Chemical Geology 184 (2002) 151–165 www.elsevier.com/locate/chemgeo

Monazite ‘‘in situ’’ 207Pb/206Pb geochronology using a small geometry high-resolution ion probe. Application to Archaean and Proterozoic rocks

Delphine Bosch a,*, Dalila Hammor b, Olivier Bruguier c, Renaud Caby a, Jean-Marc Luck d

aLaboratoire de Tectonophysique, Universite´ Montpellier II, CNRS-UMR 5568, cc 066, Place Euge`ne Bataillon, 34095 Montpellier Cedex 05, France bDe´partement de Ge´ologie, Universite´ d’Annaba, B.P. 12, El Hadjar Annaba, cService ICP-MS, ISTEEM, Universite´ Montpellier II, cc 049, Place Euge`ne Bataillon, 34 095 Montpellier Cedex 5, France dLaboratoire de Ge´ophysique, Tectonique et Se´dimentologie, Universite´ Montpellier II, CNRS-UMR 5573, cc 060, Place Euge`ne Bataillon, 34 095 Montpellier Cedex 5, France Received 19 March 2001; accepted 2 August 2001

Abstract

This paper reports the application of secondary ion mass spectrometry (SIMS) using a small geometry Cameca IMS4f ion probe to provide reliable in situ 207Pb/206Pb ages on monazite populations of Archaean and Proterozoic age. The reliability of the SIMS technique has been assessed on two samples previously dated by the conventional ID-TIMS method at 2661 F 1 Ma for monazites extracted from a pelitic schist from the Jimperding Metamorphic Belt (Yilgarn Craton, Western Australia) and 1083 F 3 Ma for monazites from a high-grade paragneiss from the Northampton Metamorphic Complex (Pinjarra Orogen, Western Australia). SIMS results provide 207Pb/206Pb weighted mean ages of 2659 F 3Ma(n = 28) and 1086 F 6Ma (n = 21) in close agreement with ID-TIMS reference values for the main monazite growth event. Monazites from the Northampton Complex document a complex history. The spatial resolution of about 30 mm and the precision achieved successfully identify within-grain heterogeneities and indicate that monazite growth and recrystallisation occurred during several events. This includes detection of one inherited grain dated at ca. 1360 Ma, which is identical to the age of the youngest group of detrital zircons in the paragneiss. Younger ages at ca. 1120 Ma are tentatively interpreted as dating a growth event during the prograde stages of metamorphism. These results demonstrate that the closure temperature for lead diffusion in monazite can be as high as 800 C. At last, ages down to ca. 990 Ma are coeval with late pegmatitic activity and may reflect either lead losses or partial recrystallisation during the waning stages of metamorphism. A third unknown sample was analysed to test the capability of the in situ method to date younger monazite populations. The sample, a pelitic metatexite from Northwestern Hoggar (Algeria), contains rounded metamorphic monazites that provide a 207Pb/206Pb weighted mean age of 603 F 11 Ma (n = 20). This age is interpreted as recording emplacement of a gabbronoritic body during amphibolite facies regional metamorphism and is representative of the late pulse of the Pan-African tectonometamorphic

* Corresponding author. E-mail addresses: [email protected] (D. Bosch), [email protected] (O. Bruguier), [email protected] (R. Caby), [email protected] (J.-M. Luck).

0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0009-2541(01)00361-8 152 D. Bosch et al. / Chemical Geology 184 (2002) 151–165 evolution in the western part of the . In situ SIMS analyses using a widely available, small geometry ion probe, can thus be successfully used to accurately determine ages for complex Precambrian monazite populations. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: SIMS; Monazite; 207Pb/206Pb geochronology; Metamorphism

1. Introduction Th content of the unknown samples (Zhu et al., 1998). Moreover, these large geometry ion probes are not yet Over the last decade, monazite, a lanthanide-rich widely available, which reduces their use as a world- phosphate, has been widely used as a geochronometer wide routine method. and this mineral is, after zircon, probably the most Electron microprobe (EMPA) has also been shown used U-rich phase in geochronology. Monazite is a to be a valuable alternative to monazite dating (Mon- common accessory mineral occurring in a wide tel et al., 1996; Cocherie et al., 1998). The main variety of rock types (sedimentary, metamorphic advantage of this technique being the very high spatial and magmatic), therefore allowing the dating of resolution of around 1 mm compared to the 20- to 30- various events such as the emplacement of magmatic mm spots used by most ion microprobes. The preci- rocks or the growth of minerals (or cooling) in sion, however, is limited to around 20 Ma, which metamorphic terranes (e.g. Parrish, 1990), or the precludes identification of monazite-forming events tracing of source region for detritus that accumulated occurring in a limited time span. in sedimentary basins. Monazite is thought to have a There is a growing interest (e.g. Poitrasson et al., relatively simple behaviour in comparison to zircon 2000) for monazite dating by laser ablation induc- and is often found in concordant position in the con- tively coupled plasma mass spectrometry (LA-ICP- cordia diagram, thus indicating closed system behav- MS) which revealed to be a very fast technique with a iour with respect to the U–Pb system. In contrast to spatial resolution comparable to secondary ion mass silicate minerals such as zircon, which have a ten- spectrometry (SIMS). The high drilling rate (ca. 0.5– dency to become metamict, monazite rarely exhibits 1 mm/s) can, however, constitute a serious drawback radiation damage of the crystal lattice in spite of very when analysing heterogeneous material. high U and Th contents (thousands of parts per In this paper, the capability of the more widely million). available, small geometry Cameca IMS4f ion microp- Recent studies, however, have highlighted com- robe for rapid, in situ, isotopic analyses of selected plexities in the behaviour of this mineral such as areas of monazite crystals has been investigated as an inheritance (Copeland et al., 1988), secondary alternative way to analyse complex metamorphic replacement (De Wolf et al., 1993; Zhu et al., 1997; populations. We present results from two well-dated Bingen and van Breemen, 1998) and Pb loss by late Archaean and Proterozoic samples and from a volume diffusion during a metamorphic event (Black third unknown sample outcropping in the Hoggar et al., 1984; Suzuki et al., 1994) or enhanced by Mountains (Algeria). In addition, the results shed damage to the crystal lattice (Hawkins and Bowring, some light on the behaviour of monazite, which has 1997). Implicit to this is the growing need for part implications for its use as a U–Pb geochronometer. grain analyses, either by conventional method or by in situ high-resolution ion microprobe. Up to now, three techniques allow in situ analyses 2. Analytical techniques of monazites for geochronological purposes. Sensitive high-resolution ion microprobes (such as SHRIMP) Separation of minerals was performed using stand- have been used to successfully determine U–Pb ages ard techniques (Wifley table and heavy liquids). After (e.g. Williams et al., 1996), but this requires character- cleaning in dilute 0.5 N HNO3 and tridistilled water, isation of monazite standards, which should match the the monazite grains were subsequently mounted in D. Bosch et al. / Chemical Geology 184 (2002) 151–165 153 epoxy resin and polished to approximately half their thickness to expose internal structures. The mounts were then carefully washed with tridistilled water, soap and alcohol and stored in a clean environment before analysis. SIMS analyses were carried out on the Cameca IMS4f ion microprobe with a spot size of about 30 mm. To avoid sample charging by the 16O À primary beam, the mounts were coated by ca. 100- nm-thick gold film. Before its introduction within the sample lock, the surface conductivity of the mount was checked to be less than 20 V and it was then held under vacuum overnight to ensure degassing. Before each analysis, a 10-min rastering was conducted to pass through the gold coat and to reach steady sputtering conditions. The primary beam currents ranged from 8 to 20 nA, the highest current being used for the youngest sample. The primary beam was accelerated onto the sample surface by a 12.65-keV potential and stability was better than 0.6%. Positive secondary ions were extracted using a 4.5-keV poten- tial and the energy window was set at 50 eV to remove low-energy ions and molecular species. The beam passed then through a double focussing mass spectrometer operated at a high mass resolution to resolve molecular and isobaric interferences in the 204–208 mass range. High-resolution mass spectrum of monazites shows that the main interferences are mostly due to REE-oxides. The most significant molecular interfer- ences are related to PrPO2, GdPO and YbO2 which 204 Fig. 1. Typical mass scans of monazites obtained with the IMS4f ion occur near the Pb (see Fig. 1a). As the measured probe at a mass resolving power of 3500. (a) Molecular ions 207 206 Pb/ Pb ratios must be corrected from common interferences in the 204 mass range spectrum. The main inter- 206 204 lead contribution by referring to the Pb/ Pb ferences are from PrPO2 and to a lesser degree to GdPO and YbO2. measured ratios, good separation of the 204Pb peak (b) Molecular ion interferences in the 208 mass range spectrum are from these neighbouring interferences is essential. related to Sm species (SmSiO2, SmCaO, SmPSi). Indeed, any unresolved interference on the small 204Pb peak will be responsible for an overcorrection of the 207Pb/206Pb ratio resulting in too young an age. integrated dead-time measured using Pb standards. A mass resolving power of 3500 is necessary to Under these operating conditions, the IMS4f ma- ensure integrity of the 204Pb as well as of the other chine achieves an overall instrumental Pb sensitivity lead isotopes (Fig. 1a and b). Increasing the mass of 3–4 cps/ppm/nA of primary beam based on resolution leads to a decrease of ions arriving to the analyses of monazites from sample W404 presented detector and, thus large contrast aperture (400 mm) in this study. This sensitivity is about five times low- and field aperture (750 mm) were used during the er than that achieved by large geometry ion probes course of this study. (e.g. Harrison et al., 1995; Williams et al., 1996) but Ion beams were measured in the peak jumping compares well with sensitivity of the Isolab machine sequence with an electron multiplier operating in operated in the SIMS mode (e.g. De Wolf et al., pulse counting mode with a 65% yield and a 30-ns 1993). 154 D. Bosch et al. / Chemical Geology 184 (2002) 151–165

