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Giant plagioclase basalts; continental flood basalt-induced remobilization of anorthositic mushes in a deep crustal sill complex

Hetu Sheth
Giant plagioclase basalts; continental flood basalt-induced remobilization of anorthositic mushes in a deep crustal sill complex
Geological Society of America Bulletin (February 2016) 128 (5-6): 916-925

Abstract

Giant plagioclase basalts, rich in plagioclase megacrysts commonly 5 cm long, form widespread lava flows and dikes in the Deccan Traps continental flood basalt province. They also occur in the Scottish Hebrides, Columbia River, and Emeishan continental flood basalt provinces, and closely similar rocks are known from Archean and Proterozoic terrains and oceanic settings. The giant plagioclase basalts have usually been interpreted as end products of magmatic cycles operating in high-level chambers, and taken as the boundaries of stratigraphic formations. A new model is proposed in which the plagioclase megacrysts grow over long (15 k.y.) time periods in a deep crustal (possibly Moho-level) sill complex, which forms tens to hundreds of thousands of years before the continental flood basalt eruptions. Megacryst flotation produces anorthositic mushes containing ferrodioritic interstitial liquids. Giant plagioclase basalts form when these components (and any melt from the crust heated by the sill complex) are remobilized by a newly injected tholeiitic magma batch, and brought to the surface or high crustal levels by dikes. Giant plagioclase basalts are thus mixed, heterogeneous rocks from the deep continental crust. The new model explains a range of geological, petrological, mineralogical, and geochemical data on the giant plagioclase basalts. Formation and remobilization of anorthositic mushes appear to be fundamental processes producing highly feldspar-phyric basalts over a wide range of tectonic settings. An important corollary of the new model is that anorthosites have continued to form well after the Precambrian, on a smaller scale, during Phanerozoic continental flood basalt events. Therefore, continental flood basalt provinces should be recognized as sites of anorthosite formation in the deep continental crust.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 128
Serial Issue: 5-6
Title: Giant plagioclase basalts; continental flood basalt-induced remobilization of anorthositic mushes in a deep crustal sill complex
Author(s): Sheth, Hetu
Affiliation: Indian Institute of Technology-Bombay, Department of Earth Sciences, Mumbai, India
Pages: 916-925
Published: 20160201
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 104
Accession Number: 2016-016636
Categories: Isotope geochemistryIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sketch map
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201609
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