Giant Radiating Mafic Dyke Swarm of the Emeishan Large Igneous Province: Identifying the Mantle Plume Centre

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doi: 10.1111/ter.12154 Giant radiating mafic dyke swarm of the Emeishan Large Igneous Province: Identifying the mantle plume centre

Hongbo Li,1,2 Zhaochong Zhang,1 Richard Ernst,3,4 Linsu L€u,2 M. Santosh,1 Dongyang Zhang1 and Zhiguo Cheng1 1State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China; 2Geologi- cal Museum of China, Beijing 100034, China; 3Department of Earth Sciences, Carleton University, Ottawa, ON K1S 5B6, Canada; 4Ernst Geosciences, 43 Margrave Avenue, Ottawa, ON K1T 3Y2, Canada

ABSTRACT In many continental large igneous provinces, giant radiating and recognized six dyke sub-swarms, forming an overall dyke swarms are typically interpreted to result from the arri- radiating dyke swarm and converging in the Yongren area, val of a mantle plume at the base of the lithosphere. Mafic Yunnan province. This location coincides with the maximum dyke swarms in the Emeishan large igneous province (ELIP) pre-eruptive domal uplift, and is close to the locations of have not received much attention prior to this study. We high-temperature picrites. Our study suggests that the Yon- show that the geochemical characteristics and geochronologi- gren area may represent the mantle plume centre during the cal data of the mafic dykes are broadly similar to those of peak of Emeishan magmatism. the spatially associated lavas, suggesting they were derived from a common parental magma. Based on the regional geo- Terra Nova, 27, 247–257, 2015 logical data and our field observations, we mapped the spatial distribution patterns of mafic dyke swarms in the ELIP,

ridge by a continent (Gower and swarms and their distributions pro- Introduction Krogh, 2002), edge-driven enhanced vide an opportunity to further test Large igneous provinces (LIPs) are mantle convection (King and Ander- the plume model for the ELIP. the product of anomalously high son, 1995, 1998; Anderson, 1998), In general, giant radiating dyke melt production rates, and thus have slab steepening and breakoff (Ke- swarms (defined to be >300 km in been linked to the arrival of a mantle skin, 2003), and meteorite impacts radius) are typically considered to starting plume at the base of the (Jones, 2005). result from the arrival of a mantle lithosphere (e.g. Richards et al., The Emeishan LIP (ELIP) has plume at the base of the lithosphere; 1989; Campbell and Griffiths, 1990; been the focus of several recent geo- they are thought to converge towards Coffin and Eldholm, 1994; Maruy- chronological, geochemical and geo- the mantle plume centre and feed ama, 1994; Kerr et al., 2000; Courtil- physical studies (e.g. Shellnutt, 2014; associated flood basalts, which are lot et al., 2003; Ernst and Buchan, and references therein). Key evidence often present only as erosional rem- 2003; Ernst et al., 2005; Saunders has been proposed to support a man- nants (May, 1971; Halls, 1982; Fah- et al., 2005; Ernst, 2014). According tle plume origin, such as the presence rig, 1987; LeCheminant and to this model, the plume centre of high magnesian picrites (Zhang Heaman, 1989; Heaman et al., 1992; region should have associated high- et al., 2006a,b) and domal uplift Ernst et al., 1995a,b, 2001; Baragar temperature picrites generated along shortly before volcanism (He et al., et al., 1996; Ernst and Buchan, the higher temperature plume axis, 2003, 2006, 2010, 2011), among other 1997b; Wilson and Head, 2002; Ernst be the locus of domal uplift shortly features. However, Ukstins-Peate and Bleeker, 2010; Ernst, 2014). before the onset of volcanism and be and Bryan (2008, 2009) recognized Therefore, a giant radiating dyke the focus of a giant radiating dyke voluminous mafic hydromagmatic swarm can be used to identify and swarm feeding the volcanism. Some deposits and submarine