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ABSTRACT The result of dating U–(Th)–Pb bearing minerals in metamorphic rocks frequently suffers from the interpretative relation of their growth to a specific metamorphic event. This problem occurs in the southern Pohorje Mountains. Therefore, granitic gneiss from this area was studied. Rims of zircon in these rocks gave U–Pb ages around 90 Ma interpreted as an indicator of Late Cretaceous metamorphism. However, in situ dating of monazite with LA–ICP–MS yielded Early Miocene U–Th–Pb ages consistent with contact metamorphism by magmas forming the Pohorje pluton. This is supported by geothermobarometry using compositions of garnet and muscovite in granitic gneiss. This method led to pressure–temperature conditions of 6.5 kbar and 625°C, in line with intrusion depths estimated for the southeastern part of the Pohorje pluton. Consequently, zircon was not suitable for dating the latest metamorphic event but monazite was. Furthermore, monazite dating suggests two main intrusion pulses of magmas differing by 2–3 Ma. [ABSTRACT FROM AUTHOR]

Despite extensive investigation, the tectono‐thermal evolution of the Archean crust in the Lewisian Gneiss Complex in NW Scotland (LGC) is debated. Most U–Pb zircon geochronological and metamorphic studies have focused on rocks from the central region of the mainland LGC, where granulite facies assemblages associated with the oldest (Badcallian) tectono‐metamorphic event at c. 2.75 Ga are overprinted by younger amphibolite facies assemblages related to the Inverian (c. 2.5 Ga) and subsequent Laxfordian (c. 1.9–1.65 Ga) tectono‐thermal events. In the southern and northern regions of the mainland LGC, deformation and metamorphism associated with the Laxfordian event are pervasive, although the timing and conditions are poorly constrained. Here, we present new field, petrographic and structural data, U–Pb zircon and titanite geochronology and phase equilibrium modelling of amphibolite samples from the northern and southern regions. Our field observations show that in both regions, pre‐Laxfordian structures are significantly reworked by steep NW‐striking fabrics that are themselves pervasively overprinted by co‐axial deformation and amphibolite facies metamorphism related to the Laxfordian event. In situ U–Pb titanite geochronology yields Laxfordian ages of 1853 ± 20 Ma in the southern region (P = 6–8 kbar and T = 640–690°C) and 1750 ± 20 Ma and 1776 ± 10 Ma in the northern region (P = 6–7.5 kbar and T = 740–760°C). While U–Pb dating of zircon rims from felsic gneisses in the central region shows a dominant Inverian metamorphic overprint at c. 2500 Ma, zircon rims in felsic gneisses from the northern and southern regions commonly yield Laxfordian dates as young as c. 1800 Ma. Combined, the results support the idea that, during the Palaeoproterozoic, the central region of the LGC acted as low‐strain domain, in which intense deformation and metamorphism were restricted to crustal‐scale shear zones. By contrast, in the southern and northern regions, early (c. 1.85 Ga) and late (c. 1.75 Ga) Laxfordian deformation and fluid‐mediated metamorphism were much more pervasive and at higher P–T conditions than previously proposed. The diachronous Laxfordian evolution of the southern and northern regions indicate that they reflect early and late snapshots of collisional to transpressional tectonics in the mainland LGC. The long‐lasting Laxfordian evolution documents the collision of the Rae and North Atlantic cratons during the Palaeoproterozoic amalgamation of the supercontinent Nuna, with implications for the palaeogeographic configuration of NW Scotland during Palaeoproterozoic Nuna. [ABSTRACT FROM AUTHOR]

Gneisses with anatectic characteristics from the Liansan island in the Sulu UHPM (ultra‐high pressure metamorphic) belt were studied for petrography, titanite U‐Pb dating and mineral geochemistry. Three origins of garnets are distinguished: metamorphic garnet, peritectic garnet and anatectic garnet, which are formed in the stages of peak metamorphism, retrograde anatexis and melt crystallization, respectively. The euhedral titanite has a high content of REE and high Th/U ratios, which is interpreted as indicating that it was newly‐formed from an anatectic melt. The LA‐ICP‐MS titanite U‐Pb dating yields 214–217 Ma ages for the titanite (melt) crystallization. The distribution of trace elements varies in response to the different host minerals at different stages. At the peak metamorphic stage, Y and HREE are mainly hosted by garnet, Ba and Rb by phengite, Sr, Nb, Ta, Pb, Th, U and LREE by allanite and Y, U and HREE by zircon. During partial melting, Y, Pb, Th, U and REE are released into the melt, which causes a dramatic decline of these element contents in the retrograde minerals. Finally, titanite absorbs most of the Nb, U, LREE and HREE from the melt. Therefore, the different stages of metamorphism have different mineral assemblages, which host different trace elements. [ABSTRACT FROM AUTHOR]

The Paleoproterozoic gneiss domes are important structures of the Khondalite Belt in the northwestern North China Craton. However, less attention has been paid to their formation and evolution, and it thus hampers a better understanding of the deformation history of the Khondalite Belt. In this paper, we conducted structural and geochronological studies on the Qianlishan gneiss domes of the Khondalite Belt. The field observations and zircon U–Pb dating results show that the Qianlishan gneiss domes consist of 2.06–2.01 Ga granitoid plutons in the core, rimmed by granulite facies metasedimentary rocks (khondalites) of the Qianlishan Group. Both of them were subjected to two major phases of deformation (D1–D2) in the late Paleoproterozoic. Of these, D1 deformation mainly generated overturned to recumbent isoclinal folds F1 and penetrative transposed foliations/gneissosities S1 at ~1.95 Ga. Subsequently, D2 deformation produced the NW(W)–SE(E)-trending doubly plunging upright folds F2 at 1.93–1.90 Ga, and they have strongly re-oriented S1 gneissosities, giving rise to the Qianlishan gneiss domes. Combined with previous studies, we argue that the Qianlishan gneiss domes were the products of the Paleoproterozoic collisional orogenesis between the Yinshan and Ordos Blocks. Additionally, the development of doubly plunging antiforms is considered an important dome-forming mechanism in the Khondalite Belt. [ABSTRACT FROM AUTHOR]

Contrary to what is currently known, archetypal zircon samples from gneisses and intrusive leucogranites in the Palaeoproterozoic Suhum Basin, SE Ghana, suggest the involvement of Neoarchean crustal material in the formation of the Palaeoproterozoic juvenile crust of the Birimian terranes in Ghana. The zircons dated using U–Pb dating methods and subjected to Lu–Hf isotopic analysis suggest that crustal‐forming events from different contemporaneous magmatic episodes within the Suhum Basin took place over a time interval of 139 Ma from 2224 to 2085 Ma. Whole‐rock Lu–Hf data obtained for the gneissic and leucogranitic rocks gave model ages (TDM2) ranging from 2789 to 2456 Ma with ɛHf(t) values ranging from −1.1 to +5.4. These model ages imply that the magmas that formed these rocks were sourced from the early Palaeoproterozoic juvenile mantle with substantial Neoarchean crustal reworking. [ABSTRACT FROM AUTHOR]

The Alxa Block is an important component of the North China Craton, but its metamorphic basement has been poorly studied, which hampers the understanding of the Alxa Block and the North China Craton. In this study, we conducted geochronological and geochemical studies on three TTG (tonalite–trondhjemite–granodiorite) gneisses and one granitic gneiss exposed in the Langshan area of the eastern Alxa Block to investigate their crustal evolution. The zircon U-Pb dating results revealed that the protoliths of the TTG and granitic gneisses were formed at 2836 ± 20 Ma, 2491 ± 18 Ma, 2540 ± 38 Ma, and 2763 ± 42 Ma, respectively, and were overprinted by middle–late Paleoproterozoic metamorphism (1962–1721 Ma). All gneiss samples had high Sr/Y ratios (41–274) and intermediate Mg# values (44.97–55.78), with negative Nb, Ta, and Ti anomalies and moderately to strongly fractionated REE patterns ((La/Yb)N = 10.6–107.1), slight Sr enrichment, and positive Eu anomalies, displaying features of typical high-SiO2 adakites and Archean TTGs. The magmatic zircons from the 2.84 Ga and 2.49 Ga TTG rocks had low εHf(t) values of −1.9–1.7, and −3.83–2.12 with two-stage model ages (TDMC) of 3.24–3.11 Ga and 3.10–3.01 Ga, respectively, whereas those from the 2.54 Ga TTG rock exhibited εHf(t) values ranging from −1.1 to 3.46 and TDMC from 3.0 Ga to 2.83 Ga, suggesting that the crustal materials of the basement rocks in the eastern Alxa Block were initially extracted from the depleted mantle during the late Paleoarchean to Mesoarchean era and were reworked in the late Mesoarchean and late Neoarchean era. By contrast, the Alxa Block probably had a relative younger crustal evolutionary history (<3.24 Ga) than the main North China (<3.88 Ga), Tarim (<3.9 Ga), and Yangtze (<3.8 Ga) Cratons and likely had a unique crustal evolutionary history before the early Paleoproterozoic era. [ABSTRACT FROM AUTHOR]

The Biesituobie A-type batholith in West Junggar in NW China of the Central Asian Orogenic Belt contains metapelite xenoliths derived from the contact metamorphic aureole. These xenoliths could be divided into two types: biotite-muscovite-andalusite hornfels and cordierite-alkaline feldspar-sillimanitecorundum gneiss, indicating prograde metamorphism. The phase equilibrium modeling on the cordierite-alkaline feldspar-sillimanite-corundum gneiss sample yielded a peak condition of P = 3.0-4.0 kbar, T = 760-800 °C. Similarly, the Na-in-cordierite geothermometer yielded a peak condition of T = 771-780 °C. The temperature of the peak metamorphism could be considered as a lower limit of the temperature of the Biesituobie A-type batholith. On the contrary, the Ti-in-zircon geothermometer applied to the Biesituobie A-type batholith zircons yields a mean temperature of 672 ± 22 °C. It suggested that the application of Ti-in-zircon geothermometer on A-type magma may involve a >100 °C underestimation. Zircon U-Pb dating indicates that the age of the contact metamorphism is between 263 and 286 Ma, consistent with the age of the pluton at 274 Ma. The result of 760-800 °C calculated from the contact metamorphic aureole of the Biesituobie batholith put a lower limit on considering the temperature condition of the A-type granite series from a metamorphic constraint. [ABSTRACT FROM AUTHOR]

