U-Pb age of the Diana Complex and Adirondack granulite petrogenesis

U-Pb isotopic analyses of eight single and multi-grain zircon fractions separated from a syenite of the Diana Complex of the Adirondack Mountains do not define a single linear array, but a scatter along a chord that intersects the Concordia curve at 1145 ± 29 and 285 ± 204 Ma. For the most concordant analyses, the²⁰⁷Pb/²⁰⁶Pb ages range between 1115 and 1150 Ma. Detailed petrographic studies revealed that most grains contained at least two phases of zircon growth, either primary magmatic cores enclosed by variable thickness of metamorphic overgrowths or magmatic portions enclosing presumably older xenocrystic zircon cores. The magmatic portions are characterized by typical dipyramidal prismatic zoning and numerous black inclusions that make them quite distinct from adjacent overgrowths or cores when observed in polarizing light microscopy and in backscattered electron micrographs. Careful handpicking and analysis of the “best” magmatic grains, devoid of visible overgrowth of core material, produced two nearly concordant points that along with two of the multi-grain analyses yielded an upper-intercept age of 1118 ± 2.8 Ma and a lowerintercept age of 251 ± 13 Ma. The older age is interpreted as the crystallization age of the syenite and the younger one is consistent with late stage uplift of the Appalachian region. The 1118 Ma age for the Diana Complex, some 35 Ma younger than previously believed, is now approximately synchronous with the main Adirondack anorthosite intrusion, implying a cogenetic relationship among the various meta-igneous rocks of the Adirondacks. The retention of a high-temperature contact metamorphic aureole around Diana convincingly places the timing of Adirondack regional metamorphism as early as 1118 Ma. This result also implies that the sources of anomalous hightemperature during granulite metamorphism are the syn-metamorphic intrusions, such as the Diana Complex.     

Sr and Nd isotope geochronology,geologic history,and origin of the Adirondack Anorthosite

We have analyzed samples from the Adirondack Marcy massif for Rb-Sr and Sm-Nd isotopes in an attempt to determine directly the primary crystallization age of a Proterozoic massif-type anorthosite rock suite. The oldest age obtained (1288 ± 36Ma) is from a 4 point Sm-Nd isochron defined by igneous-textured whole-rock and mineral separate data from a local layered sequence gradational from oxiderich pyroxenite to leuconorite. This age is older than Silver's (1969) 1113 Ma zircon age of associated charnockites, but is within the window of permissible anorthosite ages based on previous geochronology and field relationships. As such, 1288 Ma may represent the time of crystallization of the massif. For the most part, however, both Sm-Nd and Rb-Sr isotopic systems did not survive granulite facies metamorphism. Internal isochrons based on whole rocks and minerals yield ages between 995 and 919 Ma. These isotopic data suggest that the granulite fades metamorphism experienced by the massif was a prograde event that occurred a minimum of 100 Ma and as much as 350 Ma after crystallization of the massif. The relatively large range in Rb abundance, and in calculated initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ (0.7039–0.7050) and ${}^{143}\text{Nd}{}^{144}\text{Nd}$ ratios among anorthosite suite rocks, particularly those at or near the contacts of the Marcy massif is explicable by variable contamination with “crustal” materials and/or fluids, derived from surrounding acidic metaplutonic rocks, paragneisses, and marbles. Despite uncertainies caused by crustal contamination and metamorphic resetting of primary ages, Marcy samples have epsilon Nd values between +0.44 and +5.08, implying a source for the massif with long-term depletion in light rare earth elements. A probable source material would be depleted mantle.     

The evolution of the polymetamorphic basement in the Central Alps unravelled by precise U−Pb zircon dating

