K. L. Currie

A-type granites: geochemical characteristics, discrimination and petrogenesis

New analyses of 131 samples of A-type (alkaline or anorogenic) granites substantiate previously recognized chemical features, namely high SiO2, Na2O+K2O, Fe/Mg, Ga/Al, Zr, Nb, Ga, Y and Ce, and low CaO and Sr. Good discrimination can be obtained between A-type granites and most orogenic granites (M-, I and S-types) on plots employing Ga/Al, various major element ratios and Y, Ce, Nb and Zr. These discrimination diagrams are thought to be relatively insensitive to moderate degrees of alteration. A-type granites generally do not exhibit evidence of being strongly differentiated, and within individual suites can show a transition from strongly alkaline varieties toward subalkaline compositions. Highly fractionated, felsic I- and S-type granites can have Ga/Al ratios and some major and trace element values which overlap those of typical A-type granites.A-type granites probably result mainly from partial melting of F and/or Cl enriched dry, granulitic residue remaining in the lower crust after extraction of an orogenic granite. Such melts are only moderately and locally modified by metasomatism or crystal fractionation. A-type melts occurred world-wide throughout geological time in a variety of tectonic settings and do not necessarily indicate an anorogenic or rifting environment.

The reaction 3 cordierite = 2 garnet + 4 sillimanite + 5 quartz as a geological thermometer in the Opinicon Lake region, Ontario

In equilibrated metamorphic rocks containing coexisting garnet, cordierite, quartz and sillimanite, the exchange of iron and magnesium between cordierite and garnet offers a highly favourable geological thermometer and barometer, because this exchange reaction is insensitive to pressure. Thermodynamic analysis shows that this thermometer may be calibrated from knowledge of the breakdown reactions for iron and magnesian cordierite end members to garnet. The thermometer was experimentally calibrated using cordierites of intermediate composition. When applied to rocks showing petrographic evidence of equilibrium, and chemical evidence of reaction between garnet and cordierite, the thermometer yielded temperatures of 600–750:C, and pressures of 5.7–6.7 kilobars. Similar conditions are indicated by other literature data on cordierite-garnet gneisses, and are believed to represent hornblende granulite grade of metamorphism.

The Topsails igneous terrane, Western Newfoundland: evidence for magma mixing

The Topsails igneous terrane of Western Newfoundland contains a diverse suite of igneous rocks, but consists mainly of Silurian alkaline to peralkaline granites and rhyolites. The terrane exhibits evidence for the coexistence of mafic and salic magmas in the form of composite dykes and flows, sinuous, boudined mafic dykes cutting granites and net vein complexes. Field data and major and trace element chemical data suggest that these magmas mixed to produce limited volumes of more or less homogeneous hydrids.Magma mixing, a process which has received recent prominence in petrogenetic models for calc-alkaline volcanic suites, has elicited less attention than restite separation and fractional crystallization as a cause of chemical dispersion in granites. Evidence from the Topsails igneous terrane suggests the possible importance of magma mixing to granite petrogenesis and a major role for transcurrent faulting in the origin and evolution of peralkaline magmas.

The petrology of the Mont Saint Hilaire complex, southern Quebec: An alkaline gabbro-peralkaline syenite association

Abstract The Mont Saint Hilaire complex consists of an older (133 Ma) suite of layered cumulates of titanaugite, kaersutite, plagioclase and titaniferous magnetite, and two younger (122 Ma) suites, one a thick ring dyke of nepheline-olivine diorite to monzonite, and the other a pipe or funnel-like mass of peralkaline nepheline-sodalite syenite and porphyry associated with a variety of breccias. Trace element data suggest derivation of the older suite from a garnet-bearing source. Only minor amounts of possible liquid compositions are preserved in this suite. The nepheline and olivine-bearing suite followed a course of fractionation from gabbroic to monzonitic compositions involving fractionation of pyroxene, magnetite, apatite and plagioclase. Field and trace element data suggest mixing of the evolved liquid with a saline brine at crustal depths produced the strongly nepheline-normative peralkaline magma. Rich in Na and Cl, the brine was poor in other major and trace elements, and had a high initial Sr ratio. The localization and extended time of emplacement of the complex appear to be due to upward migration of a thermal anomaly from the base of a lithosperic plate.

A note on the calibration of the garnet-cordierite geothermometer and geobarometer

Experimental and theoretical considerations indicate that the distribution coefficient for iron and magnesium between coexisting garnet and cordierite increases with temperature in the assemblage cordierite-garnet-sillimanite-quartz. This conclusion is confirmed by distribution coefficients from natural garnet and cordierite from geologically well defined settings. The only published calibration which incorporates this feature is that of Currie (1971), and this is the only calibration which can be qualitatively correct although it may be wrong in detail. Other calibrations encounter catastrophes, particularly in andalusite-bearing assemblages.

Geochemical evolution of peraluminous plutons in southern Nova Scotia, Canada—a pegmatite-poor suite

Three southern Nova Scotia plutons crystallized rapidly at ∼375 Ma from magma containing both mantle and crustal components. Isotopic and chemical data suggest that the crustal contribution included both lower crustal material and Cambro-Ordovician turbidites of the host Meguma Group. Despite local evidence of mixing and mingling of magmas, the bulk of the plutons evolved by assimilation and fractional crystallisation. Evolved portions of the plutons have compositions appropriate for development of rare-metal pegmatite fields, but pegmatites are relatively rare and little differentiated. Like parental plutons, pegmatites fall into biotite+plagioclase and muscovite+potassium feldspar assemblages. The latter locally contain Mn-rich garnet+biotite, giving calculated P–T conditions of pegmatite crystallisation of ∼620°C, 0.44 GPa under water-saturated conditions. Host rocks at the time of emplacement experienced P–T conditions varying from <500° to ∼620°C at 0.44 GPa. Beryl-bearing pegmatites occur only where host rocks were below sillimanite-grade, and pegmatites of any kind are scarce in sillimanite-grade host rocks. Rarity of pegmatites can be ascribed to a combination of insufficient amounts of F, Li, and B in the magma with rapid cooling which prevented extensive fractionation and undercooling. Our data suggest rather that peraluminous suites, produced by rapid heating due to incursions of lithospheric mantle into supracrustal rocks, and typified by biotite-rich, cordierite-bearing plutons, are not favorable locales for major rare-metal-enriched pegmatite fields. Such fields appear to require relatively prolonged anatexis resulting from crustal thickening, or an areally extensive mafic underplate,