Geochronology Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol102.Issue3-4.2000:

to the southwest, suggesting a dominant horizon- tal component of movement. 2 . 1 . 8 . D 5 Fold axes vary systematically in plunge throughout the mainland exposures of the Nor- nalup Complex, consistent with folding by a later generation of regional-scale folds F 5 with half- wavelengths in the order of 20 km. F 5 folds plunge shallowly to the northwest and reflect moderate NE-SW horizontal shortening. A crenu- lation cleavage S 5 related to this deformation is developed in amphibolites in the Malcolm Gneiss.

3. Geochronology

3 . 1 . Pre6ious geochronology A comprehensive review of geochronological investigation in the Albany – Fraser Orogen is pre- sented in Nelson et al. 1995. A summary of the major studies and their implications, including recent work, is presented below. 3 . 1 . 1 . Eastern Albany – Fraser Orogen With the exception of several studies conducted in the Fraser Complex region Bunting et al., 1976; Baksi and Wilson, 1980; Fletcher et al., 1991, the age structure of the eastern part of the Albany – Fraser Orogen was unknown prior to a reconnaissance SHRIMP U-Pb zircon study by the Geological Survey of Western Australia Nel- son et al., 1995. The Biranup Complex was found to comprise Late Archaean c. 2595 – 2640 Ma basement intruded by c. 1600 – 1700 Ma and c. 1300 Ma felsic plutonic rocks. In the Nornalup Complex, pre-orogenic basement rocks outcrop only in the Malcolm Gneiss. Zircons from a metasedimentary gneiss from near Point Malcolm yielded a wide spectrum of detrital ages. Two distinct populations at 1560 9 40 and 1807 9 35 Ma, and single grain analyses ranging in age from 2033 to 2734 Ma, suggest that the sedimentary precursors to these rocks were not derived from the vicinity of the Albany – Fraser Orogen Nelson et al., 1995. The 1560 9 40 Ma population pro- vides a maximum estimate for the age of deposi- tion of the precursor sediments. Two major felsic intrusive events relating to the Albany – Fraser Orogeny were identified by Nel- son et al. 1995. Six samples of granitic gneiss representative of the widely-distributed Recherche Granite Myers, 1990; Fig. 2 yielded crystallisa- tion ages of between c. 1330 and 1283 Ma. These rocks intruded during a period of high-grade metamorphism and intense deformation recog- nised throughout the eastern Albany – Fraser Oro- gen Myers, 1995a; Nelson et al., 1995. The layered basic rocks of the Fraser Complex crys- tallised under granulite facies conditions during this event, as constrained by an Sm-Nd isochron age of 1291 9 21 Ma Fletcher et al., 1991. On the basis of this age, and Rb-Sr and Ar-Ar cool- ing ages of between 1285 and 1262 Ma Bunting et al., 1976; Baksi and Wilson, 1980; Fletcher et al., 1991, Fletcher et al. 1991 argued that the Fraser Complex intruded, was metamorphosed, and was subsequently tectonically emplaced into the upper crust all in a period of 30 Ma. The cooling ages for the Fraser Complex have also been interpreted to date the termination of high- grade metamorphism throughout the eastern part of the orogen Nelson et al., 1995. Two outcrops of undeformed granite Esper- ance Granite, Myers, 1995a gave imprecise U-Pb zircon crystallisation ages of 1138 9 38 and 1135 9 56 Ma Nelson et al., 1995. These ages were interpreted by Myers 1995a as dating a second period of tectonism and metamorphism correlating to the major c. 1190 – 1170-Ma oro- genic episode identified in the western part of the orogen. Although Esperance Granite plutons have not been recognised in the Biranup Complex, a folded 1187 9 12 Ma pegmatite dyke intruding Palaeoproterozoic gneisses at Lake Gidong Nel- son et al., 1995 may be related to this second thermo-tectonic event. 3 . 1 . 2 . Western Albany – Fraser Orogen The western part of the Albany – Fraser Orogen is dominated by late to post-kinematic granite plutons. These rocks occur mainly within the Nornalup Complex, but locally occur across the Nornalup – Biranup Complex boundary Myers, 1995b; Fig. 1b. U-Pb zircon crystallisation ages of six representative plutons range between c. 1170 and 1190 Ma Pidgeon, 1990; Black et al., 1992a. Granite intrusion was preceded by high- grade metamorphism and deformation, dated by U-Pb in zircon at about 1190 Ma Black et al., 1992a. On the basis of inherited Archaean zircons c. 3100 Ma within felsic orthogneiss, Black et al. 1992a interpreted much of the Biranup Complex to represent reworked Yilgarn Craton crust. These authors found no evidence for thermo-tec- tonic activity relating to c. 1300 Ma events in the eastern part of the orogen Fletcher et al., 1991, and therefore interpreted the c. 1190 – 1170-Ma event to be the principal period of orogenesis in the western part of the orogen. However, a 1289 9 10 Ma crystallisation age on an enderbitic pluton outcropping near Albany Pidgeon, 1990; Fig. 1b suggests the existence of an earlier event. A recent U-Pb SHRIMP study by Clark 1995 identified 1304 9 5 and 1169 9 7 Ma meta- morphic zircon populations in a granulite facies metasedimentary migmatite from near the c. 1289- Ma enderbite, indicating that the western part of the Albany – Fraser Orogen did experience high- grade metamorphism and deformation at c. 1300 Ma. K-Ar ages of 1160 – 1060 Ma obtained on horn- blende from metamorphosed basic rocks are inter- preted to date late granite emplacement and post-metamorphic uplift and cooling of the west- ern part of the orogen Stephenson et al., 1977. 3 . 