Zircon U–Pb age, trace element and Hf isotope composition of Kongling terrane in the Yangtze Craton: refining the timing of Palaeoproterozoic high-grade metamorphism

U–Pb age, trace element and Hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the Kongling terrane in the northern part of the Yangtze Craton. The zircon shows distinct morphological and chemical characteristics. Most zircon in an amphibolite shows oscillatory zoning, high Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, high formation temperature, high trace element contents, clear negative Eu anomaly, as well as HREE-enriched patterns, suggesting that it is igneous. The zircon yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2857 ± 8 Ma, representing the age of the magmatic protolith. The zircon in the other two samples is metamorphic. It has low Th/U ratios, low trace element concentrations, variable HREE contents (33.8 ≥ Lu_N≥2213; 14.7 ≤ Lu_N/Sm_N ≤ 354) and ¹⁷⁶Lu/¹⁷⁷Hf ratios (0.000030–0.001168). The data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions. The metamorphic zircon yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2010 ± 13 Ma. These results combined with previously obtained Palaeoproterozoic metamorphic ages suggest a c. 2.0 Ga Palaeoproterozoic collisional event in the Yangtze Craton, which may result from the assembly of the supercontinent Columbia. The zircon in two samples yields weighted mean two-stage Hf model ( T _DM2) ages of 3217 ± 110 and 2943 ± 50 Ma, respectively, indicating that their protoliths were mainly derived from Archean crust.     

Timing and exhumation of eclogite facies shear zones, Musgrave Block, central Australia

Timing constraints on shear zones can provide an insight into the kinematic and exhumation evolution of metamorphic belts. In the Musgrave Block, central Australia, granulite facies gneisses have been affected, to varying degrees, by mylonitic deformation, some of which attained eclogite facies. The Davenport Shear Zone is a dominant strike-slip system that formed at eclogite facies conditions ( T  ≈650  °C and P ≈12.0  kbar). Sm–Nd mineral isochrons obtained from equilibrated high-pressure assemblages, as well as ⁴⁰Ar–³⁹Ar data, show that the eclogite and greenschist facies high-strain overprints were coeval, at c .  550  Ma. Mylonitic processes do not appear to have reset the U–Pb system in zircon, but may have partially disturbed it. The thermal gradient in the Musgrave Block crust at c .  550  Ma was c .  16  °C  km⁻¹ and at c .  535  Ma was c .  18  °C  km⁻¹, based on P – T  estimates of eclogite and greenschist facies shear zones, respectively. These estimates are similar to present-day geothermal gradients in many stable continental shield areas, suggesting that the region did not undergo a significant transient perturbation of the geotherm. Therefore, in the Musgrave Block, cooling subsequent to eclogite facies metamorphism appears to have been controlled by exhumation, rather than by the removal of a heat source. Estimated exhumation rates in the range 0.2 to ≥1.5  mm year⁻¹ are comparable with other orogenic belts, rather than cratonic areas elsewhere.     

Pe trology and SHRIMP zircon geochronology of granulites from Vesleknausen,Lützow-Holm Complex,East Antarctica: Neoarchean magmatism and Neoproterozoic high-grade metamorphism

We report new petrological data and geochronological measurements of granulites from Vesleknausen in the highest-grade section of the L＆#252;tzow-Holm Complex, part of the Gondwana-assembling collisional orogen in East Antarctica. The locality is dominated by felsic to intermediate orthogneiss （charnockite and minor biotite gneiss）, mafic orthogneiss, and hornblende-pyroxene granulite, with deformed and undeformed dykes of metagranite and felsic pegmatite. Pseudosection analysis of charnockite in the system NCKFMASHTO, supported by geothermometry of mafic orthogneiss, was used to infer peak metamorphic temperatures of 750e850 ？C, approximately 150 ？C lower than those estimated for met-asedimentary gneisses from Rundv？gshetta, 6 km to the northeast. SHRIMP U-Pb analysis of zircons from feldspar-pyroxene gneiss, which corresponds to a partially molten patch around mafic orthogneiss, yielded a Concordia upper intercept ages of 2507.9 ？ 7.4 Ma, corresponding to the time of formation of the magmatic protolith to the orthogneiss. Partial melting during peak metamorphism probably took place between 591 and 548 Ma, as recorded in rims overgrew around magmatic zircon. Our results suggest that Rundv？gshetta-Vesleknausen-Strandnibba region in southwestern L＆#252;tzow-Holm Bay, where orthogneisses are dominant, consists of a single crustal block, possibly formed by ca. 2.5 Ga arc mag-matism. The Neoarchean magmatic terrane was tectonically mingled with other fragments （such as metasedimentary units in northern L＆#252;tzow-Holm Bay） by subduction/collision events during the as-sembly of Gondwana supercontinent, and subsequently underwent w850 ？C granulite-facies meta-morphosed during Neoproterozoic to Cambrian final collisional event.     

