Report 137: Basin formation by orogenic collapse: zircon U

Report 137: Basin formation by orogenic collapse: zircon U-Pb and Lu-Hf isotope evidence from the Kimberley and Speewah Groups, northern AustraliaZIRCON U–PB AND LU–HF ISOTOPE
EVIDENCE FROM THE KIMBERLEY AND
SPEEWAH GROUPS, NORTHERN AUSTRALIA
Mines and Petroleum
by JA Hollis, AIS Kemp, IM Tyler, CL Kirkland, MTD Wingate,
C Phillips, S Sheppard, E Belousova, and Y Gréau
Geological Survey of Western Australia
REPORT 137
ISOTOPE EVIDENCE FROM THE
KIMBERLEY AND SPEEWAH GROUPS,
by
JA Hollis, AIS Kemp1, IM Tyler, CL Kirkland, MTD Wingate, C Phillips,
S Sheppard2, E Belousova3, and Y Gréau3
Perth 2014
MINISTER FOR MINES AND PETROLEUM
Hon. Bill Marmion MLA
Richard Sellers
Rick Rogerson
The recommended reference for this publication is:
Hollis, JA, Kemp, AIS, Tyler, IM, Kirkland, CL, Wingate, MTD, Phillips, C, Sheppard, S, Belousova, E and Gréau, Y, 2014, Basin
formation by orogenic collapse: zircon U–Pb and Lu–Hf isotope evidence from the Kimberley and Speewah Groups, northern
Australia: Geological Survey of Western Australia, Report 137, 46p.
National Library of Australia Cataloguing-in-Publication entry:
Author: Hollis, J. A., author.
Title: Basin formation by orogenic collapse : zircon U-Pb and Lu-Hf isotope evidence from the Kimberley
and Speewah Groups, Northern Australia / JA Hollis, AIS Kemp, IM Tyler, CL Kirkland, MTD
Wingate, C Phillips, S Sheppard, E Belousova and Y Gréau.
ISBN: 9781741685657 (ebook)
Subjects: Zircon--Western Australia--Kimberley Region.
Radioactive dating--Western Australia--Kimberley Region.
Geology, Structural--Western Australia--Kimberley Region.
Other Authors/Contributors: Kemp, A. I. S., author. Tyler, I. M., author. Kirkland, C. L., author.
Wingate, M. T. D. (Michael Thomas David), author. Phillips, C., author.
Sheppard, S., author. Belousova, E. A., author. Gréau, Y., author.
Geological Survey of Western Australia, issuing body.
Dewey Decimal Classification: 549.62099465
ISSN 1834–2280
Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Locations mentioned in the text are
referenced using Map Grid Australia (MGA) coordinates, Zones 50. All locations are quoted to at least the nearest 100 m.
U–Pb measurements were conducted using the SHRIMP II ion microprobes at the John de Laeter Centre of Isotope Research
at Curtin University in Perth, Australia. Isotope analyses were funded in part by the Western Australian Government
Exploration Incentive Scheme (EIS). Lu–Hf measurements were conducted using LA-ICPMS at the ARC National Key Centre
for Geochemical Evolution and Metallogeny of Continents (GEMOC), via the ARC Centre of Excellence in Core to Crust Fluid
Systems (CCFS), based in the Department of Earth and Planetary Sciences at Macquarie University, Australia.
Copy editor: K Coyle
Desktop publishing: RL Hitchings
Disclaimer
This product was produced using information from various sources. The Department of Mines and Petroleum (DMP) and the State
cannot guarantee the accuracy, currency or completeness of the information. DMP and the State accept no responsibility and disclaim
all liability for any loss, damage or costs incurred as a result of any use of or reliance whether wholly or in part upon the information
provided in this publication or incorporated into it by reference.
Published 2014 by Geological Survey of Western Australia
This Report is published in digital format (PDF) and is available online at <www.dmp.wa.gov.au/GSWApublications>.
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Cover photograph: Flat-lying King Leopold Sandstone of the Kimberley Group, Bell Gorge, King Leopold Ranges Conservation Park
iii
Contents
Abstract ..................................................................................................................................................................1
Introduction ............................................................................................................................................................2
Analytical methods .........................................................................................................................................6
U–Pb geochronology................................................................................................................................6
Calculation of model ages .................................................................................................................7
GSWA 206101: quartz sandstone – Tunganary Formation ......................................................................7
GSWA 182106: feldspathic sandstone – Lansdowne Arkose ..................................................................9
Kimberley Group samples...............................................................................................................................9
GSWA 182104: quartz sandstone – Warton Sandstone ........................................................................17
GSWA 190639: quartz sandstone – lower Pentecost Sandstone ............................................................17
GSWA 182102: quartz sandstone – middle Pentecost Sandstone..........................................................22
GSWA 182101: quartz sandstone – upper Pentecost Sandstone............................................................22
Discussion ............................................................................................................................................................27
Provenance and source regions of detritus of the Speewah and Kimberley Groups .....................................27
Post-tectonic extensional collapse of the Halls Creek Orogen .....................................................................31
Conclusions ..........................................................................................................................................................32
Acknowledgements ..............................................................................................................................................33
References ............................................................................................................................................................33
Appendix
Hf isotope data for all standards measured during analysis of unknowns .............................................................37
Figures
1. Location map and generalized geology of the Speewah and Kimberley Basins .........................................3
2. Time–space plot showing Proterozoic events in the Kimberley region .......................................................4
3. Field photographs of analysed samples .......................................................................................................8
4. Normalized probability density diagrams of detrital zircon ages ................................................................9
5. Zircon Hf vs age diagrams .........................................................................................................................12
6. Zircon Hf vs age diagram for all analysed detrital zircons from the Speewah and Kimberley
Groups; and probability density diagram of detrital zircon ages ...............................................................30
7. Tectonic reconstruction of part of Columbia (Nuna) at 1880– 1800 Ma, and paleocurrent data
for the Speewah and Kimberley Groups ....................................................................................................32
Tables
1. U–Pb and Lu–Hf analytical data for zircons from the O’Donnell Formation, Speewah Group .................. 10
2. U–Pb and Lu–Hf analytical data for zircons from the Tunganary Formation, Speewah Group .................. 13
3. U–Pb and Lu–Hf analytical data for zircons from the Lansdowne Arkose, Speewah Group ...................... 15
4. U–Pb and Lu–Hf analytical data for zircons from the King Leopold Sandstone, Kimberley Group .......... 18
5. U–Pb and Lu–Hf analytical data for zircons from the Warton Sandstone, Kimberley Group ..................... 20
6. U–Pb and Lu–Hf analytical data for zircons from the lower Pentecost Sandstone, Kimberley Group ....... 23
7. U–Pb and Lu–Hf analytical data for zircons from the middle Pentecost Sandstone, Kimberley Group ..... 25
8. U–Pb and Lu–Hf analytical data for zircons from the upper Pentecost Sandstone, Kimberley Group .......28
iv
1
zircon U–Pb and Lu–Hf isotope
evidence from the Kimberley and
Speewah Groups, northern Australia
by
JA Hollis, AIS Kemp1, IM Tyler, CL Kirkland, MTD Wingate, C Phillips, S Sheppard2, E Belousova3, and Y Gréau3
Abstract Changes in the provenance of sedimentary successions through time can provide valuable information on the
nature and timing of tectonic processes related to basin formation and on possible sediment source regions.
The Paleoproterozoic Speewah Group and unconformably overlying Kimberley Group of northern Western
Australia are sandstone-dominated fluvial and shallow-marine sedimentary successions that also contain
siltstone, claystone, mafic volcanic rocks, and rare carbonate rocks. Both groups are intruded by sills and
dykes of the c. 1797 Ma tholeiitic Hart Dolerite, which constitutes an important time marker. Deposition
of the Speewah and Kimberley Groups occurred during and immediately following the 1835–1810 Ma
Halls Creek Orogeny, which resulted from collision of the North Australian Craton and Kimberley Craton.
However, the tectonic drivers for basin formation and associated mafic volcanism are still unclear. To
understand these drivers, we present new SHRIMP U–Pb age and LA-MC-ICPMS Lu–Hf isotope data for
detrital zircons from the Speewah and Kimberley Groups. The results reveal a marked change in provenance
at the base of the Kimberley Group. The Speewah Group is dominated by Paleoproterozoic (1880–1850 Ma)
detrital zircons with unradiogenic Hf ( Hf = –8 to +1), whereas the onset of deposition of the Kimberley
Group is marked by a dramatic increase in the proportion of mainly Neoarchean (2525–2480 Ma) detrital
zircons with radiogenic Hf ( Hf = +0.5 to +7). We propose that the Speewah Group was derived largely
from erosion of 1865–1850 Ma Paperbark Supersuite granites during the Halls Creek Orogeny. We also
propose that the increase in Neoarchean detritus in the Kimberley Group reflects post-orogenic extensional
collapse at c. 1800 Ma that resulted in uplift and erosion of currently unexposed Neoarchean basement of
the Kimberley Craton. A Neoarchean source from within (rather than distal to) the Kimberley Craton is
also supported by the isotopic character of the Paperbark Supersuite, which is consistent with magmatic
reworking of a juvenile Neoarchean source. Paleocurrent directions in the Kimberley Group, correlation of
age and isotopic characteristics of potential source rocks, and a recent paleomagnetic tectonic reconstruction
are consistent with a possible sediment source region in the Dharwar Craton, which may have been
contiguous with the Kimberley Craton at c. 1800 Ma.
KEYWORDS: lutetium hafnium dating, uranium isotopes, zircon
1 Centre for Exploration Targeting, School of Earth and Environment, The University of Western Australia
M006, 35 Stirling Highway, Crawley WA 6009
2 Brockman Mining Australia, Level 1, 117 Stirling Highway, Nedlands WA 6009
3 GEMOC, Department of Earth and Planetary Sciences, Macquarie University, North Ryde NSW 2109
Hollis et al.
2
Introduction Detrital zircon age spectra are a powerful tool for interpreting the provenance of sedimentary rocks (Fedo et al., 2003; Cawood et al., 2012). Together with Lu– Hf isotopes, the U–Pb spectra of detrital zircons can provide information on the nature and evolution of the rocks from which they were derived, and can be used to make correlations with possible sediment source regions (Valley, 2003; Kinny and Maas, 2003; Griffin et al., 2004; Veevers et al., 2005; Kemp et al., 2006; Pietranik et al., 2008; Howard et al., 2009). Changes in provenance through time can also provide valuable information on the nature and timing of tectonic processes related to basin formation, such as deposition along active margins, arrival of allochthonous terranes at continental margins, or the opening of intracratonic basins (Bingen et al., 2001; Goodge et al., 2002; Barr et al., 2003; Rojas-Agramonte et al., 2011; Cawood et al., 2012).
