The origin of eclogite-bearing granitoid gneisses and metapelites of the Chuacús Complex is investigated. This complex represents the internal basement massif of the Guatemala Suture Zone, a part of the western North America–Caribbean plate boundary. LA-ICP-MS U-Pb and trace element zircon data are combined with whole-rock Sm-Nd and Lu-Hf isotopes to re-evaluate granitoid petrogenesis and inquire into the sedimentary record. New granitoid ages of ca. 1030–1010 Ma are reported, adding to those already known of ca. 1100, 990 and 225 Ma. Stenian A-type granitoids within the bimodal Cubulco unit formed by mixing of magmas derived from late Palaeoproterozoic crust and mantle-derived melts produced in an extensional setting during Rodinia assembly. During the Tonian, an extended (or later) period of extensional tectonics produced peraluminous granitoids (Pachajob gneiss) by anatexis of rejuvenated late Mesoproterozoic crust. After a hiatus encompassing most of the Neoproterozoic, marine sedimentation occurred between the Ediacaran and the early Palaeozoic as recorded by the Palibatz schist, a sequence formed by detritus sourced from peri-Gondwanan continental areas. No evidence of middle to late Palaeozoic magmatism or sedimentation was found in the studied area. Late Triassic granitoids (Agua Caliente unit) were produced by mixing melts from late Mesoproterozoic crust with enriched mantle magmas in response to post-collisional thinning during the western Pangea breakup. This extensional stage led to considerable thinning of the Chuacús crust and its evolution into a passive margin that would be prone to subduct during the Cretaceous.

After their crystallization from magmatic melt, minerals such as accessory zircon are often subject to different kinds of chemical impairment. This includes the radiation-induced damage of the crystal lattice due to the incorporation of radioactive elements (U, Th) and the dissolution of the mineral structure due to the permanent action of aggressive fluid phases. Both phenomena can be best studied by using specific crystal sections and electron microscopy visualization techniques (backscattered electron imaging, BSEI). Crystal damage by the emission of α-particles corresponds to a continuous transformation of the regularly structured lattice into an amorphous mass. Mineral dissolution is expressed by the formation of specific corrosion pits, which may be refilled by recrystallized material.

U–Pb geochronology, Hf isotopes and trace-element chemistry of zircon grains from migmatite of the upper Sutlej valley (Leo Pargil), Northwest Himalaya, reveal a protracted geological evolution and constrain anatexis and tectonothermal processes in response to Himalayan orogenesis. U–Pb geochronology and ϵHf record separate clusters of ages on the concordia plots in the migmatite (1050–950 Ma, 850–790 Ma and 650–500 Ma). The 1050–950 Ma zircon population supports a provenance from magmatic units related to the assembly of Rodinia. A minor amount of Palaeoproterozoic grains were likely derived from the Indian craton. The potential source rock of the 930–800 Ma detrital zircons may be granitoid present in Greater Himalayan rocks themselves and the Aravalli Range, which has 870–800 Ma granitic rocks. The arc-type basement within the Himalayan–Tibet orogen recorded (900–600 Ma) igneous activity, which may depict a northeasterly extension of juvenile terranes in the Arabian–Nubian Shield. The granitoid of 800 Ma may be a potential source for 790 Ma detrital zircons owing to scatter in 206/238 dates. The 650–500 Ma zircon population suggests their derivation from the East African Orogen and Ross–Delamerian Orogen of Gondwana. The Cambrian–Ordovician magmatism during the Bhimphedian Orogeny and observed late Neoproterozoic to Ordovician detrital zircons have been derived to some extent from Greater Himalayan magmatic sources. We found no detrital zircon grains that cannot be explained as coming from local sources. One sample yielded a discordia lower intercept age of 15.6 ± 2.2 Ma, the age of melt crystallization.

