Disputed tectonic setting for the late Silurian–early Carboniferous development of the northern New England Orogen: detrital zircon ages suggest a continental margin arc association

Abstract

Devonian successions in the Calliope and Yarrol provinces of the northern New England Orogen in central to northern Queensland have a lithological assemblage and depositional environments indicative of a close association with a subduction-related volcanic arc. These rocks have been interpreted as related to island arc and backarc settings based on geochemical characteristics of magmatic rocks. We present new detrital zircon ages from the Devonian to lower Carboniferous successions of the Calliope and Yarrol province successions. Most samples have no or relatively few zircon grains extracted except for samples of quartz-rich sandstone in the Middle Devonian Erebus beds of Hunter Island, located 160 km north-northwest of Rockhampton and another sample in the Rockhampton district. The samples with abundant zircon, and some with less common zircon, have prominent peaks of mid Cambrian to Ordovician ages and are consistent with derivation from the northern Thomson Orogen where igneous rocks of this age are documented in the Charters Towers Province. These data indicate a connection between the Devonian arc-flank deposits of the northern New England Orogen and the Thomson Orogen, which had previously been well established for Carboniferous deep-marine sandstones of the subduction complex (e.g. Shoalwater Formation). These results favour an east-facing continental margin arc setting in the Devonian contra to intra-oceanic island arc settings previously proposed. Detrital zircon age spectra from sampled early Carboniferous units of northern New England Orogen association indicate a pulse of felsic magmatism of that age.

KEY POINTS

1. Devonian quartz-rich sandstones from the northern New England Orogen have abundant detrital zircon ages of mid Cambrian to Ordovician indicating a provenance connection with the Charters Towers Province of the northern Thomson Orogen.

2. These results confirm a provenance linkage to Gondwana and imply that the Devonian tectonic setting was a continental margin arc and associated forearc basinal infill.

3. Geochemical comparisons suggesting an island arc setting have overly influenced previous tectonic interpretations.

Keywords:

• Arc
• Calliope Province
• continental margin
• detrital zircon
• Devonian
• New England Orogen
• tectonics

Introduction

The New England Orogen is the most outboard, and youngest, element of the composite Tasmanides orogenic system incorporated into eastern Australian crust owing to active margin tectonism through the Paleozoic into the Triassic (Henderson & Johnson, 2016; Rosenbaum, 2018). It is divided by Mesozoic cover straddling the Queensland–New South Wales border region into northern (NNEO) and southern (SNEO) segments (Figure 1). These segments have complementary rock systems and similar histories (Jessop et al., 2019). Both have very small domains of Ordovician strata (Jell et al., 2021; Percival et al., 2011; Roder, 2015) adjoining a major crustal dislocation, the Yarrol (NNEO) and Peel (SNEO) faults, which separate extensive outboard domains of structurally complex, deep-marine strata interleaved with basaltic units, the Wandilla Province (Donchak et al., 2013) of the NNEO and Tablelands Complex (Cawood et al., 2011) of the SNEO, interpreted as a subduction complex largely of Devonian–Carboniferous age.

Figure 1. Major subdivisions of the late Silurian–Carboniferous rocks in the New England Orogen. Rectangles showing the locations of Figures 2 and 3. ACI, Alice Creek Inlier; Gb, Goodnight beds; SP, Slade Point (north of Mackay); Opcf, Ordovician Pipeclay Creek Formation and associated lower Paleozoic units southeast of Tamworth (see text).

A diverse assemblage of stratigraphic units lies inboard of these faults (Figures 1–3). The NNEO part of the assemblage consists of Late Devonian–Permian units, embraced by the Yarrol Province including the Campwyn Subprovince of coastal location to the north (Donchak et al., 2013). Formations of similar age at the western margin of SNEO exposure are part of the Baldwin–Currabubula arcs of Glen (2013). As reviewed by Jessop et al. (2019), rocks of this grouping are widely considered as an arc/forearc system complementing the subduction complex assemblage to the east. Part of the assemblage consists of older, late Silurian–Middle Devonian, stratigraphic units assigned as the Calliope Province (Blake, 2013) of the NNEO and the Cambrian(?) to early Late Devonian stratigraphic units of the Gamilaroi Terrane of the SNEO (Flood & Aitchison, 1988).

Figure 2. Geological maps of the Stanage Peninsula and Duke Islands in central Queensland showing the location of studied samples from Hunter Island (see inset). Geology after Leitch et al. (1994) and Geological Survey of Queensland (2012). See Figure 1 for location.

Figure 3. Pre-late early Carboniferous rock units in the Rockhampton region of central Queensland showing sample locations. Blank areas are undifferentiated post late early Carboniferous units. Geology after Geological Survey of Queensland (2012). See Figure 1 for location. Ord, Ordovician inlier west of the Yarrol Fault south-southeast of Calliope (see text).

Permian stratigraphic assemblages of the New England Orogen are diverse and widely scattered (Jessop et al., 2019), overlying those of the Wandilla and Yarrol provinces, the Tablelands Complex and Tamworth Belt and its component Gamilaroi Terrane, and forming a younger part of the Yarrol Province. For the New England Orogen, volcanic rocks are a prominent part of the Permian assemblages, coeval with numerous, nearby, granitoid intrusions. The lower Permian rocks are generally considered to reflect a backarc setting (Jessop et al., 2019 and references therein).

The tectonic setting of the Calliope Province, and the Gamilaroi Terrane have been subject to extensive debate. Three different constructs have been proposed within a literature that spans 50 years.

1. A forearc, continental margin setting for these assemblages has been suggested by Caprarelli and Leitch (2002), Cawood et al. (2011), Henderson (1980), Henderson et al. (1993), Korsch et al. (1990), Leitch (1974, 1975), Lindsay (1990) and Morand (1993a). Under this proposal, east Gondwana experienced continuity in an active margin setting through the mid and late Paleozoic until the opening of the Tasman Sea in the Late Cretaceous (Henderson et al., 2022; Henderson & Johnson, 2016).

2. An intra-oceanic arc origin related to east-dipping subduction, with subsequent accretion of the resulting crustal assemblage to the east Gondwana margin, has been suggested by Aitchison and Flood (1995); Buckman et al. (2015), Day et al. (1978), Flood and Aitchison (1992), Marsden (1972), and Murray et al. (1987). Geochemical attributes of mafic volcanics and hypabyssal intrusions have been employed to support the intra-oceanic arc model (Murray et al., 2012; Murray & Blake, 2005; Offler & Gamble, 2002; Offler & Huang, 2018; Offler & Murray, 2011; Stratford & Aitchison, 1997a). Under this proposal, the long-term continuity of subduction polarity relevant to east Gondwana experienced a late Silurian–mid Devonian reversal.

