First magmatism in the New England Orogen, Australia: Forearc and arc-backarc components in the Bakers Creek Suite gabbros

The New England Orogen, Eastern Australia, was established as an outboard extension of the Lachlan Orogen through the migration of magmatism into the forearc basin and accretionary prism. Widespread S-type granitic rocks of the Hillgrove and Bundarra Supersuites represent the first pulse of magmatism, followed by Iand A-types typical of circum10 Pacific extensional accretionary orogens. Associated with the former are a number of small tholeiite-gabbroic to intermediate bodies of the Bakers Creek Suite, which are a heat source for production of granitic magmas and potential tectonic markers indicating why magmatism moved into the forearc and accretionary complexes, rather than rifting the old Lachlan Orogen arc. The Bakers Creek suite gabbros capture an early (~305 Ma) forearc basalt-like component with low Th/Nb and with high Y/Zr and Ba/La, recording melting in the mantle wedge with little involvement of a slab flux and indicating forearc 15 rifting. Subsequently, arc-backarc like gabbroic magmas (305-304 Ma) were emplaced followed by diverse magmatism of mixed compositions leading up to the main S-type granitic intrusion (~290 Ma). This trend in magmatic evolution implicates forearc and other mantle wedge melts in the heating and melting of fertile accretion complex sediments and relatively long (~10 Myr) timescales for such melting.


Introduction 20
The New England Orogen (NEO) is the youngest and easternmost component in the Tasmanides accretionary orogenic system and of the Australian continental craton (e.g. Cawood 2011; Figure 1a). The NEO has similarities to its older neighbour the Lachlan Orogen, such as west-dipping subduction (e.g. Leitch 1974;1975), a general tectonic regime switching between crustal thinning and thickening (Collins 2002;Brown 2003), and granitic magmatism spanning a compositional range between peraluminous and metaluminous end members (S-type and I-type for igneous and sedimentary 25 sources respectively: Hensel et al. 1985;Chappell and White 2001;Collins and Richards 2008). However, the NEO represents outboard migration of magmatic activity into the Lachlan Orogen forearc basin and accretionary prism sediments (Jenkins et al. 2002). These sediments, derived from the 'calc-alkaline' arc rocks, inherited juvenile isotopic characters that were passed on to their derivative granitic melts by rapid subduction cycling (Kemp et al. 2009).
The termination of the Devonian-Carboniferous magmatic arc and replacement by widespread and relatively disorganised magmatism (Collins et al. 1993;Caprarelli and Leitch 1998;Jenkins et al. 2002) at ~290 Ma, culminated in the emplacement of the contrasting Bundarra and Hillgrove S-type granitoid supersuites . They differ from each other with the former being a voluminous, compositionally homogenous belt, while the latter is variably foliated, generally more mafic in composition (Shaw and Flood 1981), and is associated with high-temperature low-pressure (HTLP) metamorphic 5 complexes (Farrell 1988;Dirks et al. 1992) and small, mafic to intermediate intrusive bodies referred to as Bakers Creek Suite (Jenkins et al. 2002). Following minor magmatic activity (Roberts et al. 1995;Donchak et al. 2007;Cross et al. 2009;Phillips et al. 2011) and a temporal magmatic gap associated with orogeny , the NEO was overwhelmed by voluminous I-type magmatism from ~265 Ma . 10 The mafic mantle-derived plutons of the Bakers Creek Suite, while small and variably evolved, ultimately records the conditions of mantle partial melting and subduction zone contributions to the first magmatism in the NEO. New advances in the understanding of the geochemistry of arc-related magmas have established roles for the various mafic magmas emplaced during subduction zone initiation and migration. These include basalts with forearc (FAB; Reagan et al. 2010;Meffre et al. 2012;Ribeiro et al. 2013), backarc (BAB; Langmuir et al. 2006;Pearce and Stern 2006), and early arc tholeiite (EAT; Todd 15 et al. 2012) affinities. Each of these has distinctive trace element compositions which can potentially be recognised in paleoarc systems (Dilek andFurnes 2014, Pearce 2014). We present here a study of the geochemistry of the Bakers Creek Suite with emphasis on samples from uncontaminated, mafic plutons, and U-Pb chronology of these earliest magmatic rocks in the NEO. Further, we identify forearc and backarc components, and address the tectonic setting and mechanisms by which magmatism began in this section of an ancient extensional accretionary orogen. 20

