Geology of the Dundas-Mt Youngbuck area, Western Tasmania : the stratigraphy and structure of the regional geology, and the petrology, chemistry and petrogenesis of the ultramafic-mafic complexes in the Dundas-Mt Youngbuck area of Western Tasmania

An onlap landscape unconformity has been proved between the Precambrian Oonah Formation and Eocambrian Success Creek Group. This unconformity represents a hiatus in sedimentation as well as a structural and low grade metamorphic break. The areal distribution of the Oonah Formation has been extended from the type area to as far north as Mt Livingstone and west to Whaleback Ridge, where rocks of the Oonah Formation are gradational with rock sequences of the Arthur Lineament. The Oonah Formation consists of two parts: a lower succession of fine-grained, siliceous, indurated quartz sandstone and wacke interbedded with phyllitic mudstone and siltstone, typical of the sequence at Oonah Hill; and an upper succession of interbedded carbonate, mudstone and conglomerate with volcanic rock horizons and minor sandstone. Structurally the Oonah Formation is characterised by the presence of outcrop-scale refolded isoclinal folds. Up to five cleavages can be found in rocks of the Oonah Formation, the last two being the northerly and northwesterly cleavages associated with Devonian deformation. The Success Creek Group has been redefined, after Taylor (1954), to consist of four mappable formations. All formations crop out in Taylor's type section along the Pieman River, where they have a total measured minimum thickness of 950 m. The basal formation is a mixtite. The second formation, the Dalcoath Formation, is a sequence of interbedded, clean, shallow-water quartz sandstone with minor siltstone, pebbly sandstone and conglomerate. The Dalcoath Formation grades into the third formation which is dominated by laminated mudstone and siltstone with minor sandstone and conglomerate units. This formation is characterised by pervasive intraformational soft sediment deformation and slump structures indicative of an unstable basin of deposition. The top formation is the Renison Bell Formation whose dominant units are classic siliceous siltstone with mudstone partings. The upper member of this formation is the 'red rock' of Conder (1918). Overlying the Success Creek Group with transitional conformity is the Crimson Creek Formation. This formation consists of a turbiditic sequence of volcaniclastic lithic wacke with laminated siltstone and mudstone interbedded with tholeiitic basalt. The proportion of volcaniclastic lithic wacke and basalt in the succession increases northwards from the type area along the Pieman River towards Cleveland. Following an implied hiatus in sedimentation, during which the tectonic regime changed from tensional to compressional emplacing dismembered ultramafic bodies into basins of deposition, the Dundas Group (Elliston, 1954) was deposited. Remapping of Elliston's type area of the Dundas Group has shown that it consists of two distinct successions which are now fault juxtaposed, but may originally have been separated by a break in sedimentation or period of shallow water deposition. The lower part is sparsely fossiliferous and of Middle Cambrian age. Using the terminology of Elliston, the lower part consists of all rock units from the Judith Formation up to and including the lower part of the Brewery Junction Formation. The upper succession is a fossiliferous turbidite sequence, extending from the middle of Elliston's Brewery Junction Formation, up to and including at least the Misery Conglomerate and possibly including a white friable sandstone sequence on the western slope of Misery Hill. The Huskisson Group of Taylor (1954) is a biostratigraphic correlate of part of the Dundas Group. The group'is composed of a succession of clastic sedimentary rocks whose background sedimentation consists of laminated and thinly bedded siltstone and mudstone with minor sandstone and lithic wacke units into which numerous horizons of mass-flow conglomerate and turbidite units were deposited. The conglomeratic units in the lower 1000 m of the sequence were derived from a mixed metasedimentary and active acid to intermediate volcanic terrain, whereas those in the upper 200 m are from a dominantly metasedimentary terrain. The dominance in most of the sequence of acid to intermediate volcanic detritus is the main difference between the Huskisson and Dundas Group