3. The origin as an intra-oceanic arc related to west-dipping subduction with subsequent accretion to the east Gondwana margin, informed by the geochemistry and interpretation of mafic dykes and sills within the Gamilaroi Terrane, was proposed by Offler and Huang (2018). Under this model, west-dipping subduction marked an oceanic–continental plate boundary for part of the east Gondwana margin but diverged northwards towards the position of the New England Orogen to be located outboard of the continental margin, within oceanic crust.

Detrital zircon geochronology has the potential to provide additional constraints on how these contrasting models can be applied. Under the intra-oceanic arc proposals, significant detrital zircon sources of Gondwanan parentage would not be anticipated. Older zircon ages have been documented from oceanic island arc systems, and the issue arises as to their significance. For example, they may relate to the mixing of sediment from an older continental source with rocks of arc derivation (Glen et al., 2011), or to potential sediment derived from arc igneous rocks, which contain relict zircon reworked from older basement beneath the arc system (Buys et al., 2014; Smyth et al., 2007).

Under the continental margin forearc proposal, a contribution of grains older than the rock systems in question is to be expected. An example of how detrital zircon geochronology constrains tectonic models applies for the Permian–Triassic Gympie Province at the eastern margin of the NNEO. As demonstrated by this province, which had been long considered to represent an exotic intra-oceanic terrane subsequently docked at the east Australian continental margin, its sedimentary rocks contain detrital zircon derived from adjacent subduction complex rocks showing it to be autochthonous (Li et al., 2015).

The aim of this study is to examine the age profile of detrital zircon from samples of late Silurian–early Carboniferous lithostratigraphic units within the NNEO to provide insight into their tectonic setting.

Geological context of sampling

The Calliope Province of the NNEO includes siliciclastics, vol­canics and uncommon limestone assigned to the Erebus, Craigilee, Awoonga, Capella and Philpott subprovinces (Figures 1–3; Blake, 2013). They are geographically separated as well as by differences in lithological expression and by details of volcanic geochemistry (Murray et al., 2012) so that their palinspastic relationships are uncertain. Biostratigraphic data from corals, conodonts and brachiopods show that strata of the subprovinces overlap in age, which ranges from late Silurian to Middle Devonian (Murray et al., 2012; Figure 4). Succession and formal lithostratigraphic subdivision have been established only for the Capella Subprovince for which the stratal thickness is ∼2000 m; an informal stratigraphic succession for the Craigilee Subprovince indicates a thickness of some 2500–3000 m (Murray et al., 2012).

Figure 4. Stratigraphic relationships for subprovinces of the Calliope Province and older units of the Yarrol Provence of relevance to this study. Also shown is sampling for detrital zircon: closed circle, high zircon yield; partially filled circle, low zircon yield; open circle, no zircon yield.

Strata of the Erebus Subprovince were originally part of the Mount Holly beds of Kirkegaard et al. (1970) dated as late Silurian–Middle Devonian and later raised to formation status by Morand (1993a). However, subsequent mapping and new biostratigraphic data have resulted into division of the Mount Holly Formation into the Lower–Middle Devonian Erebus beds (Erebus Subprovince) and the Frasnian–Tournaisian Mount Alma Formation now assigned to the Rockhampton Subprovince of the Yarrol Province (Blake & Withnall, 2013). The Lochenbar, Balaclava, Mount Hoopbound formations and Three Moon Conglomerate are Rockhampton Subprovince units that are coeval with the Mount Alma Formation (Figure 4), with Yarrol Province strata of this age range outcropping extensively within the NNEO (Figures 1–3).

Volcanic and volcaniclastic strata of the Campwyn Subprovince are also considered as largely coeval with the Mount Alma Formation (Blake & Withnall, 2013) but may range to the Middle Devonian as is indicated by a fossil-bearing horizon (Fergusson et al., 1994; Henderson et al., 1994). A small, isolated domain of quartz arenite occurs within the provincial domain (Slade Point, Figure 1), but Bryan et al. (2003) considered it to be of suspect association.

Stratal domains on the Duke Islands and nearby at Stanage Bay, which were mapped as Mount Holly beds by Kirkegaard et al. (1970), and as Mount Holly Formation by Morand (1993a), are now included in the Erebus beds (Blake, 2013). As described by Leitch et al. (1994), they consist of volcaniclastic breccia, conglomerate and sandstone with subordinate intervals of mafic to felsic volcanics and limestone, reaching a thickness of some 3500 m. An interval of quartz sandstone, interbedded with limestone, occurs near the base of the succession outcropping on Hunter Island. Tabulate corals from Hunter Island limestone are no younger than Middle Devonian (Roberts, in Malone et al., 1969).

Representative sandstone samples were collected from strata of the Erebus, Craigilee, Awoonga, and Capella Creek subprovinces of the Calliope Province, including the quartz sandstone interval exposed on Hunter Island, and from older strata of the Rockhampton Subprovince (Mount Alma Formation and Rockhampton Group; Figures 2–4; Table 1). The thickness of the sandstone interval sampled on Hunter Island has not been measured. Based on mapped bedding trends (Leitch et al., 1994) sampling is likely to have spanned an interval of some 250 m. The anomalous interval of quartz sandstone (Slade Point, Figure 1) mapped within the Campwyn Subprovince was also sampled.

Table 1. List of zircon-bearing samples from the northern New England Orogen in the Rockhampton–Stanage Bay–Mackay regions.

Sample no.	Grid reference (AMG94)	Location (lat S/long E)	General location	Lithology	Unit	N (n)^a
R5	30017 734402	24°0′12ʺ 151°2′8ʺ	Dawson Highway west of Calliope	Lithic sandstone	Mount Alma Formation	7 (6)
R6	302703 734458	23°59′55ʺ 151°3′38ʺ	Dawson Highway west of Calliope	Lithic sandstone	Mount Alma Formation	11 (6)
R7	31498 734247	24°1′9ʺ 151°10′51ʺ	Dawson Highway west of Calliope	Lithic sandstone	Rockhampton Group	84 (63)
R9	29328 73437	24°0′19ʺ 150°58′4ʺ	Mt Alma Rd, northwest of Calliope	Lithic sandstone	Mount Alma Formation	14 (11)
R10	28735 736740	24°0′16ʺ 150°54′34ʺ	Gentle Annie Rd, near Ambrose	Lithic sandstone	Erebus beds	25 (17)
R15	23674 738877	23°35′25ʺ 150°25′13ʺ	Mt Morgan Range lookout	Lithic sandstone	Raspberry Creek Formation	34 (11)
R22	20281 743529	23°9′53ʺ 150°5′51ʺ	Marble Ridge Rd near yards	Lithofeldspathic sandstone	Craigilee beds	2 (0)
R28	28468 736227	23°50′11ʺ 150°53′10ʺ	Hut Creek Rd end, northwest of Ambrose	Lithofeldspathic sandstone	Erebus beds	2 (1)
R29	28563 736629	23°48′1ʺ 150°53′45ʺ	Gentle Annie Rd, northwest of Ambrose	Lithofeldspathic sandstone	Erebus beds	10 (5)
R30	27711 736401	23°49′11ʺ 150°48′43ʺ	Longmorn Rd, northwest of Ambrose	Lithofeldspathic sandstone	Erebus beds	14 (13)
SP	73152 766765	21°4′39ʺ 149°13′42ʺ	Lamberts Beach, north of Mackay	Quartz sandstone	Unassigned	125 (104)
627	20490 756730	21°58′26ʺ 150°8′33ʺ	Hunter Island, Duke Islands	Quartz sandstone	Erebus beds	70 (58)
628	20500 756750	21°58′20ʺ 150°8′31ʺ	Hunter Island, Duke Islands	Quartz sandstone	Erebus beds	65 (40)
633	20490 756800	21°58′3ʺ 150°8′33ʺ	Hunter Island, Duke Islands	Quartz sandstone	Erebus beds	56 (54)

