Rb-Sr, K-Ar and fission track ages for granites from Penang Island, West Malaysia; an interpretation model for Rb-Sr whole-rock and for actual and experimental mica data
Kwan, T.S.; Kraehenbuehl, R.; Jaeger, E.
Contributions to Mineralogy and Petrology 4: 527-542
1992
DOI: 10.1007/bf00320907
Accession: 018384790
Penang Island represents the northwestern extension of the western magmatic belt of Peninsular Malaysia. Thirty-one samples of highly evolved biotite-and biotite-muscovite granites were used in an integrated study to unravel the complex magmatic, tectonic and cooling histories of these rocks. Highly distorted RbSr whole-rock age patterns are evident. These are attributed to the partial post-magmatic Sr homogenization within the granite batholith which led to the rotation of isochrons towards younger ages and higher (87/86)Sr intercepts. The recognition of this mechanism allowed the establishment of a new RbSr interpretation model. The intrusion ages of the granites can be extrapolated based on the evolutionary trend of the initial (87/86)Sr. Including the data of Bignell and Snelling, three episodes of granite emplacement at 307±8 Ma, 251±7 Ma and 211±2 Ma are suggested for Penang and the NW Main Range. The late-Triassic intrusive induced a hydrothermal conductive convection system which affected all the granites. It is considered to be responsible for the RbSr whole-rock age distortion, the RbSr and KAr biotite age resetting and the textural and mineralogical changes in the granites. The duration of the hydrothermal convections, deduced from the RbSr whole rock ages, is about 6 Ma and 20 Ma in the northern and southern parts of Penang respectively. Fast regional cooling to 350±50°C within a time span of 1–3 Ma is recognized for the late-Triassic Feringgi intrusive from the mica ages, followed by a generally slow cooling rate of about 1°C/Ma. Fission track ages, in addition, indicate blockwise uplift along the N-S and NW-SE tending faults, thus resulting in the exposure of deeper crustal levels in southern and eastern Penang. A change in the tensional regime since Oligocene/Miocene, accompanied by a southwest tilting of the island, is indicated by the fission track apatite ages. Variable sometimes younger KAr, respectively RbSr biotite ages mainly depend on the degree of hydrothermal overprint at different crustal levels. An increase of the reaction surface by grain size reduction influences RbSr and KAr mica ages in similar ways, as has been demonstrated by experimental data.