Page 86 - Sumatra Geology, Resources and Tectonic Evolution

PRE-TERTIARY VOLCANIC ROCKS 73

Ko (1986) described diabase sheets intruded into radiolarian were the source of the tin. The Nam Salu ore body is a layer of
cherts and sediments at Cape Penjabung in the NW of Bangka iron formation, corresponding to the silicate facies of Algoma
as part of the Volcanic-Chert facies of the Pemali Group. These Type, mixed with tuff which was metasomatized into micaceous
diabases were previously mapped as volcanics by Zwierzijcki phyllite. Schwartz & Surjono (1990b) concluded that the Nam
(1933) and Verbeek (1897), but Westerveld (1936, 1937) Salu phyllite was chemically a 1:1 mixture of basalt and
describes them as intrusive sills into folded rocks and suggested silicate-facies ironstone; the bulk of their analyses (Fig. 6.10)
that they were precursors of the adjacent granite. Cobbing et al. correspond to the sub-alkaline basalt field of Winchester &
(1992) consider that they are an early basic (dioritic) facies of Floyd (1977) in a discrimination diagram using immobile
the Klabat Batholith. elements. The mineralogy of the Schachtader lode indicates it
Ko (1986) includes the lithologies described by De Roever is either a metabasalt or even a meta-serpentinite, although
(1951) and Schwartz & Surjono (1991) in the Pyritic Black Schwartz & Surjono (1990b) describe it as an altered volcaniclas-
Shale-Limestone Facies of the Pemali Group. According to tic rock.
Schwartz & Surjono (1991) the lithologies exposed in the open
pit at the Pemali Mine are deformed hornfels and skarns derived
from metasediments. However, the mineralogy (Table 6.6) and
geological setting suggest that in addition to sediments, these West Sumatra Triassic Plutonic-Volcanic Arc
metasomatic rocks also were derived from volcanic and ultramafic
rocks described at this locality by Pulunggono & Cameron (1984) Volcanic rocks associated with the West Sumatra Triassic Arc
and Suryono & Clarke (1981). are preserved in the Cubadak Formation (Rock et al. 1983), as
Similar skarns, encountered during mining, are present in the a sequence of dark green volcanic wackes interbedded with
Permian rocks on Billiton (Kelapakampit Formation of Bahruddin mudstones and siltstones containing Halobia, faulted against,
& Sidarto 1995). Of interest are the are lenticular masses of and possibly part of the carapace of the early Jurassic
?original fayalite in the Seloemar lode (Adam 1960), and the Muarasipongi Batholith, which has been dated at 197 ___ 2 Ma.
presence of fayalite as a minor constituent in the tin ores at Nam
Salu in the Klapa Kambit mine. Here, Schwartz & Surjono
(1990b) showed that Permian metavolcanics and metasediments
(Table 6.6) had been metasomatized and that tin ores had been Pahang Volcanic Belt
formed in association with Triassic granite intrusions, which
There are abundant occurrences of volcanic rocks in the Triassic of
the eastern Malay Peninsula belonging to the Pahang Volcanic
Series (Hutchison 1973). These volcanics are invariably associ-
ated with IS and A-type plutons of the Eastern Granite Province
(Central Belt) (Cobbing, pers. comm.). This association in the
R H Y O L ~ Semantan Basin (Fig. 14.11) and its continuation in the Riau
and Lingga archipelagoes (Fig. 6.8) is described here as the
0.1 Pahang Volcanic Belt (Table 6.7).
P. Karimun Besar is formed of a core of metaluminous granite
] TRACHYANDES~ of IS or A-type (Cobbing et al. 1992) which is mantled by the
contact metamorphosed Malarco Formation (Cameron et al.
U ANDESITE j/, .-/" .... "-,,,J 1982c). The presence of volcanic rocks within the graben
r
.......... u-'77 9 " _ I sediments strongly suggests that the pluton was intruded into its
0.01 9 9 ",,~ ;, i carapace of surface volcanics in a resurgent caldera. The
ANDESITE/BASALT ~' _ 9 9 nn - I Karimun Besar granite has not been dated radiometrically;
,'-
Cameron et al. (1982c) suggest a date of emplacement between
SUB-ALKALINE BASALT 9 Mid- and Late Triassic (Carnian-Norian).
In the SE of Bintan the rhyolites and trachytes which abut
o.ool A i the East Bintan batholith, intruded around 230 _+ 12 Ma (Rb-Sr
O.Ol o.1 1 lO isochron, Cobbing et al. 1992), are likely to be relics of the
volcanic carapace of this batholith.
Nb/Y
On Lingga the Lingga pluton is intruded into Triassic cherts
Fig. 6.10. Zr/TiO2-Nb/Y discrimination diagram showing fields for volcanic containing Daonella and volcanic rocks which appear to be associ-
rocks based on immobile elements (after Winchester & Floyd 1977). Both ated with this biotite-hornblende two-phase granite (Cobbing
ratios are indices of alkalinity but only Zr/TiO2 ratio represents a differentiation et al. 1992). The deformation noted by Bothe (1925a, b) may be
index. Small squares represent element ratios in the metasomatised Nam Salu due in part to later intrusion of the pluton into its own volcanic
'phyllite'. Adapted from Schwartz & Surjono (1990b). edifice.

Table 6.7. Volcanic lithologies in the Pahang Volcanic Belt in the Tin Islands Archipelagos
Island Formation Description Reference
Karimun Besar Malarco Porphyritic rhyodacites and lithic tuft's, Cameron et al. (1982c)
hornfelsed shales, ?chert,
?conglomerate and limestone
Bintan Rhyolites and trachytes Van Bemmelen (1949); Osberger (1968)
Citilim Quartzporphyrites interfingered Van Wessem (1942)
with Triassic sediments
Lingga Rhyolites, dacites, porphyrites Both6 (1925a,b)
and accompanying tufts