A Geochemical Classification for Feldspathic Igneous Rocks.pdf

AGeochemical Classification for Feldspathic Igneous Rocks B. RONALD FROST* AND CAROL D. FROST DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF WYOMING, LARAMIE, WY 82072, USA RECEIVED MAY 13, 2008; ACCEPTED OCTOBER 3, 2008 ADVANCE ACCESS PUBLICATION NOVEMBER 20, 2008 In this paper we classify the range of feldspathic igneous rocks using five geochemical variables: the FeO/(FeOtMgO) ratio or Fe-index, the modified alkali^lime index, the aluminum-saturation index, the alkalinity index, and the feldspathoid silica-saturation index.The Fe-index distinguishes between melts that have undergone extensive iron enrichment during differentiation from those that have not.The transition from tholeiite to ferrobasalt allows us to extend this boundary to silica values as low as 48 wt %.We introduce the feldspathoid silica-saturation index, which, coupled with the alkali- nity index, allows us to extend the geochemical classification to alkaline rocks. We show that most alkaline rocks are ferroan and that this probably reflects extensive fractional crystallization of olivine and pyroxene with minimal participation of Fe^Ti oxides. The expanded classification allows us to illustrate the geochemical and petrogenetic relationship of the plutonic rocks from ferroan gran- ites to nepheline syenites that commonly occur in intracratonic envir- onments. It also allows us to distinguish four families of feldspathic rocks: (1) magnesian rocks, which are exemplified by Caledonian and Cordilleran batholiths and are characterized by differentiation under oxidizing and relatively hydrous conditions; (2) ferroan rocks, which include fayalite granites, alkali granites, and nepheline sye- nites and are characterized by differentiation under reducing and relatively dry conditions; (3) leucogranites, which commonly form by crustal melting; (4) potassic and ultrapotassic rocks, which origi- nate from mantle that has been enriched in K2O. KEY WORDS: granite; rhyolite; geochemistry; classification;