The ≤1835 to >1740 Ma Speewah and Kimberley Groups comprise a largely undeformed, flat-lying succession up to 7 km thick, of sandstone-dominated fluvial and shallow- marine sedimentary rocks and mafic volcanic rocks that cover about 160 000 km2 in the Kimberley region of northwest Australia (Fig. 1). The Speewah and Kimberley Groups rest on the Kimberley Craton, but this basement is entirely concealed and, thus, the age and nature of the basement is largely unknown. Deposition of the Speewah and Kimberley Groups occurred during and immediately following the 1835–1810 Ma Halls Creek Orogeny, which was the product of collision of the Kimberley Craton with the North Australian Craton (Sheppard et al., 1997; Sheppard et al., 1999; Griffin et al., 2000a). The timing of deposition has led to the proposal that basin formation was linked to orogenesis, with the Speewah Group formed in a retro-arc setting (Sheppard et al., 2012). Furthermore, intrusion of about 250 000 km3 of the c. 1797 Ma tholeiitic Hart Dolerite into the Speewah Group and lower part of the Kimberley Group led to suggestions that plume or post-orogenic plate-margin reorganization processes played a role in the formation of the Speewah and Kimberley Basins (Griffin et al., 1994; Page and Sun, 1994; Tyler et al., 2006; Sheppard et al., 2012).
Changes in the provenance of sedimentary rocks of the Speewah and Kimberley Groups may relate to the tectonic processes that drove basin formation and sedimentation. The detrital age spectra of samples of sandstone from three formations in the Speewah Group and from three formations in the Kimberley Group are investigated. The results are used in association with other constraints on the timing and nature of tectonism, before and during sedimentation, to constrain the likely drivers of basin formation.
Geological setting
The Kimberley Craton
The Kimberley Craton, in Western Australia, is the northwestern-most part of the North Australian Craton
and comprises currently unexposed, probably Archean and Paleoproterozoic basement to the Kimberley Basin (Gellatly, 1971; Plumb and Gemuts, 1976; Graham et al., 1999; Cawood and Korsch, 2008). Although no exposed Archean crust is known from the Kimberley Craton, it is thought to form at least some of the unexposed part of the craton based on the isotopic character of younger granites and metasedimentary rocks (Tyler et al., 1999; Griffin et al., 2000a; Page et al., 2001; Downes et al., 2007). Furthermore, the interpretation of potential field data has been used to infer an Archean basement (Hancock and Rutland, 1984; Gunn and Meixner, 1998). Alternatively, it has been proposed that the Kimberley Craton comprises a series of linear, north-northeasterly trending Archean to Paleoproterozoic terranes (Gunn and Meixner, 1998; Tyler et al., 1999).
The Lamboo Province
The oldest exposed parts of the Kimberley Craton lie within the 1910–1805 Ma Lamboo Province, which comprises northwesterly and northeasterly striking orogenic belts that wrap the margins of overlying, younger sedimentary rocks of the Kimberley and Speewah Groups (Fig. 1). The Lamboo Province includes Paleoproterozoic greenschist to granulite facies metasedimentary rocks, felsic plutonic rocks (and subordinate comagmatic mafic rocks), and felsic and mafic volcanic rocks. The province is divided into three tectonostratigraphic terranes: the eastern, central, and western zones (Tyler et al., 1995). The eastern zone comprises 1910 Ma granitic and felsic and mafic volcanic basement, the Ding Dong Downs Volcanics. The basement is unconformably overlain by 1880–1845 Ma metasedimentary and volcanic rocks of the Halls Creek Group. These are thought to have formed as an 1880 Ma passive margin succession (the Biscay Formation), overlain by active margin submarine fan systems and alkaline volcanic rocks (the Olympio Formation) on the western margin of the North Australian Craton (Hancock, 1991; Tyler et al., 1995; Tyler et al., 1998; Sheppard et al., 1999; Tyler et al., 2005; Tyler, 2005), before its collision with the Kimberley Craton. The western zone is dominated by felsic and subordinate mafic and ultramafic magmatic rocks of the 1865– 1850 Ma Paperbark Supersuite, which intrude 1870 Ma metasedimentary rocks of the Marboo Formation (Tyler et al., 1999). The western zone is thought to represent the margin of the, probably Archean and Paleoproterozoic, Kimberley Craton. The Paperbark Supersuite is thought to have formed by extensive magmatic reworking of the Kimberley Craton margin following northwesterly directed subduction (Griffin et al., 2000a). The central zone is dominated by medium- to high-grade metasedimentary and mafic metavolcanic and metavolcaniclastic rocks of the Tickalara Metamorphics (Figs 1 and 2). The central zone is thought to represent an oceanic island arc, formed outboard of the Kimberley Craton margin during easterly directed subduction at c. 1865 Ma (Sheppard et al., 1999; Griffin et al., 2000a). Extension of the arc in several discrete episodes is indicated by intrusion of 1856, 1845, and 1830 Ma layered mafic to ultramafic bodies and deposition at 1845–1840 Ma of sedimentary and mafic and felsic
GSWA Report 137 Basin formation by orogenic collapse: zircon U–Pb and Lu–Hf isotope evidence
3
Figure 1. Location map and generalized geology of the Speewah and Kimberley Basins
Hollis et al.
4
Figure 2. Time–space plot showing Proterozoic events in the Kimberley region. The western, central, and eastern zones refer
to tectonostratigraphic terranes of the Lamboo Province as defined by Tyler et al. (1995).
5
volcanic rocks of the Koongie Park Formation, interpreted as a rifted arc succession (Page et al., 1994; Orth, 2002; Tyler et al., 2005).
Accretion of the oceanic arc of the central zone to the western zone (Kimberley Craton) at 1855–1850 Ma is constrained by the timing of deformation correlated between the two zones (Griffin et al., 1994; Tyler et al., 1995; Tyler et al., 1999; Thorne et al., 1999; Blake et al., 2000; Tyler, 2005). Collision of the combined western and central zones (Kimberley Craton) with the eastern zone (North Australian Craton) occurred during the 1835–1810 Ma Halls Creek Orogeny, which involved northwesterly directed subduction, deformation, and metamorphism that affected the entire Lamboo Province, and stitched together all three zones by felsic and mafic magmatic rocks of the 1835–1805 Ma Sally Downs Supersuite (Tyler et al., 1995; Tyler and Page, 1996; Blake et al., 2000; Bodorkos et al., 2000; Page et al., 2001; Sheppard et al., 2001).
Speewah and Kimberley Groups
The Speewah and Kimberley Groups together are up to 7 km thick and comprise continental and shallow-marine sandstone-dominated siliciclastic rocks with subordinate siltstone, stromatolitic carbonate, and regionally extensive basalt.
The Speewah Group overlies the western zone of the Lamboo Province and outcrops along its margin in the east and west Kimberley (Gellatly et al., 1975; Thorne et al., 1999). In the east Kimberley, the contact is either unconformable or is fault-bounded. In the west, the contact is a major west-northwesterly striking, southerly directed thrust known as the Inglis Fault (Griffin and Myers, 1988; Tyler and Griffin, 1990; Tyler and Griffin, 1993; Griffin et al., 1994). The extent of the Speewah Group beneath the Kimberley Group is unknown. The Speewah Group is typically ~1.5 km thick, although it thins significantly in the northwestern part of the basin (on Yampi 1:250 000) and is absent in the southeast (on Mount Ramsay 1:250 000). The Speewah Group is dominated by fluvial quartz and lithic sandstone, feldspathic sandstone, and arkose with interbedded siltstone. Feldspathic sandstones of the Speewah Group typically become more coarse grained towards the southeastern and western margins of the basin, consistent with provenance from these areas (Plumb et al., 1985). The Speewah Group consists of six formations: the O’Donnell Formation, Tunganary Formation, Valentine Siltstone, Lansdowne Arkose, Luman Siltstone, and Bedford Sandstone (Fig. 2). Paleocurrents for the Speewah Group, from both the northwesterly and northeasterly trending belts, indicate paleoflow from the northeast (Gellatly et al., 1970). A felsic volcaniclastic rock within the Valentine Siltstone gave a depositional age of 1835 ± 3 Ma (Page and Sun, 1994; Sheppard et al., 2012), which is thought to represent the timing of deposition of the lower Speewah Group. This indicates that deposition of the Speewah Group was synchronous with the 1835–1810 Ma Halls Creek Orogeny. The Speewah Group is thought to have been deposited in a retro-arc foreland basin behind the active arc during orogenesis (Sheppard et al., 2012).
The Kimberley Group overlies the Speewah Group, with the base of the Kimberley Group marked by a regionally extensive low-angle unconformity. The Kimberley Group is a typically ~3 km thick succession of sandstone- dominated siliciclastic sedimentary rocks. It includes quartz and feldspathic sandstone, siltstone, claystone and conglomerate, mafic volcanic rocks, and rare dolostone. The Kimberley Group comprises six formations (Fig. 2): the King Leopold Sandstone, Carson Volcanics, Warton Sandstone, Elgee Siltstone, Pentecost Sandstone, and the Yampi Formation (which may be correlative with an informal upper member of the Pentecost Sandstone). These are thought to be conformable, although there may be a paraconformity between an informal lower subgroup (the King Leopold Sandstone and Carson Volcanics) and upper subgroup (the Warton Sandstone, Elgee Siltstone, Pentecost Sandstone, and Yampi Formation) (Sheppard et al., 2012). The subdivision into lower and upper subgroups is made on the basis that the uppermost Carson Volcanics is the upper stratigraphic limit of emplacement of confirmed c. 1797 Ma Hart Dolerite. This provides a minimum age of deposition for the lower subgroup (Page and Sun, 1994; Sheppard et al., 2012). The upper subgroup may be up to 50 Ma younger, with the minimum depositional age constrained by emplacement of the 1740 Ma Wotjulum Porphyry on the Yampi Peninsula (Tyler and Griffin, 1993; Wingate et al., 2011). Paleocurrents for the Kimberley Group indicate paleoflow mainly from the north and northwest (Gellatly et al., 1970). The Kimberley Group has been interpreted to have been deposited in a broad, semi-enclosed shallow sea (Gellatly et al., 1970; Plumb, 1981). Periods of tidal influence and subaerial exposure are indicated by local evidence for submarine extrusion of the Carson Volcanics (local occurrence of pillow structures), and sand waves (Plumb and Gemuts, 1976), as well as desiccation cracks and mud-draped ripples in sandstone interbeds. Sheppard et al. (2012) interpreted the Kimberley Group as a successor basin, formed after cessation of Halls Creek orogenesis. The Moola Bulla, Red Rock, Texas Downs, and Revolver Creek Basins are probable equivalents to the Kimberley Group that outcrop across the Halls Creek Orogen (Thorne et al., 1999; Blake et al., 1999).
