Ra. Tamayo et al., Petrochemical investigation of the Antique ophiolite (Philippines): Implications on volcanogenic massive sulfide and podiform chromitite deposits, RESOUR GEOL, 51(2), 2001, pp. 145-164
The Antique ophiolite, located in Panay island (west-central Philippines),
corresponds to several tectonic slices within the suture zone between the P
hilippine Mobile Belt (PMB) and the North Palau an Block (NPB). It includes
dismembered fragments of a basaltic sequence, dominantly pillow-lavas with
minor sheet flows, rare exposures of sheeted dikes, isotropic gabbros, sub
ordinate layered mafic and ultramafic rock sequences and serpentinites. Mos
t of the ophiolite units commonly occur as clasts and blocks within the ser
pentinites, which intrude the whole ophiolitic body, as well as, the basal
conglomerate of the overlying Middle Miocene sedimentary formation.
The volcanic rock sequence is characterized by chemical compositions rangin
g from transitional (T)-MORB, normal (N)-MORB and to chemistry intermediate
between those of MORB and island are basalt (IAB). The residual upper mant
le sequence is harzburgitic and generally more depleted than the upper mant
le underlying modern mid-oceanic ridges. Calculations using whole-rock and
mineral compositions show that they can represent the residue of a fertile
mantle source, which have undergone degrees of partial melting ranging from
9-22.5 %. Some of the mantle samples display chondrite-normalized REE and
extended multi-element patterns suggesting enrichments in LREE, Rb, Sr and
Zr, which are comparable to those found in fore-are peridotites from the Iz
u-Bonin-Mariana (IBM) are system. The Antique ultramafie rocks also record
relatively oxidizing mantle conditions (Delta log f(O2), (FMQ) = 0.9-3.5).
As a whole, the ophiolite probably represents an agglomeration of oceanic r
idge and fore-are crust Fragments, which were juxtaposed during the Miocene
collision of the PMB and the NPB. The intrusion of the serpentinites might
be either coeval or subsequent to the accretion of the oceanic crust onto
the fore-are. V
Volcanogenic massive sulfide (VMS) deposits occur either in or near the con
tact between the pillow basalts and the overlying sediments or interbedded
with the sediments. The morphology of the deposits, type of metals, ore tex
ture and the nature of the host rocks suggest that the formation of the VMS
bodies was similar to the accumulation of metals around and in the subsurf
ace of hydrothermal vents observed in modern mid-oceanic ridge and back-are
basin rift settings. The podiform chromitites occur as pods and subordinat
e layers within totally serpentinized dunite in the residual upper mantle s
equence. No large coherent chromitite deposit was found since the host duni
tic rocks often occur as blocks within the serpentinites. It is difficult t
o evaluate the original geodynamic setting of the mineralized bodies since
the chemistry of the host rocks were considerably modified by alteration du
ring their tectonic emplacement. A preliminary conclusion for Antique is th
at the VMS is apparently associated with a primitive tholeiitic intermediat
e MORB-IAB volcanic suite, the chemistry of which is close to the calculate
d composition of the liquid that coexisted with the podiform chromitites.