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The fluid-fluxed melting of igneous protoliths provides an opportunity to assess the role of fluid for the formation and modification of continental crust during high-temperature metamorphism. In order to investigate such an anatexis in the Buncheon granitic gneiss, Yeongnam Massif, we combined field, microstructural, and geochemical studies with the U-Pb and oxygen isotopic analyses of zircon and monazite. The granitic gneisses comprise metatexitic migmatites containing leucosomes, although neither melanosomes nor peritectic phases are associated with neosomes. These migmatites underwent syn -kinematic wet melting in the presence of ca. 0.9–1.4 wt% H 2 O (estimated from the pseudosection modeling) via the reaction, plagioclase + K-feldspar + quartz + H 2 O = melt, that took place at ∼650–750 °C and 4–6 kbar. The leucosomes differ in the degree of Eu anomalies and have variable amounts of rare earth elements, Zr, Y, and Th. Such variations are attributed to the segregation and migration of melts as well as the entrainment of residual phases. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of zircon and monazite were determined from two granitic gneisses and three leucosomes together with an adjacent, undeformed Hongjesa granite. Zircon grains are oscillatory-zoned, except for homogeneous overgrowth rims rarely present in two leucosomes. Both zircon domains were dated at ca. 1.98–1.97 Ga and ca. 1.86 Ga, respectively, suggesting that igneous protoliths were partly recrystallized during the late Orosirian. Monazites from two gneiss samples contain submicroscopic polycrystalline inclusions of K-feldspar + quartz + apatite, reflecting the presence of former melts, and yielded a pooled 207Pb/206Pb age of 1859 ± 6 Ma (n = 30). In contrast, monazites from a leucosome are inclusion-free, and yielded the youngest age of 1840 ± 16 Ma (n = 6). This result is interpreted to represent the melt crystallization at ca. 1.84 Ga following the ca. 1.86 Ga anatexis. On the other hand, monazite from the Hongjesa granite, yielded a weighted mean 207Pb/206Pb age of 1878 ± 5 Ma (n = 11), most likely reflecting a prograde metamorphic growth. The δ18O values of ca. 1.98–1.97 Ga igneous zircon in granitic gneisses are 7.1–7.2 ‰, but those from the leucosome are reduced at 5.9–6.7 ‰. The latter is largely identical to 5.7–6.7 ‰ estimated from ca. 1.86 Ga domains of zircon in the leucosome and monazite in the granitic gneiss; in contrast, a slightly lower δ18O value of 5.46 ± 0.61 ‰ is recorded in the ca. 1.84 Ga monazite of leucosome. The relatively low δ18O values are most likely accounted for by an influx of fluid that has apparently triggered partial melting in the granitic gneiss, and the fluid itself is possibly linked to the crystallization of mafic magmas in an arc complex. Taken together, in contrast to the majority of zircons preserving the crystallization age of igneous protoliths, monazites are more susceptible to recrystallization during the fluid-present melting and permit us to decode a protracted (∼20 m.yr.) episode from partial melting to melt crystallization in the Yeongnam Massif. Such a longevity of melt-present system typifies the prolonged crustal reworking of Paleoproterozoic Korean arc situated at the eastern margin of the North China Craton. • Partial melting of granitic gneiss occurred due to an influx of external fluids. • The melting occurred at 1.86–1.84 Ga following protolith emplacement at 1.98 Ga. • The interaction with low δ18O fluids in association with the crystallization of mafic magmas. • The protracted melting typifies the crustal reworking of the Paleoproterozoic Korean arc. [ABSTRACT FROM AUTHOR]

The Southernmost Brasília Orogen surrounds the southern margins of the São Francisco Craton. It exposes a structural window with a WNW-ESE direction formed by the Archean-Paleoproterozoic basement reworked by the Campo do Meio Shear Belt. Using metamorphic modeling, geothermobarometry, and U Pb dating of zircon and monazite, this study established the pressure-temperature-time (P-T -t) evolution of the Colônia metapelite, one of the main units of the Campo do Meio Shear Belt. This revealed a clockwise path with metamorphic peak conditions at 706 ± 16 °C and 9.7 ± 0.7 kbar on a staurolite-kyanite-muscovite-garnet-biotite schist, with cooling at ∼600 °C and ∼ 6 kbar. Forward metamorphic modeling based on the whole-rock composition of this sample supports partial melting in the staurolite stability field, facilitated by water influx. Water-fluxed anatexis promoted dissolution of detrital zircon grains. These processes, occurring during prograde metamorphism, are influenced by temperature, crystal size-distribution, bulk-rock composition, and the permanence of the rock at the thermal peak. Analysis of the zircon overgrowth zones, which were linked to peak metamorphic conditions, produced an age of 605 ± 7 Ma. The monazite age of 580 ± 5 Ma was associated with later syntectonic melt crystallization. The clockwise P-T -t path of the Colônia metapelite is similar to the evolution of Neoproterozoic units from the Southernmost Brasília Orogen. However, the degree of dissolution in detrital zircon grains restricts accurate evaluation of the possible stratigraphic correlations proposed in the literature. • Staurolite stability can exceed the solidus curve in the fluid-saturated melt. • Kyanite stabilization can occur after melting into low-Al metapelites. • Water-fluxed melting promotes dissolution-recrystallization in detrital zircon. • Metamorphic ages overlap the ages of likely source areas. • Sedimentary geochronological approach for melting-bearing rocks may be unfeasible. [ABSTRACT FROM AUTHOR]