Data were collected in three blocks of 10 cycles can also arise from recrystallisation which is accom- each and the total duration for one analysis was about panied by transport and migration of elements (Pidg- 60 min. A background correction, monitored at eon, 1992). Although this generally results in a Pb- 204.10 amu, as close as possible to the 204Pb peak, free recrystallised lattice, residual radiogenic Pb can was applied to the measured Pb peaks. Typical potentially remain partially trapped in the newly analytical parameters for Pb analyses are listed in formed domain, thus leading to incomplete resetting Table 1. Common Pb corrections were based on the of the U–Pb systems and ages older than the true measured 204Pb and for all the data, the assumed age of recrystallisation. Finally, reverse discordance common Pb composition was modelled as contempo- has been also observed and is generally assumed to raneous Pb (Stacey and Kramers, 1975). Corrected derive from incorporation of intermediate daughter isotopic ratios and ages were calculated after Ludwig products into the 238U/206Pb decay chain at the time (1999). The quoted ages and related uncertainties of crystal growth (Scha¨rer, 1984). In old, Precam- are based on weighted averages of the calculated brian, monazites, unsupported thorogenic 206Pb 207Pb/206Pb ages. should be swamped by uranogenic 206Pb and thus In the absence of U–Pb and Th–Pb analyses due should not be responsible for reverse discordance. to unavailable suitable monazite standards, a review Bingen and van Breemen (1998), however, showed of the possible effects that can bias the 207Pb/206Pb that this phenomenon could be invoked to account ages is warranted. Indeed, although monazite gener- for reverse discordance of monazites as old as ally shows a high degree of concordance, a great 1 Ga. Hawkins and Bowring (1997) on the contrary number of studies have been faced to discordant proposed that reverse discordance results from dis- analyses, both normal and reverse. For example, equilibrium of the U–Pb system due to postcrystal- studies of the diffusion of Pb in monazite (i.e. lisation local enrichment of Pb by diffusion. In any Suzuki et al., 1994; Smith and Giletti, 1997) indicate case, the 207Pb/206Pb ratios do not yield reliable that diffusive Pb loss may be experienced by the ages. crystals during a high-temperature metamorphic The above discussion pertains to analyses falling event. Significant ancient diffusion-controlled Pb outside the main monazite population, for which a loss would be responsible for younger ages, with discussion about the effects of potential discordance no geological meaning, whereas present-day Pb loss on the interpretation of ages is warranted. It is clear will move the points towards the origin thus leaving that only age grouping can be considered as reflecting the 207Pb/206Pb ratio and age unaffected. In the growth or recrystallisation events and that analyses former case, 207Pb/206Pb ages should be considered yielding intermediate ages should be treated with only as minimum values. Straddling by the ion beam caution as they possibly derived from complex mon- of growth zones with different ages may also result azite grains. in intermediate ages, but this can be avoided by careful SEM imaging before analysis. Discordance 3. Results and discussion

The reliability of the method has been assessed on Table 1 two Precambrian metamorphic monazite populations Analytical parameters for Pb/Pb isotopic analyses of monazites using the Cameca IMS4f ion probe previously dated by ID-TIMS (see Bosch et al., 1996; Bruguier et al., 1999). The first sample is a late Mass Isotope Waiting time Counting (amu) for magnetic time (s) Archaean pelitic schist from the Jimperding Metamor- settling (s) phic Belt located on the western margin of the Yilgarn 203.973 204Pb 3 20 Craton (Western Australia), whereas the second sam- 204.100 background 2 20 ple is a paragneiss from the Proterozoic Northampton 205.974 206Pb 2 15 Complex of the Pinjarra Orogen (Western Australia). 206.976 207Pb 2 30 Results are reported in Table 2 and presented in 208 207.977 Pb 2 5 Figs. 2–4. D. Bosch et al. / Chemical Geology 184 (2002) 151–165 155

