Tectonic implications of Palaeoproterozoic post-collisional,
high-K felsic igneous rocks from the Kimberley region of
northwestern Australia
T.J. Griffin
a, R.W. Page
b, S. Sheppard
a,*, I.M. Tyler
aaGeological Sur
6ey of Western Australia,Mineral House,100Plain Street,East Perth,6004WA,Australia bAustralian Geological Sur
6ey Organisation,GPO Box378,Canberra2601,ACT,Australia
Received 1 March 1999; accepted 1 October 1999
Abstract
Palaeoproterozoic high-K I-type granites, high-level porphyry intrusions, and felsic volcanic rocks of the Whitewa-ter Volcanics dominate the Hooper and Lamboo Complexes in the Kimberley region of northwesWhitewa-tern Australia. The granites, porphyries and volcanic rocks are gradational into each other in the field, and they have the same mineralogy, similar major and trace element abundances, and indistinguishable SHRIMP U – Pb zircon ages of 1865 – 1850 Ma. There is evidence of widespread mingling between the granites and coeval gabbros. Magma mixing may be important in the formation of some of the mafic granites, but most of the rocks probably formed from felsic parent magmas that underwent variable degrees of fractional crystallization. The felsic igneous rocks may have formed by partial melting of intermediate to felsic, calc-alkaline rocks along the southern and eastern margins of the Kimberley Craton, following accretion of various earlier Palaeoproterozoic terranes to the craton. Therefore, models for Palaeoproterozoic high-K granites in northern Australia that invoke intracratonic rifting of a stable Archaean craton may need to be revised. Published by Elsevier Science B.V.
Keywords:Palaeoproterozoic; Granites; Felsic volcanics; Geochronology; Geochemistry; Halls Creek Orogen
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1. Introduction
Wyborn (1988) estimated that granites and fel-sic volcanic rocks of c. 1880 – 1840 Ma age in northern Australia cover at least 37 000 km2. In
the Kimberley region of northwestern Australia,
I-type granites, high-level porphyry intrusions and felsic volcanic rocks of this age cover more than 11 000 km2
. They constitute a major element of the Palaeoproterozoic Hooper and Lamboo Com-plexes (Fig. 1), and form the largest coherent felsic igneous province in northern Australia. Some of the high-level porphyry intrusions grade into adjacent felsic volcanic rocks or coarse-grained granites (Dow et al., 1964; Gellatly et al., 1975; Griffin et al., 1993), and the similarity in
* Corresponding author. Fax: +61-8-92223633.
E-mail addresses: rpage@agso.gov.au (R.W. Page), s.sheppard@dme.wa.gov.au (S. Sheppard)
T.J.Griffin et al./Precambrian Research101 (2000) 1 – 23 2
chemical compositions of the three rock types is consistent with them being cogenetic (Griffin and Tyler, 1992a).
The Palaeoproterozoic granites of northern Aus-tralia, including the pre-1840 Ma granites in the Kimberley, are mainly potassic and silicic I-types. The high K2O and SiO2contents distinguishes them
from granites at Phanerozoic convergent margins, and Wyborn (1988) and Wyborn et al. (1992) interpreted them as being generated in a series of linked intracratonic rifts within a stable Archaean craton. The source for these granites was thought to be a locally fractionated mafic underplate, em-placed craton-wide at 2300 – 2100 Ma.
The Palaeoproterozoic Hooper and Lamboo Complexes of the Kimberley region of Western Australia were interpreted by Hancock and Rut-land (1984) and Page and Hancock (1988) as an intracratonic rift. This model was largely based on the assumption that turbidite deposition was con-temporaneous throughout the Hooper and Lam-boo Complexes. However, Tyler et al. (1999) showed that turbidites in the eastern part of the Lamboo Complex were deposited at least eight million years after turbidites in the Hooper Com-plex and western part of the Lamboo ComCom-plex were deformed, metamorphosed and intruded by granite. Recent systematic regional mapping of the Palaeoproterozoic rocks in the Kimberley re-gion indicates that the two complexes consist of three fault-bounded zones (terranes) with different geological histories (Tyler et al., 1995, 1999). The presence of tectonostratigraphic terranes suggests that the evolution of the Hooper and Lamboo Complexes can be explained by modern-style plate tectonic processes. The data outlined below also argue against ensialic rifting of a stable Ar-chaean craton, and are consistent with a post-col-lisional setting for the felsic magmatism. Here we present SHRIMP U – Pb zircon data and whole-rock geochemical data, which show that the vol-canic rocks, porphyries and older granites in the Kimberley region form an 1865 – 1850 Ma felsic magmatic association. Large volumes of younger granite of 1835 – 1790 Ma age (Page and Sun, 1994; Sheppard et al., 1995) also intruded the Lamboo Complex (Fig. 1) but they are not con-sidered in this paper.
2. Regional geology
The Hooper and Lamboo Complexes initially formed in the Palaeoproterozoic between the Kimberley and North Australian Cratons (Fig. 1). The two complexes form a belt of crystalline rocks about 700 km long and at least 100 km wide along the southern and eastern margins of the Kimberley Craton. The Lamboo Complex con-sists of three parallel north-northeast trending zones (Fig. 1), which Tyler et al. (1995) inter-preted as tectonostratigraphic terranes. The
Hooper Complex is a continuation of the Western zone of the Lamboo Complex (Tyler et al., 1995). The 1865 – 1850 Ma felsic igneous rocks described here dominate the Hooper Complex and the Western zone of the Lamboo Complex. They predated final collision and suturing of the two cratons at c. 1820 Ma, which was marked by granite intrusion into all three zones of the Lam-boo Complex (Fig. 1).
2.1. Hooper Complex and Western zone of the
Lamboo Complex
The oldest rocks exposed in the Hooper Com-plex and Western zone of the Lamboo ComCom-plex are low- to high-grade turbiditic metasedimentary rocks of the Marboo Formation (Figs. 2 and 3). These rocks have a maximum age of 1870 Ma constrained by U – Pb ages of detrital zircons (Tyler et al., 1999). The Marboo Formation was intruded by a series of mafic sills collectively referred to as the Ruins Dolerite (Griffin et al., 1993). The Marboo Formation and the Ruins Dolerite were deformed and metamorphosed be-fore being overlain by felsic volcanic and volcani-clastic rocks of the Whitewater Volcanics. All three units were intruded by porphyries, granites, and gabbros.
