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The Songjianghe deposit is a newly discovered altered gold deposit in the southeastern Jiapigou-Haigou Gold Metallogenic Belt (JHGMB) in southeastern Jilin Province of NE China. The host rocks were considered to be the Mesoproterozoic Seluohe Group, and the metallogenic epoch lacked accurate isotopic constraints. To determine the age and metallogenic setting of the deposit, we describe the geologic characteristics of the deposit and present the results of petrographic and geochronologic analyses of the host rocks and ores. The ore bodies are hosted within a suite of amphibolite facies metamorphic rocks superimposed by greenschist facies indicative of retrograde metamorphism. Zircon U--Pb dating results indicate that the host rocks belong to the Jiapigou Group that formed at the end of the Neoarchean (2543--2527 Ma). Subsequently, the rocks successively underwent metamorphism during the late Neoarchean (2521--2506 Ma), retrograde metamorphism caused by the closure of the Paleo-Asian Ocean during the late Permian to Early Triassic (262--250 Ma), and extension after the closure of the Paleo-Asian Ocean during the Late Triassic (231--210 Ma). Sericite [.sup.40]Ar/[.sup.39]Ar dating results suggest that the Songjianghe deposit formed during the Late Jurassic between 157 Ma and 156 Ma. By combining these new insights with those of previous studies, we propose that the Songjianghe deposit is a mesothermal gold deposit and that mineralization occurred during the extensional period in the intermittent stage that followed the first subduction of the Paleo-Pacific Plate. All the gold deposits in the JHGMB formed from the late Permian to Early Cretaceous by multi-stage mineralization events that corresponded temporally with the tectonic evolution of the Paleo-Asian Ocean and the episodic subduction of the Paleo-Pacific Plate. Key words: geochronology, sericite [.sup.40]Ar/[.sup.39]Ar, mesothermal gold deposit, Songjianghe, Northeast China. Le gisement de Songjianghe est un gisement aurifere altere decouvert recemment dans le sud-est de la ceinture metallogenique aurifere de Jiapigou-Haigou (CMAJH), dans le sud-est de la province de Jilin du nord-est de la Chine. Les roches hôtes etaient considerees comme constituant le Groupe mesoproterozoïque de Seluohe, et l'epoque metallogenique n'etait pas delimitee par des donnees isotopiques exactes. Afin de determiner l'age et le contexte metallogenique du gisement, nous en decrivons les caracteristiques geologiques et presentons les resultats d'analyses petrographiques et geochronologiques des roches hôtes et des minerais. Les corps mineralises se trouvent dans une serie de roches metamorphiques au facies des amphibolites sur lequel est surimpose un metamorphisme retrograde au facies des schistes verts. Les resultats de la datation U-Pb sur zircon indiquent que les roches hôtes appartiennent au Groupe de Jiapigou qui s'est forme a la fin du Neoarcheen (2543--2527 Ma). Par la suite, les roches ont successivement subi un metamorphisme durantle Neoarcheen tardif (2521--2506 Ma), un metamorphisme retrograde cause parlafermeture de l'ocean Paleoasiatique du Permien tardif au Trias precoce (262--250 Ma) et une extension apres la fermeture de l'ocean Paleoasiatique durant le Trias tardif (231-210 Ma). Les resultats de la datation [.sup.40]Ar/[.sup.39]Ar sur sericite donnent a penser que le gisement de Songjianghe s'est forme au Jurassique tardif, entre 157 Ma et 156 Ma. En combinant ces nouveaux renseignements aux resultats d'etudes anterieures, nous proposons que le gisement de Songjianghe est un gisement aurifere mesothermal et que la mineralisation s'est produite durant la periode d'extension dans l'intervalle qui a suivi la premiere subduction de la plaque paleo-Pacifique. Tous les gisements auriferes de la CMAJH se sont formes du Permien tardif au Cretace precoce par des episodes de mineralisation en plusieurs etapes qui correspondaient dans le temps avec l'evolution tectonique de l'ocean Paleoasiatique et la subduction episodique de la plaque paleo-Pacifique. [Traduit par la Redaction] Mots-cles: geochronologie, [.sup.40]Ar/[.sup.39]Ar sur sericite, gisement aurifere mesothermal, Songjianghe, nord-est de la Chine., 1. Introduction The Jiapigou-Haigou Gold Metallogenic Belt (JHGMB) is located in the NW-striking collage zone between the eastern section ofthe northern margin of the North China Craton (NCC) and the [...]

Spodumene pegmatites occur in the southeastern sector of the 2.02 Ga Vredefort impact structure in South Africa. Moser et al. (published in 2011) presented an isotope dilution--thermal ionization mass spectrometry (ID-TIMS) age of 1136 ± 47 Ma for these spodumene pegmatites. This discussion questions the reliability of this age. Field relations indicate that the spodumene pegmatites are not affected by the structural fabric that affected the host amphibolites. The crosscutting impact-related veins of pseudotachylitic breccia clearly argues a preimpact age for the spodumene pegmatites. The high-uranium content of one of the two zircons used by Moser et al. to obtain the age is correlated to the Ga Pbloss event, widely observed in different rocks within the Vredefort impact structure. Des pegmatites a spodumene se trouvent dans le secteur sud-est de la structure d'impact de Vredefort (2,02 Ga) en Afrique du Sud. Moser et al. (publie en 2011) presentent, pour ces pegmatites a spodumene, un age de 1136 ± 47 Ma determine par analyse par dilution isotopique--spectrometrie de masse a thermoionisation (ID-TIMS). La presente discussion remet en question la fiabilite de cet age. Les relations sur le terrain indiquent que les pegmatites a spodumene ne sont pas touchees par la fabrique structurale qui affecte les amphibolites hotes. Les filons de breches pseudotachylitiques relies a l'impact qui les recoupent font clairement ressortir un age pre-impact pour les pegmatites a spodumene. La forte teneur en uranium de l'un des deux zircons utilise par Moser et al. pour obtenir l'age en question est correle a l'evenement de perte de Pb, vers 1,1 Ga, qui est frequemment observe dans differentes roches a l'interieur de la structure d'impact de Vredefort. [Traduit par la Redaction], Introduction Despite enduring long-time spans in complex geologic systems, the refractory behaviour of zircon is illustrated by its occurrence as >4 Ga detrital grains in 3.0 Ga metasediments from the [...]

Three conodont fossil occurrences stratigraphically above and below a rhyolitic tuff in the Lower Ordovician Crow Formation, southeast Yukon, constrain the tuff to the early Tremadocian Rossodus manitouensis Zone and possibly the Rossodus tenuis Zone. U-Pb dates were obtained from zircon from the tuff that was imaged with cathodoluminescence and chemically abraded to reduce the likelihood of dating grains that contain older components or suffered Pb loss, respectively. Six dates from grains with oscillatory parallel zoning are equivalent, with a weighted mean [sup.206]Pb/.sup.238]U date of 491.04 ± 0.13 Ma. These grains are interpreted as being primary volcanic crystals, and the date is therefore taken as the depositional age of the tuff. Four other dates from grains with sector zoning are slightly older, up to 492.0 Ma, and are interpreted as inherited or recycled from earlier volcanics. The currently defined Cambrian-Ordovician boundary is inferred to be below the tuff and separated from it by at least two conodont biostratigraphic zones that are estimated to span at least 2.25 Ma based on the top of the Rhabdinopora flabelliformis parabola graptolite zone being older than the base of the Rossodus tenuis conodont zone. We interpret the evidence in southeast Yukon to suggest that the global Cambrian-Ordovician boundary is older (>493.3 Ma) than previous estimates of ~488 and 491 Ma based upon legacy [sup.207]Pb/[sup.206]Pb zircon ages. Trois occurrences de fossiles de conodontes situees stratigraphiquement au-dessus et en dessous d'un tuf rhyolitique dans la Formation de Crow de l'Ordovicien inferieur du sud-est du Yukon placent le tuf dans la zone a Rossodus manitouensis et possiblement dans la zone a Rossodus tenuis du Tremadocien precoce. Des ges par datation U-Pb ont ete obtenus de zircons provenant du tuf qui ont ete caracterises par cathodoluminescence et abrases chimiquement afin de minimiser la possibilite de dater des grains contenant des composantes plus anciennes ou ayant perdu du Pb respectivement. Six ges U-Pb obtenus de grains presentant un zonage oscillatoire parallele sont equivalents pour un ge [sup.206]Pb/[sup.238]U moyen pondere de 491,04 ±0,13 Ma. Ces grains sont interpretes comme etant des cristaux volcaniques primaires cet ge U-Pb refletant donc l' ge de depot du tuf. Quatre autres ges obtenus de grains a zonage sectoriel sont legerement plus vieux allant jusqu' a 492,0 Ma. Il s'agirait de cristaux herites ou recycles de roches volcaniques plus anciennes. La limite Cambrien-Ordovicien telle qu' elle est actuellement definie se situerait sous le tuf dont elle serait separee par au moins deux biozones a conodonte qui representeraient au moins 2,25 Ma etant donne que le sommet de la zone a graptolite a Rhabdinopora flabelliformis parabola est plus ge que la base de la zone a conodonte a Rossodus tenuis. Les donnees pour le sud-est du Yukon suggerent que a l'echelle planetaire la limite Cambrien-Ordovicien est plus reculee (>493,3 Ma) que ce qu'indiquaient les estimations anterieures qui la situaient a ~488 et 491 Ma a la lumiere d'ges [sup.207]Pb/[sup.206]Pb existants obtenus de zircons., Introduction The Ordovician period was a time of major evolutionary turnover and diversification of marine faunas that resulted in establishment of communities that would typify the succeeding 200 million years [...]

