Fore-arc magmatic sequences associated with high Mg number andesite
lavas unconformably
overlie LeMay Group accretionary complex in Alexander Island. High-resolution
40Ar/39Ar, U–Pb zircon,
fission track and K–Ar ages demonstrate that subduction-related
fore-arc magmatism migrated northwards
along the length of Alexander Island between c. 80 Ma and c.
46 Ma. The magmatic rocks represent a third
of the western margin of the Antarctic Peninsula magmatic arc and are
critical to the understanding of the
final phase of subduction along the southern Antarctic Peninsula margin.
The onset of late Cretaceous magmatism
is recorded by poorly exposed volcanic rocks on Monteverdi Peninsula
(79.7±2.5 Ma). In central
and northern Alexander Island, the magmatic rocks can be distinguished
by the proportion, range and types
of lithofacies present, and by the periods of magmatism represented.
The volcanic rocks of the Colbert
Mountains range in age from c. 69–62 Ma and are
dominated by large volume dacitic and rhyolitic crystal-rich
ignimbrites interpreted as caldera-fill deposits. Elgar Uplands sequences
range in age from c. 55–50 Ma,
and contain approximately equal proportions of pyroclastic deposits
and less evolved (basaltic-andesite and
andesite) lavas including high Mg number andesite lavas near the base of
three sequences. The volcanic
rocks of Finlandia Foothills probably represent the youngest calc-alkaline
units on Alexander Island
(48±2 Ma). The sequence is lithologically similar to the Elgar
Uplands and also contains high Mg number
andesite lavas, but it is dominated by polymict conglomerates, with
minor lavas, which were deposited in a
graben associated with regional extension. Plutonic rocks exposed in the
Rouen Mountains, adjacent to the
Elgar Uplands, yielded a U–Pb age of 56±3 Ma which is
in discordance with a previously published Rb–Sr
age (46±3 Ma), probably due to hydrothermal perturbation of the
Rb–Sr system. Northwards migration
of magmatism was caused by the progressive collision and subduction of
three ridge segments prior to
the previously reported ridge crest–trench collisions that occurred
c. 20–30 Ma later and following which subduction ceased.