Cretaceous Kuching accretionary orogenesis in Malaysia Sarawak: Geochronological and geochemical constraints from mafic and sedimentary rocks.

The Kuching zone, the most important tectonic boundary in Borneo, separates the Sibu zone in Central Sarawak from the Schwaner Mountains granitoids in SW Borneo. It potentially provides a key window for probing the paleo-Pacific subduction process. This paper presents new geochronological, elemental and Sr Nd isotopic data for the Pakong-Serabang and Serian mafic rocks, along with detrital zircon U Pb age-data for the associated greywacke in the Sarawak Kuching zone. These mafic rocks are dated at ~77–98 Ma and show distinctive geochemical signatures. The Pakong- Serabang mafic rocks are subalkaline basalt with SiO 2 = 46.22–51.75 wt%, TiO 2 = 0.49–2.42 wt% and MgO = 6.30–10.54 wt%. They exhibit MORB-like PM-normalized patterns with depletion in LILEs and HFSEs. Their 87Sr/86Sr(t) and ε Nd (t) range from 0.70285 to 0.70728 and + 7.4 to +9.4, respectively, originating from a slab-derived fluid-modified MORB-like source. The late Cretaceous Serian Volcanics are marked by SiO 2 of 51.52–57.90 wt%, MgO of 3.50–5.06 wt% with mg-number = 36–46 and show arc-like geochemical signatures with (Nb/La) n = 0.37–0.47. They have 87Sr/86Sr(t) = 0.71188–0.71417 and ε Nd (t) = −11.1 ~ −7.3, and were derived from a mantle wedge with the input of recycled sedimentary components. The detrital zircon grains from the associated sandstones give similar age-spectra to those in East Peninsular Malaysia and SE Vietnam. Our data revealed a Cretaceous Kuching trench-arc system. In combination with other geological observations, it is concluded that NW Borneo to the southwest of the Sarawak Kuching zone was a part of the Indochina/East Peninsular Malaysia fragment in SE Asia prior to the Jurassic and re-activated as an active continental margin in the Cretaceous. Widespread accretionary orogenesis along the Kuching zone likely initiated at or before the early Cretaceous and ended no earlier than the latest Cretaceous in response to the westward subduction of the paleo-Pacific slab. • The mafic rocks in the Kuching zone of Sarawak Malaysia were dated at 77–98 Ma. • The Pakong-Serabang basalts originated from the slab fluid-modified MORB-like source. • The Serian Volcanics were derived from the recycled sediment-modified wedge source. • The Kuching Cretaceous accretionary orogenesis results from paleo-Pacific subduction. [ABSTRACT FROM AUTHOR]