T.J.Griffin et al./Precambrian Research101 (2000) 1 – 23 4
Fig. 2. Simplified geology of the central part of the Hooper Complex in the west Kimberley. Also shown are the locations and numbers of samples dated in this paper.
Hoatson, 1993; Sheppard, 1996). The granites and porphyry intrusions have been placed into the Paperbark supersuite along with the gabbro intru-sions, owing to their close temporal, spatial, and probably, genetic associations (Sheppard et al., 1997a).
Bennett and Gellatly (1970) obtained Rb – Sr isotopic data that were recalculated by Page (1976), to yield isochrons of 19129107 Ma for the Whitewater Volcanics and associated por-phyries, and 1840950 Ma from the granites of the Hooper Complex. Page et al. (1995) obtained a SHRIMP U – Pb zircon date of 185592 Ma from the Toby Gabbro, an intrusion of massive quartz-biotite bearing gabbro in the Western zone of the Lamboo Complex. Contacts between this gabbro intrusion and adjacent coarse-grained granites are marked by net-vein complexes, indi-cating that the gabbro and granites were coeval.
A second episode of deformation and metamor-phism occurred either before, or during granite intrusion. Metamorphic grade was generally low, although in the central part of both the Hooper Complex and the Western zone of the Lamboo Complex, high-grade migmatitic metasedimentary rocks and rare anatectic S-type granites are present (Griffin et al., 1993; Tyler et al., 1997).
The Hooper Complex and Western zone of the Lamboo Complex were unconformably overlain by sedimentary and mafic volcanic rocks of the Speewah and Kimberley Basins between 1835 and 1800 Ma (Page and Sun, 1994).
2.2. Central zone of the Lamboo Complex
rocks is not recognized. Protoliths to the Tick-alara Metamorphics mainly consist of mafic vol-canics and turbiditic sediments. They were intruded by the sheet-like Rose Bore Granite at c. 1863 Ma (Tyler and Page, 1996). At c. 1850 – 1845 Ma the volcanic and sedimentary rocks were in-truded by sheets of tonalite and minor trond-hjemite, and metamorphosed at high grade (Page and Sun, 1994; Bodorkos et al., 1999). Following high-grade metamorphism and deformation, the Tickalara Metamorphics were overlain by sedi-ments and mafic and felsic volcanics of the Koongie Park Formation at c. 1845 – 1840 Ma. The Central zone was extensively intruded by granite and gabbro at 1835 – 1805 Ma (Sheppard et al., 1997b).
2.3. Eastern zone of the Lamboo Complex
The Eastern zone consists of low-grade metasedimentary and mafic metavolcanic rocks of the Halls Creek Group (Griffin and Tyler, 1992b), which unconformably overlie 1920 – 1900 Ma granite and volcanic rock (Blake et al., 1999). Fluvial quartz sandstone at the base of the Halls Creek Group was overlain by mafic and minor felsic volcanic rock at c. 1880 Ma. These were in turn overlain disconformably by alkaline, in-traplate-type volcanic rocks dated at 1857 – 1848 Ma (Blake et al., 1999). Detrital zircons from a thick sequence of turbidites above the alkaline volcanic rocks indicate deposition after c. 1847 Ma (Blake et al, 1999). Therefore, the upper part of the Halls Creek Group was being deposited while rocks of the Central zone were metamor-phosed at high grade (Tyler et al., 1995). This suggests that the Eastern and Central zones were not adjacent to each other at this time.
3. Geology and SHRIMP geochronology
Detailed descriptions of techniques for SHRIMP U – Pb zircon analysis at the Research School of Earth Sciences, ANU, are given by Compston et al. (1984, 1986). Decay constants used in this paper are those recommended by the IUGS Subcommission on Geochronology (Steiger and Jaeger, 1977). Analytical data for all the samples discussed here are available from the authors on request.
3.1. Whitewater Volcanics
The exact thickness of the Whitewater Vol-canics is unknown, but it may be between 2000 and 3000 m (Sofoulis et al., 1971; Gellatly et al., 1975). The volcanic rocks consist of rhyolitic to dacitic ignimbrites with subordinate coherent lavas, and minor volcanic lithic breccia (Gellatly et al., 1975; Sheppard et al., 1997b). The lower-most part of the Whitewater Volcanics includes abundant felsic volcaniclastic sandstone and sub-ordinate conglomerate.
T.J.Griffin et al./Precambrian Research101 (2000) 1 – 23 6
The ignimbrites contain abundant recrystallized pumice fragments and glass shards, and they are commonly rich in volcanic lithic fragments (Shep-pard et al., 1997b). Eutaxitic textures are common in thin section, even where the rocks appear mas-sive in outcrop. Columnar jointing is locally well-developed. The ignimbrites contain crystals and fragments of plagioclase, sanidine, quartz, and minor chloritized mafic mineral(s). Coherent lava flows are typically massive, although flow-banding is locally present, and contain few lithic frag-ments. Many of the lavas are weakly porphyritic, but in places they are strongly porphyritic. The lavas contain phenocrysts of euhedral to subhe-dral plagioclase and sanidine, embayed and bi-pyramidal quartz, and recrystallized biotite. Least-altered samples of andesitic to dacitic com-position contain phenocrysts of hypersthene, and rare chloritized hornblende. The groundmass con-sists of very fine grained granular untwinned feldspar, quartz, sericite and chlorite. The ground-mass to all the lavas and ignimbrites is thoroughly recrystallized, and most samples are at least partly altered.
Page and Hancock (1988) reported a conven-tional U – Pb zircon age of 185095 Ma for a sample of the Whitewater Volcanics from the Lamboo Complex. The sample (8559.8005, Fig. 3) was re-analyzed by SHRIMP in this study, along with one from the Hooper Complex (sample num-ber 8759.8008, Fig. 2). Sample 8759.8008 contains a single population of concordant to sub-concor-dant zircons defining an igneous crystallization age of 185495 Ma (Fig. 4a). The zircons in sample 8559.8005 contain some cores, and SHRIMP analyses of grain 53.1 (2485 Ma) and the core of grain 51.2 (1984 Ma) reflect inheri-tance. Except for the one discordant analysis 50.1, the remaining 20 zircon data points define a crys-tallization age of 185794 Ma (x2=1.10; Fig.
4a). These two indistinguishable ages suggest that felsic volcanism was contemporaneous over a strike length of more than 600 km.
