Mantle Potential Temperature Estimates and Primary Melt Compositions of the Low-Ti Emeishan Flood Basalt

The Late Permian Emeishan large igneous province (ELIP) is considered to be one of the best examples of a mantle plume derived large igneous province. One of the primary observations that favor a mantle plume regime is the presence of ultramafic volcanic rocks. The picrites suggest primary mantle melts erupted and that mantle potential temperatures (T_P) of the ELIP were > 200°C above ambient mantle conditions. However, the picrites may represent a mixture of liquid and cumulus olivine and pyroxene rather than primary liquids. Consequently, temperature estimates based on the picrite compositions may not be accurate. Here we calculate mantle potential temperature (T_P) estimates and primary liquids compositions using PRIMELT3 for the low-Ti (Ti/Y < 500) Emeishan basalt as they represent definite liquid compositions. The calculated T_P yield a range from ~1,400 to ~1,550°C, which is consistent with variability across a mantle plume axis. The primary melt compositions of the basalts are mostly picritic. The results of this study indicate that the Emeishan basalt was produced by a high temperature regime and that a few of the ultramafic volcanic rocks may be indicative of primary liquids.

Introduction

Continental flood basalt provinces and oceanic plateaux are the remnants of atypical mantle melting in anorogenic tectonic settings (White and McKenzie, 1989, 1995; Coffin and Eldholm, 1994; Jerram and Widdowson, 2005). The significant volume (>10⁵ km³) of primarily mafic volcanic rocks in flood basalt provinces is thought to be related to anomalous thermal conditions associated with deep mantle (mantle plume) upwelling (Richards et al., 1989; Griffiths and Campbell, 1990; Ernst and Buchan, 2003; Ernst et al., 2005). However, not all flood basalts (continental or oceanic) are attributed to mantle plumes and it is possible that some are derived by decompressional melting of fertile mantle associated with tensional plate stress or lithospheric delamination induced upwelling (Sheth, 1999; Anderson, 2005; Foulger, 2010). The distinction between mantle plume-derived and non-mantle plume-derived may have implications for biological evolution as the five major mass extinctions (Ordovician-Silurian, Late Devonian, Permian-Triassic, Triassic-Jurassic and Cretaceous-Paleogene) are contemporaneous with flood basalt eruptions of which the three youngest are thought to contemporaneous with mantle plume derived flood basalts (Rampino and Stothers, 1988; Wignall, 2001; Courtillot and Renne, 2003; Phipps Morgan et al., 2004; Self et al., 2005; Sobolev et al., 2011; Rampino and Caldeira, 2018).

There are a number of geochemical, geological, and geophysical criteria used to identify mantle plume-derived continental flood basalt provinces in the geological record (Ernst and Buchan, 2003; Xu et al., 2004; Ernst et al., 2005; Campbell, 2007; Bryan and Ferrari, 2013). However, some criteria, such as uplift and doming of the crust, are more difficult to evaluate than others especially within older or dismembered flood basalt provinces. Therefore, one of the key criteria for identifying a mantle plume-related flood basalt province is estimating the mantle potential temperature (T_P) required to generate a primary melt of the lava that erupted (Herzberg et al., 2007; Herzberg and Asimow, 2008; Rey, 2015). It is expected that the thermal regime of a mantle plume is ~250°C higher than that of the ambient (1,300–1,400°C) mantle temperature (Farnetani and Richards, 1994; Herzberg and O'Hara, 2002; Herzberg et al., 2007). Ultramafic volcanic rocks are commonly found in the lower volcanic sequences of flood basalt provinces, generated by high degrees of melting under high mantle temperatures, and provide a clear record of the thermal regime (Herzberg and O'Hara, 2002; Kamenetsky et al., 2012). In cases where ultramafic volcanic rocks are unexposed or unidentified, mafic volcanic (basalt) rocks may also be able to help constrain the thermal regime, providing that they have not experienced pyroxene or plagioclase fractionation or were not derived from a volatile-rich and/or pyroxenite source (Coltice et al., 2007; Herzberg et al., 2007; Herzberg and Gazel, 2009; Hole, 2015; Whalen et al., 2015; Shellnutt and Hsieh, 2016; Yeh and Shellnutt, 2016).