2 . Metamorphic and structural context of the dated samples Six rocks samples with well-defined relation- ships to the structuralmetamorphic history of the Nornalup Complex were selected for age determi- nation Fig. 2, Table 2: 1 a post-D 3 , pre-D 4 aplite dyke from the Malcolm Gneiss; 2 a syn- D 4 pegmatite dyke from the Malcolm Gneiss; 3 a post-D 2 , pre-D 4 aplite dyke from a Recherche Granite pluton; 4 a syn-M 2a leucosome layer from a granulite facies metapelite from the Salis- bury Gneiss; and samples of quartzite 5 and schist 6 from the Mount Ragged metasedimen- tary rocks. Zircon was chosen to date the igneous crystallisation ages for samples 1 and 3, leuco- some formation in 4 and provenance ages for sample 5. Monazite was used to provide an igneous age for sample 2 as zircon was unavail- able. Metamorphic rutile crystals were dated in number 6. The morphological and internal char- acteristics of radiogenic minerals separated from the samples are described together with the iso- topic results in Section 3.4. 3 . 2 . 1 . Post-D 3 , pre-D 4 aplite dyke 95091214 , Malcolm Gneiss This metre-thick dyke cross-cuts F 3 folds defined by a pervasive S 1 gneissosity in granitic and metasedimentary rocks at Point Malcolm Fig. 2. The dyke is linear but shows signs of tectonic attenuation resulting from D 4 deforma- tion. It contains an annealed assemblage of two Table 2 Brief description of samples used for geochronology sample localities are shown in Fig. 2 a Mineral assemblage Sample Locality AMG coords Lithology StructuralMetm context Point Malcolm WC704600 1 95091214 Aplite dyke Post-D 3 pre-D 4 Qtz-Bt-Kfs-Pl-Mag 2 9411112 Little Bellinger WC648573 Pegmatite dyke Syn-D 4 Qtz-Grt-Bt-Ms-Kfs-Pl-Mag Aplite dyke WC148366 Cape Arid 3 9509243 Qtz-Bt-Kfs-Pl-Mag-Hbl-Ttn Post-D 2 pre-D 4 Qtz-Grt-Spl-Crd-Sil-Bt-Kfs-Pl 4 9611201 Syn-M 2a WB502982 Salisbury Island Migmatitic paragneiss Mt. Ragged WD437001 Quartzite Post-D 3 Qtz-Bt-Ms-Chl-Hem 5 9510101 Mt. Ragged WC431971 Mica schist Syn-M 2b Qtz-Ms-Chl-Mrg-And-Hem-Rt 9 Ky 6 9510092 a Mineral abbreviations after Kretz 1983. feldspars, quartz, and biotite. Biotite is weakly oriented sub-parallel to the margins of the dyke, which is oblique to the tectonic fabrics pre- served in the host rocks e.g. S 1 S 2 , S 3 , S 4b . The fabric is interpreted to result from igneous flow. Zircon and titanite occur as accessory minerals. 3 . 2 . 2 . Syn-D 4 pegmatite 9411112 , Malcolm Gneiss Sample 9411112 is from a pegmatite dyke hosted by a large D 4 shear zone outcropping midway between Point Malcolm and Cape Pasley Fig. 2. The shear zone contains an am- phibolite facies mineral assemblage comparable in grade to an M 2b overprint recognised in the metasedimentary rocks through which the shear zone cuts. The dyke cross-cuts the S 4b fabric in the shear zone and is boudinaged by later duc- tile movement along the shear planes late-D 4b or D 4c . Its mineralogy comprises garnet, two feldspars, biotite, muscovite and quartz. Monaz- ite occurs as an accessory mineral but zircon was not found in either thin section or sepa- rates. The pegmatite contains a fabric parallel to S 4bc defined by oriented biotite. 3 . 2 . 3 . Post-D 2 , pre-D 4 aplite dyke 9509243 , Recherche Granite This aplite intrudes into a pluton of gneissic Recherche Granite outcropping at Cape Arid Fig. 2. It comprises an annealed assemblage of two feldspars, quartz, hornblende, biotite, titan- ite and zircon. The dyke is linear over several hundred metres of outcrop and cuts the S 2 and S 2b fabrics in the host gneiss. F 3 folding in the area is at kilometre-scale so the timing of the dyke relative to D 3 is unclear. A shear fabric is developed within the dyke oblique to its margins consistent with deformation during D 4 . 3 . 2 . 4 . Syn-D 4 a migmatitic leucosome 9611201 , Salisbury Gneiss This sample was collected from an S 1S concor- dant granitic leucosome outcropping on Salis- bury Island Fig. 2. Leucosomes formed by extensive biotite dehydration partial melting of metapelitic rocks during M 2a and comprise mesoperthitic feldspars, quartz and minor gar- net 9 cordierite. Thin restitic schlieren rich in garnet, biotite, sillimanite, spinel and cordierite separate leucosome layers. Zircon and monazite are present in both leucosome and mesosome layers. The leucosomes are everywhere concor- dant with S 1S , and are locally disharmonically folded, suggesting that their formation occurred synchronous with D 4a . M 2a garnets in leucosome areas proximal to mesosome schlieren are man- tled by cordierite 9 spinel coronas, which con- tain abundant small zircon grains. 3 . 2 . 5 . Post-D 3 quartzite 9510101 , Mount Ragged Sample 9510101 was taken from near the ex- posed base of the Mount Ragged metasedimen- tary rocks at Mount Ragged. The quartzite consists almost entirely of a coarse-grained gra- noblastic aggregate of recrystallised quartz. A weak annealed S 1R foliation is defined by ori- ented haematite and minor muscovite, chlorite and biotite. In section, haematite grains com- monly form rings up to several centimetres in diameter enclosing many small quartz grains, suggesting the recrystallisation of originally much coarser grains. Zircons are sporadically distributed along the boundaries of these relic grains. 3 . 2 . 