Genesis of the Kinabalu (Sabah) granitoid at a subduction-collision junction

The Kinabalu batholith is a late Neogene granitoid in northwestern Sabah (East Malaysia) apparently marking the locus at which subducted South China Basin lithosphere interacted with roots of the northern Sabah collision suture. The exposed batholith comprises a relatively small core of biotite quartz monzodiorite (BQM) grading out to dominant hornblende quartz monzonite (HQM). Both lithologies contain mafic igneous and metasedimentary inclusions and are cut by late-stage aplite dikes. Major element data indicate the BQM (K₂O/Na₂O ratios ranging 0.72–1.03) represents a low-K type while HQM (K₂O/Na₂O ratios ranging 1.35–5.58) is a distinct high-K type. Magmaphile element distributions support this distinction, HQM showing higher K, Rb, LREE and lower Ta and Nb contents than BQM, indicating the more extensive interaction of HQM with sial. Least-squares mass balance models suggest that HQM evolved through the combined effects of fractional crystallization and crustal assimilation while BQM was dominated by fractional crystallization. However, similar plagioclase zoning patterns in both lithologies suggest they are comagmatic rather than generated by melting of separate sources. It is concluded that low-K type melts, formed by subduction-induced remelting of underplated lower crust, underwent high pressure sialic contamination with development of high-K character. These provided access to later-formed low-K melts which were less contaminated and consolidated to form the pluton core. The unusual zonation from inner low-K to outer high-K type compositions may indicate that the cessation of subduction prevented upward migration of the melting anomaly and thermal maturation of the pluton.