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

New zircon U-Pb geochronology from a peridotite suite near Ranau and the Telupid ophiolite in Sabah, eastern Malaysia, contradict previous studies, which assumed that the Sabah mafic-ultramafic rocks are largely ophiolitic and Jurassic–Cretaceous in age. We show that these rocks formed during a magmatic episode in the Miocene (9.2–10.5 Ma), which is interpreted to reflect infiltration of melts and melt-rock reaction in the Ranau subcontinental peridotites during extension, and concurrent seafloor spreading forming the Telupid ophiolite further south. Older zircons from the Ranau peridotites have Cretaceous, Devonian, and Neoproterozoic ages. Zircon Lu-Hf isotopic data suggest their derivation from a depleted mantle. However, significant proportions of crustal components have been incorporated in their genesis, as evidenced by their less-radiogenic Hf signature compared to a pristine mantle reservoir. The involvement of a crustal component is consistent with our interpreted continental setting for the Ranau peridotite and formation in a narrow backarc basin for the Telupid ophiolite. We infer that the Sulu Sea, which was expanding throughout much of the Miocene, may have extended to the southwest into central Sabah. The Telupid oceanic strand formed during the split, collapse, and rollback of the Sulu arc due to the subduction of the Celebes Sea beneath Sabah. Incorporation of the Sulu arc in the evolving Miocene oceanic basin is a potential source to explain the involvement of crustal material in the zircon evolution of the Telupid ophiolite.


INTRODUCTION
Present-day Southeast Asia was assembled from Gondwana continental blocks, volcanic arcs, and ophiolites and includes young ocean basins such as the South China and Sulu Seas (Hall, 1996(Hall, , 2013Hutchison et al., 2000, Hutchison, 2005Hall et al., 2008). It is widely considered that sedimentary basins of Sabah, northern Borneo (Malaysia), were deposited on an ophiolitic "basement" (including what is described as the "Chert-Spilite Formation"). Radiometric data (K-Ar) from the ophiolitic rocks of Darvel Bay, eastern Sabah, as well as radiolaria from cherts of the Chert-Spilite Formation, suggest Middle Jurassic to Early Cretaceous ages (Rangin et al., 1990;Jasin, 1992;Leong, 1999, and references therein;Hutchison, 2005;Jasin and Tongkul, 2013). The Telupid ophiolite of central Sabah has never been dated but has also been assumed to be of similar Mesozoic age, supported by radiolarian ages from the Chert-Spilite Formation in this area (Jasin, 1992).
Several authors have suggested genetic models for the opening and evolution of a Cretaceous oceanic basin in Sabah to explain the origin of the ophiolite (e.g., Tongkul, 1994;Jasin and Tongkul, 2013;Wang et al., 2016). Morley and Back (2008) suggested that Sabah has been a dynamically exhuming area since the Miocene, and Paleogene to Neogene compressional events have been linked to the opening of the South China Sea (Tongkul, 1994(Tongkul, , 1997Morley and Back, 2008;Morley et al., 2011;Wang et al., 2016). There is a general agreement on early Miocene collision in Sabah as a consequence of subduction of the proto-South China Sea continental crust beneath the Sabah active margin, resulting in the Sabah orogeny (Hutchison, 1996). However, for the post-early Miocene evolution of Sabah and north Borneo, there are conflicting views interpreting compressional (e.g., Rangin et al., 1990;Hesse et al., 2009;Morley et al., 2011;Sapin et al., 2011) or extensional (Hall, 2013) tectonics.
We present new U-Pb zircon geochronology data with Lu-Hf isotopic data from mafic and ultramafic rocks from central Sabah, indicating much younger ages than previously thought. These provide a robust basis for a radical change of ideas concerning the geological evolution of Sabah, which may help distinguish between conflicting hypotheses.
Our extensive fieldwork in central Sabah was focused on the mafic and ultramafic rocks, including widespread monotonous peridotites near Ranau town and a complete ophiolite suite in the Telupid area (Fig. 1). The Ranau peridotites are intensely lateritized and dominated by varieties of lherzolite with rare harzburgite (see Appendix S1 in the Supplemental Material 1 ). The lherzolites are commonly impregnated by dunite bodies ranging from microscopic veins to cylindrical or tabular bodies. Scarce blocks of mafic rocks are structurally trapped in the peridotites. There are a few rodingitized gabbroic dikes. Garnet-bearing peridotites from Sungai Mensaban, close to Ranau, are interpreted as subcontinental mantle (Imai and Ozawa, 1991). Our petrographic and geochemical data support the idea that the whole range of Ranau peridotites comprises subcontinental lithospheric mantle (SCLM) rocks and hence were never part of an ophiolite (Appendix S2). The late Miocene Kinabalu granite pluton (7.85 ± 0.08 to 7.22 ± 0.07 Ma; Cottam et al., 2010) has intruded the Ranau peridotites and the Crocker Formation (Fig. 1).
The Telupid ophiolite includes variably serpentinized peridotites dominated by lherzolites with minor harzburgites and replacive dunites ( Fig. 1). Local pods of magmatic dunite with subordinate chromite occur in these peridotites, which are crosscut by a few rodingitized gabbroic dikes. The mafic members are dismembered and comprise layered and isotropic gabbros, sheeted dikes, as well as pillow and massive basalts. Petrographic and geochemical details of the Telupid ophiolite lithologies are presented in Appendices S1 and S2. The ophiolitic rocks have normal fault contacts with the surrounding sedimentary formations.

