Peranan Subduksi terhadap Batuan Volkanik Berusia Neogen di Semenanjung Semporna: Perspektif Petrologi dan Geokimia

Abstract

The Neogene volcanic rocks of Semporna Peninsula is well known for subduction and OIB-like geochemical signature. After the Miocene subduction by the Celebes Sea, subduction-related magmatism was replaced by OIB-like magmatism. The Neogene volcanic rocks in Semporna Peninsula vary from basalt, basaltic andesite, andesite and rhyolite. Major elements and trace elements patterns suggested that the Semporna Peninsula volcanic magma underwent fractional crystallization. From the trend of primitive mantle and REE, 3 types of trends were observed: Subduction-related; depleted Nb, Ta, P, Ti, enrichment of LILE and LREE and depleted in HFSE, OIB-like; enrichment of Nb, Ta and Ti, and MORB; almost flat primitive mantle and REE. The REE patterns also shows the volcanic rocks have Eu negative and positive anomaly, which indicate plagioclase fractionation and plagioclase accumulation. According to the tectonic settings for Semporna Peninsula, the subducted-related volcanic rocks have island arc lavas and plate margin basalt setting. In Triangle FeO T -MgO-Al 2 O 3 , the subducted-related volcanic rocks scattered at the orogeny and spreading center islands areas. The tectonic settings for OIB-like (basalt, 18L5; basaltic andesite, 18L9) and MORB (basaltic andesite, 11L5) volcanic rocks are categorized into within-plate basalt and within-plate lavas which occurred in continental setting (Triangle FeO T -MgO-Al 2 O 3 ). ABSTRAK Batuan volkanik Semenanjung Semporna yang berusia Neogen terkenal dengan ciri-ciri geokimia subduksi dan menyerupai-OIB. Proses subduksi yang berlaku semasa usia Miosen menyebabkan magmatisme kaitan-subduksi diganti dengan magmatisme menyerupai-OIB. Batuan volkanik berusia Neogen di Semenanjung Semporna boleh dibahagi kepada basalt, andesit basalt, andesit dan riolit. Corak yang ditunjukkan oleh unsur major dan unsur surih mencadangkan magma batuan volkanik di Semenanjung Semporna telah mengalami penghabluran berperingkat. Berdasarkan kepada tren daripada mantel primitif dan REE, 3 jenis tren dapat dikenal pasti: Kaitan-subduksi; susutan Nb, Ta, P dan Ti, pengayaan pada LILE dan LREE dan susutan terhadap HFSE, menyerupai-OIB; pengayaan Nb, Ta and Ti, dan MORB; mantel primitif dan REE hampir rata. Pola REE turut menunjukkan batuan volkanik mempunyai nilai anomali Eu negatif dan positif, yang menjelaskan pengfraksian plagioklas dan pengumpulan plagioklas berlaku di dalam magma. Berdasarkan kepada sekitaran tektonik untuk Semenanjung Semporna, batuan volkanik kaitan-subduksi mempunyai sekitaran lava arka kepulauan dan basalt pinggir kepingan. Segitiga FeO T -MgO-Al 2 O 3 pula menunjukkan batuan volkanik kaitansubduksi tersebar pada kawasan orogeni dan pemuaian pusat kepulauan. Sekitaran tektonik untuk batuan volkanik menyerupai-OIB (basalt, 18L5; andesit basalt, 18L9) dan MORB (andesit basalt, 11L5) ialah basalt dalam-kepingan dan lava dalam-kepingan yang terbentuk di sekitaran benua (Segitiga FeO T -MgO-Al 2 O 3 ).