Emeishan Basalts, SW China: Reappraisal of the formation's type area stratigraphy and a discussion of its significance as a large igneous province

Journal of the Geological Society, Jul 2001 by Thompson, Gary M, Ali, Jason R, Song, Xieyan, Jolley, David W

Abstract: The late Permian Emeishan Basalt Formation of SW China is one of Earth's LIPs (large igneous provinces), yet its basic geology remains poorly documented. Recent work on sections close to the type area in Sichuan Province enable us in part to rectify this. Descriptions of the formation and associated units at two areas, one on the lower flanks of Mt Emei and another from a series of outcrops in Ebian County, 50-70 km to the SW, are presented. The basalt pile is 180-270 m thick and in both areas comprises 12 flows that were erupted in relatively quick succession. It rests conformably upon shallow-- marine limestones/lignites suggesting emplacement close to sea level. The upper half of the youngest basalt was intensively weathered, but not eroded, prior to it being conformably succeeded by complex body of rocks c. 30 m thick, that includes thin basalts, pyroclastic rocks, tuffs and organic-rich terrestrial sediments. This unit, which has previously been described as a sedimentary package, presumably because intense weathering has obscured the primary lithological fabric in key outcrops, is considered to mark the volcanic waning phase. Uppermost Permian and Triassic terrestrial sediments conformably overlie the terminal volcanic rocks. The sub-regional stratigraphy is compared, as best it can be, with that described from two sections 400 kin to the SE; one section matches reasonably well, the other does not, indicating that regional correlations need to be developed carefully. The information is discussed in the context of LIP generator models; several key features of the Emeishan Basalt terrain are at odds with those commonly encountered in LIP's. The most important conclusion is that the unit marks a prematurely terminated system in which full bloodied rifting leading to the development of an ocean basin never started.

Keywords: Permian, South China Block, flood basalts, mantle plumes, rifting.

Over the past three decades, large igneous provinces (LIPs) have featured prominently in the geological literature. Such terrains are characterized by massive volumes of primarily basaltic lava (typically 0.52 x 10^sup 6^ km^sup 3^) that were erupted onto Earth's surface within 1-3 million years, and are key elements in discussions on mantle plumes, rifted continental margins and global biotic crises (e.g. Storey et al. 1992; Mahoney & Coffin 1997; Courtillot et al. 1999; Courtillot 1999). One group of models proposed for LIP generation has focused on large deep mantle plumes impacting the base of the lithosphere. Of these, one sub-group involves the ascending plume head 'punching' up into the lithosphere, followed almost immediately by magma production and eruption (Richards et al. 1989; Campbell & Griffiths 1990). A second sub-group involves the plume head more slowly 'cooking' the lithosphere followed sometime later by lithospheric thinning-extension and largescale decompression volcanism (White & McKenzie 1989; Kent et al. 1992). A key aspect of the plume model variants is that they argue for considerable crustal uplift above the plume head, for example the White-McKenzie model has 1-2 km uplift above the plume head with a doming occurring over an area 2000 km in diameter. However, it must be noted that several workers (e.g. Mutter et al. 1988; Anderson et al. 1992; King & Anderson 1995; Sheth 1999; Keen & Boutilier 2000) have proposed LIP generator models that are controlled entirely by processes within the astheno-lithosphere with no involvement of a mantle plume. From the opinions and data presented at the 2000 Penrose Conference on Volcanic Rifted Margins (Menzies et al 2000), it is now clear that several models, not necessarily exclusive, can reasonably explain the nature of emplacement of the recognized LIPs from across the globe. Thus, to understand the basics of LIP formation requires a full appreciation of each province to identify the elements that are both common and unique.

The Emeishan Basalt Formation of SW China (Fig. 1) is one of the terrains mentioned in LIP discussions (e.g. Courtillot et al 1999), specifically in the context of continental rupture and its more evolved form the volcanic rifted margin. Compared to the other LIPs it is small, 0.25-0.5 x 10^sup 6^ km^sup 3^ being the respective estimates of Huang & Opdyke (1998) and Wang et al (1987). However, its close temporal relationship to both the massive Siberian LIP (3 x 10^sup 6^ km^sup 3^) and the major extinction marked by the Permian-Triassic boundary, for which a volcanism-related link is based on sections in SE China (e.g. Jin et al 2000), suggests that it could have played an important role as the Palaeozoic Earth system terminated. Although the Emeishan Basalts have been described extensively in the Chinese literature, discussion of the unit in terms of the plume/non-plume LIP models has been limited (e.g. Chung & Jahn 1995; Chung et al 1998; the paper by Courtillot et al. 1999 reviews the Emeishan LIP using mainly the work of Chung and colleagues). Although those studies presented important insights, it has to be recognized that they were based primarily upon small geochemical datasets from a handful of outcrops in one part of the province. It is also worth drawing attention to the entirely rift-based (i.e. without plume) models proposed by Luo et al. (1990) and Dmitriev & Bogatikov (1996) to explain the Emeishan Basalts' formation. The ideas behind the earlier paper were developed in the late 1970s-early 1980s and the first English version of the manuscript was submitted for publication before the various LIP generator models of 1989-1990 were published. Acknowledging a probable mantle plume involvement with the Emeishan Basalts, it remains appropriate that aspects of the rift-based models be considered in interpreting how the unit formed.