Journal of Petrology Advance Access originally published online on July 14, 2006
Journal of Petrology 2006 47(10):1997-2019; doi:10.1093/petrology/egl034
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Geochemistry of Picritic and Associated Basalt Flows of the Western Emeishan Flood Basalt Province, China
1 STATE KEY LABORATORY OF GEOLOGICAL PROCESSES AND MINERAL RESOURCES, CHINA UNIVERSITY OF GEOSCIENCES BEIJING, 100083, P. R. China
2 SCHOOL OF OCEAN AND EARTH SCIENCE AND TECHNOLOGY, UNIVERSITY OF HAWAII HONOLULU, HI 96822, USA
3 INSTITUTE OF GEOLOGY, CHINESE ACADEMY OF GEOLOGICAL SCIENCES 100037, P.R. CHINA
RECEIVED DECEMBER 13, 2005; ACCEPTED JUNE 8, 2006
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Picritic lava flows near Lijiang in the late Permian Emeishan flood basalt province are associated with augite-phyric basalt, aphyric basalt, and basaltic pyroclastic units. The dominant phenocryst in the picritic flows is Mg-rich olivine (up to 91·6% forsterite component) with high CaO contents (to 0·42 wt %) and glass inclusions, indicating that the olivine crystallized from a melt. Associated chromite has a high Cr-number (73–75). The estimated MgO content of the primitive picritic liquids is about 22 wt %, and initial melt temperature may have been as high as 1630–1690°C. The basaltic lavas appear to be related to the picritic ones principally by olivine and clinopyroxene fractionation. Age-corrected Nd–Sr–Pb isotope ratios of the picritic and basaltic lavas are indistinguishable and cover a relatively small range [e.g.
Nd(t) = –1·3 to +4·0]. The higher Nd(t) lavas are isotopically similar to those of several modern oceanic hotspots, and have ocean-island-like patterns of alteration-resistant incompatible elements. Heavy rare earth element characteristics indicate an important role for garnet during melting and that the lavas were formed by fairly small degrees of partial melting. Rough correlations of isotope ratios with ratios of alteration-resistant highly incompatible elements (e.g. Nb/La) suggest modest amounts of contamination involving continental material or a relatively low-Nd component in the source. Overall, our results are consistent with other evidence suggesting some type of plume-head origin for the Emeishan province. KEY WORDS: Emeishan; flood basalts; picrites; mantle plumes; late Permian
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The Emeishan province of southwestern China (Fig. 1) is one of two continental flood basalt provinces that formed near the end of the Permian in widely separated locations, the other being the Siberian Traps (e.g. Chung & Jahn, 1995
). Eruption of a third flood basalt province, the Panjal Traps of northwestern India, may have overlapped these two provinces in time (e.g. Bhat & Zainuddin, 1981), and at least two oceanic plateaux formed in the ensuing 
20 Myr (Lassiter et al., 1995; Genç, 2004). An even larger outburst of widespread flood basalt and plateau volcanism occurred in the Early Cretaceous, from about 145 to 110 Ma (e.g. Renne et al., 1992; Mahoney et al., 1993, 2005; Stewart et al., 1996; Duncan, 2002; Kent et al., 2002; Tejada et al., 2002; Hoernle et al., 2004). Such periods have been proposed to represent episodes of mantle overturn, which is postulated to trigger the formation and ascent of multiple plume heads (Stein & Hofmann, 1994). The plume-head or starting-plume hypothesis (e.g. Richards et al., 1989; Griffiths & Campbell, 1991; Campbell, 1998), in turn, has been the hypothesis invoked most frequently in recent years to explain individual continental flood basalts and oceanic plateaux.
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However, evidence has mounted recently that several major flood basalt provinces and plateaux lack key characteristics predicted by the plume-head model. For the late Permian flood basalts specifically, Czamanske et al. (1998)
concluded that a plume-head origin could be rejected for the Siberian Traps on the basis of a lack of any pre- or syn-volcanic uplift associated with the flood basalt event. More recent work in the West Siberian Basin, however, has yielded evidence of regional kilometer-scale uplift deemed entirely consistent with a plume-head origin (Saunders et al., 2005). Little is known about the Panjal Traps, of which only a comparatively tiny outcrop area remains (most of it in a militarized zone). The Emeishan province, in contrast, has been the focus of several recent stratigraphic, geochronological, geochemical, and geophysical studies. Evidence for >1 km of doming of the regional lithosphere shortly before volcanism has been documented, and together with associated variations in crustal thickness, upper-mantle and lower-crustal seismic characteristics, and basalt composition, has been argued to strongly support a plume-head origin (He et al., 2003; Xu et al., 2004).
Near-primary picritic rocks can provide estimates of mantle source temperature; indications of higher than normal temperature are commonly interpreted as petrological evidence for a plume source. As in other flood basalt provinces, picritic flows [here we use the MgO > 12 wt % definition of Le Bas (2000)] are rare in the Emeishan. Picritic rocks reported previously (Chung & Jahn, 1995; Xu et al., 2001; Song et al., 2001) are not lavas but intrusions in (or that can be traced into) Triassic limestone, and thus postdate the flood basalt event (Zhang & Wang, 2002b; Zhang et al., 2004). However, we recently discovered several picritic lava flows in the Lijiang area (Fig. 2; Zhang & Wang, 2002b). Here, we present chemical and isotopic data for these flows and associated basalts, and discuss the implications for mantle source composition and conditions of melting.