3.2. High-le6el porphyry intrusions of the
Paperbark supersuite
Most of the high-level porphyry intrusions
probably have a sheet-like form, although some form stocks. None of the porphyry intrusions are zoned, but the presence of intrusive contacts within some intrusions suggests that they are the product of several magma batches. Intrusions such as the Greenvale Porphyry consist of a series of sheets with differing grainsizes and slightly different compositions. Individual sheets have sharp contacts, but internally they are quite homogeneous.
The porphyries contain up to 65% phenocrysts of bipyramidal and embayed quartz, subhedral plagioclase, and tabular or rounded K-feldspar, with minor mafic minerals. Some intrusions con-tain abundant rounded phenocrysts of K-feldspar up to 4 cm in diameter, some of which have thin rims of plagioclase. Plagioclase phenocrysts in some rocks are mantled by K-feldspar. The main mafic mineral is biotite, which is commonly partly altered to chlorite. Hornblende and orthopyrox-ene may accompany biotite in rocks of intermedi-ate composition. The groundmass is composed of very fine- to fine-grained quartz and feldspar, with minor sericite, clinozoisite, epidote, biotite, chlor-ite, and accessory zircon, sphene, iron oxides, apatite and calcite. Most samples are altered or recrystallized.
Tyler et al. (1999) obtained an igneous crystal-lization age of 185895 Ma for the Richenda Microgranodiorite in the Hooper Complex. This sample contains some zircons with xenocrystic cores that yield ages of 2465, 2075 and 1940 Ma. This is the only analyzed felsic igneous rock with substantial zircon inheritance; the inherited zir-cons are probably related to the presence of abun-dant biotite-rich, metasedimentary inclusions in the pluton.
Two samples of high-level porphyry intrusions were analyzed in this study; one from the Mon-dooma Granite of the Hooper Complex (sample 8759.8007, Fig. 2) and one from the Greenvale Porphyry in the Western zone of the Lamboo Complex (sample 9452.6039, Fig. 3). The 17 ana-lyzed zircons from the Mondooma Granite define an igneous crystallization age of 186294 Ma (x2=0.98; Fig. 4b). Nineteen of the 20 analyzed
Fig. 4. U – Pb concordia plots for dated samples in the Hooper Complex and Western zone of the Lamboo Complex. Individual SHRIMP error boxes are 1sanalytical uncertainties, but pooled 207Pb/206Pb ages have uncertainties quoted as 95% confidence limits. (a) Samples of the Whitewater Volcanics from the Hooper Complex (8759.8008) and Lamboo Complex (8559.8005). (b) High-level porphyry intrusions from the Hooper Complex (Mondooma Granite, sample 8759.8007) and Lamboo Complex (Greenvale Porphyry, sample 9452.6039). (c) Coarse-grained granite samples from the Hooper Complex. Massive sample of Lennard Granite (8759.8011) and foliated sample of Lennard Granite (8759.8009). (d) Coarse-grained granite samples from the Hooper Complex. Samples of McSherrys Granodiorite (8759.8013) and Kongorow Granite (8759.8010).
igneous crystallization age of 185594 Ma (x2=
0.67; Fig. 4b). The sample also contains one older grain (309-1) indicating inheritance at c. 1960 Ma.
3.3. Granite and gabbro of the Paperbark
supersuite
The granites consist of medium- to coarse-grained, porphyritic and even-textured biotite monzogranite, with subordinate syenogranite, granodiorite and tonalite. The intrusions are not zoned, but some consist of several different rock types with intrusive contacts. The porphyritic granites contain tabular or round phenocrysts of microperthitic microcline up to 6 cm long.
T.J.Griffin et al./Precambrian Research101 (2000) 1 – 23 8
Small, rounded, fine-grained mafic enclaves are common in tonalite and granodiorite, and in some silicic intrusions. The enclaves may be either even-textured, or contain K-feldspar megacrysts similar to phenocrysts in the host granite. The enclaves have the same mineralogy and textures as gabbro intrusions thought to be coeval with the granites. Some intrusions dominated by tonalite and gran-odiorite also contain widespread mafic clots up to 3 cm in diameter. They have a decussate texture and are composed of either plagioclase and bi-otite, or plagioclase and hornblende in horn-blende-bearing host granites. Associated with these clots are scattered quartz oikocrysts with inclusions of plagioclase, biotite, and acicular apatite.
Many of the granites in the Hooper Complex were moderately to strongly recrystallized during the Mesoproterozoic. Even in weakly deformed samples from both complexes, plagioclase is partly replaced by sericite and clinozoisite, and biotite is commonly chloritized.
Coeval gabbro intrusions are composed of mas-sive and fine- to medium-grained, biotite-bearing norite, gabbronorite, gabbro and diorite. Biotite comprises about 5 – 10% of the rocks, and forms megacrysts and crystals in the groundmass. Quartz is typically interstitial to the pyroxenes and plagioclase (Sheppard et al., 1997b). Many of the intrusions contain veins and irregular patches of xenocrystic hybrid rock, granite and quartz diorite. The gabbros also grade into intrusions of hybrid rock composed of biotite-bearing norite through quartz diorite to rare tonalite. These intrusions are heterogeneous, and are character-ised by rounded xenocrysts of quartz and plagio-clase. Plagioclase xenocrysts have sieved margins and fine rims of microcline. Both plagioclase and quartz are enclosed by fine-grained orthopyroxene
and biotite. Net-vein complexes commonly mark contacts between the gabbros and hybrid rocks, and the granites. Collectively these features indi-cate that the granitic and gabbroic magmas were broadly coeval, and that the gabbros and hybrid rocks are the products of assimilation or mixing with variable amounts of granitic magma (Blake and Hoatson, 1993; Sheppard, 1996).
Four samples of the coarse-grained granites were collected for SHRIMP U – Pb zircon geochronology (Fig. 2). Two of these are monzo-granites from the extensive Lennard Granite in-trusion. One sample is an undeformed granite (sample number 8759.8011, Fig. 2) and the other is a strongly foliated granite with feldspar porphy-roclasts (sample 8759.8009, Fig. 2). The zircon crystals in both samples are euhedral. In the undeformed sample, the zircons have fine oscilla-tory zoning and no cores with identifiably differ-ent characteristics in the cathodoluminescdiffer-ent images (Fig. 5a). All 18 data points define an igneous crystallization age of 186494 Ma (x2=
1.38; Fig. 4c). The 207Pb/206Pb age for grain 12-1
is slightly older (1908921 Ma, 1s), but its
exclu-sion makes no difference to the pooled result. In the strongly deformed sample, although one of the zircon U – Pb analyses is discordant (grain 59-1; Fig. 4c), all 16 data points indicate a crystal-lization age of 186295 Ma (x2=1.22; Fig. 4c).
