Mineralogy, petrochemistry and magmatic history of Tamar lavas

The Tertiary lavas of the Tamar Trough are mostly undersaturated to near-saturated alkali olivine-basalts, with minor flows of olivine-nephelinite, nepheline-basanite, limburgite and tholeiitic olivine-basalt. Olivine forms the main phenocryst fraction in the lavas, but includes xenocrysts and late 'intergrowths, and ranges in composition from about Fou, to Fo,. Labradorite Ab zoned to about Ab, is the typical feldspar. The clino-pyroxenes are augites, passing into titan-augite and aegirineaugite in the more alkaline rocks. Nepheline is represented in the olivine-nephelinites and basanite, and analcime is a late-stage accessory in the coarser olivine-basalts. The iron ore is ilmenite or titano-magnetite, commonly altered to leucoxene, and other accessory minerals include apatite, zeolites and biotite. The finer grained lavas tend to have glassy mesostases, darkened with iron ore in a few cases, and the coarser lavas commonly show microlitic, feldspathic and zeolitic mesostases. Some of the lavas carry peridotitic xenoliths and xenocrysts, composed mostly of magnesian olivine, w1th some clino-pyroxene and spinel, and basalt at Corra Linn contains augite xenocrysts from depth, showing well developed reaction rims. Accidental xenoliths in the lavas include fused dolerite and sediments, in part replaced by clino-pyroxene. Differentiation trends can be distinguished in the Tamar suite, both between separate lavas and within individual lavas. Differentiation within thick lavas of coarse basalt has produced picritic, mesostasis-rich and pegmatitic phases, and comparisons are made with differentiated rocks of similar compositions in sills and necks elsewhere in Australia. The Tamar volcanic suite is predominantly an alkaline associa1tion, resembling the Older Volcanics of Victoria, the Auckland Basalts of New Zealand, and, to some extent, the Hawaiian alkali basalts. The Tamar eruptions commenced about Upper Eocene time, with the initial alkali basalt magma ascending in a relatively undifferentiated state, before undergoing some differentiation prior to further eruption. Olivine-nephelinite then appears to have erupted, probably in the Oligocene and possibly during waning in the volcanism, before renewed and more wide-spread eruption of olivinebasalts in about Middle Tertiary time. Fractionation of augite, and possibly olivine, or spinel, at depth may have played a part in producing the magmas for these later lavas, with some low pressure differentiation giving the coarse olivinebasalts of the capping flows The Tamar lavas form part of an alkaline volcanic. association extending to the west, and pass transitionally into an olivine-tholeiite association to the east and south-east. The parent alkali basalt magmas possibly formed from relatively restricted partial mantle melting, with segregation of magma at depths of 35-70 Kms; olivine-tholeiite parent magmas on the south-eastern outskirts possibly formed from a greater degree of melting

Aquagene volcanism in the Tasmanian Tertiary, in relation to coastal seas and river systems

Tertiary aquagene volcanics at over forty localities in Tasmania concentrate in three main regions. The North West Coast - Bass Strait islands examples are mostly related to Miocene high seas. Phreatic tuffs and flow foot breccias erupted from emergent vents (Cape Grim, Trefoil Island, Steep Island, Redpa, Brittons Swamp, Temma, N. Robbins Island and possibly Wynyard. Pillowy lavas represent subaqueous phases of emerging volcanoes (Flat Topped Bluff, S. Robbins Island and Black Pyramid Island). The aquagene volcanics supplement data on levels and depositional depths of Niocene seas and suggest relative levels now up to 110m (early Niocene), l30-l40m (early-mid Miocene) and possibly 75m (late-mid Niocene?) above present sea level

Some observations on the Dolerite Intrusion and associated structures at Golden Valley, Northern Tasmania

Investigation of an intrusion of Middle Jurassic Golden Valley showed the structure to be more complex than previously described. The intrusion appears to be located on a thrust which has brought Precambrian quartzite and schist up Ordovician conglomerate. The magnitude the movement is not easily determined' due to uncertainties concerning the thickness of the Cambrian sequence in this vicinity. The fault is considered to be a Lower Palaeozoic feature which has controlled the site of intrusion of the dolerite, possibly with concomitant Jurassic movement. The dolerite intrusion also appears to be related to a high in the Precambrian basement and possibly a Cambrian volcanic centre