Two episodes of late Neoarchean-early Paleoproterozoic and late Paleoproterozoic garnet-bearing leucogranite, named the Baotou and Jining–Liangcheng garnet-bearing leucogranite are recognized from the Khondalite Belt in the North China Craton. Geochemical studies show that garnet-bearing leucogranite has high Al2O3 content and FeOT/MgO ratio, with a large variation in the CaO content and K2O/Na2O ratio. Additionally, the garnet-bearing leucogranite is enriched in light rare earth elements and large ion lithophile elements and depleted in Nb, Ta, P, and Ti. However, there are some variations in Eu anomalies, as indicated by the Eu-enriched and Eu-depleted patterns. The sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb dating of the Baotou garnet-bearing leucogranite and Jining–Liangcheng garnet-bearing leucogranite show that their anatectic ages were 2.35–2.43 Ga and 1.90–1.92 Ga, respectively, with zircon εHf (t) values of −0.01–3.93 and −4.36–1.99, and two-stage model ages (TDM2) of 2.76–3.02 Ga and 2.57–2.83 Ga, respectively. U-Th-Pb SHRIMP analyses on zircon reveal ages of ca. 2.35–2.47 Ga for the Baotou metapelite gneiss and ca. 1.90–1.91 Ga for the Jining–Liangcheng metapelite gneiss, which represent the ages of tectonic metamorphic events related to the anatexis. Our results, combined with geological and petrographical observations, geochemistry, and geochronology, suggest that (1) Garnet-bearing leucogranites formed as a result of anatexis of metapelite gneisses; (2) They are mixtures of leucosomes, residual minerals and mantle-derived materials; (3) Two episodes of garnet-bearing leucogranites have been identified; (4) Both two episodes of garnet-bearing leucogranites corresponded to late Neoarchean-early Paleoproterozoic and late Palaeoproterozoic tectonothermal events, respectively. [ABSTRACT FROM AUTHOR]

Granitoid gneisses dominated by tonalitic–trondhjemitic–granodioritic (TTG) compositions, with metamorphic supracrustal rocks consisting of sedimentary and volcanic rocks, are widely exposed in the Eastern Hebei terrane, North China Craton (NCC). This study presents systematic zircon U–Pb geochronological and whole-rock geochemical data of the Neoarchean granitoid gneisses and supracrustal rocks in Eastern Hebei. Zircon U–Pb isotopic dating for the representative samples reveals that magmatic precursors of granitoid gneisses were emplaced between 2524 ± 7 and 2503 ± 12 Ma, and the protoliths of the pelitic granulites were deposited in the Late Neoarchean era. Both of them have been subjected to granulite facies metamorphism during 2508 ± 10 to 2468 ± 33 Ma, coeval with the intrusion of syenogranitic pegmatite (2488 ± 5 Ma). Zircon ages of 2.45–2.01 Ga obtained from the analyzed samples were considered mixed data from 2.53–2.48 Ga and 1.9–1.8 Ga and were chronologically meaningless. Paleoproterozoic metamorphic zircon ages of 1.9–1.8 Ga were usually neglected because of hardly being obtained from TTG gneisses and supracrustal rocks. The tectonic regime during the Neoarchean era was considered to be dominated by vertical tectonism in the Eastern Hebei terrane. [ABSTRACT FROM AUTHOR]

The tectonic affinities of the microcontinents in the eastern Central Asian Orogenic Belt of NE China, particularly the Jiamusi Block, have been hotly debated in the last decade. The Mashan Complex, which is the oldest exposed basement of the Jiamusi Block and consists of khondalitic rocks (felsic granulite, marble, and graphite schist) and orthogneisses, is the key geological unit to look into the tectonic affinity of the Jiamusi Block. In this study, we carried out LA-ICP-MS U-Pb dating on zircon and monazite minerals from the granitic gneisses in the Mashan Complex, to constrain the affinity of the Jiamusi Block. Magmatic zircons from two granitic gneisses yielded weighted mean 206Pb/238U ages of 517 ± 7 Ma and 527 ± 4 Ma, respectively, which are interpreted as the protolith ages. These age data verify a late Pan-African magmatic event in the Jiamusi Block. Metamorphic zircons from one granitic gneiss gave a weighted mean 206Pb/238U age of 510 ± 3 Ma, which constrained the metamorphic time of the Mashan Complex. This metamorphic age is further supported by monazite dating results – the monazites from three granitic gneiss samples yielded weighted mean 206Pb/238U ages of 489 ± 3 Ma, 498 ± 3 Ma, and 500 ± 3 Ma, respectively, which represented the metamorphic recrystallization ages of these monazites. These data consistently indicate a regional metamorphic event immediately after the late Pan-African magmatism in the Jiamusi Block. Our new data, combined with previously published late Pan-African geochronological data from the Jiamusi Block, support that the Jiamusi Block has an affinity to the Gondwana. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ca. 800 Ma I-type granites were identified in the Feidong Complex. • The I-type granitic rocks were produced by partial melting of ancient lower crust. • Ca. 800 Ma I-type granites in the Feidong Complex suggests a post-collisional extensional setting. Neoproterozoic magmatic rocks are widely distributed along the periphery of the Yangtze Block, but their petrogenesis and tectonic setting remain controversial. This study involved an integrated geochemical and geochronological analysis of mid–Neoproterozoic granitic rocks from the Feidong Complex on the northeastern margin of the Yangtze Block. LA–ICP–MS zircon U–Pb dating of two granitic gneiss samples yielded weighted mean ages of 803 ± 7 Ma and 792 ± 11 Ma. Combined with published age data from other Neoproterozoic magmatic rocks, these new dates suggest that the Feidong Complex experienced a magmatic flare–up at ca. 800 Ma. Geochemically, the granitic gneisses exhibit the characteristics of highly fractionated I–type granites. All of the studied samples have negative ε Hf (t) values of –18.20 to –8.51 and corresponding two–stage Hf model ages (T DM2) of 2.82–2.23 Ga, indicating their derivation from remelting of ancient crust. Geochemical characteristics suggest that the granitic rocks formed in a syn–collisional or post–orogenic environment. Integrating published geochemical data from other magmatic rocks along the northern margin of the Yangtze Block, we suggest that a plausible petrogenetic model for mid–Neoproterozoic magmatic rocks within the Feidong Complex may be tectonic collapse and remelting of the early to mid–Neoproterozoic collisional orogenic belt. In this model, melting represents a tectonothermal response to lithospheric extension before the breakup of the Rodinia supercontinent. [ABSTRACT FROM AUTHOR]

The studied granitic gneiss bodies of the Golpayegan metamorphic complex, located in the central part of the Sanandaj Sirjan Zone (SaSZ), in wester n Iran. Zircon U-Pb dating of two samples shows that the crystallization of the protolith occurred at 557 ± 12 Ma in the Late Neoproterozoic (Ediacaran), broadly coeval to the Neoproterozoic-Early Palaeozoic basement in other parts of Iran. Geochemically, the protolith of the gneisses probably corresponds to differentiated I-type granites with subalkaline affinities in composition. The ratios of Y/Nb >1.2 reveal an affinity to Cordilleran I-type granites for the granitic gneisses. The high Th/U ratios (2.8 to 9), low Eu/Eu* (0.13 to 0.73), with, low contents of FeO (0.55 to 1.72 wt.%), MgO (0.07 to 0.53 wt.%) and MnO (0.01 to 0.04 wt.%) and the high 87Sr/86Sr(i) ratios (0.70693 to 0.73557), negative ƐNd(t) values (−4.4 to −1.7) and Nd model ages (TDM2 = 1.35 to 1.56 Ga) suggest that the protolith may have been derived from partial melting of a pre-existing felsic crustal source (most likely differentiated granitoids). The new results reveal that the granitic source magma has been evolved in an active continental margin tectonic regime during the southward subduction of the Proto-Tethys ocean beneath the northern margin of Gondwana, like other coeval fragments dispersed in the entire Cadomian active continental margins. Also, the Cadomian crust widely extended in western Iran and it confirms these rocks have some clear affinity with Cadomian crust in the world. [ABSTRACT FROM AUTHOR]

The Central Alpine lower crustal migmatitic Gruf complex was exhumed in contact to the greenschist-grade Chiavenna ophiolite and gneissic Tambo nappe leading to a lateral gradient of ~ 70 °C/km within the ophiolite. The 14 km long, E-W striking subvertical contact now bridges metamorphic conditions of ~ 730 °C, 6.6 kbar in the migmatitic gneisses and ~ 500 °C, 4.2 kbar in the serpentinites and Tambo schists 2–4 km north of the contact. An obvious fault, mylonite or highly sheared rock that could accommodate the ~ 8.5 km vertical displacement is not present. Instead, more than half of the movement was accommodated in a 0.2–1.2 km thick orthogneiss of the Gruf complex that was heterogeneously molten. Discrete bands with high melt fractions (45–65%) now contain variably stretched enclaves of the adjacent MOR-derived amphibolite. In turn, the adjacent amphibolites exhibit tonalitic in-situ leucosomes and dikes i.e., were partially molten. The H2O necessary for fluid-assisted melting of the orthogneiss and amphibolites was likely derived from the tectonic contact metamorphism of the Chiavenna serpentinites, at the contact now in enstatite + olivine-grade. U–Pb dating of zircons shows that partial melting and diking occurred at 29.0–31.5 Ma, concomitant with the calc-alkaline Bergell batholith that intruded the Gruf. The major driving forces of exhumation were hence the strong regional North–South shortening in the Alpine collisional belt and the buoyancy provided by the Bergell magma. The fluids available through tectonic contact metamorphism led to self-enhanced magmatic weakening and concentration of movement in an orthogneiss, where melt-rich bands provided a low friction environment. Continuous heating of the originally greenschist Chiavenna ophiolite and Tambo gneisses + schists by the migmatitic Gruf complex during differential uplift explains the skewed temperature profile, with intensive contact heating in the ophiolite but little cooling in the portion of the now-exposed Gruf complex. [ABSTRACT FROM AUTHOR]

The study on the petrology, geochemistry, genesis and tectonic environment of Neoproterozoic granitic gneiss in Chaoman Forest Farm area of Daxinganling Mountains yields the age of 795.2 ± 4.3 Ma. The granitic gneiss is characterized by high Si, high alkali, rich Ca, Na and K, enrichment of LREEs and depletion of HREEs, with weak negative Eu anomaly and depletion. The magma source area is mainly crust-derived magma, and some mantle-derived magma underplating causes partial melting of rocks in the middle-lower crust, with hybridization of mantle-derived components to form mixed magma. Combined with regional data, the Neoproterozoic granitic gneiss in the study area was formed in a magmatic arc environment related to subduction. [ABSTRACT FROM AUTHOR]

The Lesser Himalayan Belt in the northern margin of Indian Shield preserves abundant sedimentary and magmatic records related to the assembly and breakup of the Columbia supercontinent. However, the Palaeoproterozoic tectonic history of the northern margin of the Indian Shield and its position in the Columbia supercontinent are still controversial. Based on detailed geological investigations in the Dailekh area of western Nepal, we conducted zircon U–Pb dating and Lu–Hf isotopic analysis for meta‐granitoids that intruded into the Lesser Himalayan Crystalline and Metasediments. The Dungeshworl granitic augen gneiss, meta‐monzogranite, and tonalitic gneiss yielded crystallization ages of 1,829 ± 12 Ma, 1,856 ± 11 Ma, and 1,852 ± 11 Ma respectively, which demonstrate that the deposition limits of both the Lesser Himalayan Crystalline and Metasediments are the Palaeoproterozoic. Zircon εHf(t) values of these meta‐granitoids range from −3.99 to +0.18, with corresponding depleted mantle two‐stage model (TDM2) ages of 2.41–2.62 Ga, indicating that they formed by reworking of Neoarchean to Palaeoproterozoic crust. Synthesizing existing geochronological, petrochemical, and zircon Lu–Hf isotopic data for metamorphic sedimentary and igneous rocks from the entire Lesser Himalayan Belt, we suggest that the Palaeoproterozoic magmatism in the northern margin of Indian Shield may be formed within subduction‐related tectonics responded to the assembly of Columbia supercontinent. [ABSTRACT FROM AUTHOR]

