Journal of Asian Earth Sciences 72 (2013) 108–117

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Journal of Asian Earth Sciences

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Revisiting Early–Middle Jurassic igneous activity in the NanlingMountains, South China: Geochemistry and implications forregional geodynamics

1367-9120/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jseaes.2012.07.010

⇑ Corresponding author at: Nanjing Institute of Geology and Mineral Resources,Nanjing 210016, China. Tel.: +86 25 84897916; fax: +86 25 84600446.

E-mail address: yhaimin@sina.com (H.-M. Ye).

Hai-Min Ye a,b,⇑, Jian-Ren Mao a, Xi-Lin Zhao a, Kai Liu a, Dan-Dan Chen a

a Nanjing Institute of Geology and Mineral Resources, Nanjing 210016, Chinab Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

a r t i c l e i n f o a b s t r a c t

Article history:Available online 23 August 2012

Keywords:Early–Middle Jurassic igneous rocksNanling MountainsGeochemistryGeodynamic implications

Early–Middle Jurassic igneous rocks (190–170 Ma) are distributed in an E–W-trending band within theNanling Tectonic Belt, and have a wide range of compositions but are only present in limited volumes.This scenario contrasts with the uniform but voluminous Middle–Late Jurassic igneous rocks(165–150 Ma) in this area. The Early–Middle Jurassic rocks include oceanic-island basalt (OIB)-type alkalibasalts, tholeiitic basalts and gabbros, bimodal volcanic rocks, syenites, A-type granites, and high-K calc–alkaline granodiorites. Geochemical and isotopic data indicate that alkaline and tholeiitic basalts andsyenites were derived from melting of the asthenospheric mantle, with asthenosphere-derived magmasmixing with variable amounts of magmas derived from melting of metasomatized lithospheric mantle. Incomparison, A-type granites in the study area were probably generated by shallow dehydration-relatedmelting of hornblende-bearing continental crustal rocks that were heated by contemporaneous intrusionof mantle-derived basaltic magmas, and high-K calc-alkaline granodiorites resulted from the interactionbetween melts from upwelling asthenospheric mantle and the lower crust. The Early–Middle Jurassicmagmatic event is spatially variable in terms of lithology, geochemistry, and isotopic systematics. Thisindicates that the deep mantle sources of the magmas that formed these igneous rocks were significantlyheterogeneous, and magmatism had a gradual decrease in the involvement of the asthenospheric mantlefrom west to east. These variations in composition and sourcing of magmas, in addition to the spatial dis-tribution and the thermal structure of the crust–mantle boundary during this magmatic event, indicatesthat these igneous rocks formed during a period of rifting after the Indosinian Orogeny rather than duringsubduction of the paleo-Pacific oceanic crust.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Mesozoic igneous activity in South China was voluminous,widespread, and occurred over a long duration, thereby represent-ing an important component of the Circum-Pacific volcanic–intrusive belt. The temporal migration of igneous activity fromsouth to north and from west to east is characterized by: (1) initiallong-duration (209–86 Ma) magmatic activity in east Guangdong,with magmatism migrating northwards to east Fujian and eastZhejiang, causing shorter-duration magmatic activity at 133–93 Ma; and (2) younging of igneous activity from west to east,coincident with an increase in intensity and scale from the interiorto the coast (Xing et al., 2002). This magmatism is thought to be re-lated to westward subduction of the paleo-Pacific Plate, although

the timing of initial paleo-Pacific Plate subduction and the influ-ence of this subduction on magmatic activity during the Mesozoicare still controversial (Zhou and Li, 2000; Li and Li, 2007; Chenet al., 2008).

Jurassic igneous rocks crop out over a total area of �64,100 km2

and represent the most abundant Mesozoic igneous rocks in theSouth China (Sun, 2006). These rocks are dominated by granitesand rhyolites (>95%) with minor basaltic rocks. Recent geochrono-logical analysis has identified two main episodes of Jurassic mag-matism: a first-stage magmatic event at 190–170 Ma, and asecond-stage event at 165–150 Ma; the majority of Jurassic gran-ites are thought to be second-stage (Li, 2000; Mao et al., 2009).The origin, petrogenesis, and metallogenesis of this extensive andmineralized second-stage magmatic event have been the focus ofmany studies (e.g., Gilder et al., 1996; Chen and Jahn, 1998; Zhouand Li, 2000; Pirajno and Bagas, 2002; Chen and Grapes, 2003; Liet al., 2003; Li and Li, 2007). In comparison, the first stage of theJurassic magmatic event has received little attention, primarily

H.-M. Ye et al. / Journal of Asian Earth Sciences 72 (2013) 108–117 109

because of its limited volume and distribution. The presence ofintraplate basalts, syenites, aluminous A-type granites and bimo-dal volcanic rocks that formed during this first stage of magmatismis suggestive of continental extension or intraplate rifting,potentially a completely different tectonic setting compared withthe second stage granitoids (165–150 Ma; Chen et al., 2002,2005; Li et al., 2003, 2004; Wang et al., 2003, 2005). However,the exact geodynamic setting that produced this continental exten-sion or intraplate rifting is controversial, especially given that thisextensional event would constrain the initiation and influence ofthe subduction of the paleo-Pacific Plate. Li and Li (2007) proposedflat-slab subduction of the paleo-Pacific Plate and ascribed the gen-eration of both Indosinian and Early Yanshanian intraplate mag-matism to slab foundering due to rollback during retreating flatsubduction. In addition, Zhou et al. (2006) suggested that Early–Middle Jurassic magmatism represents a far-field response to earlystage subduction of the paleo-Pacific Plate at the eastern margin. Incomparison, other researchers (e.g., Chen et al., 2002, 2008; Wanget al., 2003) suggest that Early–Middle Jurassic magmatism wasunrelated to paleo-Pacific subduction, and instead was caused byextension after the Indosinian Orogeny.