6 . Syn-D 4 mica schist 9510092 , Mount Ragged This sample was taken from a thin pelitic lens intercalated with massive quartzite in the same vicinity as sample 9510101. The rock contains the assemblage muscovite, chlorite, margarite, andalusite, haematite and rutile. Ilmenite, spes- sartine garnet, gahnite-rich spinel, kyanite and epidote occur as accessory phases. Small euhe- dral rutile needles post-date garnet growth and form part of a near-peak retrograde-M 2b meta- morphic paragenesis in this rock. Delicate knee- bend twins are common, suggesting that rutile growth post-dates D 4b shearing deformation in these layers. Rutile may have formed at the ex- pense of ilmenite or could have recrystallised from previously detrital grains. 3 . 3 . Methodology and analytical procedures for isotopic analysis The majority of U-Th-Pb isotopic measure- ments were made using the sensitive high resolu- tion ion microprobe SHRIMP-II at Curtin University of Technology, Perth. Zircons from sample 9611201 were analysed using the SHRIMP II facility at the Australian National University, Canberra, with the assistance of Dr R. Armstrong. All sample minerals were ex- tracted from the disaggregated rock samples and mounted in epoxy discs before being polished, gold coated and imaged. Before SHRIMP analy- sis all zircon grains were imaged by cathodolu- minescence CL using a Cambridge S360 scanning electron microscope, with an operating voltage of 20 kV, equipped with a polychro- matic CL detector. The SEM is located in the Electron Microscope Unit at the University of New South Wales. Procedures for SHRIMP U-Th-Pb isotopic analysis of zircon follow those originally de- scribed by Compston et al. 1984 and Williams et al. 1984, with subsequent modifications to analytical routines and data reduction methods outlined by Williams and Claesson 1987, Compston et al. 1992 and Williams 1998. For zircon analyses undertaken using the Perth SHRIMP-II, U-Pb ratios and U and Th concen- trations were determined relative to Sri Lankan zircon standard CZ3 564 Ma, 206 Pb 238 U = 0.0914, Nelson 1997. For analyses undertaken in Canberra, U-Pb ratios were determined rela- tive to the Duluth Complex gabbroic anorthosite standard AS3 1099.1 Ma, 206 Pb 238 U = 0.1859, Paces and Miller 1989, whilst U and Th concentrations were determined rela- tive to ANU zircon standard SL13. The procedure for monazite analysis by SHRIMP followed the method outlined by Kinny 1997, which differs somewhat from that of Williams et al. 1996, and Ireland and Gib- son 1998 in that the calibration of PbU ratios is based upon a plot of ln 206 PbUO versus UO 2 UO calibration slope 0.7, with data for unknowns normalised to Madagascan monazite standard MAD 514 Ma, 206 Pb 238 U = 0.0830, based on TIMS analyses of L.M. Heaman. An- other difference in the monazite analytical pro- cedure of Kinny 1997 is that, prior to being used for common Pb correction, 204 Pb counts are corrected for a background interference of scattered ions the size of which is directly pro- portional to the Th content of the sample. PbU ratios for rutiles were determined rela- tive to 2625 Ma-old rutile from the Windmill Hill quartzite, Jimperding metamorphic belt, Western Australia 206 Pb 238 U = 0.5025, based on TIMS analyses by L.M. Heaman. Norm- alisation of rutile unknowns was based on an observed linear covariation between 206 PbUO and UO2UO for the standard, slope 1.17. Common Pb corrections were applied using the 204 Pb correction method Compston et al., 1984, assuming the isotopic composition of Broken Hill ore Pb, except for zircon sample 96110201 which contained very low Th and so was corrected via the 208 Pb method Compston et al., 1984, using a modelled common Pb iso- topic composition appropriate to its age, and for the Mount Ragged rutile sample which con- tained no detectable Th. For rutile data in which the measured 208 Pb peak is entirely non- radiogenic, a simplified 208 Pb correction method was applied, whereby the proportion of non-ra- diogenic 206 Pb, denoted f206, is given by: f206 = 100 × 208 Pb 206 Pbmeasured 208 Pb 206 Pbcommon For both monazite and rutile analyses, the composition of the common Pb component was modelled upon that of contemporary terrestrial lead. Reproducibility of the U-Pb ratios of the standards on both machines was better than 9 2.1 in all cases. Elemental concentrations in the monazite and rutile analyses were calculated by assuming a similar sensitivity of ionising spe- cies for standards and unknowns, and are accu- rate to approximately 9 20. The decay constants used are those recommended by Steiger and Ja¨ger 1977. 3 . 4 . Isotopic results and age constraints on field relationships The processed U-Pb data are presented in Ta- bles 3 – 6. Results are presented on conventional concordia plots in Fig. 3a – f. Errors given on individual analyses in the data tables and on concordia plots are at 1s level. They are based on counting statistics, uncertainty in the common Pb correction and, in the case of PbU ratios, the uncertainties associated with normalisation to the standards. Pooled ages quoted in the text are weighted means and their errors are given at ts or 95 confidence level. Brief descriptions of the morphology of the analysed grains and, in the case of zircon, the internal characteristics as re- Fig. 3. Concordia diagrams for dated samples; error boxes shown are 1s. Inset diagrams illustrate the structural context of the samples. a Sample 95091214, Point Malcolm. The hatched analysis has not been used to determine the crystallisation age of this sample. b Sample 9411112, Malcolm Gneiss. Concordia diagrams for dated samples; error boxes shown are 1s. Inset diagrams illustrate the structural context of the samples. c Sample 9509243, Cape Arid. Hatched analyses have not been used in determining the