New zircon radiometric U/Pb ages and Lu-Hf isotopic data from the ultramafic-mafic sequences of Ranau and Telupid (Sabah, east Malaysia): Time to reconsider the geological evolution of Southeast Asia?

Abstract

Cullen and Burton-Johnson (2021) offer a reinterpretation of our zircon age data (Tsikouras et al., 2021), providing some highly arguable statements. We take this opportunity to confirm that the Ranau peridotite (RP) is not genetically associated with the Telupid ophiolite (TO). There is abundant evidence that the RP includes subcontinental lithospheric mantle (SCLM) peridotites, impregnated by magmatic fluids (Tsikouras et al., 2021). We consider that most likely the melt-rock reactions in these rocks followed rifting and extension. The Miocene zircons in our study derived from two very different sources: (1) the RP SCLM, in which the zircons are interpreted as a phase crystallized from impregnating fluids; and (2) the TO basalt and diabase (i.e., typical magmatic rocks). Only the TO is interpreted as the westernmost end of the Sulu Sea spreading. A major population of zircons from both suites yielded similar Miocene ages, clustering at around 9.2–10.5 Ma, because they formed during the same extensional phase. The Miocene zircons show similar Lu-Hf isotopic characters and εHf(t) ranges, likely indicating that melts percolating in the RP were contemporaneous with basaltic melts of the TO. Distinguishing magmatic from metasomatic zircons is not always an easy task, but there are several lines of evidence which strongly support the magmatic origin of our dated zircons. Metasomatic zircon crystals show the following morphological features (Corfu et al., 2003): (1) irregular patterns and/or bent prisms; (2) other mineral inclusions and/or inherited cores; (3) curved or diffused zoning, which does not follow the crystal outline; (4) high-U margins as embayments in low-U areas; and (5) homogenized patterns. No zircon from the RP and TO host rocks shows the above features but, in contrast, they all display well-developed growth zoning (see the Supplemental Material of our paper). In contrast to the claim by Cullen and Burton-Johnson of crust thicker than 35 km beneath Sabah, Pilia et al. (2021) reported passive seismic data and thermomechanical simulations which demonstrate significant variations in crustal thickness with crust >50 km thick beneath the Trusmadi Mountains, <35 km in Ranau, and <30 km in the Telupid area. The thicker crust is the result of Early Miocene collision, which was subsequently thinned by Late Miocene extension. Apart from the above, the five points of Cullen and Burton-Johnson can be easily refuted: