Reworking the Gawler Craton: Metamorphic and Geochronologic Constraints on Palaeoproterozoic Reactivation of the Southern Gawler Craton, Australia

Reworking the Gawler Craton: Metamorphic and geochronologic constraints on Palaeoproterozoic reactivation of the southern Gawler Craton, Australia

Rian A. Dutch, B.Sc (Hons)

Geology and Geophysics School of Earth and Environmental Sciences The University of Adelaide

This thesis is submitted in fulﬁlment of the requirements for the degree of Doctor of Philosophy in the Faculty of Science, University of Adelaide

January 2009 Chapter 2 In-situ EPMA monazite chemical dating at the University of Adelaide: Setup, procedures, comparisons and application to determining the timing of high-grade deformation and metamorphism in the southern Gawler Craton.

Abstract

Putting absolute time into structural and metamorphic analysis is a vital tool for unravelling the development of orogenic systems. Electron Probe Micro-Analysis (EPMA) chemical dating of monazite provides a useful method of obtaining good precision age data from monazite bearing mineral assemblages. presented here is a review of EPMA monazite dating theory together with a detailed description of the EPMA monazite setup and methods developed at the University of Adelaide. This includes the initial setup and optimisation of the technique on the Cameca SX51 electron microprobe, sample preparation and data reduction and analysis techniques. EPMA measurements carried out on samples of known age, from Palaeoproterozoic to Ordovician, produce ages which are within error of the isotopically determined ages, indicating the validity of the developed setup. The technique is then applied to a sample of unknown age from the southern Gawler Craton to determine the timing of high-grade metamorphism and deformation in the Fishery Bay region. Three samples from the late Archaean to Palaeoproterozoic Sleaford Complex produced EPMA monazite ages of 1707 ± 20 Ma, 1690 ± 8 Ma and 1708 ± 12 Ma indicating that the high-grade metamorphism and deformation in this region was a result of reworking during the 1725–1690 Ma Kimban Orogeny, and not the 2450–2420 Ma Sleafordian Orogeny.

2.1 Introduction 2007; Kohn, 2008). The process starts with developing a constrained tectonometamorphic Reconstructing pressure-temperature-time evolution of an area using detailed structural (P-T-t) paths is a vital tool for unravelling analysis coupled with a complete metamorphic ancient and modern orogenic systems and analysis, including the determination of P-T attempting to deduce a tectonic model for constraints thorough thermobarometry and the their development (e.g. Clarke and Powell, use of P-T pseudosections tailored to speciﬁc 1991; Harley and Fitzsimons, 1991; Spear, bulk compositions (e.g. Harley and Fitzsimons, 1993; Thoni and Miller, 1996; Mawby et al., 1991; Stuwe and Ehlers, 1997; Scrimgeour and 1999; Goscombe et al., 2003; Kelsey et al., Close, 1999; Kelsey et al., 2003; White et al., 2003; Foster et al., 2004; Goscombe et al., 2007). This tectonometamorphic framework 2005; Duchene et al., 2006; Halpin et al., then needs to be temporally constrained. The

17 Chapter 2 EPMA Monazite Dating lack of applied geochronology to proposed P-T isotopic dating. Both allow for high precision paths can lead to erroneous conclusions with isotopic ages to be determined (typically with the inferred tectonometamorphic history being errors between ± 3 – 10 Ma) with relatively an amalgam of different events, including using good spatial resolution (spot sizes between ~ assemblages which have formed during re- 5 – 60 μm). Generally they are performed on working at a later time being used to constrain mineral separates but both can in principal be the retrograde metamorphic evolution of an done in-situ allowing for spatial information to orogen (e.g. Hand et al., 1992; Holdsworth et be retained (e.g. Carson et al., 2004; Forbes et al., 2001; Dutch et al., 2005). al., 2007; Wade et al., 2008). In recognition of the importance of Electron Probe Micro-Analysis (EPMA) constraining time in geological processes, a chemical dating of monazite provides a useful number of different minerals and chemical additional or alternative method of obtaining systems have been developed to try to constrain good precision age data from monazite bearing the age of metamorphism and deformation. For mineral assemblages (e.g. Suzuki and Adachi, example, dating of accessory minerals such as 1997; Montel et al., 2000; Asami et al., 2002; zircon by the ‘kober’ or dissolution methods Pyle and Spear, 2003; Mezeme et al., 2006; using Thermal Ionisation Mass Spectrometry Santosh et al., 2008). Compared with the above (TIMS) allows for extremely precise isotopic techniques it is relatively cheap, time efﬁcient, age determinations (e.g. Cocherie et al., 1992; is readily preformed in-situ, has excellent Dougherty-Page and Bartlett, 1999; Crowley et spatial resolution (< 10μm), good age precision al., 2007; Schaltegger et al., 2008). However, (c. ± 7 – 30 Ma), and can provide a full range these methods require considerable time, can be of chemical and element zonation information expensive, and will commonly be an amalgam (Pyle et al., 2005; Williams et al., 2006). The of any age zonation in the minerals (i.e. has main disadvantage is that it is not an isotopic poor spatial resolution) and gives no textural method, so discordant data can be difﬁcult to or chemical information. Dating of major discern. phases involved in metamorphic reactions This chapter provides a review of EPMA in this way (e.g. garnet Sm-Nd) may allow a monazite dating theory together with a direct constraint on the age of metamorphism detailed description of the analytical method but suffers from many of the same issues (e.g. and procedures for EPMA monazite dating Chapter 6: Mawby et al., 1999; Thoni, 2003; developed at the University of Adelaide. In Jung et al., 2007). Accessory phase dating order to verify the accuracy of the method for by secondary ion mass spectrometry (SIMS, the available analytical facilities, a comparative e.g. Foster et al., 2000; Dutch et al., 2008) or study using previously isotopically dated Laser Ablation ICP-MS (e.g. Willigers et al., monazites (methods including ID-TIMS and 2002; Horstwood et al., 2003; Payne et al., SHRIMP) has been carried out. The results 2006; Reid et al., 2006; Payne et al., 2008) are indicate a good correspondance between the currently the most powerful tools available for EPMA ages obtained from the developed

18 Chapter 2 EPMA Monazite Dating

NOTE: This figure is included on page 19 of the print copy of the thesis held in the University of Adelaide Library.

Figure 2.1. Location and regional geology and TMI image of the southern Gawler Craton (after Schwarz, 2003). The location of Fishery Bay, the southern extension of the Kalinjala Shear Zone, is marked by the circle. setup and the isotopically derived ages. The method has then been applied to monazites of Since the EPMA monazite technique was unknown age to constrain the timing of peak first implemented by Suzuki and Adachi (1991) metamorphism in the Kalinjala Shear Zone and Suzuki et al. (1994) using the Chemical in the Fishery Bay region of the southern Isochron Method (CHIME) a number of Gawler Craton (Fig. 2.1). Additionally the different methodologies for data acquisition and method has been used to constrain the timing reduction have been developed. For a thorough of metamorphism in the Shoal Point region review of EPMA monazite geochronology (Chapter 4) and the Mine Creek region of principles and techniques see papers by Montel the Kalinjala Shear Zone (Chapter 5) in the et al. (1996), Williams et al. (1999), Pyle et al. southern Gawler Craton. (2005), Jercinovic and Williams (2005) and Williams et al. (2006). 2.2 Review of EPMA monazite dating The theoretical basis for monazite theory geochronology consists of measuring the Th, U

19 Chapter 2 EPMA Monazite Dating

and Pb concentrations in a monazite crystal and its interpretation has been established (Steiger calculating the age (τ) based on the decay rates and Wasserburg, 1966). Experimental work by of U and Th to Pb via the age equation (after Barth et al. (1994a; 1994b) has shown that for Montel et al., 1996): allanite, the Th/Pb system is more resistant to diffusion than the U/Pb system, and therefore where Pb, U and Th are in ppm and λ232, λ238, should behave in a concordant manner.

⎛ Th ⎞ ()232 ⎛ U ⎞ Diffusion of Pb in monazite is also negligible Pb = ⎜ ⎟[]e TL 1 208 +− ⎜ ⎟ .0 9928 (1) ⎝ 232 ⎠ ⎝ 238.04 ⎠ (Cherniak, 2004). Cherniak et al (2004) has

()238 ⎛ U ⎞ ()235 []e TL 2061 +−× ⎜ ⎟ .0 0072 []e TL −× 2071 demonstrated that for a cooling rate of 10 ⎝ ⎠ λ235 are the radioactive decay.238 04 constants of 232Th, °CMa-1, a 10 μm monazite grain has an effective 238U and 235U respectively. Individual ages are closure temperature of c. 900 °C, meaning that determined iteratively by entering age estimates for most geological processes Pb diffusion in into Eq. (1) with the known concentrations of U monazite is negligible. Monazite has also been and Th until the calculated Pb value matches the shown to be highly resistant to metamictisation measured Pb concentration. Because monazite (Karioris et al., 1991; Meldrum et al., 1997). can incorporate signiﬁcant amounts of Th and These arguments indicate that monazite should U, EPMA measurable amounts of radiogenic Pb remain a closed system in most geological begins to accumulate in as little as 100 Myr. settings and provide concordant, geologically The validity of the above method is meaningful ages. dependant on two assumptions: (1) that non- However, monazite often gives slightly radiogenic Pb content is negligible; and (2) different ages than zircon from within the same there has been no subsequent modiﬁcation units (e.g. Foster et al., 2000; Pyle and Spear, of the U/Th/Pb ratios except by radioactive 2003; Carson et al., 2004; Fitzsimons et al., decay. Parrish (1990) indicated that monazite 2005; Kelsey et al., 2008). This is primarily due incorporates very little non-radiogenic Pb in its to its comparatively high chemical reactivity structure, and therefore common Pb contents are (e.g. Bea and Montero, 1999; Spear and Pyle, typically < 1 ppm and are therefore negligible 2002), particularly in melt bearing systems. The for most >200 Myr old monazites. stability of monazite is primarily a function of The second assumption is that the monazite temperature and melt composition (Rapp et al., grain remains a closed system. Of the two 1987), with monazite solubility and stability decay systems active in monazite the U/Pb being a function of the destabilising effects system is best documented. Previous studies of P and REE solubility in felsic melts (Rapp (e.g. Parrish, 1990; Cherniak et al., 2004; and Watson, 1986; Rapp et al., 1987; Wolf Cocherie et al., 2005) have shown that this and London, 1995; Kelsey et al., 2008). This system typically behaves in a concordant suggests that Th-U-Pb monazite systematics, manner. The Th/Pb system, the more important and therefore the age, is less likely to be for monazite chemical dating, is poorly known controlled by diffusion than it is by the chemical despite the fact that the theoretical basis for reactivity of the monazite grains (e.g. Seydoux-

20 Chapter 2 EPMA Monazite Dating

Guillaume et al., 2002). A

2.3 Analytical method

Generally every electron microprobe is unique, with each having different variations and combinations of electron columns, spectrometers, X-ray detectors and counters. All these factors can affect the form of wavelength- dispersive spectra (WDS). Therefore monazite dating protocols must be designed for a specific B electron microprobe. The following analytical setup and procedure is for a Cameca SX51 electron microprobe (Fig. 2.2a) running SAMx software at Adelaide Microscopy, which is the major analytical facility at the University of Adelaide. 50um The SX51 uses a tungsten filament and is capable of producing beam accelerating voltages Figure 2.2. A) Cameca SX51 electron micro probe of 1–30 kV, beam currents of ~50 pA to 500 at the University of Adelaide. B) Example of sample damage caused by high beam currents and nA, with beam diameters adjustable between 1 long count times. These are caused by count times and 50 μm. The central electron column has an in excess of those used in the procedure presented X-ray takeoff angle of 40° and is surrounded by here (Image courtesy of D. Steel). four WDS spectrometers and a Gresham 10mm are maintained at approximately 1 atm while energy-dispersive (EDS) detector. detector 3 is maintained at approximately 3 All WDS spectrometers on the SX51 atm. incorporate a 160 mm radius Rowland Prior to calibration it is necessary to circle. Each spectrometer is equipped with optimise the X-ray detectors for analysis. The a number of swappable diffraction crystals X-ray detector consists of a wire anode in a with the configuration: Spectrometer 1 cylinder of P10 gas (which acts as the cathode). PET (Pentaerythrite); spectrometer 2 PET; The window allows the diffracted X-rays to spectrometer 3 LiF (Lithium fluoride) and; enter the detector. The incident X-ray photons spectrometer 4 TAP (Thallium hydrogen are absorbed by the counter gas which emits an phthalate), being used for monazite analysis. inner shell electron producing a voltage pulse The SX51 uses four P10 (Ar 90%-CH 4 in the detector. This pulse is then amplified and 10%) gas-flow proportional X-ray detectors. processed by the pulse-height analyser (PHA) Polypropylene entry windows are used on all which, amongst other things, rejects unwanted detectors. Gas pressure in detectors 1, 2 and 4 pulses. The PHA can be operated in two modes,

21 Chapter 2 EPMA Monazite Dating integral and differential. In integral mode detector. For typical monazites where elemental all pulses, including those from second and concentrations are generally < c. 25 wt% this is third-order lines in the region of interest will not a problem, but it can become an issue when be processed. In differential mode, a baseline calibrating on high concentration standards and and window value are set which filters out all is the reason for using different beam currents higher order pulses and give better peak-to- on different standards (Table 2.1). A high beam background ratios. In order to calibrate the X- current placed on a spot for a prolonged period ray counter to maximise the voltage pulse it is will also cause damage to the sample (Fig. also necessary to adjust the bias value. 2.2b). In order to maximise X-ray counts for 2.3.1 SX51 operating conditions precision while minimising the excitation volume to less than 10 μm diameter (providing The choice of accelerating voltage and excellent spatial resolution) and minimising beam current are a balance between a number sample damage for a typical analysis time of 10 of factors. Accelerating voltages in the minutes an accelerating voltage of 20 kV with a published literature that have been used for beam current of 100 nA is optimal for monazite monazite analysis (summarised in Scherrer et analysis on the Adelaide SX51 at Adelaide al., 2000) range between 15 and 25 kV. Higher Microscopy. voltages increase the intensity of the generated X-rays, giving greater analytical precision, but 2.3.2 Elements and standards it also increases the size of the X-ray excitation volume in the sample and can lead to greater Monazite is a REE-rich phosphate, (Ce, La, uncertainties in the ZAF corrections (Pyle et al., Pr, Nd, Th, Y)PO4, with a complex chemistry 2005: ZAF corrections refer to the corrections with up to 14 end members including applied to account for matrix effects related to common solid solutions between monazite- the sample composition which can effect the X- huttonite (ThSiO4) and monazite-brabantite ray spectrum produced. ZAF refers to atomic (CaTh(PO4)2) (Franz et al., 1996; Montel et number (Z), absorption (A) and fluorescence al., 2002). The number of elements required (F)). for monazite chemical age determination can The choice of beam current will also affect range from a full monazite compositional the count rate and the excitation volume. analysis (between 15 to 20 elements: Cocherie Increasing the current will increase the number of and Albarede, 2001; Spear and Pyle, 2002; X-ray emissions reaching the detector (number Cocherie et al., 2005) to just those elements of counts) while also increasing the excitation required for the age equation and those required volume in the sample. The number of counts for interference corrections (e.g. Williams and needs to be maximised for precision while not Jercinovic, 2002; Williams et al., 2006). The exceeding about 10000 cps where detector advantage of analysing a smaller number of dead-time begins to reduce the efficiency of the elements is it allows for longer counting times