extrusions, locate a mantle plume centre (e.g. alternative models of LIP formation indicative of emplacement at or Fahrig, 1987; Ernst et al., 1995b, have also been proposed (e.g. Foul- below sea-level, at Daqiao and 2001; Ernst and Buchan, 1997a,b; ger et al., 2005; Foulger and Jurdy, Binchuan, which are immediately Ray et al., 2007; Ernst, 2014; Wang 2007; see discussion in Ernst, 2014), adjacent to the ‘inner’ (maximum et al., 2014), such as the 200 Ma such as lithospheric delamination uplift) zone of the ELIP (defined by Central Atlantic Magmatic Province (e.g. Elkins-Tanton and Hager, He et al., 2003). Their observations (swarm radius >2000 km) associated 2000), over-riding of a spreading have cast doubt on the existence of with the opening of the central any pre-eruptive uplift, and even on Atlantic ocean, the 1270 Ma Mac- > Correspondence: Zhaochong Zhang, State the Emeishan mantle plume model, kenzie LIP (swarm radius 2000 km) Key Laboratory of Geological Processes although the presence of associated with the plume centre located on the and Mineral Resources, China University rifting, and local down-dropping of northern side of the Canadian shield, of Geosciences, 100083, China. Tel.: blocks, could explain their observa- and the Matachewan LIP (swarm +0086 10 82322195; fax: +0086 10 tions (e.g. He et al., 2011; see later radius >800 km) with the plume 82322176; e-mail: [email protected] discussion). The trends of dyke centre located on the southern side

© 2015 John Wiley & Sons Ltd 247 Mafic dyke swarm of the Emeishan Large Igneous Province • H. Li et al. Terra Nova, Vol 27, No. 4, 247–257 ...... of the Superior craton. Evidence low-grade metasedimentary rocks these faults (Chung et al., 1998; from Venus and Mars also supports interbedded with felsic and mafic Polyakov et al., 1998; Xiao et al., a primary radiating geometry of metavolcanic rocks. The basement is 2003a, 2004b; Hanski et al., 2004, dyke swarms (Grosfils and Head, overlain by a thick sequence of late 2010; Fan et al., 2008; Lai et al., 1994; Ernst et al., 1995b, 2001). Neoproterozoic (~600 Ma) to Perm- 2011). Field observations and mag- In the case of the ELIP, the colli- ian clastic, carbonate and metavolca- netostratigraphic data suggest that sion of Greater India and Eurasia nic rocks (BGMRSP, 1991; Yan the bulk of the ELIP was formed caused Cenozoic uplift (Leloup et al., et al., 2003). The ELIP, particularly within 1–2 Ma (Huang and Opdike, 1995; Chung et al., 1998). Conse- the Panxi region, experienced tec- 1998; Ali et al., 2002). Recent iso- quently, the lava sequence is deeply tonic uplift and denudation during tope chronological data give more eroded and numerous mafic dykes the Himalayan impingement (Zhang precise constraints on the duration of are exposed (Fig. 1a). Although the et al., 1990; Courtillot et al., 1999; the eruption as 260–257 Ma, with geochronology, geochemistry and Liu et al., 2001; Wu and Zhang, voluminous eruption at ~260 Ma petrogenesis of some mafic dykes 2012). (e.g. Wang and Zhou, 2006; Zhou associated with the ELIP have been The extensive Emeishan flood bas- et al., 2006, 2008; He et al., 2007; investigated (Guo et al., 2004; Zhou alts are exposed over an area of Fan et al., 2008; Xu et al., 2008; Li et al., 2006; Zi et al., 2008; Han ~3.0 9 105 km2 and represent a late et al., 2012; Shellnutt et al., 2012; et al., 2009; Shellnutt et al., 2008; Permian continental food basalt Zhong et al., 2014). Shellnutt and Jahn, 2011; Shellnutt event (Xu et al., 2001; Ali et al., Mafic dykes in the western part of et al., 2012; Li et al., 2012), the spa- 2005). The volcanic sequence has a the Yangtze craton can be generally tial distribution patterns of the mafic thickness ranging from several hun- grouped into three major episodes dyke swarms in the ELIP are poorly dred metres to 5 km and contains pi- based on field relations (BGMRYP, understood. In this study, we