CL Images Research Articles

Neoproterozoic continental accretion in South China: Geochemical evidence from the Fuchuan ophiolite in the Jiangnan orogen

A combined study of geochronology and geochemistry was carried out for the Fuchuan ophiolite in the eastern Jiangnan orogen, South China. The ophiolite is mainly composed of harzburgite, wehrlite, gabbro and spilite. The harzburgite is highly depleted in CaO and Al2O3, suggesting that it experienced intensive extraction of basaltic melts during mantle melting. Zircon U–Pb dating on the gabbro gave two groups of ages at 891±13Ma and 824±3Ma, respectively. The 891±13Ma zircon is less common and unzoned or weakly zoned in CL images, which is interpreted as inherited from the mantle source. The 824±3Ma zircon is dominant and mostly exhibits clear magmatic zoning, representing crystallization age of the gabbro. The harzburgite, wehrlite and gabbro have positive ɛNd(t) values of 3.29–5.69 and lower initial 87Sr/86Sr ratios of 0.7033–0.7059, whereas the spilite exhibits neutral ɛNd(t) values of −1.24 to 0.40 and elevated initial 87Sr/86Sr ratios of 0.7039–0.7068. In particular, the gabbro rich in zircon has the highest ɛNd(t) values of 5.69 and the youngest Nd model age of 1.03Ga. The zircon gave positive ɛHf(t) values of 10.2–12.1 and young Hf model ages of 0.90–0.98Ga. The positive ɛHf(t) and ɛNd(t) values for the ophiolite indicate that its mantle source is relatively depleted in radiogenic isotopes, similar to the juvenile lithospheric mantle. The gabbro and spilite exhibit the enriched MORB-like and arc-like patterns of trace element distribution, respectively, in the spidergram. This suggests their origin from different mantle sources beneath a backarc basin. The gabbro and spilite exhibit contrasting mineral δ18O values, suggesting that they suffered water–rock interaction at high and low temperature, respectively. This is also consistent with the O isotope profile of common ophiolites. Therefore, the Fuchuan ophiolite was formed in the backarc basin at about 825Ma due to partial melting of the enriched mantle wedge. And it was tectonically emplaced shortly subsequent to the mantle melting due to the closure of backarc basin in association with the accretionary arc–continent collisional orogeny. This tectonic process marks the Neoproterozoic continental growth in South China by accreting the juvenile arc terrane to the Yangtze Block.

Paleoproterozoic accretionary orogenesis in the North China Craton: A SHRIMP zircon study

The Inner Mongolia Suture Zone (IMSZ) and the Trans-North China Orogen (TNCO) incorporate the major Paleoproterozoic accretionary orogens in the North China Craton (NCC), with the Jiao-Liao-Ji Belt (JLJB) representing the third one. Here we investigate the Paleoproterozoic tectonics of the IMSZ and TNCO through zircon SHRIMP geochronology on a representative suite of rocks comprising metasediments and arc magmatic rocks. SHRIMP analysis of zircons with textures indicating extreme recrystallization under ultrahigh-temperature (UHT) conditions from the metapelites at Heling’er in the southern domain of the IMSZ reveals a single population with a weighted mean 207Pb/206Pb age of 1913±17Ma. The zircons in another UHT granulite from this locality yield a weighted mean 207Pb/206Pb age of 1910±18Ma. These data correlate with the ca. 1.92Ga age reported from zircons in sapphirine-bearing UHT granulites further north and confirm the regional extent of the Paleoproterozoic UHT metamorphism within the IMSZ. Zircons in a charnockite from the southern margin of the Khondalite Belt fringing the UHT granulites in the IMSZ show two distinct age groups: an older population with a magmatic paragenesis and a weighted mean 207Pb/206Pb age of 1932±24Ma, and a younger group of metamorphic zircons with an age of 1858±26Ma. We also report zircon ages from charnockites in two localities around Xing’he in the Huangtuyao belt belonging to the Huai’an Complex within the westernmost domain of the TNCO at the junction with the IMSZ. The charnockite from first locality carries two distinct zircon populations with the older group yielding a weighted mean 207Pb/206Pb age of 2477±2Ma and the younger population showing an age of 1807±38Ma. The internal structure as revealed from CL images and the overall high Th/U values (up to 2.42) of the older zircons suggest their magmatic affinity, whereas the younger group with extremely low Th/U (0.02–0.09) is of metamorphic origin. Zircons from the charnockite in the second locality also define two distinct age clusters with a dominant older (magmatic) group having a weighted mean 207Pb/206Pb age of 2147±11Ma and a minor younger group with an age of 1958±25Ma. The range of ages from 2477 to 2147Ma from magmatic zircons in the charnockites from the eastern periphery of the IMSZ, within the western margin of the TNCO, in combination with similar ages reported in recent studies from zircons in magmatic complexes within the IMSZ suggest a prolonged history of subduction-related arc magmatism and accretionary tectonics analogous to those in some of the Phanerozoic belts such as the Central Asian Orogenic Belt and the Western Pacific. Subsequent progressive collision and suturing of the continental blocks were accompanied by the exhumation of high-pressure (HP) and UHT metamorphic rocks. The available data from the IMSZ and TNCO suggest long-lived convergent margins associated with the southward subduction of the Yinshan Block and westward subduction of the Eastern Block in a double-sided subduction realm prior to the final amalgamation of the NCC and its incorporation within the Columbia supercontinent in the late Paleoproterozoic.

