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The Late Cretaceous Pandan Formation in Cebu Island is one of the oldest sedimentary units in the Central Philippines. The inconsistencies in geological descriptions and interpretation of the depositional environment of the Pandan Formation complicated efforts to determine the origin and tectonic history of the basement of Cebu Island. This study therefore looks into the petrological and geochemical characteristics of the Pandan Formation and their implications for the tectonic development of the Philippine Arc during the late Mesozoic. Petrographic analyses indicate significant contribution from mafic sources with additional inputs from felsic rocks, siliciclastics and metamorphic sources. Enrichment of detrital quartz from felsic volcanic and plutonic rocks, as well as from siliciclastic and metamorphic sources, has shifted the SiO2 composition of the Pandan clastics from a mafic to a more intermediate source. Whole‐rock geochemical analyses revealed low SiO2/Al2O3 = 4.21, low K2O/Na2O = 1.16, low Th/Sc = 0.13, low Th/U = 2.78, high La/Th = 4.51, significantly low REEs = ca 76.45 ppm and low LaN/YbN = 4.28. A slight negative chondrite‐normalized Eu/Eu* (0.91) anomaly and significantly high PAAS‐normalized positive Eu/Eu* (1.39) values are consistent with derivation from a young undissected magmatic arc terrane. Tectonic discrimination diagrams suggest formation in an oceanic island arc to active margin/collision zone modelled to be located at the oceanic leading edge of Australia. Rapid uplift and erosion of the magmatic arc and older allochthonous blocks gave way to the rapid deposition of the Pandan Formation in the Late Cretaceous at the subequatorial region. [ABSTRACT FROM AUTHOR]

The intermediate to low sulfidation epithermal gold deposit in Masara, Compostela Valley, Eastern Mindanao, Philippines, is associated with a diorite porphyry from the Late Miocene Lamingag Intrusive Complex. Detailed mineralogical investigation of the host rocks in the deposit reveals five major alteration zones and at least two mineralizing events. A potassic alteration zone characterized by stockwork magnetite veins, secondary biotite and magnetite and an early-stage chlorite-sericite alteration composed of quartz + chlorite + illite + sericite + pyrite ± biotite ± magnetite ± calcite assemblages are interpreted to be linked with an earlier porphyry copper system in the area. These were overprinted by late-stage chlorite-sericite alteration consisting of quartz + chlorite + illite + sericite + pyrite ± adularia ± magnetite ± epidote and sericite assemblages which generally contain quartz + illite + sericite + pyrite. These alteration overprints are inferred to be related to a younger low to intermediate-sulfidation epithermal gold mineralization which involved near-neutral fluids. Advanced argillic alteration comprising quartz + kaolinite + magnetite + dickite ± illite ± calcite was also observed in the area. Isocon analysis shows that alteration zones associated with epithermal mineralization are generally characterized by enrichment in SiO 2 and K 2 O and depletion of CaO and Na 2 O as well as additions of Au, Cu, Zn and Pb. Pearce Element Ratio (PER) analysis indicates high degrees of sericitization in the altered diorite porphyry, tuff and quartz diorite units. At least three major alteration centers are defined based on the spatial distribution of the Alteration Index of surface samples calculated from the PER analysis. The results of this study support the use of whole-rock geochemistry as a complement to traditional techniques in exploration geochemistry. [ABSTRACT FROM AUTHOR]

Remnants of a Cretaceous lithosphere are found at the peripheries of the West Philippine Basin. These Mesozoic fragments preserve arc affinity and include the Amami Plateau, the East Halmahera Ophiolite and the ophiolites along the eastern margin of the Philippine archipelago. The eastern margin of the Philippines is composed of Early to Late Cretaceous ophiolites and ophiolitic complexes that exhibit strong subduction imprints. The early Late Cretaceous Samar Ophiolite in the central Philippines forms part of this eastern belt. Recent surveys in southern Samar revealed the presence of peridotites, gabbros, and massive and pillowed flows of the Samar Ophiolite. Major, trace and rare earth element signatures of the volcanic rock samples indicate moderate to strong subduction-related influences and formation in an island arc setting. In contrast to other similarly-aged ophiolites along the eastern margin of the archipelago, those to the north of Samar Island exhibit weak to almost mid-oceanic ridge characteristics. These differences in the geochemical signatures of the ophiolites and ophiolitic complexes along eastern Philippines require a re-evaluation of a previous model suggesting that these ophiolites originated from a single oceanic lithosphere, possibly the proto-Philippine Sea Plate. [ABSTRACT FROM AUTHOR]

