average of 0.36%) in compari-son with those of the basaltic rocks from island arc(Al2O3<16%, TiO2<1%, P2O5<0.3%, Wilson 1989).These characteristics are similar to the contemporaneousChebu gabbros in the area interpreted as the analogy ofthe intraplate basaltic rocks (Li et al. 2003).

The group 2 rocks contain SiO2 of 60.24–66.71% andMgO of 2.65–5.54% with K2O/Na2O of 0.42–1.12.mg-number=0.42–0.60, Ni=24–102 ppm, Cr=84–266 ppm, CaO=3.79–7.04% and Al2O3=13.23 13.72%.They can be classified as high-mg andesitic rocks definedby Kelemen (1995) (Fig. 2a). The group 3 rocks arehighly differentiated (SiO2=72.75–77.97%) and peralu-minous with A/CNK [=molecular Al2O3/(Na2O+-K2O+CaO)] value of 1.0–1.1. They are low in FeOt(0.91–2.84%), MgO (0.17–0.46%), CaO (0.10–1.71%),TiO2 (0.169–0.43%) and P2O5(<0.1%), and high in al-kalis (K2O=3.59–7.88%, Na2O=1.55–3.13%) withNa2O/K2O of 0.22–0.52, similar to those of the con-temporaneous Zhaibei and Pitou A-type granites in thearea (Li et al. 2003; Chen et al. 1999).

In the total alkalis versus silica (TAS) diagram(Fig. 2b), the group 1, 2 and 3 rocks plot into the tho-leiitic basalts and basaltic andesites field, the andesiteand dacites field and rhyolites field, respectively. Similarclassification results were also observed in the Zr/TiO2

versus Nb/Y diagram (not shown). It was also notedthat CaO, MgO, FeOt and TiO2 decrease with magmaevolution in the group 1 and 2 rocks. Al2O3 and P2O5

generally decrease in the group 1 rocks but only showslight variation for the group 2 rocks with the increase ofSiO2 (Fig. 3). In the Harker diagram (Fig. 3), threegroups follow separate fractionation trends, suggestingthat they could not be evolved or derived from eachother.

Incompatible elements

Both the group 1 and 2 rocks exhibit fractionated REEpatterns (Fig. 4a, b). The group 1 rocks have lowerLREE contents (89–227 ppm), (La/Yb)cn ratios(3.8–7.2) and more insignificant Eu anomalies (0.65–1.15) than those of group 2 (LREE=179–349 ppm,(La/Yb)cn=8.8–11.2, Eu/Eu*=0.63–0.79). The group 3rocks share a subparallel REE pattern with pronouncedLREE and HREE enrichment and a strong negative Eu

55

anomaly (Eu/Eu*=0.15–0.57; Fig. 4c). On the primitivemantle normalized diagrams (Fig. 4d–f), the group 1rocks generally display an increasing enrichment of theelements from Lu to Rb with increasing incompatibility,apart from the significant Sr and Ba negative anomalies.Nb is depleted relative to La (0.57–0.95) and Th is en-riched relative to La (1.14–3.65). Furthermore, thegroup 2 rocks have the subparallel ‘‘spiky’’ patterns witha pronounced Nb–Ta depletion ((Na/La)PM=0.37–0.53)and weak Pb enrichment, similar to island arc-relatedvolcanics. The Sr, P and Ti depletions are also theapparent features of the group 2 rocks. Relativelysmooth spectra with negative Nb–Ta anomalies punc-tuated by strong troughs at the Ba, Sr, P and Ti posi-tions are observed for the group 3 rocks.

Isotopic composition

The initial Sr and Nd isotopic ratios have been correctedusing an average age of 172 Ma. For the group 1 rocks,the initial 87Sr/86Sr ratios are in the range of0.70602 � 0.70822 and �Nd(t) values of �1.63 to +0.11,similar to those of the contemporaneous Chebu gabbrosin the area (Li et al. 2003). On the 87Sr/86Sr(t) versus�Nd(t) diagram (Fig. 5), they are plotted within the range

defined by the Society islands-Afar-Etendeka basaltswith an affinity to EMII-type source (Hawkesworthet al. 1984). The group 2 samples show higher87Sr/86Sr(t) ratios (0.70985�0.71016) and lower �Nd(t)values (�9.44 to �7.78) than the group 1 rocks. Thegroup 3 rocks have �Nd(t) = �4.63 to �5.80 and87Sr/86Sr(t) = 0.70962 � 0.71104, similar to those of thecontemporaneous Zhaibei and Pitou A-type granites inthe region (Chen et al. 1999; Li et al. 2003).

Discussion

Group 1 and 2 rocks: derivationfrom a peridotite-plus-vein lithospheric source

Magma process

All samples have LOI of less than 3.0 wt%, and theNb/La and Th/La ratios do not correlate with LOI andEu/Eu*, suggesting insignificant alteration. Selectedelements are plotted against MgO for the group 1 and 2rocks in Fig. 6a–d. The decrease of compatible elements(Cr and Ni) reflects the early fractionation of olivine andclinopyroxene (Fig. 6a, b). This is also evidenced by thecharacteristics of MgO and FeOt (Fig. 3a, b) and

Table 1 Major and trace element concentrations of early Mesozoic volcanics in southern Jiangxi Province

Group 1 basaltic rocks

20GN47 20GN16 20GN18 20GN2 20GN20 20GN22 20GN27 20GN28 20GN76 20GN77 20GN36 20GN37 20GN39 20GN40 20GN41 20GN42 20GN43 20GN44 20GN45

