Introduction

The Emeishan flood basalts in SW China and northern Vietnam have a well-constrained Late Middle-Permian (∼260 Ma) eruptive age (Yin et al. 1992; Jin and Shang 2000; Ali et al. 2004) and include high-Ti and low-Ti basalts (Xu et al. 2001; Xiao et al. 2003), as well as alkaline rocks such as trachyandesites and trachytes (Ma et al. 2003). These basalts form part of the Emeishan Large Igneous Province (ELIP), which also includes intrusive bodies of sub-volcanic sills/dykes and layered intrusions with a variety of magmatic mineral deposits (Zhong et al. 2002, Zhou et al. 2002c; 2005; Song et al. 2003). If the intrusive bodies were part of the same igneous event that produced the ELIP, one would expect a genetic relationship between them and the extrusive rocks and a similar diversity of composition in these bodies.

The diversity of extrusive and intrusive rocks in many LIPs, such as in Siberia, is explained by variable mantle sources, mantle plume–lithosphere interaction, crustal contamination, fractionation, sulfide saturation, or a combination of these processes (Naldrett et al. 1992; Arndt et al. 1993; 1998; 2003; Lightfoot et al. 1990, 1993, 1994; Fedorenko and Czamanske 1997). There has been no attempt to link the different types of intrusions in the ELIP to one another or to the associated volcanic sequences, thus, factors that controlled the origin and diversity of the ELIP are not well understood.

In the Funing area, east of the ELIP (Fig. 1), abundant mafic intrusions in Carboniferous and Devonian strata are relatively well preserved and are spatially associated with volcanic rocks of the same age (Wu et al. 1963; YBGMR 1990). However, these rocks have traditionally not been included in the ELIP and their genetic relationship with the ELIP elsewhere remains unclear, because their age of emplacement and geochemistry have not been studied in detail.

Fig. 1
figure 1

Geological map of the Funing area, Yunnan Province, SW China (after Wu et al. 1963), showing the distribution of mafic–ultramafic intrusions and volcanic rocks

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In this paper we report the first geochronological and geochemical data for mafic rocks in Funing and use these data to examine their age and origin. Zircons from the intrusions were separated and dated using the sensitive high-resolution ion microprobe (SHRIMP) technique. The dating yielded ages of ∼260 Ma, similar to the age of the ELIP (Zhou et al. 2002b). The whole-rock major oxides, trace elements and Rb–Sr and Sm–Nd isotopic characteristics of these rocks reveal a considerable compositional diversity. Using these data, an attempt to identify the nature of the mantle sources, the compositions of parental magmas, and the processes involved in the evolution of these magmas was made.

Geological background

SW China comprises the Yangtze Block to the east, the Tibetan Plateau to the west, and the Indochina Block to the south (Fig. 1). The Yangtze Block consists of a Precambrian basement overlain by stratigraphic sequences ranging from late Mesoproterozoic to upper Jurassic and younger in age. The lower and middle parts of the sequence are basically marine sedimentary rocks, whereas the upper part consists mostly of terrestrial basin deposits (Yan et al. 2003). Along the western margin of the Yangtze Block, there are abundant Neoproterozoic granites and associated metamorphic rocks known as the Kangdian complexes, which were likely uplifted at ∼175 Ma (Zhou et al. 2002a). Farther west, in the easternmost part of the Tibetan Plateau, is the Songpan-Ganze Terrane, which is characterized by a thick (up to more than 10 km) sequence of Late Triassic strata of deep marine origin (Yin and Nie 1996).

Rocks of the ELIP crop out in the eastern part of the Tibetan Plateau and the western part of the Yangtze Block (Fig. 1) and extend over much of SW China and Northern Vietnam (Fig. 1). The ELIP comprises the Emeishan flood basalts and associated mafic–ultramafic and syenitic intrusions. The Emeishan volcanic succession varies in thickness from several hundred meters upto 5 km and includes picrites, tholeiites, and andesitic basalts, all of which are believed to have formed by melting associated with a mantle plume event (Chung and Jahn 1995; Song et al. 2001; Xu et al. 2001; Xiao et al. 2003).