Two granodiorites were also analyzed. Sample 8759.8013 comes from the McSherrys Granodior-ite, a weakly foliated body that is in tectonic contact with the Marboo Formation (Fig. 2). Zircons from this sample are small and generally only weakly zoned (Fig. 5b). Small cores are apparent in some crystals. Because of the small size of the zircons, the SHRIMP ion beam over-lapped some of these cores (e.g. analysis 26.1), but these analyses give ages indistinguishable from the
Fig. 5. Cathodoluminescence SEM images of zircon grains from granitic rocks of the Paperbark supersuite: (a) undeformed Lennard Granite (8759.8011) showing fine igneous zonation from rim to centre in most of the pristine euhedral grains, and the 30
mm-diameter SHRIMP analytical crater for grain 7.1 (207Pb/206Pb age 185498 Ma); (b) McSherrys Granodiorite (8759.8013) showing weakly zoned, broken euhedral grains. Grain 26 appears to have an unzoned core, but the overlapping SHRIMP analytical spot did not detect any measurable age difference to the rims. Image shows the 30mm-diameter SHRIMP analytical craters for
grains 26.1 (207Pb/206Pb age 187497 Ma), 27.1 (186497 Ma) and 28.1 (185697 Ma); (c) Kongorow Granite (8759.8010) showing fine igneous zonation in euhedral grains, and the 30mm-diameter SHRIMP analytical craters for grains 52.1 (207Pb/206Pb age
T
Representative analyses of 1865–1850 Ma igneous rocks from the Kimberley region, Western Australia
Coarse-grained granites Whitewater Volcanics High-level porphyry intrusions Gabbros
124609 124446a 92776a 92721
Sample 108758 99362 95422 92777a 92783a 118172 92782a 95353a 8759.8011 99308 99331
Felsic vol- Micro-gr- Micro-gran- Feldsp–qtz
Felsic vol- Micro-mon- Biotite gab- Quartz Grndiorite
Rock type Porphyry Felsic vol- Quartz gab- Porph. Porph. Micro-diorite
canic zogrt Monzgrt
canic monzgrt
canic diorite ite porphyry bro bro monzgrt
Zone 52 Zone 52 Zone 51 Zone 51 Zone 52 Zone 51 Zone 51
Zone 51 Zone 51
Zone 51 Zone 51 Zone 51
Locationb Zone 52 Zone 51 Zone 51
746380634
310680301 811880403 653681598 720881033 423281865 372281013 725181060 703680967 366980660 724380566 690381030 701580940 784580603 804780472
Wt%
69.22 72.20 75.00 51.70 52.00 56.80 65.00
66.10 69.64
72.60 76.20 72.00 75.30
SiO2 62.50 68.30
0.47
0.85 0.41 0.17 0.19 0.59 0.28 0.19 0.87 0.90 0.95 0.82 0.28 0.24 0.09 TiO2
13.64 13.50 13.00 16.70 16.80 13.70 15.90
Al2O3 16.00 14.50 13.60 12.10 14.70 15.29 13.40 12.20
1.02 0.50 2.12 1.00 1.06 1.74 1.61
1.39 0.33
0.52 0.06 0.24
Fe2O3 1.43 0.72 1.15
3.62
6.42 2.30 1.06 0.71 3.67 2.01 0.00 7.59 8.40 6.63 4.12 2.04 2.26 1.03 FeO
0.06 0.02 0.03 0.13 0.13
MnO 0.09 0.06 0.02 0.16 0.08 0.13 0.07 0.02 0.04 0.03
0.86 0.37 0.26 6.24 4.64 6.12 1.62
3.43 0.50
0.20 0.52 0.21
MgO 2.94 1.03 0.19
1.97 1.51 1.77 11.30 10.60 6.45 4.41
CaO 4.18 3.17 1.65 0.90 3.94 3.36 2.08 1.14
2.11 2.14 2.63 1.84 1.63 1.79 2.77
2.28 2.72
Na2O 1.63 2.45 2.67 2.84 2.68 2.69
2.39
2.97 4.39 5.64 4.65 4.67 5.31 4.64 0.71 0.84 2.43 2.50 4.38 4.77 5.26 K2O
0.14 0.09 0.06 0.19 0.13 0.26 0.22
0.11 0.06
0.05 0.22 0.20 0.44 0.29 0.26 0.27
0.39 –
0.22 0.29 0.19 0.25 0.21 0.29 0.21
0.19 0.36
0.21 0.18 0.12
Rest 0.25 0.19 0.18
100.45
100.75 99.82 99.55 100.24 100.02 99.28 100.70 99.71 98.31 99.60 100.51 99.34 99.35 99.11 Subtotal
0.00 0.00 0.00 0.08 0.00 0.03 0.05 0.01 0.01 0.03 0.00 0.01 0.01
‘O=S,F,Cl’ 0.02 0.00
100.02 99.20 100.70 99.68 98.26 99.59 100.50 99.31
100.45 99.35
Total 100.73 99.81 99.55 100.24 99.10
PPM
740 604 681 242 224 664 625
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Table 1 (Continued)
Coarse-grained granites Whitewater Volcanics High-level porphyry intrusions Gabbros
124609 124446a 92776a 92721
Sample 108758 99362 95422 92777a 92783a 118172 92782a 95353a 8759.8011 99308 99331
Felsic vol- Micro-gr- Micro- Feldsp–qtz Quartz Biotite Quartz Grndiorite Porph.
Rock type Porphyry Felsic vol- Felsic vol- Micro- Porph.
