Tectonophysics 617 (2014) 20–30

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Chronological constraints on the Permian geodynamic evolution ofeastern Australia

Pengfei Li ⁎, Gideon Rosenbaum, Paulo VasconcelosSchool of Earth Sciences, The University of Queensland, Brisbane 4072, Queensland, Australia

⁎ Corresponding author at: Department of Earth SciePokfulam Road, Hong Kong, China. Tel.: +852 28578522.

E-mail addresses: pengfei.li@uqconnect.edu.au, pengfe

0040-1951/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.tecto.2014.01.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 June 2013Received in revised form 5 December 2013Accepted 12 January 2014Available online 22 January 2014

Keywords:New England OrogenOroclineSlab breakoffAlumMountain VolcanicsWerrie BasaltEastern Australia

The New England Orogen in eastern Australia developed as a subduction-related orogen in the Late Devonian toCarboniferous, and was modified in the Permian by deformation, magmatism and oroclinal bending. Thegeodynamics associated with the development of the New England oroclines and the exact timing of major tec-tonic events is still enigmatic. Here we present new 40Ar/39Ar results from metasedimentary and volcanic rocksfrom the southernNewEnglandOrogen. Eight grains from fourmetasedimentary samples (Texas beds) that orig-inated in the Late Devonian to Carboniferous accretionary wedge yielded reproducible plateau ages of ~293,~280, ~270 and ~260 Ma. These results suggest a complex thermal history associated with multiple thermalevents, possibly due to the proximity to Permian intrusions. Two samples frommafic volcanic rocks in the south-ernmost New England Orogen (Alum Mountain Volcanics and Werrie Basalt) yielded eruption ages of 271.8 ±1.8 and 266.4 ± 3.0 Ma. The origin of these rocks was previously attributed to slab breakoff, following a periodof widespread extension in the early Permian. We suggest that this phase of volcanism marked the transitionfrom backarc extension assisted by trench retreat to overriding-plate contraction. The main phase of oroclinalbending has likely occurred during backarc extension in the early Permian, and terminated at 271–266 Mawith the processes of slab segmentation and breakoff.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The New England Orogen (NEO), the easternmost and youngestcomponent of the Tasmanides in eastern Australia (Fig. 1a), is character-ized by a series of orogenic curvatures (oroclines). The oroclinal struc-ture, which is recognized by the map-view pattern of Late Devonian toearly Permian tectonic units, defines an ear-shaped geometry compris-ing four bends (Fig. 1a) (Glen and Roberts, 2012; Rosenbaum, 2012;Rosenbaum et al., 2012). However, the origin of this orogenic-scalestructure and its tectonic evolution remain poorly understood. Previousmodels have considered the possibility that the New England oroclinesformed in the proximity of a transform plate boundary (Cawood et al.,2011b; Offler and Foster, 2008) or in a backarc extensional setting asso-ciated with a retreating subduction zone (Rosenbaum et al., 2012).These models are preliminary and suffer from the scarcity of data onthe magnitude of block rotations and the timing of deformation, meta-morphism and magmatism.

The aim of this paper is to establish constraints on the timing of tec-tonic processes associated with oroclinal bending in the NEO. We

nces, University of Hong Kong,

i@hku.hk (P. Li).

ghts reserved.

present new 40Ar/39Ar data from metasedimentary and volcanic rocks,which are spatially and temporally linked to the oroclines. We targettwo different tectonic units. The first set of samples is from deformedmetasedimentary rocks (Texas beds), which are characterized by astructural fabric parallel to the oroclinal structure (Texas Orocline,Fig. 1) (Lennox and Flood, 1997; Li et al., 2012a). 40Ar/39Ar ages fromthese rocks could provide a maximum age constraint on the timing oforoclinal deformation, although subsequent heating could potentiallyreset these ages. The second set of samples includes mafic volcanicrocks from the southernmost NEO (Alum Mountain Volcanics andWerrie Basalt, Figs. 1a and 2). These rocks were supposedly derivedfrom an asthenospheric source, and their origin was attributed to slabbreakoff (Caprarelli and Leitch, 2001). The eruption ages, therefore,may provide information on the timing of lithospheric-scale processesassociated with oroclinal bending (e.g. slab tearing and segmentation).

The results of this paper do not allow us to directly constrain thetiming of deformation. However, they provide new information on thethermal and magmatic history of the NEO during the early Permian.We argue that it was during this period (290–270 Ma) that the NewEngland oroclines formed (Rosenbaum et al., 2012), in a convergent en-vironment that was dominated by backarc extension (Holcombe et al.,1997a; Korsch et al., 2009b). As such, our results shed new light onthe geodynamic processes responsible for oroclinal bending.

NB

30°S

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orth Belt

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Granitoids and volcanicrocks (260-235Ma)

Granitoids(235-210Ma)

Devonian-Carboniferousforearc basin

Devonian-Carboniferousaccretionary complex

Early Permian rift basins Devonian-Carboniferousarc-related rocks

Basin

Clarence-Moreton

Surat Basin

SydneyBasin

Tab

lela

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plex

BowenBasin

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ther

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EO

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ther

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EO

GunnedahBasin

Connors-AuburnArc

Sydney Basin Newcastle

Brisbane

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Figure 1b

Queensland

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Wales

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H

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151°E 151°10′ 151°20′ 151°30′ 151°40′ 151°50′

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°S

Dominant fabricS1()

S1 trace

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* Sample locations

Texasbeds

NEO

HpNb

Hb

Emu CreekBlock

???