In the western part of the ELIP, the volcanic succession has been strongly deformed, uplifted, and eroded as a result of the Tertiary India-Eurasia collision (Boven et al. 2002; Ali et al. 2004). The distribution of the Late Middle Permian flood basalts on the eastern edge of the Tibetan Plateau indicates that the thick Triassic sedimentary sequence of the Songpan-Ganze Terrane was deposited in a basin that formed during rifting associated with the Emeishan mantle plume (Song et al. 2004). To the southeast, Permian flood basalts are known in Jinping of southern Yunnan, SW China (Xiao et al. 2003), and in Song Da of northern Vietnam (Hanski et al. 2004).

Permian mafic intrusions and basalts in the Funing area (Fig. 1) are exposed in the southeastern part of the Yangtze Block and are relatively well preserved. They are exposed in the cores of anticlines in the South China Mesozoic fold belt (Yan et al. 2003) and are hosted by Paleozoic strata. Because these intrusions are typically unaltered, they provide a rare opportunity to examine the geochemical processes involved in their formation. Mafic intrusions occur either as undifferentiated diabase sills/dykes or layered intrusions (Fig. 1). Several layered intrusions, such as the Anding and Yapai intrusions, cross-cut the host strata, whereas the mafic sills, such as the Shadou sill, are all conformable with the strata. Volcanic rocks crop out extensively in the eastern part of the area (Fig. 1).

Petrography of intrusive and extrusive rocks in Funing

Diabase sills/dykes

The Shadou intrusion, a sill-like body, about 10-km long and 200-m thick (Fig. 1), is intruded comformably into Carboniferous strata. The sill lies in a syncline, in which Middle Carboniferous limestone forms the hanging wall and Lower Carboniferous sandy shales form the footwall. Along the footwall of the intrusion, the sedimentary rocks are metamorphosed to hornfels. The body is composed of diabase, which becomes relatively coarse-grained in the center of the sill. Plagioclase and clinopyroxene are the major minerals but small amounts of olivine occur in the lower parts and magnetite is a common accessory mineral throughout.

Diabases also occur as dykes in Anding and Yapai and are petrographically similar to the Shadou sill.

Layered mafic intrusions

Both the Anding and Yapai intrusions are typical layered intrusions (Fig. 1). The Anding intrusion is 6.5-km long and about 3-km wide with a surface exposure of 15 km2, and is also intruded into Carboniferous strata. From the base upward it consists of a lower marginal zone, a middle gabbroic zone, and an upper diorite zone. The chilled marginal zone is about 60-m thick; it is composed of fine-grained gabbro and contains abundant xenoliths of the country rock. Patches of skarn are developed locally along the contact. The marginal zone is transitional to the middle gabbroic zone, which is composed mainly of clinopyroxene and plagioclase. In the lower part of the middle zone both orthopyroxene and olivine are present and form olivine-noritic gabbro. Some disseminated sulfide layers in this zone were a major exploration target (Wu et al. 1963). The uppermost zone consists of diorite composed chiefly of plagioclase, amphibole, and minor quartz.

The Yapai intrusion is similar to the Anding intrusion with the same three zones: a lower marginal zone, a middle gabbroic zone, and an upper diorite zone.

Volcanic sequence

The volcanic rocks in the Funing region overlie Carboniferous strata and are overlain unconformably by Triassic sedimentary rocks. They contain abundant xenoliths of plutonic rock derived from the intrusions and of Carboniferous strata. Many of the lava flows have well-developed, nearly vertical, columnar jointing. Sparse layers of tuff and volcanic breccia occur between some flows. The rocks are porphyritic in texture with phenocrysts of plagioclase, clinopyroxene, and sparse olivine set in a fine-grained matrix of the same composition. Chlorite-filled amygdules are present locally.

Analytical methods

SHRIMP zircon analyses

Zircon grains were separated using conventional heavy liquid and magnetic techniques, mounted in epoxy, polished, coated with gold, and photographed in transmitted and reflected light to identify grains for analysis. U–Pb isotopic ratios of zircon separates were measured using the SHRIMP II at the Curtin University of Technology in Perth, Western Australia, and at the Chinese Academy of Geological Sciences, Beijing, China. In both these laboratories, the measured isotope ratios were reduced off-line using standard techniques (see Claoué-Long et al. 1991). The U–Pb ages were normalized to a value of 564 Ma determined by conventional U–Pb analysis of zircon standards (CZ3 in the Perth lab and SL13 in the Beijing lab). Common Pb was corrected using the methods of Compston et al. (1984). The 206Pb/238U and 207Pb/235U data were corrected for uncertainties associated with the measurements of the CZ3 or SL13 standards. The 207Pb/206Pb ages given in Table 1 are independent of the standard analyses.