SiO2 51.48 55.54 51.21 47.02 45.43 48.58 54.21 52.96 50.35 50.40 53.11 53.46 53.52 53.39 52.81 50.02 50.77 51.42 50.05Al2O3 16.20 14.48 14.50 15.35 17.05 17.50 14.06 13.43 16.14 16.47 15.14 14.99 14.68 15.26 15.18 16.70 15.70 15.67 15.45Fe2O3 1.62 3.54 3.85 4.24 3.30 2.20 3.94 4.53 3.41 3.55 1.62 2.05 1.66 1.49 2.05 1.63 1.86 1.56 1.90FeO 7.48 5.17 8.50 6.80 8.38 9.08 6.00 5.60 6.43 6.15 8.08 7.73 7.92 7.90 7.80 8.03 7.92 8.18 8.70MgO 6.83 3.21 6.21 7.54 8.33 7.47 5.19 4.48 6.85 6.74 5.79 5.68 5.98 5.85 5.96 7.65 6.59 7.91 6.65CaO 7.92 7.38 8.04 9.71 10.68 8.40 7.68 9.51 10.98 10.65 7.52 7.47 8.06 7.73 7.41 8.55 9.44 8.08 9.81Na2O 2.89 2.64 2.17 2.34 2.04 1.97 3.37 2.55 1.89 1.81 2.81 2.83 2.80 2.73 2.75 2.71 2.80 2.59 2.68K2O 1.34 2.67 1.88 1.06 0.35 1.44 1.31 1.69 1.21 0.46 1.87 1.86 1.54 1.75 1.90 0.95 1.04 0.98 0.78MnO 0.15 0.17 0.11 0.18 0.19 0.13 0.19 0.22 0.18 0.20 0.15 0.15 0.15 0.15 0.15 0.15 0.16 0.15 0.17TiO2 1.30 1.56 1.99 1.76 2.21 1.71 2.03 2.48 1.40 1.53 1.62 1.65 1.58 1.59 1.61 1.39 1.58 1.32 1.60P2O5 0.27 0.27 0.56 0.22 0.55 0.66 0.43 0.48 0.18 0.18 0.30 0.30 0.26 0.29 0.29 0.23 0.27 0.21 0.25LOI 2.33 2.93 0.78 3.54 1.29 0.72 1.17 1.90 1.18 1.70 1.80 1.65 1.68 1.67 1.90 1.98 1.69 1.73 1.77mg 0.58 0.41 0.48 0.56 0.57 0.55 0.49 0.45 0.56 0.56 0.52 0.52 0.53 0.53 0.53 0.59 0.55 0.60 0.53Sc 18.4 36.5 33.6 35.4 37.9 39.9 28.6 29.8 31.5 30.0 24.2 24.9 24.7 22.6 23.4 21.1 30.1 19.6 34.7Cr 61.8 38.0 72.1 125 83.6 83.2 64.5 68.2 64.5 62.2 70.9 73.0 82.7 74.7 66.9 79.1 103 78.3 101Co 50.3 26.0 43.7 59.3 56.8 43.5 47.5 44.7 51.1 48.3 44.8 44.8 46.3 46.3 47.6 54.8 48.3 59.5 58.2Ni 97.7 16.0 33.7 70.6 79.3 70.2 45.1 47.9 48.1 44.7 64.9 66.2 62.1 70.1 65.9 126 58.5 132 50.8GaRb 71.4 140 94.2 42.2 42.3 94.8 19.5 48.2 27.9 44.6 57.4 99.0 85.0 95.7 101 45.3 49.4 55.3 35.9Sr 315 250 237 292 533 439 196 202 331 279 296 297 282 304 305 325 334 303 316Y 32.1 33.5 38.0 50.1 46.2 44.0 35.0 37.5 28.0 25.4 41.8 39.5 33.5 37.2 37.1 27.8 30.4 24.4 28.7Zr 113 137 226 210 232 272 174 188 106 108 185 172 134 161 173 119 88.5 92.6 112Nb 19.5 16.8 24.7 25.8 25.9 30.0 19.4 21.2 13.0 14.5 26.9 19.5 24.0 26.9 26.9 18.7 20.0 17.1 17.1Ba 222 458 357 76.0 266 509 75.0 210 137 224 246 245 204 236 259 168 184 135 159La 24.1 22.3 26.7 26.2 29.9 39.2 25.3 24.0 18.3 18.5 28.3 30.4 24.3 27.3 27.7 19.2 26.5 19.3 17.4Ce 51.7 48.1 64.9 60.8 67.1 90.0 56.5 54.5 39.5 39.9 61.2 64.5 52.4 58.9 58.7 41.6 44.2 41.0 38.5Pr 6.06 6.09 8.23 7.84 8.54 11.23 7.07 6.94 4.97 4.87 7.32 7.66 6.27 6.95 6.78 5.01 5.44 4.67 4.77Nd 25.0 26.6 36.2 36.3 38.2 48.9 32.0 31.4 21.0 21.5 29.7 30.9 25.0 28.5 29.0 21.2 22.9 17.9 20.6Sm 5.77 5.82 7.96 8.81 8.75 10.94 7.19 7.04 4.73 4.94 7.37 7.00 5.72 6.64 7.01 4.93 5.56 4.45 5.32Eu 1.62 2.20 2.16 3.45 3.19 3.58 2.98 2.59 1.76 1.72 1.81 1.82 1.54 1.70 1.66 1.46 1.61 1.18 1.51Gd 5.78 5.67 8.21 8.95 8.90 10.27 7.41 7.57 5.16 5.42 6.93 7.31 6.44 6.63 6.76 5.37 5.64 4.50 5.42Tb 0.98 0.91 1.23 1.36 1.40 1.49 1.12 1.14 0.87 0.83 1.12 1.24 1.02 1.11 1.05 0.84 0.91 0.73 0.88Dy 6.04 5.30 7.36 7.88 8.27 8.75 6.64 6.77 5.42 5.25 7.02 6.87 6.19 6.73 6.78 5.01 5.75 4.43 5.09Ho 1.16 0.99 1.40 1.36 1.53 1.70 1.22 1.33 1.01 0.97 1.38 1.47 1.20 1.28 1.28 0.95 1.13 0.88 1.12Er 3.49 2.72 3.88 4.14 4.54 4.93 3.50 3.75 2.95 3.17 3.97 4.28 3.45 4.01 3.85 2.94 3.19 2.65 3.08Tm 0.50 0.36 0.53 0.54 0.59 0.59 0.45 0.51 0.36 0.37 0.52 0.53 0.47 0.49 0.53 0.39 0.43 0.37 0.41Yb 3.07 2.23 3.25 3.50 3.66 4.08 2.84 3.50 2.70 2.40 3.77 3.73 3.09 3.45 3.55 2.54 2.97 2.51 2.67Lu 0.47 0.30 0.47 0.53 0.52 0.57 0.40 0.47 0.41 0.33 0.54 0.55 0.47 0.49 0.51 0.34 0.41 0.32 0.40Hf 3.35 4.66 6.16 6.92 6.41 7.75 4.79 5.16 4.64 3.20 5.88 5.03 4.20 4.96 5.13 3.39 2.74 3.26 3.36Ta 1.26 1.07 1.40 1.71 1.61 1.92 1.24 1.40 0.79 0.77 1.72 1.86 1.64 1.89 1.75 1.23 1.34 1.29 1.15Pb 9.96 6.58 8.59 6.04 5.64 11.98 5.10 4.43 4.26 4.46 10.3 10.1 8.80 9.78 11.2 6.01 6.44 8.68 5.64Th 6.59 3.55 5.61 3.57 5.14 6.89 3.84 4.41 2.80 2.70 11.8 13.2 9.66 10.2 11.0 6.95 7.32 8.72 5.61U 1.41 0.55 0.88 0.85 0.90 1.01 0.57 0.58 0.45 0.43 2.55 2.42 2.43 2.25 2.46 1.50 1.40 1.86 1.10