??

? ? ? ?

Mooki

Hunter

Wb

AmBtMEAD2

Fig. 2b

Fig. 2a

NE1058

GunnedahBasin

UndercoveredWerrie Basalt

Exposed Permian basaltsand interbedded volcanic rocks

TO

MO

NOCO

**

252Ma

253 Ma

252 Ma

256 Ma

260 Ma

257 Ma

~247 Ma

280 Ma

291 Ma

294 Ma

298 Ma295 Ma

289 Ma

Tasmanides

(a) (b)

Fig. 1. (a) Geological map of the southernNEO. The blue dashed line traces the oroclinal structure (TO: Texas Orocline; CO; Coffs Harbour Orocline;MO:Manning Orocline; NO: NambuccaOrocline). The thin black dashed lines show the general orientation of the dominant S1 fabric. Insetmap in the upper right corner shows the location of the Tasmanides and NEO in easternAustralia. Inset map in the lower right corner shows major tectonic elements within the NEO. B: Bundarra granite; H: Hillgrove Suite; Hp: Halls Peak Volcanics; Am: Alum MountainVolcanics; Wb:Werrie Basalt; Bt: Barrington Tops Granodiorite; Nb: Nambucca Block; Hb: Hastings Block. (b) Geological map of the Texas Orocline (after Li et al., 2012a) and sample lo-cations. The oroclinal structure is traced by the S1 structural fabric.The ages of intrusive rocks are from the age database summarized in Li et al. (2012b) and Rosenbaum et al. (2012).

21P. Li et al. / Tectonophysics 617 (2014) 20–30

2. Geological setting

2.1. Regional tectonic framework

The NEO developed as an ocean–continent convergent system alongthe eastern Gondwanan margin from the Late Devonian to the Triassic(Glen, 2005). The orogen is subdivided into northern and southern seg-ments, which are separated from each other by the Mesozoic Clarence–Moreton Basin (Fig. 1a). The oldest tectonic units of the NEO comprise,fromwest to east, Devonian–Carboniferous arc, forearc basin and accre-tionary complex (Leitch, 1975; Murray et al., 1987), indicating awest-dipping subduction system. In the southern NEO, the volcanic arcis rarely exposed and is interpreted to be underthrust below the forearcbasin or covered by younger sedimentary rocks (e.g., Glen and Roberts,2012). The forearc basin (Tamworth belt) and accretionary complex(Tablelands Complex) are well exposed and are overlain by earlyPermian rift-related clastic successions (Fig. 1a). Rocks in the TablelandsComplex are dominantly low grade metamorphic rocks with a penetra-tive slaty cleavage (Binns et al., 1967; Fergusson, 1982; Korsch, 1978,1981), which is absent in the overlying early Permian sedimentaryrocks (Li et al., 2012a). This indicates that the accretionary complexunderwent regional deformation and metamorphism (D1, M1) priorto the deposition of the early Permian rocks. Metamorphic conditionsduring M1 in the northern part of the Tablelands Complex (Coffs-Harbour Block, Fig. 1a) were inferred to be in the field of prehnite–pumpellyite to lower-greenschist facies (Korsch, 1978). Syn-M1mineralassemblages in the Tablelands Complex are aligned parallel to the struc-tural fabrics (S1 and L1), indicating that regional deformation occurredsimultaneously with metamorphism (Korsch, 1978; Li et al., 2012a).Subsequently, some of the rocks in the Tablelands Complex were

subjected to an overprinting metamorphic event (M2) associated withthe intrusion of Permian to Triassic granitoids (Korsch, 1978).

The Devonian to Carboniferous rocks were subjected to oroclinalbending in the Permian, which gave rise to a series of tight oroclines(Korsch and Harrington, 1987; Rosenbaum, 2012; Rosenbaum et al.,2012). Oroclinal bending took place during the early Permian, contem-poraneously with the emplacement of S-type granitoids and thedevelopment of rift basins, possibly in an extensional backarc setting(Rosenbaum et al., 2012). The timing of early Permian S-typemagmatism has been constrained to ~295–285 Ma, with minor occur-rences at ~280 Ma (Cawood et al., 2011a; Donchak et al., 2007;Rosenbaum et al., 2012). Early Permian sedimentary rocks occur in theSydney–Gunnedah–Bowen basins and in several unitswithin the south-ern NEO (e.g., Nambucca Block, Fig. 1a). Leitch (1988) has suggestedthat all early Permian sedimentary rocks in the southern NEO were de-posited in a larger rift basin, referred to as the Barnard Basin. Volcanicrocks from the base of these basins yielded U–Pb SHRIMP zircon agesof 293–291 Ma (Roberts et al., 1996) and 292.6 ± 2.0 Ma (Cawoodet al., 2011a). These ages overlap with the intrusion of the S-type gran-itoids, supporting the suggestion that the emplacement of S-typemagmas occurred in a hot backarc extensional setting, possibly in re-sponse to eastward trench retreat (Rosenbaum et al., 2012).