crystallisation age of this sample. The xenocrystic population is interpreted to be inherited from the Recherche Granite, and is quoted at 1s level. d Sample 9611201, Salisbury Island. The two groups main groups represent zircon growth under metamorphic conditions. Analyses in black do not fall into either population and have been excluded from age calculations. Concordia diagrams for dated samples; error boxes shown are 1s. Inset diagrams illustrate the structural context of the samples. e Sample 9510101, Mount Ragged. Both main populations are interpreted to be detrital. Discordant analyses have been included in the populations as they have suffered recent lead loss only. Analyses in black do not fall into either population and have been excluded from age calculations. f Sample 9510092, Mount Ragged. Fig. 3. Continued vealed by CL imaging are provided before the results for each sample. 3 . 4 . 1 . Post-D 3 , pre-D 4 aplite dyke 95091214 , Malcolm Gneiss Zircons extracted from this sample are colour- less, euhedral and squat to elongate. They range in length from 150 to 200 mm and in lengthwidth ratio from 1.5 to 2.5. Delicate oscillatory growth zoning is evident in most grains. CL inten- sity ranges from slightly darker cores to brighter rims. Hourglass and sector zoning is prominent in many grains. Crystals are bounded by large prism and pyramid faces notably {211}. Grains in this sample commonly have thin rims with dark CL response. Rims, ranging in width from 5 to 20 m m, are typically concordant to the internal zona- tion of the grains, but sometimes form embay- ments transgressive into core material, truncating core zonation. Seventeen zircon analyses fall within error of a mean 207 Pb 206 Pb age of 1313 9 16 Ma Fig. 3a. The remaining analysis 3.43 is significantly dis- cordant 9 and so has been excluded from the age calculation. The analyses contain 79 – 204 ppm U, 50 – 245 ppm Th Table 3 and ThU ratios clustering closely around an average of 0.8. The ubiquitous presence of oscillatory and sector zonation, the abundance of {211} pyramid faces, and the moderate ThU ratios strongly suggest that this zircon has a primary igneous origin. The age recorded by this zircon population is therefore interpreted to date the crystallisation of the aplite, thereby providing a lower age bound for D 3 . The D .J . Clark et al . Precambrian Research 102 2000 155 – 183 Table 3 Geochronological results obtained on zircons from samples 95091214 and 9509243 207 Pb U ppm 9 1s Label 208 Pb Pb ppm f206 a 9 1s 206 Pb Th ppm 9 1s 207 Pb 9 1s conc. b 207 Pb 206 Pb 9 1s 238 U 206 Pb Age 235 U 206 Pb 95091214 Malcolm Gneiss aplite dyke 0.00091 0.2909 0.0024 3.1 0.2236 185 0.0037 2.579 0.0536 101 1284 21 180 49 0.11 0.08363 0.00185 0.2595 0.0044 0.2238 0.0037 2.628 0.0762 0.08519 99 131 1320 42 3.2 0.11 34 116 0.08451 104 0.00144 0.1688 0.0031 0.2237 0.0038 2.607 0.0661 100 1304 33 60 25 0.31 3.3 0.08 0.08544 0.00093 0.1982 0.0021 0.2263 0.0037 2.665 0.0556 99 1326 21 3.8 180 121 45 0.00130 0.2323 0.0031 0.2209 0.0037 2.595 0.0622 0.08523 97 28 1321 29 0.10 3.1 111 87 0.08562 119 0.00152 0.1958 0.0035 0.2276 0.0038 2.687 0.0697 99 1330 34 78 30 0.31 3.14 0.08349 182 0.00102 0.2608 0.0025 0.2294 0.0038 2.640 0.0575 104 1281 24 157 49 0.21 3.19 0.00187 0.1763 0.0042 0.2237 0.0038 2.711 0.0780 0.08792 94 3.22 1381 41 0.26 24 59 99 0.18 0.08391 0.00142 0.1998 0.0032 0.2333 0.0039 2.698 0.0683 105 1290 33 3.28 118 83 31 0.00183 0.1834 0.0042 0.2262 0.0038 2.493 0.0751 0.07993 110 101 1195 45 3.37 0.34 25 64 0.08602 195 0.00128 0.3252 0.0032 0.2081 0.0034 2.468 0.0580 91 1339 29 244 50 0.57 3.43 0.08428 154 0.00140 0.2856 0.0035 0.2285 0.0038 2.656 0.0663 102 1299 32 150 42 0.28 3.46 0.00220 0.1791 0.0050 0.2242 0.0038 2.562 0.0852 0.08289 103 3.5 1267 52 0.59 20 50 80 0.00129 0.2794 0.0032 0.2266 0.0038 2.680 0.0639 3.54 99 119 1334 29 113 32 0.10 0.08580 0.00079 0.2552 0.0020 0.2287 0.0038 2.705 0.0536 0.08579 100 54 1333 18 0.04 3.62 203 171 0.08451 204 0.00098 0.2680 0.0024 0.2291 0.0038 2.670 0.0570 102 1304 23 188 55 0.09 3.63 0.08144 145 0.00125 0.2037 0.0029 0.2280 0.0038 2.560 0.0615 107 1232 30 103 37 0.37 3.66 0.00105 0.2694 0.0026 0.2294 0.0038 2.747 0.0598 0.08684 98 3.67 1357 23 0.09 34 115 127 9509243 Recherche Granite aplite dyke 0.00063 0.0877 0.0012 0.2266 0.0037 2.687 113 0.0500 492 98 1338 14 150 0.21 0.08600 1.1a 377 0.08685 0.00062 0.0175 0.0009 0.2286 0.0037 2.737 0.0509 98 1357 14 21 82 1.2b 0.03 154 0.09065 0.00103 0.2735 0.0025 0.2464 0.0041 3.080 0.0654 99 1439 22 175 45 1.2a 0.23 0.00145 0.3902 0.0040 0.2263 0.0038 2.663 0.0673 0.08536 99 1.8 1324 33 0.11 30 138 105 0.00133 0.1744 0.0029 0.2251 0.0037 2.609 1.15 0.0634 118 101 1294 31 70 29 0.30 0.08405 0.00120 0.4352 0.0033 0.2192 0.0036 2.526 0.0583 0.08358 100 69 1283 28 0.45 1.2 239 351 0.08112 129 0.00202 0.1887 0.0046 0.2277 0.0038 2.546 0.0805 108 1224 49 88 33 0.86 1.22b 0.08540 69 0.00211 0.1886 0.0048 0.2284 0.0039 2.689 0.0854 100 1325 48 46 18 0.46 1.23 0.00111 0.1908 0.0025 0.2281 0.0038 2.659 0.0597 0.08457 101 1.27 1306 26 0.00 32 80 130 0.00089 0.2793 0.0023 0.2251 0.0037 2.662 0.0547 98 1.31 1333 179 20 166 47 0.01 0.08576 0.00084 0.2209 0.0019 0.2261 0.0037 2.694 0.0541 0.08641 98 223 1347 19 1.33 0.06 57 166 0.08296 96 0.00144 0.1900 0.0033 0.2233 0.0037 2.555 0.0654 102 1268 34 63 24 0.26 1.41 0.08364 167 0.00102 0.3153 0.0027 0.2259 0.0037 2.606 0.0565 102 1284 24 176 46 0.15 1.44 0.00106 0.2347 0.0025 0.2250 0.0037 2.661 0.0579 0.08578 98 1.49 1333 24 0.14 50 154 196 0.00090 0.4004 0.0024 