SECOND AUSTRALIAN CONFERENCE ON INIS-mf—12772 GEOCHRONOLOGY
27-29 September, 1989
WORKSHOP PROGRAM AND ABSTRACTS
1989 SYDNEY
Unedited abstracts - not to be cited without permission of the author
SPONSORS:
Centre for Isotope Studies, CSBRO Division of Exploration Geoscience
Selby Anax, Sydney Advanced Analytical P/L, VG Isotech, UK Sydney ACOG 89 PROGRAM
WEDNESDAY 27 SEPTEMBER
9.45 Brian Gulson - Welcome and introductory remarks.
SESSION I. NEW DEVELOPMENTS IN ISOTOPE STUDIES METHODOLOGY, INSTRUMENTATION
Chairman; Brian Gulson
10.00 Steve Clement and Bill Compston (RSES): SIMS at high mass resolution.
10.25 Peter Kinny (RSES): New SHRIMP applications.
10.50 Chuck Douthitt and Jim Bearpark (Finnigan MAT): Recent advances in thermal ionization mass spectrometry.
11.15 Soey Sie (CSIRO): Preview of the CSIRO-AMS facility.
11.40 John Cantle (VG Instruments): A comparison of multicollector methods plus a discussion of factors relating to optimum precision in a thermal ionisation mass spectrometer.
12.05 Masahiko Honda, Ian McDougall, A. Doulgeris and Des Patterson (RSES): Analytical techniques for measurement of noble gases in terrestrial samples.
12.30 LUNCH
Chairman: Bill Compston
2.00 Graham Mortimer (RSES): A new facility in isotope geochemistry at RSES.
2.25 Ian McDougall (RSES): Precision of K-Ar and 40Ar/39Ar dating.
SESSION II. ISOTOPE STUDIES RELATING TO ECONOMIC GEOLOGY
2.50 Caroline Perkins, Ian McDougall, Jon Claoue-Long (RSES) and Paul
Heithersay (Geopeko): ^Ar/^Ar an(j u-Pb geochronology of the Goonumbla and Gidginbung gold deposits, NSW. 3.15 Jeremy Richards and Ian McDougall (RSES): K-Ar and 40Ar/39Ar age relationships at the Porgera gold deposit, PNG.
3.40 AFTERNOON TEA
Chairman: Joe Hamilton
4.05 Graham Carr and Judy Dean (CSIRO): Mantle lead signatures of Ordovician gold mineralization in the Lachlan Fold Belt of NSW. 4.30 Brian Gulson, Karen Mizon, (CSIRO), Brian Atkinson, Tony Andrew (OGS), Dave Burrows, Nick Callan, Steve Noble (U of T), Fernando Corfu (ROM): The dilem ma of the source of gold in Archean greenstone deposits.
5.30-7.30 Ice Breaker, NBTC Conference Rooms THURSDAY 28 SEPTEMBER SESSION III. ISOTOPE STUDIES RELATING TO GRANITES Chairman: Rod Brown 9.00 Rob Creaser (La Trobe): U-Pb and Sm-Nd isotopic evidence for the age and origin of mid-Proterozoic felsie magmatism of the eastern Gawler craton, South Australia. 9.25 Stirling Shaw and Dick Flood (Macquarie): Carboniferous igneous activity in the Lachlan and New England Fold Belts: parts of the same magmatic arc? 9.50 Yadong Chen (Macquarie): Sr isotopic studies on mafic enclaves and host rocks of the Glenbog, Anembo and Blue Gum plutons of the Bega Batholith. 10.15 Morning Tea Chairman; Rod Page 10.40 Shen-su Sun (BMR): Chemical and isotopic systematics of the Blue Tier Batholith, N.E. Tasmania: its bearing on the origin of the tin-bearing alkali feldspar granites. SESSION IV. FISSION TRACK STUDIES/SEDIMENTARY BASINS AND OTHERS 11.05 Andy Gleadow (La Trobe): Fission track length studies and age patterns in western Victoria, (abstract not received) 11.30 Joe Hamilton (CSIRO), Tony Fallick (SURRC), Julian Andrews (East Anglia) and Dave Whitford (CSIRO): Isotopic studies of the provenance and post depositional alteration of Jurassic clay mineral assemblages, Scotland.
11.55 Rod Brown (La Trobe): Dating and measuring regional episodes of denudation using apatite fission track analysis: an example from southern Africa. 12.20 LUNCH
SESSION V. OTHER PETROLOGICAL APPLICATIONS
Chairman: Stirling Shaw
2.00 Dave Whitford (CSIRO), Tony Crawford (U. of Tas.), Michael Korsch and Steve Craven (CSIRO): The Mount Read Voleanics, Tasmania: Sr and Nd isotope geochemistry. 2.25 Sue O'Reilly (Macquarie) and Bill Griffin (CSIRO): Isotopic signatures of mantle rocks from south eastern Australia. 2.50 Neil McNaughton (UWA), Dave Nelson, John de Laeter (Curtin) and Ian Fletcher (GSWA): The Bunbury Basalt: Kerguelen - Heard Island hot-spot volcanism along the continental margin of SW Australia. 3.15 Bill Griffin (CSIRO) and Sue O'Reilly (Macquarie): Sm-Nd dating of garnet- bearing rocks: empirical evaluation of constraints on resetting. SESSION VI. PRECAMBRIAN GEOCHRONOLOGY
Chairman: Dave VVhitford
4.05 Li Huimin (China), Rod Page, Mick Bower (BMR) and Bill Compston (RSES): Four methods of zircon dating: case study from the Proterozoic Zhongtiao Mountains, Shanxi Province, China. 4.30 Dave Nelson (Curtin), Alee Trendall (GSWA) and John de Laeter (Curtin): Sm-Nd isotopic complexities in Archaean mafic lavas and implications for Sm-Nd geochronology.
4.55 Jon Claoue-Long (RSES), R.W. King and Rob Kerrich (Saskatchewan): Dating Archaean gold deposits with the ion-probe: a Canadian example.
3.40 AFTERNOON TEA
7.00 for 7.30 pm ACOG DINNER, MACQDARIE UNIVERSITY STAFF CLUB
FRIDAY 29 SEPTEMBER
Chairman: Ian McDougall
9.00 John de Laeter (Curtin) and W.G. Libby (GSWA): 500 Ma biotite Rb-Sr dates in the Yilgarn Block near Harvey. 9.25 Des Patterson (RSES): Noble gases as tracers of volatiles in subduction zones.
9.50 R.J. Ryburn, Rod Page, John Richards, V. Laynne and E.P. Shelley (BMR): 'Ozchron' - a national database of Australian geochronology.
10.05 Discussions about ICOG-7
10.30 MORNING TEA
Visit stable/radiogenic/accelerator laboratories Dating and measuring regional episodes of denudation using apatite fission track analysis: an example from southern Africa.
Roderick Brown
Department of Geology, La Trobe University, Bundoora, 3083, Australia.
The chronology and nature of landscape development in southern Africa has stimulated debate since the beginning of the present century, and yet no consensus has been reached. Major differences in viewpoint result largely from a lack of quantitative information concerning the timing and magnitude of denudation episodes affecting the sub-continent, particularly since the break, up of Gondwana, The major discrepancies between the various proposed landscape chronologies imply that there are fundamental problems associated with dating approaches used up until now, for example, in the correlation between landsurfaces and unconformities in the offshore sedimentary record. This situation suggests that the use of a more direct dating technique may help to resolve these discrepancies. Apatite fission track analysis (AFTA) is a valuable tool for evaluating the thermal history of rocks at temperatures below ~125°C and consequently for examining the thenno-tectenic development of the upper few kilometres of the Earth's crust. As a result AFTA can be used to date major periods of denudation directly . The application of apatite fission track analysis to rocks collected from a range of elevations along the western continental margin of souihem Africa indicates that the present land surface exposes rocks that have cooled rapidly from temperatures above ~125°C to temperatures below ~60°C during the early Cretaceous. This rapid cooling of the upper crust is interpreted as the result of accelerated erosion and consequent uplift of the rock column associated with the early development of the continental margin. The timing of this episode of denudation is broadly synchronous with the break up of West Gondwana and correlates with the pattern of sedimentation derived from borehole data in the adjacent offshore basin. Furthermore, the -125°C palaeo-isothermal surface for the pre-uplift crust is recorded by the apatite age-elevation profiles as a distinct break in slope. The present day elevation at which this break occurs allows an estimate of the amount of denudation to be made. For reasonable estimates of the palaeo-geothermal gradient some kilometers of denudation must have taken place. For a palaeo-geothermal gradient of 30°C/km the caJcuatcd amount of denudation is 2.5 km. In addition, the presently available AFTA data indicates that the denudation was regional in extent ABSTRACT PR J E CANTLE. VG INSTRUMENTS
A COMPARISON OF MULT1OOLLECTOR METHODS PLUS A DISCUSSION OF FACTORS RELATING TO OPTIMUM PRECISION IN A THERMAL IONISATION MASS SPECTROMETER
Static multicollection is the simultaneous collection of multiple ion beams where each isotope i? dlocated to a designated collector. It lias been developed as a technique which removes errors due to ion beam fluctuations which can limit the precision of single collector measurements.
In the Dynamic multicollection mode, groups of isotopes are measured with a peak-jumping technique. It has been shown that dynamic mukicollection provides significant benefits in terms of reproducibilily in isotope systems that can be normalised.
In static multicollection, the precision of measurement is constrained not only by ion statistics, but also by amplifier noise and amplifier drift. In particular, the stability of the relative gain between the resistors in each collector channel can have a significant effect on the ultimate precision achievable.
The VG Sector 54 thermal ionisation mass spectrometer uses the Multi-2 collector which is unique in commercially-available niulticollectors in that the resistor and amplifier boards are kept within a special environmentally-sealed housing which is evacuated and cooled by a 'Peltier-effect' device to 10°C + 0.01°C. Furthermore ultra low temperature coefficient resistors are used with a specification of 200ppm/°C.
The design of the Multi-2 collector and the use of ultra-low temperature coefficient resistors permits extremely stable long-term gain calibrations. Stability is such thai ultimate precision in static mode should only be limited by ion statistics. MANTLE LEAD SIGNATURES OF ORDOVICIAN GOLD MINERALIZATION IN
THE LACHLAN FOLD BELT OF NSW
Graham R. Carr and Judith A. Dean
CSIRO Division of Exploration Geoscience
Lead isotopic signatures of ore deposits are used by a number of Australian mining companies as a gcochemical discriminator in their exploration programs. In the Lachlan Fold Belt (LFB), the isotopic signatures of massive sulf ide mineralization associated with greywackes and acid volcanic rocks is well established. However there has been a dearth of Pb isotopic data on other styles of mineralization, particularly the porphyry-breccia-pipe-epithermal associations which can repre- sent major resources of Cu and Au.
Studies are currently underway to establish the Pb isotopic compositions of Ordovician Au-rich deposits associated with andesitic volcanics in the Bathurst-Orange-Parkes-Temora regions of NSW. Unlike the homogeneous, crustal Pb dominated massive sulfide mineralization of the LFB, these deposits are strongly depleted in Pb relative to the orogene growth curves of Cumming and Richards (EaTth & Planet. Sci. Lett.,28, 1975,pp. 155-171) and Zartman and Doe (Tectono- physics, 75, 1981, pp. 135-162.). In addition, the 206Pb/204Pb ratios vary both within and between deposits, the least radiogenic Pb (^6Pb/2^4Pb = 17.67) so far measured occurring at the Peak Hill deposit and the most radiogenic Pb at Goonumbla and Gidginbung (206Pb/204Pb = 18.20). Copper Hill and Cargo have intermediate 206Pb/204Pb ratios. Whereas the Peak Hill Pb is con- sistent with that of average Ordovician mantle, the Goonumbla/Gidginbung Pb has a model age considerably younger than the true geological age.
207 204 On a Pb/ Pb vs 20<>PB/204 plot the deposits fall on a linear trend with a slope of . 1702 + /- .0080 and an MSWD of 0.6. It is probable that the line represents mixing of two end members. Such mixing is apparent even on the scale of single deposits, for example at Goonumbla and Copper Hill where the within-deposit isotopic variation is parallel with the broad trend. The least radiogenic end member may well be represented by the average Ordovician mantle signature of Peak Hill. The very low slope of the trend precludes the possibility that the more radiogenic end- member involved crustal contamination. In fact, both Peak Hill and Goonumbla fall on or below the Plumbotectonic mantle curve of Zartman and Doe (1981). Thus the more radiogenic end member also almost certainly represents mantle Pb.