Table 1 SHRIMP zircon U-Pb analytical results for the mafic rocks in Funing, SW China
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Whole-rock geochemical analyses

Samples were collected from the best-exposed and least-altered outcrops. The analyzed samples are believed to be representative of the major lithologies in the intrusions. The samples were cut with a diamond-impregnated brass blade, crushed in a steel jaw crusher that was brushed and cleaned with de-ionized water, and pulverized in agate mortars in order to minimize potential contamination. Major oxides were determined by wavelength-dispersive x-ray fluorescence spectrometry (WD-XRFS) on fused glass beads using a Philips PW2400 spectrometer at the University of Hong Kong. Trace elements, including REE, were determined by inductively coupled plasma mass spectrometry (ICP-MS) of nebulized solutions using a VG Plasma-Quad Excell ICP-MS at the University of Hong Kong after a 2-day closed-beaker digestion using a mixture of HF and HNO3 acids in high-pressure bombs (Qi et al. 2000). Pure elemental standard solutions were used for external calibration and BHVO-1 and SY-4 were used as reference materials. The accuracies of the XRF analyses are estimated to be ±2% (relative) for major oxides present in concentrations greater than 0.5 wt% and ± 5% (relative) for minor oxides present in concentrations greater than 0.1%. The accuracies of the ICP-MS analyses are estimated to be better than ± 5% (relative) for most elements.

Rb–Sr and Sm–Nd isotopic analyses

Isotope ratios of Sr–Nd and concentrations of Rb, Sr, Sm, and Nd were determined on a VG-354 thermal ionization magnetic sector mass spectrometer at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. The chemical separation and isotopic measurement procedures are described in Zhang et al. (2001). Mass fractionation corrections for Sr and Nd isotopic ratios were based on values of 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219. Uncertainties in Rb/Sr and Sm/Nd ratios are less than ± 2% and ± 0.5% (relative), respectively.

Analytical results

SHRIMP zircon analytical results

Two samples were selected for zircon separation. Sample FL7 is a diabase from the Shadou sill and belongs to the high-Ti group (Table 2). Sample, FL44, is a diorite from the Anding intrusion and is of the low-Ti group (Table 2).

Table 2 Major oxides and trace elements of the mafic rocks in Funing, SW China
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Zircon grains from sample FL7 of the Shadou sill exhibit a variety of textures and morphologies characteristic of magmatic origin. Twenty analyses were obtained using the Beijing SHRIMP II (Table 1). Except for one analysis that yielded an older age of around 310 Ma, all analyses, including those of cores, rims, high- and low-U zones, and crystals of different shapes, gave a single age. One analysis has a large error and is omitted from the mean. The remaining 18 analyses yielded a mean 206Pb/238U age of 260±3 Ma (Fig. 2; all uncertainties are 2σ). All 18 grains are from a single-age population of zircons, and there is no evidence of any disturbance since 260 Ma. The observed complex zonation of the zircons in FL7 most likely occurred during cooling and crystallization of the diorite. The 260-Ma age of the zircons from sample FL7 is therefore considered to be the best estimate of the crystallization age for the Shadou sill.

Fig. 2
figure 2

SHRIMP zircon U–Pb concordia plots for samples FL7 (a diabase from the Shadou sill) (a) and FL44 (a diorite from the Anding intrusion) (b) from Funing, Yunnan, SW China (see Fig. 1 for locations of each intrusion)

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Numerous igneous zircon grains were separated from diorite FL44 of the Anding intrusion. Sixteen analyses were obtained using the Perth SHRIMPII. Six xenolithic zircon grains were identified and these have 206Pb/238U ages ranging from 300 Ma to 2247 (Table 1). A group of eight analyses yielded a mean 206Pb/238U age of 258±3 Ma (Fig. 2). Thus, 258 Ma is the best estimate of the crystallization age of the Anding intrusion, slightly younger than the age of the Shadou sill (sample FL7) (Fig. 2).

