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Selected Porphyroidal and granitic rocks at Tennant Creek, Northern Territory

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Duncan, DM (1970) Selected Porphyroidal and granitic rocks at Tennant Creek, Northern Territory. PhD thesis, University of Tasmania.

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Abstract

The Lower Proterozoic sedimentary succession of the Farramunga Group,
Northern Territory, Australia is composed essentially of shales, gre~rackes
and conglomerates with minor haematite shales and cherts having an estimate
total thickness of 25,000 feet. At Tennant Creek, the lower part of the
succession contains conformable horizons, discontinuous lenses, dykes and
other isolated bodies of porphyroidal rocks. The rocks show varying
degrees of recrystallization and deformation due to an incipient greenschist
facies regional metamorphism.
Two porphyroidal bodies have been studied in detail and are generally
similar with respect to petrography, mineralogy and chemistry. They
contain between 28% and 45% of megacrysts of quartz, potassium feldspar,
plagioclase and minor biotite set in recrystallized, dominantly quartzofeldspathic
groundmasses in which relict textural patterns indicate the
former presence of close-packed shards, perlitic cracks and possible
amygdaloidal patches and identify the porphyroids as volcanic pyroclastics.
The Great Western Porphyroid forms a grossly conformable, perhaps composite,
horizon at least twelve miles in length and up to 1,500 feet thick and is
of ash-flow origin. The Creek Bed Porphyroid is half a mile long, several
tens of feet thick and appears to be a dyke.
The shape of the embayments and cavities, extensively developed in
the quartz and potassium feldspar megacrysts, appear different from the
more regular skeletal or dendritic patterns which are known to result from
irregular growth. The close association between curved grain perimeters
and embayments suggests that the two are genetically connected and is consistent
with weak magmatic corrosion of euhedral grain habits resulting the embayments and cavities. The presence of Carlsbad and Baveno twinning
in the potassium feldspar further supports an igneous derivation for these
mineral grains.
Optical and X-ray data indicate that both the potassium feldspar and
plagioclase are in a highly-ordered structural condition. In conformity
with their structural states, the compositions of the potassium feldspar
and plagioclase are consistent with recrystallization at temperatures
typical of greenschist facies metamorphic, rather than magmatic, condition.
The major element compositions of the porphyroids are similar to
those of calc-alkaline rhyolites apart from a variable alkali ratio and a
low CaO content due to the redistribution of K, Na and Ca, probably as a
result of hydrothermal alteration.
The petrographic, mineralogical and chemical similarity of the
porphyroids to high-level granites (and selected enclaves) of the neighbouring
Tennant Creek Complex suggests a genetic association.
Compositional and structural data on the feldspars are compared
throughout the various environments of porphyroid, granite and enclave.
The major element composition of the potassium feldspar and the structural
states of both the potassium feldspar and the plagioclase are generally
similar in the different rock types. This is a result of secondary process of hydrothermal alteration and/or metamorphic recrystallization,which make it impossible to correlate primary order-disorder and compositional feature
with environment of occurrence. Delicate, oscillatory zoning preserved in
the andesine from the different rock types reveals that the similar plagioclase
composition is an original magmatic characteristic. It is concluded that the close similarity of the Rb, Sr and Ba abundances in the potassium
feldspar from the various environments is a consequence mainly of the
generally similar abundances of these elements in the porphyroid, granite
and enclave bulk compositions.

Item Type: Thesis (PhD)
Copyright Information:

Copyright 1970 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).

Date Deposited: 02 May 2012 05:57
Last Modified: 11 Mar 2016 05:56
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