After a period with very little occurrence of magmatism at ~280–260 Ma (Rosenbaum et al., 2012), the southern NEO was subjected toregional ~E–W shortening (Hunter–Bowen phase) (Holcombe et al.,1997b; Korsch et al., 2009c) and arc-related magmatism (WandsworthVolcanic Group and I-type granitoids) (Bryant et al., 1997; Stewart,2001). These observations are consistent with the existence of an ad-vancing west-dipping subduction zone during the late Permian andearly Triassic (Jenkins et al., 2002; Li et al., 2012b).

Gloucester

Stratford

Bulahdelah

NE1058

Legend

Quaternary Sediments

Dewrang Group and overlain Gloucestercoal measures in the Gloucester syncline area

Alum MountainVolcanics andunderlyingconglomerate

Carboniferoussediments

Markwell Coal Measures and overlainBulahdeiah Formation in the Myall syncline area

Lakes Road Rhyolite

Burdekins Gap Basalt

Sams Road RhyoliteMuirs CreekConglomerate

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ooki Thrust

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Carboniferous rocks

Woodton Formation

Temi Formation

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Greta Coal Measures

Maitland Group

Singleton supergroup

Mesozoic toCenozoic rocks

0 10 km

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Syncline

(a) (b)

Fig. 2. (a) Simplified geological maps of the Gloucester Syncline (left) and Myall Syncline (right), based on the published 1:250,000 geological map of the Newcastle sheet (Rose et al.,1966). The AlumMountain Volcanics occur in thefloor of two basins. The stratigraphical sequence in the Gloucester Syncline is afterWard et al. (2001). The cross section and stratigraphicsequence in the Myall Syncline area is after Jenkins and Nethery (1992). (b) Geological map of the exposed part of the Werrie Basalt, based on the 1:250,000 geological map of theTamworth sheet (Offenberg, 1971), Flood et al. (1988) and Roberts et al. (2006).

22 P. Li et al. / Tectonophysics 617 (2014) 20–30

2.2. Texas beds

The Texas beds belong to the northern part of the TablelandsComplex. The rocks are deformed low-grade metamorphic rocks withprotoliths of arenite,mudstone, chert, jasper, limestone andmafic volca-nic rocks (Donchak et al., 2007; Fergusson and Flood, 1984). The internalstratigraphy of the Texas beds is not well defined due to the absence ofreliable marker layers, but four tectono-stratigraphic subunits were de-scribed by Fergusson and Flood (1984). More recently, Donchak et al.(2007) further subdivided the Texas beds into eight subunits, includingvolcaniclastic turbidites (lithic arenite and mudstone), minor chert/argillite, jasper/argillite, altered basalt, limestone, conglomerate, andconglomeratic greywacke. Early Carboniferous fossils have been foundin the Texas beds (Aitchison and Flood, 1990; Olgers et al., 1974), butit is possible that unexposed accretionary wedge rocks west of theTexas beds (Fig. 1a) are of Late Devonian, as indicated by the occurrenceof Late Devonian fossils elsewhere within the Tablelands Complex(Aitchison, 1988). The development of the accretionary wedge fromLate Devonian to Carboniferous is consistent with the occurrence ofarc-related volcanic material dated at ~360 to ~305 Ma (Cawood et al.,

Fig. 3. Photomicrographs of the analyzed samples. (a–d) Slaty cleavage in samples NE0905, Nindicates a dominant pure shear strain during cleavage development. The occurrence of biotite g(e–f) Crossed and plane polarized light photomicrographs of a basalt from the AlumMountain Vpolarized light photomicrograph (g) and a SEM–EDS image (h) of a basalt from the AlumMoununalteredplagioclase rim. (i–j) Crossed andplane polarized light photomicrographsof theWerrplagioclase; Chl: chlorite; Se: sericite.

2011a; Claoue-Long et al., 1992; Jeon et al., 2012; Korsch et al., 2009a;Roberts et al., 1995a; Roberts et al., 2006).

The penetrative structural fabric (S1) in the Texas beds is predomi-nantly sub-parallel to the axial plane of isoclinal F1 folds (Lennox andFlood, 1997; Li et al., 2012a). In a map view, the S1 fabric is curved andmimics the shape of the Texas Orocline (Fig. 1b). The presence of biotitegrains, which are aligned parallel to the S1 cleavage in the Texas beds (Liet al., 2012a), indicates up to greenschist-facies regionalmetamorphismduring the development of the S1 fabric (Turner, 1968). F2 folds withaxial plane orientations sub-parallel to the axial plane of the oroclinewere interpreted as syn-oroclinal structures (Li et al., 2012a).