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GEOLOGICAL BACKGROUND |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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General descriptions of the Emeishan province have been presented by Wang et al. (1993)
, Chung & Jahn (1995), Xu et al. (2001), Thompson et al. (2001), and Ali et al. (2005). Most of the province lies within the broad region of Cenozoic uplift caused by the collision of Greater India and Eurasia. As a consequence, the lava pile is deeply dissected. The principal remaining volcanic outcrops (Fig. 1) cover an area of about 250 000 km2, although outliers and buried remnants are present over a greater area. The lava pile is thickest in the west, where the thickest sections, located near Lijiang and Binchuan, exceed 5000 m. Total erupted volume is estimated conservatively at about 300 000–500 000 km3, making the Emeishan a medium-size flood basalt province (Yin et al., 1992; Jin & Shang, 2000; Ali et al., 2005). The lava sequence is underlain by late Carboniferous–Permian sedimentary beds atop a Mesoproterozoic to late Paleoproterozoic metamorphic basement.
In contrast to the Siberian Traps, which formed at a relatively high northern latitude, emplacement of the Emeishan flood basalts occurred near the Equator (e.g. Enkin et al., 1992). Overall, the province appears to be slightly older than the much better dated, 251 Ma Siberian Traps (e.g. Kamo et al., 2003): recent U–Pb dating of zircons in the Xinjie and Panzhihua layered mafic–ultramafic intrusions yielded ages of 259 ± 3 Ma and 263 ± 3 Ma, respectively (Zhou et al., 2002a, 2005), whereas 40Ar–39Ar ages of 254 ± 5 Ma (Boven et al., 2002) and 251–255 Ma (Lo et al., 2002) have been reported for lava flows and two late-stage intrusions.
Three volcanic stages are evident. Locally early magmatism, consisting principally of alkalic basalt, is recorded in the eastern part of the province. The major eruptive stage, present throughout the province, comprises augite-phyric, plagioclase-phyric, and aphyric tholeiitic basalt, along with corresponding basaltic pyroclastic deposits. The central part of the province preserves late-stage products of bimodal basalt–trachyte and basalt–rhyolite volcanism. Several mafic–ultramafic layered intrusions, which host the world's largest V–Ti deposits, are also exposed in the central region.
Picritic flows that we discovered recently in the Lijiang area are located in an 800 m thick lava section near Shiman and a 5500 m thick section at Daju, one of the thickest in the entire Emeishan province (see Fig. 2). In both sections, the picritic flows are intercalated with augite-phyric basalt flows (Fig. 3). At Shiman, the lowermost and uppermost picritic flows are 3–5 m thick, whereas a middle picritic flow is 15–20 m thick. In the Daju section, the lowermost picritic flow varies from 20 to 50 m thick and the others are 3–5 m thick. The two sections differ considerably in total thickness and it is unclear how, or whether, the picritic flows at Shiman correlate with those at Daju. Massive aphyric basalt and, at Daju, amygdaloidal basalt dominate the middle to upper parts of the lava sections. Plagioclase-phyric basalt, abundant in other parts of the province, is absent.
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Most of the picritic flows are highly porphyritic (with 7–15 vol. % phenocrysts). They contain abundant phenocrysts of forsteritic olivine, plus minor diopsidic clinopyroxene. The olivine phenocrysts are generally subhedral to rounded, and rarely embayed or partly resorbed (Fig. 4). Most range from 0·2 to 1 mm across, although the largest reach 4 mm. Olivine is generally replaced by serpentine, but some grains retain cores of unaltered olivine, many of which contain scattered melt inclusions. Strained, kink-banded crystals are absent. Some olivine crystals host equant, euhedral to rounded Cr-spinel a few tens of microns across. Cr-spinel is also present as solitary grains in the groundmass. The groundmass consists principally of very fine-grained, probably originally glassy, mesostasis plus lesser amounts of olivine, anhedral diopside, and tiny crystals of plagioclase. The groundmass olivine tends to be less altered than the olivine phenocrysts; some is nearly equant, but elongated skeletal forms (as large as 0·5 mm x 0·1 mm) consisting of parallel sets in optical continuity are most common.
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The phyric basalts contain 5–15 vol. % of augite phenocrysts 1–6 mm across; some form clusters. The groundmass is fine grained, with intersertal or intergranular texture, and consists predominantly of plagioclase and augite with minor iron oxides. The aphyric basalts contain a similar assemblage of minerals. Augite microphenocrysts, several tens of microns across, are present, and commonly form clots, producing a cumulophyric texture. The groundmass is composed of plagioclase laths with interstitial subophitic augite and minor anhedral magnetite.