U-Pb geochronology of hydrothermal zircon from the Mesoproterozoic Gaoyuzhuang Formation on the northern margin of the North China Block and its geological implications

This paper reports LA-ICP-MS zircon U-Pb ages and REE compositions on detrital zircons (Type 1) and hydrothermal zircons (Type 2) from low-metasedimentary rocks in the Mesoproterozoic Gaoyuzhuang Formation, Pingquan area, Hebei Province that provide important constraints on the tectonic evolution along the northern margin of the North China Block. The detrital zircons are characterized by an oscillatory magmatic core, surrounded by a narrow structureless rim in CL images. They yield 207Pb/206Pb ages ranging from 1703 to 2543 Ma with two age peaks at 2473 and 1794 Ma, which is consistent with the Neoarchean and Paleoproterozoic tectono-thermal events recorded in the NCB basement, indicating that the source of the detritus was locally derived. By contrast, the hydrothermal ones are euhedral, sector zoning or internally structureless. They have relatively higher U and Th contents and Th/U ratios (U=139–2918 ppm, Th=35–1327 ppm, Th/U (average) =0.78 vs. U=15–1044 ppm, Th=8–341 ppm, Th/U (average) =0.57 for detrital zircons). Moreover, they are also enriched in REEs relative to detrital zircons and show a lower positive Ce anomaly (ΣREE = 659–2418 ppm vs. 231–611 ppm for detrital zircon; Ce/Ce*=2–13 vs. 33–174 for detrital zircons), similar to known hydrothermal zircons derived from many locations. These characteristics, combined with our field petrographic observations, indicate that the hydrothermal zircons possibly formed from a low temperature aqueous fluid. Twelve concordant or near-concordant analyses on hydrothermal zircons yield weighted mean 206Pb/238U ages of 325–327 Ma, which provide unambiguous evidence that the northern NCB underwent late Paleozoic low temperature hydrothermal modification. This timing of this hydrothermal event is compatible with that of the late Paleozoic magmatic and metamorphic-deformational events occurred on the northern margin of the NCB, it appears likely that the hydrothermal activity resulted from southward subduction of the Paleo-Asian Oceanic plate underneath the NCB during late Paleozoic.

Multistage growth of garnet in ultrahigh-pressure eclogite during continental collision in the Dabie orogen: Constrained by trace elements and U–Pb ages

Major and trace elements for garnet and other minerals, and U–Pb ages and trace elements for zircon (as inclusions in garnet or as matrix) were determined on thin sections of UHP eclogite from the western part of the Dabie orogen. The results provide temporal constraints on growth of garnet during continental subduction-zone metamorphism. Major elements show homogeneous distribution across garnet grains, with weak or no zonation of Ca and almost flat distribution patterns of Mg and Fe. However, their REE compositions display two groups of patterns relative to chondrite. Group I garnet is characterized by steep patterns from LREE to HREE, whereas Group II garnet shows enrichment of MREE relative to LREE and HREE. These two groups of garnet also have different parageneses of mineral inclusions. Group I garnet always contains abundant inclusions of amphibole+epidote/allanite+rutile+plagioclase+quartz±zircon, whereas Group II garnet occasionally contains inclusions of omphacite±rutile±apatite±zircon. LA-ICPMS U–Pb dating and trace element analysis of the zircon provide direct constraints on the growth time of the two groups of garnet. Four types of zircon were identified based on their CL images, REE distributions and U–Pb ages. Types I and II zircons are metamorphosed after protolith magmatic zircon, with Type I being modified by solid-state recrystallization and Type II by dissolution recrystallization. Types III and IV zircons are metamorphic and grew in the stability field of garnet. They gave consistent 206Pb/238U ages of 239±6Ma and 222±4Ma, respectively, in association with Groups I and II garnets. This provides a temporal constraint on the two-stage growth of garnet, respectively, predating and postdating the peak UHP metamorphic event. Therefore, we conclude that there are two stages of zircon and garnet growth during continental collision, which occurred during the final subduction from HP to UHP phase and the early exhumation from UHP to HP eclogite facies, respectively.

