ERTH3104 Virtual Field Trip to New England Orogen

Geological Background of New England Orogen

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Overview

The New England Orogen of northeastern NSW, Australia is a tectonic collage of rocks of Paleozoic age that have developed in a long-lived convergent tectonic plate boundary. It comprises various terranes, which amalgamated, accreted or otherwise interacted during late Paleozoic - early Mesozoic time. It extends over approximately 1300 km along eastern Australia from Bowen (20.5°N) in Queensland to Newcastle (33.5°N) in NSW. Subduction probably ceased or migrated eastward in the Early Permian due to slab-rollback and was replaced by a strike-slip regime.

Traditionally, The New England Orogen (NEO) has been subdivided into three regional provinces: Yarrol, Gympie and New England. Differences in the tectonic setting and relative motions of the constituent terranes together with their subsequent post-accretionary interactions, disruption and dispersal has produced the complex geology now observed. Understanding the nature and development of various terranes and the manner in which they have been juxtaposed, is crucial to resolving the tectonic evolution of eastern Australia. Early tectonic models for the NEO were based on 1970's and early '80's understanding of tectonic processes and existing models for convergent margins. As a result of research late last century and the 1980’s concentration of Ocean Drilling Program (ODP and IODP) investigations in convergent margin basins of the western Pacific, we now have a better understanding of sedimentology, structure, geochemistry, and tectonics of modem analogues for various elements of the NEO. This increased knowledge allowed a basis on which to develop more actualistic models for the NEO. Given that many resource exploration programs are based on the prevailing tectonic interpretations for any particular region, improved understanding will have significant follow-on economic benefits to the nation.

Over two decades ago, results of bio- and chrono-stratigraphic investigations greatly improved the available database of age constraints for the NEO. These constraints, together with our improved understanding of convergent margin processes, required reassessment of existing models. However, nearly thirty years has passed and it is now timely to develop models for the evolution of individual terranes with varying degrees of detail and reliability. There is lots of potential for new studies! Models should become increasingly refined and with more data and advanced techniques, the relationships and links inferred to exist between terranes can be critically examined and tested. Simple inferred provenance links are often deceptive and there are many recent examples from California and New Zealand where this has proven to be the case. In California, various workers have long argued for a conservative model inferring strong provenance links between Sierran batholiths and Franciscan sediments. However, sophisticated paleomagnetic studies, which used amphibole geobarometry to determine the paleohorizontal of batholiths, clearly demonstrated massive orogen parallel terrane translation and disproves the inferred links of more fixist interpretations. In New Zealand, inferred provenance links between terranes of similar ages have been discredited by isotope data which demonstrate considerable inheritance in some terranes, which is not present in inferred source areas. With the benefit of age constraints, the same logic, which was used to argue for genetic relationships between several of the NEO terranes already requires that some of these inferred relationships must be rejected. Evidence for other inferred provenance links in the NEO has already been documented by several authors. The modern geological toolbox includes detrital zircons studies, which allow for better testing of former relationships between sources terranes and sediment sinks. Detailed geochemistry and isotope fingerprinting will allow rigorous testing of remaining, and future, hypotheses. While it is always nice to have a model for development of our favorite portions of an orogen, models should not become self-perpetuating and we must not lose sight of the scientific method, which requires continual rigorous testing of our hypotheses.

Bear in mind that many of the models for the NEO were developed at least two (or even three) decades ago. Subsequently, there has been relatively little study of this region but an abundance of investigations in areas to which models are related (i.e. active plate margins). For example, in 1980’s models NEO researchers only considered subduction accretion along continental margins and we now know that subduction erosion is of major importance. Early workers recognised ophiolites and former mid-oceanic ridge material but in the most recent studies a forearc origin is preferred. There is lots of scope for bringing understanding of the NEO into the 21st century.

