Carla B. Dimalanta

New rates of western Pacific island arc magmatism from seismic and gravity data

Numerous studies have been conducted in order to look into the evolution of the continental crust. Some suggest that one of the mechanisms which contribute to the growth of continental crust is arc magmatism. It is in this context that Reymer and Schubert (Tectonics 3 (1984) 63) estimated arc magmatic addition rates to the continental crust. Their results suggest that island arc magmatism was producing material at an average rate of 20–40 km3/km/Myr (volume per unit width along the strike direction of arc). The present work utilizes the most recent worldwide marine gravity data, together with improved seismic data from some oceanic island arcs in the western Pacific region. The combined gravity and seismic data allow a more accurate image of the subsurface configuration beneath the oceanic island arcs and yield better estimates of crustal volumes created during arc magmatic processes. Oceanic island arcs investigated in this study show a crustal thickness ranging from 20 to 30 km. Utilizing this thickness, the relevant crustal volume for each island arc is then estimated. Dividing the crustal volume by the age of initiation of subduction of the arc gives arc magmatic addition rates of 30–95 km3/km/Myr. The estimates presented here are nearly twice as high as the previous estimates of arc magmatic addition rates.

Extreme weather events and related disasters in the Philippines, 2004-08: a sign of what climate change will mean?

Being an archipelagic nation, the Philippines is susceptible and vulnerable to the ill-effects of weather-related hazards. Extreme weather events, which include tropical cyclones, monsoon rains and dry spells, have triggered hazards (such as floods and landslides) that have turned into disasters. Financial resources that were meant for development and social services have had to be diverted in response, addressing the destruction caused by calamities that beset different regions of the country. Changing climatic patterns and weather-related occurrences over the past five years (2004-08) may serve as an indicator of what climate change will mean for the country. Early recognition of this possibility and the implementation of appropriate action and measures, through disaster risk management, are important if loss of life and property is to be minimised, if not totally eradicated. This is a matter of urgent concern given the geographical location and geological characteristics of the Philippines.

Petrography, geochemistry, and tectonics of a rifted fragment of Mainland Asia: evidence from the Lasala Formation, Mindoro Island, Philippines

Petrological and geochemical investigations of the sedimentary Lasala formation in northwest Mindoro, Philippines, offer new insights into the origin of this geologically contentious region. Mindoro island’s position at the boundary between Sundaland and the Philippine Mobile Belt has led to variable suggestions as to how much of it is continent derived or not. The Eocene Lasala formation overlies the Jurassic Halcon metamorphics, a regionally metamorphosed suite generally thought to have formed as a result of arc-continent collision processes. The sedimentary formation consists mainly of sandstones and shales interbedded with mudstones, basalt flows, and subordinate limestones and conglomerates. Petrographic information on the Lasala clastic rocks demonstrates a uniform framework composition that is predominantly quartzose. Major oxide, trace element abundances, and various elemental ratios similarly impart a strongly felsic signature. These characteristics are taken to indicate a chiefly continental, passive margin derivation and deposition of the Lasala sediments during the Eocene. The weak indication of active margin influence is suggested to be an inherited signature, supported by paleogeographic models of the southeastern Asian margin area during the pre-Cenozoic.

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.

Magmatic and Amagmatic Contributions to Crustal Growth of an Island-Arc System: The Philippine Example

The growth of continental crust consists of contributions from both magmatic and amagmatic processes. In the Philippines, the presence of numerous active and potentially active volcanoes and ophiolite/ophiolitic complexes attests to the significant role of arc magmatism and oceanic lithosphere emplacement to crustal growth. However, an estimate of the magmatic and amagmatic contributions to crustal growth has never been attempted previously for this particular island-arc system due to the paucity of geophysical data (e.g., seismic refraction data, seismic velocity models, etc.). This study presents a synthesis of crustal thickness values based on available geophysical and geochemical data. These thicknesses are used to determine the volume of material produced by arc magmatism as well as crustal growth resulting from the amagmatic emplacement of ophiolites. Based on the computations, arc magmatism, rather than ophiolite emplacement, has contributed more significantly to crustal growth in the Philippines. Arc magmatism growth rates of 25 to 67 km3/km/m.y. are comparable to those of computed arc magmatic addition rates in other island-arc systems in the Western Pacific. Ophiolite accretion rates vary from 2 to 19 km3/km/m.y.