^a N, number of analyses; n, number of adopted age determinations.

Analytical methods

All sample processing and analyses were undertaken at the Advanced Analytical Centre at James Cook University in Townsville, Australia. Heavy mineral separations used techniques outlined in Owusu Agyemang et al. (2016). Zircon grains were subsequently picked, mounted in epoxy resin, polished to expose crystal cross-sections and subjected to cathodoluminescence imaging by scanning electron microscopy to reveal crystal zonation and imperfections as a guide to analytical site selection. U–Pb geochronology of zircon grains employed laser ablation-inductively coupled plasma-mass spectrometry using the instrumentation and methodology outlined in Todd et al. (2019) and Foley et al. (2021). All analyses employed a 30 µm laser ablation spot size, and GJ1 (609 Ma; Jackson et al., 2004) as the primary standard, combined with 91500 (Wiedenbeck et al., 2004) and Plešovice (Sláma et al., 2008) as secondary standards to ensure analytical accuracy and precision within 2%. Analytical data were processed using Iolite software (Paton et al., 2011) with a 10% age discordance filter applied to ²³⁸U/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb age estimates. All ages >1 Ga are estimates from ²⁰⁷Pb/²⁰⁶Pb, and those <1 Ga are estimates from ²³⁸U/²⁰⁶Pb. One to four outlying young zircon grain age estimates for six samples in conflict with the biostratigraphic age of strata from which the samples were collected were attributed to lead loss, which may be expected in detrital zircon studies (Sharman & Malkowski, 2020, and references therein), and discounted. Concordant data used for interpretation were graphically plotted using Isoplot software (Ludwig, 2003). Maximum depositional ages (MDAs) for individual samples were estimated from the weighted mean average of the youngest coherent zircon age population (n ≥ 3; Coutts et al., 2019; Sickmann et al., 2018) for which zircon age estimates overlap within 2σ error.

Results

The sample set overall (Table 1, Supplemental data, Table S1), except for quartz arenites, was zircon-poor (see Figure 4), similar to samples of Devonian age from the Gamilaroi Terrane documented by Korsch et al. (2010). In the current study, zircon was recovered from 14 of the 32 samples processed (Supplemental data, Table S2). Of five samples from the Craigilee Subprovince, one yielded two zircon grains (R22, Figure 3), neither of which provided a concordant age estimate, with the remainder barren. Three samples from the Awoonga Subprovince were similarly barren, as were seven of 11 samples from the Erebus beds, with one yielding just two zircon grains. Of three samples of rocks mapped within the Capella Subprovince, two were barren. Four of six samples from rocks mapped within the Rockhampton Subprovince contained zircon.

Three quartz arenite samples from the Erebus beds outcropping on Hunter Island produced abundant zircon for which age determinations are similar, with some minor differences (Figure 5). Four grain age estimates from sample 627 and one from sample 628 are Silurian–Early Devonian (ca 443–391 Ma). Age spectra for samples 627 and 633 are strikingly dominated by Ordovician grain ages (ca 485–447 Ma), which are also prominent for sample 628. Ordovician grain ages comprise 72% of the combined dataset. For sample 627, the dominant peak (30 grains) of Ordovician age plots in the interval 465–445 Ma, whereas for sample 628, ages are scattered throughout the Ordovician, and for sample 633, most of the Ordovician ages (37 grains) plot in the interval 470–455 Ma. Cambrian ages (ca 539–486 Ma) comprise 48% of accepted data for sample 628 but are sparingly represented (<6%) in the other two samples; all but four grain ages are late Cambrian (<504 Ma). Seven older grains, all early Mesoproterozoic (ca 1589–1514 Ma), are confined to sample 627 where they comprise 12% of the population. Images of the youngest grains from these three samples are provided in the Supplemental data (Figure S1).

Figure 5. Probability distribution of zircon ages (n, number of zircons), and MDA determination for R30 and ages for R29, from samples of the Erebus beds from Hunter Island (Figure 2, sample 627, 628, 633) and the Raglan area (Figure 3, samples R10, R29 and R30).

Sample R30 from the Erebus beds in its main zone of onshore outcrop provided 13 concordant zircon analyses that range in age from 409.7 ± 9.4 to 392.7 ± 9.5 Ma with a weighted mean of 403.5 ± 2.7 Ma, consistent with the biostratigraphic age assigned to this unit. Sample R10 from the same outcropping zone of Erebus beds provided an age spectrum embracing 17 grains. The dominant age grouping (12 grains) is Ordovician (481–458 Ma). The remaining four grains register Cambrian ages (ca 536–486 Ma), and the age spectrum resembles those obtained for the Duke Islands Erebus beds samples (Figure 5). Sample R29 also from this outcropping tract of Erebus beds returned only five acceptable grain ages. One grain provided an Ordovician age (ca 470 Ma), and age estimates for the remaining four are early Cambrian (ca 533 Ma) and late Neoproterozoic (ca 640–583 Ma). A single grain age from sample R28, also from this zone of Erebus beds, is Ordovician (ca 470 Ma).

Samples R5, R6 and R9 providing zircon ages (Figure 6) were collected from the Calliope district from rocks now mapped as the Mount Alma Formation, which is assigned as Late Devonian–early Carboniferous from biostratigraphic data (Murray et al., 2012). R5 and R6 each provided six grains with ages ranging from ca 369 to 343 Ma, consistent with the biostratigraphically assigned age for this unit, plus one grain of Ordovician age (ca 481 Ma). Eleven dated grains from R9 show a broad range of ages: two at ca 365 and ca 345 Ma approximating to the depositional age, five of Silurian–mid Devonian (ca 413–387) age, two that are of Ordovician age (ca 473, 458 Ma) and two of Mesoproterozoic age (ca 1568, 1508 Ma).