Middle-lower crust and mantle rocks are generally widely exposed in metamorphic core complex or gneiss dome, which is an ideal place to study the exhumation process related to regional extension and rheology. The Laojunshan metamorphic complex in southeastern Yunnan is located in a special tectonic position surrounded by the Cathaysia, Yangtze and Indochina blocks. It is composed of different metamorphic-deformation rocks and granitic intrusions. There also are many economic deposits (e.g., tin and tungsten) that are spatially and genetically associated with the formation and exhumation of the Laojunshan gneiss dome. Based on detailed analysis of macro- and microscopic structure, stress field distribution and deformation condition, the tectonic units of the Laojunshan metamorphic complex show obvious characteristics of doming, as well as of typical structural units of metamorphic core complex. It has strongly deformed metamorphic gneiss core (footwall), detachment fault system and sedimentary cover (hanging wall) with lightly metamorphism and deformation. The footwall of gneiss dome presents a strongly ductile deformation domain, accompanied by different ages of granitic intrusions. The distribution of developed foliation and lineation within granitic gneisses are arc-shaped and radial, respectively, with a nearly N-S trending from the footwall to the hanging wall. Mylonitization of deformed rocks gradually weakens and transits to orthogneiss as it moves away from the detachment fault toward the footwall. The low angle detachment fault between the footwall and the hanging wall shows an arc-like shape feature. Mylonite fabrics are preserved in the deformed rocks of the detachment fault, which are mainly composed of chloritized schist, fault breccia, cataclasite and fault gouge. A large number of normal faults are developed in detachment faults and hanging wall, and their stress fields radiate in an arc around the footwall. Zircon U-Pb ages of amphibolite and granitic gneiss from the footwall range from 445 to 420 Ma, indicating the timing of Caledonian magmatic emplacement and the main formation period of the Laojunshan gneiss dome. U-Pb ages of the zircon metamorphic rims are 241–230 Ma, representing the timing of high temperature metamorphism and shortened deformation of the Indosinian collision. In this period, the Laojunshan gneiss dome experienced the tectonic compression in association with high temperature metamorphism-deformation, which was superimposed by detachment and extensional exhumation in association with intense hydrothermal interaction and mineralization in the late stage. [ABSTRACT FROM AUTHOR]

The Chaihulanzi area in eastern Inner Mongolia is tectonically situated on the northern margin of the North China Craton (NCC). The main Precambrian lithologies of the area have been referred to the Archean Jianping Group meta‐supracrustal sequences. Based on field observations, petrographic, whole‐rock geochemical, and zircon U‐Pb geochronological results, a magmatic origin for the units is proposed. Our results show that the Chaihulanzi gneisses are mainly of granitic, dioritic and granodioritic compositions, and show typical magmatic rock textures and mineral assemblages. The dioritic and granodioritic gneisses show Na‐rich tonalite–trondhjemite–granodiorite (TTG)‐like affinity with zircon U‐Pb dates of ca. 2.57–2.59 Ga, representing a juvenile continental growth for the northeastern NCC. The granitic gneiss is indeed potassic granite and yielded a zircon U‐Pb date of ∼2.50 Ga, which is contemporaneous with the Jining–Jiaoliao microblock collision (2.53–2.49 Ga), implying another crustal growth event. The well‐developed gneissosity in 2.57–2.59 Ga dioritic and granodioritic gneisses together with the 2.5 Ga potassic granite, which crosscuts the gneissosity implies a 2.57–2.50 Ga (mainly 2.53–2.51 Ga) collisional orogeny, probably related to the Jining–Jiaoliao microblock collision. Our new geology and chronological results provide new evidence for the early Precambrian tectonic evolution of the NCC. [ABSTRACT FROM AUTHOR]

The Twakputs Gneiss is a garnetiferous, K-feldspar megacrystic, biotite granite-granodiorite orthogneiss. It represents a major unit in the Kakamas Domain of the Mesoproterozoic Namaqua-Natal Metamorphic Province extending about 250 km between Riemvasmaak in South Africa and Grünau in southern Namibia. The Twakputs Gneiss occurs as foliation-parallel, sheet-like bodies tightly infolded together with granulite-facies paragneisses into which it intrudes along with a variety of other pre-tectonic granite and leucogranite orthogneisses. These rocks were subsequently intruded by late-tectonic garnet-leucogranites, granites and charnockites. The Twakputs Gneiss is a distinctive unit characterised by large ovoid to elongate megacrysts of twinned perthitic K-feldspar, set in a coarse-grained matrix of garnet, biotite, quartz and feldspar. It contains a penetrative foliation defined by the alignment of K-feldspars and streaks of biotite that developed during the main phase D2 of the Namaqua Orogeny (~1.2 to 1.1 Ga). The foliation and an accompanying elongation lineation are more intensely developed along lithological contacts, especially at the margins of the mega-scale F3 domes and basins that refold the regional fabrics. U-Pb zircon dating of the Twakputs Gneiss has yielded concordia ages of between ~1192 and 1208 Ma. Wholerock geochemistry shows consistent major, trace and REE elemental trends, and thus reflect chemical variability from a single fractionating magma. The Twakputs Gneiss has a granitic to granodiorite composition and is strongly peraluminous. The geochemistry and the ubiquitous presence of garnet and pelitic xenoliths indicate an S-type granite protolith. The Twakputs Gneiss is the most voluminous and widespread member of the Eendoorn Suite which comprises seven textural variants of garnetiferous, K-feldspar-megacrystic granitoid orthogneiss of the same age. [ABSTRACT FROM AUTHOR]

The Sveconorwegian Orogen comprises diverse lithotectonic units formed and shaped by magmatic, sedimentary and tectonic processes at different times in the Meso- and Neoproterozoic. Evaluations of the development of each lithotectonic unit are necessary to verify the still controversial mechanisms proposed for the evolution of the orogen. The present study is focused on a particular occurrence within the Østfold Gneiss Complex of the Idefjorden lithotectonic unit. The studied locality, near Halden, south Norway, offers a 250 m-wide section that was saw-cut with a helical diamond-wire. The country rock is a high-strain shear zone in migmatitic biotite gneiss, which contains dismembered (boudinaged, sheared and flattened) fragments of garnet amphibolite and several generations of pegmatite pods and veins. Field mapping and U-Pb dating of zircon, monazite, titanite and rutile reveal the following: (1) The protolith of the host gneiss was likely greywacke, with a Mesoproterozoic detrital zircon population derived from the Gothian Stora Le-Marstrand volcanic assemblage. (2) A Gothian event of metamorphism and gneissification occurred at 1549 ± 4 Ma (2 sigma uncertainty). (3) Major Sveconorwegian metamorphism and deformation caused by SE-directed contraction of the Idefjorden unit spanned several stages between c. 1041 and 1033 Ma. (4) Local retrogression, with formation of secondary titanite, followed at around 920 Ma, likely related to emplacement of the Bohus-Idefjorden granite intrusion, and possibly linked to uplift and rotation of the shear zone and normal faulting, the last of which occurred in the brittle regime. [ABSTRACT FROM AUTHOR]

Palaeoproterozoic granites accompanied with granulite‐facies gneisses can be recognized in the Kuandian area, northern Jiao‐Liao‐Ji Belt (JLJB). This study highlights two episodes of Palaeoproterozoic magmatism, identified by granites and biotite monzogranitic gneisses herein. Zircon grains from biotite monzogranitic gneiss with homogeneous oscillatory zoning yielded a U–Pb age of 2,153 ± 16 Ma, in accordance with the age of zircon cores from the monzogranitic gneiss. Zircon grains from the two granites yielded upper intercept ages of 1,892 ± 38 Ma and 1,859 ± 36 Ma respectively. Monzogranitic gneiss samples are enriched in Th, Rb, Pb, U, and REE, but depleted in Sr and P. They have high 1,000 × Ga/Al ratios versus Zr, Ce, Nb, Y contents, which are usually interpreted as the features of classical A‐type granite that originated in a lithospheric extension setting. The ~1.89–1.86 Ga granite samples have relatively high Al, Rb, Th, U and low Sr, Zr, and Hf contents. These features, combined with the observation of minor apatite grains under the microscope are similar with the I‐type granites. In situ zircon Lu–Hf isotopic analyses revealed that ~1.89–1.86 Ga granite had a similar range and TDM2 ages with the monzogranitic gneiss, but with much lower εHf(t) values. The monzogranitic gneiss may have originated from an Archean crust, while the later granite may be derived from melting of the partial melting of the monzogranitic gneiss or the Archean crust that generated the monzogranitic gneiss. The ~1.89–1.86 Ga granites, together with the coeval S‐type granites in the JLJB, represent an orogeny process that may involve a Palaeoproterozoic subduction. [ABSTRACT FROM AUTHOR]

Mylonitic orthogneisses in the Kathmandu area, central Nepal have been investigated using whole-rock and mineral chemistry, Rb-Sr isotopes, and zircon U-Pb age dating. Zircon REE patterns determined from orthogneisses are characterized by enriched HREE patterns and the prominent Eu anomalies, consistent with a magmatic origin. The U-Pb zircon age dating and Ti-in-zircon thermometry revealed crystallization took place ca. 1.74 Ga at temperatures of 705–765℃; typical of felsic magmatism in the crust. Whole-rock data from most orthogneisses in this study and from similar rocks in previous studies span the 'syn-collisional' and 'post-collisional' fields on various tectonic discrimination diagrams, while some data also plot in rift-related magmatism fields. The peraluminous compositions, very high Sr isotopic ratios (0.865–3.585) and high Th and U concentrations for all orthogneisses in this study indicate that mylonitic orthogneisses are largely of S-type crustal origins. The new data presented herein, combined with that of previous studies, outline at least two Palaeoproterozoic magmatic episodes: 1) ca. 1.92–1.90 Ga rift-related magmatism derived from mantle melting and 2) 1.84–1.74 Ga crustal melting, resulting from burial of the Indian basement during thermal subsidence after rifting. This two-stage Palaeoproterozoic magmatism in Nepal occurred along the northern passive margin of the Indian basement during and/or after the breakup of the Columbia supercontinent. [ABSTRACT FROM AUTHOR]

