Rounded Cores Research Articles

Zircon U-Pb geochronology of the Lan Sang gneisses and its tectonic implications for the Mae Ping shear zone, NW Thailand

The Mae Ping shear zone (MPSZ), a major shear zone trending NW-SE in Thailand, is responsible for the left-lateral displacement of the N-S Triassic-Jurassic granitoid and gneiss belt. This displacement is postulated to have contributed to Cenozoic extrusion tectonics. Within the Lan Sang National Park, the MPSZ exposes several intensely deformed lithologies, collectively known as the Lan Sang gneisses. These gneisses have attracted considerable attention for their potential to substantiate the extrusion model. However, the timing of the emplacement of the orthogneiss protolith is still debated. Moreover, the origin and distribution of the Eocene syn-shearing granodiorite within this shear zone are not well understood. To shed light on the magmatic history of the MPSZ, this study utilized zircon U-Pb geochronology to systematically investigate the Lan Sang gneisses. Our findings demonstrate that these gneisses can be categorized into paragneiss and orthogneiss groups. Paragneiss samples feature zircons displaying rounded detrital cores ranging from 3,078 to 450 Ma, with metamorphic rim overgrowth of ca. 200 Ma (most Th/U <0.01). This indicates that their Paleozoic sedimentary protoliths experienced high-grade metamorphism during the Triassic-Jurassic Indosinian orogeny. On the other hand, zircon from orthogneiss samples shows that their magmatic protoliths were predominantly emplaced either around ∼200 Ma or within 45-32 Ma. The Eocene-Oligocene magmatism likely coincided with the proposed Eocene metamorphism. Since these samples were deformed by left-lateral shearing, the left-lateral motion of the MPSZ probably ended after 32 Ma. Eocene-Oligocene magmatic events have also been identified in granite, mylonite, and gneiss samples from other regions along the Sibumasu terrane, including the Three Pagodas, Klaeng, Ranong, Khlong Marui shear zones, and the Doi Inthanon area. The Eocene-Oligocene magmatism was likely linked with the movement of the shear zones and may be responsible for the regional cooling pattern. The spatial and temporal distribution of the Eocene-Oligocene magmatism within the Sibumasu terrane supports the hypothesis that the inward migration of magmatism in the overriding plate resulted from the shallowing of the Neo-Tethyan slab.

Dating initial crystallisation of some Devonian plutons in central Victoria and geological implications

Rather than being inherited as detrital grains, many of the rounded and/or embayed cores in zircon crystals from granitic rocks are probably antecrysts, formed early in the magmatic systems. Using laser-ablation, sector-field ICP-MS, we obtained internally consistent weighted-mean concordia ages of 375.3 ± 2.5 Ma and 376.9 ± 2.6 for early crystallisation of two samples from the previously undated Mount Disappointment pluton in the Melbourne Zone. Within uncertainty, our date of 376.4 ± 2.4 Ma for the Baringhup pluton of the Harcourt batholith is the same as the published and our new date for the Mount Alexander pluton, in the same batholith. Using the same techniques, we produced an improved date of 370.9 ± 6.5 Ma for the I-type Ercildoun Granite in the neighbouring Bendigo Zone of the Lachlan Fold Belt. We also obtained dates that confirm published ages for the Mount Bute Granite in the Stawell Zone, the Tynong pluton of the Tynong batholith in the Melbourne Zone and the Oberon pluton of the Wilsons Promontory batholith in the Bassian Zone. These dates confirm the Late Devonian (Fransian) age of most of this widespread plutonism, as well as the reported Emsian age of the Wilsons Promontory batholith. However, part of what has been mapped as the Mount Wombat pluton of the Strathbogie batholith could be significantly older than the rest. KEY POINTS Many rounded or embayed cores in zircon crystals are antecrysts rather than inherited detrital grains. The Mount Disappointment pluton is confirmed as Late Devonian in age, at 375.2 ± 2.5 Ma. At 378.2 ± 2.0 Ma, the Baringhup pluton of the Harcourt batholith is the same age as the Mount Alexander pluton, in the same batholith. Part of the Mount Wombat pluton in the Strathbogie batholith may be older than the rest of the pluton.

Observing the simulation behaviour of Magnesium alloy metal sandwich panel under cyclic loadings

This study aims to investigate the delamination effect of a metal sandwich panel using a four-point bending simulation under continual spectrum loading. The most recent core designs of the sandwich panel have a cavity that can increase vulnerability in terms of bonding strength under constant cyclic loading. The sandwich panel is simulated under constant cyclic loading using different core design configurations, which are rounded dimple, hemispherical dimple, and smooth surface core design. There are two types of conditions used; no pre-stress and pre-stress loads with variable stress ratios based on Gerber stress life theory. Results showed that dimple core design enhanced mechanical behaviour and fatigue life performance about 33% and 5%, respectively, compared to the sandwich panel with a smooth surface core design. It is highlighted that modification on the surface of core design could be beneficial to enhance the bonding strength performance of sandwich panels and prevent early delamination under extreme conditions such as constant cyclic loading. This study is beneficial to enhance the bonding strength for sandwich panels against extreme conditions such as high impact load and continuous cyclic load.