Detrital zircon in six surface samples of sandstone and contact metamorphic quartzite of the Magaliesberg and Rayton formations of the Pretoria Group (depositional age c. 2.20–2.06 Ga) show a major age fraction at 2.35–2.20 Ga, and minor early Palaeoproterozoic – Neoarchaean fractions. Trace-element concentrations vary widely, with Ti, Y and light rare earth elements (LREEs) spanning over three orders of magnitude. REE distribution patterns range from typical zircon patterns (LREE depletion, heavy REE enrichment, well-developed positive Ce and negative Eu anomalies) to patterns that are flat to concave downwards, with indistinct Ce and Eu anomalies. The change in REE pattern correlates with increases in alteration-sensitive parameters such as Ti concentration and (Dy/Sm) + (Dy/Nd), U–Pb discordance and content of common lead, and with a gradual washing-out of oscillatory zoning in cathodoluminescence images. U and Th concentrations also increase, but Th/U behaves erratically. Discordant zircon scatters along lead-loss lines to zero-age lower intercepts, suggesting that the isotopic and chemical variations are the results of disturbance long after deposition. The rocks sampled have been in a surface-near position (at least) since Late Cretaceous time, and exposed to deep weathering under intermittently hot and humid conditions. In this environment, even elements commonly considered as relatively insoluble could be mobilized locally, and taken up by radiation-damaged zircon. Such secondary alteration effects on U–Pb and trace elements can be expected in zircon in any ancient sedimentary rock that has been exposed to tropical–subtropical weathering, which needs to be considered when interpreting detrital zircon data.

Ancient metamorphic processes are recorded by the formation of metallic-Pb nanospheres in zircon, a product of internal Pb mobilisation and thermally driven concentration. Here, metallic-Pb nanospheres formed within an ore deposit are characterised for the first time using high-angle annular dark field scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy element-distribution mapping. Exceptional examples from the migmatite-hosted Archean–Paleoproterozoic Challenger Au deposit (Central Gawler Craton, South Australia) support widespread metallic-Pb nanosphere formation in zircon from rocks experiencing granulite-facies metamorphism. We also report new trace-element associations found with metallic-Pb nanospheres and a new mode of occurrence, in which Sc ‘haloes’ form adjacent to metallic-Pb nanospheres within the crystalline zircon lattice. This differs to previously characterised examples of metallic-Pb nanospheres associated with amorphous Si-rich glasses and unidentified Al–Ti, or Fe-bearing phases. Multiple modes of metallic-Pb nanosphere occurrences and trace-element associations suggests multiple modes of formation, probably dependant on zircon composition and metamorphic conditions. Identification of metallic-Pb nanospheres in a growing range of geological settings further highlights the mobility of Pb in zircon and the importance of detailed, nanoscale mineral characterisation, in order to constrain accurate geochronological histories for rocks within high-temperature geological environments.

The Himalaya is characterized by the presence of both pre-Himalayan Palaeozoic and syn-Himalayan Cenozoic granitic bodies, which can help unravel the pre- to syn-collisional geodynamics of this orogen. In the Bhagirathi Valley of Western Himalaya, such granites and the Tethyan Himalayan Sequence (THS) hosting them are bound to the south by the top-to-the-N extensional Jhala Normal Fault (JNF) and low-grade metapelite of the THS to its north. The THS is intruded by a set of leucocratic dykes concordant to the JNF. Zircon U–Pb laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) geochronology of the THS and one leucocratic dyke reveals that the two rocks have a strikingly similar age distribution, with a common and most prominent age peak at ~1000 Ma. To the north of the THS lies Bhaironghati Granite, a Palaeozoic two-mica granite, which shows a crystallization age of 512.28 ± 1.58 Ma. Our geochemical analysis indicates that it is a product of pre-Himalayan Palaeozoic magmatism owing to extensional tectonics in a back-arc or rift setting following the assembly of Gondwana (500–530 Ma). The Cenozoic Gangotri Leucogranite lies to the north of Bhaironghati Granite, and U–Pb dating of zircon from this leucogranite gives a crystallization age of 21.73 ± 0.11 Ma. Our geochemical studies suggest that the Gangotri Leucogranite is a product of muscovite-dehydration melting of the lower crust owing to flexural bending in relation to steepening of the subducted Indian plate. The leucocratic dykes are highly refracted parts of the Gangotri Leucogranite that migrated and emplaced along extensional fault zones related to the JNF and scavenged zircon from the host THS during crystallization.

