Rim Growth Research Articles

Post–peak petrochronology and C–O–H fluid influx in the lower crust: A case study of garnet–orthopyroxene granulites from the Rauer Islands, East Antarctica

Garnet–orthopyroxene granulites from the Rauer Islands (East Antarctica) provide a spectacular example to investigate the late fluid evolution, metamorphic duration, and behavior of monazite and zircon in response to metamorphic reactions and fluid–rock interaction. Here, we characterize the secondary fluid inclusions in peritectic garnet and orthopyroxene, which occur as multiphase inclusions along micro–fractures. Inclusions are composed of siderite, pyrophyllite, calcite, quartz and residual CO2, representing stepdaughter phases resulting from the interactions between C–O–H fluid and its hosts at variable temperatures during retrogression. Zircon grains show clear core–rim structure, which yield 206Pb/238U ages of 540–507 Ma and 527–490 Ma, respectively. Index inclusions and internal structures suggest that the cores document the timing of peak and post–peak decompression while the growth of rims corresponds to melt crystallization during the final cooling. The UPb systems in zircons are considered to have not been obviously affected by fluid or melt–mediated modification. The unusual formation of monazites in garnet–orthopyroxene granulites may be linked with the elevated phosphorus budget as a result of apatite dissolution during the prograde melting of the rocks. Detailed investigations suggest that the crystallization of monazites occurred both at peak and post–decompression stages, whose isotopic systematics has been completely reset due to melt–mediated dissolution–precipitation. The spurious dates for monazites (522–495 Ma) are highly coinstantaneous with the dating results for zircon rims, further supporting this view. Therefore, we conclude that the late carbonic fluid influx cannot result in marked U(Th)–Pb resetting in zircon and monazite. Instead, anatectic melt may have played an important role in the disturbance of isotopic systematics in monazites, especially for long–lived high–grade metamorphic terranes. Combined with previously published data, we propose that the Pan–African metamorphic event in the Rauer Islands may have reached the peak at around ∼540 Ma, followed by a protracted post–peak evolution that lasted for at least ∼50 Myr. This study highlights the importance of an integrated investigation of fluid and index mineral inclusions, as well as the chemical signatures of zircon and monazite, to interpret chronological data correctly.

Implications of high-grade metamorphism on detrital zircon data sets: A case study from the Fraser Zone, Western Australia

Detrital zircon grains provide a useful tool to determine the maximum age of sedimentary strata and can elucidate tectonic setting and basin evolution. However, high-grade metamorphism can overprint the mineral chemistry of the detrital cargo and reset both primary age and compositional information. Incomplete resetting of the U–Pb isotopic system, fluid alteration, and complex internal structures due to granulite-facies metamorphism makes dating and interpretation of the obtained zircon ages challenging. Here, we use trace element chemistry, U–Pb geochronology (LA-ICP-MS and SHRIMP), textural observations, and inclusion characterization from detrital zircon crystals in the Snowys Dam Formation (Albany–Fraser Orogen; AFO) to deconvolve its strongly overprinted sedimentary history, and to evaluate the likely geodynamic setting for basin genesis. Detrital zircons that survived metamorphic overprinting show that the main detrital load was sourced from Paleoproterozoic lithologies from neighbouring regions, with rare Archean Yilgarn crystals. The youngest detrital grains correlate with the age of plagiogranites in the Madura Province to the east of the AFO and imply a maximum depositional age of c. 1390 Ma, indicating that the Fraser Zone temporally correlates to an oceanic arc and thus best fits a back-arc basin setting. High-grade metamorphism during Stage I of the orogeny (c. 1330–1260 Ma) promoted recrystallization of pre-existing detrital zircons together with neoblastic growth of metamorphic zircon rims. At c. 1245 Ma, during early Stage II of the AFO, anatexis of the metasedimentary rocks produced rare neocrystallized magmatic zircon. Late-stage fluids are indicated by elevated LREE for these grains, which also contain a hydrous inclusion assemblage. High volume sampling techniques such as LA-ICP-MS often drilled through multiple age domains that can hinder geological interpretation. Thus, a lower volume sampling technique (e.g., SHRIMP) and verification with mineral chemistry is highly instructive in accurately resolving primary age groups in high-grade terrains.