3D Modelling and Validation of the Optimal Pitch in Commercial CORC Cables

Conductor on a rounded core (CORC®) cables with current densities beyond 300 A/mm-2 at 4.2 K, and a capacity to retain around 90 % of critical current after bending to a diameter of 3.5 cm, make them a strong candidate for high field power applications and magnets. In this paper, we present a full 3D-FEM model based upon the so-called H-formulation for commercial CORC® cables manufactured by Advanced Conductor Technologies LLC. The model presented consists of tapes ranging from 1 up to 3 SuperPower 4mm-width tapes in 1 single layer and at multiple pitch angles. By varying the twist pitch, local electromagnetic characteristics such as the current density distribution along the length and width are visualized. Measurements of macroscopical quantities such as AC-losses are disclosed in comparison with available experimental measurements. We particularly focused on the influence of the twist pitch by comparing the efficiency and performance of multiple cables, critically assessing the optimal twist pitch angle.

U-Pb isotopic geochronology of the monazite gneisses of the «zelenolevadivska suite» of the Middle Bug region

On the right bank of the South Bug river valley, opposite the village of Zelena Levada crystalline rocks are observed in separate scattered rock outcrops, more than 1 km long along the river bed. A bundle of hypersthene-biotite and biotite plagiogneisses is exposed here, which in some areas has undergone calishpatization, manifested in the form of unevenly placed porphyroblasts of potassium feldspar, thin veins of quartz-feldspar composition and migmatization. Gneisses are dissected by single veined and lenticular bodies of granites. The gneiss pack is assigned to the Zelenolevada stratum of the Dniester- Bug series. The section is considered to be a stratotype for the Zelenolevada Formation. So far, the following values of isotopic ages have been obtained for monazites from microclinized plagiogneiss and from a lenticular body of granite exposed in this area — 2031.5 ± 6.7 million years and 2030.3 ± 1.7 million years, respectively. This publication presents the dating results of monazites from calishpatized plagiogneiss. microclination is manifested in the formation of thin veins composed of quartz+microcline+plagioclase+ biotite and porphyroblasts of microcline. Optical studies of polished sections of zircon crystals revealed that the crystals of this mineral have a complex structure, they are formed by heterogeneous, often with rounded contours, light pink (brownish pink) cores and shells of milky white zircon. The presence of heterogeneous, including cores with rounded (rolled) cores indicates the primary-sedimentary (volcanogenic-sedimentary) nature of plagiogneiss. According to the results of uranium-lead isotope dating of multigrain samples of monazite, the age of monazite (weighted aver- age value based on the ²⁰⁷Pb/²⁰⁶Pb ratio) is 2031.5 ± 2.7 million years.

Morphological transition during prograde olivine growth formed by high-pressure dehydration of antigorite-serpentinite to chlorite-harzburgite in a subduction setting

Crystal morphologies are essential for deciphering the reaction history of igneous and metamorphic rocks because they often record the interplay between nucleation and growth rates controlled by the departure from equilibrium. Here, we report an exceptional record of the morphological transition of olivine formed during subduction metamorphism and high-pressure dehydration of antigorite-serpentinite to prograde chlorite-harzburgite in the Almirez ultramafic massif (Nevado–Filábride Complex, Betic Cordillera, SE Spain). In this massif, rare varied-textured chlorite-harzburgite (olivine+enstantite+chlorite+oxides) —formed after high–P dehydration of antigorite-serpentinite— exhibits large olivine porphyroblasts made up of rounded cores mantled by coronas of tabular olivine grains, similar to single tabular olivines occurring in the matrix. The correlative X-ray μ-CT and EBSD study of two varied-textured chlorite-harzburgite samples show that tabular olivine in coronas is tabular on (100)Ol with c > b >> a, and grew in nearly the same crystallographic orientation as the rounded olivine cores of the porphyroblast.Quantitative textural analysis and mass balance indicate that varied-textured chlorite-harzburgite is the result of a two-stage nucleation and growth of olivine during the progress of the high–P dehydration of antigorite-serpentinite to chlorite-harzburgite reaction. The first stage occurred under a low affinity (ΔGr) and affinity rate (ΔGr/dt) of the antigorite dehydration reaction that resulted in a low time-integrated nucleation rate and isotropic growth of olivine, forming rounded olivine porphyroblasts. With further progress of the dehydration reaction, a second stage of relatively higher affinity and affinity rate resulted in a higher time-integrated nucleation rate of olivine coeval with a shift from isotropic to anisotropic olivine growth, leading to tabular olivines. The two-stage evolution resulted in olivine porphyroblasts made up of rounded cores mantled by coronas of tabular olivine grains characteristic of varied-texture chlorite-harzburgite.Although a switch to anisotropic tabular olivine in the second stage is consistent with the relative increase in the affinity and affinity rate, these changes cannot solely account for the growth of Almirez olivine tabular on (100). Tabular olivines in komatiites and other igneous rocks are tabular on (010)Ol with either a > c >> b, or a ≈ c > > b, in agreement with experimentally determined growth rates of olivine phenocrysts under moderate to high undercooling and cooling rates. On the other hand, olivine tabular on (100) is expected in the presence of highly polymerized fluids where inhibited growth of the olivine (100) and (010) interfaces occurs, respectively, due to dissociative and molecular adsorption of water monolayers. Rounded and tabular olivines in Almirez varied-textured chlorite-harzburgite show differing trace element compositions that we interpret as due to the infiltration of external fluids during antigorite dehydration. Isothermal infiltration of highly polymerized fluids would explain the shift in the affinity and affinity rate of the antigorite dehydration reaction, as well as the olivine morphology tabular on (100) due to the inhibited growth on the (100) and, to a lesser extent, (010). Our study shows that surface-active molecules may play an essential role in shaping the morphology of growing crystals during fluid-present metamorphic crystallization.