Zircon U–Pb and Lu–Hf isotope microanalysis was conducted in (meta)-igneous units of Ammouliani island to characterize crust-forming and reworking events in the Serbo-Macedonian massif. Zircon grains from an orthogneiss of the Vertiskos unit yielded a U–Pb crystallization age for the igneous precursor at 458.8 ± 11 Ma with dominantly subchondritic ϵHf values indicating reworking of Neoproterozoic basement. A weighted mean ϵHf value of 0.7 ± 2.4 from oscillatory zoned zircon grains of the Ouranoupoli granodiorite indicates juvenile crustal input at 52.1 ± 0.6 Ma. The U–Pb–Hf zircon archive records discrete stages in the crustal evolution of the Serbo-Macedonian massif.

Zircon separates from the contact aureole of the syn-tectonic Dawros–Currywongaun–Doughruagh Complex, western Ireland, are studied to constrain the nature and timing of magmatism associated with the early stages of the Grampian Orogeny. The samples analysed come from the uppermost part of the Dalradian Supergroup in northern Connemara (the Ben Levy Grit Formation), where a laterally extensive (>10 km) package of metamorphosed siliciclastic sedimentary rocks containing heavy mineral seams crops out. The seams mainly comprise magnetite, but zircon is also present in greater than accessory quantities. The seams have been locally reworked at granulite-facies metamorphic conditions during intrusion of the Dawros–Currywongaun–Doughruagh Complex magmas. Here we combine in situ mineral chemical and U–Pb geochronological analyses of zircons from samples of these heavy mineral seams collected at different locations in the Dawros–Currywongaun–Doughruagh Complex thermal aureole. An important finding is that the zircons studied have magmatic trace-element compositions, interpreted here as a function of their growth during contact metamorphic-induced partial melting. The zircons yield a range of U–Pb spot ages whose uncertainties suggest a maximum duration of zircon growth of ˜11 Ma, between 477.1 and 466.1 Ma, though it is likely that zircon growth occurred much more quickly than this. The age constraints revealed here match well with the range of 475 to 463 Ma previously proposed for the Grampian Orogeny overall in Connemara and lend useful support to models that argue for high-intensity, relatively short-lived Grampian orogenesis in the Connemara Caledonides.

The Sargur Group has been considered to be the oldest group (>3.0 Ga) in the Archaean sequence of the Dharwar Craton in south India, whereas the rocks of the Dharwar Supergroup are younger (between 3.0 and 2.55 Ga). The supracrustal units of the Sargur Group were deposited during the Archaean period. The Banavara quartzite forms part of the supracrustal Sargur Group and contains significant amounts of chromian spinel (Cr-spinel). Here, U–Pb and Hf isotopes of detrital zircons are integrated with compositional data and X-ray refinement parameters for Cr-spinels to decipher the provenance of the metasediments. Zircons show an age spectrum from 3.15 to 2.50 Ga, and juvenile Hf isotopic compositions (ϵHf = +0.8 to +6.4) with model ages between 3.3 and 3.0 Ga. Major- and trace-element contents of the Cr-spinels do not resemble those in the Sargur ultramafic rocks, but resemble well-characterized Archaean anorthosite-hosted chromites. Cr-spinel trace-element signatures indicate that they have undergone secondary alteration or metamorphism. X-ray refinement parameters for the Cr-spinels also resemble the anorthosite-hosted chromites. We conclude that the detrital minerals were probably derived from gneissic and anorthositic rocks of the Western Dharwar Craton, and that the Sargur Group sequences have experienced a younger (2.5 Ga) metamorphic overprint.