Abstract The Early Paleozoic orogeny in the South China Block was accompanied by crustal anatexis, as indicated by the voluminous granites and migmatites. Here we report on the migmatite-granite suite within the Neoproterozoic biotite gneiss of the Baiyunshan Domain, which has directly recorded the process of anatectic melt formation, segregation and extraction. The crystallization of leucosome in migmatite was dated at ca. 438 Ma by U–Pb analysis of zircon, and the granitic vein that intruded the migmatite was dated at ca. 433 Ma. Petrological and geochemical data suggest that anatexis resulted from fluid-present melting of the biotite gneiss at low temperature and pressure conditions (~650–700 °C and ~4–6 kbar). The leucosome mainly consists of plagioclase + K-feldspar + quartz and has positive Eu anomaly (Eu/Eu* = 1.70–2.89), which is attributed to feldspar accumulation. This means that some melt has been extracted from the migmatite, and the granitic veins are likely to represent these extracted melts since they have similar trace element compositions and zircon Lu Hf isotopes to the leucosomes. The degree of partial melting and chemical differentiation to generate the migmatite and the granitic veins are revealed by mass balance calculations. Calculations suggest that the primary melt could be a result of ~40 wt% melting of the Baiyunshan biotite gneiss, and fractionated melt (~24–26 wt%) was extracted from the deeper sections to form the intrusive veins. Combined with regional dating results, it is suggested that the crustal anatexis in the Wuyi-Yunkai orogen started no later than 465 Ma with a duration of >55 Myr. Generation of the granitoids and migmatites are a result of water-fluxed melting and linked to extra heat from mantle magmas provided to the middle-lower crust. Highlights • A suite of migmatite-granitic vein is well-preserved in the Baiyunshan Domain. • Dating constrains migmatization and granite intrusion to ~438 Ma and ~433 Ma, respectively. • Leucosomes are feldspar accumulation, granitic veins are extracted interstitial melt. • Crustal anatexis lasted over 55 Myr requiring intense heat flux from the mantle. [ABSTRACT FROM AUTHOR]

Abstract Evidences of Mesoarchean crustal reworking are not very common in geological records, being only found from Precambrian terrains like Kaapvaal and Pilbara Cratons where it is preserved as anorogenic/post-collisional granitic activity of 3–3.2 Ga age. The present study focuses on a granitoid unit of similar age, found near the eastern margin of the Precambrian Singhbhum Craton of eastern India. This ellipsoidal, N-S trending, well-foliated granite-gneiss body is found within the polydeformed, metasedimenatry rocks of Singhbhum Group, belonging to the North Singhbhum Mobile Belt. LA-ICP MS U-Pb analysis of zircon grains from one sample give crystallization ages ranging from 3079.4 ± 6.8 Ma to 3115 ± 10 Ma. These rocks hereby dubbed as "Bangriposi Granite Gneiss", are composed of quartz, alkali feldspar, ferroan biotite (Fe/Fe + Mg: 0.7–0.9), titanite, illmenite, hastingsite, apatite, and numerous U-Th-REE- bearing accessory phases. They have high SiO 2 (67–77 wt%) and Na 2 O + K 2 O (8.19–9.01 wt%), low CaO (0.35–1.5 wt%), MgO (0.05–0.36 wt%) and shows enrichment of Nb, Rb, Zr, Y, Th, and REEs, and depletion of Cr, Ni, U, with high FeO t /FeO t + MgO (~0.9), Ga/Al (2.04–3.51), (La/Yb) N (6.5–13), and low Eu/Eu* (0.11–0.47). Geochemically and mineralogically they are categorized as metaluminous to weakly peraluminous (A/CNK: 0.9–1.1) ferro-potassic alkali feldspar granites. These rocks show unequivocal affinities towards 'A-type" granites and from geochemical evidences it is suggested that the parent magma was produced in response to crustal anatexis under low f O2 with P-T estimates of ~900 °C and 7–8 kbar. Geochemical modelling has revealed that the probable source was lower crustal amphibolites belonging to the Paleoarchean Older Metamorphic Group, which suffered low degrees (5–15%) of melting. Negative Ɛ Nd values (−0.5 to −1.5) are also in favour of reworking of older crust and their Nd isotopic signature bears similarities with other coeval anatectic granites. Bangriposi Granite-Gneiss, along with Mayurbhanj Granite and Bonai Granite, represent a major phase of Mesoarchean anorogenic/post-collisional granitic activity in Singhbhum Craton, indicating the onset of its stabilization. Similar Mesoarchean crustally reworked felsic units are encountered in Pilbara and Kaapvaal cratons suggesting a possibility of correlation with the hypothesized "Vaalbara" supercontinent. Highlights • Bangriposi Granite-Gneiss yields U-Pb zircon ages of 3079.4 ± 6.8 Ma- 3115 ± 10 Ma. • These ferro-potassic rocks are typical of anorogenic/post-collisional granites. • They formed by reworking of older crustal materials. • Paleoarchean amphibolites of Singhbhum Craton are the possible source rocks. • Similar Mesoarchean crustal reworking events found in Pilbara and Kaapvaal Craton. [ABSTRACT FROM AUTHOR]