A quartz sandstone bed in a dominantly conglomeratic section in the central part of the Lower Carboniferous Horton Group on Petit-de-Grat Island, offshore southern Cape Breton Island, Nova Scotia, contains [...]

Granulite-facies quartzites from the Adirondack Highlands (Grenville Province, New York) contain accessory zircon with ubiquitous metamorphic overgrowths. Detrital zircon cores are resorbed, preserve various internal zoning styles and inclusions, and have ages of 1.3 to 2.7 Ga. These ages constrain the timing of deposition of the protolith sandstone and suggest that the Adirondack Highlands were proximal or part of Laurentia during deposition. Metamorphic overgrowths formed in the quartzites during the Shawinigan orogeny (1.20-1.14 Ga). The average volume of overgrowths in eight samples ranges from 62-87%, with a positive correlation between zircon growth and feldspar content (melt productivity during metamorphism). Melt volumes and Zr solubility were too small to allow the overgrowths to have formed in one dissolution-precipitation event. Crystal-size distributions suggest zircon coarsening by the dissolution of small crystals and Zr transfer via a partial melt, and thus zircon overgrowths date anatexis. These results have implications for provenance studies, as dissolution of small zircon grains and growth of new zircon could bias age spectra of metasedimentary rocks. Keywords: Metamorphic zircon, quartzite, U-Pb geochronology, Ostwald ripening, Adirondack Highlands, Grenville Province DOI: 10.2138/am.2010.3547

Metamict zircon crystals have been thermally annealed between 500 and 1800 K and analyzed using infrared and optical spectroscopy in the spectral region of 1400-7000 [cm.sup.-1]. Recrystallization and dehydroxylation via complex proton/OH diffusion, redistribution, and incorporations of additional hydrogen-related species within the crystal structure of zircon occur at temperatures above 700 K in partially metamict zircon and above 1200 K in heavily amorphized material. Thermally induced changes in O-H stretching spectra are different between E [parallel] c and E [perpendicular to] c in weakly metamict zircon. The O-H stretching band near 3342 [cm.sup.-1] (with E [perpendicular to] c) in an untreated sample shifts to 3277 [cm.sup.- 1] at 1200 K, where the frequency of O-H stretching bands with E [parallel] c increases. Conversions of hydrogen-related species were observed and extra OH bands were found at temperatures between 1200 and 1600 K. A dramatic change of OH spectra was recorded between 1600 and 1800 K in partially metamict crystals, resulting in additional absorption features (near 3098 and 2998 [cm.sup.-1] along E [perpendicular to] e). [U.sup.4+] and [U.sup.5+] related spectra are also affected by high-temperature annealing. For highly metamict zircon, the [U.sup.4+] band near 4830 cm ~ shows an increase in intensity above 1200 K. Additional 1R bands at 2146 and 2344 cm ~ appear in the spectra of metamict zircon annealed at high temperatures. Their frequencies are consistent with stretching vibrations of CO and C[O.sub.2]. Keywords: Infrared spectroscopy, zircon, high temperature, hydroxyl species, metamictization, uranium, CO, C[O.sub.2], dehydroxylation and recrystallization DOI: 10.2138/am.2010.3567

The authors analyzed detrital zircon grains from 10 metasedimentary rock samples of the Priest River complex and three other amphibolite-facies metamorphic sequences in north-central Idaho to test the previous assignment of these rocks to the Mesoproterozoic Belt-Purcell Supergroup. Zircon grains from two samples of the Prichard Formation (lower Belt) and one sample of Cambrian quartzite were also analyzed as controls with known depositional ages. U-Pb zircon analysis by laser ablation--inductively coupled plasma--mass spectrometry reveals that 6 of the 10 samples contain multiple age populations between 1900 and 1400 Ma and a scatter of older ages, similar to results reported from the BeltPurcell Supergroup to the north and east. Results from the Priest River metamorphic complex confirm previous correlations with the Prichard Formation. Samples from the Golden and Elk City sequences have significant numbers of 15001380 Ma grains, which indicates that they do not predate the Belt. Rather, they are probably from a relatively young, southwestern part of the Belt Supergroup (Lemhi subbasin). Non-North American (1610-1490 Ma) grains are rare in these rocks. Three samples of quartzite from the Syringa metamorphic sequence northwest of the Idaho batholith contain zircon grains younger than the Belt Supergroup and support a Neoproterozoic age. A single Cambrian sample has abundant 1780 Ma grains and none younger than ~1750 Ma. These results indicate that the likely protoliths of many high-grade metamorphic rocks in northern Idaho were strata of the Belt-Purcell Supergroup or overlying rocks of the Neoproterozoic Windermere Supergroup and not basement rocks. Les auteurs ont analyse des grains de zircons detritiques de 10 roches metaseedimentaires du complexe de Priest River et de trois autres sequences au facies des amphibolites dans le centre- nord de l'Idaho afin de verifier l'assignation anterieure de ces roches au Supergroupe Belt-Purcell (Mesoproteerozolque). Des grains de zircons de deux echantillons de la Formation de Prichard (Belt infeerieur) et un echantillon de quartzite du Cambrien ont aussi ete analyses en tant que controles dont les ages de deposition etaient connus. Les analyses U-Pb sur zircon par ablation au laser et spectromeetrie de masse avec un plasma a couplage inductif revelent que six des dix echantillons contiennent des populations d'ages multiples entre 1900 et 1400 Ma et une dispersion d'ages plus anciens, semblables aux resultats obtenus du Supergroupe BeltPurcell vers le nord et l'est. Les resultats obtenus du complexe metamorphique Priest River confirment les correlations anterieures avec la Formation de Prichard. Des echantillons des sequences Golden et Elk City ont de nombreux grains datant de 1500-1380 Ma, ce qui indique qu'ils ne sont pas anterieurs au Belt, mais qu'ils proviennent probablement plutot d'une partie sud-ouest, relativement jeune du Supergroupe de Belt (sous bassin de Lemhi). Des grains ne provenant pas de l'Ameerique du Nord (1610-1490 Ma) sont rares dans ces roches. Trois exemples de quartzite de la sequence metamorphique de Syringa, au nord-ouest du batholite d'Idaho, contiennent des grains de zircon plus jeunes que ceux du Supergroupe de Belt et ils renforcent l'ideee d'un age Neoproterozoique. Un seul echantillon provenant du Cambrien a d'abondants grains de 1780 Ma et aucun plus jeune que ~1750 Ma. Ces resultats indiquent que les protolites probables de nombreuses roches a meetamorphisme eleve dans le nord de l'Idaho etaient des strates du Supergroupe de Belt-Purcell ou des roches sus-jacentes du Supergroupe de Windermere (Neeoproteerozolque) et non des roches du socle. [Traduit par la Redaction], Introduction The Mesoproterozoic Belt-Purcell Supergroup is exposed throughout much of northern Idaho, western Montana, northeastern Washington, and southeast British Columbia (Fig. 1). Its age is known to be about 1470-1400 [...]