mg-number=Mg2+ /(Mg2++Fe2+), major oxides: wt%, trace elements: ppm

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the correlation between MgO and CaO/Al2O3 (Fig. 6c).The decrease of Sr/Nd ratio with magma evolution(Fig. 6d) and negative Sr anomaly (Fig. 4d, e) indicatesthe importance of plagioclase fractionation. The deple-tions of P and Ti in Fig. 4e for the group 2 samplesmight be related to the influence of apatite and Ti–Fe-oxides. A proportional fractionation of alkaline feldsparis likely required to account for the decrease of CaO(Fig. 3) and negative Eu anomaly (Fig. 4b) shown by thegroup 2 rocks. This speculation is also consistent withpetrographic observations. However, as shown inFigs. 4 and 5, a simple fractionation fails to explain thevariability of some important elemental (REE, Th, Nb,Ta, Zr, Hf, Y) and isotopic systematics in these rocks.Therefore, the involvement of crustal components, eitherin magma source regions or crustal assimilation enroute, represents a very likely alternative.

It is difficult to assess the assimilation between inter-mediate-basic magma and crustal materials, due to thelack of knowledge of the basement and sedimentary rocksin the area. However, the data from the other areas of theSCB provide some constraints (Chen and Jahn 1998;Shen et al. 1998; Li 2000;Wang et al. 2003b). The gneissesand their derivation in the SCB, which can be regarded asthe equivalent of average crustal materials (Wang et al.

2003b; Shen et al. 1998), have Nd contents of 26–62 ppm(average of 37 ppm) that are close to those of the group 1and 2 rocks (21.0–49.9 ppm). The crustal materials in theSCB have an average �Nd(t) value of �11 (Wang et al.2003a; Shen et al. 1998; Chen and Jahn 1998), suggestingthat an unrealistic proportion of crustal assimilation(>60%) would be required to produce the group 2 rocks(�Nd(t) = �9.44 to �7.78). In the case of each group,their elemental and isotopic systematics also cannot bereasonably explained by simple crustal contaminationduring magma ascending. Assuming that the relativelyhigh mg and �Nd(t) samples (e.g., 20GN-20) can beregarded as a relatively primary magma, the variability ofHFSEs and REE concentrations for most samples for thegroup 1 rocks cannot be explained by the simple crustalcontamination of the magma (Fig. 4a, d). By contrast,simple crustal contamination of the 20GN-22 sample canexplain the REE pattern of others for group 1 (Fig. 4a),but it fails to satisfy the variability of LILEs concentra-tions and �Nd(t) values. Similar inconsistency is alsoobserved for the group 2 rocks (Fig. 4b, e). For example,simple crustal assimilation of the 20GN-65 sample canaccount for the trace elemental systematics of the group 2rocks, but the sample has the lowestmg and highest SiO2.Relatively SiO2-poor samples yield even higher

Group 2 high-mg andesite–dacites Group 3 rhyolites

20GN74 20GN70 20GN25 20GN26 20GN80 20GN82 20GN65 20GN83 20GN84 20GN85 20GN86 20GN90 20GN91 20GN92 20GN93 20GN95 20GN34 20GN35 20GN8

59.06 59.40 62.73 61.69 63.09 63.14 65.05 76.57 74.01 76.52 76.93 77.09 72.43 73.22 76.86 76.56 76.93 76.96 72.1313.35 13.16 13.46 13.45 13.10 13.01 13.14 11.72 13.13 11.56 11.94 11.74 14.05 13.22 12.03 11.75 11.09 10.46 13.482.91 2.02 2.31 2.69 3.62 4.19 2.93 1.87 1.79 1.65 1.16 0.92 1.65 1.56 1.06 0.71 2.06 2.20 1.094.38 5.07 4.53 5.03 4.13 3.43 3.80 0.35 0.25 0.30 0.63 0.17 0.25 0.18 0.10 0.20 0.75 0.78 1.755.18 5.44 3.72 3.79 3.55 3.26 2.60 0.17 0.16 0.21 0.18 0.31 0.44 0.24 0.40 0.28 0.41 0.36 0.466.90 6.37 4.94 5.20 4.75 4.41 3.74 0.28 0.45 0.53 0.10 0.36 0.14 0.14 0.25 0.39 1.19 1.71 0.662.58 3.01 2.61 2.11 2.74 2.49 3.67 1.70 1.55 1.82 1.74 2.40 1.87 2.47 1.88 3.13 1.91 1.82 2.342.45 2.59 2.26 2.53 2.56 1.09 1.55 5.75 6.99 5.73 6.11 6.13 7.91 7.88 6.14 5.99 3.85 3.59 6.320.14 0.13 0.11 0.10 0.12 0.15 0.12 0.01 0.02 0.03 0.01 0.03 0.03 0.03 0.04 0.05 0.03 0.04 0.030.82 0.80 1.17 1.15 1.01 1.33 1.15 0.17 0.17 0.18 0.16 0.22 0.39 0.34 0.25 0.19 0.42 0.43 0.320.35 0.37 0.26 0.25 0.39 0.35 0.32 0.02 0.02 0.02 0.01 0.02 0.05 0.04 0.02 0.03 0.07 0.09 0.091.52 1.48 1.70 1.74 1.04 2.50 1.87 1.27 1.31 1.33 0.90 1.40 0.67 0.56 0.84 0.63 1.09 1.36 1.100.57 0.59 0.50 0.48 0.46 0.45 0.4223.7 23.8 23.2 23.7 21.1 18.3 17.1 4.75 5.76 4.30 4.25 3.85 8.22 5.02 3.38 4.26 7.50 8.80 7.16266 249 194 195 115 107 84.4 5.39 6.36 7.31 7.38 7.25 8.35 8.01 8.05 8.32 13.86 14.23 14.1233.3 30.3 24.0 30.2 18.2 21.5 9.2 0.45 0.32 0.58 0.59 0.30 0.48 0.35 0.50 0.16 3.20 3.66 3.3999.0 102 78.2 98.2 41.5 33.6 24.2 0.97 0.86 1.15 1.86 1.50 1.73 2.18 1.56 2.69 2.99 3.07 3.00

