Cainozoic volcanism in and around Great Lake, Central Tasmania

Upper Cainozoic basaltic volcanism about Great
Lake involved the eruption of a succession of
mineralised entrail breccias, 215+ feet (65 m),
aquagene tUffs and agglomerates, 40+ feet (12 m),
unmineralised entrail breccias, 160 + feet (48 m),
and massive flows and dykes, individually up to
200+ feet (60 m) thick with sequences up to four
flows and 300+ feet (90m) thick. Associated with
the volcanics are some lacustrine and fluviatile sediments,
up to 88+ feet (27 m) thick.
The aquagene pyroclastics and entrail breccias
are confined within the present Great Lake depression,
and closely resemble hyaloclastites and bedded
breccias in the upper parts of Icelandic intraglacial
pillow lava piles. They probably represent
emergent elongate fissure volcanoes that erupted
into past high water levels in Great Lake.
The massive sub-aerial lavas erupted from centres
both within and outlying the Great Lake depression;
those within probably erupted during low or drained
water levels.
Over twenty eruptive centres can be inferred on
structural and petrological grounds and most are
aligned along intersecting NW, NNW, N, NNE
and ENE lineaments. There is some evidence of
late or post-volcanic local tilting and jointing and
of recent adjustment movements on lineaments.
The bulk of the volcanic rocks are tholeiitic
olivine-basalt, but some tholeiite and alkali olivine-basalt
occurs amongst the massive lavas. The
Great Lake volcanic association is a typical example
of the tholeiitic associations of Tasmania and falls
within a general belt of such rocks extending from
far NW Tasmania to the Derwent Valley. The
Great Lake rocks resemble to some extent basalts
of the Hawaiian province, and the known stratigraphy
suggests a somewhat similar pattern of
magmatic evolution and eruption.