LATE PERMIAN EMEISHAN FLOOD BASALTS
IN SOUTHWESTERN CHINA
XU Yigang, HE Bin, XIAO Long, MEI Houjun, XU Jifeng,
MA Jinlong and HUANG Xiaolong
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
The Emeishan flood basalt, an important large igneous province (LIP) in southwestern China, occurred around the Permian-Triassic boundary (Chung and Jahn, 1995; Xu et al., 2001; Lo et al., 2002). Its possible synchronism with the eruption of another major flood basalt, the Siberian Traps, and its relationship to very profound extinctions around the Permian-Triassic boundary have attracted a number of recent studies (Chung and Jahn, 1995; Chung et al., 1998; Courtillot et al., 1999; Song et al., 2001; Thompson et al., 2001; Wignall, 2001; Xu et al., 2001; Zhang et al., 2001; Boven et al., 2002; Zhou et al., 2002; Lo et al., 2002). The current studies include geological and sedimentary investigation on stratigraphic sequences immediately below the Emeishan basalts, radiometric dating, petrologic and geochemical studies of basalts and their related rocks and mineralisation. This paper summarizes some of recent achievements in this domain and then indicates the potential research direction in the future.
II. GEOLOGICAL BACKGROUND
The Emeishan basalts are exposed in a rhombic province of ~250,000 km2 within Yunnan, Sichuan and Guizhou Provinces (Fig.1). The Longmenshan thrust fault and the Ailaoshan-Red River slip fault are generally considered as its northwestern and southwestern boundaries, respectively. However, some basalts and mafic complexes exposed in the Simao basin, northern Vietnam and Qiangtang terrain are possibly an extension of the Emeishan LIP (Chung et al., 1998; Xiao et al., in press). Some Emeishan-type basalts traced in southwest Yunnan and northern Vietnam may be related to the mid-Tertiary continental extrusion of Indochina relative to South China along the Ailao Shan-Red River fault zone (Tapponnier et al., 1990; Chung et al., 1997). It is likely that the later tectonic events have resulted in major disruption of the former igneous province. Three sub-provinces have been divided in the previous studies, namely the west, central and east parts of the Emeishan LIP (Fig. 1, Cong, 1988; Zhang et al., 1988). The central part overlaps the ※Panxi plaeorift zone§ (Tan, 1987). The thickness of the entire volcanic sequence in these sub-provinces varies considerably from over 5000 meters in the west to a few hundred meters in the east. The province consists of dominant basaltic lavas and subordinate pyroclastic rocks. In the western sub-province, flows and tuff of trachytic and rhyolitic composition form an important member in the uppermost sequence (Huang, 1986; Chung et al., 1998; Xu et al., 2001). Such a compositional bimodality is also revealed by the associated ntrusive rocks that comprise syneites and layered gabbros. Some of syneites and gabbros are associated with massive V-Ti-Fe ore deposits (Sichuan, 1991; Yunnan, 1990).
The Emeishan volcanic successions uncomfortably overlie the late Middle Permian carbonate Formation (i.e., the Maokou limestone) and are in turn covered by the uppermost Permian in the east and the middleTriassic sediments in the west. The average lava thickness of the LIP was estimated to be about 700 m (Lin, 1985), thus the entire volume of the Emeishan basalts to be ~0.3 ℅106 km3. This represents a minimum estimate because: (1) complicated tectonic movements in Meso-Cenozoic eras in this region cut off the western extension of the LIP (Chung et al., 1998); (2) erosion must have removed a significant portion of the eruptive sequences; and (3) the associated intrusives are not taken into account.
Fig.1. Schematic map showing the distribution of the late Permian volcanic successions (black areas) in the Emeishan basalt province and adjacent regions (modified after Chung et al., 1998). The inset illustrates major tectonic unites in eastern Asia.
III. TEMPORAL AND SPATIAL DISTRIBUTION OF THE EMEISHAN FLOOD BASALTS
1. Classification of Rock Types
The Emeishan flood basalts have been divided into two major magma-types: high-Ti (Ti/Y>500) and low-Ti (Ti/Y<500) basalts on the basis of a small number of analyses for the samples from Binchuan and Ertan (Xu et al., 2001). The high-Ti group (HT) generally has higher Ti/Y (>500) and TiO2 (>3.7 wt%) than the low-Ti ones (LT) that have low Ti/Y (<500) and TiO2 (<2.5 wt%). This classification schema is confirmed by the large database recently obtained for the Binchuan basalts (Xiao et al., in press). Moreover, available data permit a further subdivision of the low-Ti group into LT1 and LT2 types on the basis of trace element characteristics (Fig. 2).
