September 2010 LIP of the Month

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Large Igneous Provinces: Sites Of The Largest Volcanic Eruptions In Earth’s History*

*This is based on the paper: “The Largest Volcanic Eruptions on Earth” to be published in Earth Science Reviews 102 (2010) 207–229; doi:10.1016/j.earscirev.2010.07.001

Scott Bryan1, Ingrid Ukstins Peate2, Stephen Self3, David Peate2, Dougal Jerram4, Mike Mawby4, Goonie Marsh5, Jodie Miller6

  1. Biogeoscience, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001 Australia. Email: scott.bryan@qut.edu.au
  2. Department of Geoscience, 121 Trowbridge Hall, University of Iowa, Iowa City, IA 52242, USA
  3. Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA United Kingdom
  4. Department of Earth Sciences, University of Durham, South Road, Durham DH1 3LE United Kingdom
  5. Department of Geology, Rhodes University, PO Box 94, Grahamstown 6140 South Africa
  6. Department of Geology, Stellenbosch University, Private Bag X1, Matieland, Western Cape 7602 South Africa

Summary

Large igneous provinces (LIPs) are sites of the most frequently recurring, largest volume basaltic and silicic eruptions in Earth history. The magma volumes, eruptive mechanisms, frequency and associated aerosol emissions of these eruptions are critical for understanding any interpreted climate forcing and environmental change by LIPs. The largest volume (>1000 km3 dense rock equivalent) and magnitude (>M8) eruptions produce areally extensive (104-105 km2) basaltic lava flow fields and silicic ignimbrites and are the main building blocks of LIPs. Available information on the largest eruptive units are primarily from the Columbia River and Deccan provinces for the dimensions of flood basalt eruptions, and the Paraná-Etendeka and Afro-Arabian provinces for the silicic ignimbrite eruptions. In addition, three large-volume (675-2,000 km3) silicic lava flows have also been mapped out in the Mesoproterozoic Gawler Range province (Australia), an interpreted LIP remnant. Magma volumes of >1000 km3 have also been emplaced as high-level basaltic and rhyolitic sills in LIPs, and may contribute substantial aerosol emissions through shallow degassing and crystallisation. The data sets indicate comparable eruption magnitudes between the basaltic and silicic eruptions, but due to considerable volumes residing as co-ignimbrite ash deposits, the current volume constraints for the silicic ignimbrite eruptions may be considerably underestimated. Magma composition thus appears to be no barrier to the volume of magma emitted during an individual eruption. Despite this general similarity in magnitude, flood basaltic and silicic eruptions are very different in terms of eruption style, duration, intensity, vent configuration, and emplacement style. Flood basaltic eruptions are dominantly effusive and Hawaiian-Strombolian in style, with magma discharge rates of ~107-108 kg s-1 producing dominantly compound pahoehoe lava flow fields. The major flood basalt eruption durations are most likely >10 yrs. Effusive and fissural eruptions have also emplaced some large-volume silicic lavas, but discharge rates are unknown, and may be up to an order of magnitude greater than those of flood basalt lava eruptions for emplacement to be on realistic time scales (<10 years). Most silicic eruptions, however, are moderately to highly explosive, producing co-current pyroclastic fountains (rarely Plinian) with discharge rates of 109-1011 kg s-1 that emplace welded to rheomorphic ignimbrites and co-ignimbrite ash deposits. Ash and aerosol injections into the stratosphere may be greater from co-ignimbrite ash clouds than eruption plumes. At present, durations for large-magnitude silicic eruptions are unconstrained; at discharge rates of 109 kg s-1, equivalent to the peak of the 1991 Mt Pinatubo eruption, the largest silicic eruptions would take many months to evacuate ~5000 km3 of magma. The generally simple deposit structure is more suggestive of short-duration (hours to days) and high intensity (~1011 kg s-1) eruptions. These extreme discharge rates would be facilitated by multiple point, fissure and/or ring fracture venting of magma. Eruption frequencies are much elevated for large-magnitude eruptions of both magma types during LIP-forming episodes. However, in basalt-dominated provinces (continental and ocean basin flood basalt provinces, oceanic plateaus, volcanic rifted margins), large magnitude (>M8) basaltic eruptions have much shorter recurrence intervals of 103-104 years, whereas similar magnitude silicic eruptions have recurrence intervals of up to 105 years. The huge volumes of basaltic and silicic magma erupted in quick succession during LIP events raises several unresolved issues in terms of locus of magma generation and storage (if any) in the crust prior to eruption, the paths and rates of ascent from magma reservoirs to the surface, and relative aerosol contributions from the flood basaltic and rhyolitic eruptions.

Introduction

It is the volume of magma emitted during individual LIP eruptions, the frequency of such large-volume eruptions, and the total volume of magma intruded and released during the main igneous pulses that make LIP events so exceptional in Earth history, and called upon to explain environmental and climatic changes and mass extinctions (e.g., Rampino & Stothers, 1998; Courtillot, 1999; Courtillot & Renne, 2003; Wignall, 2001; 2005; Self et al., 2005; Kelley, 2007). Despite the total cumulative erupted volumes and timing of LIP events being reasonably well-constrained (Coffin & Eldholm, 1994; Bryan & Ernst, 2008), our current understanding of the size, duration and frequency of individual LIP eruptions is very limited. Considerable focus has been on flood basalt eruptions in the continental flood basalt provinces, which are the best exposed and studied examples of LIPs. Almost all information on the size of individual flood basaltic eruptions comes from the many studies undertaken on the Columbia River Flood Basalt Province, which is the smallest (~0.234 Mkm3) and youngest example of a continental flood basalt province (e.g., Swanson et al., 1975; Reidel et al., 1989; Tolan et al., 1989; Self et al., 1997; Camp et al., 2003; Hooper et al., 2007). It is only recently that some understanding has been made on the magnitude of flood basalt eruptions from other flood basalt provinces (Deccan: Jay & Widdowson, 2006; Self et al., 2008; Chenet et al., 2009; NAIP: Single & Jerram, 2004).

By contrast, similarly large-volume silicic volcanic eruptions are known from a number of tectonic regimes, but which are exclusively continental in crustal setting. Extension of active continental margins, whether in narrow, rifted arc or back-arc settings (e.g., Taupo Volcanic Zone) or broader extensional belts (e.g., Basin & Range Province, western USA) and intraplate to rifted continental environments (e.g., Afro-Arabian province) have been the most productive settings for large-volume (>1,000 km3) silicic eruptions since the middle Tertiary (Mason et al., 2004). Consequently, unlike flood basalt eruptions, large-volume silicic eruptions are not exclusive to LIPs and not restricted to discrete eruptive episodes such as LIP events throughout Earth history (Thordarson et al., 2009). The presently determined average recurrence rate of one silicic eruption of Magnitude 8 or greater (Pyle, 1995; 2000) every 100,000-200,000 years (Self, 2006) reflects the contribution from sources in a variety of tectonic settings. This relatively higher frequency for large magnitude silicic eruptions means that they pose a greater hazard to human civilization than flood basaltic eruptions (Thordarson et al., 2009). What is distinctive regarding large-volume silicic eruptions from LIPs is their association with large-magnitude basaltic eruptions, their enhanced frequency and the cumulative volume of silicic magma emplaced (up to 10 Mkm3) when compared to other tectonic settings (Bryan et al., 2002; Mason et al., 2004; Bryan & Ernst, 2008).