Timing of Variscan HP-HT metamorphism in the Moldanubian Zone of the Bohemian Massif: U-Pb SHRIMP dating on multiply zoned zircons from a granulite from the Dunkelsteiner Wald Massif, Lower Austria

In an attempt to better constrain the timing of Variscan HP-HT metamorphism in the SE Bohemian Massif we have dated zoned zircons from a garnet-kyanite granulite of granitic composition from the Dunkelsteiner Wald Massif, Lower Austria, by means of sensitive high-resolution ion microprobe (SHRIMP) technique. In order to combine isotopic information with crystal growth textures, CL and BSE images were systematically taken from the dated zircons. A characteristic threefold concentric zoning was found in many zircons. This involves pre-Variscan protolithic cores followed by two distinct metamorphic/anatectic overgrowth shells of Variscan age. The inner overgrowth shell is characterized by a weak CL but bright BSE signal, and yields high contents of uranium (0.1 to 0.2 wt.%). A pooled U-Pb Concordia age for this zone is 342.0 ± 3.0 Ma (n = 11, MSWD = 0.12). The second, outer, overgrowth shell is always bright in the CL image, dark in the BSE image, and has generally low uranium contents (mostly <500 ppm). A pooled U-Pb Concordia age for this zone is 337.1 ± 2.7 Ma (n = 11, MSWD = 0.22). These results imply that the Variscan HT crystallisation history of the Moldanubian granulites took place over a period of a few million years and was not an extremely rapid subduction-exhumation process. SHRIMP measurements in the protolithic cores yield a cluster of (sub)concordant ages between ∼390 and 460 Ma and a few outliers at higher ages mostly represented by cores in cores. Core domains, which are large, homogeneous and with undisturbed igneous oscillatory zoning, yielded preferentially ages between 430 and 460 Ma. We therefore consider that granitic protolith formation took place at that time. The still older inner cores are interpreted as inherited into the granitic melt.

Early Paleoproterozoic magmatism in the Quanji Massif, northeastern margin of the Qinghai–Tibet Plateau and its tectonic significance: LA-ICPMS U–Pb zircon geochronology and geochemistry

The Quanji Massif is located on the north side of the Qaidam Block and is interpreted as an ancient cratonic remnant that was detached from the Tarim Craton. There are regionally exposed granitic gneisses in the basement of the Quanji Massif whose protoliths were granitic intrusive rocks. Previous studies obtained intrusion ages for some of these granitic gneiss protoliths. The intrusion ages span a wide range from ~ 2.2 Ga to ~ 2.47 Ga. This study has determined the U–Pb zircon age of four granitic gneiss samples from the eastern, central and western parts of the Quanji Massif. CL images and trace elements show that the zircons from these four granitic gneisses have typical magmatic origins, and experienced different degrees of Pb loss due to strong metamorphism and deformation. LA-ICPMS zircon dating yields an upper intercept age of 2381 ± 41 (2σ) Ma from monzo-granitic gneiss in the Hudesheng area and 2392 ± 25 (2σ) Ma from granodioritic gneiss in the Mohe area, eastern Quanji Massif, and 2367 ± 12 (2σ) Ma from monzo-granitic gneiss in the Delingha area, central Quanji Massif, and 2372 ± 22 (2σ) Ma from monzo-granitic gneiss in the Quanjishan area, western Quanji Massif. These results reveal that the intrusive age of the protoliths of the widespread granitic gneisses in the Quanji Massif basement was restricted between 2.37 and 2.39 Ga, indicating regional granitic magmatism in the early Paleoproterozoic, perhaps related to the fragmentation stage of the Kenorland supercontinent. Geochemical results from the granodioritic gneiss from the Mohe area indicate that the protolith of this gneiss is characterized by adakitic rocks derived from partial melting of garnet-amphibolite beneath a thickened lower crust in a rifting regime after continent–continent collision and crustal thickening, genetically similar to the TTG gneisses in the North China Craton. This suggests that the Quanji Massif had a tectonic history similar to the Archean Central Orogenic Belt of North China Craton during the early Paleoproterozoic. We tentatively suggest that the Quanji Massif and the parental Tarim Craton and the North China Craton experienced rifting in the early Paleoproterozoic, after amalgamation at the end of the Archean. The Tarim Craton and North China Craton might have had close interaction from the late Neoarchean to the early Paleoproterozoic.