Regional Geology
The Southern NEO is built upon a metasedimentary base comprising the Tamworth Belt (representing an old forearc basin) and the Tablelands Complex (an accretionary prism) separated by the Peel-Manning Fault System (Leitch 1974;Korsch 1977). Both are related to Devonian-Late Carboniferous magmatic arc rocks (Leitch 1975). In the Tablelands Complex ( Fig.   1b), high temperature and low pressure metamorphism overprints the accretion-subduction sequences (Wongwibinda and Tia 25 Complexes: Farrell 1988;Hand 1988;Dirks et al. 1992;Phillips et al. 2008;Craven et al. 2012). Subsequently, intrusion of the Hillgrove Suite biotite granites and granodiorites (±garnet, hornblende) took place, forming a discontinuous belt of scattered plutons (Flood and Shaw 1977;Shaw and Flood 1981). Spatially associated with the Hillgrove granitoids are the small plutons of the Bakers Creek Suite, a diverse group of mafic to intermediate bodies ranging from two-pyroxene (±olivine) gabbros and related cumulate rocks through hornblende-biotite diorites to mafic hornblende-bearing granodiorites 30 (Jenkins et al. 2002). The Hillgrove and Bakers Creek mafic plutons have been exhumed as a result of early Permian rifting and subsequent thrusting during the Hunter-Bowen Orogeny (Figure 1b; Landenberger et al. 1995;Shaanan et al. 2015). Also present are the voluminous and more strongly peraluminous S-type granites of the Bundarra Suite, lying in a continuous north-trending belt to the west of the Hillgrove Suite (Flood and Shaw 1977;Shaw and Flood 1981). In contrast to the Hillgrove Suite, the Bundarra Suite granites are generally non-foliated, have no mafic plutons associated with them, and are not associated with metamorphic complexes, despite generally contemporaneous intrusion ).

5
Mafic, primitive members of the Bakers Creek Suite include the small Barney House and Big Bull gabbros, while larger plutons such as the Days Creek gabbro and Apsley River Complex exhibit more complex characteristics of differentiation (e.g. samples BHC2, CC26A, G39, and GK2 respectively from Jenkins et al. 2002). Sampling was undertaken with a focus on mafic plutons such as the Barney House, Big Bull, and Days Creek gabbros. The Barney House and Big Bull gabbros are small (scale of tens to hundreds of metres) and consist of finely crystalline gabbro, often hosting plagioclase phenocrysts, in 10 contact with low-grade metasedimentary country rock. The Big Bull gabbro occurs as the most mafic member in a full spectrum of rocks varying from mafic to felsic (Sheep Station Creek Complex). In contrast, the Days Creek gabbro occurs as two larger plutons (~1 and ~2 km in length) partially surrounded by the Tobermory monzogranite (Hillgrove Suite) except at the southern margin where it borders turbidites of the Girrakool Beds. It is dominated by medium to coarse grained gabbro and contains rare pegmatite (grainsize 5-20 mm; Figure 2a). The southern pluton exhibits a doleritic (~1 mm) chilled margin 15 against turbidites which are contact-metamorphosed and exhibit rare occurrences of melting. Widespread but poorly exposed pieces of dolerite are found at various locations across both plutons, some in association with metasedimentary rocks and felsic veins containing gabbro breccia ( Figure 2b). The encompassing Tobermory monzogranite is usually coarse (average grainsize a few mm) and lacks foliation. Although most contacts are not exposed, it is often finer grained (to ~1 mm) nearer the gabbro, indicative of quenching and late emplacement relative to most other members of the Hillgrove Suite 20 (Landenberger et al. 1995). The Tobermory monzogranite is cut on the western side by younger, unrelated mid-late Permian to Triassic I-type granite .