The application of the Hf isotope composition of zircon as a geochemical tracer requires the preservation of Lu-Hf systematics within individual grains. We performed hydrothermal experiments on a self-irradiation-damaged zircon to test whether hydrothermal alteration would affect its Hf isotopic composition. Severely radiation-damaged zircon from Sri Lanka was reacted in either Teflon reactors or gold capsules at 200 [degrees]C (1080 h, autogeneous pressure), 400 [degrees]C (120 h, 1 kbar), and 600 [degrees]C (72 h, 1 kbar) in a 1 MHC1-0.2 MHF solution that was spiked with 300 ppm non-natural Hf(98.2% [sup.180]Hf) and 970 ppm Yb. Laser ablation inductively coupled plasma mass spectrometry measurements of the HI'- and U-Pb isotope compositions of the altered domains revealed that the U-Pb system of such domains was severely disturbed, resulting in a discordia pointing toward the origin of the concordia diagram, but that the Hf isotope composition was unaffected. In addition, Yb enrichment was observed in the reacted zircon domains, predominantly near the zircon-solution interface. The Yb has apparently diffused into the altered domains. The new data are fully consistent with a diffusion-controlled aqueous alteration process occurring within radiation-damaged zircon. Keywords: Zircon, alteration, Lu-Hf, radiation damage DOI: 10.2138/am.2010.3521

Radiation damage of natural and synthetic zircon grains is evaluated by Raman spectroscopy to understand annealing and stability of fission tracks. Analyses focus on a suite of 338 Paleozoic detrital zircon grains from metamorphosed strata in the Hellenic forearc that were variably annealed by a Miocene thermal event, as well as a suite of 97 synthetic zircon grains. The Raman wavenumber shift Of V3 [SiO4] reveals that radiation damage and damage distribution in this suite mainly depends on uranium concentration. In zircon with similar uranium concentration, the Raman wavenumber shift allows for the determination of radiation damage in different crystals, which is a function of effective accumulation time. Nine detrital zircons grains with moderate radiation damage were stepwise annealed at 1000 and 1400[degrees]C, which resulted in progressive removal of radiation damage revealed in an increase of [v.sub.3] [Si[O.sub.4]] peak positions. For a partly reset sample that was brought to temperatures of ~350[degrees]C in a geologic setting (Hellenic forearc), we use the Raman measurements and uranium determination to estimate a Zircon Damage Discrimination Factor ([Z.sub.RDD]), which is our attempt to estimate only radiation damage in single grains by accounting for affects of the uranium atom in the Raman wavenumber. This discrimination allows for a separation of zircon fission track (ZFT) ages of single ages based on grains that have a low track retention (high damage, fully reset grain), thus refining the age determination of cooling in a rock that shows variable resetting. Keywords: Geochronology, zircon fission track, radiation damage, uranium content, Raman spectroscopy DOI: 10.2138/am.2010.3264

Albitization is a common metasomatic process active in various geodynamic contexts. In the northern Pyrenees, there are several occurrences of albitites but, until now, only one occurrence has been dated (117 Ma, Ar--Ar dating). This paper presents new U--Th--Pb ages for several albitite occurrences throughout the Pyrenees to test whether they are contemporaneous and, if so, to specify the regional extent of the albitization event. Ages obtained from large euhedral titanite and monazite grains from distinct albitites are 110 [+ or -] 8 and 98 [+ or -] 2 Ma, respectively. The zircon U--Th--Pb isotopic system did not record this Cretaceous metasomatic event, even when grains were selected in metasomatically Zr-enriched rocks or in hydrothermal structures (millimetre-sized veins cross-cutting granitoids). We argue that the total time span of 20 Ma recorded by albitites corresponds to a long-lived hydrothermal system that was active during the rotation of Iberia around Europe, along the North Pyrenean Fault. Because albitization and talc mineralization have the same spatial and temporal distribution in the Pyrenees, we argue that these two metasomatic phenomena are two independent records of this single, regional-scale, long-lived hydrothermal system. doi: 10.1144/0016-76492009-144.

The Cordillera Darwin Metamorphic Complex in the southernmost Andes includes a basement of probable Palaeozoic age, a mid-Jurassic and younger volcano-sedimentary cover, and a suite of Jurassic granites, all of which were jointly metamorphosed during the Cretaceous. Detrital zircon ages presented here show that some of the amphibolite-facies metamorphic rocks previously mapped as basement have a Jurassic protolith. Overall the detrital zircon age patterns for samples of the Cordillera Darwin basement differ from those of the Madre de Dios Terrane of the western Patagonian Andes with which they had been correlated; instead, they are more comparable with those from the Eastern Andes Metamorphic Complex, which apparently developed in a passive margin setting. The paucity of Cambrian detrital zircons indicates that the meta-igneous basement of the Magallanes foreland basin of central and northern Tierra del Fuego was not the main source of detritus for the protolith of the Cordillera Darwin Metamorphic Complex. The possibility is envisaged that the Magallanes Fagnano transform fault boundary between the Scotia and South America plates resulted from reactivation of an older, pre-Jurassic suture zone between the basement terranes of north-central Tierra del Fuego and Cordillera Darwin. Supplementary material: Geographical coordinates of analysed samples, sensitive high-resolution ion microprobe U--Th--Pb analytical data and representative cathodoluminescence images of zircon grains analysed are available at http://www.geolsoc.org.uk/SUP18397. doi: 10.1144/0016-76492009-124.

Absolute ages for sedimentary rocks are required to construct a temporal framework in which to decipher Earth's history. The most reliable method for dating Precambrian sedimentary rocks is U-Pb geochronology of zircon in intercalated volcanic rocks. However, extracting sufficient zircons involves destructive mineral separation procedures, often requiring several kilograms of sample. We have dated felsic tufts doi: 10.1130/G30567.1

Ocean-plate stratigraphy (basalt, chert, clastics) of the Balcreuchan Group in the Ballantrae complex is repeated by layer-parallel thrusts to form duplex structures south of Bennane Head, ~5 km north of Ballantrae. Five tuff beds, all in chert, have similar U-Pb zircon ages of ca. 470 [+ or -] 10 Ma. The geometrical polarity of the duplexes and the zircon ages provide new constraints on the tectonic evolution of the accretionary wedge of the Ballantrae complex. Northwestward thrusting of the duplexes suggests that subduction was from the northwest to the southeast, and this polarity at the leading edge of the main ophiolite body is consistent with the northward younging of the volcanic arc rocks of the Ballantrae complex. doi: 10.1130/B26329.1

The Himalayan orogen has experienced intense Cenozoic deformation and widespread metamorphism, making it difficult to track its initial architecture and the subsequent deformation path during the Cenozoic India-Asia collision. To address this issue, we conducted structural mapping and U-Pb zircon geochronology across the Shillong Plateau, Mikir Hills, and Brahmaputra River Valley of northeastern India, located 30-100 km south of the eastern Himalaya. Our work reveals three episodes of igneous activity at ca. 1600 Ma, ca. 1100 Ma, and ca. 500 Ma, and three ductile-deformation events at ca. 1100 Ma, 520-500 Ma, and during the Cretaceous. The first two events were contractional, possibly induced by assembly of Rodinia and Eastern Gondwana, while the last event was extensional, possibly related to breakup of Gondwana. Because of its proximity to the Himalaya, the occurrence of 500 Ma contractional deformation in northeastern India implies that any attempt to determine the magnitude of Cenozoic deformation across the Himalayan orogen using Proterozoic strata as marker beds must first remove the effect of early Paleozoic deformation. The lithostratigraphy of the Shillong Plateau established by this study and its correlation to the Himalayan units imply that the Greater Himalayan Crystalline Complex may be a tectonic mixture of Indian crystalline basement, its Proterozoic-Cambrian cover sequence, and an early Paleozoic arc. Although the Shillong Plateau may be regarded as a rigid block in the Cenozoic, our work demonstrates that distributed active left-slip faulting dominates its interior, consistent with earthquake focal mechanisms and global positioning system velocity fields across the region. doi: 10.1130/B26460.1

A spectroscopic study on seven natural zircon crystals representing varying coloration and degree of short-range order has been conducted using Raman, optical absorption (OA), time-resolved laser-induced luminescence (TRL), and electron paramagnetic resonance (EPR) spectroscopy. There is a systematic increase in the line width of the central [Gd.sup.3+] transition in the EPR spectra with decreasing short-range order as defined by Raman spectroscopy. In the luminescence spectra, the broad-band yellow photoluminescence centered at about 560 nm is restricted to samples with a high degree of short-range order. In contrast, photoluminescence related to trivalent rare earth elements was detected in zircon with both intermediate and high degrees of short-range order, but not in one heterogeneous, intermediate to highly metamict sample. Furthermore, short-lived greenish luminescence, typical of uranyl ions, could be detected in the luminescence spectra of partly and strongly metamict zircon. Reabsorption lines related to the presence oftetravalent U were found in luminescence spectra of both well-ordered and metamict samples. These observations give evidence for the occurrence ofhexavalent U, besides the tetra- and pentavalent forms in natural zircon. It appears that pentavalent U initially present in crystalline zircon converted to tetra- and hexavalent U in highly metamict samples. For a well-ordered green zircon crystal from Caldas Minas, Minas Gerais (Brazil), spectra of [Mn.sup.2+] (or [Mn.sup.4+]) were detected by both EPR and luminescence spectroscopy, demonstrating that Mn centers may be present in this mineral. Potential color centers are discussed. Green coloration of natural zircon cannot be assigned exclusively to the presence of [U.sup.4+] as suggested previously. Furthermore, no significant influence of the [Nb.sup.4+] center on zircon coloration, including reddish color, was found. Yellow coloration may be related to the absorption due to charge-transfer bands of [Pr.sup.4+]. Keywords: Zircon, metamict, uranium, uranyl, luminescence, Raman, EPR, coloration DOI: 10.2138/am.2010.3248