22 Chapter 2 EPMA Monazite Dating

Table 2.1. EPMA monazite parameters on the SX51. Element Line Standard Xtal Spectroa PHAb T-Pkc T-Bgdc Bgd +d Bgd -d Sloped Bgd type 20 nA* Si Kα REE 1 TAP 4(LP) Int 30 30 400 1.032 linear 40 nA* U Mβ UO2 PET 1(LP) Diff 160 160 1250 0.89 linear 60 nA* Th Mα Huttonite PET 1(LP) Diff 60 30 800 1300 curved Al Kα REE 1 TAP 4(LP) Int 10 10 700 1.08 linear P Kα Apatite PET 2(LP) Int 10 10 1750 1.095 linear Ca Kα Apatite PET 1(LP) Int 20 10 500 500 linear 100 nA* Y Lα REE 1 PET 2(LP) Int 20 20 1600 0.815 linear La Lα La Glass PET 1(LP) Diff 20 20 540 1.052 linear Ce Lα Ce Glass PET 1(LP) Diff 20 20 1750 1.185 linear Pr Lβ Pr Glass LiF 3(HP) Diff 30 20 500 1 linear Nd Lβ Nd Glass LiF 3(HP) Diff 20 20 1930 0.826 linear Sm Lβ Sm Glass LiF 3(HP) Diff 30 30 300 0.97 linear Gd Lβ Gd Glass LiF 3(HP) Diff 30 15 470 500 linear Dy Lβ Dy Glass LiF 3(HP) Diff 40 40 305 0.981 linear Er Lβ Er Glass LiF 3(HP) Diff 40 40 850 0.95 linear Pb Mβ K227 PET 2(LP) Diff 240 120 4000 2695 curved * Calibration current, a spectrometer number (Low-Pressure/High-Pressure), b pulse height analyser parameter (Integral/differential), c count time (peak/background), d background sineθ positions and slope. on each element. However, there are increased Forster, 1998). They also give a large amount of analytical uncertainties introduced because of chemical data, allowing the linking of different the need to use an assumed or average total age and compositional domains to the growth element concentration for ZAF corrections. of major phases that can be used to constrain The analytical procedure adopted here is a P-T conditions (e.g. Foster et al., 2000; Pyle near complete monazite analysis. The method and Spear, 2003). Analysis of Si, Ca and Al analyses for U, Th and Pb, together with Ce, allow for quality control. Monazite typically La, many of the REEs which substitute into contains < 1 wt% Si and Ca and < 0.5 wt% Al, monazite (Pr, Nd, Sm, Gd, Dy and Er) as well as so higher concentrations of these elements can Al, Si, Ca, Y and P (A typical monazite analysis indicate; (1) a contaminated analysis, such as is presented in Table 2.2). These elements the incorporation of a micro inclusion or grain account for the compositional variation edge overlap, (2) signiﬁcant solid solution between the monazite-huttonite (Lanthanide3+ + exchange between end members away from a P5+ = Actinide4+ + Si4+) and monazite-brabantite true monazite composition or (3) hydrothermal (2Lanthanide3+ = Actinide4+ + Ca2+) exchanges or surface weathering and alteration of (Cuney and Friedrich, 1987; Franz et al., 1996; monazite (Franz et al., 1996; Poitrasson et al.,

23 Chapter 2 EPMA Monazite Dating

Table 2.2 Representative analyses of the MAD age standard monazite. MAD_1 MAD_2 MAD_3 MAD_4 MAD_5 MAD_6 MAD_7 MAD_8

O (wt%) 26.398 26.446 26.439 26.404 26.498 26.357 26.521 26.419

Al (wt%) 0.007 0.005 0.006 0.000 0.000 0.000 0.005 0.003

Si (wt%) 0.656 0.711 0.707 0.708 0.706 0.691 0.686 0.683

P (wt%) 12.105 12.055 12.063 12.102 12.106 12.033 12.119 12.032

Ca (wt%) 0.288 0.288 0.287 0.280 0.295 0.284 0.291 0.294

Y (wt%) 0.616 0.697 0.662 0.608 0.641 0.642 0.630 0.663

La (wt%) 11.979 11.814 11.836 11.770 11.824 11.833 11.832 11.854

Ce (wt%) 25.156 24.958 25.099 24.948 24.999 24.997 24.965 25.069

Pr (wt%) 2.584 2.606 2.525 2.539 2.591 2.615 2.668 2.629

Nd (wt%) 9.198 9.356 9.315 9.102 9.293 9.206 9.331 9.275

Sm (wt%) 1.974 2.104 2.034 1.996 2.107 2.138 2.134 2.139

Gd (wt%) 0.724 0.731 0.777 0.797 0.786 0.746 0.833 0.865

Dy (wt%) 0.233 0.276 0.220 0.301 0.272 0.248 0.298 0.258

Er (wt%) 0.131 0.072 0.131 0.056 0.115 0.071 0.079 0.098

Pb* (wt%) 0.162 0.162 0.177 0.171 0.163 0.159 0.182 0.162

Th (wt%) 6.318 6.628 6.567 6.627 6.626 6.578 6.620 6.632

U* (wt%) 0.150 0.156 0.159 0.152 0.137 0.151 0.155 0.141

Total 98.679 99.063 99.002 98.562 99.160 98.747 99.349 99.214

Pk(Al) 186.104 185.203 184.202 180.598 179.497 182.200 185.503 177.294

Pk(Si) 1338.722 1412.527 1410.006 1416.628 1409.165 1392.325 1374.175 1369.168

Pk(P ) 3734.987 3707.585 3707.892 3719.445 3716.377 3692.250 3707.483 3681.414

Pk(Ca) 522.418 526.781 522.267 514.192 531.095 524.675 526.781 527.082

Pk(Y ) 67.364 73.366 69.214 66.163 66.813 66.913 66.563 69.765

Pk(La) 5182.697 5098.922 5115.055 5074.184 5095.675 5101.190 5083.873 5095.623

Pk(Ce) 11418.276 11277.511 11342.667 11260.731 11289.161 11281.946 11235.030 11284.565

Pk(Pr) 225.886 225.753 222.448 222.482 226.020 226.087 226.721 227.689

Pk(Nd) 744.057 751.239 747.472 728.890 751.390 736.524 745.815 741.948

Pk(Sm) 215.840 220.245 214.371 213.203 220.579 218.844 222.014 220.746

Pk(Gd) 151.502 152.303 155.473 155.606 156.240 150.168 156.207 155.639

Pk(Dy) 121.119 124.196 120.043 125.497 123.396 121.319 125.747 123.671

Pk(Er) 127.199 126.098 128.149 125.397 127.999 127.624 124.622 126.723

Pk(Pb) 32.328 32.432 32.866 32.428 32.007 32.437 32.474 32.078

Pk(Th) 662.364 688.971 683.323 689.975 689.222 683.423 685.833 684.352

Pk(U ) 138.439 138.802 139.277 138.683 137.213 137.995 137.325 137.000

Bg(Al) 178.072 180.126 178.072 181.207 179.261 181.856 179.910 174.072

Bg(Si) 326.766 318.253 322.492 327.903 323.699 331.591 323.216 324.423

Bg(P ) 22.887 21.792 19.054 20.806 18.506 19.054 18.726 21.354

Bg(Ca) 165.182 171.388 166.984 168.335 166.433 174.041 168.185 165.733

Bg(Y ) 16.262 15.773 14.469 15.936 13.898 13.939 14.713 15.243

Bg(La) 314.514 318.414 324.052 318.941 314.514 317.781 314.725 316.042

Bg(Ce) 399.571 392.328 392.209 398.502 393.812 392.447 388.707 390.013

Bg(Pr) 59.111 58.277 60.078 59.511 59.644 58.210 55.943 59.310

Bg(Nd) 57.874 56.056 55.105 53.618 61.386 53.287 55.229 55.312

Bg(Sm) 87.421 83.797 82.471 84.024 84.088 80.368 84.153 82.600

Bg(Gd) 93.663 94.117 93.629 92.245 93.728 90.910 90.209 87.081

Bg(Dy) 102.988 102.768 102.988 102.154 102.277 102.105 102.718 103.725

Bg(Er) 117.202 120.648 118.177 121.100 119.222 122.193 118.604 119.246

Bg(Pb) 24.596 24.732 24.494 24.313 24.298 24.890 23.919 24.395

Bg(Th) 44.242 42.680 42.858 44.514 43.642 42.712 43.091 40.338

Bg(U ) 107.612 106.911 107.000 107.245 107.957 106.783 105.636 107.406

*Interference corrected, Pk-number of peak counts, Bg-number of background counts

24 Chapter 2 EPMA Monazite Dating

1996; Podor and Cuney, 1997; Forster, 1998; important to decide the location of background Poitrasson et al., 2000). Analysis of all these points around the selected peak and the shape elements typically produces totals in the range of the background curve. The best method to 99 ± 4 wt% which is within the range to give a determine which energy lines to use, where to high precision for ZAF corrections. place background positions and if there are any The use of high quality standards is potential interferences is to compile a series of essential for accurate determination of WD scans over the energy regions of interest elemental concentrations in unknowns. In order (Fig. 2.3). In order to get the best possible to minimise the effect of any interferences on precision, the highest intensity line with no or calibration, and increase count rates to produce minimal overlaps should be selected. Because good statistics, high concentration single the precision of EPMA derived monazite ages is element standards are preferred (especially for primarily a first-order function of the precision the REE’s which have multiple interferences of U and Pb analyses (monazite typically in many cases: e.g. Jercinovic and Williams, contains much more Th than either U or Pb; 2005; Pyle et al., 2005) . The compositions of Spear and Pyle, 2002), it is very important to all the standards are given in Table 2.3. The Pb accurately determine the concentrations of standard is a synthetic glass NIST K-227, UO2 these elements. for U and a natural huttonite for Th. Ca and P are Th, U and Pb are all analysed using PET calibrated using Durango apatite (Jonckheere diffraction crystals (Table 2.1). For monazite, et al., 1993; Frei et al., 2005). Si, Al and Y are the high intensity ThMα peak is free from calibrated using a multi-element synthetic glass interference and therefore provides a clean (REE1: Drake and Weill, 1972). The REE’s energy line to measure for analysis (Fig. 2.3). are calibrated using high concentration single The choice of background type (one or two element synthetic glasses (Edinburgh Materials points, linear or curved) and position will and Micro-Analysis Centre , Grant Institute of depend on the background region near the Earth Sciences, University of Edinburgh). peak of choice. For the ThMα peak the clean Age standards are not required for this background around the peak means a 2 point procedure. However, the periodic measurement curved background provides the best fit (Table of a well characterised secondary age standard 2.1). of known age is routinely carried out (the MAD The choice of line for U and Pb is more standard with a mean 206Pb/238U age of 515 Ma; difficult as both the Mα and Mβ lines are Foster et al., 2000). subjected to a number of interferences. The U region of the spectrum is subject to a number of 2.3.3 Peaks, Backgrounds and Interferences interferences from ThM lines plus steps in the background caused by the ThM5, ThM4 and Ar Once the choice of elements to be analysed K absorption edges (Fig. 2.3). The UMα line has been made, it is necessary to decide which suffers from a very large overlap by the ThMβ energy line is best to use. It is also extremely line (Fig. 2.3) and is very near the large Ar K

25 Chapter 2 EPMA Monazite Dating

Table 2.3. Standard compositions.

Durango Element K227 Huttonite UO REE1 La Ce Pr Nd Sm Gd Dy Er Apatite 2

O (wt%) 17.34 41.67 19.75 11.84 40.75 41.98 40.98 42.07 41.81 39.75 39.43 39.19 38.33 F (wt%) 2.00 Al (wt%) 6.21 5.57 5.60 5.59 5.55 6.25 6.20 6.15 6.40 Si (wt%) 8.40 8.67 23.45 26.08 24.98 26.19 25.99 22.70 22.51 22.38 21.15 P (wt%) 17.80 Ca (wt%) 38.61 13.59 12.20 11.69 12.25 12.16 13.89 13.79 13.73 14.26 Y (wt%) 4.00 La (wt%) 14.15 Ce (wt%) 16.74 Pr (wt%) 4.00 13.90 Nd (wt%) 14.48 Sm (wt%) 17.42 Gd (wt%) 18.09 Dy (wt%) 4.00 18.56 Er (wt%) 4.00 19.87 Pb (wt%) 74.26 Th (wt%) 71.59 U (wt%) 88.15 Total 100.00 100.08 100.01 99.99 100.00 99.98 99.99 100.00 99.99 100.01 100.02 100.01 100.01

absorption edge making it difficult to use. The including the ThMζ1,2, 2LaLα and YLγ lines UMβ line is overlapped by the ThMγ peak (Fig. (Fig. 2.4). The PbMβ line, although of slightly 2.3), but the peak can be resolved in a typical lower intensity, provides a better choice of monazite and the overlap empirically corrected energy line because of its cleaner spectrum. for. In geological samples, the KKα line may The PbMβ line has one first-order overlap by also overlap with the UMβ peak (Fig. 2.3). the UMζ2 line and also the second-order (2) This overlap can be avoided by not analysing CeLα line (Fig. 2.4). The higher order 2CeLα monazite grain boundaries that are in contact line can be filtered from the analysis through the with K-bearing minerals and monitoring differential mode PHA settings. However, there apparent U concentrations throughout the is a small 2CeLα escape peak (an artefact of the monazite grains. The choice of background low-pressure P10 gas-flow detector) which falls positions is hampered by the numerous ThM within the energy window of the PbMβ peak peaks and background discontinuities. In order and must be corrected for (Fig. 2.4: Pyle et al., to avoid these, a single point linear background 2005). Because the size of the 2CeLα escape is used and placed 1250 Sinθ below the UMβ peak is very small, and is proportional to the peak, after the ThM4 edge (Table 2.1; Fig 2.3). CeLα peak (and therefore the Ce concentration) The choice of Pb Mα or Mβ line is similarly it is corrected for mathematically offline by the constrained by the interferences in the PbM application of a measured correction factor of region in monazite. The higher intensity PbMα 0.0003% of the Ce concentration. The UMζ2 line suffers from a number of interferences interference is corrected for empirically on-

26 Chapter 2 EPMA Monazite Dating

18000 A UMβ Sp1(PET) 16000 Huttonite

14000 UO2

12000 UMα

10000 ThMα cts/s 8000

6000 ThMβ

4000

2000 ThMγ

0 40000 41000 42000 43000 44000 45000 46000 47000 48000 600 B ThMβ UMα Sp1(PET) 500 MAD Figure 2.3. WD scans across the U and Th region using a PET crystal. 400 UMβ A) scans on U and Th standards ThMγ ThM4-02 showing the clean ThMα line and 300 KKα

cts/s U bkg- the ThMγ overlap on the UMβ. B) scan on a natural monazite show- 200 Ar K ing the relative sizes of the interference peak on UMβ and the loca- 100 ThM5 tion of the U background position.