We present the results of a combined CA-ID-TIMS and LA-MC-ICP-MS U–Pb geochronology study of zircon and associated rutile and titanite from the Nordøyane ultra-high-pressure (UHP) domain in the Western Gneiss Region (WGR) of Norway. The dated samples include 4 eclogite bodies, 2 host-rock migmatites, and 2 cross-cutting pegmatites and leucosomes, all from the island of Harøya. Zircon from a coesite eclogite yielded an age of ca. 413 Ma, interpreted as the time of UHP metamorphism in this sample. Zircon data from the other eclogite bodies yielded metamorphic ages of ca. 413 Ma, 407 Ma, and 406 Ma; zircon trace-element data associated with 413 Ma and 407 Ma ages are consistent with eclogite-facies crystallization. In all of the eclogites, U–Pb dates from zircon cores, interpreted as the times of protolith crystallization, range from ca. 1680–1586 Ma, consistent with Gothian ages from orthogneisses in Nordøyane and elsewhere in the WGR. A zircon core age of ca. 943 Ma from one sample agrees with Sveconorwegian ages of felsic gneisses and pegmatites in the western part of the area. Migmatites hosting the eclogite bodies yielded zircon core ages of ca. 1657–1591 Ma and rim ages of ca. 395–392 Ma, interpreted as the times of Gothian protolith formation and Scandian partial melt crystallization, respectively. Pegmatite in an eclogite boudin neck yielded a crystallization age of ca. 388 Ma, interpreted as the time of melt crystallization. Rutile and titanite from 3 samples (an eclogite and two migmatites) yielded concordant ID-TIMS ages of 378–376 Ma. The results are similar to existing U–Pb data from other Nordøyane eclogites (415–405 Ma). In combination with previous pressure-temperature data from the coesite eclogite, these ages indicate that peak metamorphic conditions of ~3 GPa/760 °C were reached ca. 413 Ma, followed by decompression to ~1 GPa/810 °C by ca. 397 Ma and cooling below ca. 600 °C by ca. 375 Ma. The results are compatible with protracted UHP metamorphism followed by relatively slow exhumation. The question of whether partial melting began at UHP conditions is not resolved by this study. [ABSTRACT FROM AUTHOR]

The Tonian Goiás Stratiform Complex (TGSC, Goiás, central Brazil), is one of the largest mafic-ultramafic layered complexes in the world, emplaced during the geotectonic events that led to the Gondwana accretion. In this study, we present trace elements and in-situ U/Pb-Lu-Hf analyses of zircons and 87 Sr/ 86 Sr ratios of plagioclases from anorthosites and gabbros of the TGSC. Although formed by three isolated bodies (Cana Brava, Niquelândia and Barro Alto), and characterized by a Lower and Upper Sequence (LS and US), our new U/Pb zircon data confirm recent geochemical, geochronological, and structural evidences that the TGSC has originated from a single intrusive body in the Neoproterozoic. New Hf and Sr isotope ratios construe a complex contamination history for the TGSC, with different geochemical signatures in the two sequences. The low Hf and high Sr isotope ratios of the Lower Sequence (εHf(t) from −4.2 down to −27.5; 87 Sr/ 86 Sr = 0.706605–0.729226), suggest the presence of a crustal component and are consistent with contamination from meta-pelitic and calc-silicate rocks found as xenoliths within the Sequence. The more radiogenic Hf isotope ratios and low Sr isotope composition of the Upper Sequence (εHf(t) from 11.3 down to −8.4; 87 Sr/ 86 Sr = 0.702368–0.702452), suggest a contamination from mantle-derived metabasalts in agreement with the occurrences of amphibolite xenoliths in the US stratigraphy. The differential contamination of the two sequences is explained by the intrusion of the TGSC in a stratified crust dominated by metasedimentary rocks in its deeper part and metavolcanics at shallower levels. Moreover, the differential thermal gradient in the two crystallizing sequences might have contributed to the preservation and recrystallization of inherited zircon grains in the US and total dissolution or magmatic overgrowth of the LS zircons via melt/rock reaction processes. [ABSTRACT FROM AUTHOR]