Neoproterozoic orogenesis
The Speewah and Kimberley Groups were deformed along the basin margins during the c. 1000 to 800 Ma Yampi Orogeny and the c. 560 Ma King Leopold Orogeny, although the central parts of the basin remain undeformed. The Yampi Orogeny produced the large-scale, northerly directed thrusts and folds of the Speewah and Kimberley Groups on the Yampi Peninsula. These thrusts are traceable into the steeply dipping, northwesterly striking shear zones in the western zone of the Lamboo Province (Tyler and Griffin, 1990; Shaw et al., 1992; Tyler and Griffin, 1993; Griffin et al., 1994; Bodorkos and Reddy, 2004). The Yampi Orogeny also resulted in strike–slip faulting in the Halls Creek Orogen (White and Muir, 1989; Tyler et al., 1995; Thorne and Tyler, 1996). The King Leopold Orogeny produced southwesterly directed thrusts and folds in the King Leopold Ranges (Precipice Fold Belt), along the southwest margin of the Kimberley Basin.
Hollis et al.
6
These folds and thrusts persist up to 100 km into the basin (Fig. 1). The King Leopold Orogeny also resulted in reactivation of shear zones in the Lamboo Province in the southwest Kimberley and sinistral strike–slip faulting in the east Kimberley (Tyler and Griffin, 1990; Shaw et al., 1992; Tyler and Griffin, 1993; Griffin et al., 1994; Thorne and Tyler, 1996; Tyler, 2005).
Zircon U–Pb geochronology
and Lu–Hf isotopes
U–Pb geochronology
Analytical methods for U–Pb zircon geochronology by secondary ion mass spectrometry (SIMS) using the sensitive high-resolution ion microprobe (SHRIMP) are described in detail in Wingate and Kirkland (2013) and only a brief summary is provided here. Handpicked zircons were mounted with zircon standards in epoxy disks and polished to about half-grain thickness to expose crystal interiors. Transmitted-light, reflected-light, and cathodoluminescence (CL) images were used to target analytical locations. Zircon standard BR266 (Stern, 2001) was used for U/Pb calibration and 238U concentration. Fractionation of 207Pb*/206Pb* ratios (Pb* refers to radiogenic Pb) was monitored by comparison with the 3465 Ma OGC1 zircon standard (Stern et al., 2009). Weighted mean ages are reported with 95% (t √MSWD) uncertainties and corresponding values of mean square of weighted deviates (MSWD).
Lu–Hf isotopes
GEMOC
Hafnium isotope analyses were conducted on previously dated zircon grains in three of the eight samples presented here (GSWA 182101, 206101 [preliminary data], and 190639) over two sessions at the ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), in the ARC Centre for Excellence for Core to Crust Fluid Systems, Macquarie University (Session 1: GSWA 182101; Session 2: GSWA 206101, 190639; Appendix). Analyses were conducted using a New Wave/Merchantek LUV213 laser ablation microprobe attached to a Nu Plasma multi-collector inductively coupled plasma mass spectrometer (LA-MC-ICPMS). The analyses employed a beam diameter of approximately 55 μm and a 5 Hz repetition rate, and energies of 0.6– 1.3 mJ per pulse, which resulted in ablation pits typically 40–60 μm deep during a 30–120 s ablation. Total Hf signals were 1–6 10–11 A. The ablated sample material was transported from the laser cell to the ICPMS torch by helium carrier gas. Interference of 176Lu on 176Hf was corrected by measuring interference-free 175Lu, and using the invariant 176Lu/175Lu correction factor of
0.02669 (DeBievre and Taylor, 1993). Measurement of accurate 176Hf/177Hf ratios in zircon requires correction for isobaric interference of 176Lu and 176Yb on 176Hf. The interference of 176Yb on 176Hf was corrected by measuring the interference-free 172Yb isotope and using 176Yb/172Yb to calculate the intensity of 176Yb. The appropriate value of 176Yb/172Yb (0.5865) was determined by successively doping a JMC475 Hf standard (100 ppb solution) with various amounts of Yb, and determining the value of 176Yb/172Yb required to yield the value of 176Hf/177Hf in the undoped solution.
Zircons from the Mud Tank Carbonatite locality were analysed, together with the samples, as a measure of the accuracy of the results. Most of the data and the mean 176Hf/177Hf value (Session 1: 0.282531 ± 35; 2 , n = 55; Session 2: 0.282507 ± 48; 2 , n = 34, Appendix) are within two standard deviations (2 s.d.) of the recommended value (Griffin et al., 2007a). The average 176Hf/177Hf ratio for the 91500 zircon standard (Wiedenbeck et al., 1995; Goolaerts et al., 2004), analysed concurrently with unknown zircons, during session 1 was 0.282308 ± 4 (2 ; n = 6), which is consistent with the range of published values for the 91500 standard (Griffin et al., 2007a; Griffin et al., 2007b). The average 176Hf/177Hf ratio for Temora 2 run during session 2 was 0.282679 ± 46 (2 ; n = 14), which is identical to the accepted value (0.282680 ± 24 LA-MC-ICPMS; 2 ; Woodhead et al., 2004).
Advanced Analytical Centre – James Cook University
For the remaining five samples presented here, hafnium isotope analyses were measured using a ThermoScientific Neptune MC-ICPMS and GeoLas Pro 193nm ArF laser system in the Advanced Analytical Centre, James Cook University, Townsville, (analyst T Kemp). Instrumental operating parameters and analytical protocols followed those described by Kemp et al. (2009) and Næraa et al. (2012). Data were acquired using a 42 μm or 58 μm beam size and 4 Hz laser pulse repetition rate over a 60 s ablation period. The power density at the sample was maintained at 5–6 J/cm2. Ablation was carried out in He, which was combined with Ar and a small (~0.005 l/ min) N2 flow before transport into the ICPMS. Ablation was conducted as close as practical to the pits resulting from the previous U–Pb isotope analyses. The isobaric interference of Lu and Yb on 176Hf was corrected by monitoring the interference-free 171Yb and 175Lu intensities during the analysis and then deriving 176Yb and 176Lu using 176Yb/171Yb = 0.897145 (Segal et al., 2003) and 176Lu/175Lu = 0.02655 (Vervoort et al., 2004). Yb isotope ratios were normalized to 173Yb/171Yb = 1.130172 (Segal et al., 2003) and Hf isotope ratios to 179Hf/177Hf = 0.7325 using an exponential law. The mass bias of Lu was assumed to follow that of Yb. Data were processed offline using a customized Microsoft Excel spreadsheet; within- run outlier rejection was set at three standard errors of the mean (3 ). Reference zircons analysed concurrently with the unknowns yielded mean 176Hf/177Hf values (± 2 ) of 0.282185 ± 19 (n = 38) for FC1, 0.282496 ± 13 (n = 73) for Mud Tank, 0.282686 ± 19 (n = 36) for
7
Temora 2, 0.281614 ± 22 (n = 15) for QGNG and 0.282130 ± 15 (n = 16) for synthetic standard, identical to the solution 176Hf/177Hf values reported by Woodhead and Hergt (2005). All Hf isotope data are normalized to the accepted solution 176Hf/177Hf value of Mud Tank zircon (Woodhead and Hergt, 2005) using laser ablation data generated from this zircon during the same analytical session (see Appendix). Quoted Hf uncertainties in sample zircons combine within-run analytical errors plus the reproducibility uncertainty for Mud Tank zircon.
Calculation of model ages
Calculation of initial 176Hf/177Hf values is based on the 176Lu decay constant (1.865 10–11/y) of Scherer et al. (2001) and Hf values are based on the present-day chondritic measurement (0.282772) of Blichert-Toft and Albarède (1997). Calculation of Hf model ages (TDM
2) is based on a depleted mantle source with (176Hf/177Hf)
i = 0.279718 at 4.56 Ga and 176Lu/177Hf = 0.0384 (Griffin et al., 2000b). For each analysis, a two-stage model age is calculated, which assumes that the parent magma was produced from a volume of average continental crust with a 176Lu/177Hf ratio of 0.015 (Griffin et al., 2002). This ratio has been demonstrated to be appropriate for collision settings by evaluation of global whole-rock compilations (Kirkland et al., 2012).
Speewah Group samples
O’Donnell Formation
Sample GSWA 182107 is a medium-grained to very coarse grained, cross-laminated quartz sandstone of the O’Donnell Formation (Fig 3a, Richenda, MGA Zone 51, 731635E, 8106817N). The O’Donnell Formation is the basal unit of the Speewah Group, which unconformably overlies the c. 1855 Ma Whitewater Volcanics and granites of the 1865–1850 Ma Paperbark Supersuite, in the western zone of the Lamboo Province. The unconformable contact is typically tectonized. The O’Donnell Formation is an overall fining-upward succession of poorly sorted, coarse- grained, quartz-rich sandstone, lithic sandstone, pebbly sandstone, granule to pebble conglomerate, and chloritic siltstone and claystone. It is thought to have formed within a fluvial, probably braided, river system with high current energy. The sample was collected from a rocky slope at Inglis Gap, 1 km west of the junction of the Gibb River and Mount Hart Roads.
U–Pb geochronology is reported in Kirkland et al. (2010d). Sixty-five analyses were obtained from 65 zircons. Fifty-two analyses <5% discordant yield 207Pb*/206Pb* dates of 2718–1841 Ma (Fig. 4a, Table 1), and include a dominant age component at c. 1882 Ma (38 analyses, 73%) and several minor components between 2700 and 2400 Ma. These are interpreted as the ages of detrital sources. A conservative estimate of the maximum age of deposition is provided by the weighted mean 207Pb*/206Pb* date of 1864 ± 4 Ma (MSWD = 1.01) for the 28 youngest analyses.
