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The Watonga Formation and Tacking Point Gabbro, Port Macquarie, Aus-tralia: Insights into crustal growth mechanisms on the eastern margin ofGondwana
Solomon Buckman, Allen P. Nutman, Jonathan Aitchison, Joseph Parker,Sarah Bembrick, Tom Line, Hiroshi Hidaka, Tomoyuki Kamiichi
PII: S1342-937X(14)00097-5DOI: doi: 10.1016/j.gr.2014.02.013Reference: GR 1239
To appear in: Gondwana Research
Received date: 9 August 2013Revised date: 28 February 2014Accepted date: 28 February 2014
Please cite this article as: Buckman, Solomon, Nutman, Allen P., Aitchison, Jonathan,Parker, Joseph, Bembrick, Sarah, Line, Tom, Hidaka, Hiroshi, Kamiichi, Tomoyuki, TheWatonga Formation and Tacking Point Gabbro, Port Macquarie, Australia: Insightsinto crustal growth mechanisms on the eastern margin of Gondwana, Gondwana Research(2014), doi: 10.1016/j.gr.2014.02.013
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The Watonga Formation and Tacking Point Gabbro, Port
Macquarie, Australia: Insights into crustal growth
mechanisms on the eastern margin of Gondwana
Solomon Buckman1*
, Allen P. Nutman1, Jonathan Aitchison
2, Joseph Parker
3, Sarah
Bembrick1,2
, Tom Line4, Hiroshi Hidaka
5 and Tomoyuki Kamiichi
5
1 GeoQuEST Research Centre, School of Earth and Environmental Sciences,
University of Wollongong, Wollongong, NSW 2522, Australia
2 School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia
3 Mount Isa Mines, Mount Isa, Queensland 4825, Australia
4 Arrium Mining, Whyalla, South Australia 5600, Australia
5 Department of Earth and Planetary Systems Sciences, University of Hiroshima,
1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan
*Corresponding author: Solomon Buckman email: solomon@uow.edu.au
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Abstract
A diverse assemblage of accretionary complex, island-arc, ophiolitic and high-
pressure, low-temperature metamorphic rocks occurs within the serpentinite mélange
at Port Macquarie on the eastern extremity of the New England Orogen of eastern
Australia. New field observations, U-Pb zircon dating, petrography and geochemistry
presented here establish a more robust chronology and interpretation of these rocks.
Previously, all basalt, chert and volcaniclastic sandstones at Port Macquarie were
grouped into the Watonga Formation. Ordovician to middle Devonian radiolarians
and conodonts from ‘Watonga’ chert-basalt associations shows that they are older
than, and unrelated to, ‘Watonga’ volcaniclastic rocks like those at Green Mound
which contain volcanic/detrital zircons as young as 335 Ma that were derived from a
Carboniferous arc. Volcanic detritus with pillow lava forming a block within the
serpentinite mélange yielded 452±10 Ma igneous zircons, indicating an Ordovician
age. The Tacking Point Gabbro has an age of 390±7 Ma (Devonian) and geochemical
affinities with intra-oceanic arc igneous suites. It was intruded into deformed cherts of
the Watonga Formation giving a spatial link between an Ordovician-Devonian?
accretionary complex and adjacent Devonian island-arc. The MORB-like basalt-chert
association of the Watonga Formation and the Devonian Tacking Point gabbro
represent a mid-Paleozoic assemblage allochthonous to Gondwana, which possibly
correlate with the Djungati and Gamilaroi terranes respectively located further west in
the New England Orogen. Zircon dating shows that post-serpentinite mafic-felsic
dykes were emplaced into the Port Macquarie serpentinite at 247±20 Ma and further
disrupted. Therefore, tectonism affecting the serpentinite continued into the Early
Triassic, with final movement during the Hunter-Bowen Orogeny. Our results from
Port Macquarie are compatible with a tectonic model for the New England Orogen
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that involves episodic island-arc collisional events (Gamilaroi and Gympie terranes)
interspersed with periods of continental margin “Andean-type” magmatism and
accretion along eastern Gondwana.
Keywords: Serpentinite mélange; Watonga Formation; Tacking Point Gabbro; Port
Macquarie; New England Orogen
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1. Introduction
The New England Orogen (NEO) of Australia (Figure 1) developed along the
eastern margin of Gondwana by convergent margin accretion and amalgamation of
terranes during the Paleozoic and earliest Mesozoic (Cawood and Buchan, 2007;
Cawood et al., 2011a; Flood and Aitchison, 1988; Murray et al., 1987). Widespread
intrusion of granite plutons occurred episodically within the NEO from the latest
Carboniferous (~300 Ma) through to the Triassic (~220 Ma). The Hunter-Bowen
Orogeny is the main deformation event within the NEO (Collins, 1991) and broadly
corresponds with a magmatic hiatus from the latest Permian to Early Triassic
(Holcombe et al., 1997; Li et al., 2012). Together these resulted in the complex
tectonic collage now observed in the NEO.
For the eastern Gondwanan margin, there is considerable debate whether crustal
accretion was entirely over a single long-lived, westerly-dipping, easterly migrating
subduction zone (Cawood and Buchan, 2007; Cawood et al., 2011a) or if there were
changes of subduction polarity, with rafting-in of intra-oceanic island-arc domains of
Panthalassan origin onto the Gondwanan margin (Aitchison and Ireland, 1995;
Aitchison et al., 1992; Murray, 2007; Offler and Gamble, 2002). Furthermore, the
timing of these events is much debated. Some studies argue that most crustal
development, including the emplacement of serpentinites with blocks preserving
eclogite and blueschist facies metamorphism, occurred entirely in the early-middle
Paleozoic (Fukui et al., 1995; Och et al., 2003; Phillips and Offler, 2011), whereas
more recent dating suggests that some important crustal development as well as some
high-pressure metamorphism could have occurred as late as ~250 Ma, perhaps early
in the Hunter-Bowen Orogeny (Nutman et al., 2013).
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In this paper these controversies are explored further by U-Pb zircon dating,
petrographic and geochemical studies of the Watonga Formation, the Tacking Point
Gabbro and syn-kinematic intrusions (Leitch, 1974; Och, 2007) at Port Macquarie in
northern New South Wales (Figures 1 and 2). The Watonga Formation is one of the
easternmost occurrences of basalt-chert-turbidite associations in the NEO, and
sedimentary petrology and the dating of detrital zircons can test to what degree it was
derived from a simple (intra-oceanic) volcaniclastic source or from a complex early
Paleozoic to Precambrian Gondwanan source. The Tacking Point Gabbro is
important, because it is the easternmost body of its type in the NEO, and its age,
geochemistry and the nature of its country rocks can establish whether or not it
formed remote from the Gondwana margin. Results of our study, suggest it is possible
for the first time, to correlate key lithologies within the Port Macquarie Block with
other terranes further west in the NEO and provide a simpler and more accurate
tectonostratigraphic framework with which to advance our understanding of Paleozoic
to Mesozoic continental growth mechanisms along the eastern margin of Gondwana.
1. The New England Orogen (NEO)
The NEO extends approximately 1300 km along eastern Australia from
Bowen (20˚ S) to Newcastle (33˚ S) and has been divided into three regional
provinces; northern New England (Yarrol), southern New England and Gympie
(Figure 1). The western limit of the orogen is marked by the Hunter-Mooki fault
system, a low angle thrust fault dipping east, with the footwall containing early
Triassic rocks of the Sydney Basin (Korsch et al., 1993).
Continental rifting of Rodinia during the Neoproterozic formed the eastern
margin of Gondwana and opened the Paleo-Pacific (Panthalassan) Ocean. The closure
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of this ocean basin began in the late Neoproterozoic (~570 Ma) building the
Tasmanides or Terra Australis Orogen (Cawood, 2005) that contains a mixture of
both exotic and Gondwana margin terranes. Plate convergence and terrane accretion
along eastern Gondwana resulted in multiple orogenic events from the Cambrian to
Triassic after which New Zealand rifted away to open up the Tasman Sea.
Examination of the NEO from a tectonostratigraphic perspective (Cawood and Leitch,
1985; Flood and Aitchison, 1988; Scheibner, 1985) showed that several discrete
terranes exist, suggesting that the NEO may not have evolved as a simple,
continuously west-dipping subduction system throughout the Paleozoic, and that
exotic island arc terranes may have periodically collided and accreted to eastern
Gondwana. This occurred before evolving into an Andean-type continental margin in
the Carboniferous producing the characteristic felsic volcanic lithologies of the
Tamworth Belt (arc to fore-arc basin) and the overlapping and interleaved
Carboniferous accretionary complex known as the Anaiwan terrane or Tablelands
Complex. In this way the quantum addition of juvenile island arc material may
represent a significant component of continental growth (Aitchison and Buckman,
2012) within the Terra Australis Orogen as opposed to purely accretionary orogen
models (Cawood and Buchan, 2007; Cawood et al., 2011a). Tectonostratigraphic
nomenclature introduced by Aitchison and Flood (1990) are used here to describe the
geology.
2. Port Macquarie Geology
The Port Macquarie “Block” (Figure 1) refers to a small tract of Paleozoic rocks
which comprises a serpentinite-matrix mélange containing and associated with
lithologies that superficially resemble a dismembered ophiolitic sequence with chert,
shale, pillowed and brecciated basic extrusive rocks, mafic and ultramafic dykes,
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blueschist facies blocks and felsic dykes (Barron et al., 1976; Och et al., 2007a).
