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Mineralogy, petrochemistry and magmatic history of Tamar lavas

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Sutherland, FL (1969) Mineralogy, petrochemistry and magmatic history of Tamar lavas. Papers and Proceedings of the Royal Society of Tasmania, 103. pp. 17-36. ISSN 0080-4703

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Abstract

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.

Item Type: Article
Keywords: Royal Society of Tasmania, RST, Van Diemens Land, natural history, science, ecology, taxonomy, botany, zoology, geology, geography, papers & proceedings, Australia, UTAS Library
Journal or Publication Title: Papers and Proceedings of the Royal Society of Tasmania
Page Range: pp. 17-36
ISSN: 0080-4703
Collections: Royal Society Collection > Papers & Proceedings of the Royal Society of Tasmania
Additional Information:

Copyright Royal Society of Tasmania

Date Deposited: 28 May 2012 04:40
Last Modified: 18 Nov 2014 04:37
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