Figure 6. Probability distribution of zircon ages (n, number of zircons), and MDA determinations, from samples of the Mount Alma Formation (R5, R6 and R9) and Rockhampton Group (R7) near Calliope (Figure 3). The spectra for samples R5, R6 and R7 have matching age scales, whereas that for sample R9 is expanded to accommodate ages >390 Ma. Sample R6 also records one older zircon age (481 ± 13 Ma) that is not represented by the plot.

Sample R7, also from the Calliope district, relates to rocks mapped as Rockhampton Group. It provided 63 tightly clustered grain ages (Figure 6) ranging from ca 380 to 327 Ma, with a weighted mean average for the youngest seven of 335.3 ± 4.3 Ma, falling within the early Carboniferous biostratigraphic age range assigned to this unit.

Sample R15 from a tract of rocks east of Mount Morgan previously mapped as Raspberry Creek Formation with a biostratigraphic age of Middle Devonian (Murray et al., 2012) provided 11 clustered grain ages (Figure 7) ranging from ca 297 to ca 279 Ma. A weighted mean average of the nine youngest grains indicates a maximum depositional age of 288.6 ± 4.2 Ma (early Permian).

Figure 7. Probability distribution of zircon ages (n, number of zircons), and MDA determinations for samples of Permian age (R15, north of Mt Morgan, Figure 3 and at Slade Point, SP, north of Mackay, Figure 1).

Sample SP is from a small, isolated, anomalous domain of quartz arenite outcropping close to Slade Point, north Mackay. It contrasts with nearby volcanics at Slade Point dated as Cretaceous (Bryan & Purdy, 2013) and volcanics and volcaniclastics outcropping more extensively along the coast of the Mackay region and assigned to the Late Devonian–early Carboniferous Campwyn Volcanics. The age spectrum for 104 dated grains spans from the Archean to the Permian (Figure 7), with clusters of late Silurian–early Permian age (ca 428–283 Ma; 19%), late Neoproterozoic–Cambrian age (ca 646–496 Ma; 23%), late Mesoproterozoic age (ca 1292–1006 Ma; 16%) and late Paleoproterozoic–early Mesoproterozoic age (ca 1845–1485 Ma; 21%). A weighted mean age for the four youngest grains, which have idiomorphic shapes (Supplemental data, Figure S1), is 285.5 ± 3.6 Ma (early Permian).

Detrital zircon occurrence

Most samples of sandstones from the Calliope Province and older units of the Yarrol Province proved to be poor or lacking in zircon yield (Figure 4) suggesting they reflect an unusual source. For the units involved, siliciclastic lithologies, mainly lithofeldspathic sandstone and breccia/conglomerate, domin­ate in some intervals of mafic to felsic volcanics and limestone (Murray et al., 2012). The quartz-poor nature of their sandstones in general suggests that mafic to intermediate volcanics of proximal location, containing little zircon, were the major source. The clast composition of conglomerates and breccias reported by Murray et al. (2012) is consistent with that interpretation. Korsch et al. (2010) found that sandstone samples from the Gamilaroi Terrane, the SNEO equivalent of the Calliope Province, lacked zircon.

Rare quartz-rich sandstone within the Erebus beds, known from Hunter Island (Figure 3) and also from southwest of Rockhampton (Kirkegaard et al., 1970), represents an entirely different source, with samples from them zircon-rich.

Significance of Calliope Province detrital zircon ages

The age spectra obtained from Erebus beds samples, both quartz-rich and lithofeldspathic, are strongly dominated by Ordovician ages, with late Cambrian ages also represented (Figure 5), indicating a strong link to felsic igneous rocks of these ages. The Thomson Orogen, inboard of the NNEO, represents a potential source. Late Cambrian–Ordovician igneous rocks are widely exposed in its northern part (Henderson et al., 2020), those of Ordovician age are inferred from basement cores to be widespread in its covered central part (Fergusson & Henderson, 2015), and Ordovician granite occurs in its southern part (Purdy et al., 2018). However, Neoproterozoic–Cambrian metasediments are the dominant element of the Thomson Orogen for which zircon age spectra have distinctive signatures dominated by Pacific-Gondwana (650–500 Ma) and Grenville (1350–1000 Ma) clusters (Fergusson & Henderson, 2015; Henderson et al., 2020; Purdy et al., 2016). Thus, in contrast to the igneous rocks, a contribution from reworking of the metasediments is lacking in the quartzose sandstones of the Erebus beds.

Dominance of Ordovician ages is also shown by detrital zircon age spectra from Middle to Late Devonian strata of the Adavale Basin, cover of the Thomson Orogen located some 500 km west of the NNEO (e.g. Figure 8a). A primary source from Ordovician igneous rocks of the Thomson Orogen is indicated, but these spectra also show a significant contribution from Thomson Orogen metasediments as well as grain ages reflecting contemporary volcanism (Asmussen et al., 2023). A Thomson Orogen provenance for the quartz-rich sandstones of the Erebus beds and sandstones of the Adavale Basin is indicated, with a significant source from late Cambrian igneous rocks of the former suggesting a more northerly source.

Figure 8. Probability distribution of zircon ages (n, number of zircons) from samples in the Devonian Buckabie Formation (Adavale Basin; Asmussen et al., 2023), and lower Carboniferous(?) Shoalwater Formation at Arthur Point. See Figure 2 for location; sample SHW48 from Korsch et al. (2009); sample SHW48X from Adams and Ramsay (2022).

Early Mesoproterozoic grains are represented as a minor cluster in one sample from the Duke Island Erebus beds. Their source was the margin of the North Australian Craton, inboard of the Thomson Orogen, where igneous rocks and detrital zircon of this age are characteristic (Withnall & Hutton, 2013). Grains of this age are also represented in metasediments of the Thomson Orogen (Purdy et al., 2016), but reworking from this source can be discounted because its signature Pacific-Gondwana and Grenville age clusters are absent from this sample.

That a continental source much older than the Erebus beds themselves was involved in their provenance (Figure 5) strongly supports the contention of a continental margin arc setting for the Erebus Subprovince and by inference for other assemblages of the Calliope Province that are closely associated in space and time (Figures 3 and 4). A contemporary igneous source is indicated by zircon ages from one sample (R30) from the Erebus beds. Thicknesses estimated at 2–3 km (Leitch et al., 1994; Murray et al., 2012) and the lithological character of elements of the Calliope Province are consistent with its interpretation as near-arc basin fill.

The dominance of quartz-poor volcaniclastic strata for the Calliope Province, with rare representation of continent-derived sediment, is consistent with a forearc rather than backarc context with arc relief generally isolating the depositional setting from a continental influence. A similar circumstance applied for the Cretaceous Cordilleran margin of southwestern USA (Schwartz et al., 2021) where arc relief was instrumental in isolating forearc basins from receiving sediment of continental source. However, the detrital zircon age record for the Erebus Subprovince indicates that riverine systems draining from the continental interior at times locally breached the NNEO arc.