Provenance, protolith and metamorphic ages of jadeite-bearing orthogneiss and host paragneiss at Tavagnasco, the Sesia Zone, Lower Aosta Valley, Italy

Abstract. An eclogite-facies orthogneiss and host paragneiss from a quarry near Tavagnasco in the Lower Aosta Valley were studied in order to refine the protolith, provenance and metamorphic ages of the Eclogitic Micaschist Complex of the Sesia Zone. The orthogneiss contains jadeite with quartz + phengite + K-feldspar ± garnet + rutile + zircon, whereas the paragneiss hosts garnet + jadeite + phengite ± glaucophane + epidote + rutile + quartz. Phase diagram modeling of two representative samples yields minimum equilibration conditions of 550 ± 50 ∘C and 18 ± 2 kbar. Cathodoluminescence images of zircon from the orthogneiss show oscillatory-zoned cores that are embayed and overgrown by complex, oscillatory-zoned rims. Four concordant secondary ion mass spectrometry analyses from the cores give a weighted mean 206Pb / 238U age of 457 ± 5 Ma. The cores have Th/U = 0.1 and negative Eu anomalies indicative of an igneous protolith, which we interpret to have crystallized in the Ordovician at 780 ∘C, based on Ti-in-zircon measurements. Zircon rims yield a range of 206Pb / 238U dates from 74 to 86 Ma, and four concordant analyses define a weighted mean 206Pb / 238U age of 78 ± 2 Ma. The rims are interpreted to have grown in the eclogite facies based on their lower Th/U (0.01), less negative Eu anomalies and steeper heavy rare earth element (HREE) patterns at <600 ∘C. The paragneiss yielded a detrital zircon population with major peaks at 575–600, 655 and 765 Ma; minor older components; and a maximum depositional age of approximately 570 Ma. The prominent Neoproterozoic zircon population and Ediacaran depositional age suggest derivation from the Gondwana margin. The metamorphic zircon is consistent with the oldest eclogite-facies event in the Sesia Zone; it does not show evidence of multiple periods of rim growth or any pre-Alpine (e.g., Variscan) metamorphism.

Geochemical composition of apatite from the Zhuxi tungsten granite and the Zhenzhushan granite porphyry in the Jiangnan porphyry-skarn tungsten belt, China

Apatite is increasingly used as a tracer for petrogenetic and metallogenic processes. We present the cathodoluminescence (CL) texture, and composition of igneous apatite from the tungsten-bearing Zhuxi biotite granite and the nearby Zhenzhushan granite porphyry in the Late Mesozoic Jiangnan porphyry-skarn tungsten belt in South China. Most apatite grains show homogeneous CL-dark texture, but some apatite grains have CL-dark cores and CL-bright rims, or CL dark-bright-dark-bright (from core to rim) growth zoning bands. The CL-bright bands are Mn depleted. Apatite from the Zhuxi biotite granite has elevated W contents (mean: 0.16 ppm; n = 75) compared to the Zhenzhushan granite porphyry (mean: ≤0.02 ppm; n = 32). The CL-bright rims of apatite from Zhuxi have more W than the CL-dark cores, i.e. average 0.58 ppm W versus average 0.07 ppm W, respectively, implying that the late-stage magmatic system was enriched in W. The chondritic Y/Ho ratios (22−33) of Zhuxi apatite, and the similar or even slightly more elevated REE contents of the CL-bright rims, compared to the cores, rule out hydrothermal effects. The Zhuxi apatite samples have distinctly negative δEu, F-rich and sulfur-depleted features, which are interpreted as signature of igneous apatite from magmatically evolved granitic intrusions associated with W deposits, while the inverse trend applies to apatite from Cu porphyry systems. In-situ microanalysis of apatite may be a useful exploration tool.

Mineral Associations of Concentrates from Gold-Bearing Placers of the Miass Placer Zone (South Urals) and Possible Primary Sources of Gold

Abstract —The placer deposits of the Miass valley in the Southern Urals have been known since the beginning of the 19th century. Over two centuries of mining more than 120 tons of gold have been extracted from alluvial and alluvial–deluvial placers. Despite the industrial significance of this territory, native gold of alluvial deposits is practically unstudied and there is no data on the mineral composition of concentrates. Native gold and heavy fraction minerals from seven placers of the Miass placer zone of deluvial–alluvial and alluvial types were researched. The study of the typomorphic features of native gold and distribution of heavy fraction minerals in loose deposits makes it possible to determine the genetic types of primary sources of placer formation. Chrome-spinel and ilmenite prevail in the concentrates, magnetite, epidote, rutile, titanite, clinopyroxene, hematite, zircon, garnets, and monazite are less common. Chromespinelides of various placers are represented by chromite and subferrichromite, they are characterized by a high average content of Cr2O3 (50.9 wt.%). Chromespinelide chemical composition features show evidence of its penetration into placer sediments from rocks of the ophiolitic association. The composition and morphological features of phosphates of rare earth elements (monazite and xenotime) indicate their metamorphogenic nature and penetration into placers from the Riphean strata. Native gold is characterized by a low degree of roundness. Mineral inclusions in native gold are represented by sulfides, arsenides, bismuthides, cuprous gold, platinoids. Composition of native gold from the Miass River valley placers varies over a wide range. The main admixtures are silver (4.5–23.4 wt.%) and mercury (up to 5.14 wt.%), copper (up to 2.03 wt.%) was noted in several grains. The EBSD study of native gold internal structure showed predominance of primary crystallization structures, a low degree of deformation and growth of gold-rich rims with a fine-grained mosaic structure. Formation of gold-rich rims in gold grains in placers is explained by recrystallization of the most deformed parts of gold during transportation. Placers occurred due to the destruction of primary sources located in the upper reaches of streams or near them, which is confirmed by a low degree of grains’ roundness. The composition of native gold in combination with a set of micro-inclusions indicate the gold–sulfide-quartz type of mineralization of most primary sources.