3.1. Pelitic schist W398 Thirty SIMS 207Pb/206Pb spot analyses were per- formed on 15 grains. Intra-grain analyses generally This sample consists mostly of muscovite (25%), overlap each other except in two cases (see Table 2). pinitised cordierite (25%), red biotite (20%), quartz Analysis #4-2 gives a significantly older age (2671 F (17%), with minor sillimanite (8%), oligoclase (3%), 6 Ma) than the two other spot analyses from the same opaques (2%), and trace zircon and monazite. The grain (2653 F 8 and 2650 F 8 Ma). The discrepancy schist is medium grained with an average grain size between these two ages is attributed to preservation of of about 0.5 mm, although biotite grains vary up the first monazite population (ca. 2665 Ma), which to 1.0 mm in length. The rock possesses a sinuous underwent recrystallisation or resorption during the subcontinuous foliation defined by biotite, muscovite subsequent main monazite growth event. One spot and sillimanite, which may have been crenulated. analysis (#10-3) yields an anomalously young Granoblastic areas defined by quartz and cordierite 207Pb/206Pb corrected value of 2604 F 6 Ma, whereas occur between the biotite, sillimanite and muscovite the two other spots on this grain give ages of grains. Muscovite and biotite are intergrown, where- 2648 F 12 and 2650 F 8 Ma. This young age is not as sillimanite is present as clusters and as inclusions reproduced in the present data set and is also younger within biotite and muscovite. In places, biotite and than any reported event in this part of the Yilgarn Craton muscovite appear to overprint cordierite. Biotite + - and, in particular, than the apatite cooling age of quartz and muscovite + quartz symplectites are also 2636 F 6 Ma from a nearby syenitic body (Pidgeon et present. al., 1996). This suggests that ancient Pb loss during a The sample underwent amphibolite facies regional metamorphic event is unlikely. Moreover, the uncor- metamorphism with P–T conditions in the range of rected 207Pb/206Pb ratio is close to the expected value 650–700 C and 2–5 kbars. It contains a population (see Table 2), but the 204Pb/206Pb ratio is among the of rounded, pale yellow monazite crystals interpreted highest one, which suggests that this young age stems as metamorphic in origin. Five single crystals, pre- from an overcorrection due to high count rate on the viously analysed by ID-TIMS (Bosch et al., 1996), are 204Pb peak. This might be related to edge effects or to a slightly discordant ( < 1%) with ages ranging from small unresolved molecular contribution possibly 2652 to 2665 Ma (Fig. 2a). The ca. 10-Ma range in related to a drift in the mass calibration, shifting the 204 ages suggests that the schist contains a heterogeneous Pb measurement towards the neighbouring PrPO2 monazite population and that growth or recrystallisa- peak. This analysis was therefore discarded from the age tion occurred during several events. A neighbouring calculation. The 28 remaining analyses (Fig. 2b) have pelitic schist (W399), however, yields a homogeneous ages ranging from 2636 F 28 to 2686 F 29 Ma and give population precisely dated by ID-TIMS at 2661 F 1 aweightedmeanof0.18066F 0.00030 (2r) corre- Ma. This ca. 2660-Ma age is interpreted as reflecting sponding to an age of 2659 F 3 Ma (MSWD = 2.2). This the main monazite growth event during the prograde mean age is well within the range of ID-TIMS values stages of regional metamorphism at a temperature of (2652–2665 Ma), and identical to the 2661-Ma age for about 500 C, similarly to cases reported for meta- the main monazite growth event in this area of the morphic monazites in pelitic schists (e.g. Smith and Jimperding Belt. A cumulative probability treatment Barreiro, 1990). Monazites dated at ca. 2665 Ma are of the data (Ludwig, 1999) tends to suggest a bimodal coeval with granitic intrusions (2660–2670 Ma), distribution with mean values around 2655 and 2665 whereas the younger age of 2652 Ma corresponds to Ma. One spot age excluded (#4-2), SIMS analyses, the peak of regional metamorphism in this area however, did not detect unequivocally the different (Bosch et al., 1996; Nemchin et al., 1994; Pidgeon monazite populations identified by ID-TIMS and the et al., 1996). Homogeneity of monazite ages in sample analysed crystals would thus appear to have formed W399 contrasts with the spread observed in sample during a single growth event. The short time span W398 that suggests the growth of monazite was between the three growth episodes (ca. 15 Ma) and the controlled by local conditions and that monazites relatively low precision of the SIMS analyses (from 4 to may have been armoured against Pb loss and recrys- 32 Maatthe 2rlevel)make the agedifference difficultto tallisation. resolve. 156 D. Bosch et al. / Chemical Geology 184 (2002) 151–165

Table 2 SIMS Pb/Pb isotopic results Spot Percentage Pb Measured Corrected atomic ratiosa Th/U Apparent age (Ma) 206Pb 207Pb 208Pb 204Pb/206Pb 207Pb/206Pb 208Pb/206Pb F (%) 207Pb/206Pb F (%) 207Pb/206Pb F (2r) (2r) (2r) W398 Schist (Jimperding Metamorphic Belt, Western Australia), main monazite growth event at 2661 F1 Ma. Properties: 100–200 lm, rounded anhedral, yellow translucent 398-1-1 12.8 2.3 84.9 0.000084 0.181470 6.61 0.95 0.180441 0.70 23.8 2656.9 11.7 398-1-2 10.5 1.9 87.6 0.000340 0.182850 8.31 1.25 0.178680 1.10 29.9 2640.7 18.3 398-2-1 17.1 3.0 79.9 0.000345 0.182430 4.69 1.94 0.178193 1.72 16.8 2636.1 28.4 398-2-2 19.4 3.5 77.0 0.000123 0.182710 3.96 0.94 0.181206 0.65 14.3 2663.9 10.8 398-3-1 9.9 1.8 88.3 0.000252 0.184070 8.95 1.84 0.180986 1.19 32.2 2661.9 19.6 398-3-2 9.5 1.7 88.7 0.000248 0.183980 9.30 0.89 0.180942 0.59 33.5 2661.5 9.8 398-4-1 11.6 2.1 86.3 0.000163 0.182050 7.43 1.20 0.180055 0.50 26.7 2653.4 8.2 398-4-2 12.0 2.2 85.8 0.000107 0.183290 7.14 1.70 0.181985 0.33 25.7 2671.1 5.4 398-4-3 11.3 2.0 86.6 0.000099 0.180940 7.64 1.28 0.179724 0.48 27.5 2650.3 7.9 398-5-1 15.9 2.9 81.2 0.000158 0.183430 5.09 0.67 0.181498 0.51 18.3 2666.6 8.4 398-5-2 15.3 2.8 82.0 0.000192 0.183430 5.38 0.50 0.181085 0.31 19.3 2662.8 5.2 398-6-1 11.8 2.1 86.0 0.000332 0.185270 7.27 0.62 0.181207 1.37 26.2 2664.0 22.6 398-6-2 17.0 3.1 80.0 0.000123 0.181720 4.71 2.23 0.180216 0.57 16.9 2654.9 9.4 398-6-3 16.6 3.0 80.5 0.000302 0.184380 4.86 1.20 0.180680 0.77 17.5 2659.1 12.7 398-7-1 49.0 8.9 42.1 0.000068 0.182110 0.86 2.00 0.181278 0.25 3.1 2664.6 4.2 398-7-2 49.8 9.1 41.1 0.000081 0.182580 0.82 4.57 0.181587 0.44 3.0 2667.4 7.3 398-8-1 10.5 1.9 87.6 0.000392 0.186590 8.34 0.86 0.181802 1.25 30.0 2669.4 20.6 398-8-2 10.6 1.9 87.5 0.000393 0.183010 8.23 1.68 0.178182 1.00 29.6 2636.0 16.6 398-9 19.5 3.5 77.0 0.000290 0.183820 3.95 7.31 0.180272 0.30 14.2 2655.4 4.9 398-10-1 13.4 2.4 84.2 0.000200 0.181960 6.30 1.02 0.179512 0.72 22.6 2648.4 12.0 398-10-2 13.4 2.4 84.2 0.000070 0.180550 6.30 0.53 0.179698 0.54 22.6 2650.1 8.9 398-10-3 9.2 1.6 89.2 0.000661 0.182950 9.71 0.67 0.174798 0.40 34.8 2604.1 6.6 398-11 11.7 2.1 86.2 0.000182 0.182430 7.40 0.51 0.180204 0.85 26.6 2654.8 14.1 398-12 17.9 3.3 78.8 0.001200 0.198290 4.40 3.14 0.183682 1.75 15.8 2686.4 28.7 398-13 52.7 9.6 37.7 0.000520 0.187830 0.72 1.65 0.181478 1.26 2.6 2666.4 20.9 398-14-1 17.5 3.1 79.3 0.000260 0.182940 4.53 3.88 0.179756 0.80 16.3 2650.6 13.2 398-14-2 17.3 3.1 79.5 0.000263 0.183250 4.59 3.87 0.180027 0.84 16.5 2653.1 13.8 398-15 43.5 7.8 48.7 0.000118 0.181690 1.12 2.14 0.180243 0.55 4.0 2655.1 9.1 398-16 27.3 5.0 67.7 0.000187 0.183620 2.48 1.70 0.181333 0.82 8.9 2665.1 13.6 398-17 25.2 4.6 70.2 0.000669 0.190140 2.79 2.14 0.181974 1.95 10.0 2671.0 32.1