The slope of the trend may give some indication of the origin of this radiogenic mantle Pb, although it provides no unique solution. If considered as an isochron, the line gives an age for the last homogenization of Pb in the source rocks of 2 - 2.4Ga, assuming a mineralization age of 440Ma. Whether or not the line is an isochron, it can be argued that this "age" is a minimum for the onset of Pb isotopic heterogeneity in the mantle-Jcrived source rocks. Sr ISOTOPIC STUDIES ON MAFIC ENCLAVES AND HOST ROCKS OF THE GLENBOG, ANEMBO AND BLUE GUM PLUTONS OF THE BEGA BATHOLITH
Y. D. Chen, School of Earth Sciences, Macquarie University, NSW, 2109 D. Steel, and C. M. Gray, Department of Geology, La Trobe University, Bundoora, Victoria, 3083
Mafic enclaves from the hornblende-bearing, I-type Glenbog, Anembo and Blue Gmn plutons (occurring at the west margin of the Bega Batholith in the Lachlan Fold Belt) were analysed for Rb and Sr isotopes, along with their immediately surrounding host granites. Initial %7Srfi6Sr ratios were calculated by using ages independently determined by U-Th-Pb studies on zircons and were compared between the enclaves and hosts. Data are presented in Table 1.
Table 1. Some chemical and isotopic data for mafic enclaves and host rocks of the Glenbog, Anembo and Blue Gum Plutons
Rock type SiO2 Rb Sr (ppm) (ppm) (-
* Data are from Beams, S.D., 1980, Magmatic evolution of the southeast Lachlan Fold Beltm, Australia. Unpublished Ph.D thesis, La Trobe Univ.
Samples were taken from four localities (2 within the Glenbog Granodiorite, 1 within the Anembo Granodiorite and 1 within the Blue Gum Tonalite). The mafic enclaves analysed are all lower in SiO2 than the hosts and are composed principally of fine to medium grained plagioclase and hornblende with subordinate, variable amounts of quartz and biotite. K-feldspar is present in minor amouts in some enclaves and completely absent in others. Clino- and/or ortho-pyroxenes appear in accessory amounts only in the enclaves from the Anembo Granodiorite. Accessory 1
minerals common to all enclaves include magnetite, ilmenite, apatite, zircon, sphene and allanite. Table 1 shows that all the mafic enclaves and host granite samples have relatively high initial 87Sr/86Sr ratios (0.7073 - 0.7099). Initial 87Sr/86Sr ratios for mafic enclaves can be the same as, or lower than, or higher than for their immediately surrounding granites. For those enclaves that show difference in initial 87Sr/86Sr ratio from the host, the deviation is not very large but is beyond the error limit.
The high initial 87Sr/86Sr ratios indicate that the mafic enclaves do not represent uncontaminated, mantle-derived magma input at the generation of granite magma simply because their initial 87Sr/86Sr ratios are too high to represent mantle magma. The general similarity in initial 87Sr/86Sr ratio between the enclaves and hosts discounts the possibility that the enclaves and hosts are linked in a simple, two component mixing system (a low silica and initial 87Sr/86Sr ratio and high Sr concentration endmember, and a high silica and initial 87Sr/86Sr ratio and low Sr concentration endmember); otherwise, systematically lower initial ^Sr/^Sr ratios for the enclaves than the hosts would be expected. The following two hypotheses can both reconcile the data: (a) the enclaves and hosts could have been linked in a two component mixing mechansim but subsequently subject to considerable Sr isotope re-equilibration; (b) the enclaves and hosts are derived from the same source region. The general similarity in initial 87Sr/86Sr ratio between the enclaves and hosts are a broad reflection of the source characteristics and the slight difference between some enclaves and hosts is ascribable to heterogeneiity in the source. The two possibilities are not confidently distinguished based on the Sr isotope data alone. to
DATING ARCHAEAN GOLD WITH THE ION-PROBE: A CANADIAN EXAMPLE J.C. Claoue-Longl, R.W. King2, and R. Kerrich2 1 Research School of Earth Sciences, AN.U. 2 Department of Geological Sciences, University of Saskatchewan, Canada.
Timing relations of units of the Archaean Superior Province in Canada are generally well constrained, principally-owing to the efforts of the Toronto zircon- dating group. However, unambiguous timing constraints on the important gold mineralisation here and elsewhere have proved elusive. In part, this is because few minerals deposited by the responsible hydrothermal systems are hosts to useful isotopic clocks, and analysis has been restricted to methods (Ar, Sr-isotopes) and minerals (such as micas) of known susceptibility to alteration. Applications of these dating techniques in gold-bearing veins of the Abitibi greenstone belt, Canada, have systematically yielded very young ages, leading to recent remarkable suggestions that the major "Archaean" gold deposits of the belt might in fact be Proterozoic. This has important implications for the origin of Abitibi belt gold deposits, which constitute the richest mesothermal gold province globally, and for similar deposits in Archaean cratons in Australia and worldwide. It also carries implications for the tectonic development of the craton in which the veins are found,
Hydrothermal zircons have been found in gold-mineralised veins and altered wallrock selvages at Val d'Or, in the Abitibi greenstone belt. The zircons are paragenetically associated with vein quartz, tourmaline, mica, carbonate, scheelite, pyrite, and gold, and their existence may be associated with the intense tourmalinisation characteristic of Archaean gold deposits in this district. The SHRIMP ion-microprobe has been used to analyse hydrothermal zircons from four separate mines spatially associated with a regional batholith. Multiple stages of hydrothermal zircon growth have been unravelled, and constrain primary vein development to a short period in the Archaean. Younger (including Proterozoic) ages previously obtained for the veins, using other minerals and isotopic schemes, are misleading constraints reflecting either alteration or renewed mineral growth during much later reactivation of fluids along the same structures up to 400Ma after initial formation of the veins.
More generally, the discovery of hydrothermal zircon opens up possibilities of unravelling complex timing relations in other hydrothermal ore deposits using the ion microprobe. u
SIMS AT HIGH MASS RESOLUTION Clement, S.WJ. & Comnston. W. Research School of Earth Sciences, Australian National University, Canberra 2601, Australia. SHRIMP is a SIMS facility designed for geological applications and has made important contributions in the fields of U-Pb age determination, S isotope geochemistry, and meteoritic isotope anomalies since 1982. It is required to operate at high mass resolution to overcome isobaric interferences, and simultaneously, at maximum transmission of secon- dary ions to permit useful isotopic measurements on elements present in natural minerals at ppb concentration levels. It employs a large sector-type mass spectrometer (rM 1000 mm, rE 1272 mm), with a geometry corrected for 2nd and 3rd order image aberrations (Matsuda 1974). An improved version of S HRIMP similar in scale and layout to the original is now under construction. As before, design has involved the phase-space matching of secondary ion 'cmittance' to the 'acceptance' of the secondary mass analyser. The secondary ion emittance from a 20 |im area has been evaluated at the object slit of the mass analyser using field simulation and trajectory tracing and is shown in the diagrams below as solid lines. The range for y' and z' depends on the initial transverse energy of the ions; the limit shown corresponds to ca. lOeV, which would include more than 90% of the secondary ions. The acceptance of the analyser in the xy plane is a function of the source slit width and the angular divergence slits. As shown below (broken lines), it assumes an object slit of 70fxm and a half angle of divergence of .01 radians. The corresponding zz' acceptance is derived from the maximum permissible beam height at the magnet entrance. Quadrupcle lenses between the ion extraction system and the mass analyser have been designed for best 'matching' between emittance and acceptance, which maximizes the transmission and hence the sensitivity and precision (1/VN) for trace element analysis.
Y, Y* Matching Z, T Matching 0.02 0.010
0.01 - _ 0.005 •
0.00-
•0.01 - -0.005 •
y (micron*) X (mlMnw U-Pb AND Sm-Nd ISOT9PIC EVIDENCE FOR THE AGE AND ORIGIN OF MID- PROTEROZOIC FELSIC MAGMATISM OF THE EASTERN GAWLER CRATON, SOUTH AUSTRALIA. Robert A. Creaser Department of Geology, La Trobe University, Bundoora 3083, Victoria. Widespread emplacement of granites and acid volcanics into both Archaean and Early Proterozoic crust occurred during the Middle Proterozoic across much of the northern Gawler Craton, S.A (Hiltaba Suite granites and Gawler Range Volcanics). Granites from the Olympic Dam region of the northern Stuart Shelf are the focus of detailed isotopic (Rb-Sr, Sm-Nd, U-Pb, O) and geochemical study aimed at under- standing the nature of this widespread, felsic magmatic episode. U-Pb zircon and titanite data from granites at Olympic Dam and Moonta yield ages between 1583 and 1598 Ma, similar to recently reported ages for the Gawler Range Volcanics and related granites (Fanning et al., 1988, Prec Res, 40/41. 363; Mortimer et al., 1988, Econ Geol, 83,694). The combined data indicate eruption and emplacement of very large amounts of felsic magma occurred in a geologically short time period, between ~ 1585 and 1600 Ma. The age of the granite that hosts the Olympic Dam Cu-U-Au-REE (etc) deposit is refined to 1586 ± 2 Ma as a result of new analyses combined with the data of Mortimer et al (1988), further constraining the maximum age of ore deposition. Presently available zircon and titanite U-Pb data suggest that felsic magmatism occurred over a period of at least 10 -15 Ma in the Olympic Dam region. Sm-Nd data was determined for both the Middle Proterozoic granites and volcanics, as well as Early Proterozoic (1845 Ma) batholiths of the Eastern Gawler Craton. The Early Proterozoic (Donington) batholiths have values of £Nd(T) of -2.4 and - 2.6 and TCR ages of ~ 2200 Ma, similar to the Early Proterozoic batholiths of the Northern Australian Fold Belts, which suggests a significant component of ancient crustal material in their genesis. The Middle Proterozoic granites and volcanics have values of £Nd(T) of - 3.6 to -14.2. Granites with highly unradiogenic Nd isotopic signatures have TCR ages of 2650 Ma, similar in age to the exposed Archaean rocks of the Gawler Craton, and provide clear evidence that complete reworking of Archaean crust occurred. Most granites and volcanics have values of eNd(T) between -3.6 and -5.0 and TCR ages of ~ 2200 Ma, the same as the early Proterozoic batholiths. These data are consistent with either derivation of the Middle Proterozoic granites and volcanics from a similar source to the Early Proterozoic batholiths or reworking of mafic granites associated with the Early Proterozoic batholiths. All felsic granites are likely to represent complete reworking of existing continental crust. /3 500 Ma BIOTITE Rb-Sr DATES IN THE YILGARN BLOCK NEAR HARVEY J.R. de Laeter1 and W.G. Libby2 1. School of Physics and Geosciences, Curtin University of Technology, Perth, WA, 6001. 2. Geological Survey of Western Australia, 100 Plain Street, Perth, WA, 6000. Biotite from granite and gneiss in the southwestern part of the Yilgarn Block near Perth has been dated by the Rb-Sr geochronological technique at about 500 Ma in a be.'i at the western edge of the block. The 500 Ma belt widens from 30 km near Perth to 55 km east of Harvey. The eastern boundary of this latter belt coincides with the east margin of the Saddleback greenstone belt. A transition belt 15 to 30 km wide separates the western belt from an eastern chronological plateau where biotite dates, mainly 2300 to 2600 Ma, are not much younger than regional Rb-Sr whole-rock dates which average about 2550 Ma. In contrast to biotite dates, whole-rock dates greater than 2500 Ma persist to the western edge of the block. The 500 Ma dates probably represent uplift, perhaps related to mild orogenesis, synchronous with late Proterozoic Pan-African activity. RECENT ADVANCES IN THERMAL IONIZATIQN MASS SPECTROMETRY C. 8. Douthitt, Finnigan MAT, San Jose, CA 95134, U.S.A. J. Bfcarpark, Finnigan MAT, Sydney, AUSTRALIA M. Fanning, PRISE, ANU-RSES, Canberra, AUSTRALIA Thermal ionization mass spectrometry (TIMS) has been undergoing a quiet Renaissance over the last few years, leading to a dramatic expansion of its applications. This growth can largely be ascribed to a synergistic relationship between the geochronological community and instrument manufacturers. MAT (a Bremen, West Germany-based division of Finnigan Corporation) has been manufacturing solid source mass spectrometers for more than 35 years for a small (and select) group of geo-and cosmocheaists and nuclear chemists. Researchers using TIMS now come from fields as diverse as oceanography, hydrology, Quaternary geology, environmental geology, and archeology. University and government researchers continue to develop new applications, as well as working on enhancing ionization efficiency, improving separation chemistry, and lowering blanks. Finnigan MAT has done its part by pioneering and making corcvnercially available a steady series of innovations related to multi-sample static •tulti-collection. Current requirements include automatic acquisition of superior quality data from increasingly smaller samples with a rapid and reliable throughput; satisfactory compliance with these requirements places extraordinary demands on both the instrumentation and the user interface. Meeting the needs of the "power users" with commercially available products has benefited all TIMS users by raising the "state-of-the-practice". The current generation of instrumentation comprises two mass spectrometers of a complementary nature, the NAT 262 and the THQ (Table I). Recent enhancements to these instruments pertinent to the research needs of geochronologists include (MAT 262, #1-5,7; THQ, #6-7): 1. High efficiency ion counting: in conjunction with Faraday cup collection of major ion beams, this allows for determination of isotope ratios over a moderate dynamic range. Peak jumping involving both the Faraday cups and the electron multiplier satisfies the requirements of the ROMulans for high precision Pb analyses using a Pb 205 spike. 