Jurassic magmatism in western Gondwana produced the most voluminous
episode of continental
volcanism in the Phanerozoic era. During the Early to Middle Jurassic,
some 2.5–3 million km3 of
dominantly basalt, and to a lesser extent rhyolite, were erupted onto a
supercontinent in the early
stages of break-up. The major silicic portion of the Gondwana magmatic
province is exposed in
Patagonian South America. The volcanic rocks of Patagonia have been collectively
termed the Chon-Aike
Province and constitute one of the world's most voluminous silicic
provinces. The volcanic rocks
are predominantly pyroclastic, dominated by ignimbrite units of rhyolite
composition. Volcanic rocks
crop out sporadically across much of the once contiguous Antarctic Peninsula,
and are considered to
form an extension of the Chon-Aike Province. A continuation of the province
to include the Antarctic
Peninsula would extend its strike length along the active Pacific margin
by c. 2000 km.Volcanic rocks exposed along the east coast of the Antarctic Peninsula,
defined here as the Mapple
Formation, are also dominated by rhyolitic ignimbrite flows, with individual
units up to 80 m in thickness,
and a total thickness of c. 1 km. The ignimbrites vary in degree
of welding, from high-grade
rheomorphic ignimbrites with parataxitic textures, to unwelded, lithic-rich
ignimbrites. Rhyolite lava
flows, air-fall horizons, debris flow deposits and epiclastic deposits
are volumetrically minor, occurring
as interbedded units within the ignimbrite succession.The lithology and stratigraphy of the Jurassic volcanic rocks of the
Mapple Formation are presented,
and comparisons are made to the Chon-Aike Province. A consistent stratigraphy
of Permo-Triassic
metasedimentary rocks, unconformably overlain by terrestrial mudstone–siltstone
sequences,
which are in turn conformably overlain by largely silicic, subaerial volcanic
rocks, is present at several
localities along the Antarctic Peninsula, and at localities in the Chon-Aike
Province. Precise (zircon
U–Pb) Middle Jurassic ages exist for two volcanic formations from
the Antarctic Peninsula, and a
Middle–Lower Jurassic age has been suggested for the underlying sedimentary
formations based on
fossil flora analysis. The Antarctic Peninsula chronostratigraphy, coupled
with lithological similarities,
indicate a close relationship to those sequences of the Chon-Aike province.
A revised geochronology of Thurston Island, West Antarctica, and correlations along the proto-Pacific margin of Gondwana