Magmatic origins of calc-alkaline intrusions from the Coast Plutonic Complex, southwestern British Columbia

Abstract

Major element, trace element, and rare earth element data are presented for Permian to Tertiary calc-alkaline plutonic and volcanic rocks along a transect across the southern Coast Plutonic Complex from Vancouver to Anderson Lake. Late Jurassic to Late Cretaceous plutons are divided into two compositional suites based on mineralogy: (1) the hornblende intrusive suite (tonalite, quartz diorite, diorite, and gabbro) characterized by abundant modal hornblende and little or no K-feldspar, and (2) the K-feldspar intrusive suite (mainly granite and granodiorite) containing significant modal K-feldspar and less hornblende. Compositions of hornblende intrusive suite rocks are effectively portrayed on Pearce element-ratio diagrams utilizing axes X1 = [0.8571 Si−0.1429(Fe + Mg) + 1.2857 Ca + 1.8574 K]/Zr and Y1 = 1.1428 Ti + Al + Fe + Mg + Ca + 1.5714 Na + 0.4762 P]/Zr, because the diagram accounts for the stoichiometry of PI ± Hbl ± Bt ± Ep ± Ttn + Ap. Rocks from the K-feldspar intrusive suite are studied on diagrams using the element-ratio pair X2 = [2 Ti + Al + 3.3333 P]/Zr and Y2 = [2 Ca + Na + K]/Zr, which creates a linear trend of compositional variations controlled by the phases PI ± Kfs ± Bt ± Ttn + Ap. Mean intercepts of model trends on the element-ratio diagrams suggest differences among plutons that relate to source-region processes. For example, samples belonging to the hornblende intrusive suite represent a minimum of six batches of magma. Mean intercept values for plutons west of the Owl Lake–Harrison fault zone are significantly higher than those situated east of this structural break. These systematic differences allude to fundamental differences in the nature of Mesozoic magmatism in Wrangellia (and Harrison) terrane compared with that in Cadwallader, Bridge River, and Methow terranes, and probably in the Intermontane superterrane east of the structural break.