Model verification of material properties of wound composite rods in tensile loading

The aim of this work is to find a suitable reinforcement in terms of mutual compatibility, surface microstructure, and mechanical properties of the resulting carbon composite parts. Two types of samples in layout + 45° / 0° / ± 45° / 0° / −45° and + 45° / 0° / −45° with a total thickness of 1.8 mm, wound on a rounded polyurethane core from several carbon rovings, were experimentally tested in the area of low elastic deformation up to 1 mm. In order to obtain basic engineering constants the tensile tests of whole parts failure have been carried out. Subsequently, several material and numerical models were developed in order to describe the problem of composite tensile loading. The model was solved by two approaches, specifically a layered shell and a layered solid. Considerable differences have been found in the properties of the real (experimental) and theoretical model based on the declared properties of the individual components. Probably due to the manufacturing imperfections and the porosity in the resulting material, the values of the basic engineering constants of the laminate of the resulting laminate reach lower values. This had to be solved by adjusting the material model of the used dispersion and matrix. The real arising failures were described by using the failure criteria for several time steps of the model solution and compared to the real experiment with quite good agreement.

LA-ICP-MS elemental mapping of pyrite: An application to the Palaeoproterozoic atmosphere

A hydrothermal origin has been advocated for the genesis of rounded and subhedral-euhedral pyrite in some Neoarchaean-Palaeoproterozoic clastic sedimentary rocks, thereby limiting their value as proxies for the composition of the early atmosphere. We present Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) element maps of rounded and euhedral-subhedral pyrite grains from the basal conglomerates of the Palaeoproterozoic Mississagi Formation, Huronian Supergroup, Canada. These maps reveal the common existence of rounded detrital cores and isolated grains variably modified by hydrothermal overgrowth. The detrital origin of pyrite grains is demonstrated by the rounding of isolated grains and the presence of cores in subhedral-euhedral grains. Rounding and breakage of grains is implied to have occurred via physical abrasion during sediment transport. Furthermore, trace element maps expose truncated cores of originally rounded detrital fragments that also indicate abrasive rather than accretional or dissolutional rounding. Finally, we document a complex core that consists of multiple grains suggestive of various source materials that were co-deposited in the original placer. Textural and geochemical evidence further suggests the detrital porous pyrite had a diagenetic origin prior to fluvial transport. Grain-size and shape analysis suggests that the large porous pyrite cores and grains may have had a shorter transport distance than the, on average, smaller compact rounded grains. Transport was prolonged from tributaries into the main valley, and the high energy setting implies well- aerated Palaeoproterozoic river water. Since pyrite is unstable under oxidising conditions, the frequent survival of detrital pyrite in the fluvial Mississagi Formation after sustained transport in the catchment basin and alluvial valley as well as aerated Palaeoproterozoic river waters therefore implies that the contemporary atmosphere was oxygen deficient. The depositional age of the Mississagi Formation is between 2.45Ga (Thessalon Fm.) and 2.31Ga (Gordon Lake Fm.), and the presence of the detrital pyrite therefore suggests that the atmosphere was devoid of any significant amount of free oxygen at the time immediately preceding the Great Oxygenation Event.

Produire des lames de hache en dolérite au Néolithique en Mayenne. La carrière-atelier de Beulin à Saint-Germain-le-Guillaume

Produire des lames de hache en dolérite au Néolithique en Mayenne. La carrière-atelier de Beulin à Saint-Germain-le-Guillaume

Replacement of Calcite (CaCO3) by Cerussite (PbCO3).