2. A second stage mass filter for reduction of abundance sensitivity to the 1 ppb range; in conjunction with high efficiency ion counting, this will enable highly precise measurements of isotope ratios over an extreme dynamic range (e.g. U-Th disequilibrium dating of silicate rocks) with a conventional instrument 3. Software,for single element total evaporation experiments. 4. Intelligent design of the milltisample magazine and filament assembly allows ready implementation of new approaches (e.g. sublimation of single crystal zircon filament loadings). 5. Negative ion measurement at full 10-kV potential. 6. Integration of a multichannel analyzer with the positive and negative ion counting capabilities of the THQ allows multielement total evaporation experiments. This has particular advantage for isobaric interference-free multielement isotope dilution measurements (e.g. REE). 7. Both instruments can be controlled by either an IBM-compatible or an HP 9000 series computer, thus facilitating V"»T access to packaged data reduction and display software. TABLE I: A COMPARISON OF SELECTED FEATURES OF THE MAT 262 AND THQ MAT 262 THQ ANALYZER Magnetic sector Quadrupole (Second stage) (Quadrupole) POTENTIAL 10 kV 5 eV DETECTORS Faraday cups Faraday cup (fixed) Fixed DETECTOR MODES Faraday cup* Faraday cup* a. peak jumping a. peak jumping b. dyn^aic c. static Electron Multiplier^ Electron Multiplier1 a. peak jumping a. peak jumping 1. analog 1. analog 2. ion counting 2. ion counting a. MCA Simultaneous FC/EM2 a. dynamic ; 1. analog 2. ion counting b. static 1. analog 2. ion counting IONIZATION TI, RI3, (ED4 TI, RI3, El TOTAL EVAPORATION Single element Multi-element COMPUTER HARDWARE Compaq 386/20 Compaq 386/20 HP 9000 series5 HP 9000 series5 1 Positive and negative ions Positive ions only 3 Laser port available El is for leak checking 5 Includes HP 310 and HP 320 EM-electron multiplier; MCA-multicbannel analyzer FC-Faraday cup: TI-thermal ionization; Rl-resonance ionization; El-electron impact ionization Srn-Nd Dating of Garnet-bearing Rocks: Empirical Evaluation of Constraints on Resetting W.L. Griffin, Div. of Exploration Geoscience CSIRO, North Ryde, NSW 2113 Suzanne Y. O'Reilly, School of Earth Sciences Macquarie University, Sydney, NSW 2109 Sm-Nd dating of mineral systems is potentially very useful in studies of metamorphism, especially where garnet-bearing rocks are available. Garnet typically has a higher Sm/Nd than most common rock-forming minerals, and is stable over a wide range of T,P in a variety of rock compositions. Interpretation of the meaning of Sm-Nd mineral isochrons in polymetamorphic rocks requires knowledge of blocking temperatures and the ease of resetting of mineral systems. Two types of experiment have been carried out to evaluate these parameters. Metamorphic Resetting: In the anorthosites of the Bergen Arcs, western Norway, large (1-20 cm) coronas with concentric shells of opx-cpx-garnet formed by reaction betwen igneous olivine and plagioclase at conditions of ca. 900 °C, 10 kb [1-3]. Lineations defined by stretched coronas are cut by mangerites intrusions dated at ca. 900 Ma. During the Caledonian orogeny (ca. 400-450 Ma), the anorthosites were extensively converted to eclogite along shear zones and other fluid pathways [3,4]; estimated P-T conditions wereca. 750 °C, 15-17 kb. Detailed isotopic studies [5] of individual coronas adjacent to shear zones show no isotcpic resetting. Cpx-gnt-plag assemblages from three coronas all give isochron ages of 900-912 Ma. Minerals of the shear zones (omphacite, phengite, garnet) give Caledonian ages. One garnet separate falls above the isochron, reflecting the mixture of relict granulite-facies garnet and newly formed eclogite garnet recognized from microprobe studies. A relict corona preserved within the shear zone give a cpx-gnt age of 905 Ma, despite extensive deformation. The data show that heating to 750 °C for ca. 10 Ma is insufficient to reset adjacent cpx-gnt pairs over distances of <1 cm, even in the presence of fluids. The calculated diffusion rates for Sm and Nd in garnet are consistent with published experimental values of Harrison and Wood [6], and similar to values for the self-diffusion of Mg. Cooling/Exsolution: Basaltic diatremes at Delegate, NSW and Lake Bullenmerri, Victoria contain abundant xenofiths of garnet websterite and garnet clinopyroxenite, which have evolved from igneous spinel clinopyroxenites by exsolution of gnt ± opx during cooling. The extreme homogeneity of the phases in these rocks suggests that they were in equilibrium at the ambient T when they were entrained in the host magma, erupted rapidly to the surface, and quenched. Model-age constraints suggest that the pyroxenites formed 300-500 Ma ago. Two gnt clinopyroxenites from Delegate, with equilibration T of 1140 °C and 1060 °C, give Sm-Nd cpx-gnt ages of 146±9 and 160±4 Ma respectively, consistent with estimates of ca. 160 Ma for the intrusion of the Delegate pipe. These mineral systems were in nearly complete isotopic equilibrium at the time of eruption. Two 2-pyroxene granulite xenoliths from Delegate, with equilibration T of ca. 850 °C, give cpx-plag-WR ages of 201 ±14 and 283±26 Ma, and obviously had been below their blocking T for some time. Two garnet websterites from Bullenmerri, with equilibration T of 1100 °C and 1035 °C, give cpx-gnt ages of 27±7 and 32±14 Ma, respectively. The age of the Bullenmerri eruption is «1Ma. These data suggest that retention of radiogenic Nd in garnet-pyroxene systems, cooling slowly in the upper mantle, begins when the rocks have cooled below ca. 1100 °C; the extrapolated "blocking T" as defined by Dodson probably is between 1000 and 1100 °C under these conditions. Implications for Dating: Garnet-pyroxene systems clearly have very high blocking temperatures for Sm-Nd diffusion; diffusion rates at T < 900 °C are probably <10 -15 cm2 sec1. Heating to T less than 900 °C, even in the presence of fluids and for periods of 1-10 Ma, will not cause significant resetting of mineral systems. Only complete physical breakdown of garnet is likely to cause effective resetting; the presence of >1 generation of garnet in a rock will almost certainly lead to mixed ages. Similar considerations probably apply to pyroxene-plagioclase systems as well, if resetting between minerals on a scale of mm is this difficult, the resetting of whole-rock Sm-Nd systems during regional or thermal metamorphism, at least in granulites and eclogites, is highly unlikely. References 1] Griffin, W.L (1972) Geol. Soc. Amer. Mem. 135:37-63 2] Griffin, W.L., Mellini, M., Oberti, R. & Rossi, G. (1985) Contrib. Mineral. Petrol. 91:330-339 3] Austrheim, H. & Griffin, W.L. (1985) Chem. Geol. 50:267-281 4] Austrheim, H. (1987) Earth. Plante. Sci. Lett. 81:221-232 [5] Cohen, A.S., O'Nions, R.K., Siegenthaler, R. & Griffin, W.L. (1988). Contrib. Mineral. Petrol. 98: 303-311 [6] Harrison, W.J. & Wood, B.J. (1980) Contrib. Mineral. Petrol. 72:145-155 THE DILEMMA OF THE SOURCE OF GOLD IN ARCHEAN GREENSTONE DEPOS- ITS Brian L. Gulson+, Karen J. Mizon+, Brian Atkinson++, Fernando Corfu*, David R. Burrows**, Nicholas J. Callan**, Stephen Noble**, and Anthony J. Andrews# + CSIRO Division of Exploration Geoscience, North Ryde 2113 ++ Ministry of Northern Development, Balmertown, Ontario, Canada * Royal Ontario Museum, Toronto, Ontario, Canada ** Department of Geology, University of Toronto, Ontario, Canada # Prospectors and Developers Association, Toronto, Canada Lead isotope systematics have been used to attempt to discrim- inate between the three main theories for the genesis of gold in Archean lode gold deposits. The theories are: (i) magmatic, with the gold derivation from magmatic intru- sions often spatially associated with the deposits; (ii) metamorphic, involving gold extraction by circulating fluids derived from metamorphic dewatering of a complex metamorphic pile; and (iii) mantle degassing, which involves large scale generation of CO2 during granulitization. It was expected that the Pb isotopic patterns for the mafic rocks would have a mantle signature compared with a more evolved (crustal) signature in associated granitoids and this would allow discrimination of theories (ii) and (i). Samples were analysed from the well-documented Red Lake area in north-western Ontario, the Renabie mine in the same region, and the Lamaque mine in Quebec. The Pb isotopic compositions in whole rocks and feldspars were compared with sulfides and gold in the deposits. The isotopic patterns for the komatiites and associated mafic to falsic volcanics are identical with those for the grani- toids. This similarity is interpreted to mean either tjhat the komatiites/volcanics and granitoids were derived from the same mantle source with the granitoids being a direct partial melt of mantle material or that the granitoids vj&re formed from material that had a very short residence time in the crust. Thus Pb isotopes cannot be used to discriminate between the different theories for the genesis of Archean lode gold depos- its. A similar problem arises for the published Pb isotope data around Kambalda. The ages measured from the 207Pb/204Pb - 206Pb/204Pb plots generally approximate those derived by high precision U/Pb dating using zircons, but with larger errors. Where no miner- als such as zircon or baddelyite are available in the mafic lavas, the whole rock method offers an alternative for obtain- ing the age. In some cases, the whole rock Pb - Pb ages are up to 200 Ma younger than the zircon ages, and this is interpret- ed to reflect hydrothermal overprinting perhaps associated with the gold mineralization. Similar young apparent ages have also been reported on micas from these deposits using the 40Ar/39PtAr m^thod. Because of the difficulty in obtaining initial ratios in Archean mafic to intermediate rocks (open U/Pb systems, no K- feldspar, old age, suspect whole rock - sulfide relationships), an alternative representation of the data was 206 204 attempted. This involves the plot of Pb/ Pbi versus Model Time^ where these parameters are derived from the intersection of the linear trends for the Pb isotope data and the growth curve for Pb rich massive sulfides of Cumming and Richards (1975). The data lie on an excellent linear trend and exten- sion of the line to the axes gives values of 3200 Ma and 206Pb/204Pb of 12.5, consistent with the source age estimated from inherited zircons. Although an approximation of the radioactive decay equations, the consistency of the relative positions of the different rock groups and their measured zircon ages, is more obvious on this plot compared with the conventional 204 - based diagrams.A plot of the 206Pb/204Pb ^ versus the measured age (that derived from the linear trends on 207Pb/204Pb - 206Pb/204Pb diagrams) is even more complex; the data define two trends with the -3000 Ma rocks and other komatiites from the Superior Province defining one trend and gold/sulfide ores/altered volcanics/ ~2720 Ma granitic stocks defining another trend. However, at this stage, the true significance of these lines are somewhat elusive and require evaluation in other areas with a large Pb isotope data base and good stratigraphic controls. ISOTOPIC STUDIES OF THE PROVENANCE AiiD POST-DEPOSITIONAL ALTERATION OF JURASSIC CLAY MINERAL ASSEMBLAGES, SCOTLAND Joe HAMILTON*, Tony FALLICK**, Julian ANDREWS*** and David WHITFORD* * CSIRO Division of Exploration Geoscience, PO Box 136, North Ryde, NSW 2113, Australia ** Isotope Geology Unit, SURRC, E. Kilbride, Glasgow G75 OQU, Scotland *** School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, England The subject of this study is clay mineral assemblages from Middle Jurassic lagoonal sediments of the upper Great Estuarine Group of the Inner Hebridean islands off the west coast of Scotland. Andrews (1987) recognized four compositional groupings of<2 micron fractions from X-ray diffraction analyses. 