Page 65 - Sumatra Geology, Resources and Tectonic Evolution

52 CHAPTER 4

The contact between the Gunungkasih Complex and the dioritic rocks into gneisses and deformed the basic dykes. The
Menanga Formation in Gunung Kasih itself is obscured, due to alternation of acid and basic intrusion, with contemporaneous
rice cultivation, and in Teluk Ratai is at present inaccessible deformation, are characteristic features of the basal parts of a
as it lies within a Naval Base (Fig. 4.8). However, the latter magmatic arc, where acid and basic magmas are intruded into
contact in the Menanga River was described by Zwierzijcki an active strike-slip fault zone. This situation is similar to that
(1932) as occupied by a 'friction breccia'. On the GRDC maps which exists beneath Sumatra at the present day where the
Amin et al. (1994b) and Andi Mangga et al. (1994a) show both modern volcanic arc is built on the active Sumatran Fault Zone.
these contacts as thrusts (Fig. 4.8). However, the sense of movement along the present arc is
The Menanga Formation is interpreted by Amin et al. (1994b) dextral, in the opposite sense to the sinistral movement along
as a deep-water marine sequence with interbedded basalt lavas the Cretaceous arc.
and andesitic clastic fragments, derived from a volcanic arc, and
deposited in a trench or forearc environment. These sediments
were deformed during accretion to the Sumatran margin, Interpretations of the Woyla Group
represented by the Gunungkasih Complex. K-Ar radiometric
ages, ranging from 125 to 108 Ma (mid-Cretaceous) from horn- On completion of the Integrated Geological Survey of Northern
blende in an amphibolitic schist in the Menanga Formation, is Sumatra the DMR/BGS mapping team published an interpretion
taken as the age of accretion (Andi Mangga et al. 1994a). of the Woyla Group in Aceh (Cameron et al. 1980). It was
However, the presence of quartzite and quartz-diorite clasts suggested than the oceanic assemblage represented an ocean
suggests that the Menanga Formation was, like the Rawas and floor and its overlying pelagic sediments. The arc assemblage
associated formations in central Sumatra, derived from an was interpreted as a volcanic arc, and the associated limestones
Andean arc built on a continental basement, and was deposited as the surrounding carbonate reefs. It was suggested that the
in a forearc environment. The Menanga Formation was overthrust volcanic arc had developed on a fragment of continental crust
by the basement at a later stage. which had separated from the margin of the Sundaland continent
along a transtensional transcurrent fault, similar to the present
Intrusions in the Bandarlampung area. Near Bandarlampung Sumatran Fault System. Extension led to the formation of a
the Gunungkasih Complex is intruded by the Sulan Pluton narrow short-lived marginal basin in a process similar to that
(Fig. 4.8). The pluton is a composite body which includes which is forming the Andaman Sea or the Gulf of California at
gabbro, dated by K-Ar radiometric analysis at 151 + 4Ma the present time (Cameron et al. 1980, Fig. 4a).
(Late Jurassic), hornblende and biotite granites and granodiorite There is no direct evidence to support the suggestion that the arc
intruded by late aplogranite dykes. Granite from the Sulan assemblage was constructed on continental crust, but a number
Pluton gave an age of 113 ___ 3 Ma (mid-Cretaceous) (McCourt of circumstantial arguments have been put forward in support of
et al. 1996). this interpretation: the arc assemblage is intruded by the Sikuleh
To the north of Bandarlampung, spectacular exposures below Batholith, which it is suggested was derived from the underlying
an irrigation dam on the Sekampung River show extensive continental crust; quartz-rich rocks associated with the batholith
outcrops of granodioritic and dioritic gneiss, containing basic and shown as 'undifferentiated Woyla Group' rocks on the
xenoliths, and cut by concordant and discordant granitic and Calang map sheet (Bennett et al. 1981a) are interpreted as roof
pegmatitic veins. The granitic and granodioritic gneisses are cut pendants, uplifted from the underlying basement; and tin, recorded
by basaltic dykes, several metres thick, which contain xenoliths in stream sediment samples along the northern margin of the
of gneiss. The gneiss xenoliths show evidence of melting, and batholith, is normally restricted to continental crust (Stephenson
towards the margins of the dykes are drawn out into streaks, et al. 1982). All of these arguments are open to objection and to
which are sometimes isoclinally folded, parallel to the dyke alternative explanation.
margins. The dykes and the foliation in the gneisses both trend Unfortunately no detailed chemical analyses of the Sikuleh
in a NW-SE direction. Fold structures in the dykes and the Batholith are available. However, it is a composite body, compris-
curvature of foliation in the gneisses indicate that the dyke ing an 'Older Complex' of variably deformed and contaminated
margins have acted as strike-slip shear zones, with a sinistral gabbroic and dioritic rocks, into which is intruded a 'Younger
sense of movement. Sub-horizontal slickensides on foliation Complex' of homogeneous, largely unfoliated, biotite-hornblende
surfaces within the gneiss indicate the same sense of movement. granodiorite, with a K-Ar age of 97.7 _+ 7 Ma (Bennett et al.
Diorite from the Sekumpang exposure has been dated by the 1981b). The low values of stream sediment tin are associated
K-Ar method at 89 _+ 3 Ma (late mid-Cretaceous) (McCourt with the outcrop of the Younger Complex, which is likely to be
et al. 1996). a mantle-derived I-type granitoid body. There is no detailed
In the same area, in the Wai Triplek, greenschist facies white field or geochemical evidence in favour of the suggestion that
mica-quartz schists are intruded by metadolerite dykes. The roof pendants have been uplifted from an underlying basement;
margins of the dykes show compositional banding which is iso- they could equally well have subsided from an overlying thrust
clinally folded, in a similar fashion to the dykes in the Sekampung sheet. It is possible that the tin in stream sediments in Aceh
River. Further upstream the bed of the Wai Triplek exposes were derived directly by erosion and transport from the area to
streaky acid and basic gneisses cut by more homogeneous basic the east of the Sumatran Fault, or secondarily through Tertiary
dykes. Acid gneiss shows evidence of having been melted and sediments.
recrystallized along the dyke contacts, and quartz-feldspar veins Although there is no direct palaeontological or isotopic
fill fractures in brecciated basic dyke material, in a process of evidence for the age of the Woyla oceanic crust, and the age of
back injection. the volcanic arc is inferred only from the palaeontological
Relics of dyke rocks occurring as basic xenoliths in gneiss, and age of the fringing reefs, in the model proposed by Cameron
gneiss xenoliths enclosed in basalt dykes, indicate that et al. (1980), the marginal sea is considered to have formed by
the intrusion of basaltic dykes and granitic bodies alternated extension and rifting in the Late Jurassic and Early Cretaceous.
during the development of the gneiss complex at Sekampung. In the Late Cretaceous, compression, related to subduction on
Exposures in the Wai Triplek form part of the same gneiss the outboard side of the Sikuleh microcontinental sliver, led to
complex, but also contain fragments of the schistose continental the collapse of the marginal sea to form the imbricated oceanic
basement into which the igneous rocks were intruded. During assemblage and the accretion of the microcontinental fragment,
or shortly after intrusion, both granitic and basic rocks were with its overlying volcanic arc, against the continental margin of
affected by sinistral shearing, which converted the granitic and Sundaland.