Page 75 - Sumatra Geology, Resources and Tectonic Evolution

62 CHAPTER 5

1994). The generation of these syn- and post-collisional granites emplacement coincident with episodic movement on major
took place over an extended period from about 275 to 190 Ma, structures. There has, however been strong cataclastic deformation
with the main peak of post-collisional plutonism from 220 to of earlier granites within the Sumatran Fault Zone which seems to
200 Ma. It was during this period that most of the stannifeous have particularly affected the tin-associated granites.
granites of Sumatra and Peninsular Malaysia were emplaced. A notable feature of the Volcanic Arc granites is their extended
At about 200 Ma this phase of crustal plutonism was superseded time range, from 203 to 5 Ma. This is in marked contrast to Cor-
by volcanic arc-related plutonism and vulcanism, generated as the dilleran batholiths which generally have a more restricted time
result of the formation of a subduction zone at the southern margin range. There are consequently both similarities and differences
of the new continent which now included Sumatra and Burma. between the volcanic arc granites of Sumatra and those of the
This resulted in the production of granitic and volcanic rocks western Americas. The Sumatran granites have not yet been
in a relatively narrow zone, with an extended compositional found to have the same economic potential as similar granites in
range from diorite to monzogranite, similar to those of other other regions, but it is uncertain whether this opinion is correct,
volcanic arc terrains, such as the Cordilleras of South and North having regard to the difficulties of the terrain. Perhaps the apparent
America. The granites of Sumatra differ from those regions in lack of mineralization is associated with the lengthy time scale and
that they do not form linear granitic batholiths of great size, but the relatively small volume of granite produced.
for the most part are represented by numerous isolated plutons An additional contrast with the Cordilleran situation is that
and small batholiths, confined within a narrow belt along the whereas the youngest and more evolved granites tend to be
southwestern margin of the island. Most of these plutons present towards the back arc region, the situation in Sumatra is
are characterised by holocrystalline plutonic textures, but the reverse, with younger granites located close to the coast, and
some are deformed, and more rarely, some have several phases hence towards the subduction zone.
of polyphase deformation, perhaps resulting from a period of