The mobility of toxic elements, such as lead (Pb) can be attenuated by adsorption, incorporation, and precipitation on carbonate minerals in subsurface environments. Here, we report a study of the bulk transformation of single-crystal calcite (CaCO3) into polycrystalline cerussite (PbCO3) through reaction with acidic Pb-bearing solutions. This reaction began with the growth of a cerussite shell on top of calcite surfaces followed by the replacement of the remaining calcite core. The external shape of the original calcite was preserved by a balance between calcite dissolution and cerussite growth controlled by adjusting the Pb2+ concentration and pH. The relation between the rounded calcite core and the surrounding lath-shaped cerussite aggregates was imaged by transmission X-ray microscopy, which revealed preferentially elongated cerussite crystals parallel to the surface and edge directions of calcite. The replacement reaction involved concurrent development of ∼100 nm wide pores parallel to calcite c-glide or (12̅0) planes, which may have provided permeability for chemical exchange during the reaction. X-ray reflectivity measurements showed no clear epitaxial relation of cerussite to the calcite (104) surface. These results demonstrate Pb sequestration through mineral replacement reactions and the critical role of nanoporosity (3% by volume) on the solid phase transformation through a dissolution-recrystallization mechanism.

Biogenic overgrowth on detrital pyrite in ca. 3.2 Ga Archean paleosols

Abstract Regionally traceable paleosols in the lower Moodies Group of the Barberton greenstone belt (ca. 3.22 Ga, northeastern South Africa and Swaziland) contain locally abundant silicified nodules, originally composed of pedogenic carbonates and sulfates, interbedded with heavy-mineral laminae dominated by pyrite. Pyrite grains show rounded detrital cores and secondary idiomorphic rims with trace element concentrations and δ34S ratios clearly different from those of the cores. While cores have low Co and Ni concentrations and high Co/Ni ratios, rims show as much as 5.5 wt% of these elements and low Co/Ni ratios, reflecting the weathering of nearby ultramafic rocks. In-situ sulfur isotope analyses of pyrite cores show δ34SVCDT (Vienna Canyon Diablo troilite) values between +5‰ and −5‰, while the rims show δ34SVCDT values between −20‰ and −24.5‰, suggesting biogenic fractionation of sulfur. The close spatial association and microtextural evidence for nearly contemporaneous formation of the pedogenic sulfate nodules and the secondary pyrite rims suggests microbial processing of sulfur in the paleosols, which provided reduced and 34S-depleted sulfur for the growth of authigenic pyrite. This indicates that vadose-zone soil-forming processes in the Archean involved not only physical and chemical modification of moist, unconsolidated sediment in a terrestrial environment but also already included its microbiological modification.

Histopathological and immunohistochemical diagnosis from a B cell lymphoma in a bovine – CASE STUDY

Histopathological and immunohistochemical diagnosis from a B cell lymphoma in a bovine – CASE STUDY

On the origin of hot metasedimentary quartzites in the lower crust of continental arcs

Volcanic arcs associated with subduction zones are thought to be the primary building blocks of continents. The composition of the magmas, particularly in continental arcs, is the product of mixing between differentiation of juvenile magmas and pre-existing crustal wallrock, the former being typically mafic and the latter more silicic. Because the upper continental crust is on average thought to be more silicic than the mafic lower crust, mixing with silicic endmembers should occur primarily in the upper crust. However, we show here that the lower crust of continental arcs contains silicic metasediments. We examine garnet-bearing, granulite-facies sedimentary quartzite xenoliths from the Sierra Nevada batholith in California, a Cretaceous continental arc. The quartzites have equigranular textures and contain quartz (>50%), plagioclase (<30%), garnet (10%), and small amounts (<1%) of rutile, aluminosilicate, biotite, monazite, zircon, graphite and trace orthopyroxene. Cathodoluminescent images show zircons with rounded detrital cores mantled by metamorphic overgrowths. Hf isotopic model ages and U–Pb upper intercept ages, for a given zircon, are similar, but the zircon population shows variable protolith ages ranging from Proterozoic to Archean. In contrast, all zircons share similar lower intercept U–Pb ages (103±10Ma), which coincide with the peak of arc magmatism in the Sierra Nevada. The Precambrian protolith ages are similar to North American cratonal basement, and together with the abundance of quartz and detrital zircons, suggest that these quartzites represent ancient, passive margin sediments instead of juvenile active margin sediments in the oceanic trench and accretionary prism. Importantly, these quartzites record peak metamorphic temperatures and pressures of 700–800°C using Ti-in-quartz thermometry and 0.7–1.1GPa using garnet–aluminosilicate–plagioclase thermobarometry, indicating that these xenoliths experienced significant heating and possible partial melting in the lower crust, most likely related to arc magmatism as suggested by similarities between the lower intercept U–Pb ages and the ages of plutonism in the Sierra Nevada. Possible mechanisms by which these sediments were transported into the lower crust include continental underthrusting beneath the continental arc, underplating by buoyant slab-derived sedimentary diapirs, or viscous downflow of country rock in response to diapiric ascent of plutons. Continental underthrusting has been independently documented during the Sevier orogeny, coinciding with the peak of arc magmatism. We thus speculate that supracrustal rocks may have been underthrusted into deep crustal magmatic zones. Regardless of how these metasediments arrived in the lower crust, our observations indicate that silicic metasediments occur in the lower crust of volcanic arcs, not just in the upper crust as is commonly thought. Transport of metasediments into deep crustal magmatic zones should influence the composition of arc magmas and continental crust in general.

