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• A continuum of Paleoproterozoic heritage, seen in Nd model ages, from N to S in the Bushmanland Domain (BD), with the Buffels River shear zone simply a regional discontinuity. • The southern BD (SBD) has a greater juvenile component than the northern BD (NBD), becoming more dominant post-1.10 Ga. • Orthogneisses derived from crustal magmas with subduction-influenced signatures with mixed Meso- to Paleoproterozoic model ages. The SBD Koperberg Suite derived from low degrees of melting of enriched SCLM, but at deeper levels than those of the NBD. • The paragneisses derived from largely reworked, mixed Meso- to Paleoproterozoic felsic to intermediate sources. • Paragneisses derived from largely reworked, mixed Meso- to Paleoproterozoic felsic to intermediate sources. • SBD Koperberg Suite derived from low degrees of melting of enriched SCLM, but at somewhat deeper levels than NBD Koperberg magmas. • D 2 deformation constrained to ~ 1.11-1.09 Ga, with peak M 3 metamorphism at ~ 1.05-1.02 Ga. The timing of tectonomagmatic events and petrogenesis of ortho- and paragneisses within the high-grade southern Bushmanland Domain (SBD) of the Namaqua Metamorphic Province is, largely, poorly constrained. U-Pb age dating and whole rock lithogeochemistry of the rocks of the upper granulite facies Kliprand dome provide constraints with regards to the evolution of the SBD during the 1.2–1.0 Ga Namaquan Orogeny. The SBD has a greater juvenile component, particularly added between ∼1.1 and 1.02 Ga, compared to the northern Bushmanland Domain (NBD). This is reflected in marginally younger T DM model ages (1.60–1.95 Ga) than for the NBD (∼1.9–2.4 Ga), and higher ε Nd(t) values for magmatic rocks, particularly for the late- to post-tectonic Spektakel Suite (SS), concentrated in the SBD. The ∼1.06 Ga Klein Lieslap Charnockite of the SS was derived from reworking of radiogenic Paleoproterozoic arc crust, but has a large juvenile depleted component compared to that of the ∼1.19 Ga Grootberg Gneiss of the syn-tectonic Little Namaqualand Suite. SBD mafic magmas were derived from low degrees of partial melting of subduction-influenced enriched subcontinental lithospheric mantle. The pre-tectonic ∼1.17 Ga Oorkraal Suite shows limited crustal contamination of mantle-derived melts. The post-tectonic ∼1.05 Ga Koperberg Suite was derived from deeper mantle melting than that in the NBD, but shows less crustal contamination. The supracrustal Kamiesberg Group was derived from mixed, reworked Meso- to Paleoproterozoic crust that showed a more dominant depleted component compared to that in the NBD. Incongruent anatexis of the paragneisses gave rise to crosscutting garnetiferous quartzofeldspathic segregations, notably the strongly peraluminous ∼1.07 Ga Ibequas Leucogranite. D 2 deformation, at ∼1.11–1.09 Ga, resulted in penetrative foliation. D 3 deformation, related to the development of a sub-vertical foliation in the limbs of F 2 folds, is constrained to ∼1.07–1.06 Ga, between emplacement of the Ibequas Leucogranite and the Koperberg Suite. Peak granulite facies M 3 metamorphism occurred at ∼1.05–1.02 Ga. These age constraints are similar to those of the NBD reflecting a synchronous tectonomagmatic and kinematic history. The SBD represents a deeper crustal level with a greater mantle influence than the NBD. [ABSTRACT FROM AUTHOR]

Abstract: This study presents Lu–Hf geochronology of zoned garnet in high‐P eclogites from the North Qilian orogenic belt. Selected samples have ~mm‐sized garnet grains that have been sampled with a micro‐drill and analysed for dating. The Lu–Hf dates of bulk garnet separates, micro‐drilled garnet cores and the remnant, rim‐enriched garnet were determined by two‐point isochrons, with cores being consistently older than the bulk‐ and rim‐enriched garnet. The bulk garnet separates of each sample define identical garnet–whole rock isochron date of c. 457 Ma. Consistent U–Pb zircon dates of 455 ± 8 Ma were obtained from the eclogite. The Lu–Hf dates of the drilled cores and rim‐rich separates suggest a minimum garnet growth interval of 468.9 ± 2.4 and 452.1 ± 1.6 Ma. Major and Lu element profiles in the majority of garnet grains show well‐preserved Rayleigh‐style fractionated bell‐shaped Mn and Lu zoning profiles, and increasing Mg from core to rim. Pseudosection modelling indicates that garnet grew along a P–T path from ~470–525°C and ~2.4–2.6 GPa. The exceptional high‐Mn garnet core in one sample indicates an early growth during epidote–blueschist facies metamorphism at <460°C and <0.8 GPa. Therefore, the Lu–Hf dates of drilled cores record the early prograde garnet growth, whereas the Lu–Hf dates of rim‐rich fractions provide a maximum age for the end of garnet growth. The microsampling approach applied in this study can be broadly used in garnet‐bearing rocks, even those without extremely large garnet crystals, in an attempt to retrieve the early metamorphic timing recorded in older garnet cores. Given a proper selection of the drill bit size and a detailed crystal size distribution analysis, the cores of the mm‐sized garnet in most metamorphic rocks can be dated to yield critical constraints on the early timing of metamorphism. This study provides new crucial constraints on the timing of the initial subduction (before c. 469 Ma) and the ultimate closure (earlier than c. 452 Ma) of the fossil Qilian oceanic basin. [ABSTRACT FROM AUTHOR]

