Abstract:
Mt. Bromo -Tengger is a caldera complex located some 45 km to the south of Probolinggo, East Java, Indonesia. There are at least two calderas Ngadisari and Sandsea which are 8x8 km and 6x7 km in size respectively.
Bromo is the youngest of several young basaltic scoria cones within the Sandsea caldera. Volcanic activity has been recorded in historic time from 1804. The latest eruption was in 1980.
This study concentrates on the Sukapura ignimbrites associated with caldera formation and younger pyroclastic deposits emplaced in the Sapikerep valley. Post caldera lava flows exposed in this valley and part of the pyroclastic fall sequence are also examined. The pre caldera lavas are used as comparative study relating to the history of this caldera complex.
The volcanic rocks are calc alkaline to high -K calc alkaline series. The majority of the pre and post caldera lavas are low-K basalt to basaltic andesite, whereas the ignimbrite, pyroclastic flows and falls are high-K andesite The ages of several lava flows, dykes and pyroclastic deposits were obtained by paleomagnetism, K/Ar and 14C dating methods. Pre caldera lavas are 265,000±40 years and the Sukapura Ignimbrite is about 152,000 ± 30 years old. Other lava flows are dated at 135,000 ± 30 and 144,000± 30 years. Petrographical, chemical and grain size analyses were carried out, and part of the weathered rocks were analysed by Scanning Electron Microscope. The petrographic study indicates that plagioclase is the predominant phenocryst in basalt, basaltic andesite and andesite of the Bromo Tengger volcanic complex. Textures in the volcanic rocks include porphyritic, vitrophyric, glomeroporphyritic, porphyroaphanitic, semi eutaxitic (the welded ignimbrite), pilotaxitic and sometimes sub ophitic. The petrographical study suggests that the the abundance of plagioclase phenocrysts is decreasing with time.
The geochemical analyses show that the Bromo Tengger volcanic complex is in an island arc setting and have high Al2O3 ( 15-17 %) and low TiO2 (less than 1.2%). Enrichment of LIL elements such as K, Rb, and Ba with depletion of Ti is also consistent with island arc rocks.
The study on the ignimbrite suggests that this deposit was emplaced wet and associated with phreatomagmatic eruptions, whereas the younger pyroclastic flows were dry magmatic. The field and laboratory data suggest that the Sukapura ignimbrite is the product of the first caldera, whereas the pyroclastic flows originated from a younger cone during the post caldera period.
