Contrib Mineral Petrol (2005) 150: 146–155 DOI 10.1007/s00410-005-0006-y

O R I GI N A L P A P E R

Birger Rasmussen

Zircon growth in very low grade metasedimentary rocks: evidence for zirconium mobility at 250°C

Received: 15 November 2004 / Accepted: 14 May 2005 / Published online: 29 July 2005 Ó Springer-Verlag 2005

Abstract Zircon outgrowths are present on detrital zircon grains in many very low to low-grade metasedimentary rocks worldwide, ranging in age from midArchaean to Palaeozoic. The outgrowths comprise minute (typically <3 lm) crystals that form an irregular fringe on detrital zircon grains, and in a few cases, on diagenetic xenotime outgrowths. Textural relationships indicate that while zircon growth postdates diagenetic xenotime precipitation, it precedes or is synchronous with metamorphic xenotime formation. Unlike xenotime, zircon outgrowths are absent in unmetamorphosed sedimentary rocks, and only appear in prehnite-pumpellyite facies rocks, suggesting that zircon growth commences at temperatures of 250°C. The greater abundance of zircon outgrowths in shales than in to other sedimentary rocks may relate to higher halogen concentrations, which have been linked to enhanced zirconium mobility in hydrothermal systems. The growth of zircon in metasedimentary rocks indicates that zirconium was transported in aqueous fluids, possibly as fluorine complexes, during very low-grade metamorphism.

Introduction Zircon (ZrSiO4) is a common accessory mineral that occurs in a variety of igneous, metamorphic, and sedimentary rocks. It is one of the most widely used accessory minerals in U–Th–Pb geochronology (Ireland and Williams 2003; Parrish and Noble 2003), and using other

Communicated by T.L. Grove B. Rasmussen School of Earth and Geographical Sciences, University of Western Australia, Crawley, WA, 6009 Australia E-mail: [email protected] Tel.: +61-8-64882666 Fax: +61-8-64881037

isotopic systems (e.g., Lu–Hf, Sm–Nd and O), is an important source of information on the evolution of the crust (Amelin et al. 1999; Peck et al. 2001; Kinny and Maas 2003). A major factor in its widespread application is its remarkable physical and chemical stability demonstrated by its ability to provide precise dates even for the oldest known terrestrial rocks (Stern and Bleeker 1998; Bowring and Williams 1999) and terrestrial material (Wilde et al. 2001; Mojzsis et al. 2001), despite being subjected to long and complex metamorphic histories. Zircon in sedimentary rocks is largely derived from igneous and high-grade metamorphic rocks and has been widely used in U–Pb geochronological studies for provenance analysis (Fedo et al. 2003). The virtual ubiquity of zircon in sedimentary rocks reflects its extreme durability during weathering, transportation, and diagenesis. Zirconium is traditionally considered to be immobile under low-temperature conditions, but a number of studies indicate that zircon may form during hydrothermal alteration (Rubin et al. 1989, 1993; Kerrich and King 1993), greenschist facies metamorphism of slates (at 350°C; Dempster et al. 2004), and contact metamorphism (at 500–600°C; Fraser et al. 2004). In this study, zircon growth is shown to be a widespread process in prehnite-pumpellyite and greenschist facies metasedimentary rocks of all ages, providing compelling evidence for low-temperature zirconium mobility.

Methods and samples Over one thousand polished thin sections have been studied for outgrowths on detrital zircon grains. The aim of this work was largely to investigate authigenic xenotime growth under diagenetic and metamorphic conditions (see Rasmussen 1996, 2005). As part of this work, other mineral outgrowths were also observed, including zircon, which is the focus of this paper. The samples, which comprise unmetamorphosed and metamorphosed (prehnite-pumpellyite to amphibolite