0 40000 41000 42000 43000 44000 45000 46000 47000 Sin θ line. Because of cascading interferences, the nm) compared to the LiF crystal (2d = 4.02 nm) corrections must be done in order with the means the heavier REEs occur over a smaller

ThMγ on UMβ done ﬁrst followed by the UMζ2 Sinθ range using a PET crystal than on a LiF on PbMβ. To get clean background positions crystal (Fig. 2.5). This means more interference for the Pb region a 2 point curved background issues with the REEs using a PET crystal. This, is used (Fig. 2.4, Table 2.1). together with machine restrictions (long count The choice of REE lines is dependant on times on elements using PET crystals means which diffraction crystal is used. Using a PET the non-essential REEs can’t be efﬁciently crystal gives higher count rates for the REE analysed using a PET crystal on the SX51). As lines (Fig. 2.5). High concentrations of both a consequence the REEs are analysed using a Ce and La in typical monazites means there are LiF crystal. Because of the comparatively low no interference issues with the CeLα and LaLα concentrations of most REEs in monazite and lines and the need for accurate Ce concentrations the reduced intensity of the REE lines when for Pb overlap corrections means the high- using a LiF crystal (Fig. 2.5) on the SX51 intensity Lα lines are measured on a PET only the high-pressure X-ray counter will give crystal (Table 2.1). For higher mass REEs the enough counts to maintain precision. The high- larger D spacing in the PET crystal (2d = 8.74 pressure P10 counter also has the advantage of

27 Chapter 2 EPMA Monazite Dating

1000 A PbMβ PbMα Sp2(PET) La glass 800 Ce glass ThMζ1 K227 Huttonite UMζ2 2CeLα esc UO2 600 2CeLα UMζ1 cts 2LaLα 400

ThMζ2 200

0 56000 57000 58000 59000 60000 61000 62000 350 B 2CeLα Sp2(PET) Figure 2.4. WD scans across the 300 MAD Pb region using a PET crystal.

250 A) scans on Pb, Ce, La, U and Th 2LaLα standards showing the 2CeLα and ThMζ2 200 2CeLα escape peak and the UMζ PbMβ 2 UMζ1 UMζ2 cts interferences on the PbMβ line . B) PbMα 150 scan on a natural monazite show- Pb bkg Pb bkg ThMζ1 ing the relative sizes of the inter- 100 ferences peak on PbMβ peak and

50 the location of the Pb background positions. 0 55000 56000 57000 58000 59000 60000 61000 62000 Sin θ suppressing higher-order interferences on the Pb/s·nA·wt% is measured using a PET crystal REE’s. Because of less interference issues the and a low pressure P10 gas ﬂow proportional REE Lβ lines are used (Table 2.1). counter. Because the precision of the spot age is The remaining element lines have been dependant on the precision of the Pb, U and Th chosen because there are no interference issues analysis it is necessary to count for a number of with them and because they are the highest minutes on each of these peak and background intensity lines available. The YLα, CaKα and positions. To collect enough counts to produce PKα lines are measured using a PET crystal a statistically meaningful (and precise) peak (Table 2.1). The trace elements of Si and Al are to background ratio in a typical monazite also measured using the Kα lines using a TAP it is necessary to count on the Pb peak for crystal (Table 2.1). approximately 240 s and on each background position for 120 s each (Table 2.1). For a typical 2.3.4 Counting Times monazite with a Pb concentration of c. 2500 ppm a 1σ error of c. 110 ppm Pb is achieved (a Using the SX51 at Adelaide Microscopy 1σ counting error of < 5 %). Similarly U count at an accelerating voltage of 20 kV and a times are 160 s for peak and 160 s background beam current of 100 nA, an intrinsic detector which produces a 1σ counting error of c. 2 %. response of between 0.2 and 0.05 counts of Because of the generally high concentration of

28 Chapter 2 EPMA Monazite Dating

7000 A CeLα SmLα SP1(PET) NdLα PrLα 6000 LaLβ CeLβ 5000

4000 cts NdLβ PrLβ

3000

2000 PrLγ LaLγ

1000

0 24000 25000 26000 27000 28000 29000 30000 3000 B ErLα SP3(LiF) DyLα 2500 GdLα Figure 2.5. WD scans across 2000 SmLα NdLα the REE region. A) scan us- CeLβ ErLβ GdLβ ing a PET crystal gives large 1500

cts DyLβ SmLβ PrLα counts but occurs over a NdLβ LaLβ smaller sinθ creating numerous 1000 PrLβ overlaps. B) scan using a LiF crystal produces less counts but provides fewer interference 500 issues, particularly for the Lβ lines. 0 38000 43000 48000 53000 58000 Sin θ Th in monazite (5 – 20 wt%) the count times also be applied to mounted grain separates and are considerably lower, with a peak count time polished blocks. Because of the low levels of U of 60 s and 30 s on each of the backgrounds. and Pb in monazite it is particularly important This produces a 1σ counting error of < 0.5 to minimise contamination from the polishing %. The counting times of all other analysed process. The effectiveness of EPMA is also elements are given in Table 2.1. A typical single dependant on the polish of the sample; therefore, spot analysis of the entire 16 element package a good polish is required for accurate analyses. takes c. 10 minutes. However, these counting At the University of Adelaide, thin sections are times are not ﬁxed, and can be adjusted to suite either made in-house or externally. All polishing particular monazite compositions. is done in-house where the environment can be controlled. A three stage polishing process has 2.4 Sample Preparation been found to give the best results. First a 3 μm diamond grit on a tin plate, followed by 3 The method outlined here is generally μm diamond on a 1μm cloth lap and ﬁnally a undertaken on polished thin sections but can 1 μm diamond on a ¼ μm cloth lap. No Pb-

29 Chapter 2 EPMA Monazite Dating bearing samples are polished on these laps and as are practical within the spatial constraints the plates are cleaned and replaced periodically. required for clean analyses (i.e. at least 10 μm The samples are thoroughly washed in acetone apart and at least 10 μm from a grain boundary and deionised water between each stage. When to account for the excitation volume of the transporting the samples they are kept in sealed electron beam). The collection of a large data containers to avoid contamination. set allows for better precision ages (lower errors) as individual 1σ spot errors can be 2.5 Data Collection Strategy, Analysis and between ± 30 to 60 Ma. Individual grains are Reduction not X-ray mapped prior to analysis because: (1) to map low concentration elements in monazite Williams et al. (2006) suggest two alternative requires high beam currents (~ 150 nA) focused strategies for collecting EPMA monazite age on to small areas for long durations (dwell data. The ‘top-down’ method (e.g. Suzuki and times of over 100 ms) which causes sample Adachi, 1991; Montel et al., 1996; Cocherie damage, element migration and carbon coating et al., 1998; Montel et al., 2000; Cocherie degradation; (2) the process of mapping can and Albarede, 2001) involves collecting large be time consuming and may not produce any numbers of analyses from multiple compositional results if there are no compositional domains; domains and then using a statistical analysis to (3) not all compositional domains are also age distinguish any age populations. The ‘bottom- domains. up’ approach of Williams et al. (2006) involves The analytical method adopted here detailed measurements restricted to specific uses a full suite of elements to allow for compositional domains and therefore requires greater precision in ZAF corrections and also detailed X-ray maps of individual grains to be because it can produce geologically important acquired prior to analysis (see also Pyle and information. Spot ages and chemical zoning in Spear, 2003). Both these methods have various individual grains can be investigated by plotting pros and cons and the method developed here is various elements and spot ages (e.g. Y wt% vs. a combination of the two approaches. Th/Ce wt% or age vs. Y wt%). Grains which The approach developed here involves the show significant age/compositional variation collection of large numbers of point analyses can then be X-ray mapped without affecting the from individual samples (multiple monazite precision of the ages. grains within a thin section). Individual Prior to age calculation, the raw monazites are first located either optically or compositional data are assessed and filtered using back-scattered electron imaging either to remove suspect analyses. Generally any on a scanning electron microscope or on the data with totals below 95% or above 103% are SX51. Individual grains are then characterised excluded from the age pool because analyses texturally (e.g. located within the fabric, outside these ranges introduce uncertainties shielded within major phases etc.). Selected into the ZAF corrections (subject to monazite grains are targeted with as many point analyses composition). The raw compositional data are

30 Chapter 2 EPMA Monazite Dating then further scrutinised with analyses containing here, four previously dated monazite samples > 1 wt% Si, Ca or Al being excluded from the have been analysed using the described age pool (as discussed above monazite should protocols. The samples have all been previously contain only trace Si, Ca or Al and high values dated using isotopic methods and the isotopic may indicate a contaminated spot analysis, i.e. data is available in the literature. The analysed the excitation volume included an adjacent samples have been selected to reﬂect a variety mineral or an inclusion). of ages, from Palaeoproterozoic to Ordovician. Prior to the age calculation, the Pb concentration is corrected for the small 2CeLα 2.6.1 Sample SB17 escape peak overlap by the relationship: Sample SB17 is a grain mount of monazites Pb* = Pb(wt%)-[Ce(wt%)×0.0003] (2) from a felsic mylonitic shear zone from the southern Gawler Craton. These monazites have The individual spot ages are then calculated been isotopically dated using SHRIMP II and using the age equation (Eq. 1). Individual spot produced a concordant 207Pb/206Pb age of 1720 errors are calculated using the approach of ± 7 Ma (2σ) and a 206Pb/238U age of 1715 ± 35 Montel et al. (1996). Individual errors for U, Ma (2σ; Dutch et al., 2008; Chapter 3). The Th and Pb are calculated on the basis of the monazites are heterogeneous, with 3.6–18.0 counting (Poisson) statistics of the elemental wt% Th, 0.16–1.13 wt% U and 0.22–1.68 wt% analysis. U, Th and Pb counting errors are Pb, so represent a good test that the various calculated using the equation: peak overlap protocols are effective and provide accurate analyses of Pb and U concentrations. M(1σ) = M(ppm)×√(pk+bg)/(√ct×(pk-bg))(3) The EPMA age is 1703 ± 7 Ma (n= 60, MSWD = 0.54; Fig 6). This age is slightly younger than where M is either U, Th or Pb, pk and bg are the 207Pb/206Pb SHRIMP age but is within error the peak and background counts respectively of the 206Pb/238U age. and ct is the counting time for that element. The uncertainty is then calculated on the age 2.6.2 Sample GL4B by propagating the uncertainties on U, Th and Pb through Eq. (1). All statistical analysis Sample GL4B consists of monazites and data presentation thereafter is done using extracted from a leucosome within granulite the Microsoft Excel geochronology add-in facies metapelites from the Reynolds Range in ISOPLOT (Ludwig, 2003). central Australia. These monazites were dated isotopically using SHRIMP II and produced a 2.6 Results of Comparative Study concordant 207Pb/206Pb age of 1566 ± 3 Ma (2σ ; Rubatto et al., 2001). Chemically the monazites As a test of the validity of the EPMA are fairly homogeneous in composition with monazite setup on the Cameca SX51 outlined 3.61–5.92 wt% Th, 0.11–0.63 wt% U and

31 Chapter 2 EPMA Monazite Dating

0.32–0.51 wt% Pb. The EPMA age is 1561 ± 8 produced a 206Pb/238U weighted average age of Ma (2σ, n= 111, MSWD = 0.9; Fig. 2.6) which 450 ± 5 Ma (2σ) and a concordia age of 453 is within error of the 207Pb/206Pb isotopic age. ± 4 Ma (2σ; Payne, 2008). The monazites are homogeneous in composition with 5.9–8.13 2.6.3 MAdel Standard wt% Th, 0.17–0.42 wt% U and 0.13–0.22 wt% Pb. The EMPA monazite age is 463 ± 6 Ma (2σ, The MAdel monazite standard consists of n= 121, MSWD = 0.86: Fig. 2.6). The EPMA chips of a Pan-African aged monazite from age is within error of the SHRIMP isotopically Madagascar and is used as an age standard for determined age and the LA-ICP-MS concordia LA-ICP-MS monazite dating at the University age. of Adelaide. MAdel has been isotopically dated using TIMS at the Massachusetts Institute of 2.7 Application to an unknown Technology and has a 206Pb/238U age of 518 ± 0.25 Ma (2σ) and a 207Pb/235U age of 514 ± Here I have used the described methodology 0.3 Ma (2σ; Payne, 2008; Payne et al., 2008; and procedures for EPMA monazite dating Wade et al., 2008). MAdel was dated twice using the SX51 to constrain the timing of using EPMA, once at the start of the analytical deformation and metamorphism in the Fishery session and again at the end of the session (ﬁve Bay region of the southern Gawler Craton (Fig. days apart). The EPMA monazite ages are 516 2.1). ± 5 Ma (2σ, n= 110, MSWD = 1.136; Fig. 2.6) and 521 ± 4 Ma (2σ, n= 132, MSWD = 0.95; 2.7.1 Geological Setting Fig. 2.6) respectively. These ages are within error of the isotopically determined age and The basement geology of the southern demonstrate the stability and consistency of the Gawler Craton consists of three main rock SX51 and the developed monazite protocol. packages (Fig. 2.1). The late Archaean to Palaeoproterozoic Sleaford Complex (Thomson, 2.6.4 Sample 222 1970) consists of a series of layered ortho- and paragneisses together with the upper crustal Sample 222 consists of monazites extracted granitoids of the Dutton Suite (Fanning et al., from the matrix of garnet-bearing leucosomes 1988; Daly and Fanning, 1993; Fanning et al., from within a biotite – sillimanite – plagioclase 2007). Unconformably overlying this are the – garnet – quartz metapelite from the Harts Palaeoproterozoic Hutchison Group and Price Range Group in central Australia. These Metasediments which consist of a deformed monazites were dated isotopically via SHRIMP and metamorphosed package of shallow marine (Hand et al., 1999) and in-house LA-ICP-MS sands, silts and chemical sediments (Parker (Payne, 2008). SHRIMP isotopic analysis and Lemon, 1982; Parker, 1993; Vassallo and produced a 206Pb/338U age of 467 ± 4 Ma (2σ ; Wilson, 2001). Juxtaposed along the eastern Hand et al., 1999) while LA-ICP-MS analysis side of the Hutchison Group metasedimentaty

32 Chapter 2 EPMA Monazite Dating

2500 1780 Mean = 1560.9 ± 8.0 [0.52%] 95% conf. SB17 Mean = 1703.0 ± 6.5 [0.38%] 95% conf. GL4B 1740 Wtd by data-pt errs only, 0 of 111 rej. 2300 Wtd by data-pt errs only, 5 of 65 rej. MSWD = 0.90, probability = 0.75 MSWD = 0.54, probability = 0.999 1700 2100

1660 1900 1620 1700 1580

Age (Ma) 1500 1540

1300 1500

1100 1460 206Pb/238U age of 1715 ± 35 Ma (2σ) 207Pb/206Pb age of 1566 ± 3 Ma (2σ) 900 1420

640 Mean = 516.5 ± 4.8 [0.93%] 95% conf. Mean = 521.7 ± 3.7 [0.71%] 95% conf. MAdel-1 640 MAdel-2 Wtd by data-pt errs only, 3 of 113 rej. Wtd by data-pt errs only, 8 of 140 rej. 600 MSWD = 1.13, probability = 0.17 MSWD = 0.95, probability = 0.65 600

560 560

520 520 Age (Ma) 480 480

440 440 206Pb/238U age of 518 ± 0.25 Ma (2σ) 206Pb/238U age of 518 ± 0.25 Ma (2σ)

400 400

Mean = 463.1±5.3 [1.1%] 95% conf. 580 222 Wtd by data-pt errs only, 9 of 130 rej. MSWD = 0.86, probability = 0.87 540

500

460 Figure 2.6. EPMA geochronological results from

Age (Ma) the selected comparison samples. Plots show indi- 420 vidual analyses with 1σ error . Blue analyses were outside 95% conﬁdence and were excluded from 380 the weighted average calculations. 206Pb/338U age of 467 ± 4 Ma (2σ)

340 rocks are the deformed granitoids of the 1850 et al., 1988; Fanning et al., 2007). This event Ma Donington Suite (Mortimer et al., 1988; has been interpreted to have reached granulite- Hoek and Schaefer, 1998). facies conditions in the Carnot Gneisses (Fig. The southern Gawler Craton has been 2.1: Daly and Fanning, 1993: Chapter 4), but affected by at least two large-scale deformational/ little is known about the structural development metamorphic events. The earliest of these events of this orogen as it has been pervasively affecting the late Archaean component was the overprinted by subsequent events (Vassallo and 2440 – 2420 Ma Sleafordian Orogeny (Fanning Wilson, 2002: Chapters 3, 4).