Apart from the rocky coastline, exposure is very limited and of poor quality, hence all
our information is derived from coastal outcrops. Oceanic-derived basalt, ribbon
chert, and argillite represent the majority of blocks within the mélange (Aitchison et
al., 1994). Wilkinson (in Voisey (1969)) recorded mantle protoliths such as
harzburgite, lherzolite and orthopyroxenite from blocks within the serpentinite matrix
at Port Macquarie, typical of the “alpine” ultramafic suite. To the south the Port
Macquarie Block is overlain by Triassic fluvial sedimentary rocks and acid volcanic
rocks of the Lorne Basin (Pratt, 2010; Tonkin, 1998), which are intruded by Triassic
granites. The volcanic rocks and granites yield U-Pb ages of ca. 220 and 210 Ma
respectively (Richardson, 2013).
3.1. Port Macquarie serpentinite mélange
Eight masses of serpentinite occur along the coastal tract at Port Macquarie and are
referred to collectively as the Port Macquarie Serpentinite (Och et al., 2007a) (Figure
2). The serpentinite ranges from large linear masses, as at Town Beach (Figure 1,
Inset 1), to narrow lenses of limited lateral extent, as at northern Miners Beach. Fresh
outcrop exposures are poor away from the coast due to the development of a deep
regolith. The serpentinite matrix in some localities is highly schistose and contains
blocks of highly altered mantle protoliths such as serpentinised harzburgite/dunite,
rodingite and massive serpentinite (Och et al., 2007a). Although they are highly
modified, relict layering structure is preserved in the larger blocks (Figure 3A). Other
lithologies enveloped in the serpentinite include blueschists/eclogites (Barron et al.,
1976; Och et al., 2007a) of likely Carboniferous to-Permo-Triassic age (Nutman et
al., 2013), chert (Figure 3E) and pillow basalt interbedded with volcaniclastic
sandstones (Figure 3B). Basaltic and felsic dykes intrude the serpentinite and locally
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display an intermingled, pillowed form between the mafic and felsic melts (e.g. Town
Beach and southern headland of Oxley Beach and Tacking Point - Figure 3C). These
dykes are themselves further sheared and disrupted (Figure 3G). This suggests they
were intruded during serpentinite emplacement (synkinematic) but were disrupted by
continuing shearing within the serpentinite.
3.2. Watonga Formation
The Watonga Formation (Leitch, 1980) as currently defined, embraces a basalt-chert-
volcaniclastic sedimentary rock sequence at Port Macquarie, which has been
disrupted extensively by shearing and isoclinal folding. Its lack of stratal continuity is
typical of accretionary complexes (Leitch, 1980; Och, 2007; Och et al., 2007a). It
outcrops east of the Lake Innes Fault across to the Town Beach-Tacking Point coastal
tract (Figure 2). Chert occurs in isoclinally folded, thin ribbon beds, ranging from 1-
10 cm thick intercalated with thinner recessive dark mudstone units. Red and grey
cherts dominate but white, green and black varieties are also found. Pockets of red,
interpillow chert are common within larger basalt blocks (such as at the northern end
of Flynn’s Beach). Volcaniclastic sandstones crop out at Green Mound, east of Town
Beach, in the footwall of the Town Beach serpentinite. These are isolated from
occurrences of cherts and basalts that are assigned to the Watonga Formation. Ishiga
et al. (1988) reported Upper Silurian to Upper Devonian conodonts (Belodella spp.)
from Tacking Point and to Upper Devonian radiolarians (undescribed palaeosceniids)
from cherts of the Watonga Formation at Watonga Rock on the beach just south of
Tacking Point. However, doubts exist as to the identification of the Upper Devonian
fauna, which appear to be sponge spicules, as outlined in a discussion by Aitchison
(1989). Och et al. (2007b) extracted Middle to Late Ordovician conodonts from cherts
at Town Beach, Flynns Beach and near Tacking Point. With the exception of
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sedimentary units of Middle to Late Cambrian age (Murrawong Creek and Pipeclay
Creek formations as documented by Percival et al. (2011)) west of the Peel Fault at
Copes Creek, the Ordovician cherts at Port Macquarie represent some of the oldest
biostratigraphically dated sediments within the NEO.
The dominance of ribbon-bedded radiolarian chert overlying pillow basalts of
typical MORB geochemical character (Figure 9) as well as the absence of limestone
and only minor volcaniclastic material and pillow interstices filled with chert rather
than carbonate, indicate that these portions of the formation initially developed in a
deep marine, ocean-floor environment. The volcaniclastic sedimentary rocks
overlying or structurally adjacent the basalt-chert sequences are typically immature,
greywackes containing abundant basaltic lithic fragments and feldspar with little or no
quartz (Parker, 2010) typical of volcanic arc environments (Dickinson and Suczek,
1979).
3.3. Tacking Point Gabbro
The Tacking Point Gabbro (Och et al., 2007a) (Figure 2) is a polyphase intrusion,
with medium to coarse-grained hornblende-bearing gabbro, leucogabbro and tonalitic
suites (Parker, 2010). Although predominantly massive, some zones exhibit evidence
of compositional layering between leuco- and melano-gabbroic phases as well as
signs of magmatic mingling (Figure 3D) suggesting there were repeated injections of
mafic melt into a more felsic, fractionating magma chamber. Small patches of
pegmatitic, amphibole-rich gabbro occur within the massive gabbro indicating a
relatively hydrous magma or by interaction of an anhydrous magma with circulating
seawater (Figure 3F). The Tacking Point Gabbro contains isoclinally folded pendants
of Watonga Formation chert and basalt which in some cases have been completely
bleached and recrystallised with the original iron in the chert being reduced to
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magnetite or native iron. The intrusive rocks are only weakly deformed, with an
overprint of epidote amphibolite to greenschist facies assemblages (Parker, 2010). A
K-Ar age of 359 ± 11 Ma was obtained for actinolite in the Tacking Point Gabbro (E.
Scheibner, Geological Survey of New South Wales, personal communication, in
Aitchison and Ireland, 1995). The actinolite is a secondary mineral, thus 359 Ma
represents a minimum age of emplacement.
3.4. Post-serpentinite mafic-felsic intrusions
Barron et al. (1976) noted that the basement rocks of Port Macquarie are cut
by complex suites of felsic and basic intrusions. At Town Beach and Oxley’s Beach
the serpentinite is intruded by post-serpentinite felsic dykes, also locally with basic
components. These display a variety of relationships with the serpentinite and the
tectonic blocks within them. At Oxleys Beach (Figure 2) intrusive sheets have
irregular form with lobate chilled margins and have incorporated fragments of the
adjacent serpentinite (Figure 3G). Some of these have been disrupted by extensional
shear zones, or distorted into sigmoidal shapes by late movement in the serpentinite.
At Town Beach blocks of gabbro within the serpentinite are cut by pegmatitic felsic
sheets (Figure 2 Inset 1). The form of the intrusions indicates their emplacement was
coeval with ductile flow of the serpentinites. At Tacking Point, the gabbro is cut by
younger, polyphase, felsic-basic sheets that occupy anastomosing steep and shallow-
dipping extensional fractures (Figure 3C). Zircon dating for one of these sheets (TP1)
is reported below. The complex internal form of these sheets displaying no
transitional zone of mixing between the two melts indicates coeval emplacement of
mafic and felsic magmas and rapid cooling. Wholerock geochemistry of these
intermingled felsic and mafic sheets is shown in Table 1 and plotted in Figure 9.
These composite bodies cutting the Tacking Point Gabbro are synonymous with the
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felsic intrusive phases referred to as plagiogranites and reported in the zircon U-Pb
dating study of Aitchison and Ireland (1995). Given that field relations clearly show
that these mafic-felsic intrusions intrude and therefore post-date the schistose
serpentinite at Town Beach and the zircon populations indicate a Permo-Triassic age,
it is clear that these felsic portions are not plagiogranites in the strict definition of an
ophiolitic plagiogranite and have no direct genetic relationship to the ophiolitic
peridotite blocks within the melange. The composition of these felsic intrusions is
similar to that of an ophiolitic plagiogranite due to the melt being derived from partial
melting of components within the serpentinite melange during extension in the latter
stages of emplacement. However, to avoid confusion, we will refer to these magmatic
rocks as post-serpentinite mafic-felsic intrusions rather than plagiogranites.
4. Zircon U-Pb dating
4.1. Analytical methods and data appraisal
Zircons were handpicked and mounted with standard zircons in epoxy resin discs and
then polished to present grain cross sections for analysis. Prior to analysis,
cathodoluminescence images of the grains were acquired, in order to ascertain their
internal structure. U-Th-Pb measurements were carried out on SHRIMP II
instruments at Geoscience Australia and the University of Hiroshima. Analytical
procedures, including U-Th-Pb calibration methods, follow Williams (1998) and
Stern (1998). U abundance was calibrated using a fragment of the single crystal SL13
zircon standard (U=238 ppm). 206
Pb/238
U ratios for all samples, apart from the Town
Beach felsic intrusion zircon mount (first analysed by Aitchison and Ireland, 1995),
were calibrated using Temora 2, with a 206
Pb/238
U age of 417 Ma (Black et al., 2003).
For the re-analysis of the Aitchison and Ireland (1995) Town Beach felsic intrusion
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zircon mount, 206
Pb/238
U ratios were calibrated using SL13, with a 206
Pb/238
U age of
572 Ma (Claoué-Long et al., 1995). All errors also take into account non-linear
fluctuations in ion counting rates beyond that expected from counting statistics (e.g.,
Stern, 1998). Data were corrected for common Pb based on measured 204
Pb and
Cumming and Richards (1975) common Pb composition for the likely age of the rock.
Finalised data were appraised and plotted using ISOPLOT/EX (Ludwig, 2003).