Hf isotope data were collected from 40 zircons ranging in age from 2718–1841 Ma (Fig. 5a, Table 1). Thirty-six analyses (90%) fall within the dominant c. 1882 Ma age component and define a tight cluster of Hf compositions ( Hf = –4.9 to –1.0). The four remaining analyses in the range 2718–2335 Ma vary widely in their Hf compositions. Of these, a 2602 Ma zircon has a very radiogenic Hf isotope composition close to depleted mantle (DM).
GSWA 206101: quartz sandstone –
Tunganary Formation
Sample GSWA 206101 (preliminary data) is a grey quartz-rich sandstone with thin, coarse-grained, normally graded beds that fine upwards to fine-grained sandstone (Elgee, MGA Zone 52, 387831E, 8191784N). The Tunganary Formation conformably overlies the O’Donnell Formation and is conformably overlain by the c. 1835 Ma Valentine Siltstone. The formation comprises poorly to moderately sorted, feldspathic or lithic–quartz sandstone with interbeds of quartz arenite, pebbly lithic sandstone, and siltstone. Sedimentary structures include trough cross-bedding, cross-laminations, scour-and-fill structures, fluid escape structures, and, in the upper informal member, prominent ripples (Gellatly and Derrick, 1967). Sandstones of the Tunganary Formation are more feldspathic in the east compared with the west Kimberley (Dow and Gemuts, 1967; Gellatly and Derrick, 1967; Roberts et al., 1968; Gellatly et al., 1969; Griffin et al., 1994). Paleocurrent directions defined by scour- and-fill structures and cross-laminations are towards the north on Lissadell 1:250 000 (Thorne et al., 1999). The Tunganary Formation is thought to have formed, at least partly, in a fluvial, possibly braided, river system, probably further down the depositional profile compared with the O’Donnell Formation. The sample was collected from Speewah Metals diamond drillhole SHD08-6 (435.6 – 436.4 m).
U–Pb geochronology is reported in Kirkland et al. (2014). Seventy-one analyses were obtained from 71 zircons. Sixty-one analyses <5% discordant yield 207Pb*/206Pb* dates of 2746–1817 Ma (Fig. 4b), and include a dominant age component at c. 1854 Ma (51 analyses, 72%) and several minor components between 2750 and 2100 Ma. These are interpreted as the ages of detrital sources. A conservative estimate of the maximum age of deposition is provided by the weighted mean 207Pb*/206Pb* date of 1839 ± 4 Ma (MSWD = 0.9) for the 30 youngest analyses.
Hf isotope data were collected from 59 zircons ranging in age from 2746–1817 Ma (Fig. 5b, Table 2). These are dominated by an 1854 Ma age component, which is characterized by a wide spread in Hf isotope compositions ( Hf = –10.8 to +4.7). A few older Neoarchean and Paleoproterozoic (>2100 Ma) zircons show a wide range of Hf compositions from close to DM ( Hf = +6.7 at 2309 Ma) through to unradiogenic compositions ( Hf = –10.8 at 2510 Ma).
Hollis et al.
8
Figure 3. Field photographs of analysed samples: a) O’Donnell Formation (GSWA 182107), field of view is 4 m; b) Lansdowne
Arkose (GSWA 182106); c) King Leopold Sandstone (GSWA 182105); d) Warton Sandstone (GSWA 182104); e) middle
Pentecost Sandstone (GSWA 182102); f) upper Pentecost Sandstone (GSWA 182101)
9
Formation (GSWA 182107); b) Tunganary Formation
(GSWA 206101); c) Lansdowne Arkose (GSWA
182106); d) King Leopold Sandstone (GSWA
182105); e) Warton Sandstone (GSWA 182104);
f) lower Pentecost Sandstone (GSWA 190639);
g) middle Pentecost Sandstone (GSWA 182102);
h) upper Pentecost Sandstone (GSWA 182101)
GSWA 182106: feldspathic sandstone –
Lansdowne Arkose
Sample 182106 is a grey medium-grained feldspathic sandstone from the Lansdowne Arkose (Fig. 3b, Richenda, MGA Zone 51, 740663E, 8104738N), which conformably overlies the c. 1835 Ma Valentine Siltstone and is conformably overlain by the Luman Siltstone. The Lansdowne Arkose comprises feldspathic sandstones and arkoses with interbedded medium- to coarse-grained quartz sandstones (Dow and Gemuts, 1967; Roberts et al., 1968; Griffin et al., 1994; Thorne et al., 1999) with minor micaceous siltstones and claystones. It is dominated by
trough cross-stratification with cross-bed foresets up to 1 m thick and also preserves slumps, ripples, and clay pellets (Gellatly and Derrick, 1967; Sheppard et al., 1999; Thorne et al., 1999). Paleoflow is inferred to be from the northeast on Landsdowne 1:250 000 (Gellatly and Derrick, 1967) and mainly from the northeast and east on Lissadell 1:250 000. Bidirectional paleocurrent directions have also been observed in some horizons (Thorne et al., 1999). The Lansdowne Arkose is interpreted as a fluvial succession with marine incursions or with marine reworking of fluvial sand at some levels (Thorne et al., 1999). The sample was collected from the crest of the lowest sandstone ridge, above a creek, on the northern side of the Gibb River Road, 2.5 km west-northwest of Mount Vincent.
U–Pb geochronology is reported in Kirkland et al. (2010c). Sixty-four analyses were obtained from 64 zircons. Fifty-two analyses <5% discordant yield 207Pb*/206Pb* dates of 2517–1828 Ma (Fig. 4c), and include a dominant age component at c. 1860 Ma (32 analyses, 62%), significant components at 2511, 1954, and 1907 Ma, and several minor components in the range 2517–1828 Ma. These are interpreted as the ages of detrital sources. A maximum age of deposition is provided by the weighted mean 207Pb*/206Pb* date of 1860 ± 6 Ma (MSWD = 1.6) for the 32 youngest analyses.
Hf isotope data were collected from 48 zircons ranging in age from 2517 to 1834 Ma (Fig. 5c, Table 3). These are dominated by the 1860 Ma age component (30 analyses, 63%), characterized by Hf isotope compositions of
Hf = –5.2 to +2.1, though mainly falling within the range
Hf = –3.0 to –0.4. The small 1954 and 1907 Ma age components each have Hf compositions slightly more radiogenic than Chondritic Uniform Reservoir (CHUR) in the range Hf = –0.2 to +0.8. The 2511 Ma age component (five analyses, 10%) has Hf compositions that lie along a vertical array from unradiogenic ( Hf = –1.5) to strongly radiogenic ( Hf = +5.7).
Kimberley Group samples
Leopold Sandstone
Sample GSWA 182105 is a cross-laminated, coarse- grained, quartz-dominated sandstone of the King Leopold Sandstone (Fig. 3c, Richenda, MGA Zone 51, 749762E, 8100450N). The King Leopold Sandstone is the basal unit of the Kimberley Group, which unconformably overlies the Speewah Group. The formation comprises thick- to very thick bedded, well- or moderately sorted, cross- bedded or massive, recrystallized, quartz-dominated sandstones (quartz arenites or orthoquartzites) with minor siltstone and granule to pebble conglomerate. A basal polymictic pebble or boulder conglomerate rests on a regionally extensive, very low angle unconformity (Williams, 1969; Griffin et al., 1994; Sheppard et al., 1997; Thorne et al., 1999; Williams, 2005; Schmidt and Williams, 2008). A glacial origin for this conglomerate has been proposed on Landsdowne 1:250 000, and the basal contact is termed the ‘Bedford Surface’ (Williams, 2005;
Hollis et al.
2 3 8 U
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2 3 8 U
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s in
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12
Figure 5. Zircon Hf vs age diagrams for zircons from: a) O’Donnell Formation (GSWA 182107); b) Tunganary Formation
(GSWA 206101); c) Lansdowne Arkose (GSWA 182106); d) King Leopold Sandstone (GSWA 182105); e) Warton
Sandstone (GSWA 182104); f) lower Pentecost Sandstone (GSWA 190639); g) middle Pentecost Sandstone (GSWA
182102); h) upper Pentecost Sandstone (GSWA 182101)
13
2 3 8 U
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0 .0
0 .2
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0 .2
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0 .7
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0 .2
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0 .4
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0 .4
2 .7
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1. 3
0 .6
2 .4
0 .0
0 .2
0 .0
0 .5
2 .5
S 11
0 .0
0 .2
-3 .0
0 .3
2 .7
0 .2
-0 .7
0 .6
2 .6
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0 .2
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0 .0
0 .2
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a ta
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2 3 8 U
ƒ 2 0 4
f ±
7 7 H
f i H
0 .0
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0 .4
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0 .0
0 .2
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2 .8
0 .0
0 .2
-1 .4
0 .8
2 .6
0 .0
0 .2
-0 .9
0 .5
2 .6
0 .0
0 .2
-3 .6
0 .3
2 .8
0 .2
-3 .7
0 .5
2 .8
0 .0
0 .2
-1 .8
0 .7
2 .7
0 .0
0 .2
-6 .8
0 .8
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0 .0
0 .2
-4 .7
0 .7
2 .8
0 .0
0 .2
-1 .0
0 .4
2 .6
0 .0
0 .2
-2 .9
0 .8
2 .7
S 1.