1. Illite-smectite + illite + kaolinite 2. Smectite 3. As for 1 but with less smectite 4. Illite dominant + kaolinite + chlorite Assemblages 1 and 2 reflect the importance of source materials with the former being river transported detritus from weathered bedrock and soils in the hinterland and the latter from volcanic ashfalls. Assemblages 3 and 4 represent the products of transformation of mixed layer illite-smectite to illite rich clays after deposition. These illite-rich, smectite poor assemblages are mostly located close to the margins of Tertiary igneous intrusions. Circulation of hot meteoric fluids associated with this igneous activity effected oxygen and hydrogen isotope exchange with the clay minerals and accelerated the illite-smectite to illite transformation beyond a stage expected for the degree of burial diagenesis. Rb87/Sr86 and 87Sr/86Sr ratios are crudely correlated which reflects binary mixing between a low Rb/Sr component identified with argillized volcanic ash and a high Rb/Sr detrital and illite-rich phase. Sm-Nd model ages (Tjpjyj) range from ca. 1.6 to 1.9 Ga and reflect also binary mixing between a volcanic component (TDM - 1.6 Ga) and an older supracrustal component. Reference: Andrews, J.E., 1987. Geol. Mag. j^24, 261-271. ANALYTICAL TECHNIQUES FOR MEASUREMENT OF NOBLE GASES IN TERRESTRIAL SAMPLES M.Honda, I.McDougall, A.Doulgeris and D.Patterson Research School of Earth Sciences The Australian National University Canberra ACT 2601 Measurement of isotopic effects in noble gases has been recognized as a powerful technique for studying the early history of the solar system using meteorites and lunar samples. Precise abundance and isotope measurements of all five noble gases, helium, neon, argon, krypton and xenon, from terrestrial samples provides important information concerning the understanding or the origin and evolution of the Earth's atmosphere, crust, mantle and core. Compared with extraterrestrial samples, however, noble gas abundances in terrestrial samples are usually so low and isotopic anomalies are so small that there nave been considerable experimental difficulties in producing isotopic ratios of the required precision and accuracy. The conditions necessary for the measurement of the extremely small quantities of noble gases from terrestrial samples are: (1) low-blank, (2) high sensitivity, (3) good stability, and (4) simple operation. To meet these requirements, the system needs to be made out of stainless steel which can be baked out thoroughly and remains impermeable to atmospheric He. The metal system for extracting and purifying the gases needs to be connected on-line with a high-sensitivity, metal mass spectrometer to keep experimental procedures simple and highly reliable. The noble gas analytical system at ANU consists of five divisions: (1) gas pipette system to calibrate sensitivity and mass discrimination of the mass spectrometer; (2) resistively-heated, double -vacuum furnace system to extract noble gases from solid samples, which can be heated up to 1800 °C; (3) gas purification system with Ti-Zr bulk getters, Ti-Al alloy SAES getters and a Ti flash getter, (4) cryogenic charcoal trap assembly to collect alfnoble gases and then to desorb noble gases successively, starting with He, for isotopic analysis. The cryogenic charcoal can be cooled down as low as 14K; (5) gas analysis system with an AMETEK M200 quadrupole gas analyzer tor preanalysis of noble gas amounts, and the VG5400 mass spectrometer for noble gas isotopic measurement In the talk we will report on the design and performance of the system, which is dedicated specifically to analysis of noble gases extracted from terrestrial solid samples. It will be shown that we have achieved improved performance compared with other noble gas analysis systems currently in use. New SHRIMP applications Peter Kinny RSESANU The original SHRIMP instrument was tailored to the desired objective of performing high-precision in situ analyses of Pb/Pb and Pb/U isotopic ratios in zircons. The majority of its operational time has indeed been devoted to that application. However, the design parameters equally are suited to many other geochemical applications. For example, already well-established is the use of SHRIMP to measure 34S/32S ratios of sulphide and sulphate minerals in ore deposits (Eldridge et al, 1987). New techniques are presently being developed for U-Pb geochronoiogy of other U-bearing minerals, such as sphene, apatite, rutilc, monazite and uraninite. These will be useful in elucidating the timing of tectonothermal events in metamorphic terranes, which produce no isotopic response from zircon, and in dating rock-systems devoid of zircon, particularly many ore deposits. For each new mineral species, isobaric spectral interferences must be identified and eliminated, and a suitably homogeneous and isotopicaily-concordant standard must be found, so that in each case the matrix-specific sputtering-induced Pb/U discrimination can be determined. Determinations of trace element abundances are also underway. Of major petrological interest are the rare earth elements, which reside in the crust principally in accessory minerals. The REE are particularly strong secondary-ion emitters, so that from zircon, for example, at mass-resolution 8000R, sensitivities of 100 cps/ppm for light REE (as oxide species) are typically achieved by SHRIMP. At this resolution, M+ species are resolved from MO+, leaving only hydride species with which to contend. All 14 REE can be analysed by this method, by normalising the measured abundance of each element to the known standard. These analyses will document for zircon and other minerals trace element abundance patterns that are diagnostic of particular host-rocks and conditions of crystallisation. Such information may then be used to determine the provenance of detrital minerals. Reference: Eldridge C.S. etal. (1987) Int. J. Mass Spec, ion Proc. 76, 65-83. "3 FOUR METHODS OF ZIRCON DATING: CASE STUDY FROM THE PROTEROZOIC ZHONGTIAO MOUNTAINS. SHANXI PROVINCE. CHINA Li Huimin1, R.W. Page2 , M.S. Bower2 , and W. Compston3 1. Tianjin Institute of Geology and Mineral Resources, Tianjin, China. 2. Bureau of Mineral Resources, Canberra. 3. Research School of Earth Sciences, ANU, Canberra. A late Archaean to early Proterozoic terrane in the Zhongtiao Mountains, southern part of the North China Craton, contains both stratiform and porphyry-type copper deposits that are hosted in the several bimodal suites of volcanic and intrusive rocks. A geochronological study using the U-Pb zircon system has been undertaken in an effort to better quantify its stratigraphic and tectonic development, and also provide a time framework for the mineral deposits. The main Proterozoic .supracrustal sequence has been deformed and metamorphosed to greenschist facies, and overlies a higher grade basement gneiss terrane (Shushui Complex) that contains metavolcanic and metasedirnentary enclaves. The supracrustal sequence is divided into a number of unconformably related groups (Jiangxian Group, Zhongtiao Group, Xiyanghe Group), and in the eastern part of the region there is a separate inlier of hitherto uncertain geological relationship, the Songjiashan Group. The various sequences contain abundant felsic volcanic and volcaniclastic horizons, especially in the lower part, and are intruded by several granitic masses. Conventional multi-grain zircon geochronology has been undertaken in Chinese laboratories on afew of the volcanic rocks, and Kroner et al. (1989) have recently reported "single-grain" 207pb/206pb zircon evaporation ages. In this paper, these data are summarised and compared to our own results from (a) single zircon U-Pb analyses using a mixed 205pb.235u tracer, and (b) ion microprobe analyses on the same zircon material. Songjiashan Group Single zircon dating of fine-grained, chert-like tuffs in the separate inlier that contains the Songjiashan Group, has provided an unexpectedly old magmatic depositional age of 2556 ± 29 Ma (lc errors), with older individual grains indicating inherited ages up to 2750 Ma. Zircons from a granitic body in this inlier have coherent 207p5/206po and provide an intrusive age of 2535 ± 20 Ma. This is the first demonstration of a late Archaean history for this inlier, suggesting it is a basement block, some 300 million years older than the adjacent supracrustal development. Shushui Complex Zircon U-Pb ion microprobe measurements on a felsic volcanic enclave in this "defined" high grade basement indicate a relatively simple magmatic crystallisation at 2333 ± 5 Ma. This is in good agreement with six single zircon fractions that have a pooled 207pb/206pb age of 2316 ± 1 Ma, in addition to further inherited components at 2440 and 2510 Ma. A 207pb/206pb evaporation age of 2521 ± 3 Ma that is considered to be magmatic by Kroner et al., is clearly dominated by inheritance complexities, although the evaporated zircons originated from the same sample vial. A deformed gabbroic intrusive in this sequence contains ragged, shapeless, low-U and concordant zircons, that provide a firm minimum constraint for the Shushui Complex of 2321 ±2 Ma. .Tiangxian Group The unconformably overlying Jiangxian Group contains potassic rhyolites and tuffs that host most of the region's Cu deposits. Ion microprobe data show a clustered inherited component at 2530 ± 3 Ma, and minor older inheritance up to 2770 Ma. In this rock, the few magmatic zircons are as sparse as the inherited types, and a close 207pb/206pb grouping of the former gives the stratigraphic age of 2115 ± 6 Ma. The single-grain data for this sample do not yield an interpretable magmatic age, due to the sparscness of the population as well as grain-size limitations. We consider that this problem also plagues the older evaporation 207pb/206pb age of 2231 ± 5 Ma. Although interpreted by Kroner et al. (1989) as the magmatic age for this rock, it can again be ascribed to a blend of magmatic and inherited components. Another tuff in the Jiangxian Group sequence, 25 km to the northeast, gives a coherent pattern in single-zircon 207pb/206pb and an interpreted magmatic age of 2166 ± 26 Ma. The depositional age for this group is thus probably bracketed in the interval 2115 to 2166 Ma. A third sample from the Jiangxian Group gives an older quite anomalous single zircon age of about 2500 Ma. In fact this apparently anomalous age is substantiated by a previous multi-grain zircon age determination of 2480 ± 11 Ma, and evaporation 207pty206p0 measurements at 2506 ± 5 Ma. Assessment of the geological significance of these results requires further work. Zhongtiao Group The stratigraphically younger Zhongtiao Group contains a sequence of fine-grained sediments intercalated with porpnvritic felsic volcanics that have a single zircon 207Pb/2°6pb age of 2060 ± 3 Ma, and evidence of older xenocryst material between 2100 and 2450 Ma. The "magmatic" age determined by 207pb/206pb evaporation steps (2104 ± 5 Ma) probably reflects such complexities. Xivanghe Group An unmetamorphosed dacite in the uppermost part of the Proterozoic sequence (Xushan Formation, Xiyanghe Group) has a clearly interpretable magmatic age of 1840 ±14 Ma, and a complex pattern of 2000-2500 Ma inherited material indicated from both the single zircon and ion microprobe data. Summary This work generally shows good agreement between the single zircon and ion microprobe methods of U-Pb zircon dating, although some of the comparitive ages suggest that the respective error assignments are geologically optimistic. Further, although the Zhongtiao Mountains Proterozoic rocks are mostly in low metamorphic grades, there are a number of examples where the complex zircon systems uncovered by ion-microprobe resolution had either been undetected, or were too complex to be interpreted by the single zircon approach. Additionally, the sampling power of the ion microprobe to minimise common Pb corrections, and the ability to search for and focus on grain areas that have suffered no Pb loss is most beneficial, especially for zircon populations that contain no suitable, inclusion-free grains that can be microscopically selected for single-grain dissolution. The magmatic ages determined by the 207pb/206pb zircon evaporation method - Kroner et al. (1989) - are consistently older than those determined from both other techniques, either because the evaporated material was mixed with unseen inherited fractions and/or incorrect correction for 204pb. Despite these reservations, it is clear, as pointed out by Kroner et al. (1989), that the Zhongtiao Mountains contains an unusual Proterozoic magmatic development between 2300 and 2100 Ma, a period of very little magmatic activity so far found elsewhere in the world. Further work may demonstrate that the younger part of the Zhongtiao Mountains sequence (-1840 Ma) is akin to the early-middle Proterozoic igneous activity so prevalent in northern Australian, north American and Scandanavian Proterozoic terranes. Precision of K-Ar and 40Ar/39Ar Dating Ian McDougall Research School of Earth Sciences The Australian National University, Canberra, ACT In stratigraphic geochronology and numerical time scale studies, high resolution geochronology is especially useful. Clearly, the