Page 70 - Sumatra Geology, Resources and Tectonic Evolution

GRANITES 57

l&o I 1000
)L; 100 ~ 110 ~
,,, ",, tt
Wuntho
~ 0 200 400 600 800 1000km WPG
~ i , ~ ' : . i ~
~i~' 84184 ili~i~i!, '!:!~.~'
I O0
E o. ooOO:
Q.
0.
' L Z
!~ iii~i ~ ~,ii ~ ): :/:)~i
'
i
0
=J syn-COLG Oo "~,~. ~/"
C7 9 . ".-- ..2../
~!.?#~e ~ , "',,_ 9 . . . . . t

9
~Ol , ~lJll I I //I I ,Ill i i I I ~lltl
t0 100 1000
Log Y ppm

1000 -- b o -=_
i = ~yo COLG oo~ ~176
i
~ill ;:~:iii
_ 9 2
100
9 elDOO go //
9 "" ;'.'" / i
#
o 9 !
.J
VAG 9
ORG z
o
0
o o
lg~
10 100 1000
Log Y + Nb ppm
Fig. 5.4. (a) Nb/Y and (b) Rb/(Y + Nb) discfiminant diagrams for syn-collision
(syn-col), volcanic arc (VA), within plate (WP) and normal and anomalous ocean
ridge (OR) granites after Pearce et al. (1984). Volcanic Arc granites, South Sumatra
(filled circles; McCourt et al. 1996), the Hatapang granite (open circles; Clarke &
[aub Complex Beddoes-Stephens 1987) and Bukit Batu (squares; Gasparon & Vame 1987).

95OE 100 ~ 105 ~ ~ "~--.110~ of these granites was established by McCourt & Cobbing (1993)
and McCourt et al. (1996). Most of the currently available geo-
Fig. 5.3. Granitic provinces of Sumatra and adjacent areas (modified after chemical and isotopic data is from southern Sumatra, but Beddoe-
Cobbing et al. 1992 and McCourt et al. 1996). Stephens et al. (1987) published six whole-rock analyses from
the Muarasipongi Batholith in northern Sumatra (Fig. 5.1), with
62-68% SiO2 and an Rb-Sr isochron age of 158 4-23 Ma,
Clarke & Beddoe Stephens (1987) suggested that this suite which established its Jurassic age and volcanic arc affinity. Sato
continued southwards in central Sumatra, thus bringing stannifer- (1991) provided whole-rock geochemistry and K-Ar ages for the
ous granites of younger age into an area dominated by older tin Padangpanjang and Lassi bodies located to the northeast of
granites. The geochemical data from the Hatapang Granite Padang (Fig. 5.1). The isotopic data from these granites established
suggests that it may have some alkali affinity, since it falls
mostly within the 'Within Plate Granite' (WPG) field on Pearce a Cretaceous age of 64 Ma for Padangpanjang and 56 Ma for Lassi,
and the geochemistry confirms their volcanic arc affinity.
diagrams (Fig. 5.4a, b) and in or close to, the alkali feldspar These results are similar to those of McCourt & Cobbing (1993)
field on the QAP Le Maitre diagram (Clarke & Beddoe Stephens and McCourt et al. (1996) who provided chemical analyses and
1987) (Fig. 5.5). They also plot above the calc-alkali field of Kuno K-At ages from 13 plutons and batholiths from southern
(1969 fig. 6.7). Such an affinity is compatible with the compo-
Sumatra which, while not being a comprehensive data set, can
sitional range present in the granites of the Western Province
(Cobbing et al. 1992). be regarded provisionally as being representative of this group
for the region as a whole. That work established an age range of
203 to 5 Ma from rocks with SiO2 values ranging from 50.83 to
The Volcanic Arc Suite 76.71%. The lithological range is from gabbro to monzogranite.
This range is similar to that for Volcanic Arc and Cordilleran
It is however, the Volcanic Arc Suite (Fig. 5.1) that has provided the granitoids elsewhere and all other geochemical indices confirm
main focus for granite studies in Sumatra. The volcanic-arc affinity that affinity (McCourt & Cobbing 1993" McCourt et al. 1996).