Page 72 - Sumatra Geology, Resources and Tectonic Evolution

GRANITES 59

indicators of an I-type affinity in the scheme of Chappell & White Table 5.1. %SiO: and isotopic ages,from Sumatran Granites
(1974), and are similar in their composition to the Volcanic Arc
Pluton/Unit Sample Si02 Age (Ma) Geological age
Suite of South Sumatra (McCourt et al. 1996).
no.
Gasparon & Vame (1995) provided 20 chemical analyses from
both northern and southern Sumatra, all from the Volcanic Arc
(a) Sulit Air Granite Suite
Suite, with the possible exception of the Bukit Batu pluton Guguchina SSG8 63.28 142 + 5Bi Cretaceous
(Fig. 5.1). Eleven of these are from southern Sumatra with a 149 • 5H
range of SiO2 from 49.36 to 77.23% and five from North Sumatra Saloga SSG l 0 63.77
with a more restricted, but essentially similar range of 51.07- Belimbing SSG 12 65.09 138 +_ 4H
76.81% SiO2. They also gave eight estimated Rb-Sr ages ranging Sulit Air SSG13 63.42 183 • 4H
from 15-t-3 to 135 +_ 7 Ma, together with estimated initial 203 + 6Bi Trias
87Sr/86Sr ratios from 0.7038 to 0.7059. The extended compositional
(b) l_ztssi Granite Batholith
range is similar to that from other regions of volcanic arc related Guguk Sara• SSGl5 50.8 53 • 1.5 Eocene
plutonism and the Rb-Sr ages, although estimated, suggest an
Lass• Granite SSG20 75.3 53 • 1.4
extended period of granite plutonism. This data is also in keeping Pianggu SSG21 57.7 53 • 1.7
with field information recorded from both northern and southern Lass• Granite SSG21 a 74.9
Sumatra, that the granites have a lithological range from gabbro Leucogranodiorite SSG23 63.8 55 • 1.6
and diorite to monzogranite, similar to that in other Volcanic Arc Hornblende Diorite SSG24 61.0
terrains. Gabbro SSG25 52.6
However, the two samples from the Bukit Batu intrusion in SE Sungai Durian SSG26 68.7
Sumatra, lying to the SW of the island of Bangka and SE of Palem- Bukit Bais Gabbro SSG31 52.9 57 • 1.5
bang (Fig. 5.1), have highly anomalous compositions with > 10%
(c) Lolo Granite Pluton
combined soda and potash and ca. 60% SiO2. The isotopic data are
5 • 1.2
also markedly different, with estimated initial SVSr/86Sr ratios of Granodiorite SSG36 65.6 l l + 1 Miocene
Miocene
0.71564 and 0.71477 and an estimated age of 170 i 35 Ma. On Monzogranite SSG37 7 I. 14
the Nb vs. Y (Fig. 5.4a) and Nb + Y (Fig. 5.4b) discriminant (d) Bungo Granite Batholith
plots of Pearce et al. (1984) the data from both samples fall in Bungo North
the 'Within Plate' (WPG) field. They also have extremely high Bungo Granite SSG43 76.37 129 • 4Bi Lower Cretaceous
values of Ce, La and Zr, and these strange rocks seem to have Rantaupandang SSG44 60.76 54 • 2 Eocene
an A-type affinity but are clearly quite different from the Hatapang Rantaupandang SSG46 60.97 148 • 4 Upper Jurassic
Granite. The low silica values and high content of CaO and Na20, Muarabat SSG48 73.18
together with the presence of hornblende in one of the samples, Bt Apit SSG52 75.61
suggest a possible affinity with the volcanic arc granitoids. Bungo South
However, the wide geographical separation between Bukit Batu Sungai Siwai SSG54 70.08 169 • 5Bi Jurassic
and the outcrop of the Volcanic Arc Suite, restricted to the Dusunburu SSG55 60.39
Bar• Range, does not support this interpretation. Gasparon & Kalan SSG58 65.2 154 + 2Bi Jurassic
Varne (1995) considered these rocks to be of S-type affinity, Dusunburu S SG59 64.15
because of their high 87Sr/86Sr estimated initial ratios and esti- Dusunburu SSG59a 64.18 156 • 5H
mated age, but stated that 'they are unlike any other granitoids (e) Garba Granite Batholith
in Southeast Asia'. It is, however, possible that they may be of
Garba SSG70 71.46 86_+ 3 Bi Cretaceous
alkaline affinity. Three granites of this affinity are present in the Sungai Liki SSG72 69.46 117 • 3Bi
Tin Islands Suite (Fig. 5.2), of which Karimun and Dabo are tin
mineralized, and West Central Singkep is not (Cobb• et al. (f) Aroguru Granite Complex
1986, 1992). However, none of these granites has such an SSG82 65.6 89.2 Cretaceous
extreme composition as the Bukit Batu granite. (g) Padean Granite
A field and geochemical/geochronological study of Sumatra south
SSG80 73.69 83 • 2Bi Cretaceous
of the equator was conducted in 1992 and reported in McCourt &
SSG80a 73.53
Cobbing (1993) and McCourt et al. (1996). The data consists of 54 SSG80b 74.08
whole rock chemical analyses and 40 K-Ar ages. Nineteen plutons
SSG80c 74.61 82 • 2Bi Cretaceous
and batholiths were investigated. Material for geochemistry and geo- SSG80d 74,67
chronology was collected from three main areas extending from the SSG81 75.15 84 +_ 2 Cretaceous
latitude of Padang to the southeastern tip of Sumatra (Fig. 5.1). The
most northerly area to the east and northeast of Padang and Lake Sin- (h) Way Sulan Gabbro
SSG87 55.3 151 + 4Hb Jurassic
karak included the Sulit Air suite, the Lass• Batholith (Table 5.1 b)
and the Lolo Pluton (Table 5.1c). To the east the large Tanjung (i) Sulan Tonal#e, and the Jatibaru, Wayambang and Brant• granite plutons
Gadang pluton was sampled and geochemically analysed, but was Sulan Tonalite SSG83 69.31 111 _+ 3Bi Cretaceous
not dated because of the weathered condition of the rock. Ten SSG85 69.2 113 _+ 3Bi
samples were taken from the Bungo Batholith which lies about SMO4 69.95
200 km to the SE and were geochemically analysed and six of Jatibaru Pluton SSG88 75.6 55 • 1.5Bi Palaeocene
these were dated (Table 5.1d). The Garba Batholith about 300 km 63 • IBi
further to the southeast is not well exposed, but was partially Waybambang Pluton Tcl7A 70.3 20 i 1BiHb Miocene
sampled and dated (Table 5.1e). The remaining plutons of Brant• Pluton Sm79 70.62 86 • 3Bi Cretaceous
Aroguru, Sulan, Padean, Jatibaru, Brant• and Waybambang are H, hornblende; Bi, biotite.
located close to the southeastern tip of Sumatra (Fig. 5.1) (Tables
5.1f-i). Most of these plutons are simple, consisting of only one
granite unit, but some are more complex. Most of the plutons are On the basis of the new data these authors introduced concepts
characterised by primary magmatic textures, but some are foliated, which, while not new, had not formerly been recognized in
sometimes strongly, and some, especially Aroguru, were affected Sumatra. These were: (1) geographical persistence of granitic
by polyphase deformation. source regions over lengthy periods of time; (2) occurrence of