0.2320 0.0038 2.674 0.0546 105 1284 1.5 20 267 363 80 0.33 0.08362 0.00140 0.2610 0.0033 0.2087 0.0034 2.393 0.0595 0.08317 96 64 1273 33 1.17 1.52 254 236 0.08571 247 0.00082 0.3247 0.0022 0.2274 0.0037 2.688 0.0536 99 1332 18 275 68 0.05 1.54 0.00094 1.4 0.2111 181 0.0021 0.2245 0.0037 2.684 0.0560 96 1355 21 130 45 0.07 0.08673 0.00106 0.3062 0.0027 0.2194 0.0036 2.558 0.0560 98 1305 0.08456 24 1.6 180 192 48 0.32 a f206 = 100×common 206 Pbtotal 206 Pb. b conc = 100× 206 Pb 238 U age 207 Pb 206 Pb age. D .J . Clark et al . Precambrian Research 102 2000 155 – 183 169 Table 4 Geochronological results obtained on monazites from sample 9411112 9 1s 208 Pb 206 Pb 9 1s 206 Pb 238 U 9 1s 207 Pb 235 U 9 1s conc. b 207 Pb 206 Pb Age Label 9 1s U Th Pb f206 a 207 Pb 206 Pb 9411112 Malcolm Gneiss pegmatite 0.00019 0.67741 0.00082 0.2094 0.0043 2.297 8.75 0.048 3.06 103 1186 5 3.09 dan.3 1.21 0.07955 0.00011 0.69294 0.00074 0.1998 0.0041 2.166 dan.4 0.045 3.69 101 1162 3 10.58 3.23 0.02 0.07860 0.00022 1.25280 0.00109 0.2011 0.0041 2.184 0.046 0.07877 101 1.22 1166 5 dan.5 3.59 21.71 3.42 0.00007 1.42859 0.00101 0.2089 0.0043 2.259 0.046 dan.6 106 3.12 1158 2 18.97 3.16 0.02 0.07842 0.00013 0.30047 0.00045 0.2020 0.0041 2.213 0.046 0.07947 100 dan.7 1184 3 0.74 3.83 5.09 3.92 0.00010 0.60103 0.00061 0.1999 0.0041 2.171 0.045 101 dan.8 1167 4.13 3 11.74 3.72 0.09 0.07879 0.00010 0.58745 0.00065 0.2086 0.0043 2.264 0.047 0.07870 105 dan.9 1165 3 0.22 2.88 7.63 3.13 0.00007 0.89487 0.00072 0.2073 0.0042 2.243 0.046 dan.10 105 3.48 1159 2 14.03 3.22 0.05 0.07848 0.00012 0.42808 0.00053 0.1979 0.0040 2.145 0.044 0.07863 100 dan.11 1163 3 0.03 3.71 8.05 4.44 0.00041 2.94523 0.00485 0.2109 0.0043 2.284 0.050 106 1161 dan.12 10 0.69 9.48 0.90 1.49 0.07855 0.00014 1.44305 0.00144 0.2011 0.0041 2.183 0.045 0.07872 101 0.28 1165 4 dan.13 2.57 16.90 2.62 0.00008 0.67851 0.00063 0.2051 0.0042 2.229 0.046 103 1168 dan.14 2 3.91 10.76 3.67 0.08 0.07884 0.00014 0.70253 0.00092 0.1962 0.0040 2.144 0.044 98 1179 0.07926 3 dan.15 2.32 8.05 2.01 0.24 a f206 = 100×common 206 Pbtotal 206 Pb. b conc = 100× 206 Pb 238 U age 207 Pb 206 Pb age. D .J . Clark et al . Precambrian Research 102 2000 155 – 183 Table 5 Geochronological results obtained on zircons from sample 96110201 f206 a 9 1s 208 Pb Pb ppm 9 1s 206 Pb Th ppm 9 1s 207 Pb U ppm 9 1s Label conc. b 207 Pb 207 Pb 206 Pb Age 91s 206 Pb 238 U 235 U 206 Pb 96110201 Salisbury Gneiss migmatitic leucosome 0.00073 – – 0.2104 96-1.2 0.0024 735 2.342 0.0364 101 1214 18 34 145 1.40 0.08071 0.00042 – – 0.2049 96-2.1 0.0024 611 2.282 0.0301 99 1216 10 22 117 0.10 0.08076 0.00048 – – 0.2023 0.0023 2.253 0.0306 0.08076 98 0.10 1216 12 96-3.1 687 27 130 0.00063 – – 0.1981 0.0025 2.190 0.0345 96-4.2 97 468 1202 16 20 87 0.17 0.08019 0.00048 – – 0.2069 0.0023 2.328 0.0308 0.08159 98 96-5.2 1236 12 2.69 167 48 854 0.00050 – – 0.2118 96-6.1 0.0025 854 2.404 0.0334 99 1253 12 31 170 0.05 0.08233 0.00028 – – 0.2107 0.0025 2.352 0.0295 0.08095 101 96-8.1 1220 7 0.08 144 28 726 0.00044 – – 0.2000 0.0022 2.209 0.0287 96-9.1 98 434 1199 11 20 82 0.20 0.08010 0.00056 – – 0.2075 0.0024 2.322 0.0334 0.08116 99 96-10.1 1225 14 0.15 99 18 508 0.00034 – – 0.2176 0.0026 2.455 0.0321 96-12.1 102 775 1242 8 23 158 0.03 0.08183 0.00053 – – 0.2064 0.0025 2.297 0.0327 0.08069 100 0.05 1214 13 96-13.1 565 20 109 0.00038 – – 0.2042 0.0023 2.253 0.0289 96-14.1 100 604 1198 9 9 115 0.10 0.08003 0.00054 – – 0.1993 0.0024 2.173 0.0311 0.07911 100 96-15.1 1175 13 0.23 91 17 488 0.07 0.08097 0.00042 – – 0.2077 0.0025 2.319 0.0312 100 1221 10 34 96-16.1 157 805 0.00074 – – 0.2144 0.0027 2.389 0.0396 0.08082 103 0.07 1217 18 96-17.1 533 21 107 0.00063 – – 0.2045 0.0026 2.300 0.0360 96-18.1 97 668 1236 15 32 129 0.32 0.08159 0.00102 – – 0.1875 0.0027 2.048 0.0419 0.07924 94 96-18.2 1178 26 6.22 87 15 497 0.00125 – – 0.1855 0.0105 2.101 0.1268 96-19.1 88 783 1248 30 34 137 6.71 0.08212 0.00058 – – 0.2165 0.0027 2.434 0.0371 0.08155 102 0.02 1235 14 96-20.1 704 47 144 0.00050 – – 0.2126 0.0054 2.351 0.0633 96-5.3 103 799 1202 12 26 159 0.08 0.08021 Rims 0.00032 – – 0.2005 0.0023 2.182 0.0278 0.07894 101 96-1.1 1171 8 0.28 89 15 473 0.00036 – – 0.2038 0.0022 2.232 0.0277 96-4.1 101 480 1182 9 15 92 0.05 0.07941 0.00046 – – 0.2030 0.0027 2.206 0.0333 0.07882 102 96-5.1 1168 12 0.13 132 21 695 0.00034 96-7.1 – 488 – 0.2062 0.0023 2.269 0.0279 101 1192 8 27 95 0.11 0.07980 0.00033 – – 0.2062 0.0023 2.270 0.0275 0.07987 101 0.06 1194 8 96-11.1 779 13 150 0.00065 96-16.2 – 574 – 0.2145 0.0027 2.326 0.0373 108 1164 16 19 115 0.17 0.07867 a f206 = 100×common 206 Pbtotal 206 Pb. b conc = 100× 206 Pb 238 U age 207 Pb 206 Pb age. D .J . Clark et al . Precambrian Research 102 2000 155 – 183 171 Table 6 Geochronological results obtained on zircons from sample 9510101 and rutiles from sample 9510092 207 Pb U ppm 208 Pb 9 1s Th ppm 9 1s 206 Pb 9 1s 207 Pb 9 1s conc. b 207 Pb 206 Pb Age 91s Pb ppm f206 a Label 206 Pb 206 Pb 235 U 238 U 9510101 Mt Ragged Quartzite 0.00104 0.3236 0.0027 0.2193 1.1a 0.0055 253 2.639 0.077 94 1367 23 279 68 0.13 0.08727 0.00102 0.1998 0.0022 0.3207 0.0080 4.798 0.133 0.10853 101 361 1775 17 1.7a 0.82 134 257 0.08597 181 0.00104 0.2143 0.0024 0.2273 0.0057 2.694 0.079 99 1337 23 133 46 0.01 1.12 0.00221 0.5947 0.0063 0.3095 1.13 0.0080 87 4.643 0.161 98 1779 37 179 40 0.26 0.10879 0.00073 0.1967 0.0016 0.2018 0.0050 2.390 0.065 0.08592 89 135 1336 16 0.18 1.14 603 374 0.10803 187 0.00204 0.5124 0.0055 0.1688 0.0043 2.514 0.084 57 1767 34 269 45 0.72 1.15 0.00049 0.3696 0.0013 