Metamorphic Overgrowths Research Articles

Dating prograde fluid pulses during subduction by in situ U–Pb and oxygen isotope analysis

The Dora-Maira whiteschists derive from metasomatically altered granites that experienced ultrahigh-pressure metamorphism at ~750 °C and 40 kbar during the Alpine orogeny. In order to investigate the P–T–time–fluid evolution of the whiteschists, we obtained U–Pb ages from zircon and monazite and combined those with trace element composition and oxygen isotopes of the accessory minerals and coexisting garnet. Zircon cores are the only remnants of the granitic protolith and still preserve a Permian age, magmatic trace element compositions and δ18O of ~10 ‰. Thermodynamic modelling of Si-rich and Si-poor whiteschist compositions shows that there are two main fluid pulses during prograde subduction between 20 and 40 kbar. In Si-poor samples, the breakdown of chlorite to garnet + fluid occurs at ~22 kbar. A first zircon rim directly overgrowing the cores has inclusions of prograde phlogopite and HREE-enriched patterns indicating zircon growth at the onset of garnet formation. A second main fluid pulse is documented close to peak metamorphic conditions in both Si-rich and Si-poor whiteschist when talc + kyanite react to garnet + coesite + fluid. A second metamorphic overgrowth on zircon with HREE depletion was observed in the Si-poor whiteschists, whereas a single metamorphic overgrowth capturing phengite and talc inclusions was observed in the Si-rich whiteschists. Garnet rims, zircon rims and monazite are in chemical and isotopic equilibrium for oxygen, demonstrating that they all formed at peak metamorphism at 35 Ma as constrained by the age of monazite (34.7 ± 0.4 Ma) and zircon rims (35.1 ± 0.8 Ma). The prograde zircon rim in Si-poor whiteschists has an age that is within error indistinguishable from the age of peak metamorphic conditions, consistent with a minimum rate of subduction of 2 cm/year for the Dora-Maira unit. Oxygen isotope values for zircon rims, monazite and garnet are equal within error at 6.4 ± 0.4 ‰, which is in line with closed-system equilibrium fractionation during prograde to peak temperatures. The resulting equilibrium ∆18Ozircon-monazite at 700 ± 20 °C is 0.1 ± 0.7 ‰. The in situ oxygen isotope data argue against an externally derived input of fluids into the whiteschists. Instead, fluid-assisted zircon and monazite recrystallisation can be linked to internal dehydration reactions during prograde subduction. We propose that the major metasomatic event affecting the granite protolith was related to hydrothermal seafloor alteration post-dating Jurassic rifting, well before the onset of Alpine subduction.

U–Pb and Hf isotope records in detrital and magmatic zircon from eastern and western Dharwar craton, southern India: Evidence for coeval Archaean crustal evolution

U–Pb and Lu–Hf isotopic records in detrital and magmatic zircons from the eastern and the western Dharwar craton (EDC and WDC) suggest presence of ≥3.4Ga crustal components in both the blocks. Magmatic zircon from the western block has ages ranging between 3.1 and 3.2Ga, while those from the eastern block show a bi-modal age distribution with records of 3.0–3.2Ga protolith components as well as 2.7 and 2.5Ga metamorphic overgrowths. Detrital and magmatic zircons from granitoid conglomerate clasts of the western Dharwar block show initial ɛHf values consistent with models for the early Archaean mantle, suggesting their juvenile character. Lower ɛHf values in younger as well as detrital zircons from eastern Dharwar block reflect importance of reworking of ≥3.2Ga crust. The combined U–Pb and Hf isotope study strengthens suggestions for contemporaneous crust formation processes in both the western and eastern parts of the Dharwar craton.

Neoarchean convergent margin tectonics associated with microblock amalgamation in the North China Craton: Evidence from the Yishui Complex

