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Igneous Rock Associations 7. Arc Magmatism I: Relationship Between Subduction and Magma Genesis J

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Igneous Rock Associations 7. Arc Magmatism I: Relationship Between Subduction and Magma Genesis J Document generated on 09/29/2021 6:13 a.m. Geoscience Canada Igneous Rock Associations 7. Arc Magmatism I: Relationship Between Subduction and Magma Genesis J. Brendan Murphy Volume 33, Number 4, December 2006 Article abstract Dehydration of subducted oceanic lithosphere releases fluid into the overlying URI: https://id.erudit.org/iderudit/geocan33_4ser01 mantle wedge and initiates a chain of events culminating in the generation of magma that rises to form volcanic arcs. Arc magmas are distinct from magmas See table of contents in other settings because of the different geothermal regimes at destructive plate margins, the presence of fluids/volatiles derived from dehydration of the subducted slab and the generally compressive tectonic regime that inhibits the Publisher(s) ascent of magma, thereby promoting extensive interaction with the adjacent wall rocks. As a result, most arc magmas solidify as intrusive bodies, ranging The Geological Association of Canada from sills and dykes to large plutonic complexes. Several mechanisms facilitate the rise of arc magmas. Diapirs rising from a larger pool of buoyant magma are ISSN important in the ductile lower crust. The rapid rise and the expansion of magma results in the propagation of fractures, that facilitate stoping and 0315-0941 (print) assimilation in the brittle upper crust. Fracture zones are repeatedly exploited, 1911-4850 (digital) and the net result may be formation of a composite batholith. Water plays an important role in all stages of arc-magma evolution. Water Explore this journal lowers the temperatures that are required for the partial melting in the mantle wedge, which produces mafic magma, and in the crust, produces felsic magma. Arc magmatism is intimately related to metamorphism, although this Cite this article relationship is complicated largely because maximum pressures and temperatures are attained at different times. Arcs under compression undergo Murphy, J. B. (2006). Igneous Rock Associations 7. Arc Magmatism I:: rapid thickening, followed by erosional or tectonic exhumation. Crustal Relationship Between Subduction and Magma Genesis. Geoscience Canada, melting is triggered by a variety of processes, including relaxation following 33(4), 145–168. crustal thickening. Melting initiates at the base of the crust, but eventually occurs at shallower crustal levels. Arcs under extension have a steep geothermal gradient and underplating of the crust by mafic magma may transfer sufficient heat to induce anatexis. During a prolonged history of subduction, the dip and location of the subduction zone may vary causing the locus of arc magmatism to migrate and causing intermittent switching from compressional to extensional environments. All rights reserved © The Geological Association of Canada, 2006 This document is protected by copyright law. Use of the services of Érudit (including reproduction) is subject to its terms and conditions, which can be viewed online. https://apropos.erudit.org/en/users/policy-on-use/ This article is disseminated and preserved by Érudit. Érudit is a non-profit inter-university consortium of the Université de Montréal, Université Laval, and the Université du Québec à Montréal. Its mission is to promote and disseminate research. https://www.erudit.org/en/ GEOSCIENCE CANADA Volume 33 Number 4 December 2006 145 SERIES dykes to large plutonic complexes. Sev- SOMMAIRE eral mechanisms facilitate the rise of Le phénomène de déshydratation qui arc magmas. Diapirs rising from a larg- accompagnent la subduction de la er pool of buoyant magma are impor- lithosphère océanique relâche des flu- tant in the ductile lower crust. The ides dans le prisme mantélique sus- rapid rise and the expansion of magma jacent et initie une suite d’événements results in the propagation of fractures, qui conduit à la génération de magmas that facilitate stoping and assimilation qui forment des structures d’îles en arc. in the brittle upper crust. Fracture Les magmas d’îles en arc diffèrent des zones are repeatedly exploited, and the magmas d’autres contextes parce que Igneous Rock Associations net result may be formation of a com- les régimes géothermaux aux lieux de 7. posite batholith. destruction de marges tectoniques sont Water plays an important role différents, étant donné la présence de Arc Magmatism I: Relation- in all stages of arc-magma evolution. fluides et/ou de volatiles issus de la ship Between Subduction Water lowers the temperatures that are déshydratation de la plaque en subduc- and Magma Genesis required for the partial melting in the tion, et du régime tectonique générale- mantle wedge, which produces mafic ment compressif qui inhibe l’ascension J. Brendan Murphy magma, and in the crust, produces fel- du magma, d’où l’importance de l’in- sic magma. Arc magmatism is inti- Dept of Earth Sciences, St. Francis Xavier teraction avec la roche encaissante. En mately