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facies) sedimentary rocks, were examined by optical microscope and scanning electron microscope (SEM) using backscattered-electron (BSE) and cathodoluminescence (CL) techniques. Minerals were identified by optical microscopy and SEM-energy dispersive X-ray spectrometry (SEM-EDS). Samples include sandstone, conglomerate, siltstone, and mudstone (and their metamorphic equivalents), and vary in age from Archaean to Cenozoic. Most samples examined in this study are from Precambrian successions in Western Australia (Fig. 1). Of these, four formations were studied in detail, including the Late Archaean Mount McRae Shale (Hamersley Group), Late Archaean Jeerinah Formation (Fortescue Group), Palaeoproterozoic Maraloou Formation (Capricorn Orogen), and the Palaeoproterozoic Mount Barren Group (Albany-Fraser Orogen). The 2.65 Ga Jeerinah Formation (Fortescue Group) and 2.5 Ga Mount McRae Shale (Hamersley

Fig. 1 Map showing location of sample sites in Western Australia containing zircon outgrowths. 1 Cardup Group, 2 Stirling Range Formation, 3 Mount Barren Group, 4 Maraloou Formation, 5 Bangemall Supergroup, 6 Jeerinah Formation (Fortescue Group), 7 Whim Creek Group, 8 Gorge Creek Group, 9 Mount McRae Shale (Hamersley Group), 10 Bee Gorge Member (Wittenoom Formation), 11 Beazley River Quartzite (Lower Wyloo Group), 12 Hardey Formation (Fortescue Group), 13 Mt Belches Sandstone (Kurrawang Sequence, Eastern Goldfields Province) (Fig. 6b, e in Krapez et al. 2000)

Group) are part of the Mount Bruce Supergroup that overlies the Archaean granite-greenstone terrains of northwestern Australia. Most of the Mount Bruce Supergroup has undergone only very low-grade metamorphism, prehnite–pumpellyite in the north to greenschist in the south, and is relatively undeformed (Smith et al. 1982), although southern exposures have experienced several episodes of deformation. In the uppermost Jeerinah Formation, carbonaceous, pyritic shales predominate in a sequence also containing minor turbidite and chert (Roy Hill Shale Member). The shales contain metamorphic monazite that yields a 207Pb/206Pb age of 2192±5 Ma, interpreted as the timing of hydrothermal fluid flow (Rasmussen et al. 2001). The shales have total organic carbon (TOC) contents of up to 16.5% (Hayes et al. 1983; Brocks et al. 1999). Samples of the Jeerinah Formation are from drill-holes across the Pilbara Craton (including WRL-1, DDH-186, RHDH2A, FVG-1). The Mount McRae Shale comprises carbonaceous (TOC contents of up to 7%), pyritic shales that are interbedded with chert, carbonate turbidites, and banded iron-formation. Samples of the Mount McRae Shale are from the Mount Tom Price and Mount Whaleback (Mount Newman) iron-ore mines in the Hamersley Province. Samples from the Maraloou Formation, Capricorn Orogen, are from drill-core (KDD1). The succession consists mainly of carbonaceous shale that was deposited in a restricted marine or lacustrine environment (Krapez and Martin 1996; Pirajno and Occhipinti 2000). The shales are interlayered with pillow basalts of the Killara Formation. They have been intruded by subvolcanic dolerite sills related to lavas, which caused contact metamorphism and hydrothermal fluid flow resulting in metamorphic monazite growth. The monazite yields an age of 1,843±10 Ma, which is interpreted as the age of intrusion by the sills, and a minimum age for the Maraloou Formation (Rasmussen and Fletcher 2002). The presence of peperitic margins on the sills and the development of fine coke mosaics in intruded kerogenous shale indicate that the magma intruded wet, shallowly buried sediments. The Palaeoproterozoic Mount Barren Group in southwestern Australia comprises mainly quartzite, phyllite, and schist, with minor conglomerate and dolomite, intruded by a thick mafic sill (Thom and Chin 1984; Witt 1997; Wetherley 1998). The succession was deposited in a shallow marine environment between 1.7 Ga and 1.8 Ga, based on diagenetic xenotime (Vallini et al. 2002) and detrital zircon geochronology (Dawson et al. 2002), respectively. The rocks experienced lower greenschist to mid-upper amphibolite facies metamorphism (8 kbars and 630°C) and deformation (Witt 1997; Wetherley 1998) during collision (1.3– 1.1 Ga) in the Albany-Fraser Orogen (Clark et al. 2000). Samples of quartzite, phyllite, and quartz–mica schist are from outcrop localities, whereas samples of phosphatic sandstone are from a drill-hole (MYCD058; AMG 242,189 mE, 6,261,023 mN; Fig. 1).