阿拉善变质基底古元古代晚期的构造热事件

阿拉善地块可分为太古宙的叠布斯格杂岩(岩群)、古元古代的巴彦乌拉山杂岩(岩组)和古-中元古代的阿拉善杂岩(岩群)、波罗斯坦庙片麻杂岩和毕级格台片麻杂岩。太古宙的叠布斯格岩群和片麻岩中的锆石记录了1926Ma和1802Ma的变质事件年龄,角闪石记录了1918Ma的变质事件年龄,斜长石记录了1722Ma的冷却年龄。同时,在波罗斯坦庙变形的片麻状杂岩中的锆石记录了1839Ma的岩浆事件年龄。该区古元古代晚期的构造热事件,可初步将划分为2000～1900Ma的早期事件和1850～1800Ma的晚期事件,并在时间和特点上与大青山-集宁-大同一带变质地体中广泛发育古元古代晚期的岩浆-变质事件非常相似,进而表明,华北克拉通西部陆块中的古元古代晚期造山带(孔兹岩带)向西可延伸到阿拉善东部地区。     

Source of coesite inclusions within inherited magmatic zircon from Sulu UHP rocks, eastern China, and their bearing for fluid–rock interaction and SHRIMP dating

The 5-km deep Chinese Continental Scientific Drilling Main Hole penetrated a sequence of ultrahigh pressure (UHP)-metamorphic rocks consisting mainly of eclogite, gneiss and garnet-peridotite with minor schist and quartzite. Zircon separates taken from thin layers of schist and gneiss within eclogite were investigated. Cathodoluminescence images of zircon grains show that they have oscillatory zoned magmatic cores and unzoned to patchy zoned metamorphic rims. Zircon rims contain rare coesite and calcite inclusions whereas cores contain inclusions of both low- P minerals (e.g. feldspar, biotite and quartz) and coesite and other eclogite-facies minerals such as phengite and jadeite. The zircon cores give highly variable ²⁰⁶Pb/²³⁸U ages ranging from 760 to 431 Ma for schist and from 698 to 285 Ma for gneiss, and relatively high but variable Th/U ratios (0.16–1.91). We suggest that the coesite and other eclogite facies mineral inclusions in zircon cores were not magmatic but formed through metasomatic processes caused by fluids during UHP metamorphism, and that the fluids contain components of SiO₂, Al₂O₃, K₂O, FeO, MgO, Na₂O and H₂O. Metasomatism of the Sulu UHP rocks during continental subduction to mantle depths has partly altered magmatic zircon cores and reset isotopic systems. This study provides key evidence that mineral inclusions within magmatic zircon domains are not unequivocal indicators of the formation conditions of the respective domain. This finding leads us to conclude that the routine procedure for dating of metamorphic events solely based on the occurrence of mineral inclusions in zoned zircon could be misleading and the data should be treated with caution.     