• 3.4 Ga granitic gneisses were identified in the Southern Kongling Complex. • The 3.4 Ga granitic gneisses formed by reworking of a pre-existing Eoarchean (ca. 3.73 Ga) tonalitic crust and underwent anatexis at ca.3.0 Ga. • The ca. 2.94 Ga trondhjemitic gneisses represent neosome generated by partial melting of Paleoarchean low-K basaltic crust. The TTG gneisses, a dominant lithological assemblage within Archean cratonic provinces, provide a window into the early evolution of the continental crust. The Kongling complex in South China preserves voluminous Archean granitoids, recording the early formation and evolution history of the Yangtze Craton. We present results of a comprehensive study involving field investigation, petrographic characterization, zircon U–Pb geochronology and Lu-Hf isotope tracing, and whole-rock geochemistry for the Archean granitoid gneisses from the Southern Kongling Complex. Igneous zircons from two granitic gneisses yielded weighted mean 207Pb/206Pb ages of 3399 ± 15 Ma (MSWD = 0.99, n = 24) and 3389 ± 12 Ma (MSWD = 1.02, n = 20). Zircons from the 3.4 Ga granitic gneiss also record a ca.3.0 Ga anatexis event which is contemporaneous with the widespread granitoid gneisses in the Kongling Complex. Geochemical and zircon Hf isotope data (ε Hf (t) values of − 3.41 to + 0.77 and two-stage Hf model ages of 3.65–3.80 Ga) for the granitic gneisses support a derivation from pre-existing Eoarchean (T DM2 = 3.73 Ga) crust. LA-ICP-MS zircon U-Pb dating yielded emplacement ages of 2948 ± 17 Ma for a migmatized trondhjemitic gneiss, and 2928 ± 14 Ma for a quartz monzonite gneiss. These two gneisses have similar zircon Hf model ages (T DM2 = 3.45 Ga) and both were emplaced during a major phase of mafic magma diapiric process that significantly reworked the old crust. The occurrences of 2.94 Ga trondhjemite and associated mafic blocks suggest intra-crustal differentiation by partial melting of low-K mafic rocks. We suggest that intra-crustal remelting and differentiation played a key role in the formation, evolution, and maturation of the micro-continents in Kongling during the Paleo- to Meso-Archean. The whole-rock geochemical and zircon Hf isotopic compositions of the different generations of granitoid gneisses from the Southern Kongling Complex are similar to those of the coeval gneisses (3.45 Ga and 2.95 Ga) from the northern part of the Kongling Complex, indicating a common tectono-magmatic history shared by both segments tracing back to as early as 3.4 Ga. The Archean granitoid gneisses from the Kongling Complex record remelting and differentiation of Eo- to Paleo-Archean crusts during the Paleo- to Meso-Archean, implying that Eoarchean crust is likely to have been preserved within the Kongling complex. [ABSTRACT FROM AUTHOR]

• The Sandmata Complex preserves Neoarchaean protolith ages of 2.68–2.42 Ga. • Zircon-monazite ages indicate peak granulite facies metamorphism at 1.90–1.78 Ga. • Sandmata quartzofeldspathic gneisses record an isobaric cooling P–T path. • The complex was tectonically active up to the Neoproterozoic (∼0.90–0.78 Ga). The Sandmata Complex in the Aravalli Craton of northwestern India experienced syn- to post-collisional high-grade metamorphism and migmatization related to the 1.9–1.7 Ga Aravalli orogeny. Garnet-bearing quartzofeldspathic gneisses are an important rock type in the Sandmata Complex. Their prograde assemblage is represented by garnet cores and inclusion minerals Bt + Pl + Qz, whereas the assemblage Pl + Kfs + Qz + Bt matrix + Sil + Ilm, along with chemically homogeneous garnet porphyroblasts, represents peak metamorphism. Post-peak retrograde metamorphism is reflected in the growth of muscovite, chlorite and biotite, and diffusion adjustments in garnet rims. Mineral paragenesis, mineral chemistry, conventional thermobarometry, and phase equilibrium modelling indicate P–T conditions of 6.7–10.2 kbar at 725–830 °C for peak metamorphism, and 4.8–6.9 kbar at 450–610 °C for post-peak retrogression. The P–T conditions determined from the quartzofeldspathic gneisses define a near-isobaric cooling P–T path. In contrast, previously studied high-grade pelitic gneisses and metagranitoids from the Sandmata Complex define an isothermal decompression P–T path. U–Pb dating of magmatic zircon cores in the quartzofeldspathic gneisses provides the crystallization age of their felsic protoliths as ∼ 2.52 Ga. Zircon and monazite geochronology show that peak metamorphism occurred at ∼ 1.90–1.78 Ga and thus shortly preceded the emplacement of the Gyangarh-Asind igneous suite and Anjana granite, previously dated at ∼ 1.72–1.64 Ga. A model of oceanic crust subduction followed by continent–continent collision during the Aravalli orogeny can explain granulite facies metamorphism in the Sandmata Complex. The combined results of this and previous studies support the operation of modern-style plate tectonic processes involving subduction and continental collision during Palaeoproterozoic time. We also identify a younger (∼0.90–0.78 Ga) metamorphic episode, preserved mostly in recrystallized monazite grains elongated parallel to the final deformation fabric, likely related to the South Delhi orogeny. [ABSTRACT FROM AUTHOR]

The article presents a study on the structural geology, petrology, chronology, and geochemistry of the Shirenshan gneiss in North Qinling Terrane (NQT), China. The study evaluates the zircon uranium-lead (U-Pb) dating, the magma sources and petrogenesis, and geodynamic regimes of the gneiss. Result shows that the gneiss features A-type granite, rock production through partial melting of amphibolites, and strong interaction between crust and mantle.

• Nauseri gneiss (1.8 Ga) emplaced during the assembly of the Columbia supercontinent. • Jura granitic gneiss (0.8 Ga) and Neelum granite (0.7 Ga) were emplaced during the Pan-African Orogeny. • Jura granite (459 Ma) points to the magmatism that occurred during the Ordovician Orogeny. This study presents detailed petrography, bulk rock geochemistry, U-Pb dating, and Sr-Nd isotopes of pre-Himalayan granite gneisses and granites exposed in the Neelum Valley, NW Pakistan. To understand pre-Himalayan crustal growth history in the Indian plate, basement granitoids provide useful constraints. Two granites (the Jura and Neelum granites) and two granitic gneisses (the Nauseri and Jura gneisses) were investigated with the goal of their petrogenesis and constraining their geotectonic settings. Petrographically both granites and gneisses contain plagioclase, K-feldspar, quartz with S-type granite paragenesis of muscovite, garnet and tourmaline. Geochemically granites and gneisses are characterized by strongly peraluminous (A/NK > 1.1) character with a distinct mafic metapelitic sources. Notable negative Ta, Nb, and Ti compared to La and K, and a pronounced positive Pb anomalies in the studied rocks indicate convergence related source with a similar rare earth element pattern of upper continental crust. Two U-Pb zircon age populations, ca. 2429 ± 17 Ma and 1844 ± 11 Ma were obtained from the Nauseri gneiss whereas Jura gneiss yielded 865 ± 5.6 Ma. In contrast, Neelum and Jura granites yielded ages of 736 ± 11 Ma and 459.1 ± 3.5 Ma, respectively. Sr-Nd isotopic composition ((87Sr/86Sr)i = 0.7099 to 0.7303; εNd(t) = -8.1 to −3.6) together with the major and trace element geochemistry indicate that parental magmas of granites and gneisses were derived from the partial melting of upper continental crust during syn -collision tectonic settings. The Sr-Nd isotopes coupled with two-stage model ages further suggest incorporation of Archean and Paleoproterozoic reworked continental material that contained some mafic metapelitic component as well. Tectonically the parental magma of gneisses and granites could be linked with the global-scale orogenic events that operated during the assembly and rifting of the supercontinents of Columbia to Pangaea. Protoliths of Nauseri gneiss was possibly formed during the assemblage of Columbia supercontinent (1.8 Ga). Later, the Pan-African Orogeny provided additional pulse of magmatism that resulted Jura gneiss and Neelum granite. Moreover, the Bhimphedian orogeny in the Indian plate during Ordovician times generated the Jura granite of ca. 460 Ma. [ABSTRACT FROM AUTHOR]

The Entia Gneiss Complex (EGC) in central Australia represents a deeply exhumed Paleoproterozoic basement terrane that underwent fluid-catalysed transformation culminating in the formation of a migmatitic gneiss dome during the 450–300 Ma intracontinental Alice Springs Orogeny (ASO). Foremost amongst the changes the EGC experienced during this event was the regional-scale conversion of nominally anhydrous granulite and igneous crust to hydrous amphibolite facies migmatite, together with the emplacement of locally voluminous mica-bearing pegmatite swarms. LA–ICP–MS U–Pb dating of zircon, monazite and xenotime demonstrates that pegmatite emplacement dominantly occurred late in the Alice Springs Orogeny (c. 360–300 Ma). The oldest generation of pegmatites are transposed into the regional Palaeozoic gneissic fabric and formed at c. 1770 Ma, indicating they belong to protolith rocks of the EGC. Three subsequent generations of pegmatite have ages that range between 360 and 300 Ma. The oldest of these is parallel to the migmatitic gneissic fabric and dated at c. 360 Ma, whereas the younger two generations are largely undeformed, discordant and intruded throughout the period c. 350–300 Ma. The capability for a comparatively dry intracratonic orogenic core to repeatedly produce melt over an interval of c. 60 Myr requires explanation. We suggest crustal-scale duplex development during the latter stages of the ASO resulted in the juxtaposition of dry and mechanically strong Paleoproterozoic basement over deeply buried Neoproterozoic–Cambrian sedimentary sequences. Fluid derived from the metamorphic dehydration of these sequences led to hydration, partial melting and rheological weakening of the basement slab. Prior to its hydration at c. 360 Ma, the EGC was metamorphically unresponsive, accounting for its comparatively late timing of deformation and partial melting compared to the overall history of the Alice Springs Orogeny. The development of the crustal-scale duplex system, combined with hydrous melting of the low-density basement slab beneath high-density supracrustal rocks, resulted in a thermomechanically unstable architecture that facilitated the formation of an orogen-scale gneiss dome during the waning stages of intracontinental orogenesis. • Pegmatite emplacement in the Entia Gneiss Complex occurred late in the intracontinental Alice Springs Orogeny (360–300 Ma) • Prior to emplacement, Entia Gneiss Complex basement consisted of dry, strong and unreactive granulites and igneous rocks • Hydration, melt production and rheological weakening was driven by crustal-scale duplexing of fertile sedimentary sequences • Hydrous melting of the low-density basement slab beneath high-density supracrustal rocks facilitated gneiss dome formation [ABSTRACT FROM AUTHOR]

Clastic sedimentary rocks are important tracers to understand the evolution of the continental crust. Whole-rock major and trace element data, zircon U–Pb dating and Hf isotopic data for the paragneisses from the Jiaobei terrane are presented in this study in order to constrain their protoliths, provenance and tectonic setting. The paragneisses are characterized by enrichment in Al 2 O 3 and TiO 2 , negative DF (DF = 10.44 − 0.21SiO 2 − 0.32Fe 2 O 3 T − 0.98MgO + 0.55CaO + 1.46Na 2 O + 0.54K 2 O) values and the presence of aluminum-rich metamorphic minerals (e.g., garnet and sillimanite). Together with the mineral assemblages and zircon features, it can be inferred that the protoliths of these rocks are of sedimentary origin. The K–A (A = Al 2 O 3 /(Al 2 O 3 + CaO + Na 2 O + K 2 O), K = K 2 O/(Na 2 O + K 2 O)) and log(Fe 2 O 3 /K 2 O)–log(SiO 2 /Al 2 O 3 ) diagrams indicate that they belong principally to clay–silty rocks with some contributions from graywacke. A series of geochemical indexes, such as the widely employed CIA (CIA = [Al 2 O 3 /(Al 2 O 3 + CaO ∗ + Na 2 O + K 2 O)] × 100; molar proportions) and ICV (ICV = (Fe 2 O 3 + MnO + MgO + CaO + Na 2 O + K 2 O + TiO 2 )/Al 2 O 3 ) values, and the A–CN–K diagram for the paragneisses indicate relatively weak weathering in the source rocks and negligible post-depositional K-metasomatism. In addition, their REE patterns, low Cr/Zr (0.61–1.99), high Zr/Y (4.81–23.59) and Th/U (3.21–40.67) ratios, the low to moderate contents of Cr (197–362 ppm) and Ni (6.68–233 ppm), and source rock discrimination diagrams collectively suggest that the sediments of the protoliths of the paragneisses in the Jiaobei terrane were derived from the source with intermediate–acidic composition, probably granitic-to-tonalitic rocks. In combination with geochronological and isotopic studies on the paragneisses and the basement rocks in the Jiaobei terrane, it is suggested that the Archean–early Paleoproterozoic granitic rocks in the Jiaobei terrane possibly provided the most important source materials. In addition, the protoliths of the paragneisses might have been deposited during 2.47–2.42 Ga. High and heterogeneous ICV values of the paragneisses could imply a chemically immature source. A number of geochemical indicators and tectonic discrimination diagrams together indicate an island arc or active continent margin setting for the deposition of the protoliths of the paragneisses in the Jiaobei terrane. Together with the nearly contemporaneous igneous rocks, it can be inferred that the convergent margin setting was possibly operative during the Late Neoarchean–Early Paleoproterozoic transition in the Jiaobei terrane. [ABSTRACT FROM AUTHOR]

