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Physical volcanology of large igneous provinces
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Journal of Volcanology and Geothermal Research 172 (2008) 1 – 2 www.elsevier.com/locate/jvolgeores
Preface
Physical volcanology of large igneous provinces Large igneous provinces have been the subject of research since the earliest days of Geology. They are spectacular in size, and basaltic continental ones develop distinctive 'stepped' landscapes with erosion (hence the Deccan “Traps”, from the German term “treppen”, for step). Many publications have wrestled with the origins of the voluminous magmas erupted in large igneous provinces, but the number addressing their physical volcanology is much smaller. In this volume a selection of nine papers is presented, each of them focusing on physical aspects of large igneous provinces, with only secondary, if any, attention given to the petrological origins of the magmas erupted. Early work focused strongly on continental large igneous provinces dominated by basaltic rocks, but a diversity of large igneous provinces is now recognized both on continents and in the oceans, with the diversity fostering development of a variety of classification schemes. An important expansion has been recognition that large volumes of silicic volcanic deposits can be viewed as comprising silicic large igneous provinces. Similarly, continental breakup is commonly associated with eruption and intrusion of magma of sufficient volume, and rapidity of emplacement, to represent another type of large igneous province. Perhaps the largest, yet least well known, especially in terms of their physical volcanology. are oceanic plateaus such as the Ontong Java Plateau. Transitional between oceanic and continental provinces are those of volcanic rifted margins, with Rey and others (this volume) presenting an analysis, based largely on seismic stratigraphy, of the Gascoyne Margin of western Australia. They identify four important igneous products in their dataset. Landward flows have been drilled on a number of margins, where they consist of subaerially erupted flood basalts with no or thin intrabasalt sediment layers. Volcanic protrusions identified in seismic lines are interpreted as buildups of hyaloclastite flows and massive basalt from submarine fissure eruptions. Seaward dipping reflectors are interpreted as submarine-emplaced flood basalts, and their wedge-shaped profiles and divergent internal reflections suggest they represent synrift infilling of basins. Sill intrusions are commonly saucer-shaped bodies interpreted as large sills, like those formed in other sedimentary basins such as the Karoo. Compositionally distinctive are the silicic large igneous provinces, represented here by two manuscripts addressing emplacement of the 1.6 Ga Gawler Range Volcanics at different scales. Allen et al. (this volume) show that the GRV formed in 0377-0273/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2007.11.001
two main stages, with initial eruptions at numerous centres producing geochemically distinctive felsic lavas and lava domes of small volume; these plus ignimbrites and minor mafic to intermediate lavas form deposits less than 3 km thick. The second stage produced three voluminous felsic units that each represent over a thousand km3 of magma that apparently erupted effusively and was emplaced in long-runout lava flows. The Eucarro Rhyolite is one of these lavas, and McPhie et al. (this volume) analysed anisotropy of magnetic susceptibility (AMS) to help determine how the rhyolite was emplaced. Differences in flow directions indicated by these analyses suggest emplacement of the unit as separate lobes, if not derivation from separate vent sites. Turning to the prominent continental flood basalt provinces, a group of papers is opened by Self et al. (this volume) who qpropose that the Rajahmundry Trap lavas, found near the east coast of peninsular India, are remnants of the longest lava flows yet recognized on Earthq. The lavas are interpreted to represent two immense lava flow fields, formed by lavas that for their last 400 km of flow were confined to valleys traversing basement beyond the lava field's edge. Their distal ends reached the sea. From the Deccan focus shifts to the Ferrar province, exposed along some 3500 km along the Transantarctic Mountains. Elliot et al. (this volume) recognize three major components of the province. There are intrusive rocks comprising Ferrar Dolerite sills and dikes plus the Dufek intrusion, volcaniclastic rocks of the Prebble, Mawson and Exposure Hill Formations, and effusive lavas of the Kirkpatrick Basalt. Eruptions began with a major episode of phreatomagmatism, then evolved to a more effusive style. The authors indicate that emplacement of most sills postdated surface eruptions, and infer rift-structure control of magma distribution. Ross et al. (this volume) and Lockett and White (this volume) provide more detailed studies from one area within the Ferrar. In the former of these papers, a bewildering range of eruptive and intrusive features in the Coombs Hills–Allan Hills area is analysed to yield an integrated account of explosive, effusive, intrusive and other activity in the area, which is now among the best studied sites for mafic volcaniclastic deposits in the Ferrar LIP. Ross et al. provide a picture of the geological and morphological evolution of the pre-flood basalt landscape for this area in the early Jurassic. Lockett and White focus more specifically on the breakup and thermal state of a country-rock avalanche that was emplaced onto country rock and then buried and locally crosscut by pyroclastic deposits.