The Emeishan large igneous province (ELIP) of SW China is considered to be one of the best examples of a mantle plume derived continental flood basalt province (Chung and Jahn, 1995; Chung et al., 1998; He et al., 2003; Xu et al., 2004; Ali et al., 2005; Campbell, 2005; Shellnutt, 2014). The presence of ultramafic volcanic rocks (picrites) and a short (2–3 million years) eruptive duration are the primary evidence for a mantle plume origin of the ELIP (Ali et al., 2005; Shellnutt et al., 2012; Shellnutt, 2014; Zhong et al., 2014). Regional uplift and doming of the crust is suggested but the evidence is debated and, moreover, may not be a priori evidence for a mantle plume-derived large igneous province (c.f. Ukstins Peate and Bryan, 2008; Sun et al., 2010; Wang et al., 2014). The eruptive and mantle potential temperatures of the picrites were estimated using a variety of techniques including REE inversion, whole rock compositions, and melt inclusions (Xu et al., 2001; Zhang et al., 2006; Ali et al., 2010; Hanski et al., 2010; He et al., 2010; Tao et al., 2015). Most of the T_P estimates yield temperatures > 1,530°C (up to 1,810°C). However, melt inclusion studies of olivine demonstrate a wide range of compositions suggesting that the picrites may not represent pure liquid compositions but rather a mixture of cumulus minerals and liquid (Kamenetsky et al., 2012; Ren et al., 2017). Consequently, mantle potential temperature estimates (T_P) and primary liquid compositions based on the whole rock picrite data may not be accurate.

Most Emeishan picrites are porphyritic and can contain up to 30% phenocrysts, some of which may be antecrysts (Tao et al., 2015). In comparison, most Emeishan basalts are aphyric and more likely to be representative of liquid compositions. Thus, the Emeishan basalt that has only experienced olivine fractionation is likely to provide robust T_P and primary liquid composition estimates (Herzberg and Asimow, 2015). In this paper we calculate the mantle potential temperatures and the primary liquid compositions of the low-Ti (Ti/Y < 500) Emeishan basalt in order to constrain the thermal regime of the ELIP and the range of possible primary liquid compositions. The results can help to elucidate the composition of the primary melts that either erupted or differentiated within the crust and the thermal conditions associated with the ELIP.

Background Geology

The Middle-Late (~260 Ma) Permian ELIP covers an area of ~0.3 × 10⁶ km² along the western margin of the Archean to Paleoproterozoic Yangtze Craton of the South China Block (Ali et al., 2005; Shellnutt, 2014). Correlative dismembered units are found in the Songpan-Ganzi terrane of the Tibetan Plateau and the Phan Si Pan uplift in northern Vietnam (Figure 1). The flood basalts are volumetrically the largest unit but there are minor volumes of ultramafic (picrite) and silicic volcanic rocks (Chung and Jahn, 1995; Hanski et al., 2004; Zhang et al., 2006; Wang et al., 2007; Shellnutt and Jahn, 2010; Xu et al., 2010; Anh et al., 2011; Tran et al., 2015; Usuki et al., 2015). The picrites are restricted to the lower half of the volcanic succession whereas the basaltic-andesites and silicic volcanic rocks tend to be found in the upper half. The volcanic rocks erupted within a short duration (2–3 Ma) on top of middle Permian limestone or directly on Precambrian cratonic rocks of the Yangtze Block (He et al., 2003; Zheng et al., 2010; Shellnutt et al., 2012; Zhong et al., 2014). The plutonic units consist of ferroan alkalic granites (A-type), ore-bearing layered mafic-ultramafic intrusions and mafic dykes (Shellnutt and Zhou, 2007; Shellnutt et al., 2008; Pang et al., 2010; Li et al., 2015).

