Sains Malaysiana 30: 13-26 (2001)                                                                                                                Sains Bumi /

    Earth Sciences  

Rare-earth Elements Modelling of the Langkawi Granites

Wan Fuad Wan Hassan

Geology Program

Faculty of Science and Technology

Universiti Kebangsaan Malaysia 43600 UKM Bangi

Selangor D.E Malaysia

Azman A. Ghani

Department of Geology

Universiti Malaya

50603 Kuala Lumpur, Malaysia

Syafrina Ismail

Lestari, Universiti Kebangsaan Malaysia

43600 UKM Bangi, Selangor D.E Malaysia

ABSTRAK

Berdasarkan pola unsur-unsur nadir bumi (REE) granit Langkawi boleh dibahagikan kepada dua kumpulan iaitu granit Gunung Raya-Dayang Bunting dan granit Kuah-Pulau Tuba. Granit Gunung Raya-Dayang Bunting mempunyai anomali Eu negatif yang lemah manakala granit Kuah-Pulau Tuba mempunyai anomali Eu negatif sederhana kuat. Nisbah-nisbah Rb/Sr juga menunjukkan perbezaan apabila nisbah Rb/Sr dalam granit Kuah-Pulau Tuba jauh lebih tinggi berbanding dengan nisbah-nisbah dalam granit Gunung Raya-Dayang Bunting. Pengamatan pola-pola REE menunjukkan granit-granit tersebut mempunyai bentuk 'sayap-burung' yang lazim dengan anomali-anomali Eu negatif dari lemah hingga sederhana kuat. Pola-pola demikian serupalah dengan granit jenis-S Pantai Barat dan berbeza daripada granit jenis-l Pantai Timur. Bahan sumber mungkin bagi granit Langkawi ialah sedimen Kambria dari Formasi Machinchang. Permodelan pola-pola REE telah dilakukan dalam dua tahap iaitu secara peleburan sebaya dan penghabluran berperingkat. Hasil permodelan menunjukkan magma Gunung Raya-Dayang Bunting memerlukan darjah pemeringkatan leburan separa sedikit lebih tinggi (0.15-0.25%) berbanding dengan magma Kuah-Pulau Tuba (0.25-0.3%). Sebaliknya magma Gunung Raya-Dayang Bunting telah melalui darjah pemeringkatan lebih tinggi (16%) berbanding dengan magma Kuah-Pulau Tuba (9%).

ABSTRACT

Based on the REE patterns the Langkawi granites can be divided into two groups i.e. the Gunung Raya-Dayang Bunting granite and Kuah-Pulau Tuba granite. The Gunung Raya-Dayang Bunting granite has weak negative Eu anomalies whereas the Kuah-Pulau Tuba granite has moderately strong negative Eu anomalies. Rb/Sr ratios also show similar differences with Rb/Sr ratios in Kuah-Pulau Tuba granites being much higher than those of the Gunung Raya-Dayang Bunting granite. Examination of the REE patterns shows that they have the usual 'bird-wing' shape with weak to moderately strong negative Eu anomalies. This pattern is similar to the West coast S-type granites and is different from the I-type granites of the East coast. A possible source material for the Langkawi granite is the Cambrian sediments of the Machinchang Formation. Modelling of the REE pattern has been carried out in two stages namely batch melting and fractional crystallisation. Results show that the Gunung Raya-Dayang Bunting magma requires a slightly higher degree of partial melting fraction (0.15-0.25%) compared to the Kuah-Pulau Tuba magma (0.25-0.3%). On the other hand the Gunung Raya­/Dayang Bunting magma has undergone a higher degree of fractionation (16%) compared to the Kuah-Pulau Tuba magma (9%).

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