Weighted mean 206
Pb/238
U ages are reported at the 95% confidence level and are
rounded to the nearest million years. The analytical results are shown in the Tera-
Wasserburg diagrams of Figure 5 and 7, and are summarised in
Table 2.
4.2. Volcaniclastic sandstone block within serpentinite, Town Beach
At Town Beach (Figure 2) a volcaniclastic sandstone (sample 21-01; -31.42973°S
152.92256°E) associated with pillow lava was sampled from a <10 m broad block that
occurs within the serpentinite. The sample yielded 6 small, oscillatory zoned,
euhedral, prismatic zircons (Figure 4). These grains have high Th/U values typical of
igneous zircons. Seven analyses on four grains yielded close to concordant, mutually
indistinguishable ages (Figure 5A), with a weighted mean 206
Pb/238
U age of 455±10
Ma (MSWD=0.26; upper-middle Ordovician). This is interpreted as giving the age of
the volcaniclastic sandstone.
4.3. Green Mound volcaniclastic sandstone ascribed to the Watonga Formation
At Green Mound at the eastern end of Town Beach, quartzo-feldspathic volcaniclastic
sandstone (Parker, 2010) forms the entire headland, and tectonically underlies the
Town Beach serpentinite unit (Figure 2). Sample 21-02 (-31.43041°S, 152.92395°E)
of the Green Mound, volcaniclastic sandstone yielded only 15 grains. Based on CL
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imaging, ten of these were most suitable for U-Pb dating. Thirteen analyses were
undertaken, with the duplicate analyses performed on the some of the apparently
younger grains, to check for any loss of radiogenic Pb (Figure 5B). The youngest
grains are Carboniferous in age (360-330 Ma; grains 2, 3, 5, 6), and are oscillatory
zoned with only minor sedimentary abrasion (Figure 4). The first analysis on grain 3
has a 206
Pb/238
U age of ~283 Ma, whereas analysis 2 has an age of ~335 Ma. This
grain might have suffered local loss of radiogenic Pb, and an age of ~335 Ma is
proposed for this grain. In all other cases, duplicate analyses agreed within analytical
error. Pre-Carboniferous grains have 450-400 Ma, 550-500 Ma and Neoproterozoic
ages and tend to be more rounded (Figure 4), pointing to recycling in sedimentary
systems. The results suggest that the sediment formed proximal to a Carboniferous
(Visean?) arc, but received some influx from earlier Palaeozoic and Gondwanan
sources.
4.4. Tacking Point Gabbro
A tonalitic facies of the Tacking Point Gabbro (sample TP208/JP44; -31.472677°S
152.937647°) gave a small yield of oscillatory-zoned zircons devoid of inherited
cores and overgrowths (Figure 6). Eight analyses on five grains have high Th/U, have
mutually indistinguishable concordant ages (Figure 7B), with a weighted mean
206Pb/
238U age of 390±7 Ma. This is interpreted as the age of intrusion (Eifelian-
Emsian; Middle Devonian).
4.5. Post-serpentinite felsic-mafic intrusions
Aitchison and Ireland (1995) reported zircon dating from Town Beach for a sample
with field characteristics of a plagiogranite and also on an intrusion cutting cherts and
pillow lavas, from just north of Tacking Point. Samples from both localities yielded a
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diverse zircon population, with ages between ~3000 Ma and 250 Ma. These results
were enigmatic, because other ophiolitic plagiogranite blocks collected along the Peel
Fault gave Cambrian to Devonian ages, hence neither inherited Precambrian zircons
nor ones as young as 250 Ma were anticipated (Aitchison et al., 1994). The Aitchison
and Ireland (1995) samples were dated prior to the routine application of CL imaging
in SHRIMP U-Pb zircon dating, meaning that without detailed information on the
internal structure of these grains, these results were hard to interpret. We have
undertaken CL imaging and extra dating of the Town Beach sample zircons (Figure 6
and 7A). A minority of the grains are euhedral and prismatic, with well developed
igneous oscillatory zoning parallel to the euhedral grain margins. These yield ages of
~250 Ma (Figure 7A). Most of the grains are more rounded, with truncated internal
zoning and/or structures. Those analysed in the restudy yielded ages between ~350
and 1000 Ma. These results indicate that the intrusion probably has an age of ~250
Ma, but contains abundant inherited grains. The pattern of ages obtained by the CL-
guided analyses is congruent with the larger Aitchison and Ireland (1995) data set
from both the Town Beach and Tacking Point samples (Figure 8).
A small sample (<500 g) was taken from a thin, felsic sheet cutting the
Tacking Point Gabbro and had a low zircon yield of 30 grains. The grains are all
somewhat rounded or have been fragmented in mineral separation (Figure 6). Nine
analyses were undertaken on 7 least recrystallised grains. All have high Th/U ratios,
indicating igneous origins, and 5 have ages of 360-330 Ma, with the remaining 2
grains being Precambrian (Figure 7). None of the few grains recovered from this
sample appear to be euhedral magmatic grains, and thus the 360-330 Ma grains are
initially interpreted as the youngest inherited component.
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5. Whole rock geochemistry
5.1. XRF analytical methods
Fifteen representative gabbro samples were collected from the Tacking Point
Gabbro and cross-cutting felsic-mafic dykes (Table 1) and five samples of Watonga
Formation basalt. Samples were crushed using a TEMA chromium ring mill. Fused
buttons were made for X-ray fluorescence (XRF) major element analysis. Depending
on elemental concentrations estimated in trace element analysis, different types of
flux were used. Pure metaborate was used for high silica samples, 57% tetraborate to
43% metaborate was used for ultramafic samples, and 12% tetraborate to 22%
metaborate was used for mafic samples. 400 mg of sample was added to each flux
(300 mg for pure metaborate). Samples JP22, JP35 and JP44 were oxidised before
being fused in the furnace by adding 5 ml of lithium nitrate solution and left at 60°C
overnight. Press pellets for trace element analysis were created by mixing ~5 g of
sample with a polyvinyl acetate (PVA) binder and pressed into an aluminium cup
using a hydraulic hand press. Trace element pressed pellets were then oven dried at
60°C for 12 hours. Whole rock geochemical analysis was conducted using a
SPECTRO XEPOS energy dispersive polarisation X-ray fluorescence spectrometer at
the University of Wollongong.
A Niton 3XLt GOLDD+ handheld XRF was used to collect trace element
compositions of igneous rocks in the field to compliment the XRF data obtained from
crushed samples outlined above. Detection limits vary according to the sample
medium but for the trace elements used in tectonic discrimation plots of Figure 9 the
detection limits were; V (~40ppm), Ti (~60ppm), Zr (~3ppm) and Sr (~2ppm) which
was sufficient to discriminate using those diagrams. The analysis window of the Niton
XRF is ~6 mm making it suitable for fine-grained volcanic rocks such as the Watonga
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Formation basalts. For coarse-grained gabbros, as at Tacking Point, a single analysis
consisted of the average of three separate analyses within a 10 cm diameter. The
handheld XRF trace element data is included in Table 1 and in geochemical plots
(Figure 9) but represented as a hollow symbol rather than the solid symbols for the
standard laboratory XRF data.
5.2. Tacking Point Gabbro
The Tacking Point gabbros have a compositional range from gabbro to quartz-
dioritic compositions with SiO2 ranging from 40-49 wt.% (Table 1). They are
characterised by variable but typically high MgO (6-23 wt%), Ni (12-828 ppm) and
Cr (11-3000 ppm) concentrations. Major element compositions indicate the gabbros
are typical of a high-Mg tholeiitic suite which is also reflected in V, TiO2 and P2O5
concentrations which show a negative correlation with Mg#, suggesting increased
compatibility during fractionation (Parker, 2010). Ni, Co and Cr show a positive
correlation with Mg# which is consistent with fractional crystallisation involving
olivine and pyroxene. The high-field strength element concentrations (Nb, Zr, Ga and
Y) are low to moderate and according to various classification schemes (Pearce, 1982;
Pearce and Cann, 1973; Shervais, 1982) the Tacking Point gabbros plot well within
the typical range of island arc igneous rocks (Figure 9).
5.3. Watonga Formation basalts
Basalts collected from the Watonga Formation at Town Beach, Flynns Beach
and pendants within the Tacking Point Gabbro plot as mostly basalts to basaltic
andesites with silica values usually between 45-50 wt.% but up to 62 wt.% which
probably represent locallised alteration and silicification. The various basaltic
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discrimination diagrams (Figure 9) show that they plot in the MORB to within plate
basalt field with the exception of the Ordovician pillow basalt at Town Beach
interbedded with the volcaniclastic sandstone. It plots closer to the island-arc tholeiite
field.
5.4. Post-serpentinite mafic-felsic dykes
Barron et al. (1976) described as lamprohyric the earliest mafic intrusions
common to all basement lithologies along the Port Macquarie Town Beach to
Tacking Point tract. However, subsequent geochemical studies point to a more
boninitic affinity for similar intrusions at Tacking Point (Aitchison et al. (1994), see
analysis TP1 in Table 1). These magnesian, low titanium mafic rocks are intimately
associated with the late felsic intrusions described here (see Figure 3D). The mafic
intrusions of boninitic affinity, hint at a convergent plate boundary setting for their
formation, however the wide spread of inherited zircons within the associated felsic
sheets indicates contamination from Gondwana-derived Tablelands Complex.
Tectonic discrimination plots of a limited number of samples (Figure 9) are
inconclusive with respect to the origins of the mafic-felsic dykes intruded into
serpentinite and more work on these is needed to determine the source and
mechanisms of Permo-Triassic magma formation at Port Macquarie.