0 .0
0 .2
-8 .6
0 .4
3 .1
0 .0
0 .2
-1 .7
0 .5
2 .7
0 .0
0 .2
-2 .8
0 .3
2 .7
0 .0
0 .2
-1 .3
0 .4
2 .6
0 .0
0 .2
3 .0
0 .7
2 .5
0 .0
0 .2
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0 .5
2 .7
0 .0
0 .2
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2 .5
0 .0
0 .2
6 .7
0 .6
2 .5
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0 .2
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c o
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15
a ta
f o
L a n
2 3 8 U
ƒ 2 0 4
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0 .2
-2 .9
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-4 .4
0 .2
2 .8
0 .0
0 .2
-4 .4
0 .3
2 .8
0 .0
-1 .4
0 .3
2 .6
0 .0
0 .2
-0 .9
0 .3
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-0 .8
0 .2
2 .6
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0 .2
-1 .5
0 .2
2 .6
0 .0
-0 .5
0 .2
2 .6
0 .0
0 .2
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0 .2
2 .4
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0 .0
0 .2
-2 .2
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2 .6
0 .0
0 .2
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0 .2
2 .6
0 .0
-2 .1
0 .3
2 .7
0 .0
0 .2
-2 .6
0 .2
2 .7
0 .0
0 .2
-0 .6
0 .3
2 .6
0 .2
-2 .9
0 .2
2 .7
0 .0
0 .2
-1 .8
0 .3
2 .7
0 .0
0 .2
-0 .5
0 .2
2 .6
0 .0
0 .2
0 .6
0 .2
2 .5
0 .0
0 .2
-0 .7
0 .3
2 .6
0 .0
0 .2
-1 .8
0 .2
2 .7
0 .0
0 .2
-2 .3
0 .2
2 .7
0 .0
0 .2
-3 .0
0 .2
2 .7
2 3 8 U
f ±
7 7 H
f i H
0 .0
0 .2
-1 .7
0 .2
2 .7
1 2
0 .0
0 .0
0 .0
0 .2
-1 .6
0 .2
2 .7
9 -4
8 0
0 .2
0 .0
0 .0
0 .0
0 .2
-0 .9
0 .2
2 .6
0 .0
0 .0
0 .2
-1 .1
0 .2
2 .6
11 5
0 .2
0 .0
0 .0
0 .0
0 .2
0 .2
0 .2
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0 .2
0 .0
0 .0
0 .0
0 .2
1 3
0 .0
0 .0
0 .2
0 .5
0 .2
2 .6
11 -3
0 .2
0 .0
0 .0
0 .0
0 .2
0 .8
0 .3
2 .6
1 2
0 .0
0 .0
0 .0
0 .2
1. 8
0 .2
2 .5
0 .0
0 .0
0 .0
0 .2
1. 5
0 .2
2 .5
0 .0
0 .0
0 .0
0 .2
-0 .2
0 .2
2 .6
1 0
0 .0
0 .0
0 .0
0 .2
-2 .9
0 .2
2 .8
1 0
0 .0
0 .0
0 .0
0 .2
-2 .1
0 .2
2 .8
0 .0
0 .0
0 .2
6 .7
0 .2
2 .3
1 0
0 .0
0 .0
0 .0
0 .2
1 2
0 .0
0 .0
0 .2
0 .5
0 .2
2 .9
8 2
9 0
0 .2
0 .0
0 .0
0 .0
0 .2
2 .8
0 .3
2 .9
0 .0
0 .0
0 .9
0 .2
3 .0
1 2
0 .0
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0 .2
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0 .0
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c o
n ti
n u
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17
Schmidt and Williams, 2008). Sedimentary structures include cross-bedding, planar parallel laminations, and subangular to subrounded siltstone intraclasts indicative of high flow regime conditions, with rare overturned cross- beds implying strong bottom currents (Derrick, 1968). The typically well-sorted and mature sandstones suggest that the King Leopold Sandstone may have formed in a highly reworked, shallow-marine depositional environment with episodes of high current energy. The sample was collected from a road-cutting on the southern side of the Gibb River Road, 5.7 km east of Mount Bell.
U–Pb geochronology for this sample is reported in Kirkland et al. (2010b). Sixty-six analyses were obtained from 66 zircons. Fifty analyses <5% discordant yield 207Pb*/206Pb* dates of 2579–1840 Ma (Fig. 4d), and include significant age components at c. 2523, 2201, 1999, 1965, 1886, and 1874 Ma and several minor components in the range 2579–1840 Ma. These are interpreted as the ages of detrital sources. A conservative estimate of the maximum age of deposition is provided by the weighted mean 207Pb*/206Pb* date of 1869 ± 7 Ma (MSWD = 1.4) for the 15 youngest analyses.
Hf isotope data were collected from 49 zircons ranging in age from 2579–1840 Ma (Fig. 5d, Table 4). These are dominated by zircons in the range 1899–1840 Ma (19 analyses, 39%), although the Neoarchean age component also comprises a large proportion (13 analyses, 27%). The 1899–1840 Ma zircons yield a spread in Hf compositions ( Hf = –6.9 to +1.5), although they mainly fall in the range Hf = –4.6 to –0.4. Zircons from the 1999 and 1965 Ma age components are slightly more radiogenic than CHUR ( Hf = +1.1 to +3.3). The 2523 Ma age component has Hf compositions ranging from –0.6 to +4.0 with the majority at the more radiogenic end of that range.
Warton Sandstone
Sample GSWA 182104 is a cross-laminated and trough cross-bedded quartz sandstone of the Warton Sandstone, comprising interlayered fine-grained and coarse-grained sandstone (Fig. 3d, Richenda, MGA Zone 51, 759306E, 8101608N). It conformably overlies the Carson Volcanics and is conformably overlain by the Elgee Siltstone. The Warton Sandstone comprises medium-grained, well-sorted, quartz-dominated sandstones, feldspathic sandstones (subarkose or arkose), with minor micaceous siltstone. Regionally feldspathic sandstones are common at the base (Gellatly et al., 1969; Gellatly and Sofoulis, 1969; Plumb and Perry, 1971). The sandstones are commonly cross-bedded (Gellatly and Derrick, 1967; Gellatly and Sofoulis, 1969; Gellatly et al., 1969) with symmetrical ripples, ripple and planar parallel laminations, water escape structures, and desiccation cracks indicating differing current flow strengths and periods of subaerial exposure. Paleoflow was mainly from the west and northwest (Derrick, 1968; Gellatly and Sofoulis, 1969; Gellatly et al., 1970). Evidence for subaerial exposure suggests that the Warton Sandstone was deposited in a
tidal to shallow-marine setting. The sample was collected from outcrop above a scree slope on the southern side of the Gibb River Road, 3.9 km west-southwest of Saddler Spring.
U–Pb geochronology is reported in Kirkland et al. (2010a). Eighty-four analyses were obtained from 84 zircons. Sixty-two analyses <5% discordant yield 207Pb*/206Pb* dates of 3013–1806 Ma (Fig. 4e), and include a dominant age component at c. 2517 Ma (23 analyses, 37%), significant age components at 2035, 1900, and 1856 Ma and several minor components in the range 3013–1806 Ma. These are interpreted as the ages of detrital sources. A conservative estimate of the maximum age of deposition is provided by the weighted mean 207Pb*/206Pb* date of 1862 ± 17 Ma (MSWD = 1.5) for the 12 youngest analyses.
Hf isotope data were collected from 60 zircons ranging in age from 3013–1806 Ma (Fig. 5e, Table 5). These are dominated by Neoarchean zircons (30 analyses, 50%), mainly in the range 2580–2500 Ma. The Neoarchean component is characterized by radiogenic Hf compositions ( Hf = +1.7 to +6.7), consistent with their formation from a Neoarchean mantle-derived source. Paleoproterozoic zircons in the range 1927–1806 Ma yield a significant spread in Hf compositions, which are relatively unradiogenic, spanning Hf = –10.2 to +0.2.
lower Pentecost Sandstone
Sample GSWA 190639 is a weakly banded, quartz- rich, medium-grained quartz sandstone of the Pentecost Sandstone (Sullivan, MGA Zone 52, 245065E, 8219355N), which is the uppermost formation of the Kimberley Group and is dominated by quartz sandstone with subordinate pebbly sandstone, siltstone, and claystone. The Pentecost Sandstone is subdivided into informal lower, middle and upper members (Derrick, 1968). The lower member comprises medium- to coarse- grained quartz sandstones. The middle member contains cupriferous and ferruginous sandstone, siltstone, and shale. The upper member is characterized by upward coarsening into coarse-grained sandstone and pebbly sandstone. In the east and central Kimberley, the Pentecost Sandstone conformably overlies the Elgee Siltstone. However, in coastal regions of the west Kimberley it locally unconformably overlies the Warton Sandstone. The Pentecost Sandstone is unconformably overlain by the Paleoproterozoic Bastion Group and by the Neoproterozoic Mount House Group. This sample was collected from the lower member of the Pentecost Sandstone, which comprises cross-bedded, quartz-dominated sandstone with minor feldspathic sandstone (Derrick, 1968; Plumb and Perry, 1971). Gellatly and Sofoulis (1969) found clay pellets within the lower member in the north Kimberley (on Drysdale– Londonderry 1:250 000) and Plumb and Perry (1971) recorded a shale–pellet conglomerate in adjacent areas to the east (on Medusa Banks 1:250 000).
Hollis et al.
2 3 8 U
f ±
7 7 H
f i H
0 .0
0 .0
0 .2
-3 .1
0 .3
2 .7
0 .2
-1 .8
0 .3
2 .6
0 .0
0 .0
0 .0
0 .2
-4 .6
0 .5
2 .8
0 .0
-1 .7
0 .3
2 .6
0 .0
0 .2
-6 .9
0 .4
3 .0
0 .0
0 .2
-0 .9
0 .2
2 .6
0 .0
0 .2
-4 .1
0 .2
2 .8
0 .0
0 .2
-2 .1
0 .4
2 .7
0 .0
0 .2
-0 .4
0 .2
2 .6
0 .0
-3 .7
0 .3
2 .8
0 .0
0 .2
-2 .6
0 .3
2 .7
0 .0
0 .2
-4 .4
0 .2
2 .8
0 .0
0 .2
-2 .7
0 .3
2 .7
0 .0
0 .2
-1 .9
0 .3
2 .7
0 .0
0 .2
1. 5
0 .2
2 .5
0 .0
0 .2
-3 .0
0 .3
2 .7
0 .0
0 .2
-2 .0
0 .2
2 .7
0 .0
0 .2
-3 .2
0 .3
2 .8
0 .0
-1 .6
0 .2
2 .7
0 .0
0 .2
-2 .7
0 .2
2 .8
0 .0
0 .2
-1 .4
0 .2
2 .7
0 .0
0 .2
1. 7
0 .2
2 .5
0 .0
0 .2
1. 2
0 .4
2 .5
0 .0
0 .2
1. 1
0 .4
2 .5
S 1.