Super-eruptions have recently been defined as those yielding more than 1x1015 kg of magma (Sparks et al., 2005; Self, 2006). For rhyolitic eruptions, this is equivalent to ~410 km3 (at a magma density of 2,450 kg m-3). However, super-eruption has not yet been strictly applied to basaltic eruptions, where only 360 km3 of erupted magma is required, given their higher magma density of ~2,750 kg m-3. Here we present a compilation of known eruption volumes for the very largest (>Magnitude 8.5, equivalent to ~1,160 km3 of basalt lava, or ~1,280 km3 of dense silicic lava or ignimbrite) basaltic and rhyolitic eruptions from LIPs. This is to complement recent compilations for example, on the largest Tertiary-Quaternary silicic explosive eruptions (Mason et al., 2004), and provide a basis for improved long-term eruption rate estimates (e.g., White et al., 2006), which are currently based on sparse data from LIPs. Consequently, our understanding of what are the largest eruptions is limited and biased to late Tertiary and Recent volcanic activity. In these recent compilations, only one silicic eruption of magnitude 9 (1x1016 kg, or ~5,000 km3 of dense magma) has been recognized, which occurred 28 Ma (Mason et al., 2004). Issues investigated with the data set presented here are: What are the largest eruptions? What are the physical limits that may exist for eruption magnitude and whether magma composition imposes any limitation? How do basaltic and silicic “super-eruptions” differ in terms of eruption mechanisms, rates, durations and frequencies during LIP-forming events? And what implications do these issues have for the generation and storage of such prodigious magma volumes?

Determining the Products of Single Eruptions

Determining what deposits constitute the products of an individual eruption and assessing erupted volumes are not straightforward in LIPs given exposure problems (e.g., concealment, burial or uplift and erosion), the potential great extent of (often thin) eruptive units (104-106 km2), tectonic deformation and fragmentation, and subtle lithologic or geochemical distinction. As a consequence of these factors, each LIP will have a different prospectivity potential for delineating the products of large-magnitude eruptions (Table 1). Important tools for discriminating individual eruptive units are superposition, the presence or absence of internal palaeosoils, sedimentary or other lithologically distinct deposits (Figs 1, 2), petrography, mineral chemistry, bulk rock or juvenile component compositional characteristics, and paleomagnetic character (see Milner et al., 1995; Self et al., 1997; Jay et al., 2009; Chenet et al., 2009). Geochemistry has been useful at regional scales in defining stratigraphic units of basaltic lavas with distinctive geochemical features (e.g., Mangan et al., 1986; Devey & Lightfoot, 1986; Peate et al., 1992; Hooper, 1997; Marsh et al., 2001) but greatest success is achieved where a combination of approaches are used (e.g., Chenet et al., 2008; 2009; Jay et al., 2009). Examples of significant regional correlations include the dissected and tectonically fragmented silicic eruptive units of the Paraná-Etendeka flood basalt province (Milner & Duncan, 1987; Milner et al., 1992; 1995) and on-land ignimbrites of the Afro-Arabian province with distal co-ignimbrite ash deposited in marine basins (Ukstins Peate et al., 2003; 2008). Additionally, the recognition of key volcanic facies and facies distributions can be used to link proximal to distal units where emplacement style might change over long distances, and importantly in offshore seismic sequences where direct access is not possible (Jerram et al., 2009; Nelson et al., 2009).

Table 1. Prospectivity assessment of Late Paleozoic to Neogene continental LIPs for the future discovery and definition of large-magnitude (>M8 or >1015 kg magma) eruptive units.

LIP (age of activity, Ma)

>M8 basaltic

>M8 silicic

Comments

Columbia River (17-6)

High

High

Flat-lying to generally gently folded lava units; stratigraphy well-constrained and mapped flood basalt succession supported by detailed geochemical database. Silicic eruptive units of comparative age residing in adjoining Snake River Plain province

Afro-Arabian (31-14)

Moderate

High

Large-magnitude silicic eruptive units previously described; flat-lying stratigraphy offers promise for future recognition of products of large-magnitude flood basalt eruptions.

Sierra Madre Occidental
(38-17)

None

High

Flat-lying stratigraphy and limited erosion, but locally deeply incised providing some cross-sectional exposure. Although largely unexplored (~10% of province has been mapped in any detail), access is improving. High prospectivity for determining products of large-magnitude silicic eruptions given existence of many large-magnitude eruptive units of similar age along strike to north in western USA. Associated basaltic products are small-volume and unlikely to define large-magnitude eruptions.

North Atlantic (62-53)

Moderate

Low

Recognition of large-magnitude eruptions hampered by tectonic fragmentation, large eruptive volumes stored in offshore sequences, and deeper level of exhumation of the British Tertiary Igneous Province. Excellent exposure and field correlations of lava packages exist for West Greenland but have low total exposed volume; greater potential exists for thicker and more extensive East Greenland successions. Silicic magmatism almost absent from Greenland but is more abundant in the eroded British sequences.

Deccan (67-60)

High

Low

Flat-lying to gently warped lava pile; improving constraints on lava flow stratigraphy, ages, chemistry and palaeomagnetism offer reasonable potential for identifying large-magnitude flood basalt eruptions as much of province is intact, but mostly poorly exposed. More distal northern and western parts not yet correlated with main Deccan province. Associated silicic volcanism is small volume, and unlikely to define large-magnitude eruptions.

Madagascar (90-84)

Moderate-Low

Low

Relatively narrow, eroded and tilted, rifted margin successions with widespread erosion and weathering; geochemical database improving. Some potential for flood basalt ± rhyolite correlation between Volcan de l’Androy and Morondava Basin successions. Rhyolite and dacites recently correlated with the conjugate margin of SW India.

High Arctic (130-80)

Low

Low

Giant continental dyke swarm, sill and mafic–ultramafic intrusive province where volcanic portion has been largely removed by erosion, is partly submerged beneath Arctic Ocean and has undergone tectonic fragmentation.

Whitsunday (132-95)

None

Moderate-Low

Tilted, submerged sections along rifted margin hinder areal extent definition to eruptive units. Associated basalts are small-volume and unlikely to define large-magnitude eruptions.

SW Australia (Bunbury)-Comei (~135-100)

Low

Low

Volcanics predominantly preserved on submerged basaltic plateaux off SW Australia; onshore portion (Bunbury basalt) is limited; The volcanic portion of the Comei LIP remnant has been largely removed by erosion exposing subvolcanic intrusions.

Paraná-Etendeka (138-125)

High

High

Generally flat-lying stratigraphy, well-constrained silicic eruptive stratigraphy for Etendeka and limited erosion in Paraná; extensive geochemical database. Large areas of southern Angola on Etendeka side, and massive unexposed volumes on Paraná side mean a high potential for future identification of large-magnitude eruptions (both mafic and silicic), but generally very limited exposure in Paraná will hamper progress in this regard.

NW Australia (165-155)

Low

Low

Submerged volcanic rifted margin and marginal plateaux, with only limited drill hole and dredge sampling undertaken.

Ferrar (185-175)

Moderate-Low

Low

Extensive post-LIP erosion and limited exposure due to snow/ice sheet cover and faulting hampers areal extent definition. Sills are dominant expression of magmatism, and a number of large-magnitude intrusions already identified. Restricted exposure of silicic volcanic rocks occurs in the basal Hanson Fm, which contains a significant resedimented component, and a likely record of distal eruptions indicates a low prospectivity for large-magnitude silicic eruptions.

Chon Aike (188-153)

Low

High

Generally flat-lying stratigraphy in Patagonia, but sequences are deformed and tilted in the Andean Cordillera of Argentina. Little detailed mapping, variable exposure and faulting have hindered stratigraphic correlations. Concealment, faulting and physical weathering affect exposures in the Antarctic Peninsula. Associated basalts are small-volume and unlikely to define large-magnitude eruptions.