Whole-rock geochemical data

The mafic rocks in Funing have a wide range of chemical compositions (Table 2). Both the undifferentiated sills and layered intrusions display variable geochemical features. The volcanic rocks have similar compositions to the upper part of the layered intrusions.

The sills have highly variable TiO2 (1.55–4.44 wt%), much higher than that of the layered intrusions and volcanic rocks (<1.19 wt% TiO2). The former is termed as the high-Ti group and the latter as the low-Ti group (Table 2). The high-Ti group has a narrow range of SiO2 (44.2–47.9 wt%), whereas the low-Ti group has variable SiO2 ranging from 44.0 to 58.3 wt% (Table 2). The high-Ti group is also enriched in Al2O3, Fe2O3, and P2O5 relative to the low-Ti group. The two groups show markedly different trends in the plots of SiO2 versus oxides and MgO versus Fe2O3 and TiO2 (Fig. 3a-f). For the high-Ti group, there is a clear negative correlation between MgO and TiO2 (Fig. 3f). Although both the high- and low-Ti groups plot in distinct fields in the AFM diagram, they both have tholeiitic trends (Fig. 4).

Fig. 3
figure 3

Harker diagrams of major oxides of the mafic rocks in Funing, SW China: (a) SiO2 versus TiO2; (b) SiO2 versus MgO; (c) SiO2 versus P2O5; (d) SiO2 versus Na2O; (e) MgO versus Fe2O3(total); and (f) MgO versus TiO2

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Fig. 4
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Plots of AFM [(Na2O+K2O)-FeOt-MgO] ternary diagram for the mafic rocks in Funing, SW China

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In general, Ni has a positive correlation with MgO, whereas Cu does not correlate significantly with MgO (Fig. 5a, b). In the plots of Cu versus Ni, there is a very good positive correlation for samples from the low-Ti group (Fig. 5c). Overall, the high-Ti group has much more variable Cu/Ni ratios than the low-Ti group (Fig. 5d). The low-Ti group has a much wider range of Ni/MgO ratios than the high-Ti group (Fig. 5e). The high-Ti group has higher but variable V contents and Ti/V ratios than the low-Ti group (Fig. 5e, f).

Fig. 5
figure 5

Plots of the mafic rocks in Funing, SW China: a MgO versus Cu; b MgO versus Ni; c Cu versus Ni; d MgO versus Cu/Ni; e V versus Ni/MgO (ppm/wt%); and f MgO versus Ti/V

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The two groups also have different REE and trace element patterns. The high-Ti group has (La/Yb)cn (chondrite normalized) ratios between 6.99 and 10.45, exhibits LREE enrichment, and has positive Eu anomalies (Fig. 6a), whereas the low-Ti group has (La/Yb)cn ratios between 2.58 and 5.33, has relatively flat chondrite-normalized REE patterns, and exhibits negative Eu anomalies (Fig. 6b). The volcanic rocks have the same REE patterns as the other low-Ti rocks but their REE contents are somewhat higher. In primitive mantle-normalized spidergrams, the high-Ti group is characterized by enrichment in Ba relative to Rb and Th and especially in the Anding intrusion (except sample FL38) by marked negative Pb anomalies (Fig. 7a). Unlike the high-Ti group, the low-Ti rocks are characterized by negative Nb–Ta and Ti and P anomalies and positive Zr–Hf and Pb anomalies (Fig. 7b).

Fig. 6
figure 6

Chondrite-normalized REE patterns for the mafic rocks of the high-Ti group (a) and low-Ti group (b) in Funing, SW China

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Fig. 7
figure 7

Primitive mantle-normalized incompatible element patterns for the mafic rocks of the high-Ti group (a) and low-Ti group (b) in Funing, SW China. Normalization values are from Sun and McDonough (1989)

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Nb and Yb are positively correlated but exhibit different trends for the high- and low-Ti groups (Fig. 8a). The two groups also have different ratios of Tb/Yb, Zr/Y, Th/Yb, Ta/Yb, (Dy/Yb)cn and (La/Yb)cn, defining two different trends (Fig. 8b–d).