0.3097 0.0077 4.680 0.121 1.16 97 659 1793 8 846 260 0.04 0.10961 0.00173 0.2769 0.0040 0.3134 0.0080 4.550 0.146 0.10531 102 93 1720 30 1.2 0.34 35 93 0.10120 72 0.00252 0.3683 0.0063 0.2691 0.0070 3.754 0.143 93 1646 46 92 25 0.65 1.24 0.28 0.08392 0.00107 0.6240 0.0038 0.2224 0.0056 2.573 0.076 100 1291 25 1.25 483 231 76 0.00102 0.2097 0.0022 0.2177 0.0054 3.275 0.091 0.10911 71 346 1785 17 1.26 0.37 86 204 0.10749 244 0.00097 0.2771 0.0023 0.3145 0.0079 4.661 0.129 100 1757 16 239 92 0.22 1.29 0.00456 0.1554 0.0103 0.1096 0.0028 1.413 0.082 1.39 45 172 1498 92 71 23 3.80 0.09352 0.00131 0.2047 0.0030 0.2250 0.0057 2.564 0.080 0.08266 104 195 1261 31 1.49 0.28 49 139 0.08330 226 0.00155 0.1676 0.0035 0.1680 0.0042 1.930 0.064 78 1276 36 135 42 0.47 1.51 0.08590 49 0.00359 0.1835 0.0082 0.2209 0.0058 2.616 0.136 96 1336 81 29 12 0.23 1.52 0.00119 0.3026 0.0029 0.3199 0.0081 4.783 0.138 0.10844 101 60 1773 20 0.20 1.63 153 165 0.08768 502 0.00126 0.2844 0.0030 0.1412 0.0035 1.707 0.052 62 1375 28 451 86 0.95 1.68 0.08593 238 0.00122 0.2079 0.0028 0.2129 0.0053 2.522 0.077 93 1336 27 174 57 0.08 2.2 0.00234 0.4108 0.0058 0.2966 0.0076 4.692 0.162 0.11475 89 2.3 1876 37 0.36 42 130 107 0.08426 356 0.00090 0.1444 0.0019 0.2186 0.0055 2.539 0.072 98 1299 21 180 83 0.19 2.8 0.00235 0.1322 0.0051 0.2247 0.0058 2.595 0.105 0.08377 102 18 1287 55 0.33 2.3 74 34 0.10937 649 0.00115 0.2872 0.0027 0.1514 0.0038 2.284 0.065 51 1789 19 1139 121 0.89 2.36 0.00098 0.4184 0.0027 0.3159 0.0080 4.773 0.133 99 2.49 1793 180 16 261 75 0.18 0.10959 9510092 Mt Ragged mica schist 121 0.07797 0.00080 – – 0.1912 0.0044 2.055 0.054 99 1146 20 nd 24 mr1.1 0.62 116 0.07630 0.00085 – – 0.1931 0.0045 2.031 0.055 103 1103 22 nd 23 mr1.2 1.09 0.00100 – – 0.1962 0.0046 2.089 0.059 0.07723 103 21 1127 26 0.75 mr1.3 95 nd 0.07713 122 0.00079 – – 0.1926 0.0045 2.048 0.054 101 1124 20 nd 24 0.67 mr1.4 0.07955 77 0.00087 – – 0.2025 0.0047 2.221 0.060 100 1186 22 nd 24 2.83 mr10.1 0.00109 – – 0.2007 0.0047 2.191 0.063 0.07920 100 mr11.1 1177 27 5.25 25 nd 93 0.00077 – – 0.1958 0.0045 2.072 0.055 104 mr12.1 1115 83 20 nd 28 2.61 0.07676 0.00106 – – 0.1825 0.0043 1.976 0.056 0.07856 93 102 1161 27 mr2.1 3.50 23 nd 0.08033 94 0.00078 – – 0.1853 0.0043 2.052 0.054 91 1205 19 nd 24 2.39 mr3.1 0.00069 – – 0.1862 0.0043 2.010 0.052 mr4.1 96 80 1154 17 nd 29 1.85 0.07827 0.00141 – – 0.1983 0.0047 2.143 0.068 0.07840 101 86 1157 36 mr5.1 5.96 22 nd 0.07870 80 0.00086 – – 0.1864 0.0043 2.023 0.054 95 1165 22 nd 27 3.90 mr6.1 0.00098 mr7.1 – 121 – 0.1862 0.0043 2.015 0.056 95 1160 25 nd 25 3.06 0.07851 0.00110 – – 0.1919 0.0045 2.090 0.060 0.07903 97 23 1173 28 3.49 mr8.1 109 nd 0.07892 109 0.00108 – – 0.1910 0.0045 2.079 0.059 97 1170 27 nd 23 3.16 mr8.2 0.00143 mr8.3 – 76 – 0.1943 0.0047 2.100 0.067 99 1156 36 nd 20 4.73 0.07836 0.00119 – – 0.1904 0.0045 2.085 0.061 95 1183 0.07943 30 mr9.1 99 nd 22 3.54 a f206 = 100×common 206 Pbtotal 206 Pb. b conc = 100× 206 Pb 238 U age 207 Pb 206 Pb age. dark CL rims observed on these grains proved too thin to analyse. 3 . 4 . 2 . Syn-D 4 pegmatite 9411112 , Malcolm Gneiss Honey-yellow monazites from this pegmatite are subhedral to anhedral and equant, with lengthwidth ratios typically less than 2. Whole grains range from 100 to 250 mm in diameter and are unzoned in transmitted and reflected light. SHRIMP analyses define an approximately concordant population with excess scatter in 207 Pb 206 Pb around a mean age of 1165 9 5 Ma x 2 = 8.5. Although the data may be divided into two statistically valid populations on the basis of counting statistics, we see no geological justifica- tion to do so. The large x 2 of the population may be attributable to an underestimation of the errors in the individual measurements in monazite analy- ses Ireland and Gibson, 1998. In the present instance, the effect on the age uncertainty does not influence the geological significance of the age Fig. 4. Th contents of the population range widely, from 5.1 to 21.7, whilst ThU ratios range from 1.3 to 13.7, and average 4.4 Table 4. Typical Th contents in monazite range from 4 to 12 ThO 2 Watt, 1995 but Th-rich monazite up to 30 ThO 2 has been recorded from pegmatitic rocks Bowles et al., 1980. Th and U enrichment in such rocks has generally been considered to be controlled, at least in part, by processes involving magmatic fluids Watt, 1995. This suggests that the monazite grains from which the population of analyses were derived crystallised from the host pegmatitic melt. Mineral assemblages preserved in D 4 shear zones suggest the Malcolm Gneiss ter- rain was at upper greenschist to lower amphibo- lite-facies temperatures at the time of intrusion of the 9411112 pegmatite. This corresponds to tem- peratures much less than the estimated 725°C closure temperature for U-Pb diffusion in monaz- ite Mezger et al., 1993. The pooled age of 1165 9 5 Ma therefore records the age of crystalli- sation of the host pegmatite and provides an estimate for the timing of movement in D 4 shear zones in the Malcolm Gneiss. 3 . 4 . 