The Coto Block is one of two major units of the Zambales Ophiolite. This consists, from bottom to top, of metamorphic harzburgite, dunite, troctolite, allivalite, olivine gabbro and a high level plutonic-volcanic suite of diorite, diabase and basalt. Massive to banded harzburgites exhibit protogranular to porphyroclastic textures. They are mostly serpentinized and consist basically of olivine, orthopyroxene, spinel and minor clinopyroxene. The harzburgites are separated from the cumulate rock suite by a transitional zone of intensely fractured black serpentinized dunite which contain chromite lenses in places. The contact between the transition zone dunite and cumulate gabbro is characterized by interlayering-interfingering of dunite, harzburgite and gabbro The mafic cumulates are represented by anorthosite, troctolite, and olivine gabbro. These exhibit rhythmic layering and structures such as scour and fill, graded bedding and flame structures reminiscent of soft sediments. Intrusive relationships among the higher level units - basalt, diabase and gabbro - suggest that these rocks were more or less contemporaneous. Dike boundaries are usually defined by chilling on both sides. The basalt and diabase, which show evidence of low grade greenschist facies metamorphism, have aphyric to porphyritic and intersertal to intergranular textures. Clinopyroxene is dominant over plagioclase and olivine. Disseminations of magnetite, ulvospinel and pyrite are common. The diorites/tonalites are holocrystalline to poikilitic, and are composed of plagioclase, brown amphibole, quartz and minor clinopyroxene and magnetite. Epidote and chlorite are the dominant alteration minerals. (see Zambales Ophiolite)

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