Eclogites were relatively recently found in the Belomorian Mobile Belt (BMB) (Volodichev et al., 2004; Shchipanskii et al., 2005; Konilov et al., 2004). The very first isotopic dates (Volodichev et al., 2004; Mints et al., 2010) were obtained for these rocks in the northwestern (in the Salma and Kuru-Vaara areas) and central (Gridino area) portions of BMB and corresponded to the Archean: approximately 2.72-2.87 Ga. Because no crustal eclogites older that 2.0 Ga (Möller et al., 1995) had been known before these dates were obtained, these eclogites were regarded as unique. It is commonly believed that no crustal eclogites could be formed in the Archean because the crust was then relatively thin (Kröner, 2010), and hence, the find of crustal eclogites of Archean age in BMB called for a fundamental revision of geodynamic reconstructions of the crustal evolution and was one of the main arguments invoked to support the hypothesis that currently operating geodynamic mechanisms of plate tectonic can be extrapolated to the Early Precambrian (Rozen et al., 2008). However, these finds were practically immediately followed by serious doubts that the primary estimates of the timing of the eclogite metamorphism in the Belomorian Belt may be incorrect (Mitrofanov et al., 2009; and others). [ABSTRACT FROM AUTHOR]

Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U-Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5-30 nm nanospheres of metallic Pb. They are randomly distributed with respect to zircon crystallinity, and their association with a Ti- and Al-rich silica melt suggests that they represent melt inclusions generated during ultra-high temperature metamorphism. Metallic Pb is exceedingly rare in nature and previously has not been reported in association with high-grade metamorphism. Formation of these metallic nanospheres within annealed zircon effectively halts the loss of radiogenic Pb from zircon. Both the redistribution and phase separation of radiogenic Pb in this manner can compromise the precision and accuracy of U-Pb ages obtained by high spatial resolution methods. [ABSTRACT FROM AUTHOR]

In the Forquilha area (NE Brazil), in NW Borborema Province, high to ultra-high pressure rocks are an important geological key to understanding West Gondwana amalgamation. U–Pb geochronological data for a retrograded eclogite sample yielded an upper intercept age of ca. 1520 Ma and a lower intercept age of ca. 620 Ma. These ages most likely represent the crystallization age of the basaltic protolith and the regional metamorphism, respectively. The retrograded eclogites are enclosed in migmatized quartz-feldspathic gneiss and sillimanite (after kyanite)-garnet-biotite gneiss. Detrital U–Pb zircon data for these paragneisses show only Paleoproterozoic zircon grains with ages clustering from ca. 1800 Ma (the maximum depositional age) to ca. 2480 Ma, and frequency peaks at 2.2–2.0 Ga. Combined with Nd isotopic data from the Forquilha paragneisses, one can assume a single Paleoproterozoic source. Basement rocks of the Ceará Central and the Rio Grande do Norte domains are the most likely candidates. The absence of Meso- and Neoproterozoic zircon grains suggest that the retrograded eclogite bodies possibly do not represent slivers of oceanic rocks captured in active margin sequences during subduction. It was identified that the high-pressure rocks of the Forquilha area are in tectonic contact with high-pressure granulite facies rocks of the Ceará Complex (Independência unit) that present detrital zircon records of an active margin setting, with ages ranging from ca. 660 Ma to 2200 Ma. Metamorphism of this sequence occurred at ca. 650 Ma. Considering previous studies, field relationships, and metamorphic paragenesis, a tectonic scenario is inferred, in which the Forquilha retrograded eclogites represent Mesoproterozoic basaltic rocks of an extensional event that were metamorphosed under eclogite facies conditions during Late Neoproterozoic continental subduction/collision, and juxtaposed to an active margin sequence during the exhumation process. [ABSTRACT FROM AUTHOR]

A U-Pb-He double-dating method is applied to detrital zircons with core-rim structure from the Ganges River in order to determine average short- and long-term exhumation rates for the Himalayas. Long-term rates are calculated from the U/Pb ages of metamorphic rims of the grains that formed during the Himalayan orogeny and their crystallization temperatures, which are calculated from the Ti-in-zircon thermometer. Short-term rates are calculated from (U-Th)/He ages of the grains with appropriate closure temperatures. The results show that short-term rates for the Himalayas, which range from 0.70 ± 0.09 to 2.67 ± 0.40 km/Myr and average 1.75 ± 0.59 (1 σ) km/Myr, are higher and more varied than the long-term rates, which range from 0.84 ± 0.16 to 1.85 ± 0.35 km/Myr and average 1.26 ± 0.25 (1 σ) km/Myr. The differences between the long-term and short-term rates can be attributed to continuous exhumation of the host rocks in different mechanisms in continental collision orogen. The U/Pb ages of 44.0 ± 3.7 to 18.3 ± 0.5 Ma for the zircon rims indicate a protracted episode of ∼25 Myr for regional metamorphism of the host rocks at deeper crust, whereas the (U-Th)/He ages of 42.2 ± 1.8 to 1.3 ± 0.2 Ma for the zircon grains represent a protracted period of ∼40 Myr for exposure of the host rocks to shallower crustal level. In particular, the oldest (U-Th)/He ages of the zircon grains are close to the oldest U/Pb ages for the rims, indicating that some parcels of the rocks that contain zircons were rapidly exhumed from deep to shallow levels in the stage of collisional orogeny. On the other hand, some parcels of the rocks may have been carried upwards by thrust faults in the post-collisional stage. The parcels could be carried upwards by the thrust faults that steepen as they near the surface, or by transient movement faults so that areas of rapid exhumation became areas of slow exhumation and visa versa on a time scale of a few Myr in order to maintain the continuous exhumation. In this regard, the Ganges River must be preferentially sampling areas that are currently undergoing above average rates of uplift. [ABSTRACT FROM AUTHOR]