An investigation of rare earth elements (REE) in 15 zircon grains from lunar breccia sample 14321, combined with published analyses, has allowed lunar zircon grains to be separated into four distinctive types. Type-1 zircon is characterized by the relative depletion of light REE (LREE) resulting in a steep chondrite-normalized pattern. Type-2 zircon shows relative enrichment in the LREE compared to type-1 grains. Type-3 zircon also shows relatively high concentrations of LREE as well as a relative depletion in the heavy REE (HREE), which results in a relatively flat chondrite-normalized pattern. Type-4 zircon grains are characterized by the steepest chondrite-normalized REE pattern, with the lowest LREE and the highest HREE as well as by a distinctive positive Ce anomaly. Multiple analyses of REE in a complex impact modified zircon from breccia sample 73235 suggest a possibility that the very light REE from La to Nd were mobilized during impact. However, the main differences between the identified zircon types appear to be primary and reflect the original crystallization environment of zircon grains. These differences are not linked to major changes associated with the different suites of plutonic rocks, such as Mg- and alkali-suites, and quartz monzodiorites (QMD), but instead reflect small-scale variations in residual pockets of melt where zircon grains crystallized. For example, the presence of plagioclase in the immediate vicinity of zircon was responsible for the type-1 zircon REE pattern, whereas type-2 zircon was formed in the presence of pyroxene. The only exception is type-4 zircon, which was probably associated with some felsite and 'granite' samples representing very late differentiates of lunar mafic magmas. Keywords: Moon, zircon, REE, lunar breccias DOI: 10.2138/am.2010.3297

Solid solutions among zircon (ZrSi[O.sub.4], [I4.sub.1]/amd) and zircon-structured orthosilicates ASi[O.sub.4] (A = Hf, Th, U, Pu, or Ce) are important to a wide variety of applications: nuclear materials, geochronology, and even electronic materials. The thermodynamic mixing properties of the following seven binary solid solutions were simulated using density functional theory followed by Monte Carlo modeling and thermodynamic integration: (Zr,Hf)Si[O.sub.4], (Zr,Th)Si[O.sub.4], (Zr,U)Si[O.sub.4], (Zr,Pu)Si[O.sub.4], (Zr,Ce)Si[O.sub.4], (Hf,Pu) Si[O.sub.4], and (Th,U)Si[O.sub.4]. ZrSi[O.sub.4] and HfSi[O.sub.4] were found to form a nearly ideal solid solution, but the miscibility of the other solid solutions was limited to no more than 12 mol% of the substituting cation. The binaries were ranked by extent of miscibility: (Zr,Hf)Si[O.sub.4] > (Th,U)Si[O.sub.4] > (Zr, Pu)Si[O.sub.4] > (Zr, Ce) Si[O.sub.4] > (Hf,Pu)Si[O.sub.4] > (Zr,U)Si[O.sub.4] > (Zr,Th)Si[O.sub.4]. The extent of solid solution has been estimated for each binary. The end-members PuSi[O.sub.4], CeSi[O.sub.4], and USi[O.sub.4] were determined to be unstable relative to a mixture of Si[O.sub.2(quartz)] and crystalline Pu[O.sub.2], Ce[O.sub.2(cerianite)], or U[O.sub.2(uraninite)]. Isostructural thorite (ThSi[O.sub.4]) is calculated to be marginally stable, but its monoclinic polymorph, huttonite, which has the structure of monazite, is marginally unstable relative to Si[O.sub.2(quartz)] and Th[O.sub.2(thorianite)] at 0 K. If thermodynamic equilibrium could be reached at low temperatures, exsolution textures would appear in most zircon solid solutions perpendicular to [001], but perpendicular to a linear combination of [100] and [010] in (Hf, Pu)Si[O.sub.4]. Keywords: Solid solution, zircon, coffinite, thorite, hafnon, PuSi[O.sub.4], CeSi[O.sub.4], Monte Carlo simulation, quantum mechanics DOI: 10.2138/am.2010.3318

The standard Gibbs energy of formation of zircon was constrained by measuring the solubility of silica in [H.sub.2]O in equilibrium with zircon and baddeleyite at 800 [degrees]C, 12 kbar, by a sensitive weight- change method. Dissolution occurs incongruently according to the reaction: ZrSi[O.sub.4] = Zr[O.sub.2] + Si[O.sub.2(t)] zircon baddeleyite where Si[O.sub.2(t)] is total dissolved silica. Blank runs demonstrated that the effects of Zr[O.sub.2] solubility and/or capsule-Pt transfer were near the weighing detection limit, so weight losses or gains could be ascribed quantitatively to Si[O.sub.2] solubility. Precise Si[O.sub.2(t)] concentrations were ensured by use of three types of starting material, by approaching equilibrium from zircon-undersaturation and oversaturation, and by demonstrating time-independence of the measurements. The results yielded a Si[O.sub.2] concentration of 0.069 [+ or -] 0.002 (1 se) moles per kg [H.sub.2]O ([m.sub.s]), or a mole fraction ([X.sub.s]) of 1.23 x [10.sup.-3] [+ or -] 3.3 x [10.sup.-5]. Two runs on zircon solubility in NaCl-[H.sub.2]O solutions at 800 [degrees]C and 10 kbar showed silica solubility to decrease by nearly 1% per tool% NaCl. The standard molar Gibbs free energy of formation of zircon from the oxides at a constant P and T is given by: [DELTA][G.sup.o.sub.f,ox,zr] = RTln [[gamma].sup.ZB.sub.s] [X.sup.ZB.sub.s]/[[gamma].sup.Q.sub.s][X.sup.Q.sub.s] where ZB and Q refer, respectively, to equilibrium with zircon-baddeleyite and quartz, [[gamma].sub.s] is the activity coefficient of total silica, and the relationship between [[gamma].sub.s] and [X.sub.s] accounts for aqueous silica activity. Our results yield [DELTA][G.sup.o.sub.f,oxzr] = -18.5 [+ or -] 0.7 kJ/mol at 800 [degrees]C, 12 kbar (95% confidence), or a standard apparent Gibbs free energy of formation from the elements of -1918.3 [+ or -] 0.7 kJ/mol at 25 [degrees]C, l bar. Our value is consistent with determinations based on phase equilibrium studies, within reported error limits, but is more precise than most previous values. However, it is less negative than high-temperature determinations by calorimetry and electrochemistry. Our results indicate that solubility measurements at high T and P may be a superior method of free energy determination of other refractory silicate minerals. Keywords: Experimental petrology, phase equilibria, thermodynamics, zircon DOI: 10.2138/am.2010.3213

Zircon from a microgranular enclave in the ca. 315 Ma postcollisional Karkonosze pluton (Western Sudetes, northeastern Bohemian Massif) is characterized by unusual morphologies and reequilibration textures. Blocky, clustered, and skeletal Th-U-rich zircon grains are internally corroded along discrete boundary zones, and subsequently replaced by porous microgranular aggregates of zircon and various other minerals, including thorite. The boundary zones have textures and compositions characteristic of diffusion-controlled chemical reaction fronts, including enrichment in Ca, Ba, and light rare earth elements, whereas microgranular domains are typical of zircon replacement and regrowth by coupled dissolution and precipitation. Initial zircon crystallization occurred with the mingling of mafic magma into a cooler granitic melt, whereas zircon modification is attributed to the reaction of late magmatic fluids from the host granite with the enclave. Precise dating of reequilibrated zircon as 304 [+ or -] 2 Ma indicates that fluid activity, which is also responsible for scheelite mineralization, postdates the emplacement of the main part of the pluton by several millions of years. doi: 10.1130/G30048A.1

Nappes in the southern sector of the Southern Brasilia Belt record suturing of the Paranapanema Block and Socorro Guaxupe Arc with a subducted passive margin on the western side of the Sao Francisco Craton. We report secondary ion mass spectrometry U-Pb zircon ages that for the first time constrain the age of: (1) retrograded eclogite from a block along the tectonic contact beneath the uppermost nappe in a stack of passive margin-derived nappes; (2) high-pressure granulite-facies metamorphism in the uppermost passive margin-derived nappe; (3) high-pressure granulite-facies metamorphism in the overlying arc-derived nappe. Rare zircons from a retrograded eclogite yield a [sup.206]pb/[sup.238] U age of 678 [+ or -] 29 Ma, which we interpret as most likely to date close-to-peak-P metamorphism and to provide a minimum age for detachment of the overlying passive margin-derived nappe from the subducting plate. Zircon associated with ilmenite in samples from two structural levels in the passive margin-derived high-pressure granulite nappe yields [sup.206]pb/[sup.238]U ages of 648 [+ or -] 12 and 647 [+ or -] 11 Ma, and Ti-in-zircon crystallization temperatures from c. 860[degrees]C down to c. 785[degrees]C, but skewed toward the lower part of the range. These data indicate zircon formation during cooling from around peak T to the solidus, consistent with the high-T retrograde P-T path deduced from microstructures linked to phase assemblage fields in isochemical phase diagrams. Rb-Sr multi-mineral-whole-rock isochrons from two samples from close to the bottom of this nappe date formation of a retrograde sillimanite-bearing penetrative fabric to c. 590 Ma at temperatures of c. 750[degrees]C (based on Ti-in-quartz thermometry). Rare zircons from leucosome in high-pressure granulite from the overlying arc-derived nappe yield a [sup.206]pb/[sup.238]U age of 622 [+ or -] 28 Ma and Yi-in-zircon crystallization temperatures from c. 970[degrees]C down to c. 820[degrees]C, which we interpret to record formation of zircon during cooling from peak high-pressure granulite-facies conditions. These ages indicate that the first stage of craton amalgamation in West Gondwana may have occurred earlier than previously inferred. Supplementary material: Geochemical and geochronological data are available at http://www.geolsoc.org.uk/SUP18377. doi: 10.1144/0016-76492008-140.