2.3. Alum Mountain Volcanics and Werrie Basalt

TheAlumMountainVolcanics comprise of basalt, andesite, dacite andrhyolite (Caprarelli and Leitch, 2001). The rocks are exposed in twomajor synclines (Gloucester and Myall synclines) overlying the UpperCarboniferous strata of the Tamworth Belt (Figs. 1a and 2a, b). In theMyall Syncline (Fig. 2a), the Alum Mountain Volcanics are subdivided,from bottom to top, into Sams Road Rhyolite, Burdekins Gap Basalt and

E0929, NE0946 and NE1061. The occurrence of symmetric quartz aggregates (e.g. Fig. 3a)rains aligned along cleavage domains (Fig. 3d) indicates syn-contractionalmetamorphism.olcanics, mainly consisting of plagioclase, pyroxene and opaqueminerals. (g–h) A crossedtain Volcanics. Notice that plagioclase phenocrysts are altered to sericite and rimmed by anie Basalt. Slight alteration is represented by chlorite. Py: pyroxene;Op: opaqueminerals; Pl:

200 µm200 µm

200 µm400 µm

(e) (f)

(i) (j)

(g) (h)

Py

Pl

Op

Chl

Py

OpPl

Fresh Pl

Altered Pl Se Py

Pl

(a) (b)

(c) (d)

200 µm200 µm

200 µm

400 µm 400 µm

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24 P. Li et al. / Tectonophysics 617 (2014) 20–30

Fig. 4. 40Ar/39Ar step heating results for samples NE0905 (a–b), NE0929 (c–d), NE0946 (e–f), NE1061 (g–h), NE0958 (i–k) and MEAD2 (l–n).

25P. Li et al. / Tectonophysics 617 (2014) 20–30

Lakes Road Rhyolite (Jenkins and Nethery, 1992). The Lakes Road Rhyo-lite yielded a U–Pb SHRIMP zircon age of 274.1 ± 3.4 Ma (Roberts et al.,1995b), giving an age constraint for the Alum Mountain Volcanics. Thevolcanic rocks are overlain by younger sedimentary rocks of theDewrang Group and the Gloucester Coal Measures (Ward et al., 2001).

The Werrie Basalt is assumed to be widespread in the floor of theGunnedahBasin (Leitch, 1993), but it is exposed only in theWerrie Syn-cline (Fig. 2b). Inside theWerrie Syncline, theWerrie Basalt is interstrat-ified with Permian sedimentary rocks (Fig. 2b). Volcanic material fromthe Woodton Formation below the Werrie Basalt (Fig. 2b) yielded aU–Pb zircon age of 290.5 ± 2.8 Ma (Roberts et al., 2006), giving amaximum age constraint for the basalt and representing the oldest sed-imentary sequence in the Gunnedah Basin. The Warrigundi Igneous

Complex (Fig. 2b), which includes extrusive andesites, dacites and rhy-olites interstratified with theWerrie Basalt, yielded a whole rock Rb–Srisochron age of 269.4 ± 4.6 Ma (Flood et al., 1988).

Rocks of the AlumMountain Volcanics and theWerrie Basalt are al-tered but still display magmatic textures (Caprarelli and Leitch, 2001;Jenkins and Nethery, 1992). The absence of actinolite and epidote inthe mafic rocks indicate that the metamorphic grade associated withthe alteration processes was below greenschist facies (Caprarelli andLeitch, 2001). The basalts are fine- to medium-grained and are com-posed of plagioclase, pyroxene, olivine and opaque oxides; alterationminerals include albite, quartz, smectite, chlorite and carbonates(Caprarelli and Leitch, 2001). The geochemical signature of the AlumMountain Volcanics is characterized by a flat REE pattern, 0.54–1.07

26 P. Li et al. / Tectonophysics 617 (2014) 20–30

(La/Sm)N ratios, 0.94–2.78 (La/Yb)N ratios, and εNd values from +5.61to+8.73, indicating an origin related to large degrees of partial meltingof asthenosphericmaterial (Caprarelli and Leitch, 2001). TheWerrie Ba-salt is characterized by positive εNd values ranging from +2.05 to+6.00, also indicating an asthenospheric origin (Caprarelli and Leitch,2001). The presence of a substantial depleted mantle component inthe geochemical signature of these volcanic rocks was interpreted tobe associated with extensive melting at shallow levels due to an en-hanced mantle geotherm, possibly in response to asthenospheric up-welling and/or lateral flow following breakoff of a lithospheric slab(Caprarelli and Leitch, 2001).

3. 40Ar/39Ar geochronology

3.1. Sample description

3.1.1. Samples from the Texas bedsFour samples from the Texas beds have been dated by the 40Ar/39Ar

incremental heating method. Sample NE0905 is a slate from thesouthwestern limb of the Texas Orocline (GPS coordinate 28.77297°S/151.16850°E, Fig. 1b). It is characterized by a strong cleavage (Fig. 3a)and consists of quartz aggregates, white mica andminor calcite. The oc-currence of symmetric quartz aggregates indicates dominant pure shearstrain during cleavage formation (Fig. 3a). The cleavage domains are de-fined by fine-grained white mica and opaque minerals. In the area ofsample NE0905, at the southwestern limb of the Texas Orocline, S1cleavage is steeply dipping and strikes NE–SW (Fig. 1b).

