226 research outputs found
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
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