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

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 region. 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 Formations). 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 transport. 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."