Geology and SHRIMP geochronology

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. Fig. 3. Simplified geology of the northern part of the Lamboo Complex in the east Kimberley. Also shown are the locations and numbers of samples dated in this paper. 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 minerals. 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 1850 9 5 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 1854 9 5 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 1857 9 4 Ma x 2 = 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 1858 9 5 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 1862 9 4 Ma x 2 = 0.98; Fig. 4b. Nineteen of the 20 analyzed zircons from the Greenvale Porphyry define an 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 1s analytical uncertainties, but pooled 207Pb206Pb 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 1855 9 4 Ma x 2 = 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. Plagio- clase and quartz form phenocrysts in some tonalites and granodiorites. Even-textured rocks, and the groundmass in porphyritic rocks, consist of microcline and quartz, subhedral plagioclase, and biotite. Plagioclase displays normal and oscil- latory zoning; crystals with mottled cores are a minor but widespread component in some intru- sions. Biotite typically forms aggregates up to 5 mm in diameter, which host most of the accessory minerals: namely magnetite and minor ilmenite, apatite, zircon, monazite and allanite. Greenish- brown hornblende or blue-green ferrohastingsite is present in some tonalites and granodiorites. Primary muscovite accompanies biotite in the most silicic rocks. 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 cathodoluminescent images Fig. 5a. All 18 data points define an igneous crystallization age of 1864 9 4 Ma x 2 = 1.38; Fig. 4c. The 207 Pb 206 Pb age for grain 12-1 is slightly older 1908 9 21 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 1862 9 5 Ma x 2 = 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 m m-diameter SHRIMP analytical crater for grain 7.1 207 Pb 206 Pb age 1854 9 8 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 30 mm-diameter SHRIMP analytical craters for grains 26.1 207 Pb 206 Pb age 1874 9 7 Ma, 27.1 1864 9 7 Ma and 28.1 1856 9 7 Ma; c Kongorow Granite 8759.8010 showing fine igneous zonation in euhedral grains, and the 30 mm-diameter SHRIMP analytical craters for grains 52.1 207 Pb 206 Pb age 1857 9 10 Ma and 53.1 1859 9 10 Ma. Fig. 5. T .J . Griffin et al . Precambrian Research 101 2000 1 – 23 Table 1 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 124446 a 92776 a 92721 Sample 118172 108758 92782 a 95353 a 8759.8011 99308 99331 99362 95422 92777 a 92783 a Felsic vol- Feldsp–qtz Micro-gr- Micro-gran- Felsic vol- Biotite gab- Quartz Micro-mon- Grndiorite Rock type Porph. Porph. Micro- Quartz gab- Felsic vol- Porphyry diorite canic zogrt Monzgrt canic monzgrt canic monzgrt bro ite diorite bro porphyry Zone 52 Zone 52 Zone 51 Zone 51 Zone 52 Zone 51 Zone 51 Zone 51 Zone 51 Zone 51 Zone 51 Zone 51 Location b Zone 52 Zone 51 Zone 51 746380634 310680301 423281865 372281013 725181060 703680967 366980660 724380566 690381030 701580940 784580603 804780472 811880403 653681598 720881033 Wt 69.22 72.20 75.00 51.70 52.00 56.80 65.00 66.10 69.64 72.60 72.00 75.30 76.20 SiO 2 62.50 68.30 0.47 0.85 0.59 0.28 0.19 0.87 0.90 0.95 0.82 0.28 0.24 0.09 0.41 0.17 0.19 TiO 2 13.64 13.50 13.00 16.70 16.80 13.70 15.90 Al 2 O 3 15.29 16.00 13.40 12.20 14.50 13.60 12.10 14.70 1.02 0.50 2.12 1.00 1.06 1.74 1.61 1.39 0.33 0.52 0.06 0.24 Fe 2 O 3 1.15 0.72 1.43 3.62 6.42 3.67 2.01 0.00 7.59 8.40 6.63 4.12 2.04 2.26 1.03 2.30 1.06 0.71 FeO 0.06 0.02 0.03 0.13 0.13 MnO 0.13 0.09 0.07 0.02 0.04 0.03 0.06 0.02 0.16 0.08 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 3.36 4.18 2.08 1.14 3.17 1.65 0.90 3.94 2.11 2.14 2.63 1.84 1.63 1.79 2.77 2.28 2.72 Na 2 O 2.68 2.69 2.84 2.67 2.45 1.63 2.39 2.97 4.67 5.31 4.64 0.71 0.84 2.43 2.50 4.38 4.77 5.26 4.39 5.64 4.65 K 2 O 0.14 0.09 0.06 0.19 0.13 0.26 0.22 0.11 0.06 0.12 0.10 0.06 P 2 O 5 0.05 