24.3 25.2 21.1 21.9 17.8 20.7 20.2 18.5 19.0 17.0 17.0 26.7154 173 93.1 96.1 136 65.4 138 296 367 296 305 275 357 297 275 270 126 120 275417 362 459 499 216 282 168 27.5 31.2 28.4 25.9 52.1 68.2 44.4 50.6 9.7 253 171 13231.9 28.8 37.5 39.1 40.7 38.2 57.2 124 56.0 47.9 38.9 59.2 67.6 37.5 46.4 33.3 32.5 39.52 49.9208 207 325 329 282.2 277 347 226 253 243 221 99.4 99.3 108 89.9 98.9 246 273 29115.2 15.1 23.9 23.9 26.0 24.3 36.1 40.0 45.4 37.4 38.7 30.9 31.2 30.1 25.2 30.8 33.1 35.3 30.11302 983 856 949 687 624 899 202 255 211 218 30.5 120 85.9 67.0 28.8 679 732 111739.2 37.2 54.3 51.8 48.6 44.2 65.8 188 85.2 66.7 58.9 61.2 189 131 39.6 47.4 50.7 55.7 10376.5 74.5 108 107 103 94.6 140 369 152 103 119 140 329 258 125 115 100 114 1648.66 8.21 12. 8 12.1 11.9 11.0 16.52 40.9 17.9 13.8 12.4 16.2 42.5 28.8 11.9 11.5 11.6 13.5 21.632.8 31.3 49.5 47.7 49.1 44.3 65.4 153 65.3 51.3 43.5 63.7 159 106 44.1 41.3 42.7 51.8 81.36.75 6.75 9.60 9.84 10.04 9.18 13.21 29.0 12.8 9.54 8.54 15.1 26.8 17.9 10.0 9.66 8.74 10.1 14.81.53 1.52 2.16 1.98 2.29 2.32 3.14 1.36 0.88 0.82 0.54 0.86 2.82 1.72 0.80 0.61 1.74 1.55 2.606.51 6.00 8.71 9.06 9.38 8.38 12.24 26.1 10.9 9.08 7.16 13.3 21.1 13.2 8.67 7.69 7.50 8.94 12.80.98 0.87 1.24 1.23 1.38 1.33 1.80 3.69 1.73 1.47 1.13 2.06 2.83 1.68 1.49 1.30 1.07 1.30 1.825.56 5.28 7.10 7.26 7.87 7.56 10.82 21.3 10.9 9.30 7.24 12.1 13.7 8.48 9.20 7.59 6.12 7.52 9.611.04 0.99 1.34 1.45 1.46 1.42 2.10 3.98 2.30 1.89 1.39 2.16 2.42 1.47 1.77 1.40 1.15 1.37 1.773.49 2.89 3.80 4.01 4.40 3.68 5.92 11.8 6.89 5.68 4.42 5.99 6.11 3.81 5.09 4.31 3.31 4.14 4.890.45 0.38 0.56 0.54 0.59 0.47 0.79 1.55 1.02 0.83 0.67 0.78 0.76 0.47 0.62 0.62 0.45 0.54 0.603.03 2.64 3.46 3.52 3.93 3.02 5.25 101 7.00 5.70 4.61 4.62 4.43 2.81 3.99 4.04 3.19 3.53 3.880.39 0.40 0.50 0.51 0.55 0.40 0.78 1.50 1.04 0.83 0.69 0.63 0.61 0.38 0.52 0.54 0.44 0.53 0.606.09 6.22 8.98 8.87 7.94 8.28 9.88 9.15 9.66 9.11 8.66 4.53 3.92 4.09 3.84 4.72 7.80 8.37 8.841.12 1.02 1.47 1.53 1.52 1.49 2.05 3.12 3.32 2.93 3.02 2.88 2.07 1.97 2.94 3.19 1.65 1.89 1.9620.3 21.8 19.1 20.1 13.7 21.4 18.8 36.5 67.7 46.1 29.0 41.9 36.1 33.6 31.7 38.5 36.9 35.8 37.414.3 14.0 18.0 18.0 15.7 14.8 18.3 44.9 47.2 37.2 42.8 34.2 34.9 32.6 30.9 39.7 22.9 27.1 29.23.17 3.08 3.54 3.42 1.72 1.80 2.47 1.74 5.11 4.41 3.37 5.61 6.55 3.64 3.16 4.80 3.99 4.92 5.07

57

87Sr/86Sr(t) ratios (e.g., 20GN-76), and the sample withmore pronounced negative Nb–Ta anomaly does notnecessarily correlate with the higher SiO2 and lower mgfeatures. �Nd(t) values slightly vary irrespective of SiO2

contents for each group. Nb/La ratios are in the range of0.64–0.99 for the group 1 rocks, and display insignificantcorrelation withmagma evolution (Fig. 6e). In the case ofthe group 2 rocks, Nb/La ratios range from 0.40 to 0.56,and decrease with increasing MgO (Fig. 6e). Theseevidences clearly argue against an AFC process. There-fore, the variability of elemental and isotopic composi-tions in the group 1 and 2 rocks is more likely anindicative of crustal component involved in magmasource regions rather than crustal assimilation en route.

Source heterogeneity

Group 1 subalkaline basaltic lavas: products of an EMII-like mantle source The group 1 rocks demonstratehigher Ba/Nb (2.95–16.57), Th/La (0.14–0.42) andTh/Ta (2.09–7.10) ratios, and lower Nb/La (0.64–0.99),Ce/Pb (4.72–12.30), Ba/La (7.49–20.54) and Nb/U(8.04–36.34) ratios, in comparison with those of OIB(Hofmann et al. 1986; Sun and McDonough 1989).Such signatures are distinct from those of astheno-sphere-derived melts characterized by the enrichment ofLILEs, LREE and HFSEs with Nb/La ratio >1. Theserocks also exhibit (Nb/La)PM of 0.57–0.95 and (Th/La)PM ratios of 1.14–3.65, distinct from those of thebasaltic rocks in subduction- or arc-related settings(Nb/La<0.3). As illustrated in Fig. 7, there is no cor-relation between Nb/La and �Nd(t),

87Sr/86Sr(t), Th/Laand Ce/Pb. Therefore, such characteristics suggest that

a simple source contamination/mixing of two end-members is not a realistic petrogenetic model for thegroup 1 magma. Thus, the simplest interpretation isthat the crustal signatures of the group 1 magma di-rectly inherit from that of an enriched lithosphericmantle (EM). There are two kinds of enriched mantle(EM) sources: EMI and EMII.

