U-PB DATING OF DETRITAL ZIRCONS FROM WEST JAVA SHOW COMPLEX SUNDALAND PROVENANCE

摘要

Paleogene sedimentary rocks in southwest Java record detrital contributions from different sources with different ages. The Ciletuh, Ciemas, Bayah, Cijengkol and Cikalong Formations have depositional .ages of Middle Eocene to Early Oligocene. Samples from the Ciletuh Formation are deep marine volcanogenic sandstones whereas samples from the other formations are terrestrial and marginal marine quartz-rich sandstones. All contain abundant zircons from which U-Pb ages have been obtained by Laser Ablation ICPMS dating. Zircons yield a wide range of ages that span the Phanerozoic and Proterozoic, with rare Archean grains. Common age clusters include Late Cretaceous-Paleogene (40-80 Ma), Cretaceous (70-130 Ma), Permian-Triassic (190-270 Ma) and Late Neoproterozoic-Cambrian (480-590 Ma), although not all clusters are present in all samples. The zircon age spectra, combined with field observations, palaeocurrent measurements and light mineral analyses, are used to identify possible sediment sources.The zircon ages are interpreted to indicate a Cretaceous and Early Paleogene volcanic source for the Ciletuh Formation and Sundaland sources for all other formations. Cretaceous zircons are present in all younger (Upper Eocene and Oligocene) quartzose sedimentary rocks. The Middle Eocene volcanogenic Ciletuh Formation contains abundant Cretaceous and Paleogene zircons but few zircons of greater age. In contrast, Cretaceous zircons are almost entirely absent from the Middle Eocene Ciemas Formation. Permian-Triassic zircons in the Ciemas Formation and all other quartz-rich sandstones are interpreted to be derived from granites of this age in the Malay Peninsula and the Indonesian Tin Islands. These zircons are interpreted to be derived from Cretaceous granites that are distributed across the Sunda Shelf, and inthe Schwaner Mountains of SW Borneo. The differences in zircon populations of the quartz-rich sandstones thus reflect changing Sundaland sources with time. In the Middle Eocene sediment was derived mainly from the Tin Belt but not from Borneo, but from the late Eocene onwards a Borneo source became more important. Cambrian-Late Neoproterozoic zircons are present in all samples but their source is unknown as no rocks of this age are exposed in the region. All quartz-rich sandstones contain significant Proterozoic zircons, but unlike quartz-rich sandstones in East Java there are almost no Archean zircons.