33 Chapter 2 EPMA Monazite Dating

The 1730 – 1690 Ma Kimban Orogeny the Kimban Orogeny (Reid et al., 2007; Dutch dominates the structural architecture of the et al., 2008; Chapters 3, 4, 5). However, these southern Gawler Craton (Parker, 1993; Vassallo workers didn’t directly constrain the timing and Wilson, 2001; Vassallo and Wilson, 2002). of deformation with geochronology. Recently The crustal scale Kalinjala Shear Zone, which Duclaux et al. (2007) attributed the structure separates the Sleaford Complex and Hutchison and metamorphic features of the southern Group sequences from the Donington granitoid Gawler Craton to the c. 2440 Ma Sleafordian suite (Fig. 2.1), is the most obvious structural Orogeny on the basis of an EMPA age of c. feature of the Kimban Orogeny (Parker, 1980; 1827 Ma from a cross-cutting cordierite bearing Vassallo and Wilson, 2002: Chapter 5). The granitic vein from the northern Eyre Peninsula. Kimban Orogeny reached granulite-facies Duclaux et al. (2007) used the structural features conditions in the south and east, decreasing in as the type example of hot-weak Archaean grade to the north and west (Hand et al., 1995; orogens. Therefore constraining the timing of Tong et al., 2004: Chapters 3, 4, 5). metamorphism and deformation of the Carnot The Fishery Bay region of the southern Gneisses in the Sleaford Bay region is of vital Gawler Craton (Fig. 2.1) consists of highly significance to: (1) understanding the tectonic deformed and metamorphosed interlayered development of the southern Eyre Peninsula migmatitic garnet ± cordierite ± orthopyroxene (Dutch et al., 2007) and; (2) to inferences about paragneisses, augen and charnockitic the tectonic style during the Archaean. orthogneiss and garnet ± clinopyroxene ± orthopyroxene mafic granulites of the Carnot 2.7.2 Results Gneisses. Tong et al. (2004) determined peak metamorphic conditions from garnet-bearing Three monazite-bearing metapelitic samples mafic granulites of up to c. 10 kbar at c. 800 have been selected to constrain the timing of °C. The structure of the Fishery Bay region has metamorphism and deformation in the Carnot been described by Vassallo and Wilson (2002). Gneisses at Fishery Bay. The area is located on the western flank of the Sample FB10 is a coarse-grained garnet Kalinjala Shear Zone (Fig. 2.1) and consists of bearing metapelitic gneiss from within the an early layer parallel fabric (SS1?) which has fabric interpreted as KS1 by Vassallo and been folded by a series of inclined isoclinal and Wilson (2002) and by Duclaux et al. (2007) sheath folds (KD1 structures). These structures to be Archaean in age. All monazite grains are are then overprinted by a series of upright located within the fabric at grain boundaries tight to isoclinal folds and shear zones (KD2 adjacent to predominantly quartz, plagioclase structures). and K-feldspar but also biotite and garnet. Four Implicit in the structural and metamorphic monazite grains were analysed on the SX51 work of Vassallo and Wilson (2001; 2002) and producing a unimodal age of 1707 ± 20 Ma (n= Tong et al. (2004) is that the deformational 62, MSWD = 1.04: Fig. 2.7). Chemically the features are c. 1700 Ma in age and are a result of monazites are homogeneous and no distinctive

34 Chapter 2 EPMA Monazite Dating

0.16 2100 FB10 0.14

1900 0.12

1700 0.1 grp 1 mz 8 Y 0.08 mz 9 1500 mz 10 mz 11 Age (Ma) 0.06

1300 0.04

Mean = 1707 ± 20 [1.2%] 95% conf. 1100 0.02 Wtd by data-pt errs only, 4 of 66 rej. MSWD= 1.04, probability = 0.39 0 900 0 0.05 0.1 0.15 0.2 0.25 Th/Ce

0.9 2100 FB15 Mean = 1690.1 ± 7.5 [0.44%] 95% conf. zone 1 Wtd by data-pt errs only, 5 of 77 rej. 0.8 MSWD= 0.79, probability = 0.90 2000 0.7

1900 0.6

0.5 mz 1 1800 mz 2 Y zone 3 mz 3 0.4 zone 2 mz 4 Age (Ma) 1700 0.3

1600 0.2

1500 0.1

0 1400 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Th/Ce

0.7 FB19 zone 3 1850 0.6 zone 1

0.5 1750

0.4 mz 1 mz 2 Y mz 3 1650 0.3 mz 4 Age (Ma)

0.2

1550 Mean = 1708 ± 12 [0.73%] 95% conf. 0.1 zone 2 Wtd by data-pt errs only, 2 of 52 rej. MSWD = 1.3, probability = 0.099 0 1450 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Th/Ce

Figure 2.7. EPMA monazite geochronological results for the unknown samples from Fishery Bay. As- sociated with each age plot is an example of the types of chemical discrimination plots possible using the technique described here. Plots are Th/Ce vs. Y. Individual grains which plot within different zones may have formed in equilibrium with different phases such as garnet or xenotime. No difference in age was found between grains with different compositions.

chemical-age populations can be differentiated KS1 fabric. The shear zones are interpreted as

(Fig. 2.7). KD2 structures by Vassallo and Wilson (2002) Samples FB15 and FB19 are both highly and contain the regional lineation attributed to strained garnet-bearing metapelitic gneisses Archaean deformation by Duclaux et al. (2007). located within vertical N-S orientated dextral All monazite grains are located within the high- shear zones which crosscut and deform the strain fabric surrounded by quartz, plagioclase

35 Chapter 2 EPMA Monazite Dating and K-feldspar. structural and metamorphic development of the Four monazite grains were analysed from Carnot Gneisses in Fishery Bay to the Kimban sample FB15. These grains display some clear Orogeny. This indicates that the structural chemical differences with mz4 having high interpretation of Duclaux et al. (2007) is not Th and high Y (zone 1), mz3 contains a low consistent with the age framework of the fabrics Th, low Y zone (zone 2) and mz1 and mz2 are in the Carnot Gneiss, and that the structural indistinguishable (zone 3; Fig. 2.7). Looking at style is not indicative of Archaean tectonics. the ages of these chemical zones individually; zone 1 produces an age of 1691 ± 9 Ma, zone 2.8 Conclusions 2 produces an age of 1693 ± 58 Ma (the low Th content introduces large uncertainties on EPMA monazite dating provides an accurate the ages), while zone 3 produces 1694 ± 14 and inexpensive tool for age determination. Ma. These ages are all within error and are It is readily preformed in-situ and using the therefore either equivalent or grew within the method developed here provides extra chemical age resolution of the technique. Pooling all the information which may aid in the geological analyses (excluding the low precision ages of interpretation of the produced ages. Although zone 2) produces an age of 1690 ± 8 Ma (n= 72, EPMA monazite dating (or any low level EPMA MSWD = 0.79; Fig. 2.7). analysis) is not a trivial procedure, with the Four monazite grains from sample FB19 correct protocols, machine setup and calibration display clear chemical differences (Fig. 2.7). it is possible to achieve monazite chemical Mz1 and 2 are indistinguishable with a spread ages which are comparable with those obtained of Y values and high Th (zone 1). Mz3 has a via more traditional isotopic methods. Even similar spread of Y values but is predominantly though the precision of the above technique is low in Th (zone 2) while mz4 is a high Th not generally sufficient to resolve individual monazite but a very homogeneous low Y events within an orogenic event, it provides an content (zone 3). Zone 1 produces a pooled age efficient way to add geochronological data to of 1697 ± 18 Ma while zone 2 produces 1650 any determined P-T path and allows for rapid ± 54 Ma and zone 3 produces 1718 ± 17 Ma. verification of the ages of deformation and These ages are again within error and produce a metamorphism of the individual assemblages pooled age of 1708 ± 12 Ma (n= 50, MSWD = which are used to constrain them. 1.3; Fig. 2.7; including low Th zone 2 analyses produces an age of 1705 ± 12 Ma). Acknowledgements Although the precision offered by the EPMA monazite technique is not high enough to resolve David Steele is gratefully thanked for all chronological differences between the timing his assistance with getting me up to speed with of KD1 and KD2 deformation recorded in the the procedures (and pointing out the pitfalls) structural relationships at Fishery Bay, it does I needed to get this system operational on the provide a clear temporal constraint, linking the SX51. He is also thanked for providing me

36 Chapter 2 EPMA Monazite Dating with the single element high concentration Carson, C.J. et al., 2004. Age constraints REE glasses. The Adelaide Microscopy team on the Paleoproterozoic tectonometamorphic history of the of John Terlet, Peter Williams and especially Committee Bay region, western Angus Netting are gratefully thanked for all the Churchill Province, Canada: evidence assistance they provided. Justin Payne and Ben from zircon and in situ monazite Wade are thanked for providing the standards SHRIMP geochronology. Canadian Journal of Earth Sciences, 41(9): 1049- used for the comparative study. 1076. Cherniak, D.J., Watson, E.B., Grove, M. and References Harrison, T.M., 2004. Pb diffusion in monazite: a combined RBS/SIMS Asami, M., Suzuki, K. and Grew, E.S., study. Geochimica et Cosmochimica 2002. Chemical Th-U-total Pb dating Acta, 68(4): 829-840. by electron microprobe analysis of monazite, xenotime and zircon from Clarke, G.L. and Powell, R., 1991. Proterozoic the Archean Napier Complex, East granulite facies metamorphism in the Antarctica; evidence for ultra-high- southeastern Reynolds Range, central temperature metamorphism at 2400 Australia: geological context, P-T path Ma. Precambrian Research, 114(3-4): and overprinting relationships. Journal 249-275. of Metamorphic Geology, 9: 267-281. Barth, S., Oberli, F. and Meier, M., 1994a. Cocherie, A. and Albarede, F., 2001. An Evidence for closed-system behaviout improved U-Th-Pb age calculation of 232Th-209Pb in allanite under for electron microprobe dating hydrothermal conditions: a study of of monazite. Geochimica et altered rhyolite and granodiorite from Cosmochimica Acta, 65(24): 4509- the Atesina-Cima d’Asta volcano- 4522. plutonic complex (N Italy). United Cocherie, A., Guerrot, C. and Rossi, P., 1992. States Geological Survey Circular, Single-Zircon Dating by Step-Wise Pb 1107: 21. Evaporation - Comparison with Other Barth, S., Oberli, F. and Meier, M., 1994b. Geochronological Techniques Applied Th-Pb versus U-Pb isotope systematics to the Hercynian Granites of Corsica, in allanite from co-genetic rhyolite France. Chemical Geology, 101(1-2): and granidiorite: implications for 131-141. geochronology. Earth and Planetary Cocherie, A., Legendre, O., Peucat, J.J. Science Letters, 124: 149-159. and Kouamelan, A.N., 1998. Bea, F. and Montero, P., 1999. Behavior of Geochronology of polygenetic accessory phases and redistribution monazites constrained by in situ of Zr, REE, Y, Th, and U during electron microprobe Th-U-total lead metamorphism and partial melting determination; implications for lead of metapelites in the lower crust: behaviour in monazite. Geochimica An example from the Kinzigite et Cosmochimica Acta, 62(14): 2475- Formation of Ivrea-Verbano, NW Italy. 2497. Geochimica et Cosmochimica Acta, Cocherie, A. et al., 2005. Electron-microprobe 63: 1133-1153. dating as a tool for determining

37 Chapter 2 EPMA Monazite Dating

the closure of Th-U-Pb systems in Craton, South Australia. Australian migmatitic monazites, pp. 607-618. Journal of Earth Sciences, 55: 1063- 1081. Crowley, J.L., Schoene, B. and Bowring, S.A., 2007. U-Pb dating of zircon in the Dutch, R., Hand, M. and Reid, A., 2007. Bishop Tuff at the millennial scale. Orogen-parallel flow during Geology, 35(12): 1123-1126. continental convergence: Numerical experiments and Archean field Cuney, M. and Friedrich, M., 1987. examples: COMMENT and REPLY. Physicochemical and crystal-chemical GEOLOGY, doi: 10.1130/G24419C.1. controls on accessory mineral paragenesis in granitoids: implications Dutch, R.A., Hand, M. and Clark, C., 2005. for uranium metallogenesis. Bulletin Cambrian reworking of the southern de Mineralogie, 110(2-3): 235-247. Australian Proterozoic Curnamona Province: constraints from regional Daly, S.J. and Fanning, C.M., 1993. Archaean. shear-zone systems. Journal of the In: J.F. Drexel, W.V. Preiss and A.J. Geological Society, London, 162: 763- Parker (Editors), The geology of South 775. Australia; Volume 1, The Precambrian. . Geological Survey of South Australia, Fanning, C.M., Flint, R.B., Parker, A.J., Bulletin 54, pp. 32-49. Ludwig, K.R. and Blissett, A.H., 1988. Refined Proterozoic evolution Dougherty-Page, J.S. and Bartlett, J.M., 1999. of the Gawler Craton, South Australia, New analytical procedures to increase through U-Pb zircon geochronology. the resolution of zircon geochronology Precambrian Research, 40/41: 363- by the evaporation technique. 386. Chemical Geology, 153(1-4): 227-240. Fanning, C.M., Reid, A.J. and Teale, G.S., Drake, M.J. and Weill, D.F., 1972. New rere 2007. A geochronological framework earth element standards for Electron for the Gawler Craton, South Australia. Microprobe analysis. Chemical South Australia Geological Survey, Geology, 10: 179-181. Bulletin, 55. Duchene, S., Aissa, R. and Vanderhaeghe, Fitzsimons, I.C.W., Kinny, P.D., Wetherley, O., 2006. Pressure-temperature-time S. and Hollingsworth, D.A., 2005. evolution of metamorphic rocks from Bulk chemical control on metamorphic naxos (Cyclades, Greece): Constraints monazite growth in pelitic schists and from thermobarometry and Rb/Sr implications for U-Pb age data. Journal dating. Geodinamica Acta, 19(5): 301- of Metamorphic Geology, 23(4): 261- 321. 277. Duclaux, G., Rey, P., Guillot, S. and Menot, Forbes, C.J., Giles, D., Betts, P.G., Weinberg, R.P., 2007. Orogen-parallel flow R. and Kinny, P.D., 2007. Dating during continental convergence: prograde amphibolite and granulite Numerical experiments and Archean facies metamorphism using in situ field examples. Geology, 35(8): 715- monazite U-PbSHRIMP analysis. 718. Journal of Geology, 115(6): 691-705. Dutch, R., Hand, M. and Kinny, P., 2008. Forster, H.J., 1998. The chemical composition High-grade Palaeoproterozoic of REE-Y-Th-U-rich accessory reworking in the southeastern Gawler