Samples NE0929 (GPS coordinate 28.18959°S/151.34612°E) andNE0946 (GPS coordinate 28.22647°S/151.34553°E) are slates from thehinge area of the Texas Orocline (Fig. 1b). The rocks consist of quartz ag-gregates, fine-grainedwhitemicas and opaqueminerals (Fig. 3b, c). TheS1 fabric at the hinge zone is steeply dipping and oriented ~E–W(Fig. 1b). Quartz veins are widespread and belong to two generations,involving an earlier generation of veins folded by F1 folds with axialplanes sub-parallel to the dominant S1 fabric, and a second generationof veins that crosscut the earlier veins and were not affected by F1folds (Li and Rosenbaum, 2010). Sample NE0946 (from approximately3 km north of sample NE0929, see Fig. 1b) consists of quartz aggregatesthat were flattened parallel to the S1 fabric (Fig. 3c). The S1 cleavage isgenerally defined by fine-grained white micas.

Sample NE1061 is a biotite–quartz–schist from the eastern limb ofthe Texas Orocline (GPS coordinate 28.17832°S/151.77783°E, Fig. 1b).The S1 fabric in this area strikes ~NW–SE (Fig. 1b). The cleavage is de-fined by recrystallized biotite grains (Fig. 3d). The preferred orientationof biotite grains in this sample (Fig. 3d) indicates syn-deformationalmetamorphism.

3.1.2. Alum Mountain Volcanics and Werrie Basalt samplesIn order to constrain the timing of Permianmafic volcanism, two ba-

salt samples were dated by 40Ar/39Ar geochronology. Sample NE1058 isa fine-grained basalt from the AlumMountain Volcanics in the Glouces-ter Syncline (GPS coordinate 32.27863°S/151.91132°E, Figs. 1a and 2). Itis composed mainly of plagioclase, pyroxene, opaque minerals andminor olivine and quartz (Fig. 3e, f). Minor plagioclase phenocrysts arealtered to sericite and rimmed by an unaltered plagioclase (Fig. 3g, h).Sample MEAD2 is from a drill hole in the Gunnedah Basin (GPS coordi-nate 32.06746°S/150.24380°E; Fig. 1a) that penetrated theWerrie Basaltat depth of 1442.89–1442.96m. The sample is amedium-grained basalt,which mainly consists of plagioclase, pyroxene, chlorite, and opaqueminerals (Fig. 3i, j). The presence of chlorite indicates alteration.

3.2. Methods

Samples were crushed and washed in distilled water and absoluteethanol in an ultrasonic bath. Five to ten mica-rich grains were selectedfrom the slate samples (NE0905, NE0929 andNE0946), using a binocular

microscope. As for the biotite–quartz–schist (NE1061), visually pure bi-otite grains were selected for geochronology. From the basalt samples(NE1058 andMEAD2), the freshestwhole-rock fragmentswere selected.Following the procedure described in Vasconcelos et al. (2002), sampleswere loaded into a 21-pit aluminumdisk alongwith the neutron fluencemonitor Fish Canyon sanidine (Kuiper et al., 2008). The disks wereclosed by aluminum covers and wrapped in aluminum foil and vacuumheat-sealed into quartz vials. Samples were submitted for 14 h of irradi-ation at the B-1 CLICIT facility, TRIGA-reactor, Oregon State University,USA. Ages were calculated using the decay constants of Steiger andJäger (1977).

Samples were analyzed at the University of Queensland ArgonGeochronology in Earth Sciences (UQ-AGES) laboratory by laser incre-mental heating. For detailed information on the analytical proceduresee Vasconcelos et al. (2002). Each aliquot was heated incrementallywith a continuous-wave Ar-ion laser with a 2-mm wide defocusedbeam. The fraction of gas released was cleaned through a cryocooledcold-trap (T=−125 °C) and two C-50 SAES Zr–V–Fe getters, and ana-lyzed for Ar isotopes in aMAP 215-50mass spectrometer equippedwitha third C-50 SAES Zr–V–Fe getter. Full system blanks and air pipetteswere determined before and after analysis of each grain. The data cor-rection for mass discrimination, nucleogenic interferences, and atmo-spheric contamination followed the procedures in Vasconcelos et al.(2002). The J-factor for each aluminum disk was determined by thelaser total fusion analyses of 15 individual crystals of the neutronfluence monitor Fish Canyon sanidine.

3.3. Results

Two grains from each sample were analyzed by the laserincremental-heating 40Ar/39Ar method. Analytical data are presented inthe online Supplementary file, and step-heating spectra are shown inFig. 4. An age plateau is defined as a sequence of three or more stepsthat contain more than 50% of the 39Ar released and have individualages that are within 2σ from the mean value calculated by weightingwith inverse variance (Fleck et al., 1977). Plateau age errors are withinthe 95% confidence level (2σ), including the errors in the irradiation cor-rection factors and the error in J. However, these errors do not includethe uncertainty in the potassium decay constants. An integrated age cal-culated by combining the results from all steps of the incremental-heating analysis is also reported (Fig. 4).