148 Fig. 2 a–d Back-scattered electron (BSE) images of detrital zircon (zr) grains lined by minute, pyramidal xenotime (xt) outgrowths. Millstone Grit, Lower Carboniferous, United Kingdom. e, f Coarse xt overgrowths enclosing detrital zr grains. Mount Barren Group, Albany-Fraser Orogen, southwestern Australia

Results In unmetamorphosed sedimentary rocks, SEM-EDS analysis shows that the mineral outgrowth is almost

exclusively xenotime (YPO4) (Fig. 2a–f), and in a few cases thorite (ThSiO4). Zircon outgrowths, confirmed by SEM-EDS analysis, occur only in metamorphosed sedimentary rocks: in this study, 19 formations were

Table 1 Metasedimentary rock successions containing zircon outgrowths Formation and location

Lithology

Age

Metamorphic grade

Normanskill Formation, New York State, USA Lower Roan Group, Mufulira mine, Zambia Cardup Group, southwestern Australia Bangemall Supergroup, Western Australia (WA) Chorhat Sandstone, Lower Vindhyan, India Mount Barren Group, Albany-Fraser Orogen, WA

Palaeozoic Neoproterozoic Proterozoic Mesoproterozoic Palaeoproterozoic Palaeoproterozoic

Stirling Range Formation, Albany-Fraser Orogen, WA

Shale Quartzite Slate Shale Porcellanite Quartzite and phyllite Quartzite

Palaeoproterozoic

Wildman Siltstone, Pine Creek Orogen, northern Australia

Shale

Palaeoproterozoic

Maraloou Formation, Yerrida Basin, WA Lower Wyloo Group, Ashburton Basin, WA

Palaeoproterozoic Palaeoproterozoic

Bee Gorge Member, Wittenoom Formation, Hamersley Group, WA Mount McRae Shale, Hamersley Group, WA Jeerinah Formation, Fortescue Group, WA

Shale Quartzite and shale Shale

Prehnite–pumpellyite Greenschist Prehnite–pumpellyite Prehnite–pumpellyite Prehnite–pumpellyite Lower greenschist to amphibolite Prehnite–pumpellyite greenschist Andalusite hornfels, greenschist Prehnite–pumpellyite Prehnite–pumpellyite greenschist Prehnite–pumpellyite

Shale Shale

Late Archaean Late Archaean

Hardey Formation, Fortescue Group, WA

Quartzite

Late Archaean

Transvaal Supergroup, South Africa

Shale

Pongola Supergroup, South Africa

Quartzite

Late Archaean to Palaeoproterozoic Archaean

Gorge Creek Group, Pilbara Craton, WA Whim Creek Group, Pilbara Craton, WA Isua, Greenland

Shale Slate Schist

Mid-Archaean Mid-Archaean Archaean

Late Archaean

Prehnite–pumpellyite Prehnite–pumpellyite greenschist Prehnite–pumpellyite greenschist Lower greenschist Prehnite–pumpellyite greenschist Prehnite–pumpellyite Greenschist Amphibolite

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found to contain zircon outgrowths (Table 1). Virtually all zircon outgrowths were studied using polished thin sections, but outgrowths also occur on zircon from heavy mineral separates (see Fig. 6b in Krapez et al. 2000). Zircon outgrowths are most abundant in fine-grained lithologies (e.g., shale), where they typically occur as minute (<3 lm), irregular crystals attached to the surface of detrital zircon grains (Fig. 3a–l), but are also found in quartzites (Fig. 4a–c). The zircon crystals range from irregular to finger-like projections (Fig. 3e, h) to incomplete hexagonal prisms (Figs. 3j, l, 4a). The outgrowths commonly form frilly rims that partly surround detrital zircon. In some samples, outgrowths line the

