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 TiO 2 contents of chromites are strongly correlated with TiO 2 and other incompatible element abundances in basalts. Large differences in TiO 2 (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 TiO 2 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.

Abstract The Zambales Ophiolite in western Luzon is a large fragment of oceanic crust that was uplifted several kilometers without being obducted onto a continental margin. Deposition of pelagic limestone on the ophiolite during the late Eocene through Oligocene gave way to deposition of ophiolite-derived clastics during the early Miocene. The uplift of the western edge of the ophiolite probably was related to the initiation of subduction along the Manila Trench in the late Oligocene, but the Zambales crust predates the oldest crust in the adjacent South China Basin by about 8 million years. A new sandstone petrology method traces the uplift and erosional history of the ophiolite through the changing compositions of the ophiolite-derived clastics.