FIGURE 1

Figure 1. Simplified regional geological map of the Emeishan large igneous province showing the concentric zones (dashed red lines) of the ELIP. Reproduced from Shellnutt et al. (2008) with permission. Sample locations are identified.

The chemical subdivision of the Emeishan flood basalts into low- and high-Ti groups is primarily based on the Ti/Y (Ti/Y > 500 = high-Ti; Ti/Y < 500 = low-Ti) ratio and bulk TiO₂ wt% (TiO₂ > 2.5 = high-Ti; TiO₂ < 2.5 = low-Ti) content. Additional subgroupings are identified using the Sm/Yb ratio, Mg#, ε_Nd(t), and the fractionating mineral assemblage (Xu et al., 2001; Xiao et al., 2004). The picrites can be subdivided on the basis of the Ti/Y ratio but also using chondrite normalized rare earth element ratios (c.f. Kamenetsky et al., 2012). The basalt classification scheme is rather cumbersome as there are at least 5 types of Emeishan flood basalt (low-Ti 1, low-Ti 2, high-Ti 1, high-Ti 2, high-Ti 3). If a large database is compiled the subdivision of the basalts and picrites into two broad groups is a little misleading as it is clear there is compositional continuum from low-Ti to high-Ti varieties (Hao et al., 2004; Shellnutt and Jahn, 2011; Kamenetsky et al., 2012). Nevertheless, it can be stated that there are mafic and ultramafic volcanic rocks with lower (<2 wt%) and higher (> 2.5 wt%) TiO₂.

The emplacement of the ELIP is coeval with the end-Guadalupian mass extinction and may have contributed to the decline of biota (Bond and Wignall, 2014; Rampino and Caldeira, 2018). Based on seismic studies, the ELIP is structurally divided into three roughly concentric zones (inner, intermediate and outer), which correspond to crustal thickness estimates with the inner zone having the thickest crust that progressively thins to outer zone (Xu et al., 2004). There is some evidence to suggest the Yangtze Craton experienced uplift and doming of the crust prior to the eruption of the flood basalts, however it is debated (He et al., 2003; Ukstins Peate and Bryan, 2008, 2009; Sun et al., 2010).

Method

For this paper we compiled a database using GEOROC (http://georoc.mpch-mainz.gwdg.de/georoc/) of over 450 whole rock analyses of Emeishan basalt. The initial selection criteria for the calculation was based on high Mg# (> 50), high MgO (>6 wt%), and high bulk CaO (>11 wt%) because rocks with lower values likely experienced clinopyroxene ± plagioclase fractionation and are unsuitable for the calculation. Primary melt compositions and mantle potential temperature estimates (T_P) were calculated for Emeishan basalt using PRIMELT3 (Herzberg and Asimow, 2015). PRIMELT3 allows the user to adjust source parameters (e.g., relative oxidation state, FeOt, and MgO content) and pressure conditions that may better reflect the conditions of melt formation. Previous T_P calculations of the Emeishan ultramafic volcanic rocks used PRIMELT2 (Ali et al., 2010; He et al., 2010), however there are significant improvements in PRIMELT3 that include: correcting melt fractions, identifying the residuum mineralogy, and improved uncertainty in the thermal estimates (Herzberg and Asimow, 2015). The estimated mantle potential temperatures are considered to be accurate to ± 42°C (Herzberg and Asimow, 2015).

The bulk FeOt content of off-craton peridotite is 8.14 ± 0.9 wt% (Herzberg, 1993; Herzberg and O'Hara, 2002). The consequences of melting of an Fe-rich (FeOt = 9.0 wt%) peridotite are discussed by Herzberg and O'Hara (2002) in detail but the net result is that the primary liquids will be elevated by ~0.9 wt% FeOt at most pressures. The default setting in PRIMELT3 for bulk mantle FeOt is 8.02 wt% although it is suggested that varying the FeOt content may be useful in reproducing melt fractions from Fe-rich and Fe-poor peridotite (Herzberg and Asimow, 2015). We used the lowest possible mantle FeOt (8.02–8.41 wt%) content that yields internally consistent results (e.g., no Fe/Mg and Na/Si errors) for each sample and dismiss results that indicate the melts are generated from a volatile-rich mantle source or from pyroxenite mantle source (Table S1).