We present novel microstructural analyses of zircon from a variety of strained rocks. For the first time, multiple plastically deformed zircon crystals were analyzed in a kinematic context of the respective host shear zones. Our aim was to derive how the orientation of zircon grains in a shear zone affects their deformation, based on careful in situ observations. For sampling, we selected zircon-bearing rocks that were deformed by simple shear. Samples covered a range of P–T conditions and lithologies, including various meta-igneous and meta-sedimentary gneisses. Microstructural analyses of zircon crystals in situ with scanning electron backscatter diffraction mapping show strong geometrical relationships between orientations of: (i) the long axes of plastically deformed zircon crystals, (ii) the crystallographic orientation of misorientation axes in plastically deformed zircon crystals and (iii) the foliation and lineation directions of the respective samples. We assume that zircon crystals did not experience post-deformation rigid body rotation, and thus the true geometric link can be observed. The relationships are the following: (a) plastically deformed zircon crystals usually have long axes parallel to the mylonitic foliation plane; (b) crystals with < c > axes oriented at an angle > 15° to the foliation plane are undeformed or fractured. Zircon crystals that have < c > axes aligned parallel or normal to the stretching lineation within the foliation plane develop misorientation and rotation axes parallel to the [001] crystallographic direction. Zircon grains with the < c > axis aligned at 30–60° to the lineation within the foliation plane often develop either two low Miller indices misorientation axes or one high Miller indices misorientation axis. Host phases have a significant influence on deformation mechanisms. In a relatively soft matrix, zircon is more likely to develop low Miller indices misorientation axes than in a relatively strong matrix. These relationships are independent of zircon's grain size and shape, and reflect the strong geometric control of the macroscopic kinematic rotation axis on the crystal-plastic deformation behavior of zircon and on the geometry of its slip systems. We describe previously unknown rheological and crystallographic properties of zircon, which suggest a tool for deriving an orientation of the plastically deformed zircon crystals with respect to the associated foliation and stretching lineation. Additionally, relationships between zircon deformation microstructures and the macroscopic kinematic framework have implications for zircon geochronology. If deformation events result in zircon distortion and, as a consequence, partial or complete resetting of the zircon isotopic system, the age of deformation can be accurately dated. [ABSTRACT FROM AUTHOR]

The Lüliang Complex is located at the western margin of the middle segment of the Trans-North China Orogen, along which the Western and Eastern Blocks collided to form the North China Craton. The complex mainly consists of metamorphosed granitic plutons and supracrustal rocks, of which the latter are subdivided into the Jiehekou, Lüliang, Yejishan, Lanhe and Heichashan Groups. The Lanhe Group is composed of meta-conglomerates, quartzites, and phyllites with minor meta-basalts, whereas the Heichashan Group consists of molasse-like meta-conglomerates and coarse-grained quartzites. Geochemistry of the Yejishan meta-sedimentary rocks indicates weak source weathering and dominantly chemical immature features, whereas the Lanhe and Heichashan samples display opposite features. U–Pb ages of detrital zircons from the Lanhe Groups yield four age peaks at ~ 2180 Ma, ~ 2370 Ma, ~ 2520 Ma and ~ 2700 Ma. The former three peaks coincide with ages of the Chijianling–Guandishan TTG gneisses (2199–2151 Ma) and meta-volcanic rocks from the Lüliang and Yejishan Groups (2213–2156 Ma), age of the Gaijiazhuang porphyritic gneisses (2375–2364 Ma) and age of the Yunzhongshan TTG gneisses (2499 Ma) respectively, whereas detrital zircons forming the oldest age peak were most likely derived from the early Neoarchean crust of the Eastern Block. For the Heichashan Group, the dominant 2.2–2.0 Ga detrital zircons were probably recycled from the underlying Jiehekou Group and the minority is directly derived from the early Paleoproterozoic granitoids in the Lüliang Complex. The youngest detrital zircon age peaks of ~ 2.17 Ga and ~ 1.82 Ga place maximum depositional ages on the Lanhe and Heichashan Groups respectively, whereas the local 1.81–1.79 Ga massive granites place constraint on their minimum depositional ages. Taking into account the lithostratigraphic features, provenance and formation ages, we suggest that the Lanhe Group formed in a shrinked remnant back-arc basin and the Heichashan Group was deposited in a foreland basin. Conversion of the tectonic basin from the middle Paleoproterozoic back-arc basin to the late Paleoproterozoic foreland basin is well consistent with the model that the single collision to form the basement of the North China Craton happened at ~ 1.85 Ga. [ABSTRACT FROM AUTHOR]

