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

Abstract

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.