The relative oxidation state of the Emeishan basalt was calculated using the method of Kress and Carmichael (1991) and produced a ΔFMQ (fayalite-magnetite-quartz) range from −1 to +1 with an average of +0.1 (Shellnutt and Iizuka, 2012). Moreover, the V/Ga ratios range from ~8 to ~27, which corresponds to a relative oxidation stage ranging from ΔFMQ −2 to +1 (Mallmann and O'Neill, 2009). Consequently, each sample was calculated using an Fe₂O₃/TiO₂ ratio of 0.5 (reducing) and 1.0 (oxidizing) that reflects the range of relative oxidation state of the basalt (Table S1). The models were calculated at surface pressure (1 atm) and without water (H₂O = 0).

Results

A total of 14 samples were identified that produced meaningful results (Table S1). All samples classify as low-Ti Emeishan basalt (TiO₂ ≤ 2.2 wt%; Ti/Y < 500) and have bulk CaO > 11.3 wt%. Two samples (SC-9, HK-89) have low Mg# (~50) and Ni < 100 ppm which suggests they experienced more olivine removal than other samples as they were able to produce meaningful results (Table S1). The high-Ti basalt with high Mg#, MgO and CaO did not yield internally consistent results, which led to calculation errors (Fe/Mg error, Na/Si error) and the necessity of a volatile source or indicated pyroxene fractionation. It is thought that volatile compounds (e.g., CO₂, H₂O) may play a larger role in the genesis of the “high-Ti” basalt or that they are derived from a pyroxenite source (Xu et al., 2001; Xiao et al., 2004; Zhou et al., 2008; Shellnutt and Jahn, 2011; Kamenetsky et al., 2012).

The melt compositions under reducing conditions (Fe₂O₃/TiO₂ = 0.5) are not significantly different than the melts generated under oxidizing conditions (Fe₂O₃/TiO₂ = 1.0). The calculations indicate the primary melts are low-Ti (TiO₂ < 2 wt%) and mostly picritic (MgO = 13.3 wt% to 20.0 wt%) with four samples (HK-89, WL-13, SC-9, DJ-25) that are komatiitic/meimechitic (Figure 2). There are differences between calculated temperatures of the two oxidation state models. The estimated eruptive temperatures are consistently lower in the oxidizing (1,315–1,410°C) models compared to the reducing (1,330–1,435°C) models with temperature differences as little as ~15°C to as high as ~40°C. The estimated T_P are also lower (mean = 1470 ± 22°C) in the oxidizing (1,390–1,530°C) models in comparison to the reducing (1,410–1,560°C) models (mean = 1505 ± 22°C), but the difference between the two models is larger (~20 to ~65°C) than the difference between the eruptive temperatures (Figure 3). The estimated uncertainty of the eruptive temperatures is ±31°C indicated that the oxidizing and reducing estimates are broadly within uncertainty (Herzberg et al., 2007). The same is true for the oxidizing and reducing mantle potential temperatures estimates as they are mostly with the calculation uncertainty (±42°C). The estimated pressure of melting, with an uncertainty of ±0.3 GPa, using the maximum primary melt MgO content ranges from ~2.4 GPa (HK-94) to ~4.8 GPa (SC-9) and corresponds to a depth range of ~70 to ~145 km (Herzberg and Asimow, 2015).

FIGURE 2

Figure 2. Classification of the primary liquid composition calculated in this study using the scheme of Le Bas (2000). Data compiled from Song et al. (2001); Xiao et al. (2004); Zhang et al. (2006); Wang et al. (2007); Zi et al. (2008), and Arguin et al. (2016).