Fig. 3 The BSE images of detrital zircon grains lined by minute, irregular zr outgrowths. a–f, h and i Mount McRae Shale, Hamersley Group, northwestern Australiag and k Jeerinah Formation, Fortescue Group, northwestern Australia; j Transvaal Supergroup, southern Africa, l Mount Barren Group, Albany-Fraser Orogen, southwestern Australia

surface of fragments of a detrital zircon grain (Fig. 4d, e), indicating that zircon growth postdated sediment deposition. The outgrowths engulf matrix particles, and may form larger, planar outgrowths with a porous, inclusionrich interior (Fig. 5a–f). In contrast to the findings of Dempster et al. (2004) for slates from the Scottish Highlands, discrete authigenic zircon crystals were not identified. In shales, zircon grains are generally <30 lm in size and sub-rounded to angular in shape. Most zircon grains do not have evidence of dissolution (cf. Fig. 2i in Dempster et al. 2004). The zircon host ranges from altered, probably metamict varieties, with radial cracks

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(Fig. 3e), to grains showing no evidence of radiation damage (Figs. 3a, 5c): the zircon outgrowths do not show any affinity for metamict grains as a preferred substrate. Most grains display oscillatory growth zoning that continues to the boundary. The compositional zonation of the host is not present in the outgrowths. Rather, outgrowths display irregular zoning and are commonly darker (using BSE imaging) than host zircon, suggesting a slight difference in composition. In shales from the Whim Creek Group, Gorge Creek Group, Bee Gorge Member (Hamersley Group), and Bangemall Supergroup, some of the outgrowths are composed of Zr, Si, and Th, probably reflecting solid solution between isostructural zircon and thorite (ThSiO4). Authigenic xenotime and zircon outgrowths commonly co-exist on detrital zircon grains (Fig. 6a–j). In phyllites from the Mount Barren Group, zircon outgrowths are typically engulfed by metamorphic xenotime (Fig. 6a, b). In places, metamorphic xenotime has grown onto detrital zircon and zircon outgrowths (Fig. 6c) or is attached only to the outgrowths which have completely overgrown the detrital zircon (Fig. 6d). Where zircon outgrowths occur with diagenetic xenotime, zircon outgrowths are restricted to areas of the detrital zircon surface devoid of xenotime (Fig. 6e). In carbonaceous shales from the Maraloou Formation, minute zircon outgrowths are present on the outer margin of the innermost xenotime overgrowth, commonly intergrown or enclosed by outer zones of xeno-

Fig. 4 a The BSE image of rounded detrital zircon grain with minute zircon outgrowths. b The BSE image of zircon outgrowths, which partly enclose authigenic xt. c The BSE image of highly altered detrital zircon with two equant inclusions lined by possible zircon outgrowths (see arrows). d BSE image of two zircon grains, one of which has been fractured and comprises three fragments. e Closeup of inset in 4d showing minute zr outgrowths on fractured surfaces. a Cu-mineralized sandstone, Lower Roan Group, Mufulira mine, Zambia, b, c pebble conglomerate, Pongola Supergroup, South Africa. d, e Whim Creek Group, Pilbara Craton, northwestern Australia

time (Fig. 6g, h). The inner zone of xenotime is probably diagenetic in origin, whereas the outer zone formed during hydrothermal fluid flow initiated by the intrusion of mafic sills (Rasmussen and Fletcher 2002). In carbonaceous shales from the Transvaal Supergroup, zircon outgrowths are engulfed by inclusion-rich, metamorphic xenotime crystals (Fig. 6i, j) that are randomly distributed throughout the shale matrix.