A common feeding system of the NE and S rifts as revealed by the bilateral 2002/2003 eruptive event at Mt. Etna (Sicily, Italy)

Mount Etna volcano is often characterized by bilateral eruptive events, involving both the south (S) and the north east (NE) rifts. The last event occurred in 2002–2003 from October 27 to January 28. A detailed, stratigraphically time-controlled sampling of lavas and tephra of the southern eruptive fissure was performed in order to (1) track the petrological features of products during the eruption and (2) integrate the results with those previously obtained on the NE rift. Whole-rock composition and textural observations were implemented by major and minor element analyses of plagioclases in lavas and tephra from both sides of the volcano. Fractionation models constrained by mass balance (major and trace elements) and Rayleigh calculations suggest that magmas are linked by the same liquid line of descent by fractionating 9.11 % of a mineral assemblage of Cpx (52.69 %), Plg (21.41), and Ol (7.46 %). These new data allowed us to identify at least two feeding episodes through the southern fissure and infer that high-K2O porphyritic magmas, emitted on both the S and NE rifts, derives by fractionation from the same parent magma. However, lavas and tephra from the southern flank were slightly more primitive. Textural and petrological study of plagioclase moreover indicates that chemical–physical conditions in the deep feeding system were similar for magmas erupting from both rifts as suggested by the presence of dissolved rounded cores in both lavas. Magmas evolved differently on the S and the NE rifts only at shallow levels. Comparison with published seismotectonic data supports the idea that the main magma feeding the eruption on October 27 ascended along the same pathway at depth and was intercepted by the fracture system of the S and NE rifts at shallow depth, between 6 and 3 km b.s.l.

Geochronological and Geochemical study of Palaeoproterozoic gneissic granites and clinopyroxenite xenoliths from NW Fujian, SE China: Implications for the crustal evolution of the Cathaysia Block

U–Pb zircon dating of gneissic granite samples in northwestern Fujian Province, Southeast China, gave Neoarchean to Palaeoproterozoic ages (2.6–1.9 Ga) for the rounded zircon cores and Palaeoproterozoic upper intercept ages (1851 ± 21 Ma, 1857 ± 29 Ma, respectively) for zoned rims that have oscillatory growth zoning and yield ε Hf( t) values between −8.8 and +3.7 and T DM model ages between 2.1 and 2.6 Ga. These gneissic granites are peraluminous (A/CNK = 0.94–1.28), characterized by high SiO 2 (68–72 wt.%), Al 2O 3 (14–15 wt.%) and low CaO, MgO, FeO t , TiO 2 and P 2O 5 contents. They also possess relatively low REE contents and moderate LREE/HREE fractionation and display pronounced negative anomalies in Nb, Ta, Sr, P and Ti. All these features indicate that the precursor magmas were likely formed by partial melting of sedimentary rocks. Clinopyroxenite xenoliths with gneissosity structures are found in these gneissic granites and they have high MgO, Ni and Cr, but low contents of TiO 2, and are characterized by high CaO/Al 2O 3 (4.1–5.3), low Al 2O 3/TiO 2 ratios (8.7–9.5) and HREE depletion. These features are similar to the typical Al-depleted type komatiites. Their geochemical characteristics, such as high (Gd/Yb) N and low HREE, Y, Zr and Hf suggest partial melting of the upper mantle source with residual garnet. The negative Nb, Ta and Ti anomalies indicate a strong arc-related affinity. Our results and recently published data for granitic and mafic basement rocks in this region, reveal that 1.85 Ga was an important period for the evolution of the Cathaysia Block, possibly related to the Columbia supercontinent assembly. Large amount of granitic magma was emplaced, probably as a result of partial melting of old Neoarchean to Palaeoproterozoic materials.

500 m.y. of thermal history elucidated by multi-method detrital thermochronology of North Gondwana Cambrian sandstone (Eilat area, Israel)