Fluid action on zircon growth and recrystallization during quartz veining within UHP eclogite: Insights from U–Pb ages, O–Hf isotopes and trace elements

In-situ SIMS analyses of O and U–Pb isotopes as well as LA-(MC)-ICPMS analyses of trace elements, U–Pb, and Lu–Hf isotopes were carried out for zircon from a quartz vein and its host UHP eclogite in the Dabie orogen. The results are integrated to decipher the effect of aqueous fluid on the new growth of metamorphic zircon and on the recrystallization of protolith zircon during continental subduction-zone metamorphism. Most zircons show distinct core–(mantle)–rim structures in CL images. The rims are unzoned and have low trace element contents, low Th/U ratios of 0.003 to 0.06 and shallow MREE–HREE patterns, and insignificant negative Eu anomalies, indicating their growth from metamorphic fluid. The rims from the vein have concordant U–Pb ages of 222±6 to 212±4Ma, suggesting protracted growth during exhumation of the deeply subducted continental crust. In contrast, the rims from the eclogite gave two groups of concordant U–Pb ages at 221±3 and 208±3Ma, respectively, corresponding to two episodes of fluid action. The all rims have relatively uniformly negative δ18O values of −2.59 to −1.23‰ for the vein and −3.37 to −0.33‰ for the eclogite, suggesting their growth from negative δ18O metamorphic fluids of the same origin. The cores exhibit clearly oscillatory or patchy zonation, high trace element contents, high Th/U ratios of 0.13 to 2.12, steep MREE–HREE patterns, and significant negative Eu anomalies. They gave discordia Neoproterozoic upper-intercept U–Pb ages for the both vein and eclogite. Most cores have discordant U–Pb ages and variably positive δ18O values, with no correlation between the two variables. This suggests non-linear isotope alteration of the protolith zircon by metamorphic recrystallization in the presence of aqueous fluid. The highest δ18O value of 4.76‰ with the concordant Neoproterozoic U–Pb age is considered as a proxy for the primary δ18O value for the protolith zircon. In this regard, the cores would crystallize from slightly low δ18O magmas in the Neoproterozoic, and interacted with the negative δ18O fluids of metamorphic origin in the Triassic. In particular, the cores in the vein are the xenocrystal that was trapped by the metamorphic fluids from the eclogite. Therefore, the occurrence of the both magmatic cores and metamorphic growth in the vein suggests that sinking of the metamorphic fluids into eclogite factures brought about both the physical transport of protolith zircon and the chemical transport of element Zr during the continental collision.

U–Pb and Hf isotopic study of detrital zircons from the Yejishan Group of the Lüliang Complex: Constraints on the timing of collision between the Eastern and Western Blocks, North China Craton

The Trans-North China Orogen is a recently recognized continent–continent collisional belt along which the discrete Eastern and Western Blocks amalgamated to form the coherent basement of the North China Craton. However, there is a hot debate on the timing of the collision between the two blocks, with one school of thought arguing the collision occurred at 2.5 Ga, whereas others believe it took place at ~ 1.85 Ga. U–Pb ages and Hf isotopic data of detrital zircons from foreland basins in the Trans-North China Orogen can provide rigorous constraints on this controversial issue. One of such foreland basins is represented by the Yejishan Group in the Lüliang Complex in the middle sector of the western margin of the Trans-North China Orogen. The group consists of conglomerates, quartz wackes, sandstones and minor limestones, most of which contain large amounts of detrital zircons. CL images reveal that nearly all detrital zircons from the Yejishan Group possess oscillatory zoning, with Th/U ratios of 0.2–2.34, typical of an igneous origin. The 207Pb/ 206Pb ages of detrital zircons range between 3.24 and 1.81 Ga, with most having Paleoproterozoic ages, of which the youngest group ages are around 1.87–1.81 Ga. This indicates that the foreland basin in which the Yejishan Group was deposited must have formed not earlier than 1.87–1.81 Ga, thus supporting the collision between the Eastern and Western Blocks at ~ 1.85 Ga, not at ~ 2.5 Ga.

Multistage metamorphic events in granulitized eclogites from the North Dabie complex zone, central China: Evidence from zircon U–Pb age, trace element and mineral inclusion

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.

Geochronological evidence of Indosinian (high-pressure) metamorphic event and its tectonic significance in Taxkorgan area of the Western Kunlun Mountains, NW China

The CL images, LA-ICP-MS in situ trace elements analysis, and U-Pb dating for zircons indicate that the metamorphic ages of the sillimanite-garnet-biotite gneiss and the garnet-amphibole gneiss from eastern Taxkorgan of the Western Kunlun Mountains are 220±2 and 220±3 Ma respectively, and their protolith ages are younger than 253±2 and 480±8 Ma respectively. Two samples were collected at the same outcrops with HP mafic granulite and HP pelitic granulite. Mineral assemblage of the sillimanite-garnet-biotite gneiss (Grt+Sill+Per+Q) is consistent with that of HP pelitic granulite at early high amphibolite- granulite facies stage. Mineral assemblage of the garnet-amphibole gneiss (Grt+Amp+Pl+Q) is consistent with retro-metamorphic assemblage of HP mafic granulite at amphibolite facies stage. The dating results suggest that these HP granulites underwent peak metamorphism at 220±2 to 253±2 Ma. Thus, the Kangxiwar tectonic zone was probably formed by subduction and collision of the Paleo-Tethys Ocean during Indosinian. Protolith ages of the two samples, together with previously published U-Pb zircon dating age, suggest that the sillimanite-garnet schist-quartzite unit is a late Paleozoic unit, not a part of the Paleoproterozoic Bulunkuole Group.