The U–Pb age determinations of zircon and rutile from the Aar massif reveal a complex evolution of the Central Alpine basement. The oldest components are found in zircons of metasediments, which bear cores of Archean age; the U–Pb age of discordant prismatic zircons of the same rocks ranges between 580 and 680 Ma, an age that is typical for Pan-African metamorphism. The zircons are interpreted as Pan-African detritus with Archean inheritance. The provenance region of the Pan-African zircons is assumed to be a terrane of Gondwana-affinity, i.e. the W. African craton or the Pentevrian microplate. The Caledonian metamorphism left a pervasive structural imprint in amphibolite facies on the rocks of the Aar massif; it is dated at 456±2 and 445 Ma by zircons of a layered migmatitic gneiss and a migmatitic leucosome, respectively, both occurring in the northernmost zones of the massif. Hercynian metamorphism never exceeded greenschist-facies conditions and is recorded by zircon in a garnet-amphibolite and by rutile in a meta-psammite that yield an age of 330 Ma. Both zircon and rutile are considered to be products of retrograde mineral reactions and therefore do not date the peak conditions of Hercynian metamorphism. The Gastern granite at the western end of the Aar massif is a contaminated granite that intruded at 303±4 Ma, contemporaneously with the wide-spread late Hercynian post-collisional I-type magmatism. The study demonstrates the potential of isotope dilution U–Pb dating of single grains and microfractions in deciphering complex evolutionary histories of polymetamorphic terrains.     

Moderate pressure metamorphism and anatexis due to anorthosite intrusion,western Adirondack Highlands,New York

 Garnet-sillimanite-biotite gneiss near Port Leyden, in the western Adirondack Highlands, New York, contains mineral assemblages and textures that formed during high temperature metamorphism and anatexis at mid-crustal pressures. Evidence for melting includes thin, plagioclase-rich veins, sieve textures in biotite, and the presence of small, euhedral garnet neoblasts. Hercynite-silicate equilibria in combination with the solidus for biotite dehydration melting indicate metamorphic pressure was between 4 and 6.4 kbar at the temperature of melting (ca. 735° C). The gneiss is intruded by a small, discordant Fe-Ti oxide-apatite (nelsonite) dike. Reported field occurrences of nelsonite demonstrate its common association with anorthosite plutons. Although no anorthosite bodies are exposed in the Port Leyden region, the presence of nelsonite is evidence of anorthositic magmatism in the western Adirondacks. Post-intrusion metamorphism has caused partial apatite recrystallization and produced a weak foliated texture in the dike. U-Pb ages from zircon and monazite from both the gneiss and the nelsonite dike indicate that these rocks experienced a complex, polymetamorphic history that we interpret to reflect two thermal episodes. An older event is recorded by discordant zircons in the gneiss, which indicate a minimum age of 1129±6 Ma. A linear best fit to the data yields an upper intercept at 1166±53 Ma. This range of ages coincides with anorthosite-suite magmatism in the Adirondacks. A minimum zircon age of 1104±3 Ma was obtained from the nelsonite dike. Lead-loss or late zircon crystallization at about 1020 Ma affected the U-Pb systematics of zircon in the dike. Monazite ages from both rocks also indicate high temperature metamorphism (>700° C) between 1040 and 960 Ma. The older zircon ages and textural relations in the metapelite are viewed as evidence for anatexis at ca. 1150 Ma, and the presence of nelsonite suggests that the intrusion of anorthosite was coincident with partial melting in the gneiss. P-T estimates of metamorphism, therefore, imply that anorthosite was emplaced to about 15 km depth in the western Adirondack Highlands. Received: 13 September 1994 / Accepted: 10 May 1995     

Geochemistry and U–Pb age of zircons from the Vurechuaivench massif,Monchegorsk complex,Kola region

The paper presents data on the geochemical and geochronological characteristics of zircons from mafic rocks of part of the Monchegorsk layered complex represented by the Vurechuaivench massif. Ages of zircons (SHRIMP-II) from samples V-l-09 (anorthosite) and V-2-09 (gabbronorite) are dated back to 2508 ± 7 and 2504 ± 8 Ma, respectively. The chondrite-normalized REE patterns confirm the magmatic nature of zircons. The data unequivocally indicate that the U–Pb age of zircon from both gabbronorite and anorthosite corresponds to the age of melt crystallization in a magmatic chamber. The mantle origin of gabbroic rocks of the Vurechuaivench massif is confirmed by the REE patterns of three zircon generations with different crystallization sequences. The wide range of the Ce/Ce* ratio (9.96–105.24) established for zircons from gabbroic rocks of the Vurechuaivench massif indicates sharply oxidative conditions of zircon crystallization. For deepseated mantle rocks, these data can only be explained by significant contamination of the melt with country rock material.     

