Abstract

In mafic igneous suites typical of anorogenic igneous provinces (mid-ocean ridges, back-arc basins, continental rifts, and "hotspots"), clinopyroxenes of tholeiitic and silica-undersaturated rocks incorporate Al in clinopyroxene chiefly via the charge-coupled substitution viMgivSi2viTiivAl2 as the CaTiAl2O6 molecule. This substitution is promoted by high titania activity relative to silica in the melt, and hence is particularly characteristic of silica-undersaturated mafic rocks. Mafic igneous suites of convergent plate margins (i.e., arc tholeiites and calc-alkalic gabbros and basalts) characteristically incorporate Al in clinopyroxene largely as the CaFe3+AlSiO6 molecule. The viMgivSi ⇔ viFe3+ivAl substitution is promoted by relatively high fugacities of H2O and O2, a characteristic feature of arc-axis magmas. Recognition of cumulates that are complementary to compositions of eruptive rocks in arcs is crucial to development of valid petrogenetic models for arc magmas. Plots of tetrahedrally coordinated Al vs. octahedral Ti in augite of gabbroic and ultramafic cumulates show that augite data arrays for arc-related cumulates have a trend of substantially higher Al/Ti in clinopyroxene than do those from rift-related tholeiites. The Al/Ti ratio in clinopyroxene may be utilized to discriminate ophiolitic from nonophiolitic ultramafic-mafic allochthons in orogenic belts.

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