W404 Paragneiss (Northampton Complex, Western Australia), 1083 F 3 Ma. Properties: 100–200 lm, rounded to irregular shaped, yellow translucent 404-1-(a) 8.0 0.7 91.3 0.000312 0.091489 11.43 2.68 0.087153 2.23 38.2 1363.9 42.7 404-2-1(b) 26.7 2.1 71.3 0.000085 0.078327 2.67 2.32 0.077128 0.42 8.8 1124.5 8.3 404-2-2 9.8 0.7 89.5 0.000282 0.077925 9.13 1.02 0.073934 0.73 30.0 1039.7 14.7 404-2-3 8.4 0.6 91.0 0.000204 0.078656 10.82 0.53 0.075779 0.87 35.7 1089.2 17.3 404-3(c) 13.5 1.0 85.5 0.000288 0.077936 6.32 17.98 0.073857 0.90 20.8 1037.6 18.1 404-4 13.8 1.0 85.2 0.000391 0.078987 6.16 6.65 0.073449 1.09 20.2 1026.4 22.0 404-5 9.1 0.7 90.2 0.000476 0.079025 9.88 2.62 0.072275 0.96 32.4 993.7 19.4 404-6-1 30.8 2.3 66.9 0.000087 0.077121 2.18 2.40 0.075890 0.27 7.2 1092.2 5.4 404-6-2 31.5 2.4 66.1 0.000063 0.077012 2.10 0.82 0.076118 0.50 6.9 1098.2 10.1 404-7-1 30.1 2.3 67.6 0.000091 0.077291 2.24 0.94 0.076000 0.31 7.4 1095.1 6.3 404-7-2 31.0 2.4 66.7 0.000076 0.077114 2.15 0.51 0.076037 0.60 7.1 1096.0 11.9 404-8 25.5 1.9 72.6 0.000105 0.076202 2.85 0.84 0.074715 0.39 9.4 1060.8 7.8 404-9-1 17.3 1.3 81.4 0.000170 0.077241 4.70 2.12 0.074840 0.43 15.5 1064.2 8.7 404-9-2 12.0 0.9 87.1 0.000326 0.081815 7.26 1.61 0.077222 2.04 24.0 1126.9 40.4 404-10-1(d) 11.1 0.8 88.1 0.000320 0.080186 7.96 2.52 0.075667 2.60 26.2 1086.3 51.8 D. Bosch et al. / Chemical Geology 184 (2002) 151–165 157

Table 2 (continued) Spot Percentage Pb Measured Corrected atomic ratiosa Th/U Apparent age (Ma) 206Pb 207Pb 208Pb 204Pb/206Pb 207Pb/206Pb 208Pb/206Pb F (%) 207Pb/206Pb F (%) 207Pb/206Pb F (2r) (2r) (2r) W404 Paragneiss (Northampton Complex, Western Australia), 1083 F 3 Ma. Properties: 100–200 lm, rounded to irregular shaped, yellow translucent 404-10-2 8.9 0.7 90.4 0.000146 0.079072 10.13 2.23 0.077018 1.60 33.4 1121.7 31.8 404-11 10.4 0.8 88.8 0.000265 0.079178 8.52 1.49 0.075435 0.86 28.1 1080.1 17.2 404-12-1(e) 17.8 1.3 80.9 0.000167 0.077989 4.55 2.25 0.075623 0.52 15.0 1085.1 10.4 404-12-2 16.8 1.3 81.9 0.000115 0.076989 4.87 3.01 0.075359 1.29 16.0 1078.1 25.8 404-13 15.0 1.1 83.9 0.000208 0.078927 5.60 2.20 0.075989 0.68 18.5 1094.8 13.6 404-14 21.7 1.6 76.7 0.000271 0.079418 3.53 1.53 0.075594 1.02 11.6 1084.3 20.5 404-15 35.2 2.7 62.2 0.000077 0.076868 1.77 2.55 0.075785 0.44 5.8 1089.4 8.7 404-16 10.5 0.8 88.7 0.000179 0.078066 8.42 0.64 0.075536 0.90 27.7 1082.8 18.0 404-17-1(f) 28.9 2.2 68.9 0.000069 0.077211 2.38 0.69 0.076235 0.75 7.9 1101.3 14.9 404-17-2 12.9 1.0 86.2 0.000185 0.077866 6.71 4.78 0.075245 0.97 22.1 1075.1 19.3 404-17-3 14.8 1.1 84.1 0.000063 0.076959 5.70 5.71 0.076073 1.33 18.8 1097.0 26.6 404-18 20.7 1.6 77.7 0.000146 0.078235 3.75 1.75 0.076173 0.71 12.4 1099.6 14.1 404-19 14.7 1.1 84.2 0.000247 0.078574 5.74 2.19 0.075077 1.39 18.9 1070.6 27.8 404-20 28.4 2.1 69.5 0.000084 0.076761 2.45 5.92 0.075576 0.42 8.1 1083.9 8.4

C106 Gneiss (Hoggar). Properties: 80–125 lm, rounded anhedral, colourless to yellow translucent C106-1 23.2 1.4 75.4 0.000328 0.063840 3.25 1.86 0.059115 2.36 10.41 571.4 50.9 C106-2 24.0 1.4 74.6 0.000400 0.065676 3.11 3.90 0.059914 2.55 9.97 600.5 54.7 C106-3 26.9 1.6 71.6 0.000340 0.061724 2.66 1.90 0.058070 1.38 8.51 532.4 30.1 C106-4 28.8 1.7 69.5 0.000182 0.058962 2.42 5.14 0.058953 2.86 7.74 565.4 61.7 C106-5 24.1 1.5 74.5 0.000122 0.062905 3.09 10.50 0.060930 1.14 9.94 636.8 24.4 C106-6 27.1 1.6 71.3 0.000295 0.062388 2.63 0.93 0.059052 2.16 8.43 569.0 46.7 C106-7 25.0 1.5 73.5 0.000145 0.062527 2.94 1.55 0.060442 0.89 9.45 619.4 19.1 C106-8 23.8 1.4 74.8 0.000569 0.068650 3.14 0.71 0.060352 1.28 10.09 616.2 27.5 C106-9 22.9 1.4 75.7 0.000175 0.063015 3.31 1.98 0.060497 2.42 10.64 621.4 51.8 C106-10 27.2 1.7 71.1 0.000403 0.062005 2.61 1.01 0.061513 2.64 8.40 657.2 56.1 C106-11 39.1 2.3 58.5 0.000409 0.065362 1.50 0.58 0.059463 1.16 4.80 584.1 25.0 C106-12 39.1 2.4 58.6 0.000298 0.066007 1.50 0.58 0.060312 1.14 4.82 614.8 24.5 C106-13 41.7 2.5 55.8 0.001124 0.077400 1.34 2.46 0.059977 3.52 4.29 602.8 75.3 C106-14-1 23.2 1.4 75.5 0.000159 0.064257 3.26 1.88 0.059412 1.08 10.45 582.2 23.4 C106-14-2 25.4 1.5 73.1 0.000280 0.063127 2.88 2.65 0.059086 2.85 9.22 570.3 61.4 C106-15 28.3 1.7 70.1 0.000460 0.065102 2.48 3.10 0.058455 4.49 7.92 546.9 96.6 C106-16-1 24.3 1.5 74.2 0.000572 0.067958 3.05 4.40 0.059714 1.11 9.79 593.2 24.1 C106-16-2 24.8 1.5 73.7 0.000593 0.068230 2.97 4.63 0.059687 1.11 9.54 592.3 24.0 C106-17 23.0 1.4 75.6 0.000431 0.066112 3.29 0.77 0.059895 1.33 10.56 599.8 28.7 Th/U ratios were calculated from the radiogenic 208Pb/206Pb assuming concordance between the U–Pb and Th–Pb systems. Each analysis was labelled as follows: sample name-grain analyzed-spot number. Letters (from a to f) into brackets refer to SEM images of Fig. 4. a Lead isotopic ratios have been corrected for background and common lead.

