Late Cretaceous nature of SW Borneo and Paleo-Pacific subduction; new insights from the granitoids in the Schwaner Mountains

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 92-80 Ma 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 ppm than Group 2 with εHf(t) and δ18O values of -1.5-+5.9 and 6.1-6.6 ppm, 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 I-type 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.