http://www.sciencedirect.com/science/article/pii/S1674987112000564

High-pressure（HP） granulites widely occur as enclaves within tonalite-trondhjemitegranodiorite （TTG） gneisses of the Early Precambrian metamorphic basement in the Shandong Peninsula, southeast part of the North China Craton（NCC）.Based on cathodoluminescence（CL）,laser Raman spectroscopy and in-situ U-Pb dating,we characterize the zircons from the HP granulites and group them into three main types:inherited（magmatic） zircon,HP metamorphic zircon and retrograde zircon.The inherited zircons with clear or weakly defined magmatic zoning contain inclusions of apatites,and ²⁰⁷Pb/²⁰⁶Pb ages of 2915—2890 Ma and 2763—2510 Ma,correlating with two magmatic events in the Archaean basement. The homogeneous HP metamorphic zircons contain index minerals of high-pressure metamorphism including garnet,clinopyroxene.plagioclase,quartz,rutile and apatite,and yield ²⁰⁷Pb/²⁰⁶Pb ages between 1900 and 1850 Ma,marking the timing of peak HP granulite fades metamorphism.The retrograde zircons contain inclusions of orthopyroxene.plagioclase.quartz,apatite and amphibole.and yield the youngest ²⁰⁷Pb/²⁰⁶Pb ages of 1840—1820 Ma among the three groups,which we correlate to the medium to low-pressure granulite fades retrograde metamorphism.The data presented in this study suggest subduction of Meso- and Neoarchean magmatic protoliths to lower crust depths where they were subjected to HP granulite facies metamorphism during Palaeoproterozoic（1900—1850 Ma）.Subsequently, the HP granulites were exhumated to upper crust levels,and were overprinted by medium to low-pressure granulite and amphibolite facies retrograde event at ca.1840—820 Ma.     

Geochemistry and geochronology of the Rathjen Gneiss: Implications for the early tectonic evolution of the Delamerian Orogen

The Rathjen Gneiss is the oldest and structurally most complex of the granitic intrusives in the southern Adelaide Fold‐Thrust Belt and therefore provides an important constraint on the timing of the Delamerian Orogen. Zircons in the Rathjen Gneiss show a complex growth history, reflecting inheritance, magmatic crystallisation and metamorphism. Both single zircon evaporation (‘Kober’ technique) and SHRIMP analysis yield best estimates of igneous crystallisation of 514 ± 5 Ma, substantially older than other known felsic intrusive ages in the southern Adelaide Fold‐Thrust Belt. This age places an older limit on the start of the Delamerian metamorphism and is compatible with known stratigraphic constraints suggesting the Early Cambrian Kanmantoo Group was deposited, buried and heated in less than 20 million years. High‐U overgrowths on zircons were formed during subsequent metamorphism and yield a ²⁰⁶Pb/²³⁸U age of 503 ± 7 Ma. The Delamerian Orogeny lasted no more than 35 million years. The emplacement of the Rathjen Gneiss as a pre‐ or early syntectonic granite is emphasised by its geochemical characteristics, which show affiliations with within‐plate or anorogenic granites. In contrast, younger syntectonic granites in the southern Adelaide Fold‐Thrust Belt have geochemical characteristics more typical of granites in convergent orogens. The Early Ordovician post‐tectonic granites then mark a return to anorogenic compositions. The sensitivity of granite chemistry to changes in tectonic processes is remarkable and clearly reflects changes in the contribution of crust and mantle sources.     

Contrasting response of monazite and zircon to a high-T thermal overprint

Contact metamorphism in the aureole of the 1322 Ma Makhavinekh Lake Pluton, northern Labrador, affected monazite and zircon in the adjacent 1850 Ma metapelitic gneisses. Transformation of regional garnet and sillimanite to lower-pressure symplectitic intergrowths of cordierite, orthopyroxene, and spinel was accompanied by resorption of inherited monazite inclusions in garnet coupled with the appearance of coronitic high-Y monazite rims. In situ ion-microprobe dating is used to show that high-Y rims formed during contact metamorphism. Liberation of Y and HREE from garnet also gave rise to new xenotime growth. The coronitic nature of monazite overgrowths reflects the diffusion-controlled nature of net-transfer reactions whereas its higher Y composition reflects equilibration with xenotime at peak T (> 800 °C) conditions in the inner aureole. Very thin overgrowths on inherited zircon were also encountered, but only where zircon is surrounded by the symplectitic assemblage, reflecting liberation of Zr from garnet. Although these overgrowths are too thin to date using conventional ion-microprobe techniques, well-developed triple junctions between zircon and orthopyroxene suggests that they grew in textural equilibrium with the contact metamorphic assemblage.

