Granitophile Elements Research Articles

On discerning magmatic populations in the batholith west of Okanagan Lake, British Columbia

Seventy-five rock samples taken from seven intrusive rock units, including associated mafic inclusions and adjacent country rock, were analyzed for Ca, Σ Fe, Mg, K, Rb, Sr, Ba, Zr, Y and V by X-ray fluorescence methods. Trace-element abundances and ratios when plotted on cumulative-probability paper are generally bimodal lognormal distributions indicating that the composite intrusions consist of two distinct magmatic populations. These are represented by an early dioritic intrusive series (units 3, 4 and 5) and a later granodiorite series, (units 6, 7, 8 and 9). Log-concentration diagrams indicate that the granodiorite series has undergone considerable chemical fractionation and that the two magma series have fundamentally different petrochemical parameters. A priori contrasts of average rock-unit compositions, by analysis of variance, indicate that units 3 and 5; 6 and 9; 7 and 8 are compositionally similar but that units 5 and 6 are significantly different. Trace-element fractionation trends were controlled by crystal—melt equilibria involving plagioclase, hornblende and biotite. Ca and Sr concentrations were controlled by volving plagioclase, hornblende and biotite. Ca and Sr concentrations were controlled by temperature-sensitive plagioclase—melt equilibria, whereas K, Rb and Ba were controlled by biotite, apart from the influence of hornblende on these elements. The trace-element transition between the diorites and granodiorites can be attributed to differing source rock chemistry, or to instability of plagioclase over the melting interval of a common source rock, but a crystal fractionation control is unlikely. Trace-element data are consistent with the hypothesis that batholithic magmas were formed by equilibrium partial melting of basic source rocks at crustal depths. Granodiorites and diorites may represent partial melts derived from basic igneous rocks by about 10–30 and 30–60% percent melting, respectively. Under hydrous conditions the composition of primary melts would be controlled by plagioclase, hornblende and pyroxene multivariant equilibria, at crustal pressures. The granodioritic magmas may have undergone subsequent crystal fractionation by separation of hornblende and oligoclase. Granodiorite magmas from the West Okanagan, with initial isotopic Sr ratios of 0.704, are more fractionated in granitophile elements than those farther to the west. Mafic inclusions represent incorporated Nicola volcanic rocks that appear to have undergone Rb and Fe metasomatism.

Chemical evolution of high-grade metamorphic rocks — Anatexis and remotion of material from granulite terrains

The geochemistry of acid, sub-acid metamorphic rocks of sedimentary origin from a sequence of increasing metamorphism of the Western Italian Alps has been studied. Analysis of bulk chemistries and of pairs of geochemically coherent elements (K—Rb, K—Pb, Na—Sr, Fe—Zn, etc.) indicates that metamorphism up to amphibolite facies is essentially an isochemical process even if anatexis is active, the products of initial melting recrystallizing together with unmelted solids. Rocks of granitic composition have been strongly depleted during granulite facies metamorphism in lithophile trace elements and probably in silicate fractions rich in alkalis. Such depletions have been correlated to extensive melting of pelitic rocks under water pressure condition determined by the progressive release of H 2O from hydrated phases involved in the melting. According to fractional melting models, differentiated low-viscosity fractions of silicate melts rich in water and granitophile elements have been removed and diffused outwards under a pressure gradient established by the increase in total volume of the rocks undergoing extensive anatexis. Most of the melt which has not migrated, because of the continuous loss of water and minimum-granite fractions, enhances progressively solidus conditions and recrystallizes within the unmelted solids giving granulitic parageneses. The intimate association between melted and unmelted materials causes granulites to present small-scale elemental and isotope disequilibria. Extensive melting may deeply modify the primary distribution of elements, favouring the attainment in granulites of magmatic-like trends.

Geochemical features of a strongly fractionated alkali igneous suite

Geochemical data for 37 igneous rocks from the Paresis volcanic complex, South West Africa, are presented and discussed. Unusual abundances of certain minor constituents and variation trends of element-ratios support the view that the central rhyolites and syenitic dykes are products of a strongly differentiated magma—probably originating by palingenesis of the basement granite. Granitophile elements are strongly enriched in the rhyolite sequence but their variation in the holocrystalline dykes is more subdued. Differences in trace element distribution between rhyolites and syenitic dykes are attributed largely to the relative prominence of the forces governing diadochy. The abundances and distribution of V, Cr, Ni, Co, Sr, Ba, Ga, Zr, Pb, La, Li, Rb. and Cs are discussed in terms of their relationships to the major rock-forming constituents.