Petrology,geochemistry, and zircon U-Pb geochronology of the Zambezi Belt in Zimbabwe: Implications for terrane assembly in southern Africa

The Zambezi Belt in southern Africa has been regarded as a part of the 570-530 Ma Kuunga Orogen formed by a series of collision of Archean cratons and Proterozoic orogenic belts.Here,we report new petrological,geochemical,and zircon U-Pb geochronological data of various metamorphic rocks(felsic to mafic orthogneiss,pelitic schist,and felsic paragneiss) from the Zambezi Belt in northeastern Zimbabwe,and evaluate the timing and P-T conditions of the collisional event as well as protolith formation.Geochemical data of felsic orthogneiss indicate within-plate granite signature,whereas those of mafic orthogneiss suggest MORB,ocean-island,or within-plate affinities.Metamorphic P-Testimates for orthogneisses indicate significant P-T variation within the study area(700-780 C/6.7-7.2 kbar to 800-875 C/10-11 kbar) suggesting that the Zambezi Belt might correspond to a suture zone with several discrete crustal blocks.Zircon cores from felsic orthogneisses yielded two magmatic ages:2655±21 Ma and 813士5 Ma,which suggests Neoarchean and Early Neoproterozoic crustal growth related to within-plate magmatism.Detrital zircons from metasediments display various ages from Neoarchean to Neoproterozoic(ca.2700-750 Ma).The Neoarchean(ca.2700-2630 Ma) and Paleoproterozoic(ca.2200-1700 Ma) zircons could have been derived from the adjacent Kalahari Craton and the Magondi Belt in Zimbabwe,respectively.The Choma-Kalomo Block and the Lufilian Belt in Zambia might be proximal sources of the Meso-to Neoproterozoic(ca.1500-950 Ma) and early Neoproterozoic(ca.900-750 Ma) detrital zircons,respectively.Such detrital zircons from adjacent terranes possibly deposited during late Neoproterozoic(744-670 Ma),and subsequently underwent highgrade metamorphism at 557-555 Ma possibly related to the collision of the Congo and Kalahari Cratons during the latest Neoproterozoic to Cambrian.In contrast,670-627 Ma metamorphic ages obtained from metasediments are slightly older than previous reports,but consistent with~680-650 Ma metamorphic ages reported from different parts of the Kuunga Orogen,suggesting Cryogenian thermal events before the final collision.     

An early Miocene age for a high-temperature event in gneisses from Zabargad Island (Red Sea,Egypt): mantle diapirism?

U–Pb zircon data from a felsic gneiss located at the contact zone with the central peridotite body of Zabargad Island (Red Sea, Egypt) provide an age of 23.2 ± 5.9 Myr consistent with the ²³⁸U–²⁰⁶Pb age of the youngest concordant grain (22.4 ± 1.3 Myr). Concordant grains indicate new zircon growth and/or resetting whereas slightly discordant analyses suggest participation of an older zircon component whose age cannot be defined precisely. SEM back-scattered imaging further reveals the occurrence of zoned domains almost completely erased by complex internal structures attributed to extensive recrystallization under metamorphic conditions. The 22.4 Myr Miocene age is thus interpreted as dating a high-temperature metamorphic event. The proximity between the studied sample and the peridotite supports previous conclusions which regard parts of the peridotites from Zabargad Island as an asthenopheric mantle diapir which intruded the thinned Pan-African continental crust during the early stages of the Red Sea opening.     

扬子陆块西南缘早前寒武纪撮科杂岩记录的多期岩浆-变质事件

撮科杂岩是最近在滇中地区发现的早前寒武纪基底杂岩,对深入探究扬子陆块早期演化具有重要意义.报道了4件代表性岩石样品的锆石U-Pb年代学和Hf同位素新数据.奥长花岗质片麻岩样品的结晶年龄为2 845±33 Ma,具有正的锆石ε_Hf（t）值（1.7~4.6）和相对年轻的亏损地幔二阶段（T_DM2）模式年龄（2.97~3.12 Ga）,表明其形成于新生地壳的重熔.变二长花岗岩和片麻状花岗岩样品的结晶年龄分别为2 401±15 Ma和2 320±16 Ma,显示负的锆石ε_Hf（t）值（-6.2~-0.8）和明显老的T_DM2模式年龄（2.90~3.11 Ga）,指示其来自古老地壳物质的重熔.斜长黑云碎粒岩的变质锆石的年龄为1 948±16 Ma,结合已有变质年龄揭示一期1.96~1.95 Ga区域变质作用.扬子陆块西南缘存在太古代结晶基底,并保留了与Nuna超大陆聚合有关的多期构造-岩浆事件的记录.      

