Mount Rinjani

In Lombok, Rinjani volcano lies approximately 300 kilometres (190 mi) north of the Sunda Trench (also known as Java trench[27]) and is situated about 170 kilometres (110 mi) above the active north dipping Benioff zone.[28] Based on the composition of andesites which have very low Ni concentrations and low Mg/Mg+Fe It is suggested that the Rinjani suite is of mantle origin, but that all the andesites and dacites as well as many of the basalts have probably been modified by fractional crystallization processes.[29] It is concluded that the Rinjani calc-alkaline suite, which in many respects is typical of many suites erupted by circum-pacific volcanoes, probably originated by partial melting of the peridotite mantle-wedge overlying the active Benioff Zone beneath Lombok Island.[20][29][30] The Pleistocene-Recent calcalkaline suite from the active volcano, Rinjani is composed of a diverse range of lavas. These include: ankaramite, high-Al basalt, andesite, high-K andesite and dacite. Sr-isotopic and geochemical constraints suggest that this suite was derived from the sub-arc mantle. Geochemical models suggest that fractional crystallization is an important process in the suite's differentiation, although the series: ankaramite-high-Al basalt-andesite-dacite does not represent a continuously evolving spectrum of liquids.[31]