Discussion The irregular and delicate nature of the zircon outgrowths (see Fig. 3) precludes their survival during abrasion and transportation, and therefore indicates a post-depositional origin. This interpretation is supported by other observations, including, (1) the presence of minute zircon outgrowths on the surfaces of fractured detrital grains, (2) intergrowth with matrix particles, and (3) the growth of zircon on the outer margin of diagenetic xenotime. Syntaxial outgrowths on detrital zircon grains have been reported previously in sedimentary rocks (Butterfield 1936; Smithson 1937, 1940; Tyler et al. 1940; Bond 1948; Hutton 1950; Kilpady and Deshpande 1955; Awasthi 1961; Milner 1962). Due to the minute size of the outgrowths, the identity of the mineral was uncertain, but it was widely considered to be zircon (Pettijohn 1949; Twenhofel 1950; Packam and Crook 1960; Rajulu

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Fig. 5 a–f The BSE images of detrital zircon grains with coarse, porous zr outgrowths and irregular patches of metamorphic xt outgrowths. Mount Barren group, Albany-Fraser Orogen, southwestern Australia.

and Nagaraja 1966; Saxena 1966a, b, 1968). However, the notion that zircon could form during diagenesis was treated with some skepticism (Smithson 1940; Marshall 1967, 1968; Kalsbeek 1967). In this study, zircon outgrowths were only found in metasedimentary rocks, suggesting that zircon formation occurred during metamorphism. In samples from the Mount McRae Shale, where zircon outgrowths are particularly abundant, kerogen has organic reflectance (Ro) values that average 3.7–4.3% (Taylor et al. 2001), consistent with the rank of anthracite and maximum temperatures of 200–250°C (Tissot and Welte 1978). From the same region, zircon outgrowths in the Jeerinah Formation have also undergone prehnite-pumpellyite facies metamorphism based on mineral assemblages in underlying mafic volcanic rocks (Smith et al. 1982). These temperatures are supported by organic reflectivity (Ro) measurements on kerogen in the shales, which yield values between 4.5–6% (vitrinite equivalent; Rasmussen 2005), suggestive of maximum temperatures of 200–300°C. These observations suggest that new zircon growth may commence during very low-grade metamorphism, at temperatures as low as 200–250°C.

Metamorphic zircon has been reported in upper amphibolite and granulite facies rocks (Fraser et al. 1997; Pan 1997; Bingen et al. 2001; Whitehouse and Platt 2003), where new zircon growth is linked to a variety of metamorphic reactions involving the breakdown of Zr-bearing phases, such as garnet and hornblende (Fraser et al. 1997; Degeling et al. 2001), ilmenite (Bingen et al. 2001), biotite (Vavra et al. 1996), zirconolite (CaZrTi2O7) and igneous zircon (Pan 1997). However, in sub-upper amphibolite facies rocks, new zircon is regarded as extremely rare and ‘‘metamorphic zircon‘‘ is more commonly considered the product of recystallization of protolith zircon (Hoskin and Black 2000; Hoskin and Schaltegger 2003). The absence of new zircon in very low and low-grade rocks is thought to reflect the high stability of zircon during metamorphism and the immobile nature of Zr in low-temperature fluids. However, high Zr concentrations (up to 0.2 wt% oxide) have been recorded in authigenic titanite in Permian sandstones of the Cock of Arran, Scotland (Hole et al. 1992). The pore-filling titanite is interpreted to have formed from hydrothermal fluids, carrying Zr and other high field strength elements as halogen or carbonate complexes, generated during granitoid intrusion (Hole et al. 1992). Further evidence for Zr mobility has been recorded in contact metamorphosed and metasomatised carbonates, where accessory minerals such as zirconolite, allanite, and titanite precipitated from hydrothermal fluids generated by granitoid intrusion (Giere´ 1986; Giere´ and Williams 1992). The abundance of F-rich phlogopite, pargasite, titanian clinohumite, and fluor-apatite demonstrate that fluorine was an important component of the fluid (Giere´ 1986). Elevated fluorine concentrations have also been implicated in hydrothermal zircon growth in fluorite-mineralized limestones adjacent to granitoids (Rubin et al. 1989, 1993). Indeed, Hoskin (1999) reports concentrations of >2,000 ppm fluorine in hydrothermal zircon crystals, and suggests a possible role for F-ligands in zirconium transportation during W–Au minerlization. The greater abundance of zircon outgrowths in shales relative to sandstones does not reflect average Zr concentrations, which are similar for both rock types (200 ppm; Erlank et al. 1969; Gao et al. 1998). Rather, the observed differences may relate to the higher fluorine content in pelites, which have average concentrations between 780–940 ppm, in comparison to sandstones, where average contents range between 180–489 ppm (Fleischer and Robinson 1963; Koritnig 1969; Gao et al. 1998). Major hosts of fluorine in sedimentary rocks are clay minerals (Koritnig 1969; Saether et al. 1981), in which the fluoride ion (F ) readily substitutes for the hydroxyl ion (OH ) in crystal structures, particularly 2:1 layer silicates such as smectite, illite, and mica (Thomas et al. 1977; Chipera and Bish 2002). During burial and the onset of metamorphism, clay minerals undergo dehydration reactions resulting in the release of OH and other co-substituting anions such as F , Cl , and Br , potentially forming a highly reactive fluid. Fluoride

