Adakitic Paracale Granodiorite in southeastern Luzon, Philippines: A peek at a Proto-Philippine Sea Plate-related magmatic arc

This paper describes the geochemistry, petrogenesis and tectonic setting of a silicic pluton, the Paracale Granodiorite (PG), intruded into an ophiolitic suite in southeastern Luzon island, Philippines. Whole rock chemistry suggests that the PG samples are calc-alkaline and are characterized by light rare earth element (LREE)-enrichment and relatively weak heavy rare earth element (HREE)-depletion. They also show depletion in Nb, Ta, Zr and Ti and positive anomalies in K, Pb and Sr when normalized with the Primordial Mantle and normal-mid-ocean ridge basalt (N-MORB). The PG biotite mineral chemistry shows an affinity to calc-alkaline trends based on the FeOtot versus Al2O3, whereas in the MgO-Al2O3 plot, they exhibit transitional calc-alkaline to peraluminous characteristics. These information, along with a temperature \u3e600 °C based on biotite chemistry, and hydrous setting for the generation of the PG suggest generation in a subduction-related setting. When plotted in the Y versus Sr/Y and YbN versus (La/Yb)N, the PG samples exhibit adakitic signature. Partial melting, fluid addition and sediment participation are discerned from the geochemistry. Melting, assimilation, storage and homogenization (MASH) with limited fractionation are the dominant mechanisms of formation. The PG could represent a Late Cretaceous to Paleogene magmatic arc generated during the subduction of the proto-Philippine Sea Plate

Consumed tectonic plates in Southeast Asia: Markers from the Mesozoic to early Cenozoic stratigraphic units in the northern and central Philippines

Tectonic reconstruction models of Southeast Asia all invoke in the early Cenozoic the collision of Mesozoic oceanic plates, which have been fragmented, consumed along subduction zones or emplaced onto the overriding plate. However, with marked variations in these models, we reinvestigate the tectonic evolutionary landscape of Southeast Asia through the lens of Philippine geology. In particular, we present revisions to the more recent models by adopting the unique approach of integrating data that we have gathered for the past 17 years from the Upper Mesozoic to Lower Cenozoic stratigraphic formations in northern and central Philippines. These formations, which resulted mainly from submarine mass transport processes, evolved in response to early arc-related processes of oblique subduction, frontal wedge deformation, terrane accretion and strike slip faulting. Additional key constraints for the revisions include: (1) the timing of early Cenozoic magmatism in eastern Luzon; (2) the spatial distribution of the Upper Mesozoic to Lower Cenozoic sedimentary formations with respect to other key features (e.g. distribution of Mesozoic ophiolite fragment and continent-derived rocks) in the Philippine arc; (3) the paleolatitudinal position of Luzon and surrounding regions and; (4) the movement of the surrounding plates since the Late Mesozoic. In revising previous models, a subduction zone (proto-East Luzon Trough) separating Benham Plateau and the Philippine arc was placed to explain the spatial distribution of Eocene arc-related formational units and Mesozoic ophiolite materials comprising the accretionary complex east of Luzon at ~40 Ma period. During this time, Luzon was modeled at the southern margin of the East Asia Sea or the proto-Philippine Sea Plate. Mesozoic ophiolitic complexes that line the eastern Philippine arc as well as the ophiolitic and pelagic limestone and chert fragments included in the arc-derived, Eocene formations in Luzon could very well be traces of the now consumed East Asia Sea-proto-Philippine Sea Plate. Within the same period, we modified the Palawan Microcontinental Block (PCB), positioned at the trailing edge of the proto-South China Sea to include the whole Mindoro island and the Romblon Island Group in Central Philippines. Pieces of the consumed Izanagi Plate, the proto-South China Sea and continental-derived sediments from Asia mainland are reflected in the Mesozoic metamorphic rocks and the Eocene sedimentary formation in western Mindoro. Finally, we model Cebu, Bohol and Negros islands in Central Philippines as being at the leading oceanic edge of the Indo-Australian Plate during the early Cenozoic. With the northward movement of the Indo-Australian plate and the trench roll back of the southern margins of the Philippine Sea Plate, the accretion of the Cretaceous arc-related rocks of Cebu, Bohol and Negros onto the Philippine arc by the end of Eocene or early Oligocene becomes a possibility

Geochemistry of the Late Cretaceous Pandan Formation in Cebu Island, Central Philippines: Sediment Contributions From the Australian Plate Margin During the Mesozoic

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

Epithermal Mineralization of the Bonanza-Sandy Vein System, Masara Gold District, Mindanao, Philippines