0 .0
0 11
3 8
0 .2
1. 4
0 .2
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3 .3
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0 .2
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0 .2
2 .5
a ta
f o
K in
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e o
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ld S
a n
le 1
19
2 3 8 U
f ±
7 7 H
f i H
8 0
0 .2
0 .0
0 .0
0 .0
0 .2
-9 .5
0 .2
3 .3
5 1
0 .0
0 .2
8 11
7 9
1 2
0 .0
0 .0
-0 .3
0 .2
2 .7
1 9
0 .0
0 .0
0 .0
0 .2
-1 .4
0 .5
2 .9
1 2
0 .0
0 .0
0 .2
1. 5
0 .3
2 .7
0 .0
0 .0
0 .0
0 .2
1. 1
0 .3
2 .7
1 0
0 .0
0 .0
0 .2
-3 .8
0 .3
3 .0
9 3
0 .2
0 .0
0 .0
1 74
4 5
0 .2
0 .3
0 .3
2 .8
5 2
0 .2
0 .0
0 .0
0 .0
1 74
2 6
0 .2
3 .2
0 .2
2 .8
1 5
0 .0
0 .0
0 .0
0 .2
0 .8
0 .3
3 .0
7 0
0 .2
8 11
8 4
0 .0
0 .0
0 .2
8 11
6 2
-0 .6
0 .3
3 .1
6 -1
0 .2
0 .0
0 .0
0 .0
0 .2
1. 9
0 .3
2 .9
7 2
0 .2
0 .0
0 .0
0 .2
3 .0
0 .2
2 .8
0 .0
3 .2
0 .3
2 .8
8 2
0 .2
0 .0
0 .0
0 .2
3 .7
0 .3
2 .8
0 .0
0 .0
0 .0
0 .2
1. 1
0 .3
3 .0
0 .0
0 .0
0 .0
0 .2
3 .7
0 .3
2 .8
1 8
0 .0
0 .0
0 .2
1 2
0 .0
0 .0
0 .2
4 .1
0 .3
2 .8
0 .0
0 .0
0 .2
2 .3
0 .3
2 .9
0 .0
0 .0
0 .0
0 .2
2 .8
0 .3
2 .9
7 6
0 .2
0 .0
0 .0
0 .0
0 .2
2 .7
0 .6
2 .6
2 3 8 U
f ±
7 7 H
f i H
0 .0
0 .0
0 .0
0 .2
-6 .9
0 .3
2 .9
0 .0
0 .2
-5 .6
0 .3
2 .9
0 .0
0 .0
0 .0
0 .2
-2 .5
0 .3
2 .7
0 .0
0 .2
-1 0 .2
0 .0
0 .2
-8 .8
0 .5
3 .1
0 .0
0 .2
-4 .8
0 .2
2 .8
0 .0
0 .2
-8 .5
0 .4
3 .1
0 .0
0 .0
0 .0
-6 .4
0 .3
2 .9
0 .0
0 .2
-6 .2
0 .3
2 .9
0 .0
0 .2
-2 .1
0 .4
2 .7
0 .0
0 .2
-1 .0
0 .4
2 .6
0 .0
0 .2
-0 .8
0 .3
2 .6
0 .0
0 .2
-3 .7
0 .2
2 .8
0 .0
0 .2
0 .2
0 .4
2 .6
0 .0
0 .2
-5 .3
0 .4
2 .9
0 .0
0 .2
4 .7
0 .4
2 .4
0 .0
0 .2
-7 .5
0 .4
3 .2
0 .0
0 .2
0 .3
0 .5
2 .7
0 .0
0 .2
1. 2
0 .3
2 .7
S 11
0 .0
0 .0
0 .0
0 .2
0 .2
0 .4
2 .9
0 .0
0 .2
5 .0
0 .4
2 .6
0 .0
0 .2
2 .5
0 .4
2 .8
0 .2
1. 8
0 .3
2 .9
0 .0
0 .2
3 .4
0 .3
2 .8
0 .0
0 .2
8 11
0 0
-3 .7
0 .3
3 .2
0 .0
0 .2
3 .2
0 .2
2 .8
0 .0
3 .3
0 .4
2 .8
0 .0
0 .2
1. 7
0 .4
2 .9
2 7
11 -2
0 .2
0 .0
0 .0
0 .0
0 .2
2 .6
0 .5
2 .9
1 3
0 .0
0 .0
0 .0
0 .2
4 .0
0 .4
2 .8
3 1
0 .0
0 .0
0 .0
0 .2
3 .4
0 .3
2 .8
a ta
f o
a rt
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le 1
21
2 3 8 U
ƒ 2 0 4
f ±
7 7 H
f i H
2 1
0 .0
0 .0
0 .0
0 .2
3 .7
0 .3
2 .8
1 0
0 .0
0 .0
0 .0
0 .2
4 .1
0 .3
2 .8
11 -1
0 .2
0 .0
0 .0
0 .2
4 .4
0 .3
2 .8
9 -2
0 .2
0 .0
0 .0
0 .2
2 .5
0 .4
2 .9
9 -2
0 .2
0 .0
0 .0
0 .2
3 .4
0 .4
2 .8
9 0
0 .2
0 .0
0 .0
3 .9
0 .4
2 .8
2 0
0 .0
0 .0
0 .0
0 .2
3 .7
0 .5
2 .8
1 8
0 .0
0 .0
0 .2
3 .9
0 .4
2 .8
S 74
1 4
0 .0
0 .0
0 .2
3 .3
0 .2
2 .8
2 9
0 .0
0 11
3 0
0 .0
0 .2
-1 1.
1 6
0 .0
0 .0
0 .2
4 .6
0 .4
2 .8
1 3
0 .0
0 .2
3 .8
0 .3
2 .8
1 2
0 .0
0 .0
0 .2
4 .4
0 .3
2 .8
1 9
0 .0
0 .0
0 .0
6 .2
0 .3
2 .7
1 4
0 .0
0 .0
0 .0
0 .2
5 .8
0 .4
2 .7
9 0
0 .2
8 11
4 1
0 .0
0 .0
0 .0
0 .2
8 11
2 9
-1 .0
0 .3
3 .1
1 6
0 .0
0 .0
0 .0
0 .2
4 .8
0 .3
2 .8
1 5
0 .0
0 .0
0 .2
5 .8
0 .2
2 .7
2 0
0 .0
0 .0
0 .2
4 .7
0 .4
2 .8
1 3
0 .0
0 .0
-9 .8
0 .4
3 .2
1 4
0 .0
0 .0
0 .0
0 .2
6 .3
0 .4
2 .7
11 1
0 .2
0 .0
0 .0
0 11
2 7
0 .0
0 .2
6 .7
0 .4
2 .7
2 2
0 .0
0 .0
0 .2
6 .2
0 .4
2 .7
1 0
0 .0
0 .0
0 .0
0 .2
-0 .4
0 .3
3 .2
S 1.
1 4
0 .0
0 .0
0 .2
5 .2
0 .4
2 .8
1 0
0 .0
0 .0
0 .0
0 .2
-5 .8
0 .4
3 .6
1 0
0 .0
0 .0
0 .2
-7 .4
0 .2
3 .7
1 6
0 .0
0 .0
0 .2
4 .1
0 .5
3 .0
1 4
0 .0
0 .0
0 .0
0 .2
0 .7
0 .3
3 .4
11 8
0 .2
0 .0
0 .0
0 .2
5 .8
0 .3
2 .8
0 .0
8 .0
0 .4
2 .7
1 2
0 .0
0 .0
0 .2
8 11
7 7
7. 0
0 .3
2 .8
22
Paleocurrent directions throughout the Pentecost Sandstone are dominantly from the north and northwest (Gellatly and Derrick, 1967; Gellatly and Sofoulis, 1969; Plumb and Perry, 1971) with lesser paleocurrents from the east (Derrick, 1968). Some bidirectional paleocurrents are observed on Drysdale–Londonderry 1:250 000. The lower member of the Pentecost Sandstone is interpreted as having formed in a high current-energy shallow-marine environment, probably above fair-weather wave-base under the influence of tidal currents. The sample was collected from an outcrop adjacent to the Gibb River Road, 10.7 km east of the intersection with the Kalumbaru Road.
U–Pb geochronology for this sample is reported in Wingate et al. (2012). Sixty-six analyses were obtained from 66 zircons. Fifty-three analyses <5% discordant yield 207Pb*/206Pb* dates of 2741–1774 Ma (Fig. 4f), and include significant age components at c. 2506, 1989, 1952, 1850, and 1774 Ma, and several minor components between 2741 and 1774 Ma. These are interpreted as the ages of detrital sources. A conservative estimate of the maximum age of deposition is provided by the weighted mean 207Pb*/206Pb* date of 1796 ± 16 Ma (MSWD = 1.5) for the eight youngest analyses.
Hf isotope data were collected from 52 zircons ranging in age from 2741–1774 Ma (Fig. 5f, Table 6). The Neoarchean age component shows a spread in Hf isotope compositions, but is dominated by relatively radiogenic Hf ( Hf = +0.5 to +7.0). This is consistent with formation of these zircons from a combination of Neoarchean juvenile and older sources. The Paleoproterozoic zircons show an overall decrease in Hf with decreasing crystallization age ( Hf = +5.5 at 2199 Ma to Hf = –11.8 at 1817 Ma).
middle Pentecost Sandstone
Sample GSWA 182102 is a fine-grained cross-laminated quartz sandstone from the informal middle member of the Pentecost Sandstone (Fig. 3e, Pentecost, MGA Zone 52, 361961E, 8256457N). The middle member of the Pentecost Sandstone comprises quartz or feldspathic sandstone, with lesser siltstone and claystone. The base of the member consists of grey-green, thin-bedded, glauconitic sandstone, and siltstone with minor feldspathic sandstone (Gellatly et al., 1969; Plumb and Perry, 1971). In the north Kimberley, the middle member is dominated by quartz sandstone (quartz arenite or orthoquartzite) and contains Cu-bearing siltstone, chloritic siltstone, and glauconitic siltstone with ferruginous, glauconitic, feldspathic sandstone and micaceous shale (Roberts et al., 1968; Derrick, 1968; Gellatly and Sofoulis, 1969; Plumb and Perry, 1971). The middle member is notably more micaceous than the underlying lower member (Griffin et al., 1994). It contains ubiquitous clay pellets in the Seppelt Range (Plumb and Perry, 1971). Cross-beds and parallel laminations indicate high current energies (high flow regime and laminar flow), possibly in a lower shoreface or nearshore setting, with periods of subaerial exposure indicated by desiccation cracks (Roberts et al., 1968). This sample was collected in a road-cut on the
northern side of the Wyndham–Karunjie Road, 13.5 km west-southwest of Home Valley Homestead.