Very few zircon-bearing, kimberlite-hosted mantle eclogite xenoliths have been identified to date; however, the zircon they contain is crucial for our understanding of subcratonic lithospheric mantle evolution and eclogite genesis. In this study, we constrain the characteristics of zircon from mantle eclogite xenoliths based on existing mineralogical and geochemical data from zircons from different geological settings, and on the inferred origin of mantle eclogites. Given the likely origin and subsequent evolution of mantle eclogites, we infer that the xenoliths can contain zircons with magmatic, metamorphic and xenogenic (i.e. kimberlitic zircon) origins. Magmatic zircon can be inherited from low-pressure mafic oceanic crust precursors, or might form during direct crystallization of eclogites from primary mantle-derived melts at mantle pressures. Metamorphic zircon within mantle eclogites has a number of possible origins, ranging from low-pressure hydrothermal alteration of oceanic crustal protoliths to metasomatism related to kimberlite magmatism. This study outlines a possible approach for the identification of inherited magmatic zircon within subduction-related mantle eclogites as well as xenogenic kimberlitic zircon within all types of mantle eclogites. We demonstrate this approach using zircon grains from kimberlite-hosted eclogite xenoliths from the Kasai Craton, which reveals that most, if not all, of these zircons were most likely incorporated as a result of laboratory-based contamination.

Zircon crystals from diamondiferous kyanite gneisses of the Barchi-Kol area (Kokchetav massif, Northern Kazakhstan) have been investigated by a combined application of cathodoluminescence (CL), Raman spectroscopy and electron probe microanalysis (EPMA). The zircon crystals exhibit up to four distinct domains characterised by significantly different CL signatures and parameters of the ν3(SiO4) (1008 cm–1) Raman band (i.e. full width at half maximum, position and intensity). Extremely metamict zircon cores (Domain I) host inclusions of low-pressure minerals (quartz and graphite) and the outer mantles (Domain III) are populated by ultrahigh-pressure relicts (diamond and coesite), whereas inner mantles (Domain II) and overgrowth rim zones (Domain IV) are inclusion free. Both the zircon cores and rims have very low Ti concentrations, implying formation temperatures below 760°C. The Ti content in the inner mantles (up to 40 ppm) is indicative of temperatures in the 760–880°C range. The temperature estimates for the outer mantles are 900–940°C, indicating a pronounced overlap with the peak metamorphic values yielded by the Zr-in-rutile geothermometer for the same rocks (910–950°C). The internal textures of the zircons and the occurrence of index minerals within the distinct domains allow us to unravel the stages of the complex metamorphic history recorded in the zircon. Our data show that the zircon cores are inherited seeds of pre-metamorphic (magmatic?) origin, the inner mantles were formed on the prograde metamorphic stage, the outer mantles record ultrahigh-pressure metamorphism and the outermost rims mark the retrograde metamorphic stage. The observed zircon internal textures are thus clearly correlated with distinct growth events, and in some examples reflect a major part of the metamorphic history. It is concluded that the combined application of the CL, Raman spectroscopy and EPMA techniques to zircon offers significant potential for deciphering the metamorphic evolution of deeply-subducted rocks.