Alpine reworking of Ordovician protoliths in the Western Carpathians: Geochronological and geochemical data on the Muráñ Gneiss Complex, Slovakia

Magmatic protoliths of Ordovician age have been identified in the metamorphic rocks of the Muráñ Gneiss Complex, Veporic Unit (Central Western Carpathians). Vapor digestion single zircon U–Pb dating yields an intrusion age of 464 ± 35 Ma (upper intercept) for the granite protolith. A lower intercept age of 88 ± 40 Ma records amphibolite-facies metamorphic overprint in the Cretaceous, during the Alpine orogeny. Geochemical and isotopic data suggest crustal origin of the orthogneiss. Nd_initial are between − 2.6 and − 5.0 and T_DM^Nd between 1.3 and 1.5 Ga (two-step approach). ⁸⁷Sr / ⁸⁶Sr_initial ratios vary between 0.7247 and 0.7120, and a steep REE pattern further constrains the crustal affinity of these rocks. Associated amphibolite bodies have Nd_initial values of 6.5, ⁸⁷Sr / ⁸⁶Sr_initial ratio of 0.7017, and a flat REE pattern. They are interpreted as MORB derived metabasites. Whole-rock Pb isotope analyses define a linear array in a ²⁰⁶Pb / ²⁰⁴Pb vs. ²⁰⁷Pb / ²⁰⁴Pb diagram with an age of ca. 134 Ma, consistent with intense Alpine metamorphism and deformation.

These basement rocks of the Central Western Carpathians are interpreted as Ordovician magmatic rocks intruded at an active margin of Gondwana. They represent the eastern prolongation of Cambro–Ordovician units of the European Variscides, which were part of the peri-Gondwana superterrane and accreted to Laurussia during the Variscan orogeny. Variscan metamorphic overprint is not recorded by the isotopic data of the Muráñ Gneiss Complex. Alpine metamorphism is the most dominant overprint.     

Geochronology and geochemistry of high‐pressure granulites of the Arthur River Complex,Fiordland, New Zealand: Cretaceous magmatism and metamorphism on the palaeo‐Pacific Margin

The Arthur River Complex is a suite of gabbroic to dioritic orthogneisses in northern Fiordland, New Zealand. The Arthur River Complex separates rocks of the Median Tectonic Zone, a Mesozoic island arc complex, from Palaeozoic rocks of the palaeo‐Pacific Gondwana margin, and is itself intruded by the Western Fiordland Orthogneiss. New SHRIMP U/Pb single zircon data are presented for magmatic, metamorphic and deformation events in the Arthur River Complex and adjacent rocks from northern Fiordland. The Arthur River Complex orthogneisses and dykes are dominated by magmatic zircon dated at 136–129 Ma. A dioritic orthogneiss that occurs along the eastern margin of the Complex is dated at 154.4 ± 3.6 Ma and predates adjacent plutons of the Median Tectonic Zone. Rims on zircon cores from this sample record a thermal event at c. 120 Ma, attributed to the emplacement of the Western Fiordland Orthogneiss. Migmatitic Palaeozoic orthogneiss from the Arthur River Complex (346 ± 6 Ma) is interpreted as deformed wall rock. Very fine rims (5–20 µm) also indicate a metamorphic age of c. 120–110 Ma. A post‐tectonic pegmatite (81.8 ± 1.8 Ma) may be related to phases of crustal extension associated with the opening of the Tasman Sea. The Arthur River Complex is interpreted as a batholith, emplaced at mid‐crustal levels and then buried to deep crustal levels due to convergence of the Median Tectonic Zone arc and the continental margin.     