present crites, tholeiites and andesitic basalts 1990; BGMRSP, 1991) and isotopic the spatial pattern of the giant dyke (Chung and Jahn, 1995; Song et al., ages of Neoproterozoic, Palaeozoic swarms in the ELIP, and discuss the 2001; Xu et al., 2001; Xiao et al., (late Permian) and Cenozoic. The geometric characteristics and the 2004a; Zhang et al., 2006a). As men- Neoproterozoic dykes are predomi- implications for the postulated Emei- tioned above, the distribution of nantly exposed in the Kangding-Pan- shan mantle plume. units belonging to the ELIP has been zhihua area. Here, the host rocks, subsequently affected by Mesozoic dominated by granites, granodiorites and Cenozoic post-emplacement and tonalites, are cut by minor Geological background faulting associated with the develop- mafic-ultramafic bodies and mafic The ELIP is located to the east of ment of the Songpan-Ganze terrane dykes (Zhu et al., 2008). These dykes the Tibetan plateau and near the and the Indo-Eurasian collision have narrow widths of 1–10 m and western margin of the Yangtze (Chung and Jahn, 1995), such as the trend NE, NW or E–W. They Block, SW China (Fig. 1a). The Jinshajiang-Ailaoshan-Red River intrude the Proterozoic basement basement rocks for the ELIP locally strike-slip fault (or suture zone, Liu and show crystallization ages comprise the Palaeoproterozoic et al., 2015). Recent studies have between 792 and 752 Ma (Li et al., Kangding Complex, composed of suggested that basalts and mafic 2003; Lin et al., 2007; Zhu et al., granulite-amphibolite facies meta- complexes exposed in the Songpan- 2008; Ren et al., 2013). The late morphic rocks, the Palaeo–Mesopro- Ganzi terrane, the Qiangtang terrane, Permian mafic dykes (Emeishan- terozoic Huili Group or its the Simao basin and in northern related) will be discussed in more equivalents, and the Yanbian or Vietnam might form part of an detail below. Dykes of Cenozoic age Kunyang Groups, which consist of extension of the ELIP displaced by are found sporadically and trend

Fig. 1 (a) Map showing the distribution of the mafic dyke swarms in the ELIP. The dyke strike is drawn according to regional geological survey maps, but the length is artificially extended for clear observation. A portion of Sub-swarm V and the Jinping and Song Da picrites to the SW of the Jinshajiang-Ailaoshan-Red River fault (Appendix S1) were relocated 500 km towards the NW to correct for sinistral strike-slip movement during the Cenozoic (Tapponnier et al., 1990; Xiao et al., 2003a). Bound- aries of differential erosion are after He et al. (2006), and the star represents the focus of the converging dyke sub-swarms. Data sources: Vietnam samples (Hanski et al., 2004; Wang et al., 2007); Yanyuan samples (Xu et al., 2001; Guo et al., 2004); Miyi samples (Shellnutt and Jahn, 2011); Baima samples (Xu et al., 2001; Shellnutt et al., 2008); Panzhihua samples (Shellnutt and Jahn, 2011); Funing samples (Zhou et al., 2006; Fan et al., 2008); Wase samples (Xu et al., 2001; H. B. Li, unpublished data); Luodian samples (BGMRGP-XINGREN, 1980; Han et al., 2009); Lijiachong samples (Liao et al., 2012; H. B. Li, unpublished data); Zimushan samples (Zhang et al., 2011; H. B. Li, unpublished data); Woniusi samples (BGMRYP-BAO- SHAN, 1980; Xiao et al., 2003b); Mianning samples (Li et al., 2012); Fumin samples (Li et al., 2015). (b) Rose diagrams of the mafic dyke sub-swarms recognized in the ELIP. Number in the lower right of the rose diagram indicates the number of dykes counted, and arrows show the dominant trends (average value). The rose diagrams were produced as follows: For each sub-swarm, the number of dykes was counted in 10° steps (0–9°,10°–19°, and so on) with each dyke given equal weighting. The lengths of segments on the rose diagram were then normalized to the entire length of the sub-swarm according to the 150- km long unlabelled scale bar on the leftmost rose diagram.

