February 2020 LIP of the Month

End of magmatism related to the Emeishan Large Igneous Province

Steve Denyszyn

University of Western Australia, School of Earth Sciences, Perth, Australia

steven.denyszyn@uwa.edu.au

Figures and text modified after Zhong et al (2020) and Shellnutt et al. (2020).

The Emeishan Large Igneous Province (ELIP) in southwest China and northern Vietnam preserves one of the major continental flood basalt eruptions in the Permian (Chung and Jahn, 1995; Xu et al., 2001; Shellnutt, 2014). The associated volcanic products have been extensively studied and are inferred to be connected with the end-Guadalupian biotic crisis (Hallam and Wignall, 1999; Zhou et al., 2002; He et al., 2007; Wignall et al., 2009; Zhong et al., 2014). The ELIP is one of the most comprehensively studied large igneous provinces, but there are several issues yet to be resolved, particularly the duration of magmatism (Shellnutt, 2014). The onset of volcanism is estimated by conodont stratigraphy of interflow limestones correlative to the underlying Maokou Formation: the appearance of Jinogondolella altudaensis and J. granti constrains the eruption of the initial flows to 263–262 Ma (He et al., 2007; Sun et al., 2010). Paleomagnetic studies indicate that a single geomagnetic reversal occurred between the eruption of the lower basalt flows and the younger upper flows, suggesting that eruption occurred within 3 Ma or less (Zheng et al., 2010; Xu et al., 2018). However, high-precision radioisotopic geochronology indicates that magmatism and possibly volcanism extended beyond 3 Ma (Shen et al., 2011; Shellnutt et al., 2012). Marine Permian stratigraphic records are well-exposed and preserved in south China, which include numerous volcanic ash beds that can be used as time markers constraining deposition rates and the timing of biotic processes. They also enable the rate of volcanic emissions to be assessed, a critical element in determining the influence of magmatism on climatic change and mass extinction. Understanding of the end Permian mass extinction has been greatly enhanced in the past three decades, through use of high-precision geochronology and high-resolution biostratigraphy (Shen et al. 2019a,b; Burgess et al. 2015, and references in these). In contrast, temporal constraints on the end-Guadalupian biotic crisis and its potential primary cause, Emeishan LIP magmatism, are relatively scarce (Zhou et al., 2002; He et al., 2007, 2010; Wignall et al., 2009; Sun et al., 2010; Shellnutt et al., 2012), and firm constraints on the overall duration of Emeishan LIP magmatism and precise correlation with biodiversity changes in the sedimentary records around the Guadalupian–Lopingian boundary (GLB) are greatly needed if we are to test the hypothesized causal connection between volcanism and extinction.

Two recent studies (Zhong et al. 2020; Shellnutt et al. 2020, of which this article is largely a précis) have made significant contributions to resolving the question of the timing of termination of ELIP magmatism. The former provides new constraints on the latest volcanism as recorded in ash deposits to the northwest of the main ELIP centre; the latter provides ages of intrusive and extrusive felsic rocks in Vietnam, formerly proximal to the bulk of ELIP magmatism but later offset along a major strike-slip fault.

Previous estimates for the age of the latest volcanism were interpreted from two different silicic ashes from the sequences at Binchuan (Yunnan, China) and Puan (Guizhou, China). Chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) yielded zircon ²⁰⁶Pb/²³⁸U ages of 259.1 ± 0.5 Ma for the Binchuan ignimbrite and 259.69 ± 0.72 Ma for the Puan claystone (Zhong et al., 2014; Yang et al., 2018). However, mafic dykes and silicic plutons in the Panzhihua area are generally younger, with CA-ID-TIMS zircon ²⁰⁶Pb/²³⁸U ages from 259.6 ± 0.5 Ma to 257.6 ± 0.5 Ma (Shellnutt et al., 2012), and based on a review of U-Pb (zircon, monazite) geochronology studies from volcanic ashes across the ELIP (Huang et al. 2018) the age of Shen et al. (2011) of 257.79 ± 0.14 Ma for an ash bed in the Shangsi section, ∼300 km northeast of the ELIP, has been proposed as recording the latest ELIP volcanism. This supports the notion that Emeishan volcanism continued for millions of years after the deposition of the Binchuan and Puan ashes.

Shellnutt et al. (2020) report new ages for critical outcrops in northern Vietnam. The Tu Le Basin and Phan Si Pan uplift region were displaced from southwestern China to the southeast along the Ailaoshan–Red River shear zone during the late Paleogene (Chung et al., 1997). The Tu Le–Phan Si Pan region comprises both volcanic and intrusive rocks, including the largest (≥104 km³) contiguous silicic volcanic unit of the ELIP (Anh et al., 2011; Usuki et al., 2015). Based on structural relationships, the Tu Le rhyolite is the youngest volcanic unit in the region and may be related to neighbouring silicic plutons (Tran et al., 2015). In situ zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating of the silicic rocks in the area confirms a late Permian age, but the results span ca. 10 Ma and lack analytical precision to provide robust limits on the duration of magmatism (Pham et al., 2018). Shellnutt et al. (2020) determined zircon CA-ID-TIMS ²⁰⁶Pb/²³⁸U ages of the Tu Le rhyolite (257.1 ± 0.6 Ma to 257.9 ± 0.3 Ma) and the Muong Hum (257.3 ± 0.2 Ma) and Phan Si Pan (256.3 ± 0.4 Ma) plutons. The new data set provides unequivocal ELIP-compatible ages of the Tu Le–Phan Si Pan rocks and extends the terminal eruption age, as these represent the youngest known ages for ELIP-associated magmatic rocks.

Zhong et al. (2020) present new zircon CA-ID-TIMS ²⁰⁶Pb/²³⁸U ages for ash layers from the Chaotian and Maoershan Permian sections to the northeast of the main ELIP magmatism. These ash beds are altered, but contain single populations of zircons that yield coherent, concordant clusters of U-Pb data. Based on biostratigraphic and geochemical results also presented in Zhong et al. (2020), these layers have Hf isotopic values consistent with assignment to ELIP magmatism, and definitely postdate the end-Guadalupian extinction. The ²⁰⁶Pb/²³⁸U weighted-mean ages of 258.82 ± 0.61 Ma to 257.39 ± 0.68 Ma are in good agreement with the youngest intrusive ages of Shellnutt et al. (2012, 2020) as well as that of the Tu Le rhyolite of Vietnam (above).

Taken together, after early geochronological data indicated a prolonged (ca. 20 Ma) duration of ELIP magmatism, and initial high-precision U-Pb analyses reduced that duration to ca. 3 Ma (see Shellnutt et al. 2012 and references therein), further geochronological exploration of later, silicic components of ELIP magmatism has expanded that range somewhat, to ca. 6 Ma. This provides essential constraints on mass-balance calculations of rates of ELIP-related emissions and magmatic emplacement.

References