- Title
- Implications of structural, metamorphic and stratigraphic features in a Palaeozoic subduction complex, Southeastern NSW, Australia
- Creator
- Prendergast, Elaine I.
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2013
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The Lachlan Orogen is considered to be a Late Cambrian to Carboniferous orogen formed by subduction- accretion involving one or more subduction zones. However, not all workers agree with this model and alternative models including passive and/or strike-slip fault margin with no subduction, and obduction have been proposed. This study examines the stratigraphy, deformation and metamorphism of the Middle Cambrian to Late Ordovician Wagonga Group and Lower to Middle Ordovician Adaminaby Group rocks near Batemans Bay, southeastern Lachlan Orogen and shows that these rocks form part of the Narooma accretionary complex therefore supporting a subduction model for this part of the Lachlan Orogen. The Adaminaby and Wagonga Groups comprise a continent-derived turbidite and shale sequence and oceanic rocks of sedimentary and volcanic origin, respectively. These rock units are common components of ocean plate sequences and are commonly found in accretionary complexes. Early deformation in unconsolidated to semi-consolidated sediments of the Wagonga Group rocks, Batemans Bay resulted in the formation of syn-sedimentary folds, mélange, disaggregated bedding and faults. These features have been documented in modern, active accretionary complexes and are present in the lower slope and toe of the Nankai accretionary prism. This suggests that the Wagonga Group rocks were located in a similar position early in their incorporation into an accretionary complex. Tectonic deformation, D1, resulted in the formation of meridional isoclinal F1 folds and associated well-developed differentiated S1 cleavage that displays steep to moderate dips to the west in the Wagonga and Adaminaby Groups. D1 overprints the early, syn-depositional deformation of the Wagonga Group at Batemans Bay. A second deformation D2, produced F2 folds and a westerly dipping crenulation cleavage S2 that are slightly oblique to, and over print, D1 folds and cleavages. 40Ar/39Ar ages obtained from neocrystalline K-white mica defining S1 and S2 in the Wagonga and Adaminaby Group slates have been effected by 40Ar loss and the best estimate for the time of cleavage formation is provided by K/Ar ages of 438–435 Ma determined from the 10-20 μm fractions that are representative of the neo-crystalline white mica defining S1 and S2. However, these ages may be a minima, and the cleavage forming event during subduction is believed to be 460-440 Ma, broadly contemporaneous with the major shortening phase and active east-dipping subduction in the central Lachlan Orogen at ca. 450 - 440 Ma. Lower greenschist facies grade and intermediate pressure (P)/low temperature (T) metamorphic conditions are recorded in the Wagonga and Adaminaby Groups based on Kübler Indices and b cell parameters of K-white micas. The chemical composition of K-white micas forming S1 and S2 cleavages is similar indicating that the two cleavages formed under comparable conditions. Assuming a temperature of 300°C and a P of ~4kb, a geothermal gradient of 20°C/km operated during subduction indicating peak metamorphic conditions were attained at a depth of ~15km. These metamorphic conditions are consistent with underplating in an accretionary complex. In the Narooma accretionary complex west of Batemans Bay, similar metamorphic conditions are recorded in the deformed sediments of the Adaminaby Group. These changed subsequently as indicated by metamorphic assemblages and b cell parameters of K-white micas in pelitic rocks from the aureole of the Late Devonian Buckenbowra and Nelligen granodiorites (K-Ar =384.6 ± 7.9 Ma to 395.8 ± 8.1Ma) that intrude this group. P = 3.1kb is indicated by the b cell parameters (b = 9.021 Ǻ; n = 41) and T is assumed to be ~400°C based on the closeness of the samples to the biotite metamorphic zone boundary. This infers that the intrusions occurred at ~11km depth, after ~4km exhumation of the Adaminaby Group. A dyke swarm visible in aeromagnetic images of the Adaminaby Group appears to overprint the Buckenbowra Granodiorite suggesting that during the Devonian this area was extensional which may have been related to slab roll-back. Basalt clasts in mélange and basaltic breccia units occur in the Wagonga Group at Batemans Bay and Narooma. At Batemans Bay they are associated with Middle to Late Cambrian parautochthonous limestone beds and clasts. They were altered to prehnite pumpellyite facies assemblages by hydrothermal fluids and tectonic burial then later taken to greater depths, deformed and partially recrystallised resulting in the development of lower greenschist facies assemblages. Immobile elements reveal distinctive ocean island basalt (OIB) ‐like signatures in most basalt samples. They have an alkaline basaltic affinity, and show LREE-enriched, chondrite and LILE-enriched, primitive mantle normalised patterns. They represent plume-derived melts from a deep, HIMU-type source that has undergone low to moderate degrees of melting. The basalts formed part of a seamount and suggest that they were subducted and sequentially altered under changing P-T conditions. The stratigraphy, deformation and metamorphism of the Wagonga and Adaminaby Group rocks at Batemans Bay suggest that they have been incorporated in the lower slope and toe of an accretionary prism and subsequently underplated to depth. The low T/intermediate P conditions of metamorphism indicate they were taken to a depth of ~15km and subjected to a geothermal gradient of 20°C/km. A strike-slip fault model with no subduction, a passive margin nor an obduction model can explain this metamorphic environment.
- Subject
- subduction; Narooma accretionary complex; Kübler indices; b cell parameters; k white mica; Adaminaby Group; Wagonga Group; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/938508
- Identifier
- uon:12628
- Rights
- Copyright 2013 Elaine I. Prendergast
- Language
- eng
- Full Text
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