proper assessment of precision and accuracy of the measured ages is of considerable importance in all such work. Dating methods based upon the K-Ar decay scheme commonly are used in studies of this kind, especially for the younger half of the Phanerozoic. In the case of rapidly cooled igneous rocks that have not been reheated subsequently, the conventional K-Ar and the 40AT/39AT dating techniques commonly yield results that can be directly related to the time since eruption. With appropriate biostratigraphic control, ages measured on such rocks are valuable in the definition of the numerical time scale. Precision of age measurement by the conventional K-Ar dating method is controlled by the precision of the K and Ar determinations, made on separate splits of the sample. The long term precision of the K measurement in our laboratory is at best 0.5% (standard deviation of the population, expressed as a percentage; i.e. coefficient of variation), and for Ar it is normally also -0.5%, controlled mainly by the uncertainties in calibrating the 38Ar tracer, used for the isotope dilution measurements. Combining these errors quadratically we obtain a coefficient of variation for a K-Ar age measurement of ~0.7%. The 40Ar/39Ar technique has considerable advantages over the conventional K-Ar dating method, as the age measurement is made on a single split of the sample, thus overcoming inhomogeneity problems, and smaller amounts of sample normally can be utilized. Furthermore, as the age is proportional to the 40Ar*/39Arjr ratio, derived from isotopic ratio measurements in the mass spectrometer, a precision of better than 0.2% often can readily be achieved. It is also necessary that the fast neutron fluence the sample has received in the reactor is measured to this same order of precision, which is commonly the case. The flux monitor used for neutron fluence measurement is a mineral of known K-Ar age; even if the uncertainty of its age is ~1%, this does not effect the ability to measure highly precise ^Ar/^Ar ages. If a 40Arfi9Ai age spectrum is determined, and this has a flat undisturbed pattern, the overall age can have a very high precision, owing to an error contraction effect. This is also been in isotope correlation plots. Thus, the ^Ar/^Ar dating technique can yield ages of significantly greater precision than the conventional K-Ar dating method. A revolution currently is taking place in 4^Ar/39Ar dating, involving the use of a laser microprobe system to release Ar from single crystals. Although the first measurements employing this approach were made more than 15 years ago, it is only during the last few years that the technique has been applied to a range of dating problems that demonstrate how useful laser heating can be. With improvements in die background in mass spectrometers, critical when operated statically, it has been shown that extremely small amounts of gas can be precisely measured isotopically. Thus.very small samples can be measured using the laser heating approach. Crystal by crystal analysis enables the homogeneity of a mineral sample to be examined. In addition, high precision ages can be determined, enhanced by the ability to take full advantage of the multiple analysis approach. Precisions of the order of 0.2% standard deviation for a population have been demonstrated. An important limitation appears to be the homogeneity of the flux monitor mineral from crystal to crystal. The potential of the laser fusion technique for high precision age measurement is only just beginning to be realized. In the near future we shall see a large amount of highly precise new data appearing that is likely to greatly improve the numerical time scale. ic THE BUNBURY BASALT: KERGUELEN-HEARD ISLAND HOT-SPOT VOLCANISM ALONG THE CONTINENTAL MARGIN OF SW AUSTRALIA N.J. McNaughton1, D.R. Nelson2, J.R. de Laeter2and I.R. Fletcher3 1 Key Centre for Strategic Mineral Deposits, Dept. of Geology, University of Western Australia, Nedlands, WA, 6009. 2 School of Physics and Geosciences, Curtin University of Technology, Perth, WA, 6001. 3 Geological Survey of Western Australia, 100 Plain Street, Perth, WA, 6000. Widespread subaerial tholeiitic volcanism within the southern Perth Basin during the early Cretaceous formed two series of basalt flows known collectively as the Bunbury Basalt. Volcanism was broadly coeval with formation of the basaltic basement to the Kerguelen Plateau, a large oceanic plateau of subcontinental thickness thought to have formed, in part, from a mantle hot-spot which now underlies Heard Island. Plate tectonic reconstructions suggest SW Australia and eastern India were separating at about this time, and the Bunbury Basalt may be the earliest manifestation of the hot-spot. The two series of flows within the Bunbury Basalt have contrasting Pb-Sr-Nd isotopic signatures and trace element characteristics. The more primitive group is similar to the transitional Kerguelen basalts in 206Pb/204pb( 208pb/204Pb( 87Sr/86Sr> and £Ndi but nas distinctly higher 207pb/204Pb The seconc| group has higher LREE/HREE, evolved Sr-Nd 87 86 isotopic signatures (initial Sr/ Sr > 0.707 and £Nd < -4) and more radiogenic Pb (including higher 207Pb/204Pb). This group contains more ' xtreme isotopic ratios than almost all other previously analysed tholeiitic samples from the Indian Ocean, a feature which must relate to either the source characteristics of the early Kerguelen-Heard hot-spot, or the continental setting of the Bunbury Basalt. Comparison with Pb isotopic growth curves for the SW Yilgarn Craton and Indian Ocean ridge basalts suggest the Bunbury Basalt Pb could have been derived by mixing between these two end-members. The Sr-Nd data are also compatible with this model, although derivation of the Bunbury Basalts from an enriched mantle source cannot be discounted. A New Facility in Isotope Geochemistry at RSES Graham E. Mortimer Research School of Earth Sciences, The Australian National University. Since the inception of radiogenic isotope geochemical research at RSES over two decades ago, the scope and sophistication of the studies have steadily changed. To meet the need of such work in the 1990's and beyond, it became clear that major upgrading of existing facilities or building of new facilities was necessary. As a result, the lower floor of a new wing at RSES was set aside for new isotope laboratories, and office space to consolidate the radiogenic isotope geochemists into one area close to the labs. Design of the new laboratories aimed to cater for a wide range of applications (e.g. from single crystal zircon to ore genesis), to incorporate stringent modern safety standards and yet to keep costs within reason. This was achieved, in part, by partitioning the lab. according to function, and by designing self-contained work-stations with a shared clean-air supply rather than attempting to fill the total lab. air volume with ultra-clean air. The result is a lab. essentially in two halves, the low-level lab. (including single crystal zircon) and the general lab., sharing a common entrance but without cross-traffic. The low-level lab. work stations are down-draught exhausting, or cross-flow, HEPA-filtered units of commercial design, and the associated balance room also has HEPA-filtered air. The general lab. has a down-draught exhausting HEPA hood for more demanding, lower-level work, but the majority of the chemistry will be done in PVC-fabricated Mini-Clean-Air-Stations (MCAS) which share a common remote HEPA-filtered air supply. Individual MCAS's are dedicated by function (e.g. Rb-Sr, Sm-Nd, Boron, WR Pb), or are allocated to individuals. The general lab. also has its own balance room. A teflon-ware cleaning lab. is shared by the low-level lab and general lab., as is a furnace room (for bomb dissolutions) which is located outside the chemistry labs. A modest external plant room supplies all the air to the labs, through PVC duct-work which further exhausts to a roof installation incorporating scrubbers before venting to atmosphere. All lab air is filtered and temperature-controlled to 21°C, and the MCAS's have a shared HEPA-filtered air supply. As a safety feature, all exhaust air is flow-monitored so that in the event of any exhaust failure power to the respective work stations is shut down. Fire dampers are also incorporated in the air supply duct-work. A baffled effluent pit adjacent to the plant room neutralises waste acids before they are discharged. Winter temperature in the laboratory (and offices) is further controlled through in-built electrical heating in the concrete slabs above and below. Repeated checks show that throughout the winter and early spring the two separate halves of the lab both have temperature ranges of about 3°C, but different maxima and minima, the difference being a function of the in-slab heating variability. The two halves of die lab are supplied with two grades of reagents. All chemistry is based on sub-boiling glass-distilled acids and Milli-Q ultra-pure water, while Pb- and low-level work is further supported by reagent distillation in teflon two-bottle stills. Hydrochloric acid is shortly to be produced by a high-purity gas-bubbling system. A Millipore reverse osmosis plant feeds the Milli-Q system at the rate of about 5 litres/hour, and is the rate-limiting factor in water production, which nonetheless is more than adequate. Pb blanks measured on the Milli-Q product are ~2 pg/ml. Procedure blanks for Sr and Nd in the general lab are averaging about 500 and 70 pg, respectively, and appear to be improving slightly. Blanks measured in the MCAS's and the HEPA down-draught exhausting units are not distinguishable, suggesting the air supplies are not the limiting factors in further reducing blanks. Sm-Nd ISOTOPIC COMPLEXITIES IN ARCHAEAN MAFIC LAVAS AND IMPLICATIONS FOR SM-Nd GEOCHRONOLOGY D.R. Nelson1, A.F. Trendall2and J.R. de Laeter1 1 School of Physics and Geosciences, Curtin University of Technology, Perth, WA, 6001. 2 Geological Survey of Western Australia, 100 Plain Street, Perth, WA, 6000. There are now many examples in the literature of disparity in dates on Archaean mafic and ultramafic lavas determined by the whole-rock Sm-Nd isochron method and those determined by other techniques, such as U-Pb dating of zircon from interbedded felsic rocks. Whole-rock Sm-Nd isochron dates are commonly significantly older than those obtained by other techniques. This suggests that the mafic lavas possessed a range in initial Nd isotopic compositions which were positively correlated with Sm/Nd, thus causing rotation of the Sm-Nd isochron. There are two processes by which Archaean mafic lavas may have acquired these characteristics- crustal contamination, and derivation from geochemically and isotopically heterogeneous mantle sources. To assess the relative importance of these two processes, a detailed geochemical and Sm-Nd isotopic study of two well-preserved Archaean volcanic sequences, the Fortescue Group of the Pilbara craton, Western Australia, and the Ventersdorp Supergroup of the Kaapvaal craton, South Africa, has been undertaken. The emplacement ages of these sequences have been precisely determined by the U-Pb zircon method using SHRIMP, enabling initial Nd isotopic compositions to be accurately calculated. Mafic and felsic lavas and intrusives from both sequences possess a considerable range in REE concentrations and initial eNd values. Initial eNd values of the felsic rocks (porphyritic rhyodacites and rhyolites) are within the range found in the associated mafic rocks, suggesting that the felsic rocks are related to the mafic lavas by fractional crystallisation and have not been more extensively contaminated by continental crust. In the case of the Ventersdorp lavas and intrusives, initial eNd values are positively correlated with both 1/[Nd] (r = 0.90) and Sm/Nd (r = 0.95). If a contaminant is involved in this case, the geochemical and isotopic trends indicate that the contaminant must have possessed a considerably lower Sm/Nd ratio than that typical of modern-day or Archaean continental crust. Similar trends are found in the Fortescue Group Sm-Nd data, but the influence of other processes (such as assimilation-fractional crystallisation?) are also apparent. These results suggest that the Ventersdorp and Fortescue lavas and intrusives were derived from geochemically and isotopically heterogeneous mantle sources but that their Sm-Nd isotopic systematics may have been modified by crustal contamination processes. As little or no fractionation of Nd from Sm occurs during the large degrees of partial melting involved in generating basaltic melts, it may still be possible to obtain information about the timing of source enrichment events or the age of any source contaminant involved if the extent of crustal contamination is not too severe. 