Late Cretaceous fore–arc spreading in the northern Kuching Zone of West Borneo, SE Asia: Constraints from the Pakong Mafic Complex

Abstract

• The Pakong Complex formed at ∼ 100–78 Ma, more widely than expected. • The Pakong Complex comprises N-MORB- and E-MORB-type mafic rocks with arc-like imprints. • Fore-arc spreading resulted in the generation of the Pakong Complex. • The Cretaceous tectonic evolution of West Borneo was related to the paleo-Pacific subduction . The tectonic evolution of West Borneo and its paleogeographic correlation with other blocks/terranes in Southeast Asia during the Mesozoic remain unclear. Here we present newly obtained zircon U–Pb ages and whole-rock elemental and Sr–Nd isotopic data for the Pakong mafic rocks along the Lupar Line in the northern Kuching Zone of West Borneo. The zircon U–Pb dating results show that the Pakong gabbros were formed at 82.0 ± 1.9 Ma, while the diabases were emplaced at 78.5 ± 0.9 Ma. Geochemically, all the Pakong mafic rocks can be divided into two groups. The Group 1 samples show an affinity to N–MORB with negative anomalies in Nb, Ta and Ti, while Group 2 is characterized by E–MORB–like REE pattern with slight enrichment in LREE but similar depletion in Nb, Ta and Ti to Group 1. Isotopically, the former (ε Nd (t) = 6.23–9.87) has more depleted Nd isotopic compositions than the latter (ε Nd (t) = 4.25–5.71). The combined signatures indicate that the N–MORB–like mafic rocks (Group 1) were derived from 10 to 30% partial melting of a depleted lherzolite mantle modified by subduction–related fluids and that the E–MORB–like mafic rocks (Group 2) were the results of mixing between the Group 1 magmas and the OIB-type melts. Integrating with regional data, we propose the Late Cretaceous fore–arc spreading development along the Lupar Line in the northern Kuching Zone of West Borneo, which was induced by the rollback of the subducting Paleo–Pacific plate rather than the Proto–South China Sea slab.