Impacts may have been important mechanisms of crustal redistribution and differentiation, particularly during intense postaccretionary bombardment between 4.5 Ga and 3.9 Ga ago. Evidence of crustal processes during this period is largely provided by detrital zircons from the Yilgarn craton, Australia. Trace element compositions, crystallization temperatures, and inclusion populations of these ancient zircons have been taken as evidence for predominantly granitic source magmas, implying widespread felsic continental crust on the early Earth. However, there is little knowledge of zircons formed in impact melt sheets, a potential source for the Hadean zircons. Here we present Ti thermometry, trace elements, and inclusion populations of zircons from the 1.85 Ga Sudbury impact melt sheet (Ontario, Canada). Our results demonstrate that large variations in zircon crystallization temperature and composition will be an inevitable consequence of the evolution of such magmatic systems. We also show that zircons in mafic rocks crystallize in residual liquids of granitic composition, producing inclusion assemblages that are remarkably similar to those reported for the ancient Yilgarn grains. Thus, we conclude that the trace element compositions and inclusion populations of the Hadean zircons are consistent with crystallization from more mafic melts than previously recognized, although high crystallization temperature distributions of Sudbury zircons indicate that impact melt sheets were not a dominant source for the grains older than 3.9 Ga.

The oxygen-isotope composition of the Earth's upper mantle is an important reference for understanding mantle and crust geochemical cycles. Olivine is the most commonly used mineral for determining the influence of crustal processes on the oxygen-isotope ratio ([delta][sup.18]O) of primitive rocks, however it is an uncommon mineral in continental crust and readily alters at or near Earth's surface. Here we report the first measurements of oxygen-isotope ratios in zircon from oceanic crust exposed at a mid-ocean ridge. Measurements of [delta][sup.18]O and trace elements were made by ion microprobe on zircon in polished rock chips of gabbro and veins in serpentinized peridotite drilled from the Mid-Atlantic Ridge. The zircon grains contain both oscillatory and sector growth zoning, features characteristic of magmatic zircon. Values of [delta][sup.18]O (zircon) 5.3 [+ or -] 0.8 [per thousand] (2 st. dev., n = 68) for the population are consistent with the interpretation that these grains are igneous in origin and formed in high-temperature isotopic equilibrium with mantle oxygen. The [delta][sup.18]O values demonstrate that zircon in oceanic crust preserves primitive [delta][sup.18]O in spite of sub-solidus alteration of the whole rock. The fact that the primitive [delta][sup.18]O (zircon) values fall in a narrow range (5.3 [+ or -] 0.8 [per thousand]) strengthens the use of oxygen isotopes in zircon as a tracer to identify processes of exchange in a wide range of modern and ancient crustal environments, including subducted oceanic crust (eclogite), and also in the oldest known pieces of Earth, >3900 million-year-old detrital zircon grains from Western Australia. Keywords: Zircon, ODE MARK, oxygen isotope, oceanic crust, ion microprobe

Zircon is an excellent material to preserve the complex history of ultrahigh-pressure (UHP) metamorphic rocks, whereas raineralogical evidence of UHP conditions is mostly obliterated in matrix assemblages as a result of extensive retrograde overprinting during exhumation. Zircons from the Kokchetav UHP--HP massif contain numerous inclusions of graphite, quartz, garnet, omphacite, jadeite, phengite, phlogopite, rutile, albite, K-feldspar, amphibole, zoisite, kyanite, calcite, dolomite, apatite and monazite, as well as the diagnostic UHP minerals, such as microdiamond and coesite, which were identified by laser Raman spectroscopy. The internal structure of zircon displays a distinct zonation, which comprises an inherited core, a wide mantle and an outer rim, each with distinctive inclusion micro-assemblages. The low-pressure mineral inclusions, such as graphite, quartz and albite, are common in the inherited core and thin outer rim, whereas diamond, coesite and jadeite occupy the mantle domain. The zircon core and outer rim are of detrital and relatively low-grade metamorphic origin, whereas the mantle domain is of HP to UHP metamorphic origin. The mineral assemblages and chemistry of inclusions preserved in zircon have been used to constrain the metamorphic P--T path of the Kokchetav UHP--HP rocks, and indicate peak metamorphism al 60-80 kbar and 970-1100[degrees]C followed by nearly isothermal decompression at 10 kbar and c. 800[degrees]C. Sensitive high-resolution ion microprobe U--Pb spot analyses of the zoned zircon indicate four discrete ages of the Kokchetav metamorphic evolution: (1) a Middle Proterozoic protolith age; (2) 537 [+ or -] 9 Ma for UHP metamorphism; (3) 507 [+ or -] 8 Ma for the late-stage amphibolite-facies overprint; (4) 456-461 Ma for postorogenic thermal events. This indicates that Middle Proterozoic supracrustal protoliths of the Kokchetav UHP--HP rocks were subducted to mantle depths in the Middle Cambrian, and exhumed to mid-crustal levels in the Late Cambrian. The zonal arrangement of inclusions and the presence of coesite and diamond without back reaction imply that aqueous fluids were low to absent within zircon, and that zircon is capable of retaining minerals of each metamorphic stage. We suggest that the study of inclusions in zircon is a powerful method to clarify the multiple stages and timing of metamorphic evolution of UHP-HP rocks, the evidence for which has been more or less obliterated in the host rock.

Relaxor lead lanthanum zirconate titanate (PLZT) thin films, with compositions of (7/65/35), (8/65/35), and (9/65/35), have been investigated as optical waveguides. Resonant structures were observed in the reflected light beam that passes through these thin films after coupling with a laser-inscribed azo polymer surface relief diffraction grating. The temperature was then varied on the PLZT thin films between -20 and 70[degrees]C, and a shift in the above resonance peaks was observed that is due to a change in the refractive index of the samples. The temperature dependence of the refractive index of the tested PLZT thin films was subsequently plotted and was found to decrease linearly with an increase in temperature at different rates for all the thin-film compositions tested. OCIS codes: 310.2785, 310.3840, 310.5448, 310.6860.

Avalonia is the largest accreted crustal block in the Appalachian orogen and comprises a collection of late Neoproterozoic volcano-sedimentary sequences that are overlain by a Palaeozoic platformal sedimentary succession. Detrital zircons from the Conception Group are dominated by 570-620 Ma ages and contain a significant component generated by erosion of coeval igneous arc-volcanic rocks. Overlying samples from the Cuckold and Crown Hill formations are dominated by Neoproterozoic populations with ages between 600 and 650 Ma and are interpreted to be derived from the underlying calc-alkaline arc-plutonic rocks. Early Palaeozoic platform units are dominated by c. 620 Ma zircons with lesser Mesoproterozoic and Palaeoproterozoic zircons. The range of detrital zircon ages is inconsistent with a West African provenance and suggests that Avalonia originated along the Gondwanan margin of the Amazon craton. The influx of Mesoproterozoic and Palaeoproterozoic detritus in the Avalonian platform suggests a major change in tectonic regime. The prominent change in provenance is interpreted to be related to separation of Avalonia from Gondwana during the Early Ordovician opening of the Rheic Ocean. The Redmans Formation is interpreted to represent the rift--drift transition of the Rheic Ocean, which imposes important constraints on the palaeotectonic evolution of Avalonia. Supplementary material: U--Pb isotopic data of LA-ICP-MS analysis of detrital zircons are available at http://www.geolsoc.org.uk/SUP18346.

