Cainozoic volcanism in and around Great Lake, Central Tasmania

Abstract

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.