Research Article| July 01, 2009 500 m.y. of thermal history elucidated by multi-method detrital thermochronology of North Gondwana Cambrian sandstone (Eilat area, Israel) Pieter Vermeesch; Pieter Vermeesch † 1School of Earth Sciences, Birkbeck, University of London, Malet Street, London, WC1E 7HX, UK †E-mail: p.vermeesch@ucl.ac.uk Search for other works by this author on: GSW Google Scholar Dov Avigad; Dov Avigad 2Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Search for other works by this author on: GSW Google Scholar Michael O. McWilliams Michael O. McWilliams 3Commonwealth Scientific and Industrial Research Organization, Exploration and Mining, Kensington, WA 6151, Australia Search for other works by this author on: GSW Google Scholar GSA Bulletin (2009) 121 (7-8): 1204–1216. https://doi.org/10.1130/B26473.1 Article history received: 30 May 2008 rev-recd: 01 Oct 2008 accepted: 06 Oct 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Pieter Vermeesch, Dov Avigad, Michael O. McWilliams; 500 m.y. of thermal history elucidated by multi-method detrital thermochronology of North Gondwana Cambrian sandstone (Eilat area, Israel). GSA Bulletin 2009;; 121 (7-8): 1204–1216. doi: https://doi.org/10.1130/B26473.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Following the Neoproterozoic Pan-African orogeny, the Arabian-Nubian Shield of North Africa and Arabia was eroded and then covered by Cambrian sandstones that record the onset of platform sedimentation. We applied K-feldspar 40Ar/39Ar, zircon and apatite fission-track, and apatite (U-Th)/He thermochronology to detritus from Cambrian sandstones of southern Israel deposited at ca. 500 Ma. U-Pb detrital zircon ages from these sandstones predate deposition and record the earlier Neoproterozoic crustal evolution of the Pan-African orogens. 40Ar/39Ar ages from 50 single grains of K-feldspar yield a Cambrian mean of ca. 535 Ma. The 40Ar/39Ar age spectrum of a multi-grain K-feldspar aliquot displays diffusion behavior compatible with >560 Ma cooling later affected by a heating event. Assuming that the high-temperature domains of the K-feldspars have not been affected by subsequent (hydro)thermal events, and taking previously published K-Ar and Rb-Sr ages from other parts of the East African Orogen at face value, these ages apparently record Pan-African thermal resetting below a thick volcano-sedimentary pile similar to the Saramuj conglomerate in Jordan and/or the Hammamat in Egypt. Detrital zircon fission-track (ZFT) ages cluster around 380 Ma, consistent with previous ZFT results from Neoproterozoic basement and sediments of the region, revealing that the Cambrian platform sequence experienced a middle Devonian thermal event and low-grade metamorphism. Regional correlation indicates that during Devonian time the sedimentary cover atop the Cambrian in Israel was never in excess of 2.5 km, requiring an abnormally steep geothermal gradient to explain the complete ZFT annealing. A basal Carboniferous unconformity can be traced from Syria to southern Saudi Arabia, suggesting that the observed Devonian ZFT ages represent a regional tectonothermal event. Similar Devonian ZFT ages were reported from Arabian-Nubian Shield basement outcrops in the Eastern Desert, 500 km south of Eilat. The detrital apatites we studied all have extremely rounded cores suggestive of a distant provenance, but some grains also feature distinct euhedral, U-rich apatite overgrowth rims. Authigenic apatite may have grown during the late Devonian thermal event we dated by ZFT, coinciding with existing Rb-Sr ages from authigenic clays in the same deposits and leading to the conclusion that the Devonian event was probably hydrothermal. Like the ZFT ages, the detrital apatite fission-track (AFT) ages were also completely reset after deposition. Sixty single-grain detrital AFT ages group at ca. 270 Ma with significant dispersion. Inverse modeling of the AFT data indicate extended and/or repeated residence in the AFT partial annealing zone, in turn suggesting an episodic burial-erosion history during the Mesozoic caused by low-amplitude vertical motions. Seven detrital apatite (U-Th)/He ages scatter between 33 and 77 Ma, possibly resulting from extreme compositional zonation associated with the authigenic U-rich overgrowths. The ca. 70 Ma (U-Th)/He ages are more likely to be accurate, setting 1–2 km as an upper limit (depending on the geothermal gradient) on the post-Cretaceous exhumation of the Cambrian sandstone and showing no evidence for substantial denudation related to Tertiary rifting of the Red Sea. 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Rapid open-system assembly of a large silicic magma body: time-resolved evidence from cored plagioclase crystals in the Oruanui eruption deposits, New Zealand

