SOME ASPECTS OF THE VOLCANOLOGY AND GEOCHEMISTRY OF THE TENGGER CALDERA, JAVA, INDONESIA - ERUPTION OF A K-RICH THOLEIITIC SERIES

The Tengger Caldera together with the active Mount Bromo is situated i n Java, Indonesia. Though activity of Mt Bromo has been recorded since 1804, little modern petrographic and geochemical data is available. I n this study we provide a detailed petrographic and stratigraphic desc ription of the two major caldera units, i.e. the ''pre-caldera formati on'' and the ''post-caldera formation''. A zonal arrangement of the al kali content of the Sunda are volcanoes, i.e. an increase of the alkal i content in the lavas across the are from the trench to the backarc b asin, has been noticed for many years. In contrast to the common class ification of the geochemistry and magmatic affinity of the Javanese vo lcanoes related to the depth of the Benioff zone (100-150 km for thole iitic rocks and 150-250 km for calc-alkaline rocks), we found a tholei itic trend expressed by increasing FeO/MgO with increasing SiO2 and a n average Zr/Y ratio of 4.9 for the Tengger Caldera volcanics (Benioff zone depth ca 200km), despite high alkali contents (K2O >2wt%). These features characterize the Tengger Caldera volcanics as medium to high -K tholeiitic andesites and basaltic andesites.