Effect of Mo2C addition on the formation of core-rim structure and mechanical properties of Ti(C,N)-WC-Mo2C-(Ni, Co) cermet

Ti(C,N)-based cermets with various Mo2C contents (4, 6, 8, 10, 12 and 14 wt%) were fabricated via vacuum hot-pressing sintering. The results showed that the cermets were comprised of coarse hard phase with a core-rim structure and partial fine rimless hard phase, embedded in Ni/Co binder matrix. Mo2C inhibits the rim growth on the coarse hard phase and hinders the dissolution of the fine rimless hard phase, which improves the mechanical properties of cermets. In addition, with the increasing Mo2C content, a bimodal pattern of X-ray diffraction peaks of (Ti,W,Mo) (C,N) solid solution can be observed, which indicates that high content of Mo2C has a different effect on the W and Mo contents in the white core-grey rim and black core-grey rim hard phase. When the Mo2C addition reaches 12 wt%, the cermet exhibits better wear resistance, which is attributed to a film composed of Ti and Mo oxides formed on the sliding interface. This work provides fundamental knowledge about the formation of core-rim structure and mechanical properties of Ti(C,N)-based cermets with various Mo2C contents.

Metasediments from the lower crust reveal the history of the Picuris orogeny, southwest USA

Petrologic and geochronologic data for metapelitic lower crustal xenoliths from New Mexico (USA) and Chihuahua (Mexico) states provide evidence for both a magmatic and collisional component to the enigmatic Mesoproterozoic Picuris orogeny. These garnet-sillimanite-bearing metapelites are found within the southern Rio Grande rift at Kilbourne Hole and Potrillo Maar in southern New Mexico and northern Chihuahua. Geothermobarometry and rutile with Quaternary U-Pb dates indicate equilibration in the local lower crust, which is actively undergoing ultra-high temperature (UHT) metamorphism (Cipar et al., 2020). The samples contain older detrital zircons dating back to the Paleoproterozoic, marking their deposition at the surface. Coupled zircon U-Pb dates and trace-element ratios (e.g., Gd/Yb) show a clear transition from oscillatory-zoned, low-Gd/Yb detrital magmatic zircon to featureless, high-Gd/Yb metamorphic zircon between 1500 and 1400 Ma, marking the transition from subduction to collision during this period. Metamorphic zircon and monazite grew in two major intervals. The first, between ca. 1450 and 1350 Ma, documents the journey of the sediments to depth within the orogen and provides evidence of extended Mesoproterozoic metamorphism in the region. The second corresponds with UHT metamorphism that commenced at ca. 32 Ma and is associated with the Rio Grande rift. Whereas nearly all garnets are homogeneous in both major and trace elements, a single garnet from one sample has a core defined by abundant quartz and acicular sillimanite inclusions. The core and rim of this garnet is homogeneous in major and most trace elements, but the rim is enriched in the slowest diffusing elements, Zr and Hf, which likely indicates rim growth at higher temperatures. We interpret the garnet core to have grown at the time of emplacement of the sediments into the lower crust. Because this occurred in the sillimanite stability field and because the metamorphic zircon and monazite all have negative Eu anomalies, indicating their equilibration with feldspar (stable at depths of <45 km), we conclude that the sediments were not emplaced via subduction and/or relamination of forearc sediments, but were instead metamorphosed under warmer, shallower conditions in an orogenic setting. Collectively, the data point to a collisional orogen during the inferred timing of the Picuris orogeny. These samples may therefore define the location of the Picuris suture zone, a key feature of this orogenic event.

One-step growth of a nearly 2 mm thick CVD single crystal diamond with an enlarged surface by optimizing the substrate holder structure

The pocket substrate holder with its sidewalls perpendicular to the surface is widely used to grow CVD single crystal diamond without polycrystalline diamond rims, but the lateral growth of thick single crystal diamond is usually restricted in a one-step procedure. In this work, a modified pocket substrate holder with slanting sidewalls of the pocket was designed to avoid the growth of polycrystalline diamond rims and reserve the lateral growth space of CVD single crystal diamond, simultaneously. As a result, the growth of a 1.94 mm thick CVD single crystal diamond without polycrystalline diamond rims was realized successfully only by one-step growth for 246 h, and the lateral area gain was 1.31. The FWHM values of the diamond (004) X-ray rocking curve and Raman characteristic peak are 40.3 arcsec and 2.45 cm−1, respectively, indicating the diamond has good crystallinity.

