Adakites without a slab: Remelting of hydrous basalt in the crust and shallow mantle of Borneo to produce the Miocene Sintang Suite and Bau Suite magmatism of West Sarawak

Abstract

We present new geochronological and geochemical data for Neogene magmatism from West Sarawak. Zircon U-Pb geochronology divides Neogene magmatic rocks of West Sarawak into a Lower Miocene West Sarawak Sintang Suite with ages of c. 19 to 21 Ma, and a Middle Miocene Bau Suite with ages of c. 12 to 14 Ma. Magmatism occurred in multiple short-lived pulses from approximately 24 Ma and was coeval with magmatic activity in NW Kalimantan and East Kalimantan. The majority of, but not all, Bau Suite samples display adakitic chemistry, while the West Sarawak Sintang Suite is predominantly non-adakitic. There was no active subduction zone or subducted slab associated with this adakitic magmatism. Instead, the geochemical diversity is consistent with the Bau and West Sarawak Sintang suites representing mixtures of mafic, mantle-derived magma with felsic magma derived from remelting of hydrous, mafic rock that had been emplaced into the lithosphere of Borneo as arc basalt tens or hundreds of millions of years previously. This origin is most evident in the main Sintang Suite of central Borneo (Kalimantan) which has preserved less contaminated examples of the mafic endmember. This endmember resembles basaltic rocks from several locations across Borneo suggesting that intraplate, mantle-derived magmatism was responsible for remelting older, hydrated basaltic rocks in the crust.