6. Discussion
6.1.Redefining the Watonga Formation
The Green Mound sedimentary rocks ascribed to the Watonga Formation, are arc-
derived greywackes (Parker, 2010). They contain euhedral, 355-335 Ma zircons of
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igneous appearance likely derived from a Carboniferous arc. They also contain
generally more rounded Early Paleozoic and Precambrian zircons. This is the age
signature seen in the inherited zircon populations of the felsic sheets dated by
Aitchison and Ireland (1995). The zircon age spectrum in all these samples matches
that of Carboniferous arc-derived subduction complex assemblages throughout the
New England Orogen (Korsch et al., 2009). This links these clastic sedimentary
rocks, previously assigned to the Watonga Formation, with a regional sequence of
Carboniferous arc-related rocks – the Anaiwan terrane or more broadly the
“Tablelands Complex”, which includes anything east of the Peel-Manning Fault. As
pointed out by several researchers (e.g., (Cawood et al., 2011a; Cawood et al., 2011b)
and references therein) the Tablelands Complex developed in response to continental
‘Andean-style’ convergence which gave rise to an extensive Carboniferous
accretionary complex within the New England Orogen.
Cherts of the Watonga Formation contain Upper Ordovician conodonts (Och et
al., 2007b) and Devonian? radiolarians (Ishiga et al., 1988) and thus are clearly older
than the Green Mound (Carboniferous) volcaniclastic rocks. The extended age range
of cherts within the Watonga Formation is typical of accretionary complexes in which
progressively younger oceanic material has been off-scraped over the life-time of the
subduction complex. Furthermore, a block of pillow lava and attached volcaniclastic
sandstone within the serpentinite at Town Beach has a Late Ordovician age of 455±10
Ma, contrasting with the nearby Green Mound volcaniclastic sandstone with a
depositional age of ≤335 Ma. Kimbrough et al. (1993) reported a U-Pb zircon age of
436±9 Ma from the Pola Fogal hornblende-tonalite suite associated with serpentinites
in the Piga Barney area (Figure 1), at the southern end of the Peel Manning fault
system. This age is close to that of the 455±10 Ma arc-related volcaniclastic
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sandstone block in the serpentinite at Town Beach and hints at the presence of
Ordovician island-arc material within serpentinite mélange of the NEO. Thus
combining the U-Pb zircon dating and paleontological studies, the Watonga
Formation appears to embrace rocks of different age (455 to ≤330 Ma) and
provenance. Most of the Watonga Formation basalts that appear to underlie the cherts
plot as MORB or intra-plate basalts with occasional island-arc tholeiites. This, along
with an absence of terrigenous material interbedded with the cherts, indicates that the
Watonga Formation represents oceanic crust that was off-scraped into a sediment-
starved trench far from any continental sediment source. We suggest that the
definition of the Watonga Formation should be refined to include only the
Ordovician-Devonian? basalt-chert-siltstone associations, and thereby it can be
correlated with Djungati terrane rocks further west (Figure 1). Consequently,
Carboniferous volcaniclastic sandstones such as at Green Mound should be excluded
from the Watonga Formation, and represent either a distinct Carboniferous overlap
assemblage or accretionary complex, that can be broadly correlated with the Anaiwan
terrane (Tablelands Complex).
6.2.Tacking Point Gabbro
The Tacking Point Gabbro with a U-Pb zircon age of 390±7 Ma is Middle
Devonian. It was correlated with the Permian Clarence River Supersuite based on
geochemical similarities before any isotopic geochronology was available (Och et al.,
2007a). Instead, its age agrees within error with a 377±8 Ma U-Pb igneous zircon age
for Yarras Complex plagiogranite/tonalite (Aitchison and Ireland, 1995), ca. 60 km
west of Port Macquarie (Figure 1). As with the Tacking Point Gabbro, the Yarras
Complex is proximal to a strand of serpentinite mélange in the New England Orogen.
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The igneous age of the Tacking Point Gabbro is also close to the 394±5 Ma K-Ar age
(Sano et al., 2004) for igneous hornblende in an arc-affinity diorite block in
serpentinite of the Peel Manning fault system at Glenrock Station, ~140 km west of
Port Macquarie (Figure 1).
6.3. Ages of post-serpentinite mafic-felsic intrusions and the tectonic history of the
Port Macquarie serpentinite
The Port Macquarie area late felsic intrusions dated by Aitchison and Ireland
(1995) and studied further in this paper have an age of ~250 Ma. With their bulk
compositions (Table 1 and Figure 9) and their abundance of inherited zircons, they
are clearly not ophiolitic plagiogranites, but are low temperature partial melts
incorporating sedimentary sources. Their inheritance pattern matches that of siliceous
varieties of Carboniferous arc-related sedimentary rocks throughout the New England
Orogen (Korsch et al., 2009), including the Green Mound sample 21-02 presented
here (Figure 8). At least some of these felsic sheets were emplaced into gently-
inclined extensional fractures and are coeval with mafic mantle-derived magma
(Figure 3C). Furthermore, in the Port Macquarie serpentinite, some of these sheets
show evidence of having been sheared by subsequent movement within the
serpentinite. This indicates that ductile deformation of the serpentinite continued until
≤250 Ma. This reaffirms conclusions of Aitchison et al. (1994) that tectonic
movements in the serpentinite continued until ≤250 Ma.
The main serpentinite belts further west in the New England Orogen are all
associated with, or contain tectonic blocks of Cambrian ophiolite (Weraerai terrane),
Ordovician to Devonian island arc-related assemblages (Gamilaroi terrane) and
Ordovician to Devonian basalt-chert accretionary complex rocks (Djungati terrane).
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The Upper Carboniferous to Upper Permian zircon ages obtained for the Port
Macquarie eclogites by Nutman et al. (2013) reveal that eclogite-blueschist
metamorphism is much younger than the Ordovician K-Ar phengite ages originally
obtained by Fukui et al. (1995). This is in accord with final serpentinite emplacement
occurring at ≤250 Ma, as revealed by the U-Pb zircon dating of post-serpentinite
felsic dykes. U-Pb ages and REE chemistry of zircons from an eclogite occurrence at
Attunga (Buckman and Nutman, in prep.) indicates an upper Cambrian (~490 Ma)
age for high-pressure metamorphism in contrast to the Neoproterozoic age of
Watanabe et al. (1999). Thus the proposed Cambro-Ordovician or even late
Neoproterozoic age (Phillips and Offler, 2011; Watanabe et al., 1999) for HP/LT
metamorphism and serpentinite formation followed by prolonged (~200 Ma)
residence of HP-LT metamorphic rocks in the upper crust is untenable. These ages
indicate there is probably more than one episode of HP metamorphism preserved in
the NEO and raise serious doubts as to the validity of interpretation of previous K-Ar
ages obtained from micas. A complete re-evaluation of the tectonic history of the
New England Orogen is warranted.
6.4. Regional settings and correlations in the New England Orogen
To the west, the Port Macquarie Block is in faulted contact with the Devonian to
Carboniferous Birpai terrane (Flood and Aitchison, 1988) or Hastings terrane
(Roberts et al., 1993), which include rocks of the Yarras Complex. Following the
methodology associated with tectonostratigraphy and the discrimination of terranes,
Flood and Aitchison (1988) separated the Port Macquarie Block as a separate
tectonostratigraphic unit (the Ngamba terrane) because it could not be directly
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correlated with terranes found further west in the NEO. However our results suggest
linkages can be made.
The Tacking Point gabbro is Middle Devonian in age (390±7 Ma) and has island
arc geochemical characteristics. This suggests that it is equivalent to plutonic rocks of
the Gamilaroi terrane found further west (Figure 1). This is in accord with its
intrusion into Ordovician-Silurian oceanic cherts of similar age to those seen in the
Djungati terrane. Basalts and dolerites of the Yarras Complex display typical island
arc tholeiite characteristics (Aitchison et al., 1994) and SHRIMP U-Pb dating of
zircons from a trondhjemite intruding gabbro indicates a Late Devonian (374±8 Ma)
age. This is consistent with the nature and timing of arc-rift basalts of the Gamilaroi
terrane (Aitchison and Flood, 1994) with which the Birpai terrane can most likely be
correlated (Aitchison et al., 1994). Again, there appears to be enough similarities in
age and composition to group rocks of the Birpai terrane (Yarras Complex) with the
Gamilaroi terrane (Aitchison et al., 1999). Thus the Ngamba (Port Macquarie Block)
and Birpai (Yarras Complex) terranes are possible correlatives with the Gamilaroi
terrane and if so should be grouped together accordingly. If correct, this correlation
simplifies the nomenclature and understanding of older components in the NEO
(Figure 1).
The intrusive relationship of the Tacking Point Gabbro into the Watonga
Formation demonstrates a spatial relationship between a Devonian island-arc complex
and an adjacent accretionary complex, which had not previously been recognized. We
tentatively suggest that the Tacking Point Gabbro correlates with the Gamilaroi
terrane and the Watonga Formation with the Djungati terrane elsewhere in the NEO
based on the similarities in age and composition. The east-dipping polarity of
subduction beneath the Gamilaroi terrane initially proposed by Aitchison and Flood
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(1994) and further corroborated by Offler and Gamble (2002), provides a valid
mechanism of accreting and preserving an intra-oceanic island arc into an upper plate
position. We suggest that slab-rollback beneath the evolving Gamilaroi terrane could
explain the migration of the active portion of the island-arc (Tacking Point Gabbro)
into an older portion of the accretionary wedge (Watonga Formation or Djungati
terrane) as illustrated in Figure 10B.