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mafic dyke swarms in the ELIP converge in the Yongren area, Yunnan Province, and cover a ~800 9 780 km area, with a radius of ~400 km and a fan angle of ~145°(Fig. 1a). Each of the sub-swarms is detailed below. Sub-swarm I is exposed widely along the Kangdian Rift, and discon- tinuously forms a 30–80 km wide and 400-km long belt from Kangding in the north, through Xichang, and Miyi, to Huili in the south. Most of the dykes are tens of metres wide and can be traced for several hundred metres in length. Individual dykes can be larger: for example, the Dabanshan dyke is 5-km wide and can be traced for 16 km. The dominant trend of the dykes is N–S Fig. 2 Plots of the U–Pb dates of mafic dykes from the ELIP. The grey band repre- (Fig. 1a). Sub-swarm I mainly sents the major eruptive stage of the ELIP (Shellnutt et al., 2012; Shellnutt, 2014; intruded Devonian units, as well as Zhong et al., 2014). Ordovician, Silurian and Emeishan basalts. It is interesting to note that the orientations of the Neoproterozo- ic rocks are the same as those of the E–W or NE (Guan et al., 2006; Jia basalts (Figs 1a and 2), which ELIP dykes. These observations sug- et al., 2013). They include lamp- unequivocally indicates a temporal gest that the fossil basement trends rophyre dykes and were intruded link with them. (several N–S-striking major faults) coeval with the Himalayan orogeny To clarify the spatial distribution across this region were long lived (Munteanu et al., 2013). of the Permian mafic dykes in the and could have controlled the ELIP, we must distinguish those emplacement of the mafic dykes belonging to the Emeishan event (Chen, 1987; CGGCJ, 1988; Zhang Geochemistry, chronology and from those of older Neoproterozoic et al., 1990); a similar interpretation geometry of the mafic dyke and younger Cenozoic ages. How- has been applied to intermixed and swarms in the ELIP ever, as we cannot determine the age sub-parallel 183 Ma (Karoo) and Temporal and spatial distribution of each dyke by radioactive isotopic 1110 Ma (Umkondo) dykes of the patterns of the Permian mafic dyke methods, we use other strategies to Okavango zone in southern Africa swarms focus our attention on just the Emei- (Jourdan et al., 2004; de Kock et al., shan-related dykes. As mentioned 2014). However, for other swarms, it Palaeozoic (late Permian) mafic above, the Neoproterozoic mafic has been inferred that the local simi- dykes are widely distributed in the dykes intrude the Proterozoic to larity in trends reflects similar stress ELIP, consisting principally of dia- Archean basement. As a result, patterns at different times (e.g. Ernst base, diabase prophyrite and gabbro although we cannot entirely rule out et al., 1995a; Ernst and Buchan, diabase (BGMRSP, 1991). These the presence of late Permian mafic 1999). mafic dykes typically have average dykes intruding the Precambrian Sub-swarm II is aligned along the widths of tens of metres, and mainly strata, out of caution, only those boundary of the Sichuan and Yunan intrude the late Permian Emeishan mafic dykes intruding Palaeozoic provinces, forming a 10–15 km wide flood basalts, the late Middle Perm- strata are included in this study. The and 175-km long belt. The individual ian carbonate strata (i.e. the Qixia or younger Cenozoic dykes can be dykes are typically more than ten Maokou limestones) and/or the older avoided based on their lamprophyric metres wide, but can only be traced Palaeozoic strata. Fig. 2 plots the composition. Based on a synthesis of for 100–200 m in length. The domi- distribution of 265–240 Ma radio- regional geological data (72 regional nant trend of these dykes is NNE metric ages of mafic dykes belonging geological survey reports and 53 (Fig. 1a). They intruded the late to the ELIP. In particular, Shellnutt maps