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Peralkaline acid liquids: A petrological study

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Abstract

Electron-microprobe analyses of the feldspars and associated ferromagnesian minerals in the peralkaline volcanics, comendites and pantellerites, are presented together with new data on the major and trace-elements of the rocks and residual glasses. The feldspar phenocrysts in the pantellerites span a narrower range (Or33–Or39) than those of the comendites (Or30–Or46); both sets show only limited increase in Or outwards, and the zoning is greatest in quartz-bearing assemblages. The feldspar microlites in the residual glasses are invariably more potassic (2–4% Or) than their associated phenocrysts. In pantellerites the feldspars become more potassic as the residual liquids become more sodic; thus the most potassic feldspar is found in the most sodic (and peralkaline) pantellerite.

Of the ferromagnesian phenocrysts, aenigmatite is the most ubiquitous and is commonly associated with hedenbergite±fayalite, or ferrorichterite; in the later stages of crystallization (groundmass), it is associated with acmite, arfvedsonite and tuhualite. Aside from slight variation in Ti/Fe+Ti ratio, aenigmatite is virtually constant in composition. The pyroxenes from the different assemblages have zones which together almost span the range acmitehedenbergite. Both ferrorichterite and arfvedsonite incorporate F but not Cl, and are slightly potassic. Tuhualite exists as two varieties; one blue and potassic, the other violet and sodic; both varieties reject halogens. Using (estimated) free-energy data, a field in fo2, T space is postulated in which Fe-Ti oxides are absent; their place is taken by pyroxene and aenigmatite. The no-oxide field will be intercepted by a cooling liquid in which peralkalinity is increasing and in which fo2 is near but above the FMQ buffer.

The characteristic pattern of trace-elements in peralkaline volcanics (e.g., high Nb, Ta, Zr, Mo, Zn, Cd, R.E, etc; low Sr, Ba, Mg) are considered to be as much evidence for the peralkaline (salic) condition as of the genetic process. Several lines of evidence suggest that at liquidus temperatures, peralkaline rhyolites are essentially anhydrous.

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  1. Department of Geology and Geophysics, University of California, Berkeley, USA

    J. Nicholls & J. S. E. Carmichael

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  1. J. Nicholls
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Nicholls, J., Carmichael, J.S.E. Peralkaline acid liquids: A petrological study. Contr. Mineral. and Petrol. 20, 268–294 (1969). https://doi.org/10.1007/BF00377480

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