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minerals in peraluminous granites of metamorphism in MacRobertson and the Erzgebirge-Fichtelgebirge region, kemp lands, east Antarctica. Journal of Germany, Part I: The monazite- Metamorphic Geology, 25(6): 683-701. (Ce)-brabantite solid solution series. American Mineralogist, 83(3-4): 259- Hand, M., Bendall, B.R. and Sandiford, M., 272. 1995. Metamorphic evidence for Palaeoproterozoic oblique convergence Foster, G., Kinny, P., Vance, D., Prince, C. in the eastern Gawler Craton. and Harris, N., 2000. The significance Geological Society of Australia, of monazite U-Th-Pb age data in Abstracts, 40: 59. metamorphic assemblages; a combined study of monazite and garnet Hand, M., Dirks, P., Powell, R. and Buick, chronometry. Earth and Planetary I.S., 1992. How Well Established Science Letters, 181(3): 327-340. Is Isobaric Cooling in Proterozoic Orogenic Belts - an Example from Foster, G. et al., 2004. The generation of the Arunta Inlier, Central Australia. prograde P-T-t points and paths; Geology, 20(7): 649-652. a textural, compositional, and chronological study of metamorphic Hand, M., Mawby, J., Kinny, P.D. and Foden, monazite. Earth and Planetary Science J., 1999. U-Pb ages from the Harts Letters, 228(1-2): 125-142. Range, central Australia; evidence for early Ordovician extension Franz, G., Andrehs, G. and Rhede, D., 1996. and constraints on Carboniferous Crystal chemistry of monazite and metamorphism. Journal of the xenotime from Saxothuringian- Geological Society of London, 156 Moldanubian metapelites, NE Bavaria, Part 4: 715-730. Germany. European Journal of Mineralogy, 8: 1097-1118. Harley, S.L. and Fitzsimons, I.C.W., 1991. Pressure-temperature evolution Frei, D., Harlov, D., Dulski, P. and Ronsbo, J., of metapelitic granulites in a 2005. Apatite from Durango (Mexico) polymetamorphic terrane: the Rauer - A potential standard for in situ trace Group, East Antarctica. Journal of element analysis of phosphates, pp. Metamorphic Geology, 9: 231-243. A794-A794. Hoek, J.D. and Schaefer, B.F., 1998. Goscombe, B., Grey, D. and Hand, M., 2005. Palaeoproterozoic Kimban mobile Extrusional Tectonics in the Core of a belt, Eyre Peninsula; timing and Transpressional Orogem; the Kaoko significance of felsic and mafic Belt, Namibia. Journal of Petrology, magmatism and deformation. 45(6): 1203-1241. Australian Journal of Earth Sciences, 45(2): 305-313. Goscombe, B., Hand, M., Gray, D. and Mawby, J., 2003. The metamorphic Holdsworth, R.E., Hand, M., Miller, J.A. architecture of a transpression orogen; and Buick, I.S., 2001. Continental the Kaoko Belt, Namibia. Journal of reactivation and reworking: an Petrology, 44(4): 679-711. introduction. In: Miller, J.A., Holdsworth, R.E., Buick, I.S. & Hand, Halpin, J.A., Clarke, C.L., White, R.W. M. (eds) Continental Reactivation and Kelsey, D.E., 2007. Contrasting and reworking. Geological Society, P-T-t paths for neoproterozoic London, Special Publications, 184: 1-

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12. Antarctica. Journal of Metamorphic Geology, 21: 739-759. Horstwood, M.S.A., Foster, G., Parrish, R.R., Noble, S.R. and Nowell, G.M., 2003. Kohn, M.J., 2008. P-T-t data from central Common-Pb corrected in-situ U-Pb Nepal support critical taper and accessory mineral geochronology by repudiate large-scale channel ﬂow LA-MC-ICP-MS. Journal of Analytical of the Greater Himalayan Sequence. and Atomic Spectroscopy, 18: 837- Geological Society of America 846. Bulletin, 120(3-4): 259-273. Jercinovic, M.J. and Williams, M.L., 2005. Ludwig, K.R., 2003. User’s manual for Analytical perils (and progress) in ISOPLOT 3, A Geochronological electron microprobe trace element Toolkitfor Microsoft Excel. Berkeley analysis applied to geochronology: Geochronology Centre, Berkeley, CA, Background aquisition, interferences Special Publication No. 4. and beam irradiation effects. American Mineralogist, 90: 526-546. Mawby, J., Hand, M. and Foden, J., 1999. Sm- Nd evidence for high-grade Ordovician Jonckheere, R., Mars, M., Vandenhaute, P., metamorphism in the Arunta Rebetez, M. and Chambaudet, A., Block, central Australia. Journal of 1993. Durango Apatite (Mexico) Metamorphic Geology, 17(6): 653-668. - an Analysis of an Age Standard for the Fission-Track Dating Method. Meldrum, A., Boatner, L.A. and Ewing, R.C., Chemical Geology, 103(1-4): 141-154. 1997. Displacive radiation effects in the monazite- and zircon-structure Jung, S., Kroner, A. and Kroner, S., 2007. A orthophosphates. Physics Reviews B, ~ 700 Ma Sm-Nd gamet-whole rock 56: 13805-13814. age from the granulite facies Central Kaoko Zone (Namibia): Evidence for Mezeme, E.B., Cocherie, A., Faure, M., a cryptic high-grade polymetamorphic Legendre, O. and Rossi, P., 2006. history? Lithos, 97(3-4): 247-270. Electron microprobe monazite geochronology of magmatic events: Karioris, F.G., Gowda, K. and Cartz, L., Examples from Variscan migmatites 1991. Heavy ion bombardment and granitoids, Massif Central, France, on monoclinic ThSiO4, ThO2 and pp. 276-288. monazite. Radiation Effects Letters, 58: 1-3. Montel, J.M., Devidal, J.L. and Avignant, D., 2002. X-ray diffraction study of Kelsey, D.E., Clark, C. and Hand, M., 2008. brabantite-monazite solid solutions, Thermobarometric modelling of pp. 89-104. zircon and monazite growth in melt- bearing systems: examples using Montel, J.M., Foret, S., Veschambre, M., model metapelitic and metapsammitic Nicollet, C. and Provost, A., 1996. granulites. Journal of Metamorphic Electron microprobe dating of Geology, 26: 199-212. monazite. Chemical Geology, 131: 37- 53. Kelsey, D.E., White, R.W., Powell, R., Wilson, C.J.L. and Quinn, C.D., 2003. Montel, J.M., Kornprobst, J. and Vielzeuf, New constraints on metamorphism D., 2000. Preservation of old U-Th-Pb in the Rauer Group, Prydz Bay, east ages in shielded monazite; example from the Benmi Bousera Hercynian

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kinzigites (Morocco). Journal of Precambrian Research, 148(3-4): 275- Metamorphic Geology, 18(3): 335-342. 291. Mortimer, G.E., Cooper, J.A. and Oliver, R.L., Payne, J.L., Hand, M., Barovich, K.M. 1988. The geochemical evolution and Wade, B.P., 2008. Temporal of Proterozoic granitoids near Port constraints on the timing of high-grade Lincoln in the Gawler orogenic domain metamorphism in the northern Gawler of South Australia. In: A.I. Wyborn Craton: implications for assembly of Lesley and M.A. Etheridge (Editors), the Australian Proterozoic. Australian The early to middle Proterozoic of Journal of Earth Sciences, 55(5): 623- Australia. Precambrian Research. 640. Elsevier, Amsterdam, International, pp. 387-406. Podor, R. and Cuney, M., 1997. Experimental study of Th-bearing LaPO4 (780°C, Parker, A.J., 1980. The Kalinjala mylonite 200 MPa): Implications for monazite zone, eastern Eyre Peninsula. and actinide orthophosphate stability. Geological Survey of South Australia American Mineralogist, 82(7-8): 765- Quarterly Geological Notes, 76: 6-11. 771. Parker, A.J., 1993. Palaeoproterozoic. In: J.F. Poitrasson, F., Chenery, S. and Bland, Drexel, W.V. Preiss and A.J. Parker D.J., 1996. Contrasted monazite (Editors), The geology of South hydrothermal alteration mechanisms Australia; Volume 1, The Precambrian. and their geochemical implications. Bulletin 54- Geological Survey of Earth and Planetary Science Letters, South Australia, pp. 50-105. 145(1-4): 79-96. Parker, A.J. and Lemon, N.M., 1982. Poitrasson, F., Chenery, S. and Shepherd, Reconstruction of the early Proterozoic T.J., 2000. Electron microprobe stratigraphy of the Gawler Craton, and LA-ICP-MS study of monazite South Australia. Journal of the hydrothermal alteration; implications Geological Society of Australia, 29(1- for U-Th-Pb geochronology and 2): 221-238. nuclear ceramics. Geochimica et Cosmochimica Acta, 64(19): 3283- Parrish, R.R., 1990. U-Pb dating of monazite 3297. and its application to geological problems. Canadian Journal of Earth Pyle, J.M. and Spear, F.S., 2003. Four Sciences, 27(11): 1431-1450. generations of accessory-phase growth in low-pressure migmatites Payne, J.L., 2008. Palaeo- to Mesoproterozoic from SW New Hampshire. American Evolution of the Gawler Craton, Mineralogist, 88: 338-351. Australia: Geochronological, geochemical and isotopic constraints. Pyle, J.M., Spear, F.S., Wark, D.A., Daniel, Ph.D Thesis, University of Adelaide, C.G. and Storm, L.C., 2005. Adelaide. Contributions to precision and accuracy of monazite microprobe ages. Payne, J.L., Barovich, K.M. and hand, M., American Mineralogist, 90: 547-577. 2006. Provenance of metasedimentary rocks in the northern Gawler Rapp, R.P., Ryerson, F.J. and Miller, C.F., Craton, Australia: Implications for 1987. Experimental evidence bearing Palaeoproterozoic reconstructions. on the stability of monazite during

41 Chapter 2 EPMA Monazite Dating

crustal anatexis. Geophysical Research analysis; from sample preparation Letters, 14: 307-310. to microprobe age dating and REE quantiﬁcation. Schweizerische Rapp, R.P. and Watson, E.B., 1986. Monazite Mineralogische und Petrographische solubility and dissolution kinetics - Mitteilungen = Bulletin Suisse de implications for the thorium and light Mineralogie et Petrographie, 80(1): 93- rare-earth chemistry of felsic magmas. 105. Contributions to Mineralogy and Petrology, 94: 304-316. Schwarz, M., 2003. Lincoln, South Australia, explanatory notes: 1:250000 geological Reid, A., Vassallo, J.J., Wilson, C.J.L. and series, sheet SI53-11. Geological Fanning, C.M., 2007. Timing of the Survey of South Australia. Kimban Orogeny on southern Eyre Peninsula. South Australia, Department Scrimgeour, I. and Close, D., 1999. Regional of Primary Industries and Resources. high-pressure metamorphism during Report Book, 2007/5. intracratonic deformation; the Petermann Orogeny, central Australia. Reid, A.J., Payne, J.L. and Wade, B.P., 2006. Journal of Metamorphic Geology, 17: A new geochronological capability for 557-572. South Australia: U-Pb zircon dating via LA-ICPMS. MESA Journal, 42: Seydoux-Guillaume, A., Paquette, J.L., 27-31. Wiedenbeck, M., Montel, J.M. and Heinrich, W., 2002. Experimental Rubatto, D., Williams, I.S. and Buick, I.S., resetting of the U-Th-Pb systems in 2001. Zircon and monazite response monazite. Chemical Geology, 191: to prograde metamorphism in the 165-181. Reynolds Range, central Australia. Contributions to Mineralogy and Spear, F.S., 1993. Metamorphic phase Petrology, 140: 458-468. equilibria and pressure-temperature- time paths. Monograph series, 1. Santosh, M., Yokoyama, M., Tsutsumi, Y. Mineralogical Society of America, and Yoshikura, S.I., 2008. Electron Washington, D.C. microprobe dating of monazites from an ultrahigh-temperature granulite in Spear, F.S. and Pyle, J.M., 2002. Apatite, Southern India: Implications for the monazite, and xenotime in timing of Gondwana assembly. Journal metamorphic rocks. In: J. Kohn of Mineralogical and Petrological Matthew, J. Rakovan and M. Sciences, 103(2): 77-87. Hughes John (Editors), Phosphates; geochemical, geobiological, and Schaltegger, U., Guex, J., Bartolini, A., materials importance. Schoene, B. and Ovtcharova, M., 2008. Precise U-Pb age constraints for end- Steiger, R.H. and Wasserburg, G.J., 1966. Triassic mass extinction, its correlation Systematics in the 208Pb-232Th, to volcanism and Hettangian post- 207Pb-235U and 206Pb-238U extinction recovery. Earth and systems. Journal of Geophysical Planetary Science Letters, 267(1-2): Research, 71: 6065-6090. 266-275. Stuwe, K. and Ehlers, K., 1997. Multiple Scherrer, N.C., Engi, M., Gnos, E., Jakob, metamorphic events at Broken Hill, V. and Liechti, A., 2000. Monazite Australia. Evidence from chloritoid-