Abstract: Re–Os geochronology of fractions composed of unsized, coarse, and fine molybdenite from a pod of unusual monazite–xenotime gneiss within a granulite facies paragneiss, Hudson Highlands, NY, yielded dates of 950.5±2.5, 953.8±2.6, and 941.2±2.6Ma, respectively. These dates are not recorded by co-existing zircon, monazite, or xenotime. SEM–BSE imagery of thin sections and separated grains reveals that most molybdenite grains are composed of core and rim plates that are approximately perpendicular. Rim material invaded cores, forming irregular contacts, probably reflecting dissolution/reprecipitation. EPMA and LA-ICP-MS analyses show that cores and rims have different trace element concentrations (for example, cores are relatively enriched in W). On the basis of inclusions of zircon with metamorphic overgrowths, we conclude that molybdenite cores and rims formed after high-grade regional metamorphism. The discovery of cores and rims in individual molybdenite grains is analogous to multi-component U-Pb geochronometers such as zircon, monazite, and titanite; thus, molybdenite should be carefully examined before dating to ensure that the requirement of age homogeneity is fulfilled. [Copyright &y& Elsevier]

Late Neoproterozoic (c. 555 Ma) high-pressure–ultrahigh-temperature (HP–UHT) metamorphism has been documented for MgAl-rich migmatitic granulites from the Palni Hills in the Southern Granulite Terrane (South India). Conspicuous reaction textures indicate a clockwise P–T evolution, which is constrained through P–T pseudosection modelling and thermobarometry. The transformation of sillimanite to kyanite, which coexisted with orthopyroxene and/or garnet, records an early stage of loading. During subsequent heating to UHT conditions at deep-crustal levels (c. 1000°C, 13·0 kbar) kyanite was transformed to sillimanite, and distinct peak-temperature assemblages (orthopyroxene + sillimanite + mesoperthite + rutile ± garnet ± quartz ± sapphirine, garnet + biotite + sillimanite + spinel + corundum + rutile + plagioclase and garnet + orthopyroxene + rutile + plagioclase ± quartz) formed in specific bulk compositions through biotite-dehydration-melting reactions. A sequence of corona and sapphirine-bearing symplectite textures records subsequent isothermal decompression of the order of c. 6 kbar at persistent extreme temperatures (1010–920°C). UHT decompression is consistent with the uniformly high Al contents of porphyroblastic, coronitic and symplectitic orthopyroxene (up to 10·4 wt % Al2O3). Regrowth of garnet and biotite documents post-decompressional cooling to subsolidus conditions of <800°C at mid-crustal levels (c. 6 kbar). HP–UHT metamorphism and the clockwise P–T path of the Palni Hills granulites is attributed to a single late Neoproterozoic tectono-metamorphic event, which has been consistently dated at c. 555 Ma through laser ablation inductively coupled plasma mass spectrometry U–Pb analyses of zircon and in situ electron microprobe U–Th–total Pb analyses of monazite. The MgAl-rich granulites occur as enclaves in enderbitic orthogneiss. The intrusion of the orthogneiss in the late Archean (2534 ± 28 Ma) marks the beginning of voluminous granitoid emplacement in the Southern Granulite Terrane between 2530 and 2440 Ma, which presumably caused a first high-grade metamorphic event in the early Paleoproterozoic (2469 ± 13 Ma), recorded by zircon cores in the MgAl-rich granulites. The clockwise P–T–t evolution indicates that HP–UHT metamorphism in the central part of the Southern Granulite Terrane is related to collisional tectonics during the final assembly of Gondwana in the late Neoproterozoic. Extreme heating is ascribed to upwelling of the asthenosphere during delamination of the thickened lithospheric mantle. Fast uplift of the rocks followed by mid-crustal isobaric cooling reflects extension of the hot overthickened crust and its subsequent cooling to a normal geotherm. [ABSTRACT FROM AUTHOR]

The article offers information on the mineralogy of grothite. It states that mineralogy of titanite is explained by the new possibility of application of this mineral for radioisotope dating of the rock age and thermodynamic calculation of mineral-forming processes, as well as by the fact that some titanites contain high concentrations of rare-earth elements. It mentions that titanites with prevailing hydroxyl ion in their structure are attributed to the vuagnatite group, which is typical for metasomatic rocks and metamorphic formations of low-grade metamorphism.

Abstract: The studied ultrahigh-pressure (UHP) eclogites, located in the southwestern part of the North Dabie complex zone (NDZ) in central China, represent deeply subducted mafic lower continental crust of the South China Block and display a multiple metamorphic evolution. However, the exact timing of the UHP metamorphism in the NDZ is poorly constrained, and thus impedes our understanding of the tectonic evolution of this area. In order to constrain the ages of peak UHP metamorphism and subsequent retrogression during continental subduction and exhumation, zircon from the eclogites in the NDZ has been investigated by a combined petrological, trace element and U–Pb isotopic study. In combination with petrological data, the present zircon SHRIMP and LA-ICPMS U–Pb dating provides precise constraints on the timing of multistage metamorphic events on the eclogites in the region. The U–Pb isotope in zircon from the eclogites records the times of multiple discrete events in the history of the rock such as Neoproterozoic, 238±2Ma, 222±4–227±2Ma, 210±4–215±2Ma, 199±2Ma and 176±2–188±2Ma. Neoproterozoic ages defined by relic igneous cores represent their protolith time and the other age-groups reflect their metamorphic records by CL images, low Th/U ratios and mineral inclusions. By U–Pb age, trace element and mineral inclusion of zircon, at least two episodes of eclogite-facies metamorphism have been identified from the eclogites, best estimated at 226±3 and 214±3Ma, respectively. The younger ages of 199±2 and 176–188Ma most likely record the granulite-facies overprinting and amphibolite-facies retrogression occurring during exhumation whereas the age of 238±2Ma probably reflects the prograde metamorphic timing prior to the UHP metamorphism. Thus, Neoproterozoic mafic lower continental crust was subducted to depths greater than 120km (corresponding to the lowest pressure for diamond formation) and suffered from UHP metamorphism at 226±3Ma. Then these UHP metamorphic rocks were exhumed to about 60km depth at 214±3Ma and experienced high-pressure quartz eclogite-facies retrograde metamorphism, and subsequently to lower- and upper-crustal levels and overprinted by granulite- and amphibolite-facies metamorphism, respectively. [Copyright &y& Elsevier]