11 Isotopic Signatures of Mantle Rocks from Southeastern Australia Suzanne Y. O'Reilly, School of Earth Sciences Macquarie University, Sydney, NSW 2109 W.L Griffin, Div. of Exploration Geoscience CSIRO, North Ryde, NSW 2113 Nd and Sr isotopic studies on mantle-derived xenoliths from western Victoria reveal the imprint of at least three metasomatic episodes. The oldest occurred at 300-500 Ma, resulting in patent and cryptic metasomatism of Iherzolite mantle wall-rocks, and changing their isotopic compositions as well as Rb/Sr and Sm/Nd ratios. Another event, younger but of ambiguous timing, involved the introduction of a component with "bulk-earth" isotopic ratios, accompanied by the formation of apatite in the mantle wall-rocks. The youngest event is associated with the intrusion of basaltic magmas with eSr =-10 and eNd = 2-5. This is probably related to the same general episode of basaltic volcanism that produced the Newer Volcanics. The xenolith types that provide evidence of these processes include spinel Iherzolite mantle wall-rocks, garnet- and spinel metapyroxenites (metamorphosed equivalents of basaltic melts and differentiates thereof, intruded within the upper mantle), wehrlites (metamorphosed, younger analogues of the metapyroxenites) and granulites (derived from both lower crustal and upper mantle depths). The Sr-Nd isotopic composition of the Iherzolites (Griffin et al., GCA, 1988) form an elongate field spreading into the "enriched mantle" region. Depleted-mantle model ages cluster at 600 ±120 Ma. At 700 Ma, more than 30% of Iherzolites have eNd exceeding that of model depleted mantle; hence this represents a maximum probable age for the LREE-enrichment. Some of the metapyroxenites have eSr»150 and eNd=-9; the rest scatter toward the Iherzolite field. This distribution is interpreted as deriving from wall-rock reaction, leading to isotopic equilibration between small bodies of pyroxenite and the surrounding reservoir of Iherzolite. At 300-500 Ma the pyroxenites define a good mixing hyperbola with the Iherzolites, most of which have eSr >0 at this time. Separate plots of 143Nd/14*Nd against Nd and 87Sr/86Sr against Sr support this mixing model and also define a distinct trend for the apatite- bearing Iherzolites. The apatite component has present-day eSr and eNd =0. The wehrlites have Sr and Nd isotopic compositions that overlap the range shown by the Tertiary Newer Basalts. Their metasomatic effect is to produce some scatter around the mixing lines resulting from the metapyroxenite and apatite "episodes". The major metasomatic effects evident in the Iherzolite xenoliths are consistent with metasomatism of a mantle wedge by fluids and melts derived from subduction processes. In particular, fragments of subduction-derived basaltic melts may be represented by compositionally and isotopically unusual granulite/pyroxenite xenoliths such as those found at Delegate and Gloucester. These xenoliths define a sublinear trend on an eNd- 87Sr/86Sr plot (Fig. 1) that suggests the samples are cogenetic and related by a mixing process (O'Reilly et al, J. Petrol., 1988). The likely end-members could have: (i) eNd ^ +8, 87Sr/86Sr = 0.705 (similar to some ocean-island or sea-floor basalts altered by sea water) and (ii) eNd < -8, 87Sr/86Sr > 0.707 (reflecting long-term enrichment in Rb/Sr and LREE (old upper continental crust signature). Since petrologic evidence requires that these granulites formed near or below the crust-mantle boundary, the "upper crustal" signature was acquired at mantle depths. This may have been produced during partial melting associated with metasomatic events with subduction signatures analogous to those seen in the western Victorian Iherzolites. Phanerozoic eastern Australia has evolved by successive accretion of marginal arcs. It is thus reasonable to expect components from subducted material and Palaeozoic subduction-related volcanism in the upper mantle beneath eastern Australia. Available data suggest that such processes were active beneath the Delegate and Gloucester regions; this suggestion is currently being tested by analysis of spinel Iherzolite xenoliths from northeastern Victoria and New South Wales. 20 MORB 10 N.QId.mantle xenoliths Delegate xenoliths I 7 CD E.Australia -10 Tertiary primitive basalts Victoria mantle xenoliths -20 - .702 .704 .706 .708 .710 .712 .714 87Sr / 86Sr Fig. 1. Sr-Nd isotopic ranges for mantle xenoliths (spinel Iherzolite, spinel- and garnet-pyroxenites) from Victoria and northern Queensland and for granulites from Delegate (dashed line). NOBLE GASES AS TRACERS OF VOLATDLES IN SUBDUCTION ZONES Des Patterson Research School of Earth Sciences The Australian National University Canberra ACT 2601 The genesis and subsequent evolution magmas associated with the subduction of oceanic lithosphere at convergent plate margins is a fundamental area of geochemical research. By virtue of their geochemistry (chemical inactivity, low abundance, and incompatibility), the noble gases are direct tracers of volatile movement and behaviour in subduction environments. As such they have the potential to shed light on the relative roles of the mantle wedge, subducted lithosphere and sediments, and arc crust, in the petrogenesis and evolution of arc magmas. To date, noble gas research on samples from subduction zones have been essentially restricted to the analysis of He, and to a lesser extent Ar, in arc related fumarolic gases and hydrothermal fluids (e.g. Baskov et al, 1973; Nagao et al, 1981; Sano & Wakita, 1985; Hulston et al, 1986; Sano et al, 1987; Poreda & Craig, 1989). These workers have identified a widespread mantle derived excess of 3He with respect to 4He compared with the atmosphere (3He/4He of hydrothermal fluids is approximately six times higher than the atmospheric ratio of i.4 x 10"6). This mantle derived He isotopic ratio survives gross dilution with meteoric groundwater due to the extremely low abundance of He in the terrestrial atmosphere (a function of the continuous loss of He from the atmosphere to space). Kennedy et al (1985) demonstrated the presence of three independent noble gas components in hydrothermal fluids: atmospheric, radiogenic (4He decayed fromU + Th, 40AT decayed from 40K), and mantle derived. These components have distinctive noble gas compositions, and can be deconvolved on the basis of 36Ar/40Ar, ^e/^Ar, and ^He/^Ar ratios. Mixing of these three components will generate a planar correlation in a 3-dimensional plot of 36Ar/4OAr - 4He/40Ar - 3He/*°Ar, and the compositions of the non-atmospheric endmember components can estimated from the intercepts of the data plane with the 4He/40Ar, and 3He/40Ar axes. The application of this type of 3-dimensional analysis to published noble gas data from hydrothermal samples from Japan (Nagao et al, 1981) and New Zealand (Hulston et al, 1986), reveals a planar correlation in 36Ar/40Ar - 4He/40Ar - 3He/40Ar space. This planar correlation is interpreted as mixing of atmospheric, radiogenic, and mantle derived noble gases in the arc related hydrothermal systems of Japan and New Zealand. The estimated endmember compositions of the non-atmospheric components (radiogenic and mantle), are consistent with previous workers (Kennedy et al, 1985), and with theoretical values predicted on the basis of K/U ratios of the crust and mantle. REFERENCES Baskov et al (1973), Geochemistry International, 10:130-138. Hulston et al (1986), Proc. 8th Geotherm. Workshop, Uni. Auckland Geotherm. Inst. Kennedy et al (1985), Geochim. Cosmochim. Acta, 49:1251 - 1261. Nagao et al (1981), Earth Plan. Sci. Lett., 53:175-188. Poreda & Craig (1989), Nature, 338:473-478. Sano & Wakita (1985), J. Geophys. Res. 90:8729-8741. Sano et al (1987), Geochim. Cosmochim. Acta, 51:1855- 1860. 3* and U-Pb geochronology of the Goonumbla and Gidginbung gold deposits, NSW C. Perkins, Geology Department, Australian National University, I. McDougall, J. Claoue-Long, Research School of Earth Sciences, Australian National University, and P. Heithersay, Geopeko Exploration, Parkes, NSW Establishing the timing of mineralization and associated magmatism on both a regional and deposit scale is a fundamental research problem as well as having important implications for minerals exploration. As part of an ongoing high precision dating study of mineralization and host rocks in the Lachlan Fold Belt, NSW, the Goonumbla and Gidginbung gold deposits have been investigated. The 4"Ar-3"Ar technique of K-Ar dating has been used to determine the age of hydrothermal phases, and U-Pb dating by ion microprobe of magmatic zircons has been utilized to establish the emplacement time of host rocks to mineralization. The porphyry copper gold Goonumbla deposit near Parkes, NSW, is hosted in a number of quartz monzonite pipes which intrude a trachyte-trachyandesite volcanic succession. ^Ar-^Ar step heating experiments on hydrothermal sericite from the Endeavour 26 deposit yield spectra which rise monotonically to ages ranging from 441.2 ± 1.1 Ma to 438.7 ±1.1 Ma, taken to approximate crystallization (all ^Ar-^Ar dates are quoted at 1 s.d., with the uncertainty derived by quadratically combining precision estimates for mass spectrometer isotopic measurements and neutron fluence measurements). In one instance, the age spectrum rises to a plateau of 437.9 ± 1.1 Ma, taken to approximate the age of mineral formation. 2Qopfj_238u dates on zircons from the Nelungaloo Volcanics, which underlie the mineralized sequence, give an age of 438 ± 7 Ma (2 s.d., with the uncertainty expressed as standard deviation of the mean derived from 20 age determinations). Theepithermal goldGidgjnbung deposit near Temora, NSW, is hosted in andesitic composition volcaniclastic rocks. ^Ar-^Ar total fusion ages on hydrothermal alunite cluster in two groups, with ages ranging from 401 ± 1.3 to 406 ± 1.3 Ma (1 s.d.), and 412 ±1.3 to 417 ± 1.3 Ma (1 s.d.). Both groups of dates coincide with episodes of regional deformation (cf. Basden, 1989) which may have resetthe alunite, and 417 ± 1.3 Ma should therefore be taken as a minimum age for mineralization. 206pjj_238u dates on zircons from an andesite flow within the mine sequence yield an age of 43S ± 5 Ma (2 s.d., standard deviation of the mean derived from 26 age determinations). Results of the dating so far completed indicate an association between gold mineralization and Late Ordovician to Early Silurian magmatism (cf. Harland et al., 1989). The Nelungaloo Volcanics have been shown by U-Pb dating of magmatic zircons to be broadly contemporaneous with magmatism in the Temora area. •206pb_238u ages f^n, the ia(ter region and from Parkes are in broad agreement with K-Ar dates obtained on magmatic amphiboles from volcanic and intrusive rocks elsewhere in the Molong Volcanic Arc (D. Wybom, pers. comm.), suggesting that magmatism in this region may have been more temporally restricted than previously thought (e.g. Webby.1974). Basden, H., 1989. Geology of the Tumut 1:100,000 sheet 8527. Geol. Surv. NSW. Harland, W. B., Armstrong, R.L., Cox, A.V., Craig, L.E., Smith, A.G., and Smith, D.G., 1989. A Geologic Time Scale 1989. Cambridge University Press. Webby, B.D., 1974. The Ordovician system in south-eastern Australia. In The Ordovician System (ed. M.G. Bassett), p. 417-46. Univ. Wales Press. 33 K-Ar and 4<>Ar/39Ar Age Relationships at the Porgera Gold Deposit, Papua New Guinea. Jeremy P. Richards and Ian McDougall Research School of Earth Sciences, The Australian. National University, GPO Box 4, Canberra, ACT 2601. The world-class gold deposit at Porgera in the Highlands of Papua New Guinea is spatially associated with the locus of intrusion of a suite of small, shallow-level stocks and dykes, which comprise the Porgera Intrusive Complex. These igneous rocks have been identified by Richards (in press) as porphyritic alkali basalts, hawaiites and mugearites, and are related by crystal fractionation which occurred at depth in an unexposed parental intrusion. Hornblende, clinopyroxene and plagioclase form prominent phenocrysts in the more evolved hawaiites and mugearites, but olivine and chrome-rich diopside phenocrysts characterize the alkali basalts. Many of the intrusions display tniarolitic cavities or vesicles, and pegmatitic lenses occur locally; these observations, combined with the abundance of H2O-, F- and Cl-bearing primary mineral phases such as hornblende, biotite and fluor-apatite, suggest that the magma was volatile-rich, and that a volatile phase was exsolved during emplacement and solidification of the shallow-level stocks. Gold mineralization is associated with potassic alteration of the intrusives and the host sediments, which consist of deformed but unmetamorphosed Cretaceous siltstones and mudstones of the Chim Fm. Gold occurs in two main settings: (1) as relatively low-grade disseminations of auriferous pyrite in large volumes of sericitized and carbonated rock, and (2) as high-grade free gold intergrown with Au-Ag-tellurides and roscoelite (vanadium-rich muscovite) in banded vuggy quartz veins and hydraulic breccia cements, associated with the cross-cutting Roamane Fault. Attempts were made to clarify the age relationship between magmatism and mineralization by applying the K-Ar dating method to igneous and hydrothermal minerals. The results of this study showed highly discordant apparent ages, ranging from 6 to 14 Ma for primary hornblendes and impure mineral separates of sericite and roscoelite, but four biotite samples yielded concordant ages at 6.02 ± 0.29 Ma (2a). The discordance was such that different hornblende samples from the same intrusion, and sericites from the same orebody, varied by as much as 5 Ma in apparent age. Because of the relatively small size of the igneous complex, slow cooling cannot be considered as an explanation for these results. Instead, either the occurrence of a metamorphic (hydrothermal?) overprint at ~6 Ma on older intrusive rocks, or the presence of excess 40Ar, might explain the discordance. In order to test these two alternatives, six aliquots of the previously analyzed hornblende mineral separates were sent to the Lucas Heights reactor for neutron irradiation, and were then analyzed by the 40Ar/39Ar step-heating method. 