The Cainozoic geology of Flinders Island, Bass Strait

Cainozoic sediments and volcanic rocks superficially overlie the mountainous Palaeozoic basement of Flinders island and mainly form the coastal plains. Marine deposits include Middle Pliocene to Recent near-shore and littoral coquinoid beds, and some Quaternary beds appear related to old marine stands at about 15-18 m., 4.5-6 m. and 0.6-1.5 m. above MHWS. Quaternary dune deposits afe predominantly calcareous on the west coast and predominantly siliceous on the east coast, and show varying degrees of consolidation and soil development generally related to age. A Recent beach ridge and coastal barrier system is developed and lagoonal deposits include Pleistocene limestone and Recent peat. Non-marine gravel and grit deposits (including st.anniferous and sub-basaltic deposits) were sometimes reworked by later marine incursions. Scattered volcanic rocks include tuffs, alkali olivinebasalts and olivine-nephelinites, erupted from several centres roughly aligned along a north westerly trend. The volcanism was largely Tertiary in age and some lavas are lateritised. The Cainozoic history was initiated by faulting, tilting and uplifting of the Flinders Island block by early Tertiary time, with subsequent volcanism. During the Cainozoic, alternations of predominantly terrestrial or marine erosion and deposition on Flinders Island were related to fluctuating sea-levels, which influenced some faunal movements

Potassium-Argon ages of Tertiary volcanic rocks, Tasmania

Sixteen new K-Ar dates are presented from Tasmanian and Bass Basin basalts, more than doubling the previously published number. Eight volcanic regions are described, based on boundaries established on the range of the basalt types contained in each geographic region. Volcanism occurred within the span from Eocene to Miocene (47 to 13+ Ma), but mainly within the time range Middle Eocene to Early Miocene. Alkali basalts erupted throughout this span and are interspersed with tholeiites (22-31 Ma), fractionated alkaline rocks (22-27 Ma) and rare melilite-bearing varieties (26-35 Ma)

Devonian lamprophyres from Mt Lyell, western Tasmania

Mica concentrated from a post-cleavage lamprophyre occurring at the Prince Lyell Mine, Queenstown, has yielded a K-Ar age of 363±3 Ma (latest Devonian). This minette is the first confirmed evidence of Devonian potassic la mprophyric activity from Tasmania and places an upper constraint on Devonian ductile deformation in western Tasmania

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

Table Cape vent xenolith suite, northwest Tasmania: Mineralogy and implications for crust-mantle lithology and Miocene geotherms in Tasmania.

The Miocene Table Cape vent erupted a diverse mantle-crust xenolith suite within its fractionated nephelinitic matrix. Assemblages include mantle metaperidotites, garnet-metawebsterites and rarer garnet-metadinopyroxenitcs, garnct-mctawehrlites, metawebsterites and crusta] two-pyroxene granulites. Most metapyroxenires and granulites represent the Ti-Al-bearing augite suite and their bulk geochemistry indicates transitional olivine basalt magmatic affinities. Metasomatised, hydrous lithologies are only rarely present. Co-existing pyroxenes in the xenoliths provide re-equilibration temperature estimates from 860-1 0750C (for the whole suite) and temperature-pressure estimates for the garnet metawebsterires from 1055-1 070°C and 1.2-1.4 CPa. This gives a Miocene mantle geotherm gradient at least 80--130°C higher than the Southeast Australian (SEA) western Victorian geotherms. However, considerations of Moho from new seismic surveys below Table Cape (~.32 km) suggest that the indicated georherm is more strongly perturbed in its lower levels than at the mantle-crust transition. This localised perturbation is attributed to magma chamber in the mantle (Boat Harbour just prior to Table Cape vent activity. Tasmanian Miocene geotherms (Table Cape, Bow Hill) achieve relatively high gradients and reinforce suggestions of local variation in East Australian geothermal gradients, They illustrate the potential complexities in com paring xenolith- derived geotherms from different areas in general, both from thermometer/barometer selection and from associated magmatic heat inputs

Igneous rocks, Central Plateau

Igneous rocks of basic character dominate the Central Plateau. A great dolerite sheet of Jurassic caps the Plateau and forms its resistant surface. Later, sporadic basalt lavas of Tertiary age fill old drainage depressions cut in the Plateau. The dolerite is far more voluminous, but less varied in its chemical composition (approx. 1500 cu. km; silica range 52-60%) than the basalts (approx. 15 cu. km; silica range 36-53%). Both these rocks express important events which affected the Southern Hemisphere. The dolerite is the vast molten response to initial fracturing of the southern supercontinent, Gondwanaland, of which Tasmania is a small fragment. The basalts form part of the eastern Australian volcanic province which erupted in response to warping, stretching and increased heat flow along the continental margin as sea-floor spreading opened up the Tasman Sea and Southern Ocean, beginning about 85 million years ago