Zircon and monazite U–Pb geochronology and zircon Hf isotopes place constraints on the temporal and source relationships between crustal anatexis, magmatism, and exhumation of the Priest River metamorphic core complex, northern Idaho. Granitoids that intruded the migmatitic, pelitic Hauser Lake gneiss include the < 76.5 ± 0.1 Ma Spokane granite, 50.13 ± 0.02 Ma Silver Point quartz monzonite, c. 47.9 Ma Wrencoe granodiorite, < 46.4 ± 1.8 Ma Rathdrum granite, and a < 49.8 ± 0.4 Ma leucocratic dike. Cretaceous magmatism preceded the c. 64 Ma peak metamorphism (recorded by monazite) of the Hauser Lake gneiss, whereas discrete pulses of Eocene magmatic activity post-date the onset of exhumation by 10 Ma. The relative timing of pluton emplacement in the Priest River complex indicates that it was primarily a response to decompression rather than a cause. The mylonitized Silver Point and undeformed Wrencoe plutons bracket the end of a rapid phase of exhumation to c. 50–48 Ma. Zircon ε Hf(i) values and Lu–Hf isotope evolution indicate that the Silver Point and Wrencoe plutons crystallized from homogeneous magmas sourced from Archean–Proterozoic basement orthogneisses, whereas the Spokane granite and two leucocratic units appear to have been produced by partial melting of the Hauser Lake gneiss. Comparison of the Priest River complex with other deeply exhumed northern Cordilleran complexes indicates variability in the timing and, therefore, relative influences of partial melting and magmatism on the initiation of exhumation, which must be accounted for in numerical models of metamorphic core complex formation and evolution. [ABSTRACT FROM AUTHOR]

An investigation has been performed on three chromitite layers segregated in dunite bodies of the Phlogopite Peridotite mantle unit in the Finero complex (FPP, Ivrea–Verbano Zone, Southern Alps) aimed at providing new constraints to their origin and evolution. Field relationships, the sub-chondritic Hf isotopic composition of the zircons (εHf (188) as low as − 5.4), the heavy O isotopic composition of zircons and pyroxenes (δ 18 O up to 6.9‰), the strict similarity of the trace element composition between the clinopyroxenes and amphiboles from the chromitites and those from the phlogopite harzburgites and pyroxenites forming the typical FPP association, as well as the REE composition of zircons, which approaches equilibrium with the associate clinopyroxene, suggest that the studied chromitites were segregated from melts, highly contaminated from continental crust, during the pervasive cycle of metasomatism recorded by the FPP. An LA-ICP-HRMS survey of chromitite zircon grains has provided Early Jurassic U–Pb ages mostly between 199 ± 3 Ma and 178 ± 2 Ma, with a pronounced peak at 187 Ma. Relevant exceptions are inherited domains of two grains giving Triassic ages of 242 ± 7 Ma and 229 ± 7 Ma, and a third homogeneous zircon giving 208 ± 3 Ma. Our geochronological data and those reported in the literature show that the FPP chromitites have zircon populations with different internal CL textures, but the same sub-chondritic Hf isotopic composition, which define an overall U–Pb age span from ~ 290 Ma to 180. The segregation of the chromitite layers and the main pervasive metasomatism likely occurred in the Early Permian (in a post-collisional, transtensional setting) or before (possibly, in a subduction-related setting). The rejuvenation of the zircon ages was accompanied by a progressive disappearance of the internal zoning, interpreted as the result of a prolonged residence at mantle depths with progressive re-equilibration of the U–Pb system due to thermal perturbations. The age peak at ~ 187 Ma is argued to constrain the timing of FPP exhumation at shallower, crustal levels. This process was characterised by an important reheating event, possibly due to lithospheric hyperextension. The evolution of the FPP appears completely different than that of mantle bodies of the central IVZ (i.e., the Val Sesia-type bodies), which were emplaced within the continental crust, as part of accretionary prisms, at or before the end of the Variscan orogeny. [ABSTRACT FROM AUTHOR]

Combined U-Pb-Hf-O isotopic analyses of zircons are useful probes to investigate the origin and temporal evolution of long-lived magma reservoirs feeding large eruptions. This study presents a complete dataset of zircon Hf-O isotopes of the latest five Toba eruptions in the Quaternary. The new isotopic data differ slightly within and between eruptive units, with the majority of zircons having ε Hf values between −8 to −5 and δ18O values between 6 and 6.8. These Hf-O isotopic compositions of zircons were considered to inherit from an enriched and slightly heterogenous magma source. Specifically, zircons of the Oldest Toba Tuff (OTT) have rather restricted and constant Hf-O isotopes, which may crystallize from a reservoir protractedly supplied from the same magma source. Zircons of the Middle Toba Tuff (MTT) have a large range of δ18O values varying from ~6 to 8, with low ε Hf values extending to −14, indicating shallow-level crustal contamination. The largely negative correlations between ε Hf and δ18O for zircons from the MTT are indicative of contamination of high-δ18O crustal materials, whereas those of the positive correlations for zircons from the Youngest Toba Tuff (YTT) imply contamination of hydrothermally altered low-δ18O crustal materials. It is important that the Hf-O isotopes of all the zircons do not record any single batch of magma that was derived from partial melting of depleted mantle peridotites, but are consistent with an enriched and heterogenous mantle source hybridized by the subducted sediments, followed by contamination of low-δ18O or high-δ18O materials during their ascent to the surface or storage in the shallow magma reservoirs. The increase of zircon δ18O values towards eruptions in both the MTT and YTT suggests that the two eruptions might be accompanied by roof collapse that added high-δ18O materials to the magma. Thus, assimilation of high-δ18O crustal materials could be a possible trigger for the MTT and YTT eruptions. • Toba volcanic rocks were derived from an isotopically enriched and heterogenous mantle source. • Hf-O isotopes of the MTT zircon crystals exhibit characteristics of shallow-level crustal contamination. • The YTT magma reservoir was contaminated by both high- and low-δ18O crustal materials. • Assimilation of high-δ18O materials by roof collapse could be a likely eruption trigger for the MTT and YTT eruptions. [ABSTRACT FROM AUTHOR]

