[SVC43-P08] Stratigraphy and Unit Determination of Ranau Tuff in Sumatra, Indonesia
キーワード：Ranau Tuff, Caldera-forming eruption, Component analysis, Pumice types
Ranau Tuff is a thick massive ignimbrite produced by a large caldera-forming eruption at ± 33 ky ago. The deposit covers about 70 km around its vicinity with an approximate bulk volume of 173 km3. The remnant edifice of Ranau has an unusual shape of caldera, which is thought to be caused by the combination of pull-apart basin and caldera formation. Active tectonic setting and volcanism in the area affected the process in the magma chamber and influenced the products, especially pumice clast in ignimbrite. Although it has a huge deposit, there are only limited studies about Ranau Tuff. Therefore, this study tries to characterize the pumice by classifying them into several types and clarify the stratigraphy to obtain an insight into the magma chamber condition.
We clarified the stratigraphy of nine locations around Ranau Lake by fieldwork. For non-welded deposits, we did the grain size analysis for dried samples from each layer by sieving tools. We did the component analysis for the grains larger than 2 mm by grouping into pumice, lithic, and free-crystal with the hand-picking method. For pumice grains with the selected size range (4-8 mm and 8-16 mm), we conducted the 3D volume measurement using Medit Solutonix D700 3D scanner and read the files by Hira 3D viewer to determine the bulk density. To know the major and trace element compositions of the pumice samples, we used the X-Ray Florescence (XRF) method.
From the fieldwork, it is found that Ranau Tuff mostly shows massive and non-welded ignimbrite with no evidence of the precursory Plinian phase. We observed thin fall deposit layers (5-10 cm) within the non-welded ignimbrite layers in Location 2 (9.2 km NE from caldera rim). Component analysis and mineralogy observation for the thin sections show three dominant pumice types of the ignimbrites; type A (pyroxene-bearing), type B (quartz-bearing), and type C. The lithic components are similar in all samples. Otherwise, the free-crystal of quartz, feldspar, biotite, and quartz-biotite aggregate are found in all locations except that in location 6 (type A pumice) and less than 5% in the lower part of location 9 (type C pumice). Qualitatively, the size of vesicle and phenocryst in pumice type A and C is smaller than that in type B. Besides, bubble-bearing and hourglass melt inclusions identified only in quartz phenocryst of pumice B. Major and trace element variation diagrams also show distinct trends among these pumice types. Therefore, we call the unit with each dominant pumice type by unit A (pumice type A), unit B (pumice type B), and unit C (pumice type C). Most of the deposits we found around Ranau Lake are unit B, only location 6 (36 m NE from caldera rim) is unit A and lower part of location 9 (15.6 km NE from caldera rim) is unit C which deposited before unit B.
A different system of magma in each unit is represented by the relation between its chemical trend and the pumice occurrence. Unit A (pyroxene-bearing) has less silica and potassium content than unit B (quartz-bearing) and C. The alkali-silica diagram of unit B does not show a clear trend which possibly affected by potassium content from biotite crystal in the whole-rock samples. However, there is a clear gap among the units in the incompatible trace elements-silica variation diagram such as Zr, Ba, K, Nb and does not show a linear relationship. This fact suggests different magma batches system formed prior to Ranau caldera-forming eruption.
We clarified the stratigraphy of nine locations around Ranau Lake by fieldwork. For non-welded deposits, we did the grain size analysis for dried samples from each layer by sieving tools. We did the component analysis for the grains larger than 2 mm by grouping into pumice, lithic, and free-crystal with the hand-picking method. For pumice grains with the selected size range (4-8 mm and 8-16 mm), we conducted the 3D volume measurement using Medit Solutonix D700 3D scanner and read the files by Hira 3D viewer to determine the bulk density. To know the major and trace element compositions of the pumice samples, we used the X-Ray Florescence (XRF) method.
From the fieldwork, it is found that Ranau Tuff mostly shows massive and non-welded ignimbrite with no evidence of the precursory Plinian phase. We observed thin fall deposit layers (5-10 cm) within the non-welded ignimbrite layers in Location 2 (9.2 km NE from caldera rim). Component analysis and mineralogy observation for the thin sections show three dominant pumice types of the ignimbrites; type A (pyroxene-bearing), type B (quartz-bearing), and type C. The lithic components are similar in all samples. Otherwise, the free-crystal of quartz, feldspar, biotite, and quartz-biotite aggregate are found in all locations except that in location 6 (type A pumice) and less than 5% in the lower part of location 9 (type C pumice). Qualitatively, the size of vesicle and phenocryst in pumice type A and C is smaller than that in type B. Besides, bubble-bearing and hourglass melt inclusions identified only in quartz phenocryst of pumice B. Major and trace element variation diagrams also show distinct trends among these pumice types. Therefore, we call the unit with each dominant pumice type by unit A (pumice type A), unit B (pumice type B), and unit C (pumice type C). Most of the deposits we found around Ranau Lake are unit B, only location 6 (36 m NE from caldera rim) is unit A and lower part of location 9 (15.6 km NE from caldera rim) is unit C which deposited before unit B.
A different system of magma in each unit is represented by the relation between its chemical trend and the pumice occurrence. Unit A (pyroxene-bearing) has less silica and potassium content than unit B (quartz-bearing) and C. The alkali-silica diagram of unit B does not show a clear trend which possibly affected by potassium content from biotite crystal in the whole-rock samples. However, there is a clear gap among the units in the incompatible trace elements-silica variation diagram such as Zr, Ba, K, Nb and does not show a linear relationship. This fact suggests different magma batches system formed prior to Ranau caldera-forming eruption.