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UPb zircon geochronology and trace element analysis was applied to eclogites and (ultra)high-pressure granulites that occur as volumetrically subordinate rock bodies within orthogneisses of the Orlica-Snieznik complex, Bohemian Massif. Under favourable circumstances such data may help to unravel protolith ages and yet-undetermined aspects of the metamorphic evolution, for example, the time span over which eclogite-facies conditions were attained. By means of ion-probe and laser ablation techniques, a comprehensive database was compiled for samples collected from prominent eclogite and granulite occurrences. The [sup.206]Pb/[sup.238]U dates for zircons of all samples show a large variability, and no single age can be calculated. The protolith ages remain unresolved due to the lack of coherent age groups at the upper end of the zircon age spectra. The spread in apparent ages is interpreted to be mainly caused by variable and possibly multi-stage Pb-loss. Further complexities are added by metamorphic zircon growth and re-equilibration processes, the unknown relevance of inherited components and possible mixing of different aged domains during analysis. A reliable interpretation of igneous crystallization ages is not yet possible. Previous studies and the new data document the importance of a Carboniferous metamorphic event at c. 340 Ma. The geological significance of this age group is controversial. Such ages have previously either been related to peak (U)HP conditions, the waning stages of eclogite-facies metamorphism or the amphibolite-facies overprint. This study provides new arguments for this discussion because, in both rock types, metamorphic zircon is characterized by very low total REE abundances, flat HREE patterns and the absence of an Eu anomaly. These features strongly suggest contemporaneous crystallization of zircon and garnet and strengthen interpretations proposing that the Carboniferous ages document late-stage eclogite-facies metamorphism, and not amphibolite-facies overprinting. Keywords: eclogite, granulite, Orlica-Snieznik complex, zircon, U-Pb geochronology. doi: 10.1017/S0016756809990665

Measurement of detrital zircon U-Pb ages has become the method of choice for single crystal--based investigations of provenance for both modern and ancient sediments. Recent studies, however, demonstrated the failure of zircon to record major tectonic events in source terranes, revealing the need for a more robust provenance methodology. A direct comparison between the utility of crystallization ages of detrital zircon and monazite as provenance indicators has been made using modern river alluvium derived from known sources. While detrital zircon does not fully record the multiple coilisional phases that are the hallmark of the Appalachian orogen, detrital monazite accurately records all the major tectonic events. The physical and petrogenetic differences between zircon and monazite are the primary factors for differing detrital age spectra. Zircon, owing to its extreme refractory nature, skews detrital age spectra toward older ages and limits its ability to record low-grade thermotectonic events in orogens. Monazite recrystallizes over a broader range of metamorphic conditions than does zircon. Consequently, monazite has the potential to record metamorphic events that might otherwise be absent from the detrital zircon record, thus providing a more accurate record of source terranes in regions characterized by moderate thermal events. doi: 10.1130/G30265.1

Numerous post-metamorphic Miocene granitoids occur in the area of Lavrion, SE Attica, at the western end of the Attic-Cycladic Belt of the Aegean. U--Pb ion microprobe-dating (SHRIMP) of zircon from a granitoid sill in the hanging-wall of a regional detachment fault reveals two distinct ages: (1) 11.93 [+ or -] 0.41 Ma, obtained from inherited zircon cores with metamorphic characteristics (homogeneous cathodoluminescence, low Th/U ratios) and granulite-type (round/resorbed) morphology. This age is interpreted as the time of a likely granulite-facies metamorphism of the precursor rock. (2) 8.34 [+ or -] 0.20 Ma, obtained by oscillatory zoned zircon domains with cathodoluminescence and Th/U characteristics typical for magmatic origin. This age is interpreted as the crystallization time of the granitoid sills. Although a granulite-facies metamorphic event has not been recognized so far for rocks of the Attic-Cycladic Belt, it seems to be the most plausible hypothesis to explain both the zircon systematics and age results. This hypothesis is consistent with an extensional regime predominating in the Aegean from Late Miocene times onwards. A possible granulite-facies metamorphism can be related to magmatic underplating at the initial stages of extension, setting an upper age of c. 12 Ma for the operation of the detachment fault. The 8.34 [+ or -] 0.20 Ma zircon crystallization age is, statistically, marginally different to a previous K--Ar feldspar date of hornblende-bearing dykes (9.4 [+ or -] 0.3 Ma) and identical to a 8.27 [+ or -] 0.11 Ma K--Ar biotite date of the main granitoid stock in the area, thus being generally consistent with prior age constraints from the region. Operation of the detachment fault in the Lavrion area is therefore bracketed between c. 11.9 Ma and at least 8.3 Ma. This time range is in line with the time of operation of detachment faults suggested previously for the Cycladic islands. Carbonate-hosted replacement-type massive sulphide Pb--Zn--Ag ores are spatially associated with the detachment fault and related extensional structures in the Basal Unit. Therefore, these Pb--Zn--Ag ores probably also formed within the above time span of c. 11.9 to at least 8.3 Ma. U--Pb ion microprobe (SHRIMP) dating of zircon from an orthogneiss within the metaclastic subunit of the Basal Unit in Lavrion yielded a protolith age of 240 [+ or -] 4 Ma, consistent with ages of Triassic volcanism elsewhere in Greece. Keywords: SHRIMP-dating, granitoid sills, sulphide ore, Attic-Cycladic Belt, Lavrion granulite-facies.

The first ~600 million years of Earth history (the 'Hadean' eon) remain poorly understood, largely because there is no rock record dating from that era. Detrital Hadean igneous zircons from [...]

U-Pb sensitive high-resolution ion microprobe (SHRIMP) analyses of zircons from migmatitic metapelites in the eastern Adirondack Highlands demonstrate that substantial anatexis took place ca. 1050 Ma during the late, but still high-grade, ca. 1090-1050 Ma Ottawan orogeny. These results contrast with data from metapelites of the southwestern Adirondack Highlands and Adirondack Lowlands, which indicate that anatexis occurred ca. 1200-1160Ma, during the Shawinigan orogeny and subsequent (ca. 1155 Ma) anorthosite-mangerite-charnockite-granite (AMCG) magmatism. Ca. 1180-1150 Ma zircons from this western regime do not contain ca. 1050 Ma (Ottawan) metamorphic overgrowths. The absence of ca. 1050 Ma Ottawan anatexis and overgrowths in the Adirondack Lowlands is explained by the observation that, consistent with titanite cooling ages, post-1150 Ma temperatures did not exceed ~700[degrees]C. In the southwestern Adirondack Highlands, the absence of ca. 1050 Ma anatexis can be accounted for by earlier dehydration of metapelites during ca. 1180-1150 Ma Shawinigan-AMCG metamorphism. The occurrence of ca. 1050 Ma anatexis and formation of metamorphic zircons in the eastern Adirondacks cannot be explained by higher temperatures, because geothermometry indicates that all of the Adirondack Highlands reached a nearly uniform 750-800[degrees]C during this period. Accordingly, the occurrence of ca. 1050 Ma Ottawan anatexis in the eastern regime is interpreted to be the result of: (1) influxes of hydrous fluids at ca. 1050 Ma, or (2)decompression melting during late extensional exhumation. A recently recognized low-angle late Ottawan (ca. 1050 Ma) fault system may have provided both fluid conduits and footwall decompression. The sense of displacement along the shear zone has not yet been unequivocally determined, but preliminary investigations of kinematic indicators demonstrate normal displacement. Thus, this structure may be an analogue of the ca. 1050 Ma northwest-dipping Carthage--Colton zone normal fault system that juxtaposed the Adirondack Lowlands against the Adirondack Highlands. In this context, these fault zones form a quasi-symmetrical core complex or gneiss dome, within which the high-grade core of the Adirondack Highlands was tectonically exhumed. A similar east-dipping, along-strike normal fault in Quebec (Tawachiche shear zone) exhumed the eastern Morin and Lac Taureau terranes at ca. 1050 Ma and may merge with the eastern Adirondack shear zone described here. Keywords: Adirondack Mountains, zircons, geochronology, anatexis, core complex.

