Search

Structural and metamorphic characterisation of the subduction-accretion rocks near Walcha, NSWPaul 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 & 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, ChinaThe 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 kilometer

The Manning Orocline is one of the more controversial structures in the southern New England Orogen and has been the subject of much speculation and recent discussion. A new model for its formation has been proposed by White et al. (2016) based on a structural analysis of Permian sequences in the Manning Basin. We show that they have oversimplified the structure of the Manning Basin and have ignored critical geological elements in the southern New England Orogen in their model. Therefore, we believe their model is untenable.

The Tasmanides in eastern Australia are the most widely exposed part of the East Gondwana Paleozoic active margin assemblage. Diverse sedimentary assemblages are abundant and include: (1) extensive quartz-rich turbidites and shallow marine to fluvial successions, (2) continental margin and island arc derived sedimentary successions with abundant volcanic lithic detritus, and (3) widespread deep-marine to subaerial successions formed from reworking of older rocks. Apart from island arcs such as the Devonian Gamilaroi-Calliope Arc, most of the Tasmanides sedimentary assemblages formed along or in close proximity to the Gondwana margin. We highlight the interplay and provenance switching between the development of igneous dominated detritus related to adjoining magmatic arcs, such as the Macquarie Arc, and interactions with Gondwana derived sedimentary successions. Paleocurrents and detrital zircon ages indicate periodic influxes of mainly quartz-rich sand derived from the East Gondwana margin and adjacent interior with a common Pacific-Gondwana detrital zircon age signature (600-500 Ma), especially in the Cambrian, Early to Middle Ordovician, and Middle Triassic. A major quartzose turbidite deposit formed in an oceanic setting was accreted to the northern New England accretionary complex in the Late Carboniferous. This contrasts with the bulk of the Devonian to Carboniferous succession of the southern New England Orogen, which is dominated by volcaniclastic input with minor Gondwana-derived detritus. Surprisingly, even the base of the intraoceanic Macquarie Arc shows detrital zircon ages indicative of Gondwana input. The prevalence of Gondwana clastic input into the Tasmanides shows that much of the orogenic belt developed in an active continental margin setting with limited strike-slip displacements, and apart from offshore island arcs, lacks exotic terranes.

The Early Palaeozoic proto-Pacific Pacific margin of Gondwana was characterised by a huge turbidite submarine fan with abundant clastic detritus derived from unknown sources within Gondwana. These deposits are widespread in the Lachlan Orogen of southeast Australia and include the Ordovician Adaminaby Group. Here we show that the mudstones and sandstones of the Adaminaby Group have chemical compositions that indicate the detritus in them was derived from a felsic, continental source similar in composition to Post Archean Australian Shales (PAAS). Chondrite normalised REE patterns showing LREE enrichment, flat PAAS normalised patterns and elemental ratios La/Sc, Cr/Th, Cr/V, Th/Sc and Th/U, have been used to support this interpretation. The dominance of quartz, and to a lesser degree plagioclase and biotite in the sandstones, suggests that the source was mainly granodioritic to tonalitic in composition. Th/Yb and Ta/Yb ratios indicate that the source was probably calc-alkaline, continental and shoshonitic. In addition, the presence of detrital muscovite, low-grade metamorphic and felsic volcanic clasts, demonstrates that a low-grade metamorphic terrane and volcanic arc contributed to the detritus observed in the samples. The presence of well-rounded zircons and tourmalines, very high Zr contents, high Zr/Sc and higher Cr/V ratios in some samples particularly in the Shoalhaven River area, indicate that some of the detritus was recycled.SiO2 versus (Al2O3 + K2O + Na2O) plots suggest the source areas experienced conditions varying from humid/semi-humid to semi-arid. Textural features and weathering trends of samples from all locations follow a curved pathway on Al2O3 - (CaO* + Na2O) - K2O (ACNK) diagrams, and indicate that the clays formed from weathering had been K-metasomatised prior to penetrative deformation. Chemical indices of alteration (CIA) reveal that even the freshest sandstones are altered and others are moderately to strongly altered. Discrimination diagram

