Alkali feldspar granite
Alkali feldspar granites are the alkali feldspar-rich end.member of granites. They are felsic plutonic igneous rocks consisting essentially of quartz (20-60% of the felsic minerals) and alkali feldspar (> 90% of feldspars). Due to the high K-feldspar content, this rock tends to display intense red to pink hues. Plagioclase, when present, occurs in small amounts and with Na-rich composition. The typical color index (M) of an alkali feldspar granite ranges between 0 and 20 %. Biotite and hornblende representing the commonest mafic minerals. If M < 10% the rock can be named alaskite (i.e. a leucocratic variety of alkali feldspar granite). Some alkali feldspar granites may be so rich in alkaline elements (Na and K) to contain Na-amphibole (richterite) and/or Na-pyroxene (aegirine): in this case, they should be classified as peralkaline granites.
Alkali feldspar granite
Plutonic igneous rock
Felsic minerals:
• quarzo
• alkali feldspar
• sodic plagioclasio
Mafic minerals:
• biotite
• orneblenda
QAPF classification:
Q = 20 – 60%
Plagioclase/feldspars = 0 – 10%
Colored varieties:
• alaskite (M < 10%)
• alkali feldspar melagranite (M > 20%)
Other varieties:
peralkaline granite
Extrusive equivalent: alkali feldspar rhyolite
Bibliografia
Barbey, P., Faure, F., Paquette, J. L., Pistre, K., Delangle, C., & Gremilliet, J. P. (2019). Skeletal quartz and dendritic biotite: Witnesses of primary disequilibrium growth textures in an alkali-feldspar granite. Lithos, 348, 105202.
Kwon, S. T., Shin, K. B., Park, H. K., & Mertzman, S. A. (1995). Geochemistry of the Kwanaksan alkali feldspar granite: A-type granite. Jour. Petrol. Soc. Korea, 4(1), 31-48.
Lan, T. G., Fan, H. R., Yang, K. F., Cai, Y. C., Wen, B. J., & Zhang, W. (2015). Geochronology, mineralogy and geochemistry of alkali-feldspar granite and albite granite association from the Changyi area of Jiao-Liao-Ji Belt: Implications for Paleoproterozoic rifting of eastern North China Craton. Precambrian Research, 266, 86-107.
Lee, M. R., Waldron, K. A., & Parsons, I. (1995). Exsolution and alteration microtextures in alkali feldspar phenocrysts from the Shap granite. Mineralogical Magazine, 59(394), 63-78.
Mackenzie, D. E., Black, L. P., & Sun, S. S. (1988). Origin of alkali-feldspar granites: An example from the Poimena Granite, northeastern Tasmania, Australia. Geochimica et Cosmochimica Acta, 52(10), 2507-2524.
See also
Alkali feldspar granite – Alexstrekeisen.it
Peralkaline granite – Alexstreisen.it.
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