An oceanic arc interpretation to account for detrital zircon predating its development would require a zircon source from incorporated older basement as an inlier or as substate (Buys et al., 2014; Smyth et al., 2007) and can be discounted. Ordovician sedimentary rocks are represented from the NNEO but are of insignificant extent (see below). In addition, arc derivation of quartz-rich sandstone from a mafic to intermediate igneous source is implausible.

Significance of Late Devonian–early Carboniferous Yarrol Province detrital zircon ages

Abundant detrital zircon ages from a sampled Rockhampton Group sandstone clustered at 360–328 Ma, together with small populations of dated grains from two samples of the Mount Alma Formation ranging from 353 to 347 Ma (Figure 6), suggest an early Carboniferous pulse of volcanism. A unimodal age of approximately 351 Ma from 63 of 64 detrital zircon grains analysed for a sandstone sample from the D’Aguilar Subprovince of the NNEO Wandilla Province subduction complex (Korsch et al., 2009) registers the same volcanic pulse. A supporting record is shown by detrital zircon clusters of similar age from five volcani­clastic Wandilla Formation samples also from the NNEO subduction complex (Coastal Subprovince) reported by Korsch et al. (2009). Early Carboniferous grain ages comprise 54% of the combined dataset. The Late Devonian–mid Carboniferous Drummond Basin developed at the inboard margin of the NNEO (see Figure 1) contains mafic to felsic volcanics ranging in age from ca 356 to ca 344 Ma in its lower part, and the dominant age cluster for detrital zircon from the succeeding basin fill is at 351 Ma (Sobczak, 2019). These records suggest that continental arc volcanism located immediately west of, and lapping into, the NNEO was strongly active in the early Carboniferous, resulting in a substantial tract of positive relief subject to erosion.

One sample of the Mount Alma Formation contains some detrital zircon grains that are older than the NNEO. Similar detrital zircon ages apply for a sample of the volcaniclastic Alice Creek beds that outcrop as a small inlier within Mesozoic cover some 100 km west of Brisbane (Rosenbaum et al., 2021). Blake and Withnall (2013) assigned this unit to the Yarrol Province, confirmed by an MDA of ca 382 Ma (Late Devonian coeval with part of the Mount Alma Formation) obtained from this sample. A cluster of older, mainly Ordovician, grain ages are also apparent in the data. That a minor continental source was involved in the provenance of both the Mount Alma Formation and the Alice Creek beds is indicated, consistent with continental margin depositional setting. Morand (1993a) noted muscovite, also consistent with a continental provenance, from siltstone horizons within strata now incorporated the Mount Alma Formation.

Significance of detrital zircon samples of Permian age

The MDA of ca 285 Ma for a sample (SP) of quartzose sandstone within the mapped distribution of the Campwyn Subprovince shows this unit to be an early Permian outlier, much younger than the surrounding rocks. The sample shows a very wide age spread of pre-Permian zircon ages (Figure 7). Those of Silurian to Carboniferous (440–300 Ma) age are likely to have been sourced from older parts of the NNEO. Groupings of Ordovician (480–440 Ma), Pacific-Gondwana (650–500 Ma) and Grenville (1350–1000 Ma) ages are likely to have been sourced from the Thomson Orogen (see above). Grains of late Paleoproterozoic–early Mesoproterozoic and older age are well represented and are likely to have been derived from the southeastern North Australian Craton. The age spectrum of this sample is dominated by grains that pre-date the NNEO. Provenance involving a riverine system reaching deep into the continental interior, traversing the continental margin zone marked by early Permian magmatism and basin formation (Jessop et al., 2019), is indicated.

Sample R15 from rocks mapped as the Middle Devonian Raspberry Creek Formation of the Capella Subprovince has detrital zircon only of Permian age and provides an MDA of ca 289 Ma (Figure 7). This result indicates that part of the Raspberry Creek Formation as mapped for its western distribution (Blake et al., 2006) is in error.

Discussion

Given the new perspectives obtained from detrital zircon from the Calliope Provence and older units (Frasnian–Tournaisian) of the Yarrol Province, it is instructive to review their wider context from previously published data and interpretation for NNEO and the orogen more broadly.

Age of the Calliope Province and Gamilaroi Terrane

Very small, fault bounded inliers of Ordovician strata occur within the New England Orogen. These older rocks abut a major structural dislocation extending throughout the orogen, named the Yarrol Fault in the NNEO and its equivalent, the Peel–Manning Fault in the SNEO. This structure marks a boundary near the leading edge of the late Paleozoic continental margin that separates outboard subduction complex assemblages from inboard, more diverse assemblages in part coeval with the subduction complex and including older assemblages of the proposed island arc (Jessop et al., 2019). Ordovician strata mapped for an area, some 7 km², located 29 km south-southeast of Calliope occurs in the NNEO (Jell et al., 2021; Murray et al., 2012). A domain of Ordovician strata covering some 15 km² occurs 28 km southeast of Tamworth in the SNEO (Figure 1; Cawood, 1976; Percival et al., 2011). Three formations are recognised. The oldest unit, Murrawong Creek Formation, contains limestone conglomerate clasts yielding Cambrian fossils. Based on very limited detrital zircon data (two ages), its age is assigned as Ordovician. A sample from the succeeding Pipeclay Formation has a substantial detrital zircon population (43 ages) with a tightly constrained age (443.4 ± 4.3 Ma) close to the Ordovician–Silurian boundary (Roder, 2015). This age determination conflicts with that of the Haedon Formation, considered to succeed the Pipeclay Formation, but assigned a middle Ordovician (Darriwilian) age from conodont determinations (see Percival et al., 2011).

Biostratigraphic age control for assemblages of the Calliope Province of suggested island arc association in the NNEO places them as latest Silurian (Pridoli) to Middle Devonian (Murray et al., 2012; Figure 4) consistent with the detrital zircon MDA of ca 405 Ma obtained here for a sample from the Erebus beds (Figure 5). For the SNEO, the island arc assemblage is embraced by the Tamworth Group, which includes Ordovician units (see above) but is in very large part composed of formations assigned as Devonian (Emsian–Frasnian) and possibly Silurian (Stratford & Aitchison, 1996, 1997b). The oldest Devonian unit, the Drik Drik Formation, contains allochthonous limestone blocks containing late Ordovician (Eastonian) fossils (Percival et al., 2011). An elongate domain extending for some 250 km, described as the Dunmore terrane by Percival et al. (2011), abuts, and lies east of, the Peel Fault. It also contains fossiliferous allochthonous limestone blocks, some of which provide an early Silurian maximum depositional age and others that are of Late Ordovician (Eastonian) age. A similar relationship is known for the Calliope beds (Awoonga Subprovince) outcropping near the Ordovician inlier in the NNEO where conglomerate contains fossiliferous limestone clasts of both Devonian and Late Ordovician age (Jell et al., 2021).