Abstract: The geological evolution in the southwestern Chinese Tianshan orogen in NW China is poorly understood because of controversial geochronological results of the low-temperature eclogites. It is likely due to Sm–Nd and Rb–Sr isotopic disequilibrium in different mineral fractions, excess 40Ar in the high-pressure K-bearing minerals, and multi-stage growths of zircon in U–Pb system. Rutile, as a typical eclogite-facies mineral with high U/Pb ratio, is a feasible candidate to constrain the eclogite-facies metamorphic event. In this study, we measured U–Pb rutile age data using SIMS (Cameca IMS 1280) for low-temperature eclogites from southwestern Chinese Tianshan orogen, NW China. We demonstrate that SIMS is a powerful technique for in-situ U–Pb dating of rutile with U content as low as 1ppm. SIMS U–Pb rutile dating yielded an age of 318±7Ma, which may record the time when the rocks cooled down to the closure temperature of Pb in rutile (ca. 500°C), but should be very close to the time of eclogite-facies metamorphism in terms of common fast exhumation of coesite-bearing eclogite. This new proof strongly suggests a late Carboniferous collision between Kazakhstan–Yili block and Central Tianshan microcontinent. [ABSTRACT FROM AUTHOR]

Abstract: Garnet-bearing micaschists and paragneisses of the Yaounde Group in the Pan-African Central African Orogenic Belt in Cameroon underwent a polyphase structural evolution with the deformation stages D1–D2, D3 and D4. The garnet-bearing assemblages crystallized in course of the deformation stage D1–D2 which led to the formation of the regional main foliation S2. In X Ca–X Mg coordinates one can distinguish several zonation trends in the garnet porphyroblasts. Zonation trends with increasing X Mg and variably decreasing X Ca signalize a garnet growth during prograde metamorphism. Intermineral microstructures provided criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The syndeformational P–T path sections calculated from cores and rims of garnets in individual samples partly overlap and align along clockwise P–T trends. The P–T evolution started at ∼450°C/7 kbar, passed high-pressure conditions at 11–12kbar at variable temperatures (600–700°C) and involved a marked decompression toward 6–7kbar at high temperatures (700–750°C). Th–U–Pb dating of metamorphic monazite by electron microprobe (EMP-CHIME method) in eight samples revealed a single period of crystallization between 613±33Ma and 586±15Ma. The EMP-monazite age populations between 613±33Ma enclosed in garnet and 605±12Ma in the matrix apparently bracket the high temperature–intermediate pressure stage at the end of the prograde P–T path. The younger monazites crystallized still at amphibolite-facies conditions during subsequent retrogression. The Pan-African overall clockwise P–T evolution in the Yaounde Group with its syndeformational high pressure stages and marked pressure variations is typical of the parts of orogens which underwent contractional crustal thickening by stacking of nappe units during continental collision and/or during subduction-related accretionary processes. [Copyright &y& Elsevier]

We applied SHRIMP zircon U-Pb age dating to ultrahigh-temperature (UHT) sapphirine-bearing orthopyroxene garnet (SOG) granulite and biotite-hornblende (Bt-Hbl) gneiss in the Schirmacher Hills, East Antarctica. In the Bt-Hbl gneiss, concordant ages of 657 ± 7.1 and 807 ± 9.1 Ma were obtained from zircon overgrowth rims and zircon cores, with oscillatory and irregular zoning, respectively. The zircon overgrowth rims (657 ± 7.1 Ma) with low Th/U ratios from the Bt-Hbl gneiss are interpreted as having a metamorphic origin. Oscillatory-zoned and/or irregularly zoned zircon cores may have crystallized during an igneous event at 807 ± 9.1 Ma; 800-Ma igneous events have not previously been recognized in central Dronning Maud Land (DML) inland nunatak. Zircons in the SOG granulite yielded a concordant age of 643 ± 6.5 Ma, using analyses of sector-zoned and simple-zoned grains. These zircons have relatively high Th/U ratios with a narrow range, and they occur in association with garnet breaking down to form cordierite. The 643 ± 6.5-Ma age obtained from these zircons is interpreted as the timing of crystallization from a high-Th/U partial melt soon after peak metamorphism. The combination of a ca. 800-Ma igneous age and 660-640-Ma metamorphic ages obtained from Schirmacher Hills is different from that of other neighboring parts of central DML. In addition, a metamorphic PT path involving ultrahigh temperatures at early and subsequent isobaric cooling (IBC) stages at around 650 Ma has not previously been known in the central DML nunatak region. The Ca. 650-Ma UHT metamorphic event probably occurred in a back-arc tectonic setting and predates the main collisional event of central DML (ca. 550-500 Ma). [ABSTRACT FROM AUTHOR]

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]