Karoo (190-178)

Moderate

Low

Tilted stratigraphy in Lebombo section precluding areal extent definition to silicic eruptive units. Lesotho Formation basalts offer most prospectivity but is complicated by the likely eruption from a widespread network of fissures precluding the formation of a single large, long run-out flow. Potential exists for defining large-magnitude intrusions or coupled lava-feeder intrusive units.

Central Atlantic (205-191)

Low

Low

Basaltic lavas predominantly occurring as tectonically fragmented remnants within rift basins of North America and northwest Africa, with a substantial volcanic volume removed by erosion. Most onshore preservation is as continental dyke swarms, sills and mafic–ultramafic intrusions; a higher prospectivity exists for defining large-magnitude intrusions or coupled lava-feeder intrusive units.

Siberian Traps (254-248)

Moderate

Low

Large areas of province are buried/unexposed with extensive subcrops of basalt occurring beneath the West Siberian Basin. Limited detailed mapping in many areas due to exposure and seasonality of outcrop access. Wide extent and large volumes of mafic volcaniclastic deposits and intrusions offer possibility for large-magnitude basaltic hydromagmatic eruptions and sills, respectively. Improving geochemical and geochronological database. Silicic igneous rocks are volumetrically minor, and unlikely to yield large-magnitude eruptive units.

Emeishan (261-251)

Moderate

Low

Tilted and faulted sections, significant erosional dissection, and lack of detailed lava litho- and chemostratigraphy. Associated silicic volcanism is small-volume, although the Wangpo bed offers potential. Mafic volcaniclastic deposits may offer more potential than associated flood basalt lavas.

Tarim (292- ~270)

Low

Low

Majority of the igneous rocks buried by a thick succession of post-Permian strata and folded and faulted by Cenozoic deformation. Province extent and volume estimates largely based on geophysical data.

 




Figure 1. Examples of intervening deposits recording time breaks between basaltic LIP eruptions. A) Gently folded Columbia River Basalt lavas exposed at Sentinel Gap (46° 48.750' N 119° 56.910' W), Washington, USA, showing part of the younger Wanapum Formation lavas over lying upper part of the voluminous Grande Ronde Formation lavas and separated by the Vantage sediment inter-bed (white layer, up to 20 m thick). Total height of cliff is ~ 300 m. B) Contact between lavas of the Ambenali Formation of the Wai Sub-group (Deccan LIP, 17° 53.710' N 73° 42.715' E) with a massive base of the upper lava's core (note, virtually no lower crustal zone) resting on a mechanically eroded/brecciated S-type pahoehoe top of an upper crustal zone of lower flow with relatively thick red-weathering zone of silty-clayey material between the breccia clasts. C) Aeolian cross-bedded sandstone interbedded with olivine-phyric compound pahoehoe flood basaltic lavas of the Paraná-Etendeka LIP, Huab Outliers, Namibia (20° 39.161' S 14° 09.350' E).



Figure 2. Examples of intervening deposits recording time breaks between silicic eruptions in LIPs. A) Intracaldera lacustrine sedimentary succession developed between two major ignimbrite and caldera-forming eruptions. This well-bedded, fine-grained, low-energy sedimentary package overlies a capping lag breccia unit to a welded rhyolitic ignimbrite, and is itself overlain by a flow banded rhyolite lava flow/dome complex with a marginal hyaloclastite and fluidised sediment contact zone; Whitsunday Island (20° 18.545'S 149° 2.873'E), Whitsunday silicic LIP. B) Normally graded, clast-supported and matrix-poor, resedimented pyroclastic unit overlying a reddened top to a fine-grained rhyolitic ignimbrite at left and capped by a welded, fine-grained base to a rhyolitic ignimbrite at right. Contacts marked by yellow dashed lines (21° 41.305' N 103° 52.520' W; San Martin de Bolaños mine, drill hole Z-425 @ 147.3- 141.4 m depth; Sierra Madre Occidental silicic LIP, Mexico).

A complication in unravelling the products of an individual eruption in LIPs is caused by factors such as the great areal extent of volcanism, multiple vent activity, potentially long crustal transport distances, and in particular, the duration of flood basalt eruptions. Given these factors, synchroneity of contrasting magmas (Fig. 3) and of widely separated eruptions are a distinct possibility. Because of the high frequency of large eruptions during a LIP-forming event, the products of the majority of eruptions cannot be discriminated on the basis of current radiometric dating techniques. Heterogeneous granitoids produced by mingled gabbroic and granitic magmas in associated intrusive complexes (e.g., Messum; Fig. 3A) record the simultaneous existence of mafic and silicic magmas in LIP plumbing systems (Vogel, 1982; Ewart et al., 2002). Consequently, it remains unclear where interbedded units do exist whether this interstratification implies either a substantial timebreak or simultaneous (basalt-rhyolite or basalt-basalt) eruptions from different vent regions within the province.




Figure 3. Textural features illustrating the synchroneity of mafic and silicic magmas in LIPs. A) Mafic and silicic magma interaction producing ‘organ-pipes’ (21° 25.72' S 14°16.89' E), the result of multiple diapirs of granite that have intruded upwards through unsolidified mafic diorite in the easternmost moat of the Messum igneous complex (Paraná-Etendeka LIP; see also Ewart et al., 2002). B) Outsized, ductile deformed and folded juvenile mafic spatter clast within the lava-like Jozini rhyolite (Karoo LIP), Namaacha Falls, Mozambique (25° 57.55' S 32° 02.1' E). Pen is 15 cm long. C) Outsized, juvenile mafic scoria clast within a densely welded rhyolitic ignimbrite, with the eutaxitic fabric subhorizontal in photo (Whitsunday silicic LIP, Cid Island, northeast Queensland; 20° 16.341'S 148° 55.083'E). Pen lid width is 0.8 cm.

Two pertinent examples of the above occur in the Paraná-Etendeka flood basalt province (Fig. 4): 1) two basaltic magma types (Urubici and Gramado) are interbedded in the southern Paraná (Peate et al., 1999); and 2) two discontinuous and chemically different basalt lavas are interbedded within the Goboboseb Quartz Latite of the Etendeka Province (Milner & Ewart, 1989; Milner et al., 1992; 1995). In the Paraná example, compositionally identical Urubici-type basaltic lava flow units 9 and 10, exposed along the coastal escarpment, were divided into two units as they are separated by a Gramado-type basaltic lava flow near Urubici. We suggest that units 9 & 10 were part of a single eruptive event (sourced from a vent region to the north where Urubici-type magmas predominate) that coincided with the simultaneous eruption of a Gramado flow field from the south.



Figure 4. Potential examples of contemporaneous eruptions in LIPs. A) Local correlation of Urubici lava flows in the São Joaquim area (Peate et al., 1999) with local topographic relief controlling the emplacement of early lavas flow units (3-8). PE, Perico; AB, Aguas Brancas; UR, Urubici; SM, Morro do Igreja; CO, Corvo Branco; RR, Rio Rufino sections of Peate et al.(1999). B) Compositional plot of MgO-TiO2 showing the similarity of basaltic lava units 9 & 10 and their difference to other Urubici high-Ti lava flow units. The basaltic lava interbedded with the Urubici units 9 & 10 in the UR profile is sample DUP-38 (from Peate & Hawkesworth, 1996) that is an evolved Gramado-type basalt with 3.3 wt% MgO and 1.68 wt% TiO2. C) Simplified map of southern Etendeka showing the distribution of the Etendeka flood volcanic succession and schematic stratigraphic sections for the Awahab and Goboboseb Mountain area (modified from Jerram et al., 1999) where two compositionally different mafic lava units separate units 1 and 2 of the Goboboseb quartz latite. In the Goboboseb Mountains, the Copper Valley icelandite (CVI, Ewart et al., 1998a) lies stratigraphically between Goboboseb units 1 and 2, whereas chemically different, low-Ti-type basalts separate these quartz latite units in the Awahab area. QL, quartz latite.

