Empat pluton granitoid terasing di Johor: Satu tinjauan petrokimia dan genesis

702001-101043-844-B
Author : Askury Abd. Kadir
Publication : Bulletin of the Geological Society of Malaysia
Page : 85-106
Volume Number : 31
DOI : https://doi.org/10.7186/bgsm31199206

Bulletin of the Geological Society of Malaysia, Volume 31, July 1992, pp. 85 – 106 

 

Empat pluton granitoid terasing di Johor: Satu tinjauan petrokimia dan genesis

(Four isolated granitoid plutons in Johor: An overview on their petrochemistry and genesis)

ASKURY ABD. KADIR

Geological Survey of Malaysia, Ipoh, Perak

 

Summary: Four isolated granitoid plutons in Johor viz. Gunung Ledang (Mt. Ophir), Bt. Mor, Lenga (Ma‘Okil) and Batu Pahat were studied petrographically and chemically. Texturally, they range from medium- to coarse-grained porphyritic- to non-porphyritic biotite granite to fine-grained biotite granite or microgranite. Late phase intrusions included pegmatites in Bt. Mor and granophyres in Lenga during the middle-Triassic to Tertiary(?) respectively.

The ages of these granitoids range from Permian (173 Ma) to Cretaceous (78 Ma) according to available Rb-Sr data.

The Q-A-P, Q-P, A-B, Q-B-F and Qz-Ab-Or plots show that these granitoids do not vary considerably in composition as is evident by their limited SiO2 contents (67 to 79%). They fall within the granite and adamellite fields and are commonly peraluminous in nature.

All of these are characterized as ilmenite-series granites, with values of magnetic susceptibility of less than 15 x 10-3 SI unit. The Gunung Ledang, Bt. Mor and Lenga plutons are categorized as I-type granites, which were apparently derived from an igneous source through partial melting at the mantle. However, the perquartzose Batu Pahat pluton can be categorized as an S-type granite which was presumably derived from sedimentary rocks by the process of partial remelting or anatexis.

Petrochemical evidence based on Harker and other variation diagrams shows that the plutons are not genetically related to one and another. Magmatic differentiation trends are suggested and restricted only within individual plutons. These four granitoid plutons were probably emplaced separately from different magmatic sources. They are believed to represent high level epizonal intrusions that had crystallized under relatively low pressure from minimum melt.

 

Ringkasan: Empat pluton granitoid terasing di Johor terdiri daripada Gunung Ledang, (Mt. Ophir), Bt. Mor, Lenga (Ma‘Okil) dan Batu Pahat. Berdasarkan petrografi, didapati julat komposisi granitoid amat terbatas atau tidak meluas merangkumi granit biotit berbutiran sederhana ke kasar, sama ada berporfir atau tak-berporfir hingga granit biotit berbutiran halus atau mikrogranit. Perejahan fasa akhir diwakili oleh pegmatit di Bt. Mor dan granofir di Lenga masing-masing merejah semasa tengah Jura dan Tertier(?).

Usia granitoid berdasarkan kepada data-data Rb/Sr yang sedia ada berjulat dari Perm (173 Ma) ke Kapur (78 Ma).

Plot-plot Q-A-P, Q-P, A-B, Q-B-F dan Qz-Ab-Or menggambarkan granitoid tidak melibatkan spektrum penjenisan yang meluas dengan kandungan SiO2 di antara 67 hingga 79%, terbatas kepada komposisi granit (+ adamelit) serta bersifat peralumina.

Keempat-empat pluton mencirikan granit siri-ilmenit dengan nilai ‘magnetic susceptibility‘ kurang daripada 15 x 10-3 unit SI, serta mempunyai nilai mol FeO/Fe2O3 yang tinggi. Sementara itu, pluton Gunung Ledang, Lenga dan Bt. Mor mencirikan granit jenis-I yang lebih dominan, berpunca daripada igneus melalui proses peleburan separa juzuk mantel. Tetapi, pluton Batu Pahat yang bersifat granit perkuarzos dengan kandungan alumina yang tinggi tergolong dalam granit jenis-S, berpunca daripada batuan sedimen melalui proses peleburan semula atau anateksis. 

Bukti-bukti petrokimia yang berdasarkan plot-plot variasi bagi mengenalpasti tahap pembezaan magma, didapati tiada perkaitan genetik secara langsung di antara satu sarna lain. Tahap pembezaan magma cuma berlaku dalam pluton masing-masing dan amat terbatas serta tiada keterusan (continuity). Ciri-ciri berkenaan menunjukkan rejahan pluton-pluton terjadi secara berasingan dari punca magma yang berbeza dan dipercayai mereka merejah pada aras tinggi atau epizon.

https://doi.org/10.7186/bgsm31199206