Textural analysis and crystal size distribution of plagioclase from Ciremai's a'ā lava: Interpretation magmatic processes and crystallization time

Abstract

Ciremai Volcano is an active volcano in West Java, Indonesia. Based on geological observation, this volcano has erupted as central and flank eruptions. One of flank eruption products can be observed on Sukageri area. The products consist of 'a'ā lava and pyroclastic fall deposits. This research is focus on 'a'ā lava characteristics using petrology methods including textural analysis and crystal size distributions. The 'a'ā lava of Sukageri area has ±11.5m thick and it spreads out into 4 km to the southward of Sukageri area. Section of the 'a'ā lava can be divided into three sections; upper, middle, and lower part. The upper and lower part characterized by autobreccia, and middle part has sheet-like shear joint structure. Petrography analysis shows that the 'a'ā lava consist of vesicular (16,04-47,85%), phenocryst of plagioclase (10,6-12.4%), pyroxene (1,66-2.78%), olivine (0.87-3,25%), and opaque mineral (0,3-0,42%). Groundmass (65,29-83,53%) of the 'a'ā lava consists of microcrystalline of plagioclase, pyroxene, and glass. The differences of crystal size indicate the changing on crystallization time. Plagioclase texture of the 'a'ā lava compose of fine-sieve, zoning, resorption surface, synneusis, glomerocrysts, and broken crystals. Those textures are indication of changing of magmatic system which caused by hotter and more primitive of magma injection or magma degassing. Crystal size distribution (CSD) analysis shows 2 trends and concave curve which indicate a progressive change in? T during magma ascent. One trend shows a steep slope (- 21,05- -27,61), with higher y-axis intercept (10,24-11,69), indicating of nucleation density, and smaller crystal size. The other trend shows phenocrysts with gentle slope (-2,22- -4,25) and lower y-axis intercepts (1,4-4,25). Crystallization time was calculated from equation T=(-1/G*slope)/31536000), with assumption of growth rate (G) is 10 8 and the difference of trend between phenocryst and microcrystalline. Those calculation show that phenocryst crystallization time of the 'a'ā lava is 0,75-1,43 years and whereas for microcrystalline is 41-54 days. The existence of microcrystalline indicates that before magma was erupted, it stayed on conduit for ±41 days. After the eruption, the 'a'ā lava needs ±13 days for cooling and solidifying.

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