Analytical Methods
Selected 30 m thin sections of samples were polished and carbon coated for X-ray analysis of mineral phases by scanning electron microprobe (SEM) at the University of Newcastle (UoN) using a Phillips XL30 SEM, with Oxford ISIS Energy 25 Dispersive Spectrometer (EDS); 15 kV accelerating voltage; 3 nA beam current. Bulk-rock samples were crushed by tungsten-carbide mill and diluted in lithium borate flux at 1050 °C to produce a glass disc. Major oxide and some trace elements were analysed by X-ray fluorescence spectrometry (XRF) at the UoN (Spectro X'Lab 2000 XRF system with: EDS; Pb anode tube; polarised beam; multiple targets). All Fe is reported as FeO. Glass discs from this study and from Jenkins et al. (2002) were sectioned and polished for trace element analysis by laser ablation inductively coupled plasma 30 mass spectrometry at the Research School of Earth Sciences, Australian National University (ANU), using a quadrupole Agilent 7500s coupled to a 193 nm ArF Excimer laser (Eggins 2003). Samples were run against NIST glasses and either 43 Ca or 29 Si were used as internal standards depending on bulk silica content (using CaO or SiO 2 from XRF) and data reduced using an in-house spreadsheet.
Magmatic zircon 238 U/ 206 Pb ages of gabbroic and dioritic samples were determined at the ANU using Sensitive High-mass Resolution Ion Micro Probes (SHRIMP). The gabbroic samples (Barney House and Days Creek Gabbro) were analysed 5 using SHRIMP-RG against the reference standard TEMORA, while dioritic samples (Bakers Creek Complex and Charon Creek Diorite) were analysed using SHRIMP-I against the AS3 reference material. Rejection of analyses was made on the basis of measureable common Pb, loss of Pb, or contribution to an unreasonably high MSWD. In reviewing other U-Pb data for the NEO in the literature, it is noted that they were obtained against a range of reference materials over many years. The comparative study of Black et al., (2003) showed that some zircon ion-probe reference materials yielded small biases, with 10 ages calculated against AS3 being ~1% too high, and ages calculated against SL13 being variably (although on average, ~1%) too low. To account for this, we made corrections of -1% to our AS3 ages and +1% to SL13 ages assembled in our age compilation; other relevant AS3 ages in the literature were verified by other standards (Roberts et al. 2004;. Although these corrections are significant in terms of precision, they have little influence on tectonic conclusions. ~An 80 but up to An 86 in cores). Phenocryst rims are sodic (to An 50 ) and texturally interlock with the fine gabbro groundmass.
Ilmenite and rare magnetite (sometimes intergrown) is usually associated with amphibole and phlogopite, often being mantled by them or included in their interstitial domains.
Coarsely crystalline gabbro (mm to cm crystals) is more typical of the Days Creek gabbro, with orthocumulate or 10 mesocumulate textures ( Figure 3d) comprising plagioclase, rare resorbed olivine, high-Ca clinopyroxene, very rare low-Ca orthopyroxene, ilmenite and amphibole (latter often secondary). Massive coarse grained gabbro also has rare granular texture ( Figure 3e). Plagioclase (An 80-47 ) ranges from isolated, equant euhedral crystals to subhedral crystals in an interlocking network, defining the ortho-or mesocumulate texture. They are commonly normally zoned, and rarely exhibit oscillatory zoning or scissor deformation twins. Olivine is Fo 65-59 , is anhedral or embayed. Secondary clinozoisite and serpentinite after 15 plagioclase and olivine was not observed in fine grained gabbros but is present in coarse grained samples. Pyroxenes are subhedral or interstitial and rarely optically continuous across multiple domains. In coarse gabbros, high-Ca clinopyroxene is diopside-augite (average Mg# 79) while low-Ca orthopyroxene is very rare, possibly because of uralitisation (Mg# 70 with exolved clinopyroxene at Mg# 78). Very fine orthopyroxene exsolution is also present in clinopyroxenes. Amphibole is present in abundances approximately equal to that of pyroxenes and occurs as primary interstitial magnesio-hornblendes 20 (pale brown and green; average Mg# 63) and secondary fibrous or radiating irregular actinolitic-hornblende, magnesiohornblende or tschermakitic-hornblende (green to green-blue varieties; average Mg# 60). Anhedral or interstitial ilmenite shares intercumulus spaces with pyroxenes and amphiboles. 1.6). U-Pb data are given in Figure 6 and Supplement 2.
