• Bhakti H.Harahap Pusat Survei Geologi



Neogene magmatic product in Pangkalan Kotabaru region are plotted by using TAS diagram (total alkali versus silica), and they have wide variations of rocks namely basaltic trachy andesite, basaltic andesite and dacite. On diagram SiO2 versus K2O, most of the Neogene andesite and dacite are classified as high-K calc-alkaline and rare normal calc- 2 alkaline and tholeiite, and on AFM diagram the samples are concentrated around the dividing line between calcalkaline and tholeiitic fields. In general, the Neogene volcanic rocks from this area are characterized by high aluminium (Al2O3) that varies from 15.50 - 18 wt%, low MgO (2-5 wt%), alkali (Na2O+K2O) content less than 8 wt% and low TiO2 (<1 wt%). While the trace element of the rocks is summarized on the spider diagram. Here their patterns are showing enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and heavy REE (HREE). Moreover, these patterns show an Nb negative anomaly that resembles the typical characteristic of arc volcanism. The geochemical characteristic of the volcanic rocks from this area have lower concentrations of Ba, Sr and La than other active continental margins like the Andes volcanic rocks. Like many other subduction related rocks, this volcanism characterizes a very evolved magma with MgO contents ranging from 1-5wt% and low Mg# (MgO/MgO+FeOtotal) at maximum of 40. The island of Sumatera is on the western side of the Sunda Craton (Sundaland) where India-Australia Oceanic Plate is being obliquely subducted beneath the island since Eocene. The Neogene volcanic rocks from Pangkalan Kotabaru region have been interpreted to be directly related to this subduction process.


Key words: magmatism, geology, Pangkalan Kotabaru, petrology, Neogene


Download data is not yet available.


Bailey, D.K.,1983. The chemical and thermal evolution of rift. Tectonophysics 94 : 385 - 397.

Cameron, N.R., Clarke, M.C.G., Aldis, D.T., Aspden, J.A. and Djunuddin, A., 1980. The geology evolution of Northern Sumatera. Proceeding 9th Ann. Conv. IPA. Jakarta: 149-187.

Clarke, M.C.G., Kartawa, W., Djunuddin, A., Suganda, E. dan Bagdja, M., 1982 a. Peta geologi lembar Pakanbaru, Sumatera, skala 1: 250 000. Pusat Penelitian dan Pengembangan Geologi Bandung.

Clarke, M.C.G., Kartawa, W., Djunuddin, A., Suganda, E. dan Bagdja, M., 1982 b. Geologi lembar Pakanbaru, Sumatera, skala 1: 250 000. Pusat Penelitian dan Pengembangan Geologi Bandung.

Cox, K.G. and Hawkesworth, C.J.,1985. Geochemical stratigraphy of the Deccan Traps, at Mahabaleshwar, Western Ghats, India, with implications for open system magmatic process. J. Petrol. 26:355-77.

Hamilton, W., 1979. Tectonics of the Indonesian region; US. Geol. Survey Proff. Paper 1078

Harahap, B.H., and Abidin, Z.A., 2006. Petrology of lava from Maninjau Lake, West Sumatera. Journal of Geological Resources XVI (6):359-370.

Harahap, B.H., 2006. Petrology of the Upper Miocene Volcanics on the Western Barisan Mountain Ranges Lubuk Sikaping Region West Sumatera. Bulletin Geologi, 38, 3:81-108. Department of Geology, Institut Teknologi Bandung.

Hartono, U., 1995. The major, trace, rare earth element geochemistry of the Lawu Volcano, Central Jawa. Journal of geology and mineral resources 50: 12-29.

Irvine, T. N. and Baragar, W.R.A. ,1971. A guide to the chemical classification of the common rocks. Can. J. Earth Sci. 8: 523-48.

Jakes, P. and White, A.J.R., 1972. Major and Trace Element Abundance in Volcanic Rocks of Orogenic Areas. Geological Society of America Bulletin 83:29-40.

Kay, R.W., 1980. Volcanic arc magmas: implication of a melting-mixing model for element recycling in the crustupper mantle system. J. Geol. 88: 497-522.

Le Bas, M.G., Le Maitre, R.W., Strckeisen, A. and Zanettin, B., 1986. Chemical classification of volcanic rocks based on total alkali silica diagram. J. Petrol. 27(3): 745-750.

Miyashiro, A., 1974. Volcanic rock series in island arcs and active continental margins. American Journal of Science, 274: 321-355

Peccerillo, A. and Taylor, S.R., 1976. Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastamonu area, Northern Turkey. Contrib. Mineral. Petrol. 58: 63-81.

Posavec, M., Taylor, D., Leeuwen, T. van and Spector, A., 1973. Tectonic controls of volcanism and complex movements along the Sumatera Fault System. Geol. Soc. Malaysia Bull. 6: 43-60.

Silitonga, P.H. dan Kastowo, 1995. Peta geologi lembar Solok, Sumatera Barat, sekala 1:250.000. Pusat Penelitian dan Pengembangan Geologi Bandung.

Sun, S.S. and McDonough, W.F., 1989. Chemical and isotopic systematics oceanic basalts: implications for mantle composition and processes. In: Saunders, A.D. and Norry, M.J. (eds) Magmatism in the cean Basin. Geological Society, London, Special Publications 42: 313-345.

Taylor, S.R., 1968. Geochemistry of Andesites. In: Origin and Distribution of the Elements, Ahrens, L. H. Ed, 59-583, Pergamon Press. Oxford.

Wilson, M., 1989. Igneous Petrogenesis a global tectonic approach. Harper Collins Academic.