In contrast to monazite, inherited zircon remained intact during contact metamorphism, exhibiting no change in morphology (other than the growth of thin rims) or internal zoning throughout the aureole. However, inherited sector-zoned zircons of anatectic origin display evidence for intracrystalline Pb redistribution in the inner aureole. In these samples, ion-microprobe analyses encountered heterogeneous Pb signals and a dispersion of ²⁰⁷Pb / ²⁰⁶Pb dates away from the well constrained 1850 Ma age of regional metamorphism. Whereas analyses from the outer aureole faithfully record the age of regional metamorphism, those from the inner aureole are normally and reversely discordant and distributed along a line collinear with a 1850 to 1322 Ma discordia. This disturbance is correlated with proximity to the pluton implying that Pb was mobile in the zircon lattice during contact metamorphism. Most grains are characterized by apparent Pb loss from low-U domains and apparent Pb gain in higher-U domains. These data are interpreted to reflect recovery of strained crystalline domains leading to expulsion of Pb* that was able to efficiently diffuse into higher-U domains that were partly amorphous prior to rapid reheating in the inner aureole.     

Metamorphic phase equilibria modelling and zircon U–Pb geochronology of ultrahigh-temperature cordierite granulites from the Madurai Block,India: implications for hot Gondwana crust

The Madurai Block (MB) is the largest Precambrian crustal block in the Southern Granulite Terrane (SGT) of India and hosts rare cordierite- and orthopyroxene-bearing granulites. Investigations based on field study, petrology, metamorphic P–T estimation, and detrital zircon geochronology of these granulites are crucial for understanding the ultrahigh-temperature (UHT) metamorphism and crustal evolution in this block. Here we investigate the petrology and zircon U–Pb geochronology of two new localities of cordierite granulites at Kottayam (southern MB; SMB) and Munnar (central MB; CMB). Petrographic observations and phase equilibria modelling results indicate that these rocks experienced UHT metamorphism with the peak temperature exceeding 950℃ and involving clockwise P–T paths. The prograde mineral assemblages define the P–T conditions of 6.8–8.7 kbar and 750–875℃. The peak conditions are estimated using pseudosection modelling and geothermometry, which yield P–T estimates of 7.1–9.1 kbar and 955–985℃. The retrograde cooling and decompression are inferred at 860–790℃ and <6.5 kbar, respectively. Partial melting played an important role during metamorphism and contributed to the overgrowth around detrital zircons. The melt production process was probably related to biotite dehydration melting, and was mainly triggered by heating, with or without the effect of decompression. Detrital zircons in cordierite granulite samples from the two localities show similar age distributions and have dominantly Neoproterozoic ages (1024–760 Ma). The zircon cores show oscillatory zoning with a wide range of Th/U ratios (0.01–0.96), implying complex protoliths from multiple Neoproterozoic provenances from both southern and central domains of the MBs. Zircon rims and homogeneous bright zircons yield mean ages of 549 ± 5 Ma, 536 ± 6 Ma, and 544 ± 6 Ma, which are interpreted to represent zircon overgrowths during the post-peak cooling and decompression process. The timing of peak UHT metamorphism is constrained as 549–599 Ma, which coincides with the assembly of the Gondwana supercontinent.     