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ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Mineral compositions in selected samples were measured by electron microprobe at the Chinese Academy of Geological Sciences and the University of Cardiff, following Bloomer et al. (1982)
. Bulk-rock major and trace element compositions were determined on agate- and alumina-ground powders at the Chinese Academy of Geological Sciences and the University of Hawaii. Major elements were determined by X-ray fluorescence spectroscopy using the methods of Norrish & Chappell (1977), and trace elements by inductively coupled plasma–mass spectrometry following Dulski (1994) and Neal (2001). Isotope ratios of Nd, Pb, and Sr and associated isotope-dilution concentrations were measured at the University of Hawaii (see Sheth et al., 2003) on small, handpicked chips of acid-cleaned rock to avoid alteration and olivine phenocrysts (e.g. Peng & Mahoney, 1995). Because such chips may not be strictly representative of bulk-rock mineralogy, particularly in coarser-grained or patchily altered samples, we use the isotope-dilution data here only for age-correcting isotope ratios. |
RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Electron microprobe data
Analyses of olivine, clinopyroxene, and Cr-spinel in picritic samples are listed in Table 1. Mg-number [100 x Mg/(Mg + Fe), molar] in the olivine varies from 84·5 to 91·6. The more Mg-rich olivine crystals are visually indistinguishable from those with Mg-number <90. Most olivine crystals contain chromite and melt inclusions, and all have significant amounts of NiO, CoO, Cr2O3, and CaO.
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Olivine with Mg-number >90 is a common feature of komatiites (e.g. Arndt et al., 1977
; Lesher, 1989). It also has been reported in picritic rocks from several continental flood basalt provinces (e.g. Larsen & Pedersen, 2000; Thompson & Gibson, 2000) but is found only rarely in ocean ridge basalts (Donaldson & Brown, 1977) and ocean island basalts. In Hawaiian lavas, Mg-number in olivine phenocrysts does not exceed 91·3 (Garcia et al., 1995), and in Réunion lavas, 90·6 (Fretzdorff & Haase, 2002).
In the sample for which Cr-spinel was analyzed (SM-14), Mg-number varies from 26 to 35, Cr-number [100 x Cr/(Cr + Al), molar] from 73·1 to 75·2, TiO2 from 1·85 to 1·96 wt %, and total iron as FeO (FeO*) from 30·36 to 32·92 wt %. These compositions are distinct from those of Cr-spinel in abyssal peridotites (Dick & Bullen, 1984) and picritic continental flood basalts (e.g. West Greenland; Larsen & Pedersen, 2000) but are broadly similar to Cr-spinel in some Archaean komatiites (Arndt et al., 1977; Nisbet et al., 1977; Lesher, 1989; Barnes & Roeder, 2001).
Clinopyroxene phenocrysts in picritic sample SM-14 have high TiO2 (as high as 2·52 wt %), CaO, and MgO contents, with Mg-number between 72·1 and 80·5. They are classified as Ti-rich diopside. Those in phyric basalt samples DJ-33 and DJ-34 are augitic. In comparison, basalts from other areas of the Emeishan province contain augite (Wang et al., 1993).
Bulk-rock major and trace element composition
The MgO content of the picritic lavas ranges from 26·99 to 12·25 wt % and their Mg-number varies from 81 to 66 (Table 2). The associated basalt flows have Mg-number between 66 and 55 at MgO contents of 11·92–6·83 wt % (except for sample DJ-1, with an Mg-number of 34 at MgO 4·17 wt %). These values are higher than reported for many basalts from other areas of the Emeishan province (Cong, 1988; Xu et al., 2001; Zhang & Wang, 2002a). A general decrease in Ni content with decreasing Mg-number implies removal of significant amounts of olivine between picritic and basaltic compositions.
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Trends of increasing SiO2, Al2O3, TiO2, and Na2O with decreasing Mg-number (Fig. 5) are also consistent with olivine fractionation, whereas the steady increase in Al2O3 and lack of pronounced FeO* enrichment indicate little or no removal of plagioclase, consistent with the absence of plagioclase phenocrysts in these lavas. Clinopyroxene fractionation appears to have commenced around Mg-number of 62–64 (
10 wt % MgO), where CaO values reach a maximum. With one exception, samples with Mg-number >64 have rather high CaO/Al2O3 (0·95–1·17), suggesting a relatively high pressure of melting (Hirose & Kushiro, 1993; Baker & Stolper, 1994). On a total alkalis vs SiO2 diagram (not shown), values for many of the samples suggest a somewhat alkalic character.
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However, some care must be taken in interpreting both the major and trace element data. The samples are variably altered in thin section, and this is reflected in, for example, their LOI (weight loss on ignition to 1000°C) values, which range from 1·49 to 7·51 wt %. Consistent with the general replacement of olivine phenocrysts by secondary phases, the most olivine-rich samples have the highest LOI values (e.g. note the correlation of LOI with Mg-number; Fig. 5). Although post-eruptive alteration clearly has not destroyed the overall relationships among most of the major elements, it is probably the reason for the lack of any correlation between K2O and Mg-number (or the other major element oxides) and, together with excess phenocrysts in some samples, probably has broadened several of the other arrays in Fig. 5.