Transition-layer of core–rim structures and β→α transformation in SiC ceramics

Similar to Si3N4 ceramics, β→α phase transformation in SiC ceramics plays a key role in tailoring the microstructures thus optimizing related properties. SiC microstructures are dominated with the core–rim structures by AlN-solution, and by EBSD analysis, α-lamellae were revealed as stacking-faults (SF) and twin-boundaries (TB) in β-grains, co-existing with the core–rim structures as α/β→α’/β’ transformation by sintering. The structural transformation can proceed much further by gas-pressure-sintering than hot-pressing with only RE2O3 agents, while the latter retain a high-density of SF/TB in the metastable β-SiC grains. By high-angle secondary-electron imaging, nanoscale transition-layer (TL) was observed as an inter-phase to fully separate the core and rim, which is created by a transitory equilibrium in the solution–reprecipitation process. The enrichment of AlN or RE in TL demonstrates their segregation to core surface until reaching the super-saturation and before the growth of rims. With higher AlN or RE solution and after sintering, a shear stress can develop from TL contour to drive the expansion of SF/TB in Martensitic transition, especially under an external isotropic pressure. The combinations of β→α transformation, core–rim structures and viscous liquid-phase enable the comprehensive assessment of sintering–microstructure–property–performance relationship of SiC ceramics, as demonstrated for their creep behaviors and fracture toughness.

Deciphering the tectonometamorphic history of subducted metapelites using quartz‐in‐garnet and Ti‐in‐quartz (QuiG–TiQ) geothermobarometry—A key for understanding burial in the Scandinavian Caledonides

AbstractThe Seve Nappe Complex is a subduction‐related high‐grade metamorphic unit that was emplaced onto the margin of Baltica during Caledonian orogenesis. In this paper, the tectonometamorphic evolution of the Lower Seve Nappe in the Scandinavian Caledonides was characterized with the help of the continuous Collisional Orogeny in the Scandinavian Caledonides (COSC‐1) drill core, using a combination of various P–T estimation techniques based on garnet–quartz mineral pairs (quartz‐in‐garnet and Ti‐in‐quartz [QuiG–TiQ]), conventional thermobarometry and thermodynamic modelling of phase equilibria. This multi‐method approach yields complementary results and delivers critical data to constrain a comprehensive pressure–temperature–deformation–time (P–T–D–t) evolutionary path for the metasedimentary rocks of the Lower Seve Nappe. In the garnetiferous metasedimentary rocks, quartz inclusions in garnet preserve the P–T conditions of three distinct garnet growth stages corresponding to three metamorphic stages Ms1 to Ms3, including prograde and peak metamorphic conditions. Ms1 and Ms2 stages were constrained via quartz inclusions in garnet core and mantle. They are relatively close in the P–T space and could be considered as one single continuous prograde event occurring at epidote–amphibolite facies conditions of 460–520°C and 0.6–0.85 GPa. The growth of the garnet outermost rim defines the Ms3 stage at amphibolite facies conditions of 590–610°C and 1.13–1.18 GPa and corresponds to the peak metamorphic conditions. The microstructural analysis shows that the finite ductile strain pattern of the Lower Seve Nappe results from the superposition of four deformation phases. The initial phase D1 is defined by the S1 foliation that is still preserved as a curved inclusion trail in the garnet core. The D2 phase initiated contemporaneously with garnet core growth and the development of muscovite–biotite–plagioclase S2 foliation. Garnet outermost rim growth marks the end of the prograde path and peak metamorphic conditions. This stage is overprinted by the D3 phase and Ms4 stage associated with the development of the main regional metamorphic and mylonitic fabric S3 associated with C′‐type shear bands along the retrograde path. Ms4 stage, which was constrained using traditional thermobarometric techniques, corresponds to the chemical re‐equilibration of the metasedimentary minerals and occurred under amphibolite facies conditions at ~570–610°C and 0.78–1.00 GPa. The D3 phase is then generally weakly to strongly overprinted by later lower grade deformation D4 phase at greenschist facies conditions (Ms5). 40Ar/39Ar ages of syn‐kinematic white mica and biotite indicate that the final stage of the thrusting of the Lower Seve Nappe and thus the timing of its emplacement onto the Offerdal Nappe occurred at c. 423 Ma. Collectively, these results are consistent with previous estimates of the timing and conditions of metamorphism derived from the Lower Seve Nappe especially in west‐central Jämtland. However, application of QuiG–TiQ thermobarometry demonstrated that quartz inclusions in garnet can preserve different aspects of garnet growth, which are not accessible by traditional methods especially in complex terranes, and therefore provided new significant insights into the Lower Seve prograde evolution.