北秦岭西段冥古宙锆石(4.1～3.9Ga)年代学新进展

2007年王洪亮等报道在北秦岭西段火山岩中获得一粒年龄为4079±5Ma的冥古宙捕虏锆石。之后,对这一发现开展了深入的调查研究,我们除利用SHIMP技术方法对原4079Ma的锆石进行验证外,新获得了两粒~(207)Pb/~(206)Pb年龄为4007±29Ma和3908±45Ma捕获的变质成因锆石,表明早在4.0Ga已经有变质作用的发生,这或许说明在冥古宙时期地球已经具有相当规模和厚度的地壳。同时开展的岩石学研究表明,蕴含古老锆石的母岩属于火山碎屑熔岩类而不是火山熔岩。     

华北克拉通古元古代晚期地壳演化和荆山群形成时代制约——胶东地区变质中-基性侵入岩锆石SHRIMP U-Pb定年

胶东地区的荆山群呈近东西向环绕太古宙TTG花岗质片麻岩展布,主要由成熟度高的含石墨变泥砂质岩石、钙硅酸岩和大理岩组成,变质程度达高角闪岩相-麻粒岩相,具孔兹岩系性质。变质中-基性岩侵入到荆山群。它们的侵位时代对于探讨华北克拉通东部元古宙构造演化以及对荆山群沉积时代的制约,都有重要意义。锆石SHRIMP U-Pb定年结果表明,遭受低级变质的闪长岩(S0835)岩浆锆石年龄为1852±9Ma (MSWD=2.1),遭受中高级变质的辉长岩(S0816)变质锆石年龄为1865±11Ma (MSWD=0.76)。结合区域资料,可得出如下结论:(1)荆山群孔兹岩系形成于古元古代晚期(2.2~1.9Ga);(2)古元古代期间,胶东地区从挤压体制转入伸展体制的时间在1.87Ga之前。在华北克拉通中西部的恒山、大青山地区,存在1.97~1.92Ga辉长岩,在1.92~1.83Ga期间发生变质,与本文研究结果类似。这表明华北克拉通中-西部和东部具有类似的古元古代演化历史。     

Metamorphism in the Adirondacks. I. Petrology, Pressure and Temperature

Grenville Supergroup sediments and suites of pre- and syn-tectonicigneous rocks have been metamorphosed to the upper amphiboliteand granulite facies in the Adirondacks of northern New Yorkduring the Grenville orogeny about one billion years ago. Magnetite-ilmenite, alkali feldspar-plagioclase, calcite-dolomiteand garnet-clinopyroxene thermometry indicate that metamorphictemperatures (T) increase from about 650 ?C in the area westand northwest of Gouverneur to 700–750 ?C near Coltonand along the Lowlands-Highlands boundary to 750–800 ?Cin areas within and around the Marcy anorthosite massif. Thepresence of grossular-rich garnet + quartz without wollastonite+ plagioclase in calc-silicate rocks and the apparent absenceof metamorphic ferropigeonite in charnockites restrict maximummetamorphic T to less than 800–850 ?C. Metamorphic pressures (P), determined from coexisting pyrite-pyrrhotite-sphalerite,garnet-rutile-sillimanite-ilmenite-quartz, fayalite-quartz-ferrosilite,fayalite-anorthite-garnet, ferrosilite-anorthite-garnet-quartz,kyanite-sillimanite, anorthite-garnet-sillimanite-quartz andthe stability of akermanite, are 6?5–7?0 kb near Gouverneurand increase to 7?5–8?0 kb in the central Adirondack Highlands. The above P-T data deduced from diverse mineralogical/chemicalsystems are interpreted as peak or near-peak conditions forAdirondack metamorphism. The compositions of thin retrograderims on garnets indicate a post-peak-metamorphic P-T path forthe Adirondacks with appreciable cooling (200–300?) beforedecompression. Peak and retrograde P-T conditions inferred forthe Adirondacks are similar to numerous other granulite terranessuggesting that similar tectonothermal events are necessaryfor the formation of many granulite belts.     

Detrital zircon ages from southern Norway – implications for the Proterozoic evolution of the southwestern Baltic Shield