The Shyok Suture in western Himalaya preserves a record of the opening and closure of the Mesotethys Ocean between the Shyok ophiolite and Karakoram terrane prior to the India–Eurasia collision. The formation age of the Shyok ophiolite was unknown, which impeded correlation with similar rocks along the Shyok Suture in Pakistan and corresponding sutures in Tibet. We report the first zircon U–Pb ages of a newly documented suite, here named the Changmar Complex. The Changmar Complex gabbronorite and plagiogranite yielded SHRIMP U–Pb zircon Late Jurassic ages of 159.4 ± 0.9 Ma and 151.9 ± 1.5 Ma. Their highly positive initial εHf values (+14.9 to +16.9) indicate a juvenile mantle origin, without continental crust influence on the magma source. The Shyok ophiolite represents either: (1) a separate island arc that preceded formation of the Cretaceous–Eocene Ladakh Arc; or (2) the oldest magmatism and early stage of the Ladakh Arc. Intrusive and extrusive mafic rocks from the Shyok Suture analysed in this study have typical supra-subduction zone enrichment characteristics in their geochemistry and are classified as part of the volcanic-arc ophiolite. The U–Pb age and Hf isotopic signatures for the Shyok ophiolite are similar to the Late Jurassic Matum Das tonalite within the Kohistan Arc; we therefore suggest that they are part of the same intra-oceanic island-arc system that formed in the Mesotethys Ocean prior to Late Jurassic time.

Permian–Triassic boundary (PTB) volcanic ash beds are widely distributed in South China and were proposed to have a connection with the PTB mass extinction and the assemblage of Pangea. However, their source and tectonic affinity have been highly debated. We present zircon U–Pb ages, trace-element and Hf isotopic data on three new-found PTB volcanic ash beds in the western Hubei area, South China. Laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircons yields ages of 252.2 ± 3.6 Ma, 251.6 ± 4.9 Ma and 250.4 ± 2.4 Ma for these three volcanic ash beds. Zircons of age c. 240–270 Ma zircons have negative εHf(t) values (–18.17 to –3.91) and Mesoproterozoic–Palaeoproterozoic two-stage Hf model ages (THf2) (1.33–2.23 Ga). Integrated with other PTB ash beds in South China, zircon trace-element signatures and Hf isotopes indicate that they were likely sourced from intermediate to felsic volcanic centres along the Simao–Indochina convergent continental margin. The Qinling convergent continental margin might be another possible source but needs further investigation. Our data support the model that strong convergent margin volcanism took place around South China during late Permian – Early Triassic time, especially in the Simao–Indochina active continental margin and possibly the Qinling active continental margin. These volcanisms overlap temporally with the PTB biocrisis triggered by the Siberian Large Igneous Province. In addition, our data argue that the South China Craton and the Simao–Indochina block had not been amalgamated with the main body of Pangea by late Permian – Early Triassic time.

We present a new dataset of zircon U–Pb ages that document igneous activity in the SW Japan arc during middle Miocene time and discuss its relationship with the opening of the Japan Sea, Philippine Sea plate migration, and subduction of the young hot lithosphere of the Shikoku Basin. Precursory magmatism, characterized by dike and stock intrusions, started c. 15.6 Ma in both Kyushu and the Kii Peninsula. Most plutonism occurred between 15.5 and 13.5 Ma in an area 600 km long and 150 km wide. No along-arc trend was recognized in the U–Pb ages of igneous activity near the trench. Our data indicate that all near-trench middle Miocene igneous activity occurred immediately after the opening of the Japan Sea ceased, i.e. after 16 Ma, implying that melt extraction and the emplacement of granites in the near-trench region had some influence on the back-arc opening. Our data also imply that the trench–trench–trench-type triple junction between the Japan arc and the Izu–Bonin–Mariana arc must have reached the east side of the Kii Peninsula by 15.6 Ma. The wide distribution of contemporaneous magmatic activity along the arc requires a trench-parallel heat source, such as the subduction of a trench-parallel ridge or a young and highly segmented ridge–fracture zone system in addition to the hot wedge mantle condition related to the opening of Japan Sea.