Rhyolitic pumices in the 26.5 ka Oruanui eruption (Taupo volcano, New Zealand) contain an average of 10 wt% crystals. About 2 wt% of the crystal population is feldspar crystals that display bluish–grey cloudy cores, the colour being imparted by exsolved needles of rutile. The volume of cloudy-cored feldspars thus amounts to ~1.0 km3 in a total magma volume of ~530 km3. The cored feldspars show great variability in detail, but in general have a rounded cloudy core bounded by a zone rich in glass and mineral inclusions, that was then overgrown by a euhedral clear rim. Sr-isotopic variations in eight representative crystals were measured on micromilled samples of selected growth zones in the cores and rims, and linked to feldspar compositions through microprobe traverses. The cloudy cores range from 87Sr/86Sr = 0.70547 to 0.70657, with compositions of An43 to An78. The overgrowth rims display wider variations: inner parts show extreme ranges in composition (maxima 87Sr/86Sr = 0.70764 and An78), while outer parts in seven of eight crystals are zoned, with outward-decreasing Sr-isotopic and An values to figures that are in accord with the bulk pumice and other, clear-feldspar values, respectively. The three parts of the crystals represent distinct regimes. The cloudy cores are inherited from an intermediate plutonic protolith that has been subjected to melting. The inner overgrowth rims were crystallised from a high temperature, relatively radiogenic melt derived from Mesozoic-Palaeozoic metasedimentary rocks (“greywacke”). The outer euhedral rims reflect mixing into and continued growth within the growing Oruanui magma body. The cloudy-cored feldspars also contain rare zircon inclusions. Twenty one zircons were recovered by HF digestion of a bulk sample of cloudy feldspars and analysed by SHRIMP for U–Th isotopes with which to calculate model ages. Eighteen of 21 crystals returned finite ages, the model-age spectrum of which is similar to the age spectra from free zircons in Oruanui pumices. Assembly of the Oruanui magma body was not only rapid (over ~40 kyr, as shown by other data) but involved a wide open system, with significant contributions from partly-melted intermediate-composition igneous intrusions (cloudy cores) and greywacke melts (inner overgrowths) being introduced into the magma body up to the point of eruption. Such open system behaviour contrasts with that proposed in models for comparably voluminous silicic magmas derived dominantly by fractionation (such as the Bishop Tuff) where the magma and its crystal cargo were better insulated thermally and chemically from country-rock interaction.

U-Pb Zircon and Sm-Nd isotopic study of the huangtuling granulite, dabie-sulu belt, China: Implication for the paleoproterozoic tectonic history of the yangtze craton

SIMS data demonstrate Archean (2.9 to 2.7 Ga) and Paleoproterozoic (1991 ± 43 Ma) ages for rounded detrital zircon cores and unzoned rims respectively in a graulite sample collected from Huangtuling, Dabie-Sulu orogenic belt, China. The granulite rocks in this locality have pelitic compositions and represent an exposure of the northern margin of the Yangtze Craton. The rounded zircon cores show oscillatory zoning and have Th/U ratios (0.17 − 0.78) systematically higher than those of the unzoned luminescent rims (0.005 − 0.03). The age of the detrital zircon cores and whole rock Nd isotope model age suggest that the Huangtuling granulite was possibly derived from a provenance dominated by Archean rocks similar to the Kongling Complex exposed 200 km in the southwest. Multiple phases of metamorphism have been identified for the granulites, with the dominant granulite-facies mineral assemblage, defining an early clock-wise metamorphic P-T path. Orthopyroxene, plagioclase and whole-rock material define a linear trend in a ^147^Sm/^144^Nd − ^143^Nd/^144^Nd diagram, corresponding to an age of 1926 ± 146 Ma. This result and the U-Pb data for the zircon rims suggest that a collisional event may have occurred along the northern margin of the Yangtze Craton at ∼2.0 Ga, inviting further study on whether the Yangtze Craton was involved in the assembly of the recently-proposed Paleo-Mesoproterozoic supercontinent Columbia.

Zircon geochronology and Sr, Nd, Pb isotope geochemistry of granitoids from Bayuda Desert and Sabaloka (Sudan): Evidence for a Bayudian event (920–900 Ma) preceding the Pan-African orogenic cycle (860–590 Ma) at the eastern boundary of the Saharan Metacraton

