Document generated on 10/02/2021 12:12 a.m. Geoscience Canada Igneous Rock Associations 4. Oceanic Island Volcanism I Mineralogy and Petrology John D. Greenough, Jaroslav Dostal and Leanne M. Mallory-Greenough Volume 32, Number 1, March 2005 Article abstract Oceanic islands tend to occur at the young ends of hotspot trails because they URI: https://id.erudit.org/iderudit/geocan32_1ser01 record the passage of oceanic plates over rising convection cells (plumes) in the mantle, or the propagation of cracks in the lithosphere. Basaltic volcanism on See table of contents oceanic islands is generally unexplosive and, although potentially destructive, poses less threat to human life than volcanism in other tectonic environments. However, the possibility of giant tsunamis from the catastrophic gravitational Publisher(s) collapse of islands is of real concern for major cities surrounding the ocean basins. The Geological Association of Canada Two series of magmas are recognized in oceanic islands. Tholeiites form at lower pressures than alkali basalts, from higher percentages of decompression ISSN melting. The former contain a low-Ca pyroxene and the latter can crystallize 0315-0941 (print) nepheline. Furthermore, minerals common to both series (chromite, olivine, 1911-4850 (digital) augite, plagioclase, magnetite and ilmenite) are compositionally distinct reflecting fundamental chemical differences between the two magma series. Explore this journal Mineral compositions vary as magmas evolve in sub-volcanic, lithospheric magma chambers by assimilation and differentiation. Magmas assimilate wall rocks in these chambers. Time-scales for differentiation (mostly crystal fractionation) are generally less than a few thousand years. Early olivine, Cite this article pyroxene, chromite and immiscible sulfide formation cause compatible Greenough, J. D., Dostal, J. & Mallory-Greenough, L. M. (2005). Igneous Rock elements (e.g., Ni, Co, Cr) to decline rapidly as differentiation proceeds. Associations 4. Oceanic Island Volcanism I Mineralogy and Petrology. Plagioclase dramatically removes Sr at intermediate stages and alkali feldspars Geoscience Canada, 32(1), 29–45. sequester Ba and Rb as late-stage trachytes and phonolites form in alkaline magmas. The high-field-strength elements are generally incompatible but locally decline reflecting apatite (P) and Fe-Ti oxide (Ti, Nb, Ta) removal. Studies of layering in individual lava flows suggest that rising volatiles may effect mass transfer of complexed ions during differentiation in magma chambers. All rights reserved © The Geological Association of Canada, 2005 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/ GACGeoCanV32No1.qxp 3/18/2005 3:14 PM Page 29 GEOSCIENCE CANADA Volume 32 Number 1 March 2005 29 SERIES environments. However, the possibility océaniques au-dessus de cellules de con- of giant tsunamis from the catastrophic vection ascendantes (panaches) dans le gravitational collapse of islands is of manteau ou de la propagation de fissures real concern for major cities surround- de la lithosphère. En général, le volcan- ing the ocean basins. isme basaltique des îles océaniques n’est Two series of magmas are recog- pas explosif, et bien qu’il puisse être nized in oceanic islands. Tholeiites form destructeur, il présente moins de danger at lower pressures than alkali basalts, pour les humains que le volcanisme from higher percentages of decompres- d’autres environnements tectoniques. sion melting. The former contain a low- Cependant, la formation possible de Ca pyroxene and the latter can crystallize tsunamis géants provoqués par l’effon- nepheline. Furthermore, minerals com- drement d’îles constitue un danger réel Igneous Rock Associations mon to both series (chromite, olivine, pour les grandes agglomérations situées augite, plagioclase, magnetite and au pourtour des basins océaniques. 4. ilmenite) are compositionally distinct On distingue deux séries magma- Oceanic Island Volcanism I reflecting fundamental chemical differ- tiques dans les matériaux constitutifs des Mineralogy and Petrology ences between the two magma series. îles océaniques. Les tholéiites qui se Mineral compositions vary as forment à des pressions plus faibles que magmas evolve in sub-volcanic, lithos- les basaltes alcalins, et plus souvent sous John D. Greenough1, Jaroslav pheric magma chambers by assimilation des conditions de fusion par décompres- Dostal2, and Leanne M. Mallory- and differentiation. Magmas assimilate sion, et les basaltes alcalins. Les pre- Greenough3 wall rocks in these