Among the trace elements, Rb and Ba display considerable variability relative to alteration-resistant elements such as Th and Nb (Fig. 6a and b); as with K2O, this largely appears to be an alteration effect. Large troughs or, in a few cases, peaks at Sr also are present in many of the primitive-mantle-normalized element patterns in Fig. 6. Strontium is compatible in plagioclase yet, as noted, plagioclase phenocrysts are absent. Europium is also compatible in plagioclase, but the lavas lack significant Eu anomalies. Unlike Eu, Sr does not correlate with alteration-resistant elements (or with isotope ratios). Thus, Sr concentrations appear to have been affected considerably by alteration, as has been documented in altered subaerial basalts elsewhere (e.g. Lindstrom & Haskin, 1981; Clague & Frey, 1982; Fleming et al., 1992). Peaks at Pb are present in most of the patterns. The peaks are of highly variable size, and the extent to which they are primary features is unclear. Only weak correlations are seen between Pb and the alteration-resistant incompatible elements, suggesting that alteration has modified Pb concentrations significantly. Some of the patterns in Fig. 6 show a small P trough and/or variability in U relative to Nb and Th, which may likewise be an alteration effect.
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Patterns of alteration-resistant incompatible elements for the picritic and basaltic lavas are similar in overall shape (Fig. 6c and d), but element concentrations are higher in the basaltic samples, consistent with their more evolved nature. All of the lavas are enriched in the more highly incompatible elements relative to moderately incompatible ones; for example, primitive-mantle-normalized (La/Yb)P varies from 5·2 to 18·7. In comparison, values for most basalts from other parts of the Emeishan province range between 2·9 and 11·4 (Xu et al., 2001
; Zhang & Wang, 2002a; Xiao et al., 2004). Unlike some other areas of the province, sizeable troughs at Nb and Ta relative to La and Th are not observed among our samples; rather, many have Nb–Ta peaks. Overall, patterns of the alteration-resistant elements are very similar to those of many oceanic island basalts.
Xu et al. (2001) used a Ti/Y ratio of 500 to subdivide their Emeishan samples into low- and high-Ti (or -Ti/Y) types. By this definition, all of our lavas, which have Ti/Y between 502 and 767, are of the high-Ti/Y type. However, the TiO2 contents of the samples (1·14–3·16 wt %, except for two with higher values) are lower than those of the high-Ti/Y basalts described by Xu et al. (2001) and Xiao et al. (2004), which have TiO2 >3·5 wt %. In part, this difference reflects the generally less evolved nature of the rocks of the Lijiang area.
High-Ti and low-Ti suites are fairly common in other continental flood basalt provinces (e.g. Carlson, 1991; Sharma, 1997; Arndt et al., 1998; Pik et al., 1998; Kieffer et al., 2004; Melluso et al., 2006). As with our samples, many of the high-Ti suites have broadly ocean-island-like incompatible-element characteristics. Other distinctive features of such high-Ti rocks tend to be a variably alkalic character and higher FeO* contents than those of ocean-ridge basalts at similar MgO. High-Ti, high-MgO rocks in the Siberian Traps (picritic basalt and meimechite) and Ethiopian Traps (picritic basalt) exhibit some similarities to the Lijiang area picritic lavas, but generally have higher TiO2 and FeO* for their MgO contents (e.g. Arndt et al., 1998; Pik et al., 1998).
Nd, Sr, and Pb isotope ratios
The lavas define a relatively small range of age-corrected Nd(t) (t = 250 Ma) from +4·0 to –1·3 (Table 3; Fig. 7a). There is no difference between picritic and basaltic samples, as the range for both is the same within error (Fig. 8a). Values of (87Sr/86Sr)t vary from 0·70344 to 0·70524. Given that both Rb and Sr concentrations have been affected by alteration, some of the age-corrected Sr isotope ratios are no doubt too high or too low. In Fig. 7a, the Lijiang data lie within or slightly to the low-87Sr/86Sr side of the larger array defined by other Emeishan basalts. Basalts from other parts of the province exhibit a total Nd(t) and (87Sr/86Sr)t range of +4·8 to –4·8 and 0·70393–0·70759, respectively. This range is notably smaller than those for many other continental flood basalts, including the Karoo, Deccan Traps, Columbia River, Paraná, and Siberian Traps, although it is larger than found for the intra-oceanic Caribbean or Ontong Java plateaux (see Hawkesworth et al., 1984; Carlson & Hart, 1988; Mahoney, 1988; Kerr et al., 1997; Peate, 1997; Sharma, 1997; Tejada et al., 2004, and references therein).
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The range of age-corrected Pb isotope ratios is also relatively small compared with those of other continental flood basalts: (206Pb/204Pb)t varies from 17·93 to 18·88, (207Pb/204Pb)t from 15·51 to 15·59, and (208Pb/204Pb)t from 37·93 to 38·86. In Fig. 7c and d, the data lie slightly above the estimated 250 Ma field for modern East Pacific Rise mantle, and are broadly similar in this respect to Indian Ocean hotspot and ridge basalts. Previously published Pb isotope data for the Emeishan province are for seven samples from sections near Binchuan and Yongsheng (Zhang & Wang, 2003
) to the south and east of the Lijiang area. With one exception, these data overlap or lie close to our data points in Fig. 7.