U--Pb ion microprobe detrital zircon provenance investigations of pre-Mesozoic sediment samples from southern Novaya Zemlya and northern Taimyr, Arctic Russia, provide new constraints on the tectonic evolution of northern Eurasia. Sediments from both areas previously assigned a Neoproterozoic age are shown to be Palaeozoic ([less than or equal to] 500 Ma), changing our understanding of the timing of tectonic events and their regional correlation. Samples from Novaya Zemlya provide an a priori Baltica signature, and dominant cumulative probability age peaks between 610 and 530 Ma are consistent with derivation from Timanide sources. Samples from northern Taimyr also contain the unique Timanian 'fingerprint' and are also inferred to be derived from the continental slope of northeastern Baltica, implying that the North Kara block was a part of Baltica in the early Palaeozoic. These data constrain the timing of deformation, metamorphism and development of an unconformity in southern Novaya Zemlya to a restricted period no earlier than late Cambrian--early Ordovician and at least 50 Ma after the peak Timanian event. The unconformity on Novaya Zemlya can be correlated in style and timing with an unconformity on Severnaya Zemlya and may record post-Timanian extensional collapse and onset of spreading in the Uralian Ocean. Supplementary material: U--Th--Pb ion-microprobe analytical results are available at http://www.geolsoc.org.uk/SUP18340.

Detrital zircon analysis of sandstones interbedded with c. 464 Ma ignimbrites in the lower Mweelrea Formation of the South Mayo Trough, western Ireland, suggests Ordovician source-rock provenance that corresponds to two distinct volcanic-arc phases on the Laurentian margin. East-derived sandstones contain a suite of zircons with a mean age of c. 487 Ma that suggests derivation from the Cambrian to early Ordovician Baie Verte Oceanic Tract arc--ophiolite complex, locally represented by the Lough Nafooey arc rocks and the Clew Bay Complex. Zircons from south-derived sandstones within the Bunnacunneen conglomerate fan have average ages of c. 467-474 Ma, and correspond to the Notre Dame arc and locally the Connemara metagabbro and orthogneiss suite. Granite clasts in the Bunnacunneen conglomerate are similar to the Connemara orthogneiss suite, in terms of both their geochemistry and their age (c. 471 Ma). The southerly derived sedimentary strata also include Archaean and Proterozoic zircon age spectra consistent with a Dalradian source. A southern provenance from the Notre Dame arc and Dalradian rocks suggests that the Connemara terrane lay to the south of the South Mayo Trough during middle Llanvirn times, from at least 464 Ma. Supplementary material: U--Pb laser ablation multicollector inductively coupled plasma mass spectrometry data for detrital zircons and details of analytical methods for U--Pb LA-MC-ICPMS and U--Pb isotope dilution thermal ionization mass spectrometry analyses are available at http://www.geolsoc.org.uk/SUP18331.

[1] The Salton Trough is the northernmost segment of the active Gulf of California oblique rift system. The main rift-related structure in the western Salton Trough is the low-angle west Salton detachment fault (WSDF). Footwall and hanging wall apatite and zircon (U-Th)/He ages record a significant contrast in thermal history across the WSDF, confirming Pliocene normal slip along the fault. Apatite (U-Th)/He ages record rapid exhumational cooling from ~5 to ~2 Ma, consistent with the timing of accelerated tectonic subsidence recorded in WSDF hanging wall sedimentary strata and consistent with the initiation age of dextral plate boundary slip on the southern San Andreas fault in the Salton Trough. Our results indicate that the WSDF caused at least 2.3-4 km of exhumation and >8-10 km of approximately E directed horizontal extension since ~5 Ma. Slip rate of the WSDF at Yaqui Ridge was likely in the range of ~2.3-5 km/Ma from ~7 to ~2 Ma. The WSDF may have been active well before ~5 Ma. Middle Miocene apatite (U-Th)/He ages from samples at high elevations have steep age-elevation gradients and suggest that exhumation may have initiated by ~12 Ma, about when subduction ceased along a large segment of the Baja California margin and consistent with the age of onset of extension in northeastern Baja California. doi: 10.1029/2007TC002172.

Complex textures in zircon associated with Ta oxides have been used to assess the processes at the origin of zircon crystallization and associated Ta mineralization in pegmatites, in particular the role of magmatic vs. hydrothermal processes. The Tanco pegmatite is used as an example because its zircon is devoid of post-magmatic alteration and its petrogenesis is well constrained. Zircon in primary units is metamict with high U-Pb-Th contents. By contrast, in secondary assemblages affected by late-magmatic micaceous alteration, zircon is devoid of radiogenic elements, but it may contain abundant [Ta.sub.2][O.sub.5] (up to 4.7 wt%). The incorporation of Ta into zircon may occur through coupled substitution mechanisms involving other minor elements such as P, Al, Mn, or Li. The presence of Ta accounts for the distorted structures in the Ta-rich zircon, as revealed in high-resolution TEM images. Four zircon types occur sequentially in single zircon crystals, which permits a new model for zircon growth and evolution in rare element pegmatites to be advanced: (1) zircon (Z1) is Ta-rich and crystallized possibly before discrete Ta phases; (2) Z1 was overgrown by regularly zoned zircon Z2 and Z3, which show lower Ta and extreme Zr/Hf fractionation (Hf[O.sub.2] up to 38.9 wt%), suggesting crystallization from a highly fractionated melt, possibly at the onset of Ta mineralization; and (3) close to the solidus, the aqueous fluid at the origin of micaceous alteration corroded the distorted structure of Z1, and a low-Ta zircon (Z4) replaced Z1 by dissolution-reprecipitation, whereas Z2 and Z3 resisted this alteration. Tantalum was no longer stable in the zircon structure and crystallized as Ta-oxide inclusions in the reequilibrated zircon (Z4). Keywords: Zircon, tantalum mineralization, hydrothermal alteration, pegmatite, Tanco

Zircons from nine Palaeoproterozoic granitoid intrusions within the southern part of the Fennoscandian Shield have been studied by laser ablation inductively coupled plasma source mass spectrometry to obtain U-Pb ages (in the range 1.88-1.68 Ga) and Hf isotope compositions. Six granitoids are from the 1.85-1.65 Ga Transscandinavian Igneous Belt; during that period more than [10.sup.6] [km.sup.3] of granitoid magma intruded the pre-existing crust. The large majority of magmatic zircons from the nine granitoids have a limited initial range, [sup.176]Hf/[sup.177]Hf = 0.2816-0.2818, and define an evolutionary trend given by an initial value of [[epsilon].sub.Hf(1.88Ga)] [approximately equal to] +2 [+ or -]3 at an average [sup.176]Lu/[sup.177]Hf=0.015. These data show that a geographically extensive, long-lived, relatively homogeneous, and dominant magma source resided within 2.1-1.86 Ga Svecofennian juvenile crust between 1.88 and 1.68 Ga. Zircon xenocrysts (1.91-1.98 Ga) with initial [[epsilon].sub.Hf] = +0 to +2.5 from one of the intrusions provide additional evidence for such a long-lived crustal source of granitic magmas in central Fennoscandia. The granitoids were emplaced during a period of active mafic underplating that supplied heat to the anatectic zone in the lower-middle crust, but little or no mantle-derived Hf to the granitic magmas, in contrast to many mixed intermediate rocks. Supplementary material: U-Pb analyses, age calculations and Lu-Hf data are available at http:// www.geolsoc.org.uk/SUP18330.

U-Pb dating of detrital zircons shows that the provenance of Cretaceous-Palaeogene sandstones on Hatton and Edoras banks (SW Rockall Plateau) comprises magmatic rocks dated at c. 1800 Ma and c. 1750 Ma, respectively. Their depositional setting, first-cycle mineralogy and unimodal detrital zircon populations suggest that these sandstones are of local origin. The zircon age data are therefore considered to provide constraints on these poorly understood areas of the Rockall Plateau. The U-Pb dates are directly comparable with U-Pb zircon crystallization ages from granitoid rocks reported from the Ketilidian Belt of southern Greenland and from the Rhinns Complex of western Britain. Hf isotopic data from the Edoras Bank sample are consistent with derivation from a juvenile Palaeoproterozoic block. In conjunction with previously reported Sm-Nd [T.sub.dm] model ages from the Ketilidian Belt, Rockall Bank and the Rhinns Complex, these data extend the known distribution of a large juvenile Palaeoproterozoic terrane spanning the southern NE Atlantic. In contrast, Hf isotopic data from the Hatton Bank sample imply a large contribution from Archaean crust. The zircon population from Edoras Bank also contains sparse Mesoproterozoic grains, providing evidence for the presence of volumetrically minor Grenville-age intrusions in the southern part of the Rockall Plateau.

