Preliminary study of the origin of mafic mineral megacrysts in volcanic rocks in the southwestern part of Kulon Progo Mountains, Indonesia

Abstract

The Bagelen district in the southwestern part of Kulon Progo Mountains has a unique aspect in the form of the occurrence of mafic mineral megacrysts in volcanic rocks. This research is conducted to determine the type, the distribution, and the origin of those megacrysts, which are abundant in pyroclastic rocks. Detailed geological mapping on an area of 4 x 4 km with a scale of 1: 25,000 and petrographic analysis were done to explain the geological aspects that led to the presence of mafic mineral megacrysts in the study area. Petrographic analysis shows that the types of mafic mineral megacryst are hornblende, clinopyroxene, and plagioclase. Their features under the microscope observation are oscillatory zoning, half oscillatory zoning, sieve, and breakdown texture. These minerals are found as single crystal fragments and also as clinopyroxene and hornblende cumulate fragments in pyroclastic breccia. Based on mineralogical composition and texture, these mafic mineral megacrysts were formed by fractional crystallization process in the form of crystal settling mechanism. These minerals were initially accumulated in the bottom of magma chamber, then carried upward by rapid magma rising in association with explosive eruption event of Mount Ijo. These minerals were then transported on the surface through the mechanism of pyroclastic flow and were deposited in the valley to form an alluvial fan morphology in the southwestern part of Kulon Progo Dome.