Two grains (6761-01 and 6761-02) from sample NE0905 yielded con-cordant plateau ages of 293±2 and 293.9±1.7Ma, respectively (Fig. 4a,b). SampleNE0929 yielded relatively younger but also compatible plateauages (270± 2Ma for grain 6762-01; and 271.5 ± 1.7 Ma for grain 6762-02; Fig. 4c, d). Two grains (6764-01 and 6764-02) from sample NE0946also yielded compatible plateau ages of 282.7 ± 1.7 and 280.3 ±1.7 Ma, respectively (Fig. 4e, f). The two biotite grains (6906-01 and6906-02) from sample NE1061 are internally consistent and show dis-tinct plateau ages of 260.2 ± 1.7 and 260.9 ± 1.7 Ma (Fig. 4g, h).

Two grains (6765-01 and 6765-02) from sample NE1058 (AlumMountain Volcanics) yielded two concordant plateau ages of 271.7 ±1.7 and 273 ± 3 Ma (Fig. 4i, j). The plateau ages are in good agreementwith the age of the inverse isochron age of 271.8± 1.8Ma (Fig. 4k). Thetwo grains (7291-01 and 7291-02) from sampleMEAD2 (Werrie Basalt)yielded concordant plateau ages of 262± 6 and 268±3Ma (Fig. 4l, m).These ages are compatible with the isochron age of 266.4 ± 3.0 Ma, themost likely age for this unit (Fig. 4n).

4. Discussion

4.1. Interpretation of dated samples

The four samples from the Texas beds yielded distinct plateau agesranging from 293.9 ± 1.7 to 260.2 ± 1.7 Ma. The samples analyzedyielded two distinct types of spectra: hump-shaped spectra (sample

(a)

(b)

Fig. 5. (a) An age probability diagramwith all 40Ar/39Ar age steps of four Texas beds sam-ples; (b) An age probability diagram of intrusive rocks in the area of the Texas beds(Fig. 1b). Ages of intrusive rocks are from the age database summarized in Li et al.(2012b) and Rosenbaum et al. (2012). Note that the ~260Ma 40Ar/39Ar peak age is corre-lated with the ~260–250 Ma intrusive event.

27P. Li et al. / Tectonophysics 617 (2014) 20–30

NE0905, Fig. 4a, b) suggesting the co-existence of different generationsof micas (Wijbrans and McDougall, 1986); and ascending spectra(samples NE0929, NE0946, NE1061, Fig. 4c–h), suggesting partial reset-ting of the K–Ar clock (McDougall and Harrison, 1999). We interpretthat the hump-shaped spectra of sampleNE0905 resulted fromprogres-sive degassing of syn-deformation mica intimately intergrown withrecrystallized mica formed during subsequent thermal events. The as-cending age spectra obtained for samples NE0929 and NE0946 alsoshow a slight drop in ages for the highest temperature steps (Fig. 4and Supplementary data Table), possibly suggesting the presence ofsmall quantities of highly retentive partially recrystallized micas. Theamount of gas extracted from these high-T steps is small and it is diffi-cult to confirm that these gas fractions actually come from different res-ervoirs. Nevertheless, the samples showing the slight drop in age at thehigh-T steps yielded well-defined plateaus containingmore than 60% ofthe total 39Ar release of the grain, which we interpret as the cooling agefor the sample. The other two grains that show ascending spectra(6906-01 & -02, sample NE1061) defined plateaus containing morethan 60% of the 39Ar contained in the sample, and yielded reproducibleplateau ages of 260.2± 1.7 and 260.9± 1.7Ma.We interpret these pla-teau ages as reliable constraints on the timing of cooling below the clo-sure temperature of the main K-bearing phases (biotite).

The fact that we dated eight grains from four different samples andobtained reproducible plateau ages indicating four distinct crystallizationand/or cooling ages (~293, ~280, ~270, and ~260 Ma), suggests that thethermal history of the Texas bedswas complex and subjected tomultiplethermal events. Alternatively, it is possible that the emplacement of vo-luminous granitoids at 260–250 Ma (Fig. 5) may have partially resetthe K–Ar clock throughout the region. A large number of granitoids in-truded the Texas beds during the Permian and the Triassic, with majorintrusive events at ~295 and ~255 Ma (Figs. 1, 5 and 6) (Bryant et al.,1997; Cawood et al., 2011a; Li et al., 2012b; Rosenbaum et al., 2012;Shaw and Flood, 1981). The ~260 Ma thermal event recorded in ourdated samples is consistent with the initiation of the late Permian toearly Triassic intrusions in the Texas beds (Figs. 5 and 6). Therefore, itis possible that the emplacement of these intrusions was responsiblefor the thermal disturbances associated with Ar loss or partial recrystal-lization of fine-grained mica grains in the Texas beds. A regional-scalestatic thermal metamorphism in the adjacent Coffs-Harbour Block ofthe Tablelands Complex (Fig. 1a) was attributed to the emplacement ofthese granitoids (Korsch, 1978). Similarly, low-grade regional metamor-phism may have also affected the Texas beds. Further support for thissuggestion, however, would require a regional thermochronologicalstudy targeting the link between the age distribution and the proximityto the various regional plutons.

