Page 71 - Sumatra Geology, Resources and Tectonic Evolution

58 CHAPTER 5

I I
[]
[]

~ ~ ~

//0 / ~ 0 0 .........
,-i eS~ [fill -----~'--
.,: o / 0 ~ J'~ ~ O0
o
§
o4 "~ IO e i fi~ ..~,I
Z
8. +.
A/ 8,,/' v 7 v ' v Ov v 9ov O\,7x,,x 10 ~,
Fig. 5.5. South Sumatran Volcanic Arc Granites (filled circles), Hatapang 0 t t ............
Granite (open circles and Bukit Batu Granites (squares) plotted on the QAP 50 60 70 80
modal diagram of Le Maitre (1989).
SiO2(Wt%)
Fig. 5.7. Compositions of the granites of southern Sumatra plotted on a total
alkalies vs. SiO2 diagram, dashed lines denote the calc-alkaline field of Kuno
They also plot within the volcanic arc field on Pearce diagrams
(McCourt & Cobbing 1993; McCourt et al. 1996) (Fig. 5.4) and (1979). Symbols as in Figure 5.5
in the calc-alkali field in Figs 5.6 & 5.7.
At about the same time Gasparon & Varne (1995) published a the fault, which was initiated during the Miocene, and it is most
study of selected granites and volcanic rocks from widely dis- likely that the deformation developed as a result of emplacement
persed localities from the whole of Sumatra. They provided 16
processes. Barber (2000, p. 732) has suggested that it was
analyses of granitic rocks ranging from 50 to 77% SiO2. Eleven emplaced in an active sinistral strike-slip shear zone. Elsewhere
of these analyses were from southern Sumatra and seven from along the West Sumatra Fault, particularly to the north of
northern Sumatran granites, including the Sikuleh Batholith at Padang, strong cataclastic deformation has been observed from
the northwestern tip of the Island (Fig. 5.1) from which two plutons which were fully crystalline before the initiation of the
samples were taken, a monzogranite and a granodiorite. This is fault. This is particularly the case for some K-feldspar megacrystic
a large, complex and in part deformed and foliated batholith, for granites which are representative of the tin-associated granites.
which until now only been field observations have been available.
The data of Gasparon & Varne (1995) confirms the volcanic arc
nature of all these granitoids.
The majority of granitoids of the volcanic arc suite are unde- Comparison of recent work
formed, or only weakly foliated. Some however, are strongly
deformed and some show clear evidence for deformation during Clarke & Beddoe-Stephens (1987) provided 17 chemical analyses
crystallisation. During field work in 1992 five phases of synplu- from the Hatapang Pluton in North Sumatra (Figs 5.1, 5.5 & 5.7),
tonic deformation were recognised from the Aroguru Pluton in ten of these were of granites and seven from greisens and veins.
southern Sumatra (Fig. 5.1). This body lies close to the present The pluton is an oval body of 6 x 4 km 2 located about 70 km to
the SE of Lake Toba. The granite is a coarse K-feldspar megacrys-
trace of the West Sumatra Fault Zone, it is however older than
tic rock with a marginal zone of about 100 m width consisting of
microgranites, aplites, pegmatites and greisens, grading into
normal granite. The greisens are strongly mineralized with cassi-
FeO* terite, wolframite and other minerals, and there is a wide aureole
of several hundred metres containing microgranite and pegmatite
veins and dykes. Chemical analyses of the main porphyritic facies
have silica values ranging from 73 to 77% SiOz. The granite has a
Rb-Sr isochron age of 80 Ma and an initial ratio of 0.7151. The
authors established an S-type affinity for the granite, and
because of its age, suggested that it might be a representative of
the Western Granite Province established by Beckinsale (1979).
The nearest representatives of the Western Province are at
Phuket in Peninsular Thailand. Gasparon & Varne (1995) have
questioned this interpretation on the basis of the Rb-Sr initial
ratio, which they intimate is too low for a Western Province
granite. Cobbing et al. (1992), however, reported ages and
initial ratios from the Western Province in Burma which are com-
parable with that for Hatapang, supporting the interpretation of
Clarke & Beddoe-Stephens (1987).
Beddoe-Stephens et al. (1987) studied the Muarasipongi Batho-
Na=O+K20 MgO lith in North Sumatra in connection with the skarn mineralization
developed in contact limestones of that region, and published six
Fig. 5.6. Compositions of the Volcanic Arc granites of southern Sumatra plotted chemical analyses with a silica range of 62-68% SiO2 and a
on the AFM diagram of Irvine & Baragar (1971). Rb-Sr isochron age of 158 +_ 3 Ma which were interpreted as