Fig. 8
figure 8

Plots of the mafic rocks from Funing, Yunnan Province, SW China: (c) Yb versus Nb; (d) Zr/Y versus Tb/Yb; (a) Ta/Yb versus Th/Yb; and (b) (La/Yb)cn versus (Dy/Yb)cn

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Rb–Sr and Sm–Nd isotopic compositions

Samples from Funing display large variations in the Rb–Sr and Sm–Nd isotopic compositions (Table 3). The low-Ti group has (87Sr/86Sr)i (initial) ratios ranging from 0.710 to 0.715, much more variable and higher than the high-Ti group (0.706–0.707) (Table 3). Although Rb–Sr isotopic variations can be partly due to the mobile nature of Rb and Sr during alteration (e.g., Rollison 1993), the large differences between the two groups of rocks reflect their different origins. On the other hand, the Sm–Nd isotopic compositions within the high and low-Ti groups are relatively constant. The initial ɛNd values range from −1.5 to −0.6 for the high-Ti group and from −9.6 to −4.0 for the low-Ti group. Both the ɛNd values and (87Sr/86Sr)i ratios show a good correlation with trace elemental ratios (Fig. 9a–d). There is a positive correlation between Th/Nb and (87Sr/86Sr)i ratios (Fig. 9a), but a negative correlation between Ce/Pb and (87Sr/86Sr)i ratio (Fig. 9b). The high-Ti group has lower Th/Nb and Rb/Nb but higher (143Nd/144Nd)i ratios than the low-Ti group (Fig. 9c, d).

Table 3 Rb-Sr and Sm-Nd isotopic analytical results of the mafic rocks in Funing, SW China
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Fig. 9
figure 9

Plots of (87Sr/86Sr)i versus Th/Nb (a) and Ce/Pb ratios (b) and (143Nd/144Nd)i versus Th/Nb (c) and Rb/Nb ratios (d) for the mafic rocks in Funing, Yunnan Province, SW China

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All ɛNd values show an overall negative correlation with (87Sr/86Sr)i ratios (Fig. 10a). The high-Ti group plots in the field of the Emeishan flood basalts (Fig. 10a).

Fig. 10
figure 10

Plots of (143Nd/144Nd)i versus 87Sr/86Sri (a and b) and modeling of crustal contamination of the mafic rocks in Funing, SW China. Field of the Emeishan flood basalts are based on data from Chung and Jahn (1995), Xu et al. (2001), Xiao et al. (2003), and Song et al. (2004). Fields of mantle array, MORB, EM1, EM2, HIMU, UC (upper crust) and LC (lower crust) are from Hart (1988) and Weaver (1991)

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Discussion

Petrogenesis of the Funing mafic rocks

The plutonic and volcanic mafic rocks in Funing form a widespread sub-volcanic and volcanic system (Fig. 1). The plutonic suite includes undifferentiated diabase sills and layered intrusions. Although geochemically the rocks belong to the high- and low-Ti groups, they have similar crystallization ages within the uncertainties (Fig. 2). The volcanic rocks are compositionally identical to the upper part of the layered intrusions. The close spatial and temporal association of the volcanic and plutonic rocks indicates that they formed from the same magmatic event. However, significant geochemical differences between the high- and low-Ti groups indicate that a variety of processes were involved in their formation, as invoked for LIPs elsewhere (Naldrett et al. 1992; Arndt et al. 1993, 1998, 2003; Lightfoot et al. 1990, 1993, 1994; Fedorenko and Czamanske 1997). Such processes include different degrees of fractional crystallization, different degrees of crustal contamination, and different mantle sources.

Fractional crystallization

Fractional crystallization (FC) appears to have played a major role in the compositional evolution of the Funing mafic rocks (Fig. 11). Compositional variations within the low and high-Ti groups can be easily explained by this process. For example, the negative Sr and Eu anomalies of the low-Ti group (Figs. 6 and 7) are likely to have resulted from plagioclase fractionation. The large variations of SiO2 and MgO are also due to FC. The andesitic basalts are similar to the diorites in the layered intrusions, suggesting that they formed from an evolved magma that formed the upper portion of the layered intrusions.