Archean tectonic history of the North China Craton (NCC) involved complex processes of amalgamation of microcontinents along multiple subduction zones prior to the consolidation of the major crustal blocks and their assembly into unified cratonic architecture. Here we report a suite of granitoids, diabase, metabasalts, volcanic tuff, banded iron formations and quartzite from the Yishui Complex along the southern margin of the Jiaoliao microblock within the Eastern Block of the NCC. The geochemical features of the magmatic suite are consistent with calc-alkaline magmatism in a convergent margin setting. In tectonic discrimination diagrams, the mafic suite shows variable IAB, MORB and OIB affinities typical of rocks formed in an arc-related subduction environment. Zircon grains in most of the rocks from Yishui Complex display core–rim texture with the cores showing magmatic crystallization and the narrow structureless rims corresponding to metamorphic overgrowth. The 207Pb/206Pb ages of magmatic zircons show 2504±19Ma for the volcanic tuff, 2581±21Ma for the granitoid, 2501±19Ma for the metavolcanics, 2537±38Ma for the pyroxenite, and 2506±13Ma for the diabase. Metamorphism is constrained from the 2451±18Ma and 2466±23Ma age groups in the metavolcanics and (meta-) pyroxenites. Zircons from BIF show multiple population with the oldest showing a spot age of 2503Ma, followed by a number of distinct groups of Paleoproterozoic zircons corresponding to later thermal events. The oldest population of magmatic zircons from the quartzite shows 207Pb/206Pb mean age of 2495±24Ma. The dominantly positive εHf(t) values of the magmatic zircons from the Yishui suite are broadly consistent with a depleted mantle source with only minor input of crustal components. Their Hf crustal residence ages (TDMC) range from 2586 to 3181Ma and Hf depleted mantle model ages (TDM) are in the range of 2548–2927Ma. The data indicate that magma production involved Meso- to Neoarchean juvenile sources within a continental arc setting, suggesting the Jiaoliao microblock as one of the ancient continental nuclei in the NCC. We trace the continuity of a Neoarchean subduction system along the western and southern margins of the Jiaoliao microblock with convergence of the Qianhuai and Xuhuai microblocks towards the Jiaoliao microblock with subduction–accretion–collision during the Archean–Proterozoic transition.

Paleoproterozoic magmatic and metamorphic events link Yangtze to northwest Laurentia in the Nuna supercontinent

Zircons from granitic gneisses in North Vietnam have magmatic cores dated at 2.28–2.19 Ga, and constitute the first reported evidence of continental crust with these ages in the Yangtze Block of the South China Craton. Overgrowths on zircon rims indicate two periods of metamorphism at 1.97–1.95 Ga and ∼1.83 Ga. These events, along with a previously reported ∼2.36 Ga metamorphic overgrowth on ∼2.9 Ga crystallized zircons from the same region, suggest a sequence of events similar to that recorded for the northwestern region of Laurentia and possibly Siberia, which are associated with assembly of the Nuna supercontinent. These include the 2.4–2.3 Ga Arrowsmith Orogen and a range of events in the interval 2.32–1.80 Ga, including accretionary magmatism in northwestern Laurentia and Siberia (2.32–2.07 Ga), the Thelon orogeny (2.02–1.96 Ga) and the 1.85–1.80 Ga collision between the Superior and Hearne–Rae cratons during the Trans-Hudson Orogen in Laurentia, and the Akitkan Orogen in Siberia (2.03–1.86 Ga). Subsequent attempted breakup of Nuna may be represented by ca. 1.80 to 1.59 Ga consanguineous extension related sedimentation and magmatism in the southwestern Yangtze Block and northwestern Laurentia. These correlations favor location of the Yangtze Block adjacent to northwest Laurentia, and possibly Siberia, within the Nuna supercontinent.

Maximum sedimentation ages and provenance of metasedimentary rocks from Tinos Island, Cycladic blueschist belt, Greece

U–Pb zircon ages of five metasedimentary rocks from the Lower Unit on Tinos Island (Cycladic blueschist belt, Greece) document supply of detritus from various Proterozoic, Paleozoic and Mesozoic source rocks as well as post-depositional metamorphic zircon formation. Essential features of the studied zircon populations are Late Cretaceous (70–80 Ma) maximum sedimentation ages for the lithostratigraphic succession above the lowermost dolomite marble, significant contributions from Triassic to Neoproterozoic source rocks, minor influx of detritus recording Paleoproterozoic and older provenance (1.9–2.1, 2.4–2.5 and 2.7–2.8 Ga) and a lack or paucity of zircons with Mesoproterozoic ages (1.1–1.8 Ga). In combination with biostratigraphic evidence, the new dataset indicates that Late Cretaceous or younger rocks occur on top of or very close to the basal Triassic metacarbonates, suggesting a gap in the stratigraphic record near the base of the metamorphic succession. The time frame for sediment deposition is bracketed by the youngest detrital zircon ages (70–80 Ma) and metamorphic overgrowths that are related to high-pressure/low-temperature overprinting in the Eocene. This time interval possibly indicates a significant difference to the sedimentation history of the southern Cyclades, where Late Cretaceous detrital zircons have not yet been detected.