An ion-microprobe (SIMS) U-Pb zircon dating study on four samples of Precambrian metasediments from the high-grade Bamble Sector, southern Norway, gives the first information on the timing of discrete crust-forming events in the SW part of the Baltic Shield. Recent Nd and Pb studies have indicated that the sources of the clastic metasediments in this area have crustal histories extending back to 1.7 to 2.1 Ga, although there is no record of rocks older than 1.6 Ga in southern Norway. The analysed metasediments are from a sequence of intercalated, centimetre to 10-metre wide units of quartzites, semi-metapelites, metapelites and mafic granulites. The zircons can be grouped in two morphological populations: (1) long prismatic; (2) rounded, often flattened. The BSE images reveal that both populations consist of oscillatory zoned, rounded and corroded cores (detrital grains of magmatic origin), surrounded by homogeneous rims (metamorphic overgrowths). The detrital zircons have ²⁰⁷Pb/²⁰⁶Pb ages between 1367 and 1939 Ma, with frequency maxima in the range 1.85 to 1.70 Ga and 1.60 to 1.50 Ga. There is no correlation between crystal habit and age of the zircon. One resorbed, inner zircon core in a detrital grain is strongly discordant and gives a composite inner core-magmatic outer core ²⁰⁷Pb/²⁰⁶Pb age of 2383 Ma. Two discrete, unzoned zircons have ²⁰⁷Pb/²⁰⁶Pb ages of 1122 and 1133 Ma, representing zircon growth during the Sveconorwegian high-grade metamorphism. Also the μm wide overgrowths, embayments in the detrital cores and apparent “inner cores” which represent secondary metamorphic zircon growth in deep embayments in detrital grains, are of Sveconorwegian age. The composite-detrital-metamorphic zircon analyses give generally discordant ²⁰⁶Pb/²³⁸U versus ²⁰⁷Pb/²³⁵U ratios and maximum ²⁰⁷Pb/²⁰⁶Pb ages of 1438 Ma. These data demonstrate the existence of a protocrust of 1.7 to 2.0 Ga in the southwestern part of the Baltic Shield, implying a break in the overall westward younging trend of the Precambrian crust, inferred from the southeastern part of the Baltic Shield. Received: 8 April 1997 / Accepted: 14 July 1997     

Volcanism,mineralization and metamorphism at the Xitieshan Pb–Zn deposit,NW China: Insights from zircon geochronology and geochemistry

The footwall volcanic rocks of the Ordovician Tanjianshan Group in the world-class Xitieshan Pb–Zn deposit have experienced prolonged arc volcanism followed by strong metamorphism and deformation. This has resulted in a complex thermal history and led to ambiguity in interpretation of zircon geochronological results. An integrated study involving textural characterization, CL imaging, trace element analysis, Ti-in-zircon thermometry and LA-ICPMS U–Pb dating has provided tight constraints on the age and genesis of the zircon groups in the volcanic rocks. The temperature of metamorphism and deformation indicated by metacryst minerals and micro-structures in the volcanic rocks ranges from 550 to 650 °C, which partially overlaps with the lower temperature range of zircon crystallization (600–750 °C) calculated using the Ti-in-zircon thermometer. Cathodoluminescence images and trace element compositions confirm a magmatic origin for the zircons, which have also been variably altered by metamorphic fluids. Two ranges of U–Pb ages, 475–470 Ma and 460–450 Ma, have been obtained on typical magmatic zircons and are interpreted to represent pre-mineralization arc volcanism in the Xitieshan deposit. A younger age group of 440–430 Ma for the fluid-modified zircons is considered to record post-ore metamorphism during the North Qadaim Orogeny. Thus, we propose that the original exhalative ores at the Xitieshan Pb–Zn deposit formed at 450–440 Ma.     

U–Pb dating and composition of inclusions in zircon from ophiolitic gabbro of the Klyuchevsk massif (Middle Urals): Results and geological interpretation

The U–Pb (SHRIMP) dating of zircon from the layered complex of ophiolitic gabbro in the Klyuchevsk massif yielded an age of 456 ± 6 Ma corresponding within the limits of error to zircon dates obtained for other petrographic varieties from this massif. The investigation of the composition of silicate inclusions in dated zircon grains revealed that they are represented by typical metamorphic minerals: albite, zoisite, and secondary amphiboles. The data indicate that zircon was crystallized during metamorphic transformations of gabbroids and its U–Pb age (Late Ordovician–Silurian) is characteristic of all rocks in the ophiolite association of the Klyuchevsk massif indicating the age of metamorphism, not their formation time.     

