Anaphanitic (fine-grained) igneous rock is classified as trachybasalt when it has a silica content of about 49% and a total alkali metal oxide content of about 6%. This places trachybasalt in the S1 field of theTAS diagram. Trachybasalt is further divided into sodium-richhawaiite and potassium-richpotassic trachybasalt, withwt%Na2O >K2O + 2 for hawaiite.[3][4][5] The intrusive equivalent of trachybasalt is monzonite.[6]
Trachybasalt is not defined on theQAPF diagram, which classifies crystalline igneous rock by its relative content offeldspars andquartz.[3][4][5] However, theU.S. Geological Survey defines trachybasalt as amafic volcanic rock (composed of over 35% mafic minerals) in which thequartz-feldspar-feldspathoid fraction of the rock is less than 20% quartz and less than 10% feldspathoid, and in whichplagioclase is between 65% and 90% of the total feldspar content.[7]
^abPhilpotts, Anthony R.; Ague, Jay J. (2009).Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 139–143.ISBN9780521880060.
^Allaby, Michael (2013). "trachybasalt".A dictionary of geology and earth sciences (Fourth ed.). Oxford: Oxford University Press.ISBN9780199653065.
^Orlando, Andrea D'Orazio; Armienti, Pietro; Borrini, Daniele (29 August 2008). "Experimental determination of plagioclase and clinopyroxene crystal growth rates in an anhydrous trachybasalt from Mt Etna (Italy)".European Journal of Mineralogy.20 (4):653–664.Bibcode:2008EJMin..20..653O.doi:10.1127/0935-1221/2008/0020-1841.
^Goff, Fraser; Kelley, Shari A.; Goff, Cathy J.; McCraw, David J.; Osburn, G. Robert; Lawrence, John R.; Drakos, Paul G.; Skotnicki, Steven J. (2019). "Geologic map of the Mount Taylor volcano area, New Mexico".New Mexico Bureau of Geology and Mineral Resources Geologic Map.80.
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