The petrology of some young subvolcanic and volcanic rocks from West Kalimantan, Indonesia

The northern part of West Kalimantan, which is the northern part of Sundaland, has been a continental margin since the Cretaceous time. The southern part of West Kalimantan is occupied by a belt of Cretaceous granitoids Nest Borneo Basement), while the western part is composed of Jurassic-Cretaceous sediments, and Late Palaeozoic granitic and metamorphic rocks. The northern and central parts are now a sedimentary basin and a melange belt. There is a distinct unconformity between the Cretaceous and the Tertiary. Three episodes of volcanic activity (Cretaceous, Tertiary and Quaternary) have been recognized in western and central West Kalimantan. The Cretaceous volcanics are mainly basalts and dacites and minor andesites. The basalts are characterized by the presence of quartz xenocrysts and biotite in the groundmass. They have higher Si02 and lower TiO2' P205' La and Zr contents than the Tertiary volcanic rocks. These volcanics have been metamorphosed to greenschist facies. They are mainly distributed in the western part of West Kalimantan. These volcanics have an island-arc tholeiite character, which may be a response to the southward subduction recorded in the Lupar Line and the Boyan Melange. The Tertiary volcanics are largely dacites with less common rhyolites, and minor basalts and andesites. They are mainly distributed in the central West Kalimantan (CWK). The basalts are tholeiitic, and the dacites, andesites and rhyolites are calcalkaline. A few rhyolites from the southern province of the CWK are alkaline and in the central province trondhjemite occurs. The basalts from the northern province have a .different source from those of the southern province. The Tertiary volcanics contaminated by lower crustal material have a substantially different chemical signature from north to south. In the southern province, the Tertiary volcanics intrude into the belt of Cretaceous granitoids. These volcanics have the most siliceous character and the highest enrichment in K-type elements. In the northern province the Tertiary volcanics intrude the Boyan Mlange belt and the Tertiary sediments of the Kentungau Basin. The andesites and dacites have a high TiO2 content and the spidergrarn patterns are similar to those of ocean-island basalt which is contaminated by sediments. In the central province the Tertiary volcanics intrude the thick sedimentary succession of the Melawi Basin and the Cretaceous turbidite sediments. The volcanics found here are dacites and rhyolites. The Tertiary volcanics from CWE: may be related to the southeast subduction beneath the Sarawak accretionary prism. Mineral evidence indicates magma mixing has taken place in th dacites. The Quaternary lavas from Mt Niut are trachybasalts and basaltic andesites. The lavas display the geochemical features of continental tholeiites which are similar to transitional mid-oceanic ridge basalt (MORB).