Garnet-rich granulite xenoliths from the Hannuoba basalts, North China: Petrogenesis and implications for the Mesozoic crust-mantle interaction

Garnet-rich granulite xenoliths collected from the Hannuoba basalts, the North China craton (NCC), were studied to reveal the Mesozoic crust-mantle interaction. These xenoliths are characterized by low SiO2 (37.7 wt.%–46.0 wt.%) and high Al2O3 (10.8 wt.%–17.9 wt.%) contents. Their Mg# (60–75, Mg#=100×Mg/(Mg+Fe), atomic number) are relatively low for their low SiO2 contents. They have low rare-earth element (REE) contents and LREE-rich REE patterns, and show remarkable enrichments in Sr relative to the adjacent REE. Some of them exhibit convex REE patterns with a maximum at Nd and remarkably positive Eu anomalies. Taking into account their high garnet mode (generally >30%), these features suggest that they are high-pressure metamorphic products of low-pressure cumulates (e.g., gabbro) after it had been depressed into the garnet stability field. They have evolved Nd and Sr isotopic compositions (143Nd/144Nd=0.511 763–0.512 173, 87Sr/86Sr=0.705 34–0.706 99) and fall in the trend defined by the >110 Ma Mesozoic basalts and high-Mg# andesites from the NCC. Zircon U-Pb dating by LA-ICP-MS shows a wide age range from 83 to 2 581 Ma, most of which cluster in 83–134 Ma. CL images of some Mesozoic zircons from the granulites show typical features of igneous zircons, providing direct evidence for the Mesozoic underplating event in this area. Neither peridotite-derived basaltic underplating model nor residue model of ancient lower crust after lithospheric thinning alone can reasonably explain the above features of the garnet-rich granulite xenoliths. Combined with the previous research, we propose that most of the granulite xenoliths from the Hannuoba basalts are products of the Mesozoic magmatic underplating and mixing with the pre-existing lower crust (i.e., AFC process). However, the melts could be mostly derived from partial melting of basaltic layers that were previously subducted (a fossil oceanic slab) or underplated into the base of the lithospheric mantle, or from partial melting of Archean lithospheric mantle that was variably hybridised by melts derived from foundered lower crustal eclogite, although it cannot be excluded that some of the melts were derived from depleted mantle peridotite. In other words, parent melts of most granulite xenoliths could share the same petrogenesis as the >110 Ma Mesozoic basalts from the NCC.

Insights into silicic melt generation using plagioclase, quartz and melt inclusions from the caldera-forming Rotoiti eruption, Taupo volcanic zone, New Zealand

The Rotoiti (~120 km3) and Earthquake Flat (~10 km3) eruptions occurred in close succession from the Okataina Volcanic Centre at ~50 ka. While accessory mineral geochronology points to long periods of crystallization prior to eruption (104–105 years) and separate thermal histories for the magmas, little was known about the rates and processes of the final melt production and eruption. Crystal zoning patterns in plagioclase and quartz reveal the thermal and compositional history of the magmatic system leading up to the eruption. The dominant modal phase, plagioclase, displays considerable within-crystal zonation: An37–74, ~40–227 ppm MgO, 45–227 ppm TiO2, 416–910 ppm Sr and 168–1164 ppm Ba. Resorption horizons in the crystals are marked by sharp increases (10–30%) in Sr, MgO and XAn that reflect changes in melt composition and are consistent with open system processes. Melt inclusions display further evidence for open system behaviour, some are depleted in Sr and Ba relative to accompanying matrix glass not consistent with crystallization of modal assemblage. MI also display a wide range in XH2O that is consistent with volatile fluxing. Quartz CL images reveal zoning that is truncated by resorption, and accompanied by abrupt increases in Ti concentration (30–80 ppm) that reflect temperature increases ~50–110°C. Diffusion across these resorption horizons is restricted to zones of <20 μm, suggesting most crystallization within the magma occurred in <2000 years. These episodes are brief compared to the longevity (104–105 year) of the crystal mush zones. All textural and compositional features observed within the quartz and plagioclase crystals are best explained by periodic mafic intrusions repeatedly melting parts of a crystal-rich zone and recharging the system with silicic melt. These periodic influxes of silicic melt would have accumulated to form the large volume of magma that fed the caldera-forming Rotoiti eruption.

