Baguio Mineral District: An oceanic arc witness to the geological evolution of northern Luzon,Philippines

The Baguio Mineral District exposes rock formations that evince the geological and tectonic evolution of this district from a subduction‐related marginal basin to an island arc setting. Available onshore and offshore data are consistent with an Early (onset phase) to Middle (developed phase) Miocene arc polarity reversal from the east (termination of subduction along the proto‐East Luzon Trough) to the west (initiation of subduction along the Manila Trench). Geophysical modeling and geochemical data calculation showed a 30 ± 5 km crustal thickness for the mineral district. Subduction‐related multiple arc magmatism and ophiolite accretion contributed to crustal thickening. Recent information on the Oligo–Miocene Zigzag and Klondyke formations in the mineral district reveal that the marginal basin, where these rocks were deposited, has received eroded materials from adjacent terrains characterized by siliceous lithologies. Furthermore, adakitic rocks, high permeable zones and extensional zones which are exploration markers applied to identify possible mineralization targets, are prevalent in the mineral district. The geological evolution that the district had undergone mimics the evolution of island arcs worldwide in general and northern Luzon in particular.     

Geology and geochemistry of the Shuanggou ophiolite (Ailao Shan ophiolitic belt), Yunnan Province, SW China: Evidence for a slow-spreading oceanic basin origin

The early Carboniferous Shuanggou ophiolite lies in the middle segment of the Ailao Shan orogenic belt between the South China Block to the north and the Indochina Block to the south. The ophiolite consists of meta-peridotite, gabbro, diabase and basalt, capped by radiolarian-bearing siliceous rocks. No layered gabbros or sheeted dikes have been observed. The meta-peridotite underwent low degrees of partial melting, consistent with the low magma budget of this oceanic lithosphere. Whole-rock rare earth element analyses of gabbro indicate a geochemical affinity with normal mid-ocean ridge basalts, consistent with the crystallization order of plagioclase followed by clinopyroxene recognized in the gabbros. The ophiolite is believed to have formed in a small, slow-spreading oceanic basin. Collision of the Indochina Block with the South China Block in the late Paleozoic was responsible for the closure of the oceanic basin and emplacement of the ophiolite in the Ailao Shan orogenic belt.     

Abnormal weather events in 2009, increased precipitation and disastrous impacts in the Philippines

The Philippines has recently experienced distinct changes in the weather patterns with disastrous results. These changes which were distinctly felt in 2009 included: 1. too much precipitation throughout the year; 2. some areas received a lot of rain while other parts of the country went through dry spell and drought conditions; and 3. abnormalities and variance in weather patterns (e.g. multiple entry of a tropical cyclone during an El Niño event; longer duration of tropical cyclone; deviations from the normal tropical cyclone path). The country, with its disaster risk management program in place, has managed to bring down the cost of damage and number of casualties due to weather-related disasters. However, in some instances, disaster risk response was made difficult due to other factors (e.g. degraded ecosystem, ill-managed land use and risk denial by people and communities). In general though, the resiliency and ability to recover by the people devastated by these disasters and the availability of community-based support systems provided the best means of coping with these catastrophic events. This is important as climate change is projected to bring more variations in the country’s weather and climate patterns which, as of now, are already adversely affecting the people.     

Textural Evidence for the Chromite‐Oversaturated Character of the Melt Involved in Podiform Chromitite Formation

Well‐preserved oval‐shaped dunite clots occur within the exceptionally fresh massive podiform chromitites from the Coto Block of the Zambales Ophiolite Complex, the Philippines. The dunite/chromitite boundary shows an interlocking texture; olivine inclusions in chromites in the podiform chromitites show the same optical extinction with larger adjacent olivines in the dunite clots. This texture was formed by the reaction between chromite‐oversaturated melt and its dunite inclusions. The existence of such type of melt was previously only hypothesized to explain the origin of layered and podiform chromitites but is now confirmed by this discovery.     

Diverse protolith ages for the Mindoro and Romblon Metamorphics (Philippines): Evidence from single zircon U–Pb dating

Within the north‐eastern part of the Palawan Continental Terrane, which forms the south‐western part of the Philippine archipelago, several metamorphic complexes are exposed that are considered to be rifted parts of the Asian margin in South‐East China. The protolith age(s) and correlations of these complexes are contentious. The largest metamorphic complex of the Palawan Continental Terrane comprises the Mindoro Metamorphics. The north‐eastern part of this metamorphic complex has recently been found to be composed of protoliths of Late Carboniferous to Late Permian protolith age. However, meta‐sediments exposed at the westernmost tip and close to the southern boundary of the exposure of the Mindoro Metamorphics contain detrital zircons and with U–Pb ages, determined by LA–ICP–MS, in the range 22–56 Ma. In addition, zircons as young as 112 Ma were found in a sample of the Romblon Metamorphics in Tablas. As the youngest detrital zircons provide an upper age limit for the time of deposition in meta‐sediments, these results suggest that the Mindoro and Romblon Metamorphics comprise protoliths of variable age: Late Carboniferous to Late Permian in NE Mindoro; Eocene or later in NW Mindoro; Miocene at the southern margin of the Mindoro metamorphics; and Cretaceous or later on Tablas. The presence of non‐metamorphic sediments of Late Eocene to Early Oligocene age in Mindoro (Lasala Formation), which are older than the youngest metasediments, suggests that metamorphism of the young meta‐sediments of Mindoro is the result of the collision of the Palawan Continental terrane with the Philippine Mobile Belt in Late Miocene. Similarities of the age spectra of zircons from the Eocene to Miocene metamorphics with the Eocene to Early Miocene Lasala Formation suggest that the protoliths of the young metamorphics may be equivalents of the Lasala Formation or were recycled from the Lasala Formation.     