Micro-canic Monzgrt
canic
canic diorite granite porphyry monzogrt gabbro gabbro diorite monzgrt monzgrt Zone 52 Zone 52 Zone 51 Zone 51 Zone 52 Zone 51 Zone 51
Zone 51 Zone 51
Zone 51 Zone 51 Zone 51
Locationb Zone 52 Zone 51 Zone 51
423281865 372281013 725181060 703680967 366980660 724380566 690381030 701580940 784580603
310680301 811880403 653681598 720881033 746380634 804780472
51.1 – 52.9 25.0 22.7 35.9 32.3
49.9 31.9 – 30.9 38.5
51.7 37.8
24.0 Nd
9.8 – 10.0 5.8 5.3 7.4 6.7 –
Sm 4.7 6.9 9.8 10.9 6.2 6.1 11
1.7 – 1.4 1.7 1.4 1.5 1.5
1.0 –
Eu 1.4 1.3 0.7 0.9 4.9 10
9.7 – 7.3 4.6 6.1 6.4 5.6
Gd 3.1 6.7 9.3 8.5 5.3 – 1 0.5
7.6 – 5.9 3.9 5.8 4.4 4.8
3.9 –
Dy 2.3 5.3 8.0 8.0 5.9 9.9
1.5 – 1.3 0.8 1.4 0.9 1.0 – 1 2.1
Ho B0.5 1.0 1.5 1.8 0.9
4.5 – 3.3 2.2 3.3 2.2 2.5
2.0 –
4.6 3.2 6.1
Er 0.9 2.8 4.4
4.4 – 3.0 2.0 3.3 2.1 2.5 – 2.9 5.9
Yb 1.1 2.7 4.0 4.5 2.0
0.7 – 0.4 0.3 0.7 0.3 0.4 – 0.4 0.9
0.3 Lu B0.5 0.4 0.5 0.7
aGeochemistry sample taken from same site as dated sample: 92777=8759.8008; 92783=8759.8015; 92776=8759.8007; 92782=8759.8013; 95353=8759.8010; 124446=9452.6039.
bLocations are given in Australian Map Grid co-ordinates to the nearest 100 m. Numbers 8759.XXXX are Australian Geological Survey Organisation samples, the remainder are Geological Survey of Western Australia
T.J.Griffin et al./Precambrian Research101 (2000) 1 – 23 12
Table 2
Sm–Nd whole-rock data for 1865–1850 Ma igneous rocks in the Kimberley regiona
Rock unit
Sample Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/ 2s T(Ma) oNd
i TDM
144Nd
7.32 41.01 0.1079 0.511372
Mondooma Ganite 7
8759.8007 1862 −3.5 2542
Lennard Granite
8759.8011 6.34 42.49 0.0903 0.511144 6 1864 −3.7 2460
6.62 36.35 0.1102
8759.8009 Lennard Granite 0.511386 7 1862 −3.7 2577
6.87 38.77 0.1070 0.511405
Kongorow Granite 6
8759.8010 1852 −2.7 2474
Neville Granodiorite
108532 4.83 24.38 0.1198 0.511542 10 1860 −3.0 2589
Paperbark Granite
113416 8.30 45.79 0.1096 0.511438 10 1854 −2.7 2487
7.80 39.00 0.1229 0.511541
Whitewater Volcanics 15
92777 1855 −3.8 2679
91085 Whitewater Volcanics 6.90 39.00 0.1083 0.511354 8 1855 −4.0 2577 2.80 12.20 0.1373 0.511800
Wombarella Quartz 9
95313 1850 −2.2
Gabbro
Wombarella Quartz
92721 4.00 19.60 0.1247 0.511711 8 1850 −1.0
Gabbro Toby Gabbro
8333.0052 4.69 21.97 0.1292 0.511754 14 1855 −1.1
aSamples 8759.XXXX were analyzed at the Australian National University. Samples of the Whitewater Volcanics and Wombarella Quartz Gabbro were analyzed by Ian Fletcher at Curtin University of Technology. Samples 108532 and 113416 were analyzed at LaTrobe University. Sample 8333.0052 is from Sun and Hoatson (in press). All data have been recalculated relative to La Jolla=0.511860. TDM calculated assuming mantle depletion beginning at 4.56 Ga, and using 143Nd/144Nd=0.513144 and 147Sm/144Nd=0.2136 for present day depleted mantle.
remainder of the analyses. Therefore, the cores are not significantly older than the rest of the zircon crystal. Nineteen of the 20 analyses, with the exception of analysis 33-1, form a single popu-lation (x2=1.08) that defines an igneous
crystal-lization age of 186194 Ma (Fig. 4d).
Sample 8759.8010 (Fig. 2) is from the Kon-gorow Granite, a thoroughly recrystallized gneis-sic granodiorite containing garnet. The zircons from this sample show concentric oscillatory zon-ing; some crystals contain unzoned cores. How-ever, analyses that overlap these cores (e.g. 52.1 and 53.1, Fig. 5c) give ages indistinguishable from analyses from zoned rims, indicating that the cores are not significantly older. All 17 analyses from this rock plot on or near the concordia curve, defining an igneous crystallization age of 185294 Ma (x2=1.33; Fig. 4d). This result is
significantly younger (different at the 1% proba-bility level using Student’st-test) than other dated granites in the Hooper Complex. However, the intrusion shares the same field relationships as the other dated granites, and is inseparable from ages for the high-level porphyries and Whitewater Volcanics.
Collectively, the Whitewater Volcanics, and high-level porphyry intrusions and granites of the Paperbark supersuite represent a period of volu-minous felsic magmatism at 1865 – 1850 Ma. However, the data presented do not allow us to determine whether the volcanic rocks have the same age range as the granites, or if they repre-sent a more restricted episode of magmatism. All of the dated samples appear to have a paucity of significantly older inherited zircon crystals.
4. Major and trace element and isotope chemistry
two Whitewater Volcanics, and three gabbros) were also analyzed for their Nd isotopic compo-sitions (Table 2).
4.1. Major and trace element chemistry
The rocks range from about 46 – 80 wt% SiO2,
with a pronounced minimum at 58 wt% SiO2
(Fig. 6). All of the analyses below 59 wt% SiO2
belong to intrusions of gabbro and hybrid rock, or mafic enclaves in granite plutons, with the exception of three granite samples (Fig. 7). These three samples have been moderately to strongly overprinted by low- to medium-grade metamorphic assemblages, and their relationship to the remainder of the granites is unclear. The volcanic rocks and high-level porphyry intru-sions do not contain any mafic compositions. Among the felsic rocks, the volcanic rocks are overall slightly more mafic than the granites and high-level porphyries (Fig. 6). The porphyries and granites have a peak at :70 – 72 wt% SiO2,
whereas the volcanic rocks have a peak at :68 wt%.
