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Structural and metamorphic evolution of the Arthur Lineament, northwestern Tasmania, Australia

Holm, OH 2002 , 'Structural and metamorphic evolution of the Arthur Lineament, northwestern Tasmania, Australia', PhD thesis, University of Tasmania.

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The aim ofthis project is to investigate the origin and deformational history ofthe Arthur
Lineament, using amulti-disciplinary approach. The field-based research concentrated on
the structural deformation ofthe study area. Laboratory-based research included
microstructural investigations, whole-rock geochemistry to investigate the mafic igneous
rocks in the study area, and metamorphic petrology and geochronology to characterise the
timing and nature ofthe metamorphic events that affected the lineament and surrounding
TheArthur Lineament is a Cambrian age, high strain metamorphic belt located innorthwest
Tasmania, Australia. It is east-dipping, NE-trending, 5to 10 km wide and approximately
110 km long. The lineament consists ofphyllitic to schistose lithologies that have been
subject to medium- to high strain, and variably metamorphosed to blueschist, or greenschist
grade. The high strain belt comprises several different units. The major lithological unit in
the southern Arthur Lineament isthe Late Neoproterozoic "eastern" Ahrberg Group, which
comprises metamorphosed psammite and psammopelite units, transitional to tholeiitic
metabasalts and mafic volcanogenic metasediments. The southern Arthur Lineament also
contains the Bowry Formation, which contains basalt/dolerite-derived tholeiitic amphibolites
that are intruded by minor deformed 777±7 Ma granitoids, aunit correlated with the Ahrberg
Group (AGC) and Neoproterozoic high strain turhidite sequences (Bumie and Oonah
Whole rock geochemistry ofthe metabasites in the "eastem" and "western" Ahrberg Groups,
the Bowry Formation and the AGC have been examined in order to enable an understanding
ofthe tectonic setting, into which the igneous rocks were emplaced. The "eastem" and
"western" Ahrberg Group sequences, and the AGC, are interpreted to be lithostratigraphic
correlates. The units have sequences oftransitional tholeiites, that are enriched relative to
MORB, and are increasingly tholeiitic towards the top ofthe stratigraphic pile. The Bowry
Formation also comprises transitional tholeiites, however they are more evolved than the
other units, and have REE pattems only slightly enriched relative to MORB.
The main orogeny to affect northwestem Tasmania was inthe Early to Middle Cambrian.
This event (the Tyennan Orogeny) consisted ofthree deformational episodes. The first two
episodes (CaD, and CaDz) were high strain events, related to arc-continent collision. These involved the emplacement of allochthonous slices, and the structuralrepetitionof units
within the lineament. CaDi produced isoclinalfolding, a schistose axial planar fabric and
shearingparallel to the CaFi axial plane. CaD2 wasmorepervasive than CaDi, and
produced tightto isoclinal folds with a schistose axial planarfabric. The CaSi and CaS2
axialplanar foliations are commonly sub-parallel, and the fold axesare close to parallel. The
orientation of CaFi and CaF2 folds changes from the low to high strain domains. In the low
strain domains, CaFi folds are variable in their orientation. CaF2 folds in the low strain
domains are gently east- and west-plunging, with south-dipping axial planes. In the high
strain domains, CaFi and CaF2 folds have east-southeastdipping axial planes, and southplunging
fold axes. This is consistent in both thenorth andthe southof the lineament.
Structural analysis suggests that the change in orientation is due to a strongly rotational shear
component during the CaD2 event, and thisisinterpreted as evidence forsouth-directed
The subsequent CaDs event, in the latest Cambrian, involvedeast-directed thrust. It
produced asymmetric folds with gentlywest-dipping longlimbsand steeply east-dipping
short limbs. The CaFs are gently south plunging, with gently west-dipping axial planes and
associated thrust faults. During the Devonian, and associated with extensive granitoid
intrusion, two episodes of open, east-west trending folding occurred.