From Green Mound, we identify fragments of Carboniferous volcaniclastic
sedimentary rocks associated with the serpentinite mélange at Port Macquarie. The
Green Mound rocks are similar to units broadly correlated with the Carboniferous
Tablelands Complex (Fergusson, 1984) to the north, or correlatives such as the
Anaiwan terrane (or Yarrowitch Block - (Leitch et al., 1990) to the west, and the
Myall Block to the south (Roberts and Engel, 1987; Skilbeck and Cawood, 1994). As
with the Hastings Block, a distinct change in provenance occurs between the
predominantly basaltic Silurian-Devonian units to andesitic to felsic provenance at the
Devonian-Carboniferous boundary (Roberts et al., 1995). The first evidence of
Gamilaroi terrane arrival at the margin of Gondwana is the observation made by
Flood and Aitchison (1992) of Lachlan-derived quartzite clasts within the latest
Devonian Keepit Conglomerate which overlies the youngest unit in the Gamilaroi
terrane - the Baldwin Formation. This is consistent with a switch from an oceanic
island arc to a continental arc tectonic setting (Figure 10C-D).
In terms of the overall northward displacement of the Port Macquarie serpentinite
relative to the Weraerai terrane bodies at the southern end of the Peel Fault system,
Cawood and Leitch (1985) suggested that the Hastings (Birpai) terrane was once
connected with the southern end of the Tamworth belt (Gamilaroi terrane) and
Roberts et al. (1993) suggested the block underwent 130° dextral rotation and
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northward displacement via sinistral faulting. Collins et al. (1993) further developed
this model to include a phase of oroclinal bending to accompany the sinistral faulting.
Skilbeck and Cawood (1994) reverted to a model involving syn-subduction, dextral
strike-slip activity along an irregular continental margin during the Late Devonian to
Carboniferous in order to shift the Tamworth Belt from an intra-oceanic position
outboard of the Gondwana continental margin to be juxtaposed with the margin. The
underlying assumption with all of these models is that the dislocation is due to lateral,
horizontal displacements alone. Here we introduce a new hypothesis to explain
apparent oroclines within the New England Orogen involving vertical rather than
lateral displacements. We follow on from the model of Aitchison and Flood (1994)
which proposed that the exotic, Panthalassan, Cambrian-Devonian terranes
(Gamilaroi + Djungati + Weraerai) were accreted westward over the Gondwanan
continental margin by the latest Devonian (Figure 10A-C). Following the collision
and flip in subduction polarity in the latest Devonian, eastern Gondwana developed as
a continental ‘Andean’-style convergent margin (Figure 10D). Subsequently an
accretionary complex (Anaiwan terrane) grew on top of the previously obducted
Weraerai + Gamilaroi + Djungati terranes throughout the Carboniferous. The
collision of the Permo-Triassic Gympie terrane (Figure 10E-F) initiated the Hunter-
Bowen Orogeny at about the Permo-Triassic boundary (~251 Ma) (Aitchison and
Buckman, 2012; Harrington and Korsch, 1985; Nutman et al., 2013). Although the
Gympie terrane only crops out along a small portion of the Australian eastern
seaboard it has been correlated with similar age and composition rocks of island arc
affinity in both New Zealand and New Caledonia (Aitchison, 1993; Spandler et al.,
2005). This suggests that the Gympie terrane was laterally extensive and that the
collision and accretion of this island-arc was a major tectonic event that was driving
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mechanism for the Hunter-Bowen Orogeny. We propose that the Hunter-Bowen
compressional event is responsible for exhuming portions of the Weraerai +
Gamilaroi + Djungati terranes from under their Carboniferous carapace, along thrust
zones such as the one observed at Town Beach. Thus, the northward (sinistral)
displacement of the Port Macquarie and Hastings Blocks and the apparently
contradictory dextral displacement of the Coffs Harbour Block, is apparent only, and
due more in part to vertical displacements of an extensive, thin-skinned sheet of
oceanic plus island arc crust that underlies the Tablelands Complex, rather than
extensive lateral movements with the opposite sense of motion. In this way there is no
need to invoke large-scale ‘oroclinal’ folding (Cawood et al., 2011; Li and
Rosenbaum, in press; Offler and Foster, 2008; Rosenbaum et al., 2012) or significant
sinistral faulting (Schmidt et al., 1994) to explain the repetition of Hastings and Port
Macquarie blocks in the southern New England. Indeed, a simple geological section
across the postulated Texas or Manning oroclines would show that the steeply dipping
limbs several hundred kilometres either side of the megafold axis must extend to a
depth of many hundreds of kilometres. Given that the maximum thickness of
continental crust is ~70 km beneath continental collision zones, these oroclines must
be detached along a low-angle décollement rather than be a simple series of mega-
synclines and anticlines extending several hundreds of kilometres below the surface.
The orocline concept was initially proposed (Korsch, 1981; Korsch and Harrington,
1987) to explain apparent large scale bending of lineaments observed in regional
magnetic surveys. However, strong geological evidence for such a mega-fold is
lacking and the entire concept of an orocline in the NEO raises many questions as
raised by Lennox et al. (2013). One critical question revolves around a valid
mechanism for oroclinal folding. In the Himalaya there is an obvious mechanism of
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collision of two continents with an irregular continental margin boundary to produce
the large indent and oroclinal mountain belts either side of the Indian continent. No
such continent-continent collision has occurred within the New England or Lachlan
orogens. For these reasons, we look to a simpler, alternative and testable model that
can be supported by modern-day analogues.
Our results support the interpretation of Aitchison and Buckman (2012), that the
eastern margin of Gondwana grew by a combination of; (a) the addition of exotic
island arc complexes (i.e. the exotic Macquarie Arc in the latest Ordovician to Early
Silurian the Gamilaroi terrane in the latest Devonian and finally the Gympie terrane at
about the Permo-Triassic boundary) plus; (b) coeval or interspersed Andean-style arcs
at the continental margin. Each island-arc collision is associated with a major
contractional deformation event – the Benambran, Kanimblan and Hunter-Bowen
orogenies respectively. A switch in subduction polarity follows arc accretion and this
marks the onset of classic continental-margin accretionary orogenesis and the
intrusion of voluminous S- and I-type granites.
This style of episodic continental growth via arc-continent collisions is observed
in Phanerozoic systems, such as, Taiwan (Huang et al., 2008), Oman (Searle et al.,
2004), the ancient southern margin of Tethys in Ladakh (Corfield et al., 1999), and
Cyprus (Robertson, 2004) or on the Kamchatka Peninsula in the NW Pacific
(Hourigan et al., 2009). In these examples, intra-oceanic island arcs are drawn
towards continental margins via the consumption of intervening oceanic crust and
emplaced onto continents due to their position on the overriding plate. Evidence of
supra-subduction zone rocks in upper plate positions that have over-ridden continental
margins are recorded not only in the Phanerozoic but also within Precambian terranes
such as the Paleoproterozoic Nagssugtoqidian Orogen, Greenland (Kalsbeek and
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Nutman, 1996; Nutman et al., 2008). Obducted intra-oceanic arc systems are thin-
skinned or rootless systems commonly accompanied by a basal melange structurally
overlying a continental margin sequence. Oblique convergence can result in lateral
translations and duplexing of portions of these tectonic packages (McCaffrey, 1992).
Widespread strike-slip faulting during the Early Permian was introduced by Aitchison
and Flood (1992) to help explain the development of strike-slip basin deposits
(Manning Group) along the Peel-Manning Fault System. Strike-slip faulting may have
some influence on the apparent northward (sinistral) translation of the Hastings and
Port Macquarie blocks (Manning orocline) in the southern NEO but it is at odds with
the large-scale dextral Demon Fault associated with the Texas orocline to the north.
This structural contradiction is difficult to reconcile via oroclinal bending or strike-
slip faulting alone. However, compression and vertical movements along thrust faults
can result in complex, doubly plunging outcrop patterns that have an apparent
opposite sense of displacement in plan view. We suggest that the Weraerai, Gamilaroi
and Djungati terranes are thin-skinned terranes overlain by a Carboniferous to
Permian accretionary complex but in places like Port Macquarie and Coffs Harbour
these terranes have been exhumed during the Hunter-Bowen Orogeny and the
Carboniferous carapace eroded away.
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7. Conclusions
(1) The Tacking Point Gabbro has a middle Devonian age of 390±7 Ma and
geochemical affinities with intra-oceanic arc igneous suites. Therefore, along with its
chert and basalt country rocks, it represents a mid-Paleozoic assemblage
allochthonous to Gondwana (Gamilaroi + Djungati terrane affinity).
(2) The Watonga Formation, as it is currently defined, contains rocks of different age
and origin including an Ordovician, MORB-like basalt-chert-volcaniclastic
association to a Carboniferous volcaniclastic sequence. We conclude that the
Watonga chert-basalt associations are older than, and unrelated to, the Carboniferous
arc-derived volcaniclastic rocks like those at Green Mound. We propose that the name
Watonga Formation should be reserved for only the older chert + basalt components
while the Carboniferous Green Mound rocks be grouped with the Tablelands
Complex (Anaiwan terrane).
(3) Felsic dykes intruded the Port Macquarie serpentinite at 247±20 Ma. These post-
date serpentinite melange emplacement but are late- to syn-kinematic. This, along
with the revised younger Upper Carboniferous to Upper Permian age (probably
≤251±6 Ma) of the Port Macquarie eclogites (Nutman et al., 2013) indicates that the
tectonic evolution of the serpentinite at Port Macquarie continued into the early
Triassic during the Hunter-Bowen Orogeny.
(4) The accrued data enable correlations between the Port Macquarie Block and the
more extensive terranes of the NEO further west. We propose that the NEO contains a
large, far-travelled oceanic and island arc terrane that was emplaced over the
Gondwanan margin during the latest Devonian to Early Carboniferous.