The >1000-km-long Oligocene—Miocene left-lateral Red River shear zone (RRSZ) and metamorphic belt and the Pliocene—active right-lateral Red River fault (RRF), stretching from SE Tibet to the South China Sea. has been cited as one of the primary examples of a lithospheric scale strike-slip fault that has resulted in syn-kinematic metamorphism and partial melting and accommodated several hundred to a thousand kilometers of horizontal motion as a result of the indentation of India into Asia. Alternatively we interpret the metamorphic complexes along the RRSZ as exhumed metamorphic core complexes of older rocks, subsequently affected by Oligocene–Early Miocene leftlateral shear and localized partial melting(leucogranite dykes), Miocene low-angle normal faulting along margins (Range Front faults), and Pliocene active dextral strikeslip faulting (RRF). Along the Ailao Shan (ALS) and Diancang Shan (DCS) ranges in Yunnan, SW China, early amphibolite facies metamorphic rocks were intruded by Kfeldspar orthogneisses of Triassic age (Indosinian). LA-ICP-MS U-Pb zircon dating reveals a complex history with zircon cores showing evidence of Indosinian (∼239–243 Ma) to Neoproterozoic magmatism. Zircon rims show an Oligocene (∼26 Ma) magmatic or metamorphic overprint. Biotite granodiorites and syenites of mantle origin intruded the gneisses during the Oligocene (35 Ma). Later biotite leucogranites intruded the orthogneisses and migmatite host rocks before a signifi cant phase of tight to isoclinal folding. Ductile, left-lateral strike-slip shear fabrics were superimposed on all lithologies at high temperature (∼500–550 °C) for the ALS and lower temperatures (∼250–150 °C) after peak metamorphism and after granite intrusion. A few very small biotite (±Grt ± Tur) leucogranite veins and dykes crosscut the ductile strike-slip shear fabrics at Yuanjiang, in the Ailao Shan. Low-angle normal faulting along the margins of the metamorphic massif accommodated fi nal exhumation of the Red River gneisses. Using published U-Th-Pb and 40Ar/39Ar ages of granites along the shear zone, the age of left-lateral ductile shearing along the RRSZ can be constrained as between the earlier deformed leucogranites (31.9–24.2 Ma) and the later crosscutting dykes (21.7 Ma) with exhumation-related cooling continuing until ∼17 Ma. [ABSTRACT FROM AUTHOR]

Abstract: Results of more than 400 in situ U–Th–Pb and Lu–Hf zircon isotope analyses combined with cathodoluminescence images and field relationships reveal that the oldest magmatic rocks of the Limpopo Belt (Musina area) formed during the Palaeoarchaean between 3.40 and 3.27Ga, experienced a first anatexis at 3.24–3.07Ga (M1) and were overlain by sediments mainly between 3.24 and 3.0Ga. Subsequently, the infra- and supracrustal rocks together underwent two high-grade metamorphic overprints, a first during the Neoarchaean at 2.65Ga (M2), and a second at 2.025Ga (M3). Hafnium isotope data additionally reveal that the Palaeoarchaean magmatic rocks result from substantial recycling of much older crust. This is well reflected by nearly chondritic ɛHfint obtained from primary magmatic zircon domains of four Sand River TTG-gneiss samples (ɛHf3.27–3.28Ga =−1.3 to −1.8), from leucosomes within the Sand River Gneisses (ɛHf3.40Ga =−2.0; ɛHf3.28Ga =−1.5), and from two meta-anorthosite samples of the Messina layered intrusion (ɛHf3.35Ga =0.1–1.4). Crustal recycling is also supported by hafnium isotope data of detrital zircon grains from two quartzite samples. These data reflect a general increase of the ɛHfint between 3.65 and 3.2Ga from ca. −4.0 to 0.0, indicating that an Eo- to Palaeoarchaean crust was mixed with juvenile mantel material in the hinterland of the Limpopo Belt. The present study also demonstrates that Hf isotope data provide a powerful tool, which makes it possible to distinguish zircon domains formed during distinct magmatic or metamorphic events, from such affected by a pseudomorphic alteration, even if the investigated zircon grain underwent multiple alteration processes. [Copyright &y& Elsevier]

The article discusses the method of isotope-geochronological timing of rock metamorphism at the boundary zone of Aldan Shield and the Dzhugdzhuro-Stanovoi Folded Area in the area of Nagornyi Village and Kholodnikan Pass in Russia. The method indicated an overthrust-sheet structure that developed in the zone and a tectonic plate that moderately points to the south. Further observations showed that the metamorphic rocks are deformed linearly or folded isoclinally in the latitudal direction parallel to the whole junction zone.