Fig.2. Classification of the Emeishan basalts in terms of Ti/Y versus Mg#, Sm/Yb and Th/Nb.
In general, the LT1 basalts have higher Mg# (67〞61) than the LT2 basalts (Mg# = 54〞48) (Fig. 2). A more evolved nature is found for the HT basalts for which Mg# varies between 58-44. Distinction between the LT1 and LT2 lavas is also clear in the plot of Th/Nb and Sm/Yb versus Ti/Y, which highlights the high contents of highly incompatible elements in the LT1 lavas (Fig. 2). LT1 lavas have Sm/Yb (3>Sm/Yb>2) ratios higher than LT2 (<2) and lower than the HT lavas (>3), and higher Th/Nb (>0.17) ratios than the LT2 and HT basalts (Th/Nb <0.17). The measured and age-corrected 87Sr/86Sr and 143Nd /144Nd ratios of the Emeishan basalts define an array that lies near the ※mantle correlation line§ but is distinctly displaced to the right (Xu et al. 2001). In general, the HT basalts have relatively higher Nd(t) and lower 87Sr/86Sr(t) values than the LT basalts.
2. Melting Conditions
Xu et al. (2001) adopted the fractional melting inversion of McKenzie and O'Nions (1991), incorporating the modification proposed by White et al. (1992), to quantitatively determine the melting conditions. The inversion utilizes averaged REE concentrations to estimate the melt distribution as a function of depth, the total integrated melt fraction, and the total melt thickness, equivalent to the thickness of basaltic crust produced. Only samples with MgO>6wt% were used in REE inversion to minimize the effect of fractionation. The comparison between the melt distributions inferred from the LT lavas and the predicted one from isentropic decompression of mantle indicates a mantle potential temperature of >1550 ∼C (Fig. 3a). The melt distributions also show melting starting at a depth of 140 km. The upper limit of melting in the inversion is 60 km, typical of basalts generated beneath stretched and thinned continental lithosphere. The maximum melt fraction estimated from inversion is of about 16% (Fig. 3b). In contrast, the maximum melt fraction estimated from inversion of the HT lavas is significantly low (1.5%). The melting starts at ~100 km and the upper limit of melting in the inversion is 75 km. These results suggest that the HT lavas were generated at a higher depth by a smaller degree of partial melting from mantle than the LT lavas. The potential temperature of the mantle involved in melt generation of the HT lavas is also relatively lower (<1500 ∼C) compared to that for the LT lavas. Therefore, any transition from the LT to HT lavas is believed to be associated with change in melting conditions.
3. Plume Signature in the Emeishan Basalts
A starting plume model has been put forward for the generation of the Emeishan basalts by Chung et al. (1998) mainly based on two arguments: (1) The immense volume of magma produced in a rather short time span requires a large thermal anomaly within the mantle. (2) Forsterite (Fo) contents in olivine phenocrysts in picrites vary from 83 to 89 (Xu and Chung, 2001; unpublished data). This indicates various amount of accumulated olivines in the Emeishan picrites and primary magmas have MgO of >16%. The high potential temperature (ˇ1550 ∼C) is also revealed by the REE inversion results presented in this study. The plume model is supported by compositional evidence, which is not equivocal for some important CFBs (e.g., Parana, Peate, 1997; Karoo, Ellam and Cox, 1991).
Fig.3. Melt distribution for the Emeishan basalts (LT and HT) estimated from REE inversion. Thin dashed line with labeled temperatures is the predicted melt distribution from isentropic decompression of mantle (White and McKenzie, 1995).
Most HT and LT2 samples exhibit the trace element ratios that overlap with the field of oceanic island basalts (OIB). They show smooth trace element patterns that are very similar to OIB except for weak Nb anomaly in some samples (Fig.4). The pronounced negative Sr anomaly in HT lavas suggests that they have been affected by extensive fractionation of plagioclase. Samples with low trace element abundance also show high Nd(t) (4.6〞4.8) and low 87Sr/86Sr(t) (0.7042〞0.7046) values, thus likely reflecting the isotopic signature of the least-contaminated Emeishan plume head. A similar Sr-Nd isotopic composition of the plume head has also been proposed on the basis of analyses of the high-Ti/Y picritic lavas (Chung and Jahn, 1995). This indicates a depleted mantle source for the HT lavas, consistent with the relative depletion of Rb and Ba compared with Th, Ta and La in the HT samples (Fig. 4).