Single crystals of synthetic reidite and natural radiation-damaged zircon from Okueyama, Japan were investigated using X-ray diffraction. The pressure-induced zircon–reidite transition is described by the twisting and translations of SiO4 tetrahedra with disappearance of the SiO4–ZrO8 shared edges. The lattice of radiation-damaged zircons expands mainly from α-decays of radioactive elements such as U and Th. Although old radiation-damaged zircons show anomolous lattice distortion, young radiation-damaged zircons do not show such distortions. These distortions are caused by thermal recovery that suppresses the Si4+–Zr4+ repulsion between the SiO4 tetrahedron and ZrO8 dodecahedron. These changes in structure can be understood by considering the cation–cation repulsion between the SiO4–ZrO8 shared edges.

The Stac Fada Member of the Stoer Group, within the Torridonian succession of NW Scotland, is a melt-rich, impact-related deposit that has not been conclusively correlated with any known impact structure. However, a gravity low approximately 50 km east of the preserved Stac Fada Member outcrops has recently been proposed as the associated impact site. We investigate the location of the impact structure through a provenance study of detrital zircon and apatite in five samples from the Stoer Group. Our zircon U–Pb data are dominated by Archaean grains (> 2.5 Ga), consistent with earlier interpretations that the detritus was largely derived from local Lewisian Gneiss Complex, whereas the apatite data (the first for the Stoer Group) display a single major peak at c. 1.7 Ga, consistent with regional Laxfordian metamorphism. The almost complete absence of Archaean-aged apatite is best explained by later heating of the > 2.5 Ga Lewisian basement (the likely source region) above the closure temperature of the apatite U–Pb system (c. 375–450°C). The U–Pb age distributions for zircon and apatite show no significant variation with stratigraphic height. This may be interpreted as evidence that there was no major change in provenance during the course of deposition of the Stoer Group or, if there was any significant change, the different source regions were characterized by similar apatite and zircon U–Pb age populations. Consequently, the new data do not provide independent constraints on the location of the structure associated with the Stac Fada Member impact event.

The Caledonides of Britain and Ireland include terranes attributed to both Laurentian and Gondwanan sources, separated along the Solway line. Gondwanan elements to the south have been variably assigned to the domains Ganderia and East Avalonia. The Midland Platform forms the core of East Avalonia but its provenance is poorly known. Laser ablation split-stream analysis yields information about detrital zircon provenance by providing simultaneous U–Pb and Lu–Hf data from the same ablated volume. A sample of Red Callavia Sandstone from uppermost Cambrian Stage 3 of the Midland Platform yields a U–Pb age spectrum dominated by Neoproterozoic and Palaeoproterozoic sources, resembling those in the Welsh Basin, the Meguma Terrane of Nova Scotia and NW Africa. Initial εHf values suggest that the Neoproterozoic zircon component was derived mainly from crustal sources < 2 Ga, and imply that the more evolved Palaeoproterozoic grains were transported into the basin from an older source terrane, probably the Eburnean Orogen of West Africa. A sample from Cambrian Stage 4 in the Bray Group of the Leinster–Lakesman Terrane shows, in contrast, a distribution of both U–Pb ages and εHf values closely similar to those of the Gander Terrane in Newfoundland and other terranes attributed to Ganderia, interpreted to be derived from the margin of Amazonia. East Avalonia is clearly distinct from Ganderia, but shows evidence for older crustal components not present in West Avalonia of Newfoundland. These three components of the Appalachian–Caledonide Orogen came from distinct sources on the margin of Cambrian Gondwana.