Fig. 11
figure 11

Plots of SiO2 versus (87Sr/86Sr)i (a) and (143Nd/144Nd)i ratios (b) and TiO2 versus (87Sr/86Sr)i (c) and (143Nd/144Nd)i ratios (d) for the mafic rocks in Funing, Yunnan province, SW China. FC, fractional crystallization; and AFC, assimination and fractional crystallization

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The large variations of Cu and Ni with constant Cu/Ni ratios in the low-Ti group are controlled by the segregation of sulfide, because both Cu and Ni similarly prefer sulfide melts over silicate magmas (e.g., Naldrett 2004). The constant V and variable Ni/MgO ratios of the low-Ti group are a reflection of sulfide segregation without the involvement of magnetite. In the high-Ti group the large variations of TiO2 and P2O5 are due to variable accumulation of magnetite and apatite. In contrast to the low-Ti group, sulfide is not an important phase in the high-Ti group and the variable Cu/Ni ratios in these rocks indicate mafic mineral fractionation/crystallization. TiO2 and V are controlled mainly by magnetite. The large variation of V with constant Ni/MgO ratios in the high-Ti group (Fig. 5) is consistent with magnetite accumulation with no sulfide involvement. The positive Sr and Eu anomalies are probably due to accumulation of plagioclase.

Crustal contamination

The different trends of the low- and high-Ti groups in the Harker diagrams (Fig. 3) cannot be produced by FC of the same magma. The pronounced differences in some incompatible elements and their ratios between the high- and low-Ti groups suggest that they were derived from different magmas with different degrees of crustal contamination, because these elemental ratios are insensitive to alteration, melting conditions, and FC. Because of the relatively high (87Sr/86Sr)i and low ɛNd values of the low-Ti group, which contains abundant xenolithic zircon, contamination appears to have played an important role. The low-Ti group is much richer in some incompatible elements and is characterized by pronounced negative anomalies of Nb–Ta and Ti–P (Fig. 7). Because continental crust is poor in these elements (e.g., Rollison 1993), the distinctive negative anomalies (Fig. 7) can be accounted for by extensive crustal contamination.

The larger scatter of both (87Sr/86Sr)i and ɛNd values of the low-Ti group is consistent with variable degrees of crustal contamination (Fig. 10). Samples with high (87Sr/86Sr)i ratios and low ɛNd values have higher SiO2 contents and Th/Nb ratios and lower TiO2, clearly demonstrating an evolution involving assimilation and fractional crystallization (AFC) (Fig. 11).

In the plot of ɛNd versus (87Sr/86Sr)i, the low-Ti group lies between the enriched mantle (EM2) and upper crust (UC), away from the “mantle array” (Fig. 10b). This requires a significant degree of crustal contamination either in the mantle source or during magma ascent and differentiation. Assuming that the crustal contamination occurred during emplacement of the magmas, modeled degrees of crustal contamination are high (>20%). Although crustal contamination would have increased the rocks with ratios of LILE (large ion lithophile elements, such as Th, Rb, and Ba) and HFSE (high-field strength elements, such as Nb, Ta, Zr, and Hf), rocks of the low-Ti group have ratios of Th/Nb (>0.7) and Th/La (>0.3) even higher than the average upper continental crust (Fig. 12). A possible explanation is that it was derived from an enriched mantle source previously metasomatized by crustal fluids, such as fluids derived from altered rocks of oceanic crust with high Th/Nb ratios (∼42) (Dorlendorf et al. 2000) (Fig. 12). There are two types of EM sources: EM1 and EM2. An EM2-type source has higher Th/La and Zr/Nb ratios than an EM1-type source. It is generally characterized by (87Sr/86Sr)i ratios (>0.7065) and intermediate (143Nd/144Nd)i ratios (Hart 1988; Weaver 1991). Therefore, the low-Ti group rocks have (87Sr/86Sr)i ratios and ɛNd values displaying an affinity to an EM2-like source. We use average compositions of the lower and upper crusts in our modeling. Assuming an EM2-like parental magma, most of the low-Ti samples had 3–5% upper crustal contamination and ∼2% lower crustal contamination with the least contaminated sample having 1.2% upper crustal contamination (Fig. 10).