4. Plume-lithosphere Interaction
LT1 lavas show negative Nb anomaly suggesting that components other than plume must have been involved in generation and evolution of the Emeishan basalts. The most likely components are from the lithosphere. There is still hotly debate about the way by which the lithosphere contributes to magma generation. Either contamination of plume magmas by lithosphere-derived melts (Arndt et al., 1993) or whole scale melting of the subcontinental lithospheric mantle (CLM, Gallagher and Hawkesworth, 1992) has been proposed.
Fig.4. Primitive mantle-normalized trace element concentrations in the Emeishan basalts. Normalizing values are from Sun and McDonough (1989).
The chemical and isotopic composition of the LT samples may inherit that of the CLM. However, the thickness of the LT lavas in the Emeishan LIP is more than 3000 m. These lavas were emplaced during a relatively short time span (Huang and Opdyke, 1998; Lo et al., 2002). It is thus difficult to imagine that such a large volume magma was generated by melting of lithospheric mantle which is stable for a long time period in a non-convective state. The thermomechanic model suggests that only a small amount of melts can be produced from the lithospheric mantle by conduction of heat from mantle plume (McKenzie and Bickle, 1988; Arndt and Christensen, 1992). The generation of the large amount of CFB is likely confined to the convective asthenosphere or plume. It has been argued that the melting temperature of the lithosphere may be considerably reduced by the presence of volatile phases and melting of hydrous CLM would be more readily than the volatile-free plume (Gallagher and Hawkesworth, 1992). If this model applies to the Emeishan LIP, the LT lavas should be hydrous. However, primary biotites are only found in some HT lavas but not in the LT samples (Xu et al., 2001). The geochemical variation of the LT1 lavas can therefore be accounted for by crustal contamination of plume-derived magmas. This is consistent with recent Os isotopic studies that suggest an insignificant role of the lithospheric mantle in the genesis of the Emeishan basalts.
5. Geodynamic Implication of Temporal and Spatial Distribution of Two Rock-Types
The temporal and spatial relationship between the LT and HT lavas in the Emeishan LIP is critically important in understanding the thermal structure of the Emeishan plume and the nature of the plume-lithosphere interaction. Following generalization can be made in terms of data available so far. In the western part of the Emeishan LIP, the thick lava succession is dominated by the LT basalts with subordinate HT lavas in the uppermost sequence (Fig.5). In contrast, in the eastern part of the Emeishan LIP, the basalts are uniformly of HT type. Because HT and LT lavas have different REE composition and may reflect different melting depth, their temporal and spatial relationship may reflect a variable lithospheric thickness beneath the Emeishan province and mantle potential temperature during the Permian-Triassic Period (Xu et al., 2001; Xu and Chung, 2001).
Fig.5. Variation in TiO2 in the Emeishan basalts with stratigraphic height.
(a) western province; (b) eastern province.
Given the presence of picritic lavas and thickest preserved lava succession (>5000 m) in the western province, the plume center was likely in the western province (Chung et al., 1998). In this axial area of the plume, the mantle temperature was high enough to initiate melting at relatively great depth (garnet stability) and continue to the shallow level (spinel stability). The melts derived from such a large degree of melting of mantle peridotites in the absence of garnet evolved to form the LT basalts (low REE abundance and low Sm/Yb ratio). Due to upward heat transfer from the mantle, the overlying continental crust was incubated so that became warmer and favorable for being assimilated into the magma chambers. This assimilation accounts for the lithospheric signature observed in most LT lavas. In contrast, in the periphery of the LIP (i.e., Guizhou Province) the lithospheric thickness was greater and the geotherm was lower than those underneath the axial area. This eventually led the melting column to have confined within the garnet stability field and a relatively low degree of melting. The low degree melts (with garnet residue in the source) evolved during ascent to form the HT basalts of high Sm/Yb ratios. Due to the relatively cooler geothermal structure in the lithosphere, the magmas may have arisen more rapidly without pronounced assimilation by crustal materials.
