The geochemistry, tectonic and palaeogeographic setting of the Karing Volcanic Complex and the Dusunbaru pluton, an early Permian volcanic-plutonic centre in Sumatra, Indonesia

Abstract

The Karing Volcanic Complex and the Dusunbaru Pluton comprise an Early Permian volcanic-plutonic centre in the Volcanic-Plutonic Arc on the western margin of the Kluet-Kuantan basin of the West Sumatra Block in Sumatra, Indonesia. The Karing Volcanic Complex is mostly composed of ‘Monotonous Intermediate Type’ tuffs which interfinger with the volcaniclastics and coastal and marine sediments of the Mengkarang Formation. Eight lithological Intervals each commencing with tuffs overlain by volcaniclastics and sediments, have been mapped in the Merangin river section. The climax of the volcanicity occurred in Interval IV. In the waning stages of volcanism basalt flows were extruded and the volcanic pile was eroded by lahars and debris flows. Zircon CA-IDTMS dates from tuff beds at the base and top of the section bracket the volcanic activity as having taken place over a period of 630,000 years within the Asselian Stage of the Permian. The Dusunbaru pluton is composed of Gabbroids, Granitoids with subordinate Metabasalts and Metatuffs occurring as xenoliths. These lithologies are present as lithic clasts within the Karing Volcanic Complex tuffs, and it is considered that the Dusunbaru Pluton represents a former magma chamber from which the tuffs were vented. The Mildly-alkaline to Sub-alkaline Metabasalts and Gabbroids, were derived from the N-MORB sector of the lithospheric ‘Mantle Array’ while the compositions of the Granitoids (including gabbroid-granitoid hybrid rocks) were influenced by the reaction of basic magmas with the lower crust. The Sub-alkaline and High–K Late Basalt Lava flows were derived from the melting of basalt below the crust-mantle interface. The rock suites were variably hydrated as a result of alterations. Magmatic alteration has affected the Metabasalts, Gabbroids, Granitoids, their xenoliths and the Late Basalts. Hydrothermal alteration has weakly affected the aubaerial tuffs and strongly changed the mineralogy of some of the Late Basalt flows, in particular the entablatures. The chemistry of the Dusunbaru Pluton is of Cordilleran-type, typical of continent margin volcanic-plutonic arcs. The Dusunbaru Pluton was likely originally emplaced at the interface between the Kuantan Formation (Visean to Permian) and a Proterozoic gneissic basement. A late hydrothermal event/tectonic event during which the Dusunbaru Pluton was faulted against the Karing Volcanic Complex, is attributed to the collision of the Woyla oceanic terrains with Sumatra in the late-mid Cretaceous. The stratigraphy of the West Sumatra Volcanic Arc and the Kluet-Kuantan Basin resembles that of the Sukhothai Volcanic Arc and associated basin sediments in the Indochina Block. A palaeogeographical reconstruction suggests that the West Sumatra Block originally was an appendage to the Indochina Block at a palaeolatitude between 5 and 15° S.