Results of zircon geochronology and Sr, Nd and Pb isotope investigations carried out on granitoid rocks from the boundary of the Saharan Metacraton with the Arabian–Nubian Shield (ANS) reveal a protracted, partly pre-Pan-African geodynamic evolution and the existence of crustal terranes of different ancestries. (Meta)granitoids and gneisses in north-central Bayuda Desert record a 920–900 Ma orogenic event, hitherto unrecognized in northeastern Africa. This early Neoproterozoic Bayudian event is restricted to a pre-Pan-African crustal terrane which appears only slightly affected by Pan-African tectogenesis and deformation, named here the Rahaba–Absol terrane. Within this terrane, amphibolite-facies metamorphism occurred at 921 ± 10 Ma in the El Melagi muscovite–biotite gneiss of the Rahaba Series, probably during a collisional phase; followed by the intrusion of the Absol medium-K granite–granodiorite pluton discordantly emplaced at 900 ± 9 Ma into high-grade schists and amphibolites of the metavolcanosedimentary Absol Series. Nd T DM model ages (2040–2430 Ma) and ages of rounded zircon cores (1060, 1980, 2540 and 2675 Ma) of the El Melagi gneiss indicate a predominantly late Archaean to Palaeoproterozoic source region and suggest a latest Mesoproterozoic to early Neoproterozoic depositional age of its pelitic precursor sediment. The post-collisional Absol pluton has isotope characteristics ( ɛ Nd 900 of −0.3 to −4.3, T DM model ages: 1300–1830 Ma, SrIR 900 of 0.7028–0.7055, 207Pb/ 204Pb of 15.66–15.81) indicating assimilation of old pre-Neoproterozoic crust. The Rahaba–Absol terrane is part of the Saharan Metacraton and is in tectonic contact with the high-grade metamorphic Kurmut terrane of eastern Bayuda Desert. The Kurmut terrane has juvenile Neoproterozoic isotope characteristics, its granitoids record only Pan-African orogenic events and it is therefore considered a part of the Arabian–Nubian Shield. Within the Kurmut terrane the Dam Et Tor medium-K epidote–biotite gneiss gave a concordant zircon age of 858 ± 9 Ma, interpreted as the crystallization age of the volcanic/subvolcanic precursor of the gneiss. Isotopic data of the Dam Et Tor gneiss ( ɛ Nd 858 of 5.7–6.1, T DM model ages: 860–900 Ma, SrIR 858 of 0.7026–0.7028, 207Pb/ 204Pb of 15.46) indicate a depleted mantle source and an oceanic arc magmatic environment. The age of the amphibolite-facies metamorphism and deformation of the Dam Et Tor gneiss and its enclosing metavolcanosedimentary Kurmut Series is best approached by a Sm–Nd isochron age (806 ± 19 Ma; Küster and Liégeois [Küster, D., Liégeois, J.P., 2001. Sr, Nd isotopes and geochemistry of the Bayuda Desert high-grade metamorphic basement (Sudan): an early Pan-African oceanic convergent margin, not the edge of the East Saharan ghost craton? Prec. Res., 109, 1–23]). Both the Rahaba–Absol and the Kurmut terrane of the Bayuda Desert are intruded by post-collisional high-K granitoid magmatism of late Pan-African age, with no intermediate events recorded. The An Ithnein pluton that intrudes the pre-Pan-African Rahaba–Absol terrane is dated at 597 ± 4 Ma. The isotope characteristics of this pluton ( ɛ Nd 600 of −1.5 to 2.4, T DM model ages: 910–1190 Ma, SrIR 600 of 0.7011–0.7038, 207Pb/ 204Pb of 15.59–15.69) and that of the Nabati pluton intruding the Kurmut terrane ( ɛ Nd 600 of 0.1–4.5, T DM model ages: 750–1050 Ma, SrIR 600 of 0.7032–0.7036, 207Pb/ 204Pb of 15.56–15.60) indicate a derivation from a juvenile crustal source or the mantle with limited assimilation of older crust. Post-collisional late Pan-African granite plutons at Sabaloka, south of Bayuda Desert, are distinguished into high-K and shoshonitic suites. The Banjedid high-K pluton was emplaced at 605 ± 4 Ma, while the Babados shoshonitic pluton intruded at 591 ± 5 Ma. Magmatic zircons from both plutons have rims with low Th/U ratios, suggesting a HT metamorphic event contemporaneous with magma emplacement. Both granitoid suites have isotopic characteristics ( ɛ Nd 600 of −8.3 to −2.1, T DM model ages: 1160–1560 Ma, SrIR 600 of 0.7029–0.7069, 207Pb/ 204Pb of 15.72–15.81) indicating the assimilation of older pre-Neoproterozoic crust. Sabaloka is thus clearly identified as a part of the Saharan Metacraton but is distinguished from the Rahaba–Absol terrane by a strong Pan-African high-grade metamorphic imprint [Kröner, A., Stern, R.J., Dawoud, A.S., Compston, W., Reischmann, T., 1987. The Pan-African continental margin in northeastern Africa: evidence from a geochronological study of granulites at Sabaloka, Sudan. Earth Planet. Sci. Lett. 85, 91–104] indicating the likely existence of a discrete Sabaloka terrane.

Zoned Zircon from Eclogite Lenses in Marbles from the Dabie‐Sulu UHP Terrane, China: A Clear Record of Ultra‐deep Subduction and Fast Exhumation

Abstract: Eclogite lenses in marbles from the Dabie‐Sulu ultrahigh‐pressure (UHP) terrane are deeply subducted meta‐sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited (detrital) and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence (CL) imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones (domains) to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark‐luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high‐pressure (HP) metamorphic conditions of T = 571–668°C and P = 1.7‐2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white‐luminescent images. It contains coesite eclogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782–849°C and P &gt; 5.5 GPa. Domain 3, with gray‐luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low‐pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600–710°C and P = 0.7‐1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245 ± 4 Ma for domain 1, 235 ± 3 Ma for domain 2 and 215 ± 6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244 ± 4 Ma, 233 ± 4 Ma and 214 ± 5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10–11 Ma and exhumation of the rocks occurred over a period of 19–20 Ma. Thus, subduction from ∼ 55 km to &gt; 160 km deep mantle depth took place at rates of approximately 9.5–10.5 km/Ma and exhumation from depths &gt;160 km to the base of the crust at −30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin Iithosphere followed by ultrafast exhumation driven by buoyancy forces after break‐off of the UHP slab deep within the mantle.