Timing of late Neoarchean to late Paleoproterozoic events in the North China Craton: SHRIMP U–Pb dating and LA-ICP-MS Hf isotope analysis of zircons from magmatic and metamorphic rocks in the Santunying area,eastern Hebei

Santunying is an important area for revealing nature of the late Neoarchean tectono-magmato-thermal events in the eastern Hebei part of the North China Craton. It is mainly composed of meta-intrusive rocks. Supracrustal rocks sporadically occur in the meta-intrusive rocks. The meta-intrusive rocks are subdivided into the Santunying tonalitic gneiss, Qiuhuayu tonalitic-trondhjemitic gneiss, Xiaoguanzhuang dioritic gneiss and Qingyangshu meta-gabbro. Respectively, SHRIMP U–Pb zircon dating on fourteen samples yielded weighted mean ²⁰⁷Pb/²⁰⁶Pb ages of 2525–2537, 2532–2546, 2530–2544 and ∼2531 Ma for magmatic zircons from them. Dioritic gneiss of the Xiaoguanzhuang gneiss contain abundant 2544–3487 Ma xenocrystic zircons. SHRIMP U–Pb dating on a garnet-biotite gneiss sample yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2537 Ma for detrital zircons. All rocks underwent strong metamorphism, deformation and anatexis, resulting in formation of leucosomes and residues, with some leucosomes concentrating to form large veins. They record a strong late Neoarchean event by metamorphic zircon ages of 2489–2519 Ma. Some rocks also record metamorphic zircon ages of 1772–1843 Ma. Magmatic zircons from the magmatic rocks show large variations in ε_Hf(t) values ranging from −1.7 to +8.7. Combined with early studies, conclusions are: 1) Intrusive rocks with the involvement of mantle-derived materials have a narrow range of magmatic zircon ages from 2525 to 2546 Ma, and supracrustal rocks were formed during the same period. 2) Ancient crustal remnants (>2600 Ma) are present, consistent with the late Neoarchean arc magmatism involving older continental crust, similar to Phanerozoic Andean margins. 3) The Archean basement underwent a strong tectonothermal event at the end of the Neoarchean, with the metamorphic zircon ages being 10–30 million years younger than the timing of magmatism, a common feature of the North China Craton. 4) A late Paleoproterozoic tectonothermal event widely occurred in the western part of eastern Hebei, which is linked with regional ductile deformation.     

Polyphase zircon in ultrahigh-temperature granulites (Rogaland, SW Norway): constraints for Pb diffusion in zircon

SHRIMP U–Pb ages have been obtained for zircon in granitic gneisses from the aureole of the Rogaland anorthosite–norite intrusive complex, both from the ultrahigh temperature (UHT; >900 °C pigeonite‐in) zone and from outside the hypersthene‐in isograd. Magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis. A sample from outside the UHT zone has magmatic cores with an age of 1034 ± 7 Ma (2σ, n = 8) and 1052 ± 5 Ma (1σ, n = 1) overgrown by M1 metamorphic rims giving ages between 1020 ± 7 and 1007 ± 5 Ma. In contrast, samples from the UHT zone exhibit four major age groups: (1) magmatic cores yielding ages over 1500 Ma (2) magmatic cores giving ages of 1034 ± 13 Ma (2σ, n = 4) and 1056 ± 10 Ma (1σ, n = 1) (3) metamorphic overgrowths ranging in age between 1017 ± 6 Ma and 992 ± 7 Ma (1σ) corresponding to the regional M1 Sveconorwegian granulite facies metamorphism, and (4) overgrowths corresponding to M2 UHT contact metamorphism giving values of 922 ± 14 Ma (2σ, n = 6). Recrystallized areas in zircon from both areas define a further age group at 974 ± 13 Ma (2σ, n = 4). This study presents the first evidence from Rogaland for new growth of zircon resulting from UHT contact metamorphism. More importantly, it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c. 950 °C for at least 1 Myr. Magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information, a result inconsistent with some experimental data on Pb diffusion in zircon which predict measurable Pb diffusion under such conditions. The implication is that resetting of zircon ages by diffusion during M2 was negligible in these dry granulite facies rocks. Imaging and Th/U–Y systematics indicate that the main processes affecting zircon were dissolution‐reprecipitation in a closed system and solid‐state recrystallization during and soon after M1.     