chambers. Time- mières contiennent un pyroxène à faible 1Department of Earth and Environmental scales for differentiation (mostly crystal teneur en calcium et l’autre permet la Sciences, University of British Columbia - fractionation) are generally less than a cristallisation de la néphéline. De plus, Okanagan, 3333 University Way, Kelowna, few thousand years. Early olivine, pyrox- les minéraux communs à ces deux séries BC, V1V 1V7. E-mail: [email protected] ene, chromite and immiscible sulfide (chromite, olivine, augite, plagioclase, Tel: 250 762 5445 ext. 7520; Fax: 250 470 formation cause compatible elements magnétite et ilménite) ont des composi- 6005 (e.g., Ni, Co, Cr) to decline rapidly as tions distinctes qui reflètent les dif- 2Department of Geology, Saint Mary’s differentiation proceeds. Plagioclase dra- férences de composition chimique University, Halifax, NS, B3H 3C3. E-mail: matically removes Sr at intermediate intrinsèques de ces deux séries magma- [email protected] stages and alkali feldspars sequester Ba tiques. 3Department of Geology, University of and Rb as late-stage trachytes and Les compositions minérales vari- Toronto, 22 Russell St., Toronto, ON, M5S phonolites form in alkaline magmas. The ent puisque les magmas changent de 3B1. E-mail: [email protected] high-field-strength elements are general- composition dans les chambres magma- ly incompatible but locally decline tiques lithosphériques pré-volcaniques SUMMARY reflecting apatite (P) and Fe-Ti oxide (Ti, par assimilation et différentiation. Les Oceanic islands tend to occur at the Nb, Ta) removal. Studies of layering in magmas absorbent les roches des murs young ends of hotspot trails because individual lava flows suggest that rising d’enceinte dans ces chambres. Les they record the passage of oceanic volatiles may effect mass transfer of échelles de temps de ces différentiations plates over rising convection cells complexed ions during differentiation in (principalement de fractionnement (plumes) in the mantle, or the propaga- magma chambers. cristallin) s’étalent généralement sur tion of cracks in the lithosphere. Basaltic quelques milliers d’années. D’abord, la SOMMAIRE volcanism on oceanic islands is generally formation d’olivine, de pyroxène, de unexplosive and, although potentially Les îles océaniques se trouvent générale- chromite et de sulfures immiscibles destructive, poses less threat to human ment à l’extrémité la plus jeune des entraîne un appauvrissement rapide du life than volcanism in other tectonic traînées de points chauds parce qu’elles magma en éléments compatibles (Ni, sont la marque du passage de plaques Co, Cr par ex.). Puis, aux stades inter- GACGeoCanV32No1.qxp 3/18/2005 3:14 PM Page 30 30 médiaires, la formation de plagioclases and eruptive style from the activity at indicates that collapses could imperil réduit considérablement la teneur en Sr plate margins. These sub-sea and subaer- major cities bordering the ocean basins et, aux stades finaux, la formation de ial land forms represent <1 % of all (McGuire, 1996). Thus, studies of feldspaths alcalins en extrait le Ba et le igneous rocks on Earth and ~2% of all oceanic islands have both intellectual Rb alors que se forment les trachytes et volcanic activity in the deep ocean and hazard-alleviation significance for les phonolites à partir de magmas basins. However, enormous emphasis humans. alcalins. Les éléments les plus fortement has been placed on them for under- This is the first of two comple- polarisés sont généralement incompati- standing igneous processes and the evo- mentary papers on oceanic island bles mais, localement, leur teneur lution of Earth. Separated from contam- basalts. It reviews the mineralogy, classi- décline, reflétant ainsi la formation d’ap- inating continental crust, and unaltered fication, tectonic significance and lithos- atite (P) et d’oxydes de Fe-Ti (Ti, Nb, by mountain-building metamorphic and perhic processes (e.g., differentiation) Ta). Les résultats d’études sur la forma- tectonic processes (Basaltic Volcanism affecting OIB magmas. The second tion en couche de coulées de laves Study Project = BVSP, 1981, p. 161), paper (Greenough et al., in press) exam- indiquent que l’ascension gravitation- oceanic island basalts (OIB) provide ines the mantle processes affecting OIB nelle des volatils pourrait avoir un effet insights into mantle composition, magma generation, including the influ- sur le transfert de masse d’ions complex- magma formation processes and magma ence of source mineralogy and the es durant la différenciation