IV. SEDIMENTERY EFFECT OF MANTLE PLUME UPLIFT
The mantle-plume theory predicts a considerable lithospheric uplift and doming in response to anomalous thermal upwelling (Cox, 1989; Campbell and Griffiths, 1990; White and McKenzie, 1995; Pirajno franco, 2000). This uplift would leave recognizable effects on the sedimentation, such as localized shoaling, thinning of strata over the uplifted area, and erosional unconformity between the basalts and underlying sedimentary sequence (Rainbird and Ernst, 2001). These sedimentary patterns have been observed in many plume-related LIPs, thereby providing a reliable and independent means of identifying the role of mantle plume in the generation of LIP (Rainbird, 1993; Dam et al., 1998; Williams and Gostin, 2000).
In order to better characterize the crustal processes prior to the eruption of the Emeishan basalts, He et al. (in press) examined the nature of the strata underneath the flood basalts (i.e., the Maokou Formation) and the contact between them. Systematic correlation and comparison of biostratigraphic units of the Maokou limestone reveal a domal thinning of the strata in the Emeishan large igneous province (LIP). The thinned carbonates are capped by a subaerial unconformity, which in many cases are manifested by karst paleotopography, paleoweathering zone, and locally by relict gravels and basal conglomerates. Provenance analysis suggests that the gravels were mainly from the uppermost Maokou Formation. Therefore, the stratigraphic thinning likely resulted from differential erosion due to uplift. Iso-thickness contour of the Maokou Formation further delineates an uplifted area of subcircular shape (Fig. 6), which is very similar to the crustal doming above an upwelling mantle plume as predicted by theoretical modeling. The duration of this uplift is estimated to be less than 3 M.y. and the extent of uplift is greater than 1000 m in the core of the uplift (He et al., in press). The rapid uplift suggests that plume impact rather than plume incubation was responsible for the formation of the Emeishan LIP. The sedimentary records therefore provide an independent supporting evidence for the starting plume initiation model for the generation of the Emeishan LIP. Furthermore, extent of stratigraphic thinning and unconformity suggest that the center of the Emeishan plume is located at the western Emeishan LIP, an inference also was reached by independent petrologic studies (Xu et al., 2001).
V. AGE OF THE EMEISHAN BASALTS AND ITS RELATIONSHIP TO MASS EXTINCTION AT P-Tr BOUNDARY
There were two mass extinction events in the Late Permian: one that occurred at the Permo-Triassic (P-Tr) boundary (251 Ma) and a second, smaller mass extinction that occurred 5每8 Ma earlier at the end of the Guadalupian (Wignall, 2001). Many workers have argued that there is a causal relationship between large-scale volcanic activity and mass extinctions (e.g., Courtillot et al., 1994). Two large igneous events occur at the Permian-Triassic boundary, namely the Siberian Traps in Russia and the Emeishan flood basalts in SW China. While precise Ar-Ar dating provided firmed evidence for a causal relationship between the Siberian flood basalts and the greatest mass extinction at the Permian每Triassic boundary (Renne et al., 1995), the proposed relationship between the Emeishan volcanism and the end-Quadalupian mass extinction (Courtillot et al., 1999; Wignall, 2001) and its genetic links with the siliceous tuffs at the P每Tr boundary in South China remains speculative, because the age of eruption of the Emeishan basalts is poorly constrained.
Fig.6. The iso-thickness contour of the Maokou Formation (after He et al., in press) delineates a subcircular shape suggesting a doming uplift prior to the eruption of the Emeishan basalts.
An accurate determination of the age and duration of the Emeishan basalts is hindered because they have been pervasively affected by a low to mediate temperature metamorphism. For instance, Boven et al. (2002) have performed 40Ar/39Ar dating on the lavas and intrusive rocks from the Emeishan LIP, but unfortunately they did not obtain plateau age. These authors suggested that the Emeishan basalts may have experienced a pervasive metamorphism probably during subsequent tectonization as a consequence of terrane amalgamation.
Lo et al. (2002) presented the first set of high-precision 40Ar/39Ar plateau ages of volcanic and intrusive rocks from the Emeishan Traps. The results define a main stage of the flood magmatism at ~251每253 Ma and a subordinate precursory activity at ~255 Ma. This time span is generally coeval with, or slightly older than, the age of the P每Tr boundary estimated by the ash beds in the Meishan stratotype section and the main eruption of the Siberian Traps. These authors argued that the eruption of the Emeishan Traps, rather than eruption of the Siberian Traps, accounted for the formation of the P每T boundary ash beds in South China. They suggested that the Emeishan flood magmatism may have triggered rapid release of large volumes of methane and carbon dioxide that could have been responsible for the global 汛13C excursion and associated environmental crisis leading to the mass extinction at the P每T boundary. However, using the Sensitive High-Resolution Ion Microprobe to analyze zircons, Zhou et al. (2002) have established the age of the Xinjie intrusion in the Emeishan igneous province, believed to be 259㊣3 Ma. This age is coincident with a proposed mass extinction event at 256每259 Ma. These authors therefore argued for a temporal link between the Emeishan large igneous province and the end-Guadalupian mass extinction.