Magmatic and amagmatic contributions to crustal growth in the Philippine island arc system: Comparison of the Cretaceous and post-Cretaceous periods

There have been few studies on both magmatic and amagmatic processes that contribute to crustal thickening. As a result, estimates of the rates at which these processes have taken place are rather limited. With the recent availability of arc magmatic addition and ophiolite accretion rates from geochemical and geophysical data, it is now possible to look at the variations of these rates in time and space especially with respect to the Cretaceous period. Arc magmatic addition rates in the Philippines vary from 25 to 60 km3/km/m.y. whereas accretion rates for the different ophiolite complexes range from 2 to 30 km3/km/m.y. Low accretion rates by arc magmatism and ophiolite accretion (<25 km3/km/m.y. and <10 km3/km/m.y., respectively) are obtained for the Cretaceous compared to post-Cretaceous rates. Moreover, lower ophiolite accretion rates are obtained for the eastern ophiolites whereas the reverse is noted for the western ophiolites (e.g., Zambales, Mindoro, Antique, Palawan, NE Zamboanga). The Cretaceous period, as suggested by the presence of several emplaced Cretaceous ophiolites was characterized by a dominant process of oceanic crust formation, followed by basin closure and lithospheric emplacement. The higher arc magmatic addition rates after the Cretaceous are attributed to the successive subduction-related magmatic episodes, which led to thickened crust in Central Luzon and the Bicol-Negros-Northeastern Mindanao area. The temporal and spatial variations noted in the ophiolite accretion rates could be a function of the active margin-related tectonic processes that took place along the western side of the Philippine archipelago after the Cretaceous period.

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.

Translation and docking of an arc terrane : geological and geochemical evidence from the southern Zambales ophiolite complex, Philippines

Abstract The Zambales Ophiolite Complex is made up of three massifs: the Masinloc, Cabangan and San Antonio Massifs. Field, petrographic and geochemical analyses show that the Cabangan and San Antonio Massifs are genetically related to the Coto (transitional mid-ocean ridge–island arc) and Acoje (island arc) blocks of the Masinloc Massif, respectively. The Subic Bay Fault Zone, a left-lateral fault zone, separates the San Antonio Massif island arc terrane from the transitional mid-ocean ridge–island arc-like sheeted diabase dikes–pillow basalts of the Cabangan Massif. The San Antonio Massif is a rifted terrane from the Acoje block which was translated southward to its present position through the West Luzon Shear–Subic Bay Fault Zone. Tectonized clinopyroxenite and gabbronorite hills, which mimic the physical and geochemical characteristics of the Acoje block and the San Antonio Massif ultramafic–mafic cumulate rocks, were left behind along the western side of the Cabangan Massif during the translation of the arc massif southward. This scenario can account for the present-day configuration of the Zambales Ophiolite Complex.

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.

Upper mantle-lower crust dikes of the Zambales Ophiolite Complex (Philippines) : Distinct short-lived, subduction-related magmatism

Abstract The residual harzburgite–lherzolite suites in the Coto and Acoje blocks of the Zambales Ophiolite Complex are cut by numerous dikes that range from clinopyroxenites to diabases, gabbros and diorites. Diabase dikes extensively cut the Coto block harzburgites while some clinopyroxenite to gabbro dikes are noted in the Acoje block lherzolite–harzburgite–dunite suite. The upper mantle–lower crust dikes in both blocks do not extend to their respective overlying sheeted dike–sill complexes. The whole-rock geochemistry of the mafic dikes intruded into the Coto and Acoje block peridotites show subduction-related signatures. These island-arc dikes are not unextracted frozen melts from their respective abyssal peridotite-like country rocks. They represent post-ophiolite, short-lived magmatic events that tapped geochemically different mantle source regions in a subduction-related marginal basin setting.