3.2. Paragneiss W404 The gneiss is composed of quartz (25–40%), micro- perthitic microcline (25–35%), andesine (10–20%), This sample is a quartzofeldspathic gneiss yielding phlogopite (5–15%) and garnet (5–15%), with minor a granoblastic texture, although where there is a high ilmenite ( < 5%) and graphite ( < 5%), and trace mus- proportion of phlogopite, one, and in some cases two, covite, zircon, monazite and rutile. The sample expe- foliations can be recognised. Both foliations are rienced granulite facies metamorphism with peak defined by sparsely distributed phlogopite and ilmen- temperatures and pressures of 850 F 50 C and 5–6 ite, and phlogopite pressure shadows around garnets. kbars. 158 D. Bosch et al. / Chemical Geology 184 (2002) 151–165

main metamorphic population. This value falls in the age spectrum (1150–1450 Ma) given by a group of detrital zircons from this paragneiss (Bruguier et al., 1999) and this, along with the rounded shape of the core, suggests a detrital origin for the original crystal. Preservation of this old age clearly implies that the U–Th–Pb systems of the original monazite crystal has not been reset and survived high-grade granulitic conditions with peak temperatures and pressures in the range 800–900 C and 5–6 kbars. These con- ditions are well above the nominal closure temper- ature proposed by Copeland et al. (1988) and is another example of the robustness of the U–Th–Pb system in monazite (De Wolf et al., 1993; Bingen and van Breemen, 1998). SEM imaging of some monazite crystals show evidence for recrystallisation/resorption along irregu-

Fig. 2. Isotopic results for monazites from the W398 pelitic schist (Jimperding Metamorphic belt, Western Australia). (a) Concordia diagram for ID-TIMS analyses. U–Pb analyses from metamorphic monazites extracted from W399 schist are also shown. Black boxes: W399; shaded boxes: W398. Boxes are 2r errors. (b) SIMS 207Pb/206Pb diagram. Error bars are 2r.

Five single grains previously analysed by ID-TIMS (Bruguier et al., 1999) provided an age of 1080 F 5Ma (Fig. 3a). Discordant analyses were interpreted as reflecting disturbances of the original monazite and, on the ground of one concordant analysis, a more precise age of 1083 F 3 Ma was proposed for the metamorphic growth of these minerals. This age is identical to the zircon age of a mafic granulite (1079 F 3 Ma) inter- preted as dating granulite facies metamorphism. Twenty-nine SIMS analyses were performed on 20 grains (Fig. 3b). The age spectrum is complex and analyses conducted at different places of monazite grains do not overlap completely at the 2r level. The F Fig. 3. Isotopic results for monazites from the W404 paragneiss oldest age (1364 42 Ma) is from grain #1 which (Northampton Metamorphic Complex, Western Australia). (a) appears to contain a partly recrystallised central Concordia diagram for ID-TIMS analyses. Boxes are 2r errors. rounded core (Fig. 4a) about 200 Ma older than the (b) SIMS 207Pb/206Pb diagram. Error bars are 2r. D. Bosch et al. / Chemical Geology 184 (2002) 151–165 159