An EMII-type source is generally characterized by87Sr/86Sr ratios in excess of 0.7065 and intermedi-ate143Nd/144Nd ratios (Hart 1988; Weaver 1991). It haslower Ba/Nb and Ba/La ratios, but higher Th/La andZr/Nb ratios, than those of an EMI-type mantle source(Hart 1988; Thompson 1983; Palacz and Saunders1986; Weaver 1991). The group 1 rocks have 87Sr/86Srratio of 0.70602 � 0.70812, �Nd(t) value=�1.63 to+0.11, Nb/La=0.64–0.99 and Zr/Nb=4.43–9.07, dis-playing an affinity to an EMII-like source (Fig. 5). Themafic magmatism characterized by an EMII-like sourceusually favors an origin involving subducted or recy-cled sediments (Weaver 1991). However, numerouspaleomagnetic, structural and geochemical data fromthe SCB do not support the development of a youngsubducted slab during early Mesozoic (Engebretsonet al. 1985; Wang et al. 2003a, b; Li et al. 2003), andthus argue against the subduction-modified lithosphericmantle scenario during early Mesozoic. Recently, Liet al. (2003) reported the elemental and Sr–Nd isotopiccompositions of the early Mesozoic gabbro-syenitesuite in the region (see Fig. 1b for locations), andconsidered that their sources might have been an en-riched lithospheric mantle metasomatised by OIB-typemelts. As shown in Fig. 5, the contemporaneous Chebugabbros (172 Ma) have isotopic compositions similar tothe group 1 rocks, and uncontaminated syenites

Table 2 Results of Sr–Nd isotopic analyses for the volcanic sequence in southern Jiangxi Province

Sample Sm Nd Rb Sr 147Sm/144Nd 87Rb/86Sr 143Nd/144Nd(2r) 87Sr/86Sr(2r) 87Sr/86Sr(t) � Nd(t)

Group 120GN27 7.19 31.98 19.50 195.58 0.136 0.289 0.512557(10) 0.708220(20) 0.707520 �0.2620GN76 4.73 21.00 27.88 331.26 0.136 0.244 0.512511(10) 0.708707(19) 0.708121 �1.1720GN22 10.94 48.94 154.78 239.14 0.135 1.877 0.512486(10) 0.709734(18) 0.708224 �1.6320GN20 8.75 38.17 42.33 333.34 0.139 0.386 0.512557(7) 0.708426(16) 0.707873 �0.3220GN18 7.96 36.15 94.22 236.91 0.133 1.153 0.512494(11) 0.709504(18) 0.706724 �1.4320GN16 5.82 26.64 140.39 249.71 0.132 1.630 0.512572(15) 0.710342(30) 0.706403 0.1120GN36 7.37 29.72 57.35 295.69 0.150 0.562 0.512557(15) 0.707762(19) 0.706413 �0.5720GN42 4.93 21.24 49.32 325.23 0.147 0.429 0.512573(14) 0.707130(20) 0.706092 �0.1820GN46 8.38 31.93 71.88 275.08 0.159 0.757 0.512567(12) 0.707833(20) 0.706007 �0.5620GN47 5.77 24.99 71.43 315.22 0.140 0.657 0.512554(10) 0.708300(30) 0.706712 �0.40Group 220GN74 6.75 32.81 154.26 417.20 0.124 1.072 0.512074(14) 0.713496(25) 0.710905 �9.4420GN70 6.75 31.31 173.10 362.39 0.130 1.385 0.512125(12) 0.714215(59) 0.710868 �8.5720GN25 9.60 49.49 93.13 458.74 0.117 0.589 0.512151(11) 0.711413(20) 0.709990 �7.7820GN26 9.84 47.73 96.10 499.19 0.125 0.558 0.512143(13) 0.711509(30) 0.710160 �8.1020GN82 9.18 44.32 65.40 281.77 0.125 0.673 0.512103(10) 0.711476(20) 0.709850 �8.89Group 320GN90 15.07 63.69 255.28 72.063 0.143 10.27 0.512311(10) 0.735862(30) 0.711040 �5.2220GN92 17.93 105.63 277.22 70.401 0.103 11.42 0.512296(15) 0.738321(20) 0.710728 �4.6320GN34 8.74 42.66 225.51 203.26 0.124 3.22 0.512260(10) 0.717908(21) 0.710134 �5.8020GN8 14.75 81.27 275.24 130.34 0.110 6.12 0.512229(11) 0.724415(18) 0.709617 �6.0920GN93 10.03 44.09 274.48 50.60 0.138 15.73 0.512263(12) 0.748624(23) 0.710612 �6.03

58

(165 Ma, i.e., shortly after the eruption of the group 1rocks) exhibit an increasing OIB contribution. Therealso exists a small volume of alkaline basalts (168 Ma)as asthenosphere-derived melt in the Antang area ofcentral Jiangxi Province (location shown in Fig. 1a,Wang et al. 2004). In southern Jiangxi Province, nocontemporaneous (or earlier) OIB-type magma hasbeen discovered so far. This suggests that the group 1rocks and contemporaneous Chebu gabbros are likelyrepresentative of a potential end-member of the latermafic magmatism. Accordingly, the geochemical vari-ations of the group 1 rocks can be attributed to thesource heterogeneity of an EMII-like lithosphericmantle.

Group 2 rocks: derivation from metasomatised veins in themantle Most high-mg rocks on the earth were producedduring the subduction of a young and hot oceanic crust,such as those in Cascades, West Mexico, southern Chileand West Aleutian (Kelemen 1995; Morra et al. 1997).As such, high-mg rocks are generally considered to bederivations from the incremental interaction of youngslab melt with mantle peridotite (Kelemen 1995; Defantand Drummond 1990; Yogodzinski et al. 1994; Shinjo1999), and are characterized by the depleted Sr–Ndisotopic signatures, whereas the group 2 rocks exhibithigher 87Sr/86Sr(t) ratios (0.70985�0.71016) and lower�Nd(t) values (�9.44 to �7.78). They also have SiO2=60.24–66.71%, Al2O3=13.23–13.72%, Yb= 2.64–

SiO2

Normal arcs lavas

High mg andesites

0.0

0.2

0.4

0.6

0.8

40 45 50 55 60 65 70 75 80

Group 1Group 2

SiO2

0

K2O

+N

a 2O

4

6

8

10

12

40 45 50 55 60 65 70 75 80

basaltbasaltpicritic basaltic

andesite

andesite

basanite

Trachyandesite

tracybasalt

trachyandesitebasaltic

Trachydacite

Subalkaline

Alkaline

dacite

rhyolite

Group 1

Group 3Group 2

mg

num

ber

(a) (b)Fig. 2 a SiO2 versus mg numberplot. b SiO2 versus K2O +Na2O diagram. The fields forhigh-mg andesites (between 54and 65 wt% SiO2) and normalarc rocks are defined byKelemen (1995), and theclassification scheme in (b) isafter Middlemost (1994)