Neoproterozoic ophiolite and related high-grade rocks of the Baikal–Muya belt, Siberia: Geochronology and geodynamic implications

We report zircon for from ophiolitic and high-grade rocks of the Neoproterozoic Baikal–Muya belt of Siberia that occupies an arc-shaped area on the southeastern margin of the Siberian craton. It consists of arc-related plutonic, metavolcanic and metasedimentary rocks as well as fragmented ophiolites and high-grade metamorphic assemblages. Magmatic zircons from two plagiogranite dyke samples of the Mamakan ophiolite complex in the Sredne–Mamakan massif of the eastern Baikal–Muya belt yielded similar and concordant SHRIMP mean 206Pb/238U ages of 640.0±4.1 and 650±6Ma, respectively, that reflect the time of dyke emplacement and from which we suggest an age of ca. 645Ma as the most likely time of ophiolite formation.Enderbitic gneisses of the North Baikal area, in the western part of the Baikal–Muya belt, contain complex zircon populations that reflect variable recrystallization, Pb-loss and metamorphic overgrowth during granulite-facies metamorphism. LA-ICP-MS dating of these zircons yielded inconclusive results that led us to undertake a detailed study of cathodoluminescence images combined with U–Pb SHRIMP dating. Well-preserved magmatic domains in zircons from enderbite sample 2821 yielded concordant results with a mean 206Pb/238U age of 640±5Ma, slightly higher but broadly comparable with the data obtained by LA-ICP-MS. The zircon populations of two more enderbitic gneiss samples are more complex, and their LA-ICP-MS data constitute broad swaths along concordia between ca. 840 and 600Ma, reflecting two end-member isotopic components, namely an igneous crystallization event at ca. 800Ma and a Pb-loss and recrystallization event at ca. 600Ma. SHRIMP analyses of magmatic zircon domains of these samples yielded concordant data with identical mean 206Pb/238U ages of 826±7.5Ma and 826±8Ma, respectively, whereas low-U metamorphic rims crystallized at 640±7Ma. Newly crystallized ball-round metamorphic zircons in one sample produced a mean 206Pb/238U age of 640±6Ma. We suggest that the protoliths of the enderbitic gneisses crystallized at 826±7.5Ma and experienced granulite-metamorphism at 640±6Ma. The LA-ICP-MS analyses are fully compatible with this interpretation.Our geochronological data and previously published ages for Neoproterozoic igneous rocks of the Baikal–Muya belt define two age groups at 830–780 and 650–640Ma. We interpret that the older group reflects the evolution of a large arc system in the Baikal–Muya belt and the eastern Sayan–northwestern Mongolia region, whereas the younger group documents collision between the above arc system and the southern margin of the Siberian craton.

Unraveling the tectonic evolution of a Neoproterozoic–Cambrian active margin in the Ribeira Orogen (Se Brazil): U–Pb and Lu–Hf provenance data

The Neoproterozoic–Ordovician Central Ribeira Orogen (southeastern portion), in SE Brazil, presents two contrasting tectonic domains in its southern portion: (a) the arc domain constituted of Neoproterozoic to Paleozoic magmatic rocks and low P-high T metamorphic para (São Fidelis Group)- and ortho-derived units (in Oriental Terrane); and (b) the basement domain, constituted of a Paleoproterozoic and Neoproterozoic medium P-high T metamorphic para (Palmital–Búzios succession)- and ortho-derived units (in Cabo Frio Tectonic Domain). Our work focuses on paraderived rocks sequences from both domains. The provenance analysis using U–Pb and Lu–Hf in zircon grains is presented here as an effective tool to unravel the paleogeography and nature of the pre-collisional sedimentary basins. We performed 505 analyses (U–Pb) on detrital zircon grains and some metamorphic overgrowths from six paragneiss samples. Besides, 141 analyses (Lu–Hf) in six samples only on the detrital zircon grains domains. All samples present a main peak from Neoproterozoic sources (750–570Ma) and the other minor peak in the Stenian/Tonian periods (1200–850Ma), this indicate an orogenic contribution for this basin. Scarce register from the Mesoproterozoic and two peaks in the Archean/Paleoproterozoic (2.6 and 1.9Ga) are recognized as a contribution from an ancient continent. The Lu–Hf data reveals a juvenile source for the detrital zircon grains from Buzios succession while Palmital and São Fidelis Group units show a main crustal signature for their detrital zircon population. Based on the U–Pb and Lu–Hf data presented here, plus petrological data, geological correlations, and compilation of data from literature, we propose a tectonic model for the origin of para-derived rocks from the eastern part of the Ribeira Orogen. In at least two stages, starting with an extensional environment of ca. 600Ma in a back-arc basin (Buzios succession deposition) and continuing as an active margin between 570 and 550Ma in the fore-arc and prism basin (São Fidelis Group and Palmital succession).

