The Transition to Potassic Alkaline Volcanism in Island Arcs: The Ringgit--Beser Complex, East Java, Indonesia

Abstract

The origin of potassic lavas with within-plate characteristics in island arc settings is unclear. The volcanic complex of Ringgit-Beser, situated in eastern Java, has erupted lavas of both normal island arc calc-alkaline type and atypical potassic lavas, including some highly magnesian lavas. The occurrence of these primitive lavas gives an unusual insight into the source characteristics of the potassic lavas. The lavas from Ringgit-Beser have a wide range of K2O (1-1—6-4 wt.%) and MgO contents (180-1-6 wt.%). The most magnesian lavas have high Ni and Cr contents. The calc-alkaline lavas have incompatible trace element patterns typical of island arc lavas with enrichments in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and heavy REE (HREE). The potassic lavas may be divided into two series on the basis of Ba and Nb contents, with the enriched potassic (EK) series having higher Ba and Nb contents for a given MgO content than the potassic (K) series. The EK and K series lavas have some incompatible trace element ratios similar to within-plate lavas (e.g., high Ce/Pb, low LILE/HFSE ratios, and low B/Be). However, both the EK series and K series lavas have negative Ti and Zr anomalies, and the EK series lavas have high Ba/La similar to arc lavas. There is little distinction in Sr and Nd isotopes between the K and EK series, but the calc-alkaline lavas have lower 87Sr/86Sr