A detailed electron microprobe (EMP) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) study of zircon from six types of miaskitic and agpaitic alkaline pegmatite from the Larvik Plutonic Complex, Oslo rift valley, Norway, was undertaken to shed light on the pegmatite petrogenesis. Detailed rare earth element (REE) analyses indicate important differences between the zircon from each type of pegmatite. Primary zircon from miaskitic Stavern-, Tvedalen- and Stålaker-type pegmatites has a mean ΣREE = 704 ppm, is depleted in LREE and has a significant positive Ce anomaly (Ce/Ce* = 44–67) and negative Eu anomaly (Eu/Eu* = 0.15–0.18). Secondary Tvedalen-type zircon is REE-enriched (ΣREE = 5035 ppm), with a flatter REE pattern, Ce/Ce* = 0.97 and a Eu anomaly similar to primary Tvedalen-type zircon (Eu/Eu* = 0.21). Secondary zircon from agpaitic Langesundsfjord-type pegmatites display a distinctive flat REE pattern characterized by overall REE enrichment (ΣREE = 967), Ce/Ce* = 1.92, and a minor negative Eu anomaly (Eu/Eu* = 0.37). Zircon from agpaitic Bratthagen-type pegmatites occurs as both altered primary and secondary phases and is strongly enriched in REE relative to other zircon (ΣREE = 4178 and 8388, respectively). Primary Bratthagen-type zircon has a similar REE pattern to miaskitic zircon, with a steeper HREE profile and smaller Ce and Eu anomalies (Eu/Eu* = 0.73; Ce/Ce* = 6.22). Secondary Bratthagen-type zircon is strongly enriched in LREE compared to primary zircon, does not display a positive Ce anomaly and has Eu/Eu* = 0.56. The altered primary and secondary Bratthagen-type zircons have elevated Th/UN ratios, suggesting a different melt source for Bratthagen-type agpaitic pegmatites. Zircon from external pegmatites has trace-element signatures similar to Stavern-, Tvedalen- and Staålaker-type primary zircon with Ce/Ce* = 214 and Nb/Ta and Th/U ratios that are similar to those of secondary Langesundsfjord- and Bratthagen-type zircon. It is suggested that the parental melt of the external pegmatites is the same as the miaskitic pegmatites, but that it has undergone alteration by hydrothermal fluids derived from the host basalt, or by post-magmatic F-rich fluids which mobilize Nb and Th. On the basis of texture, morphology and geochemistry, two populations of zircon can be recognized: (1) primary zircon from miaskitic pegmatites; and (2) secondary zircon from post-magmatic, hydrothermal assemblages. The U–Th–Pb isotope analyses indicate that the secondary and altered zircon are depleted in 238U, and enriched in LREE. Interaction of a post-magmatic hydrothermal fluid with an externally derived meteoric fluid is suggested to have influenced the REE signatures, and in particular the Eu and Ce anomalies of the late-stage zircons.

The sizes, distributions and shapes of zircon grains within variably deformed granite gneiss from the western Alps have been studied. Zircon shows numerous indicators of a metamorphic response in both the host gneiss and a 5 cm wide continuous ductile shear zone, within which the zircon grain sizes range from <1 urn to >50 μm. However, the very fine grain sizes are virtually absent from grain boundaries. Within this zone, zircons consistently have more rounded and embayed margins, which are interpreted as evidence of dissolution in response to fluid influx during shearing. Zircons are preferentially located near metamorphic muscovite in both the host gneiss and the shear zone and tend to show the poorest crystal shape, indicating that fluids linked to the formation and presence of muscovite may enhance both the crystallization of zircon and its subsequent dissolution. Larger zircon crystals typically show a brittle response to deformation when adjacent to phyllosilicates, with fractures consistently perpendicular to the (001) mica cleavage. The variety of metamorphic behaviour observed for zircon indicates that it may be highly reactive in sub-solidus mid-crustal metamorphic environments.

The macroscopic behaviour of minerals is not always directly related to their crystalline structure at the atomic scale but often depends explicitly on mesoscopic (nanometer–micrometer) features. This paper reviews various cases where the macroscopic phenomena differ from those of the bulk, with structural and chemical variations related to: domain walls, leading to enhanced or reduced transport properties; surfaces controlling growth morphologies; and radiation-damaged minerals where the interface between the amorphous and crystalline phase is believed to play a key role in hydrothermal leaching behaviour. Minerals explicitly discussed are: quartz, agate, hydroxylapatite, cordierite and metamict zircon.