3 . Post-D 3 , pre-D 4 aplite dyke 9509243 , Recherche Granite Most zircons from this sample are uniform in morphology and range from 150 to 280 mm in length. Lengthwidth ratios range from 2.5 to 4. Crystals are well-faceted, inclusion-free and colourless. Prominent steep {211} pyramidal faces are common. CL imaging reveals bold oscillatory growth zoning with dark cores grading into bright rims. Very thin B 10 mm dark rims are often present. They are concordant with the oscillatory zonation of the cores. Two grains in this sample are morphologically distinct from the remainder 1.1 and 1.2. They are squat and contain rounded, irregular cores showing signs of metam- ictisation patchy CL response. The outer mar- gins of core regions appear strongly resorbed. The cores are enveloped by thick 20 – 60 mm euhe- dral, faceted rims, which show bold concentric zonation consisting of broad bands of bright and dark CL response. Thin outer rims of dark CL response envelope the thicker inner-rims, without truncation of zonation, and may represent contin- uous growth. Fourteen zircon analyses define the main popu- lation in this sample and scatter about a mean 207 Pb 206 Pb ratio corresponding to an age of 1313 9 16 Ma Fig. 3c. Most analyses contain 95 – 270 ppm U and 60 – 370 ppm Th Table 3. ThU ratios range from 0.5 to 1.5 with a cluster- ing of seven analyses around 0.7. All the analysed grains from this population are elongate, are bounded by well-developed crystal faces, and show prominent oscillatory zoning. This habit is consistent with their growth in a magma Vavra, 1994. The pooled age of 1313 9 16 Ma is there- fore interpreted to represent the igneous crystalli- sation age for the host aplite dyke. Analyses 1.22b and 1.52 are discordant 8 and 4, respectively and have a relatively high f206 0.86 and 1.17, respectively and so were excluded from the age calculations. Rim analyses 1.1a and 1.2b on the two mor- phologically distinct grains have much lower Th U ratios 0.3 and 0.06, respectively than those in the main population, mainly due to significantly higher U contents Table 3. This characteristic suggests that the rims of these grains formed in a Fig. 4. Time-space diagram constructed for the Nornalup Complex. The four units compared are shown in Fig. 2. The height of an ‘event block’ is schematic; large boxes for intrusive events represent larger volumes of magma, cf. smaller boxes. Question marks indicate uncertain interpretations. The S symbol represents the formation of bedding surfaces. SHRIMP error bars are at 95 confidence levels for pooled analyses and 1s for single analyses. Geochronological data from Nelson et al. 1995 and Myers 1995a included in the diagram are mentioned in the text. Four of the five single grain xenocrystic analyses shown for the Recherche Granite were obtained from a gneissic sample of Recherche Granite located at Cape Arid Clark, D., unpublished data. chemical environment of different ThU composi- tion to the main population of analyses. The two analyses have a pooled 207 Pb 206 Pb age of 1345 9 10 Ma 1s, Fig. 3c which is consistent with the 1330 9 14 Ma crystallisation age obtained on the Recherche Granite pluton that the dyke intrudes Nelson et al., 1995. Thin, dark CL rims on these grains too thin to analyse may represent a second period of zircon growth within the aplitic magma. Analysis 1.2a from the irregular core of grain 1.2 contains 175 ppm Th, 154 ppm U and a ThU ratio of 1.1. A distinct 207 Pb 206 Pb ratio consistent with an age of 1442 9 22 Ma suggests that this core is xenocrystic to the aplite. 3 . 4 . 4 . Post-D 3 migmatitic leucosome 9611201 , Salisbury Island Two distinct morphological types of zircons occur in this sample. Elongate grains averaging 150 – 250 mm long occur dispersed throughout the leucosome portion of the migmatite. Length width ratios range from 1.5 to 3.5. These zircons are clear, subhedral and are strongly oscillatorily zoned. The second group consists of clear equant grains soccerballs averaging 150 – 200 mm in di- ameter. The grains are bounded by many high-or- der facets. This group is also abundant in the leucosome and shows strong oscillatory zoning. Grains from both groups are typically mantled by unzoned rims 5 30 – 50 mm thick of slightly brighter CL-response than the cores. Core zona- tion is truncated by rim material in rare instances. Very thin bright-CL outer rims truncate zonation in some grains. Eighteen zircon analyses of cores from both morphological groups in this sample define a pop- ulation with a mean 207 Pb 206 Pb age of 1214 9 8 Ma Fig. 3d. Uranium contents range from 434 to 854 ppm and average 637 ppm Table 5. Thorium contents are low and range from 9 to 48 ppm, averaging 26 ppm. ThU ratios are ex- tremely low 0.01 – 0.07, which is consistent with the coeval growth of monazite with this zircon. The well-developed planar boundaries and oscilla- tory zonation exhibited by these grain cores sug- gests that they formed within the granitic leucosome. The 1214 9 8 Ma age is therefore in- terpreted to date the onset of crystallisation of the leucosome and thus constrains the timing of M 2a . Six rim analyses from both elongate and equant grains fall within error of a mean 207 Pb 206 Pb age of 1182 9 13 Ma Fig. 3d. Uranium and thorium contents are lower but comparable to the core material Table 5, while ThU ratios range from 0.02 to 0.06, averaging 0.03. Similar chemistry and the absence of zonation is reported to be consistent with zircon growth under meta- morphic conditions Williams et al., 1996. Fraser et al. 1997 demonstrated that zircon growth in high-grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of Zr-bearing phases such as garnet. Fluid-present D 4b shearing, which resulted in the extensive re- placement of peak assemblages by M 2b biotite + sillimanite + quartz, provides a likely candidate for such a zirconium-liberating event. Hence, the 1182 9 13 Ma rim age is interpreted to record the timing of D 4b shearing, which then provides a lower age bound for high-grade activity and sub- sequent decompression in the Salisbury Gneiss. Two analyses 6.1 and 12.1 fall statistically outside the two major populations in this sample based on a x 2 -test and so were not included in age calculations. 