Large-scale Mechanical Redistribution of Orthopyroxene and Plagioclase in the Basement Sill, Ferrar Dolerites, McMurdo Dry Valleys, Antarctica: Petrological, Mineral-chemical and Field Evidence for Channelized Movement of Crystals and Melt
The Jurassic Ferrar dolerite sills of the McMurdo Dry Valleys, Antarctica represent the plumbing system for flood basalt eruptions associated with the breakup of Gondwana. Among the Ferrar sills, the 350–450 m thick cumulate-textured Basement Sill is differentiated into a Lower Marginal Zone (LMZ) g...
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Oxford University Press
2007
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fthighwire:oai:open-archive.highwire.org:petrology:48/12/2289 2023-05-15T13:55:26+02:00 Large-scale Mechanical Redistribution of Orthopyroxene and Plagioclase in the Basement Sill, Ferrar Dolerites, McMurdo Dry Valleys, Antarctica: Petrological, Mineral-chemical and Field Evidence for Channelized Movement of Crystals and Melt Bédard, Jean H. J. Marsh, Bruce D. Hersum, Taber G. Naslund, H. Richard Mukasa, Samuel B. 2007-12-01 00:00:00.0 text/html http://petrology.oxfordjournals.org/cgi/content/short/48/12/2289 https://doi.org/10.1093/petrology/egm060 en eng Oxford University Press http://petrology.oxfordjournals.org/cgi/content/short/48/12/2289 http://dx.doi.org/10.1093/petrology/egm060 Copyright (C) 2007, Oxford University Press Original Papers TEXT 2007 fthighwire https://doi.org/10.1093/petrology/egm060 2008-05-01T04:29:14Z The Jurassic Ferrar dolerite sills of the McMurdo Dry Valleys, Antarctica represent the plumbing system for flood basalt eruptions associated with the breakup of Gondwana. Among the Ferrar sills, the 350–450 m thick cumulate-textured Basement Sill is differentiated into a Lower Marginal Zone (LMZ) gabbronorite, a thick Lower Zone (LZ) orthopyroxene–plagioclase orthocumulate pyroxenite, a strongly layered mela- to leuco-gabbronorite Middle Zone (MZ), a thick Upper Zone (UZ) gabbronorite with ferrogabbroic pods, and an Upper Marginal Zone (UMZ) gabbronorite. Textures and mineral compositions in the LZ pyroxenite and MZ–UZ gabbronorites are nearly identical, the main distinction being the greater relative proportion of plagioclase in the MZ–UZ gabbronorites, and of pigeonite in the UZ. Most orthopyroxene in the LZ, MZ and UZ occurs as sub-euhedral, normally zoned primocrysts, commonly with rounded plagioclase inclusions. Plagioclase is usually sub-euhedral and normally zoned, but can contain sodic cores interpreted to be xenocrystic. Orthopyroxene and feldspar compositions thoughout the sill are generally fairly uniform, and resemble the compositions of micro-phenocrysts in the chilled margins. We infer that the sill was filled by a c. 1250°C slurry of orthopyroxene + plagioclase phenocrysts or primocrysts that subsequently unmixed in response to buoyancy forces. The LZ websterite contains numerous anorthosite to gabbronorite schlieren, veins and pipes (< 2 m diameter), which we interpret as fossil segregation channels. Textures and mineral compositions in these felsic channels are very similar both to UZ and MZ gabbronorites, and to the groundmass separating accumulated orthopyroxene primocrysts in the LZ and MZ. We infer that plagioclase-charged, hydrous pore melt from the pyroxenite may have segregated, pooled and ascended through these conduits to feed growth of the UZ gabbronorite. Detailed mapping shows that the pipes are separated by about 15 m on average. Calculations suggest that this number density of ... Text Antarc* Antarctica McMurdo Dry Valleys HighWire Press (Stanford University) McMurdo Dry Valleys Journal of Petrology 48 12 2289 2326 |
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English |
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Original Papers |
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Original Papers Bédard, Jean H. J. Marsh, Bruce D. Hersum, Taber G. Naslund, H. Richard Mukasa, Samuel B. Large-scale Mechanical Redistribution of Orthopyroxene and Plagioclase in the Basement Sill, Ferrar Dolerites, McMurdo Dry Valleys, Antarctica: Petrological, Mineral-chemical and Field Evidence for Channelized Movement of Crystals and Melt |
topic_facet |
Original Papers |
description |