The volcanic rocks, porphyries and granites are I-types with high K2O, Rb, Y, Th, and
K2O/Na2O, and low Sr, Sr/Y and K/Rb (Fig. 7;
Sheppard et al., 1997c). They also have high
contents of large ion lithophile elements (LILE) such as Ba, Rb, K and Th, relative to the high field strength elements (HFSE) such as Zr, Y and Nb (Sheppard et al., 1997c). The three groups of rocks share similar rare earth element (REE) abundances, although the granites show a wider range. A minority of granites and vol-canic rocks have positive Eu anomalies or very small negative Eu anomalies (Fig. 8). These rocks contain cumulate feldspar, and are proba-bly cumulate rocks. Granites of the Paperbark supersuite are similar to Palaeoproterozoic gran-ites from elsewhere in northern Australia (e.g. Wyborn, 1988).
The gabbros and hybrid rocks of the Paper-bark supersuite range from about 46 to 55 wt% SiO2 (Fig. 6). There are no trends apparent
within the group, and for some elements (e.g. Sr, Fig. 7), the gabbros and hybrid rocks do not plot on the same overall trend as the granites. The gabbros have REE contents that overlap with those of the granites, but chondrite-nor-malised patterns for the gabbros are less frac-tionated ([La/Yb]N=5) than for the granites
([La/Yb]N=10 – 30) (Fig. 8b). The gabbros plot in the tholeiite field on an AFM diagram (Shep-pard et al., 1995), and have incompatible trace element abundances similar to many continental tholeiites (Sun and Hoatson, in press).
The mafic enclaves within the coarse-grained granites have similar compositions to the intru-sions of gabbro and hybrid rock of the Paper-bark supersuite (Fig. 9a), but with on average, higher K2O, Ba, Rb, Zr and Th contents. These
relative abundances may be explained by con-tamination of the enclaves by the host granite magma. One of the enclaves has a large positive Eu anomaly (Fig. 9b) consistent with incorpora-tion of feldspar phenocrysts from the granite host. No analyses were obtained from the smaller mafic clots in some granodiorite and tonalite intrusions.
There is compositional heterogeneity within the granites and the porphyries. For example, analyses from the Greenvale and Castlereagh Hill Porphyries do not plot on trends defined by the Bickleys Porphyry and the Mondooma Granite (Fig. 10). Some of the porphyries (e.g.
T.J.Griffin et al./Precambrian Research101 (2000) 1 – 23 14
Fig. 7. Harker diagrams for the Whitewater Volcanics, porphyries, and granites and gabbros of the Paperbark supersuite.
Mondooma Granite) have a large range in SiO2
contents, whereas others, such as the Greenvale Porphyry, have a very restricted range. Trends for individual granite intrusions (not shown) show more scatter than for the porphyries, but it is still apparent that not all intrusions plot on the same trends. The presence of different chemical trends in the intrusive rocks indicates that the volcanic rocks, porphyries and granites are not comag-matic; that is, they do not represent a single magma lineage.
4.2. Nd isotope compositions
Neodymium isotope compositions for six sam-ples of coarse-grained granite, two samsam-ples of the Whitewater Volcanics, and three samples of bi-otite-bearing gabbro are shown in Table 2 and Fig. 11. The samples of granite and Whitewater Volcanics define a narrow range in initialoNdfrom
−2.9 to −4.2. The values at either end of this range are almost within error of each other, given an uncertainty of about 90.5 oNd units. In
values of −1.1 to −2.4. Depleted mantle model ages for the granites range from about 2500 – 2600 Ma (or 2230 – 2320 Ma using the calculation of McCulloch, 1987), similar to most other Palaeoproterozoic granites of northern Australia. Fig. 11 shows that the granites must contain a large proportion of older crust, but they cannot be derived by wholesale melting of Archaean crust, which McCulloch (1987) also noted.
5. Petrogenesis
The gabbros of the Paperbark supersuite crys-tallized from tholeiitic magmas separate from the felsic magmas that formed the coeval Whitewater Volcanics, porphyries and coarse-grained granites. The presence of disseminated biotite and quartz in
the gabbro intrusions, and the orthopyroxene-rich nature of many of the rocks, suggests that they assimilated some granitic magma. In addition to abundant field evidence of mingling and limited hybridization at the level of emplacement (Blake and Hoatson, 1993; Sheppard, 1996), there is evidence for limited mixing before intrusion. The relatively uniform distribution of quartz and bi-otite in the gabbros indicates that some assimila-tion occurred before emplacement. The abundance of quartz and plagioclase xenocrysts in the hybrid rocks show no spatial relationship to enclosing granite veins, suggesting that most of the xenocrysts were incorporated at depth (Shep-pard, 1996).
Although mixing was important in the genera-tion of the hybrid rocks, there is little evidence for mixing in most of the granites. This conclusion is
T.J.Griffin et al./Precambrian Research101 (2000) 1 – 23 16
Fig. 9. (a) Samples plutons of gabbro and hybrid rock, and mafic enclaves in granites normalized to average E-MORB of Sun and McDonough (1989); (b) chondrite-normalized rare earth element plot for plutons of gabbro and hybrid rock, and mafic enclaves in granites. Normalising values from Nakamura (1974).
fractionation from intermediate granites. Some silicic intrusions (e.g. the Greenvale Porphyry) do not plot on the same trends as the intermediate intrusions (Fig. 10), and the two therefore, cannot be related by crystal fractionation. The presence of positive Eu anomalies in some of the granites and volcanic rocks suggests that they are the cumulate products of fractional crystallization. The presence of sharp internal contacts in many granite and porphyry intrusions indicates that they were constructed from a number of magma batches. The magma batches may be of similar or contrasting compositions. Collectively the data are consistent with variable amounts of crystal fractionation superimposed on a range of primary intermediate to silicic magmas.
There are few constraints on the age of the source for the 1865 – 1850 Ma granites, porphyries and felsic volcanic rocks in the Kimberley; no basement is exposed, and the felsic igneous rocks appear to contain very few xenocrystic zircons. Experimental data indicate that potassic and sili-cic I-type granites, such as those studied here, can be generated by melting of calc-alkaline igneous rocks at moderate pressure (:8 – 10 kbar) and at temperatures of 900°C or more (Conrad et al., 1988; Rutter and Wyllie, 1988; Singh and Johan-nes, 1996). The granites in the Kimberley were accompanied by intrusion of numerous gabbro plutons, and it was probably the initial emplace-ment of these mafic magmas into the lower crust that induced crustal melting (e.g. Huppert and Sparks, 1988). High temperatures of partial melt-ing may explain the paucity of inherited zircon in felsic igneous rocks of the Kimberley. At tempera-tures greater than about 850°C, only the largest zircons in the source are likely to survive melting (Watson, 1996).
