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Sains Malaysiana 31: 11-21 (2002) Sains Bumi / Earth Science
Mafic Dykes from the Perhentian Islands: Subdivision and Preliminary Textural and Geochemical Study
Azman Abdul Ghani Department of Geology University of Malaya 50603 Kuala Lumpur Malaysia
ABSTRACT
Mesozoic mafic dykes in the Perhentian islands and their surrounding area can be divided into two based on their field occurrence, i.e. the older and younger dykes. The older dykes are synplutonic to their felsic host and the younger dykes post date their felsic host. Synplutonic features shown by the older dykes are recrystallisation of the dyke with the production of amphibolite or hornfelsic texture, back veining into the dyke and dismemberment of the dyke into trains of ameboid enclaves. All these features suggest that hot mafic dyke magma intruded into mobile semi solid felsic magma. The younger dykes, which are more abundant, are mainly doleritic in composition and are similar to those found throughout the Eastern Belt of Peninsular Malaysia. They are made up of plagioclase, clinopyroxene, amphibole, iron ore and chlorite. Geochemical study shows that the younger dykes formed in a continental environment within plate tectonic setting.
ABSTRAK
Daik mafik berusia Mesozoik di Pulau Perhentian dan kawasan sekitarnya boleh dibahagikan kepada dua berdasarkan kepada kewujudan lapangan iaitu daik tua dan daik muda. Daik tua adalah sinplutonik dengan hos felsiknya manakala daik muda adalah lebih muda daripada hos felsiknya. Ciri-ciri sinplutonik yang ditunjukkan oleh daik tua adalah penghabluran semula daik dengan pembentukan tekstur amfibolit dan honfelsik, 'back veining' ke dalam daik dan pemisahan daik kepada enclave ameboid. Kesemua ciri-ciri ini mencadangkan bahawa magma panas daik mafik telah menerobos ke dalam magma felsik yang separa menghablur. Daik muda, yang lebih banyak dijumpai, berkomposisi doleritik dan sama dengan daik-daik yang di temui di Jalur Timur, Semenanjung Malaysia. Mineralogi daik ini terdiri daripada plagioklas, klinopiroksin, amfibol, mineral opak dan klorit. Kajian geokimia menunjukkan bahawa daik muda ini terbentuk di sekitaran benua dalam kepingan tektonik.
RUJUKANREFERENCES
Azman A. Ghani. 1992. Petrologi dan geokimia kawasan Pulu Perhentian dan pulau-pulau sekitarnya. BSc. Thesis, Univ. of Malaya. Azman A. Ghani. 1998. Occurrence of synplutonic dyke from the Perhentian Kecil Island Besut Terenggnau. Warta Geologi 24(1): 65-68. Azman A. Ghani. 2000a. Mesozoic mafic dykes from Eastern Belt (Part 1): Textural study of the older dykes. Proc. Dynamic Stratigraphy & Tectonics of Peninsular Malaysia (3rd seminar). The Mesozoic of Peninsular Malaysia, 48-54. Azman A. Ghani. 2000b. Mesozoic mafic dykes from Eastern Belt (Part 2): Geochemistry of the younger dykes. Proc. Dynamic Stratigraphy & Tectonics of Peninsular Malaysia (3rd seminar), The Mesozoic of Peninsular Malaysia, 96-105. Azman A. Ghani. 2001. Petrology and Geochemistry of granite and syenite from Perhentian Island, Peninsular Malaysia. Geosciences Journal 5(2): 123-137. Azman A. Ghani & Khoo, T.T. I 998. Field relation and petrology of igneous rocks in the Perhentian Island and its surrounding area, Besut Terengganu. Warta Geologi 24(4): 175-185. Azman A. Ghani, Grapes, R. & Khoo, T.T. 1998. Geochemistry of dolerite dykes from the Perhentian and Redang islands, Besut Terengganu. Programme and Abstract GEOSEA 98, Ninth Regional Congress on Geology, Mineral and Energy resources of Southeast Asia, 214-215. Barr S. M. & James, D. E. 1990. Trace element characteristics of Upper Cenozoic basaltic rocks of Thailand, Kampuchea and Vietnam. Journal of Southeast Asian Earth Sciences 4(3): 233-242. Barr, S. M., Tantisukrit, c., Yaowanoiyothin, W. & Mac Donalds, A. S. 1990. Petrology and tectonic implication of Upper Paleozoic volcanic rocks of the Chiang Mai belt, Northern Thailand. Journal of Southeast Asian Earth Sciences 4 (1): 37-47. Bussell, M. A. 1991. Enclaves of the Mesozoic and Cenozoic granitoids of the Peruvian Coastal batholith. In J. Didier & B. Barbarin (eds) Enclaves and granite petrology. Elsevier, 263 - 276. Cobbing, EJ. & Mallick, D.I.J. 1987. Southeast Asia granite project: Field report for Peninsular Malaysia. BGS, 129 p. Haile, N. S., Beckinsale, R. D., Chakraborty, K. R., Hanif Hussein, A. & Hardjono, T. 1983. Paleomagnetism, geochronology and petrology of the dolerite dykes and basaltic lavas from Kuantan, West Malaysia. Geological Society of Malaysia Bulletin 16: 71-85. Kumar, S.c. 1988. Microgranular enclaves in granitoids: agents of magma mixing. Journal of Southeast Asian Earth Sciences 2: 109-121. Mac Donald, S. 1967. The geology and mineral resources of north Kelantan and north Terengganu. Geological Survey District Memoir 10: 202. Meschede, M. 1986. A method of discriminating between different types of midocean ridge basalt and continental tholeiites with the Nb-Zr-Y diagram. Chemical Geology 56: 529 - 554. Moore, J.G. & Hopson, C.A. 1961. The independence dyke swarm in eastern California. American Journal of Sciences 259: 241-259. Pearce, J. A. & Cann, J. R. 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth and Planetary Science Letter 19: 290-300. Pitcher, W. S. 1991. Synplutonic dykes and mafic enclaves. In J. Didier and B. Barbarin (eds); Enclaves and granite petrology. Elsevier, 383-391 Roddick, J. A. & Armstrong, J. E. 1959. Relict dykes in the Coast Mountains near Vancouver, B.C. Journal of Geology 67: 603-613. Sita Ram. G., Chakraborty, K.R. & Sharifah Barlian Aidid. 1980. Instrument neutron activation analysis for rare earth elements in dolerite dykes of Kuantan area, Peninsular Malaysia. Geological Society of Malaysia Bulletin 13: 87-92. Vernon, R.H. 1991. Interpretation of mikcrostructures of microgranitoid enclaves. In J. Didier and B. Barbarin (eds) Enclaves and granite petrology. Elsevier, 227-291. Winchister, J.A. & Floyd, O. 1976. Geochemical magma type discrimation: applications to altered and metamorphosed basic rocks. Earth and planetary Science Letter 28: 459-469.
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