FIGURE 3

Figure 3. Comparison of crystallization (T°C) and mantle potential temperature ($T_{\text{P}}^{°}$C) estimates of the ELIP. The results of this study are shown as red circles and white circles with red trim (ELIP basalt). The Al-in-olivine crystallization temperature estimates (diamonds) are for intermediate and low-Ti Emeishan picrite inclusions (Xu and Liu, 2016). The mantle potential temperature estimates of picrite T_P (squares) are estimated using PRIMELT2 (He et al., 2010). The olivine-liquid T_P (vertical black lines) estimates for low-Ti Emeishan picrite using melt fractions of (F) of ≈0.35 and ≈0.2 (Tao et al., 2015). Crystallization and mantle potential temperature estimates (blue circles) of mid-ocean ridge (MOR) basalt are also shown (Shellnutt and Hsieh, 2016).

Discussion

Thermal and Compositional Estimates of the Emeishan Primary Magmas

The crystallization temperatures based on the primary melt compositions of the low-Ti basalt in this study are between 1,300 and 1,450°C (oxidizing models = 1,315–1,410°C; reducing models = 1,330–1,435°C) and similar to the estimates using Al-in-olivine for komatiites and basalts from Gorgona, Baffin Island, SE Greenland and Madagascar but much higher than estimates (<1,300°C) for mid-ocean ridge basalt (Coogan et al., 2014; Shellnutt and Hsieh, 2016). The Al-in-olivine thermometer method produced a range of crystallization temperatures from 1,188 ± 56°C to 1,440 ± 63°C (low-Ti picrites = 1249 ± 58°C−1356 ± 61°C) for all compositions of the ELIP picrites (Xu and Liu, 2016). The lower temperature estimates are from olivine which have lower Fo-values (<86), lower Al₂O₃ (<0.05 wt%) and lower NiO (<0.32 wt%) contents and likely indicate a cooling trend within the system (Figure 3). Xu and Liu (2016) suggest that the higher crystallization temperatures of the ELIP picrites can only be reconciled within the context of a mantle plume model.

Mantle potential temperature estimates using olivine-liquid equilibration estimates of the “low-Ti” picrites range from 1,536 to 1,296°C but, depending on the melt fraction used, may be as high as 1,740–1,810°C (Figure 3; Tao et al., 2015). Our T_P estimates (~1,400 to ~1,550°C) are generally higher but broadly match the results using a melt fraction (F) of ≈0.35. However, we think the maximum temperature estimates based on olivine-liquid equilibration and F-value of ≈0.2 are too high (1,740–1,810°C) as the suggested temperature range overlaps with and possibly exceeds estimates for Archean thermal regimes (Lee et al., 2010). It is possible that the differences between the T_P estimates using PRIMELT3 and olivine-liquid equilibration may be due to the origin of the olivine within the picrite (i.e., antecryst, xenocryst). Although Xu and Liu (2016) did not report mantle potential temperature estimates based on their results, Matthews et al. (2016) using the same method for olivine from Iceland volcanic rocks suggests the highest observed crystallization temperature (1399 ± 20°C) constrains the mantle potential temperature to ${1,480}_{-30}^{+37°}$C. This suggests the highest crystallization temperature estimate (1,440 ± 63°C) of Xu and Liu (2016) probably corresponds to a mantle potential temperature > 1,500°C and thus could be representative of hot thermal regime.

A comparison of the calculated primary melt compositions of this study with those derived from melt inclusions of the low-Ti picrites show that, in spite of the differences in the applied methods, the results are broadly similar (Figure 4). The primary melt composition estimates from the Dali picrite inclusions have considerable variation but those from the Binchuan picrites, specifically the estimates of Ren et al. (2017) for sample EM43, are closer to the estimates of this study. Furthermore, it is clear that most low-Ti picrites are different from the primary melt compositions, supporting the notion that they are not pure liquid compositions. The group of picrites with low SiO₂ (<45 wt%) and high Al₂O₃ (9–12 wt%) were collected to the west of Xichang near Muli are described as having porphyritic textures with phenocrysts of clinopyroxene and plagioclase indicating they may represent mixtures of cumulate clinopyroxene and plagioclase and lava (Li et al., 2010). There are a few picrites (HK-5, HK-92; YJ-66; EM-83; PU-03) that overlap in all major elements indicating they may be closer to primary liquid compositions. In fact samples YJ-66 and PU-03 yield T_P estimates of 1,500–1,530°C (Fe₂O₃/TiO₂ = 1.0), and 1,536–1,547°C (Fe₂O₃/TiO₂ = 0.5) which are consistent with the high T_P estimates from the basalt (Table S1).

