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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