[Display omitted] • Clockwise P–T evolution is recorded by the South Altyn Tagh felsic gneiss. • The felsic gneiss was exhumed from eclogite-facies to granulite-facies within 20 Myr. • HP–HT eclogite-facies records were preserved in the weakly deformed felsic gneiss. • The HP mineralogical records were further eliminated by the strong deformation. Felsic gneisses are usually the dominant rock types in most high-pressure (HP) and ultrahigh-pressure (UHP) terranes, although their peak metamorphic conditions and complete P–T paths are rarely known. This lack of knowledge is attributed to the high variance of mineral assemblages at HP/UHP conditions of the felsic gneisses, or to the effects of retrograde-related overprinting. Thus, it is difficult to distinguish whether felsic gneisses underwent HP/UHP metamorphism. In this paper, we focus on the felsic gneiss that has been previously documented as having formed under HP granulite-facies metamorphism with peak P–T of > 1.15 GPa and > 860 °C from the Danshuiquan locality in the South Altyn HP – UHP belt. The Danshuiquan felsic gneiss is strongly deformed and is spatially associated with weakly deformed garnet-rich felsic gneiss. On the basis of detailed microstructure observations on mineral assemblages and phase equilibria modelling, three stages of metamorphic evolution for the weakly deformed garnet-rich felsic gneiss are identified. The first stage is represented by garnet cores, revealing a prograde P–T path from 1.5 GPa/750 °C to 2.1 GPa/900 °C. The second stage is characterized by garnet mantles with multiphase inclusions, defining HP eclogite-facies metamorphic conditions of 2.2–2.6 GPa/950–1100 °C. The third stage is represented by the composition of garnet rims, biotite, and plagioclase, suggesting an HP granulite to amphibolite facies decompression cooling process to 1.1–1.3 GPa/710–770 °C. In contrast, the strongly deformed felsic gneiss just records a deformation-related decompression process from 1.1 to 1.2 GPa/750–760 °C to 0.87–0.98 GPa/760–770 °C, and finally to 0.83 GPa/740 °C. Zircon U–Pb dating yields c. 920 Ma protolith ages, c. 500 Ma eclogite-facies ages, and c. 480 Ma granulite to amphibolite facies retrograde ages for the Danshuiquan felsic gneisses. Our results show that the Danshuiquan felsic gneisses underwent a similar metamorphic evolution process to that of Yinggelisayi HP–UHP rocks in the eastern segment of the South Altyn Tagh. Most HP mineralogical records tend to be eliminated by the strong deformation at crustal depths (∼30 km), meaning that the peak metamorphic conditions of the studied felsic rocks in UHP metamorphic terranes are previously underestimated. [ABSTRACT FROM AUTHOR]

This report presents the results of local U-Pb zircon dating (SIMS SHRIMP II) for a sample of migmatite gneiss dredged on the western slope of Alpha Ridge in the Arctic Ocean in the course of the 'Arktika-2012' Russian polar expedition. The distribution of U-Pb ages of the examined zircon points to the Early Precambrian origin of this gneiss, for the bulk of the zircon was crystallized at least 3450 Ma ago from a magmatic melt under acidic volcanism at the primary crust formation. Zircon of the second generation was crystallized 3300 Ma ago under the remelting of acid volcanics and appearance of migmatite gneisses under the amphibolite facies of metamorphism. Most likely, a partial recrystallization of zircon and formation of microfolded structures and foliation took place 3000 Ma ago at the stage of rocks deformation. The latest zircon was formed 1900 Ma ago from the crust fluid or melt under the low-gradient metamorphism. In view of the possibility of the appearance of the treated clastogenic gneiss fragment under current oceanic erosion, the obtained results allow one to affirm that the occurrence of a fragment of the most ancient sialic continental crust formed at least 3450 Ma ago is possible at the submarine rises of the Arctic Ocean (Alpha Ridge and the Mendeleev Rise). [ABSTRACT FROM AUTHOR]

New data on the U-Pb (SHRIMP II) age are obtained for aluminous gneisses of the Kola Group of the Baltic shield. Gneisses are typically ascribed to metasedimentary rocks with detrital zircons. Our work interprets the isotopic data based on the magmatic (tonalite) origin of gneisses and zircon that was established from study of the morphology and geochemistry of zircons. The age of crystallization of the protolith is 2.9 Ga. The existence of two stages of Archean granulite metamorphism is confirmed: an early stage (2.9 Ga) and later granulite metamorphism (2707-2656 Ma). [ABSTRACT FROM AUTHOR]

The 30 km-long, N–S-trending Forquilha eclogite zone, occurs within a Paleoproterozoic block mainly composed of gneisses and migmatites, in the Ceará Central domain, Borborema Province, NE Brazil. The Forquilha eclogite zone contains lenses of high to ultra-high pressure metamafic rocks, found as granulites and amphibolites associated with kyanite-sillimanite gneisses. Three samples of clinopyroxene-garnet amphibolite yielded the U–Pb zircon ages of 1566 ± 9 Ma, 1547 ± 37 Ma and 1532 ± 24 Ma, interpreted as the timing of igneous crystallization of the mafic protolith. Additionally, zircon grains of a leucocratic layer of a metamafic rock and a retrograded eclogite provided the less precise U–Pb ages of 1613 ± 40 Ma and 1454 ± 120 Ma, respectively. Lu–Hf and Sm–Nd model ages provided T DM (Hf) between 1.55 and 1.81 Ga with positive ε Hf values of +7.50 to +10.48, and T DM (Nd) ranging between 1.57 and 1.92 Ga with positive ε Nd values of +1.84 to +4.36. It is believed that part of the rocks of the Forquilha eclogite zone were emplaced as mafic dikes in an extensional setting at ca . 1.57 Ga. [ABSTRACT FROM AUTHOR]

The South Yellow Sea basin in eastern China has experienced a multi-stage tectonic evolution history. The major structures were created when the basin was a foreland basin during the Mesozoic. However the geological evolution of the basin has not yet been corroborated by direct evidence from the underlying basement rocks. Qianliyan Island in the southern Yellow Sea provides an opportunity to study the formation and evolution of the basin by means of direct geochronological and geochemical evidence. On Qianliyan Island, basement rocks are exposed that consist of granitic gneiss, felsic gneiss and minor mylonite, and lenses of eclogite. Major and trace element characteristics of these four types of gneiss indicate that they originated from crustal material, varying in composition from pelite to greywacke. SHRIMP U-Pb zircon dating results of a felsic gneiss sample show that this rock crystallized between 659 and 796 Ma and underwent a metamorphic overprint at 229 ± 4 Ma. This age pattern resembles that of gneisses from the ultra-high-pressure terrain in the Dabie–Sulu belt. We conclude that the study area was part of the northern margin of the Yangtze Block during the Neoproterozoic. Neoproterozoic magmatic activity occurred along this margin and the basement sequence underwent Triassic metamorphic overprint during the northward subduction of the Yangtze Block beneath the North China Block. We further conclude that the deformation associated with this metamorphic event led to the formation of the southern Yellow Sea foreland basin. [ABSTRACT FROM PUBLISHER]

Dioritic--trondhjemitic gneisses of Neoarchean age are widely distributed throughout the Gualanyu--Shangying area in Eastern Hebei, North China Craton. These orthogneisses consist of dioritic, trondhjemitic, and quartz-rich trondhjemitic gneisses, which were subjected to amphibolite- to granulite-facies metamorphism. LA-ICP-MS zircon U--Pb isotopic dating reveals that the magmatic precursors of these rocks were contemporaneously emplaced from 2535 ± 23 to 2513 ± 8 Ma, reflecting an important Neoarchean dioritic--trondhjemitic magmatic event followed by sequential Proterozoic tectonothermal events between 2472 ± 60 and 2307 ± 50 Ma. Dioritic and trondhjemitic gneisses exhibit similar zircon ɛHf(t) values of +1.5 to +5.2 and +2.9 to +5.3, respectively, indicating their depleted mantle affinity. Geochemical analysis reveals that the dioritic gneisses have low SiO2 contents (55.4-63.0%), various MgO contents (2.07-4.78%) and Mg# (36.07-63.56), and high (La/Yb)N ratios (5.45-15.19). In contrast, the trondhjemitic gneisses exhibit higher SiO2 contents (67.4-73.8%) and (La/Yb)N ratios (12.86-76.73), and lower MgO contents (0.66-2.67%), although their Mg# (36.29-61.41) are similar to those of dioritic gneisses. These geochemical and zircon Lu--Hf isotopic features, together with geochemical modeling, suggest that the dioritic magmas were derived mainly from the partial melting of a depleted mantle wedge that was metasomatized by slab-derived melts or fluids, and that the trondhjemitic magmas were formed by fractional crystallization of the dioritic magmas. Quartz-rich trondhjemitic gneisses exhibit extremely high SiO2 contents (77.4-84.6%) with zircon ɛHf(t) values ranging from -3.4 to +4.1, suggesting that their precursors formed after the partial melting of a mixed magmatic source. It is suggested that the precursors comprised a mafic member derived from a mantle wedge altered by oceanic slab melts or fluids, and a felsic member derived from the partial melting of ancient crustal material. These new data, combined with previous studies of contemporaneous metavolcanic rocks in Gualanyu-Shangying area and surrounding areas, suggest that the magmatic precursors of these Neoarchean dioritic--trondhjemitic gneisses formed in a subduction-related tectonic environment at an E-W strike convergent plate margin. Furthermore, the Eastern Block of the North China Craton had a major lateral accretion of crustal growth in the north margin in the Neoarchean, whereas vertical crustal growth and recycling under plume tectonic backgrounds occurred in the interior of the Eastern Block. [ABSTRACT FROM AUTHOR]