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(5) It is likely that HP-LT metamorphism and serpentinite emplacement at Port
Macquarie and in the Great Serpentinite Belt to the west occurred at different times
and are unrelated.
Acknowledgments
Support to S. Buckman and A.P. Nutman was via the University of Wollongong
GeoQuEST Research Centre, a Japan travel grant to Allen Nutman from the
Australian Academy of Sciences, the University of Hiroshima, University of
Wollongong support for Honours students J. Parker, T. Line and S. Bembrick. We
thank S. Meffre and P-F. Li for their constructive comments in the review process.
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Table Captions
Table 1 Whole rock XRF geochemical analyses of Port Macquarie rocks. Date
sources: Parker (2010).
Table 2 SHRIMP U-Th-Pb summary data.
Figure captions
Figure 1 Geology of the New England Orogen grouping units into the terrane scheme
of Aitchison and Flood (1990) and showing the location of the Port Macquarie study
area.
Figure 2 Geological outcrop map of the Port Macquarie Town Beach to Tacking Point
coastal tract showing the occurrence of major lithologies. Inset 1 shows the detailed
geological mapping of the Town Beach/Green Mound area.
Figure 3 Field relations showing:
A) Layered peridotite and pillow basalt blocks within the serpentinite mélange at
Town Beach. The pillow basalt block is interbedded with interpillow volcaniclastic
material ascribed to the Watonga Formation from which zircons were extracted
(sample 21-01, 152.92256°E, 31.42973°S).
B) Volaniclastic sandstone (21-02) taken from Green Mount Point east of Town
Beach (152.92395°E, 31.43041°S).
C) Mafic-felsic sheet cutting the Tacking Point Gabbro which itself is cut by dolerite
dykes (31°28'24" S 152°56'15" E).
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D) Tacking Point gabbro showing evidence of magmatic mingling between coarse-
grained leucocratic- and melanocratic-gabbro components (31.473544°S
152°937352). Note the younger (~250 Ma) composite mafic-felsic sheet at the top of
the outcrop.
E) Highly deformed chert of the Watonga Formation forming inclusions within the
Tacking Point Gabbro (-31.474043°S 152.937351°). These attest to pre-Devonian
deformation, probably during accretion.
F) Pegmatitic gabbro patches within the Tacking Point Gabbro similar to the sample
from which Devonian zircons were extracted (TP208/JP44; -31.472677°S
152.937647°).
G) Felsic intrusion cutting through the schistose serpentinite mélange which is itself
sheared, indicating syn-kinematic emplacement with the serpentinite (-31.433160°S
152.924380°).
H) A low-angle mylonitic shear zone (looking south) on the contact between the
Town Beach serpentinite mélange to the west and cherts of the Watonga Formation to
the east. S-C fabrics show a clear west over east sense of motion suggesting the
serpentinite mélange has been thrust over the Carboniferous volcaniclastic rocks
found to the east at Green Mound.
Figure 4 Cathodoluminescence images of zircons extracted from (A) volcano-
sedimentary block 21-01 within the Port Macquarie serpentinite mélange and (B)
from Green Mound sedimentary rock 21-02.
Figure 5 238
U/206
Pb – 207
Pb/206
Pb (radiogenic) plots, with analytical errors depicted at
the 2 σ level. (A) Sample 21-01 and (B) sample 21-02.
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Figure 6 Cathodoluminescence images of zircons extracted from (A) Tacking point
Gabbro sample TP, (B) Town Beach late felsic sheet sample ‘Town Beach’ and (C)
Tacking Point late felsic sheet sample 12-01.
Figure 7 238
U/206
Pb – 207
Pb/206
Pb (radiogenic) plots, with analytical errors depicted at
the 2 σ level. (A) Tacking Point Gabbro sample TP, (B) late felsic sheet sample
‘Town Beach’ and (C) late felsic sheet sample 12-01.
Figure 8 Zircon age histograms for regional New England Orogen quartz rich ‘broken
formation’ samples (n=815 from (Korsch et al., 2009)), Green Mound sample 21-02
and for the Port Macquarie and Tacking Point felsic igneous rocks, using data
presented here and by Aitchison and Ireland (1995).
Figure 9 A) Ti/V diagram of Shervais (1982); B) Zr-Ti diagram of Pearce (1982); C)
Ti-Zr-Sr and D) Ti/Zr diagrams of Pearce and Cann (1973). Data sourced from Table
1 (Parker, 2010) and Aitchison et al. (1994). *Portable XRF trace element data shown
in Appendix A.
Figure 10 A schematic tectonic reconstruction of the eastern margin of Gondwana
from the Cambrian to the Triassic highlighting the episodic nature of island arc
collision events.
A) Ordovician initiation of island arc development (possible proto-Gamilaroi terrane)
on top of Late Cambrian ophiolitic basement (possible Weraerai terrane). During the
Early Silurian eastern Gondwana is experiencing widespread deformation
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(Benambran Orogeny) associated with collision of the Macquarie Arc (Aitchison and
Buckman, 2012).
B) Active island arc development in the Gamilaroi terrane including the intrusion of
the Tacking Point Gabbro into the Ordovician cherts of the Watonga Formation.
During the same time (Upper Silurian to Middle Devonian) the continental margin of
Gondwana is experiencing widespread intrusion of voluminous S- and I-type granites
indicating that subduction was occurring contemporaneously beneath eastern
Gondwana and the Gamilaroi terrane. While the polarity of subduction beneath
Gondwana could only have been to the west, subduction beneath the Gamilaroi
terrane is not fully understood, however it’s preservation in an upper plate position in
the NEO suggests it was situated on the overriding plate and therefore subduction was
to the east beneath it.
C) The west dipping subduction zone beneath Gondwana stalls by the Middle
Devonian as the Gamilaroi terrane arrives at the Gondwana margin, thus switching off
production of granites in the Lachlan Orogen. Accretion of the Gamilaroi terrane onto
Gondwana in the latest Devonian results in a subduction flip and a brief continental
magmatic hiatus (Late Devonian to Early Carboniferous).
D) An Andean-type continental margin developed along eastern Gondwana
throughout the Carboniferous. The magmatic core of the continental arc is marked by
the Bathurst Granite suite within the eastern Lachlan Orogen and the felsic volcanics
of the Currabubula-Connors-Auburn arc and Tamworth Belt (fore arc basin) in the
western NEO. The Tablelands Complex develops as a continental accretionary
complex dominated by volcanic detritus shed off the Carboniferous continental arc
but with a significant Gondwanan inheritance.
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E) The island arc Gympie terrane docks onto Gondwana at about the Permo-Triassic
boundary initiating the Hunter-Bowen Orogeny and thrusting elements of the NEO
over the Permo-Triassic Sydeny Basin and possibly emplacing the serpentinite
mélange and associated eclogites at Port Macquarie.
F) Another subduction flip follows the Hunter-Bowen Orogeny resulting in another
eastward magmatic jump before the continental subduction zone resulted in
emplacement of the Triassic coastal suite granites (220-210 Ma).
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Table 1. Whole rock XRF geochemical analyses of Port Macquarie rocks. Data sources: Parker (2010).
Sample# JP40 JP39 JP21 JP15 0821-01 JP44 JP17 JP23 JP24 JP32 JP34
Lat. S 152.9267 152.9266 152.9370 152.9256 152.9226 152.9376 152.9376 152.9376 152.9376 152.9376 152.9376
Long. E -31.4407 -31.4411 -31.4748 -31.4378 -31.4297 -31.4727 -31.4727 -31.4727 -31.4727 -31.4727 -31.4727
Location
Flynns
B.
Flynns
B.
Flynns
B.
Flynns
B.
Flynns B.
Tacking
Pt.
Tacking
Pt.
Tacking
Pt.
Tacking
Pt.
Tacking
Pt.
Tacking
Pt.
Rocktype
Wat. Fm.
Basalt
Wat. Fm.
Basalt
Wat. Fm.
Basalt
Wat. Fm.
Basalt
Wat. Fm.
