Re-evaluating vertical motion preceding the Emeishan continental flood basalt province, SW China

Scott Bryan^1,2 & Ingrid Ukstins Peate³

¹Centre for Earth and Environmental Science Research, Kingston University, Penrhyn Road, Kingston Upon Thames, Surrey KT1 2EE, UK

²The University of Queensland, Sustainable Minerals Institute, WH Bryan Mining and Geology Research Centre, St Lucia, Queensland 4072, Australia; s.bryan@uq.edu.au

³Department of Geoscience, 121 Trowbridge Hall, University of Iowa, Iowa City, Iowa 52242, USA

Introduction

A tenet of mantle plume theory is that continental flood volcanism should be preceded by domal uplift of ~1000 m in amplitude with a radius of about 400 km (see “The case for mantle plumes” by Ian Campbell; Campbell, 2005; 2007; Saunders et al., 2007). This domal uplift occurs in response to the impact of a buoyantly rising plume head on the base of the overlying continental lithosphere and as it flattens to form a disc up to 2,500 km across. Documenting dynamic uplift in response to mantle plumes is difficult for many igneous provinces (Sheth, 2007). The Emeishan large igneous province (LIP) in Southwest China has been critical in this documentation, as it has been claimed to provide field evidence of pre-volcanic, kilometre-scale lithospheric doming in response to mantle plume impact on the lithosphere (Campbell, 2005, 2007; He et al., 2003; 2006; Xu et al., 2007). Such domal uplift and the magnitude of uplift reported, has not yet been demonstrated for any other LIP.

The Emeishan LIP is arguably the most appropriate LIP to provide critical insight into this prediction of mantle plume theory because of the excellent base level constraints provided by the Permian shallow marine limestone platform underlying the flood basalts. A critical line of evidence is the breccias containing basaltic and limestone clasts at the contact between the flood basalts and limestones, which were interpreted as being alluvial fan deposits shed from a ~1 km domal high (He et al., 2003; 2006). These deposits and the stratigraphic relationships between the limestones and flood basalts have recently been reinterpreted (Ukstins Peate & Bryan, 2008).

Emeishan Geology

The Emeishan LIP is located on the western margin of the Yangtze craton in southwest China, with an estimated area of ~2.5 x 10⁵ km² and an estimated preserved extrusive volume of ~0.3 x 10⁶ km³ (Xu et al., 2001; Figure 1). Sensitive high-resolution ion microprobe U/Pb ages indicate that it is about 260 Ma, equivalent to the Middle–Late Permian boundary and the end-Guadalupian mass extinction event (Zhou et al., 2002; He et al., 2007). The Emeishan volcanic succession comprises predominantly basaltic lavas with a total thickness ranging from several hundred metres up to 5 km (Chung et al., 1998; Xu et al., 2001). However, there is considerable compositional variability across the province and up section. In the west, the Emeishan lavas show a remarkable diversity of rock types including picrite, basalt, basaltic andesite, rhyolite–trachyte and basaltic pyroclastic rocks, while tuffs of trachyte and rhyolite composition occur in the upper volcanic sequences (Huang, 1986; Chung et al., 1998). Compositional bimodality is also observed in the associated intrusive rocks that comprise syenites and layered gabbros; some syenites and gabbros are associated with massive V–Ti–Fe ore deposits (Sichuan, 1991). In contrast, the lavas emplaced in the east part of the Emeishan LIP are uniformly tholeiitic–alkali basalts. They are characterized by rather high TiO₂ (3.6–5 wt%) and low MgO (<6 wt%; Lin, 1985; Huang, 1986), suggesting an evolved basaltic magma composition (Xu et al., 2001).

Figure 1. Schematic geologic map of the Emeishan LIP showing the distribution of mafic volcaniclastic deposits. Inset shows the location of the Emeishan LIP in China. A second sampled section with mafic volcaniclastic deposits is located at Xiluo. Grey shaded areas at the periphery of the LIP illustrate the areal extent (after He et al. 2006) of rift basins, carbonate mass flows and submarine lavas with intercalated syn-volcanic marine limestone (Thomas et al., 1998). Pillow lavas at Binchuan (star near Dali) are located in the core of inferred domal uplift. Grey shading on the map represents inferred zones of differential uplift: inner, middle and outer (after He et al. 2003, 2006).

Contact Relationships Between the Middle Permian Limestones and Emeishan Flood Basalts

The late Middle Permian Maokou Formation immediately underlies the Emeishan LIP and is a bedded to massive, fossiliferous limestone generally ranging in thickness from 250 to 600 m (He et al., 2003). The Maokou Formation, and the underlying Quixia Formation, are the main constituents of the Middle Permian carbonate platform in south China, representing a clean, shallow water (20–50 m depth) oceanic reef environment that formed in response to extensive transgression and basin subsidence from the early Lower to Upper Permian (Wang et al., 1994; Feng et al., 1997). The Maokou Formation has been divided into three biostratigraphic units on the basis of fusulinid foraminifera: lower (Neoschwagerina simplex), middle (Neoschwagerina craticulifera–Afghanella schencki) and top (Yabeina-Neomisellina) and 16 composite sections were interpreted to show thinning and preferential erosion of the upper biostratigraphic units (He et al., 2003).