4

8

12

16

2

4

6

MgO

Al 2O

3B

2O5

SiO2 SiO2

FeO

CaO

TiO

2

8

10

12

12

14

16

18

20

3

6

9

12

0.0

1.0

2.0

3.0

40 45 50 55 60 65 70 75 800.0

0.2

0.6

0.4

0.8

40 45 50 55 60 65 70 75 80

(a) (b)

(f)(e)

(d)(c)

Group 1

Group 3Group 2

Fig. 3 SiO2 versus major oxidesdiagrams of the early Mesozoicvolcanic sequence in southernJiangxi Province. All majoroxides are normalized to 100%on a volatile-free basis. Symbolsare same as those in Fig. 3a

59

5.25 ppm, Y=28.8–57.2 ppm, Sr=168–499 ppm, Sr/Y=3.0–13.1, La/Yb= 12.4–15.7 and negative Sr and Euanomalies. These features are distinct from those oftypical adakites, which are advocated to be the deriva-tions from a subducted oceanic slab or interaction of thedelaminated lower crust with lithospheric mantle ornewly accreted lower crust (Xu et al. 2002; Defant andDrummond 1990).

The lower degree partial melting of hydrous refrac-tory peridotite is another petrogenesis scenario for high-mg rocks (Hirose 1997; Baker et al. 1994). Higher La/Yband Gd/Yb ratios of the group 2 rocks in comparisonwith those of group 1, appear to favor this possibility.However, the group 2 rocks display distinct Sr–Nd iso-topic compositions from the group 1 rocks derived froman EMII-like source. They also have closely temporaland spatial association with the group 1 basaltic rocks,and only represent a very small volume in the volcanicsequence. It is therefore proposed here that the genera-tion of the group 2 rocks are more likely related to themetasomatised veins in the peridotites rather than thehydrous refractory peridotite (Santos et al. 2002;Chalot-Prat and Boullier 1997). The metasomatisedveins are normally centimeter-scale thick, and generallyconstitute <10% of the volume of the peridotite (Santoset al. 2002). Relatively lower K2O/Na2O ratios for thegroup 2 rocks may indicate the greater importance ofclinopyroxene and amphibole in the veins (Foley 1992;Witt-Eickschen et al. 1998). These pyroxenitic and/or

Zhaibei & Pitou A-type granites

Zhaibei & Pitou A-type granites

Roc

ks/P

rimiti

ve m

antle

Roc

ks/P

rimiti

ve m

antle

RbBaTh U Nb Ta La Ce PbSr Nd P Zr Hf SmEu Ti Y Yb Lu1

10

100

100020GN-74 20GN-70 20GN-25

20GN-2620GN-80 20GN-82 20GN-65

Rb BaTh U NbTa La CePb Sr Nd P Zr Hf SmEu Ti Y YbLu1

10

100

100020GN-20 20GN-72 20GN-2 20GN-2220GN-42 20GN-43 20GN-45 20GN-76

20GN-77

20GN-47 20GN-41 20GN-36 20GN-3720GN-39 20GN-40 20GN-18 20GN-28

20GN-16

Group 2

Group 1

(f)(e)

Group 1

(a) (d)

20GN-83 20GN-8520GN-86 20GN-90 20GN-9120GN-92 20GN-93 20GN-9520GN-34 20GN-35

20GN-8

Roc

ks/C

hond

rite

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Group 30.1

1

10

100

1000 20GN-8320GN-85

20GN-8620GN-90

20GN-91

20GN-92

20GN-9320GN-95

20GN-3420GN-3520GN-8

Roc

ks/P

rimiti

ve m

antle

Rb Ba Th U NbTa La Ce PbSr Nd P Zr Hf SmEu Ti Y Yb Lu

Group 3

Roc

ks/C

hond

rite

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

20GN-74 20GN-70 20GN-25

20GN-2620GN-80 20GN-82 20GN-65

Group 2

(b)

Roc

ks/C

hond

rite

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

20GN-20 20GN-72 20GN-2 20GN-2220GN-42 20GN-43 20GN-45 20GN-76

20GN-77

20GN-47 20GN-41 20GN-36 20GN-3720GN-39 20GN-40 20GN-18 20GN-28

20GN-16

(e)

Gneisses in the SCB

Granulite

Gneisses in the SCB

Granulite

Granulite

Gneisses in the SCB

Granulite

Gneisses in the SCB

Gneisses in the SCB

Granulite

Gneisses in the SCB

Granulite

Fig. 4 Chondrite-normalizedREE patterns and primitivemantle-normalized spidergramsfor the early Mesozoic volcanicrocks in southern JiangxiProvince. Normalized valuesfor chondrite and primitivemantle are from Taylor andMcLennan (1985) and Sun andMcDonough (1989),respectively. The data for theaverage gneisses and granulitesin the SCB are from Wang et al.(2003b). The fields for theAntang alkaline basalts(168 Ma) in central and theZhaibei and Pitou A-typegranites in southern JiangxiProvince are from Wang et al.(2004) and Li et al. (2003),respectively

Walvis R

idge

Kenya

KerguelenAfar

-12

-8

-4

0

4

8

12

0.702 0.704 0.706 0.70887Sr/86Sr(t)

0.710 0.712

MORB

OIB

Society islands

Etendeka

Nd(

t)

Zhaibei and Pitou A-type granitoid

-160.714

Group 1 basaltic rocksGroup 2 high mg rocksGroup 3 rhyolites

Antang alkaline basalts (168 Ma)Quannan Syenites (165 Ma)

Chebu gabbros (171 Ma)

LC

/MC

in

Sou

th C

hina

Syenites contaminated by the continental crust en route

EMII

EMI

Samoa

Fig. 5 87Sr/86Sr(t) versus �Nd(t) diagram for the early Mesozoicvolcanic sequence in southern Jiangxi Province. The isotopiccompositions of the lower/middle crust (LC/MC) in South China,represented by the Shangxi and Shuangqiaoshan Groups, are fromChen and Jahn (1998) and Chen et al. (1999). The fields for MORB,OIB, Afar, Society Islands, Etendeka, Kenya, Walvis Ridge andKerguelen mantle sources are from Hawkesworth et al. (1984),Hart (1988) and Weaver (1991). The Zhaibei and Pitou A-typegranites (172 Ma) in southern Jiangxi Province are from Chen et al.(1999) and Li et al. (2003). The field for syenites with contaminatedcontinental crust en route in southern Jiangxi (165 Ma) is from Liet al. (2003). The ranges for the Chebu gabbros (172 Ma) andQuannan syenites (165 Ma) in southern Jiangxi are from Li et al.(2003), and that of the Antang alkaline basalts (168 Ma) in centralJiangxi are from Wang et al. (2004)

60

amphibolitic veins might be the crystallized productsfrom a previously (possibly the early Paleozoic) meta-somatised fluid/melt derived from a deeper mantlesource or mantle wedge (Suen and Frey 1987). Themetasomatism of fluid/melt likely resulted in majorelement additions, and involvement in LILEs, LREEand Sr–Nd isotopes with respect to dominantly perido-titic wall rocks.

