Mineralogical and geochemical constraints on magmatic evolution at Tengger caldera, Indonesia

A detailed mineralogical and geochemical study was carried out on stratigraphically wellconstrained samples at Tengger Caldera Volcanic Complex (TCVC) to investigate the petrological and geochemical evolution of Tengger Caldera and the genetic relationship of the post-caldera volcanic cones to the pre-caldera magmatic system. The TCVC deposits are separated into the pre-caldera, caldera-forming and post-caldera eruptive episodes that span 820,000 years. These deposits range in composition from basalt, basaltic andesite to trachyandesite and are tholeiitic to calc-alkaline in nature. The volcanic rocks define two geochemical groups based on major and trace element compositions: a low silica group and high silica group, with the initial caldera-forming eruption sequence characterised by low silica content. The post-caldera centres of Batok and Bromo show affinity to the low silica and high silica groups, respectively. Bromo rocks contain a more restricted range in plagioclase composition, towards lower An content compared to Batok. Bromo samples contain phenocrysts of pigeonite, which is absent elsewhere in the system. Thermobarometry estimates of crystallisation temperatures and pressures reveal two storage regions beneath the caldera: a shallow region at 7 to 11 km and a deeper region at 19 to 27 km. Magma feeding Bromo shows relativity deeper storage compared to that at Batok.