Publications

New geochronological data from SW Borneo indicate that supposed basement rocks of the Pinoh Metamorphic Group are Cretaceous rather than Palaeozoic as accepted until now. They are intruded by Cretaceous granites in the Schwaner Mountains. The Southern Schwaner Zone (SSZ) intrusives are granites and alkali-granites. Petrological data and major and trace element compositions indicate that they are within-plate granitoids (WPG) with a mix of S- and I-type characteristics. U-Pb ages of zircons from SSZ granitoids record magmatism at c. 185 Ma and c. 75 Ma. Granitoid rocks from the Northern Schwaner Zone (NSZ) are I-type tonalites and diorites. Trace element compositions indicate an arc magmatic character. U-Pb ages of zircons from Schwaner granitoids demonstrate that NSZ magmatism took place from c. 120 Ma to c. 80 Ma. Unmetamorphosed and metamorphosed volcanic rocks from the NSZ are tuffs, ignimbrites, and lavas of intermediate to acid composition. Trace element compositions indicate a volcanic arc character and they chemically resemble the NSZ granitoids. U-Pb ages of zircons record volcanism from c. 130 Ma. Metamorphic rocks (the Pinoh Metamorphic Group) are pelitic schists and hornfelses, with occasional quartzites and metabasites. Major element geochemistry points to an igneous protolith. Trace element plots suggest volcanogenic sediments which chemically resemble the NSZ granitoids. All metamorphic rocks contain Cretaceous zircons. Their ages overlap with zircons from volcanic and plutonic rocks, suggesting that rocks of the Pinoh Metamorphic Group were derived from a volcanogenic protolith which was subjected to thermal metamorphism related to granitoid intrusion. WPG magmatism at c. 185 Ma is considered to mark extensional rifting of a SW Borneo block from the northern margin of Australia. This block moved northwards during the Cretaceous forming an intra-oceanic arc above a subduction zone in the early Cretaceous which provided the source of the NSZ arc granitoids. The block docked with Sundaland at about 90 Ma.