FIGURE 4

Figure 4. Comparison of the calculated primary melt compositions of the low-Ti Emeishan basalt (circles) with the whole rock compositions of the low-Ti picrites (blue crosses), the primary melt composition derived from inclusions of sample EM43 (squares), and inclusions from the Dali picrites (green field = Ren et al., 2017; orange field = Hanski et al., 2010). (A) FeOt wt.% vs. MgO wt.%, (B) Na₂O wt.% vs. SiO₂ wt.%, (C) K2O wt.% vs. SiO₂ wt.%, (D) Al₂O₃ wt.% vs. SiO₂ wt.%, (E) CaO wt% vs. SiO₂ wt.%, and (F) TiO₂ wt.% vs. SiO₂ wt.%. The olivine accumulation and relative pressure arrows are taken from Arndt (2003). The low-Ti picrite data are from Kamenetsky et al. (2012); the EM43 (low-Ti picrite) and Dali picrite inclusions are from Ren et al. (2017). The picrites reported from Muli are plagioclase (pl) and clinopyroxene (cpx) phyric (Li et al., 2010). All data are normalized to 100%.

Mantle Plume Structure of the Emeishan Large Igneous Province

The mantle potential temperature estimates of the oxidizing and reducing models range from ~1,400 to ~1,550°C. The lower end temperatures are consistent with ambient mantle conditions (1,300–1,400°C) whereas the higher end temperatures are anomalously (~1,550°C) high (Green and Falloon, 2005; Campbell, 2007; Herzberg et al., 2007). The relatively large temperature range (~150°C) is not that unusual as other large igneous provinces (Deccan, East Africa, North Atlantic, Caribbean, Central Atlantic Magmatic Province) and oceanic islands (Hawaii, Cook) have a similar range (Herzberg and Asimow, 2008; Rooney et al., 2012; Hole, 2015; Shellnutt et al., 2018). In many cases the temperature variability is attributed to the thermal structure of a mantle plume, where hot primary melts are derived from the plume axis and cooler primary melts are derived from the periphery (Herzberg and Gazel, 2009; Hole and Millet, 2016). Therefore, we suggest that the high T_P estimates of the basalts are indicative of an anomalously hot thermal regime for the ELIP and consistent with the expected conditions associated with a mantle plume (Campbell, 2007). The rocks that yield lower T_P estimates (HK-93, HK94, SCHL-48) may either be due to melts derived from the peripheral regions of the plume axis or melt extraction during the waning stages of volcanism. Rare earth element ratios for the three samples that produced the lowest T_P estimates (1,410–1,440°C) have the lowest chondrite normalized La/Yb ratios (1.7–1.8), although sample SCHL-48 has high ratio (5.1), and low Dy/Yb ratios (1.6–2.1) which favor a spinel peridotite source. In comparison, the samples that produced high T_P estimates (1,480–1,560°C) have La/Yb_N ratios from 3.7 to 5.1, Dy/Yb ratios of 1.9–2.4 which indicates the possibility of residual garnet in the source and thus originated from greater depth (Davidson et al., 2013).