Abstract: Zircon and monazite U–Pb data document the geochronology of the felsic crust in the Mozambique Belt in NE Mozambique. Immediately E of Lake Niassa and NW of the Karoo-aged Maniamba Graben, the Ponta Messuli Complex preserves Paleoproterozoic gneisses with granulite-facies metamorphism dated at 1950±15Ma, and intruded by granite at 1056±11Ma. This complex has only weak evidence for a Pan-African metamorphism. Between the Maniamba Graben and the WSW–ENE-trending Lurio (shear) Belt, the Unango and Marrupa Complexes consist mainly of felsic orthogneisses dated between 1062±13 and 946±11Ma, and interlayered with minor paragneisses. In these complexes, an amphibolite- to granulite-facies metamorphism is dated at 953±8Ma and a nepheline syenite pluton is dated at 799±8Ma. Pan-African deformation and high-grade metamorphism are more intense and penetrative southwards, towards the Lurio Belt. Amphibolite-facies metamorphism is dated at 555±11Ma in the Marrupa Complex and amphibolite- to granulite-facies metamorphism between 569±9 and 527±8Ma in the Unango Complex. Post-collisional felsic plutonism, dated between 549±13 and 486±27Ma, is uncommon in the Marrupa Complex but common in the Unango Complex. To the south of the Lurio Belt, the Nampula Complex consists of felsic orthogneisses which gave ages ranging from 1123±9 to 1042±9Ma, interlayered with paragneisses. The Nampula Complex underwent amphibolite-facies metamorphism in the period between 543±23 to 493±8Ma, and was intruded by voluminous post-collisional granitoid plutons between 511±12 and 508±3Ma. In a larger context, the Ponta Messuli Complex is regarded as part of the Palaeoproterozoic, Usagaran, Congo-Tanzania Craton foreland of the Pan-African orogen. The Unango, Marrupa and Nampula Complexes were probably formed in an active margin setting during the Mesoproterozoic. The Unango and Marrupa Complexes were assembled on the margin of the Congo-Tanzania Craton during the Irumidian orogeny (ca. 1020–950Ma), together with terranes in the Southern Irumide Belt. The distinctly older Nampula Complex was more probably linked to the Maud Belt of Antarctica, and peripheral to the Kalahari Craton during the Neoproterozoic. During the Pan-African orogeny, the Marrupa Complex was overlain by NW-directed nappes of the Cabo Delgado Nappe Complex before peak metamorphism at ca. 555Ma. The nappes include evidence for early Pan-African orogenic events older than 610Ma, typical for the Eastern Granulites in Tanzania. Crustal thickening at 555±11Ma is coeval with high-pressure granulite-facies metamorphism along the Lurio Belt at 557±16Ma. Crustal thickening in NE Mozambique is part of the main Pan-African, Kuunga, orogeny peaking between 570 and 530Ma, during which the Congo-Tanzania, Kalahari, East Antarctica and India Cratons welded to form Gondwana. Voluminous post-collisional magmatism and metamorphism younger than 530Ma in the Lurio Belt and the Nampula Complex are taken as evidence of gravitational collapse of the extensive orogenic domain south of the Lurio Belt after ca. 530Ma. The Lurio Belt may represent a Pan-African suture zone between the Kalahari and Congo-Tanzania Craton. [Copyright &y& Elsevier]

Abstract: Kyanite–staurolite–garnet micaschists from the Plankogel unit, Saualpe–Koralpe region, Eastern European Alps, document a poly-metamorphic evolution. Textures indicate two stages of garnet growth, interpreted as (i) relics of an early, pre-Alpine metamorphic event and (ii) newly-grown garnet of Alpine age. The age of the first crystallisation event during prograde Permian metamorphim is constrained by Sm–Nd dating of the almandine-rich garnet core domains. Mineral–wr ages of nine high-Sm/Nd garnet single crystal core fractions from three distant localities (Laaken, SE Koralpe; Plankogel, NW Saualpe; Friesach, E Gurktal Alps) range between 275.0±2.7 and 264.1±3.6 Ma. The pooled mean regression age of just the garnet fractions is 269.6±6.2 Ma (MSWD=4.3; n =9), with an initial Nd isotope composition of εNd(270)=−8.6. Leaching experiments and whole rock analyses revealed slight Nd isotope disequilibrium between garnet porphyroblasts, REE-rich inclusions, and the matrix of the polyphase mineral assemblage. Kyanite inclusions in the inner and sillimanite in the outer parts of the garnet cores, decreasing grossular and increasing pyrope contents indicate variable pressure/temperature conditions during Permian garnet growth. Modelling of mineral assemblages in pressure–temperature pseudosections suggests that the first-generation, Permian garnet crystallized at about 0.46 GPa and 540 °C, whereas the eo-Alpine garnet documents distinctly higher pressures of 1.0 GPa and 600 °C. The data document regional prograde metamorphism in metapelites, at low or moderate pressure conditions, during the Middle Permian. Crustal-scale extensional processes and heat input from the mantle, induced by ongoing Paleotethys subduction and Neotethys opening in the course of the separation of Europe and Africa-related Adria were responsible for this low-P tectonothermal event. [Copyright &y& Elsevier]

U-Pb zircon dating indicates that the Balugaon Anorthosite Massif of the Chilka Lake Domain, Eastern Ghats Belt, India, crystallized at 983.0 ± 2.5 Ma, ∼200 m.yr. earlier than previously inferred from ferrodiorite that intruded the massif. An integration of the new date with published structural data indicates that the anorthosite intruded a crust that was deformed twice before the intrusion. Furthermore, U-Th-Pb chemical dating of monazite cores (714 ± 11 Ma) and rims (655 ± 12 Ma) yields results consistent with progressive high-grade metamorphism of the anorthosite, and monazite + allanite rims (463 ± 22 Ma) around apatite indicate low-grade pan-African thermal activity. The 714- 655-Ma dates possibly correspond to convergence and oblique collision between the Eastern Ghats-Rayner Block and western Australia in the mid-Neoproterozoic. [ABSTRACT FROM AUTHOR]

The Lanterman Fault Zone, a major terrane boundary in northern Victoria Land, displays a polyphase structural evolution. After west-over-east thrusting, it experienced sinistral strike-slip shearing. Sheared metabasites from the Wilson Terrane (inboard terrane) preserve a record of retrograde metamorphic evolution. Shearing took place under amphibolite-facies metamorphic conditions (roughly comparable to those reached during regional metamorphism) which later evolved to greenschist-facies conditions. In contrast, the Bowers Terrane (outboard terrane) preserves a prograde metamorphic evolution which developed from greenschist-facies to amphibolite-facies metamorphism during shearing, followed by greenschist-facies metamorphism during the late deformational stages. Laser step-heating 40Ar–39Ar analyses of syn-shear amphibolite-facies amphiboles yielded ages of 480–460 Ma, in agreement with a ∼480-Ma age obtained from a biotite aligned along the mylonitic foliation. These ages are younger than those (∼492 to ∼495 Ma) obtained from pre-shear amphibole relics linked to regional metamorphism of the Wilson Terrane. Results attribute the structural and metamorphic reworking during shearing to the late stages of the Cambrian-Ordovician Ross Orogeny and to the Middle-Late Ordovician probably in relation to the beginning of deformation in the Lachlan Orogen, thus precluding any appreciable impact of the Devonian-Carboniferous Borchgrevink event in the study area. [ABSTRACT FROM AUTHOR]