The Jurassic Ferrar dolerite sills of the McMurdo Dry Valleys, Antarctica represent the plumbing system for flood basalt eruptions associated with the breakup of Gondwana. Among the Ferrar sills, the 350–450 m thick cumulate-textured Basement Sill is differentiated into a Lower Marginal Zone (LMZ) gabbronorite, a thick Lower Zone (LZ) orthopyroxene–plagioclase orthocumulate pyroxenite, a strongly layered mela- to leuco-gabbronorite Middle Zone (MZ), a thick Upper Zone (UZ) gabbronorite with ferrogabbroic pods, and an Upper Marginal Zone (UMZ) gabbronorite. Textures and mineral compositions in the LZ pyroxenite and MZ–UZ gabbronorites are nearly identical, the main distinction being the greater relative proportion of plagioclase in the MZ–UZ gabbronorites, and of pigeonite in the UZ. Most orthopyroxene in the LZ, MZ and UZ occurs as sub-euhedral, normally zoned primocrysts, commonly with rounded plagioclase inclusions. Plagioclase is usually sub-euhedral and normally zoned, but can contain sodic cores interpreted to be xenocrystic. Orthopyroxene and feldspar compositions thoughout the sill are generally fairly uniform, and resemble the compositions of micro-phenocrysts in the chilled margins. We infer that the sill was filled by a c. 1250°C slurry of orthopyroxene + plagioclase phenocrysts or primocrysts that subsequently unmixed in response to buoyancy forces. The LZ websterite contains numerous anorthosite to gabbronorite schlieren, veins and pipes (< 2 m diameter), which we interpret as fossil segregation channels. Textures and mineral compositions in these felsic channels are very similar both to UZ and MZ gabbronorites, and to the groundmass separating accumulated orthopyroxene primocrysts in the LZ and MZ. We infer that plagioclase-charged, hydrous pore melt from the pyroxenite may have segregated, pooled and ascended through these conduits to feed growth of the UZ gabbronorite. Detailed mapping shows that the pipes are separated by about 15 m on average. Calculations suggest that this number density of ... |
format |
Text |
author |
Bédard, Jean H. J. Marsh, Bruce D. Hersum, Taber G. Naslund, H. Richard Mukasa, Samuel B. |
author_facet |
Bédard, Jean H. J. Marsh, Bruce D. Hersum, Taber G. Naslund, H. Richard Mukasa, Samuel B. |
author_sort |
Bédard, Jean H. J. |
title |
Large-scale Mechanical Redistribution of Orthopyroxene and Plagioclase in the Basement Sill, Ferrar Dolerites, McMurdo Dry Valleys, Antarctica: Petrological, Mineral-chemical and Field Evidence for Channelized Movement of Crystals and Melt |
title_short |
Large-scale Mechanical Redistribution of Orthopyroxene and Plagioclase in the Basement Sill, Ferrar Dolerites, McMurdo Dry Valleys, Antarctica: Petrological, Mineral-chemical and Field Evidence for Channelized Movement of Crystals and Melt |
title_full |
Large-scale Mechanical Redistribution of Orthopyroxene and Plagioclase in the Basement Sill, Ferrar Dolerites, McMurdo Dry Valleys, Antarctica: Petrological, Mineral-chemical and Field Evidence for Channelized Movement of Crystals and Melt |
title_fullStr |
Large-scale Mechanical Redistribution of Orthopyroxene and Plagioclase in the Basement Sill, Ferrar Dolerites, McMurdo Dry Valleys, Antarctica: Petrological, Mineral-chemical and Field Evidence for Channelized Movement of Crystals and Melt |
title_full_unstemmed |
Large-scale Mechanical Redistribution of Orthopyroxene and Plagioclase in the Basement Sill, Ferrar Dolerites, McMurdo Dry Valleys, Antarctica: Petrological, Mineral-chemical and Field Evidence for Channelized Movement of Crystals and Melt |
title_sort |
large-scale mechanical redistribution of orthopyroxene and plagioclase in the basement sill, ferrar dolerites, mcmurdo dry valleys, antarctica: petrological, mineral-chemical and field evidence for channelized movement of crystals and melt |
publisher |
Oxford University Press |
publishDate |
2007 |
url |
http://petrology.oxfordjournals.org/cgi/content/short/48/12/2289 https://doi.org/10.1093/petrology/egm060 |
geographic |
McMurdo Dry Valleys |
geographic_facet |
McMurdo Dry Valleys |
genre |
Antarc* Antarctica McMurdo Dry Valleys |
genre_facet |
Antarc* Antarctica McMurdo Dry Valleys |
op_relation |
http://petrology.oxfordjournals.org/cgi/content/short/48/12/2289 http://dx.doi.org/10.1093/petrology/egm060 |
op_rights |
Copyright (C) 2007, Oxford University Press |
op_doi |
https://doi.org/10.1093/petrology/egm060 |
container_title |
Journal of Petrology |
container_volume |
48 |
container_issue |
12 |
container_start_page |
2289 |
op_container_end_page |
2326 |
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1766262053166645248 |