Petrochemical investigation of the Antique ophiolite (Philippines): Implications on volcanogenic massive sulfide and podiform chromitite deposits

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.