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The Tasmanian ultramafic-gabbro and ophiolite complexes

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Rubenach, MJ (1973) The Tasmanian ultramafic-gabbro and ophiolite complexes. PhD thesis, University of Tasmania.

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Abstract

A string of ophiolite and ultrnmafic gabbro complexes in western
and northern Tasmania constitutes the Main Belt, while a second group in
the Adamsfield area are composed entirely of ultramafics. These complexes
are associated with Eocambrian and Cambrian sequences of mudstones, wackes,
conglomerates and volcanic rocks lying between or flanking Proterozoic
blocks. The Heazlewood River Complex, the largest and least dismembered
in the Main Belt, is an ophiolite consisting of layered ultramafics intruded
by gabbros and overlain by extrusive rocks. Dolerite dyke swarms intrude
the ultramafics and part of the volcanic sequence, while tonalites,
trondhjemites and granophyres intrude all other rock types. The rock
contacts of the ultramafics with the Eocambrian Luina Beds are faulted and/
or have formed by solid flow of serpentinite. Contacts between the extrusive
rocks and the Luina Beds are poorly exposed, but on the basis of established
faults and melange zones it is tentatively concluded that the ophiolite is
an allochthon.
The Nineteen Mile Creek Dunites occurring along the NW margin of
the Heazlewood River Complex are tectonites, some of which may be deformed
cumulates. The rest of the ultramafics in the Hleazlewood River Complex
and the other Main Belt bodies are dominantly layered orthopyroxenites,
harzburgites, lherzolites and dunites, many of which contain interstitial
plagioclase. Considered together, the layering styles, textures, and
compositional ranges (Fo79-89, En79-89) of these rocks are consistent with
a cumulative origin. It is suggested that nucleation (as opposed to
mechanical sorting processes) and postcumulus diffusion are important
controls on the layering.
The layered ultramafics and gabbros of the Main Belt are believed
to have formed by crystal accumulation from tholeiitic or high-Ilg
tholeiitic magmas. Lack of olivine-plagioclase reactions suggest crystallization
occurred at pressures less than 6 kb.
The extrusive rocks of the Heazlewood River Complex are probably
mainly basalts, but high-Ng basalts ao well as some intermediate and acid
types also occur. The dolerites, which have suffered less than the
volcanics from the pervasive hydrothermal or burial metamorphism, are
quartz tholeiites which grade into granophyres and tonalites. Although
similar to Cainozoic mid-ocean ridge basalts in Cr and Ni contents and
being relatively low in the incompatible elements, the dolerites and
volcanic rocks are dominantly quartz tholeiites rather than olivine tholeiites and
have unusually low Ti02 contents. Nevertheless it is concluded that
the Heazlewood River Complex (and perhaps the other Main Belt complexes)
may have formed as Eocambrian lithosphere at a mid-ocean ridge, marginal
sea, or similar spreading environment. Detritus of ophiolite rocks in
Cambrian sedimentary rocks in a number of localities suggests that the
Main Belt complexes were tectonically emplaced prior to the commencement
of Middle Cambrian sedimentation. It is suggested that the lenses of
foliated amphibolites which occuur along the contacts of several complexes,
may have formed by metamorphism of ophiolite rocks prior to, or in the
early stages of the initial tectonic emplacement.
As a result of a series of faulting, serpentinization and tectonic
re-emplacement events, several of the Tasmanian Complex now occur as
serpentinite lenses strung out along major fault zones, and in contact with
sedimentary rocks as young as Silurian.
The first serpentinization phases affecting the complexes preferentially
replaced dunites and harzburgites by massive lizardite
serpentinites, and altered gabbro layers and dykes to rodingites or
amphibole-prehnite rocks. Characteristic massive and sheared varieties
of green waxy-lustred serpentinites formed in later phases associated
with deformation. These green serpentinites are lizardite-chrysotile
mixtures, and in many localities contain cross-fibre asbestos veins which
typically surround residual pyroxenite kernals.

Item Type: Thesis (PhD)
Copyright Information:

Copyright 1973 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:

Appendix 1 appears to be the equivalent of an Accepted Manuscript of an article published by Taylor & Francis in Australian journal of earth sciences in March 1974, available online: http://www.tandfonline.com//doi/abs/10.1080/00167617408728837

Date Deposited: 03 Feb 2015 03:26
Last Modified: 24 Oct 2016 22:33
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