A temporal link between the Emeishan large igneous province (SW China) and the end-Guadalupian mass extinction
Introduction
Volcanic eruptions associated with large igneous provinces (LIPs) are, in many cases, considered synchronous with crises in global climate and with mass extinctions [2], [3], [4], [5], [6]. It is well documented that the Siberian traps basalts were erupted at the end of the Permian at 251 Ma [7], [8]. Eruption of the voluminous Emeishan flood basalts (EFB) in SW China was also previously thought to correlate with the Permo-Triassic (P/T) boundary volcanic layer in the Meishan section, Zhejiang province, China. Thus, the EFB, along with the Siberian traps, have traditionally been considered to be products of synchronous mantle plumes that caused biological extinction at the P/T boundary [9], [10]. However, the thin (10 cm) volcanic layer in the Meishan section at the P/T boundary is intermediate-acidic in composition and is spread over a vast area including much of North China [9] and SE Siberia [11], whereas typical lithologies of the EFB are basalt and basaltic andesite. The provenance of the volcanic layer needs to be re-interpreted, and the correlation of the EFB with the P/T boundary extinction appears inappropriate [1], [2]. A few geochronological studies (mainly K–Ar) on the EFB have yielded ages ranging from 211 to 350 Ma [12], [13]. The wide spread in ages may reflect post-eruption alteration or assimilation of older continental crust, but is still questionable, as the ages have large analytical uncertainties. On the other hand, geological relationships suggest that the EFB erupted at the end-Guadalupian (i.e. 258 Ma) [14], [15] and is thought to be contemporaneous and comparable with the Panjal volcanics in NW India (see Reviews in [2]). Thus, precise dating of the EFB is important for interpreting the events that occurred in the Late Permian or at the P/T boundary. In an attempt to establish the age of the Emeishan LIP (ELIP), we have carried out precise Sensitive High-Resolution Ion Microprobe (SHRIMP) dating of zircon from the Xinjie intrusion, which is believed to be contemporaneous with the lavas. Our work shows that the main phase of the EFB erupted significantly earlier than the P/T boundary and that its age correlates well with a proposed Late Permian mass extinction at about 256–259 Ma [16]. We suggest that eruption of the EFB was the cause of this extinction, and that this event predates the well acknowledged P/T boundary extinction by 5–8 Myr.
Section snippets
Geological background
The ELIP, which consists of massive flood basalts and numerous contemporaneous mafic intrusions, is exposed over a large part of SW China from the eastern margin of the Tibetan Plateau to the western margin of the Yangtze Block (Fig. 1). This region was reworked during the collision between the Indian and Eurasian continents [17]. On a regional scale, the Xianshuihe and Red River strike–slip faults demonstrate displacements (Fig. 1).
Within the Yangtze Block, the Mesoproterozoic strata are
Geology of the Xinjie intrusion
The Xinjie intrusion is a mafic/ultramafic sill, 2 km long and 1–1.5 km wide, that intruded the Emeishan basalts (Fig. 1, inset). The sill exhibits igneous layering and may be divided into three cyclic layers: I, II, and III. Layer I is composed, from the base upward, of peridotite, olivine pyroxenite, gabbro, and quartz gabbro. Layer II comprises olivine pyroxenite and fine-grained gabbro, whereas layer III comprises pyroxenite and quartz gabbro. The different rock types within individual
Analytical methods
Zircons were separated from Xinjie gabbro using conventional heavy liquid and magnetic techniques. The zircons were mounted in epoxy, polished, and coated with gold. The mounts were then photographed in transmitted and reflected light for identification of analyzed grains. Cathodoluminescence images were obtained on a Philips XL30 scanning electron microscope. The instrumental techniques for isotopic analysis of zircons using the SHRIMP II at the Curtin University of Technology are similar to
Origin and timing of the ELIP igneous activity
Continental flood basalt (CFB) magmatism has been attributed to the ascent of mantle plume heads, possibly originating close to the core–mantle boundary [24], [25], which resulted in short-lived, highly productive magmatic events (eruptive rates of 0.1 to >1.0 km3/yr) [26]. Chung et al. [10] (see also [15]) have proposed that the EFB originated from a mantle plume that reached the base of the South China block coincident with the P/T boundary. Production of the basalts of the ELIP was a rapid
Conclusions
This study suggests that the mafic/ultramafic intrusions in the western margin of the Yangtze Block are parts of the ELIP and are feeders to the lavas. The intrusions and lavas appear to be derived from melts produced by the same mantle plume at 259 Ma. There is a clear temporal relationship between the ELIP and the end-Guadalupian extinction at about 259 Ma. The mass extinction at this time may reflect climatic and other environmental changes related to eruption of the EFB. However, further
Acknowledgements
This study was fully supported by Grants from the Research Grant Council of the Hong Kong SAR, China (HKU7301/99P and HKU7101/01P) to M.F.Z./J.M./C.M.L./R.R.K. Zircon analyses were carried out on the Sensitive High Resolution Ion Micro Probe mass spectrometer (SHRIMP II) operated by a consortium consisting of Curtin University of Technology, the University of Western Australia, and the Geological Survey of Western Australia, with the support of the Australian Research Council. We thank
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