Geochemical study of pyroclastic rocks in Maninjau Lake, West Sumatra

Maninjau area situated in Barisan Mountain range was a central of volcanic eruption during Plistocene-Holocene. Eruption of Maninjau volcano resulted in andesitic lava flows, and pyroclastic flows. Maninjau deposites accummulated around Maninjau Lake are covered by andesitic rocks. This Plistocene - Holocene volcano might have a close link with Semangko fault (Sumatra Fault). Pyroclastic rocks in the study area predominantly consist of pumice deposites with some crystals of plagioclase ± quartz ± pyroxene + hornblende + andesitic, lithics and glass shards. These deposites are slightly unwelded and mostly weathered. On the basis of its chemical composistion, Maninjau deposite can be divided into Ryolitic Pumiceous Tuff (Ignembrite Maninjau) and Andesitic Pumiceous Tuff (Tephra) deposites. Ignembrite Maninjau deposite is characterized by the occurrence of high-K rhyolite, whereas Tephra deposite is able to be grouped into calc-alkaline andesite. In general, these deposites have major elements with various silisic contents; this sugests that Maninjau deposites originated from heterogeneous magmas, and was erupted from a magma chamber. The relationship between SiO2 and another oxide shows negative correlation except Na2O. Pumice deposites contain various high trace elements, suggesting that the source of magma was likely from the subducted slab of oceanic plate underneath an active continental margin.