Fig. 4. SEM (BSE) imaging of selected monazite crystals from the W404 paragneiss (Northampton Metamorphic Complex, Western Australia). Brightness is correlated with Th content such that the brighter the area, the higher the Th content and the higher the calculated Th/U ratio. The location of the SIMS analyses are circled. (a) Image of monazite #1 showing a homogeneous high-Th rim surrounding a central rounded core. (b) Complex monazite grain #2 showing an irregular, patchy, low-Th core resorbed by inward-directed high-Th fronts. Note the high Th content along the fracture in the left portion of the grain. (c) Homogeneous anhedral grain #3 containing a high-Th rim in the lower part of the grain and a central low-Th core. (d) Same as grain #2 with a bright rim in the right upper part. High-Th areas possibly reflect Th exsolution during resorption of the core. (e) Simple homogeneous grain #12. (f) Same as grain #12 with a possible low-Th core preserved in the left part. 160 D. Bosch et al. / Chemical Geology 184 (2002) 151–165 lar intra-grains discontinuities which look like inward- element migration was a more efficient way for directed reaction fronts (see Fig. 4b and d). These resetting of the U–Pb isotope system of pre-existent processes resulted in a patchy replacement of a low- monazite than volume diffusion of Pb, although Th core by a bright, high-Th, material and is some- admittedly without information on how potentially times accompanied by Th exsolution in the core (see discordant the data are, this cannot be warranted. the bright dots in Fig. 4d), possibly occurring in the Most analyses (21 out of 29) yield ages ranging from first stages, and before completion, of this secondary 1061 F 8 to 1101 F 15 Ma and can be combined to replacement. This suggests redistribution of the ele- give a weighted mean 207Pb/206Pb ratio of ments at least on a local (subgrain) scale. Preservation 0.07566 F 0.00021 (MSWD = 5.2) corresponding to of chemically distinct zones, however, implies that Th an age of 1086 F 6Ma(2r). This age is slightly older, did not homogenise completely within the grain. One but similar to the 1083 F 3 Ma ID-TIMS reference spot analysis conducted on the low-Th core of grain value and reflects the main monazite growth event #2 gave an age of 1125 F 8 Ma, which although close to the peak of granulite facies metamorphism. younger than the core analysis of grain #1, is still Younger ages are also present in the monazite age significantly older than the main monazite growth spectrum. These include ages in the range 1026–1040 event. Since losses of elements from the crystal lattice Ma (#2-2, #3, #4) and one analysis at 994 F 19 Ma may have accompanied the replacement processes, (#5). Grain #2, with three distinct ages of ca. 1125, this intermediate age may reflect partial lead losses 1040 and 1090 Ma, again illustrates the subgrain from an old core. Thus, it cannot be ruled out that complexity of this composite population. Ages in measurement was not influenced by a component of the range 1020–1040 Ma are difficult to relate to inherited Pb from a ca. 1360-Ma-old core that has any known geological activity in the complex, biased the age to be too old. This age, however, is although Rb–Sr ages of 1020 and 1037 Ma have reproduced by analyses #9-2 (1127 F 40 Ma) and been reported for granulites by Compston and Arriens #10-2 (1122 F 32 Ma), the latter being associated (1968) and Richards et al. (1985). These ages were with an internal structure consistent with analysis first interpreted as dating granulite facies metamor- #2-1. It is unlikely that each analysed domain from phism, but given the susceptibility of the Rb–Sr three different grains had lost the same amount of Pb system to fluid intervention, they can as well be to produce identical 207Pb/206Pb ratios and ages. related to pegmatitic activity in the complex. An Moreover, the lack of analyses yielding ages inter- alternative interpretation is that the in situ 207Pb/206Pb mediates between 1360 and ca. 1125 Ma, suggests ages of 1020–1040 Ma reflect discordance associated that 1125 Ma may constitute a true age grouping and with lead losses (Black et al., 1984; Suzuki et al., is thus unlikely to derive from crystal zones having 1994), or that the beam straddled zones of different suffered lead losses during partial secondary replace- ages. This interpretation is supported by SEM images ment. An alternative interpretation is to consider that of analysis #2-2 where the beam appears to have they reflect an early growth event during the prograde struck a crack underlined by bright, high-Th, dots stage of metamorphism. This hypothesis again implies and analysis of grain #3 that clearly straddled a high- that radiogenic lead in monazite can be preserved at Th domain. We thus favour the interpretation that ages temperatures above 800 C and that this mineral can in the range 1020–1040 Ma reflect discordance and thus potentially be used to calculate burial and heating have no geological significance. However, they sug- rates to provide key information on the tectonothermal gest a younger disturbance event. The youngest age evolution of ancient orogen even for rocks subjected from grain #5 (994 F 19 Ma) is identical to the zircon to high-grade metamorphic conditions. The large age from an undeformed pegmatitic dyke of 989 F 2 grain size of the analysed monazites (100–250 mm) Ma (Bruguier et al., 1999) and could be related to may be responsible for Pb retention under these high- pegmatite intrusion and fluid infiltration in the quartz- temperature conditions as suggested by experimental ofeldspathic gneissic sequence. Deformation in the modelling by Smith and Giletti (1997). The sharp age complex ceased by this time (ca. 990 Ma) but ana- discontinuities and complex internal structures, how- tectic conditions and related fluid flows were still ever, suggest that recrystallisation and the associated active at least on a local scale. We speculate that these D. Bosch et al. / Chemical Geology 184 (2002) 151–165 161 conditions were sufficient to trigger partial recrystal- green spinel, relict corundum being present in an lisation or Pb loss by diffusion in some monazite adjacent sample. Garnet–biotite thermometry (Ferry grains during fluid–mineral interaction in the waning and Spear, 1978) gives for this sample, temperatures stages of regional metamorphism and reflect the of 800–820 C for garnet core/primary biotite inclu- susceptibility of monazite to fluid flows even in the sion pairs, and of only 540–560 C for garnet rim/ late stage of metamorphism. Such conclusion is sup- biotite (for P fixed at 4 kbars). The garnet–cordierite ported by U–Pb dating of metapelites that indicates pair gives consistent temperatures of 740 C for the growth of metamorphic monazites associated with same pressure. Amphibole–plagioclase geothermom- pegmatite intrusion (Lanzirotti and Hanson, 1995). etry (Blundy and Holland, 1990) from the adjacent The scarcity of these young ages also indicates that amphibolitised gabbro–norite gives rather high tem- monazites from one single rock can respond differ- perature of equilibration ranging from 880 to 970 C. ently under similar conditions, possibly because of The analysed monazite grains appear as metamor- relatively mild conditions and/or shielding by host phic blasts included in cordierite. Nineteen SIMS minerals. analyses were conducted on 17 monazite crystals (Fig. 5). Analyses yielded a spectrum of 207Pb/206Pb 3.3. Metapelitic gneiss C106 (Egatalis/In Tassak area, ages from 532 to 657 Ma. Of the 19 analyses, the NW Hoggar) youngest was considered as an outlier and rejected from calculation. The remaining 18 analyses can be The studied sample comes from the Egatalis/In combined to provide a weighted mean 207Pb/206Pb Tassak area of Northwestern Hoggar and was col- ratio of 0.05998 F 0.00030 (MSWD = 1.8) corre- lected as part of a comprehensive study to determine sponding to an age of 603 F 11 Ma. The low MSWD the magmatic and metamorphic Precambrian evolu- value suggests that the sample contains a single tion of this part of the Pan-African belt of the Tuareg monazite population, although, on this basis alone, it Shield. The western branch of the Pan-African belt in cannot be ruled out that heterogeneous grains with NW Hoggar (Algeria) comprises fresh granulite facies composite age pattern are present as indicated by rocks in the Egatalis area considered as the deepest scattering of the 207Pb/206Pb ages. In this age range, crustal level exposed south of the Tassendjanet terrane the time resolution of the SIMS technique does not (Caby, 1970, 1987; Black et al., 1994). In this belt, allow distinguishing possible second-order events, late low-pressure high-temperature metamorphic con- which are not separated by about 20 Ma. However, ditions are progressively evidenced westward by the these results indicate that monazite, as young as late overprint of kyanite-bearing mineral assemblages by andalusite. The passage towards the sillimanite zone is observed in schists and aluminous quartzites of Late Paleoproterozoic age (Caby and Andreopoulos- Renaud, 1983) beneath the syn-kinematic Tin Edehou granodiorite–tonalite composite pluton that has the geometry of a gently E-dipping, 2- to 4-km-thick sheet. The C106 sample is a coarse-grained pelitic meta- texite collected at a few metres from the root of a gabbronoritic body. It contains the very fresh mineral assemblage quartz, garnet (alm 73-83, pyr 17-10, gro 02, spe 8-5 from core to rim), brown biotite, cordier- ite, plagioclase (An 24%), perthitic K-feldspar, Fe spinel (1% ZnO), ilmenite and graphite as major phases. The leucosomes contain both antiperthitic plagioclase and perthitic K-feldspar, and fresh cordier- Fig. 5. SIMS 207Pb/206Pb diagram for monazites from the C106 ite containing numerous inclusions of sillimanite and metapelitic gneiss (Egatalis zone, Hoggar). Error bars are 2r. 162 D. Bosch et al. / Chemical Geology 184 (2002) 151–165

