Late Cretaceous Nature of SW Borneo and Paleo-Pacific Subduction: New Insights from the Granitoids in the Schwaner Mountains

Abstract

Cretaceous igneous rocks in SW Borneo are significant for investigating the Paleo-Pacific subduction and regional correlation. This paper presents new zircon ages and Sr–Nd–Pb–Hf–O isotopic and whole-rock geochemical data for the Cretaceous granitoids in the Schwaner Mountains. These granitoid samples yield formation ages of 92–80Ma and can be geochemically subdivided into Group 1 biotite granite and granodiorite and Group 2 monzogranite. Group 1 has higher εHf(t) values of +1.2–+14.5 and lower δ18O values of 5.2–6.6‰ than Group 2 with εHf(t) and δ18O values of −1.5–+5.9 and 6.1–6.6‰, respectively. Group 1 samples show variable εNd (t) values of −1.9–+1.7, (206Pb/204Pb)i ratios of 18.63–19.29, (207Pb/204Pb)i ratios of 15.62–15.69, and (208Pb/204Pb)i ratios of 38.67–39.49 and are the products of partial melting of newly underplated mafic rocks with a component of crustal rocks. Group 2 with slightly negative εNd (t) values of −1.1–−1.5 was originated from partial melting of metasedimentary rocks with a juvenile mafic component. These granitoids were formed in a continental arc setting related to the Paleo-Pacific subduction during the Late Cretaceous and can compare with those contemporaneous Itype granites along the Kuching zone in NW Borneo. Those Late Cretaceous granitoids with A-type affinities from the easternmost Peninsular Malaysia and Singapore to the east of Sundaland might be formed in a back-arc extensional setting after the Paleo-Pacific subduction.