Continental margin (Andean-style) convergent margin

The Tamworth Belt or Block includes Devonian-Carboniferous lithologies thought to represent a forearc basin (Noda, 2016) deposited adjacent to a continental margin arc. Rocks associated with this belt lie between the Hunter-Mooki and Peel-Manning fault systems (PMFS) to the wet and east respectively. These are interpreted to be a western continental magmatic arc-frontal arc sequence or forearc basin of (mostly) Carboniferous age. The forearc basin is dominated by volcaniclastic units interstratified with shallow water limestones and radiolarian-bearing tuffs. In the west of the belt there are abundant ignimbritic rocks indicating the proximity of an Andean style arc (McPhie, 1983).

Intra-oceanic island (Izu-Bonin-Marians, IBM) style arc

Gamilaroi terrane (Nundle area, NSW): The name 'Gamilaroi terrane' was introduced for intra-oceanic island arc-related rocks found within the Tamworth Belt but forming a separate lithostratigraphic identity. It is the westernmost terrane of the NEO and contains Silurian-Devonian age volcanogenic sediments including deep marine high-density mass-flow conglomerates, turbidite sandstones and altered felsic tuffs. Volcanic rocks including meta-andesites, rhyolites, dacites and basalts (spilites), intercalated with volcanogenic sediments (Aitchison and Flood, 1995). These rocks were first recognised and mapped over a century ago by Professor W.N. Benson who used a bicycle for field transportation (Barkas and Aitchison, 1994). Subsequently, a well-established lithostratigraphy has been determined between Tamworth and Nundle, and can be correlated to other areas including the northern New England orogen in the Rockhampton area, QLD. The eastern boundary of the Gamilaroi terrane is marked by the PMFS. At Barry station to the east of Nundle strike-slip faulting has resulted in structural repetition of the terrane (Aitchison et al., 1997). The Gamilaroi terrane was originally thought to encompass the entire Tamworth Belt (but further geochemical data indicate that two arc related sequences are present: a Devonian intra-oceanic island arc devoid of any continental (Gondwana) sediment influx, and a Carboniferous continental margin arc). The collision of the Gamilaroi terrane with Gondwana is marked by the presence of westerly-derived submarine canyon fill, including quartzite derived from the Lachlan Fold Belt within the Keepit Conglomerate (Flood and Aitchison, 1992). This indicates the maximum age of an overlap sequence that unconformably overlies the Gamilaroi terrane and links the NEO to Gondwana.

The Gamilaroi terrane contains rocks, which appear to have developed in a variety of intra-oceanic island arc-related environments. Volcaniclastic sedimentary rocks including deep-water marine high-density mass-flow conglomerates, volcaniclastic turbidite sandstones, altered felsic tuffs and tuff turbidites are found throughout the terrane. The volcaniclastic rocks arc intercalated with meta-andesites, rhyolites and dacites together with meta-basalts originally mapped by Benson (Benson, 1913a, b, c, 1915a, b, 1917, 1918; Benson and Chapman, 1918; Benson et al., 1920). Support for the island arc-related environments comes from the geochemistry of the volcanic rocks (Cawood and Flood, 1989; Morris, 1988; Offler, 1982; Offler et al., 1988; Stratford and Aitchison, 1997a) and framework mineralogy of the sandstones (Cawood, 1983).

The lithostratigraphic succession is well established between Tamworth and Nundle. This succession can be broadly correlated with sequences in other areas throughout the terrane. As the terrane developed in a dynamic tectonic setting, various facies occur and recur at different stratigraphic levels in different areas. Flood & Aitchison (1992) have suggested that uppermost Devonian and younger strata which unconformably overlie the Gamilaroi terrane, comprise a form of foreland basin sequence which overlaps the Gamilaroi terrane and the Lachlan orogen (the pre-Carboniferous eastern limit of Gondwana) to the west. Initiation of the foreland basin followed collision of the Gamilaroi terrane with the Gondwana continental margin. lt is marked by an unconformity, major facies change and the first appearance of sediments containing conspicuous, mineralogically-mature, Gondwana-derived quartzite clasts.