Hualong Complex, South Qilian terrane: U–Pb and Lu–Hf constraints on Neoproterozoic micro-continental fragments accreted to the northern Proto-Tethyan margin

The Neoproterozoic Hualong Complex dominates the South Qilian terrane. It contains mafic to felsic magmatic rocks, cherts and turbiditic sediments, some of which have been metamorphosed to amphibolite grade and is traditionally regarded as a pre-Cambrian micro-continental block rifted from the South China plate. New U–Pb geochronology of morphologically complex zircons sheds light on the history of the complex. Garnet-bearing amphibolite and quartzite have detrital zircon U–Pb main peak ages of 1.47–1.78Ga with youngest U–Pb ages of 967Ma and 964Ma. They are intruded by Neoproterozoic orthogneisses that crystallized between ca. 850 and 940Ma. Psammitic paragneisses are dominated by 940–780Ma detrital zircon populations with a youngest U–Pb weighted mean age of 721±3Ma and a main peak at 906Ma.Metamorphic overgrowths on older zircons record amphibolite facies metamorphism of the Hualong Complex and correspond to the emplacement of the younger plutonic rocks during the Early Paleozoic northward subduction of Proto-Tethyan oceanic lithosphere. 426Ma and 455Ma detrital zircon populations that originated from synkinematic granitoids dominate quartz–mica schist.Negative to positive ɛHf(t) values for both 940–850Ma granitic orthogneisses and intruding 460–410Ma granites suggest a crustal–mantle mixture source. Hf TDM1 model ages and inherited zircon U–Pb ages are the same as those for garnet-bearing amphibolites and psammitic gneiss indicating that late magmatic rocks were formed by melting of the accreted Neoproterozoic rocks.

First bedrock samples dredged from submarine outcrops in the Chukchi Borderland, Arctic Ocean

The Chukchi Borderland, a prominent bathymetric feature within the Arctic Ocean, has been interpreted as a fragment of an undeformed continental platform sequence rifted from the passive margin of Arctic Canada. Dredges collected for the U.S. Extended Continental Shelf project aboard the icebreaker U.S. Coast Guard Cutter Healy (cruise number HLY0905) recovered hundreds of kilograms of broken crystalline basement lithologies consisting of mylonitically deformed biotite-bearing amphibolite, garnet-bearing feldspathic gneiss, and augen-bearing orthogneiss from the Chukchi Border land. Metamorphic zircon within the amphibolite and associated leucogranitic seams within these rocks yielded U-Pb zircon ages between ca. 480 and 530 Ma. Garnet-bearing feldspathic gneisses contain variably discordant Mesoproterozoic zircon, ca. 600 Ma igneous zircon, and ca. 485–505 Ma metamorphic overgrowths. While we interpret these gneisses as deformed and metamorphosed granitoids, they could, instead, have a very immature sedimentary protolith. The youngest rocks sampled were K-feldspar augen orthogneisses that yield ca. 430 Ma zircon crystallization ages. Whole-rock geochemistry and Sr-Nd isotopic data indicate that the orthogneisses are I-type calc-alkaline granitoids. All of the basement rocks including the orthogneisses are variably metamorphosed and mylonitized. Collectively, the U-Pb age, geochemistry, and fabric of the dredged Chukchi Borderland basement samples indicate that they represent Neoproterozoic–Ordovician orogenic crust and Silurian arc batholithic rocks. This geologic origin is inconsistent with the Neoproterozoic to early Paleozoic passive margin history of western Arctic Canada to which the Chukchi Borderland has been previously correlated. We alternatively propose that the basement of the Chukchi Borderland is related to the peri-Laurentian composite terranes of Pearya and western Svalbard that have similar geologic histories.

The age of Au–Cu–Pb-bearing veins in the poly-orogenic Ubendian Belt (Tanzania): U–Th–total Pb dating of hydrothermally altered monazite

The age of gold-copper-lead mineralization in the Katuma Block of the Ubendian Belt remains controversial because of the lack of radiometric ages that correlate with the age of tectonothermal events of this polyorogenic belt. Previous studies reported whole rock and mineral Pb-Pb ages ranging between 1,660 and 720 Ma. In this study, we report U-Th-total Pb ages of monazite from hydrothermally altered metapelites that host the Au-Cu-Pb-bearing veins. Three types of chemically and texturally distinct types of monazite grains or zones of grains were identified: monazite cores, which yielded a metamorphic age of 1,938 +/- 11 Ma (n = 40), corresponding to known ages of a regional metamorphic event, deformation and granitic plutonism in the belt; metamorphic overgrowths that date a subsequent metamorphic event at 1,827 +/- 10 Ma (n = 44) that postdates known eclogite metamorphism (at ca. 1,880 Ma) in the belt; hydrothermally altered poikilitic monazite, formed by dissolution-precipitation processes, representing the third type of monazite, constrain the age of a hydrothermal alteration event at 1,171 +/- 17 Ma (n = 19). This Mesoproterozoic age of the hydrothermal alteration coincides with the first amphibolite grade metamorphism of metasediments in the Wakole Block, which adjoins with a tectonic contact the vein-bearing Katuma Block to the southwest. The obtained distinct monazite ages not only constrain the ages of metamorphic events in the Ubendian Belt, but also provide a link between the metamorphism of the Wakole metasediments and the generation of the hydrothermal fluids responsible for the formation of the gold-copper-lead veins in the Katuma Block.