It is additionally inferred that the 01GN-70 and �74samples, which are typified by SiO2 of 59.06–59.40%,mg of 0.58–0.61, MgO of 5.28–5.54%, Cr of 249–266 ppm and Ni of 99–102 ppm, likely representhigh-mg primary andesitic magma derived from themetasomatised veins. The other samples might representthe fractionated products of such high-mg primaryandesitic magmas.

Group 3 rhyolites: melting of a hybridized sourceand subsequent fractionation

In southern Jiangxi Province, rhyolites represent �50%of the total volume of the early Mesozoic volcanicproducts. They have high Zr (99–290 ppm), Hf(3.9–9.7 ppm) concentrations and Ga/Al (2.76–3.87)ratios, and low Ba, Sr, P and Ti contents. These rockshave compositions similar to the contemporaneousZhaibei and Pitou A-type granitoids in the region (Chenet al. 1999; Li et al. 2003) and average A-type granites(White and Chappell 1983). In the Ga/Al versusZr+Nb+Ce+Y plot (Fig. 8), they fall into the A-typegranite field. Four models might be considered to ex-plain the petrogenesis of A-type magma. These include:(1) the anatexis of continental crust; (2) the partialmelting of granulites or granulitic residues (Whalen et al.1987); (3) extensive differentiation of the mantle-derivedmelts in shallow magma chambers; and (4) the partialmelting of mafic lower crust underplating at the base ofthe crust (Trua et al. 1999).

The anatexis scenario can be firstly discarded on thebasis of the following characteristics. The metamorphicbasement in the SCB and their derivations have87Sr/86Sr(t) of 0.7120 � 0.7250 and �Nd(t) of �12 to �16(Chen and Jahn 1998; Shen et al. 1998), which do notmatch those of the rhyolites. The Al2O3 contents of therhyolites are in the range of 10.6–13.7%, lower thanthose of crust-derived melts (>13.5%, Zen 1988;Skjerlie and Johnson 1992). Furthermore, the hotter andanhydrous A-type rocks could not be generated by thefractionation of anatectic magma, because their contin-uous fractionation would lead to the decrease in tem-perature accompanied by increasing H2O (Turner et al.1992).

Cr

MgO

Pl

(d)

0.3

0.6

0.9

1.2

1.5

(Sr/N

d)P

M

(e)

0.2

0.4

0.6

0.8

1.0

1.2

2.0 4.0 6.0 8.0 10.0

(Nb/

La)

PM

OlPl

Cpx

(c)

0.2

0.4

0.6

0.8

1.0

CaO

/Al 2

O3

(b)

50

100

150

200

250

300

(a)

Group1

Group2

40

80

120

160

Ni

Fig. 6 Plots of MgO versus Cr and Ni (a, b), and MgO versus CaO/Al2O3 (c), (Sr/Nd)PM (d) and (Nb/La)PM (e) for the group 1 andgroup 2 rocks in the early Mesozoic volcanic sequence fromsouthern Jiangxi Province. Symbols are the same as in Fig. 6a

b

61

Several granulite xenoliths hosted by late Mesozoic–Cenozoic basalts in the SCB have isotopic compositionssimilar (Yu et al. 2002) to the group 3 rocks. However,

the Rb, Ba, Y and LREE contents of the granulites aretoo low (Taylor and McLennan 1985) to attain the ob-served abundances of A-type rocks under a reasonabledegree of partial melting (Trua et al. 1999). In addition,the melting of granulites should display quite low Rb/Sr,K2O/MgO, high Na2O (>4.3%) and positive Euanomaly (Rapp and Watson 1995), but such featuresare not observed in these rhyolites. In order to achievethe incompatible element abundances of the A-typemagma, a very low degree melting of granulites andsubsequently inconspicuous fractionation are required(Trua et al. 1999). But this is inconsistent with the factthat these rhyolites experienced extensive crystallization(see the discussion below). Thus, it is unlikely that thepartial melting of the granulites is responsible for thegenesis of the group 3 rocks.

The extensive fractionation of contemporaneousmafic magmas is now one of the two remainingalternative reasons for the genesis of the group 3rocks. The extremely low concentrations of certainelements depicted above pinpoint the role of the majorfractionating minerals: Sr and Eu were extracted byplagioclase and K-feldspar; Ba by alkaline feldspar; Pby apatite and Ti by Ti–Fe oxides. One or more fer-romagnesian phases must have additionally been frac-tionated to reduce the MgO and FeO contents. Morefractionated samples should qualitatively have lowerSr, Ba, Eu*, P and Ti and higher LREE concentra-tions. However, there is no correlation between theSiO2 and Sr, Ba, La and Eu* contents of the group 3rocks. The variations of the Sr, Zr, Th, Ba and Laconcentrations are far greater than what would beexpected from low-pressure crystal fractionation alone(Fig. 9). In Fig.5, the group 3 rocks are plotted alonga trend toward the field of the Shangxi group (equiv-alent to the middle-lower crust). This suggests a con-tribution of the crustal materials to the genesis of the

0.7055

0.7060

0.7065

0.7070

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-0.50

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Nd(

t)87

Sr

/S

r(t)

Th/

La

Ce/

Pb

Nb/La

(b)

(c)

(d)

(a)86

Fig. 787Sr/86Sr(t), �Nd(t), Th/La and Ce/Pb against Nb/La plots

for the group 1 basaltic rocks in southern Jiangxi Province

Zhaibei & PitouA-type granites

50

100

1000

1

10000*Ga/Al3 6

300

2

I-,S-&M-typegranites

A-typegranites

Zr+

Ce+

Y+

Nb(

ppm

)

4 5

Fig. 8 Plots of early Mesozoic group 3 rhyolites in southernJiangxi Province in the Zr+Nb+Y+Ce versus 10000Ga/Aldiagram of Whalen et al. (1987), showing an affinity to A-typegranite. The field for the Zhaibei and Pitou A-type granites in thearea is from Li et al. (2003) and Chen et al. (1999)

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group 3 rocks. Thus, it seems unlikely that the group 3rhyolites are the direct product of strong fractionationof contemporaneous mantle-derived magma. A morereasonable interpretation is that the group 3 rocks arethe melting product of the underplating mafic magmacontaminated by the crustal basement, which subse-quently underwent fractionated crystallization. Tofurther test this hypothesis, these rocks were plotted inthe Zr, La and Sr diagrams (Fig. 9a, b) from simpleequilibrium melting calculations. It is noted that all of

the samples do not simply follow the partial melting orfractionated crystallization trends. An extensive degreeof partial melting followed by an extensive degree offractionation can account for the variations of Zr, Laand Sr contents. This also holds true for involvementof the above-mentioned fractionation and petrologicalobservation of alkaline feldspar, plagioclase, apatiteand Fe–Ti minerals in their genesis.