The Masara Gold District in southeastern Mindanao island is an area of prolific hydrothermal copper and gold mineralization. This study documents the mineralization characteristics of the NW-trending Bonanza-Sandy epithermal veins to constrain possible hydrothermal fluid sources and ore-forming mechanisms. Epithermal mineralization in the NW veins is divided into three main stages: Stage 1 - massive quartz-sulfide; Stage 2 - massive to amorphous quartz-carbonate (calcite); and Stage 3 - colloform-cockade quartz-carbonate (bladed rhodochrosite). Stage 1 is the main gold mineralization phase, with chalcopyrite, pyrite, sphalerite and galena occurring with native gold and tellurides. Stages 2 and 3 contain invisible gold in the sphalerite, galena, pyrite and chalcopyrite. The deposit exhibits mineralization characteristics typical of intermediate sulfidation epithermal deposits based on the dominant chalcopyrite-pyrite mineral assemblage; illite-muscovite-chlorite alteration mineralogy that point to neutral pH conditions; and sphalerite composition of 2.26 to 8.72 mol% FeS in Stage 1 and 0.55 to 1.13 mol% FeS in Stage 2. The K-Ar age date of illite separates from highly altered diorite porphyry of the Lamingag Intrusive Complex yielded an Early Pliocene age (5.12 ± 0.16 Ma). Hydrothermal fluid exsolved from the magma that formed the Lamingag Intrusive Complex probably formed the ore-forming Stage 1 veins. Stages 2 and 3 involved the deposition of quartz and carbonate veins possibly by boiling hydrothermal fluids. Precious and base metal deposition was controlled by the Masara Fault Zone. Exploration markers for gold mineralization in the Masara Gold District and vicinity include the presence of Lamingag Intrusive Complex and massive sulfide veins

Mesozoic rock suites along western Philippines: Exposed proto-South China Sea fragments?

An ancient oceanic crustal leading edge east of mainland Asia, the proto-South China Sea crust, must have existed during the Mesozoic based on tectonic reconstructions that accounted for the presence of subducted slabs in the lower mantle and the exposed oceanic lithospheric fragments strewn in the Philippine and Bornean regions. Along the western seaboard of the Philippine archipelago, numerous Mesozoic ophiolites and associated lithologies do not appear to be genetically associated with the younger Paleogene-Neogene ocean basins that currently surround the islands. New sedimentological, paleomagnetic, paleontological, and isotopic age data that we generated are presented here, in combination with our previous results and those of others, to reassess the geological make-up of the western Philippine island arc system. We believe that the oceanic lithospheric fragments, associated melanges, and sedimentary rocks in this region are exhumed slivers of the proto-South China Sea ocean plate

Slab rollback and microcontinent subduction in the evolution of the Zambales Ophiolite Complex (Philippines) : A review

New radiolarian ages show that the island arc-related Acoje block of the Zambales Ophiolite Complex is possibly of Late Jurassic to Early Cretaceous age. Radiometric dating of its plutonic and volcanic-hypabyssal rocks yielded middle Eocene ages. On the other hand, the paleontological dating of the sedimentary carapace of the transitional mid-ocean ridge – island arc affiliated Coto block of the ophiolite complex, together with isotopic age datings of its dikes and mafic cumulate rocks, also yielded Eocene ages. This offers the possibility that the Zambales Ophiolite Complex could have: (1) evolved from a Mesozoic arc (Acoje block) that split to form a Cenozoic back-arc basin (Coto block), (2) through faulting, structurally juxtaposed a Mesozoic oceanic crust with a younger Cenozoic lithospheric fragment or (3) through the interplay of slab rollback, slab break-off and, at a later time, collision with a microcontinent fragment, caused the formation of an island arc-related ophiolite block (Acoje) that migrated trench-ward resulting into the generation of a back-arc basin (Coto block) with a limited subduction signature. This Meso-Cenozoic ophiolite complex is compared with the other oceanic lithosphere fragments along the western seaboard of the Philippines in the context of their evolution in terms of their recognized environments of generation

Interpretation of Ground Magnetic Data in Suyoc, Mankayan Mineral District, Philippines

Ground magnetic surveys conducted in Suyoc, Mankayan, Benguet led to the delineation of features related to epithermal and porphyry copper targets in the area. High reduced to equator (RTE) anomalies are observed in areas with epithermal mineralization. The high RTE anomalies are attributed to hydrothermally altered rock with quartz veins. The previously recognized porphyry copper prospect in Palasaan (Mohong Hill) is characterized by low RTE anomaly surrounded by a high RTE anomaly. One explanation for this signature is the possible presence of a magnetic core and the destruction or absence of magnetite in the alteration haloes at the periphery of a porphyry prospect. Areas such as Mangga and Liten exhibit the same magnetic signatures. This distinct magnetic pattern coupled with observed alteration and mineralization signatures led to the interpretation of prospective blind porphyry deposits in these localities. Results of the study reveal the applicability of ground magnetic data in characterizing and extracting a potential area of mineralized zones at a regional scale

Mineralization parameters and exploration targeting for gold- copper deposits in the Baguio (Luzon) and Pacific Cordillera (Mindanao) Mineral Districts, Philippines: A review