Proterozoic in age, can be successfully investigated al., 1998). In the Tassendjanet terrane of Western with the Cameca IMS4f ion probe. Hoggar, the Tin-Zebane dyke swarm, which includes The represent the northernmost dykes and stocks of gabbros, has been dated at exposure of the Trans-Saharan Pan-African belt which 592 F 8 Ma (Hadj-Kaddour et al., 1998). All these formed by aggregation and suturing of continental mafic–ultramafic rocks were emplaced during asthe- fragments along the eastern margin of the West nospheric upwelling related to a rapid lithospheric African craton. Further south in Northern Mali, thinning which affected most of the Hoggar (Cottin et UHP metamorphism has been dated at ca. 620 Ma al., 1998; Hadj-Kaddour et al., 1998). The 603 F 11 (Jahn et al., in press), whereas granulite facies meta- Ma monazite age is consistent with the 592 F 8Ma morphism in the Dahomeyide belt of Western Africa Rb–Sr age of the Tin-Zebane dyke swarm, and the and amphibolite facies metamorphism in the mobile closeness of the C106 metapelite sample with a belt of Nigeria have been dated at 610–620 Ma gabbro–norite suggests that monazite may date (Bruguier et al., 1994; Attoh, 1998; Affaton et al., some point in the retrograde path of the regional 2000). The 620- to 610-Ma age range thus appears to metamorphism following metamorphic peak that correspond to an active period of high-grade meta- was, in the present case, closely related to crystallisa- morphism along the eastern margin of the West tion of the gabbronoritic magma at ca.  10–12 km African craton with subduction of continental litho- depth. Taking into account the 620-Ma U–Pb zircon sphere, collision and suturing of continental fragments age of the syn-kinematic diorite–granodiorite associ- resulting in the formation of the Gondwana super- ation typical of the same belt farther south in Mali continent at the end of the Proterozoic. The temporal (Caby and Andreopoulos-Renaud, 1989), this age is relationship between the 603 F 11 Ma monazite age representative of the late pulse of Pan-African tecto- and regional metamorphism in Northwestern Hoggar nometamorphic evolution in the western part of the is unclear. Although slightly younger, this age over- Tuareg shield. laps the period of high-grade metamorphism along the western margin of the and may be related to the climax of regional metamorphism, 4. Conclusions which peaked at 800–900 Cinthispartofthe Hoggar Mountains. This age is also consistent with TheCamecaIMS4fionmicroprobehasbeen the migmatisation age of 609 F 17 Ma of the Aleksod successfully used to determine in situ 207Pb/206Pb eclogites further west (Barbey et al., 1989). However, ages on monazite crystals from Archaean to Late at In Tassak, the progressive passage from two-mica Proterozoic ages. Molecular interferences can be sep- schists to kinzigites and metatexites only occurs over arated with a mass resolving power of ca. 3500 and an a distance of 150–300 m. This rather sharp thermal energy filtering of 50 eV. Large field and contrast paleogradient is related to the synmetamorphic apertures have been used to increase the number of emplacement of a 300- to 500-m-thick sheet of ions arriving at the detector resulting in an overall gabbronoritic composition (Caby, 1987). This sug- instrumental sensitivity of 3–4 cps/ppm of Pb/nA of gests that monazites record a later event that accom- primary beam. Measurements can be routinely panied emplacement of the gabbronoritic body. achieved with a spatial resolution of ca. 30 mm. As In the southern part of Central Hoggar (Laouni a single measurement requires only 60 min of data Terrane), troctolites, olivine-bearing gabbros and nor- acquisition, a comprehensive study of a monazite ites (Cottin et al., 1998) have been emplaced at the population can be performed in a few days even for end of regional metamorphism within syn-kinematic heterogeneous monazite populations such as those Pan-African granitoids dated at 630–600 Ma by often present in metamorphic rocks. Bertrand et al. (1986). In this area, field relationships Two sets of Proterozoic and Archaean monazites, have been used to bracket emplacement age of the analysed earlier by ID-TIMS technique, yielded mafic–ultramafic intrusions between 600 and 520 nearly identical SIMS and TIMS ages for the main Ma, the latter corresponding to the age of the N–S monazite growth events. SIMS analyses provided elongated late orogenic Taourirt granites (Paquette et 207Pb/206Pb weighted mean ages with uncertainties D. Bosch et al. / Chemical Geology 184 (2002) 151–165 163 ranging from 3 Ma (2r) for Archaean monazites to 6 ampton Complex, pegmatite intrusion during the Ma (2r) for Grenvillian population and to 11 Ma (2r) waning stage of metamorphism was possibly respon- for Pan-African monazite grains. In addition, SIMS sible for discrete disturbances or even partial recrys- analyses make it possible to analyse separately dis- tallisation in pre-existing minerals. In the close tinct age domains present within one single grain. vicinity of magmatic bodies, as documented by the Although second-order events that are not separated Hoggar sample C106, resetting of the U–Pb isotope by ca. 20 Ma cannot be resolved successfully, the age system can reach completion, possibly due to fluid spectrum given by SIMS analyses shows a greater flows and high-temperature gradients. complexity and the technique makes it possible to The polycyclic growth and complexity of monazite unravel complex age patterns presented by composite has opposite consequences, as it can constitute a monazite populations. The technique is thus capable serious drawback to the use of this mineral in U–Pb of dating discrete events that are not represented by geochronology or, on the contrary, provide a wealth of new growth or complete recrystallisation of pre-exist- information that can open up new perspectives in our ing grains. In situ analyses of 207Pb/206Pb ratios in understanding of metamorphic processes, providing such monazite populations can therefore provide a tools can be developed to unravel such within-grain wealth of information on the timing of metamorphic complexity. events and have implications for U–Pb systematic in monazite. In the examples presented above, monazite crystals show complex internal structures, comparable Acknowledgements to those commonly observed for zircons (e.g. Hanchar and Miller, 1993). As for zircon, new growth and We thank J. Kieffer and E. Lebeau from the recrystallisation appear to be a very efficient phenom- ‘‘Service Commun National du SIMS de l’Universite´ enon in monazite and substantiate the usefulness of in de Montpellier II’’ for their help when running the situ analyses. In metamorphic environments, monazite samples. Helpful and constructive reviews by Antonio growth can occur at several periods during the whole Lanzirotti, Alexander Nemchin, and Roberta Rudnick metamorphic history, from the prograde stages to the are greatly appreciated. RR retrograde part of the metamorphic path. In amphib- olite facies metapelites, monazite grows during the prograde stage and, due to the robustness of the U– References Th–Pb systems, generally preserves information on this part of the P–T–t path. In such environments and Affaton, P., Kro¨ner, A., Seddoh, K.F., 2000. Pan-African granulite providing textural relationship can be established, formation in the Kabye massif of northern Togo (West Africa): monazite can be used to estimate burial and heating Pb–Pb zircon ages. Int. J. Earth Sci. 88, 778–790. rates. In granulite facies rocks, recrystallisation of Attoh, K., 1998. High-pressure granulite facies metamorphism in crystals inherited from source regions or grown during the Pan-African Dahomeyide orogen, West Africa. J. Geol. 106, 236–246. the prograde stages of metamorphism is almost com- Barbey, P., Bertrand, J.M., Angoua, S., Dautel, D., 1989. Petrology plete, but preservation of radiogenic lead in monazite and U–Pb geochronology of the Telohat migmatites, Aleksod, domains substantiates robustness of the U–Pb system central Hoggar, Algeria. Contrib. Mineral. Petrol. 10, 207–219. in this mineral which was not completely reset by Bertrand, J.M., Michard, A., Boullier, A.M., Dautel, D., 1986. peak temperature of 800–900 C. In situ analysis of Structure and U/Pb geochronology of Central Hoggar (Algeria): a reappraisal of its Pan-African evolution. Tectonic 5, 955–972. such domains constitutes a window to look back into Bingen, B., van Breemen, O., 1998. U–Pb monazite ages in am- parts of the metamorphic history, which are generally phibolite- to granulite-facies orthogneiss reflect hydrous mineral inaccessible due to blotting out of primary assemb- breakdown reactions: Sveconorwegian Province of SW Norway. lages. Monazite populations can even show a greater Contrib. Mineral. Petrol. 132, 336–353. complexity due to possible new growth, recrystallisa- Black, L.P., Fitzgerald, J.D., Harley, S.L., 1984. Pb isotopic com- position, colour and microstructure of monazites from a poly- tion or partial Pb loss during the retrograde part of metamorphic rock in Antarctica. Contrib. Mineral. Petrol. 85, metamorphism, which is often dominated by an 141–148. important magmatic activity. In the case of the North- Black, R., Latouche, L., Lie´geois, J.P., Caby, R., Bertrand, J.M., 164 D. Bosch et al. / Chemical Geology 184 (2002) 151–165