0.08 0.14 – – 1.68 0.81 – – 0.52 – – – – – – – – H 2 O + – – 0.11 0.03 – – 0.07 – – – – – – – – H 2 O − 0.05 0.22 0.20 0.44 0.29 0.26 0.27 0.39 – – – – 0.16 CO 2 0.24 – 1.75 2.27 – – 0.81 1.13 – 2.29 1.24 0.36 1.01 0.73 1.36 0.95 0.91 LOI 0.01 B 0.01 B 0.01 0.06 0.08 0.02 0.02 S B 0.01 0.05 0.01 0.02 0.03 B 0.01 B 0.01 B 0.01 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 0.18 0.19 0.25 100.45 100.75 100.02 99.28 100.70 99.71 98.31 99.60 100.51 99.34 99.35 99.11 99.82 99.55 100.24 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’ 0.00 0.02 100.02 99.20 100.70 99.68 98.26 99.59 100.50 99.31 100.45 99.35 Total 99.10 100.73 100.24 99.55 99.81 PPM 740 604 681 242 224 664 625 471 1 472 Ba 561 106 449 567 1 049 563 83 212 21 17 B 10 242 278 353 34 50 – – – – Cr 13 12 8 18 33 73 40 28 62 3 18 2 7 Cu 81 7 22 20 19 18 7.2 20 20 9.9 20 24 19 17 13 21 17 20 Ga 34 31 – 19 27 13 9 22 29 13 30 22 18 6 Li 12 – B 0.1 B 5 B 5 0.1 B 5 B 5 B 5 4 8 B 2 2 B 5 Mo B 6 4 10 11 13 2.3 14 9 4.7 12 20 5 11 13 11 12 14 Nb 7 6 B 2 36 107 64 12 Ni 43 25 16 8 21 10 2 40 30 4.5 24 B 15 B 0.5 B 15 18 16 23 21 29 55 Pb 34 45 43 127 137 196 267 229 25 42 105 130 121 218 319 142 258 290 Rb 17 16 13 – 24 30 8 10 18 9 5 4 8 4 Sc 34 6 2.5 B 5 B 5 0.6 B 5 B 5 B 5 3 B 4 – 7 Sn B 5 6 2 193 199 168 83.7 165 304 119 396 260 265 138 48 217 113 86 Sr 27 26.7 24 B 10 6.9 B 10 B 10 Th 10 17 27 40 19 34 36 B 10 6 6.4 4 B 2 0.8 B 2 B 2 2 2 3 10 14 U 7 11 5 46 111 34 19 9 165 167 158 60 15 12 4 23 4 7 V 40 40.4 32 20 25 22 24 Y 16 28 30 64 30 45 45 20 72 36 27 77 80 86 74 56 35 30 29 13 24 Zn 89 58 194 211 270 151 160 79 134 176 246 159 138 111 215 186 170 Zr 55.9 – 67.9 22.5 25.6 38.3 36 La 54 36.8 48.1 45.3 56.9 81.3 54.3 38.6 117.6 – 121 42.7 48.2 67.1 61.9 67 94 52.6 76.8 81.4 Ce 102 136 96.6 – 6.2 13.8 – – 5.5 6.7 – – – – – – – – Pr T .J . Griffin et al . Precambrian Research 101 2000 1 – 23 11 Table 1 Continued Coarse-grained granites Whitewater Volcanics High-level porphyry intrusions Gabbros 124609 124446 a 92776 a 92721 Sample 118172 108758 92782 a 95353 a 8759.8011 99308 99331 99362 95422 92777 a 92783 a Felsic vol- Grndiorite Micro-gr- Quartz Micro- Biotite Feldsp–qtz Porph. Quartz Rock type Micro- Felsic vol- Micro- Porph. Felsic vol- Porphyry canic Monzgrt canic canic monzgrt monzogrt gabbro monzgrt porphyry gabbro granite diorite diorite Zone 52 Zone 52 Zone 51 Zone 51 Zone 52 Zone 51 Zone 51 Zone 51 Zone 51 Zone 51 Zone 51 Zone 51 Location b Zone 52 Zone 51 Zone 51 423281865 372281013 725181060 703680967 366980660 724380566 690381030 701580940 784580603 310680301 804780472 811880403 653681598 720881033 746380634 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 6.1 4.7 11 6.9 9.8 10.9 6.2 1.7 – 1.4 1.7 1.4 1.5 1.5 1.0 – Eu 4.9 10 0.9 0.7 1.3 1.4 9.7 – 7.3 4.6 6.1 6.4 5.6 Gd – 3.1 1 0.5 6.7 9.3 8.5 5.3 7.6 – 5.9 3.9 5.8 4.4 4.8 3.9 – Dy 5.9 9.9 8.0 8.0 5.3 2.3 1.5 – 1.3 0.8 1.4 0.9 1.0 – 1 2.1 Ho B 0.5 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 B 0.5 0.4 0.5 0.7 a Geochemistry 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. b Locations 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 sample numbers. Sample 8759.8015 dated by Tyler et al., 1999. Table 2 Sm–Nd whole-rock data for 1865–1850 Ma igneous rocks in the Kimberley region a Rock unit Sample Sm ppm Nd ppm 147 Sm 144 Nd 143 Nd 2s T Ma o Nd i T DM 144 Nd 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 0.511386 Lennard Granite 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 6.90 Whitewater Volcanics 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 a Samples 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. T DM calculated assuming mantle depletion beginning at 4.56 Ga, and using 143 Nd 144 Nd = 0.513144 and 147 Sm 144 Nd = 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 x 2 = 1.08 that defines an igneous crystal- lization age of 1861 9 4 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 1852 9 4 Ma x 2 = 1.33; Fig. 4d. This result is significantly younger different at the 1 proba- bility level using Student’s t-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