Ongoing reappraisal of Gamilaroi terrane lithostratigraphy, aided by data from radiolarian studies and comparisons with sedimentation rates of modem intra-oceanic arc environments, permits development of a more appropriate model for the development of the terrane. Many aspects of the existing tectonic models for the NEO, while developed on the basis of all available existing data at the time, are simplistic and could be greatly improved if new data become available. Previous interpretation of the Gamilaroi terrane was based on widespread belief in a fixed relationship between all NEO terranes throughout their evolution. The Tamworth Belt has been considered to represent a long-lived (possibly Cambrian to Permian) forearc basin. Although rare Cambrian and Ordovician limestone clasts are present in conglomerates, the oldest demonstrably in situ rocks in the belt are those of a Silurian-Devonian rifled intra-oceanic island arc assemblage (Gamilaroi terrane).

This island arc assemblage is discrete from younger, overlying, continental margin-arc rocks of Carboniferous age. Significantly, as the Gamilaroi terrane was an intra-oceanic island-arc, it could not have developed as part of the continental margin of Gondwana during the Devonian and thus it must be allochthonous to eastern Australia. By definition all other terranes of similar or older ages outboard of the Gamilaroi terrane must now also be regarded as suspect. Subduction complex rocks to the east of the PMFS have been used previously to provide a basis for inferring subduction polarity. However, these rocks constitute part of a separate terrane, which was not necessarily genetically related to the Gamilaroi terrane during the Devonian. Indeed, radiolarian fossil data indicate that their development was not coeval. No unambiguous evidence, which could indicate the polarity of the Gamilaroi terrane arc, is known. In an attempt to resolve the question of polarity Aitchison and Flood (1995) examined modern settings where continents and arcs are converging, through the subduction of intervening oceanic crustal material. In these areas, subduction is occurring beneath the arc rather than the continental margin (e.g. Timor and Australia, Taiwan and China). By analogy they suggest that oceanic crust intervening between the Gamilaroi terrane and Gondwana was subducted towards under the western margin of the Gamilaroi terrane arc. This contrasted with existing models in which various authors, who suggested that the geology of the NEO is the product of magmatic arc development associated with a west-dipping subduction zone active along the eastern margin of Australia through the Paleozoic. Although both Devonian and Carboniferous volcanic rocks are recognised in the Tamworth Belt. There is a marked change in the nature and locus of volcanic activity. The Gamilaroi terrane, a Devonian lithotectonic entity, which forms part of the Tamworth Belt, predates younger Carboniferous Gondwana margin calc-alkaline arc volcanic rocks. Carboniferous volcanic rocks developed along the Gondwana margin and were associated with west-dipping subduction beneath this margin. However, they postdate Gamilaroi terrane/Gondwana continent collision and amalgamation. A flip in subduction polarity may have occurred in a manner similar to that associated with the Cenozoic Papua New Guinea/ Australian continent collision. Several other factors point to eastward subduction of intervening crust beneath the Gamilaroi terrane island arc. This inferred west-facing arc system would place the Gamilaroi terrane in an upper plate position facilitating its accretion and preservation through obduction during collision with Gondwana. Deep seismic surveys carried out by the Bureau of Mineral Resources have revealed the presence of probable Lachlan orogen layered rocks structurally under lying westward overthrust Gamilaroi terrane rocks north of Tamworth (Korsch et al., 1986). Conversely, if Gondwana had been in the upper plate position then we would not expect to find significant evidence of the Gamilaroi arc. Further evidence for Gamilaroi terrane having been thrust over an older basement is becoming available through studies of inheritance in magmatic zircon crystals in igneous rocks of the Carboniferous arc. While there is no evidence for any inherited older cores in zircon crystals from the one analysed Gamilaroi terrane sample, zircons from overlying, Carboniferous, continental margin, volcano-sedimentary overlap assemblage rocks, contain evidence of both Silurian and Precambrian inheritance. Both the SHRIMP and deep-seismic results supported interpretation of Gamilaroi terrane as having overridden the eastern margin of Gondwana. Twenty years ago, Aitchison and Flood (1995) proposed a model for development of the Gamilaroi terrane in a west-facing intra-oceanic island-arc setting during the Silurian to Devonian in which the arc was over-riding eastward-subducting oceanic crust (Aitchison and Flood, 1995). Meta-felsic and basaltic lithologies, which are common in the Barry-Glenrock-Pigna Barney district, represent portions of this island-arc. During the Middle to Late Devonian, the arc experienced a period of rifting. Mass wasting of volcaniclastic sediments into arc-rift basins together with limestone blocks shed off uplifted older basement arc edifices resulted in development of the Tamworth Croup. These sediments were intruded by dolerites and are intercalated with eruptive basalts. Near the end of the Devonian the Gamilaroi terrane, which was in an upper plate setting, collided with and overrode, or was obducted onto, the leading edge of older crustal rocks of Gondwana. Collision resulted in a subduction flip and during the Early Carboniferous a new east-facing subduction complex developed east of accreted Gamilaroi terrane rocks in association with west-dipping subduction. Collision is marked by a hiatus in volcanic activity followed by a major westward shift in the locus of volcanism.

