Unformatted text preview: cting beneath the
Sandwich plate at a rate of 70–85 km/Myr (Pelayo &
Wiens, 1989). Earthquake data indicate that the subducting plate dips at 45–55° to the west, probably steepening slightly in the southern part of the subduction zone
(Brett, 1977). The age of the subducting plate is 28–35 Ma
below the southern part of the arc, and 50–60 Ma below
the northern part of the arc (Sclater et al., 1976; Barker &
Lawver, 1988). The sedimentary cover on the subducting
plate consists of some 200 m of siliceous sediments in
the south, and some 400 m of calcareous and siliceous
sediments in the north (Barker, 1995). More than 95%
of this sediment input to the trench is subducted, and NUMBER 8 AUGUST 2002 there is no signiﬁcant sediment accretion at the trench
(Vanneste & Larter, 2002).
The South Sandwich arc consists of 11 main islands
forming a distinctly curved island arc, 500 km in length.
Most of the islands have abundant evidence of recent
volcanic activity, and all are entirely volcanic in origin.
The volcanoes belong to the tholeiitic and (relatively
rare) calc-alkaline series, and the arc is regarded as a
classic example of the primitive stages of island arc
development (Baker, 1968, 1990; Pearce et al., 1995). All
the arc magmas are depleted in high ﬁeld strength
elements (HFSE) such as Ti, Zr, Hf, Nb and Ta, and
heavy rare earth elements (HREE), relative to normal
MORB, and are therefore thought to be derived from
mantle wedge material that had experienced melt extraction before the magma generation events (Hawkesworth et al., 1977; Pearce et al., 1995). All the magmas
are enriched (relative to MORB) in highly incompatible
ﬂuid-mobile trace elements, such as Pb, U, Ba and Rb,
derived from the subducting plate. Pearce et al. (1995)
suggested that variations in incompatible trace element
abundances were a result of dynamic melting processes
within the sub-arc mantle and that the Nd and Sr isotope
covariations of the island arc sample suite indicate the
involvement of a subduction component derived from
the subducted sediment and altered oceanic crust. We
argue below that the sediment-derived component has
locally inﬂuenced the back-arc magmatism. Previous geochemical studies
Previous investigations of lavas from the East Scotia
Ridge were conﬁned to eight dredge sites (D20–D24,
D56, D57, D60) sampled by R.R.S. Shackleton in 1974
and 1981, as well as volcanic rocks from the axis and
the lateral ﬂanks of segment E2 and the northern part
of E3, sampled by R.R.S. James Clark Ross in 1996 (Fig.
2). Dredge sites were located on the northern tip of
segment E3 (D20), east of the southern tip of E2 (D21),
west of segment E5 (D22), close to the northern tip of
segment E9 (D24), in the centre of segment E9 (D23),
and east of the northernmost segment E1 (D56). Apart
from the recently sampled segment E2 and the northern
part of E3, only the dredge stations D20 and D23 are
located in the neo-volcanic axis of the East Scotia Ridge.
The petrographic characteristics, major and trace element geochemistry, volatile content, and Sr, Nd, Pb, O
and C isotope...
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This document was uploaded on 02/01/2014.
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