Reconstructing the Mesozoic-early Cenozoic evolution of northern Philippines: Clues from palaeomagnetic studies on the ophiolitic basement of the Central Cordillera

The first reliable palaeomagnetic data from the Cretaceous to Eocene ophiolitic basement rocks in the Philippines are presented. A total of 12 drill core sites from five localities in the Central Cordillera in northern Luzon, Philippines were sampled. Eight drill core sites were from pillow basalts, and four were from diabase feeder dykes. Combining the characteristic remanent magnetization direction from these sites gives a mean in situ direction of Dec = 162.2°, Inc = -21.4° (α95 = 17.0°, k = 21.1) and a tilt-corrected direction of Dec = 159.3°, Inc = -12.5°, α95 = 6.0°, k = 162.5. Along with other lines of palaeomagnetic evidence, this clustering suggests that the magnetization is primary and that the ophiolitic basement rocks of the Central Cordillera formed at subequatorial latitudes (6.3°N ± 3.1°). This information further suggests that the basement rocks of northern Luzon were formed close to where the island was during the early Cenozoic. These rocks could represent relicts of the proto-Philippine Sea Plate. © 2009 The Authors Journal compilation © 2009 RAS.link_to_subscribed_fulltex

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. © 2007 Elsevier Ltd. All rights reserved.link_to_subscribed_fulltex

Behavior of major and trace elements during Ore deposition: Example from the low-sulfidation Pantingan gold system, Mount Mariveles, Bataan, Philippines

The evaluation of the relatively fresh host rock and altered rock samples associated with the Pantingan Gold System exposed in Mount Mariveles, Bataan yield several notable observations that are useful in pinpointing potential gold pathfinder elements. Geochemical and petrologic analysis showed that the altered rocks can be subdivided into rocks that underwent propylitic alteration (group 1), argillized rocks with silica contents similar to those of the fresh host rocks (group 2), argillized but not strongly silicified rocks (group 3) and argillized and strongly silicified rocks (group 4). Selected element ratio patterns in the altered rocks and gold concentrations in gold-bearing quartz veins vary between the rock groups. Moreover, mass balance calculation also reflected the geochemical observations pertaining to the gains and losses of SiO2, Fe2O3 + MgO, CaO + Na2O and K2O, which are believed to be chemical reactions (i.e. breakdown of plagioclase, silica inundation or leaching, sulfide and calcite formation) caused by the influx of hydrothermal fluids. © 2007 The Authors Journal compilation © 2007 The Society of Resource Geology.link_to_subscribed_fulltex

Rare earth element geochemistry of the zigzag - Klondyke Sedimentary Rock Formations: Clues to the evolution of the Baguio Mineral District (Luzon), Philippines

Petrographic analysis of the Oligocene-Miocene Zigzag Formation and Miocene Klondyke Formation of the Baguio Mineral District reveals a transitional arc source for the former and an undissected arc source for the latter. Whole rock geochemistry of these sedimentary rock formations show affiliation of the Zigzag Formation to active continental margin whereas the Klondyke Formation appears to have been derived from an oceanic island arc source. A different aspect of the geology of the district is further gleaned from the whole rock rare earth element geochemistry of these rocks. Samples from the Zigzag Formation are characterized by higher REE concentrations compared to the Klondyke Formation samples. The REE data indicate that the sedimentary rocks of the Klondyke Formation had geochemically-less fractionated igneous lithologies as their progenitor whereas the older Zigzag Formation was derived from a more differentiated source. This new set of information contributes to the understanding of the evolution of this district from a marginal basin to an island arc setting.link_to_subscribed_fulltex

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

The Early Cretaceous Southeast Bohol OphioliteCansiwang 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; NMORB) 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 ophiolite's 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.link_to_subscribed_fulltex

Geology, geochemistry and U-Pb SHRIMP age of the Tacloban Ophiolite Complex, Leyte Island (Central Philippines): Implications for the existence and extent of the proto-Philippine Sea Plate

The oceanic basement of the Central Philippines is exposed in ophiolitic massifs the age and origin of which remain debated. The Tacloban Ophiolite Complex (TOC) outcrops as a NW-SE trending massif in the northeastern portion of Leyte Island, Central Philippines. It is unconformably overlain by sedimentary sequences dated to Late Miocene-Pliocene and Pleistocene volcaniclastic deposits on its eastern and western flanks, respectively. Field, petrographic and trace element data suggest a subduction-related origin for this ophiolite. Sensitive High Resolution Ion Microprobe (SHRIMP) U-Pb dating of zircons from a gabbro yielded Early Cretaceous magmatic age for the TOC, which is very much older than a previously reported whole rock K-Ar derived Eocene age. The Early Cretaceous age of the TOC limits its possible progenitor to the proto-Philippine Sea Plate. Correlation with other Cretaceous ophiolites in Central Philippines reveals the possible extent of the proto-Philippine Sea Plate remnants now exposed onland.link_to_subscribed_fulltex

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 elastic 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 supra-subduction 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.link_to_subscribed_fulltex