Electron microprobe chemical dating was carried out on monazites enclosed in two generations of mineral paragenesis of St + Ky + Grt and Sil + And + Gil + St, respectively, from the Qinling Group. Two different ages, 520±23 Ma and 435±9 Ma, were obtained from these monazites. This indicates that the Qinling Group experienced a metamorphism during the Early Cambrian, which is probably in response to the Late Pan-African subduction-accretion tectonothermal event. [ABSTRACT FROM AUTHOR]

The Haiyangsuo Complex in the NE Sulu ultrahigh-pressure (UHP) terrane has discontinuous, coastal exposures of Late Archean gneiss with amphibolitized granulite, amphibolite, Paleoproterozoic metagabbroic intrusives, and Cretaceous granitic dikes over an area of about 15 km2. The U–Pb SHRIMP dating of zircons indicates that theprotolith age of a garnet-biotite gneiss is >2500 Ma, whereas the granulite-facie metamorphism occurred at around 1800 Ma. A gabbroic intrusion was dated at ∼1730 Ma, and the formation of amphibolite-facies assemblages in both metagabbro and granulite occurred at ∼340–460 Ma. Petrologic and geochronological data indicate that these various rocks show no evidence of Triassic eclogite-facies metamorphism and Neoproterozoic protolith ages that are characteristics of Sulu-Dabie HP-UHP rocks, except Neoproterozoic inherited ages from post-collisional Jurassic granitic dikes. Haiyangsuo retrograde granulites with amphibolite-facies assemblages within the gneiss preserve relict garnet formed during granulite-facies metamorphism at ∼1.85 Ga. The Paleoproterozoic metamorphic events are almost coeval with gabbroic intrusions. The granulite-bearing gneiss unit and gabbro-dominated unit of the Haiyangsuo Complex were intruded by thin granitic dikes at about 160 Ma, which is coeval with post-collisional granitic intrusions in the Sulu terrane. We suggest that the Haiyangsuo Complex may represent a fragment of the Jiao-Liao-Ji Paleoproterozoic terrane developed at the eastern margin of the Sino-Korean basement, which was juxtaposed with the Sulu terrane prior to Jurassic granitic activity and regional deformation. [ABSTRACT FROM AUTHOR]

Garnet in metapelites from the Wölz and Rappold Complexes of the Austroalpine basement east of the Tauern Window typically shows two distinct growth zones. A first garnet generation usually forms the cores of garnet porphyroblasts and is separated by a prominent microstructural and chemical discontinuity from a second garnet generation, which forms rims of variable width. Whereas the rims were formed during the Eo-Alpine metamorphic overprint, the garnet cores represent remnants of at least two pre-Eo-Alpine metamorphic events. The pressure and temperature estimates obtained from garnet isopleth thermobarometry applied to the first growth increments of the pre-Eo-Alpine garnet cores from the Wölz and Rappold Complexes cluster into two distinct domains: (i) in the Wölz Complex, incipient growth of the first-generation garnet occurred at 4 ± 0.5 kbar and 535 ± 20 °C, (ii) in the Rappold Complex, incipient growth of the oldest garnet cores took place at 5.3 ± 0.3 kbar and 525 ± 15 °C. The Eo-Alpine garnet generation started to grow at 6.5 ± 0.5 kbar and 540 ± 10 °C. According to radiometric dating, the low-pressure garnet from the Wölz complex was formed during a Permian metamorphic event. The first-generation garnet of the Rappold Complex is probably of Variscan age. [ABSTRACT FROM AUTHOR]

New results on the pressure–temperature–time evolution, deduced from conventional geothermobarometry and in situ U-Th-total Pb dating of monazite, are presented for the Bemarivo Belt in northern Madagascar. The belt is subdivided into a northern part consisting of low-grade metamorphic epicontinental series and a southern part made up of granulite facies metapelites. The prograde metamorphic stage of the latter unit is preserved by kyanite inclusions in garnet, which is in agreement with results of the garnet (core)-alumosilicate-quartz-plagioclase (inclusions in garnet; GASP) equilibrium. The peak metamorphic stage is characterized by ultrahigh temperatures of ∼900–950 °C and pressures of ∼9 kbar, deduced from GASP equilibria and feldspar thermometry. In proximity to charnockite bodies, garnet-sillimanite-bearing metapelites contain aluminous orthopyroxene (max. 8.0 wt% Al2O3) pointing to even higher temperatures of ∼970 °C. Peak metamorphism is followed by near-isothermal decompression to pressures of 5–7 kbar and subsequent near-isobaric cooling, which is demonstrated by the extensive late-stage formation of cordierite around garnet. Internal textures and differences in chemistry of metapelitic monazite point to a polyphasic growth history. Monazite with magmatically zoned cores is rarely preserved, and gives an age of c. 737 ± 19 Ma, interpreted as the maximum age of sedimentation. Two metamorphic stages are dated: M1 monazite cores range from 563 ± 28 Ma to 532 ± 23 Ma, representing the collisional event, and M2 monazite rims (521 ± 25 Ma to 513 ± 14 Ma), interpreted as grown during peak metamorphic temperatures. These are among the youngest ages reported for high-grade metamorphism in Madagascar, and are supposed to reflect the Pan-African attachment of the Bemarivo Belt to the Gondwana supercontinent during its final amalgamation stage. In the course of this, the southern Bemarivo Belt was buried to a depth of >25 km. Approximately 25–30 Myr later, the rocks underwent heating, interpreted to be due to magmatic underplating, and uplift. Presumably, the northern part of the belt was also affected by this tectonism, but buried to a lower depth, and therefore metamorphosed to lower grades. [ABSTRACT FROM AUTHOR]