Our samples from the AlumMountain Volcanics yielded two concor-dant plateau ages of 271.7± 1.7 and 273± 3Ma, which are compatible(within error)with theU–Pb SHRIMP zircon age of 274.1±3.4Ma fromthe overlying Lakes Road Rhyolite (Fig. 2a) (Roberts et al., 1995b). Sim-ilarly, the new age of 266.4 ± 3.0 Ma from theWerrie Basalt is compat-ible with the 269.4 ± 4.6 Ma whole-rock Rb–Sr isochron age from theWarrigundi Igneous Complex (Flood et al., 1988). These ages areinterpreted as eruption ages of the basalts. However, based on thewhole-rock ages obtained in this study, it is difficult to assess whethermagmatism in the Alum Mountain Volcanics and the Werrie Basalt oc-curred synchronously. Previous studies have shown that whole-rock40Ar/39Ar dating of basalt may yield relative large age ranges (up to10Ma), while singlemineral 40Ar/39Ar dating for the same units suggesta much shorter period of magmatism (Hofmann et al., 2000; Jourdanet al., 2003; Jourdan et al., 2007). This discrepancy between total rockand single mineral separates has not been identified in other 40Ar/39Arstudies (e.g. Renne et al., 1992; Thiede and Vasconcelos, 2010). But toerr on the conservative side, we conclude that mafic magmatism musthave occurred in the southernmost NEO at 273–263 Ma; whether ornot these ages represent a single magmatic event remains an openquestion.

4.2. Tectonic implications

As discussed in the previous section, our 40Ar/39Ar results provideevidence for early Permian heating and magmatism in the southernNEO. Unfortunately, due to possible resetting of the 40Ar/39Ar system,we were unable to constrain the exact timing of fabric development inthe Texas beds. This penetrative structural fabric is recognized ubiqui-tously in the Late Devonian–Carboniferous accretionary wedge rocksof the Tablelands Complex, and its strike orientation clearly followsthe curvature of the Texas and Coffs-Harbour oroclines (Fig. 1b)(Korsch, 1981; Lennox and Flood, 1997; Li et al., 2012a). It is thereforeinterpreted as a pre-oroclinal structural fabric, which likely developedduring the Carboniferous when the Tablelands Complex was still posi-tioned in an accretionary complex setting (Fig. 7a). The forearc basinassociated with this supra-subduction environment is represented inthe rocks of the Tamworth Belt and Emu Creek Block (Fig. 1a), whichsimilarly to the fabric in the accretionary complex, are curved aroundthe Texas Orocline. Rocks of the Tamworth Belt and the TablelandsComplex also seem to follow the curvature of theManning Orocline far-ther south (Fig. 1a) (Glen and Roberts, 2012; Li and Rosenbaum, 2014;Rosenbaum, 2012), although this interpretation is still a matter of de-bate (Lennox et al., 2013).

The main phase of oroclinal bending has most likely occurred in theearly Permian (Rosenbaum et al., 2012), and our new 40Ar/39Ar ages

270

280

290

300

260

Ma

Sla

bS

lab

rollb

ack

Sub

duct

ion

adva

nce

Per

mia

n

Tablelands ComplexGunnedah Basin

BarnardBasin

Faulting

Bun

darr

a an

dH

illgr

ove

suite

s

Forelandthrust

Oro

clin

al d

efor

mat

ion

Lopi

ngia

nG

uada

-lu

pian

Cis

ural

ian

Contraction

TamworthBelt

I-ty

peG

rani

toid

s

and uplift

brea

koff

Contraction

NE

0946

NE

0929

NE

1061

B

BoggabriVolcanics

Werrie Basalt+Wi(MEAD2)

Alum Mt(NE1058)

WandsworthGroup

volcanicsGr+Ma+Si

Alum Rock volvanics

Halls Peak volvanics

Sedimentary basin Volcanic rocks Granitoids

?

Thermal events

Fig. 6. Spatial and temporal diagram of Permian tectonic events. B: Barrington Top Granodiorite. Wi: Warrigundi igneous Complex; Gr: Greta Coal Measures; Ma: Maitland Group;Si: Singleton supergroup.

28 P. Li et al. / Tectonophysics 617 (2014) 20–30

provide additional constraints on the geodynamic evolution during thisperiod. The samples from the Texas beds were subjected to thermal dis-turbances associatedwith the emplacement of S-type and I-type granit-oids at ~295 and ~255 Ma, respectively (Figs. 5b and 6). The belt ofS-type granitoids,which seem to follow the shape of the Texas,Manningand Nambucca oroclines (Fig. 1a) (Rosenbaum et al., 2012), were likelyderived from melting of supracrustal rocks of the Tablelands Complex(Jeon et al., 2012; Phillips et al., 2011). It appears, therefore, that theheat flowwas relatively high at ~295Ma, thus indicating that the Table-lands Complex must have changed its tectonic position in the earlyPermian, from a relatively cold forearc region (i.e. accretionary com-plex) to a hot arc or backarc environment. This is indicative to an east-ward trench retreat (Jenkins et al., 2002).