40Ar/39Ar age spectra obtained from the irradiated samples are saddle-shaped, and descend from high apparent ages (in excess of 100 Ma in several cases) in low temperature gas releases, to minima at between 6 and 8 Ma, then rise again in age in the high temperature gas releases. Three of the samples contain plateau-like segments in the central, saddle-portions of their releases, and the best defined plateau, from sample 88-8, consisting of six steps comprising 48% of the 39Ar release, yields an apparent age of 5.96 ± 0.25 Ma (2a). This age is identical to the conventional K-Ar age of biotites, but is almost 1 Ma younger than JJC total fusion age of the sample. Saddle-shaped age spectra are generally considered to be diagnostic of the presence of excess 40Ar, and are quite different in appearance to the argon-loss profiles expected from metamorphically overprinted samples. The minima in the saddles represent maximum estimates of the age of the sample, but may closely approach or reach tne true age in samples containing only small amounts of excess 4<^Ar. Thus we conclude that the age of intrusion of the Porgera Igneous Complex is 6.0 ± 0.3 Ma (2a), and that older apparent ages are the result of entrapment of variable amounts of excess 40Ar. The source of this argon is unknown, but evidence for a high volatile content of the magma suggests that it may be of direct mantle origin, and that entrapment in hornblende phenocrysts may have occurred during devolatilization of the magma after emplacement at shallow crustal levels. In the light of these results, it was decided to investigate further the age of the alteration minerals associated with gold mineralization. Because of the small grain-size of the secondary micas (generally less than a few microns in length), the ^Ar/^Ar method is inappropriate due to loss of 39Ar by recoil during irradiation. The samples previously analyzed were 75-53 |i.m composite grains of quartz with sericite (identified as disordered 1M illite by XRD) or roscoelite, and several separates contained over 75% quartz. K-Ar analysis of a separate of hydrothermal quartz from one of these samples yielded an apparent age of 33 Ma, and it was therefore concluded that excess 40Ar was present in this mineral, perhaps located in fluid inclusions. In order to exclude the quartz contaminant, mineral separates of clay-sized illite and roscoelite were prepared, and these were analyzed by the conventional K-Ar method. The results from five different samples, including two which had previously yielded apparent ages of 11 and 12 Ma from impure separates, cluster between 5 and 6 Ma. This close grouping of apparent ages, and the fact that two completely different minerals (illite and roscoelite) are concordant, suggests that the results are geologically meaningful, and that mineralization at Porgera occurred at between 5 and 6 Ma. However, we caution against over-interpretation of the data due to the possibilities of 40Ar*-loss from the fine-grained clay particles and the persistent presence of minor amounts of excess 40Ar. We therefore conclude simply that gold mineralization occurred within 1 Ma of the time of emplacement of the Porgera Igneous Complex. The implications of this result for metallogenesis will be discussed elsewhere. if 'OZCHRON' - A NATIONAL DATABASE OF AUSTRALIAN fiEOCHRONOLOGY R.I. Ryburn, R.W. Page, J.R. Richards, V. Laynne and E.P. Shelley Bureau of Mineral Resources, Canberra, A.C.T. So you have just published another frontier paper occupying pages 3047-3059, volume 298 of the Journal of Geewiz. Wonders! Some people will actually read your paper from start to finish, and will become quite familiar with its data and interpretations. However, such users probably will be few - in fact, only a small number of readers will ever get past the abstract. Even fewer will be aware of your results in 1995, and not too many at all by the year 2015 ....et . Can those isotopic results that you sweated over at great personal and capital cost remain effective and accessible, or are they doomed to stratification under the scientific overburden as the years roll on? The establishment of national databases is seen as a bridge between that obscure scientific article, whether published some year or two ago, or some decade or two ago, and today's geological user who might like access to and use of your otherwise cobwebbed results. Furthermore, coordinated geoscience database activity can help avoid rediscoveries of the wheel and unnecessary duplication of scarce, expensive scientific effort Perhaps more importantly, the increased use of geographic information systems in modem research and exploration projects, demands access to all types of geoscienufic data that can be manipulated on computer facilities. One of BMR's database projects is concerned with the development of a useful system for storing and retrieving information from the ever-burgeoning growth market related to geochronology of Australian rocks. Up until now, the only compilations of such data have been in BMR publications, the most recent covering the period 1966 to 1970. There is a need for ongoing, up-to-date national syntheses, as there have been widespread geological events of great resource significance that have affected either sedimentary basins or basement provinces. The present aim is to establish and maintain such a database, so that expensively acquired, often complex geochronological data are readily available to government, industry and other geoscience organisations, in order to facilitate various resource assessment, research, or exploration activities. The OZCHRON database has been developed on BMR's Data General MV/20000 computer using the Oracle relational database management system. OZCHRON uses many tables in common with other BMR databases. For example, information on rock sample is entered into the same input form and table as the PETCHEM geochemical database, and some authority tables such as rock types, geological regions, and 1:100 000 map sheets are common to several databases on the system. Each geochronological method covered by the OZCHRON database employs one input form and one or two tables. The software allows data to be organised as tables, the way in which geochronological data are usually published. Where a number of analytical results are required to arrive at one age determination, as in Rb-Sr whole-rock isochron work, there are two tables, one for analytical results and one for the pooled age result and interpretation. The analytical results are generally joined to a pooled-result number and to the sample information by a combined index on originator plus sample number - the originator being the person/organisation responsible for collecting the sample and/or publishing the results. This dual indexing system allows published sample numbers to be preserved and obviates the need for an additional numbering system. The only requirement is that the sample numbers from any one originator must be unique within his or her numbering system. OZCHRON presents users with a number of potential benefits. Its relational structure permits virtually any query and a large variety of interrogation and reporting routines. With appropriate software, U-Pb concordia plots and other geochronological plots wiil be able to be generated directly from the database. Using BMR's in-house MAPDAT software, the location of dated samples with geographic coordinates can be plotted on any map in Australia on any projection and scale. The data can easily be moved to other systems should this become necessary. The current first stage of OZCHRON work is an attempt to capture recently published and up-to-date geochronological data on Australia's early to middle Proterozoic terranes. Overall it is estimated that some 2000 isotopic analyses (about 200 age determinations) are produced each year by the 9 Australian laboratories. The backlog, including existing compilations of isotopic data, amounts to more than 20,000 analyses. No doubt all of us have appropriate standards that we apply to the collection, compilation and presentation of our isotopic data, and these are normally satisfactory to journal referees and editors. Few of us, however, either publish or privately record sufficiently accurate locality information about the sampled geological locality. However, a national geochronological database ihat can be usefully integrated with other digital geoscientific data, relies on proper locality documentation (eg. 1: 100 000 grid reference), and you are urged to consider this longer term responsibility towards those who would be pleased to use your otherwise obscure contribution sometime in the 21st century. CARBONIFEROUS IGNEOUS ACTIVITY IN THE LACHLAN AND NEW ENGLAND FOLD BELTS: PARTS OF THE SAME MAGMATIC ARC ? Stirling E. Shaw and Richard H. Flood School of Earth Sciences, Macquarie University, N.S.W. 2109 In New South Wales, Carboniferous calc-alkaline rocks occur as a broad magmatic arc some 400 km long by 200 km wide consisting of an eastern volcanic zone along the western margin of the New England Fold Belt and a western plutonic zone within the north-eastern part of the Lachlan Fold Belt (Fig. 1). The eastern volcanic rbcks form part of a volcanogenxc sedimentary sequence and consist of middle-late Visean (estimated 345 Ma) basaltic-andesite and dacite (flows? sills?), dacite tuffs (K/Ar 319, 312 Ma Roberts 1984) and extensive ignimbrite flows (Rb/Sr biotxte 305 Ma, Shaw pers. com.; K/Ar 302, 293 Ma, McPhie 1984). The western plutonic rocks vary from gabbro/diorite (340 Ma) through granodiorite to late-stage leucocratic-adamellite and spherulitic rhyolite dykes (312 Ma). A histogram of 34 biotite Rb/Sr ages is shown in Fig. 2. NEW ENGLAND FOLD BELT 3± 8 N = 6 • NUMBER 4- LACHLAN FOLD 2 BELT 0 305310315320325330335340345 Age M« Fig. 1 Structural elements of Fig. 2 Histogram of Carboniferous NE N.S.W. showing Carboniferous granitoid ages, Lachlan Fold Belt. granitoids (black areas) and Rb/Sr biotite-whole rock. volcanic rocks (inverted V's). if Comparisons of the age and geochemistry, including rare-earth elements (Wall, 1984; T. H. Green, pers. comm.) (Fig. 3), suggest that although the volcanic and plutonic rocks occur within separate fold belts, there is a general similarity of both age and composition. Initial 87sr/86sr ratios of the plutonic rocks vary from 0.7042 to 0.7067 but reduce to 0.7042 and 0.7051 if only the hornblende-bearing variants are chosen. Eleven plagioclase separates from vi-trophyric lavas and devitrified ignimbrites give initial 87sr/86sr ratios of 0.7039 to 0.7052. Differences in mineralogy, geochemistry and initial 87sr/86sr ratios of some of the more evolved Carboniferous plutonic rocks such as a muscovite-bearing pluton at Burraga indicate possible minor amounts of crustal involvement and/or fractionation. In the Lachlan Fold Belt, the Carboniferous magmatic arc cuts across the regional structural north-south trend and the elongation of Silurian and Devonian batholiths. If the Wologorong Batholith is dominantly S-type, as considered by Shaw and Flood (1982), then the Siluro-Devonian I-S line would lie to the east of the Batholith and would be intersected by the Carboniferous magmatic arc. Alternatively, Chappell et al. (1988) consider the Wologorong Batholith to be I-type and position the I-S line further to the west. Initial 87sr/86sr ratios of the Devonian I-type Marulan Batholith and S-type Wologorong Batholith that occur within the Carboniferous magmatic arc are 0.7065 and 0.7110 respectively (Shaw, pers. comm.). The values are significantly higher than the 0.7042 to 0.7051 of the hornblende-bearing Carboniferous granitoids, which indicates that the nature of the source rocks and/or the depth to magma generation changed between the Siluro-Devonian and the Carboniferous. (a) 1000 LACfLAN FOLD BELT CARBONIFEROUS GRANITOIDS \QQ Fig. 3 Rare-earth element CHONDNORM plots of Carboniferous igneous rocks: 10 (a) granitoids from the Lachlan Fold Belt (data from Wall, 1984) CtPrW Sm Eu Od Tb Dj Ho E> tt Lu (b) volcanic rocks from REE the New England Fold Belt (data from T. H. (b) Green, pers. comm.) NEW ENGLAND FOLD BELT CARBONIFEROUS VOLCANICS 100 CHONDNORH 10 CtPrNd &• Eu W Tb Of Ho & Yb REE The alignment of Carboniferous volcanic and plutonic activity to the western margin of the New England Fold Belt, together with the temporal and compositional equivalence of these rock types, are factors which indicate that they were probably parts of the same magmatic belt. Whether collision and accretion of the New England Fold Belt to the Australian continent was involved, or whether the arc was a consequence of tectonic processes on already juxtaposed