The robust nature of the mineral zircon, combined with our analytical ability to readily acquire in-situ uranium-lead (U-Pb), lutetium-hafnium (Lu-Hf) and oxygen (O) isotopic data, has resulted in a rapid rise in the use of zircon isotopic datasets for studying both the generation of continental crust and its growth through Earth history. In such studies there has been a strong focus on developing methods to determine the timing and/or proportion of juvenile magmatic addition to the continental crust. One widespread approach to determine the timing of crustal growth has been the construction or fitting of ‘reworking arrays’ to regional Hf isotopic datasets. Simple stochastic models are presented which highlight that in many cases apparent reworking arrays are much more likely to represent a process of on-going dilution and refertilisation of ancient crust, consistent with “Hot Zone” models of granitoid generation and the need to refertilise lower crustal reservoirs to maintain magmatism. A new compilation of magmatic rock zircon Lu-Hf and O isotope data is used to demonstrate that the use of mantle-like O isotope data as a screening tool for “meaningful” Hf model ages is also unlikely to be reliable, with independently constrained data indicating that as few as 14% of Hf model ages provide a meaningful indicator of the timing of crustal growth. The limitations of Hf model ages are discussed with regard to existing approaches for continental growth and we demonstrate that popular inverse modelling approaches suffer from a bias created by both the use of model ages and numerical artefacts. In an effort to address some of the limitations within existing models, we develop stochastic models based on joint calibration of multiple datasets which allow for more unique solutions. [ABSTRACT FROM AUTHOR]

Voluminous felsic volcanic magmas were formed in Central Europe at the Carboniferous/Permian boundary in numerous pull-apart basins; one of which is the Saale Basin, which holds the Halle Volcanic Complex (HVC), the focus of this study. The rhyolites in the HVC formed laccoliths and scarce lavas, and occur in two different textural types: fine and coarse porphyritic. Zircon isotope and trace element composition was analysed in four units, two per each textural type. Zircon from the different units shows similar ranges in εHf (− 4.1 to − 8.1) and δ 18 O values (6.51–8.26), indicating similar sources and evolution processes for texturally diverse rhyolites from the HVC. Scarce inherited zircon ranges from ~ 315 Ma to ~ 2100 Ma with the major groupings around 315–550 Ma. These ages are typical for Devonian arc magmatic activity (350–400 Ma) and Cadomian igneous rocks (500–600 Ma), which occur in the basement presently underlying the HVC. Therefore, the source of the rhyolites was multicomponent and probably represented by a basement composed of various crystalline rocks. Trace elements in zircon show similar distributions in all analysed samples, which is broadly consistent with zircon cores crystallizing in a less evolved magma undergoing limited fractional crystallization, whilst the zircon rims crystallized from a magma undergoing extensive fractional crystallization of major and accessory minerals. Interestingly, comparison of the zircon composition in HVC rhyolites and other rhyolites worldwide shows that the observed trends are similar in such rhyolites despite the values being different. This may suggest that most of the zircon in rhyolites crystallizes at a similar stage in the rhyolite magma evolution, from magmas undergoing extensive crystallization of major phases and apatite. The implication is that most of the zircon represents late stage crystallization, but also that antecrystic component may be present and preserve information on the development of precursor magmas, probably within mush-rich magma bodies. [ABSTRACT FROM AUTHOR]