A number of studies have been undertaken on certain aspects ofthe 190–170 Ma Early–Middle Jurassic magmatism (e.g., Chenet al., 2002; Li et al., 2003; He et al., 2007; Yu et al., 2010), yet acomprehensive analysis and interpretation of the compositionalassociations, geochemical characteristics, and Sr–Nd isotope sys-tematics of this event does not as yet exist. In this paper, we pro-vide a comprehensive synthesis of the Early–Middle Jurassicigneous record, the majority of which has previously been pub-lished in regional Chinese journals. The aim of this study is toexamine the spatial and temporal variations in the geochemistryof these igneous rocks, determine the mantle sources and compo-sitional evolution of the magmas that formed these rocks, and con-strain the geodynamic setting of this magmatic event.

2. Geological settings

The Nanling Mountains are an E–W-trending mountain systemcomprising the Yuechengling, Dupangling, Mengzhuling, Qitianlingand Dayuling mountains. These mountains lie along the Hunan–Guangxi, Hunan–Guangdong, and Jiangxi–Guangdong province bor-ders and are the watershed of the Yangtze and Pearl rivers (Fig. 1).Since the Early Paleozoic, this region has experienced multiple tec-tonic events, including the Caledonian and Indosinian orogeniesand the Yanshanian tectono-magmatic event, leading to the forma-tion of mineralized granitoids of various ages. The Indosinian Orog-eny occurred in the Middle Triassic and represents the final completeamalgamation of the Yangtze and Cathaysian plates, which led to theformation of the South China Continent (Chen et al., 2002).

The region is dominated by granitic rocks with only small vol-umes of outcropping basaltic rocks, and is an important area forW, Sn, Bi, REE, U, Cu, Pb, and Zn mineralization. Granite-hostedmineralization in the Nanling Mountains has been the focus of re-search for decades, primarily due to the presence of world-class W,Sn, and Bi ore deposits (Chen and Jahn, 1998; Zhou and Li, 2000;Pirajno and Bagas, 2002; Li et al., 2003, 2004). Batholiths withinthe Nanling Mountains are predominantly composed of biotiteand A-type granites, and granodiorite, with less voluminous EarlyJurassic intraplate basalts, syenites, aluminous A-type granites,and bimodal volcanic rocks in the south Hunan, south Jiangxi, westFujian, and north Guangdong areas.

Early–Middle Jurassic magmatic activity is limited both in spatialdistribution and scale. Igneous rocks associated with this magmaticevent occur in the southern parts of Hunan and Jiangxi provinces, thewest of Fujian Province, and the north of Guangdong Province alongthe E–W-trending Nanling Tectonic Belt, with minor outcrops along

the Jiangshan–Shaoxing Fault zone (Fig. 1, Appendix A). The follow-ing four rock associations have been identified. (1) Basaltic and gab-broic rocks in the east of Hunan Province and the south of JiangxiProvince, including the Changchengling basalts (178 ± 3.6 Ma; Zhaoet al., 1998), Baoanyu alkaline basalts (174.3 ± 0.8 to 176.2 ± 0.9 Ma;Li et al., 2004), and Huilongyu lamprophyres (169.1 ± 2.7 to172.2 ± 2.7 Ma; Wang et al., 2003) in eastern Hunan, the Chebu(172.9 ± 4.3 Ma; Li et al., 2003) and Chenglong gabbros(182.3 ± 1 Ma, He et al., 2010) in southern Jiangxi, and the Xialangabbros (196 ± 2 Ma; Yu et al., 2009) in eastern Guangdong. (2) Bi-modal volcanics, consisting of equal volumes of basalts and rhyoliteslocated in a volcanic basin in the south of Jiangxi Province and thesouthwest of Fujian Province, including the Changpu–Baimianshivolcanics in Xinwu (173 ± 5.5 Ma; Chen et al., 1999), the Dongk-eng–Lingjiang volcanics in Longnan (178 ± 7.2 Ma, Chen et al.,1999; 173.7 ± 2.5 to 174.9 ± 3.9 Ma, Zhang et al., 2002), and thePankeng volcanics in Yongding (170 ± 0.8 Ma; Deng et al., 2004).(3) Alkaline syenites and A-type granites associated with bimodalvolcanics in south Jiangxi, including the Tabei (178.2 ± 1.5 Ma; Heet al., 2010) and Huangbu syenites (179.3 ± 1.0 Ma; He et al.,2010), and the Pitou (178.6 ± 1.5 Ma; He et al., 2010), Zhaibei(171.6 ± 4.6 Ma; Li et al., 2003), and Keshubei A-type granites(189 ± 3 Ma; Li and Li, 2007). (4) High-K calc-alkaline rocks of south-eastern Hunan Province and the Dexing area of northeast JiangxiProvince, including the Shuikoushan, Baoshan, and Tongshanlinggranodiorites (172.3 ± 1.6 Ma, 173.3 ± 1.9 Ma, and 177.1 ± 1.6 to181.5 ± 8.8 Ma, respectively; Wang et al., 2002) in southeast HunanProvince, the Dexing and Fujiawu granodiorite–porphyries in north-east Jiangxi Province (171 ± 3 Ma; Wang et al., 2004), and the Tang-quan granodiorites (182.9 ± 3.6 Ma; Mao et al., 2004) in southwestFujian Province. Each of these associations has a distinct spatial dis-tribution (Fig. 1); for example, most basaltic rocks are located adja-cent to the Binzhou–Linwu Fault, whereas the bimodal volcanics,A-type granites, and syenites are situated near the Shaowu–Heyuanand Zhenghe–Dapu faults.