With our Th/U criteria, the recalculated age of the Rockvale Granodiorite is 296.7 ±2.3 Ma (MSWD 1.8), ~4 Myr older than the age given by Cawood et al. (2011) (Supplement 3). We revisited the original U-Pb age for the Rockvale Granodiorite reported by Kent (1994), which at 303 ±3 Ma, is older than other igneous rocks of the Southern NEO. His rejection criteria 25 were fundamentally in accord with ours, although the age may suffer from variable bias from the SL13 standard which would have depressed the age. If bias was, in this case, insignificant (SL13 behaviour is not consistent and sometimes does not bias ages at all; Black et al., 2003) then a discrepancy of 1.0 Myr remains between Kent (1994) and the age recalculated from the data of Cawood et al. (2011). If, however, bias is present then the age could be up to ~306 Ma, with associated discrepancy of up to ~4 Myr. Hence, U-Pb data for the Rockvale Granodiorite remains poorly understood.  Woodhead et al. 2001). Alternately, Y/Zr can be used to identify previously depleted mantle sources (e.g. Arculus et al. 2015). Despite the geochemical similarity of these elements to V/Ti under typical subduction zone redox conditions (trivalent and tetravalent respectively) there is clear distinction between main-group Bakers Creek Suite melt compositions and anomalous melts in Y/Zr space (Figure 7b; samples DC65, DC104 and D12). These have much lower Zr 20 (and Hf) for similar Y contents, which is a characteristic shared by forearc type basalts, e.g. Izu-Bonin FAB (Reagan et al. 2010;Ribeiro et al. 2013;Arculus et al. 2015).  (Woodhead et al. 2001). Anomalous samples are therefore associated with a peculiar elevated Ba/La (Figure 8b). 5 Additionally, DC104 has much lower Cs (and K 2 O) than the others, despite similar levels of Rb in main-group and anomalous Bakers Creek Suite samples. This strongly indicates decoupling of Rb from other trace alkalis Cs and K 2 O, as well as from Ba, elements that are ordinarily associated in sub-arc settings, e.g. via phengite and paragonite melting (Spandler and Pirard 2013).

10
A direct comparison is made of the multi-element plots for the melt compositions of the Bakers Creek Suite, with the FAB basalts of Reagan et al. (2010) and Ribeiro et al. (2013) in Figure 9, for similar major element compositions (especially for MgO, in the range ~6.6-8.6 wt. %). They share some relative and absolute abundance trace element characteristics, especially those of anomalous composition (D12, DC104, and DC65). Low abundances of certain slab-flux elements such as Th and U, the light to mid-REEs especially La and Ce (and consequently low LREE/HREE), and the HFSE Zr and Hf, all 15 indicate involvement of a FAB component in otherwise arc-or backarc-like basaltic compositions.
The trace element geochemistry of Bakers Creek Suite samples therefore indicates divorcement of some components ordinarily associated in subduction zone associations (e.g. Reagan et al. 2010;Ribeiro et al. 2013). The unusual compositions found in the chilled margin of the Days Creek Gabbro may represent early forearc style magmas, with the 20 more extreme characteristics, especially high Y/Zr and low Th/Yb. As chilled margins, these have been specifically sampled in the field and might be less often captured by random undersea sampling (e.g. Arculus et al. 2015). Such magmas were overwhelmed by later arc-backarc style magmas (main group Bakers Creek) and the larger Days Creek gabbro might be an example of a feeder pipe, capturing early and late components.

Chronology of earliest NEO magmatism 25
The oldest dated Southern NEO intrusives clearly comprise the gabbroic plutons of the Bakers Creek Supersuite (Barney House and Days Creek Gabbro); they are clearly resolved by U-Pb dating from other intrusive bodies. Other early samples which are not so clearly resolved include the largest compositionally-intermediate pluton of the Bakers Creek Supersuite (Bakers Creek Complex), as well as isolated members of the S-type Hillgrove Supersuite (Tia and Rockvale Granodiorites, with the age of the latter not well known but still older than 294 Ma; possibly also the Blue Knobby Monzogranite and 30 Henry River Granite). The Tia Granodiorite constrains the age of the HTLP Tia Complex (Phillips et al., 2008)  As the magmatic pulse accelerated, diverse compositions continued with intrusion of the Jibbinbar Granite at ~298 Ma (Cross et al. 2009) followed by the Rockisle Granite, Dorrigo Mountain Complex, and Mount You You Granite at ~295 Ma . This diverse magmatism is also reflected at the same time in volcanic rocks, with the I-type Halls Peak Volcanics and various basaltic flows near the base of the newly opened Barnard Basin (Cawood et al. 2011), and the 5 Alum Rock Volcanics (Roberts et al. 1996).
