Rodolfo A. Tamayo

Subduction-related magmatic imprint of most Philippine ophiolites: implications on the early geodynamic evolution of the Philippine archipelago

The basement complexes of the Philippine archipelago include at least 20 ophiolites and ophiolitic complexes. These complexes are characterised by volcanic sequences displaying geochemical compositions similar to those observed in MORB, transitional MORB-island arc tholeiites and arc volcanic rocks originating from modern Pacific-type oceans, back-arc basins and island arcs. Ocean island basalt-like rocks are rarely encountered in the volcanic sequences. The gabbros from the ophiolites contain clinopyroxenes and plagioclases showing a wide range of XMg and An values, respectively. Some of these gabbros exhibit mineral chemistries suggesting their derivation from basaltic liquids formed from mantle sources that underwent either high degrees of partial melting or several partial melting episodes. Moreover, some of the gabbros display a crystallization sequence where orthopyroxene and clinopyroxene appeared before plagioclase. The major element compositions of coexisting orthopyroxenes and olivines from the mantle peridotites are consistent with low to high degrees of partial melting. Accessory spinels in these peridotites display a wide range of XCr values as well with some of them above the empirical upper limit of 0.6 often observed in most modern mid-oceanic ridge (MOR) mantle rocks. Co-existing olivines and spinels from the peridotites also exhibit compositions suggesting that they lastly equilibrated under oxidizing mantle conditions. The juxtaposition of volcanic rocks showing affinities with modern MOR and island arc environments suggests that most of the volcanic sequences in Philippine ophiolites formed in subduction-related geodynamic settings. Similarly, their associated gabbros and peridotites display mineralogical characteristics and mineral chemistries consistent with their derivation from modern supra-subduction zone-like environments. Alternatively, these rocks could have, in part, evolved in a supra-subduction zone even though they originated from a MOR-like setting. A simplified scenario regarding the early geodynamic evolution of the Philippines is proposed on the basis of the geochemical signatures of the ophiolites, their ages of formation and the ages and origins of the oceanic basins actually bounding the archipelago, including basins presumed to be now totally consumed. This scenario envisages the early development of the archipelago to be largely dominated by the opening and closing of oceanic basins. Fragments of these basins provided the substratum on top of which the Cretaceous to Recent volcanic arcs of the Philippines were emplaced.

A cretaceous supra-subduction oceanic basin source for Central Philippine ophiolitic basement complexes: Geological and geophysical constraints

The Central Philippines is made up of several Cretaceous oceanic lithospheres that were generated and emplaced in a variety of geologic setting and manner. The Antique Ophiolite Complex exposed along the western side of Central Philippines is associated with blueschists, which suggests tectonic erosion accompanied subduction during its emplacement. Mélanges are associated with the Southeast Bohol Ophiolite Complex and the Cebu ophiolitic rocks suggesting that subduction kneading was a major process during the emplacement of these oceanic fragments. The geology and geochemistry of the Tacloban Ophiolite Complex in Northern Leyte, the Malitbog Ophiolite Complex in Southern Leyte and the dismembered ophiolitic sequence in southern Samar imply that subduction played a role in the generation and emplacement of these Cretaceous oceanic lithospheres. Whole rock major and trace element, along with spinel mineral chemistry, favor a suprasubduction zone origin for these ophiolites. The ophiolites become younger towards the east (present-day geographic setting) indicating that convergence was accentuated by trench jumping. The similarities in the petrologic features, geochemical signatures and ages displayed by these oceanic lithosphere fragments in Central Philippines indicate that they were probably derived from a single Cretaceous oceanic basin which could have been a part of the proto-Philippine Sea Plate.

Mineralization Controls in Island Arc Settings: Insights from Philippine Metallic Deposits

Abstract A review of the gold-copper, volcanogenic massive sulfide and ultramafic-hosted (i.e., chromitite, nickel sulfide, platinum-group minerals) deposits in the Philippines is presented. It is critical that a thorough understanding of the spatial and temporal relationship among magmatism, structures and mineralization must be gained if the correct evaluation of the economic potential of a particular deposit is to be done. Structural features conducive to precious and base metal mineralizations are associated with shear zones, extensional jogs and collision zones. In Northern Luzon, alkali and adakitic magmatism are considered good markers for gold-copper mineralization. Volcanogenic massive sulfide deposits are hosted by either ophiolites of marginal basin origin or metamorphic terranes. Exploration works on these deposits have been geared in determining the gold content of the massive sulfides. Chromitite deposits are related with ultramafic rock-hosted deposits. Their occurrence is attributed to crystallization, magma mixing and mantle-melt interaction processes in subduction-related settings. The multiple stages of partial melting responsible for the formation of supra-subduction zone ophiolites result in the generation of second to third stage melts that are enriched in nickel sulfides and platinum group minerals. On the basis of structural, geochemical and tectonic controls, Panay, Mindoro and Central Mindanao and the Sierra Madre, Leyte, and Samar are good exploration targets for precious and base metal deposition in the western and eastern sides of the Philippines, respectively.

