Title : Magma Genesis in Kabanjahe Region

Continental Margin Arc of Sumatra

Keywords:

petrology, geochemistry, Kabanjahe,

continental margin arc, tectonic

Journal’s Name & Address : Jurnal Geologi Indonesia, Vol. 6 No. 2 Juni 2011: 105-127

Author : Bhakti H. Harahap - PSG, Badan Geologi.

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Introduction : -The location of the researched area is shown on Figures 1 dan 2. The volcanic rocks from Kabanjahe Region, Karo Regency, North Sumatra Province is a product of Sibayak, Sipiso-piso, and Toba eruption which give an age of <0.074 Ma (Chesner and Rose, 1991) and display a wide range in chemical compositions, from basic to acid where basaltic andesite and andesite lavas are the main product. -Previous studies on magmatic rocks were briefly reported by Aldiss et al. (1983) and Cameron et al. (1982) during regional geological mapping of northern Sumatra. -Nineteen samples were collected from Kabanjahe Region, eleven samples are derived from Sibayak Mount and Berastagi areas, six samples derived from Sipiso-piso Mount, area and two samples from northeast coast of Lake Toba.

Methodology : -Volcanic rocks in Kabanjahe region, Karo Regency, North Sumatra Province, are products of old Toba Caldera, Sibayak Volcano, and Sipiso-piso Volcano. -The lava in this area belongs to a high-K, calc-alkaline series, with particular high Nb concentrations. -Data show that the rock was originated from magma of a continental origin formed at a subduction zone environment. -This article performs a petrological aspect, especially based on geochemical analysis including major elements, trace elements, and rare earth elements. -The results are plotted into a general and specific classification used in petrology.

Result : A.Geology -Sumatra Island is the northwest oriented physiographic expression, lying on the western edge of Sundaland, a southern extension of the Eurasian Continental Plate (Figure 1). -The main body of the island is composed of Paleozoic sediments and igneous rocks intruded by granitic plutons, and overlain extensively by Lower Tertiary sediments and Late Tertiary volcanics. -The Kabanjahe Region is covered by various types of lithology and age (Figure 4). -The oldest unit in this area is the Kluet and and Bahorok Formations that are Late Carboniferous to Early Permian in age. -The Kluet Formation is unconformably overlain by the Oligocene Bruksah Formation comprising sandstone and conglomerate -The Takur-takur unit mostly distributed around Mount Sibayak comprises andesite, dacite, and pyroclastics. -All the above units are intruded by the Plio-Pleistocene Mendem Microdiorite. -The Barus Centre spreading to the east of Mount Sibayak comprises andesitic lavas and pyroclastics. -Alluvial deposits have been mapped along the coast of the Lake Toba, composed of gravels, sands, muds, fanglomerates, and diatomaceous earth. B. Petrology -The petrographical determination of the volcanic rocks, i.e. basalt, andesite, dacite, and rhyolite have been based on the chemical nomenclature diagram proposed by Le Bas et al. (1986). -Geochemical signature of volcanic rocks from Kabanjahe Region will be determined by classification approached and composition of geochemical pattern on major elements, trace elements, and rare earth elements. C. Petrography -Eleven samples from the Kabanjahe Region, of which eight samples (07BH01A, 02A, 03AB, 04A, 05A, 06A, and 07A) belong to the Sibayak Unit and three samples (07BH08A, 09A, and 10A) come from the Singkut Unit -The basaltic andesite and andesitic lavas of the Sibayak Unit are dark grey, vesicular, and amygdales, fine-grained, and strongly porphyritic in texture with phenocrysts of plagioclase, hornblende, clinopyroxene, and biotite. -The basalt and basaltic andesite from Sipiso-piso has strongly porphyritic texture with phenocrysts of plagioclase and pyroxene in groundmass of glass and microlith of plagioclase and pyroxene (Figure 7). -The dacite from Sipiso-piso has strongly porphyritic texture with phenocrysts of plagioclase, K-feldspar, hornblende, and biotite in groundmass of microlith of plagioclase, K-feldspar and glass (Figure 8). -The basaltic andesites of Haranggaol Volcanic Formation is strongly porphyritic in texture with phenocrysts of twin clinopyroxene and plagioclase in groundmass of glass and microlith of plagioclase and clinopyroxene.