The entire population of magmatic oscillatory zoned zircons in a migmatitic granite from the Tjarnesjo intrusion, southwest Sweden, reveal fine-scale brittle fracturing. The oscillatory zoned fragments are rotated but not dispersed. Fractures between individual fragments are sealed by newly formed CL-bright zircon. Hydraulic fracturing is the most probable mechanism. The internal structure of fractured zircons and the LREE-enriched, low Th character of CL-bright zircon both suggest that cracks between oscillatory zoned zircon fragments were rapidly sealed after fracturing by CL-bright zircon, precipitated from hydrothermal fluids. Zircon fracturing and crack-sealing has been dated by SIMS ion-probe and U-Th-Pb isotopes to 920 [+ or -] 5] Ma (lower intercept age, 2[sigma], MSWD = 1.09) with a limit for the youngest possible age of 960 [+ or -] 16 Ma ([sup.207]Pb/[sup.206]Pb, 2[sigma], MSWD = 0.23) dated by sector-zoned rims forming overgrowths on the fractured cores. Keywords: Zircon, hydrothermal, cathodoluminescence, U-Pb, Sveconorwegian, SIMS, EBSD, fracturing, hydraulic

In situ zircon U-Pb and Hf-isotopic data have been determined for mafic microgranular enclaves and host granitoids from the Early Cretaceous Gudaoling batholith in the Liaodong Peninsula, NE China, in order to constrain the sources and petrogenesis of granites. The zircon U-Pb age of the enclaves (120 [+ or -] 1 Ma) is identical to that of the host monzogranite (120 [+ or -] 1 Ma), establishing that the mafic and felsic magmas were coeval. The Hf isotopic composition of the enclaves [[epsilon.sub.Hf](t) = +4.5 to -6.2] is distinct from the host monzogranite [[epsilon.sub.Hf](t) = -15.1 to -25.4], indicating that both depleted mantle and crustal sources contributed to their origin. The depleted mantle component was not previously revealed by geochemical and Nd and Sr isotopic studies, showing that zircon Hf isotopic data can be a powerful geochemical tracer with the potential to provide unique petrogenetic information. Some wallrock contamination is indicated by inherited zircons with considerably older U-Pb ages and low initial Hf isotopic compositions. Hafnium isotopic variations in Early Cretaceous zircons rule-out simple crystal-liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, mixing of mantle-derived mafic magmas with crustal-derived felsic magmas, coupled with assimilation of wall rocks, is compatible with the data.

Ion microprobe (SIMS) dating of zircon from the Miranda do Douro orthogneiss, Central Iberian Zone, Hercynian Iberian belt, defines an Early Ordovician U-Pb age of 496.0 [+ or -] 2.6 Ma (95% conf., MSWD = 1.14) for magmatic zircon crystallization in its granitic protolith. The age contrasts with an earlier, conventional (ID-TIMS) U-Pb zircon age of 618 [+ or -] 9(95% conf.) Ma, now thought to be an artefact of the complex zircon population. Individual SIMS ages for zircon from the rock range from 2700 to 180 Ma and comprise inherited and magmatic zircon, both concordant and common Pb-enriched, and younger, reset ages. The ID-TIMS study seems to have misinterpreted this heterogeneous population as a cogenetic suite consisting of magmatic zircon and its age-reset equivalents produced by recent Pb loss. The 496 [+ or -] 3 Ma SIMS age represents the weighted average for 26 magmatic zircon domains located by careful inspection of cathodoluminescence, secondary electron and optical microscopy images of ~ 700 individual zircon crystals. Inherited zircon is widespread, ranging in age from 2700 to 550 Ma, with age clusters, which are statistically indistinguishable from those known from pre-Hercynian granitic basement material elsewhere in the Central Iberian Zone. Including the present age information, 582 [+ or -] 4 Ma (95% conf., MSWD = 1.02, n = 13) and 619 [+ or -] 9 Ma (95% conf., MSWD = 0.93, n = 7) appear as prevailing inherited zircon age components in basement intrusions in the Central Iberian Zone. Keywords: age determination, Hercynian, zircon, SIMS, Iberian Peninsula, Ordovician, granite.

In contrast to most large-volume silicic magmas in continental arcs, which are thought to evolve as open systems with significant assimilation of preexisting crust, the Kos Plateau Tuff magma formed dominantly by crystal fractionation of mafic parents. Deposits from this ~60 [km.sup.3] pyroclastic eruption (the largest known in the Aegean arc) lack xenocrystic zircons [secondary ion mass spectrometry (SIMS) U-Pb ages on zircon cores never older than 500 ka] and display Sr-Nd whole-rock isotopic ratios within the range of European mantle in an area with exposed Paleozoic and Tertiary continental crust; this evidence implies a nearly closed-system chemical differentiation. Consequently, the age range provided by zircon SIMS U-Th-Pb dating is a reliable indicator of the duration of assembly and longevity of the silicic magma body above its solidus. The age distribution from 160 ka (age of eruption by sanidine [sup.40]Ar/[sup.39]Ar dating; Smith et al., 1996) to ca. 500 ka combined with textural characteristics (high crystal content, corrosion of most anhydrous phenocrysts, but stability of hydrous phases) suggest (1) a protracted residence in the crust as a crystal mush and (2) rejuvenation (reduced crystallization and even partial resorption of minerals) prior to eruption probably induced by new influx of heat (and volatiles). This extended evolution chemically isolated from the surrounding crust is a likely consequence of the regional geodynamics because the thinned Aegean microplate acts as a refractory container for magmas in the dying Aegean subduction zone (continent-continent subduction). Keywords: igneous petrology, rhyolite, zircon geochronology, magma residence time, Aegean arc.

New ion microprobe U/Pb dates from zircon in deformed orthogneiss and migmatite and an undeformed granite in Mabja Dome are the first to constrain the timing of peak metamorphism, and onset and duration of mid-crustal ductile extension, in southern Tibet at 35.0 [+ or -] 0.8 Ma and ~12-19 million years. The structural, metamorphic, and intrusive histories in midcrustal rocks exposed in these north Himalayan gneiss domes are similar to those recorded in the Greater Himalayan sequence, suggesting that middle crust was continuous from beneath southern Tibet southward to the high Himalaya. Strain compatibility indicates that 35 Ma ductile extension in mid-crustal rocks of southern Tibet was accommodated to the south at shallow crustal levels via normal slip along the southern Tibetan detachment system, the oldest age estimate for slip along this normal fault zone, and extrusion of its footwall. If gravitational collapse is an additional important process driving extension, then southernmost Tibet may have been at or near maximum elevation by late Eocene--early Oligocene time. Keywords: Tibet, U/Pb dating, zircon, north Himalayan gneiss domes, age dating, extensional tectonics, ductile flow.

