CRETACEOUS CRUST IN SW BORNEO: PETROLOGICAL, GEOCHEMICAL AND GEOCHRONOLOGICAL CONSTRAINTS FROM THE SCHWANER MOUNTAINS

摘要

New geochronological data from SW Borneornindicate that supposed basement rocks of the PinohrnMetamorphic Group are Cretaceous rather thanrnPalaeozoic as accepted until now. They are intrudedrnby Cretaceous granites in the Schwaner Mountains.rnThe Southern Schwaner Zone (SSZ) intrusives arerngranites and alkali-granites. Petrological data andrnmajor and trace element compositions indicate thatrnthey are within-plate granitoids (WPG) with a mixrnof S- and I-type characteristics. U-Pb ages ofrnzircons from SSZ granitoids record magmatism atrnc. 185 Ma and c. 75 Ma. Granitoid rocks from thernNorthern Schwaner Zone (NSZ) are I-type tonalitesrnand diorites. Trace element compositions indicaternan arc magmatic character. U-Pb ages of zirconsrnfrom Schwaner granitoids demonstrate that NSZrnmagmatism took place from c. 120 Ma to c. 80 Ma.rnUnmetamorphosed and metamorphosed volcanicrnrocks from the NSZ are tuffs, ignimbrites, and lavasrnof intermediate to acid composition. Trace elementrncompositions indicate a volcanic arc character andrnthey chemically resemble the NSZ granitoids. U-Pbrnages of zircons record volcanism from c. 130 Ma.rnMetamorphic rocks (the Pinoh Metamorphic Group)rnare pelitic schists and hornfelses, with occasionalrnquartzites and metabasites. Major elementrngeochemistry points to an igneous protolith. Tracernelement plots suggest volcanogenic sedimentsrnwhich chemically resemble the NSZ granitoids. Allrnmetamorphic rocks contain Cretaceous zircons.rnTheir ages overlap with zircons from volcanic andrnplutonic rocks, suggesting that rocks of the PinohrnMetamorphic Group were derived from arnvolcanogenic protolith which was subjected tornthermal metamorphism related to granitoidrnintrusion.WPG magmatism at c. 185 Ma is considered tornmark extensional rifting of a SW Borneo block fromrnthe northern margin of Australia. This block movedrnnorthwards during the Cretaceous forming an intraoceanicrnarc above a subduction zone in the earlyrnCretaceous which provided the source of the NSZrnarc granitoids. The block docked with Sundaland atrnabout 90 Ma.