For flood basalt eruptions, the term lava flow field refers to the aggregate product of a single eruption and can be formed of one or more lava flows, each the product of a vent or a group of vents along a fissure segment (Self et al., 1997). Consequently, all flood basalt lava fields are compound pahoehoe lavas composed of 1000's of lava sheet lobes, and innumerable smaller lobes, stacked/superposed in some places and laterally arranged in others. By contrast, the stratigraphy of silicic eruptive units appears much simpler, as exemplified by the Paraná-Etendeka quartz latites (Fig. 5), which crop out as relatively featureless extensive sheets with a simple internal structure and jointing, leading to the interpretation as single flow or cooling units (Milner et al., 1992). A simple deposit structure, an extremely chemically uniform or distinctive character and chemical distinction from other silicic units are used here to support the interpretation of silicic units in flood basalt provinces such as the Paraná-Etendeka, Karoo and Afro-Arabian provinces as being the products of individual eruptions.


Figure 5. Examples of large-volume silicic eruptive units from LIPs. A) View northeast towards Tafelkop in the Etendeka (21° 07.13'S 14° 17.3'E) that is capped by two ~30 m sheet-like quartz latite rheoignimbrite units (Goboboseb I, Gb 1; Goboboseb II, Gb 2). B) Closeup of base of Goboboseb I in contact with brecciated basaltic lava at base of photo illustrating a typical ‘valley-fill’ geometry. A basal, discontinuous topography-filling, massive quartz latite facies is overlain by an intensely platy jointed facies (PJF) that is more sheet-like in geometry (boundary between depositional facies marked by dashed line). Location of section shown in A. C) Afro-Arabian main silicic sequence of pyroclastic flow and fall deposits from Bayt Baws, located near Sana’a, Yemen (15° 16.5' N 44° 11.3' E). Main eruptive units, from base to top are: 1) Jabal Kura’a Ignimbrite; 2) Escarpment Ignimbrite; 3) Green Tuff; 4) SAM Ignimbrite; 5) caldera-collapse breccia Iftar Alkalb; conglomerate separates the Escarpment Ignimbrite and Green Tuff (Ukstins Peate et al., 2005). The ignimbrites exhibit distinct tabular morphology, which is recognizable across 100’s of km around the Sana’a basin and towards the western escarpment. Total thickness of section is 115 m.

Magnitude of LIP eruptions

LIPs are dominantly basaltic igneous events and the primary volcanic building blocks are extensive (103-105 km2), sheet-like lava flow fields (Fig. 6; e.g., Self et al., 1997; Jerram, 2002; White et al., 2009). Detailed work in the NAIP has identified lava flows at a number of scales, many of the units being only several 10s of km3 in examples from the Skye main lava series (Single & Jerram, 2004), up to much larger sequences as imaged in offshore seismic. Within these offshore sequences, lavas can be seen to have flowed long distances from subaerial to subaqueous environments producing large hyaloclastite sequences (Jerram et al., 2009). Additionally, in continental LIPs (flood basalt provinces and volcanic rifted margins), mafic volcaniclastic deposits (Ross et al., 2005; Ukstins Peate & Bryan, 2008), sill and dyke intrusions (Jerram, 2002; Elliot et al., 2008) and silicic ignimbrites (Bryan et al., 2002; Bryan, 2007) are also, areally and volumetrically significant and important architectural components (White et al., 2009).





Figure 6. Examples of large-volume basaltic units from LIPs. A) Deccan lavas from the Wai Sub-Group at Arthur Seat in the Western Ghats near Mahabaleshwar, India (17° 58.780' N 73° 38.225' E), showing several major basalt lava sheet lobes. The main parting in the middle of the photo is the base of a 60 to 70-m-thick sheet lobe. Below this, three thinner (~20 m thick) sheet lobes show sloping, vegetated upper crustal zones and cliff-forming lava cores. The core-upper crust level in the thick sheet lobe is a horizon with overhangs, and the top of the lobe is the lower of two small, prominent cliff lines. Above this is a vegetated slope capped by another cliff-line (near top of photo), which is another flow-field composed of smaller (~100 m long x 10 m thick) inflated pahoehoe lobes without thick cores. B) Overview of the Sand Hollow flood basalt flow (Table 2) illustrating internal morphology of a single, large-magnitude, ~60 m thick sheet lobe. An upper and lower zone of wider-spaced joints in core of lobe are separated by a more closely spaced central jointed zone. Darker, slightly banded, and gullied part approximates the upper crustal zone of the lobe (top removed by erosion); base is in vegetated slope. Palouse Falls (46° 39.730' N 118° 13.554'W), Columbia River LIP, USA. C) Coastal exposure of basaltic lava flow unit showing well-developed colonnade and entablature jointing, near Trongisvágur, Suðuroy Island, Faroes (North Atlantic LIP). D) Overview of the ~300 m thick Finger Mountain Sill of Jurassic Ferrar Dolerite, Upper Taylor Glacier, Antarctica (77° 44.45' S 160° 42.78' E), intruded into the Beacon Sandstone Formation, part of the Dry Valleys nested sill complex and plumbing system for the Kirkpatrick flood basalts (Ferrar LIP).

Table 2. Catalogue of the largest known mafic eruptive units from LIPs (≥1,300 km3) ordered in terms of eruptive volume. The Mesozoic to Cenozoic LIPs are the best studied and preserved and the catalogue is biased to these more modern examples. For the basaltic eruptions, the Columbia River flood basalt province may contain more than 300 individual basalt lava flows that have an average volume of 500-600 km3 (Tolan et al., 1989). Eruptive volumes are dense rock equivalent. Eruption magnitude is based on Pyle (1995, 2000) using a magma density of 2,700 kg m-3. Note that the eruptive volume for the Mahabaleshwar-Rajahmundry Traps eruptive unit is an upper end of a plausible range of eruptive volumes for this pahoehoe lava flow field that reached across the Indian subcontinent (see Self et al., 2008).

Eruptive Unit

LIP

Eruptive Age (Ma)

Minimum Eruptive Volume (km3)

Lithology & Thickness (m)

Magnitude

Composition
(wt% SiO2)

References

Mahabaleshwar- Rajahmundry Traps (Upper)

Deccan

64.8

9,300

Basalt lava (20-50)

 9.4

High-Ti tholeiitic Basalt (48.1)

Self et al. (2008)

McCoy Canyon flow (Sentinel Bluffs Member, Grande Ronde N2)

Columbia River

15.6

4,278

Basalt lava (10-60)

9.06

Tholeiitic Basalt (53.6)

Reidel (2005); Landon & Long (1989)

Umtanum flow1 (Grande Ronde N2)

Columbia River

~15.6

~2,750

Basalt lava (~50)

8.87

Tholeiitic Basalt (54.7)

Reidel et al. (1989)

Sand Hollow flow (Frenchmans Springs member, Wanapum Basalt)

Columbia River

15.3

2,660

Basalt lava (~40)

8.86

Tholeiitic Basalt (51.8)

Beeson et al. (1985) Tolan et al. (1989)

Pruitt Draw flow (Teepee Butte Member, Grande Ronde R1)

Columbia River

16.5

2,350

Basalt Lava (30-100)

8.80

Tholeiitic Basalt (53.0)

Reidel & Tolan (1992); Reidel (1983)

Museum flow (Sentinel Bluffs Member, Grande Ronde N2)

Columbia River

15.6

2,349

Basalt Lava (10-80)

8.80

Tholeiitic Basalt (54.2)

Reidel (2005); Landon & Long (1989)