3 . 4 . 5 . Post-D 3 quartzite 9510101 , Mount Ragged Zircons in this sample range from subhedral elongate grains up to 320 mm in length exhibit- ing strong concentric zonation in transmitted light and CL, to rounded grains \ 100 mm in length filled with apatite inclusions. Many are metamict to varying degrees and are brown in colour, while others are colourless. All show pitting and have irregular surfaces, consistent with detrital trans- port. CL imaging reveals a surprising conformity of internal zonation patterns. Most grains pre- serve concentric oscillatory zoning with no evi- dence of inherited cores. The zonation commonly truncates against fracture surfaces. The intensity of the CL response varies markedly between grains. No grains preserve evidence for more than one major period of zircon growth, although some grains show evidence of small palaeofrac- tures having healed. Zircon analyses from this sample fall into two main age groupings Fig. 3e. Several discordant analyses were included in the age calculations for both populations. Their inclusion is justified be- cause the lower intercept of the discordia they lie upon is the present, indicating recent lead loss. Primary 207 Pb 206 Pb ratios are therefore retained. Percentages of common 206 Pb are generally less than 0.5 for these analyses Table 6. The younger population, comprising nine analyses, has 207 Pb 206 Pb ratios, which are within error of a single value and indicate an age of 1321 9 24 Ma. Analyses contain between 54 and 600 ppm U, and 34 – 484 ppm Th Table 6. U and Th contents average 257 and 207 ppm, re- spectively. Apart from a few outliers the ThU ratios of analyses from this population cluster fairly closely around a mean value of 0.8. To- gether with the oscillatory zonation noted in CL images, these data suggest that the analyses sample zircon formed in an igneous rock. The older population, comprising seven analy- ses, forms a discrete group with a pooled 207 Pb 206 Pb age of 1783 9 12 Ma. Two analyses 1.24 and 2.3 fall outside the older population. Both are discordant and were not included in age cal- culations. Analyses from this population are more heterogeneous with respect to chemistry. Uranium concentrations range from 71 to 660 ppm and thorium concentrations from 93 to 1126 ppm Table 6. The average U and Th concentrations 261 and 322 ppm, respectively do not differ significantly from those of the younger population. ThU ratios show no sig- nificant cluster and vary from 0.6 to 2.1. The chemistry of the zircons does not provide con- clusive evidence as to their origin but as oscilla- tory zonation is present in the majority of grains an igneous origin is most plausible. The rounded, fractured and abraded surfaces of zircons from both populations indicates detri- tal transport. The zircons are therefore inter- preted to be detrital grains in the sedimentary precursor to the quartzite and have the U-Pb isotopic characteristics of their igneous source rocks. The younger population of 1321 9 24 Ma sets a maximum age for the deposition of the sediments that formed the protolith of the quartzite. The dominantly clean quartzitic na- ture of the Mount Ragged metasedimentary rocks precludes a volcanic or volcanoclastic origin and instead suggests granitic source rocks shed off the uplifted and eroding Albany – Fraser Orogen. The high oxidation state of the metasedimentary rocks, characterised by the sta- bility of haematite, suggests deposition in a shal- low and oxygenated environment. 3 . 4 . 6 . Syn-D 4 mica schist 9510092 , Mount Ragged Rutile crystals from this sample are a lustrous brown-red colour, euhedral in shape and vary from elongate crystals lengthwidth 4 – 7 up to 500 mm in length to equant plates 250 mm in length. The width of needles varies from 50 mm in the most elongate grains to several hundreds of micrometres. The grains show no evidence of zonation in transmitted light. All 18 rutile analyses from this sample fall within error of a mean 207 Pb 206 Pb ratio corre- sponding to an age of 1154 9 15 Ma Fig. 3f. The percentage of common 206 Pb in the analyses is high ranging from 0.6 to 6.0, see Table 6 but is lower than usual for this mineral by virtue of atypically high uranium concentrations, which range from 76 to 122 ppm and average 98 ppm Table 6. The metamorphic mineral as- semblage in the schist, the abundance of planar crystal faces on rutile grains, and their tendency to form delicate knee-bend twins suggests that they grew as a part of a post-kinematic parage- nesis, which slightly post-dates peak metamor- phism. Based on considerations of mineral assemblage, peak metamorphic temperatures are unlikely to have far exceeded 500°C. The cool- ing rate for the metasedimentary rock is un- known but can be assumed to be in the order of several degrees or more per million years. At this cooling rate, and for rutile grains of the size analysed, the closure temperature must be in ex- cess of 420°C Mezger et al., 1989. The rutiles therefore crystallised near to their closure tem- perature, so it is expected that the age recorded is close to the actual crystallisation age. The 1154 9 15 Ma age therefore provides a minimum estimate for the timing of peak metamorphic conditions in the Mount Ragged metasedimen- tary rocks.

4. Discussion