3. Geochemistry

Published major and trace element compositions, and isotopicdata for representative samples of the Early–Middle Jurassic igne-ous rocks are presented in Appendices B, C, and D, respectively.

3.1. Basaltic and gabbroic rocks

Basaltic and gabbroic rocks that formed during the Early–Middle Jurassic magmatic event can be grouped into two types: atholeiitic series and an alkaline series (Fig. 2). Tholeiitic series rockscontain low total alkali (Na2O + K2O 6 4 wt%) and P2O5 concentra-tions (<0.3 wt%), with high Al2O3 concentrations (>15 wt%); thesetholeiites include the Changchengling basalts in Hunan Province,the Chebu gabbros in the south of Jiangxi Province, and the Xialangabbros in the east of Guangdong Province. Alkaline basalt seriesrocks contain high total alkali (Na2O + K2O > 4 wt%), and P2O5 con-centrations (>0.3 wt%), and low Al2O3 concentrations (<15 wt%);alkaline series rocks include the Baoanxu alkaline basalts, andthe Huilongyu lamprophyres in the east of Hunan Province.

On chondrite-normalized (C: Sun and McDonough, 1989) REEspidergrams (Fig. 2), both tholeiitic and alkaline series rocks havestraight rare earth element (REE) patterns with light REE (LREE)enrichment, with alkaline series (RREE = 176.7–229.8 ppm, (La/Yb)N = 12.5–16.9) samples having higher total REE concentrations,and increased LREE enrichment, when compared with tholeiiticseries samples (RREE = 62.3–156.6 ppm, (La/Yb)N = 3.25–6.0). Boththoleiitic and alkaline rocks have moderately negative to slightlypositive Eu anomalies, with dEu values ranging from 0.75 to 1.1.

Fig. 1. Distribution of Early-Middle Jurassic igneous rocks in the Nanling Mountains, South China.

Fig. 2. SiO2 vs. K2O + Na2O diagrams, chondrite-normalized REE patterns and primitive mantle-normalized spider diagrams for the Early-Middle Jurassic igneous rocks of theNanling Mountains, South China. s = alkaline basalts; d = tholeiitic basalts; h = basaltic bimodal volcanics; j = rhyolitic bimodal volcanics; } = A-type granites; � = syenites;N = high-K. Chondrite normalization values are from Sun and McDonough (1989) and primitive mantle normalization values are from McDonough et al. (1992).

110 H.-M. Ye et al. / Journal of Asian Earth Sciences 72 (2013) 108–117

H.-M. Ye et al. / Journal of Asian Earth Sciences 72 (2013) 108–117 111

In primitive mantle-normalized (PM: McDonough et al., 1992)trace element spidergrams (Fig. 2), tholeiitic rocks are enrichedin incompatible elements, particularly in large ion lithophile ele-ments, and are weakly depleted in Ta and Nb, whereas alkalinerocks are enriched in high field strength elements, such as Ta andNb, relative to REEs. The Chebu tholeiitic gabbros are slightly de-pleted in Sr, P, and Ti, indicating that these gabbros formed frombasaltic magmas that had undergone only limited fractional crys-tallization. In comparison, the Baoanxu alkaline basalts are de-pleted in K and Rb relative to Sr and Ba, reflecting the presenceof phlogopite relic in the mantle source region.

In a eNd(t) vs. (87Sr/86Sr)i diagram (Fig. 3), alkaline series rocks plotalong the mantle array. The Baoanxu alkaline basalts plot in the firstquadrant, with (87Sr/86Sr)i values of 0.7035–0.7040 and eNd(t) valuesof 5.88–6.10, close to the field defined by primitive mantle end-member, and indicating that the magmas from which these basaltsformed were derived from the asthenospheric mantle. In compari-son, the Huilongyu lamprophyres are more enriched, with(87Sr/86Sr)i values of 0.7043–0.7049, and eNd(t) values from �1.3 to�1.7. The tholeiitic basalts have (87Sr/86Sr)i values of 0.7065 to0.7082, and eNd(t) values from �0.82 to 4.4, plotting outside of themantle array and point to enriched-mantle II end-member (EMII).

3.2. Bimodal volcanics

The basic end member of the bimodal volcanic sequences in thestudy area contain low total alkali concentrations (Na2O + K2-

O = 2.8–4.2 wt%), similar to those of continental tholeiitic basalts(Fig. 2). In comparison, the silicic end member of the bimodal volcan-ics are characterized by high SiO2 (74.89–76.85 wt%), Al2O3 (11.72–13.77 wt%), K2O (4.56–4.37 wt%), and total alkali concentrations(Na2O + K2O = 7–7.31 wt%), with low CaO contents (0.23–0.68 wt%).

These two end-members have comparable total REE concentra-tions (106–241 ppm of basalts and 132–224 ppm of rhyolites,respectively), and similar LREE-enriched REE patterns. However,the basalts have a higher degree of fractionation between LREEsand heavy REEs (HREEs) than the rhyolites, with (La/Yb)N valuesof 5.6–7.3 for the basalts and (La/Yb)N values of 3.2–6.3 for the rhy-olites. The rhyolites have uniform pronounced negative Eu anoma-lies (dEu = 0.09–0.27) whereas the basalts show insignificant Euanomalies (dEu = 0.92–1.01).

