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In this paper we classify the range of feldspathic igneous rocks using five geochemical variables: the FeO/(FeO + MgO) 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. [ABSTRACT FROM PUBLISHER]

The 2.95–2.82 Ga quartzofeldspathic gneisses and granitoids in the Bighorn, western Owl Creek, and northeastern Wind River uplifts in the central Wyoming Province include low-K tonalite–trondhjemite–granodiorite (TTG) and high-K granodiorite–granite (GG) rocks. Both types of granitoids were intruded contemporaneously, although TTGs are more abundant in the older gneisses. The TTG suite consists of calcic to marginally calc-alkalic rocks that straddle the boundaries between metaluminous and peraluminous and between ferroan and magnesian compositions. Rare-earth element (REE) patterns of these rocks may be highly fractionated with low heavy rare-earth element (HREE) contents and modest to absent Eu anomalies but may also be less strongly HREE depleted. These rocks do not represent first-generation continental crust: most have unradiogenic Nd and radiogenic 207Pb/204Pb isotopic compositions that require the incorporation of isotopically evolved sources. The GG suite has compositions that are transitional between Archean TTG and modern, continental margin calc-alkalic rocks. The GG suite is characterized by higher alkali contents relative to CaO than the TTG suite and higher K/Na ratios but exhibits a similar range in REE patterns. The Nd, Sr, and Pb isotopic compositions of the GG suite are slightly less variable but lie within the range of those of the TTG suite. We interpret them as having a source similar to that of the TTG, perhaps forming by partial melting of preexisting TTG. The shift from TTG-dominated to GG-dominated continental crust was a gradual transition that took place over several hundred million years. Clearly subduction-related calc-alkalic magmatism is not recognized in the Wyoming Province prior to 2.67 Ga. [ABSTRACT FROM AUTHOR]

A composite intrusive igneous complex in the central mountain range of Queen Maud Land (Thor Range), Antarctica, displays characteristic features of anorogenic granites. A suite of massive intrusives and various sets of dykes and satellite intrusions are ferroan, alkalic to alkali–calcic, and weakly peraluminous. An early set of plutons consists of charnockitic alkali-granites; a later group of plutons comprises fayalite Qtz-syenites. Coarse mesoperthite is the dominant mineral in all rocks, quartz is abundant and plagioclase is a minor mineral. Olivine (fayalite) is the characteristic mafic mineral, but subcalcic augite and occasionally pigeonite or orthopyroxene are present. In most samples, amphibole is the dominant mafic mineral and its composition is close to end-member hastingsite. It contains high concentrations of F and Cl. Some samples contain igneous fluorite. Thermobarometry suggests a temperature of 900 ± 25°C and a pressure of 0·4 ± 0·1 GPa for the crystallization conditions of the pyroxene–olivine assemblages. The solidus temperature of 800–850°C for both suites of plutonic rocks is typical of water-deficient granitic melts. The estimated low water activity of 0·3–0·5 at solidus conditions is consistent with the high halogen content of the bulk-rocks and their constituent minerals. In the absence of an aqueous fluid, the halogens remained in the minerals at the solidus. Oxygen fugacity stayed below QFM in all igneous rocks above solidus. This is typical of melts derived from partial melting of mafic source rocks. The igneous rocks were locally affected by at least three distinct episodes of hydration. As the melt approached solidus conditions, fayalite and pyroxene were locally transformed into hastingsite as a result of increasing fugacity of volatile components. Fayalite-free and fayalite-bearing igneous rocks are arranged in banded structures. Subsolidus hydration locally modified the igneous rocks and transformed pyroxene- and fayalite-bearing granites into biotite-granites and hornblende-granites in which all evidence of former high-T history was erased. This local hydration of igneous rocks occurred in response to uptake of H2O that had been given off by gneissic xenoliths as a result of progressing, continuous, dehydration reactions. The reactions in the gneiss xenoliths were driven by contact metamorphism. This exchange of H2O between igneous and metamorphic rocks occurred in a fluid-absent regime at temperatures of about 750°C. Late reaction veins formed by hydraulic fracturing of the plutonic rocks and indicate the presence of a low-density fluid phase at amphibolite facies conditions. [ABSTRACT FROM AUTHOR]

This geochemical classification of granitic rocks is based upon three variables. These are FeO/(FeO + MgO) = Fe-number [or FeOtot/(FeOtot + MgO) = Fe*], the modified alkali–lime index (MALI) (Na2O + K2O – CaO) and the aluminum saturation index (ASI) [Al/(Ca – 1·67P + Na + K)]. The Fe-number (or Fe*) distinguishes ferroan granitoids, which manifest strong iron enrichment, from magnesian granitoids, which do not. The ferroan and magnesian granitoids can further be classified into alkalic, alkali–calcic, calc-alkalic, and calcic on the basis of the MALI and subdivided on the basis of the ASI into peraluminous, metaluminous or peralkaline. Because alkalic rocks are not likely to be peraluminous and calcic and calc-alkalic rocks are not likely to be peralkaline, this classification leads to 16 possible groups of granitic rocks. In this classification most Cordilleran granitoids are magnesian and calc-alkalic or calcic; both metaluminous and peraluminous types are present. A-type granitoids are ferroan alkali–calcic, although some are ferroan alkalic. Most are metaluminous although some are peraluminous. Caledonian post-orogenic granites are predominantly magnesian alkali–calcic. Those with <70 wt % SiO2 are dominantly metaluminous, whereas more silica-rich varieties are commonly peraluminous. Peraluminous leucogranites may be either magnesian or ferroan and have a MALI that ranges from calcic to alkalic. [ABSTRACT FROM PUBLISHER]

Abstract: Mesoproterozoic tholeiitic and alkalic magmatism is widespread in the southern segment of the Eastern Ghats Belt, India, and is particularly well recorded in three cospatial plutons: Chimakurti, a pluton consisting of clinopyroxenite, anorthosite, and gabbro; Errakonda, a ferrosyenite pluton; and the Uppalapadu nepheline syenite. U–Pb zircon ages of ferrosyenite and nepheline syenite suggest that these cospatial plutons were intruded at approximately 1352 Ma, and Nd and Sr isotope compositions suggest that each pluton experienced varying degrees of crustal assimilation. Petrography, mineral chemistry, geochemistry and isotopic compositions suggest that the clinopyroxenite, anorthosite, and gabbro of the Chimakurti suite were derived from a high-Al tholeiitic parental magma. The ferrosyenite exhibits A-type characteristics and are interpreted as a differentiate or partial melt of a tholeiitic source. The Uppalapadu nepheline syenites were derived from an alkaline (basanitic) parental magma. These three plutons illustrate the spectrum of basaltic magmas that may be involved in producing intraplate intrusive complexes. Tholeiitic magmas emplaced at depth in the crust may differentiate to produce massif anorthosites and related A-type differentiates, represented in the Eastern Ghats Belt by the Chimakurti and Errakonda plutons. More alkalic basalts, such as basanites, will evolve to nepheline syenites like those of the Uppalapadu pluton. The 1352 Ma magmatism in the Eastern Ghats is unusual in that it documents a spatial and temporal association of both tholeiitic and alkaline endmembers of continental rift magmatism. [Copyright &y& Elsevier]