Apatite fission track analysis in the Argentera massif: evidence of contrasting denudation rates in the External Crystalline Massifs of the Western Alps

Apatite fission track dating from a central transect in the Argentera massif (southernmost External Crystalline Massif = ECM) yielded ages between 8.05 ± 0.6 and 2.4 ± 0.2 Myr, with a positive age/altitude correlation above 3 Ma, 1200 m. Recognising a thermal peak at c . 250°C, 33 Ma, based on stratigraphic, metamorphic and ³⁹Ar/⁴⁰Ar data, the present results suggest a slow cooling rate (8–5°C) for the Argentera massif during the Oligocene–early Pliocene. This rate compares with that from the Pelvoux massif, but contrasts with those observed in the northern ECM (Mont-Blanc and Aar: up to 14°C Myr⁻¹) for the same time interval. This can be related to the different location of the ECM within the collided European margin. At about 3–4 Ma, the denudation rate would have increased up to c . 1 mm yr⁻¹ in the Argentera massif, reaching the same value as in the Belledonne and northern ECM, likely a consequence of Penninic thrust inversion.     

U---Pb, Sr and Nd evidence for grenvillian and latest proterozoic tectonothermal activity in the spitsbergen caledonides, arctic ocean

Within the Caledonian complexes of northwestern Spitsbergen, high PT formations provide U---Pb zircon ages of 965±1 Ma of a metagranite and 955±1 Ma of a corona gabbro, indicating the influence of Grenvillian activity in the area. Various isotopic systems suggest that these rocks were partially derived by reworking of ancient crust (as old as Archaean). Eclogites and felsic agmatite indicate latest Proterozoic magmatic or metamorphic events (625₋₅⁺² and 661±2 Ma, respectively) by U---Pb zircon dating. The eclogitic metamorphism age is not fully constrained and ranges between 540 and 620 Ma; this occurred prior to the superimposed Caledonian metamorphism, indicated by a part of the K---Ar and Rb---Sr mineral cooling ages. The new data and other evidence of Precambrian tectonothermal activity on Svalbard suggest that the Early Palaeozoic and Late Proterozoic successions exposed elsewhere on Svalbard may also be underlain by Grenvillian or older basement rocks. Relationships to other Grenvillian and older terrains in the Arctic are reviewed.     

Fifty-five-million-year history of oceanic subduction and exhumation at the northern edge of the Caribbean plate (Sierra del Convento mélange, Cuba)

Petrological and geochronological data of six representative samples of exotic blocks of amphibolite and associated tonalite-trondhjemite from the serpentinitic mélange of the Sierra del Convento (eastern Cuba) indicate counterclockwise P–T paths typical of material subducted in hot and young subduction zones. Peak conditions attained were ∼750 °C and 15 kbar, consistent with the generation of tonalitic partial melts observed in amphibolite. A tonalite boulder provides a U-Pb zircon crystallization age of 112.8 ± 1.1 Ma, and Ar/Ar amphibole dating yielded two groups of cooling ages of 106–97 Ma (interpreted as cooling of metamorphic/magmatic pargasite) and 87–83 Ma (interpreted as growth/cooling of retrograde overprints). These geochronological data, in combination with other published data, allow the following history of subduction and exhumation to be established in the region: (i) a stage of hot subduction 120–115 Ma, developed upon onset of subduction; (ii) relatively fast near-isobaric cooling (25 °C Myr⁻¹) 115–107 Ma, after accretion of the blocks to the upper plate lithospheric mantle; (iii) slow syn-subduction cooling (4 °C Myr⁻¹) and exhumation (0.7 km Myr⁻¹) in the subduction channel 107–70 Ma; and (iv) fast syn-collision cooling (74 °C Myr⁻¹) and exhumation (5 km Myr⁻¹) 70–60 Ma.     

A zircon study from the Rhodope metamorphic complex, N-Greece: Time record of a multistage evolution

Zircons from an eclogite and a diamond-bearing metapelite near the Kimi village (north-eastern Rhodope Metamorphic Complex, Greece) have been investigated by Micro Raman Spectroscopy, SEM, SHRIMP and LA-ICPMS to define their inclusion mineralogy, ages and trace element contents. In addition, the host rocks metamorphic evolution was reconstructed and linked to the zircon growth domains.