The major phase of pre-Hunter-Bowen magmatism in the Southern NEO occurred with the climactic emplacement of S-type granites and granodiorites of the Bundarra Supersuite at ~292-285 Ma, and many larger plutons of the Hillgrove Supersuite at ~293-288 Ma (e.g. Hillgrove Monzogranite). Some diversity in magmatic compositions continued throughout this period, 10 with emplacement of the ungrouped Kaloe Granodiorite (Cawood et al. 2011), Bullaganang Granite (Donchak et al. 2007 and Gandar Granodiorite , as well as our own AS3-corrected age for the Charon Creek Diorite (Bakers Creek Suite).
S-type magmatism persisted until ~280 Ma for the Cheyenne Complex of the Hillgrove Supersuite, and possibly until ~282 15 Ma for part of the Banalasta Monzogranite of the Bundarra Supersuite (Phillips et al., 2011). This last emplacement of Stype magma was contemporaneous with another burst of I-type magmatism in the form of the Alum Mountain Volcanics (~274 Ma; Roberts et al. 1995) and the more conspicuous low-K, HREE-depleted Greymare Granodiorite (Donchak et al. 2007) similar to the Clarence River Supersuite, and finally the ~267 Ma Barrington Tops Granodiorite (Cawood et al. 2011).
This chronology is illustrated in Figure 10 (see also Supplement 4); thereafter, magmatism following the Hunter-Bowen 20 Orogeny is reviewed by Li et al. (2012).

Tectonic Implications
Magmatism in the long lived subduction zone of the Lachlan Orogen ceased at ~305 Ma (Claoué-Long and Korsch 2003;Roberts et al. 2004;Jeon et al. 2012; Figure 10  generated high-T metamorphic complexes and the earliest S-type granitic melts of the Hillgrove Suite at ~302-300 Ma. These mixed with Bakers Creek Suite gabbroic melts producing a full spectrum from mafic to felsic compositions ( Figure   11d). Peak melting of fertile greywackes, probably due to underplating of mafic melts, led to the main flux of S-type granites at ~290 Ma (Figure 11e). While modelling of such processes usually indicates relatively short timescales of ~1 Myr or less for abundant felsic melt production (Annen and Sparks 2002;Solano et al. 2012), the chronology constructed for the NEO 5 implies that melt mobilisation takes significantly longer, perhaps due to rapid stratification of mafic and felsic melts (preventing later mafic melts from ascending) and importantly involving high melt fractions. -Rifting, extension of the overlying sedimentary complex, melting of the mantle wedge, and transport of the resulting melts 15 were responsible for the high temperature, low pressure metamorphic complexes in the mid-crust and abundant S-type granitic magmas at depth (~300 Ma and onwards), with peak migration and emplacement of the latter at ~290 Ma.
-Ultimately, formation of a new orogen (New England) from the forearc of the Lachlan Orogen (Tamworth Belt and Tablelands Complex Accretionary prism) occurred by forearc rifting.      (Arevalo and McDonough 2010). Finely and coarsely crystalline gabbros (triangles and squares respectively) that approximate melt compositions with ~8 wt. % MgO in green; anomalous melt compositions in pink; Eastlake monzogranite (Hillgrove Suite) in yellow. Range of compositions for basaltic melts, cumulates, and higher silica hybrid melts are given by green, blue, and orange fields respectively.   Reagan et al. 2010;Timm et al. 2011;Escrig et al. 2012;Ribeiro et al. 2013;Todd et al. 2013;Kemner et al. 2015). Ti-V fields from Shervais (1982) and Pearce (2014).