Rosalia flow (Priest Rapids Member, Wanapum Basalt)

Columbia River

14.5

1,900

Basalt lava (~50)

8.70

Tholeiitc Basalt (50.5)

Tolan et al. (1989)

Joseph Creek flow (Teepee Butte Member, Grande Ronde R1)

Columbia River

16.5

1,850

Basalt Lava (20-90)

8.70

Tholeiitic Basalt (52.3)

Reidel & Tolan (1992)

Ginkgo Basalt (Frenchmans Springs member)

Columbia River

15.3

1,600

Basalt lava (30->150)

8.64

Tholeiitic Basalt (51.5)

Tolan et al. (1989); Reidel et al. (1994); Beeson et al. (1985)

Rosa Member (Wanapum Basalt)

Columbia River

14.5

1,300

Basalt lava (3-50)

8.55

Tholeiitic Basalt (50.2)

Tolan et al. (1989); Self et al. (1997)

Stember Creek flow (Sentinel Bluffs Member, Grande Ronde N2)

Columbia River

15.6

1,192

Basalt lava (5-50)

8.51

Tholeiitic Basalt (53.5)

Reidel (2005); Landon & Long (1989)

   1. Reidel et al. (1989) recognised the Umtanum unit comprised 2 lava flow units with a cumulative volume of >5,500 km3, but an average of 2,750 km3 for each lava flow.

 

Table 3. Compilation of the largest known silicic eruptive units from LIPs (>1,500 km3 dense rock equivalent) ordered in terms of eruptive volume/magnitude. Eruption magnitudes (Pyle, 1995; 2000) are based on magma densities of 2,500 kg m-3 for the Paraná-Etendeka quartz latites, and 2,400 kg m-3 for the Afro-Arabian and Gawler Range rhyolites. Areal extents of correlated eruptive units from the Paraná-Etendeka province are shown in Figure 7. For comparison, the 74 ka Toba eruption from Indonesia evacuated the equivalent of 2,700 km3 of magma, and the Fish Canyon Tuff at 4,500 km3 dense rock equivalent volume, is the most commonly cited example of the largest known ignimbrite eruption (see references in Mason et al., 2004). The ~132 Ma eruptions of the PAV-B/Springbok and Guarapuava-Tamarana/Sarusas quartz latites in the Paraná-Etendeka Province evacuated ~1.7 and ~2.1 times more magma than the Fish Canyon Tuff, respectively.

Eruptive Unit

LIP

Eruptive Age (Ma)

Minimum Eruptive Volume (volume basis) km3 DRE1

Lithology & Thickness (m)

Magnitude

Composition
 (wt% SiO2)

References

Guarapuava (P2O5 <0.44 wt%)-Tamarana/Sarusas2

Paraná-
Etendeka

132

8,587 (O)

Rheoignimbrite
(50-120)

9.33

High-Ti Quartz Latite (67.2/64.3)

Marsh et al. (2001); Ewart et al. (2004); Peate et al. (1992); Nardy et al. (2008)

Santa Maria/Fria

Paraná-
Etendeka

~132

7,808 (O)

Rheoignimbrite
(100-200)

9.29

Low-Ti Quartz Latite (70.5)

Marsh et al. (2001); Ewart et al. (2004); Garland (1994); Nardy et al. (2008)

Guarapuava (P2O5 >0.44 wt%)/Ventura2

Paraná-
Etendeka

~132

7,571 (O)

Rheoignimbrite
(45- >70)

9.28

High-Ti Quartz Latite (65.5)

Marsh et al. (2001); Ewart et al. 2004); Nardy et al. (2008)

PAV B-Caxias do Sul /Springbok3

Paraná-
Etendeka

132

6,866 (O)

Rheoignimbrite (250)

9.23

Low-Ti Quartz Latite (68.2)

Milner et al. (1995); Marsh et al. (2001); Renne et al. (1996); Whittingham (1991)

PAV F-Caxias do Sul/ Grootberg

Paraná-
Etendeka

~132

5,651 (O)

Rheoignimbrite (100)

9.15

Low-Ti Quartz Latite (68.2)

Milner et al. (1995); Marsh et al. (2001); Nardy et al. (2008)

PAV A-Jacui/Goboboseb II4

Paraná-
Etendeka

132

4,348 (O)

Rheoignimbrite (70)

9.04

Low-Ti Quartz Latite (67.5)

Milner et al. (1995); Marsh et al. (2001); Ewart et al. (1998); Nardy et al. (2008)

Ourinhos/Khoraseb

Paraná-
Etendeka

~132

3,929 (O)

Rheoignimbrite (60-140)

8.99

High-Ti Quartz Latite (68.0)

Marsh et al. (2001); Ewart et al. (2004); Garland (1994)

PAV G-Anita Garibaldi /Beacon

Paraná-
Etendeka

132

3,452 (O)

Rheoignimbrite
(40-140)

8.94

Low-Ti Quartz Latite (66.6)

Milner et al. (1995); Marsh et al. (2001)

Iftar Alkalb - Tephra 4W

Afro-
Arabian

29.5

2,667 (O+A)

Ignimbrite lag breccia (70->150), co-ignimbrite ash

8.81

Rhyodacite (~68)

Ukstins Peate et al. (2005; 2008)

SAM Ignimbrite - Tephra 1W63

Afro-
Arabian

29.5

2,330 (O+A)

Welded to nonwelded ignimbrite (≤25), co-ignimbrite ash

8.75

Rhyolite
(72.5)

Ukstins Peate et al. (2003, 2005; 2008)

Moonaree Dacite

Gawler Range

1591

2047 (O)

Dacitic lava (~250)

8.7

Rhyodacite (66.8-69.2)

Allen et al. (2003, 2008)

Palmas BRA-21/Wereldsend

Paraná-
Etendeka

132

1,875 (O)

Rheoignimbrite
(35-480)

8.67

Low-Ti Quartz Latite (69.1)

Milner et al. (1995); Marsh et al. (2001)

Jabal Kura'a Ignimbrite - Tephra 5W

Afro-
Arabian

~29.6

1,627 (O+A)

Welded ignimbrite
(5-9), co-ignimbrite ash

8.59

Rhyolite
(71.9)

Ukstins Peate et al. (2003, 2005; 2008)

Sana'a Ignimbrite - Tephra 2W63

Afro-
Arabian

~29.5

1593 (O+A)

Welded ignimbrite (5), co-ignimbrite ash

8.58

Rhyolite (74.6)

Ukstins Peate et al. (2005; 2008)

  1. Volume estimates are based on preserved outflow volume (O), ash fall volume (A); geochemical correlations between onshore silicic units volumes and Indian Ocean deep-sea tephra layers (Ukstins Peate et al., 2003) have enhanced eruptive volume estimates from the Afro-Arabian LIP. In contrast, no distal ash layers have yet been identified that correlate to the Paraná-Etendeka rheoignimbrites, but their eruptive volumes could be 2 to 3 times greater than the listed estimates if distal tuffs are identified. Given the effusive eruptive nature for the Moonaree Dacite, no additional erupted volume is expected to reside as ash fall deposits.
  2. The Chapecó quartz latite suite of Bellieni et al. (1986) has been subdivided into three by Nardy et al. (2008) with the Tamarana quartz latites distinguished by having intermediate TiO2 and P2O5 contents, which best correlates with the Sarusas quartz latite of the Etendeka, but as mapped, consists of multiple cooling units based on the work of Ewart et al. (2004a, b). The high P2O5 (>0.44 wt%) Chapecó group quartz latites are interpreted here to correlate with the Ventura quartz latite of the Etendeka province.
  3. The volume estimate has been revised slightly upward from Milner et al. (1995) using the reconstruction map of Nardy et al. (2008).
  4. Although generally considered as one silicic unit, units 1 and 2 of the Goboboseb quartz latite are locally separated by basalt (Fig. 4C; Milner & Ewart, 1989), and may be the products of two but very closely spaced eruptions. Geochemical comparisons (Milner et al., 1995; this study) indicate the PAV-A quartz latite of the Paraná most closely corresponds to Goboboseb unit 2; field studies in the Etendeka (Milner, 1988; Ewart et al., 1998) indicate Goboboseb unit 3 is only locally preserved in the Goboboseb Mountains and proximal to Messum.