Subduction (accretionary) complexes and ocean plate stratigraphy (OPS)

Rocks associated with two types of subduction complexes occur in the NEO. The Djungati terrane is a subduction complex dominated by ‘off-scraped material from the down-going plate whereas the Anaiwan terrane is dominated by trench-fill material (volcaniclastic continental margin arc-derived sediments). In both terranes we are able to observe classical OPS successions.

From Isozoki 2009 abstract for GSA annual Conference in Portland Oregon MC Blake Symposium: “Subduction processes usually destroy ocean floors, whereas smaller pieces of ancient ocean floors are occasionally preserved within accretionary complexes (ACs) along active continental margins. ACs are mostly composed of coarse-grained terrigenous clastics with minor oceanic material, such as MORB greenstones, deep-sea chert, seamount-derived OIB/atoll carbonates etc. Owing to the tectonic accretion under horizontal compression regime, ACs are characterized by structurally repeated, out-of-sequence sedimentary rocks, sometimes with chaotically-mixed features (mélange/olistostrome). Ocean Plate Stratigraphy (OPS) represents a pre-accretion primary stratigraphy of AC components that consists of MORB, deep-sea pelagic-hemipeagic sediments, and trench-fill turbidites, in ascending order. OPS records a travel log of an oceanic plate from its birth at MOR to its demise at trench, and an age of subducted plate that formed an ancient AC. In other words, an OPS provides an identity code for each ancient AC. OPS analysis is critical in reconstructing tectonic history of ancient subduction-related (Miyashiro-type) orogens because it is the only information source for ancient ridge-trench systems that disappeared. As demonstrated in SW Japan, OPS analysis can clarify tectonic history and internal structures of accretionary orogenic belts; 1) onset time of subduction, 2) discrimination of AC belts, 3) sub-horizontal nappe pile, and 4) episodic subduction of MOR. Additional fruit of OPS study is the first detection of the mid-oceanic deep-sea Permo-Triassic boundary that recorded the biggest mass extinction in the Phanerozoic. The basic concept of OPS came earlier with plate tectonics in the early 1970s; however, the newly developed high-resolution microfossil (conodont, radiolarians) stratigraphy enabled the practical reconstruction of OPS for on-land exposed ACs in SW Japan for the first time in the world in the late 1970s (cf. Isozaki et al., (1990); Matsuda & Isozaki, (1991)).”

The Djungati terrane is a dismembered terrane dominated by red ribbon-bedded radiolarian chert estimated to be up to 100 m thick, conformably underlain by meta-basalts that together comprise the Woolomin Group. The red cherts are overlain by green siliceous tuffs, arc derived sediments and olistoliths of chert, basalt and limestone. Limestones fossils that range from Ordovician to Lower Devonian. Using radiolarian biostratigraphy, it has been shown that there were marked disparities in age compared with previous interpretations of the New England Geology. Radiolarians extracted from the red ribbon-bedded cherts belong to species that existed from the latest Silurian to Middle Devonian. Radiolarian faunas from the uppermost levels of the red chert assemblage indicate that deposition continued until Late Devonian time (Aitchison et al. 1992). Radiolarians indicate that the Wisemans Arm Formation facies accumulated in the latest Devonian to Early Carboniferous and are the youngest units in the Djungati terrane.