An area of thinned Maokou Formation 800 km in radius was defined that formed the basis for delineating circular uplift within the Emeishan LIP. Mantle-plume-induced uplift was estimated assuming an average thickness of 350 m for the Maokou Formation, an inferred erosion of up to 500 m and adding the thickness of the clastic wedge and underlying lavas (600 m). The uplifted area was broken into three zones on the basis of the amount of inferred erosion (Table 1; Figure 1). The fluvial conglomerates and irregular contact between the upper Maokou Formation and the Emeishan flood basalts were interpreted by He et al. (2003) to represent an erosional unconformity formed in response to the pre-volcanic doming.

Table 1. Summary of dimensions and magnitude of dynamic uplift inferred for the Emeishan LIP (from He et al., 2003).

The earliest phase of flood basalt eruption was restricted to the northeastern part of the province, where lavas vary from 160 to 485 m in thickness (He et al., 2003, 2006). This is overlain by a layer of deposits described as conglomeratic with gravel to boulder-sized volcanic and limestone clasts with abundant fossiliferous material. In other parts of the province, this clastic deposit directly overlies the Maokou Formation limestones and underlies the main flood basalt lavas. The clastic unit forms a wedge in the northeastern part of the province, up to ~170 m thick, 30–80 km wide and 400 km in length, with limestone blocks up to 1 m in the thickest part of the deposit and fining to 3–5 cm diameter in the thinner, distal deposits (He et al., 2003; 2006; Figures 1 and 2). This clastic wedge was interpreted to be alluvial fan conglomerate formed in response to pre-volcanic domal uplift and erosion of the Maokou Formation. The Xichang–Qiaojia Fault (Figure 1), forming the western border of the clastic wedge, was interpreted to be a doming-generated normal fault that provided extra faulted topography for erosion and generation of coarse clastic material, as well as acting as a conduit for the early flood basalt magmas.

Figure 2. Stratigraphy of mafic volcaniclastic deposits, Daqiao. Rock lithologies are: lava and mafic volcaniclastic deposits (MVDs), with clast size distribution of A=ash, L=lapilli and B=block/bomb. Volcanostratigraphic column represents textural variations in lava flows and MVDs through the Daqiao section. a, Fine-grained, fining-upwards sequence of basaltic pyroclastic surge deposits with accretionary lapilli. The notebook is 19 cm long in all photographs. b, Basaltic bomb with bomb sag in underlying thin (3 cm) fine-grained basaltic ash layer. c, Limestone-cored basaltic bomb in breccia dominated by angular limestone fragments. d, Autobrecciated lava flow top with overlying accretionary lapilli-bearing tuff filling interstices and forming lag deposits in topographic lows. The ruler in the centre of the photo is 8 inches long.

Domal uplift, however, is not the only interpretation to explain thickness variations in the Maokou Formation. The regional variations equally reflect syn-depositional normal faulting that resulted in greater thicknesses of limestone accumulation towards the unstable margins of the Yangtze craton. Clear thickening of Neoschwagerina zones occurs towards the craton margins, and is unrelated to any later uplift, erosion and truncation inferred by He et al. (2003). The same lateral thickening trends are mirrored in siliciclastic and limestone formations that overlie the Emeishan flood basalts (He et al., 2006). The thickest limestone sections (1.35 km) along the western craton margin also correspond to a region of carbonate mass flow deposition at the top of the Maokou Formation, previously interpreted to have formed in response to normal faulting, and narrow, deeper marine rift basins developed along the southeastern margin (He et al., 2006; Figure 1).

An issue not addressed in previous studies is that Himalaya-related deformation increases westwards across the region complicating the Emeishan stratigraphy through faulting and steep tilting. In addition, modern reefs show significant primary relief (tens of metres: for example, reef flat versus lagoonal and inter-reef) such that the uneven contact between the Maokou limestone and Emeishan flood basalts may reflect natural topography rather than erosion and karst formation. An important constraint is that all carbonate-bearing clastic sedimentary deposits immediately beneath the Emeishan flood basalts across the inferred inner and intermediate uplifted zones were deposited in a marine environment (He et al., 2006). Any karst formation may have more recent origins given the subaerial tropical exposure of the limestones since the Mesozoic era. Karst formation also provides no constraint on the magnitude of uplift, only subaerial exposure.