A combined study of mineral inclusions, U–Pb ages and trace elements was carried out on zircon from migmatites in the Dabie orogen. The results provide insights into multistage anatexis of ultrahigh-pressure metamorphic rocks in the continental collision orogen. Zircon grains in thin sections and mounts record four episodes of magmatic, metamorphic and anatectic events: (1) middle Neoprotrozoic U–Pb ages for domains that contain Qz ± Ap ± Pl inclusions and exhibit high Th/U ratios (> 0.1), steep HREE patterns with marked negative Eu anomalies, dating protolith emplacement; (2) late Triassic U–Pb ages of 212 ± 5 to 219 ± 4 Ma for domains that contain Cpx ± Grt inclusions and show low Th/U ratios (< 0.1), flat HREE patterns without negative Eu anomalies, dating quartz eclogite-facies metamorphism during the early exhumation of deeply subducted continental crust; (3) U–Pb ages of 192 ± 4 to 200 ± 4 Ma for domains that contain multiphase solid inclusions of Qz + Pl + Hem + Cal, Qz + Kfs + Ep, Qz + Bt + Ap and Qz + Pl + Bt, and exhibit low Th/U ratios (< 0.1), steeper HREE patterns with negative Eu anomalies and high Nb and Ta contents, dating the first episode of anatexis in association with granulite-facies overprinting during the late exhumation; and (4) early Cretaceous U–Pb ages of 124 ± 1 to 140 ± 4 Ma for domains that contain Qz inclusion and show variable Th/U ratios, typical magmatic REE patterns, dating the second episode of anatexis which occurred during the Cretaceous in the postcollisional stage. The zircon domains in the two episodes of anatexis exhibit a large range of U–Pb ages, suggesting protracted durations of anatexis in both exhumational and postcollisional stages. There are considerable differences in the compositions of zircons between the two episodes of anatexis, suggesting differential behaviors of their anatexis. The first episode of anatexis is caused by dehydration melting in association with decompressional exhumation, resulting in coprecipitation of zircon and REE-rich minerals such as allanite/epidote and monazite from the anatectic melt. The second episode of anatexis is caused by both dehydration and hydration melting due to breakdown of monazite and hydrous minerals such as allanite/epidote and amphibole with local focus of aqueous solutions. This leads to variable Th/U ratios but relatively consistent magmatic zircon-like trace element compositions, with the widespread occurrence of poikilitic amphibole in the leucosome. In either case, the anatectic zircons record the partial melting of ultrahigh-pressure metamorphic rocks at different times and P–T conditions. [ABSTRACT FROM AUTHOR]

A comprehensive database of zircon composition in West Australian magmatic rocks reveals negative correlations between both U and Th zircon/whole rock ratio and the zircon saturation temperature, with the observed change with temperature less for U (zircon/whole rock) than for Th (zircon/whole rock) . This observation implies a systematic increase in the zircon/rock ratio with falling crystallisation temperature, a result which replicates findings from experimental partition coefficient studies. Under equilibrium conditions there is a trend to lower zircon Th/U with increasing melt temperature which can be attributed to lattice strain. However, within a fractionating magma, Ti-in-zircon temperatures yield the opposite relationship of lower zircon Th/U in cooler melts. This is due to zircon growth under non-equilibrium conditions with greater incompatibility of Th relative to U, and the removal and segregation of mineral precipitates. These observations can be used as a tool to determine whether zircon growth was in a liquid of similar composition to the observed whole rock. We present an equation that estimates the degree of fractionation between the whole rock composition and the zircon parental liquid. This parameter demonstrates the dissimilarity between the liquid from which the zircon grew and the whole rock composition and aids in distinguishing mesostasis growth in fractionated melt versus cumulate growth in less fractionated magma. We use this ratio to investigate zircon growth in igneous rocks of the Musgrave Province. For a suite of c. 1200 Ma magmas that become progressively more fractionated, based on whole rock La/Sm, the fractionation index demonstrates increasing compositional differences between the whole rock and the zircon growth liquid. In the most extreme case independent petrographic evidence indicates mesostasis growth of zircon, whereas in the least fractionated melt zircon growth is established to be close to equilibrium with a zircon saturation temperature of c. 900 °C likely being accurate. In contrast, zircon crystals from a rhyolite of the c. 1070 Ma Giles Supervolcano have distinctive compositional discordances indicative of antecrystic components. The fractionation factor in this rock implies some zircon growth under higher temperature conditions than the whole rock zircon saturation temperature. [ABSTRACT FROM AUTHOR]

Zircon and xenotime occur in tourmaline-rich hydrothermal pockets in the auriferous lode of Passagem de Mariana, a world-class gold deposit. Zircon grains show pristine oscillatory zoning, but many of them are altered, exhibiting porous domains filled with graphite. Uranium–Pb dating of zircon, using in-situ laser ablation–inductively coupled plasma–mass spectrometry, yields ages between 3.2 and 2.65 Ga, which match those for detrital zircon of the footwall quartzite of the > 2.65-Ga-old Moeda Formation. Discordant analyses point to zircon-age resetting during the Brasiliano orogeny at ca. 500 Ma. This interpretation is supported by U–Pb dating of euhedral xenotime immediately adjacent to altered zircon within the same tourmaline pocket. The xenotime grains give a Concordia age of 496.3 ± 2.0 Ma, which is identical to that determined for monazite of a quartz–hematite vein-type deposit (i.e., jacutinga lode) in the region (Itabira), another important mineralisation style of gold. The occurrence of relatively abundant inherited detrital zircon, but absence of rock fragments in the tourmaline pocket investigated here, implies that detrital material was completely replaced by tourmaline. The graphite overprint on the altered detrital zircon attests to a reducing fluid, which was likely formed by fluid–rock interaction with carbonaceous phyllite of the Batatal Formation, the host rock of the Passagem lode. [ABSTRACT FROM AUTHOR]