P-T path from garnet zoning in pelitic schist from NE Sardinia, Italy: Further constraints on the metamorphic and tectonic evolution of the north Sardinia Variscan belt

Mylonitic micaschists in the south-eastern sector of the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain were investigated for the reconstruction of their metamorphic evolution and P-T history. Micaschists underwent polyphase ductile deformation consisting of an old D1 deformation (~345–340 Ma) associated to shearing and folding and to a penetrative S1 axial plane foliation. The S1 foliation is progressively transposed by the D2 phase, which is associated with upright up to NE verging folds and dextral shear zones. Micaschists are characterized by abundant centimetric garnet crystals with strong compositional zoning. The garnet porphyroblasts (~15 vol%) are associated with plagioclase, quartz, biotite, staurolite, white mica, chloritoid and retrograde chlorite. Garnet presents an internal foliation identified by the isoorientation of quartz inclusions sometimes arranged into a sigmoidal pattern suggesting rotation of the garnet during growth, discordant respect to the external S2 foliation. The S2 foliation is identified by the preferred orientation of micas and chlorite and by the alternance between quartzo-feldspathic and micaceous layers. The garnet core contains several small inclusions of quartz, rutile, apatite, and minor monazite and zircon. Additional inclusions, observed in the garnet domain around the core are ilmenite, chloritoid, staurolite and white mica. EMPA analyses reveal an even more complex chemical zoning consisting of garnet core, garnet mantle, and rim. The compositional isopleths that match the composition of the garnet core intersect with isopleths for the highest Si contents in K-white mica at about T 430–490 °C, P 1.3–1.4 GPa. Garnet rim isopleths and isopleths for the lowest Si contents in K-white mica indicate P-T conditions close to 560–630 °C/0.6–1.1 GPa. The resulting P-T path is clockwise, subdivided into two separate stages. The first stage is a prograde segment suggested by the garnet core to mantle compositional variation whereas the second reflects garnet rim growth during exhumation. • Garnet crystals up to 1 cm are characterized by core, mantle, rim, and outer rim • Clockwise P-T path shows a prograde segment up to pressure peak (1.7GPa) • High – pressure conditions pre-date Barrovian metamorphism in Sardinia

Temperature and basinal fluid controls on feldspar diagenesis, Lower Cretaceous sandstones, Scotian Basin, Canada

The passive-margin Scotian Basin accumulated thick Upper Jurassic–Lower Cretaceous deltaic sandstones and shales, which were influenced by high heat flow in the mid-Cretaceous and deformed by salt tectonics. This study takes advantage of the complex thermal history and record of saline fluid flow in the basin to determine the relative importance of burial depth, temperature, salinity of basinal brines, overpressure, and sandstone permeability in controlling the dissolution of detrital feldspar and growth of authigenic K-feldspar and albite. Feldspar grains and volcanic lithic clasts from two transects, with differing thermal and salt tectonic histories, were studied by scanning electron microscope (SEM) backscattered electron images, energy-dispersive spectroscopy, SEM cathodoluminescence (CL) imaging, and hot-cathode CL microscopy. Published fluid inclusion measurements are integrated with burial history curves and timing of deformation on salt detachment surfaces to provides a new synthesis of the post-Jurassic thermal and saline fluid flow history of the basin. Surface-controlled dissolution of K-feldspar and growth of authigenic K-feldspar rims occur from 1.5 to 3.1 km. More destructive transport-controlled dissolution, down to 3.9 km, is more effective in thick-bedded permeable sandstones that acted as fluid pathways. Albitization of detrital K-feldspar and sodic plagioclase, and of feldspars in volcanic lithic clasts was facilitated by high Na+ content of formation waters migrating preferentially through such pathways at the time of silica and carbonate cementation, with strong albitization coinciding with transport-controlled dissolution of K-feldspar. Authigenic albite partially fills pores created by K-feldspar dissolution. Overpressured conditions enhance albite authigenesis, as a result of slower fluid advection. Episodic release of hot, saline, overpressured fluids in the zone of surface-controlled dissolution of K-feldspar favoured albitization but had no detectable effect on K-feldspar dissolution. Basin hydrology plays a definitive role in feldspar diagenesis, producing patterns that cannot be interpreted as a simple consequence of temperature and activity of Na+.

Two-stage exhumation of deeply subducted continental crust: Insight from zircon, titanite, and apatite petrochronology, Sulu belt of eastern China

Abstract The rates and mechanisms by which deeply subducted continental crust was exhumed back to the surface are not well understood, but can be better characterized using multimineral petrochronology. Here, we combine zircon, titanite, and apatite U-Pb ages from leucogranite and phengite gneiss with a pressure–temperature (P–T) path from eclogite to provide robust quantitative constraints on cooling and exhumation of the Sulu belt, a large ultrahigh-pressure metamorphic terrane in eastern China. The leucogranite, which formed during exhumation, is enriched in light rare earth elements (REE) relative to heavy REE and in large ion lithophile elements relative to high field strength elements, similar to hydrous crustal melts. Whole-rock Sr-Nd isotope compositions indicate that the leucogranite was not directly derived from the host phengite gneiss, but was more likely sourced from deeper in the exhuming crust. For the gneiss, mantles on inherited zircon yield an age of 230 ± 2 Ma and a temperature of 802 ± 36 °C based on a minimum pressure of 2.9 GPa, which records the minimum timing and P–T of initial decompression. Overgrowths on inherited zircon from the leucogranite constrain crystallization to 224 ± 1 Ma, coeval with the growth of zircon rims in the gneiss, at a temperature of 764 ± 42 °C and a pressure within the quartzeclogite facies. Titanite and apatite define single populations with lower concordia intercept ages of 222 ± 3 Ma and 198 ± 7 Ma, at temperatures of 720 ± 30 °C and ∼450 ± 100 °C, respectively, recording the timing of passage through the quartz-eclogite to the amphibolite facies and then the transition to the upper greenschist facies. Although the data yield a nearly constant cooling rate of 10.9−3.6+4.5 °C/m.y., exhumation was completed in two stages. The first stage from coesiteeclogite facies to ∼1.2 GPa, corresponding to the depth of the Moho, occurred at a rate of 7.5−2.6+5.8 km/m.y. Thereafter, exhumation into the mid-crust occurred at a much slower rate of 0.87−0.71+0.86 km/m.y. The first stage of faster exhumation was accompanied by migration of leucogranite melt along foliation in the gneiss, which would have decreased the average density and weakened the crust, enhancing the rate of return flow.