Mafic Hydromagmatic Deposits at the Base of the Emeishan LIP

The Daqiao section of the Emeishan LIP is located 200 km northeast of Kunming and proximal to the core of maximum inferred pre-volcanic domal uplift (Figure 1). This section (Figure 2) contains a complete record of the early stages of volcanic activity from the initial basaltic lavas overlying Maokou limestone through the clastic section interpreted as alluvial fan conglomerates (He et al., 2003), to the main stage of subaerially-emplaced flood basalts. Overlying a ~22 m thick stacked sequence of pahoehoe and a’a sheet flows are the clastic deposits where interstices in the vesicular autobrecciated top of the uppermost lava are infilled with a tan fine-grained basaltic ash with abundant accretionary lapilli (Figure 2). Overlying this basal accretionary lapilli tuff are volcaniclastic units that are highly diverse in composition, morphology and thickness. Individual beds contain varying amounts of basaltic lava and limestone fragments from >90% limestone to >90% basalt, but a volumetrically greater basaltic component occurs up-section. Basaltic and massive to fossiliferous limestone fragments range from <1 mm up to 50 cm in size, and blocky basalt fragments are similar to lavas underlying the mafic volcaniclastic deposits.

Many beds, however, contain attenuated dense, glassy mafic juvenile clasts, suggesting a primary volcaniclastic origin. These basaltic fragments exhibit structures such as a fluidal morphology  or limestone clasts forming indentations (Figure 3), indicating that they were molten or ductile on deposition. Thin-section observation confirms a wide variety of volcanic clast textures from blocky (vesicular to massive glassy fragments and crystalline basaltic and gabbroic lithic clasts) to fluidal glassy clasts with vesicular to ductile and welded textures (Figure 3). This range of petrographic textures and vesicularity indicates that hydromagmatic eruptions ejected molten magma as well as fragments of previously erupted lava occurring as young country rock. Bombs and cored bombs are abundant, and range from basaltic with ductile impact structures and bomb sags developed in underlying bedded units to cored bombs with both basaltic and limestone lithic fragment cores (Figure 2). Fine-grained units become more abundant towards the top of the section, and again contain abundant accretionary lapilli. Importantly, free fossils of shell and crinoid are also found within these clastic deposits (Figure 3), indicating these organisms were not bound and lithified at the time of volcanic eruption.

Figure 3. Textural features of Daqiao mafic volcaniclastic deposits and Binchuan pillow lavas. A, Clast-rich breccia bed with blocky to variably attenuated basaltic clasts and blocky limestone clasts (Daqiao). Note indentation and deformation of basalt clast by limestone fragment. The pen is 15 cm long in all photographs. B, Basaltic breccia with angular basaltic and limestone clasts. The round limestone fragment in the centre of the photo is a crinoid stem (Daqiao). C, Accretionary lapilli (up to 3 cm) with coarse-grained ash and basaltic lithic fragments in core and fine-grained ash rims (Daqiao). D, Mafic hydromagmatic deposit (thin section, plain light) with free microfossils (crinoid and foraminifer) in a basaltic lithic and glass-rich matrix. Basaltic ash partly infills foraminifer (centre right), indicating it was unbound at the time of deposition (basal Daqiao). E, Pillow lavas with glassy rinds (now altered) with syn-depositional limestone in the interstices and overlying the pillows (Binchuan). Click here or on Figure for enlargement.

Further styles of hydromagmatic volcanism occur in the inner zone where maximum domal uplift has been argued. The Binchuan locality of Xiao et al. (2003) (near Dali, Figure 1) contains a >10-m-thick sequence of pillow lavas with well-developed chill rinds (Figure 3), conformably overlain by limestone. Limestone also partly fills the interstices of the underlying pillow lava mounds (Figure 3), indicating that this is a primary depositional contact and the units have not been tectonically juxtaposed.

Reinterpretation of conglomerates as mafic hydromagmatic deposits

The presence of pyroclastic textures such as accretionary lapilli, volcanic bombs with bomb sags and ductile deformation of basaltic clasts throughout the section unequivocally identifies the ‘alluvial fan conglomerate wedge’ at Daqiao as the products of hydromagmatic volcanic eruptions. A similar range of volcanic textures is observed in other sections of this ‘clastic facies’ at Xiluo (Figure 1), demonstrating that hydromagmatic volcanism was widespread and dominated the initial eruptions of the Emeishan LIP. This is seen in several other LIPs such as the Siberian Traps (see, for example, Ross et al. 2005), the East Greenland flood basalts (Ukstins Peate et al., 2003) and Ferrar flood basalts (White & McClintock, 2001; Ross & White, 2005). Regionally, pre-, syn- and post-volcanic sedimentation in southwest China was dominated by shallow marine carbonate platform deposition such that marine limestones are intercalated with distal and proximal Emeishan lavas (Figure 1). The shallow marine setting of southwest China in the Permian, the ubiquitous presence of fossiliferous limestone, coupled with free fossils within the mafic volcaniclastic deposits themselves (Figure 3), suggests that the source of the water fuelling hydromagmatic eruptions was marine, precluding kilometre-scale pre-volcanic uplift. Furthermore, the occurrence of free fossils within the mafic volcaniclastic deposits demonstrates that unbound and probably living reefal material was being incorporated into these hydromagmatic deposits, precluding uplift and karstification of the Maokou limestone before the onset of volcanism.

