Page 83 - Sumatra Geology, Resources and Tectonic Evolution

70 CHAPTER 6

(a) BASALT BASALTIC ANDESITE DACITE 100
3 ANDESITE
SHOSHONITIC SERIES HIGH K -..@. ..... B ..... 0... ",.,. /0 .....
CALC-ALKALINE ........ ....~?? ............................
K20
(wt%) 2 10
/ /
CALC-ALKALINE

/ /
LOW K SERIES I ~ I I I I I I I I I I I
(a) La Ce Pr Nd Sm Eu Gd Dy Er Yb
45 53 57 63 68 o ............. 9 SILUNGKANG FORMATION
Si20 (wt%) o o PALEPAT FORMATION
Volcanics
1000
9 Silungkang Formation
(b) FeO 9 Palepat Formation
LU
~- 100 ID.O__O.-O ...... O._ O
m
nr-
a
Z
0
-i- 10-
o
o ,0"
O 1
n-
0.1 I I I I I I I I I I I I I I
(b) Ba Rb Th K Nb La Ce Sr Nd P Sm Zr Hf Tb
Fig. 6.7. (a) Chondrite-normalized REE patterns for the Silungkang and Palepat
Formations. (b) Chondrite-normalized spidergram for the Silungkang and
Palcpat Formations. Adapted from Suwarna et ell. (2000).

Metavolcanics and serpentinites in the Medial
Sumatra Tectonic Zone
Na20+K20 MgO
The Medial Sumatra Tectonic Zone ('Line' of Hutchison 1994) is
Fig. 6.6. (a) Potassium-silica diagram for the Silungkang and Palepat a wide zone of deformed rocks which separates the West Sumatra
Formations. (b) AFM diagram for the Sih, ngkang and Palepat Formations. Block from Sibumasu (East Sumatra Block). The zone is best
Adapted from Suwarna et al. (2000). known north of the equator where Rock et al. (1983) and Clarke
et al. (1982b) described the outcrops of the intensely deformed
Pawan and Tanjungpuah formations (Table 6.5). The Pawan
48-58%, with a rhyolite sample at 85%, and in the Palepat For-
mation is 47-62%. The composition of the rock samples analysed Member consists of fine-grained chloritic metavoicanics inter-
bedded within intensely folded muscovite, chlorite and tremolite
varied between basalt and andesite (Fig. 6.6a), showing both schists, often with carbonate. The tremolite schists are deformed
tholeiitic and calc-alkaline differentiation trends (Fig. 6.6b).
K20 contents in the Palepat Formation are higher than those and metamorphosed ultrabasic rocks, and probably originated
as tectonic slivers of ophiolite. To the SE, to the west of the
in the Silungkang Formation and fall in the potassic alkaline
field, while K20 values in the Silungkang Formation are lower Tigapuluh Mountains, Andi-Mangga et al. (2000) found serpenti-
nites within slates of the Ganggsal Formation. The Ganggsal
and the rocks more calc-alkaline. The magnesium number
(Mg# = 100 Mg/Mg + Fe 2+) for the Silungkang Formation was Formation (refer to Fig. 4.6) is intensely deformed compared to
the other rock units in the Tigapuluh Mountains (Simandjuntak
calculated at 40-56, while the range for the Palepat Formation
is 31-56, indicating that the basalts were out of equilibrium et al. 1991) and may be the SE continuation of the Medial
Sumatra Tectonic Zone.
with the mantle (Mg# -- 68-75) due to the fractional crystalliza-
tion of olivine and pyroxene. Chondrite-normalized REE patterns
(Fig. 6.7a) for two samples from each formation have moderate
Eu anomalies, indicating some plagioclase fractionation. The Bentong-Billiton Accretionary Complex
rock/chondrite normalization diagram (spidergram) (Fig. 6.7b)
shows that the range of values for the two formations overlap, The 'Bentong-Billiton Accretionary Complex' is an assemblage
but the samples from the Silungkang Formation show a greater of deformed and imbricated basic volcanics, ultrabasic rocks and
range and fall between the normal and enriched values for sediments in Peninsular Malaya and the Tin Islands of Indonesia,
MORB. Suwarna et al. (2000) concluded that the analysed occurring between the Sibumasu and the Indochina blocks
samples showed evidence for fractionation, differentiation and (Fig. 6.8). The complex includes the Bentong-Raub Suture
possibly contamination processes, and noted that the volcanics (Line) in Peninsular Malaysia (Metcalfe 2000). The continuation
had geochemical similarities with those from an island arc of the suture into Indonesia has been a source of speculation
setting, although a continental margin, fault-related, origin has (see Metcalfe 1996). However, Barber & Crow (2003) suggest
also been proposed. that the 'suture' is a broad zone of imbrication passing from the