The high-Nd(t), high-(206Pb/204Pb)t end of the Emeishan data array lies close to (Fig. 7a and b) or overlaps (Fig. 7c and d) the estimated 250 Ma field for the sources of several modern Indian Ocean hotspots (Re–Cr–Am field). At the highest Nd(t) values, it approaches the field for the sources of some Pacific ‘C-type’ (Hanan & Graham, 1996) hotspots, such as those feeding the Easter Seamount Chain and Nazca Ridge. Interestingly, the majority of data for the other major late Permian flood basalt province, the Siberian Traps, define a broadly similar array to that of the Emeishan at the high-Nd(t) end [though with few representatives at Nd(t) > +2; Fig. 7a and b], and also fall slightly above the estimated 250 Ma East Pacific Rise source field in Fig. 7c and d. However, the Siberian Traps data extend to much lower (206Pb/204Pb)t and Nd(t) and higher (87Sr/86Sr)t than yet found for the Emeishan province.
Rough correlations are present between Nd(t) (and to a lesser extent, Sr and Pb isotope ratios) and ratios of alteration-resistant incompatible elements in which at least one of the elements is highly incompatible (e.g. Fig. 8c–f). Lower values of Nd(t) are associated with higher La/Yb, La/Sm, Th/Nd, etc. and with lower Nb/La; in other words, with greater relative enrichment in highly incompatible elements and with decreasing Nb–Ta peak size in the patterns of Fig. 6. Most of our analyses are for the thick Daju section, and in this section there is also a crude correlation with stratigraphic position, in that the stratigraphically higher lavas tend to have higher Nd(t) (Fig. 8b), Nb/La, etc. In contrast, ratios of moderately incompatible elements, such as Sm/Yb and Ti/Y, do not correlate with stratigraphic position or with isotope ratios; the highest values of Sm/Yb, Ti/Y, etc. tend to be found in samples with intermediate Nd(t).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Original liquid composition
The olivine phenocrysts with Mg-number >90 contain 0·22–0·45 wt % CaO and 0·03–0·17 wt % Cr2O3. Olivine in mantle peridotites is characterized by much lower Ca and Cr contents (e.g. Gurenko et al., 1996
; Thompson & Gibson, 2000). Along with the presence of melt inclusions and lack of strain texture, these features indicate that the Mg-rich olivines in our picritic lavas crystallized from melt and are not accidental xenocrysts of mantle olivine.
Under equilibrium conditions, olivine composition reflects the composition of the magma from which the olivine crystallized. Thus, the composition of olivine in picritic rocks can be used to estimate parental magma composition (e.g. Simkin & Smith, 1970; Larsen & Pedersen, 2000; Révillon et al., 1999). In Fig. 9a, olivine Mg-number is plotted vs bulk-rock MgO content. Also shown are curves representing the loci of equilibrium values for liquids containing 8–13 wt % FeO; these values correspond to the FeO content of parental magmas, assuming 15% of the total magmatic iron oxide content is ferric. If values for the most Mg-rich olivine compositions from a sample plot on an olivine–liquid equilibrium curve at approximately the same FeO content as present in the bulk rock, the MgO content of the rock is close to that of the liquid from which the olivine crystallized. Data points falling below the curves indicate excess (cumulus) olivine or olivine that formed later as the magma was cooling.
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Both cases are represented by the Emeishan picritic rocks. Data for samples DJ-26, DJ-31, and DJ-34–1 all fall below the equilibrium curves in Fig. 9a, indicating that these samples do not represent primitive compositions. (However, they are not simply basalts with excess olivine phenocrysts, because the basalts have no olivine phenocrysts, and contain augite rather than the diopside found in the picritic lavas.) In contrast, the most magnesian olivine values measured for sample SM-14 lie within the region defined by the FeO curves. The highest Mg-number (91·6) lies near the 11 wt % FeO curve. This rock, with 22·40 wt % MgO (normalized to a major element total of 100 wt % on a volatile-free basis), has an appropriate FeO* content (12·92 wt %), again assuming 15% of total iron oxide is ferric (see Figure 9b). We conclude that sample SM-14 is close to an unfractionated melt of the mantle source. Sample SM-15 has a very similar bulk composition.
An independent estimate of initial magmatic MgO content may be made from the Ni content of olivine in SM-14 (3300 ppm). Assuming olivine in the source mantle has 3500 ppm Ni, a likely upper limit (Korenaga & Kelemen, 2000), implies a melt with only marginally greater MgO than that of SM-14. For aphyric samples that have lost only olivine, Korenaga & Kelemen's (2000) Ni-in-olivine method of estimating initial magmatic MgO content mathematically adds equilibrium olivine to the composition of a sample until an olivine Ni content of 3500 ppm is reached. Our highest-MgO aphyric sample, DJ-36, has only 7 wt % MgO; as noted above, clinopyroxene crystallization generally appears to have begun around 10 wt % MgO in the Lijiang area magmas. Nevertheless, application of this approach to sample DJ-36 yields an estimated initial magmatic MgO content of 21·2 wt %, reasonably close to that of SM-14.
