Contrasting Behavior of P, Ti, and Nb in a Differentiated High-Alumina Olivine Tholeiite and a Calc-Alkaline Andesitic Suite

Abstract

Crystallization differentiation in a low-K 2 O, high-Al 2 O 3 olivine tholeiite (Hat Creek, California) yields segregation veins of basaltic andesite composition, and residual, interstitial glasses of dacite and rhyolite composition. P, Ti, and probably Nb, are progressively enriched in segregation veins and residual dacitic glass by crystallization of olivine, plagioclase, augite, and magnetite. P and Ti are depleted in residual rhyolitic glass by crystallization of magnetite, ilmenite, and apatite. By contrast, in a typical orogenic calc-alkaline suite (from Mount Jefferson, Oregon), P, Ti, and Nb are depleted with increasing K and Si. The most likely minerals capable of producing decreasing P, Ti, and Nb with increasing K are apatite, Fe-Ti oxides, amphibole, biotite, and sphene. There is no direct evidence of these minerals in Mount Jefferson basalt and andesite but they are present in calc-alkaline gabbro and tonalite and may occur at a shallow (crustal) depth beneath Mount Jefferson. Alternatively, mixing of dacite-rhyolite with basalt-andesite may account for the Mount Jefferson trend. By physical and chemical analogy with segregation veins and residual interstitial glasses, basaltic andesite and dacite-rhyolite magmas may segregate from complementary rock at depth, and mix during ascent to the surface.