The eclogite contains relicts of a high pressure stage (ca. 700 °C and > 17.5 kbar) characterised by matrix omphacite with Jd_40–35. This assemblage was overprinted by a lower pressure, higher temperature metamorphic event (ca. 820 °C and 15.5–17.5 kbar), as indicated by the presence of clinopyroxene (Jd_35–20) and plagioclase. Biotite and pargasitic amphibole represent a later stage, probably related to an influx of fluids. Zircons separated from the eclogite contain magmatic relicts indicating Permian crystallization of a quartz-bearing gabbroic protolith. Inclusions diagnostic of the high temperature, post-eclogitic overprint are found in metamorphic zircon domain Z2 which ages spread over a long period (160 – 95 Ma). Based on zircon textures, zoning and chemistry, we suggest that the high-temperature peak occurred at or before ca. 160 Ma and the zircons were disturbed by a later event possibly at around 115 Ma. Small metamorphic zircon overgrowths with a different composition yield an age of 79 ± 3 Ma, which is related to a distinct amphibolite-facies metamorphic event.

The metapelitic host rock consists of a mesosome with garnet, mica and kyanite, and a quartz- and plagioclase-bearing leucosome, which formed at granulite-facies conditions. Based on previously reported micro-diamond inclusions in garnet, the mesosome is assumed to have experienced UHP conditions. Nevertheless, (U)HP mineral inclusions were not found in the zircons separated from the diamond-bearing metapelite. Inclusions of melt, kyanite and high-Ti biotite in a first metamorphic zircon domain suggest that zircon formation occurred during pervasive granulite-facies metamorphism. An age of 171 ± 1 Ma measured on this zircon domain constrains the high-temperature metamorphic event. A second, inclusion-free metamorphic domain yielded an age of 160 ± 1 Ma that is related to decompression and melt crystallization.

The similar age data obtained from the samples indicate that both rock types recorded a high-T metamorphic overprint at granulite-facies conditions at ca. 170 – 160 Ma. This age implies that any high pressure or even ultra-high pressure metamorphism in the Kimi Complex occurred before that time. Our findings define new constraints for the geodynamic evolution for the Alpine orogenic cycle within the northernmost Greek part of the Rhodope Metamorphic Complex. It is proposed that the rocks of the Kimi Complex belong to a suture zone squeezed between two continental blocks and result from a Paleo-ocean basin, which should be located further north of the Jurassic Vardar Ocean.     

扬子地台西缘结晶基底的时代

对扬子地台西缘结晶基底变质地层以及岩浆片麻岩中变质地层残片或包裹体中锆石SHRIMP U-Pb同位素年龄研究表明,在康定地区辉长–闪长质片麻岩中,表壳岩包体—糜棱岩化的长英质片岩和宝兴地区黑云斜长角闪岩的年龄分别为816±8.6 Ma和826±13 Ma,代表了它们的原岩——酸性火山岩和火山凝灰岩的形成年龄;泸定地区变质岩层中长英质糜棱岩和斜长角闪岩的年龄分别为816±9 Ma和818±8 Ma,代表了中性火山岩和基性火山岩的形成时代;茨达地区斜长角闪岩和角闪黑云斜长片麻岩的年龄分别为830±7 Ma和827±10 Ma,代表了其原岩火山–沉积建造的形成时代。以上数据表明,所谓的结晶基底都是新元古代的产物,在形成时间上与盐边群、盐井群等褶皱基底的相一致,只是变质程度上略有差异。因此该区并不存在古老（太古宙—古元古代）的结晶基底。     

The isotopic composition of zircon and garnet: A record of the metamorphic history of Naxos, Greece