related to metamorphism, conséquence, la plupart des magmas University, Antigonish, NS, Canada, although this relationship is complicat- d’îles en arc forment des intrusifs, vari- B2G 2W5, e-mail: [email protected] ed largely because maximum pressures ant des filons-couches, aux dykes aux and temperatures are attained at differ- complexes plutoniques. Plusieurs SUMMARY ent times. Arcs under compression mécanismes favorisent l’ascension des Dehydration of subducted oceanic lith- undergo rapid thickening, followed by magmas d’îles en arc. Ainsi, l’ascen- osphere releases fluid into the overly- erosional or tectonic exhumation. sion de diapirs à partir d’une grande ing mantle wedge and initiates a chain Crustal melting is triggered by a variety accumulation de magma moins dense of events culminating in the generation of processes, including relaxation fol- que l’encaissant est-il un phénomène of magma that rises to form volcanic lowing crustal thickening. Melting initi- important dans la croûte inférieure arcs. Arc magmas are distinct from ates at the base of the crust, but even- ductile. Cette ascension rapide jointe à magmas in other settings because of tually occurs at shallower crustal levels. l’expansion du magma provoque la the different geothermal regimes at Arcs under extension have a steep geo- propagation de fractures, ce qui facilite destructive plate margins, the presence thermal gradient and underplating of l’agrégation et l’assimilation de l’encais- of fluids/volatiles derived from dehy- the crust by mafic magma may transfer sant dans la portion supérieure cas- dration of the subducted slab and the sufficient heat to induce anatexis. Dur- sante de la croûte. Fréquemment, le generally compressive tectonic regime ing a prolonged history of subduction, magma injecte ces zones de fractures, that inhibits the ascent of magma, the dip and location of the subduction ce qui conduit parfois à la formation thereby promoting extensive interac- zone may vary causing the locus of arc d’un batholite composite. tion with the adjacent wall rocks. As a magmatism to migrate and causing L’eau joue un rôle important à result, most arc magmas solidify as intermittent switching from compres- toutes les étapes de l’évolution des intrusive bodies, ranging from sills and sional to extensional environments. magmas d’îles en arc. La présence 146 d’eau abaisse les températures de sented here. The products of arc mag- fusion partielle dans le prisme man- matism, from the pyroclastic and lava télique, ce qui conduit à la formation deposits that erupt at the surface to the de magmas mafiques, et de magmas intrusive rocks in the plumbing system felsiques dans la croûte. Le magma- at depth, are described, as are the vary- TETHYS tisme d’île en arc est intimement lié au ing processes governing the ascent of SEA métamorphisme, bien que cette rela- arc magma and the controversies asso- tion soit compliquée, surtout parce que ciated with the mechanisms of intru- les pressions et les températures maxi- sion. The critical role of water, derived males sont atteintes à des moments from dehydration of the subducted PERMIAN 225 million years ago différents. Les magmas d’îles en arc en slab, is considered in all stages of arc- situation de compression s’épaississent magma evolution, from the generation rapidement, puis sont érodés ou of the magma in the mantle and crust, exhumés tectoniquement. La fusion de during its rise toward the surface, to its la croûte est déclenchée par une variété final cooling and crystallization. This de processus, dont la détente accompa- article concludes with an examination gnant l’épaississement crustal. La of the varying relationships between fusion se produit d’abord à la base de arc magmatism, metamorphism and la croûte, mais elle atteint éventuelle- deformation during subduction-related ment des niveaux moins profonds de la orogenesis. CRETACEOUS 65 million years ago croûte. Les magmas d’île en arc en sit- uation d’extension ont un gradient PLATE TECTONIC SETTING Asia géothermal très pentu, et le placage de Plate tectonics plays a crucial role in N.A. la croûte par un magma mafique peut y the generation of arc magmas. On a Africa apporter assez de chaleur pour provo- globe of constant volume, the creation India quer l’anatexie. Si l’histoire de la sub- of lithosphere at constructive margins S.A. Australia duction se prolonge, le contexte d’un (where two plates move apart) is bal- système d’îles en arc peut alterner anced by the destruction of lithosphere épisodiquement d’un régime en com- at destructive margins (where two Antarctica pression à un régime en extension. plates collide). This balance is apparent PRESENT DAY on paleocontinental reconstructions for Figure 1. Reconstructions over the INTRODUCTION: A REVIEW OF THE the past 200 million years, which show past 200 million years showing how TECTONICS OF DESTRUCTIVE PLATE that the Atlantic Ocean expanded by expansion of the Atlantic Ocean was MARGINS seafloor spreading as the Pacific and accompanied by contraction of the This is the first of two articles on arc Tethys oceans contracted by subduc- Pacific and Tethys oceans.
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