This geochemical classification of granitic rocks is based upon three INTRODUCTION variables. These are FeO/(FeO + MgO) = Fe-number [or Although granitoids are the most abundant rock types FeO/(FeO + MgO) = Fe∗], the modified alkali–lime index in the continental crust, no single classification scheme (MALI) (Na2O + K2O – CaO) and the aluminum saturation has achieved widespread use. Part of the problem in index (ASI) [Al/(Ca – 1·67P + Na + K)]. The Fe-number granite classification is that the same mineral assemblage, (or Fe∗) distinguishes ferroan granitoids, which manifest strong iron quartz and feldspars with a variety of ferromagnesian enrichment, from magnesian granitoids, which do not. The ferroan minerals, can be achieved by a number of processes. and magnesian granitoids can further be classified into alkalic, Granitoids can form from differentiation of any hyalkali–calcic, calc-alkalic, and calcic on the basis of the MALI persthene-normative melt and from partial melting of and subdivided on the basis of the ASI into peraluminous, metamany rock types. Furthermore, granitic melts may be luminous or peralkaline. Because alkalic rocks are not likely to be derived solely from crustal components, may form from peraluminous and calcic and calc-alkalic rocks are not likely to be evolved mantle-derived melts, or may be a mixture peralkaline, this classification leads to 16 possible groups of granitic of crustal and mantle-derived melts. Because of this rocks. In this classification most Cordilleran granitoids are magnesian complexity, petrologists have relied upon geochemical and calc-alkalic or calcic; both metaluminous and peraluminous classifications to distinguish between various types of types are present. A-type granitoids are ferroan alkali–calcic, although granitoids. Approximately 20 different schemes have evolved over the past 30 years [see Barbarin (1990, 1999) some are ferroan alkalic. Most are metaluminous although some are for a summary thereof]. Most of these schemes are either peraluminous. Caledonian post-orogenic granites are predominantly genetic or tectonic in nature. This paper is an attempt magnesian alkali–calcic. Those with <70 wt % SiO2 are domto present a non-genetic, non-tectonic geochemical clasinantly metaluminous, whereas more silica-rich varieties are comsification scheme that incorporates the best qualities of monly peraluminous. Peraluminous leucogranites may be either the previous schemes, and to explain the petrologic magnesian or ferroan and have a MALI that ranges from calcic to processes that makes this scheme work. alkalic.

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A geochemical classification for granitic rocks

Assembly, configuration, and break-up history of Rodinia: A synthesis

Review of global 2.1-1.8 Ga orogens: implications for a pre-Rodinia supercontinent

/pdf/a-paleo-mesoproterozoic-supercontinent-assembly-growth-and-2zk1cgkgki.pdf

A Paleo-Mesoproterozoic supercontinent: assembly, growth and breakup

A-type granites and related rocks: Evolution of a concept, problems and prospects

We recognize eight types of ferroan granitoid that can be distinguished on the basis of major element chemistry.These include alkalic granitoids that may be metaluminous or peralkaline, alkali-calcic granitoids that may be metaluminous, peraluminous or peralkaline, calc-alkalic granitoids that may be metaluminous or peraluminous, and rare calcic ferroan granitoids. These granitoids may form by two distinct end-member processes. Extreme differentiation of basaltic melts results in ferroan granitoids that are either peralkaline alkalic and alkali-calcic, or metaluminous alkalic, alkali-calcic, and calc-alkalic, with alkalinity increasing with increasing pressure of differentiation. Partial melting of tonalitic to granodioritic crust produces alkali-calcic to calc-alkalic granitoids that are metaluminous at low pressures and peraluminous at high pressures. It is likely that a combination of these two processes plays some role in the formation of most ferroan granitoids. Most granitoids that are referred to as ‘ ferroan. However, the term ‘ has become more confusing than edifying because it has been applied to a broad spectrum of granitoid compositions with varying petrogenesis. For this reason we recommend that the term ‘ be discontinued and ‘ferroan’ used in its stead.

http://geofaculty.uwyo.edu/cfrost/pdfs/frost & frost (2011).pdf

On Ferroan (A-type) Granitoids: their Compositional Variability and Modes of Origin

Reduced rapakivi-type granites are the most iron enriched and reduced (i.e., least oxidized) of the “anorogenic” granite association. The low oxygen fugacity and chemical composition of these granites severely limit their sources. In this paper we argue that reduced rapakivi-type granites and their eruptive equivalents, high-potassium fayalite rhyolites, are derived from mafic sources, because tholeiitic magmas and their derivatives have the required low oxygen fugacity. Reduced, rapakivi-type granites are produced either by extreme differentiation of basaltic melts or by partial melting of underplated basalts and their differentiated equivalents. They form in extensional environments where the asthenosphere is present at shallow depths. We envision three stages in the origin of these rocks: (1) tholeiitic melts are emplaced at the base of the crust, (2) continued introduction of heat partially remelts these tholeiitic rocks, and (3) the hot, dry melts so produced migrate into the middle crust to produce rapakivi batholiths or erupt as rhyolites. Partial melting of felsic continental crust may accompany the intrusion of rapakivi-type magmas, thereby producing the other metaluminous and peraluminous granite compositions of the anorogenic suite.

Reduced rapakivi-type granites: The tholeiite connection

Sphene (titanite): phase relations and role as a geochronometer

In this paper we classify the range of feldspathic igneous rocks using five geochemical variables: the FeO/(FeO þMgO) 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 alkalinity 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 granites to nepheline syenites that commonly occur in intracratonic environments. 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 syenites 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 originate from mantle that has been enriched in K2O.

/pdf/a-geochemical-classification-for-feldspathic-igneous-rocks-2yih9ah1va.pdf

B. Ronald Frost

Papers

A geochemical classification for granitic rocks

On Ferroan (A-type) Granitoids: their Compositional Variability and Modes of Origin

Reduced rapakivi-type granites: The tholeiite connection

Sphene (titanite): phase relations and role as a geochronometer

A Geochemical Classification for Feldspathic Igneous Rocks