<p>Provenance studies using sedimentary geochemistry, particularly whole-rock, trace element, and rare earth element (REE) analyses, have proven useful in complex terranes such as the Philippines. In Northern Luzon, much of the geological understanding of the Central Cordillera comes from geological investigations and exploration in mining areas. While geochemical studies have mainly focused on igneous rocks, recent works on sedimentary rocks from the Baguio District, such as the Late Oligocene to Early Miocene Zigzag Formation and the Middle Miocene to Late Miocene Klondyke Formation, have identified the source rock composition and tectonic affinity of these units, using sedimentary geochemistry techniques. Building on these methods, trace and REE geochemistry was applied to Oligocene–Miocene clastic units in southwestern Mountain Province. The results revealed mafic igneous source rock compositions and suggested that the sediments were deposited in a basin with an oceanic island arc affinity. Field investigation identified a Middle Miocene unconformity supported by paleontological and geochronological data. This event is attributed to the arc polarity reversal proposed by previous studies in the region. These findings provide additional insights into the geodynamic evolution of southwestern Mountain Province during the Oligocene–Miocene period and contribute to a broader understanding of the tectonic development of the Central Cordillera.</p>