Abstract: Re–Os dating of molybdenite from small deposits is used to define crustal domains exhibiting ductile versus brittle behaviour during gravitational collapse of the Sveconorwegian orogen in SW Scandinavia. A 1019±3 Ma planar quartz vein defines a minimum age for brittle behaviour in central Telemark. In Rogaland–Vest Agder, molybdenite associated with deformed quartz and pegmatite veins formed between 982±3 and 947±3 Ma in the amphibolite-facies domain (three deposits) and between 953±3 and 931±3 Ma west of the clinopyroxene-in isograd (two deposits) in the vicinity of the 0.93–0.92 Ga Rogaland anorthosite complex. The data constrain the last increment of ductile deformation to be younger than 0.95 and 0.93 Ga in these two metamorphic zones, respectively. Molybdenite is the product of an equilibrium between biotite, oxide and sulfide minerals and a fluid or hydrated melt phase, after the peak of 1.03–0.97 Ga regional metamorphism. Molybdenite precipitation is locally episodic. A model for gravitational collapse of the Sveconorwegian orogen controlled by lithospheric extension after 0.97 Ga is proposed. In the west of the orogen, the Rogaland–Vest Agder sector is interpreted as a large shallow gneiss dome, formed slowly in two stages in a warm and structurally weak crust. The first stage at 0.96–0.93 Ga was associated with intrusion of the post-collisional hornblende–biotite granite suite. The second stage at 0.93–0.92 Ga, restricted to the southwesternmost area, was associated with intrusion of the anorthosite–mangerite–charnockite suite. Most of the central part of the orogen was already situated in the brittle upper crust well before 0.97 Ga, and did not undergo significant exhumation during collapse. In the east of the orogen, situated against the colder cratonic foreland, exhumation of high-grade rocks of the Eastern Segment occurred between 0.97 and 0.95 Ga, and included preservation of high-pressure rocks but no plutonism. [Copyright &y& Elsevier]

Poly-deformed and poly-metamorphosed glaucophane-eclogite mega-boudins beneath the Samail Ophiolite, Oman record an early subduction-related high-P metamorphism as well as subsequent overprinting deformation and metamorphism related to exhumation. Previously published Rb/Sr ages of 78 Ma and 40Ar/39Ar ages of 82–79 Ma record the major NE-directed shearing event that partially exhumed the eclogites to a shallower crustal level. New Sm/Nd garnet–garnet leachate–whole rock isochron data from garnet-bearing eclogite assemblages in the As Sifah subwindow in NE Oman are 110±9 Ma (DG02-87D); 5-point isochron) and 109±13 Ma (DG02-86E; 3-point isochron). On the basis of microfabric and field structural relationships these ages are interpreted to reflect the timing of prograde, peak high-P metamorphism in the rocks structurally beneath the Samail Ophiolite. This metamorphism clearly predates the age of formation of the obducted Samail oceanic lithosphere (97–94 Ma) as well as the subsequent obduction onto the margin (80–70 Ma). A U–Pb SHRIMP zircon age from small (<200 μm in length) zircons with herring-bone textured zoning from DG02-87D indicate that rapid zircon growth associated with high-Si phengites occurred at 82±1 Ma. Zircon growth is possibly related to liberation of Zr on garnet breakdown during decompression metamorphism under high-P conditions with exhumation. These data require that crustal stacking models attendant with ophiolite obduction are inappropriate to explain the Oman high-P metamorphism. [Copyright &y& Elsevier]

The paper documents a late Palaeoproterozoic episode of metamorphism and anatexis in the Suomuja¨rvi Complex, an Archaean enclave in the Palaeoproterozoic Central Lapland Granitoid Complex of northern Finland. U–Pb ages were obtained from monazite and titanite in four Archaean granitoid rocks, one quartzite and a massive granite. The results show that growth of monazite and titanite in these rocks spanned a period of about 15 million years between 1780 and 1765 Ma, presumably reflecting complex processes of metamorphic (re-) crystallisation, invasion by granitic dykes, veins and fluids and post-peak-metamorphic retrogression. The timing of this activity corresponds to post-orogenic granitoid magmatism throughout the Baltic Shield. In this region, between the Lapland-Kola and that of Svecofennian orogens, it may represent a retarded response to their coeval, ca. 1.9 Ga convergent collision. The results support a proposed affinity of the Suomuja¨rvi Complex with the Belomorian rather than the Archaean Karelian Province. [Copyright &y& Elsevier]

Presents a 40Ar/39Ar thermochronological analysis of Mesoproterozoic metamorphism in the Colorado Front Rage. Presence of staurolite, andalusite, cordierite, and garnet porphyroblasts overprinting earlier assemblages; Interpretation of muscovite and biotite ages; Observation of thermal effects for plutons, suggesting pervasive, regional crustal heating.

Dates an episode of intense postimpact metamorphism in the core of the Vredefort impact structure of South Africa, using U-Pb geochronology of single grains of zircon and monazite. Geologic setting and sample descriptions; Indlandsee leucogranofels; Norite; Age of impact and core metamorphism; Causes of thermal imprint.

Presents the results of a uranium-lead sphene dating of metamorphism in the Red Mountain pluton, Laramie Mountains, Wyoming. Boundaries of study area; Red Mountain pluton and its aureole; Sample descriptions; Strategies for interpreting uranium-lead data from low uranium materials.

Documents the thermal history of rocks surrounding the 1.43 Ga Priest pluton in Manzano Mountain, central New Mexico, using metamorphic studies and 40Argon/39Argon dating. Proterozoic geology of the central and southern Manzano Mountains; Evidence for polymorphism preserved by rocks surrounding the Priest pluton; Regional metamorphic conditions.