The Cenozoic sedimentary sequences of southern Samar Island in eastern Philippines were examined to understand the unroofing history of an ancient arc terrane. Petrographic and geochemical data revealed varying degrees of inputs from the ophiolite basement and differences in modal compositions. The sedimentary units are mostly made up of lithic fragments. The Late Oligocene to Early Miocene Daram Formation contains more chert and volcanic fragments whereas the late Middle Miocene to Early Pliocene Catbalogan Formation is dominantly composed of ultramafic components. These variances are correspondingly reflected in the geochemical signatures of these two sedimentary formations. The Catbalogan Formation clastic rocks have higher volatile-free MgO and Fe 2 O 3 values (average: 8.4% for both oxides) compared to the Daram Formation samples (average: 5.1 and 6.3%, respectively). Geochemical variations are also reflected in the Co, Cr and Ni values: the Catbalogan Formation samples reflect higher concentrations (Co: 15–57 ppm; Cr: 231–1094 ppm; Ni: 84–484 ppm) compared to the Daram Formation samples (Co: 24–32 ppm; Cr: 234–418 ppm; Ni: 212–323 ppm). These observations suggest that the Daram Formation eroded and transported more of the crustal portions of the ophiolite, while the younger Catbalogan Formation represents a later exhumation and subsequent erosion of the ultramafic section. An oceanic island arc (OIA) setting is proposed for the two formations based on several tectonic discrimination diagrams (e.g., Th-La-Sc, La vs. Th). The OIA signature is further supported by their smooth chondrite-normalized rare earth element (REE) patterns with no obvious Eu anomaly as well as LREE enrichment which are typical of sediments deposited in OIA setting. Based on the dominantly ophiolitic provenance of the Daram and Catbalogan formations, the post-emplacement history of the nearby Samar Ophiolite is constrained during the Late Oligocene to Early Pliocene period. [ABSTRACT FROM AUTHOR]

This paper presents the first field, geochemical and geophysical information on the recently recognized Early Cretaceous Balud Ophiolitic Complex (BOC) in the island of Masbate in the Central Philippines. Mapping of the western limb of the island revealed that only the upper crustal section of the BOC is exposed in this area. Geochemically, the pillow basalts are characterized by transitional mid-oceanic ridge basalt-island arc tholeiitic compositions. Gravity surveys yielded low Bouguer anomaly values that are consistent with the highly dismembered nature of the BOC. Short wavelength, high amplitude magnetic anomalies registered across the study area are attributed to shallow magnetic sources. This is taken to support the model that the ophiolitic complex occurs as thin crustal slivers that are not deeply-rooted in the mantle. Comparing BOC with other ophiolites in the Central Philippines, such as those in the islands of Sibuyan, Leyte and Bohol, suggests the possibility of a common or contiguous source for similarly-aged and geochemically composed crust-mantle sequences in the region. [ABSTRACT FROM AUTHOR]

Samar island in the eastern part of Central Philippines is underlain by a complete ophiolite suite, the Samar Ophiolite. We present the first geochronological and paleomagnetic data for the Samar Ophiolite. Whole rock K–Ar dating of two basalt samples yielded an age of 100.2 ± 2.7 Ma and 97.9 ± 2.8 Ma. Thirteen sites in four localities yielded characteristic remanent magnetization with in situ direction of D = 340°, I = − 24°, k = 15, α95 = 11° and tilt-corrected direction of D = 342°, I = − 27°, k = 15, α95 = 11°. These values suggest that the ophiolitic basement rocks of Samar formed in the Late Cretaceous at a paleolatitude of 14°S ± 6°. The paleolatitude is several degrees south of the sub-equatorial positions calculated for the three other Mesozoic ophiolites of the Philippine Mobile Belt (PMB) whose paleomagnetism had been previously studied. The PMB ophiolites in eastern and central Philippines share a common age, geochemistry and paleolatitude with the Halmahera Ophiolite, suggesting that they originated from a Mesozoic supra-subduction zone that spanned a few degrees north of the equator to around 15°S. [ABSTRACT FROM AUTHOR]

Abstract: Whole-rock geochemical analysis of samples from the clastic sequences in Northwestern Panay revealed different compositional ranges for the Buruanga Peninsula and Antique Range. The results provide additional evidence that Northwestern Panay consists of two distinct terranes as a result of the arc-continent collision between the Palawan Microcontinental Block and the Philippine Mobile Belt. Major element compositions of clastic rocks from the Buruanga Peninsula (Saboncogon Formation) revealed that they belong to a continental margin tectonic setting whereas the Antique Range sedimentary rocks (Lagdo Formation) show an affinity to an oceanic island arc setting. The Fragante Formation of the Antique Range shows some similarities in tectonic setting and provenance with Buruanga Peninsula clastic rocks, which suggests a Buruanga Peninsula sediment contribution during its time of formation. Furthermore, provenance analyses from combined major and trace element data indicate a more silicic source for the Buruanga Peninsula clastics and a more mafic derivation for the sedimentary rocks of the Antique Range. These findings are consistent with previous studies suggesting that the Buruanga Peninsula forms part of the Palawan Microcontinental Block and the Antique Range belongs to the Philippine Mobile Belt. The Palawan Microcontinental Block is believed to be a drifted continental fragment from the Mesozoic East Asian accretionary complex in the southern part of the Eurasian mainland, which represents the continental margin setting and felsic source of the Buruanga Peninsula clastic rocks. Meanwhile, the oceanic island setting and mafic source of the Antique Range clastic rocks represent the volcanic rocks of the Philippine Mobile Belt that were formed as a result of the subduction of the leading oceanic edge of the Palawan Microcontinental Block. [Copyright &y& Elsevier]