Orthopyroxene rim growth during reaction of (Co, Ni, Mn, Zn)-doped forsterite and quartz: Experimental constraints on element distribution and grain boundary diffusion

Mantle metasomatism is an important process in subduction zones in which fluids from the dehydrating oceanic slab interact with the overlying upper mantle resulting in a chemical alteration of the mantle. Consequently, this fluid-rock interaction may influence the mantle rock's physical properties such as the deformation behavior. In order to study element redistribution during mantle metasomatism in the laboratory, we used the simplified model reaction olivine + quartz = orthopyroxene, where olivine acts as representative for the upper mantle and quartz as proxy for the metasomatizing agent. We conducted piston-cylinder experiments at 1.5 GPa and 950 to 1400 °C, lasting between 48 and 288 h, on samples containing a mixture of quartz and one set of synthesized forsterite samples doped with either Co, Ni, Mn, or Zn. Additionally, we tested the influence of either nominally anhydrous or hydrous experimental conditions on the chemical distribution of the respective dopant element by using either crushable alumina or natural CaF2 as pressure medium. Results of the chemical analyses of the recovered samples show dopant specific partitioning between doped forsterite and orthopyroxene independent of the confining pressure medium; except for the runs in which Ni-doped forsterite samples were used. The observed Ni- and Co-enrichment in forsterite samples may be used to identify mantle rocks that underwent mantle metasomatism in nature.

U‐Pb Zircon Geochronology From the Northern Cordillera, Central Yukon, With Implications for Its Tectonic Assembly

AbstractThe tectonic assembly of the Northern Cordillera is currently disputed and directly impacts Paleozoic‐to‐recent paleogeographic and plate tectonic reconstructions of North America. In this study, we present new U‐Pb zircon geochronology from the allochthonous Yukon‐Tanana terrane and the parautochthonous Cassiar terrane of the Northern Cordillera from south‐central Yukon, Canada. Our data provide new constraints for the assembly of the Northern Cordillera in this region. Metasedimentary samples from the Ingenika Group (Cassiar terrane) and the Snowcap and Finlayson assemblages (Yukon‐Tanana terrane) yielded detrital zircon age spectra that are comparable to known northwest Laurentia age spectra. One of our samples from the Snowcap assemblage yields a detrital zircon age spectrum that is anomalous for northwest Laurentia, but comparable to Early Paleozoic strata deposited in the Nevada‐Idaho‐Utah region. Zircon rim growth and Pb‐loss recorded by detrital zircon in the Snowcap assemblage and Ingenika Group samples record metamorphism at 370 ± 4 Ma and between 171 ± 5 and 135 ± 3 Ma. Late Devonian metamorphism and magmatism possibly corresponds to rifting of the Snowcap assemblage from the Laurentian margin. Middle Jurassic‐Early Cretaceous metamorphic zircon rim ages from the Cassiar terrane record metamorphism during collisions between the Intermontane superterrane (including the Yukon‐Tanana terrane) and Laurentia (Early to Middle Jurassic), and between the Insular superterrane and Laurentia (Middle to Late Jurassic). Our study suggests that collision between the Yukon‐Tanana terrane and the Laurentian margin began no earlier than ∼205 Ma.

Reactivated shear zones: A case study in a tectonic superposition zone between the Southern Brasília and Ribeira orogens, southeastern Brazil

