Significant volcanic contribution to some quartz-rich sandstones, East Java, Indonesia

Quartz-rich sedimentary rocks are commonly assumed to be the eroded products of cratons or recycled orogens. However, active or eroded acidic volcanic regions can also be an important, but commonly overlooked, source of quartz. Cenozoic sandstones from East Java, Indonesia, illustrate this point. They are rich in quartz, and it has long been assumed that they are the product of erosion of a continental source. However, new work using a variety of provenance indicators shows that the sandstones contain a significant, previously overlooked, volcanic component. A number of factors have contributed to their character: quartz-rich source regions, eruptive volcanic processes, and tropical weathering. Ternary discriminant diagrams, such as QFL plots which use the ratios of quartz, feldspars, and lithic grains to interpret provenance from cratonic, volcanic, and recycled orogen hinterland, may mislead, particularly in tropical volcanic settings. The quartz from acidic volcanic sources is commonly overlooked because it is commonly assumed that quartz has a continental crustal source. Volcanic eruptive processes can lead to crystal enrichment in rapidly eroded ash and sediments. Intense chemical weathering can have considerable impact on the composition of sedimentary rocks by selectively removing labile minerals and lithic grains. The resulting deposits may be texturally immature but compositionally mature, and rich in resistant minerals such as quartz and zircon. In tropical settings the widely held view that quartz-rich sandstones are mature sediments representing multiple phases of recycling may in many cases be incorrect.