Rhyolite sample have negative Ba, Sr, P, and Ti, and positive La,Ce, Nd, Sm, and Y anomalies on primitive mantle-normalized traceelement spidergrams (Fig. 2). The high Si, K, and total REE concen-trations, low Ca concentrations, FeOt/MgO ratios, and alkaline nat-

Fig. 3. (87Sr/86Sr)i vs. eNd(t) diagram for the Early-Middle Jurassic igneous rocks ofthe Nanling Mountains, South China. s = alkaline basalts; d = tholeiitic basalts;h = basaltic bimodal volcanics; j = rhyolitic bimodal volcanics; } = A-type gran-ites; � = syenites; N = high-K calc-alkaline granodiorites.

ure of the rhyolites suggests a relationship to A-type granites (Eby,1990). In comparison, the basalts are characterized by weak deple-tions in high field strength elements (Nb and Ta) and slight enrich-ments in large ion lithophile elements (K, Rb, Ba, and Th). Inprimitive-mantle-normalized spidergrams, the basalts have a con-vex-upward pattern, similar to continental rift basalts.

Basalts and rhyolites of the Baimianshu bimodal volcanics havediffering isotopic compositions; the basalts have Sr–Nd isotopecompositions similar to the tholeiitic basalts discussed above(eNd(t) = �0.15–0.13, (87Sr/86Sr)i = 0.7083–0.7090). In a (87Sr/86Sr)i

vs. eNd(t) diagram (Fig. 3), the bimodal basalts plot on the right sideof the mantle evolution curve; in contrast, the rhyolites have high(87Sr/86Sr)i ratios, with values from 0.7256 to 0.7431, and negativeeNd(t) values from �11.9 to �11.7.

3.3. Alkaline syenites and A-type granites

Syenite plutons in the study area have high total alkali (Na2-

O + K2O = 10.6–12.0 wt%) and Al2O3 (15.03–17.42 wt%) and lowTiO2 concentrations (0.22–0.58 wt%). In SiO2 vs. total alkali (TAS)diagrams, the syenites plot in the alkaline (Fig. 2) and high-Kshoshonite (Fig. 4) fields. However, there are also geochemical dif-ferences between individual plutons. For example, the Huangbusyenite pluton contains high K2O concentrations (5.35–6.38 wt%),whereas the Taibei syenite has high Na2O concentrations (5.66–6.78 wt%). In addition, the syenites contain uniformly high LREEconcentrations, with high (La/Yb)N ratios (5.5–1.8). However, thesyenites have a very wide range of dEu values (0.3–1.8) that corre-late negatively with SiO2 concentrations, with negative Eu anoma-lies associated with high SiO2 concentrations, and positive Euanomalies in samples with low SiO2 concentrations; these varia-tions may relate to plagioclase fractionation/accumulation. Inprimitive mantle-normalized trace element spidergrams (Fig. 2),syenites are enriched in large ion lithophile elements, such as Rb,Ba, K, and Th, and high field strength elements, such as Nb, Ta,Zr, and Hf, and are significantly depleted in Sr, P, and Ti. Thesecharacteristics are probably indicative of fractional crystallizationof pyroxene, plagioclase, apatite, and Fe–Ti oxides (Almeida et al.,2002). The syenites also have depleted Nd and variable Sr isotopiccompositions (Fig. 3). The Huangbu syenite has eNd(t) values of3.61–1.20 and (87Sr/86Sr)i values of 0.7028–0.7068, whereas the Ta-bei syenites have a relatively narrow Sr–Nd isotopic compositionalrange, with (87Sr/86Sr)i values of 0.70412–0.70543 and eNd(t) valuesof 3.14–3.52.

Fig. 4. K2O vs. SiO2 diagram for the syenites and the high-Kcalc-alkaline granod-iorites. � = syenites; N = high-K calc-alkaline granodiorites; boundaries are accord-ing to Gill (1987).

Fig. 5. (a) Zr + Nb + Ce + Y concentrations vs. FeO⁄/MgO ratios diagram and (b) Zr + Nb + Ce + Y vs. 10,000 ⁄ Ga/Al diagram, showing an affinity to A-type granite (after Whalenet al., 1987; } = A-type granites).

112 H.-M. Ye et al. / Journal of Asian Earth Sciences 72 (2013) 108–117

A-type granites in the study area are characterized by high SiO2

(>71 wt%), K2O (4.5–5.9 wt%), total alkali (Na2O + K2O = 7.9–9.38 wt%), and RREE concentrations (270–718 ppm), low CaOconcentrations (<1.8 wt%), and high FeOt/MgO (9.6–145) and(La/Yb)N ratios (5.8–28.5), these features almost identical to thoseof the Kuiqi miarolitic arfvedsonite in Fujian Province (Hong et al.,1987) and the granitic Mumbulla Suite of the Lachlan Fold Belt inAustralia (Whalen et al., 1987). In primitive-mantle-normalizedtrace element spidergrams (Fig. 2), A-type granites have negativeBa, Sr, P, and Ti anomalies, and positive La, Ce, Nd, Sm, and Y anom-alies. When compared with I- and S-type granites, A-type granitesin the study area have lower concentrations of large ion lithophileelements (Rb, Th, and U), but relatively high concentrationsof total REEs and high field strength elements (Nb, Zr, and Y).In Zr + Nb + Ce + Y vs. FeOt/MgO and Zr + Nb + Ce + Y vs.10,000 � Ga/Al diagrams (Whalen et al., 1987; Fig. 5), all of thegranite samples plot within the A-type granite field. In addition,A-type granites within the study area have relatively high initialSr isotopic compositions, with (87Sr/86Sr)i values from 0.70805 to0.711, and negative eNd(t) values from �6.29 to �4.27.

