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This paper examines the compositions of early cumulus minerals, particularly chromite, from two different sections (Acoje and Coto) in the Zambales Range ophiolite, Luzon, Philippines, and then infers compositional characteristics of the magmas from which they were derived.

The cumulus chromite compositions are compared with compositions of phenocrysts that occur in different types of basaltic glasses. Alkali and transitional alkalic basalts, midocean-ridge basalts, and primitive island arc basalts (boninites) are considered. Chromite, one of the earliest minerals to crystallize from basaltic magma and common in the basalt cumulates of ophiolites, is found to be the most useful mineral to compare cumulate occurrences with phenocryst occurrences. Data from basalt-chromite pairs suggest that the TiO2 contents of chromites are strongly correlated with TiO2 and other incompatible element abundances in basalts.

Large differences in TiO2 (0.05 to 1.2 percent) occur in the basal cumulate chromites from the different sections in the Zambales ophiolite. Ti-rich chromites occur at the base of the Coto section. Their compositions are similar to compositions of chromite phenocrysts in alkalic and transitional alkalic basalts. These data suggest that the early magma from which the Coto cumulates were derived had alkalic characteristics. In contrast, chromites from the Acoje section of the Zambales ophiolite are depleted in TiO2 and appear to have crystallized from a magma depleted in magmaphilic elements. Clinopyroxene compositions from the respective sections can be similarly interpreted. Thus, compositional variability of oceanic basalts might be identified in ophiolite sections by careful examination of cumulate mineral compositions and sequences.

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