Emeishan large igneous province, SW China
Introduction
For more than 60 years, the Permian Emeishan (or Omeishan) basalts in Sichuan, Yunnan and Guizhou Provinces have featured in lithostratigraphic descriptions of China's geology (Lee, 1939, Yang, 1986, Liu and Xu, 1994a, Liu and Xu, 1994b, Enos, 1995). In northern Vietnam, tectonic lenses of the same basalt sequence (Camthuy Formation) and associated rocks are present (Tien, 1993, Tien, 2000, Shi and Shen, 1998), displaced several hundred kilometers to the southeast by Oligo-Miocene sinistral motion along the Ailao Shan-Red River Fault. Modern work on the Emeishan Basalts started in the late 1970s, and in the 1980s and early 1990s, a number of prominent Sino-Western palaeomagnetism teams were particularly active, studying the rocks for plate tectonic modeling purposes, trying to exploit the generally excellent magnetic signature of basalt flows (e.g., McElhinny et al., 1981; Chan et al., 1984; Huang et al., 1986; Ma et al., 1993; Van der Voo et al., 1993). However, none of the investigations had a LIP focus, and were instead aimed at understanding South China's motion history within the wider context of East Asia's Late Palaeozoic through Mesozoic assembly (e.g., McElhinny et al., 1981, Lin et al., 1985, Enkin et al., 1992). Luo et al. (1990) were the first to describe in the English language press, and within a modern geotectonic framework, how the basalts may have been generated (by rifting). A rift model was also proposed by Dmitriev and Bogatikov (1996). Coffin and Eldholm (1994) mentioned the flood basalt terrain in their synthesis of LIPs. Chung and Jahn (1995) and Chung et al. (1998) suggested, after also carrying out geochemical studies, that the Emeishan Basalts formed above a mantle plume, and these works were instrumental in raising awareness of the province among the wider geological community.
Since 2000, a substantial number of LIP-focused Emeishan basalt papers have been published, particularly in the fields of geochemistry and geochronology. As regards LIP studies, answers to the following questions are of most interest: How big is the igneous province? What is its basic stratigraphy and that of the associated deposits? What is its age? How long did volcanism last? Does its age coincide with one of the past mass extinctions, and if so are they linked? How was the province generated and was a mantle plume involved? Does the new data from the Emeishan province add to, or refute, recent suggestions for Earth having experienced particularly enhanced mantle plume and related continental rifting activity in the latter part of Permian–early Triassic? Does the province possess any other notable features? This review of the Emeishan LIP will attempt to answer these questions, drawing together information presented in the recent and in-press publications, as well as older work.
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Aerial extent and volume of the Emeishan province
Estimating the original extent and size of the Emeishan igneous province is not as simple as for younger, better preserved large igneous provinces, and the values obtained usually involve corrections to account for erosion, buried portions and structural dismemberment. The summary geological map of Boven et al. (2002; redrawn here in Fig. 1) is a useful guide for assessing the various proposed area/size estimates. The main outcrop occupies a rhombic shape with an area of ∼2.5×105 km2.
Stratigraphy of the Emeishan LIP and associated deposits
The basic stratigraphy of the Emeishan Basalts and underlying, laterally equivalent and overlying units has been known for some time (e.g., Lee, 1939: 454–498; Yang, 1986: 114–125; Liu and Xu, 1994a, Liu and Xu, 1994b: 129). The volcanic rocks are of “latest Middle” to “earliest Late” Permian age (“Guadalupian” is the series name applied to the Middle Permian—see Jin et al., 1997, for the current Permian stratigraphic nomenclature). Syntheses placing the Emeishan LIP in the context of the
Age of the province: radiometric dating
Aside from their huge volumes, a common feature of large igneous provinces is that they formed over geologically short intervals, the volcanism often taking place in one or two pulses (Courtillot and Renne, 2003). The combined size and rate of emplacement is why many scientists suggest that LIPs have played a key role in many aspects of Earth history, particularly mass extinction events (e.g., Wignall, 2001; Courtillot and Renne, 2003). Thus, as the profile of the Emeishan Basalts has risen,
Duration of volcanism: magnetostratigraphy and field observations
Magnetostratigaphy has provided key emplacement-duration/stratigraphic data for a number of large igneous provinces, partially overcoming the limits imposed by the inherent errors of the high-resolution age-dating techniques and the common lack of chronstratigraphically useful fossils, e.g., Rochette et al. (1998) on the Ethiopian Traps. As was noted above, palaeomagnetists played an important role in China's early modern geological investigations, with a number of tectonically orientated works
Geochemical evidence
Using geochemistry, and drawing upon the work of Glennie (1984) on the Permian rift basins of the NE Atlantic-North Sea region, Luo et al. (1990) were the first to explain how the Emeishan Basalts may have been generated (the English-language paper resulted from a series of earlier papers that had appeared in the Chinese press). They argued for a protracted period of extension (Devonian–Permian) along the western Yangtze Platform, which culminated with the Emeishan basalt eruptions. (Note, that
Link to the end-Guadalupian extinction
Courtillot et al. (1999) were the first to suggest that the Emeishan province may have been responsible for the end-Guadalupian mass extinction, one of the Phanerozoic's largest (e.g., Stanley and Yang, 1994; Wang and Sugiyama, 2000; Wignall, 2001). Although there is strong evidence indicating that the Emesihan LIP was emplaced at about the Middle/Late Permian boundary (e.g., Ali et al., 2002; Zhou et al., 2002a), the scenario linking it to the end-Guadalupian biotic crisis has not yet been
Conclusions
Prior to 2000, our knowledge of the Emeishan Basalt Formation was somewhat rudimentary because only a limited number of LIP-focused studies had been reported from the terrain. It could be summed up as “a ∼0.3×106 km3 unit that may have been generated in the late Middle to early Late Permian by a mantle plume impacting the western edge of the Yangtze platform/South China Block”. Since 2000, there has been a considerable effort, by several research teams, to improve our understanding of the unit.
Acknowledgements
Ching-Hua Lo, Dan Zhu, Dan-Ping Yan, Phan Cu Tien, Sun-Lin Chung, Yigang Xu, Yugan Jin, Yunliang Wang, Bruce Bohor, Christopher Wilson, Clark Burchfiel, David Jolley, Friedrich Heller, Ian Metcalfe, Jonathan Aitchison, Paul Enos, Paul Robinson Paul Wignall and Scott Hughes are thanked for sharing ideas and information. Godfrey Fitton and Henriette Lapierre are thanked for their thoughtful reviews. Andrew Kerr provided particularly valuable editorial input.
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