Zircon provenance in meta-sandstones of the São Roque Domain: Implications for the Proterozoic evolution of the Ribeira Belt, SE Brazil

Detrital zircons from quartzites of the São Roque Domain were subject of studies of crystal growth and stratigraphy, using in situ LA-ICPMS trace element geochemistry and U–Pb dating to deduce their provenance and contribute to understand the tectonic evolution of the Proterozoic metasedimentary successions of southeast Brazil. Trace element chemistry of most detrital cores indicates derivation from plagioclase-rich felsic rocks (e.g., tonalites to granodiorites); however, a small proportion may derive from gabbroic sources, as indicated by a combination of features (strong positive Ce anomaly, high LuN/SmN, low U/Yb; concave-down shape of the intermediate REE patterns). Thin metamorphic overgrowths are chemically distinct (e.g., enriched in several trace elements, especially the LREE), and were dated at 584±47Ma, reflecting the Ediacaran metamorphism and granite magmatism that affected the São Roque Domain. The detrital zircon U–Pb ages for five quartzite samples from different stratigraphic positions within the São Roque Domain are restricted to the Paleoproterozoic and Archean (∼1730–3440Ma). A striking feature of the dataset is a main peak at ∼2.2Ga which is characteristic of all samples; the age and abundance of Archean peaks allow distinguish two groups of samples: Jaraguá and Japi, with 41–43% Archean zircons and a peak at 2.6–2.7Ga; and Voturuna, Pirucaia and Serra da Viúva, with 16–21% Archean zircons and a peak close to the Archean-Paleoproterozoic boundary (2.4–2.5Ga). A limited contribution of late Paleoproterozoic (Statherian) sources places an upper limit for the deposition of the sedimentary successions that is consistent with the estimated age of the metaconglomerate-metarkose rift sequence of the São Roque Group (Boturuna Formation), based on U–Pb dating of interlayered meta-trachydacites (1.75Ga). Our data demonstrate that the different sedimentary successions of the São Roque Domain have an essentially continental (cratonic) provenance which is very similar to those of other sequences in the Ribeira Belt (Açungui and possibly also Embu Domain). The ∼2.2Ga age of the volumetrically most important sources, also indicated by the age of granite pebbles in metaconglomerates, is a diagnostic signature, consistent with the age of regional basement (e.g., orthogneisses from few basement nuclei in the Açungui Domain). Similarities in provenance and tectonic evolution of the metasedimentary sequences are consistent with the interpretation that the Ribeira Belt is part of the reworked border of the São Francisco Paleoplate, displaced southwestward as a result of significant dextral orogen-parallel tectonics during the Neoproterozoic collision, and may imply that a suture zone between this paleoplate and another stable continental area to the SW lies below the Phanerozoic sediments of the Paraná Basin.

Sr and Nd isotopic compositions of apatite reference materials used in U–Th–Pb geochronology

Apatite is an important common U- and Th-bearing accessory mineral in igneous, metamorphic and clastic sedimentary rocks. The advent of in situ U–Th–Pb apatite geochronology by the SIMS and LA-(MC)-ICP-MS methods has demonstrated the importance of having uniform and homogeneous reference materials. Recently, it has been shown that Sr and Nd isotopic data combined with U–Pb age and trace element concentration data can provide important constraints on apatite paragenesis because this phase usually exhibits high Sr and REE concentrations but has low Rb/Sr ratios which result in negligible corrections for the ingrowth of radiogenic Sr. However, as apatite can potentially have complex internal structures resulting from multiple thermal events, such as inherited cores and metamorphic overgrowths, requires that the Sr and Nd isotopic data should be measured with high spatial resolution. However isobaric interferences hamper the precise determination of Sr or Nd isotopic compositions in LA-MC-ICP-MS analysis. In this work we undertook in situ measurements of Sr and Nd isotopic compositions of eleven apatite reference materials (AP1, AP2, Durango, MAD, Otter Lake, NW-1, Slyudyanka, UWA-1, Mud Tank, McClure Mountain and SDG) commonly used in U–Th–Pb geochronology. Our obtained Sr and Sm–Nd isotopic compositions for these apatite samples are consistent with those values obtained by solution-based methods (isotope dilution and ion chromatography) using MC-ICP-MS or TIMS, which demonstrates the reliability and robustness of our analytical protocol.