New data are presented related to the evolution of major shear zones in the Neoproterozoic Southern Brasília and Ribeira orogens, considering their attitude, kinematics and metamorphic conditions. The integration and analysis of geological maps, aerogeophysical and new geochronological U-Pb data, allowed to unravel the kinematics and reactivation of these shear zones. The first major event related to the collision of the São Francisco and Paranapanema paleocontinents is represented by the Alterosa Suture Zone (ASZ), interpreted as the suture of the Southern Brasília Orogen, which is a low to medium angle shear zone with mylonitic fabric indicative of granulite facies conditions. Geochronological analyzes in zircon from this zone provided three age peaks of 630 Ma, 610 Ma and 600 Ma associated with the collision of this orogen. A second regional event related to the Ribeira Orogen was responsible for the formation of steep NE-SW shear zones of low to high metamorphic grade and the local reactivation of the ASZ. Metamorphic zircon from one of these shear zones yielded an age of 574 ± 2 Ma, with minor rim growth and/or lead loss at 553 ± 2 Ma and 524 ± 4 Ma. The age of 574 ± 2 Ma fits well in the peak activity period of the Ribeira Orogen (600-560 Ma). The ages of 553 ± 2 Ma and 524 ± 4 Ma probably reflect reactivation. Several of these shear zones show evidence of original sinistral and later dextral kinematics. The paper also presents a new structure resulting from this dextral reactivation of former sinistral shear zones, which is named here “arrow tip structure”.

The effect of fluorine on reaction-rim growth dynamics in the ternary CaO-MgO-SiO2 system

AbstractGrowth of reaction rims is mainly controlled by a change in physical parameters such as pressure and temperature, a change in the chemical composition of the system, and/or by the presence of volatiles. In particular, the effect of volatiles other than water on reaction-rim growth remains poorly understood. To accurately model metamorphic and metasomatic processes, a quantification of the effect of volatiles on reaction-rim growth dynamics is necessary but hitherto missing.In this study, reaction rims were experimentally grown in a series of piston-cylinder experiments in the ternary CaO-MgO-SiO2 system at 1000 °C and 1.5 GPa with 0–10 wt% F for 20 min. In the fluorine-free system, a rim sequence of wollastonite (Wo) | merwinite (Mer) | diopside (Di) | forsterite (Fo) | periclase (Per) formed, complying with the stable phase configuration at water-saturated conditions. As soon as 0.1 wt% F was introduced into the system, humite group minerals (HGMs) and monticellite (Mtc) appeared, resulting in the multilayer rim sequence Wo | Mer | Mtc | Fo + HGMs | Per. In experiments with fluorine concentrations ≥0.5 wt%, cuspidine (Csp) appears in the layer sequence and represents the major fluorine sink. Our data show that the addition of fluorine may stabilize the fluorine-bearing phases cuspidine and HGMs to higher temperatures, which is in agreement with previous studies (Grützner et al. 2017). However, the appearance of the nominally anhydrous minerals (NAMs) monticellite and åkermanite (Ak) at this P-T condition suggests that the addition of fluorine may also affect the stability of nominally fluorine-free minerals. This may be explained by the effect of fluorine on the Gibbs free energies of fluorine-bearing phases, which in turn affects the relative Gibbs free energies and thus the stabilities of all phases. An increase in absolute rim thickness from 11.8(21) to 105.6(22) µm (1σ standard deviations in parentheses) in fluorine free and 10 wt% F experiments, respectively, suggests that fluorine enhances absolute component mobilities and thus results in faster rim growth rates. Additionally, due to the presence of fluorine, a change in relative component mobilities results in microstructural changes such as a phase segregation of diopside and cuspidine at high-fluorine (≥3 wt% F) concentrations.These results not only imply that reaction rims may be used as a tool to infer the amount of fluorine present during metamorphic reactions but also that we need to consider the role of fluorine for a correct interpretation of the P-T-t history of metamorphic and metasomatic rocks.

Timing of Grenville magmatism in the southern Blue Ridge, North Carolina, USA: New in situ zircon U-Pb geochronology and implications for timing of Rodinian orogenesis in eastern Laurentia

Mesoproterozoic metaplutonic rocks from the southern French Broad massif in southwestern North Carolina preserve Grenville magmatic and metamorphic ages (1.28–1.0 Ga). New in-situ zircon U-Pb measurements from eight basement lithologies, obtained by laser ablation and ion-probe techniques, delineate Elzevirian (1283–1250 Ma), Shawinigan (1191–1126 Ma), and Ottawan (1034–1008 Ma) magmatism and metamorphism. Magmatism produced dominantly calc-alkaline plutons with normative compositions of quartz diorite, quartz monzodiorite, granodiorite, and monzogranite. The majority of magmatism in the field area occurred between ca. 1190 and 1125 Ma, a period correlative to the Shawinigan phase of the Grenville orogeny. Syn- to post-emplacement metamorphism of these units is evidenced by complex zircon U-Pb systematics in all Shawinigan age plutons. Ottawan magmatism, expressed by emplacement of the Max Patch granite and protoliths of the Earlies Gap granitoid gneiss, is less voluminous than Shawinigan magmatism; a difference that mirrors their relative proportions elsewhere in the orogen. Ottawan metamorphism is recorded by growth of new zircon rims around magmatic core domains in several samples and spans a period of ca. 25 m.y. from ca. 1035 to 1010 Ma. The timing of the older and younger magmatic suites from the southern French Broad massif correlate to well-established times of Shawinigan and Ottawan magmatism and metamorphism in other Grenville-age terranes within the U.S. (e.g., Appalachian inliers, Adirondacks).