6. Comparison with Phanerozoic granites
The composition of felsic igneous rocks is largely a function of the source composition and mineralogy, but granites from the same tectonic setting commonly share some broad similarities. Sheppard et al. (1997a) noted that the rocks stud-ied here are more silicic and potassic than consistent with a lack of field or textural evidence
for magma mixing in the volcanic rocks and porphyries. An exception however, may be some of the tonalite- and granodiorite-dominated intru-sions that contain widespread mafic clots. The mineralogy and textures of these clots are similar to microgranitoid enclaves described by Vernon (1991) and attributed to magma mixing. These intrusions may have formed in part by mixing (i.e. large-scale homogenization) of mafic and felsic magmas.
The compositional gap between 55 and 61wt% SiO2 in the Paperbark supersuite is incompatible
Fig. 10. Harker variation diagrams for some of the individual porphyry intrusions.
Phanerozoic batholiths developed at convergent continental margins, such as the Peninsular Ranges batholith of California or the Coastal batholith of Peru. In addition, the 1865 – 1850 Ma Kimberley granites have Ba- and Sr-depleted, and Y-undepleted mantle-normalized patterns, which contrast with the Sr-undepleted and Y-depleted patterns of most Cordilleran granites (Wyborn et al., 1992). Most of the Cordilleran batholiths also display a range in initial oNd values, including
some with positive initial oNd values, indicating a
juvenile component in the granites (e.g. Pankhurst et al., 1988).
The compositions of the granites in the Hooper and Lamboo Complexes are broadly similar to high-K post-collisional granites from the pre-Cordilleran of South America (Mpodozis and Kay, 1992; Rapela et al., 1992), the southern Alps in Italy (Rottura et al., 1998), Dabieshan in east-central China (Ma et al., 1998), and the Badjal intrusive suite in far east Russia (Grigoriev and Pshenichny, 1998). These post-collisional intru-sions range from gabbro to syenogranite, but are generally dominated by monzogranite and gran-odiorite. In common with the Paperbark super-suite the rocks are high-K calc-alkaline to
shoshonitic in composition (Fig. 12).
Mantle-normalized patterns for granites of the Paperbark supersuite are similar to granites from the Badjal intrusive suite in Russia and Group III granites from Dabieshan (Fig. 13). These rocks have comparable abundances for most elements, and are all characterized by negative Ba and Sr anomalies, and are undepleted in Y relative to
T.J.Griffin et al./Precambrian Research101 (2000) 1 – 23 18
Fig. 12. Comparison of granites of the Paperbark supersuite with Phanerozoic high-K post-collisional granites. Field for Paperbark supersuite includes\95% of analyses. Data for Dabieshan granites from Ma et al. (1998); Badjal intrusive suite from Grigoriev and Pshenichny (1998); Triassic granites of Patagonia from Rapela et al. (1992); Triassic granites of Chile from Mpodozis and Kay (1992). Fields in (a) from Peccerillo and Taylor (1976).
Na. These characteristics are consistent with par-tial melting in the stability field of plagioclase. The sample from the Group II granites of Da-bieshan is characterised by small positive ‘spikes’ in Ba and Sr, and a strong depletion in Y. This pattern may be produced by extensive fractiona-tion of amphibole without plagioclase as sug-gested by Ma et al. (1998), or equilibration of the melt with amphibole or garnet either in the residue, or en route to the level of emplacement. Nevertheless, the felsic igneous rocks of the Kim-berley differ from Phanerozoic post-collisional granites in some respects. For example, the Kim-berley rocks have lower Na2O (and thus higher
K2O/Na2O ratios), and higher Rb and Y
con-tents than most post-collisional granites (Fig. 12). In addition, the Kimberley rocks show a positive correlation between Y and SiO2, whereas
post-collisional granites generally show a weak negative correlation (Fig. 12). The behaviour of Y in the Kimberley granites may reflect the ab-sence of hornblende in these rocks. Other chemi-cal differences may be related to the particular source composition for the Kimberley granites.
The Phanerozoic high-K post-collisional gran-ites generally have moderately or strongly negative initial oNd values indicating that they contain a
7. Tectonic setting
If the 1865 – 1850 Ma felsic igneous rocks did form in a post-collisional setting, then the colli-sion was not that of the Kimberley and North Australian Cratons, because in the Lamboo Com-plex the felsic igneous rocks are confined to the Western zone (Fig. 1). Furthermore, the rocks of the Eastern zone were not first deformed until sometime between c. 1845 and 1820 Ma (Shep-pard et al., 1999). The three north-northeast trending zones that make up the Lamboo Com-plex have different geological histories up to c. 1820 Ma, when they were all stitched by granite plutons (Tyler et al., 1995). This suggests that they were not in their current relative positions until about 1820 Ma; that is, about 30 m.y. after emplacement of the Paperbark supersuite and Whitewater Volcanics.
Tectonic models for the Kimberley during the Palaeoproterozoic are hampered by the lack of recognized basement anywhere in the Hooper Complex and the Western and Central zones of the Lamboo Complex. The 1865 – 1850 Ma felsic igneous rocks in the Kimberley, like other Palaeoproterozoic high-K, I-type granites in northern Australia, were thought by Wyborn (1988) and Wyborn et al. (1992) to have formed following underplating and rifting of a stable
Archaean craton. It has generally been assumed that the Kimberley Craton is Archaean in age, similar to other basement inliers exposed in north-ern Australia (Hancock and Rutland, 1984).
Turbiditic sediments of the Marboo Formation, which were sourced from the now unexposed Kimberley Craton (Hancock, 1991), mainly con-tain early Palaeoproterozoic detrital zircons (c. 2500 – 1910 Ma) (Tyler et al., 1999). The composi-tion of the sedimentary rocks suggests that they were derived from a mixture of mature continen-tal crust and recycled orogens or magmatic arcs (Tyler et al., 1999). Thus, the Kimberley Craton probably was largely formed and/or reworked by magmatic/orogenic events in the early Palaeoproterozoic, rather than consisting of a sta-ble Archaean craton. This conclusion is consistent with magnetic and gravity data that indicate the presence of several northeast-trending terrains in the basement to the Speewah and Kimberley Basins, which overlie the Kimberley Craton (Gunn and Meixner, 1998). These workers also identified an extensive linear gravity and magnetic high under the southeastern margin of the Speewah and Kimberley Basins. This feature can be explained by a sheet or series of lens-like slices of ultramafic rock dipping at about 30° to the northwest (Gunn and Meixner, 1998). Therefore, we suggest that the southeastern margin of the Kimberley Craton may have been a convergent margin, with subduction of oceanic crust to the northwest, during much of the early Palaeoproterozoic (Fig. 14a, c) before about 1900 Ma.