Growth of zircon with respect to that of garnet has been studied using a combination of petrography, U–Pb dating and oxygen isotope analysis. The aim is to document the mechanism and pressure–temperature conditions of zircon growth during metamorphism in order to better constrain the Tertiary metamorphic history of Naxos, Greece. Two metamorphisms are recognised: (1) an Eocene Franciscan metamorphism (M1) and (2) a widespread Miocene Barrovian metamorphism (M2) that increases from greenschist facies up to partial melting. An amphibolite sample contains zircon crystals characterised by a magmatic core and two metamorphic rims, denoted as A and B, dated at 200–270, 42–69, and 14–19 Ma, respectively. The first metamorphic rim A (δ¹⁸O = 7 ± 1‰) preserves the δ¹⁸O value of the magmatic core (6.2 ± 0.8‰), whereas rim B is characterised by higher δ¹⁸O values (7.8 ± 1.8‰). These observations indicate the formation of A rims by solid-state recrystallisation in a closed system with regard to oxygen and those of B in an open system. Compositional zoning in garnet is interpreted as the result of decompressional heating. Zircon B rims and garnet rims display similar δ¹⁸O values which indicates a contemporaneous growth of garnet and zircon rims during the Miocene Barrovian event (M2). Calcic gneiss and metapelite samples contain zircon crystals with single metamorphic overgrowths aged 41–57 Ma. δ¹⁸O values measured in zircon overgrowths (11.8 ± 1.4‰) from the calcic gneiss are similar to those measured in garnet rims (11.4 ± 1.1‰) from the same rock. This suggests that garnet rims and zircon overgrowths grew during the high pressure–low temperature event in equilibrium with prograde fluids. In the metapelite sample, δ¹⁸O values are similar in garnet cores (14.8 ± 0.2‰) and in zircon metamorphic overgrowths (14.2 ± 0.5‰). As zircon overgrowths have been dated at ca. 50 Ma by U–Pb, garnet cores and zircon overgrowths are interpreted to have grown during the high pressure event.

As demonstrated here for the island of Naxos, correlating the crystallisation of zircon with that of metamorphic index minerals such as garnet using stable isotope composition and U–Pb determination is a powerful tool for deciphering the mechanism of zircon growth and pin-pointing zircon crystallisation within the metamorphic history of a terrain. This approach is potentially hampered by an inability to verify the degree of textural equilibrium of zircon with other mineral phases, and the possible preservation (in metamorphic rims) of isotopic signatures from pre-existing zircon when they form by recrystallisation. Nevertheless, this study illustrates the application of this approach in providing key constraints on the timing and mechanism of growth of minerals important to understanding metamorphic petrogenesis.     

肥东杂岩锆石U-Pb年龄和Lu-Hf同位素:对扬子克拉通统一结晶基底的限制

扬子克拉通广泛存在太古宙-古元古代的结晶基底,但关于扬子克拉通早前寒武纪基底形成和演化的基本问题尚未完全解决,特别是基底是由多个块体拼合而成还是一个统一的结晶基底尚无定论.对扬子北缘张八岭隆起内肥东杂岩的片麻岩、斜长角闪岩及侵入其中的花岗岩进行了同位素地质年代学和地球化学研究,发现片麻岩具有相似的原岩年龄,分别为2 449±17 Ma、2 444±15 Ma、2 495±23 Ma、2 478±24 Ma,斜长角闪岩和A型花岗岩的原岩年龄分别为2 032±35 Ma和747±4 Ma,表明肥东杂岩出露有扬子克拉通晚太古-古元古代的结晶基底.片麻岩锆石Hf同位素组成相似,ε_Hf（t）平均值为-3.6±0.5,两阶段Hf模式年龄（T_DM2）范围主要在3.3~3.0 Ga,平均值为3 185±31 Ma,说明这一期约为2.45~2.50 Ga的岩浆活动主要是对扬子北缘古-中太古代地壳的再造.扬子北缘的鱼洞子群和陡岭杂岩都经历了相同性质的岩浆事件,同时扬子克拉通广泛出现约2.50 Ga的碎屑锆石和捕获锆石,表明扬子克拉通结晶基底是统一的.      

塔里木西南大陆边缘原特提斯洋构造演化

通过对塔里木西南地区奥陶系-泥盆系的钻井和露头碎屑岩样品开展锆石U-Pb年代学、地球化学和重矿物分析,探讨塔西南大陆边缘原特提斯洋的俯冲增生演化过程.碎屑锆石记录了482~443 Ma、438~425 Ma和414~406 Ma三期寒武纪以来的构造-热事件,以及840~750 Ma的新元古代裂谷岩浆事件.奥陶系-泥盆系碎屑岩具有高的SiO₂含量和相对低的Al₂O₃和TFe₂O₃+MgO含量,以及弱-中等的Eu负异常、LREE相对富集和HREE分布较平坦的特征.地球化学图解反映晚奥陶统碎屑岩来源于大陆岛弧和活动大陆边缘环境,早志留世-中泥盆世期间物源区可能以大陆岛弧和被动陆缘环境为主,还间断出现少量活动大陆边缘环境,晚泥盆世以后主要呈现被动陆缘环境.重矿物组合指示早志留世至中泥盆世期间,中-酸性岩浆活动加剧,而基性岩浆活动趋弱,晚泥盆世以后陆壳基底或造山带已大规模隆升.塔西南边缘原特提斯洋构造演化以奥陶纪向北俯冲、志留纪弧后洋盆关闭-褶皱造山和泥盆纪后碰撞伸展为特征.      