at a scale of 1:200 000 or Middle Permian carbonate Fm. (i.e. et al. (2012) reported high-precision 1:50 000) and our field observations, the Qixia or Maokou limestone) and CA-TIMS zircon U-Pb ages of the we provide the first compilation of also Emeishan basalts. Miyi mafic dykes with a narrow age mafic dyke swarms associated with Sub-swarm III extends from the range from 260 to 257 Ma, which is the ELIP (Appendix S1). middle of Guizhou (Guiyang) to the coincident with the major eruptive As shown in Figure 1a, these mafic east of Yunnan (Dongchuan), and is stage of the ELIP (Shellnutt, 2014). dykes can be divided into six sub- 30–150 km wide, extending for Overall, the ages of these mafic dykes swarms, termed Sub-swarm I, II, III, 425 km. It is composed of a large match well those of the Emeishan IV, V and VI. The giant radiating number of mafic dykes, which are

250 © 2015 John Wiley & Sons Ltd Terra Nova, Vol 27, No. 4, 247–257 H. Li et al. • Mafic dyke swarm of the Emeishan Large Igneous Province ...... dozens of metres to a few hundred for example, their LOI (weight loss those of the Emeishan basalts and metres in width (the Lijiachong and on ignition to 1000°C) values, which OIB in the Sr–Nd diagram (Fig. 4). Yangliu dykes are 300 m in width). are generally less than 3%. Thus, the In summary, the geochemical charac- In length, they can be traced for a major and trace element geochemical teristics of the mafic dykes are in few hundred metres to several kilo- characteristics of the mafic dykes can good agreement with those of the metres (even up to 15 km; the Jin- be used to probe their magmatic Emeishan flood basalts, suggesting gmenkou dyke). The dominant trend affinity and petrogenesis. Geochemi- that they are cogenetic. of these dykes is E–W (Fig. 1a). The cally, with one exception (Miyi, mafic dykes in Sub-swarm III pre- Shellnutt et al., 2008), the mafic Implications for mantle plume dominantly intrude the Emeishan dykes in the ELIP are characterized basalts and Maokou Fm. by sub-alkaline tholeiitic affinity. In many Palaeozoic and Proterozoic Sub-swarm IV is distributed from Their Mg-number varies from 77 to LIPs, dyke swarms are better pre- the Fumin to Funing areas, Yunnan 37, and most do not exceed 63, served than the volcanic components province, and is 15–25 km in width which suggests that they represent after erosion and tectonic events, and ~170 km in length. The dykes in evolved magmas. The TiO2 contents because they are sub-vertical features the Fumin area vary considerably, are within a wide range between hidden beneath the lavas and pre- from 70 m to 2 km in width, and 0.80% and 4.58%. Xu et al. (2001) served in the basement rocks (e.g. Fah- can be traced for 6–10 km in length. used TiO2 contents of 2.5% to divide rig, 1987; Ernst et al., 1995a,b). They The dominant trend of these dykes is the Emeishan basalts into low-Ti are part of the LIP plumbing system, nearly E–W to NW (Fig. 1a). These (LT) and high-Ti (HT) types, which which consists of giant continental dykes intruded the Upper Devonian show a spatial chemical variation, dyke swarms, sills and layered intru- to Early Permian Maokou Fm. with the dominant LT in the western sions (Ernst and Buchan, 1997a, 2001). In addition, although the Jinshaji- part of the ELIP and the HT in the Although uplifted, vertically eroded ang-Ailaoshan-Red River large eastern part of the ELIP (Xiao et al., and partially segmented since the late strike-slip fault system is traditionally 2004a). By this definition, the mafic Permian (Leloup et al., 1995; Chung thought to mark the southwest dykes can also be classified into et al., 1998), the ELIP essentially pre- boundary of the ELIP, there are those two types. Moreover, the dykes serves the initial spatial distribution some dykes along both sides of this are comparable to the nearby