Queensland Geological Record 2009/01

Geology of the Late Triassic Agnes Water Volcanics, Central Queensland

The Agnes Water Volcanics are a major component of a north-north-west
trending belt of Late Triassic Volcanics that extend from south-east
to central Queensland. The sequence is divided into three stratigraphically
separate volcano-sedimentary facies associations. Basal deposits are
grouped as the Mafic to Intermediate Facies Association and comprise
basaltic to andesitic lavas, thin dacitic pyroclastic deposits and
laterally-restricted fluvial sediments. These rocks have similar compositions
to continental arc volcanics and exhibit geochemical variation consistent
with fractionation processes. They were emplaced in a relatively high-relief
terrain dominated by stratovolcanoes where prevailing effusive eruptions
were occasionally interspersed with minor explosive eruptions and
reworking of volcanic material.

Emplacement of the overlying Dacitic Facies Association involved a
period of dacitic dome growth contemporaneous with fluvial sedimentation,
followed by a major explosive eruption. This led to collapse of a
large (~20km diameter) caldera and emplacement of thick dacitic to
rhyolitic ignimbrite. These rocks do not appear genetically related
to the earlier-erupted mafic to intermediate volcanics or the later-erupted
rhyolitic volcanics.

Upper parts of the sequence comprise rhyolitic ignimbrite and lavas
grouped as the Rhyolitic Facies Association. Emplacement of these
rocks followed a period of erosion and sedimentation and involved
a second major explosive eruption that produced a larger caldera (~30km
diameter) and a thicker, more extensive rhyolitic ignimbrite. Caldera
structural margins are apparent on regional geophysical images; these
define the Agnes Water Caldera Complex. At ~229Ma, shortly following
the second caldera forming event, lava domes and cryptodomes were
emplaced during a period of effusive rhyolitic volcanism. The geochemistry
of these rocks is consistent with a close relationship to the Mafic
to Intermediate Facies Association. The rhyolites may be moderate
degree partial melts of basaltic to andesitic material emplaced in
the upper crust during the mafic to intermediate phase of magmatism.

The 'continental margin' type magmatism and volcanism expressed
by the Agnes Water Volcanics is observed in other volcanic units of
possible Late Triassic age. This period is generally regarded as a
transition from continental convergent margin magmatism to extension-related
magmatism. Further investigation of stratigraphy/volcanic architecture
and detailed geochemistry and geochronology are required to understand
the timing and dynamics of this event.

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Last Updated 12 June 2009