云开地块印支期构造热事件叠加改造:来自片麻岩中多矿物U-Pb年代学的证据

云开地块显生宙的构造演化一直是华南板块的研究热点之一.通过对云开地块变质基底中片麻岩进行独居石、磷灰石和锆石LA-ICP-MS U-Pb定年,获得了434±6 Ma（MSWD=3.0）和437±12 Ma（MSWD=6.0）的岩浆锆石年龄,结合镜下观察,认为其代表的是片麻岩原岩（花岗质岩石）的形成时代,与区域上加里东期构造-岩浆-变质-深熔事件的时限基本一致,为武夷-云开加里东造山作用的产物;磷灰石经过普通铅校正后分别给出了241±13 Ma（MSWD=2.2）和242±6 Ma（MSWD=1.4）的变质年龄,独居石则分别获得了236.2±0.8 Ma（MSWD=0.66）和229.4±1.2 Ma（MSWD=2.4）的变质年龄,这些年龄结果与云开地块构造岩云母Ar-Ar年龄相一致,暗示云井地块经历了印支期（230~240 Ma）构造热事件的叠加改造.对片麻岩中黑云母电子探针数据分析显示其具有相对高的Al、Ti和Mg含量,X_Mg值为0.37~0.45,属于铁质黑云母,其Ti饱和温度计给出温度范围为650~700℃（±50℃）,与独居石估算温度580~640℃（±25℃）在误差范围内基本一致,暗示印支期构造热事件对云开地块的叠加改造温度达到了700℃.综上所述,认为云开地块现今的构造格局是在加里东期造山作用基础上叠加了印支期构造塑造而成,叠加温度可能达到了700℃;此外,在利用矿物对温压计恢复加里东期变质作用P-T轨迹时应着重考虑印支期构造热事件对矿物成分的再造作用.      

北天山温泉群的地质特征、时代和构造意义

北天山温泉群长期以来被认为是古元古代的变质岩。最新的野外调查和锆石SHRIMP U-Pb 测年结果表明,温泉群可 以划分为三个岩石构造单元：（1）前早新元古代变质火山岩和变质沉积岩,主要包括斜长角闪岩、云母片岩、石英片岩、 黑云母片麻岩、大理岩等;（2）早新元古代混合岩和正片麻岩;（3）早古生代未变质变形的辉长岩和闪长岩。上述三种岩 石组合类型均被后期二云母花岗岩（脉）所侵入。温泉县以南的混合岩和花岗片麻岩中锆石的SHRIMP U-Pb 年龄分别为 926±12 Ma 和907±11 Ma,与天山地区出露的新元古代花岗岩类的时代基本一致。结合前人对花岗片麻岩Nd 同位素组成 的研究,花岗片麻岩应为古老地壳物质部分熔融的产物,而同期的混合岩化作用则是新元古代地壳加厚和部分熔融的直接 地质证据。辉长岩和闪长岩侵入到温泉群花岗片麻岩和斜长角闪岩中,其中闪长岩的锆石SHRIMP U-Pb 年龄为452±7 Ma, 并含有1.1 Ga 和1.4 Ga 的继承锆石。根据前人的研究成果,本区早古生代辉长岩和闪长岩具有岛弧岩浆岩的地球化学特征, 可能与准噶尔-巴尔喀什洋的俯冲作用有关,这一俯冲增生作用最终导致伊犁北部与哈萨克斯坦陆块在志留纪拼贴造山, 并使温泉群前寒武纪变质岩与侵入岩发生变质变形作用。