Melting conditions
We use sample SM-14 to estimate the conditions under which the primary picritic magmas were generated. From the equation T(°C) = 17·86 x MgO(wt %) + 1061 (Nisbet et al., 1993), an estimated eruption (1 atm) temperature of about 1460°C is obtained. Our samples lack hydrous primary minerals (amphibole, mica, etc.), so the magmatic water content was probably relatively low. For this 1-atm temperature, a recent model of adiabatic pressure–temperature paths for primary ultramafic magmas (Herzberg & O'Hara, 2002) suggests an initial melting temperature of about 1630–1680°C and pressure of 4·2–5·0 GPa (Fig. 10). Magma formation at relatively high pressures is consistent with the high CaO/Al2O3 (see above) and FeO* (12·70, 12·94 wt %), and relatively low SiO2 (45·64, 46·15 wt %) and Al2O3 (8·12, 8·15 wt %) of SM-14 and SM-15 (Walter, 1998). Arndt (2000) noted that the temperature estimated in this manner may commonly have an error of as much as 100°C (with correspondingly large errors in estimated pressure), because of uncertainties in water content and oxygen fugacity. Additional sources of error are the depth of magma segregation from the source (Nisbet et al., 1993), the proportion of crystals in the ascending magma, and the nature of the melting process itself. Another empirical estimate of source potential temperature and pressure can be made from, respectively, T(°C) = 2000[MgO/(SiO2 + MgO)] + 969 and ln[10P](GPa) = 0·00252T – 0·12SiO2 + 5·027 (Albarède, 1992), yielding a temperature of about 1630°C and pressure of 3·9 GPa, in broad agreement with the values above. The olivine saturation-surface model of Putirka (2005, equations A and B) suggests a similar initial temperature of 1650–1690°C. A high temperature is also supported by the high Cr content of the Cr-spinel; a value of 1673°C is obtained from the equations of Fabries (1979) and Engi & Evans (1980), using the analysis with the highest Cr-number in Table 1.
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, a rough estimate of temperature and pressure at the start of melting. Numbers on paths to the left of the solidus indicate wt % MgO of liquids. Ranges of estimated mantle pressure and temperature for Gorgona komatiites (K) and picritic basalts, West Greenland picritic lavas, modern Hawaii and Iceland, and MORB (mid-ocean ridge basalt) are from Herzberg & O'Hara (2002)Recent estimates of mantle potential temperature for the sources of Hawaii and Iceland are in the same range as above, whereas those for normal asthenosphere are 150–230°C lower (Putirka, 2005
). Thus, the source temperature estimated for the Lijiang area picritic flows appears consistent with some type of mantle thermal plume, whether ‘standard-type’ (e.g. Griffiths & Campbell, 1991), non-Newtonian (e.g. Larsen & Yuen, 1997), or thermochemical (e.g. Davaille et al., 2005), as the magma source. Interestingly, from inversion of lanthanide rare earth element (REE) data, Xu et al. (2001) also estimated a rather high potential temperature (>1550°C) for the source of their (low Mg-number) low-Ti/Y Emeishan basalts.
A pressure of 4 GPa corresponds to a depth of about 130 km, within the garnet stability field (e.g. Walter, 1998). Indeed, the variation in the middle to heavy REE among the Lijiang area lavas is relatively large, comparable with that in the light REE, and indicative of an important role for garnet during melting (unlike the light REE, the heavy REE are variably compatible in garnet). For example, primitive-mantle-normalized (Sm/Yb)P and (La/Sm)P both vary by slightly more than a factor of two, in the range of 2·92–6·74 and 1·69–3·74, respectively.
In a diagram of (Tb/Yb)P vs (Yb/Sm)P, melting of garnet peridotite produces a markedly different trajectory from melting of spinel peridotite (Fig. 11a). The Lijiang area data lie closer to a melt path for garnet peridotite than to one for spinel peridotite. With the assumptions used to construct Fig. 11a, the data are consistent with a 60–100% contribution to the melt from melting in the presence of garnet, and with rather small amounts of partial melting, 2–7%. We caution that these values should not be taken literally, because the positions of the curves in Fig. 11a can vary with a different choice of melting model, distribution coefficients, source composition, and/or mineral proportions. Moreover, given the range in Nd(t) among our samples, the assumption of a single model source composition is not necessarily valid (see below). However, the conclusion that much of the melting occurred in garnet-facies mantle and that the amount of partial melting was rather modest is difficult to avoid. It seems clear that compared with an intra-oceanic plateau such as the Ontong Java, for which mean melt percentages of 25–30% have been estimated (Fitton & Godard, 2004; Herzberg, 2004), the Lijiang area magmas formed through much smaller amounts of partial melting at rather great depth. High-Ti, high-MgO lavas in the Siberian, Ethiopian, and Deccan Traps also have been interpreted to represent rather small percentages of melting involving substantial contributions from garnet peridotite (e.g. Arndt et al., 1998; Pik et al., 1998; Melluso et al., 2006). In contrast, low-Ti/Y lavas at Binchuan and Yongsheng appear to reflect a greater role for melting at shallower depths in the spinel stability field (Xu et al., 2001; Zhang & Wang, 2002a), whereas rare low-Ti/Y, high-MgO (komatiitic) lavas in Vietnam, which might also be related to the Emeishan event, are interpreted as melts from a significantly more depleted and garnet-free source (Hanski et al., 2004) (see Fig. 11b).