2
Preface
Twin to the Ferrar province, in age and pre-separation geography, is the Karoo province in southern Africa. Maes et al. (this volume) studied with magnetic methods the ~ 1 km-thick Insizwa sill, a well-exposed Karoo intrusion comprising several distinct units. One outcome of the work is evidence that caution needs to be exercised when applying AMS techniques to the reconstruction of magmatic flow, because magnetic fabrics of the sill do not systematically mimic its mineral fabrics. Elsewhere in the Karoo, the Sterkspruit vent complex analyzed by McClintock et al. (this volume) shows some basic similarities with the Coombs Hills one, but also important differences that provide a wealth of detail regarding early eruption processes in the Karoo. Magma arriving through many feeder dikes and stocks fed many small vents that coalesced as the complex developed. Their deposits overlap both within and outside the complex, with an abrupt end to volcaniclastic eruptions indicated by sharply overlying lacustrine and lavadelta deposits at the lowest part of the crater; basalt lavas subsequently flooded the region. The papers in the volume are ordered beginning with those addressing the Deccan, Ferrar and Karoo continental basaltic
provinces. Analysis of the Gascoyne volcanic rifted margin's development follows, and the volume closes with the two papers about the Gawler Range silicic province. We thank the authors for their contributions to this volume, and appreciate their patience as it has slowly evolved from its inception as an accompaniment to a thematic session at AGU's 2004 Western Pacific Geophysics Meeting. As it turned out, the session didn't materialize; LIP volcanologists clearly prefer manuscript production to the rigors of a geophysical meeting in Honolulu.
Guest Editors J.D.L. White University of Otago, New Zealand E-mail address: [email protected]. I.P. Skilling University of Pittsburgh, USA E-mail address: [email protected].
Journal of Volcanology and Geothermal Research 172 (2008) 1 – 2 www.elsevier.com/locate/jvolgeores
Preface
Physical volcanology of large igneous provinces Large igneous provinces have been the subject of research since the earliest days of Geology. They are spectacular in size, and basaltic continental ones develop distinctive 'stepped' landscapes with erosion (hence the Deccan “Traps”, from the German term “treppen”, for step). Many publications have wrestled with the origins of the voluminous magmas erupted in large igneous provinces, but the number addressing their physical volcanology is much smaller. In this volume a selection of nine papers is presented, each of them focusing on physical aspects of large igneous provinces, with only secondary, if any, attention given to the petrological origins of the magmas erupted. Early work focused strongly on continental large igneous provinces dominated by basaltic rocks, but a diversity of large igneous provinces is now recognized both on continents and in the oceans, with the diversity fostering development of a variety of classification schemes. An important expansion has been recognition that large volumes of silicic volcanic deposits can be viewed as comprising silicic large igneous provinces. Similarly, continental breakup is commonly associated with eruption and intrusion of magma of sufficient volume, and rapidity of emplacement, to represent another type of large igneous province. Perhaps the largest, yet least well known, especially in terms of their physical volcanology. are oceanic plateaus such as the Ontong Java Plateau. Transitional between oceanic and continental provinces are those of volcanic rifted margins, with Rey and others (this volume) presenting an analysis, based largely on seismic stratigraphy, of the Gascoyne Margin of western Australia. They identify four important igneous products in their dataset. Landward flows have been drilled on a number of margins, where they consist of subaerially erupted flood basalts with no or thin intrabasalt sediment layers. Volcanic protrusions identified in seismic lines are interpreted as buildups of hyaloclastite flows and massive basalt from submarine fissure eruptions. Seaward dipping reflectors are interpreted as submarine-emplaced flood basalts, and their wedge-shaped profiles and divergent internal reflections suggest they represent synrift infilling of basins. Sill intrusions are commonly saucer-shaped bodies interpreted as large sills, like those formed in other sedimentary basins such as the Karoo. Compositionally distinctive are the silicic large igneous provinces, represented here by two manuscripts addressing emplacement of the 1.6 Ga Gawler Range Volcanics at different scales. Allen et al. (this volume) show that the GRV formed in 0377-0273/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2007.11.001