Here we report the discovery of mosaic zircons in Hole U1473A on the Atlantis Bank in the Southwest Indian Ridge. Oxygen isotope, U Pb dating, and geochemical analyses of the zircons were carried out directly in petrographic thin-sections. The mosaic zircons are from diorite and oxide gabbro at shallow depths, whereas the nonmosaic zircons occur in oxide gabbronorite and tonalite at greater depths. The majority of zircons have δ18O of 5.5 ± 0.1‰, which are mantle-like values, reflecting formation by simple fractional crystallization. No correlation exist between U Pb ages, δ18O values, and the mosaic microstructures. The mosaic subdomains in a single zircon are well correlated with some trace elements and fractures. Therefore, the variable trace element contents were produced during crystallization, and associated with secondary brittle deformation due to movement on mid-ocean ridge detachment faults. The pervasive brittle deformation weakened the originally rigid zircon and triggered fracturing. The fractures became highly permeable pathways, allowing rapid grain-boundary diffusive loss of Pb, which resulted in the relatively young age of 11.42 ± 0.41 Ma for the mosaic zircons as compared with 12.16 ± 0.14 Ma for the non-mosaic zircons. The zircon trace element patterns are all indistinguishable from global oceanic zircons. However, the trace element abundances and ratios require a significantly depleted normal-type mid-ocean ridge basalt mantle source. • Mosaic zircons were observed from Atlantis Bank of Southwest Indian Ridge. • Mosaic texture reflects the distribution of trace elements and post-growth deformation. • Dates from Hole U1473A are older than the predicted age. • Young dates results from Pb-loss due to the fractures within single zircon grain. [ABSTRACT FROM AUTHOR]

U-Pb and Hf-isotope analyses of zircon xenocrysts from drilling-sampled Carboniferous volcanic rocks suggest the presence of unexposed Eoarchean crust beneath the Junggar Basin (NW China), southwestern Central Asian Orogenic Belt (CAOB). The oldest zircon population with U-Pb ages of 3.8 Ga shows chondritic ε Hf (t) of −0.7 to +0.7, and Hf model ages of 3900–3952 Ma (T DM) and 3973–4062 Ma (T crust). Three slightly younger populations at 3.7, 3.62 and 3.45 Ga have ε Hf (t) of −5.7 to +2.4, T DM of 3.6–3.9 Ga and T crust of 3.7–4.1 Ga. The youngest xenocrysts yield U-Pb ages of 2.55 Ga with ε Hf (t) values of +3.7 to +8.0. Crustal mineral inclusions and the trace-element compositions of all these zircons show that they probably crystallized from granitic magmas. The observation of deeply hidden Archean crust significantly older than the exposed Phanerozoic upper crust suggests that the juvenile origin in CAOB may be overestimated, and the study of crustal growth rates through time for young orogens will require revision. By comparing the Hf isotopic signatures of ancient zircons (>3.4 Ga) worldwide, we infer that there may have been a great change in early crustal evolution at the Hadean–Archean transition (~3.9 Ga), possibly linked to the Late Heavy Bombardment (LHB). • The Junggar Basin volcanic rocks captured abundant Archean zircon xenocrysts. • Our results imply unexposed Eoarchean crust beneath the Junggar Basin. • Global Hf isotopes reveal a great change in early crustal evolution at ~3.9 Ga. [ABSTRACT FROM AUTHOR]

IODP Hole 1256D penetrates a complete section of upper oceanic crust spread from the East Pacific Rise at a full rate of up to 220 mm/yr. Here we present U-Pb zircon data from Hole 1256D gabbroic rocks using isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and secondary ion mass spectrometry (SIMS). Three gabbro samples yielded Th-corrected weighted mean 206Pb/238U SIMS dates of 15.03 ± 0.14 Ma, 15.13 ± 0.35 Ma, and 15.23 ± 0.12 Ma. An ID-TIMS Th-corrected weighted mean 206Pb/238U date of 15.191 ± 0.040 Ma from the first of the three samples provides verification and refinement of the timing of magmatism. The U-Pb dates agree with the predicted age of the crust based on marine magnetic anomalies, suggesting the gabbros most likely formed by intrusion and crystallization at or near the ridge axis. Zircons from two of the studied samples have anomalously high Th/U ratios, likely reflecting oxidation of U prior to or during zircon crystallization. The elevated ƒO 2 of late stage melts in the axial melt lens may be related to progressive crystallization, assimilation of hydrothermally altered sheeted dikes and gabbros, and/or contamination of the melt lens by seawater derived saline brines. • TIMS and SIMS zircon U-Pb dates from in situ Miocene-age East Pacific Rise spread gabbros indicate axial melt emplacement. • In the context of global data, elevated Th/U values are consistent with oxidation of the axial melt lens. • The analyses support previous conclusions from petrologic and magnetic analyses of the Hole 1256D core. [ABSTRACT FROM AUTHOR]