This controversy may stem from the different samples and dating techniques used by different research groups. While Zhou et al. (2002) used a SHRIMP to date zircons separated from intrusive rocks, Lo et al. (2002) employed an Ar-Ar technique to date the eruptive rocks. Therefore, it may be too early to make an assessment on these results, because (1) caution should be exercised when compared ages obtained from 40Ar/39Ar technique and U-Pb zircon, (2) the genetic relationship between intrusion and basalts is not evaluated. Moreover, samples from the lower part of the lava succession is not dated yet.
Stratigraphic relationship between the Emeishan basalts and Permian sedimentary rocks provides some constraints on the age of eruption of the Emeishan basalts. The Permian strata in south China may be divided, in ascending order, into Liangshan (Lower Permian), and Qixia and Maokou (Middle Permian), and Wujiaping and Changxing (Upper Permian) Formations. The Emeishan basalts cover the Middle Permian Maokou Formation and are capped by the uppermost Permian Xianwei Formation and Longtan Formation (equivalent to the Wujiaping Formation) in the eastern part of the province and by Triassic sediments in the central part. As discussed previously, the western and central sub-provinces may be the plume impact site. Consequently, the absence of the late Permian sequence in this area may be related to the uplifted topography compared to that in the western sub-province. We therefore suggest that the Emeishan basalts were erupted prior to the P-Tr boundary. The boundary between the middle Permian and late Permian corresponds to the Capitanian/Kazanian stage (Harland et al., 1990). Therefore, the eruption age of the overlying magmas should be between 254〞258 Ma. Further study is clearly needed to determine the age of the Emeishan volcanic emission accurately, and to test the validity of the assumed short duration of the eruption.
(1) The late Permian Emeishan basalt, covering large areas in southwestern China, is one of the oldest Phanerozoic LIP in the world. The province is composed of dominant basaltic lavas and subordinate silicic rocks at the uppermost part of the lava sequence. Based on geochemical variation, the Emeishan LIP lavas can be classified into two major magma-types (i.e., LT and HT). The chemical variation of HT and LT lavas cannot be described as simple crystal fractionation from a common parental magma. Based on the inversion modeling, it is proposed that the parental magmas of the LT lavas were produced by a larger degree (16%) of partial melting of mantle plume at shallower depth (depth to top of melting column <60 km) than those of the HT lavas (1.5%, >75 km).
(2) The available data provide some constraints on the temporal and spatial variation of the Emeishan flood basalts. The LT lavas are largely confined to the lower sequence of volcanic successions in the western part of the Emeishan LIP. This magma-type is overlain by the HT lavas. To the east in the Guizhou Province of the LIP's margin, the lavas consist uniformly of the HT magma-type. This temporal variation in the basalt chemistry (i.e., from LT to HT) indicates not only a deepening of the melting column, but also the transition from the peak to the waning stage of the mantle plume activity. The spatial variation in rock type, on the other hand, is likely controlled by the difference of lithospheric thickness and mantle geothermal structure. Consequently, the LT lavas were generated probably in the plume axis region where the geotherm was higher and the lithosphere was thinner. In melting due to lower temperature and thicker lithosphere.
(3) A mantle plume model is further supported by a rapid domal uplift immediately preceding eruption of the basalts, inferred from systematic biostratigraphic correlation and examination of the Maokou Formation underneath the Emeishan basalts. Extent of stratigraphic thinning and unconformity suggest that the center of the Emeishan plume is located at the western Emeishan LIP, consistent with that reached by independent petrologic studies.
(4) The age of the Emeishan basalts is not well constrained at present and needs further studies. This is relevant to the causal relationships between the Emeishan flood volcanism and mass extinction events around the P-Tr boundary.
Acknowledgements: The study benefited from financial supports by the National Natural Science Foundation of China (40234046), the Chinese Ministry of Science and Technology (G1999043205) and Chinese Academy of Sciences (KZCX2-101).
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