Development and Degradation of Archean Lavas, Abitibi Area, Canada, in Light of Major Element Geochemistry

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Abstract
The lavas of the Abitibi area, Ontario and Quebec, display evidence of two major periods of metamorphic recrystallization since their eruption 2.75 billion years ago. The first, represented by widespread development of pumpellyite and prehnite, particularly in porous rocks, preserves original textures and, in some coarsely crystalline types, the original mineralogy of the volcanics. The distribution of this assemblage throughout the region investigated, and its appearance as detrital remnants in clasts of subsequent conglomerates, suggest it developed during or soon after eruption and has since been overprinted by higher grade events in many places. Greenschist facies assemblages are dominant in parts of the lavas located at edges of the lava exposures or near large plutonic intrusives developed during the Kenoran orogeny about 2.6 billion years ago. Amphibolites are also present but occupy only a small percentage of the total area. On local scales, in the cm range, permeable rocks underwent metamorphic differentiation such that Na, K, and minor Mg was removed while Ca was introduced into most highly altered types. Nearby rocks were enriched in Na and Mg, but depleted in Ca. FeOt, TiO2, MnO, Al2O3, and SiO2, in addition to less coherent MgO, are reliable guides to original compositions if altered lavas are eliminated by arbitrary but systematic screens.

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