Late-orogenic Sveconorwegian massif anorthosite in the Jotun Nappe Complex,SW Norway,and causes of repeated AMCG magmatism along the Baltoscandian margin

The age and tectonometamorphic history of massif anorthosite in the Jotun Nappe Complex, SW Norway, were investigated by zircon and titanite U–Pb ID-TIMS. The anorthosite contains sparse zircons showing complex U–Pb systematics reflecting events dated at 965 ± 4 and 913 ± 2 Ma, and a pronounced Caledonian metamorphic overprint. The oldest age is interpreted as the protolith age of the massif anorthosite. We propose that the Jotun anorthosite is related to 970–960 Ma magmatism in the Western Gneiss Region and coeval, orogen-perpendicular extension. Conversely, a 930 Ma high-grade metamorphic event in the Jotun Nappe Complex and the related Lindås Nappe is likely related to formation of the autochthonous ca. 930 Ma Rogaland anorthosite complex. We suggest that the two late- to post-orogenic AMCG events reflect two instances of lithospheric foundering below the orogen separated by ca. 20–30 my. The 913 ± 2 Ma metamorphic episode appears to date a heating event restricted to the outermost edge of the Western Gneiss Region. Leucosome formation in high-grade gneisses geographically close to the Jotun anorthosite is dated at 892 ± 4 Ma and suggested to reflect CO₂-rich (?) fluid flux along shear zones.     

A massif‐type (~1.86 Ga) anorthosite complex in the Yeongnam Massif,Korea: late‐orogenic emplacement associated with the mantle delamination in the North China Craton

To unravel the petrogenesis of a massif‐type anorthosite in terms of the crust‐mantle geodynamics, we dated zircons separated from six samples from the Sancheong‐Hadong (SH) complex, Korea, using a sensitive high‐resolution ion microprobe. The weighted mean ²⁰⁷Pb/²⁰⁶Pb age of two anorthosites is 1862 ± 2 Ma, whereas the ages of the hornblende gabbro and granitic gneiss are 1873 ± 4 Ma and 1875 ± 5 Ma respectively. Zircon rims from mafic granulite and migmatitic gneiss yielded ages of 1860 ± 5 Ma and 1858 ± 4 Ma, respectively, implying that the granulite facies metamorphism and anatexis are associated with anorthosite emplacement. Our results, together with available Re–Os data, are compatible with the ~1.9–1.86 Ga collisional orogeny prevalent in the North China Craton and the Korean Peninsula, and suggest that orogenesis was accompanied by mantle delamination beneath the craton. It is thus likely that the SH anorthositic rocks are a product of late‐orogenic magmatism during the post‐collisional extension‐dominated phase of orogeny.     

U–Pb ID-TIMS dating of igneous and metaigneous rocks from the El-Sibai area: time constraints on the tectonic evolution of the Central Eastern Desert,Egypt

This paper presents new ID-TIMS U–Pb zircon and titanite ages from the El-Sibai gneiss complex in the Eastern Desert of Egypt. The zircon data support previous studies, indicating that the protoliths of the gneissic (oldest) units in the area were emplaced during the East African orogeny, and do not represent an older pre-Neoproterozoic, reworked cratonic basement. The crystallization ages of three compositionally distinct orthogneiss protoliths are c. 685, 682 and 679 Ma, respectively. A U–Pb titanite age from one orthogneiss overlaps with the protolith age, indicating that the gneisses did not undergo post-magmatic high-temperature metamorphism. The gneissic textures of the rocks are therefore interpreted to reflect syn-emplacement deformation. This, and evidence for static amphibolite facies metamorphism in country-rock metavolcanics, lead us to conclude that the gneisses of El-Sibai do not represent an exhumed middle crustal gneiss dome, but are part of the island arc affined allochthon into which they were emplaced synchronously with NW-ward nappe translation. We also report ages from rocks cross-cutting the gneisses and the surrounding island arc affined assemblages that yield the hitherto youngest robust pre-Cretaceous intrusive ages in the Eastern Desert. The dated rocks are an anorthosite and a cross-cutting syenogranite giving ages of c. 541 and 540 Ma, respectively. We consider this late magmatic pulse to be anorogenic, most likely reflecting a separate extensional event involving asthenospheric upwelling and decompression melting of the mantle.