 

Deposit dense-rock-equivalent volumes from known large-volume LIP eruptions are listed in Tables 2 & 3. These should be considered as minima. Constraints on flood basalt eruption magnitudes come mostly from the well-studied Columbia River Basalt (CRB) province, which is the product of many, dominantly pahoehoe flow fields varying in size from 1 to >2,000 km3 in volume (Tolan et al., 1989). The recent study of Reidel (2005) of proposed chemically correlated flow types such as the McCoy Canyon or Cohassett (Table 2) of the Grande Ronde Basalt Formation, indicates much larger volume flows may have been emplaced during the interval when >60% of the volume of the CRB province was erupted (Tolan et al., 1989; Camp et al., 2003). Also, studies on the Ambenali and Mahabaleshwar Formations of the Deccan indicate single formations have volumes similar to the entire CRB Group (~230,000 km3, Camp et al., 2003), with volumes of individual flow fields ranging from ~2,000 to >8,000 km3 (Self et al., 2006; 2008).This general upper magnitude for flood basalt lavas is similar to the dimensions of associated sills in flood basalt provinces, such as the enormous Peneplain Sill in the Dry Valleys, Antarctica, (19,000 km2; Gunn & Warren, 1962) with an estimated volume of 4,750 km3, the Dufek-Forrestal intrusions (10,200-11,880 km3; Ferris et al., 1998), and the 1,500 to 5,000 km3 Palisades Sill of the Central Atlantic Magmatic Province (Husch, 1990; Gorring & Naslund, 1995).

The magnitude of silicic eruptions in LIP events, by contrast, has received little attention with a presumption that the flood basalt eruptions were larger in volume and more likely to perturb climate (cf. Cather et al., 2009). The potential scale of silicic eruptive units emplaced during LIP events was first realised by the work of Milner et al. (1992, 1995) in the southern Etendeka continental flood basalt province and through correlations with its rifted counterpart, the Paraná province in South America. This database of extremely large silicic eruptions during LIP events (Table 3) has recently been expanded following detailed studies in the northern Etendeka (Marsh et al., 2001; Ewart et al., 2004b) and Afro-Arabian province (Ukstins Peate et al., 2003, 2005; 2008).

Areal extents of correlated quartz latite units from the Paraná-Etendeka LIP are shown in Figure 7. The areal extents of the quartz latite units indicated by cross-South Atlantic correlations of Milner et al. (1995) and Marsh et al. (2001) are huge (>0.1 Mkm2), ranging up to 0.17 Mkm2 and greatly exceeding areas of the largest mapped flood basalt lavas from the CRB Province (0.01-0.1 Mkm2; Tolan et al., 1989; Self et al., 1997). Lateral extents of the Paraná-Etendeka quartz latites are also correspondingly large (up to 650 km; Milner et al., 1995; Marsh et al., 2001) and equivalent to the longest run-out lengths of flood basalt lavas (Tolan et al., 1989). As emphasised in previous correlations (Milner et al., 1995; Marsh et al., 2001), a distinct asymmetry exists in the quartz latite distributions, occupying more area on the South American continent, as well as there being a strong linear distribution to most of the quartz latite units with a preferred NW-SE orientation (Fig. 7). Additionally, a spatial variation exists with the high-Ti quartz latites restricted to the northern area of the Paraná-Etendeka province (Bellieni et al., 1986; Peate et al., 1992; Marsh et al., 2001; Ewart et al., 2004b).



Figure 7. Map of the pre-rift juxtaposition of South America and Africa showing the Paraná-Etendeka LIP (modified from Peate et al., 1992 and Nardy et al., 2008) and the areal extents of correlated silicic eruptive units discussed in text and listed in Table 3. Dashed lines around São Paulo and Curitiba are dyke swarms.

Table 3 demonstrates that dense rock equivalent eruptive volumes for many silicic eruptive units in LIPs range between 1,000 and 4,000 km3 (>M8.5), and are at least equivalent to the dominant erupted volumes for the largest flood basalt lavas from the CRB Province (Table 2). Importantly, the data indicate the Paraná-Etendeka LIP has been the site of up to five silicic eruptions (Table 3) larger in magnitude than the Fish Canyon Tuff (>4,500 km3 dense rock equivalent), the most commonly cited example of the largest known ignimbrite eruption (Mason et al., 2004).

In addition to large explosive silicic eruptions, recent studies of the Mesoproterozoic Gawler Range Volcanic Province have delineated three >M8 silicic lava eruptions from this LIP remnant. The large areal extent of these units has been aided by eruptions from multiple point or fissure vents and contemporaneous eruption of chemically distinct magma batches (Allen et al., 2008; McPhie et al., 2008). However, despite the similar eruption magnitudes (Table 3), these lavas have run-out lengths that are over an order of magnitude less than the ignimbrite eruptive units. As with basaltic sills intruded into the flood basalt piles, the 2.47 Ga Woongarra Rhyolite sill of the Hammersley Basin in Western Australia (Trendall, 1995) reinforces the point that large-volume batches of silicic magma are emplaced at high stratigraphic levels and available for effusive or explosive eruption during LIP events. The Woongarra Rhyolite is a composite sill of 15,400 km3 emplaced at depths of a few hundred metres from the paleosurface. Sill emplacement occurred in two pulses thought to be separated by a few hundred years, with the pulses recording injection of 6,200 and 9,200 km3 of magma (Trendall, 1995).

In terms of cumulative erupted volumes, the silicic LIPs (Bryan et al., 2002; Bryan, 2007; Bryan & Ernst, 2008) contain the largest volumes (~0.25-3 Mkm3) of silicic volcanic rock and are equivalent to many continental flood basalt provinces. Because of this large and rapidly emplaced cumulative volume, the silicic LIPs must also be host to high frequency, large volume (>1,000 km3 and >M8) eruptions (Mason et al., 2004; Bryan & Ernst, 2008). However, at present, there are virtually no constraints on the dimensions of individual eruptions from the silicic LIPs. Mid-Tertiary examples from the western USA (e.g., the Fish Canyon Tuff and other monotonous intermediates) give some insight into the potential magnitude of silicic LIP eruptions, as these were erupted at the same time and along strike to the north of the ~0.4 Mkm3 Sierra Madre Occidental silicic LIP, the largest ignimbrite-dominated volcanic province in North America (Swanson et al., 2006; Ferrari et al., 2007). Several examples of similarly crystal-rich and potentially large-volume, caldera-related rhyolitic ignimbrites occur within the Sierra Madre Occidental (e.g., Copper Canyon and Vista Tuffs, Swanson et. al., 2006).

