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.

 

 

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