The Philippines preserves evidence of the superimposition of tectonic processes in ancient and present‐day collision and subduction zone complexes. The Baguio District in northern Luzon, the Palawan–Central Philippine region and the Mati–Pujada area in southeastern Mindanao resulted from events related to subduction polarity reversal leading to trench initiation, continent‐arc collision and autochthonous oceanic lithosphere emplacement, respectively. Geological data on the Baguio District in Northern Luzon reveal an Early Miocene trench initiation for the east‐dipping Manila Trench. This followed the Late Oligocene cessation of subduction along the west‐dipping proto‐East Luzon Trough. The Manila Trench initiation, which is modeled as a consequence of the counter‐clockwise rotation of Luzon, is attributed to the collision of the Palawan microcontinental block with the Philippine Mobile Belt. In the course of rotation, Luzon onramped the South China Sea crust, effectively converting the shear zone that bounded them into a subduction zone. Several collision‐related accretionary complexes (e.g. Romblon, Mindoro) are present in the Palawan–Central Philippine region. The easternmost collision zone boundary is located east of the Romblon group of islands. The Early Miocene southwestward shift of the collision boundary from Romblon to Mindoro started to end by the Pliocene. Continuous interaction between the Palawan microcontinental block and the Philippine Mobile Belt is presently taken up again along the collisional boundary east of the Romblon group of islands. The Mati–Pujada Peninsula area, on the other hand, is underlain by the Upper Cretaceous Pujada Ophiolite. This supra‐subduction zone ophiolite is capped by chert and pelagic limestones which suggests its derivation from a relatively deep marginal basin. The Pujada Ophiolite could be a part of a proto‐Molucca Sea plate. The re‐interpretation of the geology and tectonic settings of the three areas reaffirm the complex geodynamic evolution of the Philippine archipelago and addresses some of its perceived geological enigmas.

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Abstract. We have identified in the Philippine Archipelago 230 samples of Late Miocene to Quaternary intermediate and evolved magmatic rocks or glasses, the compositions of which plot within the adakitic field defined by Defant and Drum‐mond (1990) using Sr/Y ratios versus Y contents. These rocks belong to four different subductions, along the Manila Trench (Batan, Northern Luzon, Central Luzon), the Negros and Sulu Trenches (Negros and Western Mindano), the Cotobato Trench (Southern Mindanao) and the Philippine Trench (Eastern Mindanao). Lavas from Central Mindanao overlie the deep remnants of the Molucca Sea Plate, and were emplaced in a post‐collision setting. All these samples show a significant depletion in Y and HREE with respect to their “normal” calc‐alkaline equivalents, suggesting that garnet was either a residual phase during partial melting or a fractionating mineral during differentiation or assimilation coupled with fractional crystallisation (AFC). However, only 19 samples out of our set (i.e., 8 %) display very high Sr/Y ratios (100–250). Our preferred model for the genesis of these “typical adakites” is ca. 20 % partial melting of subducted altered oceanic metabasalts converted to eclogite. This melting process could have been triggered by water from the underlying serpentinites. Most of the samples, termed “intermediate adakites”, display major and trace element chemical features intermediate between those of the former group and those of normal calc‐alkaline lavas. We show that magma mixing between slab‐derived adakitic magmas and mafic mantle‐derived melts accounts for most of the trends linking typical and intermediate adakites, although an additional contribution of mantle is required in some cases.

The basement complexes of the Philippine archipelago include at least 20 ophiolites and ophiolitic complexes. These complexes are characterised by volcanic sequences displaying geochemical compositions similar to those observed in MORB, transitional MORB-island arc tholeiites and arc volcanic rocks originating from modern Pacific-type oceans, back-arc basins and island arcs. Ocean island basalt-like rocks are rarely encountered in the volcanic sequences. The gabbros from the ophiolites contain clinopyroxenes and plagioclases showing a wide range of XMg and An values, respectively. Some of these gabbros exhibit mineral chemistries suggesting their derivation from basaltic liquids formed from mantle sources that underwent either high degrees of partial melting or several partial melting episodes. Moreover, some of the gabbros display a crystallization sequence where orthopyroxene and clinopyroxene appeared before plagioclase. The major element compositions of coexisting orthopyroxenes and olivines from the mantle peridotites are consistent with low to high degrees of partial melting. Accessory spinels in these peridotites display a wide range of XCr values as well with some of them above the empirical upper limit of 0.6 often observed in most modern mid-oceanic ridge (MOR) mantle rocks. Co-existing olivines and spinels from the peridotites also exhibit compositions suggesting that they lastly equilibrated under oxidizing mantle conditions. The juxtaposition of volcanic rocks showing affinities with modern MOR and island arc environments suggests that most of the volcanic sequences in Philippine ophiolites formed in subduction-related geodynamic settings. Similarly, their associated gabbros and peridotites display mineralogical characteristics and mineral chemistries consistent with their derivation from modern supra-subduction zone-like environments. Alternatively, these rocks could have, in part, evolved in a supra-subduction zone even though they originated from a MOR-like setting. A simplified scenario regarding the early geodynamic evolution of the Philippines is proposed on the basis of the geochemical signatures of the ophiolites, their ages of formation and the ages and origins of the oceanic basins actually bounding the archipelago, including basins presumed to be now totally consumed. This scenario envisages the early development of the archipelago to be largely dominated by the opening and closing of oceanic basins. Fragments of these basins provided the substratum on top of which the Cretaceous to Recent volcanic arcs of the Philippines were emplaced.