3.4. High-K calc-alkaline rocks

In a K2O + Na2O vs. SiO2 diagram, the majority of high-K gran-ites in the study area are classified as granodiorites, with a fewother high-K granites samples plotting as quartz monzonites anddiorites (Fig. 2). In a K2O vs. SiO2 diagram (Fig. 5), the majority ofhigh-K granite samples plot within the high-K calc-alkaline seriesfield, with a few samples plotting within the potassic basalt seriesfield, reflecting the calc-alkaline nature of these rocks.

Representative samples of high-K granites from the study areahave total REE concentrations ranging from 103 to 164 ppm, andpatterns that decrease from LREEs to HREEs in chondrite-normalizedspidergrams (Fig. 2), with moderately negative Eu anomalies(dEu = 0.69–0.86) and variable (La/Yb)N values (7.1–23.7). In primi-tive mantle-normalized trace element spidergrams (Fig. 2), samplesof high-K granodiorites from different locations have very similarpatterns that are characteristically enriched in the large ion litho-phile elements. All of these samples are depleted in Nb and Ta, andhave Nb/La ratios between 0.50 and 0.75, similar to those of islandarc-related potassic rocks, but different from those of Nb- and Ta-undepleted potassic basaltic rocks from the southeast of GuangxiProvince (Li et al., 2000). The granodiorites exhibit depletion of Pand Ti except Shuikoushan granodiorites, suggesting that the latterwere influenced by fractional crystallization of apatite and ilmenite.

In a eNd(t) vs. (87Sr/86Sr)i diagram (Fig. 3), granodiorites fromthe Dexing area of northeast Jiangxi Province have relativelyhigh eNd(t) values of �1.14 to 1.8, and low (87Sr/86Sr)i ratios of0.7044–0.7047, whereas the Tangquan granodiorites from thesoutheast of Fujian Province have relatively low eNd(t) values from�9.7 to �10.1, and high (87Sr/86Sr)i ratios of 0.7079–0.7082.

4. Discussion

4.1. Source of magmas

4.1.1. Basaltic and gabbroic rocksChen et al. (2008) defined a mixing trend of the Mesozoic basalts

in South China sourced from between OIB-like asthenospheric man-tle, and continental lithospheric mantle sources, based on variationsin La/Nb ratios and eNd(t) values (Fig. 6). This mixing trend is nearlyidentical to others constructed for Emeishan flood basalts, and bas-alts in the southwestern Basin and Range Province in the westernUnited States by mixing of OIB-like asthenospheric and lithosphericmantle (DePaolo and Daley, 2000; Xu et al., 2007). Basaltic and gab-broic rocks from the study area plot along this trend, suggesting thatthey were produced by mixing of magmas derived from these twomantle end-members. The Ningyuan alkaline basalts have Sr andNd isotopic compositions close to those of the most depleted mid-ocean ridge basalts (MORB; Fig. 3), and plot close to the OIB-likeasthenospheric mantle end of the mixing trend in Fig. 6, indicatingthat these basalts formed from magmas derived from highly de-pleted asthenospheric mantle. Moreover, the Ningyuan alkaline bas-alts have a discordant relationship between depleted isotopiccompositions and enriched incompatible element concentrations.For example, the alkaline basalts in the study area have eNd(t) val-ues > 0 (5.88–6.1), and fSm/Nd values < 0 (from�0.17 to�0.38). Giventhe known evolution of Nd isotope compositions over time, if a mag-ma with an fSm/Nd value < 0 undergoes a long period of evolution,143Nd/144Nd ratios that originally had chondritic values (i.e., werederived from the uniform (CHUR) reservoir) should be relativelylow eNd(t) values < 0. However, this is inconsistent given the compo-sition of the Ningyuan basalts, which have eNd(t) values > 0. Thisinconsistency cannot simply be interpreted by the evolution of mag-ma. In addition, high Nb/La ratios rule out the possibility of crustalcontamination, indicating that this inconsistency in Nd isotope sys-tematics most probably relates to short-lived metasomatism of themantle source region (Norry and Fitton, 1983). Amphibolitized alka-line olivine basalt xenoliths and the replacement of embayed augitesby amphibole in spinel lherzolite are both indicative of metasoma-tizing agents in the mantle. This suggests that the Ningyuan alkalinebasalts were formed from asthenosphere-derived magmas thatmixed with a minor amount of magma derived from metasomatizedlithospheric mantle.

The Huilongyu lamprophyres and tholeiitic basalts have moreenriched Sr–Nd isotopic compositions than the Ningyuan alkalinebasalts (Figs. 3 and 6), indicating a gradual decrease in the propor-tion of asthenospheric mantle-derived magma. The tholeiitic bas-alts have high (87Sr/86Sr)i ratios (0.7065–0.7082) and are slightlyNb-depleted relative to La, reflecting crustal contamination.

It should be noted that none of these data plot at either the sub-continental lithosphere ends of the mixing line and enriched litho-sphere (West Great Basin) area in a La/Nb vs. eNd(t) diagram

Fig. 6. eNd(t) vs. La/Nb diagram showing isotopic variations in basaltic and gabbroicrocks, and syenites of Chen et al. (2008); s = alkaline basalts; d = tholeiitic basalts;� = syenites.