Fig. 12
figure 12

Plots of the mafic rocks from Funing, Yunnan Province, SW China: a Th versus Th/Nb and b Th/La versus Nb/U. Values of lower crust (LC), middle crust (MC), and upper crust (UC) are from Rudnick and Gao (2003), N-MORB and primitive mantle from Sun and McDonough (1989) and altered oceanic fluids (AOF) from Dorendorf et al. (2000)

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Mantle sources of the high-Ti group and degrees of partial melting

Magmas from which the high-Ti group was generated were relatively uncontaminated, and thus samples of this group are the most suitable for examining mantle source compositions. The dynamic melting inversion (DMI) method established by Zou and Zindler (1996) provides an effective means of estimating the source composition of mafic rocks and their degrees of partial melting (Zou et al. 2000). Based on the dynamic melting model, the source composition and degrees of partial melting can be determined independently without assuming one, in order to calculate the other. Therefore, the results obtained are expected to reflect more accurately the melting process. According to Zou et al. (2000), requirements for applying this method include: (1) the selected mafic rocks should have the same isotopic composition; (2) highly incompatible elements in general are used as a reference because their ratios with less-highly incompatible elements should be large and variable, implying that the selected samples formed from different degrees of partial melting; (3) the ratios of various element abundances (R) should vary systematically according to their bulk distribution coefficients; and (4) samples selected should represent liquid compositions typically with high Mg#. The high-Ti Shadou and Anding sills are un-differentiated. This interpretation is confirmed by the narrow range of REE concentrations in the high-Ti rocks. Thus their weighted mean concentrations can represent the liquid compositions. Therefore, the mean concentrations of trace elements, La, Ce, Nd, Sm, and Tb, are used for modeling (Table 4). Eu was not used for the calculations because it is sensitive to plagioclase fractionation during evolution of the magmas.

Table 4 Mantle source compositions and degrees of partial melting for the Shadou and Anding mafic intrusions
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The concentration ratios (Qa) of a highly incompatible element La with a less incompatible element Ce (Qb) are as follows:

$$ \hbox{Q}_{\rm a} = \hbox{Q}_{\rm La} = 27.61/12.51=2.207 $$
$$ \hbox{Q}_{\rm b} = \hbox{Q}_{\rm Ce} = 59.94/27.71=2.163 $$

Applying these trace element ratios to the equations of Zou et al. (2000), degrees of partial melting are obtained for Anding and Shadou magmas of 6.32 (f1) and 8.21 (f2), respectively. Similarly, using La concentration ratio Qa as a reference and Nd, Sm, and Tb to obtain Qb, we obtained additional three sets of f1 and f2 values (Table 4). Thus, the source compositions of these elements were obtained according to the method of Zou et al. (2000).

The calculated compositions of the mantle source for the rocks of both Shadou and Anding are LREE-enriched (Table 4). The magmas from which the Shadou and Anding rocks formed were generated by different degrees of partial melting: the high-Ti magmas of Shadou were formed by 9.5% melting whereas the high-Ti magmas of Anding were formed by 7.3% melting. These magmas were little modified by interaction with continental crust.

An integrated petrogenetic model

The high-Ti group has a limited range of Sr and Nd isotopic ratios and highly enriched LREE, TiO2, and P2O5. The high La/Yb ratios and the depletion of HREE can be explained by partial melting in the garnet stable field. These features are thus consistent with the derivation from an enriched, asthenospheric, OIB-type mantle source, supporting an origin related to a mantle plume.

Rocks of the high-Ti group have higher alkalis (Na2O) and LREE than rocks of the low-Ti group. Therefore, both the low- and high-Ti groups cannot be generated from the same magma, because crustal contamination as recorded in the low-Ti group should increase the alkaline and LREE components. Crustal contamination of normal mantle-derived magmas during magma emplacement cannot explain the observed isotopic and geochemical differences between the low- and high-Ti groups from a consideration of mass balance. The much lower La/Yb ratios and relatively flat HREE suggest derivation of the low-Ti group from a shallow, lithospheric, enriched mantle. An enriched EM2-like source is proposed for the generation of the low-Ti group. An EM2-like source is usually explained to have formed by the previous subduction (Weaver 1991). The Yangtze Block was surrounded by oceanic subduction in Neoproterozoic time (Zhou et al. 2002a). A similar EM2-like mantle source has also been invoked for the Emeishan flood basalts (Song et al. 2001).