The Taita Hills Galana-River region is a key area to demonstrate the polycyclic nature of the Mozambique Belt in SE Kenya. On the basis of petrological and tectonic data, this area is composed of two different granulite-facies terranes, which are separated by the 20-30 km wide Galana Shear Zone. The Taita Hills and adjoining Sagala Hills exhibit a metamorphic overprint at 630-645 Ma, similar to areas in Tanzania. An emplacement age for the magmatic precursor rocks of 850-960 Ma was derived from zircon cores. Sm-Nd garnet--whole-rock analyses give an age of 585 Ma, interpreted as the cooling age after 630-645 Ma metamorphism. Nd crustal residence ages are between 1000 and 1500 Ma. The Galana Shear Zone east of the Taita Hills contains strongly deformed tonalitic migmatites with interlayered pegmatites that date a younger tectonometamorphic event at 560-580 Ma. East of the shear zone only a young metamorphic age of 550 Ma was found. The Nd model ages are c. 1500 Ma to c. 2900 Ma. In a continental configuration prior to Gondwana break-up our study area was located close to Madagascar, where several large shear zones are observed. One of these shear zones (Ranotsara Shear Zone) may be a continuation of the Galana Shear Zone.

U--Pb detrital zircon ages in variably metamorphosed, dominantly fine-grained clastic successions are used in northeastern Australia to identify two major successions along the East Gondwana margin. The older succession is of probable Late Neoproterozoic age and is considered part of a passive margin associated with rifting at c. 600 Ma. Most detrital zircons have ages in the range 1000-1300 Ma and were probably derived from an extension of a Late Mesoproterozoic (1050-1200Ma) orogenic belt from the central Australian Musgrave Complex located 1500 km to the west. No evidence has been found for 600-800 Ma rifting of a Rodinian supercontinent and therefore it is suggested that breakup must have occurred well outboard of the present Early Palaeozoic East Gondwana margin. The younger succession is of Early Palaeozoic age and contains the distinctive 500-600 Ma detrital zircon signature that is widespread in East Gondwana in addition to some samples with ages in the range 460-510 Ma consistent with local igneous sources. The younger succession is related to the active margin of Gondwana that developed on the former passive margin in a back-arc setting, and the source of 510-600 Ma zircons is considered to be a composite of rift-related and back-arc volcanic sources.

Large volumes of silicic magma were produced on a very short timescale in the nested caldera complex of the SW Nevada volcanic field (SWNVF). Voluminous ash flows erupted in two paired events: Topopah Spring (TS, >1,200 [km.sup.3], 12.8 Ma)-Tiva Canyon (TC, 1,000 [km.sup.3], 12.7 Ma) and Rainier Mesa (RM, 1,200 [km.sup.3], 11.6 Ma)-Ammonia Tanks (AT, 900 [km.sup.3], 11.45 Ma; all cited ages are previously published [sup.40]Ar/[sup.39]Ar sanidine ages). Within each pair, eruptions are separated by only 0.1-0.15 My and produced tufts with contrasting isotopic values. These events represent nearly complete evacuation of sheet-like magma chambers formed in the extensional Basin and Range environment. We present ion microprobe ages from zircons in the zoned ash-flow sheets of TS, TC, RM, and AT in conjunction with [[delta].sup.18]O values of zircons and other phenocrysts, which differ dramatically among subsequently erupted units. Bulk zircons in the low-[[delta].sup.18]O AT cycle were earlier determined to exhibit ~1.5 [per thousand] core-to-rim oxygen isotope zoning; and high-spatial resolution zircon analyses by ion microprobe reveal the presence of older grains that are zoned by 0.5-2.5 [per thousand]. The following U-Pb isochron ages were calculated after correcting for the initial U-Pb disequilibria: AT (zircon rims: 11.7 [+ or -] 0.2 Ma; cores: 12.0 [+ or -] 0.1 Ma); pre-AT rhyolite lava: (12.0 [+ or -] 0.3 Ma); RM: 12.4 [+ or -] 0.3); TC: (13.2 [+ or -] 0.15 Ma); TS: (13.5 [+ or -] 0.2). Average zircon crystallization ages calculated from weighted regression or cumulative averaging are older than the Ar-Ar stratigraphy, but preserve the comparably short time gaps within each of two major eruption cycles (TS/TC, RM/AT). Notably, every sample yields average zircon ages that are 0.70-0.35 Ma older than the respective Ar-Ar eruption ages. The Th/U ratio of SWNVF zircons are 0.4-4.7, higher than typically found in igneous zircons, which correlates with elevated Th/U of the whole rocks (5-16). High Th/U could be explained if uranium was preferentially removed by hydrothermal solutions or is retained in the protolith during partial melting. For low-[[delta].sup.18]O AT-cycle magmas, rim ages from unpolished zircons overlap within analytical uncertainties with the [sup.40]Ar/[sup.39]Ar eruption age compared to core ages that are on average ~0.2-0.3 My older than even the age of the preceding caldera forming eruption of RM tuff. This age difference, the core-to-rim oxygen isotope zoning in AT zircons, and disequilibrium quartz-zircon and melt-zircon isotopic fractionations suggest that AT magma recycled older zircons derived from the RM and older eruptive cycles. These results suggest that the low-[[delta].sup.18]O AT magmas were generated by melting a hydrothermally-altered protolith from the same nested complex that erupted high-[[delta].sup.18]O magmas of the RM cycle only 0.15 My prior to the eruption of the AT, the largest volume low-[[delta].sup.18]O magma presently known. Keywords Paintbrush tuff * Timber Mountain tuff * Oxygen isotopes * Geochronology * Isotope zoning * Zircon * Yucca Mountain

The Seiland Igneous Province (SIP) of northern Norway comprises a suite of mainly gabbroic plutons, with subordinate ultramafic, syenitic and felsic intrusions. Several intrusions from the Seiland Igneous Province have been dated by ID-TIMS U-Pb zircon and monazite analyses. The Hasvik Gabbro on the island of Soroy, previously assigned an age of 700 [+ or -] 33 Ma by Sm-Nd, yields a U-Pb zircon age of 562 [+ or -] 6 Ma, within error of the Storelv Gabbro (569 [+ or -] 5 Ma) and a diorite associated with the Breivikbotn Gabbro (571 [+ or -] 4 Ma). Various intrusions on the Oksfjord peninsula give nearly identical ages of 565 [+ or -] 9 Ma (gabbro), 566 [+ or -] 4 Ma (monzonite), 565 [+ or -] 5 Ma (monzodiorite), 570 [+ or -] 9 Ma (norite), and 566 [+ or -] 1 Ma (orthopyroxenite). These ages overlap with those from Soroy, and define a single and short-lived period of gabbroic (to felsic) magmatism for the region between 570 and 560 Ma, pre-dating a subordinate episode of alkalic magmatism at 530-520 Ma. The U-Pb ages contradict the previous geochronological interpretation for the Finnmark area, which implied a period of 250 m.y. for the emplacement of the SIP intrusions. The new age data also clearly distinguish the Seiland intrusions, emplaced into the Soroy Group metasediments of the Kalak Nappe Complex, from several older granitic intrusions (c. 850 to 600 Ma) that cut the Soroy Group farther east and south. The coincident ages of the different Seiland intrusive bodies also contradict the previous structural model for the area, which posits that the different gabbro bodies were emplaced at intervals, with compressional deformation affecting the gabbros between periods of intrusion. The short time span between the main plutonic phases strongly suggests that the mechanism for the emplacement of mafic magma operated in a single, probably extensional, tectonic regime. The mafic intrusions were later deformed and metamorphosed to at least amphibolite facies, most likely by the Scandian (420 Ma) phase of the Caledonian Orogeny. Keywords: Seiland, age, magmatism, zircon, Caledonides.