H.-M. Ye et al. / Journal of Asian Earth Sciences 72 (2013) 108–117 113

(Fig. 6). This suggests that the composition of the 190–170 MaEarly–Middle Jurassic mantle was not influenced by subduction-modified continental lithosphere material.

4.1.2. Bimodal volcanicsThe basic portions of the bimodal volcanic rocks within the

study area have almost the same trace element and Sr–Nd isotopiccompositions as the tholeiitic rocks described in Section 4.1.1. Gi-ven this, we infer that these basic volcanics originated from mag-mas derived from the same mantle source as the tholeiites, and,as such, the discussion is not repeated here. In comparison, the fel-sic portion of the bimodal volcanic rocks is characterized by A-typerocks. The petrogenesid of A-type granites has been a topic of hotdebate and a number of petrogenetic models have been proposed,including: (1) melting of felsic crust (Patiño Douce and Beard,1995; Patiño Douce, 1997; King et al., 1997); (2) direct fraction-ation of mantle-derived tholeiitic and alkaline magmas (Turneret al., 1992; Mushkin et al., 2003); and (3) mixing of crustally-derived felsic and mantle-derived mafic magmas (Frost and Frost,1997; Frost et al., 1999; Mingram et al., 2000). Although A-type fel-sic volcanic rocks can be produced by extreme differentiation ofcoeval tholeiitic parental magmas, the differing Sr–Nd isotopiccompositions of the tholeiitic and felsic samples discussed herediscount this possibility. Derivation from mixing between crustaland mantle sources is also untenable for the Early–Middle JurassicA-type felsic volcanic rocks, as their extremely enriched Sr–Nd iso-topic (87Sr/86Sr)i values of 0.7132–0.7256 and eNd(t) values of�11.7 to �11.9 preclude sourcing from the mantle. This indicatesthat the A-type felsic volcanic rocks were most probably derivedfrom melting of highly evolved ancient continental crust.

On Zr/Y vs. Zr and (Y + Nb) vs. Rb tectonic discrimination dia-grams, samples from the basic end member of the Early–MiddleJurassic bimodal volcanics plot within the within-plate basalt field(Fig. 7). In comparison, the acidic end-member samples plot withinthe post-collisional and within-plate granite fields (Fig. 7). Thissuggests that the basaltic–rhyolitic volcanics in the western Fujianand southern Jiangxi basins are orogeny-related and were formedduring extension and rifting of the continental crust. Given that bi-modal volcanic rocks are often genetically associated with crustalextension, the formation of bimodal volcanic rocks can be consid-ered a signature of craton breakup or orogenic belt extension. Onepossible genetic model for the Early–Middle Jurassic bimodal vol-canic rocks of Nanling Mountains, as evidenced by eNd(t) and(87Sr/86Sr)i data (Fig. 3), is as follows: upwelling basaltic magma

derived from partial melting of a weakly depleted region of themantle was strongly contaminated by assimilation of crustal mate-rial, and acted as a heat source, simultaneously providing enoughheat energy to induce partial melting of the surrounding crustalmaterial; this crustal-derived magma erupted contemporaneouslywith the mantle-derived basaltic magma, producing bimodal vol-canic rocks.

4.1.3. Alkaline syenites and A-type granitesThe enrichment in K of shoshonitic magmas can be produced by

fractional crystallization of orthopyroxene of basaltic magmas(Meen, 1987). However, the concentration of K2O in syenites fromthe study area decreases with increasing SiO2 concentrations(Fig. 4), indicating that the strongly K-enriched alkaline syeniteswere not resulted from fractional crystallization of basaltic magmas.Melting experiments indicate that decompression and dehydrationmelting of phlogopite- and pargasite-bearing metasomatized lherz-olite at pressures of <1.5 GPa may originate primitive shoshoniticmagmas (Conceiçäo and Green, 2004). The high and invariant K2Oconcentrations (4.5–5.9 wt%) indicate that K-enriched mineralphases, most possibly phlogopite and pargasite, should have beenpresent during partial melting of the mantle source for these sye-nites. Syenites may have been derived from the metasomatizedlithospheric mantle by upwelling asthenospheric mantle-derivedmelt in an extensional tectonic setting. This metasomatism resultedin increased concentrations of incompatible elements and an in-creased abundance of K-rich mineral phases, such as phlogopiteand pargasite, in the lithospheric mantle, with subsequent decom-pression and dehydration melting of this metasomatized lithospher-ic mantle producing alkaline syenite parental magmas.

The A-type granites in the study area can be further classified asA2-type granites using the Nb–Y–Ce and Nb–Y–3 � Ga triangulardiagrams of Eby (1990) Fig. 8. Although these A-type granites couldhave been produced by extreme differentiation of basaltic parentalmagmas (e.g., Turner et al., 1992; Bonin, 1996), this is not likely be-cause similar or larger volume basic rocks with A-type granites donot coexist in the study area. Melting experiments indicate thathigh-silica A-type granitic magmas most likely form by dehydra-tion melting of hornblende-bearing continental crust at pressuresof <4 kbar (Patiño Douce, 1997). A-type magmas are usually gener-ated in non-compressive tectonic regimes such as extensional orrifting environments, where thinning of the crust means magmaticheat advection can reach shallow crustal levels. This model appearsto be appropriate for generation of A-type granites in the studyarea. Attaining the required melting temperatures (>900 �C) toform A-type granite magmas at relatively shallow crustal levels re-quires heat and/or mass transfer from mantle-derived, hot basalticmagmas; the contemporaneous basalts and gabbros within thestudy area may represent a heat source of this type.

