Archean subqueous high-silica rhyolite coul¨¦es: Examples from the Kidd-Munro Assemblage in the Abitibi Subprovince

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• 作者：Benjamin J.A.  ; Moulton^a ;   ; ^{bja.moulton@utoronto.ca} ; Anthony D.  ; Fowler^a ;   ; ^{afowler@uottawa.ca} ; John A.  ; Ayer^a ;   ; ^b ;   ; ^{john.ayer@ontario.ca} ; Ben R.  ; Berger^b ;   ; ^{ben.berger@ontario.ca} ; Patrick  ; Mercier-Langevin^c ;   ; ^{pmercier@nrcan.gc.ca}
• 关键词：Coulé ; e ; Rhyolite ; Southern Abitibi greenstone belt ; Kidd Creek ; Subaqueous flow ; Alteration
• 刊名：Precambrian Research
• 出版年：2011
• 出版时间：September 2011
• 年：2011
• 卷：189
• 期：3-4
• 页码：389-403
• 全文大小：3867 K

文摘

Subaqueous high-silica rhyolite in Prosser Township, southern Abitibi Subprovince, Ontario, consists of intercalated, thin (<15 m; low aspect ratio < 0.3) aphyric lavas with brown felsic tuffs, overlain by a pyroclastic pumice- and spatter-rich tuff breccia and are capped by quartz¨Cporphyritic rhyolite. The quartz¨Cporphyritic flows contain ?0 % phenocrysts whereas ¡®aphyric?flows have only sparse phenocrysts, typically 1 % . The aphyric flows are characterized by features that are typical of subaqueous rhyolite flows, including well developed flow-top breccia, basal peperite, flow-banding, and hyaloclastite dominated breccia.

Mass balance calculations and textural analysis demonstrate that the high-silica aphyric rhyolites were originally glassy having been recrystallized to a pseudo-spherulitic quartz¨Calbite mixture which was subsequently sericitized and chloritized. The REE were moderately mobile (10?0 % ) whereas the high field strength elements were less mobile (<10 % ). The major elements within the flow interior are shown to have been relatively immobile and represent a reasonable estimate of the protolith composition.

The viscosity of the aphyric lava protolith was estimated to be 7.8 ¡Á 10⁶ Pa s based on an eruption temperature of ?50 ¡ãC calculated from measurements using the Ti-in-zircon saturation thermometer. This viscosity estimate is similar to the calculated viscosity (? ¡Á 10⁷ Pa s) of the subaerial N¨¢mshraun rhyolite flow, Iceland. The Prosser and N¨¢mshraun rhyolites have comparable dimensions and eruption temperature. It is concluded that the low aspect ratio of the Prosser Township flows may have been controlled by a number of environmental factors including paleo-slope, magma rheology, eruption rate and the insulative effects of the flow breccia.