We present a U-Pb detrital zircon age and provenance study of a sequence of metasedimentary rocks in the northwestern Thor-Odin high-grade gneiss dome within the Omineca crystalline belt of the Canadian Cordillera. Despite strong overprint by defor mation and metamorphism, we successfully analyzed the age and provenance of six samples collected at various structural levels, using U-Pb detrital zircon laser-ablation- inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis. The Thor-Odin dome consists of Paleoproterozoic basement and a metasedimentary cover sequence of previously unknown age and tectonic signifi cance. Our results indicate that the oldest units of this sequence may be Paleoproterozoic, and some of the oldest known metasedimentary rocks in the Canadian Cordillera, originally deposited on top of Laurentian basement rocks. The youngest rocks are Devonian and deposited shortly before the onset of widespread Late Devonian to early Mississippian igneous activity in the Selkirk Domain or Kootenay arc. The cover sequence of the Thor-Odin dome thus preserves some of both the oldest and the youngest (meta)sedimentary rocks deposited between the formation of supercontinent Columbia and the onset of igneous activity and convergence that marked the beginning of Cordilleran deformation and metamorphism. Parts of as many as ∼1.4 b.y. of sedimentary history are preserved in the Thor-Odin dome, implying that much information on the sedimentary history of the Canadian Cordillera may be hidden in other Cordi lleran gneiss domes. [ABSTRACT FROM AUTHOR]

Abstract: Previous investigations have indicated that the Changning–Menglian (CMSZ) and Longmu Co–Shuanghu (LSSZ) suture zones represent the Paleo-Tethys Ocean of northern Tibet and western Yunnan of China, respectively. However, our understanding of the Paleo-Tethys suture zone in eastern Tibet seems to be incomplete. The North Lancangjiang suture zone (NLSZ) has been hypothesized to mark the Paleo-Tethys Ocean in eastern Tibet. The present study reports new zircon U–Pb ages and Hf isotope data, and whole-rock major and trace element data from the gneissic granites from the Jitang complex in the Leiwuqi area of the NLSZ in order to verify this hypothesis. Six samples of gneissic granites yielded weighted mean 206Pb/238U ages of 249–237Ma. The geochemical data show that the gneissic granites are characterized by high SiO2 (70.79–77.75wt.%); K2O (2.00–6.13wt.%); low A/CNK values (<1.1); enrichment in Rb, Th, Ba, and Pb; depletion in Nb, Ta, Sr, Ti, and P; and negative Eu anomalies. These features suggest that the gneissic granites were high-potassium, calc-alkaline, and I-type granites. Moreover, the zircon εHf(t) values (from −2.7 to +2.5) and TC DM model ages (1447–1009Ma) indicate that they were generated by the partial melting of the Meso-proterozoic juvenile crust. These gneissic granites were interpreted to have formed in the post-collisional tectonic setting related to the closure of the Paleo-Tethys Ocean. By combining previously published and the new data, we propose that the NLSZ connects the LSSZ and CMSZ in the Tibetan plateau, and marks a Paleo-Tethys Ocean basin in eastern Tibet. [Copyright &y& Elsevier]

[Display omitted] • The Hengshanli dome experienced three stages of Paleoproterozoic deformation. • Partial melting of the Liaoji granites of 2147 Ma at 1864–1846 Ma during the D 2 shearing. • Sub-horizontal shearing in association of diapirism dominated the formation of dome. • Doming was accompanied by middle and lower crustal vertical and lateral flow. • The JLJOB experienced early E-W extension and contraction, late N-S contraction. A group of granitic gneiss domes that may provide constraints on the tectonic evolution of early Precambrian orogenic belts are well preserved in the LiaoJi segment (or LJOB-Liaoji orogenic belt) of the Jiao-Liao-Ji Orogenic Belt (JLJOB), eastern North China Craton. To deeply understand the genesis of the domes and how the crustal mass flowed in collisional orogenic belt, we carried out structural analysis of the Hengshanli dome in the western part of the LJOB. Detailed analyses of deformation structures, microstructures and fabrics, and geochronology of syntectonic granitic dykes demonstrate that three episodes of structural deformation contributed to the formation of the dome structure. During D 1 , a pure shear-dominated deformation, penetrative foliations (S 1), symmetrical lenses or boudinages, intrafolial folds (F 1) and mineral lineations (L 1) developed in the interlayered rocks of viscosity contrasts. The D 2 , the WNW-ESE orientated sub-horizontal simple shear-dominated deformation, is characterized by asymmetrical A-type folds (F 2), stretching lineations (L 2) , mylonitic foliations (S 2), localized ductile shear zones, axial planar foliations, rotated porphyroblasts, L>S tectonites and S-C fabrics that indicate high-strain simple shear deformation. A D 3 brittle deformation is evidenced by E-W directed open folds (F 3) by flexural folding mechanism. LA-ICP-MS U-Pb dating of zircons from the syntectonic granitic dykes reveals that the main deformation event occurred from ca. 1946 to 1844 Ma. The dating also reveals that the Liaoji granites at the core were emplaced at ca. 2147 Ma and experienced anatexis at ca. 1864 Ma and 1846 Ma. It is suggested that the mechanism of doming is a combination of middle and lower crustal sub-horizontal flow, diapirism and folding superposition. Convergent flow of remobilized magma at depth and sub-horizontal simple shearing of metasedimentary rocks triggered middle and lower crustal flow in association with diapirism during the doming. The granitic gneiss domes are the products of dominant lateral and subsidiary vertical crustal mass flow under E-W directed compressional regime in the LJOB. We suggest that the orogenic belt experienced peak deformation at ca. 1946-1844 Ma, accompanied with syntectonic metamorphism and magmatism at nearly the same time. [ABSTRACT FROM AUTHOR]

High-grade terranes showing multiple episodes of leucogranite magmatism are ideal to assess the fertility of crustal protoliths. Two episodes of Paleoproterozoic leucogranite magmatism occur in the Mahalapye Complex high-grade terrane, at the northern edge of the Kaapvaal Craton. The older one (leucogranite 1) at ∼2.05 Ga represented by leucosomes, thin veins and dykes is related to the melting of ∼2.06 Ga biotite-bearing diorite gneiss. In comparison, the younger episode (leucogranite 2) at ∼2.03 Ga forms massive bodies. Leucogranite 1 is associated with the migmatitic diorite gneiss and crosscut by the ∼2.04 Ga granodiorite. Leucogranite 2 contains xenoliths of diorite gneiss and granodiorite, as well as inclusions of quartz-poor and quartz-rich metapelites (Bt + Crd + Pl + Kfs + Spl ± Sil ± Grt + Ilm + Qz). Phase equilibria modelling of a representative quartz-poor metapelite indicate that its partial melting occurred at ∼810 °C and ∼4.8 kbar in the presence of aqueous fluid. The peraluminous, ferroan and alkalic composition of the predicted melt is comparable to the chemistry of leucogranite 2. The larger volume of anatectic melt for the estimated P-T condition corresponds to the occurrence of leucogranite 2 as massive bodies. The lower temperature of in-situ anatexis (<750 °C) at a deeper level (∼7.3 kbar) and lack of additional fluid, likely did not generate significant melt fraction during the formation of leucogranite 1. The contrast in the fertility of the two protoliths with respect to production of leucogranite melts was also related to different setting of melting. Melting of diorite gneiss is related to the local influence of mafic dykes and sills, whereas melting of metapelite is related to regional collision. It follows that high pressures, water-deficient regimes and localized heat sources produce limited volumes of partial melt, whereas high temperatures, fluid-present regimes and a large-scale heat source produces higher volumes of melts. [Display omitted] • Two episodes of leucogranite (LG1, LG2) magmatism in the Mahalapye Complex. • Dehydration melting of diorite gneiss (∼730 °C/∼7.2–7.5 kbar) formed the less dominant LG1. • Fluid-present melting of metapelite (∼810 °C/∼4.8 kbar) formed the dominant LG2. • Phase equilibria modelling indicates large content of anatectic melt for LG2 than LG1. • Temperature, presence/absence of fluid, scale of heat source, affect fertility of protoliths. [ABSTRACT FROM AUTHOR]

Abstract: The Western Gneiss Region (WGR), Norway, is dominated by migmatitic gneiss that contains inclusions of eclogite, some of which contain evidence for ultrahigh-pressure metamorphism. To evaluate geochemical and age relationships between host migmatite and eclogite, we obtained LA–ICP–MS U–Pb dates and trace-element analyses for zircon from a variety of textural types of leucosome, from layer-parallel to crosscutting. Zircon textures (euhedral, oscillatory- and sector-zone grains) indicate a likely magmatic origin of the leucosomes. Caledonian U–Pb zircon dates from zircon rim and near-rim regions are as old as 410–406Ma, coeval with previously determined ages of high- and ultrahigh-pressure metamorphism of WGR eclogite. Trace-element analyses obtained simultaneously with U–Pb dates indicate crystallization of zircon under garnet-present conditions in the majority of leucosomes. Other zircons, including those from crosscutting pegmatite, yield younger ages (as young as 385Ma), coinciding with dates determined for amphibolite-facies retrogression of eclogite; trace-element analyses suggest that these zircons grew under plagioclase-present (garnet-absent) conditions. Combined age and trace-element data for leucosome zircons record the transition from high-pressure (garnet-present, plagioclase-absent) crystallization to lower-pressure (plagioclase-present) crystallization. If the euhedral zircons that yield ages coeval with peak or near-peak UHP metamorphism represent crystallization from anatectic leucosomes, these results, combined with field and petrographic observations of eclogite–migmatite relationships, are consistent with the presence of partially molten crust in at least part of the WGR during continental subduction. The decreased viscosity and increased buoyancy and strain weakening associated with partial melting may have assisted the rapid ascent of rocks from mantle to crustal depths. [Copyright &y& Elsevier]

The article estimates the age of the rocks of Solzan granitoid massif of the Khamar-Daban Province in Baikal, Russia using uranium-lead dating method. It analyzes the composition of the massif in the eastern which include granodiorite-gneiss and granite with amphibole, biotite, or garnet in dependence on the composition of host gneiss. It examines the accessory zircons that were extracted from samples representing biotite and garnet-biotite biotite leucogranites of layered bodies from the eastern part of the massif. It cites the uranium-lead dating of zircons which showed that magmatic zircons from granite melts in the Solzan massif started to crystallize 516-515 million years ago.