Discharge Rates of LIP Eruptions

The two very different vent configurations and eruptive styles (Table 4) play a significant role in the orders of magnitude difference in discharge rates for the basaltic and silicic eruptions in LIPs. For the flood basalt eruptions, early workers inferred fast eruption rates to emplace the huge volumes of magma in a matter of days to a few weeks (Shaw & Swanson, 1970; Swanson et al., 1975; Mangan et al., 1986; Tolan et al., 1989). Determining the discharge rate has obvious implications in terms of the requirement for vast plumbing systems and magma reservoirs for the magma to be delivered quickly to the surface. The identification of lengthy fissure systems of 70-200 km long in the CRB Province (e.g., Swanson et al., 1975) provided the mechanism for the rapid evacuation of a huge magma reservoir with the constraint that the mass discharge rate was not so high as to generate a large eruption column. However, the vent system for only one moderate-sized flood basalt eruption (Roza) has been studied in detail, and the nature and extent of activity along other fissure vent systems and dykes for other flood basalt flow fields remain poorly known.

 


Table 4. Summary of the key characteristics of basaltic and silicic eruptions from LIPs. The reader is referred to White et al. (2009) who have summarised the key characteristics of explosive mafic eruptions from LIPs.

Eruption Characteristic

Basaltic Lava Eruptions

Silicic Ignimbrite Eruptions

Silicic Lava Eruptions

Eruptive volumes

Predominantly 100's to 5,000 km3; up to 9,300 km3

Predominantly 100's to 5,000 km3; up to 9,000 km3

1 to 3,000 km3

Vent-types

Fissures

Calderas, volcano-tectonic rifts/fissures

point-source/domes, fissures

Eruptive styles

Effusive to Hawaiian-Strombolian

Explosive, Plinian to pyroclastic fountaining

Effusive

Magma discharge rates

103-104 m3 s-1; 103-104 kg s-1

>106 m3 s-1; 109-1011 kg s-1

?

Eruption Durations

yrs to 10's yrs

hours/days to weeks

?

Emplacement Styles

Dominantly pahoehoe lava flow fields

Pyroclastic density currents, fallout from co-ignimbrite ash plumes; rare plinian fallout

Dominantly non-particulate flow (fluidal to blocky lavas)

 

The studies of Self and coworkers on the Roza flood basalt eruption have demonstrated that the eruptive volume can be accounted for by an ~10 year duration at an averaged effusion rate of ~4,000 m3 s-1 (1.12 x 107 kg s-1), equivalent to the peak rate of the 1783-1784 Laki basaltic eruption in Iceland (Self et al., 1997). However, given the ~150 km length of the total fissure system for the Rosa flow, at these effusion rates, only part of the fissure system can have been active at any one time. The eruption rate, if averaged over the fissure length, would be exceedingly low (~0.0267 m3/s/m length of fissure) and result in magma freezing in the dykes/fissures in transit to the surface. Such magma freezing may be one mechanism by which effusion becomes concentrated or localised along the fissure system. Voluminous sheet lava flows, therefore, do not require the rapid extrusion of mafic magma at rates much higher than in historic eruptions except for the few cases of young flood volcanism, nor do long fissures imply high eruption rates as only segments of the fissure may be active at any one time (cf. Hooper et al., 2007).

Few constraints are available for discharge rates of silicic eruptions during LIP events. This is in part due to the lack of information on the nature of source vents and the virtual absence of Plinian fall deposits in LIPs from which most constraints on magma discharge rates for silicic explosive eruptions are made (e.g., Wilson et al., 1978; Sparks, 1986; Wilson & Walker, 1987; Carey & Sigurdsson, 1989). The large-volume silicic eruptive units in LIPs are dominantly ignimbrite or rheoignimbrite (e.g., Milner et al., 1992; Bryan et al. 2002; Ukstins Peate et al. 2005; Bryan, 2007) and the general lack of widespread Plinian fall deposits suggest that in general, mass discharge rates are sufficiently high (>108-109 kg/s) to prevent stable and buoyant Plinian eruption columns forming at the onset of, and during eruptions. Other factors that would contribute to co-current eruption dynamics include large, wide or multiple vents, such as fissures or along ring faults, and lower gas content (particularly for the rheomorphic ignimbrite examples) that in turn help lower eruption velocity (Wilson et al., 1980; Woods, 1995; Freundt, 1999). In general, the formation of single cooling units and in some cases, absence of internal erosion surfaces or sedimentary deposits produced by epiclastic processes within ignimbrite sheets have been interpreted to indicate ignimbrite emplacement within a period of no more than a few hours or days (e.g., Christiansen, 2001).

The inferred intensities for a number of prehistoric and historic Plinian eruptions vary between 1.6 x 106 kg s-1 to 1.1 x 109 kg s-1 (Carey & Sigurdsson, 1989), whereas the magma discharge rate for the June 15, 1991 eruption of Mt Pinatubo was between 4 x 108 and 2 x 109 kg s-1 (Koyaguchi, 1996). Assuming a discharge rate of 1 x 109 kg s-1 and given the deposit volumes of the large silicic eruptive units in Table 3, the estimated duration of these eruptions varies from ~44 days for the smallest volume ignimbrite listed (Sana'a Ignimbrite, Afro-Arabian LIP) to ~248 days for the Guarapuava-Tamarana/Sarusas quartz latite (Paraná-Etendeka LIP). If mass eruption rates were an order of magnitude lower, then eruption durations would be in the order of 1-10 years, and approach the inferred durations of the flood basalt lava eruptions. The duration of the first pulse of the Woongarra Rhyolite sill emplacement has been estimated at ~240 years, equating to a magma discharge rate of 5 x 106 kg s-1 (Trendall, 1995), which approaches discharge rates of Quaternary sub-Plinian eruptions (Carey & Sigurdsson, 1989). In such long-lived eruptions, we would expect to see unsteadiness in the eruption that would be reflected in bedding or multiple eruptive units in the deposits, which are rarely observed (cf. Sarusas quartz latite, Ewart et al., 2004b). Alternatively, if eruption durations approach those from well-documented Quaternary eruptions (ie. hours to days), then eruption intensities of 1010 to 1011 kg s-1 are required. Such high eruption intensities without the development of a tall Plinian eruption column can be achieved by multiple vents or ring fracture fissure eruptions. The simple deposit structures of the silicic eruptive units thus supports the notion for high eruptive fluxes (109-1011 kg s-1) and short duration (<1 month) eruptions. These higher rates are supported by recent work on giant ash clouds (Baines & Sparks, 2005) that suggest eruption intensities approach 1010 kg s-1 resulting in durations between 2-10 days for the largest eruptions (M8-9). Additional supporting examples for short durations include the 450 km3 Bishop Tuff eruption from Long Valley, California, about 770,000 years ago that has been estimated to have lasted about 4 days (Wilson & Hildreth 1997), and the compositionally zoned 2,200 km3 Huckleberry Ridge Tuff erupted from the Yellowstone volcanic field, which occurs as a single cooling unit and likewise been suggested to result from an eruption that took days at most (Christiansen, 2001).

Frequency of Large-Magnitude (>M8) Eruptions From LIPs

It is both the volume of magma emitted during individual eruptions in LIP events and the total volume of magma released (>0.1-80 Mkm3) that make LIP events so exceptional in Earth history (Self et al., 2005; Self, 2006). It is this combination of large erupted volumes and high frequency that lead to the rapid construction of thick (1->3 km) areally extensive plateaus (0.1-2 Mkm2), which internally, show few signs of major time breaks, erosion surfaces and regional unconformities (Fig. 6A). The high-frequency of large-magnitude eruptions also distinguishes LIP events from other tectonic settings and processes where igneous rocks are formed. Importantly, without LIP-forming igneous events, basalt super-eruptions would not have occurred through Earth history, but in contrast, silicic super-eruptions have occurred independently of LIP events (Sparks et al., 2005).