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bearing parageneses in the Nine-Mile Tong, L., Wilson, C.J.L. and Vassallo, J.J., Mine region. Journal of Petrology, 2004. Metamorphic evolution and 38(9): 1167-1186. reworking of the Sleaford Complex metapelites in the southern Eyre Suzuki, K. and Adachi, M., 1991. Peninsula, South Australia. Australian Precambrian provenence and Silurian Journal of Earth Sciences, 51: 571-589. metamorphism of the subonosawa paragneiss in the South Kitakami Vassallo, J.J. and Wilson, C.J.L., 2001. terrane, Northeast Japan, revealed by Structural repetition of the Hutchison the chemical Th-U-total lead isochron Group metasediments, Eyre Peninsula, ages of monazite, zircon and xenotime. South Australia. Australian Journal of Geochemical Journal, 25: 357-376. Earth Sciences, 48(2): 331-345. Suzuki, K. and Adachi, M., 1997. Th, U and Vassallo, J.J. and Wilson, C.J.L., 2002. Pb analytical data of monazites used Palaeoproterozoic regional-scale in the paper “Denudation history of non-coaxial deformation; an example the high T/ P Ryoke metamorphic belt, from eastern Eyre Peninsula, South southwest Japan; constraints from Australia. Journal of Structural CHIME monazite ages of gneisses and Geology, 24(1): 1-24. granitoids”. The Journal of Earth and Planetary Sciences, Nagoya University, Wade, B.P., Hand, M., Maidment, D.W., 44: 103-111. Close, D.F. and Scrimgeour, I.R., 2008. Origin of metasedimentary and igneous Suzuki, K., Adachi, M. and Kajizuka, I., 1994. rocks from the Entia Dome, eastern Electron microprobe observations of Arunta region, central Australia: a U- Pb diffusion in metamorphosed detrital Pb LA-ICPMS, SHRIMP and Sm-Nd monazites. Earth and Planetary Science isotope study. Australian Journal of Letters, 128(3-4): 391-405. Earth Sciences, 55(5): 703-719. Thomson, B.P., 1970. A review of the White, R.W., Powell, R. and Holland, T.J.B., Precambrian and Lower Palaeozoic 2007. Progress relating to calculation tectonics of South Australia. of partial melting equilibria for Royal Society of South Australia, metapelites. Journal of Metamorphic Transactions, 94: 193-221. Geology, 25(5): 511-527. Thoni, M., 2003. Sm-Nd isotope systematics Williams, M.L. and Jercinovic, M.J., 2002. in garnet from different lithologies Microprobe monazite geochronology; (Eastern Alps): age results, and an putting absolute time into evaluation of potential problems for microstructural analysis. Journal of garnet Sm-Nd chronometry. Chemical Structural Geology, 24: 1013-1028. Geology, 194: 353-379. Williams, M.L., Jercinovic, M.J., Goncalves, Thoni, M. and Miller, C.H., 1996. Garnet P. and Mahan, K., 2006. Format and Sm-Nd data from the Saualpe and philosophy for collecting, compiling the Koralpe (Eastern Alps, Austria): and reporting microprobe monazite chronological and P-T constraints ages. Chemical Geology, 225: 1-15. on the thermal and tectonic history. Journal of Metamorphic Geology, 14: Williams, M.L., Jercinovic, M.J. and Terry, 453-466. M.P., 1999. Age mapping and dating of monazite on the electron microprobe;

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deconvoluting multistage tectonic histories. Geology (Boulder), 27(11): 1023-1026. Willigers, B.J.A., Baker, L.A., Krogstad, E.J. and Peate, D.W., 2002. Precise and accurate in situ Pb-Pb dating of apatite, monazite and sphene by laser ablation multiple-collector ICP-MS. Geochemica Et Cosimichimica Acta, 66(6): 1051-1066. Wolf, M.B. and London, D., 1995. Incongruent dissolution of REE- and Sr- rich apatite in peraluminous granitic liquids: differential apatite, monazite and xenotine solubilities during anatexis. American Mineralogist, 80: 765-775.

Appendix 2.1. Sample Locations

Sample Easting Northing FB 10 563489 6136971 FB 15 563486 6136923 FB 19 563359 6136738 Datum WGS 84 Zone 53

44 Chapter 3 High-grade Palaeoproterozoic reworking in the southeastern Gawler Craton, South Australia. Published as: Dutch, R., Hand, M. and Kinny, P., 2008. High-grade Palaeoproterozoic reworking in the southeastern Gawler Craton, South Australia. Australian Journal of Earth Sciences, 55: 1063-1081.

Abstract

SHRIMP U-Pb geochronology and monazite EPMA chemical dating from the southeast Gawler Craton has constrained the timing of high grade reworking of the early Palaeoproterozoic (c. 2450 Ma) Sleaford Complex during the Palaeoproterozoic Kimban Orogeny. SHRIMP monazite geochronology from mylonitic and migmatitic high-strain zones that deform the c. 2450 Ma peraluminous granites indicates that they formed at 1721 ± 3 and 1725 ± 2 Ma. These are within error of EPMA monazite chemical ages of the same high-strain zones which range between 1691 Ma and 1736 Ma. SHRIMP dating of titanite from peak metamorphic (10 kbar at 730°C) maﬁc assemblages give ages of 1712 ± 8 and 1708 ± 12 Ma. The post-peak evolution is constrained by partial to complete replacement of garnet – clinopyroxene bearing maﬁc assemblages by hornblende – plagioclase symplectites, which record conditions of c. 6 kbar at 700°C, implying a steeply decompressional exhumation path. The timing of Palaeoproterozoic reworking corresponds to widespread deformation along the eastern margin of the Gawler Craton and the development of the Kalinjala Shear Zone.

3.1 Introduction affected the craton (Fanning et al., 1988; Daly et al., 1998; Ferris et al., 2002; Fanning et al., The Gawler Craton, in southern Australia 2007). (Fig. 3.1), has a protracted tectonic, Two of the most widespread tectonic metamorphic and magmatic history spanning intervals represented on the Gawler Craton the late Archaean to the Mesoproterozoic are the Late Archaean to earliest Proterozoic (e.g. Daly et al., 1998; Fanning et al., 2007). Sleafordian Orogeny (c. 2440-2420 Ma, Despite being a region of exceptional economic Fanning et al., 1988; Daly and Fanning, 1993; potential the evolutionary framework for the Fanning et al., 2007) and the Palaeoproterozoic Gawler Craton is still poorly constrained. Many Kimban Orogeny (c. 1730–1690 Ma, Fanning of the ambiguities surrounding the evolution of et al., 2007). Previous workers (e.g. Fanning the Gawler Craton revolve around the timing, et al., 1988; Daly and Fanning, 1993) have spatial distribution and thermobarometric suggested that the Late Archaean core of the conditions of the tectonic events, which have Gawler Craton has been ubiquitously affected

Dutch, R., Hand, M. and Kinny, P.D. (2008) High-grade Paleoproterozoic reworking in the southeastern Gawler Craton, South Australia. Australian Journal of Earth Sciences, v.55 (8) pp. 1063-1081, December 2008

NOTE: This publication is included on pages 45 - 74 in the print copy of the thesis held in the University of Adelaide Library.

It is also available online to authorised users at:

http://dx.doi.org/10.1080/08120090802266550

Chapter 4 Tectonothermal evolution of reworked Archaean granulite-facies metapelites in the southern Gawler Craton, Australia.

Abstract

The Shoal Point region of the southern Gawler Craton consists of a series of reworked granulite- facies metapelitic and metaigneous units belonging to the Late Archaean Sleaford Complex.

Structural evidence indicates three phases of fabric development with D1 retained within boudins,

D2 consisting of upright open to isoclinal folds producing an axial planar fabric and D3, a highly planar vertical high-strain fabric that overprints the D2 fabric. Chemical EMPA monazite and garnet

Sm-Nd geochronology constrain the D1 event to the c. 2450 Ma Sleaford Orogeny while the D2 the

D3 events are constrained to the 1730–1690 Ma Kimban Orogeny. P-T pseudosections constrain the metamorphic conditions for the Sleafordian Orogeny to between 4.5–6 kbar and 750–780 °C. Subsequent Kimban-aged reworking reached peak metamorphic conditions of 8–9 kbar at ° between 820–850 C during the D2 event. This was followed by near isothermal decompression to ° metamorphic conditions <6 kbar and 790–850 C associated with the development of the D3 high- strain fabric. The P-T-t evolution of the Shoal Point rocks reﬂects transpressional exhumation of lower crustal rocks during the Kimban Orogeny and the development of a regional transpressional ﬂower structure.

4.1 Introduction 1999; Pyle and Spear, 2003; Rutherford et al., 2006; Sheppard et al., 2007; Simmat and Unravelling the tectonic evolution of Raith, 2008). The lack of direct geochronology polydeformed terranes is commonly an can lead to a misreading of the geological extremely difﬁcult task (e.g. Butler et al., record, for example with temporally distinct 1997; Holdsworth et al., 1997; Mawby et al., metamorphic events being linked to create a 1999; Holdsworth et al., 2001; Rutherford et false P-T evolution (e.g. Dirks et al., 1991; Hand al., 2006; Sarkar et al., 2007; Sheppard et al., et al., 1992; Dutch et al., 2005). This process is 2007; Simmat and Raith, 2008). It requires an made more difﬁcult in highly deformed terrains integrated approach, where detailed structural where the original fabric and the overprinting mapping and observations are coupled with fabric both formed under similar metamorphic petrologic and metamorphic analysis, all conditions. constrained by geochronology. The application The Sleaford Complex (Thomson, 1970; of detailed geochronology is vital in any attempt Thomson, 1980), in the southern Gawler Craton to differentiate between different events within (Fig. 4.1), is an example of a highly deformed a reworked rock package (e.g. Mawby et al., and polymetamorphic terrain, with the added

Dutch, R. et al. Tectonothermal evolution of reworked Archaean granulite-facies metapelites in the southern Gawler Craton, Australia. Not yet published.

NOTE: This publication is included on pages 75 - 114 in the print copy of the thesis held in the University of Adelaide Library.

Chapter 5 The tectonothermal evolution of the crustal scale Kalinjala Shear Zone, southern Gawler Craton, Australia

Abstract

The Kalinjala Shear Zone, in the southern Gawler Craton, is a crustal scale structure which forms the main structural element of the poorly exposed Kimban Orogen. Samples from the core and the flank of the shear zone record a similar structural development with an initially dextrally transpressive system resulting in a layer parallel migmatitic gneissic to mylonitic KS1 fabric which was subsequently deformed and reworked by upright folds and discrete KD2 east-side-down sub- solidus mylonitic shear zones during east-west compression. In-situ EPMA monazite and garnet Sm-Nd geochronology constrain the timing of deformation and metamorphism along the Kalinjala Shear Zone to the craton-wide Kimban Orogeny between 1720 and 1700 Ma. Metamorphic P-T analysis and pseudosections constrain the peak M1 conditions in the core of the shear zone to 10–11 kbar at c. 800 °C reflecting deep crustal conditions at depths of up to 30 km. On the flank ° of the shear zone the M 1 conditions reached 6–7 kbar at 750 C followed by sub-solidus reworking ° during KD2 at conditions of 3–4 kbar at 600–660 C, suggesting a maximum burial of < 24 km. Cooling rates determined from garnet zoning profiles suggest that the core of the shear zone cooled at rates in excess of 40–80 °CMa-1 while the flank underwent much slower cooling at < 10°CMa-1. The rapid cooling and inferred decompression in the core of the shear zone reflects rapid burial and exhumation of lower-crustal material into the mid-crust during the Kimban Orogeny along the Kalinjala Shear Zone. The absence of evidence for extension indicates that differential exhumation and the extrusion of lower-crustal material into the mid-crust was driven by transpression along the Kalinjala Shear Zone and highlights the role of transpression in creating large variations in vertical exhumation over relatively short lateral extents.

5.1 Introduction deformation and metamorphism (e.g. Thoni and Miller, 1996; Hand et al., 1999; Mawby Understanding the tectonic evolution of et al., 1999; Goscombe et al., 2003; Kelsey et orogenic belts requires an holistic approach, al., 2003; Muller, 2003; Duchene et al., 2006; where the structural development of the orogen Yin, 2006; Rigby et al., 2008). This holistic is coupled with a thorough metamorphic approach is especially important in poorly analysis of the P-T evolution, including cooling exposed terranes, where as much information and exhumation rates, together with direct as possible is required to attempt to unravel the geochronological constraints on the timing of tectonic history. A number of studies describing

115

Dutch, R. et al. The tectonothermal evolution of the crustal scale Kalinjala Shear Zone, southern Gawler Craton, Australia Not yet published.

NOTE: This publication is included on pages 115 - 155 in the print copy of the thesis held in the University of Adelaide Library.

Chapter 6 Retention of Sm-Nd isotopic ages in garnets subjected to high-grade thermal reworking: Implications for diffusion rates of major and rare earth elements and the Sm-Nd closure temperature.

Abstract

Garnet is a vital mineral for determining constrained P-T-t paths as it can give both the P-T and t information directly. However, estimates of the closure temperature of the Sm-Nd system in garnet vary considerably leading to significant uncertainties in the timing of peak conditions. In this study five igneous garnets from a 2414 ± 6 Ma garnet–cordierite bearing S-type granite, subjected to high-T reworking, have been dated to examine their diffusional behaviour in the Sm-Nd system. Garnets of 8, 7, 6, 6 and 2.5 mm diameter were compositionally profiled and then dated, producing two point Sm-Nd isochron ages of 2412 ± 10 Ma, 2377 ± 5 Ma, 2370 ± 5 and 2365 ± 8 Ma and 2313 ± 11 Ma respectively. A direct correlation exists between grain size and amount of resetting highlighting the effect of grain size on closure temperature. Major element EPMA and LA-ICP- MS REE traverses reveal homogoneous major element profiles and relict igneous REE profiles. The retention of REE zoning and homogenisation of major element zoning suggests that diffusion rates of REEs are considerably slower than that of the major cations in garnet. The retention of REE zoning and the lack of resetting in the largest grains suggests that Sm-Nd closure temperature in garnet is a function of grain-size, thermal history and REE zoning in garnet.

6.1 Introduction descriptions (e.g. Foster et al., 2000; Williams and Jercinovic, 2002; Pyle and Spear, 2003; Reconstructing the pressure-temperature-time Gibson et al., 2004; Dutch et al., 2005; Kelsey (P-T-t) evolution of metamorphosed rocks forms et al., 2007; Dutch et al., 2008). Attempting to a vital part of unravelling tectonic processes such establish a link between the timing of accessory as orogenesis (e.g. Brown, 1993; Mawby et al., mineral growth and particular points on a given 1999; Foster et al., 2004; Duchene et al., 2006; P-T path remains a challenge due to a number Halpin et al., 2007; Kelsey et al., 2007; Jessup et of uncertainties (e.g. Lanzirotti and Hanson, al., 2008; Kohn, 2008). The traditional approach 1996; Carson et al., 2002; Fitzsimons et al., to date the timing of high-grade metamorphism 2005; Kelly et al., 2006; Harley and Kelly, is to date U-Th rich accessory minerals (e.g. 2007; Hinchey et al., 2007; Kelsey et al., 2008). zircon, monazite, titanite, baddeleyite, apatite) For example, the mechanism and temperature and attempt to link the age of these minerals to at which accessory minerals crystallise or the thermobarometric results and petrographic recrystallise is often difﬁcult to asses or poorly

157

Dutch, R. and Hand, M. (2009) Retention of Sm–Nd isotopic ages in garnets subjected to high-grade thermal reworking: implications for diffusion rates of major and rare earth elements and the Sm–Nd closure temperature in garner. Contributions to Mineralogy and Petrology, published online 10th July 200, 20 pp.