Uranium-Lead Radiometric Dating
The methodological details for the U-Pb dating are described in Appendix S3. Zircons were separated from several rock types and are mostly euhedral to subhedral with aspect ratios (length:width) ranging from 1:1 to 4:1 (Fig. S2). Generally, the most elongated crystals occur in the mafic samples rather than the ultramafic ones. Most of the zircons display oscillatory zoning, and a few show planar zoning and are variably luminescent under cathodoluminescence (Fig. S2). Table 1 presents concordant ages (<15% discordance) from two Ranau peridotites (34 spots from 29 zircon crystals), and one diabase (8 spots from 7 crystals) and two basalts (29 spots from 27 crystals) from the Telupid ophiolite. Excluding the inherited ages, 54 spots (out of 71) from all samples yielded consistently Miocene ages (Fig. 2). From the subcontinental Ranau peridotites, sample SB 120B yielded 12 spots with inherited Cenomanian and two spots with inherited Early to Middle Devonian ages. Sample SB 120A has one inherited Neoproterozoic age. The vast majority of the zircons in the Telupid ophiolitic basalts and diabases are Miocene. The basaltic pillow lava SB 130C had two spots with inherited Middle Triassic ages.

Lutetium-Hafnium Isotopic Data
The Lu-Hf isotopic data for the 57 zircon spots with concordant U-Pb ages are listed in Table S2. The analytical methodology is described in Appendix S3. The Miocene zircons from the Ranau peridotites (samples SB 120A and SB 120B) yielded a large variation in their ε Hf(t) values (−5.2 to +3.5) and Nd depleted mantle model, T DM2 , ages (868-1427 Ma). An outlier has ε Hf(t) = −16.8 and a T DM2 = 2164 Ma (Fig. 2E). A Cretaceous (Aptian-Turonian) age is recorded in zircons from sample SB 120B with narrower ranges of ε Hf(t) (+5.9 to +9.6) and T DM2 model ages (543-778 Ma) relative to the Miocene ones. An outlier of this age group shows ε Hf(t) = +2.3 and an older T DM2 of 1016 Ma. Two Devonian zircon spots have more radiogenic ε Hf(t) of +8.4 and +13.1 and T DM2 model ages of 851 and 554 Ma, respectively. One zircon core in sample SB 120A has a Neoproterozoic age, a highly radiogenic ε Hf(t) (+15.1) similar to the depleted mantle value, and a T DM2 model age of 668 Ma (Fig. 2E).

