Structural and metamorphic characterisation of the subduction-accretion rocks near Walcha, NSW
Creators: name: Lennox, Paul
name: Offler, Robin
name: Yan, Jie
Publisher: name: UNSW Sydney
Year: 2019

description: Structural and metamorphic characterisation of the subduction-accretion rocks near Walcha, NSW Paul G. Lennox(1), Robin Offler (2), Jie Yan (3,1) 1. School of BEES, The University of New South Wales, Sydney, Australia (p.lennox@unsw.edu.au) 2. NSW Institute of Frontier Geosciences, School of Environmental &amp; Life Sciences, University of Newcastle, Newcastle, NSW 2308, Australia 3. State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang, Jiangxi 330013, China The southern New England Orogen from west to east, consists of a buried volcanic arc, concave to the east forearc basin (Tamworth Belt) and a subduction–accretion complex (Tablelands Complex) associated with a west-dipping subduction zone. Many authors propose that these three belts were deformed into a megafold in the north (Texas-Coffs Harbour Orocline) and two smaller scale, megafolds (Manning-Hastings/Nambucca Orocline) in the south. In this study, structural, petrographic and x-ray diffraction studies have been undertaken on deformed, HTLP rocks associated with the Tia Complex and LTHP subduction-accretion rocks of the Tablelands Complex with the aim to test whether (1) the Manning Orocline existed and (2) the structure in the accretion-subduction complex is as simple as some authors have suggested. Mapping of the subduction-accretion rocks west of Walcha reveals dominantly S2 fabrics which have been crenulated and folded during D3. Bedding is tightly folded into mesoscopic folds (F2) which plunge gently southsouthwest axial plane to which is a steeply dipping axial surface cleavage S2. The sharp changes in structural vergence of F2 folds suggest fault-bounded packages of rock consistent with their origin in a subduction-accretion complex. Petrographic analyses shows that the western most exposures were subjected to HTLP overprinting associated with the Tia Granodiorite in contrast to those closer to Walcha which lack this overprint. The rocks up to 10 kilometers north of Walcha, assigned to the Sandon Beds are considered by Li et al. (2013) to contain only one fabric (S1). Our studies indicate that they record a structurally more complex history resulting in a variety of cleavages developing including pencil cleavage, penetrative cleavage and spaced crenulation cleavage. The variability in the orientation of the spaced crenulation cleavage in adjacent outcrops suggest it has been re-folded. Thin section examination reveals limited evidence for the HTLP event suggesting that the affects of this event is patchy. The rocks east of Walcha consist of two packages separated along the north-south trending Tia Falls Line (Tia River-Oxley Hwy junction). Petrogaphic and x-ray diffraction analyses of K-white mica, limit the HTLP event to sequences W of Yarrowitch. E of Yarrowitch, the meta-chert-slate dominated sequences are multiply deformed, show fabrics produced during the 3 deformation events that have been formed under lower greenschist facies conditions. In addition, b cell parameter determined from K-white micas (9.040-9.045) reflect the HPLT event. Thus the structural-metamorphic history recorded is far more complex than previously suggested resulting in multiple tectonic fabrics and a variety of mineral assemblages produced during the two major events. The assumption that the Manning Orocline exists based on the variation in orientation of one tectonic fabric is therefore not tenable.
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