The setting and relationship of Ordovician strata to those of Silurian–Devonian age in the New England Orogen are unresolved, as is their relationship to Ordovician assemblages of the Lachlan Orogen for which complex relationships apply, and the interpretation of setting is contested (Fergusson & Colquhoun, 2018). However, the reworking of blocks and clasts of Ordovician age into Silurian–Devonian strata suggests a close spatial association within the New England Orogen applied when strata of these two ages were deposited.

Structural context of the Calliope Province and Gamilaroi Terrane

Docking of an island arc with a continental margin must be marked by a major fault dislocation. If the zone of contact were masked by a covering sequence, an unconformable relationship would result with the age of docking constrained by the age of the assemblages involved. It is therefore useful to review the New England Orogen to identify the age and relationships of the island arc assemblage proposed for it, relative to those assigned to the continental margin. Timing of the proposed collision for the NNEO is subject to different views, and for the SNEO lacks supporting evidence.

A Late Devonian tectonic event is recognised for the NNEO. It is registered by an angular unconformity between strata of the Capella Creek Subprovince (Raspberry Creek Formation) and the Mount Hoopbound Formation and nonconformity with the Mount Morgan Trondhjemite, which is hosted by the former (Figure 4). The older age limit of this event is Frasnian, constrained by a SHRIMP zircon age for the Mount Morgan Trondhjemite of 379.9 ± 3.27 Ma (Murray et al., 2012), supported by similar K–Ar and Ar–Ar age estimates summarised by these authors and a 381.0 ± 4.7 Ma SHRIMP zircon age reported by Golding et al. (1994). A Frasnian biostratigraphic determination for the lower part of the Mount Hoopbound Formation (Murray et al., 2012) places the unconformity within this age. Murray and Blake (2005) considered this tectonic event to mark island arc collision with the continental margin.

Based on the geochemical similarity between mafic volcanics of the Calliope Province and those of the succeeding Late Devonian–basal Carboniferous units, Murray et al. (2012) considered that arc collision with the continental margin occurred late in the deposition of the Mount Alma Formation, presumably in the latest Devonian. However, structural evidence or unconformity expression to support this view is lacking.

Division of the Gamilaroi Terrane from younger covering strata is uncertain, as no separation based on structural data or recognition of unconformity is apparent. The Keepit Conglomerate was considered to represent the oldest cover by Flood and Aitchison (1992), who assigned it as Famennian based on biostratigraphic data, confirmed by a detrital zircon age of ca 366 Ma (Korsch et al., 2010). The underlying Mostyn Vale Formation, biostratigraphically dated as Frasnian (Wright et al., 1990), was assigned to the Gamilaroi Terrane by Flood and Aitchison (1992) and Offler and Murray (2011), but Glen (2013), based on field relationships, considered it as cover. If the latter interpretation were accepted, then the separation in time of both the Gamilaroi Terrane and the Calliope Province from younger cover would be broadly coeval at ca 377 ± 5 Ma.

A crustal shortening event of Late Devonian (Frasnian) age imposed widespread penetrative fabric development in the Mossman Orogen (Henderson & Fergusson, 2019; Withnall & Henderson, 2012). These authors considered it an expression of Tabberabberan orogenesis although marginally later than that recognised for the Lachlan Orogen where crustal shortening extended from ca 400 to ca 380 Ma (Fergusson, 2017). A potentially diachronous Tabberabberan compressional event may therefore have affected the Tasmanides, inclusive of the New England Orogen, along their entire length.

Morand (1993b) considered general folding and fabric development in the NNEO to result from Hunter–Bowen orogenesis of Permian age. Subsequent regional mapping (Murray et al., 2012) is consistent with that view.

Character and setting of NNEO late Silurian–Devonian stratigraphic units

Offler and Murray (2011) considered the Craigilee Subprovince and Upper Devonian strata of the Lochenbar and Mount Hoopbound formations to represent oceanic island arc assemblages, an interpretation that implies their construction as positive relief features built by volcanism. This conclusion overlooks their lithostratigraphic content. All elements of the Calliope Province and older units of the Yarrol Province are dominated by siliciclastic rocks, predominantly sandstone, conglomerate and breccia, with only a subordinate contribution of volcanics (Donchak et al., 2013; Murray et al., 2012). Most are considered to represent shallow marine facies (Murray et al., 2012).

Partitioning of these assemblages into a collage of arc, remnant arc, backarc and mid ocean ridge assemblages as proposed by Murray and Blake (2005), Offler and Murray (2011) and Murray et al. (2012) cannot be supported based on lithological content, which shows them to be basin fill. In overview, late Silurian–early Carboniferous stratigraphic assemblages of the NNEO (Figure 4) represent an ongoing episode of basin development, interrupted briefly by the Frasnian unconformity and accompanying intrusion of the Mount Morgan Trondhjemite, that continued into the early Carboniferous spanning an interval of some 100 Ma.

Given that the delivery of sediment to form units of the Calliope Province and older Yarrol Province (Figure 5) episodic­ally involved a continental source, it could be argued that this could have extended onto ocean floor to a near-continent, west-verging oceanic arc system as proposed for the New England Orogen by Offler and Huang (2018). Deep ocean transit of sands would be required. However, as assemblages of the Calliope Provence and older units of the Yarrol Province are commonly of shallow marine facies (Murray et al., 2012), this interpretation is untenable.

Rocks of the Campwyn Subprovince mainly range in age from Frasnian to Tournaisian as shown by zircon geochronology and biostratigraphic data (Blake & Withnall. 2013; Bryan et al., 2004). However, fossil occurrence also indicates an age range extending to the mid Devonian (Fergusson et al., 1994; Henderson et al., 1994), pre-dating the age attributed to arc accretion (Figure 4). The Campwyn Volcanics are dominantly mafic to felsic volcanic and volcaniclastic rocks that record arc-adjacent basin infill. The volcaniclastic component was transported from the west and its source considered to be of continental margin location (Bryan et al., 2003), consistent with a continental forearc interpretation for the setting of the subprovince (Fergusson et al., 1994).

Other NNEO quartzose sandstone occurrences and source

In addition to those of the Erebus Province, younger units of quartzose sandstone occur in the NNEO. They are extensively represented by the Shoalwater Formation north of Rockhampton, the Neranleigh-Fernvale beds south of Brisbane and also the Goodnight beds west of Maryborough (Figure 1). These units relate to the Wandilla Province (Figure 2), interpreted as a subduction complex, and are of Carboniferous age (Donchak et al., 2013). Additionally, at Arthur Point and the surrounding area in the Stanage Bay region, a unit of quartz-rich sandstone and mudstone (tentatively identified as part of the Shoalwater Formation by Leitch et al., 1994) has been mapped within the Yarrol Province west of the northern extension of the Yarrol Fault (Figure 2; Leitch et al., 1994).