U–Pb geochronology for this sample is reported in Kirkland et al. (2010f). Sixty-one analyses were obtained from 61 zircons. Fifty analyses <5% discordant yield 207Pb*/206Pb* dates of 3109–1740 Ma (Fig. 4g), and include a dominant age component at 2525 Ma, significant age components at c. 1969, 1868, and 1787 Ma, and several minor components in the range 3109–1740 Ma. These are interpreted as the ages of detrital sources. A conservative estimate of the maximum age of deposition is provided by the weighted mean 207Pb*/206Pb* date of 1796 ± 24 Ma (MSWD = 1.53) for the seven youngest analyses.
Hf isotope data were collected from 27 zircons ranging in age from 3109–1740 Ma (Fig. 5g, Table 7). The Neoarchean age component (2525 Ma, eight analyses, 30%) is characterized by relatively radiogenic Hf zircon ( Hf = +1.5 to +5.2), consistent with formation of these zircons from a Neoarchean mantle-derived source. The Paleoproterozoic zircons show an overall decrease in Hf with decreasing crystallization age ( Hf = +2.4 at 2255 Ma to Hf = –8.3 at 1785 Ma), similar to that seen in results for the lower Pentecost Sandstone (GSWA 190639).
upper Pentecost Sandstone
Sample GSWA 182101 comprises cross-laminated quartz sandstone from the informal upper member of the Pentecost Sandstone (Fig. 3f, Pentecost, MGA Zone 52, 378820E, 8255170N). The informal upper member of the Pentecost Sandstone comprises interlayered quartz sandstone, pebbly sandstone, siltstone, and claystone (Derrick, 1968; Roberts et al., 1968; Thorne et al., 1999). In the west Kimberley region, the upper member is absent from Lennard River 1:250 000 (Griffin et al., 1994) and is correlative with the Yampi Formation on Yampi 1:250 000 (Tyler and Griffin, 1993). The upper Pentecost Sandstone appears to lack the feldspathic sandstone found in the lower members and is dominated by coarse- grained, trough and planar cross-bedded, and commonly symmetrically rippled quartz-dominated sandstone, which coarsens upwards to coarse-grained sandstone and pebbly sandstone (Derrick, 1968; Roberts et al., 1968; Thorne et al., 1999). This coarsening upwards suggests a shoaling shoreface or nearshore, high current-energy depositional setting. The sample was collected from the western side of the Wyndham–Karunjie Road, 7.3 km south-southeast of Home Valley Homestead.
U–Pb geochronology for this sample is reported in Kirkland et al. (2010e). Sixty-one analyses were obtained from 61 zircons. Fifty analyses <5% discordant yield 207Pb*/206Pb* dates of 3102–1796 Ma (Fig. 4h), and include a dominant age component at 1866 Ma (40%) and significant age components at 2512, 2497, and 2482 Ma. These are interpreted as the ages of detrital sources. A conservative estimate of the maximum age of deposition is provided by the weighted mean 207Pb*/206Pb* date of 1862 ± 5 Ma (MSWD = 1.4) for the 20 youngest analyses.
23
2 3 8 U
f ±
7 7 H
f i H
0 .0
0 .0
0 .0
0 .2
-8 .8
0 .2
3 .0
S 1.
0 .0
0 .2
-9 .0
0 .5
3 .0
0 .0
0 .0
-5 .1
0 .6
2 .8
0 .0
0 .2
-4 .4
0 .4
2 .8
0 .0
-4 .5
0 .6
2 .8
0 .0
0 .2
0 .0
1 74
2 7
0 .2
-7 .9
0 .9
3 .0
0 .0
0 .0
0 .0
0 .2
-3 .1
0 .4
2 .7
0 .0
0 .2
-1 1.
0 .0
0 .2
-3 .1
0 .5
2 .7
S 7.
0 .0
0 .2
-6 .2
1. 1
2 .9
0 .0
0 .2
-4 .0
0 .9
2 .8
0 .0
0 .2
-1 .3
0 .4
2 .6
0 .0
0 .2
-4 .6
0 .8
2 .8
0 .0
0 .2
-5 .2
0 .5
2 .9
0 .0
0 .0
0 11
2 1
0 .0
0 .2
-3 .5
0 .4
2 .8
0 .0
0 .2
-1 .5
0 .6
2 .6
0 .0
0 .2
-0 .5
0 .4
2 .6
0 .0
0 .2
-1 .7
0 .3
2 .7
0 .0
0 .2
0 .0
0 .2
2 .6
0 .0
0 .0
0 .0
0 .2
-5 .0
0 .5
2 .9
0 .0
0 .2
0 .2
0 .7
2 .6
0 .0
0 .2
4 .8
0 .8
2 .3
0 .0
0 .2
2 .8
0 .4
2 .4
0 .0
0 .2
-5 .5
0 .5
3 .0
0 .0
0 .2
-1 .8
0 .8
2 .7
0 .0
0 .2
-4 .0
0 .5
2 .9
0 .0
0 .2
8 11
7 7
0 .0
0 .0
0 .0
0 .2
1. 2
0 .7
2 .6
6 6
0 .0
0 .0
0 .0
0 .2
0 .3
0 .7
2 .6
0 .0
0 .0
0 .0
0 .2
-0 .1
0 .3
2 .7
1 3
0 .0
0 .0
0 .2
8 11
1 5
a ta
f o
lo w
e r
2 3 8 U
f ±
7 7 H
f i H
0 .0
0 .0
0 .0
0 .2
-0 .3
1. 1
2 .7
1 3
0 .0
0 .2
5 .5
1. 0
2 .4
4 2
0 .0
0 .0
0 .0
0 .7
0 .3
2 .9
1 2
0 .0
0 .0
0 .0
0 .2
1. 9
0 .3
2 .9
S 11
0 .0
0 .0
0 .0
1. 0
0 .3
3 .0
1 2
0 .0
0 .0
0 .2
6 .1
1. 5
2 .6
0 .0
0 .0
0 .0
0 .2
6 .9
0 .7
2 .6
9 2
0 .2
0 .0
0 .0
2 11
4 5
0 .2
0 .7
0 .4
3 .0
0 .0
0 .0
0 .0
0 .2
5 .3
0 .5
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11 0
0 .2
0 .0
0 .0
0 .0
4 .1
0 .3
2 .8
0 .0
0 .0
0 .2
6 .5
3 .2
2 .6
0 .0
0 .0
0 .0
3 .1
0 .5
2 .9
11 3
0 .2
0 .0
0 .0
0 .0
0 .2
4 .1
0 .5
2 .8
8 1
0 .0
0 .0
0 .2
8 11
7 8
0 .5
0 .9
3 .0
1 4
0 .0
0 .0
0 .0
0 .2
5 .2
0 .3
2 .7
0 .0
0 .0
0 .2
-2 .0
1. 5
3 .2
0 .0
0 .0
0 .0
0 .2
7. 0
0 .5
2 .7
1 2
0 .0
0 .0
0 .0
0 .2
-3 .7
0 .7
3 .4
0 .0
0 .0
0 .2
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0 7
2 .7
0 .5
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0 .0
0 .0
0 .0
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0 .3
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3 .2
c o
n ti
n u
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25
2 3 8 U
f ±
7 7 H
f i H
0 .0
0 .0
0 .0
0 .2
-7 .7
0 .6
2 .9
0 .0
0 .0
0 .0
0 .2
-8 .3
0 .6
3 .0
0 .0
-2 .2
0 .5
2 .6
0 .0
0 .2
-7 .2
0 .5
3 .0
0 .0
0 .2
0 .4
0 .6
2 .5
0 .2
-3 .4
0 .6
2 .8
0 .0
0 .2
0 .5
0 .4
2 .6
0 .0
0 .2
1. 2
0 .6
2 .5
0 .0
-1 .2
0 .5
2 .7
0 .0
0 .0
2 11
1 2
0 .2
1. 7
0 .3
2 .5
0 .0
0 .2
-1 .4
0 .5
2 .8
0 .0
0 .0
0 .0
0 .2
2 .4
0 .5
2 .6
0 .0
0 .2
1. 3
0 .4
2 .7
0 .0
0 .2
2 .4
0 .6
2 .7
0 .0
0 .0
0 .0
0 .2
1. 1
0 .5
2 .8
0 .0
4 .0
0 .3
2 .7
0 .0
0 .2
3 .5
0 .4
2 .8
1 8
0 .0
0 .0
0 .0
0 .2
3 .2
0 .6
2 .8
9 -1
1 2
1 0
a ta
f o
m id
d le
2 3 8 U
f ±
7 7 H
f i H
7 0
6 4
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2 3
1 5
0 .0
0 .0
0 .0
0 .2
3 .0
0 .5
2 .9
7 2
0 .2
0 .0
0 .0
0 .0
0 .2
1 0
0 .0
0 .0
0 .0
0 .2
3 .3
0 .6
2 .8
9 2
0 .2
0 .0
0 .0
0 .0
0 .2
2 .0
0 .6
2 .9
11 0
8 0
0 .2
0 .0
0 .0
0 .0
0 .2
3 .8
0 .5
2 .8
1 2
0 .0
0 .0
0 .0
0 .2
1. 5
0 .6
3 .0
1 2
11 0
2 1
0 .0
0 .0
0 .0
0 .2
-3 .4
0 .4
3 .4
9 0
0 .0
0 .0
2 11
4 0
0 .2
8 11
5 1
4 .4
0 .3
2 .9
7 3
0 .2
0 .0
0 .0
0 11
4 9
0 .0
0 .2
1. 9
1. 0
3 .4
c o
n ti
n u
e d
27
Hf isotope data were collected from 24 zircons ranging in age from 3102–1796 Ma (Fig. 5h, Table 8). These are dominated by the 1866 Ma age component, which is characterized by a significant spread in unradiogenic Hf values ( Hf = –10.4 to –0.2). The few analysed Neoarchean grains (2589–2502 Ma, 5 analyses, 21%) show a significant spread from Hf = –1.5 to +6.8 (lying on the DM model curve), although four of the five are significantly more radiogenic than CHUR.
Discussion
Kimberley Groups
The onset of deposition of the Kimberley Group marks a significant change in provenance compared with that of the Speewah Group. The Speewah Group is dominated by Paleoproterozoic detrital zircons (1880–1850 Ma), with Archean grains representing ≤8% of the detrital zircons (Fig. 4). The onset of sedimentation in the Kimberley Group coincides with a dramatic increase in the proportion of mainly Neoarchean (2525–2480 Ma) detrital zircons (Fig. 4) from 8% in the Lansdowne Arkose of the Speewah Group (GSWA 182106) up to 24% in the King Leopold Sandstone of the Kimberley Group (GSWA 182105) and then a further increase to 54% in the overlying Warton Sandstone (GSWA 182104). The proportion of Archean relative to Paleoproterozoic detrital zircons in the overlying Pentecost Sandstone is variable, but remains at similarly high levels. These data indicate that the unconformity at the base of the Kimberley Group marks a significant change in provenance and possibly in the basin-forming tectonic processes.