The calc-alkaline, gneissic El Shalul granite is the westernmost gneiss dome or core complex within the Arabian–Nubian Shield. Previous studies have indicated that it represents either a window into the underlying pre-Neoproterozoic Sahara metacraton or a melt derived from the metacraton. U–Pb LA-ICP-MS dating of magmatic zircons from two samples of the variably foliated El Shalul pluton gives ages of 637 ± 5 Ma and 630 ± 6 Ma, excluding it from representing exhumed cratonic rocks. The ages are, however, indistinguishable from the age of the Um Ba'anib pluton, constituting the core of the Meatiq Gneiss Dome, as well as several other plutons in the Eastern Desert, indicating an important magmatic pulse in the Arabian–Nubian Shield in Late Cryogenian time. Major and trace element data indicate a within-plate setting. Bulk rock Nd-isotope and Hf-isotope data on zircons from the El Shalul pluton indicate derivation of the primary melt from a relatively juvenile source, either the lower crust of a mid-Neoproterozoic volcanic arc or as a result of fractionation of a mantle-derived mafic melt. Sm–Nd bulk rock isotopic data indicate a model age of c. 720 Ma for the protolith from which the melt was derived. Time-corrected Hf-isotope data obtained on the magmatic zircons indicate that the bulk of the source rock was extracted from the mantle around 810 Ma. [ABSTRACT FROM AUTHOR]

Abstract: The Huai’an gneiss terrane is a typical Archean terrane within the Trans-North China Orogen of the North China Craton (NCC) and is composed of Tonalite–Trondhjemite–Granodiorite (TTG) suite and dioritic granulite facies gneisses. Precise SHRIMPII and Cameca IMS1280 zircon U–Pb ages demonstrate that the Huai’an gneisses formed around 2.55–2.50Ga and underwent granulite facies metamorphism at 1.85–1.80Ga. The TTG gneisses are compositionally equivalent to high-SiO2 adakites, with high SiO2 and Al2O3 concentrations, strongly fractionated rare earth element (REE) patterns ((La/Yb)N =10–54) with positive or negligible Eu anomalies (Eu/Eu*=0.9–3.4), low heavy REE (HREE) and Y concentrations, high Sr (435–938ppm) concentrations, and high Sr/Y ratios (48–261). High Mg numbers (33–60) and high compatible element concentrations (e.g., Cr=11–183ppm and Ni=11–82ppm) indicate that the magmas that formed these gneisses were derived from interaction between adakitic melts and mantle material. The dioritic gneisses have compositions similar to high-Mg diorites or sanukitoids, with high Al2O3 (14.46–17.61wt%), MgO (3.0–6.4wt%), Cr (100–304ppm), and Ni (57–109ppm) concentrations, and fractionated REE patterns with weak negative or no Eu anomalies (Eu/Eu*=0.8–1.0). All the TTG and dioritic gneiss samples analysed during this study have negative Nb, Ta, and Ti anomalies, compared to neighbouring elements in spider-diagrams, with the majority having high positive ɛ Nd(t) and ɛ Hf(t) values, close to values of the depleted mantle. These geochemical characteristics strongly suggest that protoliths of the Huai’an TTG and dioritic gneisses originated in a juvenile crust-forming event at 2.55–2.50Ga through partial melting of subducted oceanic crustal material; these magmas also assimilated mantle peridotite material during their ascent. This indicates that ∼2.5Ga mantle-derived magmatism led to significant crustal growth in the North China Craton, and was probably associated with subduction. [Copyright &y& Elsevier]

Abstract: The early history of the Yangtze Block has not been well constrained yet, due to the scarce outcrops of Archean to Paleoproterozoic rocks. In this study, we report an integrated study of zircon U–Pb age, major and trace element data and Sr–Nd–Hf isotope compositions for gray gneisses from the Houhe complex in the northwestern part of the Yangtze Block. Zircon U–Pb dating yields a weighted mean 207Pb/206Pb age of 2081±9Ma for a gneiss. This age is interpreted as the formation age of the Houhe complex and thus the Houhe complex represents the oldest rocks found in the western part of the Yangtze Block. The gray gneisses from the Houhe complex range in SiO2 contents from 58.63% to 68.59% and Na2O from 3.88% to 5.28%, and have relatively high Fe2O3 contents of 2.86–6.69%, Al2O3 of 16.01–18.88%, and MgO of 0.97–2.65%. These rocks show low Sr (149–390ppm), Cr (9.07–45.1ppm) and Ni (4.97–21.3ppm) contents, but high Y (12.9–32.7ppm) and Yb (0.95–2.25ppm). They are characterized by a relative enrichment in LILEs and LREEs, but a depletion in HFSEs. These features are similar to those of calc-alkaline granitoids, suggesting that their formation might be related to a subduction-related process or remelting of preexisted arc rocks. Compiled age spectra of Archean to Paleoproterozoic zircon grains reveal that the western and eastern parts of the Yangtze Block have similar Archean and early Paleoproterozoic age ranges, indicative of the occurrence of an old continental nucleus in both parts. On the other hand, there are large amounts of ca. 2050–2400Ma zircon grains revealed in the western part of the Yangtze Block, implying that there might be a microcontinent with an active-type continental margin during the Paleoproterozoic times in the western part of the Yangtze Block. Because of the poor outcrops of Archean to Paleoproterozoic rocks in the Yangtze Block, it is no possible to establish the exact nature of such an active margin yet. The occurrence of ca. 2.0Ga khondalitic and metasedimentary rocks has been documented in the eastern part of the Yangtze Block, implying that there might be a passive-type continental margin in the eastern part of the Yangtze Block. At ca. 2.0Ga, the collision between the western microcontinent and the eastern part may have resulted in the final arc-continental collision and the amalgamation of the Yangtze Block. [Copyright &y& Elsevier]

Abstract: Three samples of garnet–kyanite paragneiss from the Variscan Ulten Zone (Northern Italy) were studied in detail for U–Th–Pb monazite dating. Monazite in these gneisses is abundant, shows highly variable grain size and occupies different textural positions: within the matrix, as inclusion in garnet and kyanite, within apatite aggregates. Monazite shows different deformation features as a function of the textural position: enclosed (shielded) monazite is generally more fractured than matrix (unshielded) monazite. The integration of textural information with deformation features and in situ U–Th–Pb analyses by LA–ICP-MS indicates that there is no direct correlation between textural site and monazite ages. Old ages of 351–343Ma, determined on portions of large matrix (unshielded) monazite and on rare domains of monazite shielded by garnet, have been related to a prograde stage of the Variscan metamorphic evolution of the Ulten Zone. Ages of 330–326Ma, which are related to the thermal peak, are recorded by small matrix monazite, external domains of large matrix monazite, and by (domains of) fractured monazite enclosed in garnet and kyanite. Large, old unshielded grains formed as blasts during the prograde metamorphic history and survived the peak metamorphism during which crystallisation/re-crystallisation partially occurred. [Copyright &y& Elsevier]

Zircon grains in rocks collected from the Itsaq Gneiss Complex, southwest Greenland, were analyzed for U-Pb and Lu-Hf in the same grain using isotope dilution - thermal ionization mass spectrometry (TIMS) and multicollector - inductively coupled plasma - mass spectrometry (MC-ICP-MS). Grains were pretreated using chemical abrasion or air abrasion to assure that only zircon material unaffected by the migration of parent and daughter elements was analyzed. The data are consistent with derivation of all studied rocks from a single enriched mantle source or mafic crustal protolith with 176Lu/177Hf of 0.022 ±0.003 that was repeatedly melted and produced tonalitic magmas. The assessment of the primary mantle source from which this mafic protolith was derived, at or before 3.85 Ga, greatly depends on the assumed composition of the bulk silicate Earth. Using the currently accepted Lu-Hf bulk Earth parameters based on the analysis of chondrites yields εHf(T) of 0 to +1 for the 3.80-3.86 Ga rocks, suggesting that the protolith was derived from mantle that underwent moderate depletion shortly before 3.9 Ga. However, using alternative models of the bulk silicate Earth composition, i.e., that account for the possible irradiation-induced accelerated decay of 176Lu in the early Solar System, and (or) loss of the products of early planetesimal or planetary differentiation, can lead to widely variable interpretations of the enrichment or depletion history of the mantle source of the Itsaq protolith. Des grains de zircon dans des roches prélevées du complexe gneissique Itsaq, dans le sud-ouest du Groenland, ont été analysés pour U-Pb et Lu-Hf dans un même grain au moyen de la dilution isotopique dans un spectromètre de masse à ionisation thermique (TIMS) et par spectrométrie de masse à plasma inductif après ablation au laser (MC-ICP-MS). Les grains ont été prétraités par abrasion chimique ou par abrasion à l'air afin de s'assurer que seul le matériel du zircon non affecté par la migration d'éléments parents ou de filiation est analysé. Les données concordent avec la provenance de toutes les roches étudiées d'une source unique enrichie du manteau ou d'un protolithe mafique de la croûte, avec un rapport 176Lu/177Hf de 0 022 ±0 003, qui a été mis en fusion de manière répétée et qui a produit des magmas tonalitiques. L'évaluation de la source primaire de manteau de laquelle ce protolithe mafique a été dérivé, à ou vers 3,85 Ga, dépend grandement de la composition en silicate présumée de l'ensemble de la Terre. L'utilisation des paramètres actuellement acceptés de Lu-Hf pour l'ensemble de la Terre, lesquels sont basés sur l'analyse des chondrites, donne une valeur de εHf(T) entre 0 et +1 pour les roches âgées de 3,80-3,86 Ga, suggérant que le protolithe provienne du manteau qui a subi un appauvrissement modéré peu avant 3,9 Ga. Toutefois, en utilisant d'autres modèles pour la composition en silicate de l'ensemble de la Terre, c.-à-d. qui tiennent compte de la possible accélération de la désintégration de 176Lu, induite par irradiation dans le système solaire précoce et (ou) la perte de produits de la différenciation planétisimale ou planétaire, peut conduire à des interprétations grandement variables de l'historique d'enrichissement ou d'appauvrissement de la source dans le manteau du protolithe Itsaq. [ABSTRACT FROM AUTHOR]

Reconstructing tectonic histories involving continental collision, subduction, and exhumation at plate-tectonic rates of ~1 cm/year, requires precise U-Pb zircon geochronology. The Western Gneiss Region has exceptional exposures of high-pressure (HP) and ultra-high-pressure (UHP) rocks. The strategy adopted here involved sampling eclogite and associated late unstrained pegmatites to acquire the time of eclogite crystallization and subsequent exhumation, respectively. The oldest eclogite sampled is 415 ±1 Ma from layered, probably UHP eclogite at Tevik, Averøya, also with a garnet-hornblende assemblage at 410 ±1 Ma. The Flem Gabbro eclogite margin, with implied UHP conditions, is 410 ±2 Ma. Hornblende eclogite at Seth, Lepsøya, never at UHP, is 412 ±2 Ma. These compare to Devonian ages of 401 ±1 Ma for overgrowths on Proterozoic baddeleyite in Selnes Gabbro, 402 ±2 Ma for coesite eclogite at Hareidlandet, 405-400 Ma for coesite eclogite at Flatraket, and 405 ±2 Ma for near-UHP eclogite at Hjelmelandsdalen. The 415 Ma eclogite at Tevik compares to granitic pegmatite in the same outcrop at 395.2 ±1.3 Ma and to pegmatite in eclogite at Aspøya at 395.3 ±2 Ma. The 410 Ma age at Flem compares to nearby pegmatite in eclogite at 396 ±4 Ma. Collectively, these results imply 14-20 million years between deep eclogite crystallization at ~130 km and return to amphibolite-facies conditions at ~30 km, with crystallization of locally derived granitoid melts. Nearby garnet-pyroxenite records older ages (~430) and greater depths (~200 km), but on similar exhumation paths at ~0.4-0.7 cm/year. La reconstruction d'historiques tectoniques impliquant des collisions continentales, de la subduction et de l'exhumation à des taux tectoniques d'environ 1 cm/année demande une géochronologie U-Pb sur zircon précise. La région du gneiss occidental de la Norvège présente des affleurements exceptionnels de roches ayant subi des pressions hautes (HP) et ultra-hautes (UHP). La stratégie adoptée ici était d'échantillonner l'éclogite et des pegmatites associées tardives n'ayant pas subi de contraintes afin d'obtenir respectivement le moment de cristallisation de l'éclogite et son exhumation subséquente. L'éclogite échantillonnée la plus ancienne a 415 ±1 Ma et provient d'une éclogite, probablement formée à ultra-haute pression, située à Tevik, Averøya, avec un assemblage grenat-hornblende datant de 410 ±1 Ma. La bordure éclogite du gabbro Flem, possédant des conditions UHP, est âgée de 410 ±2 Ma. L'éclogite à hornblende à Seth, Lepsøya, n'ayant jamais subi de UHP, date de 412 ±2 Ma. Elles se comparent avec des âges dévoniens de 401 ±1 Ma pour des accroissements sur une baddeleyite datant du Protérozoïque dans le gabbro Selnes, de 402 ±2 Ma pour l'éclogite coésite à Hareidlandet, de 405-400 Ma pour l'éclogite coésite à Flatraket et de 405 ±2 Ma pour l'éclogite presque UHP à Hjelmelandsdalen. À Tevik, l'éclogite datant de 415 Ma peut être comparée à une pegmatite granitique dans le même affleurement et datant de 395,2 ±1,3 Ma et à une pegmatite dans l'éclogite à Aspøya datant de 395,3 ±2 Ma. L'âge de 410 Ma à Flem se compare bien à celui de la pegmatite dans une éclogite avoisinante datant de 396 ±4 Ma. Ensemble, ces résultats signifient une période de 14 à 20 Ma entre la cristallisation en profondeur de l'éclogite, à ~130 km, et le retour aux conditions du faciès des amphibolites à ~30 km, avec la cristallisation de granitoïdes en fusion de provenance locale. La pyroxénite à grenat avoisinante enregistre des âges plus anciens (~430 Ma) et une plus grande profondeur (~200 km) mais avec des cheminements d'exhumation similaires d'environ 0,4-0,7 cm/année. [ABSTRACT FROM AUTHOR]

