Age and petrology of the Usun Apau and Linau Balui volcanics: Windows to central Borneo’s interior

• The Usun Apau dacites are ca. 4 Ma. Usun Apau and Linau Balui basalts are ca. 2 Ma.
• Both suites result from a small percentage of melting of a garnet-bearing mantle.
• Their OIB-like mantle source was LILE-enriched probably during much older subduction.
• Isotope ratios indicate assimilation of old, possibly Precambrian, continental crust.
• Isotopic ratios are similar to Pliocene volcanics of the SCS and Sulu Arc.

The Usun Apau plateau lies in a remote area of Sarawak along the Tinjar Line, which defines the onshore part of a suture between the Luconia and Dangerous Grounds blocks. Reconnaissance studies in late 1950s established that the plateau is composed of a bimodal suite of young volcanic rocks, but no further work exists to constrain the age and petrogenesis of the Usun Apau volcanics. We present and discuss new data from a suite of volcanic rocks recently collected from the Usun Apau region. These data include 40Ar–39Ar age dates of mineral separates, major and trace element geochemistry, and Sr, Nd, Pb isotope geochemistry. The Usun Apau plateau is constructed largely of dacite and andesite erupted between 3.9 and 4.1 Ma. Minor basaltic dikes and flows (ca. 2.1 Ma) represent a distinctly younger episode of volcanism that is similar in age and character to the Linau Balui basalts about 100 km SE of the plateau. Although the trace element and isotopic suites from both areas indicate the parental melts were generated from a garnet-bearing, LILE-enriched, non-HIMU OIB-like mantle source, depletion in the HREEs and a negative Nb anomaly impart some characteristics of an island arc-type source contribution. The Usun Apau and Linau Balui volcanics are too young to be directly linked to subduction beneath Borneo; indicating a source region possibly modified by an older episode of subduction. Sr, Nd, Pb inter-isotope correlations plot within the same arrays as Pliocene basalts from the Southern Sulu Arc (500 km NE) which suggests much of northern and central Borneo is underlain by similar lithosphere. Assimilation–fractional crystallization modeling indicates that differentiation of the Usun Apau dacite magmas included assimilation of continental crust with very low 143Nd/144Nd. Modeling different basement compositions as contaminants yielded non-unique results. Triassic Malay granite and different Archean granites represent plausible types of assimilants; whereas crust of Dangerous Grounds and Kontum Plateau do not.