Late Paleoproterozoic medium-P high grade metamorphism of basement rocks beneath the northern margin of the Ordos Basin, NW China: Petrology, phase equilibrium modelling and U–Pb geochronology

Basement rocks from drill cores at the northern margin of the Ordos Basin were investigated by integrating petrography, geochronological analysis and mineral equilibria modelling in the Na2OCaOK2OFeOMgOAl2O3SiO2H2OTiO2O system. The P–T conditions constrained by mineral compositional isopleths on calculated pseudosections indicate that the rocks underwent medium-pressure high grade metamorphism. The pelitic rocks reached granulite facies with peak P–T conditions of 780–810°C at c. 6.3–7.8kbar. SHRIMP U–Pb analyses of detrital zircons indicate Paleoproterozoic protoliths for the basement rocks. NanoSIMS U–Pb analyses of metamorphic overgrowths on zircon grains yield upper intercept ages of 1964±21 and 1960±21Ma for pelitic (QT1-2) and psammitic (LT1-2) samples, respectively. The basement rocks are petrologically, geochronologically and metamorphically comparable with rocks from the Khondalite Belt, which posits the existence of khondalite series rocks beneath the northern margin of the Ordos Basin. Therefore, the Paleoproterozoic mobile belt between the Yinshan and Ordos blocks is likely to be wider than previously thought.

An occurrence of the post-orogenic Triassic strata on Deokjeok Island, western Gyeonggi massif, Korea

The U-Pb isotopic compositions of zircon from a metasandstone cobble and a granitic dyke in Deokjeok Island, western Gyeonggi massif, Korea, were measured using a SHRIMP-II ion microprobe to constrain the depositional age of cobble-bearing metasedimentary strata. The metasandstone cobble, collected from a metaconglomerate of the slighltly-metamorphosed sedimentary unit (SMU), yielded 50 to 200 μm long zircon crystals showing variable internal structures together with rounded corners. Sixty spots were analysed from 60 detrital crystals, including two spots of Permian metamorphic overgrowth dated at 255 ± 4 Ma (1σ) and 263 ± 4 Ma (1σ), respectively. Except for the Permian ages, their U-Pb age distribution pattern is characterized by a few major populations at ~400 Ma, ~1.0 Ga and ~2.5 Ga, and several minor peaks between 400 Ma and 2.5 Ga. This pattern is identical to that of the Devonian Taean Formation widespread along the western Gyeonggi massif. On the other hand, the ~260 Ma overgrowth age in the cobble could represent the zircon fraction formed in the sedimentary protolith of the cobble during the Permo-Triassic collisional orogeny. Zircon crystals from a granitic dyke intruding the SMU generally showed the oscillatory zonation, and yielded the weighted mean 206Pb/238U age of 225 ± 3 (tσ), corresponding to its intrusion age. It is thus likely that the cobble-bearing strata might have deposited during the Triassic prior to 225 Ma. Moreover, the SMU unit may represent post-orogenic sediementary strata filling the extensional basin developing along the suture at the western margin of the Gyeonggi massif.

Zircon ages and Nd–Hf isotopes in UHT granulites of the Ider Complex: A cratonic terrane within the Central Asian Orogenic Belt in NW Mongolia

The Ider Complex of the Tarbagatai Block in northwestern Mongolia is part of a Precambrian microcontinental terrane in the Central Asian Orogen Belt and has experienced a polymetamorphic tectono-metamorphic evolution. We have investigated an enderbitic gneiss, derived from a quartz diorite and a charnockite, derived from a leucogranite, and zircon SHRIMP data reveal late Archaean protolith ages of 2520–2546Ma for these rocks. Metamorphic overgrowth on these zircons as well as newly-formed metamorphic zircons document a high-temperature metamorphic event (T=930–950°C) at about 1855–1860Ma. Nd whole-rock isotopic systematics show these and other gneisses of the Ider Complex straddling the CHUR-line in a Nd isotope evolution diagram, suggesting both crustal reworking and input of some juvenile material, with Nd model ages ranging between ca. 2.5 and 3.1Ga. Hf-in-zircon isotopic data provide a similar pattern and also yielded Archaean Hf crustal model ages. The metamorphic zircons seem to have inherited their Hf isotopic composition from the igneous grains, suggesting a complex process of dissolution, transportation, and re-precipitation involving a fluid phase during high-grade metamorphism. The zircon age patterns do not make it possible to unambiguously assign the Tarbagatai Block to any of the cratons bordering the Central Asian Orogenic Belt, since age peaks at ca. 2520–2550 and ca. 1860Ma are common in the Siberian, North China and Tarim cratons.

First report of Paleoproterozoic incipient charnockite from the North China Craton: Implications for ultrahigh-temperature metasomatism