New geochemical, metamorphic, and isotopic data are presented from high-pressure metamorphic rocks in the southern New England Orogen (eastern Australia). Conventional and optimal thermobarometry are augmented by U-Pb zircon and 40Ar/39Ar phengite dating to define pressure-temperature-time (P-T-t) histories for the rocks. The P-T-t histories are compared with competing geodynamic models for the Tasmanides, which can be summarized as (i) a retreating orogen model, the Tasmanides formed above a continuous, west dipping, and eastward retreating subduction zone, and (ii) a punctuated orogen model, the Tasmanides formed by several arc accretion, subduction flip, and/or transference events. Whereas both scenarios are potentially supported by the new data, an overlap between the timing of metamorphic recrystallization and key stages of Tasmanides evolution favors a relationship between a single, long-lived subduction zone and the formation, exhumation, and exposure of the high-pressure rocks. By comparison with the retreating orogen model, the following links with the P-T-t histories emerge: (i) exhumation and underplating of oceanic eclogite during the Delamerian Orogeny, (ii) recrystallization of underplated and exhuming high-pressure rocks at amphibolite facies conditions coeval with a period of rollback, and (iii) selective recrystallization of high-pressure rocks at blueschist facies conditions, reflecting metamorphism in a cooled subduction zone. The retreating orogen model can also account for the anomalous location of the Cambrian-Ordovician high-pressure rocks in the Devonian-Carboniferous New England Orogen, where sequential rollback cycles detached and translated parts of the leading edge of the overriding plate to the next, younger orogenic cycle.

The Cambrian-Ordovician Wagonga Group contains basalts at Melville Point and Barlings Beach, 20 km south of Batemans Bay, New South Wales. At Melville Point, the succession has basal altered basalts overlain by chert and interbedded siliceous mudstone of the Wagonga Group, in turn overlain by turbidites and chert of the Adaminaby Group with a latest Cambrian to earliest Ordovician age. By contrast, at Barlings Beach, basalt is associated with highly disrupted chert (tectonic mélange), various slivers of mudstone and turbidites, and turbidites of the Adaminaby Group. Immobile elements in the basalts show consistent patterns that allow the magmatic affinity and tectonic setting to be determined in spite of pervasive hydrothermal alteration and subsequent lower greenschist facies metamorphism that accompanied strong folding and multiple foliation development. The Melville Point basalts show Ti/V ratios transitional between arc and MORB and therefore may have formed in either a forearc or backarc basin setting. However, these rocks have higher Ti/V ratios, LREE, Th and Nb than found in forearc basalts and are therefore considered to have formed in a backarc basin setting. In contrast to Melville Point, most basalts at Barlings Beach have a geochemical signature distinctive of ocean island settings like those reported from elsewhere in the Wagonga Group. We believe these rocks developed in a Cambrian backarc basin setting. In the Early to Middle Ordovician, much of the ocean basin was inundated by quartzose turbidites followed by basin destruction with accretion/underplating at a Late Ordovician-early Silurian Benambran subduction zone and formation of the Narooma Accretionary Complex.

Structural and K–Ar dating studies of gouge in N–S, NNE and E–W-trending faults in four locations in the Sydney–Hunter region are reported. The fault zones are manifest as joint swarms and highly brecciated zones containing gouge with authigenic illite produced as a result of fluid infiltration. Strike-slip movement accompanied by minor dip-slip, normal movement occurred on the NNE faults, with dip slip on N–S and E–W-trending faults. In this study, gouge from a NE-trending, steep, SE-dipping fault showing dip-slip movement at Cut 10, on the Hunter Expressway and from an E–W-trending, steep south-dipping, normal fault at the Westside Open Cut, Lake Macquarie have been analysed. K–Ar dating of illite and illite–smectite in fractions extracted from fault gouges in areas unaffected by a thermal overprint reveals ages varying from 166 to 119 Ma for the <2 μm and finer fractions, and a mean age of ca 120 Ma for the <0.4 μm fraction. In the Sydney area and the Westside Open Cut coal mine, Lake Macquarie, the ages obtained from similar size fractions both for the gouge and for the host rocks are younger (134–76 Ma; av. <0.4 μm = 111 Ma). The data indicate influence of a thermal overprint associated with subsurface magmas emplaced during the early stages of the rifting of eastern Gondwana during the early Cretaceous.

Well-defined metamorphic zones are developed in pelitic and psammitic rocks of the Late Neoproterozoic to Cambrian Anakie Metamorphic Group of the Anakie Inlier, central Queensland. They are defined by the incoming of biotite, garnet, and andalusite, with or without staurolite. Mineral assemblages indicate that low pressure-high temperature metamorphism is associated with D1, medium pressure-high temperature metamorphism with D2, and retrograde, low pressure-low temperature metamorphism with D3. A mean b cell parameter of 9.035 obtained from K-white micas in the lowest-grade rocks suggests upper intermediate pressure conditions during D2. The timing of the growth of the index minerals indicates that isotherms retreated to the southwest during D2. Phase diagram calculations for both Al-saturated and Al-undersaturated metapelites containing D1 associations indicate pressure-temperature (P-T) conditions of 0.4 GPa and 560°C. These changed to 0.64-0.79 GPa and 580°-640°C as a result of crustal thickening. The samples thus record a history of heating (D1), followed by near-isothermal compression (D2). This P-T pathway shows that contraction rather than extension, as suggested by some authors, occurred during D2. The contractional event is suggested to have taken place during the Delamerian Orogeny as a result of convergence and collisional processes along the former passive margin of Gondwana. © 2011 by The University of Chicago. All rights reserved.