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Mantle source and contamination
The high-
Nd(t), high-(206Pb/204Pb)t end of the array defined by the Lijiang area data in Fig. 7 points to a mantle source isotopically similar to that of several present-day oceanic hotspots (allowing for isotopic evolution via radioactive decay of parent nuclides in mantle reservoirs since 250 Ma). Likewise, patterns of alteration-resistant incompatible elements are ocean-island-like (Fig. 6c and d). These characteristics suggest a source in the convecting mantle or in lithospheric mantle not long isolated from it. In addition to various versions of the plume-head model, some non-plume hypotheses (e.g. Alt et al., 1988; Sheth, 2005) may accommodate such signatures, and we therefore cannot rule out the possibility of a non-plume mechanism. However, the combination of hotspot-type geochemical characteristics with the evidence for a high mantle-source temperature discussed above and the apparently very strong evidence for a large amount of regional lithospheric uplift immediately preceding flood volcanism (He et al., 2003; Xu et al., 2004) qualitatively appears consistent with some type of plume-head origin.
The Lijiang-area data points trending in Fig. 7 toward lower Nd(t) and (206Pb/204Pb)t and higher (87Sr/86Sr)t could indicate low-Nd material in the source mantle or variable contamination of magmas with Mesoproterozoic or late Paleoproterozoic lithosphere (crust or mantle) overlying the high-Nd source region (recent dating of a granulite in the vicinity gives an age of 1140 Ma; Xu et al., 2002). The low-Nd(t) material was relatively enriched in highly incompatible elements and had lower Nb/La than the high-Nd source mantle (Fig. 8c–e). Many continental crustal rocks and siliceous marine sediments have low Nb/La and incompatible-element patterns with pronounced troughs at Nb–Ta, together with strongly negative Nd values (e.g. Rudnick & Fountain, 1995; Plank & Langmuir, 1998). Non-picritic basalts with pronounced Nb–Ta troughs and more negative Nd(t) than any of the Lijiang area lavas are present elsewhere in the Emeishan province, and have been interpreted to represent moderate to substantial lithospheric inputs (Xu et al., 2001; Zhang & Wang, 2002a; Xiao et al., 2004). In contrast, our lowest-Nd(t) samples barely reach negative Nd territory, and lack significant Nb–Ta troughs in their incompatible-element patterns. Thus, the contribution of low-Nb–Ta continental lithospheric or subducted-sediment-derived material in the mantle source of the Lijiang area lavas appears likely to have been comparatively minor. However, we cannot rule out the possibility of a role for a continental lithospheric end-member with relatively high (though negative) Nd and Nb/La. Granitic gneisses from a Neoproterozoic (800 Ma) phase of magmatic-arc activity are present in the region; Nd isotope data are not available, but these rocks have large Nb–Ta troughs in their incompatible-element patterns (e.g. Zhou et al., 2002b). It is possible that other rocks formed in the same event had relatively high Nd at 250 Ma; however, for the most part such arc-related crust would also be expected to possess incompatible-element patterns with marked Nb–Ta troughs. In any case, no correlation between isotope ratios and SiO2 is present among the Lijiang area lavas, implying that bulk assimilation of high-SiO2 continental crust was comparatively minor. The absence of plagioclase phenocrysts suggests the magmas spent little time at crustal levels. Consistent with this inference, application of Putirka et al.'s (1996) geobarometer to the clinopyroxene data in Table 1 suggests the clinopyroxene phenocrysts equilibrated at depths between about 30 and 70 km.
Temporal evolution
In the thick Daju section, for which we have the most isotopic data, the stratigraphically higher lavas tend to have higher Nd(t) (Fig. 8b), indicating that the contribution of low-Nd material waned overall with time. This could represent the gradual depletion of easily melted lithospheric wall-rock in the magmatic conduit system, or the melting out of a more fusible component in the mantle source itself. The variation in garnet-sensitive ratios such as (Yb/Sm)P and (Tb/Yb)P (Fig. 11a) testifies to melting over a range of depths in the mantle source. More fusible, low-Nd material in the source should ordinarily begin melting at greater depths than more refractory components (e.g. Ito & Mahoney, 2005), which in turn could lead to correlations of garnet-sensitive element ratios with Nd(t) in the melts. The lack of such correlations may favor a lithospheric influence, because melting in the source could be largely decoupled from assimilation of low-Nd lithospheric wall-rock.
In any case, lithological evidence suggests that the Lijiang area lavas represent an early stage in the flood basalt episode. In particular, plagioclase-phyric basalts are absent in the olivine- and clinopyroxene-dominated Daju and Shiman sections, whereas to the south and east (Binchuan and Yongsheng, respectively), plagioclase-phyric lavas overlie a sequence of clinopyroxene-phyric basalts (Zhang et al., 2004). Still farther east, quartz tholeiites overlie a succession of plagioclase-phyric flows (Hou et al., 1999). If these regional differences correspond to a temporal sequence, an early transition from relatively low-Nd(t) to higher-Nd(t) ( +4, ±1) compositions was repeated at least once during the Emeishan episode, because the stratigraphically higher lavas in the Binchuan and Ertan areas tend to have the highest Nd(t), as in the Lijiang area (Xu et al., 2001; Zhang & Wang, 2003; Xiao et al., 2004). At present, it is not clear how to interpret these results in terms of lithospheric thinning with time, the location of magma sources relative to any postulated plume axis, development of separate magma-plumbing systems, etc. For example, in addition to temporal changes in magma and/or mantle source composition, peak volcanism may have migrated with time in response to plate motion over a hotspot.