sequences. Structurally, all folds and cleavage surfaces found within the Success Creek Group, Crimson Creek Formation, Dundas and Huskisson Groups and correlates are consistent with having been formed during' Devonian deformation. Litho- and biostratigraphic correlates of part of the Gordon Sub-Group Limestone succession occur around the outer edges of the Huskisson Syncline. On the southern nose of the Huskisson Syncline the limestone sequence is in faulted contact with the Huskisson Group, but a hiatus in sedimentation from the middle of the Late Cambrian to the middle Middle Ordovician is indicated. Along the eastern side of the Huskisson Syncline the transition between the limestone sequence and basal Eldon Group correlate successions appears to be conformable. Within the study area there exists an area of Tertiary basalt, Devonian granite and numerous gabbroic phases. Volcanic activity associated with the Eocambrian-Cambrian successions was dominantly tholeiitic to andesitic in character. Three separate phases of volcanism were differentiated. The first is the tholeiitic volcanism associated with the Crimson Creek Formation, the second is a succession of high-magnesian andesite lavas considered to be the second stage melting of the same source as the Crimson Creek Formation tholeiitic basalts. The third phase is a low-titanium tholeiite suite which interdigitates with the basal conglomerate units of the Dundas Group. The ultramafic rocks of western Tasmania fall into three groups. A succession of Layered Pyroxenite-Dunite (LPD); a succession of Layered Dunite-Harzburgite (LDH); and a succession of Layered Pyroxenite-Peridotite and associated Gabbro (LPG). These three ultramafic rock successions can be recognised both by field characteristics and mineral chemistry. Comparison of mineralogy and mineral chemistry with experimental work indicates that the ultramafic rocks formed at high temperatures and low pressures, the parental melt having low oxygen fugacity and a low water content. All three of the ultramafic successions are consistent with being cumulate bodies formed as crustal magma chamber products, each from one of the three different magma events found within the Eocambrian-Cambrian successions within the Dundas Trough. The LPD succession is a monotonous, finely layered sequence containing orthopyroxene of En89_89; olivine of F087-89; minor chrome diopside (Ca:Mg:Fe = 47:49:4), and chrome spinel with an average 100 x Cr/(Cr + Al) ratio of 65 and a variable 100 x Mg/(Mg + Fe 2+) ratio with an average of 43. The LDH succession is well layered, contains a tectonic foliation parallel to layering, the result of flattening of the mineral grains, and contains olivine of F098-94; orthopyroxene of En93 - 94 with very low to undetectable Al 20 3and CaO contents. Chrome spinel grains have a 100 x Cr/(Cr + Al) ratio of 87-93 and a variable 100 x Mg/(Mg + Fe 2+) ratio with an average of 49. The LPG succession consists of multiple intrusions which formed an - orthopyroxene-rich layered sequence, an olivine-rich sequence and a gabbroic unit. Locally the ultramafic sequences are plagioclase bearing, and the orthopyroxene-rich sequence contains numerous primary structural features. The orthopyroxene-rich sequence is intruded by the olivine-rich sequence which locally contains chromite-rich zones. No relic olivine cores were encountered; orthopyroxene is En89-37, chrome diopside (Ca:Mg:Fe = 48:48:4), and there are two compositions of chrome spinel. Within the ultramafic rocks chrome spinel has a 100 x Cr/(Cr + Al) ratio with an average of 60, and an 100 x Mg/(Mg + Fe 2+) average ratio of 48. In the chrome-rich zones within the olivine-rich sequence spinel grains have an average 100 x Cr/(Cr + Al) of 69 and an average 100 x Mg/(Mg + Fe 2 +) ratio of 40. The gabbroic phase is a two-pyroxene gabbro with a variable grainsize and texture. The shallow water nature of the original sediments which infilled the proto-Dundas Trough; a gradual deepening of an elongate basin, probably less than 70 km wide, into which tholeiitic lavas and associated turbiditic sedimentary rocks formed; the low pressure melting required to form the ultramafic rocks; high-magnesian andesite lavas associated with terrigenous rocks; and the presence of areas of Precambrian rocks in the Ramsay River and Dundas areas, added to previous data, reinforces the implication that the Dundas Trough represents part of a failed continental rift zone.