4.1.4. High-K calc-alkaline rocksGranodiorites within the study area have (La/Yb)N and (Dy/Yb)N

ratios, and negative Eu anomalies, that do not correlate with SiO2

concentrations, indicating that magmatic evolution was not con-trolled by the fractionation of plagioclase. La concentrations ingranodiorites positively correlate with La/Sm ratios (Fig. 9), sug-gesting that the geochemical characteristics of the granodioriteswere controlled by partial melting (e.g., at different depths, andin different proportions) of the crust (Borg and Clynne, 1999). Neg-ative Nb anomalies and low Nb/La ratios (0.36–0.75) could beinherited from either fluid metasomatized mantle, or could origi-nate from contamination of magmas by continental crust (Wilson,1989). As all of the granodiorites, barring those in the Tangquanarea, are located in the hinterland of South China, it is very unlikelythat these geochemical characteristics could relate to fluid-metasomatism.

Fig. 7. (a) Zr vs. Zr/Y discrimination diagram of Pearce and Norry (1979). (b) Rb vs. Y + Nb discrimination diagram of Pearce et al. (1984) and Pearce (1996). Abbreviations areas follows: ORG = ocean-ridge granitoid; GPG = within-plate granitoid; VAG = volcanic granitoid; COLG = collisional granitoid; h = basaltic bimodal volcanics; j = rhyoliticbimodal volcanics.

Fig. 8. (a) Nb–Y–Ce and (b) Nb–Y–3 � Ga ternary diagrams used to subdivide A-type granites into A1 and A2 subdivisions; diagram after Eby (1990).

Fig. 9. La vs. La/Sm diagram showing variations in high-K calc-alkaline granodioritecompositions; diagram after Eby (1990).

114 H.-M. Ye et al. / Journal of Asian Earth Sciences 72 (2013) 108–117

Granodioritic magmas are thought to be generated by eitherintensive fractional crystallization of mantle-derived melts, evolu-tion of underplating basaltic magmas, anatexis of crustal rocks, ormixing of magmas from differing source regions (Arnaud et al.,1992; Thompson, 1996; Wang et al., 2003). Early–Middle Jurassicgranodiorites contain high concentrations of SiO2, and low concen-trations of MgO, with pronounced negative Nb and Ta anomalies,and high incompatible element concentrations, suggesting thatthese magmas were not directly derived from either asthenosphericor enriched lithospheric mantle. Generation of granodioritic mag-mas by anatexis of crustal granulites is also not likely because: (1)175 Ma lower crustal granulites in the south of Hunan Province haveeNd(t) values from �5.8 to �6.0 (Kong et al., 2000), distinctly differ-ent from granodiorites in the study area (�1.14 to 1.8); and (2)

experimental studies show that magmas originated by anatexis ofgranulite facies lower-crustal material is characterized by weaklyaluminous/peraluminous, positive Eu anomalies, negative Th andU anomalies, low K2O concentrations, and high Na2O concentrations(>4.3 wt%; Sen and Dunn, 1994; Rapp and Watson, 1995).

In a (La/Yb)n vs. Yb diagram (Fig. 10), Early-Middle Jurassichigh-K calc-alkaline granodiorites plot between the melting curvesfor plagioclase hornblendite and 10% garnet hornblendite (Defantand Drummond, 1990), indicating that these granodiorites cannotsimply have been generated by anatexis of granulite facies lower-crustal material, although the involvement of granulite faciesmaterial should not be ruled out. One alternative explanation isthat the Early–Middle Jurassic granodiorites were derived frommelting of a mixed source, for example a combination of under-plating basaltic magma and lower crustal material. Granodioritesfrom the southeast Hunan, Dexing, and northeastern Jiangxi areashave isotopic compositions indicating they contain significantamounts of mantle material, with Sr and Nd isotope compositionsplotting close to the primitive mantle field, whereas granodioritesfrom the Tangquan area have more enriched Sr and Nd isotopiccompositions (Fig. 3). This suggests that the granodiorites fromsoutheast Hunan, and granodiorite porphyries from Dexing andnortheastern Jiangxi, contain more mantle-derived material com-pared with the Tangquan granodiorites from southeastern Fujian,which contain more lower crustal material.

4.2. Geodynamic setting

During the Early–Middle Jurassic (190–170 Ma), the dominantgeotectonic regime in Southeast China underwent an importantchange. The location of this magmatic activity along the E–W-trending Nanling tectonic belt indicates a north–south-trendingextensional regime. When compared with the anomalously en-

Fig. 10. (La/Yb)N vs. YbN diagram showing variations in high-K calc-alkalinegranodiorite compositions; diagram after Defant and Drummond (1990).

