Timelines of induced subduction zones in inverted spreading centers: Records in the Central Palawan Ophiolite, Philippines

Petrological studies on the evolution of the nascent island arc and the slab-mantle wedge interface provides clues on process that occur during incipient stages of subduction. In this study, the layered mafic-ultramafic sequence of the Central Palawan Ophiolite (CPO) and radiometric dates for its metamorphic sole are presented. The CPO is a Late Eocene-Early Oligocene fossil oceanic lithosphere which experienced Tethyan-type subduction following a mid-ocean ridge inversion. Mafic-ultramafic sequences of the CPO are exposed in Simpocan (gabbronorites) and Bacungan (olivine websterites, clinopyroxenites, dunites and minor gabbronorites). These rocks represent the lower crust to upper mantle cumulate section of the CPO fossil island arc based on the low forsterite (Fo_86-88) and NiO contents (= 0.10-0.27 wt. %) of their olivines, high spinel Cr# (= 0.55-0.65), and high anorthite contents (An_89-94) of plagioclases comprising the sequence. This is further supported by P-T estimates for the equilibration of these cumulates at 880-940°C, 5-7 kbars. The conditions indicate that the mafic-ultramafic cumulates represent magmas which stagnate at or near the Moho Transition Zone of the CPO oceanic crust.

In order to constrain the timing of the subduction event, zircons were separated from the metamorphic sole of CPO referred to as the Dalrymple Amphibolite. Specifically, U-Pb ages (concordia intercept age = 35.20 ± 0.26 Ma) were obtained for the metamorphic overgrowth rims of the matrix sample B214-2G. These rims have lower Th/U ratios (= 0.04-0.34) than the inherited cores (0.25-3.35). Detailed investigation by previous works on the petrogenesis and P-T-D history of a kyanite-garnet-biotite-hornblende schist (sample B214-2G) preserved peak metamorphic conditions (~700 °C, 13kbars) and was not significantly affected by the later retrograde metamorphism of the mélange complex. Our results reveal the timing of prograde metamorphism of the subducting slab at moderately low P/T gradients (~16 °C/km) linked with incipient subduction. Together with compiled radiometric ages for the CPO by previous works (= ~35.2424 – 40.01 Ma), these results indicate that CPO-related rifting of the proto-South China Sea persisted even as subduction has begun at ~35 Ma. Furthermore, the similar age between the protolith of the Amphibolite (= 35.242 and 35.862 Ma) obtained in an earlier work with our weighted mean age of metamorphism (=35.46 ± 0.18 Ma) supports a rapid rate of reversal from spreading to subduction in induced subduction zones (~1 Ma). Similar observations on nascent arcs have thus far largely been limited to the Cretaceous Semail Ophiolite and its metamorphic sole. Since the paleogeothermal gradient preserved CPO mélange complex is significantly cooler than Semail and other ophiolites, our results may also indicate the short timeframe (~1-2 Ma) needed to cool the slab-mantle interface from the very low P/T gradients during very early stages of subduction initiation (>25°C / km), to geothermal gradients more comparable to hot subduction zones (~16 °C/km).