X-ray excited luminescence and photoluminescence of Bi4(GeO4)3 glass-ceramics

Bi4(GeO4)3 glass materials have been characterized by X-ray excited luminescence, photoluminescence and cathodo-luminescence measurements. The materials were obtained by crystallization at different temperatures and their spectroscopic parameters were compared before and after crystallization. Thermoluminescence curves recorded after electron irradiation of BGO glass behave similarly to BGO crystals, showing several peaks between 408 K (135 °C) and 610 K (337 °C). The differences between the Bi4(GeO4)3 crystals and glass materials are believed to result from the random distribution of GeO4 tetrahedra around Bi3+ ions which influences the photoluminescence and TL parameters. The CL images of glass-ceramic samples obtained by partial crystallization at 600 °C show luminescent crystalline structures, which are probably responsible for the increase in scintillation efficiency.

REE-assisted U–Pb zircon age (SHRIMP) of an anatectic granodiorite: Constraints on the evolution of the A Silva granodiorite, Iberian allochthonous complexes

The A Silva granodiorite is a plutonic body intruded into the metasediments of the upper unit of the Órdenes Complex (Variscan belt, NW Spain). These metasediments represent the middle section of a magmatic arc located in northern Gondwana. The A Silva granodiorite has been classically considered a late Variscan granite. In this work, new field mapping, structural analysis, and SHRIMP U–Pb zircon dating indicate the granodiorite is significantly older. However, the data indicate a concordant age range between 540 and 460 Ma, and therefore CL images are not useful toward the interpretation of the geochronological results. This issue can be unravelled by using the hafnium and rare earth element composition of zircon in the assessment of the age. In this way, we determined that the age distribution was the result of lead loss, rather than a real age scatter or inheritance, and we could obtain a 206Pb/ 238U crystallization age of 510.28 (+ 1.57, − 1.44) Ma using the TuffZirc algorithm. This age together with the well-preserved field relationships of the host rock permit us to interpret the A Silva granodiorite as multiple sheets intruded into a sequence of metatexitic host rocks after crustal thickening and subsequent decompression that developed coeval with partial melting during the latest stages of a regional extensional event. Taken together with the underlying Monte Castelo gabbro (499 ± 2 Ma), the whole plutonic complex reaches 8 km in thickness and forms an antiformal stack structure in a shear parallel (N–S) cross-section. This structure could be responsible for previously described, localized granulite facies metamorphism. The presence of a late Cambrian magmatic event has been widely reported in other areas of northern Gondwana and it is related to the opening of the Rheic Ocean.

Timing of granulite-facies metamorphism in the eastern Himalayan syntaxis and its tectonic implications

We present geochronological evidence in the eastern Himalayan syntaxis (Namche Barwa) for high-pressure (HP) granulite-facies metamorphism and explain its importance for understanding both the deep continental subduction of the Indian plate beneath Asia and its subsequent exhumation. The timing of peak and retrograde metamorphism in part constrains these processes but is debated. We present zircon U–Pb and trace element data on granulite-facies rocks. Zircon cores and rims from a weakly retrograded mafic granulite ( P = 14–18 kbar, T ≈ 800 °C) yield U–Pb ages of 24.0 ± 0.3 Ma and 18.8 ± 0.3 Ma, respectively. Zircon cores and rims from an orthogneiss, the host of the mafic granulite, yield U–Pb ages of 490 ± 3 Ma and 24.2 ± 0.4 Ma, respectively. An amphibolitized mafic granulite gives a U–Pb zircon age of 17.0 ± 0.4 Ma. Combined with petrography, zircon CL images, Th/U ratios and REE patterns, we suggest that the peak metamorphic age for the HP granulite is at ∼ 24 Ma and subsequent moderate- and low-pressure retrograde metamorphism occurred at 19–17 Ma, indicating reasonably rapid exhumation for the HP granulite. The ages of detrital zircons from a metasedimentary rock, another host rock of the mafic granulite, range from ∼ 0.6 to ∼ 2.0 Ga with peak at 0.8–1.2 Ga. The protolith depositional ages for the metasedimentary rock are constrained to be between 490 and ∼ 600 Ma. Our data suggest that the granulite terrane in the eastern Himalayan syntaxis has an affinity with the Greater Himalayan Series. The HP granulite-facies metamorphic events in the eastern Himalayan syntaxis are distinct from ultrahigh-pressure (UHP) metamorphic events in the western Himalayan syntaxis in both age and depth of burial. However, the metamorphic history of the eastern and western Himalayan syntaxes becomes similar after ∼ 24 Ma. The Namche Barwa granulites appear to result from the underthrusting of the Indian plate lithologies beneath the Lhasa block during progressive collisional processes, followed by extrusion and/or exhumation that result from a slab breakoff of the Indian plate during the Miocene.