volcanicl. Gabbro Gabbro Gabbro Gabbro Gabbro Gabbro
SiO2 45.6 46.4 48.7 61.8 50.5 39.4 41.6 43.1 44.4 44.9 45.3
TiO2 0.9 1.9 1.4 0.9 0.7 1.8 0.2 0.3 0.2 0.5 0.5
Al2O3 17.2 15.0 17.9 13.9 14.1 13.9 17.0 11.3 14.3 12.4 18.9
Fe2O3 9.0 13.0 9.3 9.5 8.7 20.1 9.0 9.6 7.8 10.1 8.1
MnO 0.2 0.2 0.2 0.1 0.1 0.3 0.2 0.2 0.1 0.2 0.1
MgO 6.8 7.5 7.1 3.2 3.5 6.4 12.3 16.7 14.5 14.6 6.9
CaO 11.8 8.0 10.0 1.2 9.1 9.4 12.3 13.1 10.9 9.6 13.8
Na2O 2.5 3.7 3.4 5.5 6.3 1.6 1.0 0.2 0.7 1.3 2.0
K2O 0.3 0.4 0.9 0.3 0.2 0.7 0.4 0.0 0.6 0.9 1.3
P2O5 0.1 0.2 0.2 0.2 0.2 0.1 0.0 0.1 0.1 0.2 0.1
SO3 0.1 0.3 0.1 0.9 0.1 1.5 0.0 0.0 0.0 0.0 0.1
LOI 5.8 4.1 2.3 2.9 6.7 3.9 3.7 4.9 3.5 2.9 4.0
TOT. 100.1 100.6 101.2 100.3 100.2 99.1 97.7 99.6 97.3 97.8 101.1
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S <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1
Cl 178.0 234.0 402.0 174.0 39.0 158.1 42.3 25.0 23.6 60.1 59.8
V 234.0 382.0 269.0 262.0 243.0 966.0 122.0 164.0 137.0 210.0 334.0
Cr 331.0 231.0 328.0 64.0 73.0 11.0 760.0 1250.0 957.0 1381.0 203.0
Co 55.0 58.0 45.0 35.0 21.0 < 3.0 68.0 105.0 80.0 85.0 40.0
Ni 78.0 61.0 56.0 52.0 15.0 12.0 269.0 450.0 366.0 410.0 45.0
Cu 88.0 80.0 45.0 137.0 292.0 231.0 16.0 8.0 11.0 17.0 100.0
Zn 57.0 99.0 55.0 108.0 58.0 116.0 64.0 55.0 46.0 75.0 50.0
Ga 11.0 16.0 15.0 14.0 12.0 19.0 12.0 9.0 9.0 12.0 15.0
As < 1 2.0 < 1 14.0 < 1 4.0 1.0 < 0.5 < 0.2 1.0 0.0
Br < 1 < 1 1.0 1.0 < 1 1.0 0.0 < 0.5 < 0.5 < 0.5 < 0.5
Rb 4.0 6.0 17.0 4.0 2.0 19.0 9.0 1.0 17.0 21.0 28.0
Sr 101.0 77.0 260.0 87.0 913.0 268.0 355.0 204.0 219.0 264.0 445.0
Y 20.0 33.0 22.0 24.0 19.0 14.0 7.0 7.0 5.0 10.0 9.0
Zr 51.0 121.0 84.0 92.0 61.0 25.0 < 1.0 15.0 9.0 18.0 19.0
Nb 3.0 9.0 8.0 3.0 2.0 2.0 < 0.2 1.0 0.0 1.0 1.0
Mo < 1 < 1 < 1 < 1 < 1 <1 <1 <1 <1 <1 <1
Sn < 3 < 3 < 3 < 3 < 3 < 3.0 < 3.0 0.0 2.0 < 3.0 0.0
Sb < 3 < 3 < 3 < 3 < 3 <1 <1 <1 <1 <1 <1
Cs < 3 < 3 < 3 < 3 < 3 <1 <1 <1 <1 <1 <1
Ba 38.0 57.0 270.0 62.0 115.0 128.0 76.0 13.0 91.0 210.0 173.0
La < 2 12.0 < 2 24.0 19.0 < 2.0 12.0 16.0 11.0 < 2.0 14.0
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Ce < 2 16.0 < 2 < 2 15.0 < 2.0 < 2.0 < 2.0 14.0 < 2.0 < 2.0
Hf 4.0 5.0 3.0 4.0 0.0 <1 <1 <1 <1 <1 <1
Ta 6.0 5.0 3.0 9.0 27.0 2.0 < 1.5 4.0 4.0 < 1.0 < 1.0
W 72.0 54.0 70.0 69.0 2.0 2.0 < 1.0 2.0 < 1.0 0.0 1.0
Pb 2.0 1.0 1.0 6.0 8.0 3.0 3.0 2.0 1.0 0.0 2.0
Th < 1 1.3 < 1 1.4 2.1 < 1.0 < 1.0 1.4 < 1.0 0.7 < 1.0
U < 1 < 1 < 1 < 1 < 1 0.5 < 1.0 0.9 0.8 0.8 0.6
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Table 1 cont.
Sample# JP26 JP43 JP28 JP22 JP38 JP25 JP33 JP35 HONS240 JP36a JP36b
Lat. S 152.9376 152.9376 152.9376 152.9376 152.9376 152.9376 152.9376 152.9376 152.9237 152.9376 152.9376
Long. E -31.4727 -31.4727 -31.4727 -31.4727 -31.4727 -31.4727 -31.4727 -31.4727 -31.4314 -31.4727 -31.4727
Location
Tacking
Pt.
Tacking
Pt.
Tacking
Pt.
Tacking
Pt.
Tacking
Pt.
Tacking
Pt.
Tacking
Pt.
Tacking
Pt.
Tacking
Pt.
Tacking
Pt.
Tacking
Pt.
Rocktype Gabbro Gabbro Gabbro Gabbro Gabbro Gabbro Gabbro Gabbro
Post-serp.
bas. dyke
Post-serp.
bas. dyke
Post-serp.
fels. dyke
SiO2 45.4 45.6 45.8 46.2 46.6 46.7 46.8 48.8 40.7 52.0 70.9
TiO2 0.5 0.5 0.5 0.4 0.5 0.2 0.3 0.8 0.6 0.9 0.3
Al2O3 14.4 20.8 13.1 16.1 6.8 24.3 6.1 11.2 12.8 14.4 14.7
Fe2O3 7.9 7.1 8.8 7.7 10.8 3.4 8.8 9.0 10.5 9.4 2.4
MnO 0.2 0.1 0.2 0.1 0.2 0.1 0.1 0.2 0.5 0.2 0.0
MgO 10.7 6.2 12.2 9.1 15.8 5.5 22.5 8.6 16.2 7.1 0.8
CaO 9.8 14.0 9.1 11.8 15.1 11.9 8.9 10.9 14.2 7.2 3.5
Na2O 2.1 2.6 2.1 1.6 0.6 1.5 0.2 2.3 0.2 5.1 6.5
K2O 1.2 0.5 1.0 0.8 0.1 3.1 0.1 0.5 0.1 0.3 0.1
P2O5 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.3 0.2 0.3 0.1
SO3 0.0 0.5 0.0 1.2 0.0 0.1 0.0 0.4 0.0 0.0 0.0
LOI 1.0 4.4 1.9 4.1 4.1 2.5 2.9 2.9 5.1 4.5 4.4
TOT. 93.4 102.3 94.8 99.2 100.9 99.2 97.3 96.0 101.0 97.7 103.9
S <1 <1 <1 <1 <1 <1 <1 <1 23.2 <1 <1
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Cl 128.9 138.6 151.1 19.4 233.3 22.8 161.0 226.1 507.6 198.2 6.9
V 228.0 376.0 240.0 301.0 363.0 105.0 123.0 399.0 296.4 332.0 54.0
Cr 691.0 164.0 664.0 400.0 1124.0 268.0 2995.0 471.0 770.0 190.0 109.0
Co 54.0 14.0 56.0 46.0 90.0 14.0 130.0 < 3.0 61.8 37.0 < 3.0
Ni 287.0 46.0 241.0 122.0 234.0 72.0 828.0 157.0 200.9 68.0 9.0
Cu 64.0 91.0 54.0 146.0 142.0 12.0 64.0 1434.0 89.2 113.0 65.0
Zn 58.0 35.0 63.0 50.0 71.0 23.0 47.0 77.0 89.7 70.0 12.0
Ga 11.0 15.0 11.0 13.0 6.0 16.0 5.0 16.0 11.7 11.0 13.0
As 1.0 < 0.5 1.0 < 0.2 1.0 1.0 19.0 1.0 1.4 1.0 < 0.5
Br 1.0 1.0 0.0 < 0.5 < 0.5 0.0 < 0.5 1.0 <1 < 0.5 0.0
Rb 28.0 11.0 23.0 20.0 2.0 76.0 5.0 10.0 <1 2.0 1.0
Sr 532.0 621.0 478.0 356.0 69.0 470.0 28.0 535.0 804.1 253.0 338.0
Y 13.0 6.0 12.0 9.0 14.0 5.0 7.0 26.0 15.7 24.0 20.0
Zr 22.0 10.0 20.0 12.0 16.0 9.0 13.0 64.0 34.7 72.0 156.0
Nb 1.0 1.0 1.0 1.0 1.0 0.0 0.0 2.0 1.3 4.0 5.0
Mo <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1
Sn 1.0 1.0 1.0 1.0 < 3.0 3.0 2.0 1.0 <3 5.0 10.0
Sb <1 <1 <1 <1 <1 <1 <1 <1 <3 <1 <1
Cs <1 <1 <1 <1 <1 <1 <1 <1 <4 <1 <1
Ba 314.0 75.0 294.0 147.0 < 2.0 562.0 < 2.0 123.0 <2 48.0 47.0
La 10.0 < 2.0 < 2.0 < 2.0 < 2.0 6.0 < 2.0 < 2.0 8.4 < 2.0 18.0
Ce < 2.0 < 2.0 < 2.0 17.0 < 2.0 14.0 < 2.0 18.0 <2 34.0 29.0
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Hf <1 <1 <1 <1 <1 <1 <1 <1 1.3 <1 <1
Ta 56.0 1.0 < 1.0 2.0 6.0 < 1.0 5.0 < 1.0 <1 10.0 < 1.0
W 1.0 1.0 < 0.8 1.0 < 0.8 1.0 < 0.1 6.0 <1 1.0 3.0
Pb 2.0 5.0 2.0 2.0 0.0 2.0 1.0 4.0 <1 < 0.3 2.0
Th < 1.0 < 1.0 < 0.6 1.1 < 1.0 1.1 < 1.0 1.7 1.6 1.8 3.7
U 0.4 < 0.5 1.1 < 0.5 < 1.0 1.4 < 1.0 < 1.0 <1 0.7 0.6
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Table 2 SHRIMP U-Th-Pb summary data
Labels site U/ppm Th/ppm Th/U 204Pb/206Pb f206% 238U/206Pb 207Pb/206Pb age (Ma)* %disc
21-01 volcaniclastic block within serpentinite, Town Beach
1.1 e,osc,p 211 102 0.49 0.0004 ± 0.0003 0.672 14.052 ± 0.395 0.0518 ± 0.0047 443 ± 12