NOTE: This publication is included on pages 157 - 186 in the print copy of the thesis held in the University of Adelaide Library.

It is also available online to authorised users at:

http://dx.doi.org/10.1007/s00410-009-0418-1

Chapter 7 Summary

7.1 Introduction The methods developed and utilised in this study are also directly relevant to any The aim of this thesis was to develop a better region with limited exposure. Targeted in-situ understanding of the timing, distribution and microanalysis and geochronology combined tectonothermal evolution of the major orogenic with metamorphic phase-equilibria modelling systems expressed in the southern Gawler as applied here can help refine the P-T-t Craton. This process involved the development evolution of poorly exposed and constrained of in-situ Electron Microprobe (EPMA) terraines world-wide. monazite geochronology and a study into the diffusion properties of the major and rare earth 7.2 Thesis Summary elements in garnets subjected to reworking to appraise the use of the Sm-Nd geochronometer 7.2.1 In-situ EPMA monazite Geochronology in reworked assemblages. Both of these as a tool for constraining event timing in techniques were utilised to constrain the timing reworked assemblages of deformation and metamorphism in the complexly reworked rock units of the southern Chapter 2 presented a comprehensive Gawler Craton. The structural and metamorphic analytical procedure and protocol for in-situ evolution of the southern Gawler Craton EPMA monazite geochronology developed on was then constrained using a combination of a Cameca SX51 Electron Microprobe housed these geochronological techniques, together at Adelaide Microscopy in the University with detailed petrographic, metamorphic and of Adelaide. In-situ EPMA monazite structural analysis, applied to specific regions geochronology is particularly suited to of interest. The specific regions were chosen as constraining ages within complexly reworked they represent the different lithologies across units. The small spatial resolution allows the southern Gawler Craton, from the upper- for the dating of small mineral grains and crustal Sleaford Complex units of the Dutton the in-situ nature of the analysis allows for Suite in Coffin Bay to the lower-crustal units individually dated grains to be microstructurally of the Sleaford Complex at Shoal Point to the constrained. overlying metasediments of the Hutchison In order to verify the reliability and validity Group at Port Neill and Mine Creek. The results of the technique, a comparative study using of this study form a significant component of a four previously isotopically dated samples larger collaborative effort to develop a coherent was conducted. The results showed a good and constrained geological framework for the correlation between the isotopically derived evolution of the Gawler Craton. ages and the ages obtained via EPMA monazite

187 Chapter 7 Summary geochronology, with a discrepancy of less than Coffin Bay Peninsula and the P-T conditions 1%, indicating the robustness of the technique. of the metamorphism recorded in the deformed The technique developed here has the metabasic dykes. added advantage of collecting a large amount EPMA monazite and SHRIMP monazite of additional chemical data from individual and titanate geochronology indicates that grains These data allow for the ability to link a series of predominantly N-NE trending the growth of accessory phases with major migmatitic and mylonitic shear zones developed phases in the assemblage, thereby providing an on the Coffin Bay Peninsula between 1725 additional control in constraining the timing of and 1700 Ma during the Kimban Orogeny. metamorphic mineral growth. Even though the Granulite facies metamorphism at this time age precision of the EPMA monazite technique is recorded in deformed and metamorphosed is generally not sufficient to resolve individual mafic dykes, which intrude the peraluminous ‘events’ within a single orogeny, it provides Sir Isaac Granite. Peak metamorphism was an efficient way to add geochronological data followed by near-isothermal decompression to any determined P-T path, and allows for during the exhumation and juxtaposition of rapid verification of the ages of deformation these lower-crustal rocks against the upper- and metamorphism for the individual crustal Price Metasediments along a regional assemblages which are used to constrain them. scale interpreted shear system. These results It is particularly useful in regions such as the imply that the metamorphic architecture of the Gawler Craton, which has undergone a number Kimban Orogen in the south-eastern Gawler of widely spaced (in geological time) reworking Craton is defined by panels of dramatically events, for differentiating between the effects contrasting metamorphic grade, reflecting large of each orogenic event and therefore reducing local gradients in the degree of exhumation. the risk of producing an invalid, amalgamated P-T path. 7.2.3 Reworking the Sleaford Complex

7.2.2 Reworking the southeastern Gawler Chapter 4 presents an integrated geological Craton study into the evolution of the Carnot Gneiss from the Shoal Point region of the southern Chapter 3 presented EPMA and SHRIMP U- Gawler Craton. A combination of structural Pb monazite geochronology from shear zones mapping and quantitative metamorphic and SHRIMP U-Pb titanite geochronology from modelling of metapelitic assemblages was maﬁc assemblages which constrain the timing used to constrain the evolution of this region. of high-grade reworking in the upper-crustal This was then temporally constrained by a Dutton Suite of the late-Archaean Sleaford combination of in-situ EMPA monazite and Complex. This was coupled with structural garnet Sm-Nd geochronology. mapping and thermobarometric analysis to The results show that the late Archaean determine the geological evolution of the Sleaford Complex orthogneisses and

188 Chapter 7 Summary metapelitic units at Shoal Point contain a are compared with those from the ﬂank of the complex polymetamorphic history representing shear zone at Mine Creek, highlighting the two granulite-facies events. The earliest event variation in metamorphic pressures across the is recorded by granulite-facies assemblages shear zone. contained within boudins, protected from The results of targeted in-situ EPMA subsequent deformation. Geochronology monazite geochronology has constrained the places this metamorphic event at c. 2455 Ma timing of deformation and metamorphism which is interpreted to be the early stages of along the crustal-scale Kalinjala Shear Zone the Sleafordian Orogeny. The metamorphic to the craton-wide Kimban Orogeny between conditions of between 4.5–6 kbar and 750–780 1720 and 1700 Ma. In this region the Kimban °C represent the ﬁrst quantitative constraints Orogeny developed as a dextrally transpressive on the Sleafordian Orogeny from the southern system resulting in a layer parallel migmatitic

Gawler Craton. gneissic to mylonitic KS1 fabric which was Subsequent deformation has been subsequently deformed and reworked by temporally constrained to the 1730–1690 Ma upright folds and discrete KD2 east-side-down Kimban Orogeny. Early Kimban deformation sub-solidus mylonitic shear zones during east- resulted in upright, open to tight, shallowly west compression. Metamorphic P-T analysis south plunging folds producing a weak KS1 and pseudosections constrain the peak M1 axial planar fabric. This was associated with conditions in the core of the Kalinjala Shear peak metamorphic conditions of 8–9 kbar at Zone to 10–11 kbar at c. 800 °C reflecting between 820–850 °C. This was followed by lower crustal conditions. Forty kilometres to late Kimban-aged sub-horizontal east-west the south at Mine Creek the M 1 conditions only flattening producing isoclinal folds and vertical, reached 6–7 kbar at 750 °C followed by sub- planar high-strain zones which overprint and solidus reworking during KD2 at conditions of ° crosscut the KS1 fabric. This was associated with 3–4 kbar at 600–660 C. Rapid cooling and near isothermal decompression to metamorphic inferred decompression of the Port Neill section conditions <6 kbar and 790–850 °C. units reflects rapid burial and emplacement of lower-crustal material into the mid-crust during 7.2.4 Evolution of the Kalinjala Shear Zone the Kimban Orogeny along the Kalinjala Shear Zone while slow cooling and high geothermal Chapter 5 presents an integrated structural, gradients in the flank of the shear zone at Mine metamorphic and geochronological study from Creek suggest slower exhumation and longer two well exposed areas of the crustal scale residence times at mid-crustal depths. The Kalinjala Shear Zone. Pressure-temperature absence of evidence for extension indicates that constraints from thermobarometry and P-T the differential exhumation and the extrusion pseudosections together with cooling rates of lower-crustal material into the mid-crust derived from garnet zoning profiles, from the was primarily driven by transpression along the core of the Kalinjala Shear Zone at Port Neill Kalinjala Shear Zone.

189 Chapter 7 Summary

garnet, in disagreement with results published 7.2.5 Garnet Sm-Nd isotope systematics from experimental studies. The ability of garnet to record growth ages is a function of grain- Chapter 6 presents a natural diffusion size, thermal history and, importantly, on the experiment exploring aspects of the closure way the REEs are zoned within garnet. Garnets temperature of the Sm-Nd system in garnet with medium to heavy REE enriched cores are and the comparative rates of diffusion of major more likely to retain their peak ages compared and RE elements in garnet. The Point Sir Isaac with garnets that show medium to heavy REE granite contains a magmatic garnet – cordierite enriched rims. This suggests that the utility of bearing assemblage that formed during granite Sm-Nd dating of garnet is strongly controlled crystallisation. The age of granite crystallisation by the growth behaviour of medium to heavy has been constrained to 2414 ± 6 Ma by LA- REE partitioning accessory minerals. ICP-MS monazite U-Pb geochronology. The Sir Isaac Granite underwent thermal reworking 7.3 Constraints on the Southern Sleafordian at conditions of c. 750 °C and 10 kbar during the Orogeny Kimban Orogeny at c. 1720 Ma. Five individual garnets of different sizes were analysed by major Previous workers have been divided on element and REE profiling and isotopically the age of granulite-facies metamorphism dated using the Sm-Nd system. recorded in the late Archaean Sleaford The results show that the Sm-Nd garnet ages Complex basement. Some workers believed from magmatic garnets display a systematic the assemblages were entirely Sleafordian in relationship with grain-size. The largest age and that the Kimban overprint was lower garnet (8 mm diameter) effectively records grade (e.g. Fanning et al., 1981; Fanning et al., the magmatic age of 2412 ± 10 Ma, while the 1988; Daly and Fanning, 1993) while others smaller garnets preserve slightly younger ages have suggested the assemblages are purely (2377 ± 5 Ma for a 7 mm grain, 2370 ± 5 and a result of the Kimban Orogeny and there is 2365 ± 8 for two 6 mm grains and 2313 ± 11 no metamorphic evidence for a high-grade for a 2.5 mm grain). This suggests the Sm-Nd Sleafordian event (e.g. Vassallo and Wilson, system remained approximately closed during 2002; Tong et al., 2004). high-grade reworking. A study by Duclaux et al. (2007) used Major elements in garnet show homogeneous, numerical models to investigate the evolution flat zoning profiles. Isopleths for composition in of warm continental lithosphere during garnet on a modelled P-T pseudosection indicate convergence and subsequent unloading, using that the major elements represent Sleafordian the southern Gawler Craton as their primary (magmatic) compositions. REE profiles show field example. They posited that the north- that the garnets preserve primary, growth zoned south trending fabrics in the southern Gawler REE patterns, suggesting that REE diffusion is Craton were the result of lateral, orogen- considerably slower than the major elements in parallel, collapse during the waning stages of

190 Chapter 7 Summary

Sleafordian-aged deformation. Their argument Gawler Craton) which occurred at c. 2450– is constrained by an EPMA age of 2479 ± 20 Ma 2443 Ma, constrained by SHRIMP zircon for monazite included within porphyroblastic (Fanning, 2002) and monazite (McFarlane, garnet and an age of 1827 ± 10 Ma for a late, 2006). Metamorphic constraints from the un- cordierite-bearing, granitic dyke from an reworked Challenger Gneiss indicate peak outcrop of the Sleaford Complex in the central metamorphic conditions of 800–850 °C at 7.5 Eyre Peninsula. From this they suggest that 2479 ± 1.5 kbar (Tomkins and Mavrogenes, 2002), ± 20 Ma is the timing of fabric development which are consistent with conditions of 750–800 and that deformation had ceased prior to 1827 °C at 4.5–5.5 kbar obtained by Teasdale (1997) ± 10 Ma. Dutch et al. (2007) suggested that the from the western Mulgathing Complex. These 2479 ± 20 Ma age for garnet included monazite P-T conditions equate to geothermal gradients indicates high-grade metamorphism during the in excess of 30 and 43 °Ckm-1 respectively. Sleafordian Orogeny but contested that the bulk These results suggest the Sleafordian Orogeny of the structural and geochronological data from was a regional event, characterised by elevated the southern Gawler Craton indicate that these geothermal gradients, which affected the entire north-south orientated fabrics were the result of Archaean core of the Gawler Craton. the Palaeoproterozoic Kimban Orogeny. The results of the current study support 7.4 The Evolution of the Southern Kimban the assertions of Dutch et al. (2007) and the Orogeny interpretations of Vassallo and Wilson (2002) and Tong et al. (2004) which suggest that the The results of this study allow us to place north-south orientated fabrics in the southern constraints on the tectonothermal evolution of Gawler Craton are a result of the Kimban the Kimban Orogeny. Recent geochronological Orogeny and that the Sleafordian Orogeny has data (Fig. 7.1) from across the Gawler Craton been thoroughly overprinted. (Teasdale, 1997; Hopper, 2001; Betts et al., Granulite-facies metapelitic assemblages 2003; Swain et al., 2005; Payne et al., 2008) contained within boudins in the Kimban-aged and into Antarctica (Goodge et al., 2001; Zeh et fabrics have retained their primary 2455 ± 7 Ma al., 2004; Duclaux et al., 2008) indicate that the Sleafordian-aged metamorphic assemblages. Kimban Orogeny was a signiﬁcant craton wide A P-T pseudosection for this assemblage thermal event. So a clear understanding of the indicates peak metamorphic conditions of tectonothermal evolution of this orogenic event between 4.5–6 kbar and 750–780 °C. These P- is vital for our understanding of the evolution T conditions equate to a minimum geothermal of the Gawler Craton, and Proterozoic Australia gradient of c. 47 °Ckm-1 (based on 1 kbar = as a whole. 3.5 km of burial), indicating metamorphism The geochronology presented in this thesis occurred at signiﬁcantly elevated geothermal constrains the timing of the Kimban Orogeny gradients. This correlates well with high-grade to between 1725–1690 Ma in the southern metamorphism in the Christie Gneiss (northern Gawler Craton. The structural data presented

191 Chapter 7 Summary

132°0'0"E 133°0'0"E 134°0'0"E 135°0'0"E 136°0'0"E 137°0'0"E 138°0'0"E

8 1720 ± 8 Ma 1727 ± 7 Ma 1728 ± 36 Ma 9 9 28°0'0"S

1701 ± 9 Ma 9

29°0'0"S 1713 ± 10 Ma 12 9 1736 ± 14 Ma

Karari Fault Zone 30°0'0"S 9, 111691 ± 10 Ma 1712 ± 10 Ma 10 11 1690 ± 9 Ma

31°0'0"S 10 1727 ± 7 Ma

32°0'0"S ¹ interpreted boundaryinterpreted Gawler Craton to 33°0'0"S Kalinjala Shear Zone 025 50 100 150 200 Kilometers c. 1700 Ma 7

34°0'0"S Key to symbols 1 6 1717-1700 Ma SHRIMP monazite 1724-1706 Ma EPMA monazite SHRIMP zircon 1733-1687 Ma 2 1710-1701 Ma Pb/Pb garnet 3 4 5 LA-ICP-MS monazite 1707-1690 Ma 1713 ± 5 Ma 35°0'0"S