Metamorphic investigations show that twometamorphic events occurred in the Arthur
Lineament. The earliest event corresponds to CaDi. This event has been pervasively
overprinted by the CaD2 event, whichhaslargely obliterated CaDi metamorphic textures.
Where observed, CaDi occurred at greenschistgradein allochthonous units. However in
units that are interpreted to be allochthonous, in theBowryFormation and withinpart of the
"eastern" Afrrberg Group, CaDi occurredat blueschist gradewith amphibole coresrecording
conditions of approximately 700-1000 MPa and 450-500°C. Syn-CaD2 metamorphism
varies across the Arthur Lineament. Early in the CaD2 event in the western parts of the
lineament, amphibole compositions indicate thatpressures were around500MPa, and the
metamorphic grade was in the upper greenschist facies. However in parts of the Bowry
Formation and the AGC, early CaD2 temperatures were around 460 ± 40°C, and pressures
were around 500-600 MPa. In these rocks, hornblende was stable in the metabasites, and
syn-CaD2 garnet, indicative of localised temperatures of approximately 500°C (greenschistamphibolitefacies
transition) were found. Duringlate CaD2, pressures were around300-400
MPa and the metamorphic grade was lower greenschist facies. Outside ofthe lineament,
metamorphic conditions duringCaDi and CaD2 didnot exceedgreenschist grade, with
temperatures of approximately 400°C and pressures of around 350 MPa. Metamorphic monazites in northwestern Tasmania andKingIslandwere investigated to
define the extent ofthe Cambrian deformation, and provide insight into other metamorphic
events. King Island has experienceda complexmetamorphic history. There is no evidence
ofthe Cambrian deformation, however a pronounced 1270 Ma regional metamorphic event,
contact metamorphism related to the 760 MaWickham Orogeny, and localised, mylonite
zone-related metamorphismat 380 Ma were observed. In the Arthur Lineament, the
dominantmetamorphic event occurredbetween510-540 Ma. Monazite growthin
metasedimentarysamples from within the lineament is attributed to the early-CaDa event.
However, awayfromthe lineament the Cambrian event is much weakerand significantly
older monazites of detrital origin, are common. No strong Devonian peak was observed.
In summary, in northwestem Tasmania, following theCambrian arc-continent collision, the
blueschistgrade (CaDi) allochthonous slices wereobducted and transported to the southof
the subductionzone, in a progressive deformational event. Other parautochthonous rocks
were also transported to the south duringthis high strainevent, with associated shearingand
folding occurring at greenschist grademetamorphic conditions. Sequential to this, a near
coaxial high straindeformational event took place, detaching other parautochthonous slices
that were variablymetamorphosed to greenschist andupper greenschist-amphibolite facies
(earlyCaD2), and overthrusting the different grade slices to the south. During these events,
strain-related rotation of the developingfolds took place, resulting in a change in orientation
ofthe fold axes from east-west to north south, and foliations from south-dipping to eastdipping,
from thelow strainzones into thehigh strain zones. The revised metamorphic
geochronology of the ArthurLineament indicates the earlyCaDi eventoccurred around 505-
510 Ma. DuringCaDx and early CaDj, the allochthonous and parautochthonous lithologies
were structurally emplacedon top of the less deformed, autochthonous units. In the final
stage of assembling the tectonostratigraphic 'pile', the various sliceswere stacked together,
and lower greenschist facies metamorphic conditions were developeduniformly in all
allochthonous, parautochthonous and autochthonous blocks(late CaD2). Subsequent to this,
the CaDs event caused the tilting ofthe tectonostratigraphy, to complete the evolution of the
Arthur Lineament.

Item Type: Thesis - PhD
Authors/Creators:Holm, OH
Keywords: Geochemistry, Metamorphism (Geology)
Copyright Holders: The Author
Copyright Information:

Copyright 2002 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:

Includes CD-ROM in back pocket. Thesis (Ph.D.)--University of Tasmania, 2002. Includes bibliographical references

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