Continental and Oceanic Crust Recycling-induced Melt-Peridotite Interactions in the Trans-North China Orogen: U-Pb Dating, Hf Isotopes and Trace Elements in Zircons from Mantle Xenoliths

We present the first finding of continental crust-derived Precambrian zircons in garnet/spinel pyroxenite veins within mantle xenoliths carried by the Neogene Hannuoba basalt in the central zone of the North China Craton (NCC). Petrological and geochemical features indicate that these mantle-derived composite xenoliths were formed by silicic melt^lherzolite interaction. The Precambrian zircon ages can be classified into three age groups of 2·4^2·5 Ga, 1·6^2·2 Ga and 0·6^1·2 Ga, coinciding with major geological events in the NCC. These Precambrian zircons fall in the field of continental granitoid rocks in plots of U/Yb vs Hf and Y. Their igneous-type REE patterns and metamorphic zircon type CL images indicate that they were not crystallized during melt^peridotite interaction and subsequent high-pressure metamorphism.The 2·5 Ga zircons have positive eHf(t) values (2·9^10·6), whereas the younger Precambrian zircons are dominated by negative eHf(t) values, indicating an ancient continental crustal origin.These observations suggest that the Precambrian zircons were xenocrysts that survived melting of recycled continental crustal rocks and were then injected with silicate melt into the host peridotite. In addition to the Precambrian zircons, igneous zircons of 315 3 Ma (2 ), 80^170 Ma and 48^64 Ma were separated from the garnet/spinel pyroxenite veins; these provide evidence for lower continental crust and oceanic crust recycling-induced multi-episodic melt^peridotite interactions in the central zone of the NCC. The combination of the positive eHf(t) values (2·91^24·6) of the 315 Ma zircons with the rare occurrence of 302^324 Ma subduction-related diorite^granite plutons in the northern margin of the NCC implies that the 315 Ma igneous zircons might record melt^peridotite interactions in the lithospheric mantle induced by Palaeo-Asian oceanic crust subduction. Igneous zircons of age 80^170 Ma generally coexist with the Precambrian metamorphic zircons and have lower Ce/Yb and Th/U ratios, higher U/Yb ratios and greater negative Eu anomalies.The eHf(t) values of these zircons vary greatly from ^47·6 to 24·6.The 170^110 Ma zircons are generally characterized by negative eHf(t) values, whereas the 110^100 Ma zircons have positive eHf(t) values.These observations suggest that melt^peridotite interactions at 80^170 Ma were induced by partial melting of recycled continental crust. The 48^64 Ma igneous zircons are characterized by negligible Ce anomalies, unusually high REE, U and Th contents, and positive eHf(t) values.These features imply that the melt^peridotite interactions at 48^64 Ma could be associated with a depleted mantle-derived carbonate melt or fluid.

Zircon LA-ICPMS study and petrogenesis simulation of Dahuabei pluton in the Wulashan area, Inner Mongolia

Dahuabei pluton in the Wulashan area, Inner Mongolia, is an alkaline and peraluminous granitic pluton. LA-ICPMS U–Pb dating of zircons indicates that this pluton crystallized at 330 ± 10 Myr. The pluton did not experience apparent zircon fractionation. The consistent temperatures between Zr saturation (765 °C) and the average zircon–rutile Ti equilibrium (780 °C), together with zircon ages and their CL images, indicated that there were no inherited zircons in this pluton. Through the simulation of the dry-system crystallization process of magma cooling at 200 MPa and 1100–709 °C, the phase diagram and composition evolution tendency of different phases were obtained. The magma reached its liquidus at 1069 °C. Sanidine was the first crystallized phase; at 709 °C the residual magma was less than 4%, and the range of 1069–719 °C should be the most suitable approach to the real condition. It is shown that at least some part of the pluton underwent such isobaric equilibrium crystallization processes during its crystallization.

LA-ICP-MS U–Pb zircon ages of the Qianlishan Complex: Constrains on the evolution of the Khondalite Belt in the Western Block of the North China Craton