two main stages, with initial eruptions at numerous centres producing geochemically distinctive felsic lavas and lava domes of small volume; these plus ignimbrites and minor mafic to intermediate lavas form deposits less than 3 km thick. The second stage produced three voluminous felsic units that each represent over a thousand km3 of magma that apparently erupted effusively and was emplaced in long-runout lava flows. The Eucarro Rhyolite is one of these lavas, and McPhie et al. (this volume) analysed anisotropy of magnetic susceptibility (AMS) to help determine how the rhyolite was emplaced. Differences in flow directions indicated by these analyses suggest emplacement of the unit as separate lobes, if not derivation from separate vent sites. Turning to the prominent continental flood basalt provinces, a group of papers is opened by Self et al. (this volume) who qpropose that the Rajahmundry Trap lavas, found near the east coast of peninsular India, are remnants of the longest lava flows yet recognized on Earthq. The lavas are interpreted to represent two immense lava flow fields, formed by lavas that for their last 400 km of flow were confined to valleys traversing basement beyond the lava field's edge. Their distal ends reached the sea. From the Deccan focus shifts to the Ferrar province, exposed along some 3500 km along the Transantarctic Mountains. Elliot et al. (this volume) recognize three major components of the province. There are intrusive rocks comprising Ferrar Dolerite sills and dikes plus the Dufek intrusion, volcaniclastic rocks of the Prebble, Mawson and Exposure Hill Formations, and effusive lavas of the Kirkpatrick Basalt. Eruptions began with a major episode of phreatomagmatism, then evolved to a more effusive style. The authors indicate that emplacement of most sills postdated surface eruptions, and infer rift-structure control of magma distribution. Ross et al. (this volume) and Lockett and White (this volume) provide more detailed studies from one area within the Ferrar. In the former of these papers, a bewildering range of eruptive and intrusive features in the Coombs Hills–Allan Hills area is analysed to yield an integrated account of explosive, effusive, intrusive and other activity in the area, which is now among the best studied sites for mafic volcaniclastic deposits in the Ferrar LIP. Ross et al. provide a picture of the geological and morphological evolution of the pre-flood basalt landscape for this area in the early Jurassic. Lockett and White focus more specifically on the breakup and thermal state of a country-rock avalanche that was emplaced onto country rock and then buried and locally crosscut by pyroclastic deposits.
2
Preface
Twin to the Ferrar province, in age and pre-separation geography, is the Karoo province in southern Africa. Maes et al. (this volume) studied with magnetic methods the ~ 1 km-thick Insizwa sill, a well-exposed Karoo intrusion comprising several distinct units. One outcome of the work is evidence that caution needs to be exercised when applying AMS techniques to the reconstruction of magmatic flow, because magnetic fabrics of the sill do not systematically mimic its mineral fabrics. Elsewhere in the Karoo, the Sterkspruit vent complex analyzed by McClintock et al. (this volume) shows some basic similarities with the Coombs Hills one, but also important differences that provide a wealth of detail regarding early eruption processes in the Karoo. Magma arriving through many feeder dikes and stocks fed many small vents that coalesced as the complex developed. Their deposits overlap both within and outside the complex, with an abrupt end to volcaniclastic eruptions indicated by sharply overlying lacustrine and lavadelta deposits at the lowest part of the crater; basalt lavas subsequently flooded the region. The papers in the volume are ordered beginning with those addressing the Deccan, Ferrar and Karoo continental basaltic
provinces. Analysis of the Gascoyne volcanic rifted margin's development follows, and the volume closes with the two papers about the Gawler Range silicic province. We thank the authors for their contributions to this volume, and appreciate their patience as it has slowly evolved from its inception as an accompaniment to a thematic session at AGU's 2004 Western Pacific Geophysics Meeting. As it turned out, the session didn't materialize; LIP volcanologists clearly prefer manuscript production to the rigors of a geophysical meeting in Honolulu.
Guest Editors J.D.L. White University of Otago, New Zealand E-mail address: [email protected]. I.P. Skilling University of Pittsburgh, USA E-mail address: [email protected].