General term for granitic rocks typically occurring in rift zones and in the interiors of stable continental plates. They are usually mildly alkaline granites with low CaO and Al2O3, high Fe / (Fe+Mg), high K2O / Na2O and K2O values and consist of quartz, K-feldspar, minor plagioclase and Fe-rich biotite, and sometimes alkali amphibole. The prefix A stands for anorogenic. A-type granites are formed above volcanic “hot spot” activity and have peculiar mineralogy and geochemistry. These granites are formed by melting of the lower crust under conditions that are usually extremely dry. The rhyolites of the Yellowstone caldera are examples of volcanic equivalents of A-type granite.
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