The basalts in this study are principally from the western portion (Binchuan, Lijiang, Panzhihua) or the displaced western portion (Song Da) of the ELIP, therefore a complete spatial-thermal characterization cannot be fully developed. However, the basalts examined from different areas (Heishitou, Yanghe, Longlin, Xilin, Bama, Baise, Tianyang) of the eastern ELIP (Sichuan, Guizhou, Guangxi) are mostly of the “high-Ti” variety, but even those that are “low-Ti” typically have characteristics (CaO ≤ 10 wt%; MgO < 6 wt%) indicative of clinopyroxene (± plagioclase) fractionation (Qi and Zhou, 2008; Lai et al., 2012; Li et al., 2017; Liu et al., 2017).

The hypothetical ELIP plume-center, based on dyke orientation, is suggested to be located south-southwest of Panzhihua (~60 km) near Yongren County (Yunnan) (Li et al., 2015). The radial distribution of dykes is compelling evidence that favors a plume-center in southern Sichuan-northern Yunnan, but the western margin of the ELIP is highly tectonized and dismembered (Song et al., 2006; Zi et al., 2010; Usuki et al., 2015; Li et al., 2016). For example the Song Da region of Northern Vietnam was displaced ~600 km to the SE along the Ailaoshan-Red River fault system during the Oligocene (Chung et al., 1997). Therefore, the original areal distribution of basalts in the west prior to accretion and deformation of the Songpan-Ganzi terrane is poorly constrained (Weislogel et al., 2010). However, the fact that the basalts that successfully yielded primary melt compositions are exclusively from the western region (inner zone) of the ELIP is unlikely to be random. The implication is that the low-Ti Emeishan basalt located in the west are closer to primary melt compositions than those located in the east. It is possible that the “western” basalt erupted closer to the hypothetical plume-center.

The eastern low-Ti Emeishan basalt may have erupted further from the plume-center and thus had more time to fractionate other silicate minerals (i.e., clinopyroxene and plagioclase) during their transit through the crust or that they were derived from a pyroxenitic source. The pressure estimates based on MgO content of the primary melt compositions from the reducing models (Fe₂O₃/TiO₂ = 0.5) ranges from ~2.4 GPa to ~4.8 GPa and covers an equivalent depth range of ~70 km (~70 to ~145 km). The high pressure melts, which have the highest T_P, correspond to a region beneath the lithosphere of the Yangtze Craton that has high seismic velocity (Vp) zone and is interpreted to be solid residue after melt extraction of the Emeishan mantle plume (Liu et al., 2001; Xu et al., 2004). In contrast, the low pressure melts, which have the lowest T_P, correspond to a high velocity lower crust region beneath inner zone of the ELIP, which is interpreted to be a region of magmatic underplating (Xu et al., 2004; Chen et al., 2015). It is possible that the region of underplating served as a zone where primary melts initially differentiated before forming layered mafic-ultramafic intrusions within the middle-upper crust (Hou et al., 2012; Zhang et al., 2012; Bai et al., 2014; Shellnutt and Wang, 2014). The high T_P estimates, melt extraction pressure estimates, geographic location of the picrites and basalt that yield high T_P estimates, and the spatial distribution of the basalt, all suggest that the structure of the ELIP was the likely consequence of a mantle plume.

Conclusions

The primary melt compositions of the low-Ti Emeishan basalts are chemically similar to picritic melts and broadly similar to primary melt compositions determined from melt inclusions of the low-Ti Emeishan picrites. A few of the Emeishan picrites overlap with the primary melt compositions, indicating that some may be representative of primary liquids. The highest T_P estimates yielded temperatures >1,550°C, indicating that the thermal regime of ELIP reached temperatures >250°C above ambient mantle conditions, consistent with the expected thermal regime of a mantle plume.

Author Contributions

JS designed the project, calculated the models, interpreted the results and wrote the paper. TP collected and processed the samples. All authors contributed to the discussion of results.

Funding

This study was supported by the Ministry of Science and Technology (Taiwan) grant (106-2116-M-003-007) to JS. TP was supported by the Society of Economic Geologists Student grant program (2015).

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We thank George Zellmer and David Peate for their insightful comments that improved the manuscript and editorial comments by Scott Whattam.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/feart.2018.00067/full#supplementary-material

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