From Rifting to Arc–Continent Collision: The Low P/T Metamorphic History of the Romblon Metamorphic Complex

ABSTRACT

Rifted continental fragments provide critical records of metamorphic conditions and processes associated with the transition from continental extension to subsequent convergence along active margins. In particular, the shallower crustal sections of rifted crust that later collide with island arcs are comparatively less well preserved and, consequently, remain understudied relative to the deeper crustal sections. Furthermore, distinguishing metamorphic signatures associated with rifting from those related to subsequent collision remains challenging due to overprinting of earlier mineral assemblages and structures. In this work, we investigate the Romblon Metamorphic Complex (RMC) in the Philippines, an exposed fragment of continental crust located at the easternmost edge of the Palawan–Mindoro Microcontinental Block (PCB), which records both extensional and collisional processes along the western Pacific margin. The RMC consists mainly of interlayered metapelites and metapsammites exposed in the three main islands of the Romblon Island Group (RIG), namely Tablas, Romblon and Sibuyan. We integrate petrography, Raman spectroscopy of carbonaceous material, mineral thermometry, phase equilibrium modelling and zircon U–Pb geochronology to reconstruct its pressure–temperature–deformation ( P‐T‐D ) evolution. The results reveal two distinct metamorphic stages. Stage 1 represents a regional greenschist‐facies event (~520 °C, up to 6.0 kbar) associated with extension‐related deformation, preserved by sedimentary layering and structures such as boudinage. Stage 2 records localised amphibolite‐facies metamorphism (~610 °C, 3.2–4.7 kbar) preserved proximal to the contact between the RMC and the overlying Sibuyan Ophiolite Complex (SOC), reflecting later tectonic modification during arc–continent collision. Detrital zircon U–Pb ages from metasedimentary rocks indicate a predominantly South China affinity, with youngest zircon populations of Early Eocene age (~55.1 ± 1.4 Ma). This work demonstrates that rifted continental fragments, such as the RMC, offer insights into the effects of continental rifting and subsequent tectonic reworking during collision. In particular, the two‐stage metamorphic evolution preserved in the RMC metasedimentary rocks provides empirical constraints on the thermal conditions of shallow, low pressure (i.e., <6 kbar) crustal regions during rifting (= 26 °C/km) and subsequent arc–continent collision (= 43–63 °C/km). Our detailed investigation of the RMC further highlights the importance of integrating P‐T‐D analyses with geochronological approaches to distinguish imprints of multiple tectonic events preserved in metamorphic terranes.