Abstract: The Paleoproterozoic Jiao-Liao-Ji Belt lies in the Eastern Block of the North China Craton, with its southern segment extending across the Bohai Sea into the Jiaobei massif. High-pressure pelitic and mafic granulites have been recently recognized in the Paleoproterozoic Jingshan Group (Jiaobei massif). New SHRIMP U–Th–Pb geochronology combined with cathodoluminescence (CL) imaging of zircon has been applied to the determination of the timing of the metamorphism of the high-temperature and high-pressure granulites and associated gneisses and marbles. Metamorphic zircons in these high-pressure granulites, gneisses and marbles occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores. Metamorphic zircons are all characterized by nebulous zoning or being structureless, with high luminescence and relatively low Th/U values. Metamorphic zircons from two high-pressure mafic granulites yielded 207Pb/206Pb ages of 1956±41Ma and 1884±24Ma. One metamorphic zircon from a garnet–sillimanite gneiss also gave an apparent 207Pb/206Pb age of 1939±15Ma. These results are consistent with interval of ages of c. 1.93–1.90Ga already obtained by previous studies for the North and South Liaohe Groups and the Laoling Group in the northern segment of the Jiao-Liao-Ji Belt. Metamorphic zircons from a high-pressure pelitic granulite and two pelitic gneisses yielded weighted mean 207Pb/206Pb ages of 1837±8Ma, 1821±8Ma and 1836±8Ma respectively. Two diopside–olivine–phlogopite marbles yielded weighted mean 207Pb/206Pb ages of 1817±9Ma and 1790±6Ma. These Paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c. 1.85Ga produced from the Ji''an Group in the northern segment of the Jiao-Liao-Ji Belt and c. 1.86–1.80Ga obtained for the high-pressure pelitic granulites from the Jingshan Group in the southern segment. As this metamorphic event was coeval with the emplacement of A-type granites in the Jiao-Liao-Ji Belt and its adjacent areas, it is interpreted as having resulted from a post-orogenic or anorogenic extensional event. [Copyright &y& Elsevier]

Abstract: U–Pb zircon geochronology of two Permo-Triassic granites (samples OT-52 and OT-272 with ages of 229±8Ma and 256±2Ma, respectively) in the Unazuki area, Hida Metamorphic Belt, southwest Japan, revealed the presence of Eoarchean to Paleoproterozoic inheritance. Inheritance is consistent with both samples showing low zircon saturation temperatures for their bulk compositions. In OT-52, dark in CL, low Th/U zircon domains have a mean 207Pb/206Pb age of 1940±17Ma, which is consistent with an age of 1937±6Ma for anatexis in the Precambrian Busan gneiss complex in Korea. Eoarchaean inherited zircons with 207Pb/206Pb ages from ca. 3750 to 3550Ma are common in OT-272 but are few in OT-52, suggesting a source from rocks with affinities to those in the Anshan area in the northeast China part of the North China Craton. On the other hand, a Hida Metamorphic Belt metasedimentary gneiss into which the granites were intruded contains ca. 1840, 1130, 580, 360, 285 and 250Ma zircons (). These ages suggest that the Unazuki Mesozoic granites did not originate from proximal Hida Metamorphic Complex rocks, but instead from unrelated rocks obscured at depth. The predominance of Eoarchean to Paleoproterozoic age components, and the marked lack of 900–700Ma components suggest that the source was the (extended?) fringe of the North China Craton, rather than from Yangtze Craton crust. The Mesozoic evolution of Japan and its linkages to northeast Asia are discussed in the context of these results. [ABSTRACT FROM AUTHOR]

Abstract: Field studies integrated with cathodoluminescence petrography and SHRIMP U–Pb dating of zircons from >150 orthogneisses and metatonalites from the Eoarchaean Itsaq Gneiss Complex (southern West Greenland) shows that only a minority contain ≥3840Ma zircons, whereas the majority carry only younger ones. Rocks containing ≥3840Ma zircons vary from very rare single-phase metatonalites to more common complexly banded tonalitic migmatites. The former metatonalites have simple oscillatory-zoned ≥3840Ma zircon with limited recrystallisation and overgrowth, whereas the more common migmatites have much more complicated zircon populations with both ≥3840Ma and 3650–3600Ma oscillatory-zoned zircon, more extensive recrystallisation and widespread complex core-rim multiple growth relationships. With only 100–160ppm Zr in the tonalites and likely melt generation temperatures of >1000°C, the experimentally determined zircon solubility–melt composition relationships established by other workers shows that the precursor melts to the Itsaq Complex tonalites were strongly undersaturated in zircon, thus any entrained xenocrystic zircon would have been rapidly dissolved. Therefore, the ≥3840Ma oscillatory-zoned zircons crystallised out of tonalitic melt and gives magmatic age of the rock in which they occur. With an established igneous age of ≥3840Ma established from such relationships, we interpret the correlated variation between the field nature of these rocks and their zircon petrography/age structure as due to superimposition onto ≥3840Ma tonalite protoliths of variable amounts of heterogeneous strain, heterogeneous distribution of melt patches formed during in situ anatexis at up to ∼800°C, plus granitic veining. This explains why geologically simple metatonalites have simple zircon populations, whereas complex orthogneisses have complex zircons. The large amount of integrated field, geochemical and zircon data rule out an alternative interpretation, that the ≥3840Ma zircons represent an igneous xenocrystic component, present in younger rocks to varying degrees. If this were true, then the structurally simple (less reworked) rocks should still display complex zircon populations. Gneisses with ≥3840Ma zircon are commonest on Akilia and neighbouring islands, in Itilleq fjord (∼65km east Akilia) and on the north of Ivisaartoq (∼150km northeast of Akilia). These include from Itilleq a 3891±6Ma gneissic tonalite (with minor neosome)—which is currently the oldest rock recognised in the Itsaq Gneiss Complex. Overall, the ≥3840Ma tonalites are a widespread and unevenly distributed in the Itsaq Gneiss Complex, and they are a volumetrically minor component compared with ∼3800, 3750 and 3700Ma tonalite generations. Using the subset of our data covering Itilleq and the neighbouring fjords, migmatite samples with ≥3800Ma igneous zircon are mutually exclusive from migmatite samples with ∼3700Ma igneous zircon. This suggests that prior to an amalgamation event followed by 3660–3600Ma high-grade metamorphism, ≥3840Ma tonalites might have resided in a terrane discrete from ∼3700Ma tonalites. This is in accord with interpretation of the non-migmatised part of the Complex in the Isua area, where a terrane of ∼3800Ma tonalites with a minor associated ≥3840Ma component and a terrane with ∼3700Ma tonalites were tectonically juxtaposed at ∼3660Ma. [Copyright &y& Elsevier]

Abstract: U–Pb zircon ages from trondhjemitic gneisses of the Badplaas domain, located in the southwestern parts of the Mesoarchaean Barberton granitoid-greenstone terrain of South Africa, document a hitherto largely unrecognized plutonic event. The geochronological results indicate the addition of juvenile, felsic crust over a period of 60 Myr between ca. 3290 and 3230Ma and prior to the main collisional event in the granitoid-greenstone terrain at 3230Ma. The timing and duration of plutonism, together with the structural and compositional heterogeneity of the Badplaas domain suggest that the Badplaas domain represents part of a convergence-related magmatic arc. On a regional scale, the spatial and temporal relationships between plutonism, metamorphism and deformation are interpreted to preserve an almost complete inventory of a Mesoarchean arc-trench system. This includes (1) the 3290–3230Ma convergence-related magmatic arc, (2) a largely coeval deposition of the back-arc type volcano-sedimentary 3260–3225Ma Fig Tree Group of the Barberton greenstone belt, (3) relics of the underplated crust preserved in high-P, low-T rocks to the immediate east of the magmatic arc, juxtaposed against (4) high-T medium-P rocks of the overlying plate along the major crustal structure of the Inyoni shear zone. These findings corroborate the notion that thermal and rheological conditions on the early Earth supported, at least locally, crustal growth and the extraction of buoyant felsic crust along convergence-related magmatic arcs. [Copyright &y& Elsevier]

Abstract: The Kokchetav Massif of northern Kazakhstan is the best-known metamorphic diamond locality among numerous ultrahigh-pressure (UHP) terranes in the world. At the Kumdy-Kol deposit, diamondiferous rocks are interbedded with granitic gneisses, and biotite gneisses; some have been migmatized. Some granite gneisses and migmatites were formed by partial melting of diamondiferous rocks. To verify such suggestion, sensitive high resolution ion microprobe (SHRIMP) U–Pb dating of zoned zircons from migmatites at the Kumdy-Kol region was performed to constrain the age of partial melting of the Kokchetav UHP metamorphic rocks. Most age data from core and rim domains of zircon separates are concordant. The apparent 206Pb/238U ages for core and rim domains of zirconsare nearly identical within analytical error. All SHRIMP analyses of zircons from four samples fall in the range 508–538 Ma with the weighted mean age for all zircon domains at 526±2.1 (MSWD=1.7). Our data show that migmatization of UHP pelites occurred later than the peak metamorphism (537±9 Ma) and the decompression partial melting took place during exhumation of diamondiferous rocks from mantle depths to amphibolite-facies conditions at mid-crustal levels (507±8 Ma). [Copyright &y& Elsevier]