Generally, it has been assumed that signal intensity variations in back-scattered electron (BSE) images of minerals are mainly controlled by chemical heterogeneity. This is especially true for images of single crystals, where effects of different crystal orientations with respect to the incident beam on the observed BSE are excluded. In contrast, we show that local variations of the structural state within single-crystals (i.e., degree of lattice order or lattice imperfectness) may also have dramatic effects on the back-scattering of electrons. As an example, we present BSE images of single-crystals of natural zircon, ZrSi[O.sub.4], whose intensity patterns are predominantly controlled by structural heterogeneity, whereas effects of chemical variations are mostly negligible. In the case of natural zircon, structural heterogeneity affecting the BSE patterns is predominantly due to heterogeneous accumulation of radiation damage. We attempt to explain our observations with lowered penetration and channeling and, thus, enhanced back-scattering of electrons in more radiation-damaged internal zones and microareas. Back-scattered electron contrast of natural zircon is, therefore, considered as a special case of electron channeling contrast. This phenomenon seems to have been generally underappreciated in the discussion of BSE images of radiation-damaged minerals thus far. Keywords: Electron microscopy, BSE imaging, Raman spectroscopy, zircon

The Svecofennian Domain of the of the Fennoscandian Shield constitutes a considerable volume of Palaeoproterozoic crustal growth, 2.1-1.86 Ga ago, in between the Archaean craton in the NE and the 1.85-1.65 Ga Transscandinavian Igneous Belt (TIB) in the south and west. The Bergslagen area is a classical ore province located in the southwestern part of the Svecofennian Domain of south-central Sweden. Its northern part is dominated by volcanic and plutonic rocks ofa magmatic arc with continental affinity, while the SE part is made up by a sedimentary basin. The Bergslagen area shows a metamorphic zonation from lower to middle amphibolite facies in the north to upper amphibolite facies and locally granulite facies in the south; a small greenschist area exists in the west. Identifying the age spectra of inherited components, magmatic crystallization, as well as metamorphic episodes, provide important constraints on the geodynamic evolution of this centrally located piece of the Shield. U-Pb zircon SIMS data presented in this paper complement the previous, regionally scattered TIMS data from this area. Magmatic zircons from two felsic metavolcanic rocks and two amphibolites (metagabbros) yield 1888 [+ or -] 12, 1892 [+ or -] 7 and 1887 [+ or -] 5, 1895 [+ or -] 5 Ma, respectively; i.e. within the 1.91-1.86 Ga range previously obtained for Early Svecofennian magmatism in Bergslagen. An augen gneiss from southern Bergslagen, assigned to the earliest TIB generation, yield an intrusive age of 1855 [+ or -] 6 Ma. Metamorphic monazites from the same rock indicate that deformation and elevated thermal activity prevailed 1.83-1.82 Ga ago (TIMS). Metamorphic zircons in high-grade metasedimentary rocks from the south and west yield ages of 1793 [+ or -] 5 and 1804 [+ or -] 10 Ma, in accordance with ages for regional peak metamorphism and migmatite formation found elsewhere in the southern Svecofennian province of Sweden. More importantly, a few zircon crystals and overgrowths in rocks from the north indicate an early metamorphic episode at c. 1.87 Ga, indicating that Bergslagen has experienced two major metamorphic events. Detrital and inherited zircons span the range 2.781.90 Ga, with an apparent gap at 2.45-2.1 Ga, which further emphasize previous observations of a major juvenile (< 2.1 Ga) and a minor Archaean provenance. This, and in particular the 1.94-1.91 Ga crystals present in the c. 1.89 Ga amphibolites, support the suggestion of a former Palaeoproterozoic pre-1.91 Ga crust in the Bergslagen area. Keywords: geochronology, zircon, SIMS, TIMS, Bergslagen, Svecofennian, magmatic, inherited, metamorphic.

Zircons have been extracted from garnet peridotite and its wall rock (gneiss), which was intersected by the pre-pilot hole of the Chinese Continental Scientific Drilling project (CCSD-PP1) in the Sulu ultrahigh-pressure (UHP) metamorphic belt. The peridotitic zircons record early Mesozoic UHP metamorphism ([sup.206]Pb/[sup.238]U age of 223.5 [+ or -] 7.5 Ma), but their Hf isotope compositions indicate that the protolith of the peridotite is at least Mesoproterozoic in age (minimum depleted-mantle ages [[T.sub.DM]] of 1.4 Ga). Zircons from the gneiss also reflect the Mesozoic metamorphism, with a cluster of [sup.206]Pb/[sup.238]U ages at 224.5 [+ or -] 11.5 Ma; a trail of discordant grains indicates a protolith age older than 800 Ma, consistent with [T.sub.DM] model ages of younger than 1.2 Ga. The peridotitic zircons have trace-element patterns similar to kimberlitic and/or carbonatitic zircons, while those from the gneiss have affinities with zircons from syenites/monzonites. The differences suggest that the metasomatic agents that affected the peridotitic zircons were derived from the asthenospheric mantle rather than from subducted continental crust. The strong depletion of the CCSD-PP1 peridotite in basaltic components, and the relatively unradiogenic Hf isotopic compositions (e.g., -16.3 to ~-13.8 [[epsilon].sub.Hf]) of the peridotitic zircons, indicate that the peridotitic body is a fragment of refractory Archean mantle that experienced Mesoproterozoic metasomatism and represents a tectonic intrusion into younger crust. Keywords: peridotitic zircon, Hf isotope, trace element, U-Pb dating, Sulu UHP terrane, China.

The release of Zr, U, and Pb from nearly metamict zircon and its recrystallized analog and of Zr from fully crystalline and slightly radiation-damaged zircon in [H.sub.]O + HCl [+ or -] Si[O.sub.2] fluids was investigated in situ at temperatures between 200 and 500[degrees]C using a hydrothermal diamond-anvil cell and time-resolved synchrotron-radiation XRF analyses. Dissolution of nearly metamict zircon proceeded much faster than that of zircon with little or no radiation damage and resulted in a 1.5 to 2 log units higher Zr molality in 6 to 7 m HCl fluids. Extensive recrystallization of the almost fully amorphous material started at 260 to 300[degrees]C in [H.sub.2]O + HCl, and at about 360[degrees]C if quartz was added, and was coupled with a decrease of the Zr concentration in the fluid by more than an order of magnitude. Recrystallization in 7 m HCl had little effect on the aqueous U and Pb concentrations, whereas addition of quartz caused a more sluggish decrease of the Zr concentration in the fluid upon recrystallization and lowered the release of U. The data presented here support the interpretation that enhanced Zr mobility in low-grade metamorphic rocks may be related to dissolution of metamict zircon by aqueous fluids and illustrate the significance of the silica activity on the kinetics of dissolution and recrystallization during zircon-fluid interaction. Keywords: High-temperature studies, zircon-aqueous fluid interactions, hydrothermal diamond-anvil cell, in-situ SR-XRF analysis, dissolution kinetics, order-disorder, metamictization, recrystallization, fluid phase, [H.sub.2]O + HCl [+ or -] Si[O.sub.2], recrystallization kinetics

The trace element compositions of Hadean zircons have been used in two ways to argue for the existence of Hadean continental crust. One argument is based on low crystallization temperatures of Hadean zircons that have been determined using a novel geothermometer based on the Ti content of zircons in equilibrium with rutile. The second argument is based on using the trace element abundances in zircons to calculate their parental melt compositions, especially the rare earth elements. Here we demonstrate that zircons that grow from a melt formed by basalt differentiation at modern mid-ocean ridges cannot be unambiguously distinguished from Hadean zircons on either of these grounds. Thus, we conclude that the trace element compositions of Hadean zircons are permissive of models that do not include the generation of continental crust in the Hadean. Keywords: Hadean, mid-ocean ridge, zircon, continental crust.