152 Fig. 6 a The BSE image of an angular detrital zircon grain lined by irregular zircon outgrowths, some of which are enclosed by metamorphic xt. b Detrital zircon grain lined by zr outgrowths and metamorphic xt. c, d Zircon grain with welldeveloped zr outgrowths upon which metamorphic xt has nucleated and grown. e Fragment of rounded zircon grain with minute zr outgrowths and diagenetic xt. Note the absence of zircon outgrowths on the detrital zircon surface with diagenetic xenotime. f Thin, elongate, zircon outgrowths enclosed by metamorphic xenotime. g An irregular, detrital xenotime grain enclosed by diagenetic xenotime (xt1) upon which zr outgrowths and hydrothermal xenotime overgrowths (xt2) have precipitated. h detrital zircon grain engulfed by diagenetic and hydrothermal xenotime (xt1 and xt2, respectively). Note presence of discrete zr crystals included in outer xenotime layer. i close-up of a detrital zircon grain lined by zr outgrowths that are partly engulfed by metamorphic xt. j An irregular detrital zircon grain lined by zr outgrowths. The zircon is enclosed in inclusion-rich, metamorphic xt cement. a–d, f Mount Barren Group, Albany-Fraser Orogen, southwestern Australia, e Jeerinah Formation, Fortescue Group, northwestern Australia, g, h Maraloou Formation, Capricorn Orogen, northwestern Australia, i, j Transvaal Supergroup, southern Africa

ions are probably released at higher temperatures than OH because they tend to form stronger bonds within 2:1 layer silicates (Chipera and Bish 2002). The dissolution of apatite and carbonate, which are also significant hosts of F and Cl , may contribute additional halogens to the pore water. Fluorine is often concentrated in contact aureoles or metasomatic rocks (Fleischer and Robinson 1963; Ronov et al. 1974), where it has been linked with hydrothermal Zr mobility and new zircon growth (Giere´ 1986; Rubin et al. 1989, 1993).

Similarly, it is conceivable that the production of F (as well as Cl , Br ) complexes during very low-grade metamorphism, increased Zr solubility and mobility in shales. Once the Zr was mobilized, detrital zircon grains facilitated zircon precipitation by providing a suitable isostructural substrate for nucleation and growth. The source of the Zr is problematic. Zirconium concentrations in river water (30–170 parts per trillion [ppt]) and ocean water (2–30 ppt) are very low (Broecker and Peng 1982; McKelvey and Orians 1993;