In areas between Woolomin and Barry Station the Djungati terrane consists of red ribbon- bedded chert, green tuffs and arenites overlain by olistostromal deposits interbedded with green tuffs that comprise the Wisemans Arm Formation. The entire sequence is repeated by numerous thrust faults. A unit which occurs in a fault sliver east of the Barry homestead is dominated by basalt flows, limestone and green arenites and contains an Early Devonian coral fauna. This unit is interpreted as an accreted seamount forming part of the Wisemans Arm Formation. Radiolarians are locally abundant in the red ribbon-bedded cherts and green-grey tuffaceous cherts of the Djungati terrane. They provide an important dating tool for use with the complex strata of the NEO.

Ages obtained for in-situ tuff and chert assemblages, on the basis of radiolarian biostratigraphy, are considerably younger than Ordovician-Silurian ages, previously inferred from faunas contained within allochthonous limestone blocks. Radiolarians show that petrographic similarities between epiclastic strata within well-dated interpretations of New England geology. Ages obtained for in-situ tuff and chert assemblages, on the basis of radiolarian biostratigraphy, are considerably younger than the Ordovician-Silurian age, previously inferred from faunas contained within allochthonous limestone blocks. The plate of images shows radiolarians extracted from the red ribbon-bedded cherts of the Woolomin Group. From both radiolarian and conodont fossils found within the Djungati terrane timing of development of the red ribbon-bedded cherts can be constrained to the Late Silurian to Late Devonian (Frasnian). Overlying green siliceous tuffs of the Wisemans Arm Formation are believed to have developed from the latest Devonian to Early Carboniferous. This kind of rock association and the way in which it is assembled is reminiscent of what one could expect to find in a subduction complex associated with a convergent plate margin with relatively low sedimentation rates ± subduction erosion. Recent studies show that the evolution of subduction complexes is not as simple as early models suggest and the topic of subduction accretion vs. erosion is hotly debated (Stern, 2011; Stern and Scholl, 2010).

Ophiolite: the (Cambrian) Weraerai Terrane

Ophiolites are interpreted to represent slices of ancient oceanic crustal material exposed on land. However, their geochemistry indicates that they are not typical mid-ocean ridge crustal material rather the associated magmas have been influenced by a subduction component. One of the most famous ophiolites is that exposed in Cyprus and back in 1973 Akiho Miyashiro created an uproar when he suggested it might have formed in an island arc (Miyashiro, 1973). The famous geochemist Julian Pearce followed up on this hypothesis (Pearce et al., 1984) and gradually it has become widely accepted. Ocean Drilling of the forearc regions of modern intra-oceanic island arcs (Izu-Bonin-Marians and Tonga-Kermadec) has provided confirmation (Dilek and Furnes, 2014).

Weraerai terrane: Early last century Benson was the first to map the general extent of the rocks he termed the Great Serpentine Belt of N.S.W. Ultramafic rocks and serpentinite mélanges are usually found as narrow elongate semi-continuous zones of rock that outcrop from Warialda southwards to Nundle then south-east to the coast near Taree. The zone reaches a maximum width of 1 km and extends a distance of over 300 km along the Peel Manning Fault system. These rocks are today recognized as a separate lithotectonic entity, the Weraerai terrane (Aitchison et al., 1994). It is usually reduced to a mélange that is dominated by a highly schistose serpentine matrix with a steeply dipping foliation. The fabric anastomoses around blocks of ophiolitic rocks such as harzburgite, dunite, pyroxenite, gabbro, dolerite, plagiogranite and basalt. In the early days of plate tectonics these rocks were interpreted as "alpine-type" ultramafic rocks injected into higher crustal levels as a series of cold intrusions during the Permian. On the basis of petrological and geochemical studies, it has been established that these rocks are dismembered fragments of an ophiolite sequence. Most lithologies of the Weraerai terrane have undergone low-grade metamorphism to prehnite-pumpellyite facies. Rare blocks of high P/ low T metamorphic rocks including blueschist, eclogite and high T/ low P amphibolites are present within the serpentinite mélange (Manton et al., 2017).