Development of the East Gondwana passive margin and when it occurred are constrained by the composition of low grade mafic schists and U-Pb ages of detrital zircons in psammitic schists from the Bathampton Metamorphics in the Anakie Inlier of central Queensland. These rocks show considerable variation in light lithophile elements due to post-magmatic processes. They have flat heavy rare earth element patterns, low-TiO2 (<2 wt%) contents and their immobile element Ti, V, Y, La, Nb, Th and Zr values, indicate that they have an NMORB- like magmatic affinity. However, they differ from N-MORB in that they show light rare earth depleted patterns and lower incompatible trace element contents. Their relative low abundance and association with metasediments suggest they formed in a magma-poor rifted margin setting. They are associated with psammitic rocks with detrital zircon ages indicating probable deposition in the late Neoproterozoic at ca 600 Ma. A magma-poor rifted margin in northeastern Australia differs from the volcanic passive setting that occurred in southeastern Australia at this time. These findings support development of the East Gondwana margin at 600 Ma that may have been related to rifting of a microcontinent off East Gondwana well after the breakup of Rodinia at ca 750 Ma.

Fault kinematics can be successfully determined using structures resembling those in S-C mylonites that are developed in serpentinites within fault zones. Serpentinites containing such structures occur in faults zones bounding the Hastings Terrane, southern New England Fold Belt. Fault kinematic data constrain the various models for emplacement of the Hastings Terrane.

Carboniferous to Lower Permian successions along the western border of the Tamworth Belt between Wallabadah and Muswellbrook were remapped to clarify the stratigraphy and establish a boundary between the Werrie and Rouchel blocks. The boundary, located at the Waverley Fault, separates Carboniferous sequences containing different formations and volcanic members. SHRIMP AS3 dating of volcanic members indicates that successions within the Rouchel and Gresford blocks were deposited, uplifted and eroded at different times. The lacustrine Woodton Formation in the Werrie block, previously considered Carboniferous, is earliest Permian (Asselian) from palaeobotanical and SHRIMP AS3 evidence. Stratigraphic and palaeoenvironmental differences between the Werrie and Rouchel blocks suggest that they were not directly juxtaposed during the greater part of the Carboniferous, supporting palaeomagnetic evidence that blocks within the Tamworth Belt are allochthonous. Superposition of the western extremity of the Waverley Fault by Lower Permian (Sakmarian) formations and intrusion of folded and faulted Devonian to Lower Permian successions by the Barrington Tops Granodiorite (ca 280 Ma) indicate that the Werrie, Rouchel and Gresford blocks were subjected to tectonism before the Late Permian Hunter-Bowen Orogeny.

Geological mapping in the Carroll-Nandewar region near the northwestern margin of the Tamworth Belt has identified an Upper Carboniferous to Lower Permian succession, the Willuri Formation, containing coarse volcaniclastic sediments and interbeaded dacitic to rhyolitic pyroclastics, lavas and a possible dome or coulée. The Currangandi Group (new) accommodates Upper Carboniferous volcaniclastic formations in the northern Tamworth Belt, including the Willuri Formation. Volcanic rocks in the Willuri Formation are distributed in four stratigraphically distinct, intertonguing or overlapping packages: the Kaputar, Piney Range, Tulcumba and Gunnan packages. According to SHRIMP (AS3) dating most of the succession ranges in age from ca 320 to ca 308 Ma (Namurian-Westphalian). Dacific and rhyolitic lavas, apparently in sequence with the succession on the northwestern part of Tulcumba Ridge, are 297-287 Ma or younger (Early Permian). They appear to disconformably overlie the Carboniferous succession and could be either part of the Willuri Formation or related to the Boggabri Volcanics. Volcanic centres near Maules Creek (Kaputar centre) and Wean (Wean centre) appear to have been the sources for volcanics in the Kaputar package and most parts of the Rocky Creek region to the northeast, and the Tulcumba and probably the Piney Range packages, respectively. In the Gunnan package a single major ignimbrite was derived from a volcanic centre in the Werrie pyroclastic field, but the provenance of other pyroclastics is unknown. Correlations based on SHRIMP ages and the distribution of pyroclastics indicate that the Willuri Formation is equivalent to most of the Currabubula Formation in the Werrie Syncline, and to the Rocky Creek Conglomerate and Lark Hill Formation in the Rocky Creek region. Boundaries of the Werrie and Rocky Creek pyroclastic fields are adjusted to incorporate a new pyroclastic field based on the Wean volcanic centre and covering the Tulcumba and Piney Range packages