The Baguio Mineral District in Luzon, Philippines is known to host several world-class epithermal gold – porphyry copper deposits. The interplay of tectonic setting, magma composition, structural control and hydrothermal systems contributed to the generation of these deposits. Ridge subduction (Scarborough seamount) resulting to flat subduction and a transpressional regime could also be related to the formation of epithermal gold - porphyry copper deposits in Baguio. Subduction processes leading to the formation of calc-alkaline rocks associated with high water pressure, oxygen fugacity and late sulfur saturation are almost always associated with the gold-copper deposits in the district. Compared to the Baguio Mineral District, less exploration work, mine development and production were done in the Pacific Cordillera Mineral District, Mindanao in southern Philippines. It is worth noting, however, that both mineral districts show similarities and overlapping features in terms of geological, geophysical and geochemical characteristics. This leads one to conclude that the Pacific Cordillera Mineral District has ore deposits waiting to be discovered

Bouguer Anomaly of Central Cebu, Philippines

Cebu Island in Central Philippines consists of a Cretaceous basement complex capped by mostly Tertiary sedimentary units. Recent mapping conducted in Central Cebu revealed limited exposures of lithologies, especially those comprising the basement complex. The gravity method was utilized to generate Bouguer anomaly maps for Central Cebu. These geophysical maps provide the first images of the subsurface extent of the basement units. A prominent nearly circular gravity anomaly high is consistently observed in the Bouguer anomaly maps coinciding with the location of dense basement and intrusive rocks. However, field mapping revealed the very limited surface exposure of these units. The gravity highs recognized in the residual anomaly maps may correspond to the larger extent of the intrusive units at depth. The broad gravity high observed in the regional anomaly map may define the extent of the subsurface distribution of the Cretaceous basement complex

Petrogenetic constraints on magma fertility in the Baguio Mineral District, Philippines: Probing the mineralization potential of the igneous host rocks in the Sangilo epithermal deposit

In the Baguio Mineral District (BMD), porphyry Cu ± Au deposition and associated epithermal mineralization are attributed to the highly evolved magmatism during the Pliocene. It has been well-documented that the interaction between silicic crustal melts and primitive mantle melts formed water-rich, oxidized magmas that resulted to hydrothermal mineralization. However, there are very few studies on the Early to Middle Miocene calc-alkaline magmatism which is considered to be barren of mineralization. This magmatic event is represented by phases of the Central Cordillera Diorite Complex (CCDC), which also serve as host rocks to the Sangilo epithermal deposit. The Sangilo quartz-carbonate veins in the BMD are hosted by an Early Miocene hornblende diorite (22.33 ± 0.63 Ma) intruded by a Middle Miocene quartz diorite (15.91 ± 0.6 Ma) which are, in turn, penetrated by Pliocene basaltic andesite dikes. The Miocene magmatic units with hybrid crust-mantle source affinity were formed from varying degrees of interaction within the MASH (mixing, assimilation, storage, and homogenization) zone during the formation of the CCDC. The basaltic andesite dikes, part of the Pliocene Mafic Dike Complex, represent direct differentiates of basaltic melts that experienced ponding at the base of the lower crust before ascending to shallow crustal levels. Based on the assessment of the physico-chemical conditions, three distinct magmatic events were identified: a barren Early Miocene event, a fertile Middle Miocene event and a fertile Pliocene event. The Middle Miocene fertile magmatism is attributed to further development of the MASH zone under the Luzon arc from the Early to Middle Miocene. On the other hand, the enhanced fertility during the Pliocene is associated with the subduction of the Scarborough Ridge. In the Baguio Mineral District (BMD), porphyry Cu __ Au deposition and associated epithermal mineralization are attributed to the highly evolved magmatism during the Pliocene. It has been well-documented that the interaction between silicic crustal melts and primitive mantle melts formed water-rich, oxidized magmas that resulted to hydrothermal mineralization. However, there are very few studies on the Early to Middle Miocene calc-alkaline magmatism which is considered to be barren of mineralization. This magmatic event is represented by phases of the Central Cordillera Diorite Complex (CCDC), which also serve as host rocks to the Sangilo epithermal deposit. The Sangilo quartz-carbonate veins in the BMD are hosted by an Early Miocene hornblende diorite (22.33 __ 0.63 Ma) intruded by a Middle Miocene quartz diorite (15.91 __ 0.6 Ma) which are, in turn, penetrated by Pliocene basaltic andesite dikes. The Miocene magmatic units with hybrid crust-mantle source affinity were formed from varying degrees of interaction within the MASH (mixing, assimilation, storage, and homogenization) zone during the formation of the CCDC. The basaltic andesite dikes, part of the Pliocene Mafic Dike Complex, represent direct differentiates of basaltic melts that experienced ponding at the base of the lower crust before ascending to shallow crustal levels. Based on the assessment of the physico-chemical conditions, three distinct magmatic events were identified: a barren Early Miocene event, a fertile Middle Miocene event and a fertile Pliocene event. The Middle Miocene fertile magmatism is attributed to further development of the MASH zone under the Luzon arc from the Early to Middle Miocene. On the other hand, the enhanced fertility during the Pliocene is associated with the subduction of the Scarborough Ridge