Magmatic dyke systems of the South Shetland Islands volcanic arc (West Antarctica): reflections of the geodynamic history

The Antarctic Peninsula forms part of a magmatic arc at least since
Jurassic times. Magmatic dykes are essential elements of such arcs
and intrude along zones of instability. In contrast to other
hypabyssal intrusions and the effusive products of arc activity,
dykes do not only reflect the geochemical characteristics of their
magma source but also the tectonic parameters at the time of their
emplacement. The South Shetland Islands form an archipelago located
at the northern tip of the Antarctic Peninsula and belong to this
arc. Areas of up to 100,000 m2 have been mapped at several
locations of these islands, mainly on King George and Livingston
Island. A structural analysis of the dykes and the host rocks was
carried out, and about 250 dykes were sampled for further studies.
As deduced from field relationships, on Livingston Island six
different intrusive events could be distinguished, on King George
Island up to seven. This subdivision into different intrusive
events is also well reflected by the geochemical data. Analysis of
the structural data of the dykes and their host rocks shows, that
the tectonic stress field was not only very similar throughout the
archipelago, but that moreover only minor changes of this stress
field occurred during the time of dyke emplacement. This holds for
all investigated areas in the South Shetland Islands. The
geochemical data (ICP-MS) reveal, that most dykes of the South
Shetland Islands belong to a calc-alkaline, arc-related suite,
ranging from basalts to highly differentiated rhyolites. However,
especially during early stages of intrusive activity in the
respective areas, also tholeiites occur. Isotopic data (Sr, Nd, Pb)
prove a strong crustal component during initial stages of magmatic
activity, especially on Hurd Peninsula (Livingston Island). This
crustal component decreased with time, accompanied by an increase
of sedimentary input into the subduction zone. The high amount of
crustal contamination during the initial stages was probably due to
a still unstretched continental crust. Besides the continental
crust underlying the South Shetland Islands, partial melts from the
subducted sediments, fluids derived from the subducting plate and a
depleted, heterogeneous mantle wedge contributed to arc magma
genesis. According to Ar-Ar datings on plagioclase separates and
K-Ar (WR) age determinations, dyke intrusion was restricted to the
Paleocene and Eocene. The dykes started to intrude around the
Cretaceous/Paleogene boundary at Livingston Island. Only around the
Thanetian/Ypresian boundary, dyke intrusion commenced also further
NE at Nelson and King George Island, culminating during the
Lutetian at 47-45 Ma in all investigated areas. Dyke intrusion then
ceased in the latter areas but still continued at Livingston Island
until the Priabonian. Combining the information given by the
tectonic and geochemical datasets, the time interval covered by the
dykes obviously marks a period of geodynamic stability. This
includes a stable geometry of the subduction zone and the
corresponding parameters (subduction direction and velocity) during
that time, as well as stable magma sources. The contribution of the
respective sources (sediments, slab, mantle, crust) varied, but the
sources themselves remained the same. Very primitive, olivine
tholeiitic dykes sampled on Penguin Island as a by-product of this
work yielded an unexpectedly high Ar-Ar age (Tortonian), thus
questioning the onset of rifting in Bransfield Strait during the
Pliocene, as believed so far.