The assembly of crustal blocks within the North China Craton (NCC) at the terminal stage of craton building during Paleoproterozoic witnessed major subduction-collision events. Here we illustrate a case where hot and anhydrous magmas emplaced in the lower crust during the early Paleoproterozoic subduction realm in the NCC led to the formation of orthopyroxene-bearing dehydrated zones of incipient charnockites within the adjacent TTG (tonalite-trondhjemite-granodiorite) suite. The coarse greenish anhydrous zones of incipient charnockite show an assemblage of antiperthitic plagioclase+orthopyroxene+K-feldspar+quartz with minor clinopyroxene, ilmenite+magnetite and secondary biotite. The peak P–T conditions of the incipient charnockite are estimated as 890–970°C and 3.5–6.5kbar based on mineral chemical analysis and pseudosection modeling in the system NCKFMASHTO, suggesting ultra-high temperature metasomatism, and the ‘hottest incipient charnockites’ so far reported. Zircons from the massive charnockite show oscillatory-zoned magmatic cores with high Th/U values rimmed by unzoned and luminescent thin metamorphic overgrowths. A concordant group of magmatic zircons yields weighted mean 207Pb/206Pb age of 2401±29Ma, and a lower intercept age of ca. 1834±81Ma. A similar concordant group of zircons from the TTG yields a weighted mean 207Pb/206Pb age of 2446±18Ma and a weighted mean lower intercept age of 1834±47Ma. The ca. 2.45Ga age from the TTG and 2.40 age from the charnockite are taken to represent the magma emplacement ages, and the identical 1.83Ga from zircons in both rocks denoting the timing of late Paleoproterozoic metamorphism. Lu–Hf analysis of magmatic cores of the zircons from the charnockite and the TTG show clear positive ɛHf(t) values (1.2–5.9), suggesting juvenile sources.The formation of incipient charnockite zones close to the contact with the charnockite pluton is consistent with the quantitative numerical models that predict CO2 migration in anhydrous silicate melt and metasomatic fluid from a rapidly cooling pluton. The ultra-high temperature metasomatism and solid-state reaction generating the incipient charnockites provide important insights on CO2 advection from lower crustal magmas during Paleoproterozoic orogeny in the North China Craton.

Zircon U-Pb geochronological and Hf isotopic constraints on the Precambrian crustal evolution of the north-eastern Yeongnam Massif, Korea

In situ U-Pb dating and Hf isotopic analyses were conducted on zircons extracted from Paleoproterozoic metasedimentary rocks and peraluminous (meta)granitoids in the north-eastern Yeongnam Massif, Korea. Combined with previous results, analyses from detrital zircons from the metasedimentary rocks yield a predominant age population of ca. 2.5Ga with subordinate clusters at ca. 2.7, 2.3, and 2.1Ga, and minor points older than 2.8Ga. The detrital zircons frequently have textureless rims that were overgrown at ca. 2.03–1.85Ga, indicating post-depositional thermal overprints associated with the intrusion of neighbouring granitoids and metamorphism. The (meta)granitoids are divided into three lithologic groups of banded or augen biotite gneisses (group I: Pyeonghae and Buncheon gneiss), massive cordierite or two mica granitic gneisses (group II: Icheonri and Hongjesa granitic gneiss), and a garnet-bearing leucogranite (group III: Imwon leucogranite). The best estimates of the timing of the emplacement of the first two groups are indistinguishable within their error ranges; 1980±22Ma (Pyeonghae gneiss), 1966±15Ma (Buncheon gneiss), 1985±14Ma (Icheonri granitic gneiss), and 1975±16Ma (Hongjesa granitic gneiss). The upper intercept discordia ages of ca. 1.86Ga indicated by the metamorphic overgrowth rims of zircons from the Buncheon gneiss and the Icheonri granitic gneiss agree with the emplacement age of the Imwon leucogranite (1867±6Ma). A close genetic link between the (meta)granitoids and metasedimentary rocks is demonstrated by the comparable age pattern of inherited zircon cores in the former with that of detrital zircons in the latter. The lower intercept ages of zircons indicate repeated Pb loss events in the Neoproterozoic to Paleozoic, although their exact tectonic meaning is still unclear. Most zircons have negative ɛHf values corresponding to two-stage Hf model ages (T2DM) from 3.4 to 2.7Ga, demonstrating Neoarchaean to Paleoproterozoic reworking of the Paleo- to Neoarchaean crust. Zircons from group I metagranitoids display a narrow T2DM range (2.74±0.09Ga). The Neoarchaean Hf model ages are also reduced by high-ɛHf zircons from the metasedimentary rocks and group II metagranitoids (T2DM=ca. 2.75Ga), and most zircons from the Imwon leucogranite (T2DM=2.62±0.06Ga). The protoliths of group I metagranitoids are considered to be I-type granites that was derived by infracrustal melting at depth. In contrast, the scattered Hf model ages of zircons from group II metagranitoids are suggestive of crystallisation from heterogeneous S-type magmas derived from the partial melting of supracrustal rocks. It is concluded that the Neoarchaean Era (ca. 2.75–2.62Ga) marks the most important stage of crustal formation in the north-eastern Yeongnam Massif. The Paleoproterozoic (ca. 2.50–1.98Ga) magmas from which the zircons crystallised were principally a product of crustal reworking. These Hf isotopic features generally match those reported for zircons from the North China Craton and the eastern part of the Cathaysia Block in the South China Craton, but the zircon ages determined here do not allow an indisputable correlation of the north-eastern Yeongnam Massif with Paleoproterozoic terranes in eastern China.