Early Paleozoic sandstones in the Southern Uplands terrane of Scotland were deposited during closure of the Iapetus Ocean between Laurentia and Avalonia. Their tectonic setting and sources are controversial, and different authors have supported subduction-accretion, extensional continental-margin development, or back-arc basin settings. We report new U-Pb detrital zircon ages from five Late Ordovician sandstones from the Northern Belt of the Southern Uplands and test models of their tectonic setting. The U-Pb zircon age distributions are dominated by peaks characteristic of sources in Laurentia and include grains as old as 3.6 Ga, older than any previously recorded in the British Caledonides SE of the Laurentian foreland. Discordant grains in one sample suggest derivation via erosion of metasedimentary rocks incorporated in the Grampian-Taconian orogen. Rare Neoproterozoic grains, previously interpreted as originating from a peri-Gondwanan terrane, may be derived from igneous rocks associated with Iapetan rifting. Only rare zircons are contemporary with the depositional ages. The results are difficult to reconcile with extensional continental-margin and back-arc models, but they support an active continental-margin subduction-accretion model. Close similarities with distributions from the Newfoundland Appalachians are consistent with sinistral transpression during closing of the Iapetus Ocean. Keywords: tectonics, Caledonides, Southern Uplands, detrital zircon geochronology, provenance.

Lu-Hf depleted mantle model ([T.sub.DM]) ages, obtained by analysis of zircons previously dated by U-Pb methods, demonstrate that ca. 1.75 Ga bimodal Paleoproterozoic rocks in the Gunnison-Salida region of central Colorado, and by extension in much of the southwestern U.S., were formed by partial melting of preexisting crustal rocks, the ages of which mostly range from 1.83 to 1.87 Ga. Some calculated Lu-Hf [T.sub.Dm] ages greater than 2.0 Ga indicate that even older crust was probably involved, consistent with the limited presence of ca. 2.5 Ga xenocrystic zircons in some rocks. These results suggest that rocks related to the Trans-Hudson and Penokean orogens are cryptically present much farther to the south than previously believed. Coupled with the bimodality of the volcanic suite in central Colorado, and indeed in much of the southwestern U.S., these results indicate that, in contrast to current juvenile arc-accretion models, melting of older crust related to extensional tectonics played an important role in the genesis of many magmatic rocks between 1.6 and 1.8 Ga. Keywords: Laurentia, crustal evolution, Colorado, Paleoproterozoic, zircon, depleted mantle.

A new approach to zircon dating is described that potentially offers a considerable improvement in the accuracy of [[207].sup.Pb]/[[206.sup.Pb] ages on a wider variety of samples and at less cost than that generally available from conventional techniques. Zircon is first preheated in a vacuum to evaporate Pb from altered domains, leaving predominantly Pb with isotopes that preserve the primary age of the sample. Refractory Pb is removed from the zircon by thermal extraction at higher temperature directly into a silica-melt ionization activator within a thermal ionization mass spectrometer. This produces strong ion beams that allow [[207].sup.pb]/[[206/sup.Pb] isotope ratios to be measured to high precision ([+ or -][10.sup.-4]) with almost negligible contamination from common Pb. Isotope fractionation appears to be more reproducible than with conventional solution analysis, allowing routine age determinations with precision of ~[+ or -] 0.2 m.y. for the Precambrian. As a test application, zircon from a noritic boundary phase of the Sudbury impact melt gives 1849.53 [+ or -] 0.21 Ma, while a phase from several hundred meters higher in the noritic layer is resolvably younger at 1849.11 [+ or -] 0.19 Ma (95 % confidence errors). The enhanced precision and ease of application of this method should greatly increase the scientific power and availability of zircon dating. Keywords: geochronology, zircon, U-Pb, Precambrian, Sudbury, thermal extraction-thermal ionization mass spectrometry, TE-TIMS.

We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from a lower crustal paragneiss in the Neoarchean Superior province. Detrital igneous zircon cores with [[delta].sup.18]O values of 5.1 [per thousand]-7.1 [per thousand] record creation of primitive to increasingly evolved crust from 2.85 [+ or -] 0.02 Ga to 2.67 [+ or -] 0.02 Ga. Sharp chemical unconformity between cores and higher [[delta].sup.18]O (8.4 [per thousand]-10.4 [per thousand]) metamorphic overgrowths as old as 2.66 [+ or -] 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 [+ or -] 0.01 Ga included ~80 m.y. of high-temperature (~700-650 [degrees]C), nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512 [+ or -] 8 Ma (~ 600 [degrees]C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted regional metamorphism, and the consequent advances now possible in reconstructing planetary dynamics with zircon zoning. Keywords: zircon, U-Pb, oxygen isotopes, ion probe, lower crust, Kapuskasing, Archean.

Zircon crystals from lateritic soils at Nsimi, Cameroon, were investigated using electron microprobe analysis (EMPA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Raman spectroscopy to determine the extent of radiation damage from alpha-decay events. The soils belong to a small watershed developed on granitic rocks of the Congo craton (2.9 Ga). Interactions with fluids are evidenced by significant CaO (up to 1.5 wt%), [Al.sub.2][O.sub.3] (up to 2.9 wt%), and [Fe.sub.2][O.sub.3] (up to 2.9 wt%) concentrations in U[O.sub.2] rich regions (0.05 to 1 wt%) of the zircon. Regional heating up to 500 [degrees]C, related to the Pan-African orogeny about 0.6 Ga ago, has lead to the recrystallization of the radiation-damaged grains and the formation of a nanoporous microstructure. The correlation observed between the presence of dissolution features and the actual damage state of zircon shows that zircon dissolution occurs under tropical weathering conditions and with preferential dissolution of the highly radiation-damaged regions. Congruent dissolution of zircon and the limited mobility of Zr are supported by the absence of zirconium oxide precipitates in the fractures of weathered grains of zircon. Keywords: Zircon, Nsimi, laterite, nanopore, zirconium, metamict, radiation damage, TEM

The geochemistry of kyanite and orthopyroxene eclogites (7 samples) indicate that they are gabbroic cumulates. Incompatible trace elements in these rocks occur at low concentrations compared to regionally associated eclogites that are compositionally similar to basaltic magmas (11 samples). Eclogites with cumulate protoliths commonly contain 100 ppm Zr, >4 ppm Sm, and >0.4 ppm Lu for basaltic eclogites. Because of low Zr concentrations, igneous and metamorphic zircons are rare or absent in these eclogites. Samarium and Lu concentrations are also low in the kyanite and orthopyroxene eclogites, but they have parent/daughter Sm/Nd ratios of 0.23-0.51 and Lu/Hf ratios of 0.22-0.60, higher than most associated basaltic eclogites at 0.22-0.38 and 0.11-0.18, respectively. These results suggest that kyanite and orthopyroxene eclogites are poor targets for zircon geochronologic work, but are good targets for Sm/Nd and Lu/Hf mineral/whole rock geochronology because of their high parent/daughter ratios. Keywords: Norway, kyanite eclogite, geochronology, zircon, gabbro, cumulate, orthopyroxene eclogite, samarium-neodymium, lutetium-hafnium

Early Palaeozoic volcanic suites are widespread throughout the Variscan Belt, and have commonly been ascribed to incipient rifling along the northern periphery of Gondwana in Cambrian to Ordovician times. Their distribution across Europe defines the present-day extent of Gondwana-derived terranes and constrains the timing of their separation from the Gondwanan margin. The Kaczawa Mountains in the West Sudetes, at the eastern termination of the Variscan Belt, include bimodal rift-related rocks, but their protolith age and, hence, their significance have been highly uncertain. We have applied secondary ionization mass spectrometry zircon geochronology to a metarhyodacite and a metatrachyte from this suite, yielding ages of 502.4 [+ or -] 2.6 Ma and 485.7 [+ or -] 1.6 Ma, respectively. This constrains the initial rift magmatism to c. 500-485 Ma in this part of the European Variscides. The rift-related Early Palaeozoic volcanism thus seems to have been broadly synchronous throughout the peri-Gondwanan terranes of Europe.

The Huqf Supergroup in Oman contains an exceptionally well-preserved and complete sedimentary record of the Middle to Late Neoproterozoic Era. Outcrops of the Huqf Supergroup in northern and central Oman are now well documented, but their correlation with a key succession in the Mirbat area of southern Oman, containing a sedimentary record of two Neoproterozoic glaciations, is poorly understood. Integration of lithostratigraphic, chemostratigraphic and new U Pb detrital zircon data suggests that the Mirbat Group is best placed within the Cryogenian (c. 850 635 Ma) part of the Huqf Supergroup. The c. 1 km thick marine deposits of the Arkahawl and Marsham Formations of the Mirbat Group are thought to represent a stratigraphic interval between older Cryogenian and younger Cryogenian glaciations that is not preserved elsewhere in Oman. The bulk of detrital zircons in the Huqf Supergroup originate from Neoproterozoic Parent rocks. However, older Mesoproterozoic, Palaeoproterozoic and even Archaean zircons can be recognized in the detrital population from the upper Mahara Group (Fiq Formation) and Nafun Group, suggesting the tapping of exotic sources, probably from the Arabian Nubian Shield.