Detrital zircon age data for these units (Adams & Ramsay, 2022; Carson et al., 2007; Korsch et al., 2009) show, in addition to a zircon age grouping interpreted as approximating to the depositional age, a scatter of Devonian–Ordovician ages, minor clusters of late Neoproterozoic–Cambrian ages (Pacific-Gondwana grouping) and late Mesoproterozoic ages (Grenville grouping), plus a scatter of older Precambrian ages (e.g. Figure 8). This spectrum of zircon ages is consistent with general sampling of rock systems within the Thomson Orogen inboard of the NNEO (Fergusson et al., 2017; Henderson et al., 2020; Shaanan, Rosenbaum, & Sihombing, 2018).

Two units comprise most of the subduction complex mapped as the Coastal Subprovince of the Wandilla Province (Figure 1). The quartzose Shoalwater Formation is broadly coeval with the abutting volcaniclastic Wandilla Formation (Korsch et al., 2009) of more westerly location. Sampled detrital zircon from the latter was >90% of Carboniferous age (Korsch et al., 2009) indicating a broadly contemporaneous, and dominating, arc source. One explanation for the juxtaposition of these contrasting units proposed by Leitch et al. (2003) suggests a proximal arc source for the Wandilla Formation but a distal, northern, continental interior source for the Shoalwater Formation with deep-marine, axial trench transport south to the site of deposition. Other potential explanations by Korsch et al. (2009) are northern deposition of the Shoalwater Formation and tectonic transport to its present location or a distal inboard, continental source for the unit that involved a transport pathway through a breach in the arc.

Given the across-arc transport apparent for quartzose sandstone of the Erebus beds, the latter explanation has enhanced credence. It could be that an offset in age separated deposition of the Wandilla and Shoalwater formations, as implied by their spatial separation within the inferred subduction complex. An episode of arc quiescence at the time of Shoalwater Formation deposition may have facilitated sediment transport from a continental source.

Late Silurian–Devonian magmatic records for other elements of the Tasmanides

The broader context on Tasmanide magmatism coeval with that recorded by the Calliope Province and Gamilaroi Terrane bears on potential relationships and setting (Figure 9). Siluro-Devonian granitoids of the Pama Igneous Association are extensively developed as part of the Mossman Orogen, with sediment sourced from igneous rocks reflected in volcaniclastic strata and detrital zircon from Silurian and Devonian sedimentary assemblages of this age in the Broken River Province (Henderson, Donchak, et al., 2013; Henderson & Fergusson, 2019) and detrital zircon from the Hodgkinson Province (Adams et al., 2013; Shaanan, Rosenbaum, & Sihombing, 2018).

Figure 9. Map of the Tasmanides in eastern Australia showing units referred to in the text and the record of late Silurian – Devonian magmatism for eastern Australia, compiled from Glen (2005), Fergusson (2010), Jell (2013) and Purdy et al. (2018). BHB, Broken Hill Block; BB, Burdekin Basin; GP, Greenvale Province; KB, Koonenberry Belt (part of the Delamerian Orogen); LB, Lolworth Batholith; PIA, Pama Igneous Association; RaB, Ravenswood Batholith; RB, Retreat Batholith; US, Ukalunda Shelf.

Extensive Devonian granitoids are represented in the Thomson Orogen, represented by the Lolworth and Ravenswood batholiths of the Charters Towers Province and the Retreat Batholith of the Anakie Province. A dominantly igneous source is apparent for strata of Givetian–Frasnian age in the Burdekin Basin (Henderson, 2013). Devonian volcanics and volcaniclastics are represented in units of the Ukalunda Shelf (Henderson, Withnall, et al., 2013) and the Adavale Basin (Asmussen et al., 2018; McKillop, 2013). Detrital zircon ages for Adavale Basin samples indicate a persistent Devonian igneous contribution to its sediment sources (Asmussen et al., 2023). Late Silurian–Devonian igneous rocks, including volcanics and granitoids, are extensively developed and widespread in the southern Thomson Orogen (Purdy et al., 2018).

Silicic igneous rocks, both granitoids and volcanics of late Silurian–Middle Devonian age, are widespread in the eastern Lachlan Orogen (Fergusson, 2010; Glen, 2005). The belt they define extends to northeastern Tasmania where Devonian granitoids are developed (Black et al., 2005). These rock systems have been widely considered as the product of an active arc–backarc setting (e.g. Collins & Richards, 2008; Fergusson, 2010; Glen, 2013). Their northern limit is some 160 km inboard of the Gamilaroi Terrane.

Thus, late Silurian–Devonian felsic igneous rocks in the Tasmanides inboard of the New England Orogen (Figure 9) and its offshore northerly extension to the Queensland Plateau (Shaanan, Rosenbaum, Hoy, et al., 2018) extend throughout eastern Australia. These data strongly suggest the addition of igneous rocks generated by westerly directed subduction along the east Gondwana margin, coeval with the generation of the Calliope Province. A double subduction plate geometry during the Devonian was proposed by Offler and Murray (2011) as an explanation. West-directed subduction was considered to have generated Devonian igneous rocks on the east Gondwana margin. East-directed subduction was considered to have generated the island arc and related assemblages that were subsequently accreted to the east Gondwana margin.

Geochemistry of mafic volcanics as an indicator of tectonic setting

Although several considerations as noted above question the interpretation of the late Silurian–Devonian elements of the NNEO as an oceanic island arc accreted to the east Gondwana margin, the interpretation of geochemical signatures of mafic volcanics has held sway in tectonic interpretation (Murray et al., 2012, and references therein; Offler & Murray, 2011).

A continental setting was inferred for the NNEO from the geochemistry of mafic rocks by Morand (1993a) and Bryan et al. (2001). However, Offler and Murray (2011), Murray and Blake (2005) and Murray et al. (2012) expanded geochemical datasets to provided Th/Yb and Nb/Yb ratios and measurement of immobile elements V, Ti, Cr and Y. Based on this more comprehensive data, they concluded that intra-oceanic island arc and associated backarc basin settings, related to east-dipping subduction, dominated the NNEO from the late Silurian to the Devonian. Analyses of some mafic rocks in the Three Moon Conglomerate and Capella Creek Subprovince showed MORB-like geochemical signatures considered to relate to incipient rifting in a volcanic arc setting.

Based on the geochemistry of mafic volcanics, Offler and Murray (2011) interpreted the Craigilee Subprovince to be of intra-oceanic arc origin, with other elements of the late Silurian–Middle Devonian Calliope Province representing remnant arc or backarc basin assemblages. The overlying Late Devonian–Tournaisian Mount Hoopbound and Lockenbar formations were also interpreted as intra-oceanic arc assemblages and correlative formations of the Yarrol Province (Figure 4) interpreted as representing backarc basin fill, with or without an arc component.