Sampling Method for the Determination of Total Inorganic Carbon (TIC) in Strongly Acidic Waters

Samples from extremely acidic waters should be taken using the introduced syringe technique avoiding atmospheric contact to minimize CO2 evaporation from the samples. The syringe technique inclusive manual injection enables the determination of TIC in acid waters with high precision and accuracy. If a TOC analyser with a small sample volume is used, the vials should be filled directly inside the water sampler without air bubbles, covered with aluminium foil and transported at ambient water temperatures. If an autosampler is used, partial selection of the vials increases the stability of the results. Care should be taken that samples can be analysed within a short time.     

Characterization of the proto-Philippine Sea Plate:Evidence from the emplaced oceanic lithospheric fragments along eastern Philippines

The proto-Philippine Sea Plate(pPSP)has been proposed by several authors to account for the origin of the Mesozoic supra-subduction ophiolites along the Philippine archipelago.In this paper,a comprehensive review of the ophiolites in the eastern portion of the Philippines is undertaken.Available data on the geology,ages and geochemical signatures of the oceanic lithospheric fragments in Luzon(Isabela,Lagonoy in Camarines Norte,and Rapu-Rapu island),Central Philippines(Samar,Tacloban,Malitbog and Southeast Bohol),and eastern Mindanao(Dinagat and Pujada)are presented.Characteristics of the Halmahera Ophiolite to the south of the Philippines are also reviewed for comparison.Nearly all of the crust-mantle sequences preserved along the eastern Philippines share Early to Late Cretaceous ages.The geochemical signatures of mantle and crustal sections reflect both mid-oceanic ridge and suprasubduction signatures.Although paleomagnetic information is currently limited to the Samar Ophiolite,results indicate a near-equatorial Mesozoic supra-subduction zone origin.In general,correlation of the crust-mantle sequences along the eastern edge of the Philippines reveal that they likely are fragments of the Mesozoic pPSP.     

Geology of the southern Zambales Ophiolite Complex, Luzon, Philippines

Abstract Field, geochemical and geophysical evidence show that the southern Zambales Ophiolite Complex attained its present-day configuration through the juxtapositioning of an arc terrane (San Antonio massif) to a back-arc crust (Cabangan massif). The San Antonio massif manifests island arc-related characteristics (i.e. spinel XCr [Cr/(Cr + Al)] >0.60; mostly plagioclase An92–95; pyroxene crystallizing ahead of plagioclase; orthopyroxene as an early, major crystallizing phase) which cannot be directly parental to the Cabangan massif transitional mid-ocean ridge basalt to island arc tholeiitic volcanic carapace. The two massifs are believed to be separated by a left-lateral strike–slip fault, the Subic Bay Fault Zone. Apart from the presence of highly sheared, allochthonous outcrops, the Subic Bay Fault Zone is generally defined by northwest–southeast trending magnetic and bouguer anomalies. The San Antonio massif was translated southward from the northern part of the Zambales Ophiolite Complex through the Subic Bay Fault Zone. This resulted into its suturing with the Cabangan massif and could have led to the formation of the present-day Subic Bay.     

Comparative study of platinum-group elements in the Coto and Acoje blocks of the Zambales Ophiolite Complex, Philippines

Abstract The Zambales Ophiolite Complex (ZOC), Philippines, includes two geochemically distinct, ophiolitic assemblages: the high-Al chromitite-bearing Coto Block and high-Cr chromitite-bearing Acoje Block. This paper reports a comparative platinum-group element (PGE) study of these two blocks. The PGE data were obtained using Ni-sulfide fire assay preconcentration combined with inductively coupled plasma mass spectrometry (ICP-MS) measurement. Podiform chromitites in the Acoje Block have higher Cu, Ir, Ru and Rh contents than their equivalents in the Coto Block, although chromitites from both have similar Pt and Pd contents. The PGE mantle-normalized patterns of dunites from the two blocks are also different: dunites from the Coto Block are depleted in Pt, whereas those from the Acoje Block have a relatively flat pattern. The data demonstrate that Coto and Acoje Blocks have different origins in terms of their source region and partial melting processes. This study implies that the ZOC is a paired ophiolite belt formed in an island arc and back-arc basin environment.     

The Cansiwang Melange of Southeast Bohol (Central Philippines): Origin and tectonic implications

Abstract The Cansiwang Melange underlies the Southeast Bohol Ophiolite Complex (SEBOC) and is composed mainly of sheared ophiolite-derived blocks such as harzburgites, microgabbros, basalts and cherts in a pervasive serpentinite matrix. Available field, as well as geophysical evidence show that this melange unit is not diapiric, nor does it have a sedimentary origin considering that it lacks slump and flow structures. A tectonic origin for the Cansiwang Melange is favored in view of the numerous thrust faults, which cut across the exposures, as well as the tectonic contacts that the melange has with the overlying and underlying formations. The presence of the Cansiwang Melange in between the SEBOC and the Alicia Schist provides evidence that the amphibolite of the Alicia Schist do not correspond to the metamorphic sole of SEBOC. Similar to what is recognized in the Josephine Ophiolite, this suggests a 'cold' emplacement of the ophiolite over the Alicia Schist. The Cansiwang Melange represents an accretionary prism product which marks the location of an ancient subduction zone in what is now Central Philippines.