H.-M. Ye et al. / Journal of Asian Earth Sciences 72 (2013) 108–117 115

riched Sr and Nd isotopic compositions of the second-stagemagmatic rocks, the first-stage magmatic rocks are relativelyisotopically depleted, indicative of the involvement of bothasthenospheric and lithospheric mantle material during partialmelting (Fig 3, Appendix D). The tectonic environment during theformation of Early–Middle Jurassic basalts is well documented,with the majority of researchers agreeing that they are within-platebasalts (Chen et al., 2002, 2005; Kong et al., 2000; Li et al., 2003;Xing et al., 2002; Zhao et al., 1998; Deng et al., 2004). In MiddleJurassic volcanic basins of the southern parts of Fujian and Jiangxiprovinces, basalts are associated with rhyolites in a bimodal volca-nic sequence within ‘‘graben-like rift basins’’ (Shu et al., 2004). Theconfirmation of the presence of Early–Middle Jurassic A-type gran-ites in the southern part of Jiangxi Province is also suggestive of anextensional environment in this area (Chen et al., 2008; Li and Li,2007). Shu et al. (2004) suggested that an E–W-trending ‘‘intracon-tinental rift belt’’ (250 km long and 60–80 km wide) runs fromYongding in Fujian Province through Xunwu, Longnan, and Quan-nan in Jiangxi Province, to Shixing in Guangdong Province. Theyalso documented a large number of synsedimentary growth faultsin Early–Middle Jurassic sedimentary sequences (Shu et al., 2004).These faults are E–W trending, have plough-shaped fracturesurfaces, and controlled the deposition of Early–Middle Jurassicsedimentary sequences, indicating N–S-trending extensional tecto-nism during the Early–Middle Jurassic. The presence of bothmagmatic activity and synsedimentary faulting strongly suggeststhat the eastern parts of South China were in an extensional tec-tonic regime during the Early–Middle Jurassic, which developedalong the E–W-trending Nanling Tectonic Belt.

The origin of extensional tectonism-related thermal events, andthe geodynamic setting of deep-seated tectonics during the Early–Middle Jurassic remain controversial. Zhou et al. (2006) suggestedthat Early Jurassic magmatism was genetically associated with thepaleo-Pacific tectonic regime, and represented a tectonic responseto far-field stresses at the plate margins. In comparison, Li and Li(2007) suggested that the Early Jurassic magmatism resulted fromfoundering during rollback, and foundering of the retreating flat-subducting plate. In contrast, other researchers (Chen et al., 2005,2008; Yu et al., 2010) suggest that tectonism related to post-orogenic continental break-up after the Indosinian Orogenydominated the Early Yanshanian tectonic regime in the Nanling re-gion. It is certain that two different igneous belts have existed inSouth China since the Mesozoic, relating to active continental mar-gin-type magmatism in the Zhejiang–Fujian–Guangdong coastalarea, and intracontinental rifting-type magmatism in the Hunan–

Jiangxi–Guangdong area. However, the relationship between thesetwo different igneous belts remains highly controversial. It is stillunknown whether these two separate magmatic events representdifferent stages of the same tectonic system, two cogenetictectonic systems with a causal relationship, or merely spatial over-lapping of two unrelated tectonic systems. Considering that the190–170 Ma Early–Middle Jurassic magmatic event occurred alongan E–W-trending belt, with the majority of magmatism controlledby the E–W-trending Nanling Tectonic Belt, we infer that thesetectono-magmatic activities were caused by extension of the lith-osphere and intracontinental rifting following compressionaldeformation related to the Indosinian Orogeny, with no direct ge-netic relationship to oceanic plate subduction. This interpretationis further evidenced by Paleozoic and Early Mesozoic sedimentarybasin evolution and tectonic deformation (see detailed discussionsby Zhang et al. (2009) and Xu et al. (2009)). These Paleozoic andEarly Mesozoic sequences underwent two phases of deformation:an early E–W-trending folding and thrusting event, and a laterNNE-striking thrusting event. These two phases of deformationindicate a significant change from the Tethyan to the Pacifictectonic system in South China at �170 Ma (Zhang et al., 2009;Xu et al., 2009).

5. Conclusions

Early–Middle Jurassic (190–170 Ma) magmatic activity occurredin the Indosinian tectonic interior; the limited magmatism associ-ated with this event was concentrated within an E–W-trending foldbelt in the south of Hunan, the south of Jiangxi, the west of Fujian,and the north of Guangdong provinces in South China. Igneous rocksthat formed during this event can be classified into the followingfour groups. (1) Basaltic and gabbroic rocks derived from meltingof an OIB-like asthenospheric mantle source with variable interac-tion with the lithospheric mantle. (2) Bimodal volcanic rocks con-taining basic rocks derived from basaltic magmas generatedduring mixing between asthenospheric and lithospheric mantlesources, and felsic crustal-derived A-type rhyolites. (3) Syenitesformed by decompression and dehydration melting of metasoma-tized lithospheric mantle that interacted with upwelling astheno-spheric mantle material, and A-type granites derived fromdehydration melting of crust during heating by contemporaneousintrusion of mantle-derived magmas. (4) High-K calc-alkalinegranodiorites derived from mixing between underplating basalticmagmas and lower crustal materials. The spatial variations in lithol-ogy, geochemistry, and isotopic systematics of this Early–MiddleJurassic magmatic event suggest a gradual decrease in the involve-ment of asthenospheric mantle material from west to east.

The 190–170 Ma magmatic activity in the Nanling Mountainsdiffers significantly from later 165 to 150 Ma magmatic activityin the same area, and has affinities with rifting-related lavaserupted through thick continental crust, suggesting that this tec-tonic–magmatic activity occurred in an extensional tectonic re-gime accompanied by mafic underplating, and is thereforeunrelated to paleo-Pacific subduction.

Acknowledgements

Prof. Zhi-Gang Lu is thanked for his assistance during the prepa-ration of the initial manuscript. This work was supported by the Chi-nese Geological Survey (Project 1212010611805, 1212011085446),the National Nature Sciences Foundation of China (Project40703009), and the International & Science Technology CooperationProgram of China (Project 2011DFA22460).

116 H.-M. Ye et al. / Journal of Asian Earth Sciences 72 (2013) 108–117

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jseaes.2012.07.010.

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