Age constraints: SHRIMP ion microprobe data indicate that magmatic zircons from plagiogranite of the Weraerai terrane give an age of 530 ± 6 Ma (Aitchison et al., 1992). This makes the rocks of the Weraerai terrane the oldest in eastern Australia. Such an age is not in agreement with tectonic models suggesting that progressively younger crust was accreted onto the eastern margin of Gondwana throughout the Paleozoic. Ordovician K/Ar ages (469-482) have been obtained from blueschist and amphibolite rocks at Pigna Barney. It has been suggested that younger, thin-skinned terranes may have been thrust westward over the continental freeboard of eastern Australia during the Paleozoic. There is no evidence for erosion of these Cambrian ophiolites into New England sediments until the Early Permian.

Geochemistry: Whole rock analyses of the basalts and gabbros of the Weraerai terrane at Barry indicates these rocks are depleted in TiO2, and contain relatively high MgO and CaO values. Geochemical analyses indicate highly refractory mantle source origins as shown in a plot of log Cr/log Y. This is similar to ophiolites generated elsewhere in supra-subduction zone settings. Some samples analysed from Barry yielded detectable quantities of Cr. When compared to the Gamilaroi terrane igneous rocks, using log Cr/log Y relations the mafic igneous rocks of the Weraerai terrane are derived from a much more fertile mantle source. Geochemical analyses reveal low incompatible element contents and relatively high Mg contents indicating similarities with depleted or Ti-poor ophiolite sequences common in modern forearc settings. The fact that the Gamilaroi terrane igneous rocks are derived from a more fertile mantle source than the Weraerai terrane igneous rocks dispels the idea that the Weraerai terrane forms part of the Gamilaroi terrane forearc basement. If this was the case it would be expected that the younger (Late Silurian) Gamilaroi terrane igneous rocks would be derived from a refractory mantle source and the Weraerai terrane from a fertile source.

Interpretations of the Weraerai terrane: The Weraerai terrane is locally juxtaposed against the Gamilaroi terrane, Djungati terrane and/or the Manning Group. Leitch (1980) suggested that the mafic rocks of the Weraerai terrane were part of an igneous complex formed at the inception of arc activity and constituted the forearc basement. Geochemical data indicate a highly refractory magmatic source for the Weraerai terrane igneous rocks similar to contemporary ophiolite sequences and therefore not related to the Gamilaroi igneous rocks. SHRIMP data indicates the Weraerai terrane ophiolitic rocks formed in the earliest Cambrian (Aitchison et al., 1992), which is in marked contrast to the Late Silurian age of the Gamilaroi intrusives. There is no evidence of serpentinite detritus in the NEO sedimentary rocks until the Early Permian (Manning Group), indicating emplacement as cold intrusions due to extensional/transtensional regimes. However, the origin and mechanism of emplacement of these ophiolites remains highly controversial. Ordovician ages have been obtained from blueschist and amphibolite blocks within the Weraerai terrane at Glenrock, Pigna Barney and elsewhere. These ages are younger than the Weraerai terrane plagiogranites but older than the Gamilaroi or Djungati terrane. The blueschists probably formed in a subduction event that predates much of the development of the NEO. Strike-slip movement and localized transtension resulted in these rocks being emplaced and drawn out along the PMFS. A possible origin for these ophiolitic rocks that has been suggested is the Lachlan Fold Belt, which is believed to underlie the NEO. One problem with this idea is that few sedimentary rocks such as the Cambrian-Ordovician sedimentary rocks, which are abundant within the Lachlan Fold Belt are found within the serpentinite.

Convergent margin magmatism: S- and I-type granites

New England Orogen has some classic localities, which the study of formed a basis for Chappell and White’s famous work on S- and I- type granites (sometimes also referred to as ilmenite and magnetite series granites).

S-type Granites: Two temporally and geochemically distinct phases of S-type plutonism are recorded at ~303 Ma (Hillgrove Supersuite) and ~280 Ma (Bundarra Supersuite). Hillgrove Supersuite Granites of the Hillgrove Supersuite are commonly intensely foliated. Although they have low CaO, Na2O and Sr, and are strongly reduced (locally contain graphite), other typical S-type characteristics are poorly developed; (i) they are not particularly peraluminous; ferromagnesian minerals include dominant aluminous russet red biotite, and minor almandine- rich garnet, actinolitic amphibole and cummingtonite, (ii) are comparatively mafic and compositionally diverse (65-75 wt % SiO2), (iii) P2O5 is negatively correlated with SiO2, as for I-type granites, (iv) isotopically relatively primitive for S-type granites (87Sr/86Sr = 0.705-0.706). Such granites are generally interpreted to have been derived from young volcano-sedimentary materials. The granites record subtle regional variation is sediment chemistry.

