Katili, J .A., i 973. Geochronology of West Indonesia and its implicatioti on plate tectonics: .Tectono:.physics, 19: 195-212. The results of radiometric dating of granitic rocks around Kotanopan near the west coastof-Ce0ntr~l umatra indicate an average age of 45 million years.
Granites from the Lissi Mass in the Padang Highlands, Central Sumatra, and the Larnpong Mass, uth Sumatra, possess radiometric ages of ca. 112 and ca. 88 m.y ., respectively. Granites and other• rocks from the offshore areas north of Java indicate an average age of 100 rn.y. Late Cretaceous granitic rocks are present in the islands of the Sunda Shelf namely Anambas (ca.•
rn.y.), Tembelan (ca. 85 m.y.) and Natuna (ca. 75 rn.y.). Late Paleozoic granites possessing ages of ca. 276-298 rn.y. are encountered in the basement rocks r Djambi, South Sumatra. ._ utcorhe _of thfs radiometric age dating proves to be significant for i_t permits a fresh a"alysis of
e geological evolution of Indonesia based on the plate-tectonics concept.
The Tertiary volcano-plutonic arcexposed along the west coast of Sumatra can be traced .tothe uth coast of Java. The corresponding subduction zone can be found in the islands west of Sumatra d the submarine ridge south of JavaThe Late Cretaceous plutonic belt of Sumatra does not continue to Java• but passes north of it, run:
• g however parallelto the subduction zone of Java. These two zones merge in the Mera tus Mountains
f Southeast Kalimantan.
Sumatra was already a volcano-plutonic arc during Permian time, suggesting that since this Period
e margins of at least four lithospheric plates have remained near the side of the active Sumatran arc .. The presence of Permian volcanic and granitic rocks in the Malay Peninsula and West Kalimantan,
d the results of the radiometric age determination of granitic rocks from the islands §ituatedin the unda Shelf area, point to the existence of other Permian and Cretaceous volcano-plutonic arcs east
d north of the arcs previously described in Sumatra and Java. Thus a double volcano-plutonic arc rith opposing Benioff zones must have existed during Permian and Cretaceous time in this area.
TI-,e Schwaner Mountains of West K'alimantan are considered to be the place where volcano-plutonic
cs of different ages have merged together. The correlative subduction .zones have to lie sought in the lied Danau Formation of West Kalirnantan and the northern part of'the Kuching zone, the Sibu
ne of Serawak situated northcf the Schwaner Mountains.
The evolution and complex geology of the western part of lndon~sia can only be understood by
e supposition of the existence of mega plates and sub-plates generated from spreading centers• situated the Indian Ocean and presumably in the area of the South China Sea, respectively.

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