The North-East Greenland eclogite province is divided into a western, central and eastern block by the sinistral Storstrommen shear zone in the west and the dextral Germania Land deformation zone in the east. A family of steep, NNW-striking dextral mylonite zones in the Danmarkshavn area are geometrically and kinematically similar to the ductile Germania Land deformation zone, located 25 km to the east. Amphibolite facies deformation at Danmarkshavn is characterized by boudinage of eclogite bodies within quartzofeldspathic host gneisses, pegmatite emplacement into the boudin necks and subsequent deformation of pegmatites parallel to gneissosity, a widespread component of dextral shear within the gneisses, and localization of strain into 10-50 m thick dextral mylonite zones. The gneisses and concordant mylonite zones are cut by a swarm of weakly to undeformed, steeply dipping, E-W-striking pegmatitic dykes. Oscillatory-zoned zircon cores from two boudin neck pegmatites give weighted mean [sup.206]Pb/[sup.238]U sensitive, high mass resolution ion microprobe (SHRIMP) ages of 376 [+ or -] 5 Ma and 343 [+ or -] 7 Ma. Cathodoluminescence images of these zircons reveal complex additional rims, with ages from ranging from c. 360 to 320 Ma. Oscillatory-zoned, prismatic zircons from two late, cross-cutting pegmatites yield weighted mean [sup.206]Pb/[sup.238]U SHRIMP ages of 343 [+ or -] 5 Ma and 332 [+ or -] 3 Ma. Zircons from the boudin neck pegmatites record a prolonged growth history, marked by fluid influx, during amphibolite facies metamorphism beginning at c. 375 Ma. The cross-cutting pegmatites show that dextral deformation in the gneisses and ductile mylonite zones had stopped by c. 340 Ma. Ultrahigh-pressure metamorphism in the eastern block at 360 Ma requires that the Greenland Caledonides were in an overall contractional plate tectonic regime. This, combined with 20% steep amphibolite facies lineations in the eclogites, gneisses and mylonites suggests that dextral transpression may have been responsible for a first stage of eclogite exhumation between 370 and 340 Ma. Keywords: eclogite, exhumation, Greenland Caledonides, U-Pb SHRIMP geochronology, zircon.

The free energy of formation of zircon (ZrSi[O.sub.4]) from its oxides was determined between 1100 and 1300 K by an electrochemical method, in which values of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] defined by the two assemblages [Fe.sub.2]Si[O.sub.4]-Fe-Si[O.sub.2] (fayalite-iron-quartz) and [Fe.sub.2] Si[O.sub.4]-Fe-Zr[O.sub.4]-ZrSi[O.sub.4] were each measured using oxygen concentration cells with calcia-stabilized zirconia solid electrolytes. The difference in [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] between these two assemblages corresponds to the reaction Zr[O.sub.2] + Si[O.sub.2](qz) = ZrSi[O.sub.4]. The results, when analyzed using calorimetric data for the entropies and high-temperature heat capacities of ZrSi[O.sub.4], Zr[O.sub.2], and Si[O.sub.2](quartz), yields [[DELTA].sub.f,ox][H.sup.0.sub.298K] = -24.0 [+ or -] 0.2 kJ/mol for ZrSi[O.sub.4], in good agreement with the calorimetric value of Ellison and Navrotsky (1992). ZrSi[O.sub.4] is predicted to decompose to Zr[O.sub.2] plus Si[O.sub.2] (cristobalite) at 1938 K, assuming a temperature of 1430 K for the martensitic phase transition between the tetragonal and monoclinic forms of Zr[O.sub.2] (baddeleyite), with an enthalpy of transition of 8.67 kJ/mol. The same experimental approach was used also to determine the free energy of formation of hafnon (HfSi[O.sub.4]). The entropy of hafnon ([S.sup.0.sub.298K] = 93.6 J/mol x K) is similar to that for zircon, but the enthalpy of formation is slightly more exothermic ([[DELTA].sub.f.ox]/[H.sup.0.sub.298K] = -25.0 [+ or -] 0.2 kJ/mol). The cells with either ZrSi[O.sub.4] + Zr[O.sub.2] or HfSi[O.sub.4] + Hf[O.sub.2] produce an anomalous excursion in EMF when the temperature of the [alpha]-[gamma] transition in Fe metal at 1184 K is traversed; this excursion takes >12 hours to decay back to the equilibrium value. This behavior is presumably related to strain caused by the volume change of the [alpha]-[gamma] transition. The redetermination of the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] of the [Fe.sub.2] Si[O.sub.4]-Fe-Si[O.sub.2](qz) equilibrium (the quartz-fayalite-iron or QFI oxygen buffer) carried out in the course of this study gave results in reasonable agreement with previous work, but with a different slope vs. temperature, implying a slightly higher value of [S.sup.0.sub.298K] for [Fe.sub.2]Si[O.sub.4] than the currently accepted calorimetric datum (i.e., 153.5 vs. 151.0 [+ or -] 0.2 J/K x mol). Keywords: Zircon, hafnon, free energy of formation, fayalite, thermodynamic data

Sensitive high-resolution ion microprobe (SHRIMP) U--Pb ages for detrital zircons from the Caspian region reveal the age ranges of basement terrains that supplied the sediment. One sample from the modern Volga river has groupings at c. 340-370 Ma, c. 900-1300 Ma and c. 1450-1800 Ma, with a small number of older zircons. This is consistent with derivation from the Precambrian basement of the East European Craton, and Palaeozoic arcs in the Urals. Mid- and Late Proterozoic components may be derived from beyond the present Volga drainage basin, such as the Sveconorwegian orogen. A Bajocian sandstone from the Greater Caucasus has 73% zircons that post-date 350 Ma. Ages cluster at c. 165-185 Ma, c. 220-260 Ma, c. 280-360 Ma and c. 440-460 Ma. This pattern suggests derivation from Palaeozoic basement of the Greater Caucasus itself and/or the Scythian Platform, and igneous rocks generated at a Jurassic arc in the Lesser Caucasus. Four samples from the Lower Pliocene Productive Series of the South Caspian Basin have common Phanerozoic grains, and groups between c. 900-1300 Ma and 1500-2000 Ma. Each sample contains zircons dated to c. 2700 Ma. The overall age patterns in the Productive Series samples suggest a combination of East European Craton and Greater Caucasus source components.

The Songpan-Garze fold belt, located in the eastern part of the Tibetan Plateau, covers a huge triangular area bounded by the Yangtze (South China), the North China and the Tibetan Plateau blocks. In the northeastern part of the Songpan-Garze fold belt, the Yanggon and Maoergai granitoids provide insights into regional tectono-magmatic events, basement nature and tectonic evolution. U--Pb zircon SHRIMP dating shows that the Yanggon and Maoergai granitoids have magmatic crystallization ages of 221 [+ or -] 3.8 Ma and 216 [+ or -] 5.7 Ma, respectively. Both the granitoids display adakitic geochemical signatures, suggesting that their magma was derived from partial melting of thickened lower crust. Pb--Sr--Nd isotopic compositions for granitoids reveal that there is an unexposed Proterozoic basement in the Songpan-Garze belt, which has an affinity with the Yangtze block. During development of the Paleo-Tethys ocean, the basement of the Songpan-Garze belt would be a peninsula approaching the Paleo-Tethys ocean from the Yangtze block.