The Tia Granodiorite is a ca. 300 Ma biotite granodiorite, a member of the S-type Hillgrove Suite characterised by the presence of microcline, biotite, and muscovite but lacking hornblende typical of many younger members of the New England Batholith. It is cut by widespread mostly planar but in places folded pegmatite dykes and aplite and quartz veins.

NEB I-type granites: I-type granites in the NEB are primarily Late Permian–Early Triassic2 in age. Three major supersuites are recognized. Moonbi Supersuite: The Moonbi Supersuite (MSS) consists of strongly oxidised, high-K hornblende- biotite (±augite) granites that are characterised by abundant pink K-feldspar phenocrystals (megacrysts), titanite and magnetite. Overall, Moonbi granites have high K2O, Rb, Sr, Ba, Th, U, P2O5 and Pb. Nevertheless, geographically significant geochemical variations exist; southern MSS granites have distinctly higher K2O, Rb, Sr, Ba etc, are more mafic and have lower Y and HREE than the northern MSS intrusions. The latter are dominated by more highly fractionated leucogranites. Moonbi granites have geochemical similarities with those of the Sierra Nevada batholith and show distinct chemical overlaps with Carboniferous LFB granites.

Subduction zone metamorphism [link]

The coast of Port Macquarie provides a beautiful exposure of mafic and ultramafic rocks, including serpentinites and high-pressure assemblages. The origin of these rocks, possibly associated with an ophiolitic mélange (Aitchison et al., 1994), is Cambrian-Ordovician (Aitchison and Ireland, 1995). A similar lithological assemblage occurs in a number of other localities throughout the southern New England Orogen most noticeably along the length of the PMFS. Farther south, the southwards continuation of the PMFS is unclear, but patches of serpentinites are found in the area of the Manning Orocline and around the Hastings Block (Lennox and Offler, 2009). Therefore, it is possible that the same serpentinite belt that crops out along the Peel-Manning Fault System is folded around the Manning Orocline and the Hastings bend before reappearing in Port Macquarie. It is noted, however, that the correlation between the different serpentinite outcrops is not straightforward. For example, some of the ophiolitic-related rocks on the western side of the Hastings Block are probably Silurian-Devonian (Aitchison et al., 1994), i.e., considerably younger than the Cambrian-Ordovician Port Macquarie rocks.

Detailed descriptions of the geology of Port Macquarie have been provided by numerous workers (Barron et al., 1976; Buckman et al., 2015; Och et al., 2007; Och et al., 2003). The rocks are characterised by blocks of massive serpentinites and mafic rocks surrounded by a serpentinite schist. The serpentinite assemblage in both blocks and matrix is lizardite and chrysotile. Some of the blocks preserve cumulative textures and show primary magmatic layering. A protolith of harzburgite, lherzolite and orthopyroxenite has been suggested for the serpentinite blocks. High-pressure rocks occur as blocks within a metamorphic mélange, and include retrogressed eclogite- and blueschist-facies metamorphic assemblages of almandine, omphacite, ±lawsonite, ±glaucophane and ±quartz Blocks of tremolite marble and omphacitite are also found. Some layers of garnet-omphacite-lawsonite-quartz are intercalated with phengite glaucophane schist. The matrix is chlorite-actinolite schist, most likely associated with metasomatic interactions between the mafic and ultramafic rocks.