Zircon U-Pb and geochemical signatures in high-pressure, low-temperature metamorphic rocks as recorders of subduction zone processes, Sikinos and Ios islands, Greece

Zircon U-Pb dating is a powerful and widely used geochronologic technique to constrain the timing and rates of magmatic and high and lower-grade metamorphic processes, as well as sediment provenance. Zircon trace element (TE) compositions also record magmatic and metamorphic processes during zircon growth. In this study, zircon laser ablation split-stream (LA-SS)-ICP-MS U-Pb and TE depth-profiling and novel two-dimensional zircon mapping techniques are used in combination with oxygen isotope analyses (secondary ion mass spectrometry, SIMS) to reconstruct the timing and metamorphic conditions recorded by recrystallization and growth of zircon rims, which provide valuable insight into the petro-tectonic evolution of high-pressure/low-temperature (HP/LT) metamorphic rocks formed in subduction zones. These techniques are applied to zircon grains from HP/LT metamorphic rocks of the Cycladic Blueschist Unit (CBU) and Cycladic Basement (CB) on Sikinos and Ios islands, Greece, which experienced metamorphism and deformation associated with subduction and subsequent back-arc exhumation. Zircon records multiple episodes of non-magmatic zircon rim growth at ~50 Ma and ~ 26 Ma. Eocene metamorphic rims are associated with HP/LT metamorphism and are observed in both units, suggesting likely juxtaposition prior to or during subduction and associated HP metamorphism. The similarity between TE concentrations and δ18O values of the Eocene rims and their corresponding cores is an indicator for re-crystallization and precipitation as a mechanism of zircon growth. In contrast, Oligocene zircon rims appear to be restricted to a < 0.5 km thick zone along the CB-CBU contact, characterized by garnet break-down, and show HREE enrichment and higher ẟ18O values in the rims compared to the cores, consistent with a model suggesting metasomatic infiltration of fluids derived from dehydrating sedimentary rocks during progressive subduction and underplating prior to back-arc extension. This metamorphism appears to be static in nature and does not support major late Cenozoic reactivation of the contact as an extensional shear zone during back-arc extension.

The Origin of Rhyolitic Magmas at Krafla Central Volcano (Iceland)

AbstractWe present a detailed petrologic study of rhyolites from seven eruptions spanning the full (∼190 ky) history of rhyolitic volcanism at Krafla volcano, northeast Iceland. The eruptions vary widely in size and style, but all rhyolites are crystal-poor (&amp;lt;6 modal%: plagioclase + augite ± pigeonite ± orthopyroxene ± titanomagnetite ± fayalite) and have similar evolved compositions (73.7–75.8 wt% normalized whole-rock SiO2) and trace element patterns. Macrocryst rim compositions from each eruption cluster within a narrow range and are appropriate for equilibrium with their carrier melt. Crystal cores and interiors display complex growth patterns and commonly host resorption surfaces, but compositional variations are slight (e.g. typically &amp;lt;10 mol% An for plagioclase, Mg# &amp;lt;10 for pyroxene), and consistent with an overall trend of cooling and differentiation by crystal fractionation. Although most crystal core and interior compositions are broadly appropriate for equilibrium with melts similar to their host whole-rock, variable growth histories, juxtaposition of grains with distinct trace element compositions, and scatter in melt inclusion compositions indicate mixing of antecrysts from compositionally similar evolved melts and/or assimilated felsic mush or intrusions before final rim growth. Evidence for mafic recharge (e.g. coupled increases in An and Fe in plagioclase) is absent in most crystals; rhyolite storage and fractionation thus occurred largely in isolation from the underlying mafic system. Comparison of observed matrix glass compositions with published experimental work on melting of altered (meta)basalts casts doubt on previous models favouring rhyolite generation by partial melting of hydrothermally altered basalts, instead supporting recent isotopic and modelling arguments for a crystallization-driven process [Hampton, R. L. et al. (2021). Journal of Volcanology and Geothermal Research 414, 107229]. MELTS fractional crystallization and assimilation-fractional crystallization (AFC) models at 1 kbar predict liquid major and trace element compositions similar to Krafla rhyolites after ∼60–70 vol% crystallization of a quartz tholeiite melt representative of the evolved crystal-poor basalts commonly erupted within Krafla caldera. We thus suggest that stalling and crystallization of these evolved basalts at shallow depth forms crystal mushes from which evolved (broadly dacitic to rhyolitic) melts are extracted. These melts ascend and mix with other compositionally similar melt bodies and/or assimilate felsic intrusive material in the uppermost crust. The Daly gap between ∼57 and 71 wt% SiO2 at Krafla is consistent with preferential extraction of evolved melts from quartz tholeiite mushes in the ∼50–70% crystallinity window. Residual solid (cumulate) compositions predicted by MELTS are exclusively mafic, hence efficient silicic melt extraction from quartz tholeiite mushes may also explain the apparent compositional bimodality in some Icelandic plutonic suites.