Detrital zircon U-Pb-Hf and trace element analyses reveal basement of western Luzon, Philippines originates from Mesozoic proto-South China Sea (SCS) rifted continent

The combination of zircon U-Pb geochronology, trace element geochemistry and Hf isotopes have become an extremely popular tool for provenance studies and paleogeographic reconstructions. Here, we present an integrated study of zircon U-Pb-Hf isotopic and geochemical constraints from two basement complexes in western Luzon, Philippines. Basement rocks from these areas offer insights into the geologic and tectonic development of northern Philippines and its correlation with adjacent areas in southeast Asia. We also include Mesozoic rocks from Taiwan for comparison. Exposed in western Luzon, the Zambales Ophiolite Complex (ZOC) preserves a complete ophiolite sequence that spans almost the entire Zambales range. Further north of the ZOC, the Dos Hermanos Mélange (DHM) is considered a tectonic mélange and forms the basement complex in NW Luzon. The presence of both magmatic and inherited zircons from this unit poses essential questions concerning their origin and provenance. Igneous zircons (n=34) from a gabbroic clast in this mélange gave a weighted mean ²⁰⁶Pb/²³⁸U age of 114.85 ± 0.85 Ma interpreted as the crystallization age of the gabbro. The ε_Hf(t) values between -25.4 to -3.5 suggests a crustally contaminated mantle-derived magma formed in a continental setting. Older zircons from the same sample show inherited ages clustering at ca. 235 Ma (n=3), 760 Ma (n=2), 1860 Ma (n=8), and 2460 Ma (n=3). These older zircons have heterogenous ε_Hf(t) values from -25.2 to +2.0 suggesting a strong crustal contribution.  The results also suggest the involvement of ancient crustal material with Yanshanian (200-60 Ma), Indosinian (250-200 Ma), and Paleoproterozoic (2800-1600 Ma) age populations, consistent with a provenance comparable to the detrital zircons from the Cathaysian Block in southeast China. Conversely, detrital zircons from a mica schist (another clast in the mélange) yielded two prominent age groups peaking at 187 Ma (n=15) and 225 Ma (n=90). These zircons have ε_Hf(t) values from +15.6 to +11.1 suggesting derivation from juvenile crust in the Late Triassic time. By contrast, detrital zircon grains from the sediments overlying the ZOC record two significant ages peaking at 43 Ma (n=21) and 107 Ma (n=3). The Eocene zircons are characterized by very high ε_Hf(t) values (+11.8 to +15.9) indicative of primitive magmas that represent juvenile additions to the crust. The older Cretaceous zircons, on the other hand, have slightly lower ε_Hf(t) values (+0.2 to +10.6) pointing to a less juvenile composition. Interestingly, these older zircons have similar geochemical and isotopic compositions as the zircons in the gabbro from the DHM. Our study provides further evidence for the presence of continental fragments beneath western Luzon. Combining these with literature data, we propose that the Mesozoic rocks from the DHM and ZOC were formed in the same tectonic setting which represents an old piece of continent that rifted off the South China continental margin during the opening of the South China Sea (SCS). This resulted in the subduction of the proto-SCS beneath the Philippine Sea Plate (PSP) and eventually collided with the rest of the western PSP in the Cenozoic.