U--Pb detrital zircon ages (LAM-ICPMS) are reported for 20 greywackes and sandstones from seven major tectono-stratigraphic terranes of the Eastern Province of New Zealand (Cretaceous to Carboniferous) to constrain sediment provenances. Samples are mainly from three time horizons: Late Permian, Late Triassic and Late Jurassic. Age datasets are analysed as percentages in geological intervals, and in histogram and cumulative probability diagrams. The latter discriminate significant zircon age components in terms of terrane, sample stratigraphic age, component age, precision and percentage (of total set). Zircon age distributions from all samples have persistent, large Triassic-Permian, and very few Devonian--Silurian, populations, features which exclude a sediment provenance from the early Palaeozoic, Lachlan Fold Belt of southeast Australia or continuations in New Zealand and Antarctica. In the accretionary terranes, significant Palaeozoic (and Precambrian) zircon age populations are present in Torlesse and Waipapa terranes, and variably in Caples terrane. In the forearc and back-arc terranes, a unimodal character persists in Murihiku and Brook Street terranes, while Dun Mountain--Maitai terrane is more variable, and with Caples terrane, displays a hybrid character. Required extensive Triassic--Permian zircon sources can only be found within the New England Fold Belt and Hodgkinson Province of northeast Australia, and southward continuations to Dampier Ridge, Lord Howe Rise and West Norfolk Ridge (Tasman Sea). Small but significant Palaeozoic (and Precambrian) age components in the accretionary terranes (plus Dun Mountain--Maitai terrane), have sources in hinterlands of the New England Fold Belt, in particular to mid-Palaeozoic granite complexes in NE Queensland, and Carboniferous granite complexes in NE New South Wales. Major and minor components place sources (1) for the older Torlesse (Rakaia) terrane, in NE Queensland, and (2) for Waipapa terrane, in NE New South Wales, with Dun Mountain--Maitai and Caples terrane sources more inshore and offshore, respectively. In Early Jurassic Late Cretaceous, Torlesse (Pahau) and Waipapa terranes, there is less continental influence, and more isolated, offshore volcanic arc sources are suggested. There is local input of plutonic rock detritus into Pahau depocentres from the Median Batholith in New Zealand, or its northward continuation on Lord Howe Rise. Excepting Murihiku and Brook Street terranes, all others are suspect terranes, with depocentres close to the contemporary Gondwanaland margin in NE Australia, and subsequent margin-parallel, tectonic transport to their present New Zealand position. This is highlighted by a slight southeastward migration of terrane depocentres with time. Murihiku and Brook Street terrane sources are more remote from continental influences and represent isolated offshore volcanic depocentres, perhaps in their present New Zealand position. Keywords: U--Pb geochronology, detrital zircon, provenance, New Zealand, Palaeozoic terranes, Mesozoic terranes.

The first in situ Hf and U-Pb isotope analyses of zircon separates from Mesozoic granites in southern Tibet identify a significant, previously unknown stage of magmatism. Igneous zircons (n = 34) from a granite within the Gangdese batholith show a weighted mean [sup.206]Pb/[sup.238]U age of 188.1 [+ or -] 1.4 Ma and [[epsilon].sub.Hf](T) (the parts in [10.sup.4] deviation of initial Hf isotope ratios between the zircon sample and the chondritic reservoir) values between +10.4 and +16.8, suggesting predominantly Early Jurassic intrusive activity with a juvenile mantle contribution. Of 40 inherited zircons from two Cretaceous S-type granites in the northern magmatic belt, 23 delineate a slightly older [sup.206]Pb/[sup.238]U age cluster between 188 and 210 Ma. These zircons have [[epsilon].sub.Hf](T) values from -3.9 to -13.7, yielding crustal Hf model ages from ca. 1.4 to 2.1 Ga, suggesting a major episode of crustal growth in Proterozoic time and remelting of this crust in Early Jurassic time. Combining these with literature data, we interpret the Jurassic Gangdese magmatism as an early product of the Neo-Tethyan subduction that played a long-lasting role in the tectonic evolution of southern Tibet prior to the India-Asia collision. Keywords: zircon, Hf isotope, U-Pb dating, Tibet, Neo-Tethyan subduction.

Eastern Australian sediments of Cambrian, Ordovician, Silurian-Devonian, Triassic, and Neogene ages are known to be dominated by zircons dated 700-500 Ma ('Southwest Pacific--Gondwana igneous component') by the U--Pb SHRIMP method, and thought to be derived from Antarctica, as suggested also by paleogeographical evidence. To extend the characteristics of the provenance we subjected SHRIMPed zircons from the Middle Triassic Hawkesbury Sandstone and four Neogene beach sands to LAM--ICPMS analysis for rock type and Hf-isotope [T.sub.DM] model ages. These data confirm the demonstration (from ages alone) that the beach sands were recycled from the Hawkesbury Sandstone. All five samples have a substantial fraction of 700-500 Ma zircons derived from alkaline rocks with [T.sub.DM] of 2.0-1.0 Ga. We analysed zircons from the nearest exposed alkaline rock of appropriate age in Antarctica: the 550-500 Ma Koettlitz Glacier Alkaline Province of the Ross orogen of the Transantarctic Mountains, emplaced during contemporary transtension. The rock types and [T.sub.DM] of the Koettlitz Glacier Alkaline Province zircons match those of the eastern Australian samples but only over the restricted range of 550-500 Ma. Rocks of 700-550 Ma age and alkaline type are unknown in Antarctic exposures. Rocks of this age and type, however, abound in the Pan-Gondwanaland orogens. The nearest of these is the Leeuwin Complex of the Pinjarra orogen of southwestern Australia, which connects through the Prydz--Leeuwin Belt with the Mozambique Orogenic Belt. In turn, the Leeuwin Complex is dominated by 700-500 Ma alkaline rocks generated by contemporary transtension. We find that zircons from a beach sand at Eneabba, 450 km north of the Leeuwin Complex, are dominated by alkaline types, so confirming demonstration by age alone that the sand came from the Leeuwin Complex. An important inference is that Pan-Gondwanaland terranes are potential provenances of sediment with zircons of ages 700-500 Ma, model ages 2.0-1.0 Ga, and alkaline affinity. We compiled the age spectra of our samples with others of the Southwest Pacific Gondwana igneous component in southern Australia and reviewed their paleogeographical data. The Early Cambrian turbidites of the Kanmantoo Group were deposited from an axial northward paleocurrent that flowed across adjacent Antarctica in Wilkes Land. The Ordovician turbidite was deposited in fore-arc and abyssal fans likewise from a northward paleocurrent that flowed parallel to the uplifted Ross--Delamerian orogen. The Ordovician--Silurian-Devonian Mathinna Supergroup was deposited in a submarine fan system that prograded to the east-northeast from high ground in Antarctica. The Hawkesbury Sandstone was deposited in a braided-fiver system on a surface that sloped to the north-northeast from high ground in Antarctica. Together with the zircon data, the paleogeographical evidence in the Cambrian, Ordovician, Ordovician--Silurian--Devonian, and Triassic all point to a provenance in Wilkes Land, Antarctica of Pan-Gondwanaland (700-500 Ma) terranes with alkaline rocks. Keywords: Australia; Antarctica; detrital zircons; 700-500 Ma; Pan-Gondwanaland; Hf-isotopes; trace elements; [T.sub.DM]; paleogeography; alkaline provenance

Clastic sediments deposited in foreland basins and offshore fans allow the evolution of an orogen to be reconstructed--provided their source regions are properly characterized. Isotopic data from the lithotectonic units of the eastern Himalaya (Bhutan) indicate clear isotopic differences directly comparable with the equivalent units from the central Himalaya. Zircons from metaquartzites of the High Himalayan Series that range in age from 980 to 1820 Ma and an orthogneiss intruded at 825 [+ or -] 9 Ma bracket deposition to between 816 and 980 Ma. Model ages derived from Nd isotopes of associated metapelites range from 1700 to 2200 Ma. In contrast, zircons from a metaquartzite from the Lesser Himalayan Series range from 1850 to 2550 Ma. A Paleoproterozoic deposition age (ca. 1750 Ma) is inferred from the age of a metarhyolite associated with the sediments. Model Nd ages of metapelites from the Lesser Himalayan Series range from 2500 to 2600 Ma. The Bhutanese Himalayan units can be correlated more than 1000 km westward along strike from catchments of the Brahmaputra to the headwaters of the Sutlej on the basis of their structural position, provenance, and isotopes. Because current discharge from the Brahmaputra carries the dominant sedimentary flux into the Bengal Fan, results from this work validate the interpretation of isotopic variations observed in offshore deposits, in terms of the unroofing history of the component lithologies of the Himalaya. Keywords: Himalaya, U-Pb ages, zircon, isotope geochemistry.