Of the three zones in the Lamboo Complex, the contrasts are particularly marked between the Central and Eastern zones (see Fig. 3, Sheppard et al., 1999; Fig. 3, Bodorkos et al., 1999). In the Eastern zone, metasedimentary and mafic metavolcanic rocks of the Halls Creek Group were deposited between c. 1880 and 1845 Ma on continental crust of the North Australian Craton (Fig. 14b, e). The long interval over which they were deposited, the absence of deformation and metamorphic events during this time, and the transition from continental to deeper marine sedi-mentation collectively indicate a passive margin setting (Sheppard et al., 1999). In addition, 1857 –
T.J.Griffin et al./Precambrian Research101 (2000) 1 – 23 20
Fig. 14. Possible tectonic settings of the felsic igneous rocks in the Kimberley region with (a) – (c) the Tickalara Metamorphics of the Central zone as an oceanic island arc/back arc, and (d) – (f) the Tickalara Metamorphics as an ensialic marginal basin.
1848 Ma alkaline volcanic rocks in the Halls Creek Group have Nd-isotopic compositions and incompatible element ratios similar to ocean is-land basalt ‘hot-spot’ type sources in intraplate settings (Taylor et al. 1995).
In the Central zone mafic volcanic and turbidite protoliths to the Tickalara Metamorphics were deposited between about 1867 – 1863 Ma. The maximum age is provided by detrital zircons that dominate the population in the turbidites (Page and Sun, 1994; Bodorkos et al., 1999), and the minimum age is provided by intrusion of a leuco-granite sheet (Tyler and Page, 1996). Metabasalts from the Tickalara Metamorphics consist of de-pleted and enriched types (Sheppard et al., 1999);
the depleted samples resemble oceanic island-arc/
At 1850 – 1845 Ma protoliths to the Tickalara Metamorphics were intruded by tonalite and trondhjemite sheets and metamorphosed at high grade. The sheets are characterised by high Na2O,
CaO, and Sr, and low K2O, Rb, LREE, Th, and
U, and have a chemistry similar to Phanerozoic tonalites and trondhjemites at continental margins associated with subduction or subsidiary back-arc spreading, and island arcs (Sheppard et al., 1997a). Initial oNd values for these rocks indicate
that they were not derived by melting of juvenile mafic rocks alone, and that the source contained a significantly older component (Sun and Hoatson, in press). This is compatible with an ensialic mar-ginal basin model for the Tickalara Metamor-phics. The deformation and metamorphism at 1850 – 1845 Ma may record the closure of the basin (Fig. 14f). Alternatively, if the Tickalara Metamorphics had formed an island arc, the Nd isotopes may reflect structural interleaving of fel-sic crust following accretion of the arc to the Kimberley Craton (Fig. 14c).
After 1845 Ma, northwestward-directed sub-duction underneath the Kimberley Craton contin-ued under the former marginal basin, or resumed following accretion of the arc to the craton. Sub-duction was terminated by collision of the Kim-berley Craton, including the combined Western and Central zones, with the North Australian Craton. This final collision was marked by intru-sion into all three zones of the Lamboo Complex of high-K granite.
Our tectonic model is somewhat similar to that proposed for the Triassic to Early Jurassic pre-Cordilleran granites of South America. These were formed in a broadly extensional setting along a continental margin following accretion of various terranes and continental fragments to the margin of Gondwana (Kay et al., 1989; Mpodozis and Kay, 1992). Subduction probably ceased fol-lowing accretion of a large continental fragment. A subsequent combination of slab collapse and lithospheric delamination was associated with de-compressional mantle melting along the continen-tal margin. The granites appear to be related to the initial stages of supercontinent rifting (Kay et al., 1989; Rapela et al., 1992). The tectonic setting of the Badjal intrusive suite in far east Russia
(Grigoriev and Pshenichny, 1998) may also be analogous to the 1865 – 1850 Ma granites and felsic volcanic rocks of the Kimberley. Intrusion of the Badjal intrusive suite (and eruption of coeval volcanic rocks) in the Late Cretaceous followed accretion of a microcontinent and sev-eral island arcs to the Asian continent at the end of the Early Cretaceous or start of the Late Cretaceous.
In the Kimberley, intrusion of mafic magma into the crust was probably a key factor in the generation of the silicic magmas. As in the exam-ples of Phanerozoic post-collisional magmatism, mantle melting may have been triggered by de-lamination of the lithosphere. Partial melting in the lower crust was caused by underplating of mafic magma and the upwelling of hot aestheno-spheric mantle.
The model outlined above, for the 1865 – 1850 Ma felsic igneous rocks of the Kimberley, is con-sistent with the presence of tectonostratigraphic terranes in the Lamboo Complex (Tyler et al., 1995). These terranes are indicative of the opera-tion of modern-style plate tectonics in the Palaeoproterozoic, and are an important con-straint on any tectonic models.
The interpreted post-collisional setting for the 1865 – 1850 Ma high-K granites and volcanic rocks of the Kimberley, implies plate tectonic processes similar to those operating today. Similar models have been proposed for other Palaeoproterozoic high-K granites in northern Australia in the Arunta Inlier (Sun et al., 1995; Zhao and McCulloch, 1995) and the Mount Isa Inlier (McDonald et al., 1997). An intracratonic model, or any simple model, for all the Palaeoproterozoic high-K granites of northern Australia is probably inappropriate. We also sug-gest that there is no requirement that all of the high-K granites in northern Australia formed in exactly the same tectonic setting.
Acknowledgements
T.J.Griffin et al./Precambrian Research101 (2000) 1 – 23 22
greatly to the SHRIMP data aquisition. We thank Hugh Smithies for his thorough reviews of the manuscript, John Myers for suggestions that im-proved the paper, and Ed Mikucki for numerous discussions. Reviews by W. Collins and A. Nem-chin were much appreciated. This paper is pub-lished with permission of the Director of the Geological Survey of Western Australia, and the Executive Director of the Australian Geological Survey Organisation.
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