Anaiwan terrane: Coffs Harbour Block

Rocks of the Anaiwan Terrane, a clastic trench-fill sediment-dominated subduction complex crop out along the NSW Coast near Coffs Harbour. The Coramba beds are of Late Carboniferous age (ca. 323 Ma) on the basis of detrital zircon ages (Korsch et al. 2009b). Hillgrove Plutonic Suite rocks that intrude the southern Coffs Harbour Block and have an age ca. 305 Ma and provide an upper age constraint (Korsch et al. 2009b). The structure of the southern Coffs Harbour Block has been studied by Korsch (1981b) and consists of a northward younging and folded succession. Structural thickening by strike-parallel thrust faults is inferred to account for the exceptional stratigraphic thicknesses but most of the succession appears coherent with well-developed layers apart from areas of soft-sediment disruption as at Mullaway headland (Korsch 1972). Mélanges have been mapped in the Coffs Harbour Block to the west of Grafton (Fergusson) but are largely missing from the coherent turbidite units of the southern Coffs Harbour Block.

Red Rock headland: From Coffs Harbour we will follow the Pacific Highway north to the turnoff to Red Rock (ca. 32 km). We will visit outcrops on the headland. The Red Rock headland at the eastern end of the beach contains exposures of the Redbank River beds. The first outcrops consist of altered mafic volcanic rocks and chert whereas chert and jasper form the headland. The siliceous rocks consist of thin alternating beds (1-3 cm) and contain abundant spherical radiolarians (including Holoeciscus formanae of Late Devonian age). Within chert and jasper, F1 folds are tight to isoclinal with axial surfaces dipping moderately to the east. F2 warps reorient the F1 structures.

From the trig station on top of the headland there is a panoramic view in all directions. To the south the prominent headland of Woolgoolga contains east-west striking strata of the Coramba beds whereas to the north they are striking northerly (Korsch 1981b). Red Rock is at the hinge of a regional syncline that forms the eastern closure of the Texas-Coffs Harbour megafold (Korsch 1981b; Murray et al. 1987). The Texas-Coffs Harbour megafold was first recognized during regional mapping work undertaken by Olgers and Fold in the Texas region of SE Queensland. It is a z-shaped orocline in the subduction complex of the New England Fold Belt. It is also apparent on magnetic and gravity maps. The scale of the structure implies considerable duplication of the subduction complex and the mode of formation and timing of development are disputed issues. To the west the prominent line of hills are formed by north-trending resistant conglomerate and sandstone of the Mesozoic Clarence-Moreton Basin.

Coramba beds (Upper Carboniferous): Interbedded mudstone and greywacke of the Coramba beds are exposed along the headland at the end of Pollack Esplanade. These rocks have well developed layers of the Bouma sequence and were formed in a deep marine setting by turbidite deposition. F1 folds, with near vertical axial surfaces and gentle plunges towards 100° and 280°, are abundant. The folds are open to close with rounded hinges and are symmetric. A strongly anastomosing spaced cleavage is developed in mudstones and is axial planar to F1 folds (Korsch 1972, 1981b).

Farther to the east at the western end of a small cove occur gray-green fine-grained siliceous rocks interbedded with mudstones. These are probably tuffaceous rocks that have formed by the accumulation of silicic ash derived from a distant major ignimbrite eruption (Leitch 1981). Beyond the eastern end of the cove occur some tension gash arrays containing fibrous quartz.

At the northeastern point of the headland, northerly dipping and younging thin-bedded turbidites are underlain by massive greywacke. Massive greywacke is common in the Coramba beds (Korsch 1972). 50 m south of the rocky point at the eastern end of the headland, and across a major fissure (caution required for crossing to avoid a 10 m deep chasm), thin-bedded turbidites dip moderately to the south and appear to be in fault contact with the massive greywacke at the hinge of a map-scale F1 anticline. The anticline has a steep axial surface and occurs along trend from the abundant F1 mesoscopic folds encountered at the start of the traverse.

The Lochaber Greywackes: The route travels east along the Oxley Highway and after 15 km crosses into the Lochaber Greywackes (Devonian-Carboniferous). To access stop (mostly scenic/comfort stop but a chance to see some greywackes too) turn right (northeast) about 3 km northwest of the Oxley Highway crossing of Stony Creek at the road to Apsley Falls and proceed to the lookout. This is mainly a scenic stop, the falls being a prominent one of many that mark the transition from tablelands to steeply dissected gorge along the eastern fall of the New England Tablelands. The rocks of the falls and the surrounding gorge are part of the Lochaber Greywackes.

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