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The alkaline rocks of Port Cygnet, Tasmania

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Ford, Ramsay John (1984) The alkaline rocks of Port Cygnet, Tasmania. PhD thesis, University of Tasmania.

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Abstract

The Port Cygnet alkaline complex consists of small scattered
concordant and discordant intrusions of Cretaceous age emplaced in
Jurassic dolerite and Permian fluvioglacial sediments in the vicinity
of Port Cygnet, southern Tasmania.
The alkaline rocks are miaskitic and fall into two main groups:
one is a group of slightly oversaturated syenites with a porphyritic
texture rich in phenocrysts of oligoclase (syenite porphyries), and
the other consists of nepheline syenites, characteristically with a
flow structure and containing phenocrysts of euhedral sanidine crystals,
with feldspathoids in the groundmass, and occasionally as phenocrysts.
These are the sanidine rocks or sanidine porphyries.
The primary magma was a potash-rich upper mantle or lower crustal,
partial melt (of a mafic alkali parent) probably equivalent ultimately
to a tinguaite which began crystallizing in a magma chamber with
subsequent successive injections of magma containing coarser sanidine
phenocrysts. . This gave rise to the nepheline syenites.
The intrusion of the potassic magma was accompanied by volatiles
with evidence of the presence of water, carbon dioxide, sulphur
dioxide and hydrogen sulphide, demonstrated by the presence of both
primary and secondary reaction mineral products.
The porphyritic syenites seem to have been derived by the mixing
of a potassic upper mantle or lower crustal partial melt, with a
crustal partial melt of the feldspathic matrix of an amphibolite
followed by subsequent initial crystallization at depth before
injection into the country rocks.
The model is based on microscopic and textural characteristics,
chemical compositions, mineralogy and initial strontium isotope ratios.
Variation of the syenite porphyries due to addition of
recrystallized amphibole from the parent amphibolite formed a horn-blende porphyry. Fractionation of the syenitic melt has resulted
in a group of mixed feldspar phenocryst rocks showing potash feldspar
reacting with plagioclase phenocrysts in a brown matrix.
Assimilation of mineralized carbonates by the magma has produced
one rock unique in having phenocrysts of spessartite and epidote.
Sulphur from the country rocks was mobilized by this intrusion.
At Regatta Point, on the western shore of Port Cygnet, the
.potassic magma reacted with Jurassic dolerite to form hybrid rocks.
These were most likely produced within the temperature range of
710 °C to 810 °C and a log £0 2 in the range -11 to -13.
The alkaline intrusion was probably associated with the break-up
of Gondwanaland.

Item Type: Thesis (PhD)
Keywords: Syenite, Petrology
Copyright Holders: The Author
Copyright Information:

Copyright 1984 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

5 maps in pocket in v. 2. Includes bibliography. Thesis (Ph.D) - University of Tasmania, 1984

Date Deposited: 09 Dec 2014 01:11
Last Modified: 11 Mar 2016 05:56
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