Figure 7.1. 1st VD magnetic intensity image of the Gawler Craton with the distribution of Kimban Orog- eny metamorphic ages. 1, Dutch et al. 2008 (Chapter 3); 2, Chapter 4; 3, Dutch et al. 2007 (Chapter 2); 4, Jagodzinski et al. 2006; 5, Reid et al. 2007; 6, Chapter 5; 7, Fanning et al. 2007; 8, Hopper 2001; 9, Payne et al. 2008; 10, Howard et al. 2008; 11, Teasdale 1997; 12, Finlay 1993

192 Chapter 7 Summary here from across the southern Gawler Craton entire orogen may have behaved in a triclinic complement and extend the work of Vassallo manner, with a shear component parallel to, and and Wilson (2001; 2002). These data suggest a non-coaxial shortening component normal to, the Kimban Orogeny involved two phases of the strike of the orogen. deformation. KD1 is characterised across the The metamorphic evolution of the southern region by the development of north-south Kimban Orogeny is characterised by large trending upright tight to isoclinal folds and variations in the amount of exhumation over meso- and macroscopic sheath folds produced short lateral extents (Fig. 7.2). The metamorphic during a dextral top-to-the-north shearing evolution of the Coffin Bay, Shoal Pt, Fishery event synchronous with north-south stretching. Bay (Tong et al., 2004) and Port Neill regions Fold plunges parallel a primary north-south all display peak medium-pressure high- trending linear fabric defined by prolate-shaped temperature (MPHT) metamorphic conditions mineral aggregates and aligned platy minerals. of 730–850 °C at between 8–11 kbar followed This linear fabric is predominantly shallowly by near isothermal decompression to low- plunging but in places plunges steeply, to the pressure high-temperature (LPHT) conditions north or south. In places a strongly developed, below 6 kbar (Fig. 7.2). This is in stark high-grade, mylonitic fabric is developed which contrast to the Mine Creek region which never grades to a weakly developed axial planar experienced conditions above 6 kbar, and the fabric, both of which are primarily sub-vertical Price Metasediments and Hall Bay Volcanics in orientation. This phase of deformation is which contain andalusite and chloratoid bearing interpreted to have formed in an overall dextral assemblages indicating pressures < 4 kbar (Fig. transpressional system with a component of 7.2). The metamorphic grade decreases to low- sub-horizontal constriction. P amphibolite-facies conditions in the northern The second phase of Kimban-aged Eyre Peninsula (Parker, 1993). These varying deformation (KD2) is characterised by curvilinear metamorphic pressures indicate variations in high-strain to mylonite zones and tight to exhumation on the order of 17–21 km (based on isoclinal upright to overturned folds, grading to 3.5 km per 1 kbar) across horizontal scales of open to tight upright and overturned folds in the 20–30 km, suggesting a nearly 1:1 relationship granitic lithologies of the Dutton Suite, which between vertical exhumation and horizontal overprint the KD1 fabrics. Conjugate shear extent. sets and symmetrically folded mafic dykes Due to the limited outcrop in the southern suggest that the KD2 deformation involved a Gawler Craton it is difficult to accurately component of east-west orientated flattening constrain a model for the development of across the strike of the Kimban Orogeny. the Kimban Orogeny. The lack of evidence

Mineral stretching lineations in KD2 high-strain for post orogenic extension suggests that fabrics vary from steeply to shallowly plunging the exhumation was primarily driven by to predominantly the south. The variation in transpressional deformation during the lineation plunge across the orogen suggests the development of the Kimban Orogeny itself.

193 Chapter 7 Summary

KEY TO SYMBOLS granulite-facies outcrop (>8 kbar peak) granulite-facies outcrop/drill hole (<6 kbar peak) sub amphibolite-facies outcrop/drill hole (<4 kbar peak) interpreted KF1/KF2 fold axis

Kalinjala SZ 34°0'0"S major shear zone

Pt Neill

Drummond Pt SZ < 4 kbars Mine Creek Coffin Bay

Dutton Suite Price MS KD 1 Donnington syn- 2 KF1-2 KF2 1710 ± 6 Ma Shoal Pt KF2 Suite KF1 1701 ± 7 Ma KF2 1702 ± 4 Ma Fishery Bay 1706 ± 3 Ma 35°0'0"S

135°0'0"E 136°0'0"E

10 Pt Neill Peak 10 Coffin Bay c. 1720 Ma KD1 1724-1706 Ma >40°C/Ma 5 KD 5 2 (kbar) P P (kbar) P

10 Fishery Bay2 400 T (°C) 800 400 T (°C) 800 KD1

5 KD2

P (kbar) P 1690-1700 Ma

10 Shoal Pt 10 Mine Creek 1730-1720 Ma KD1 400 T (°C) 800 c. 1710-1720 Ma KD1 KD Key 5 1687 - 1705 Ma 2 5 KD 2 <10°C/Ma P (kbar) P Shear direction (kbar) P

Interpreted PT path c. 1700 Ma Inferred PT path 400 T (°C) 800 400 T (°C) 800 Figure 7.2. Collection of P-T and geochronology data for the southern Gawler Craton displaying the variation in degree of exhumation along strike of the Kimban-aged structural fabrics. The P-T paths suggest the Kimban Orogeny developed along a clockwise P-T path in the southern Gawler Craton. 1 Geo- chronolgy of Reid et al. 2007; 2 P-T data of Tong et al. 2004.

194 Chapter 7 Summary

Initial KD1 deformation resulted in the Gawler Craton is one small part of the reactivation of pre-existing structures into larger Gawler Craton which, in terms of the predominantly dextral, sub-vertical high-strain metamorphic and temporal evolution, has been and mylonite zones, including the crustal scale largely unexplored and would be logical goals Kalinjala Shear Zone, and regional scale open for future work. to isoclinal upright folding during regionally (1) The recent discovery of Kimban dextral transpression (see Fig. 4.16b; Vassallo metamorphic ages from the northern and Wilson, 2001; Vassallo and Wilson, 2002). and western Gawler Craton (Fig. This resulted in basement-involved basin 7.1) have opened up the possibility to inversion burying of much of the basement to constrain the metamorphic evolution of the lower crust and producing the MPHT peak the Kimban Orogeny across the Gawler metamorphic conditions. Craton. At present there has been no

The KD2 phase of deformation, dominated investigation into the metamorphic by sub-horizontal east-west ﬂattening, led to evolution of these units. The methods the development of structures characterised by developed and utilised in this thesis are isoclinal folding and the development of planar highly applicable to regions such as the curvi-linear shear- and mylonite zones (see Fig. northern Gawler Craton, where there is 4.16c; Vassallo and Wilson, 2001; Vassallo and little or no outcrop and all geological Wilson, 2002). Triclinic movement along these investigations must be done through zones has resulted in the exhumation of lower- available drill core samples. crustal units into the mid crust during KD2. The (2) At present there has been little Kimban Orogeny in the southern Gawler Craton exploration of the extent of the Kimban is therefore expressed by an obliquely exposed overprint in the central Gawler Craton. transpressional ‘ﬂower structure’ geometry Rigorous dating of fabrics in this area (e.g. Vassallo and Wilson, 2001; Vassallo and via targeted geochronology may help Wilson, 2002; Chetty and Rao, 2006). This to reveal the extent of the Kimban model provides a mechanism to produce the Orogeny in the Central Gawler Craton. apparent rapid lateral variation in metamorphic (3) These points above, together with grade observed across the southern Gawler further geochronological, structural and Craton, which characterises the metamorphic metamorphic work in Antarctica may architecture of the southern Kimban Orogeny. help to produce further correlations between the southern Australia and 7.5 Future Work Antarctica and further unravel the Kimban Orogeny, providing a clearer The work presented here provides an large scale model for its evolution important contribution to developing a and what it means for Proterozoic consistent constrained geological framework reconstructions. for the Gawler Craton. However, the southern (4) A number of workers have recently

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proposed the possibility that the ages. Precambrian Research, 167: 316- Gawler Craton forms an orocline (e.g. 338. Fraser and Reid, 2008; Stewart and Dutch, R., Hand, M. and Reid, A., 2007. Betts, 2008). Targeted geochronology Orogen-parallel flow during continental in areas such as the western Gawler convergence: Numerical experiments and Archean field examples: Craton, together with structural and COMMENT and REPLY. GEOLOGY, metamorphic analysis, such as used in doi: 10.1130/G24419C.1. this thesis, may help provide links or Dutch, R., Hand, M. and Kinny, P., 2008. High- preclude this hypothesis. grade Palaeoproterozoic reworking in (5) The recent discovery of 3150 Ma granitic the southeastern Gawler Craton, South rocks in the southern Gawler Craton Australia. Australian Journal of Earth Sciences, 55: 1063-1081. (Fraser, 2008) leads to the possibility of other undiscovered timelines existing Fanning, C.M., 2002. Gawler Craton: synthesis of recent SHRIMP U-Pb geochronology. in the Gawler Craton. A systematic Gawler Craton 2002: State of Play, geochronological exploration will help Department of Primary Industries and to resolve the ambiguities in the existing Resources Workshop Proceedings age framework of the Gawler Craton. (CD). Office of Minerals and Energy Resources, South Australia.

References Fanning, C.M., Flint, R.B., Parker, A.J., Ludwig, K.R. and Blissett, A.H., 1988. Refined Betts, P.G., Valenta, R.K. and Finlay, J., 2003. Proterozoic evolution of the Gawler Evolution of the Mount Woods Inlier, Craton, South Australia, through U-Pb northern Gawler Craton, southern zircon geochronology. Precambrian Australia; an integrated structural and Research, 40/41: 363-386. aeromagnetic analysis. Tectonophysics, 366(1-2): 83-111. Fanning, C.M., Reid, A.J. and Teale, G.S., 2007. A geochronological framework Daly, S.J. and Fanning, C.M., 1993. Archaean. for the Gawler Craton, South Australia. In: J.F. Drexel, W.V. Preiss and A.J. South Australia Geological Survey, Parker (Editors), The geology of South Bulletin, 55. Australia; Volume 1, The Precambrian. . Geological Survey of South Australia, Fanning, C.M., Oliver, R.L. and Cooper, J.A., Bulletin 54, pp. 32-49. 1981. The Carnot Gneisses, southern Eyre Peninsula. Quarterly Geological Duclaux, G., Rey, P., Guillot, S. and Menot, Notes - Geological Survey of South R.P., 2007. Orogen-parallel flow during Australia, 80: 7-12. continental convergence: Numerical experiments and Archean field Finlay, J., 1993. Structural interpretation of the examples. Geology, 35(8): 715-718. Mount Woods Inlier. Monash University. B.Sc (Honours) thesis (Unpublished). Duclaux, G. et al., 2008. Superimposed Neoarchaean and Palaeoproterozoic Fraser, G., Foudoulis, C., Neumann, N., tectonics in the Terre Adelie Craton Sircombe, K., McAvaney, S., Reid., A. (East Antarctica): Evidence form Th- and Szpunar, M., 2008. Foundations of U-Pb ages on monazite and 40Ar/39Ar South Australia discovered. Aus Geo

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News, 92: 10-11. Bulletin 54- Geological Survey of South Australia, pp. 50-105. Fraser, G. and Reid, A., 2008. The Kararan Orogeny - a link between the proterozoic Payne, J.L., Hand, M., Barovich, K.M. North Australian and South Australian and Wade, B.P., 2008. Temporal cratons? Australian Earth Sciences constraints on the timing of high-grade Convention: GSA Abstract No, 89: metamorphism in the northern Gawler 104. Craton: implications for assembly of the Australian Proterozoic. Australian Goodge, J.W., Fanning, C.M. and Bennett, Journal of Earth Sciences, 55(5): 623- C.V., 2001. U-Pb evidence of similar 640. to 1.7 Ga crustal tectonism during the Nimrod Orogeny in the Transantarctic Reid, A., Vassallo, J.J., Wilson, C.J.L. and Mountains, Antarctica: implications Fanning, C.M., 2007. Timing of the for Proterozoic plate reconstructions. Kimban Orogeny on southern Eyre Precambrian Research, 112(3-4): 261- Peninsula. South Australia, Department 288. of Primary Industries and Resources. Report Book, 2007/5. Hopper, D.J., 2001. Crustal evolution of Palaeo- to Mesoproterozoic rocks in the Peak Stewart, J.R. and Betts, P., 2008. A Gawler and Denison Rangers, South Australia, Orocline? Yalbrinda-Oolabinnia Shear University of Queensland, Brisbane, Zone Connections. Australian Earth 216 pp. Sciences Convention: GSA Abstract No, 89: 233. Howard, K., Dutch, R., Hand, M., Barovich, K.M. and Reid, A., 2008. Unravelling the Swain, G.M., Hand, M., Teasdale, J., Rutherford, Fowler Domain: new geochronological L. and Clark, C., 2005. Age constraints data from the western Gawler Craton, on terrane-scale shear zones in the South Australia. South Australia. Gawler Craton, southern Australia. Department of Primary Industries and Precambrian Research, 139(3-4): 164- Resources. Report Book, 2008/10. 180. Jagodzinski, E. et al., 2006. Compilation of Teasdale, J., 1997. Methods for understanding SHRIMP U-Pb data fo rhte Gawler poorly exposed terrains: The interpretive Craton, South Australia, 2005. geology and tectonothermal evolution Geochronology Series Report Book of the western Gawler craton. Ph.D. 2005-1. South Australia. Department Thesis, University of Adelaide, of Primary Industries and Resources. Adelaide, 182 pp. Report Book, 2007/21. Tomkins, A.G. and Mavrogenes, J.A., 2002. McFarlane, C.R.M., 2006. Palaeoproterozoic Mobilization of gold as a polymetallic evolution of the Challenger Au Deposit, melt during pelite anatexis at the South Australia, from monazite Challenger deposit, South Australia: A geochronology. Journal of Metamorphic metamorphosed Archean gold deposit. Geology, 24(1): 75-87. Economic Geology and the Bulletin of the Society of Economic Geologists, Parker, A.J., 1993. Palaeoproterozoic. In: J.F. 97(6): 1249-1271. Drexel, W.V. Preiss and A.J. Parker (Editors), The geology of South Tong, L., Wilson, C.J.L. and Vassallo, J.J., 2004. Australia; Volume 1, The Precambrian. Metamorphic evolution and reworking

197 of the Sleaford Complex metapelites in the southern Eyre Peninsula, South Australia. Australian Journal of Earth Sciences, 51: 571-589. Vassallo, J.J. and Wilson, C.J.L., 2001. Structural repetition of the Hutchison Group metasediments, Eyre Peninsula, South Australia. Australian Journal of Earth Sciences, 48(2): 331-345. Vassallo, J.J. and Wilson, C.J.L., 2002. Palaeoproterozoic regional-scale non- coaxial deformation; an example from eastern Eyre Peninsula, South Australia. Journal of Structural Geology, 24(1): 1- 24. Zeh, A., Millar, I.L. and Horstwood, M.S.A., 2004. Polymetamorphism in the NE Shackleton Range, Antarctica: Constraints from petrology and U- Pb, Sm-Nd, Rb-Sr TIMS and in situ U-Pb LA-PIMMS dating. Journal of Petrology, 45(5): 949-973.

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