[1] To better understand the driving mechanisms behind the transition from collision to extension in a convergent orogen, data from multiple low-temperature thermochronometers were obtained from the Simav detachment fault (SDF), the earliest developed major extensional structure recognized in the western Anatolide orogen of western Turkey. Twenty-two zircon fission track (FT), 26 apatite FT, 12 apatite (U-Th)/He ages, and 26 apatite FT track length analyses are presented. The data establish that the SDF was a major extensional fault active between ~25 and ~ 19 Ma. The coincidence in timing of magmatism with cessation of SDF activity implies the detachment became locked owing to doming induced by magma emplacement. Zircon FT ages away from the influence of Miocene magmatism record rapid footwall cooling between ~25 and ~21 Ma at a slip rate of up to ~l 5 km/Myr and demonstrate that active ductile deformation along the SDF migrated northward with time. Apatite FT ages from the footwall of the SDF are spatially invariant over > 100 km and consistently ~2-3 Myr younger than the zircon fission track ages from the same samples. These data are consistent with a regional, but relatively rapid erosion-linked cooling phase that removed ~2-3 km of overburden following cessation of SDF activity but before deposition of sedimentary rocks on the detachment surface at 16.4 Ma. Postcollision extension of the Anatolide orogen with development of the SDF can be best explained as the result of postcollision magmatism and thermal weakening of the crust inducing the extensional reactivation of an earlier major out-of-sequence thrust. Citation: Thomson, S. N., and U. Ring (2006), Thermochronologic evaluation of postcollision extension in the Anatolide orogen, western Turkey, Tectonics, 25, TC3005, doi:10.1029/2005TC001833.

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

Zircons gneisses and migmatites collected from the Antarctic Peninsula have different core-rim hafnium isotope ratio relationships depending on whether evidence for zircon dissolution is present or absent. Two samples contain inherited zircon that is partially dissolved. In these samples, the [sup.176]Hf/[sup.177]Hf rations of the inherited zircon and new magmatic zircon rims are, on average, indistinguishable and consistent with in situ melting. In such cases the hafnium isotopic composition of the melt was probably strongly influenced by the dissolved zircon component at the source. Variation in [sup.176]Hf/[sup.177]Hf within the magmatic zircon rims from grain to grain suggests that Hf isotopes were only partially homogenized during melt migration; alternatively, zircon growth may have taken place within small volumes of partial melt. Other samples do not preserve textural evidence for zircon dissolution during melt generation; in these samples the [sup.176]Hf/[sup.177]Hf values of the inherited zircon and new magmatic zircon rims are different. The zircon rims apparently suggest a source of less evolved hafnium than that contained within the inherited zircon. Whether this relates to a separate juvenile source or, alternatively, is derived from minerals other than zircon at the source, cannot be resolved. Inherited zircon, irrespective of age, has been strongly influenced by the reworking of a juvenile Late Mesoproterozoic source, suggesting that such crust underlies the Antarctic Peninsula. Our results therefore suggest that Hf isotope analyses provide great potential for future studies investigating the source and processes involved in the generation of crustal melts.

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.

A vast sequence of quartz-rich sandstone was deposited over North Africa and Arabia during Early Palaeozoic times, in the aftermath of Neoproterozoic Pan-African orogeny and the amalgamation of Gondwana. This rock sequence forms a relatively thin sheet (1-3 km thick) that was transported over a very gentle slope and deposited over a huge area. The sense of transport indicates unroofing of Gondwana terranes but the exact provenance of the siliciclastic deposit remains unclear. Detrital zircons from Cambrian arkoses that immediately overlie the Neoproterozoic Arabian-Nubian Shield in Israel and Jordan yielded Neoproterozoic U-Pb ages (900-530 Ma), suggesting derivation from a proximal source such as the Arabian-Nubian Shield. A minor fraction of earliest Neoproterozoic and older age zircons was also detected. Upward in the section, the proportion of old zircons increases and reaches a maximum (40 %) in the Ordovician strata of Jordan. The major earliest Neoproterozoic and older age groups detected are 0.95-1.1, 1.8-1.9 and 2.65-2.7 Ga, among which the 0.95-1.1 Ga group is ubiquitous and makes tip as much as 27 % in the Ordovician of Jordan, indicating it is a prominent component of the detrital zircon age spectra of northeast Gondwana. The pattern of zircon ages obtained in the present work reflects progressive blanketing of the northern Arabian-Nubian Shield by Cambrian-Ordovician sediments and an increasing contribution from a more distal source, possibly south of the Arabian-Nubian Shield. The significant changes in the zircon age signal reflect many hundreds of kilometres of southward migration of the provenance. Keywords: zircon, Cambrian-Ordovician, Arabian-Nubian Shield, provenance.

Zircon from lower crustal xenoliths erupted in the Navajo volcanic field was analyzed for U-Pb and Lu-Hf isotopic compositions to characterize the lower crust beneath the Colorado Plateau and to determine whether it was affected by ~1.4 Ga granitic magmatism and metamorphism that profoundly affected the exposed middle crust of southwestern Laurentia. Igneous zircon in felsic xenoliths crystallized at 1.73 and 1.65 Ga, and igneous zircon in mafic xenoliths crystallized at 1.43 Ga. Most igneous zircon has unradiogenic initial Hf isotopic compositions ([[epsilon].sub.Hf]= +4.1-+7.8) and 1.7-1.6 Ga depleted mantle model ages, consistent with 1.7-1.6 Ga felsic protoliths being derived from 'juvenile' Proterozoic crust and 1.4 Ga mafic protoliths having interacted with older crust. Metamorphic zircon grew in four pulses between 1.42 and 1.36 Ga, at least one of which was at granulite facies. Significant variability within and between xenoliths in metamorphic zircon initial Hf isotopic compositions ([[epsilon].sub.Hf]=-0.7 to + 13.6) indicates growth from different aged sources with diverse time-integrated Lu/Hf ratios. These results show a strong link between 1.4 Ga mafic magmatism and granulite facies metamorphism in the lower crust and granitic magmatism and metamorphism in the exposed middle crust.

Mafic-layered intrusions and sills and spatially associated andesitic basalts are well preserved in the Funing area, SW China. The 258 [+ or -] 3 Ma-layered intrusions are composed of fine-grained gabbro, gabbro and diorite. The 260 [+ or -] 3 Ma sills consist of undifferentiated diabases. Both the layered intrusions and volcanic rocks belong to a low-Ti group, whereas the diabases belong to a high-Ti group. Rocks of the high-Ti group have FeO, Ti[O.sub.2] and [P.sub.2][O.sub.5] higher but MgO and Th/Nb ratios lower than those of the low-Ti group. They have initial [sup.87]Sr/[sup.86]Sr ratios (0.706-0.707) lower and [epsilon]Nd (-1.5 to -0.6) higher than the low-Ti equivalents (0.710-0.715 and -9.6 to -4.0, respectively). The high-Ti group was formed from relatively primitive, high-Ti magmas generated by low degrees (7.3 -9.5%) of partial melting of an enriched, OIB-type asthenospheric mantle source. The low-Ti group may have formed from melts derived from an EM2-like, lithospheric mantle source. The marie rocks at Funing are part of the Emeishan large igneous province formed by a mantle plume at ~260 Ma.