1.2 e,osc,p 176 58 0.33 <0.0001 ± 0.0011 0.002 13.405 ± 0.425 0.0563 ± 0.0166 464 ± 14
2.1 e,osc,p 561 243 0.43 0.0001 ± 0.0001 0.171 13.701 ± 0.327 0.0546 ± 0.0020 454 ± 10
2.2 e,osc,p 398 117 0.29 <0.0001 ± 0.0002 0.002 13.881 ± 0.484 0.0557 ± 0.0027 448 ± 15
3.1 e,osc/rex,p 120 37 0.31 0.0008 ± 0.0005 1.204 13.987 ± 0.526 0.0481 ± 0.0080 445 ± 16
4.1 e,osc,p 317 163 0.51 <0.0001 ± 0.0003 0.002 13.662 ± 0.538 0.0572 ± 0.0052 455 ± 17
4.2 e,osc,p 186 80 0.43 <0.0001 ± 0.0002 0.002 13.728 ± 0.430 0.0551 ± 0.0039 453 ± 14
21-02 volcaniclastic Watonga Formation, Green Point
1.1 e,osc,p 832 142 0.17 0.0001 ± 0.0001 0.114 14.959 ± 0.448 0.0555 ± 0.0014 417 ± 12
1.2 e,osc,p,fr 641 176 0.27 <0.0001 ± 0.0001 0.002 14.326 ± 0.262 0.0555 ± 0.0018 435 ± 8
2.1 e,osc,p 265 132 0.50 <0.0001 ± 0.0000 0.002 18.326 ± 0.504 0.0553 ± 0.0015 343 ± 9
2.2 e,osc,p,fr 311 248 0.80 <0.0001 ± 0.0000 0.002 18.011 ± 0.523 0.0553 ± 0.0020 348 ± 10
3.1 e,osc,p 810 621 0.77 0.0005 ± 0.0002 0.877 22.282 ± 0.927 0.0517 ± 0.0039 283 ± 12
3.2 m,osc,p 215 127 0.59 <0.0001 ± 0.0001 0.002 18.701 ± 0.637 0.0541 ± 0.0019 336 ± 11
4.1 e,osc,p 433 297 0.68 <0.0001 ± 0.0000 0.067 11.718 ± 0.267 0.0608 ± 0.0011 528 ± 12
5.1 m/c,hb,ov 274 225 0.82 <0.0001 ± 0.0000 0.002 9.203 ± 0.250 0.0619 ± 0.0012 665 ± 17
6.1 e,osc,p,fr 824 247 0.30 0.0004 ± 0.0000 0.635 6.324 ± 0.123 0.0738 ± 0.0010 1035 ± 27 -9
7.1 m/c,osc,p,fr 329 432 1.31 0.0002 ± 0.0002 0.387 17.679 ± 0.688 0.0496 ± 0.0031 355 ± 13
8.1 m,osc,p 202 198 0.98 0.0002 ± 0.0002 0.317 18.124 ± 0.625 0.0519 ± 0.0029 346 ± 12
9.1 e,osc,p 225 155 0.69 0.0002 ± 0.0002 0.323 15.100 ± 0.492 0.0533 ± 0.0032 413 ± 13
10.1 e,osc,p,fr 199 42 0.21 0.0001 ± 0.0000 0.096 11.303 ± 0.569 0.0581 ± 0.0025 546 ± 26
TP208 Tacking Point Gabbro tonalite, north of Tacking Point
1.1 m,osc,p 598 858 1.43 0.0001 ± 0.0000 0.152 16.381 ± 0.414 0.0526 ± 0.0012 382 ± 9
1.2 e,osc,p 458 580 1.27 <0.0001 ± 0.0000 0.011 15.985 ± 0.361 0.0535 ± 0.0014 391 ± 9
2.1 m,osc,p,fr 166 128 0.77 0.0002 ± 0.0004 0.246 15.950 ± 0.479 0.0523 ± 0.0062 392 ± 11
2.2 e,osc,p,fr 239 125 0.53 <0.0001 ± 0.0002 0.002 16.181 ± 0.497 0.0547 ± 0.0040 387 ± 12
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3.1 m,osc,p,fr 409 395 0.97 0.0001 ± 0.0001 0.194 16.187 ± 0.597 0.0532 ± 0.0020 386 ± 14
4.1 m,osc,eq,fr 130 68 0.52 <0.0001 ± 0.0001 0.034 15.920 ± 0.381 0.0581 ± 0.0021 393 ± 9
5.1 e,osc,p,fr 431 402 0.93 0.0002 ± 0.0001 0.354 16.219 ± 0.676 0.0528 ± 0.0020 386 ± 16
5.2 m,osc,p,fr 368 339 0.92 0.0000 ± 0.0001 0.002 16.178 ± 0.405 0.0552 ± 0.0018 387 ± 9
Town Beach' felsic sheet cutting gabbro in the serpentinite (extra CL-guided analyses of Aitchison and Ireland (1995)
sample) 1.1† r,osc,p 2268 3225 1.42 0.0071 ± 0.0003 13.106 32.450 ± 0.563 0.0492 ± 0.0059 196 ± 3
1.2† r,osc,p 984 723 0.73 0.0059 ± 0.0004 10.892 24.422 ± 0.678 0.0451 ± 0.0071 259 ± 7
1.3† c,osc,p 498 313 0.63 0.0024 ± 0.0002 4.449 19.027 ± 0.254 0.0527 ± 0.0032 330 ± 4
1.4† r,osc,p 843 726 0.86 0.0047 ± 0.0003 8.558 25.836 ± 0.486 0.0599 ± 0.0050 245 ± 5
2.1 r,h,ov 298 49 0.17 0.0001 ± 0.0001 0.148 12.176 ± 0.168 0.0561 ± 0.0014 509 ± 7
2.2 c,osc,ov 491 386 0.79 <0.0001 ± 0.0000 0.011 5.986 ± 0.137 0.0785 ± 0.0010 1159 ± 26 -14
2.3 r,h,ov 356 66 0.18 0.0005 ± 0.0002 0.915 12.273 ± 0.222 0.0567 ± 0.0025 505 ± 9
3.1 e,osc,fr 5 20 3.80 0.0024 ± 0.0007 4.504 1.712 ± 0.103 0.1247 ± 0.0140 2024 ± 214 47
4.1 m,osc,p 412 146 0.35 0.0001 ± 0.0000 0.131 9.877 ± 0.148 0.0606 ± 0.0010 622 ± 9
4.2 e,osc,p 434 57 0.13 0.0001 ± 0.0000 0.153 10.458 ± 0.150 0.0587 ± 0.0008 589 ± 8
5.1 e,osc,p,fr 462 518 1.12 0.0003 ± 0.0001 0.518 20.236 ± 0.386 0.0538 ± 0.0023 311 ± 6
6.1 m,osc,p 309 168 0.54 0.0002 ± 0.0001 0.307 26.109 ± 0.408 0.0511 ± 0.0016 242 ± 4
12-01 felsic sheet cutting Tacking Point Gabbro, north of Tacking Point
1.1 e,osc,p 128 80 0.63 0.0005 ± 0.0004 0.897 15.191 ± 0.377 0.0545 ± 0.0060 411 ± 10
2.1 m,osc,p,fr 408 554 1.36 0.0018 ± 0.0003 3.213 16.934 ± 0.368 0.0302 ± 0.0047 370 ± 8
3.1 c?osc,p 959 796 0.83 0.0015 ± 0.0001 2.647 21.130 ± 0.431 0.0401 ± 0.0021 298 ± 6
3.2 c?osc,p 678 304 0.45 0.0004 ± 0.0002 0.641 18.653 ± 0.344 0.0493 ± 0.0027 337 ± 6
4.1 e,osc,eq 524 120 0.23 0.0001 ± 0.0000 0.221 9.745 ± 0.194 0.0614 ± 0.0014 630 ± 12
5.1 m,osc/h,p 177 162 0.91 0.0005 ± 0.0002 0.974 18.423 ± 0.383 0.0505 ± 0.0042 341 ± 7
5.2 m.osc/h,p 211 181 0.86 0.0008 ± 0.0002 1.446 18.924 ± 0.428 0.0482 ± 0.0029 332 ± 7
6.1 m,osc/h,p 494 193 0.39 0.0026 ± 0.0004 4.717 17.842 ± 0.402 0.0311 ± 0.0058 352 ± 8
7.1 c,osc,ov 155 121 0.78 <0.0001 ± 0.0002 0.002 3.900 ± 0.236 0.1519 ± 0.0088 2367 ± 102 -38 grain analytical sites: e=end, m=middle, r=rim, c=structural core, p=prism, eq=equant, ov=oval, fr=grain fragment
cathodoluminescence petrography: osc=oscillatory zoning, h=homogeneous, hb=homogeneous and bright, hd=dull homogeneous, rex=recrystallised
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f206% is the percentage of 206Pb of non in situ radiogenic origin, based on measured 204Pb and Cumming & Richards (1975) common Pb compositions
all analytical errors are given at the 1σ level
* ages are given after correction for common Pb. For Neoproterozoic and Phanerozoic grains 206Pb/238U ages are given. For pre-Neoproterozoic grains are given 207Pb/206Pb ages (in italics) and the degree of discordance (%disc)
between the 206Pb/238U and 207Pb/206Pb ages † sites with overestimated common Pb due to isobaric interference under the 204Pb measuring position
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Highlights
Tacking Point Gabbro is Devonian not Permian and Watonga Formation is
confirmed as Ordovician.
Sandstone at Green Mound is Carboniferous and therefore not a part of the
Watonga Formation.
Dykes cutting the serpentinite are ~250 Ma constraining the emplacement age
of the mélange.