Tectonic implication: an intracontinental rifting model

Based on the discussions in the preceding sections, it canbe concluded that the group 1 basaltic rocks are likelyderived from an EMII-like lithospheric mantle. Thegroup 2 high-mg andesite–dacites are genetically relatedto the melting of the metasomatised veins related to theprevious subduction in the lithospheric mantle. A two-stage scenario, that is, the partial melting of hybridizedsource between underplating mantle-derived rocks andthe crustal materials followed by extensive fractionalcrystallization, represents a possible petrogenesis modelfor the group 3 rocks. These data have shed some newlights on the early Mesozoic tectonics of the SCB, whichhas been an issue of long-term debate (Hsu et al. 1990;Rodgers 1989; Charvet et al. 1994; Zhou and Li 2000;Gilder et al. 1996; Wang et al. 2003a, b; Li et al. 2003,2004; Li 2000). A rifting tectonic model in response tothe lithospheric extension can be proposed to accountfor the petrogenesis of the early Mesozoic volcanicsequence in the region (Li et al. 2003).

In the case of passive rifting model, the continentallithospheric mantle is physically replaced by theasthenosphere mantle. The prerifting continental litho-spheric mantle unlikely contributes to the generation ofbasaltic magmas. In the case of active rifting model, thebasaltic magma would be derived exclusively from thepre-rifting lithospheric source, or the chemical interac-tion between the asthenospheric and pre-rift lithosphericmantle (Altherr et al. 1990; Chung et al. 1994). It is oftendifficult to evaluate whether an extension process ispassive or active. However, the group 1 rocks and theChebu gabbro (172 Ma) in southern Jiangxi Provinceexhibit an affinity to an EMII-like source, and theuncontaminated syenites of 156–165 Ma have a greatercontribution of OIB component. This evidence appearsto support an active rifting model in which intraconti-nental lithospheric extension was triggered by thermalerosion in response to the asthenospheric upwelling. Asa consequence, the EMII-like lithospheric mantle wasmelted to produce the group 1 magma, and the meta-somatised veins in the source to generate the group 2rocks. Accompanying the erosion of the basal litho-sphere, some preexisting lithospheric faults might bereactivated, and this may further lead to the propagationof lithospheric fractures. These faults/cracks would actas conduits for transporting heat energy upward andresult in the elevation of ambient temperatures in thecontinental crust. The elevation of crustal temperature

FCFC

Partial melting curve

Nb(

ppm

)(a)

Zr(ppm)

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Fractional crystallization

Partial melting Fractional crystallization

Partial melting

DSr=2.80; DZr=0.06

CNb CSr=7pm; =450 ppmO O

DSr=9.80; DZr=0.70

CNb CZr=7 pm; =30 ppmO O

DNb=0.18; DZr=0.06 DNb=0.30; DZr=0.70

Fig. 9 Zr versus Sr and La diagrams (a, b), illustrating the two-stage model for the petrogenesis of the group 3 rhyolites insouthern Jiangxi Province. The modal equilibrium partial meltingcurves (solid) have been drawn using Zr, Nb and Sr contents of 30,7 and 450 ppm in the protolith. The phase proportions deducedfrom the major elements calculations and mineral-melt partitioncoefficients are referred from Villemant et al. (1981) and Furmanet al. (1991). The fractionated crystallization (FC) curves (dashed)have been drawn using products of 10 and 30% melting of theprotolith. The phase proportions observed in 20GN-35 rhyoliteand mineral-melt partition coefficient are from Villemant et al.(1981), Mahood and Stimac (1990) and Ewart and Griffin (1994).The numbers on the partial melting and FC curves refer topercentages of melting and fractionation, respectively. The bulkpartition coefficient used in the calculations of the partial meltingand fractionated crystallization are illustrated in (a) and (b). Theresults show that 5–30% partial melting followed by �30–50%fractionation might be the most likely process to generate the group3 rhyolites

63

could then induce the melting of the underplating maficrocks hybridized with crustal materials to form thegroup 3 A-type magma at shallower levels. Experimentalwork has demonstrated that the mineral assemblage ofA-type magma can only be produced in high tempera-ture environments (>900�C) with low H2O contents(Clements and Vielzeuf 1987; Thompson 1983). Thissuggests that the group 3 rocks represent highly frac-tionated products from high-temperature and relativelyanhydrous magmas.

It is generally considered that there are twoadditionally essential, intrinsic conditions for generatinghigh-mg rocks (Tatsumi and Maruyama 1989; Crawfordet al. 1989). These include (1) an extensional tectonicregime, either by a sudden increase in the rate of thinningor by sudden switch from convergence to extension, and(2) a thermal regime with high temperatures. In light ofthe first condition, the occurrence of the group 2 high-mgmagma in southern Jiangxi Province probably indicates asudden change of the tectonic regime in the SCB at�172 Ma. This speculation is supported by the facts that:(1) the Indosinian intracontinental orogenic event wasdominated by the development of thrust and nappe untilthe end of J1 (Li 1998; JBMGR 1989; Chen 2001); (2) anextensional regime with the development of the meta-morphic core complexes, extensive granitic magmatismand red-bed fault-basins became predominant since J3 (Li2000; Li et al. 2003, 2004). To meet the second condition,a thinner lithospheric mantle would be required. There-fore, an intraplate active rift setting provided favorabletectonic conditions for the petrogenesis of the earlyMesozoic volcanics in southern Jiangxi Province.

Acknowledgements We would like to thank Jorg Geldmacher andWolf-Christian Dullo for their critical and constructive review andcomments. We also thank Drs. Y.H. Zhang and X.H. Li for helpfuldiscussions and suggestions, and X. Chen for his help duringfieldwork. This study was supported by the projects from NationalNature Sciences Foundation of China (40334039, 49973021), theChinese Academy of Sciences (GIGCX-03-01, KZCX2-102), theMinistry of Science and Technology of China (G1999043209) andthe Elemental and Isotopic Laboratory of GIG, CAS.

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