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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
Additional Information: Copyright Royal Society of Tasmania
Date Deposited: 28 May 2012 04:40
Last Modified: 18 Nov 2014 04:37
URI: http://eprints.utas.edu.au/id/eprint/14228
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