Thick piles of low-Ti/Y tholeiites make up the bulk of sections in the Binchuan and Ertan areas, from which Xu et al. (2001, 2004) and Xiao et al. (2004) concluded that the western Emeishan province is composed predominantly of low-Ti/Y basalts. These basalts have flatter REE patterns than the high-Ti/Y Lijiang area lavas. Compositions are too evolved to make major-element-based estimates of initial temperature, but application of McKenzie & O'Nions' (1991) REE inversion led Xu et al. (2001) to conclude that the low-Ti/Y basalts represent comparatively high-degree partial melts formed at high potential temperature (>1550°C); the setting was inferred to be above the axis of a plume head. Minor high-Ti/Y basalts were found in the uppermost parts of the Binchuan and Ertan sections. They have broadly ocean-island-like incompatible-element signatures rather similar to those of the Lijiang area lavas, but concentrations of incompatible elements are significantly higher. These high-Ti/Y basalts were interpreted, again on the basis of REE inversion, to be very small-degree partial melts (<1·5%) formed at greater mantle depths and lower potential temperatures (<1500°C) during the waning stages of volcanism, probably from a cooling plume mantle (Xu et al., 2001). Sections in the eastern and southern parts of the province are dominated by high-Ti/Y basalts, which Xu et al. (2004) attributed to small-degree melting near the periphery of the plume head, where the lithospheric lid was thicker. Somewhat similar secular variations have been observed in a portion of the 55 Ma East Greenland flood basalt pile, and are also interpreted in terms of cooling of a plume and variation in lithospheric thickness (Tegner et al., 1998). In the Maymecha River basin of the Siberian Traps, alkalic, high-Ti picritic basalts and meimechites lie above a sequence of low-Ti tholeiitic lavas. This late high-Ti group is interpreted to have formed by small amounts of partial melting of garnet-facies mantle at even greater depths (Arndt et al., 1998) than those we estimate for the Lijiang area lavas.
High-Ti lavas, some of which are picritic, are also present in the lower portions of the Siberian Traps sequence (e.g. Lightfoot et al., 1993; Sharma, 1997). Our work shows that a thick sequence of high-Ti/Y lavas with ocean-island-like source characteristics is present in the western part of the Emeishan province. As noted above, these lavas are likely to be earlier in the volcanic succession than the low-Ti/Y basalts at Binchuan and Ertan. Formation of the Lijiang area magmas by modest amounts of partial melting, largely in garnet peridotite mantle, can be accommodated straightforwardly in the context of a plume-head model if melting occurred beneath a thick lithospheric lid before plume-related or other stress on the overlying lithosphere led to major lithospheric thinning (e.g. see Campbell, 1998). Lithospheric thinning might have been rapid in the case of a plume with non-Newtonian rheology (Larsen & Yuen, 1997).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Olivine phenocrysts with Mg-number as high as 91·6 in picritic lavas of the Lijiang area crystallized from liquids. We estimate that the picritic lavas formed from melts with about 22 wt % MgO generated by relatively small amounts of partial melting, much of which appears to have occurred in the presence of residual garnet. Initial source temperature is estimated to have been as high as 1630–1690°C. The high temperature is consistent with, although not proof of, a plume-head origin, whereas the importance of garnet during melting implies a rather thick lithospheric lid. As such, the Lijiang area lavas may represent an early stage in the Emeishan flood basalt episode, before major lithospheric thinning had occurred.
The basalt flows that make up the bulk of the Lijiang area sequence are isotopically indistinguishable from the picritic lavas, and appear to be derived from picritic magmas by removal of olivine and clinopyroxene. Isotopic and incompatible-element characteristics of the lavas indicate an Nd(t) +4 mantle source similar to those of several present-day oceanic hotspots. An isotopically broadly similar source may have been involved in the generation of the nearly contemporaneous Siberian Traps, although they were formed in a location far from the Emeishan flood basalts. The absence of plagioclase phenocrysts in both the picritic and basaltic lavas implies that the Lijiang area magmas spent little time at shallow lithospheric levels prior to eruption, which may help explain the lack of evidence for significant contamination. However, correlations between isotope ratios and ratios of highly incompatible elements suggest that some interaction of ascending magmas with continental crust or lithospheric mantle occurred. Alternatively, relatively low-Nd, low-Nb/La material may have been present in the mantle source.
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ACKNOWLEDGEMENTS |
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We are grateful to D. Pyle, D. VonderHaar, and N. Hulbirt for assistance with the work at SOEST, and to K. Johnson for discussions on primary magmas. We thank reviewers N. Arndt, D. Peate and A. Saunders, and editor W. Bohrson for their thoughtful and constructive comments. This research was supported by Program for New Century Excellent Talents in University (NCET-04-0728), NSF-China grant (40273020), and US NSF grant EAR98-0531.
*Corresponding author. E-mail: zcchang{at}cugb.edu.cn
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