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Godfrey et al. 1996). In sedimentary rocks, potential sources include unstable Zr-bearing detrital grains (e.g., ilmenite; Erlank et al. 1969; Bingen et al. 2001), Zr complexes adsorbed onto fine-grained particles, and detrital zircon. Crystalline zircon is extremely insoluble and is stable during hydrothermal alteration and medium- to high-grade metamorphism. However, radiation damage to the crystal structure from the decay of U and Th can increase Zr solubility and lead to etching and dissolution of affected regions by reaction with HF vapour (Krogh and Davis 1975). Damaged regions are susceptible to leaching of Zr, Si, and Pb during lowtemperature hydrothermal alteration (Geisler et al. 2001, 2003). Hydrothermal dissolution of high-U (metamict) zircon grains is a common process in lowgrade metamorphic rocks and typically produces U–Pb dates that are younger than the formation age (e.g., Krapez et al. 2000; Pickard 2002). Most samples studied here contain detrital zircon grains, and many of these contain regions affected by radiation damage, and thus metamict zircon represents a likely source of Zr (cf. Dempster et al. 2004). However, most detrital zircon grains do not appear to have been affected by dissolution, so other sources, such as altered detrital Fe–Ti oxide grains, may also have contributed trace quantities of Zr. Detrital baddeleyite (ZrO2) and zirconolite were not observed in any of the samples studied here. Both minerals are relatively common accessory minerals in mafic igneous rocks (Heaman and LeCheminant 1993; Rasmussen and Fletcher 2004), and may be expected as trace constituents in some sedimentary rocks. Their absence may reflect post-depositional alteration and dissolution, in which case, both minerals are potential sources of Zr. The scale of Zr mobility is difficult to assess, but the presence of zircon outgrowths on non-metamict detrital zircon grains suggests movement beyond the grain-scale. Widespread growth of new zircon in prehnitepumpellyite and greenschist facies rocks may provide a new chronometer of low-temperature metamorphism. Zircon is the most widely used accessory mineral in U– Th–Pb geochronology, but its application has been restricted to dating magmatic and high-grade metamorphic rocks. The development of isotopic techniques for dating zircon outgrowths may help constrain the timing of low-temperature metamorphism and hydrothermal alteration. However, a potential limitation is the minute size of outgrowths (typically <5 lm), necessitating in situ techniques such as electron microprobe or ion microprobe dating. Chemical dating by electron microprobe has the required spatial resolution to date the outgrowths (1–3 lm), but the technique generally produces low-precision dates and is not ideal for zircon. The spatial resolution of ion microprobes is >5 lm, which is not sufficiently small to analyse most zircon outgrowths. Perhaps the successful application of low-grade zircon dating awaits the development of in situ isotopic techniques with 1–2 lm spatial resolution (e.g., Stern et al., in press).

Conclusions Zircon outgrowths are relatively common in prehnitepumpellyite and greenschist facies metasedimentary rocks. Petrographic textures suggest that the zircon outgrowths form after diagenetic xenotime and are synchronous with or predate metamorphic xenotime formation. The widespread growth of zircon in metapelites indicates that Zr is mobile during very low-grade metamorphism (250°C). The Zr was probably derived from hydrothermal leaching of metamict zircon and ilmenite grains and was transported as halogen complexes, of which fluorine was probably the most important. The highly reactive halogen compounds are thought to have been produced during dehydration of clay minerals and dissolution of apatite associated with very low-grade metamorphism. Because Zr is a critical diagnostic element in petrogenetic studies of igneous rocks, the widespread mobility of Zr during very lowgrade metamorphism and hydrothermal alteration indicates that caution should be employed in its use in the interpretation of trace element abundances and element ratios. With the further development of in situ isotopic techniques, the growth of zircon during very low grade metamorphism may also provide a valuable mineral chronometer of low-temperature processes. Acknowledgements I thank Ian Fletcher, Mary Gee, Bryan Krapez, Janet Muhling, Steve Sheppard, and Richard Stern for discussion and comments, and the staff of the Centre for Microscopy and Microanalysis, University of Western Australia, for technical expertise. Tim Blake, Roger Buick, Gavin England, and Bryan Krapez provided access to samples. This work was supported by an Australian Research Council fellowship and grant. The manuscript was improved by reviews from Geoff Fraser and Paul Hoskin.

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