A number of additional lines of evidence support the suggestion thatthe southern NEO was mostly in a backarc setting during the earlyPermian. These include observations onwidespread extension through-out the whole NEO (Holcombe et al., 1997a), bimodal volcanism(Asthana and Leitch, 1985; Cawood et al., 2011a; Leitch, 1969), andthe deposition of rift-related sedimentary basins (Korsch et al., 2009b;Leitch, 1988). In the southernNEO, the onset of volcanismand sedimen-tation, at ~293–291 Ma (Cawood et al., 2011a; Roberts et al., 1996), co-incides exactly with the timing of emplacement of S-type granitoids(Fig. 6). We envisage, therefore, a backarc environment in the earlyPermian (Fig. 7b), involving the deposition of the Barnard andBowen–Sydney–Gunnedah basins as extensional back arc basins, theemplacement of S-type granitoids, and bimodal volcanism (Figs. 6 and7b). This geodynamic setting is possibly comparable to modern conver-gent environments, such as the Mediterranean and southwest Pacific,where the origin of oroclines is intimately linked to retreating subduc-tion systems and overriding-plate extension (Faccenna et al., 2004;Hall, 2002; Lonergan and White, 1997; Rosenbaum and Lister, 2004;Royden, 1993; Schellart et al., 2002). We therefore suggest that theearly Permian extensional setting in the southern NEOwas the primarycause for oroclinal bending.

Our new 40Ar/39Ar ages show that basaltic volcanism in the south-ernmost NEO (AlumMountain Volcanics andWerrie Basalt) did not co-incide with the main pulse of extensional tectonism at 295–290 Ma.Rather, volcanism in the Alum Mountain and Werrie Basalt took place20–30 Ma later, shortly before the onset of contractional deformation(Hunter–Bowen phase), which affected the NEO from 265 to 230 Ma(Holcombe et al., 1997b). This phase of volcanism, therefore, mayhave marked the transition from an extensional backarc setting in aretreating subduction zone to overriding-plate contraction duringtrench advance (Fig. 7b–d). The suggestion that the Alum Mountainand Werrie Basalt were derived from asthenospheric upwelling associ-ated with slab-breakoff (Caprarelli and Leitch, 2001) supports the ideathat the transition from early Permian extension to Hunter–Bowen con-traction involved lithospheric-scale reorganization of the subductionzone. Accordingly, we propose that slab-breakoff at 271–266 Mamarked the termination of oroclinal bending, prior to the reestablish-ment of an Andean-type subduction system at ~260 Ma (Fig. 7c–d).

5. Conclusions

We presented new 40Ar/39Ar ages from four samples in the Texasbeds (Tablelands Complex) and two samples from mafic volcanicrocks in the southernmost NEO (Alum Mountain Volcanics and WerrieBasalt). Results from the Texas beds yielded multiple plateau ages of~293, ~280, ~270 and ~260 Ma. Combined with argon age spectra, wesuggest that these rocks underwent a complex thermal history, possiblyinvolving thermal disturbances associated with Permian intrusiveevents. Basalts from the Alum Mountain Volcanics and Werrie Basaltyielded eruption ages of 271.8 ± 1.8 and 266.4 ± 3.0 Ma, respectively.The origin of these rocks has been attributed to asthenospheric upwell-ing during slab breakoff (Caprarelli and Leitch, 2001), following a periodofwidespread backarc extension.We think that this period ofmafic vol-canism, at 271–266Ma, marked the termination of oroclinal bending ina retreating subduction system.

Continentalarc

Tablelands Complex

Subduction

SubductionRollback

Tamworth Belt

Sydney-Gunnedah Basin

S-typegranitoids

Barnardbasin

E

EW

W

Bimodalvolcanism

Forelandbasin

Arc-relatedmagmatic rocks

New offshoretrench

(backarc extension)

Hunter-Bowen contraction)(

EW

Subduction advance

?

?

(b) ~300-275 Ma

(a) ~360-300 Ma

Subductiontrench

Marker oforogenic system

(d) ~265-230 Ma

WerrieBasaltW

WerrieBasalt Alum Mt Volcanics

Breakoff ofretreating slab

Delaminated slab

Alum MtVolcanics

?

(c) ~275-265 Ma

?

?

E

Arc (unexposed)

Fig. 7.A schematic tectonicmodel showing the possible evolution of the subduction system in amap view (left) and a cross section (right). (a) Late Devonian to Carboniferous subduction;(b) early Permian backarc extension and oroclinal bending; (c) mid Permian slab breakoff; (d) late Permian to early–mid Triassic contraction.

29P. Li et al. / Tectonophysics 617 (2014) 20–30

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.tecto.2014.01.013.

Acknowledgments

This work is funded by the Australian Research Council (DP0986762and DP130100130). The manuscript benefited from constructive com-ments by Laurent Jolivet, Fred Jourdan and an anonymous reviewer.We thank Benjamin Cohen for assistance with sample preparation,Xiaodong Deng for discussion of data interpretation, and Russell Korschfor beneficial discussions on the evolution of the accretionary complexin the southern NEO. Larissa Gammidge and Alexander Tutt-Branco(Division of Resources & Energy, NSW) are thanked for assistance inobtaining the sample from the Werrie Basalt. Comments by StephenJohnston, Uri Shaanan and Thiago Piacentini on early versions of themanuscript are appreciated.

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