Emei- pattern of its feeder dykes. As noted fault system (Tapponnier et al., shan basalts in terms of composition, above, the giant mafic dyke swarms, 1990). Sub-swarms V and VI and, interestingly, share the same which are contemporaneous and co- (Fig. 1a) have been reconstructed spatial TiO2 chemical distributions as magmatic with the Emeishan basalts, after correction for the inferred the Emeishan basalts, i.e. low and are composed of six sub-swarms that 500 km of post-Emeishan sinistral high Ti dykes in the west and eastern converge at the Yongren area, Yunnan strike-slip offset along the Jinshaji- parts of the ELIP respectively Province, indicating the position above ang-Ailaoshan-Red River faults (Fig. 1a). the centre of the head of the mantle (Xiao et al., 2003a). All mafic dykes show uniform plume responsible for the ELIP. Impor- Sub-swarm V is exposed from the chondrite-normalized rare earth ele- tantly, this position is identical to the west of Sichuan province to Binchu- ment (REE) patterns, which are location of the maximum crustal uplift an in Yunnan province. Dykes in characterized by enrichment in light obtained by delineating the differential Sub-swarm V are about 200 m to REE relative to heavy REE and the erosion zones, which resulted from a 2.5 km in width and can be traced absence of any significant Eu anoma- large amount of regional lithospheric for 500 m to 4 km in length. They lies (Fig. 3a). The dykes have similar uplift immediately preceding the Emei- intruded the Emeishan basalts or primitive mantle normalized trace shan flood volcanism (He et al., 2003, Upper Carboniferous units, and have element patterns (Fig. 3b), character- 2006, 2010, 2011; Xu et al., 2004). a dominant trend of NNW–NW ized by the enrichment of LILE and Recently, the differential erosion zones (Fig. 1a). HFSE, and an absence of negative have been confirmed via the residual Sub-swarm VI extends for approx- Nb-Ta and Th-U anomalies. Both gravity and density model of the ELIP, imately 200 km from Ninglang in chondrite-normalized REE patterns with a higher density anomaly À Yunnan Province to Miyi in Sichuan and primitive mantle normalized (+0.06 g cm 3) in the inner zone than À Province. Mafic dykes in Sub-swarm trace element patterns are very simi- inthemiddle(+0.04gcm 3) and outer À VI are generally dozens of metres lar to those of the Emeishan basalts (+0.03 g cm 3) zones (Deng et al., wide and can be traced for several and ocean island basalts (OIB). The 2014). The two lines of evidence (radi- 87 86 hundred metres in length. Dykes ( Sr/ Sr)i values of the mafic dykes ating mafic dyke swarm and sedimen- intrude the Qixia or Maokou Fm. vary from 0.70375 to 0.70759, tary unconformity pattern underlying and Emeishan basalts. The dominant whereas the eNd (t) values range from the basalts) indicate the same location trend of these dykes is NW (Fig. 1a). À1.97 to +5.13 with most values for the centre of the Emeishan plume, between À1.97 and +2.70 (Xu et al., thereby strengthening the hypothesis of Geochemistry of the Emeishan mafic 2001; Zhang and Wang, 2003; Xiao the existence of the Emeishan mantle dykes et al., 2004a; Hou et al., 2005; Jiang plume and the location of its centre. et al., 2007; Shellnutt and Jahn, As noted above, Ukstins-Peate and The Emeishan mafic dykes are rela- 2011). The initial Sr isotopic and eNd Bryan (2008, 2009) challenged the idea tively fresh, and this is reflected in, (t) values of the mafic dykes overlap of significant uplift prior to volcanism,

(a) (b)

Fig. 3 (a) Chondrite-normalized REE patterns and (b) primitive mantle-normalized trace element patterns of the mafic dykes of the ELIP and the Emeishan basalt. Normalized values and data of average OIB from Sun and McDonough (1989). Emei- shan flood basalt values after Shellnutt and Jahn (2011) and references therein.

