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International Research exhibits an island arc signature whereas the Coto Block is of transitional mid-oceanic ridge basalt to island arc signature. The difference between these two blocks is further highlighted by the chromite types they host. The Acoje Block contains metallurgical chromitites, in contrast to that of Coto Block’s refractory type. Because of these differences, the ZOC has been modeled as an arcback-arc pair that amalgamated to the Philippine archipelago prior to the rifting of South China Sea Basin (Hawkins and Evans, 1983; Yumul et al., 1998). The Middle Miocene Cabaluan Formation is widely exposed along the Cabaluan River and the mountainous terrain along the road leading to the Acoje Mines. It is mainly composed of thick to very thick conglomerate beds with peridotite and chert clasts derived from the underlying Acoje Block of the ZOC (Figure 2). Broken shell and coral fragments sporadically occur as detrital fragments within these deposits, while huge chert blocks interpreted as olistoliths also occur within the conglomerates. These cherts are believed to have been derived from the sedimentary carapace of the Acoje Block. Figure 2. Photo of overlying sedimentary rocks. In Tocon Sta. Cruz, Zambales, the contact of the west-dipping Cabaluan Formation with the Acoje Block of the Zambales Ophiolite Complex is observed. The Candelaria Limestone unconformably overlies the Cabaluan Formation and is composed of thickly bedded, buff-colored karstic and reefal limestones. This unit distinctly juts out of the terrain as it forms a north-south trending ridge to the west of the Cabaluan Formation and the Acoje Block. Figure 1. Geological map of the northern portion of western Zambales Range. The Zambales Ophiolite Complex is overlain by three sedimentary formations: Cabaluan Formation, Candelaria Limestone and Sta. Cruz Formation. The overlying sedimentary rocks Recent investigations were done on the sedimentary rocks that overlie the ZOC. Three sedimentary formations were recognized: Cabaluan Formation, Candelaria Limestone and Sta. Cruz Formation. Both the Cabaluan Formation and the Candelaria Limestone occur to the west of and geographically proximal to the Acoje Block. In contrast, the Sta. Cruz Formation exposes along the west of all the older formations, including both the Acoje and the Coto Blocks. The younger middle Late Miocene Sta. Cruz Formation was observed unconformable to the Candelaria Limestone and is also assumed to unconformably overlie all the other formations. It is composed of sandstone and mudstone interbeds that exhibit compositions: tuffaceous in some layers and calcareous in others. The sandstones contain broken shell fragments, detrital quartz and significantly fewer peridotite fragments as compared to the Cabaluan Formation. Two distinct ophiolites of the Zambales Ophiolite Complex Examination of the chert blocks contained within the Cabaluan Formation, presumably derived from the deep marine sedimentary carapace of the Acoje Block, suggests a Middle Jurassic to Early Cretaceous age (Figure 3); this age is significantly older than what was previously reported for the ZOC (i.e. Eocene). With contrasting geochemical signatures of the Acoje and Coto Blocks and different INTERRIDGE NEWS 28 VOL.21, 2012
International Research Figure 3. Photomicrographs of some radiolarians extracted from chert olistoliths of the Cabaluan Formation. Scale bar = 100µm. INTERRIDGE NEWS 29 VOL.21, 2012
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