Peralkaline granites exhibit the hallmark features of A-type igneous rocks, but strongly differentiated chemistry and intense hydrothermal alteration camouflage their ultimate origins. We present the first in situ Hf isotope data from zircons of peralkaline granites, aimed at clarifying the protoliths of these plutons and their genetic relationship to associated metaluminous/weakly peraluminous granites. This study used rocks of the Devonian Narraburra Complex in southeastern Australia, and found that correlations between Hf isotopes and trace element ratios reveal fundamentally different origins for the nonperalkaline and peralkaline granites. The latter have a depleted mantle-like ancestry, whereas a weakly peraluminous rock formed from melts of older arc crust that were modified by interaction with juvenile, probably alkaline magmas. Juxtaposition of crust- and mantle-derived magmas reflects the high heat flow and lithosphere-scale faults associated with continental extension, and explains the diversity of A-type granites. Keywords: peralkaline, granite, zircon, Hf isotope.

Until recently it has been widely accepted that protoliths to metasediments of the Harts Range Metamorphic Complex (central Australia) were deposited prior to c. 1.75 Ga and form part of the Patacoproterozoic Arunta Inlier. However, new sensitive high-resolution ion microprobe U-Pb analyses of detrital zircon, together with recently published data, suggest that they were deposited coeval with c. 545-520 Ma sediments from the adjacent, little metamorphosed Neoproterozoic to Palaeozoic Centralian Superbasin. Protoliths of the Harts Range Metamorphic Complex were deposited in the Irindina sub-basin, an early- to mid-Cambrian rift located between the present-day Amadeus and Georgina Basin remnants of the Centralian Superbasin. Deposition occurred during a widespread and long-lived interval of extension in parts of central Australia associated with eruption of the voluminous Kalkarinji Continental Flood Basalts. The Harts Range Metamorphic Complex was metamorphosed to upper amphibolite- to granulite-facies conditions within c. 40 Ma of deposition of its sedimentary protoliths. Keywords: SHRIMP, zircon provenance, central Australia, metamorphism, tectonics.

New U--Pb zircon ion-microprobe ages from the alluvial conglomerates and flood plain sediments of the Botany Bay Group demonstrate that sedimentation occurred at c. 167 Ma, coeval with rift-related silicic volcanism in the northern Antarctic Peninsula. In contrast, rift-related volcanism and sedimentation in the southern Antarctic Peninsula (Latady Basin) occurred at c. 183 Ma. The new data indicate that syn-rift sedimentation and volcanism was diachronous from south to north, consistent with early opening of the Weddell Sea embayment by anti-clockwise rotation of the Antarctic Peninsula in the Mid-Jurassic. A definitive date for the Botany Bay Group floras has important implications for Southern Hemisphere biostratigraphic correlations. Keywords: Antarctica, Middle Jurassic, Gondwana, biostratigraphy, absolute age.

A newly discovered pre-Variscan cover basement relationship in NE Sardinia is defined by differences in structural and metamorphic histories. The oldest metamorphic fabrics in the basement are absent from a series of metaplutons that are inferred to be comagmatic with metavolcanic rocks in the cover. Cover and basement are further distinguished by contrasting metapelite compositions. Superimposed metamorphism is represented by pre-Variscan garnet and Variscan plagioclase porphyroblasts that appear in the field as a 'mixed' garnet + albite zone. In situ U-Pb zircon dating of meta-igneous rocks from cover and basement by laser-ablation inductively coupled plasma-mass spectrometry gives ages of 458 [+ or -] 7 Ma for the Tanaunella orthogneiss, 456 [+ or -] 14 Ma for the Lode orthogneiss, and 474 [+ or -] 13 Ma for the Lula porphyroid. The internal Sardinian Variscides thus incorporate a mid-Ordovician magmatic belt developed upon an older basement: correlation is proposed with a tectonostratigraphic domain that is at present exposed in the Alps. Within the mid-Ordovician peri-Gondwanan reahn, the internal Sardinian Variscides appear to have developed along an active margin, whereas the foreland correlates with the Ibero-Aquitaine domain and is related to the evolution of a passive margin during rifting and detachment of Avalonia to form the Rheic Ocean. Keywords: Sardinia, Variscan Orogeny, U-Pb, zircon, palaeogeography.

This study reports U-Pb ages for zircons from the Los Ranchos Formation, which is part of the primitive island are sequence, the oldest volcanic rocks in the Greater Antilles. Zircons were analyzed from three samples: quartz porphyry (quartz keratophyre) from the lower part of the formation (Quita Sueno Member), fragmental quartz porphyry from the upper part of the formation (Pueblo Viejo Member), and the Cotui quartz diorite stock that intrudes the lower part of the formation. The lower part of the Los Ranchos Formation, represented by the Quita Suefio sample, formed at either 113.9 [+ or -] 0.8 or 118.6 [+ or -] 0.5 Ma depending on interpretation of the data, whereas the upper part, represented by the Pueblo Viejo sample, formed ca. 110.9 [+ or -] 0.8 Ma. The Cotui stock (RD-73-601) was emplaeed ca. 111.8 [+ or -] 0.6 Ma or 112.9 [+ or -] 0.9 Ma depending on whether eight grains reflect subtle inheritance. These results show that the Los Ranchos Formation was emplaced during the Aptian-Albian transition, that the Cotui stock could have supplied magma to the volcanic sequence and fluids to the Pueblo Viejo gold-silver deposit in the upper part of the formation, and that carbonaceous sediments in the upper part of the formation formed at the same time as ocean anoxic event 1b (Paquier). Caribbean plate tectonic models involving invasion of anomalous Pacific crust into the Caribbean region appear to agree best with this age because they provide a mechanism for the change from primitive island arc (PIA) to calc-alkaline magmatism and account for the restricted ocean circulation necessary to generate regional anoxic conditions. Keywords: Los Ranchos Formation, U-Pb, island-arc tholeiite, Hispaniola, Caribbean, plate tectonics.

Granulites including a charnockite suite, mafic granulites, pelitic granulites, metanorite dykes and their retrograde varieties occur as discontinuous shear zone-bounded bodies within the Archaean basement comprising a granite gneiss-amphibolite-metasedimentary rock association in the central part of the Aravalli Mountains, northwest India. The entire suite, named the Sandmata Complex, preserves a complex history of tectonothermal evolution. Except for their strongly foliated margins, the granulite bodies are largely massive. Partial melting in the 'country rocks' led to the development of migmatite gneisses close to the contact of the granulite, a feature not as common in the rocks further away from the granulite contact. Geothermobarometry of massive granulites indicates [T.sub.max] > 900 [degrees]C and [P.sub.max] ~ 7.5 kbar. The retrograde granulites, which formed at lower amphibolite/upper greenschistfacies conditions, experienced channelized hydration reactions concomitant with shearing. These rocks locally appear as hornblende-biotite-bearing foliated granulite with or without Cpx or Opx. The rocks seem to have followed an inverse PTt path and have undergone an earlier phase of near-isobaric cooling. Our single zircon Pb-Pb ages indicate that the exhumation of granulites to the shallower amphibolite-facies levels with concomitant melting in the country rocks took place between 1690 Ma and 1621 Ma. Assuming that the granulite-facies metamorphism took place at around 1725 Ma, we relate the entire process of granulite metamorphism and exhumation covering an age range between 1725 and 1621 Ma to the rift basin opening stages of the Delhi Orogenic cycle that culminated at c. 1450 Ma. Keywords: Aravalli Mountains, geothermobarometry, granulite exhumation, zircon geochronology.