Re-evaluation of Plume-Induced Uplift

The re-interpretation of the Daqiao and along strike ‘conglomerate’ sections as mafic volcaniclastic deposits, and the presence of thick basaltic pillow lavas interbedded with limestone in the basal to lower parts of the Emeishan LIP stratigraphy have significant implications for the accuracy of numerical and fluid-dynamic models that predict pre-volcanic kilometre-scale plume uplift (e.g., Griffiths & Campbell, 1991; Farnetani & Richards, 1994; Campbell, 2007). The initial stage of volcanism in the Emeishan, and within the inner zone of predicted maximum uplift, was strongly dominated by voluminous and widespread hydromagmatic eruptions, most likely the result of basalt magma injection and eruption through an active shallow marine carbonate platform. The wedge-shaped geometry of the mafic volcaniclastic deposits (Figure 1) is strongly controlled by the Xichang–Qiaojia fault, suggesting syn-depositional normal faulting. The estimated volume of this clastic wedge is about 1,200–5,000 km³ (He et al., 2003; Figure 1), indicating that the Emeishan LIP contains volumetrically significant mafic volcaniclastic deposits generated at the initiation of flood volcanism.

These clastic deposits, when interpreted as alluvial fan conglomerates, were one of the lines of evidence used to support a phase of domal uplift and erosion of the Maokou Formation. Because they are hydromagmatic deposits, erupted and emplaced at or near sea level, not only do they not reflect uplift on the edge of the inner zone, they also do not provide any evidence for uplift and erosion of inner zone Maokou Formation limestones. Pillow basalt lavas intercalated with marine limestone and clastic sedimentary rocks in the lower to middle parts of the volcanic succession, from the core to the distal edges of the flood basalt province (Figure 1), corroborate evidence from the mafic volcaniclastic deposits and further support the widespread extent and persistence of shallow marine palaeodepositional environments during initial emplacement of the Emeishan LIP. Volcano-stratigraphic evidence thus strongly indicates that the initial stages of Emeishan LIP volcanism did not involve kilometre-scale rapid pre-volcanic uplift, because of the requirement to maintain:

1. an active carbonate reef system, and
2. both widespread explosive (producing mafic volcaniclastic deposits) and non-explosive (pillow lavas) seawater–magma interaction during the early stages of flood basalt volcanism.

The geographic location and positioning of the voluminous mafic volcaniclastic deposits, pillow lavas and marine sedimentation in the Emeishan LIP stratigraphy consequently do not support the zonal definition of a broad uplifted dome (Table 1), as suggested by previous studies.

The limited uplift (<500 m) observed in continental LIPs such as the Emeishan, Afro-Arabia (a few tens of metres (Ukstins Peate et al., 2005), the North Atlantic (~300–400 m, Ukstins Peate et al., 2003) and Deccan (Sheth, 2007) indicates that significant plume-head-induced uplift, as predicted by many numerical and fluid dynamic models, is not a commonly observable feature of LIP events. Previous modelling of plume–lithosphere interaction requiring kilometre-scale uplift may be an artefact of the model assumptions about lithospheric rheology generating inaccurate predictions of surface evolution (Burov & Guillou-Frottier, 2005). Alternative models of continental volcanism do not require uplift with LIP emplacement, and instead suggest that magmatism can be simultaneous with topographic subsidence due to lithospheric gravitational instabilities (e.g., Hales et al., 2005; Elkins-Tanton, 2007).

In summary, correct lithological identification, detailed field analysis and an understanding of volcanic processes and environments are critical elements in studies of LIPs that can provide geologic evidence to test the predictions of mantle plume theory. The Emeishan  'conglomerate wedge’ is a sequence of mafic volcaniclastic deposits and the product of hydromagmatism; the requirements for formation are incompatible with interpretations of kilometre-scale pre-volcanic dynamic uplift as predicted by numerical and fluid-dynamic modelling of mantle-plume-induced volcanism. We find that the initial erupted products of the Emeishan LIP event were emplaced at or around sea level, and that modest positive relief then developed in response to the rapid accumulation of the volcanic pile. Importantly, these types of deposit provide some uniquely powerful constraints on the palaeoenvironmental conditions immediately preceding and during the early stages of an extinction linked LIP event.

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