fold belts is unknown but it is considered that in relation to each other both fold belts were either in their present configuration at the start of igneous activity some 345 Ma ago or else later movement has juxtaposed two segments of the same arc. It is uncertain how far the Lachlan Fold Belt extends eastwards beneath the Sydney Basin, although the presence of quartz-rich metasedimentary clasts from a Tertiary basaltic diatreme at Mogo Hill that are considered to be of Lachlan Fold Belt association (Emerson and Wass, 1980) and the presence of an inlier of Carboniferous adamellite (Rb/Sr biotite 344 Ma) in the Sydney Basin at Mt Bright and Mt View just west of Maitland suggest that a significant proportion of the basement underlying the Sydney Basin is part of the Lachlan Fold Belt. References Chappell, B. W., White, A. J. R. and Hine, R. 1988. Granite provinces and basement terranes in the Lachlan Fold Belt, southeastern Australia. Australian Journal of Earth Sciences, 3!5_, 505-521. Emerson, D. W. and Wass, S. Y. 1980. Diatreme characteristics - evidence from the Mogo Hill Intrusion, Sydney Basin. Australian Society of Exploration Geophysics Bulletin, 11^, 1-20. McPhie, J. 1984. Permo-Carboniferous silicic volcanism and palaeogeography on the western edge of the New England Orogen, north-eastern New South Wales. Australian Journal of Earth Sciences, 2L» 133-146. Roberts, J. 1984. The Carboniferous of the World II: Australia, Indian sub-continent, South Africa, South America and North Africa, ed. C. M. Diaz. I.U.G.S. Publication No. 20, pp. 9-145. Instituto Geologico y Minero de Espana. Shaw, S. E., Flood, R. H. and Riley, G. H. 1982. The Wologoron Batholith, New South Wales, and the extension of the I-S line of the Siluro-Devonian granitoids. Journal of the Geological Society of Australia, 73_, 41-48. Wall, T. 1984. Genetic significance of rare earth elements in granitoids of the north east Lachlan and New England Fold Belts, New South Wales. B.A. (Honours) Thesis (unpubl.), Macquarie University, N.S.W. Preview of the CSIRO-AMS Facility S.H. Sie Heavy ion Analytical Facility CSIRO Division of Exploration Geoscience P.O. Box 136, N. Ryde 2113 Abstract An AMS (Accelerator Mass Spectrometry) system is being constructed at the CSIRO HIAF laboratory at North Ryde. The system is designed to enable cosmogenic isotope based chronology, and eventually will be developed to enable ultra trace measurements in mineralogical samples. The HIAF laboratory was commissioned in 1983 with the mission of developing applications of accelerator based analytical methods in minerals research in aid of the minerals industry. An essential requirement in order to achieve this is the development of the proton microprobe, completed in 1985, to enable applications of ion beam analysis (PIXE,RBS,NRA) on mineralogical samples, the fundamental constituent of geological samples. AMS, a relatively new addition to the accelerator based methods, derives its ultra-sensitivity (1 in 10**+12 to 10**+15) from the reduction if not destruction of possible interfering molecular ions and the application of particle identification techniques which are feasible only at MeV energies. In this method, secondary ions sputtered from the specimen in the ion source are accelerated to the required energies. The ultra sensitivity enables direct counting of rare isotopes, such as cosmogenic radioisotopes, and can be applied to measurements of traces of elements through one of its isotopes. The research interest of the Division lies mainly in the applications of lOBe and 36C1 dating to problems relevant to exploration and mining. The scope of published work on applications of lOBe, ranging from soil transport and erosion studies, rock exposure ages on to petroleum dating offers exciting prospects in research of ore and petroleum genesis as aids in exploration. Similarly, the information from 36C1 as tracers and chronometer of groundwater can be an important supplement in hydrogeochemical methods exploration, and water management in arid areas such as typically found around mines. 14C dating will also be made available for other research. The AMS capability to detect very low levels (ppb) of trace elements has thus far been demonstrated with submillimeter sputtering beam, which is not adequate for most measurements requiring monomineralic analysis. A microprobing sputter source must be developed for successful applications of AMS as trace analyzer. The HIAF-AMS project will proceed in two stages, with the first resulting in standard AMS capability, viz. lOBe, 14C and 36C1 dating on "bulk" samples. The second stage involves the development of the microprobing Cs sputter source. The first stage of the CSIRO-AMS facility is virtually completed and is undergoing tests. CHEMICAL AND ISOTOPIC SYSTEMATICS OF THE BLUE TIER BATHOLITH, N.E. TASMANIA: ITS BEARING ON THE ORIGIN OF THE TIN-BEARING ALKALI FELDSPAR GRANITES* Shen-su Sun, Bureau of Mineral Resources, Geology and Geophysics, Canberra Two competing hypotheses have been proposed for the origin of the tin-bearing alkali feldspar granites (AFG) i.e., (1), the magmatic-metasomatic hypothesis: the AFG is derived by fractional crystallization of its neighbouring more mafic granite followed by fluid exsolution and sub-solidus alteration; (2), the independent magmas hypothesis: the AFG are generated independently and are genetically unrelated to their neighbouring more mafic granite. Each hypothesis might be applicable to different specific cases. In the case of AFG of the Blue Tier Batholith both hypotheses have been promoted. Higgins et al (Lithos, 1985) and Higgins (GCA, in press) favour the magmatic- metasomatic connection, whereas Mackenzie et al (GCA, 1988, in press) support an independent origin of the AFG. There are disagreements regarding field relationship, emplacement ages of the Poimena biotite granite (380 Ma) and Lottah and Mt Paris AFG (370 Ma?) and interpretation of the chemical varition trends. Even differences between "altered" felsic Poimena granite and "metasomatized" Lottah AFG are not simply semantic. An integrated chemical-isotopic approach reveals some clear systematics which, in my opinion, favours an independent origin of the Lottah and Mt Paris AFG. On Y versus Ga/Al and Nd plots, Poimena granite and Lottah and Mt Paris AFG's have distinct fields and discontinuous, step-wise variation trends. As a result of magmatic differentiation Y content in the Poimena granite decreases from -45 to -18 ppm and lOOOx Ga/Al increases from -2.2 to -2.7. Phyric Lottah AFG have much higher Y (-60 ppm) and lOOOx Ga/Al (-3.6). Through magmatic-metasomatic processes Y content in AFG progressively decreases to <2 ppm and lOOOx Ga/Al increases to -6. Poimena granite samples have initial ENd (at 370 Ma) -5.0 to -6.5. Samples with lower values (-6.5) are located near AFG. In contrast, phyric Lottah and Mt Paris AFG with high Y contents (50-60 ppm) have distinctly higher ENd -2.4. Aphyric AFG and greisen have lower £Nd values, -3.5 to -4.0 except for one sample from Mt Paris (-6.1). A systematic decrease of E-Nd from phyric AFG to aphyric AFG and greisen is consistent with the idea that during metasomatic alteration and mineralization fluids derived from the country rocks (Poimena granite and Mathinna beds withfiNd -11) have been mixed with the magmatic fluids. Y/Ho ratios of the Poimena granite samples are close to chondritic (28). In contrast Lottah and Mt Paris AFG have higher Y/Ho ratios up to 100 suggesting different complexing behavior between these two elements during magmatic differentiation/ metasomatic modification involving a fluid phase. Different complexing capacity also affects REE resulting in clear kinks in the chondrite normalized REE patterns of the aphyric AFG. The magmatic-metasomatic hypothesis favouring a genetic connection between the Lottah and Mt Paris AFG and Poimena granite will be defended by Higgins in the February 1990 AGC in Hobbart. *This abstract is based on a paper by Sun, Higgins and McCulloch (in prep.) on the same subject. THE MOUNT READ VOLCANICS, TASMANIA: Sr and Nd ISOTOPE GEOCHEMISTRY David J Whitford1 Anthony J Crawford2 Michael J Korsch1 Stephen J Craven1 (1) CSIRO Division of Exploration Geoscience North Ryde NSW (2) Department of Geology University of Tasmania The Cambrian Mount Read Volcanics of western Tasmania are of considerable interest because of their association with significant base and precious metal mineralization. This belt forms the western margin of the Dundas Trough, and borders the Precambrian Tyennan Block. The Volcan- ics comprise a basalt-andesite-dacite-rhyolite suite with abundant interbedded pyroclastic, epi- clastic and shale horizons. Rapid facies changes, complex internal stratigraphy and structure, paucity of fossils, variable alteration, low-grade regional metamorphism and poor exposure have frustrated a detailed understanding of their regional geological and tectonic significance. The Mount Read Volcanics have been subdivided stratigraphically into the Central Volcanic Complex, and the younger Western Sequence and Tyndall Group. The Central Volcanic Complex is dominated by intermediate tofelsic rocks with abundant ignimbrites. The Western Sequence contains abundant basalts but also includes a range of intermediate to rhyolitic lavas and large quartz-feldspar "porphyrite" bodies. The Tyndall Group is dominated by felsic tuffs and lavas. Three distinct volcanic suites can be recognized on the basis of major and trace element geochem- istry: two high-K calc alkaline suites and an unusual shoshonitic association. The volcanic rocks appear to have been erupted in a series of extensional basins following an arc-continent collision. The Murchison and Darwin Granites are chemically similar to the felsic volcanic rocks and appear to intrude the Central Volcanic Complex and the Tyndall Group respectively although de- tailed stratigraphic relationships remain unclear. The Noddy Creek Volcanics. south of Mac- quarie Harbour, are thought to correlate with the Mount Read Volcanics. Tholeiitic basalts and dolerites underlie the Western Volcanic Sequence at Miners Ridge. Thole- iitic rocks comprising the Henty Dyke Swrrm intrude the Central Volcanic Complex. Basic lavas with tholeiitic affinities also occur in the Hcnty Fault Wedge, but their relationships with the other tholeiitic suites remains unknown. Basaltic and andesitic rocks from the Central and Western groups and a basalt from the Noddy Creek Volcanics have Epsilonj^ (T = 500Ma) values generally within the range + 1 to -2. Relic calcic clinopyroxene phenocrysts have similar Epsilon^j (T = 500Ma) values of + 1- + 2. The high- K calc alkaline and shoshonitic suites cannot be distinguished isotopically. The more felsic rocks have lower Epsilon^j (T = 500Ma) values to -8.5. Samples with the lowest epstlon values are from the quartz-feldspar "porphyritc" bodies of the Western Volcanic Sequence. There is a suggestion that the western and Central Volcanic belts define distinct trends on plots of Epsilon^j versus selected major and irace element abundances. The Tyndall Group and the related graniles are isolopically similar to the more felsic volcanic rocks, particularly those from the Central Volcanic Complex. The felsic rocks have old model ages, relative to a depleted model mantle, of up to 1.8Ga. The low Epsilon^j values together with the inverse correlations between Epsilon^^ and major and trace element abundances reflect the involvement of old continental crust in their formation. The tholeiitic rocks are distinctive with Epsilonj^j (T = 500Ma) values ranging from +5.1 to +9.2. The isotopic composition of Sr is variable with age-corrected Sr/ Sr ratios ranging from < 0.7 to >0.72. Both the high and low values probably reflect the Devonian regional metamorphism insofar as it has obviously affected both 87Sr/^6Sr ratios and Rb and Sr abundances. There is a clustering of ratios from 0.7065-0.7095 that may reflect primary values. Isotopic studies of relic calcic clinopyroxene phenocrysts derived from some of the basalts and andesites from the Central and Western Sequences confirm the relatively high Sr/ Sr primary ratios. The unusual isotopic character of the Mount Read Volcanics imposes some constraints on petro- genetic and tectonic models. The felsic rocks are clearly not simple differentiation products of the more mafic rock's and old continental rocks were probably involved in their formation. Some of the rhyolites may represent partial melts of such crust perhaps in response to coeval mafic volcan- ism. Many of the intermediate andesites and dacites appear to reflect mixing of mantle- and crust- derived components. Whether the basalts and more mafic andesites directly reflect mantle compositions or whether they have been contaminated is not completely clear. The occurrence of subduction in the Cambrian, inferred from the trace element geochemistry of samples from the Mount Read Volcanics, together with other geological evidence, offers a mechanism to enrich the mantle source. In this context it may be significant that the Jurassic dolerites, widespread in •- Tasmania and Anarctica, have similar isotopic characteristics to those observed in the Mount Read Volcanics.