A number of factors contribute to the geochemical character of primitive arc melts with variation in their influence leading to a wide range of basalts in individual arcs (Schmidt & Jagoutz, 2017; Wang et al., 2022), including the Cascades as a continental example (Pitcher & Kent, 2019). In addition, oceanic arc magmas that have incorporation of buried continental crust may retain geochemical signatures that are typical of oceanic arc rocks (Buys et al., 2014) bringing into question the reliability of geochemical signatures as diagnostic of a setting involving only oceanic crust. Rock assemblages interpreted as representing a Cambrian–mid Ordovician forearc (Burianek et al., 2022) are similar to those of late Silurian–basal Carboniferous age developed in NNEO and include basalts with comparable geochemical signatures.

The Lachlan Orogen, inboard of the SNEO, mainly experienced crustal extension in the late Silurian–Middle Devonian (Collins, 2002; Glen, 2013), and the northern Tasmanides also experienced extension during this time (Abdullah & Rosenbaum, 2018; Rosenbaum, 2018). It is suggested here that the NNEO, at the outboard perimeter of the Tasmanides, consisted of extended, thin continental crust through this time interval. That extension continued after a brief Frasnian contraction is suggested by continuing shallow marine accumulation of basin infill within the NNEO through part of the Late Devonian into the early Carboniferous (Figure 4). As noted by du Bray and John (2011), extensional regimes and associated structural conduits promote relatively rapid crustal transit of mafic melts that are less prone to crustal contamination as a consequence. In our view, the geochemical character of NNEO late Silurian–early Carboniferous mafic volcanics reflects this circumstance with decompressional melting induced by crustal extension, as well as the influence of other factors contributing to the generation of primitive compositions in continental margin arcs (Schmidt & Jagoutz, 2017), resulting in oceanic arc-like and MORB-like compositions.

Evidence of subduction polarity

Murray and Blake (2005) separated analyses of Late Devonian mafic volcanics from the Mount Hoopbound and Lochenbar formations and the Three Moon Conglomerate into two suites. One was considered to best match basalt analyses from modern island arcs, whereas the other, mainly of more easterly distribution, was thought to best match basalts from spreading backarc basins. These authors concluded that the geochemical signatures of these formations became more arc-like to the west, consistent with an east-dipping subduction polarity (see also Offler & Murray, 2011).

Conversely, Offler and Huang (2018) advocated that west-dipping subduction polarity applied in the origin of the Gamilaroi Terrane and Calliope Province. Their interpretation was based on the occurrence of sedimentary rocks of late Silurian–Devonian age within the Tablelands subduction complex, located east (outboard) of the Gamilaroi Terrane of overlapping age, which they interpreted as of intra-oceanic arc association.

The continental margin model for the tectonic setting of the NNEO suggested here (Figure 10b), informed by detrital zircon age data, represents a variation of the model advocated by Offler and Huang (2018). They suggested that the plate boundary involved was of continental margin location for southeast Australia and related to Devonian backarc emplacement of igneous rocks in the Lachlan Orogen (e.g. Collins, 2002). Under their proposal, the plate boundary migrated to an oceanic setting further north where it was involved in generating the Gamilaroi Terrane and Calliope Province of intra-oceanic arc association.

Figure 10. (a) Devonian sediment transport pathways delivering continent-derived zircon to the NNEO and Adavale Basin. See text for discussion. (b) Proposed late Silurian–Devonian tectonic setting of the NNEO.

It is suggested here that a continental margin location of the plate boundary and west-dipping subduction applied to east Gondwana throughout the history of the New England Orogen.

Conclusions

Detrital zircon from the late Silurian–Middle Devonian Erebus beds of the Calliope Province and Mount Alma Formation from the older (Frasnian–Tournaisian) part of the Yarrol Province of the NNEO were in part sourced from the Thomson Orogen and North Australian Craton. This provenance combined with the lithological composition of these units shows that they origin­ated in close proximity to a continental margin arc, not as part of an oceanic arc system generated by east-dipping subduction as has previously been proposed. Late Silurian–Devonian igneous rocks attributed to subduction are very widely distributed in eastern elements of the Tasmanides adjoining the NNEO, consistent with their continental margin occurrence as proposed herein for NNEO.

The primitive character of mafic volcanic and hypabyssal rocks of the NNEO is considered to have been generated on continental margin crust thinned by extension impeding an evolution involving crustal contamination and similar to primitive mafic rocks documented from the western North American Cretaceous continental arc represented by the Cascades. Lithofacies represented by the Calliope Province and older part of the Yarrol Province are dominated by siliciclastics. Their accumulation represents basin fill spanning the late Silurian to the early Carboniferous, some 100 Ma, briefly interrupted by a short-lived intra-Frasnian, contraction best attributed to expression of Tabberabberan orogenesis that had widespread effects in the Tasmanides.

Detrital zircon for the Erebus beds in small part reflects contemporary volcanism but was largely derived from Ordovician igneous rocks. These are extensively developed in the northern Thomson Orogen, which can be identified as the source. A similar dominance of Ordovician detrital zircon is characteristic of Devonian sandstones of the Adavale Basin developed within the Thomson Orogen to the west of the NNEO. The enhanced representation of Ordovician igneous rocks in the northern Thomson Orogen, and the representation of older zircon grains in Erebus Formation samples attributed to a Mesoproterozoic source from the North Australian Craton, suggests a southeasterly sedimentary transport vector episodically applied during deposition of the Erebus beds (Figure 10a).

Detrital zircon sampled from early Carboniferous strata of the Yarrol Province is strongly dominated by ages of 360–328 Ma, suggesting that a NNEO pulse of felsic magmatism occurred in this time span, a conclusion supported by the detrital zircon age record from elements of the NNEO Carboniferous subduction complex and from infill of the Drummond Basin at the inboard margin of the NNEO as well as the widespread occurrence of volcaniclastics in early Carboniferous strata of the Yarrol Province.

Unexpected early Permian ages were obtained from two sandstone samples from NNEO units mapped as of Late Devonian–early Carboniferous age. One of these, representing the occurrence of anomalous quartzose sandstone in the Campwyn Subprovince, records a dominantly cratonic source, providing evidence of long-distance early Permian sedimentary transport from deep within Gondwana during a temporary absence of volcanic relief across the NNEO.

Acknowledgements

We thank Dr Yi Hu in the Advanced Analytical Centre ICPMS facility (JCU) who assisted in the collection of the detrital zircon U/Pb analytical data. Katarzyna Sobczak kindly provided access to her PhD thesis. AJES journal reviewers Kim Jessop and David Purdy are thanked for their input, which significantly improved the presentation of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are openly available in the Supplemental data Tables S1, S2 and Figure S1.