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Northwestern Ilocos Norte in Luzon, Philippines, exposes cherts, peridotite and a variety of metamorphic rocks including chlorite schist, quartzo-feldspathic schist, muscovite schist and actinolite schist. These rocks are incorporated within a tectonic mélange, the Dos Hermanos Mélange, which is thrust onto the turbidite succession of the Eocene Bangui Formation and capped by the Upper Miocene Pasuquin Limestone. The radiolarian assemblages constrain the stratigraphic range of the cherts to the uppermost Jurassic to Lower Cretaceous. Stratigraphically important species include Eucyrtidiellum pyramis ( Aita), Hiscocapsa acuta ( Hull), Protunuma japonicus ( Matsuoka & Yao), Archeodictyomitra montisserei ( Squinabol), Hiscocapsa asseni ( Tan), Cryptamphorella conara ( Foreman) and Pseudodictyomitra carpatica ( Lozyniak). The radiolarian biostratigraphic data provide evidence for the existence of a Mesozoic basinal source from which the cherts and associated rocks were derived. Crucial to determining the origin of these rocks is their distribution and resemblance with known mélange outcrops in Central Philippines. The mélange in the northwestern Ilocos region bears similarities in terms of age and composition with those noted in the western part of the Central Philippines, particularly in the islands of Romblon, Mindoro and Panay. The existence of tectonic mélanges in the Central Philippines has been attributed to the Early to Middle Miocene arc-continent collision. This event involved the Philippine Mobile Belt and the Palawan Microcontinental Block, a terrane that drifted from the southeastern margin of mainland Asia following the opening of the South China Sea. Such arc-continent collision event could also well explain the existence of a tectonic mélange in northwestern Luzon. [ABSTRACT FROM AUTHOR]

Alternating chert–clastic sequences juxtaposed with limestone blocks, which are units typical of accretionary complexes, constitute the Buruanga peninsula. New lithostratigraphic units are proposed in this study: the Unidos Formation (Jurassic chert sequence), the Saboncogon Formation (Jurassic siliceous mudstone–terrigenous mudstone and quartz-rich sandstone), the Gibon Formation (Jurassic(?) bedded pelagic limestone), the Libertad Metamorphics (Jurassic–Cretaceous slate, phyllite, and schist) and the Buruanga Formation (Pliocene–Pleistocene reefal limestone). The first three sedimentary sequences in the Buruanga peninsula show close affinity with the ocean plate stratigraphy of the North Palawan terrane in Busuanga Island: Lower–Middle Jurassic chert sequences overlain by Middle–Upper Jurassic clastics, juxtaposed with pelagic limestone. Moreover, the JR5–JR6 (Callovian to Oxfordian) siliceous mudstone of the Saboncogon Formation in the Buruanga peninsula correlates with the JR5–JR6 siliceous mudstone of the Guinlo Formation in the Middle Busuanga Belt. These findings suggest that the Buruanga peninsula may be part of the North Palawan terrane. The rocks of the Buruanga peninsula completely differ from the Middle Miocene basaltic to andesitic pyroclastic and lava flow deposits with reefal limestone and arkosic sandstone of the Antique Range. Thus, the previously suggested boundary between the Palawan microcontinental block and the Philippine Mobile Belt in the central Philippines, which is the suture zone between the Buruanga peninsula and the Antique Range, is confirmed. This boundary is similarly considered as the collision zone between them. [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]