From a volcanological viewpoint, geochronological studies of LIPs, summarized in studies such as Rampino & Stothers (1988), Courtillot & Renne (2003), Kelley (2007), Bryan & Ernst (2008) and Chenet et al. (2008), have revealed two main features relevant to the timing of eruptions and LIP formation. These are that: 1) much (70-90%) of the eruptions are produced during one or two main pulses of eruptive activity; and 2) that the pulse or pulses, or even the whole duration of activity in the LIP, can be very brief geologically, <5 Ma, and possibly even < 1 Ma in some cases. For LIPs of any age, the errors on the age estimates cannot resolve individual formations within these pulses of activity, and certainly cannot resolve individual eruptions. In many cases, the errors encompass the age range of almost all eruptive units from top to bottom of the LIP pile (e.g., Barry et al., 2010).

For the flood basalt eruptions, evidence from the CRB LIP suggests that during the main pulse and emplacement of the Grande Ronde and Wanapum Basalt Formations (~16-5-15.3 Ma), a high frequency of the largest magnitude eruptions existed producing the most voluminous flow fields (Tolan et al., 1989). For example, the Grande Ronde Basalt forming >60% of the total volume of the CRB LIP (Camp et al., 2003), comprises at least 110 individual eruption packages or flow fields (Barry et al., 2010) with an averaged volume of 1,238 km3 (Tolan et al., 1989). New 40Ar/39Ar dates for Grande Ronde lavas reveal they were emplaced within a maximum time range of 0.42 ± 0.18 Myr (Barry et al., 2010), corresponding to an averaged frequency of ≥M8 eruptions of 220/Myr or one ≥M8 eruption every ~4,200 yrs. For larger continental flood basalt provinces such as the Deccan, recent studies (Self, 2006; Self et al., 2008; Chenet et al., 2009) indicate that individual formations emplaced over similar time scales of hundreds of thousands of years, have volumes either equivalent to the main phase lavas (Grande Ronde Formation, ~0.15 Mkm3) or to the entire CRB LIP (~0.23 Mkm3, Camp et al., 2003). Therefore, LIP formations must also be characterised by even higher frequencies of M8 and larger eruptions.

For most LIPs other than the CRB, however, the number of eruptions is unknown. Even within the CRB, the number is not known precisely but is probably around 200. Fitting the number of eruptions within a 1-2 Ma timeframe still gives average eruption intervals of 1,000s - 10,000 yrs (Self et al., 2006; Barry et al., 2010). While the accumulated lava pile, and the thickness added by each eruption, is impressive, considered from a modern or historic perspective, LIP eruptions probably were not necessarily ‘hyperactive’, even during the main pulse. Much more needs to be known about the rates of lava production and the lengths of hiatuses between eruptions in LIPs, and various approaches are being undertaken to estimate this (Chenet et al., 2008), but such studies are in their infancy.

The recent compilation on large-volume silicic explosive eruptions by Mason et al. (2004) revealed 42 known eruptions of >M8 over the past 36 Ma. This yielded a minimum time-averaged estimate of eruption frequency of 1.1 events/Myr since the beginning of the Oligocene, but over this time, such large magnitude eruptions have clustered in two pulses at 36-25 Ma and 13.5 Ma to present. The older pulse corresponds to two LIP events: the Afro-Arabian and Sierra Madre Occidental provinces but with most dimensional data for eruptions drawn from related large-volume silicic ignimbrite volcanism to the north of the Sierra Madre Occidental in the Great Basin region of western U.S.A. (e.g., Gans et al., 1989; Best & Christiansen, 1991). During these pulses, large eruption frequencies were slightly higher at ~2 events/Myr. However, virtually no data are currently available on the magnitude of individual silicic eruptions within the Sierra Madre Occidental, and eruption frequencies will have been much higher for this period. In contrast to the eruptive record for the last 35 Myrs, large eruption frequencies were at least 9 events/Myr during the Paraná-Etendeka LIP event, and for the Afro-Arabian province, the equivalent of 12 events/1 Myr. Consequently, the frequency of silicic super-eruptions is greater during LIP events than when compared to global, long-term averaged frequencies of silicic super-eruptions. Importantly though, based on available age data for the continental flood basalt provinces, these high frequencies appear sustained only for very brief periods of ≤1 Myr.

Generation & Storage of Large Magnitude LIP eruptions

The eruption of such exceptionally voluminous magmas and the often remarkable chemical homogeneity in on-land deposits have led to the general interpretation that LIP eruptions require very large magma reservoirs, which are rapidly evacuated. The very large volumes for individual eruptions from LIPs raise a number of space-volume issues in terms of the storage and dimensions of, and connectivity and interactions between holding chambers within the crust. Not only do issues of space arise from the storage of such huge volume magma bodies in the crust, but additional complications arise in a temporal sense because many >M8 basaltic and silicic eruptions may occur within 1 Myrs, some or all of which may be genetically unrelated and represent new episodes of large-volume magma generation. For example, a number of large volume (>1,000 km3), closely-spaced (≤105 yrs) and genetically related silicic eruptions (Goboboseb and Springbok quartz latites) occurred from the Messum igneous complex, and which overlapped in space and time with flood basaltic eruptions (Ewart et al., 1998b; 2002). How do these magma reservoirs spatially overlap and how does this impact on the thermal and rheological character (e.g., de Silva & Gosnold, 2007) of the crust? The architecture and spatial-temporal relationships of flood basaltic and rhyolitic magma reservoirs in the crust during LIP events remain poorly understood. In our paper we have considered that the variety of large-volume basaltic and silicic eruptions in LIPs can be envisaged in terms of four end-member magma petrogenetic pathways (Fig. 8). These pathways reflect the varying importance of crustal storage for low-pressure crystallisation, crustal assimilation and large-scale partial melting.


Figure 8. Conceptual crustal view (not to horizontal scale) of four end-member petrogenetic pathways for large-magnitude (basaltic and silicic) eruptions, principally in continental LIPs. The effects of crustal thinning on magma generation are not included in this depiction. Sills of mantle-derived basaltic magma are injected principally near the crust–mantle boundary. Over time this underplated basaltic magma produces seismic (top of basaltic underplate) and petrologic (base of underplate) mohos. Basaltic magmas may be either (A) extracted rapidly from melting source regions in the mantle and erupted at the surface, or (B) pond in lower crustal magma chambers where the magmas become subject to open system processes (assimilation, magma mixing, melt extraction) as well as fractional crystallisation. Low-Ti-type flood basalt magmas (B1; e.g., Tafelberg–Gramado lavas, Ewart et al., 1998a) have typically experienced assimilation of intermediate to silicic composition lower crust, whereas high-Ti-type flood basalt magmas (B2; e.g., Khumib–Urubici lavas, Ewart et al., 2004a) have typically experienced magma mixing ± remelting of the newly formed underplate. In both cases, additional upper crustal storage (B3) can result in further crustal assimilation, crystallisation and magma degassing. High-temperature (>950°C) silicic magmas are genetically related to the low- and high-Ti type flood basaltic magmas and large-scale crustal assimilation characterises low-Ti (C1) silicic magmas (e.g., Goboboseb and Springbok quartz latites, Ewart et al., 1998b), whereas high-Ti silicic magmas (C2) show little to no evidence for the involvement of silicic crust in their petrogenesis (e.g., Chapecó-type rhyolites, Garland et al., 1995; Ewart et al., 2004b), and appear to be rapidly extracted from magma chambers residing in the lower crust. Thermal and mass fluxes of basalts to mid to upper crustal depths result in either the remobilisation of felsic cumulate piles or partially crystallised batholiths (D1) akin to the model for the Fish Canyon Tuff proposed by Bachmann et al. (2002). Additionally, remelting of differentiated and solidified granitic intrusions (D2, e.g., Alacrán Tuff of the Sierra Madre Occidental, Bryan et al., 2008) can also occur, producing moderate to large volumes of relatively low-temperature (<850°C), high-K2O rhyolites.

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