D. Geochemistry: -The volcanic rocks of Kabanjahe Region are characterized by a wide range in composition and their moderate to high-K calc-alkaline character. -They mostly fall on the calc-alkaline field on Figure 10 and calc-alkaline and tholeiitic field on Figure 11. E. Major Element -The volcanic rocks from Kabanjahe Region display a relatively wide range of silica (SiO2) contents (50.80 - 74.35 wt.%) -The volcanic rocks from Kabanjahe Region have very low Mg value (Mg/Mg+Fe total) ranging from 24.00 to 42.00. -The rocks are moderate to highly orthoclase normative (6.74 - 18.44%) except one sample of 25.00% and high anorthite normative 12.33 - 35.2 %, except one sample (4.69%). F. Trace Element -Trace element analyses of the volcanic rocks from Kabanjahe Region are represented in Table 3. -Chrome (Cr) and nickel (Ni) are easily affected by removal or addition of ferromagnesian phases such as olivine. -The abundances of vanadium (V) show an inverse correlation with SiO2 (Figure 14h),but is not followed by Cr and Ni. -The volcanic rocks from Kabanjahe, the part of the pattern from P to Cr (i.e. the immobile elements) lies parallel to but a lower level than MORB, which plots as a horizontal line at 1.0, as the data are MORB-normalized. In contrast, Rb, Ba, K, Th, and Nb (and to lesser extend La-Ce) are enriched above this level. G. Ree -Rare Earth Element (REE) analyses of the lava basalt - rhyolite from Kabanjahe Region are pre-sented in Table 4 and the result is summarized in spider diagrams (Figures 18a,b,c,d). -La, Ce, and Nd contents range from 12 - 30.62 (except one sample 6.74), 22.11 - 49.61, and 7.53 - 23.59 ppm, respectively. -The lavas are characterized by enrichment in LREE with (La/Yb)N varying from 4 - 9.

Discussion and Conclusion: 1.Discussion -Active continental margin and island arcs in the world have many features in common. -Sunda Arc of Sumatra where the studied area is located is an active continental margin in which oceanic crust flooring the Indian Ocean is moving northeasterly and being obliquely subducted at a Benioff zone along the western margin of Asian Plate. -The data presented in this paper clearly confirm a well established concept that arc magmas have a complex multi source origin, including the subduction components and mantle wedge. -The rocks collected from the Quaternary volcanic units from Kabanjahe Region range in composition from basic to acid (basal to rhyolite) and belong to an arc magma of moderate to high K-calc-alkaline, low abundance of TiO2, P2O5, and MgO with high-Nb and low Sr. -The negative Sr anomaly is most probably a consequence of the low-pressure fractional crystal-lization of plagioclase. -The presence of normative corundum is one of evidences indicating their magma was formed by partial melting of continental crust. -Phenocrysts of biotite, hornblende, clinopyroxene and also ratio of K2O/Na2O = 0.61 - 1.1 are typical of calc-alkaline of continental margin. -The trace element similarities of Kabanjahe rocks to OIBs and MORB generally suggest the presence of an OIB and MORB source. 2.Conclusion

-The volcanic rocks from Kabanjahe region in northern Sumatra are ranging in composition from basalt to rhyolite. - The volcanic products which are located 50 km east of the SFS are dominated by basaltic andesite with a high Nb, low Sr and P of medium to high K-calc-alkaline character. - A continuous range in Harker diagram indicates they are cognate. - The volcanic rocks from continental margin of Kabanjahe region have also high elements concentrations of Rb, Th, and Ce concentration. - Regarding the pattern of the rock samples in REE pattern-diagram (Figures 18a - d), the rock pattern represents relatively flat pattern of HREE (Lu-Yb-Tm-Er-Ho-Tb). - The differences depend on the degree of the primitive magma and the crystallization processes Comments : - why subduction modified OIB component makes NB high on volcanic rock? - what makes Lava in this area include the calc-alkaline series K-high?