1994. Pan-African displaced terranes in the Tuareg shield (cen- swarm (Pan-African Tuareg Shield, Algeria): prevalent mantle tral ). Geology 22, 641–644. signature and late agpaitic differentiation. Lithos 45, 223–243. Blundy, J.D., Holland, T.J.B., 1990. Calcic amphibole equilibria and Hanchar, J.M., Miller, C.F., 1993. Zircon zonation patterns as re- a new amphibole–plagioclase geothermometer. Contrib. Miner- vealed by cathodoluminescence and backscattered electron im- al. Petrol. 104, 208–224. ages: implications for interpretation of complex crustal histories. Bosch, D., Bruguier, O., Pidgeon, R.T., 1996. Evolution of an Ar- Chem. Geol. 11, 1–15. chean metamorphic belt: a conventional and SHRIMP U–Pb Harrison, T.M., Mackeegan, K.D., Lefort, P., 1995. Detection of study of accessory minerals from the Jimperding Metamorphic inherited monazite in the Manaslu leucogranite by 208Pb/232Th Belt, Yilgran Craton, Western Australia. J. Geol. 104, 695–711. ion microprobe dating: crystallization age and tectonic implica- Bruguier, O., Dada, S.S., Lancelot, J.R., 1994. Early Archean com- tions. Earth Planet. Sci. Lett. 133, 271–282. ponent ( > 3.5 Ga) within a 3.05 Ga orthogneiss from Northern Hawkins, D.P., Bowring, S.A., 1997. U–Pb systematics of mona- Nigeria: U–Pb zircon evidence. Earth Planet. Sci. Lett. 125, zite and xenotime: case studies from the Paleoproterozoic of the 89–103. Grand Canyon, Arizona. Contrib. Mineral. Petrol. 127, 87–103. Bruguier, O., Bosch, D., Pidgeon, R.T., Byrne, D., Harris, L., 1999. Jahn, B.M., Caby, R., Monie´, P., in press. Precambrian UHP eclo- U–Pb chronology of the Northampton Complex, Western Aus- gites from Northern Mali, West Africa: age of UHP metamor- tralia—evidence for Grenvillian sedimentation, metamorphism phism, nature of protoliths and tectonic implications. Chem. and deformation and geodynamic implications. Contrib. Miner- Geol. al. Petrol. 136, 258–272. Lanzirotti, A., Hanson, G.N., 1995. U– Pb dating of major and Caby, R., 1970. La chaine pharusienne dans le NW de l’Ahaggar accessory minerals formed during metamorphism and deforma- (Sahara central; Alge´rie): sa place dans l’orogene`se du Pre´cam- tion of metapelites. Geochim. Cosmochim. Acta 59, 2513– brien supe´rieur en Afrique. PhD Thesis, University of Montpel- 2526. lier, 289 pp. Ludwig, K.R., 1999. User’s Manual for Isoplot/Ex, version 2.06. Caby, R., 1987. The Pan-African belt of west Africa from the Sa- Berkeley Geochronology Center Spec. Publ., no. 1a, 49 pp. hara to the Gulf of Benin. In: Schaer, J., Rodgers, P. Montel, J.M., Foret, S., Veschambre, M., Nicollet, C., Provost, A., (Eds.), Anatomy of Mountain Ranges. Princeton Univ. Press, 1996. Electron microprobe dating of monazite. Chem. Geol. Princeton, NJ, pp. 129–170. 131, 37–53. Caby, R., Andreopoulos-Renaud, U., 1983. Age a` 1800 Ma du Nemchin, A.A., Pidgeon, R.T., Wilde, S.A., 1994. Timing of Late magmatisme sub-alcalin associe´ aux me´tase´diments monocycli- Archean granulite facies metamorphism in the Southwestern ques dans la chaıˆne Pan-Africaine du Sahara central. J. Afr. Yilgarn Craton of Western Australia: evidence from U–Pb ages Earth Sci. 1, 193–197. of zircons from mafic granulites. Precambrian Res. 68, 307– Caby, R., Andreopoulos-Renaud, U., 1989. Age U–Pb a` 620 Ma 321. d’un pluton synoroge´nique de l’Adrar des Iforas (Mali). Con- Paquette, J.L., Caby, R., Djouadi, M.T., Bouchez, J.L., 1998. U–Pb se´quences pour l’aˆge de la phase majeure de l’oroge`ne Pan- dating of the Pan-African in the Tuareg shield: the post- Africaine. C. R. Acad. Sci. Paris 308, 307–314. collisional syn-shear Tioueine pluton (western Hoggar, Algeria). Cocherie, A., Legendre, O., Peucat, J.J., Kouamelan, A.N., 1998. Lithos 45, 245–253. Geochronology of polygenetic monazites constrained by in situ Parrish, R., 1990. U–Pb dating of monazite and its application to electron microprobe Th–U–total lead determination: implica- geological problems. Can. J. Earth Sci. 27, 1431–1450. tions for lead behaviour in monazite. Geochim. Cosmochim. Pidgeon, R.T., 1992. Recrystallisation of oscillatory zoned zircon: Acta 62, 2475–2497. some geochronological and petrological implications. Contrib. Compston, W., Arriens, P.A., 1968. The Precambrian geochronol- Mineral. Petrol. 110, 463–472. ogy of Australia. Can. J. Earth Sci. 5, 561–583. Pidgeon, R.T., Bosch, D., Bruguier, O., 1996. Inherited zircon and Copeland, P., Parrish, R.R., Harrison, T.M., 1988. Identification of titanite U–Pb systems in an Archean Katrine syenite from inherited radiogenic Pb in monazite and its implications for U– southwestern Australia: implications for U–Pb stability of titan- Pb systematics. Nature 333, 760–763. ite. Earth Planet. Sci. Lett. 141, 187–198. Cottin, J.Y., Lorand, J.P., Agrinier, P., Bodinier, J.L., Liegeois, J.P., Poitrasson, F., Chenery, S., Shepherd, T.J., 2000. Electron microp- 1998. Isotopic (O, Sr, Nd) and trace element geochemistry of the robe and LA-ICP-MS study of monazite hydrothermal altera- Laouni layered intrusions (Pan-African belt, Hoggar, Algeria): tion: implications for U– Th – Pb geochronology and nuclear evidence for post-collisional continental tholeiitic magmas var- ceramics. Geochim. Cosmochim. Acta 64, 3283–3297. iably contaminated by continental crust. Lithos 45, 197–222. Richards, J.R., Blockley, J.G., De Laeter, J.R., 1985. Rb–Sr and Pb De Wolf, C.P., Belshaw, N., O’Nions, R.K., 1993. A metamorphic isotope data from the Northampton Block, Western Australia. history from micron-scale 207Pb/206Pb chronometry of Archean Bull. Proc.-Australia Inst. Min. Metall. 290, 43–55. monazite. Earth Planet. Sci. Lett. 120, 207–220. Scha¨rer, U., 1984. The effect of initial 230Th disequilibrium on Ferry, J.M., Spear, F.S., 1978. Experimental calibration of the par- young U–Pb ages: the Makalu case, Himalaya. Earth Planet. titioning of Fe and Mg between biotite and garnet. Contrib. Sci. Lett. 67, 191–194. Mineral. Petrol. 66, 113–117. Smith, H.A., Barreiro, B., 1990. Monazite U–Pb dating of staur- Hadj-Kaddour, Z., Lie´geois, J.P., Demaiffe, D., Caby, R., 1998. The olite grade metamorphism in pelitic schists. Contrib. Mineral. alkaline–peralkaline granitic post-collisional Tin Zebane dyke Petrol. 105, 602–615. D. Bosch et al. / Chemical Geology 184 (2002) 151–165 165

Smith, H.A., Giletti, B.J., 1997. Lead diffusion in monazite. Geo- sode of early Mesoproterozoic metamorphic fluid flow in the chim. Cosmochim. Acta 61, 1047–1055. Reynolds Range, Central Australia. J. Met. Petrol. 14, 29–47. Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead Zhu, X.K., O’Nions, R.K., Belshaw, N.S., Gibb, A.J., 1997. Sig- isotope evolution by a two stage model. Earth Planet. Sci. Lett. nificance of in situ SIMS chronometry of zoned monazite from 6, 15–25. the Lewisian granulites, NW Scotland. Chem. Geol. 135, 35– Suzuki, K., Adachi, M., Kajizuka, I., 1994. Electron microprobe 53. observations of Pb diffusion in metamorphosed detrital mona- Zhu, X.K., O’Nions, R.K., Gibb, A.J., 1998. SIMS analysis of U– zites. Earth Planet. Sci. Lett. 128, 391–405. Pb isotopes in monazite: matrix effects. Chem. Geol. 144, 305– Williams, I.S., Buick, L.S., Cartwright, I., 1996. An extended epi- 312.