Field relationships indicate that the high-Ti sills/dykes are older than the low-Ti intrusions (e.g., Wu et al. 1963). Their new SHRIMP zircon U–Pb ages are also supportive of such age relationship, although both types of intrusions have ages within their uncertainties (Fig. 2). It is therefore believed that the heat source needed for the melting of the lithospheric mantle was provided by a mantle plume.

Implications for ELIP magmatism

On the basis of geological correlations, the Emeishan flood basalts have a well-constrained age of end-Guadalupian (∼260 Ma) (Yin et al. 1992; Jin and Shang 2000), although radiogenic dates for volcanic rocks are not available, because of post-eruption alteration and metamorphism (Boven et al. 2002; Ali et al. 2004). This eruptive age is consistent with SHRIMP zircon U–Pb ages of 259±3 Ma for the Xinjie mafic–ultramafic intrusion near Miyi (Zhou et al. 2002b) and 262±2 Ma for an olivine gabbroic dyke near Panzhihua (Guo et al. 2004).

Although mafic rocks in Funing were not previously considered to be part of the ELIP, the new SHRIMP zircon ages presented here for the Shadou and Anding intrusions are similar to ages for the ELIP elsewhere. The geochemistry of the volcanic rocks in Funing is identical to the diorite of the layered intrusions. They are broadly comparable to low-Ti basalts of the ELIP elsewhere, although they show heavier crustal contamination. In the absence of any other known magmatism of this age and composition in the region, it is suggested that the mafic rocks in Funing were produced by the same mantle plume that generated the ELIP. Much of SW China is covered by Triassic strata and it is possible that large portions of the ELIP are not exposed (Yan et al. 2003). If so, the real extent of the ELIP would be much greater than previously thought, perhaps on the order of 1×106 km2 (Song et al. 2004). This interpretation extends the inferred distribution of the ELIP and supports our contention that a major igneous event took place at ∼260 Ma.

Mafic–ultramafic intrusions in the ELIP are spatially and temporally associated with the Emeishan flood basalts, which include both high- and low-Ti varieties (Xu et al. 2001) and alkaline rocks (Ma et al. 2003). Although genetic links between the intrusive and extrusive suites within the ELIP have not yet been established, the low-Ti-layered intrusions in Funing are geochemical analogies to the volcanic rocks, strongly supporting the interpretation of a similar origin. The same mantle plume is therefore believed to have been responsible for the formation of the entire suite.

The identification of both the low- and high-Ti groups in Funing suggests a diversity in the plutonic rocks, similar to that of the volcanic rocks of the ELIP. The diversity of plutonic rocks in Funing is similar to that of the Siberian Traps, which contain alkaline complexes and tholeiitic and picritic intrusions associated with flood basalts. Intrusions in the Noril’sk-Talnakh region of Siberia with distinctive petrology and chemical compositions are considered to have formed from magmas of variable composition (Naldrett et al. 1992; Lightfoot et al. 1990; 1993; Fedorenko and Czamanske 1997; Arndt et al. 1993, 1998), produced from mantle-derived melts that experienced different degrees of crustal contamination (Naldrett et al. 1992; Lightfoot et al. 1993, 1994; Arndt et al. 2003). Similarly, geochemical differences between the two types of intrusions in the Funing area indicate that a variety of processes were involved in the formation of the ELIP. These involved not only a rising mantle plume that transported mass and energy from the asthenospheric mantle to the continental crust but also extensive crustal contamination and derivation from enriched mantle source regions.

Conclusions

The ELIP is composed of the Emeishan flood basalts and a variety of mafic–ultramafic intrusions. The intrusions have compositions and isotopic signatures similar to those of the volcanic rocks, indicating derivation from the same mantle source. The Funing intrusions include high- and low-Ti groups, have ages identical to those of ELIP plutonic bodies and the associated volcanic rocks, and show the same diversity of compositions as the Emeishan flood basalts, strongly suggesting that they are all part of the same magmatic event at ∼260 Ma. The high-Ti group in Funing formed from relatively uncontaminated mafic melts produced by low degrees of partial melting of an enriched, OIB-type, asthenospheric mantle source an EM2-like source and was, whereas the low-Ti group was derived from heavily crustally contaminated. These two melt types then evolved along different paths by FC.