Volcanic-hypabyssal rock geochemistry of a subduction-related marginal basin ophiolite: Southeast Bohol Ophiolite-Cansiwang Mélange Complex, Central Philippines

The Early Cretaceous Southeast Bohol Ophiolite-Cansiwang Mélange Complex and the Alicia Schist form the basement of southeastern Bohol Island in central Philippines. New geochemical data show that four discrete groups constitute the volcanic and associated hypabyssal rocks of the ophiolite-mélange complex: boninitic rocks (BON), enriched and normal mid-ocean ridge basalt-like rocks (E-MORB; N-MORB) and high-magnesian andesites (HMA). Of these four groups, the BON are the most depleted in REEs and with the most pronounced negative Nb anomalies. Both MORB-like types exhibit subduction-zone influence as reflected in their slight negative Nb anomalies. Characteristically with flat and LREE-depleted patterns, the HMA samples appear to mimic N-MORB patterns but with lower REE concentrations. This geochemical diversity is best explained by a suprasubduction zone environment of formation as is also evident from field geological information. Formation of the Cansiwang Mélange is believed to have been concurrent with the ophiolites emplacement by subduction-accretion along a forearc margin. This tectonic boundary was later jammed into inactivity with the entry of the Alicia Schist that most likely was an oceanic bathymetric high. The intercalation of both tuffaceous materials and pelagic chert with the pillow basalts are consistent with a marginal basin tectonic setting.

Onramping of Cold Oceanic Lithosphere in a Forearc Setting: The Southeast Bohol Ophiolite Complex, Central Philippines

Geochemical and mineralogical data show that the Early Cretaceous Southeast Bohol Ophiolite Complex on Bohol island, Central Philippines, was generated in a subduction-related marginal basin. The volcanic-hypabyssal rocks show negative Nb, Zr, and Ti anomalies, suggesting an island-arc affinity. This signature is consistent with the high anorthite contents of the gabbro and norite plagioclases, although the ultramafic rocks contain low XCr [Cr/(Cr + Al) < 0.60] spinels. The ophiolite is thrust on top of a tectonic melange, the Cansiwang Melange, which in turn is thrust over the Alicia Schist, a metamorphosed oceanic crust sliver made up of chlorite schist, amphibolite schist, and quartz-sericite schist. The presence of serpentinite between the ophiolite and the Alicia Schist indicates that the amphibolite schist cannot represent the emplacement-related metamorphic sole of the Southeast Bohol Ophiolite Complex. The composition of the associated serpentinite sole (Cansiwang Melange) of the ophiolite complex suggests that this crust-mantle sequence was emplaced at a relatively low temperature (<500°C). The onramping of the Southeast Bohol Ophiolite Complex as a forearc ophiolite, followed by its collision and suturing with the Alicia Schist, occurred during the Late Cretaceous. This ophiolite could represent a fragment of the Cretaceous proto-Philippine Sea plate.

Geology of southeast Bohol, central Philippines: accretion and sedimentation in a marginal basin

Recent field mapping has refined our understanding of the stratigraphy and geology of southeastern Bohol, which is composed of a Cretaceous basement complex subdivided into three distinct formations. The basal unit, a metamorphic complex named the Alicia Schist, is overthrust by the Cansiwang mélange, which is, in turn, structurally overlain by the Southeast Bohol Ophiolite Complex. The entire basement complex is overlain unconformably by a ∼2000 m thick sequence of Lower Miocene to Pleistocene carbonate and clastic sedimentary rocks and igneous units. Newly identified lithostratigraphic units in the area include the Cansiwang mélange, a tectonic mélange interpreted as an accretionary prism, and the Lumbog Volcaniclastic Member of the Lower Miocene Carmen Formation. The Cansiwang mélange is sandwiched between the ophiolite and the metamorphic complex, suggesting that the Alicia Schist was not formed in response to emplacement of the Southeast Bohol Ophiolite Complex. The accretionary prism beneath the ophiolite complex and the presence of boninites suggest that the Southeast Bohol Ophiolite Complex was emplaced in a forearc setting. The Southeast Bohol Ophiolite Complex formed during the Early Cretaceous in a suprasubduction zone environment related to a southeast‐facing arc (using present‐day geographical references). The accretion of this ophiolite complex was followed by a period of erosion and then later by extensive clastic and carbonate rock deposition (Carmen Formation, Sierra Bullones Limestone and Maribojoc Limestone). The Lumbog Volcaniclastic Member and Jagna Andesite document intermittent Tertiary volcanism in southeastern Bohol.

Geology and geochemistry of the Rapu-Rapu Ophiolite Complex, eastern Philippines: Possible fragment of the proto-Philippine Sea plate

The Cretaceous Rapu-Rapu Ophiolite Complex is a dismembered marginal basin ophiolite formed at an intermediate to fast spreading center. Limited subduction is manifested by the crystallization of pyroxene before plagioclase in the layered ultramafic cumulates. However, mafic cumulate rocks which exhibit the crystallization of plagioclase before pyroxene, consistent with low-water pressure conditions, could have formed in a mid-ocean ridge-like setting. Bulk-rock and trace-element analyses of volcanic-hypabyssal and gabbroic rocks show the predominance of a marginal basin geochemical signature. Spinel XCr ([Cr/Cr+Al] <0.60) suggests that the Rapu-Rapu peridotite and pyroxenite have mid-ocean ridge affinities. Fractional crystallization, and mantle-melt interaction together with melt replenishment, characterize the evolution of the ultramafic-mafic cumulate sequence; cryptic variation analysis reveals that the magma chamber operated as an open system. The Rapu-Rapu Ophiolite Complex, together with other fragments of the oceanic lithosphere exposed along the eastern Philippines, was probably derived from the proto—Philippine Sea plate.