We report U-Pb electron microprobe (zircon and monazite) and Secondary Ion Mass Spectrometry (SIMS) U-Pb (zircon) ages from a granulite-facies metapelite and a garnet-biotite gniess from Chittikara, a classic locality within the Trivandrum Block of southern India. The majority of the electron-microprobe data on zircons from the metapelite define apparent ages between 1500 and 2500 Ma with a prominent peak at 2109 [+ or -] 22 Ma, although some of the cores are as old as 3070 Ma. Zircon grains with multiple age zoning are also detected with 2500-3700 Ma cores, 1380-1520 mantles and 530-600 Ma outer rims. Some homogeneous and rounded zircon cores yielded late Neoproterozoic ages that suggest that deposition within the Trivandrum Block belt was younger than 610 Ma. The outermost rims of these grains are characterized by early Cambrian ages suggesting metamorphic overgrowth at this time. The apparent ages of monazite grains from this locality reveal multiple provenance and polyphase metamorphic history, similar to those of the zircons. In a typical case, Palaeoproterozoic cores (1759-1967 Ma) are enveloped by late Neoproterozoic rims (562-563 Ma), which in turn are mantled by an outermost thin Cambrian rim (~ 515 Ma). PbO v. Th[O.sub.2]* plots for monazites define broad isochrons, with cores indicating a rather imprecise age of 1913 [+ or -] 260 Ma (MSWD = 0.80) and late Neoproterozoic/Cambrian cores as well as thin rims yielding a well-defined isochron with an age of 557 [+ or -] 19 Ma (MSWD = 0.82). SIMS U-Pb isotopic data on zircons from the garnet--biotite gneiss yield a combined core/tim imprecise discordia line between 2106 [+ or -] 37 Ma and 524 [+ or -] 150 Ma. The data indicate Palaeoproterozoic zircon formation with later partial or non-uniform Pb loss during the late Neoproterozoic/Cambrian tectonothermal event. The combined electron probe and SIMS data from the metapelite and garnet-biotite gneiss at Chittikara indicate that the older zircons preserved in the finer-grained metapelite protolith have heterogeneous detrital sources, whereas the more arenaceous protolith of the garnet-biotite gniess was sourced from a single-aged terrane. Our data suggest that the metasedimentary belts in southern India may have formed part of an extensive late Neoproterozoic sedimentary basin during the final amalgamation of the Gondwana supercontinent. Keywords: electron microprobe dating, monazite, Southern Granulite Terrane, India, SHRIMP, zircon.

Ion-microphone U-Pb zircon geochronology demonstrates that magmatic rocks that cut the Slishwood Division are early Ordovician in age and relate to the earliest stages of the Grampian Orogeny. These tonalite and granite intrusions yield ages of 474 [+ or -] 5 Ma, 472 [+ or -] 6 Ma, 471 [+ or -] 5 Ma and 467 [+ or -] 6 Ma. A Sm-Nd mineral isochron of 457 [+ or -] 36 Ma confirms an Ordovician age for one of the tonalite bodies and demonstrates that they were affected by Ordovician metamorphism whereas biotite Rb-Sr mineral ages from two of the tonalite bodies show that the Slishwood Division had cooled below c. 350[degrees]C at 449 [+ or -] 7 Ma. The early orogenic tonalite and granite instrusions are mylonitized in structures that overthrust the Dalradian Supergroup on the Slishwood Division during major nappe formation and, thus, date the onset of such deformation at or before 471 [+ or -] 5 Ma, the age of the youngest instrusion where such field relations can be demonstrated. Keywords: Dalradian, Caledonides, absolute age, zircon, granite.

Integrated textural and chemical characterisation of zircon is used to refine the U-Pb geochronology of the Archaean, ultra-high temperature Napier Complex, east Antarctica. Scanning electron microscope characterisation of zircon and the rare earth element compositions of zircon, garnet and orthopyroxene are integrated to place zircon growth in an assemblage context, thereby providing tighter constraints on the timing of magmatic and metamorphic events. Data indicate that magmatism occurred in the central and northern Napier Complex at ca. 2,990 Ma. A regional, relatively low-pressure metamorphic event occurred at ca. 2,850-2,840 Ma. Mineral REE data from garnet-bearing orthogneiss indicate that ca. 2,490-2,485 Ma U-Pb zircon ages provide an absolute minimum age for the ultrahigh temperature (UHT) foliation preserved in this rock. Internal zircon zoning relationships and estimated zircon-garnet DREE values from paragneiss suggest that an absolute minimum age of ultra-high temperature metamorphism is ca. 2,510 Ma, but that it is more likely to be older than ca. 2,545 Ma. We suggest that the high proportion of published zircon U-Pb data with ages between ca. 2,490-2,450 Ma reflects late, post-peak zircon growth and does not date the timing of peak UHT metamorphism.

We propose that the late Paleoproterozoic igneous and deformational history preserved in the northern midcontinent United States can be explained by a change in subduction-polarity from geon 18 south-dipping subduction during Penokean accretion to geon 17 north-dipping subduction as convergence continued after Penokean orogenesis. New U-Pb zircon ages indicate that late to post-Penokean magmatism occurred at ca. 1800, 1775, and 1750 Ma and generally migrated southeastward across the newly accreted Penokean terrane. We suggest that geon 17 Yavapai slab rollback caused continental arc magmatism to step southeastward between 1800 and 1750 Ma. As the slab steepened, reduced compressional stresses and magma-induced thermal weakening allowed for collapse of the overthickened portions of the Penokean crust. Postcollapse crustal stabilization (the 1750-1650 Ma Baraboo interval) was followed by geon 16 Mazatzal arc accretion further south. The 1900-1600 Ma tectonic history of the north-central United States, not surprisingly, records events related to the southward growth and tectonic development of the southern Laurentian margin. New and published [sup.40]Ar/[sup.39]Ar mineral ages delineate the northern and western extent of geon 16 Mazatzal deformation. Interestingly, only little exhumed crust intruded by a small volume of shallow-level ca. 1750 Ma plutons (and associated rhyolites) was deformed significantly during geon 16. In contrast, more deeply exhumed crust and crust pervasively invaded by a large volume of post-Penokean magma (i.e., East-Central Minnesota Batholith) were largely unaffected by Mazatzal deformation and reheating. We suggest that posttectonic intrusions and crustal thinning were an important step in strengthening and stabilizing the crust in the southern Lake Superior region. Keywords: Penokean orogen, U-Pb zircon ages, geon 17 magmatism, Yavapai slab rollback, crustal stabilization.

SEM back-scattered electron images of zircon separates from a 2.9 Ga granitic dyke from the Acasta Gneiss Complex, Slave Province, reveal primary igneous oscillatory zoning which is transgressively overprinted by irregular alteration domains. Electron microprobe analyses show that altered zircon domains are depleted in Zr and Si relative to unaltered zircon while retaining a constant Zr/Si ratio. SIMS trace element analyses indicate that LREE are preferentially enriched in the altered domains. Altered zircon contains elevated concentrations of Ba (up to 580 ppm), Ca (up to 2% weight of the element) and common Pb (> 50 ppb). The presence of LREE, Ca, Ba and common Pb in the zircons is interpreted to be the result of secondary alteration by an aqueous fluid, and not by primary incorporation during crystallization. We propose that the alteration-related elements are located in amorphous zircon domains.