and (2) the removal of magma from the mantle underlying the plume head region and dispersion away from this region can lead to enhanced subsidence of the overlying crust and mantle lithosphere (Camp- bell and Griffiths, 1990; Campbell, 2001). This subsidence near the centre of the mantle plume will be enhanced, because this is the area of the greatest volume and degree of partial melting, and transgression may accompany the subsidence (Campbell and Griffiths, 1990). The subsidence will appear as rift basins, block faults, grabens, or syn- clines, which have been recognized in the Siberian LIP (Czamanske et al., 1998), Wrangellia LIP (Richards et al., 1991), the Deccan Traps (the Cambay Basin and Narmada Graben, Raju et al., 1971; Choubey, 1971), the North Atlantic LIP (Nadin et al., 87 86 e Fig. 4 ( Sr/ Sr)i vs. Nd (t) plot for the mafic dykes of the ELIP. Data sources: 1997; Jones et al., 2001), etc. Emeishan basalt (Xu et al., 2001; Song et al., 2004, 2008; Xiao et al., 2004a; Wang However, in other areas, the Emei- et al., 2007; Fan et al., 2008; Qi and Zhou, 2008; Shellnutt et al., 2008). DMM, shan flood basalts rest unconform- EMI, EMII are from Zindler and Hart (1986), Hart (1988) and Weaver (1991). ably on the early Middle Permian Maokou Fm. at Shangcang, Binchu- an, and there is a palaeosol at the thereby potentially undermining the uplift of the Earth’s surface begins boundary (BGMRYP-DALI, 1973), Emeishan plume-head model. How- 10–20 Ma before the onset of volca- suggesting uplift and erosion in these ever, a domal uplift and local rifting nism, which can be followed by local areas. Thus, we infer that the above can be accommodated in the mantle subsidence. Theories to explain the descriptions can be accommodated in plume model. subsidence include: (1) as the plume the mantle plume hypothesis (Grif- On the basis of the plume-head or head spreads beneath the continental fiths and Campbell, 1991). In other starting-plume hypothesis (e.g. Rich- lithosphere, the mantle buoyancy words, uplift will lead to the uncon- ards et al., 1989; Griffiths and anomaly is dispersed over a larger formities on the top of the Maokou Campbell, 1991; Campbell, 1998; area, leading to subsidence above the Fm., whereas the ‘hydromagmatic Leng and Zhong, 2010), significant plume axis (Griffiths et al., 1989); deposits’ could be localized in the rift

252 © 2015 John Wiley & Sons Ltd Terra Nova, Vol 27, No. 4, 247–257 H. Li et al. • Mafic dyke swarm of the Emeishan Large Igneous Province ...... basins caused by the subsequent with the location of the plume centre Yunnan Province, Regional Geology of localized subsidence (Song et al., based on domal uplift modelling and Dali Region, Scale 1:200.000. China 2004; see also He et al., 2011). the location of picrites. Ministry of Geology and Mineral Based on the plume hypothesis, Resource, Beijing. picrites are the early melting prod- Campbell, I.H., 1998. The mantle’s Acknowledgements chemical structure: insights from the ucts of the hottest portions of the melting products of mantle plumes. In: plume head (Hill, 1991; Gill et al., This study was financially supported by The Earth’s Mantle: Composition, 1992; Coffin and Eldholm, 1994; Ar- the 973 program (2012CB416806) and Structure and Evolution (I.N.S. Jackson, ndt, 2000; Thompson and Gibson, Natural Science Foundation of China ed), pp. 259–310. Cambridge University 2000; Campbell, 2005, 2006). Thus, (No. 40925006). We thank Dr. Victor Press, New York. picrites should also be most abun- Puchkov, Dr. Kenneth Buchan and Pro- Campbell, I.H., 2001. Identification of fessor Michael Watkeys for constructive dant towards the hot centre of the ancient mantle plumes. In: Mantle comments on the manuscript. plume head and become less abun- Plumes: Their Identification Through dant towards the margins, as inter- Time (R.E. Ernst and K.L. Buchan, eds), pp. 5–21, Geological Society of preted, for example, for the Karoo References America Special Paper 352, Geological LIP (Campbell, 2005). As shown in Ali, J.R., Thompson, G.M., Song, X.Y. Society of America, Boulder, Figure 1a, the Emeishan picrites are and Wang, Y., 2002. Emeishan basalts Colorado. exposed predominantly in the west– (SW China) and the ‘end-Guadalupian’ Campbell, I.H., 2005. 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