The Qianlishan Complex is located in the westernmost part of the Khondalite Belt, a continent–continent collisional belt along which the Yinshan Block in the north and the Ordos Block in the south amalgamated to form the Western Block, which then collided with the Eastern Block along the Trans-North China Orogen to form the North China Craton. The complex is dominated by high-grade supracrustal rocks and minor S-type granites, of which the supracrustals consist of graphite-bearing sillimanite-garnet gneiss, garnet quartzite, felsic paragneiss, calc-silicate rock and marble. CL images reveal the existence of detrital and metamorphic zircons in major rocks of the Qianlishan Complex. In most cases, detrital zircons occur as either single grains with oscillatory zoning or oscillatory zoning cores, typical of igneous origin, which are surrounded by metamorphic overgrowth rims that are structureless, high bright and low in Th/U ratio. Detrital zircons from the Qianlishan Complex yield nearly concordant 207Pb/ 206Pb ages ranging from 2.3 to 2.0 Ga, suggesting that the protoliths of the high-grade supracrustal rocks in the Qianlishan Complex were deposited at some time after 2.0 Ga. Metamorphic zircons yield two age populations with one at ∼1.95 Ga and another at ∼1.92 Ga, of which the former is interpreted as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the later is considered to be the age of subsequent post-orogenic extensional event. Minor S-type granites were emplaced at ∼1.88 Ga, as a result of partial melting of supracrustals at the stage of the exhumation of the Qianlishan Complex. These new zircon ages, combined with structural and metamorphic considerations, enable resolution of the tectonothermal events involving the collision between the Yinshan and Ordos Blocks to form the Western Block, followed by the post-collisional extension and subsequent exhumation of the Khondalite Belt.

Microinclusions in monocrystalline octahedral diamonds and coated diamonds from Diavik, Slave Craton: Clues to diamond genesis

Detailed in-situ analyses of trace-element patterns, N contents, N aggregation state and δ 13C define two populations of gem-quality diamonds in the A154 kimberlite; these probably formed at different times and from different media. Four octahedra with 500–1200 ppm N and 12–35% B aggregation represent an older population; flat REE patterns and low Ba + K indicate that they crystallised from fluids similar to those in many monocrystalline diamonds worldwide. The second population includes coated octahedra and uncoated etched octahedra. On two etched stones, abundant flat-based trigons bottom out 10–15 μm below the faces of the stones, in a layer with abundant microinclusions (0.5 μm diameter) enriched in LREE, Ba and HFSE. Four coated octahedra with 800–1200 ppm N and 11–20% B aggregation have trace-element patterns consistent with growth from a fluid with high LREE + Ba, similar to that in the microinclusion-rich layers of the etched octahedra; the contents of LREE and Ba, and Ba/La, increase toward the rims. The coats on these stones typically have higher Ba than the outer cores, but Ba/La may be either higher or lower. δ 13C is relatively constant (− 5.5 to − 4.7‰) across one etched octahedron but drops to − 9.7‰ in the microinclusion layer. The core of a coated stone has δ 13C like that of the microinclusion layer (− 9.8 to − 10.5‰), but δ 13C drops to − 13.9‰ in the coat. In the coated stones, core-to-rim decreases in the degree of N aggregation are not correlated with N content, but are accompanied by increases in Ba and LREE. This pattern, the lack of δ 13C zoning in the one stone analysed, and homogeneous CL images suggest that the coated stones have grown continuously from a range of evolving fluids. The opaque coats appear to represent a final late stage of accelerated growth in some diamonds, rather than random overgrowths on pre-existing diamonds. Zoning patterns and FTIR data are consistent with the growth of the coated and etched gem-quality stones at 1200–1250 °C in the deep lithosphere shortly before their entrainment in the kimberlite.

Post-orogenic granites in Pingwu region, Northwest Sichuan: Evidence for North China block and Yangtze block collision during Triassic

The Nanyili (南一里), Laohegou (老河沟), and Shaiziyan (筛子岩) granitic intrusions are located in the southern margin of the Bikou (碧口) block in Pingwu (平武) area, Northwest Sichuan (四 川). The petrography and geochemical characteristics of the granitic intrusions as well as their source and tectonic settings are reported and discussed in this article. The Laohegou and Shaiziyan granites are with high SiO2 (69.89 wt.%–73.05 wt.%) and Al2O3 contents, and A/CNK=1.04–1.12. They are typical strongly peraluminous granites, with supersaturation in Al and Si. The abundance of ΣREE varies in the range of (33.13−89.12)×10−6. The rocks show an LREE enrichment pattern and obvious Eu negative anomaly. The trace element geochemistry is characterized evidently by a negative anomaly of Ta, Nb, Ti, etc. and a positive anomaly of Rb, Ba, Sr, etc.. Zircons of the Nanyili granite have higher Th/U ratios, and their CL images have internal oscillatory zoning, suggesting that the zircons of the samples are igneous in origin. The LA ICP-MS zircon U-Pb isotopic concordia diagram yields an age of 223.1±2.6 Ma (MSWD=1.4), which indicates that the granodiorite intrusions formed in the early Late Triassic. The Nanyili, Laohegou, and Shaiziyan granites have the characteristics of post-collisional granites and are regarded as post-orogenic granites. Thus, the granite intrusions are interpreted as syn-collisional granites that resulted from the crustal thickening caused by the collisions between the North China plate and the Yangtze plate during the Indosinian. The granitic intrusions formed in a transitional environment from syn- (compressional environment) to post-collision (extensional environment).