Detrital tourmaline as an indicator of source rock lithology: an example from the Ropianka and Menilite formations (Skole Nappe, Polish Flysch Carpathians)

Tourmaline populations studied from the Campanian–Maastrichtian part of the Ropianka (Upper Cretaceous–Paleocene) and Menilite (Oligocene) formations of the Polish Carpathians, represent a mixture of first-cycle and polycyclic grains. The tourmalines of the deposits studied display very strong resemblance in terms of optical features and chemical composition. They belong mostly to the schorl-dravite series with a minor contribution of tourmalines of foititic or Mg-foititic composition. Euhedral tourmalines originated from metasedimentary rocks, while the rounded grains crystallised in Li-poor granitic rocks or in pegmatites, Al-poor and Al-rich metasedimentary rocks. Most of the tourmalines studied crystallised during a single igneous or metamorphic event. However, tourmalines forming in evolving chemical conditions as well as polymetamorphic grains (having a metamorphic detrital core and metamorphic overgrowths) are also present. The chemical composition of the metamorphic tourmalines studied indicates their formation in medium-grade metamorphic conditions. This is supported by the crystallisation temperature of the garnet-biotite inclusion present in one of rounded metamorphic tourmalines from the Ropianka Formation. The euhedral grains derive from metasediments, directly from a massif located close to the Skole Basin. The scarcity of euhedral grains in the tourmaline populations studied suggests that their source rocks were poor in these minerals. The direct sources of rounded tourmalines (most probably polycyclic grains), may have been Paleozoic and Mesozoic sedimentary rocks of the Skole Basin foreland or crystalline rocks of remote source areas. The initial igneous and metamorphic host rocks of the tourmalines may have been crystalline domains of the Bohemian Massif and/or the crystalline basement of Brunovistulicum.

Multistage garnet in high-pressure metasediments: Alpine overgrowths on Variscan detrital grains

Garnet is commonly found in metamorphic and magmatic rocks, where its growth records the pressure-temperature history of the rock. Petrologists pay particular attention to multistage garnets, because they document the superposition of successive metamorphic stages and/or cycles in orogenic belts. Due to its considerable resistance to mechanical and chemical weathering, garnet is also commonly present as detrital grains in modern and ancient sediments. Therefore, some multistage garnets could potentially result from overgrowths on detrital grains. We report the first discovery of this type of metamorphic overgrowth on detrital garnet grains in a high-pressure metasediment (Money Unit, western Alps). The studied garnet crystals are found in metamorphic rocks derived from matrix-supported conglomerates. Garnet cores display morphological characteristics of the detrital grains and testify that crystals have been corroded as a consequence of abrasion and dissolution. Furthermore, they display a range of compositions incompatible with their growth in a single rock type, implying their provenance from diverse sources. Garnet overgrowths developed on detrital grains during a blueschist facies event, as indicated by chloritoid and rutile inclusions.

Proterozoic granulite formation driven by mafic magmatism: An example from the Fraser Range Metamorphics, Western Australia

Elevated heat flow and mafic magmatism during lithospheric extension have often been invoked as a mechanism to drive high-temperature low-pressure metamorphism that produces granulite facies mineral assemblages. Typically, however, evidence of the contemporaneous heat source, such as coeval mafic magmatism, is absent. In this study, we present pressure–temperature (P–T) pseudosection analysis combined with U–Pb isotopic data from zircon and monazite that constrain both the conditions and timing of granulite facies metamorphism in the Fraser Range Metamorphics of the Albany-Fraser Orogen in southern Western Australia. These results also elucidate the extremely rapid timing of, sequentially, deposition of sedimentary protoliths, mafic magmatism, partial melting, and metamorphism within the Fraser Zone during the Mesoproterozoic. The youngest detrital zircons, together with the magmatic ages of intrusive rocks, constrain the depositional age of the protoliths to the Fraser Range Metamorphics to between 1334 and 1293Ma. Peak metamorphic conditions at c. 1290Ma were c. 850°C at pressures of 7–9kbar. Peak metamorphism was followed by a period of isobaric cooling at pressures of c. 9kbar. U–Pb zircon ages from leucosomes and metamorphic overgrowths in the metapelitic rocks indicate crystallization of partial melts at 1290Ma, essentially coincident with the emplacement of mafic rocks at 1292Ma. In situ analyses of both matrix hosted monazite and monazite inclusions in garnet yield ages between 1285 and 1268Ma, with no significant age difference between monazite in the two textural positions. Cooling of the Fraser Zone below the Rb-Sr biotite closure temperature (∼400°C) occurred at 1260Ma. Cooling and strengthening of the Fraser Zone rendered it less susceptible to subsequent tectonic events that affected rocks to the north and south of this resistant lozenge. Only rare geochronological evidence of later events can be resolved in recrystallised monazite rims dated at 1234±17Ma.