The informal lower and upper subgroups of the Kimberley Group were defined on the basis of the presence of c. 1797 Ma Hart Dolerite sills in the lower subgroup, which comprises the King Leopold Sandstone and Carson Volcanics. Dolerite sills in the upper subgroup (Warton Sandstone, Elgee Siltstone and Pentecost Sandstone) have not been dated and may belong to a possibly younger mafic suite, based on their distinct whole-rock Nd isotopes (Sheppard et al., 2012). Therefore, the upper subgroup of the Kimberley Group could be as young as 1740 Ma, the age of the intrusive Wotjulum Porphyry. The detrital zircon data presented here provide no tighter constraints on the timing of deposition of the upper compared with the lower subgroup. However, the data show that there was no significant change in provenance between the lower and upper subgroup, each of which contains significant proportions of Archean detrital zircons, relative to the Speewah Group. These data suggest that the lower and upper subgroups may represent a single succession formed in an evolving basin with no significant break in deposition.
The Paleoproterozoic age component (1880–1850 Ma) present in each of the Speewah and Kimberley Group samples has a characterist ic spread in relatively unradiogenic Hf compositions, in the range
Hf = –12 to +5, with the majority of analyses in the range –8 to +1 (Figs 5 and 6). This range in Hf compositions suggests formation of these zircons by magmatic reworking of both isotopically juvenile and unradiogenic sources of at least Paleo- to Neoarchean age (Figs 5 and 6). The majority of results for zircons in this age range are consistent with zircon formation in magmas with crustal sources extracted from Neoarchean mantle (Figs 5 and 6). These data are consistent with whole-rock Nd data for magmatic rocks of the Paperbark Supersuite that are also characterized by unradiogenic compositions ( Nd = –1.1 to –4.2), consistent with their formation by magmatic reworking of a large proportion of Neoarchean crust (Griffin et al., 2000a; Sheppard et al., 2001). Therefore, granites of the 1865–1850 Ma Paperbark Supersuite, or equivalents, from the western zone of the Lamboo Province, are permissible sources for detrital zircons of this age in the Speewah and Kimberley Groups. The older Paleoproterozoic detrital zircons, constituting the main Paleoproterozoic age component in the Speewah and Kimberley Group samples (up to c. 2400 Ma), may also have been sourced from the western zone of the Lamboo Province. This could have occurred by sedimentary reworking of detrital zircons from the Marboo Formation, which contains significant detrital age components at 2387, 2339, 2184, 2057, 1983, and 1875 Ma (Tyler et al., 1999; Kirkland et al., 2013). However, zircon Hf data for the Marboo Formation are not currently available for comparison.
Neoarchean to earliest Paleoproterozoic (2525–2480 Ma) detritus forms a minor age component in samples from the Speewah Group, but becomes significantly more prevalent in the lower units of the Kimberley Group. This age component also has a distinctive Hf character, typified by zircons with radiogenic Hf in the range Hf = +0.5 to +7, which covers the compositional range from CHUR to DM (Figs 5 and 6). This range in Hf compositions suggests formation of these zircons by magmatic reworking of mainly isotopically juvenile Neoarchean, but also older, sources (of at least Mesoarchean age; Figs 5 and 6). A spread of radiogenic Hf compositions in zircons along DM in the range 2660–2500 Ma suggests that juvenile crust formation was significant throughout the late Neoarchean in the source region for this detritus (Figs 5 and 6). Notably, Hf model ages for most of the Neoarchean detrital zircons are consistent with (a) zircon Hf model ages for 1898–1850 Ma detrital zircons from all of the Speewah and Kimberley Group samples, and (b) Nd model ages for the inferred main source of this Paleoproterozoic detritus: granites of the 1865–1850 Ma Paperbark Supersuite. Thus, the data are consistent with derivation of the two main detrital age components (1880–1850 Ma and 2525– 2480 Ma) by magmatic reworking of the same Neoarchean source.
Hollis et al.
2 3 8 U
f ±
7 7 H
f i H
0 .0
0 .0
0 .0
0 .2
-2 .5
0 .6
2 .6
0 .0
0 .2
-7 .6
0 .8
3 .0
0 .0
0 .0
-3 .8
0 .9
2 .8
0 .6
3 0
.0 7
1 8
5 3
1 2
0 .0
0 .0
0 .2
-4 .8
0 .9
2 .8
0 .0
0 .0
0 .2
-8 .2
1. 1
3 .0
0 .0
0 .2
-5 .6
0 .6
2 .9
0 .0
0 .2
-3 .0
0 .6
2 .7
0 .0
0 .2
-4 .1
0 .8
2 .8
0 .0
0 .2
-1 0 .4
0 .0
0 .0
0 .0
0 .2
-0 .2
0 .4
2 .6
0 .0
0 .0
0 .0
0 .2
-4 .9
0 .5
2 .9
S 11
0 .2
2 0
.0 2
1 8
7 1
7 4
0 .3
8 0
.0 6
1 8
7 5
1 4
0 .0
0 .2
-3 .9
0 .5
2 .8
0 .0
0 .0
0 .2
-6 .2
0 .8
2 .9
0 .0
0 .2
3 .3
0 .3
2 .4
0 .2
4 0
.0 8
1 9
9 6
1 2
0 .0
0 .0
0 .2
-4 .9
0 .9
3 .0
0 .4
2 0
.0 1
2 0
3 4
6 0
0 .2
0 .0
0 .2
2 .1
0 .3
2 .5
0 .6
1 0
8 3
0 .4
6 4
a ta
f o
u p
p
29
2 3 8 U
ƒ 2 0 4
f ±
7 7 H
f i H
2 2
0 .0
0 .0
0 .0
0 .2
1. 4
0 .5
2 .9
9 2
0 .2
0 .0
0 .0
0 .0
0 .2
0 .9
0 .5
2 .9
1 7
0 .0
0 .0
0 .2
8 11
0 2
-4 .1
0 .8
3 .3
5 -1
5 1
7 1
7 -1
9 0
1 0
0 .0
0 .0
0 .0
0 .2
4 .2
0 .8
2 .8
6 0
0 .7
5 3
0 .0
0 .0
0 11
7 1
0 .0
0 .2
4 .4
0 .6
2 .8
1 0
0 .0
0 .0
0 .0
0 .2
6 .8
0 .6
2 .6
S 7.
8 1
0 .2
8 11
3 1
0 .0
0 .0
0 .0
0 .2
-1 .5
0 .7
3 .2
7 0
0 .2
0 .0
0 .0
0 .0
0 .2
2 .3
0 .5
3 .3
30
Figure 6. a) Zircon Hf vs age diagram for all analysed
detrital zircons from the Speewah and Kimberley
Groups. Bracketing Hf evolution lines are drawn for
Lu/Hfi = 0.015; b) Probability density diagram of
detrital zircon ages showing the variation in Hf
through time
Similarly, the array in Hf compositions for Neoarchean to Paleoproterozoic zircons in sample GSWA 182102 (middle Pentecost Sandstone) in particular, may reflect an Hf evolution trend from a common Neoarchean source. A best fit of the data based on an initial 176Lu/177Hf of 0.0125–0.015 (Rudnick and Fountain, 1995; Vervoort and Patchett, 1996; Amelin et al., 1999; Chauvel et al., 2014) indicates formation of these zircons from a c. 2.7–2.6 Ga mantle-derived source. Figure 6 illustrates that average crust-formation ages derived from all detrital zircon analyses fall mainly in the range 2.9–2.6 Ga. If the Paperbark Supersuite was derived from magmatic reworking of the same Neoarchean crust that is directly represented in the detrital zircon cargo of the Speewah and Kimberley Groups, this indicates that this Neoarchean crust is, or was, part of the Kimberley Craton, rather than a distal source unrelated to the Kimberley Craton.
Paleocurrent directions consistently indicate that paleoflow was from the north and northwest during deposition of the Kimberley Group (Gellatly et al., 1970). Therefore the source region for both Paleoproterozoic and Neoarchean detritus was most likely also from the north and northwest, because there is no evidence for significant subsequent horizontal tectonic movement of the Kimberley region, aside from tectonic reactivation along the basin margins during the Neoproterozoic Yampi and King Leopold Orogenies. The paleocurrent information suggests that the western zone of the Lamboo Province, and the Neoarchean
crust from which it was derived, once extended north of the present exposure of the Kimberley Basin, and therefore probably also forms part of the basement to the Kimberley Basin. The extension of Paleoproterozoic domains beneath the Kimberley Basin has been previously proposed (Plumb and Gemuts, 1976; Gunn and Meixner, 1998; Tyler et al., 1999) and is consistent with whole-rock Nd isotope data for a granite xenolith from the Aries kimberlite, close to the centre of the Kimberley Basin, which has an age and isotopic character consistent with granites of the Paperbark Supersuite (Downes et al., 2007).
There are two possible source regions for Neoarchean detritus to the north and northeast of the Kimberley region at c. 1800 Ma: the Pine Creek Orogen and the Dharwar Craton. Tectonic reconstructions based on paleomagnetic data place both of these regions adjacent to the Kimberley Craton in the period 1880–1800 Ma (Zhang et al., 2012). Both regions also contain Neoarchean rocks of the same age as the Neoarchean detritus in the Speewah and Kimberley Groups. The Archean basement to the Pine Creek Orogen comprises 2670, 2640, and 2550–2510 Ma granites and granitic gneisses (Page et al., 1980; Williams and Compston, 1983; Cross et al., 2005; Hollis et al., 2009; Carson et al., 2011; Kositcin et al., 2013). The youngest of these granites, 2550–2510 Ma, are volumetrically dominant and are the same age as most of the Neoarchean detritus in the Speewah and Kimberley Groups. However, the Hf isotopic character of the 2550–2510 Ma basement to the Pine Creek Orogen is significantly less radiogenic (Hollis et al., 2010) than Neoarchean detrital zircons in the Speewah and Kimberley Groups ( Hf = +0.5 t

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Report 137: Basin formation by orogenic collapse: zircon U