DISCUSSION
New radiometric data from Sabah contradict previous assumptions of a Cretaceous age for the Telupid ophiolite and Ranau peridotite mafic and ultramafic rocks. The detailed petrogenesis of the Ranau peridotites is beyond the scope of this paper, but we show evidence for a SCLM origin (garnet-bearing peridotites, refertilization, extremely Al-rich spinels, very small degrees of melting; see Appendices S1 and S2), analogous to, e.g., the Ronda (southern Spain) or Lanzo (Italian Western Alps) peridotites (e.g., Garrido and Bodinier, 1999;Piccardo, 2010).
Inherited Cretaceous, Devonian, and Neoproterozoic zircons in the Ranau peridotites indicate a complex evolution of these rocks, typical of SCLM, which normally undergoes multiple tectonic and magmatic episodes. The complex history is also reflected in the variable ε Hf(t) ratios and T DM2 model ages of these zircons, indicating variable proportions of pristine mantle and crustal components in their genesis. We suggest that infiltrating melts scavenged these crystals from older sources during their ascent. The majority of the zircons from the Ranau peridotites crystallized during the Miocene. It is unlikely that the zircons were derived from depleted peridotites because these rocks contain incompatible elements and high Si contents. We suggest that the dated zircons are products of crystallization from melts produced during rifting that infiltrated the peridotites, and hence indicate that the melt-peridotite reaction occurred in the Miocene. The investigated zircons from the Ranau peridotites show a wide range of ε Hf(t) values that are lower than those of depleted mantle, with some even negative (Fig. 2E). This observation rules out an origin of the zircons via simple fractionation and supports the hypothesis that both a depleted mantle and a crustal source were involved in their crystallization. The magma generating these zircons was likely influenced by crustal components, which had a nonradiogenic Hf signature (e.g., Griffin et al., 2000Griffin et al., , 2002, compatible with a subcontinental lithospheric origin. The broad range of the calculated T DM2 model ages further supports a hypothesis of multiple sources. The consistent Miocene ages of zircons from the Telupid ophiolitic rocks contradict previous assumptions of Jurassic-Cretaceous ages. Their typical mid-oceanic ridge basalt (MORB) affinities (Appendix S2), variable ε Hf(t) (from very radiogenic to nonradiogenic), and highly variable T DM2 model ages suggest that melts were derived from a depleted source with variable and considerable degrees of influence from continental source(s). Spreading and rifting that began in the late middle Miocene is the most plausible explanation for melting and concurrent infiltration in the Ranau peridotites. We propose that the crustal involvement indicates that the Telupid ophiolite most likely represents remnants of a narrow ocean basin, analogous to the Rocas Verdes ophiolite in western Chile (e.g., Stern and De Wit, 2003). Continental breakup and opening were likely favored by extensional tectonics and rifting in a marginal basin, triggered by nearby subduction and convection in the mantle wedge. Inherited zircons from the 13.8-13.5 Ma Bay Peak and Mount Capoas granites (Palawan Island, Philippines) provide support that crustal melting did occur during the early stages of the Sulu Sea opening, induced by subduction rollback (Suggate et al., 2014). It is likely that contribution of crustal melts persisted until the late Miocene and affected the rifted Ranau peridotites. The absence of any major tectonic contact between the Telupid ophiolite and the Ranau peridotites (e.g., ophiolite mélange, major cataclastic zones, amphibolite sole) is in line with continental rifting and opening of a narrow oceanic basin without any subsequent major compressive tectonic event.
The new ages presented here demonstrate that the Telupid ophiolite is a separate unit from the Jurassic-Cretaceous Chert-Spilite Formation, which has commonly been included as part of the ophiolites of Sabah. During the interval of ca. 9-10 Ma, seafloor spreading occurred in the Telupid region, associated with rifting in the Ranau area, thus strongly favoring a major extensional regime in the post-early Miocene evolution of Sabah (Hall, 2013). This concept fits well with opening of Sulu Sea, where extension started ca. 19 Ma and ceased at the end of the Miocene, with a major increase in subsidence rate at ca. 11-9 Ma (Huang et al., 1991), interpreted as reflecting subduction rollback due to the northward subduction of the Celebes Sea below the Sulu arc (Hall, 2013). If this hypothesis is correct, then the region of Telupid was the westernmost end of the Sulu Sea (Fig. 3). The reported less-radiogenic Hf signatures of the studied rocks indicating crustal contamination may reflect the split, collapse, and melting of the Sulu arc in the evolving oceanic basin.

CONCLUSIONS
Uranium-lead (U-Pb) ages of zircon from mafic and ultramafic rocks of Sabah are predominantly late Miocene (9.2-10.5 Ma). Older zircons indicate events in the Cretaceous, Middle Triassic, Devonian, and Neoproterozoic. The moderately nonradiogenic Hf signature of the Miocene zircons indicates crystallization of the dated zircons from melts derived from a depleted mantle reservoir, which subsequently incorporated variable amounts of crustal material. The Miocene zircons constrain timing of extension of two regions in Sabah, with the opening of a narrow oceanic basin (Telupid ophiolite) and the concurrent impregnation of the adjacent SCLM Ranau peridotites. Our new data favor interpretations of an extensional, not compressional, regime during the middle and late Miocene in northern Borneo. The Miocene opening of the Sulu Sea continued southwest into western and central Sabah, probably resulting from subduction rollback during formation of the Dent-Semporna-Sulu arc.