Garnet Growth Research Articles

A depleted mantle source for Neoproterozoic continental rifting in the Seve Nappe Complex, Kebnekaise region, northern Swedish Caledonides

The Central Iapetus Magmatic Province (CIMP) is a large igneous province (LIP) emplaced in the Baltican and Laurentian paleocontinents that marks the onset of the Caledonian Wilson Cycle. Paleozoic magmatism of the CIMP is preserved in both northeastern America and northern Europe. This study investigates rocks belonging to the hyper-extended margin of Baltica currently found in the Seve Nappe Complex of the Scandinavian Caledonides. Specifically, U-Pb zircon geochronology and whole-rock geochemistry are applied to a migmatitic variety of the Vierručohkka amphibolite of the Mårma Terrane, to the Aurek gabbro, and amphibolite of the Aurek Assemblage exposed in the Seve Nappe Complex in the Kebnekaise region, northern Swedish Caledonides. U-Pb zircon geochronology yields crystallization ages of 626 ± 7 Ma for the protolith of the Vierručohkka amphibolite, and 614 ± 2 Ma and 609 ± 1 Ma for the emplacement of the Aurek gabbro and amphibolite protolith, respectively. A younger age of 599 ± 3 Ma is recorded in the Vierručohkka amphibolite and is interpreted as the age of partial melting and migmatization. The geochemical signatures of the rocks demonstrate crustal assimilation during the emplacement of their protoliths and modification due to prograde metamorphic processes during Caledonian subduction. The Vierručohkka amphibolite and the Aurek Assemblage samples display upper and lower crustal assimilation, respectively. Trace elements (Dy, Sm, Lu, and Y) record the growth of metamorphic garnet, while elevated TiO2 contents record the crystallization of metamorphic rutile. Nevertheless, high field strength elements (HSFE) and ∆Nb suggest a depleted mantle source for the magmas of the protoliths of the investigated rocks. Altogether, geochemical data indicate that the igneous activity recorded in the Vierručohkka amphibolite and the Aurek Assemblage between c. 626–609 Ma is related to continental rifting processes associated with the opening of the Iapetus Ocean.

Xenolith Petrochronology (San Luis Potosi, Mexico) Constrains Heat Sources for Cenozoic Ultrahigh‐Temperature Metamorphism in the Lower Crust

AbstractUltrahigh‐temperature (UHT; >900°C) metamorphism drives crustal differentiation and is widely recognized in the rock record, but its geodynamic causes are debated. Previous work on granulite‐facies metapelite xenoliths from San Luis Potosí, Mexico suggests the lower crust experienced a protracted UHT metamorphic event that coincided with the onset of regional extension. To determine the duration, conditions, and heat sources of UHT metamorphism recorded by these xenoliths, this study characterizes the major‐element, trace‐element, and U‐Pb isotopic systematics of quartz, rutile, feldspar, garnet, and zircon by in situ electron microprobe (EPMA) and laser‐ablation inductively coupled‐plasma mass spectrometry (LA‐ICP‐MS), and augments these data with detailed petrography, thermobarometry, phase equilibria modeling, and diffusion modeling. Thermobarometry and phase equilibria modeling suggest peak metamorphic conditions exceeded 0.7 GPa and 900°C. Zircon petrochronology confirms >15 Myr of UHT conditions since its onset at ∼30 Ma. A small population of zircon record elevated temperatures following transition from backarc compression to extension during the waning stages of orogenesis (60–37 Ma). Garnet preserves trace‐element zoning and mineral inclusions consistent with suprasolidus garnet growth and subsequent compositional modification by intracrystalline rare‐earth element diffusion during protracted heating, with diffusion chronometry timescales in agreement with zircon data, followed by fluid‐driven remobilization of trace elements along now‐healed fractures within ∼1 Myr of eruption. In sum, these data are most compatible with lithospheric mantle attenuation or removal as the dominant heat transport mechanism driving synextensional UHT metamorphism and crustal melting, which has bearing on models for crustal differentiation and formation of modern and ancient granulite terranes globally.

Timing of multiple fluid pulses recorded by garnet and accessory minerals in metarodingites

Metarodingites are commonly garnet-bearing metasomatized mafic dykes within serpentinized mantle rocks. Garnet in metarodingites has the potential to preserve compositional and chronological information about the entire metamorphic-metasomatic evolution, from ocean floor formation to deep subduction. In this study, we investigate the chemical and chronological record of garnet, titanite, and zircon from metarodingites and garnet veins in chloritized blackwall, from the Zermatt-Saas zone in the Western European Alps. Garnet in the chlorite-rich metasomatic rind (blackwall) displays re-crystallization and new growth textures and therefore represents a second generation of garnet (Grt2), after the one in the metarodingite core (Grt1). Major and trace element mapping shows that Grt2 is richer in andradite and Ti-andradite components compared to the more grossular-rich Grt1 in the metarodingite. Moreover, we found that in both Grt1 and Grt2, Ti and U content are positively correlated, with U abundances up to 1.5 μg⋅g−1. Grt2 does not show rare earth element (REE) zonation, suggesting that REE transport was facilitaed by the presence of a fluid phase. We propose that the metasomatizing fluids from which Grt2 precipitated, were serpentinite-derived high-temperature brines capable of transporting Fe3+, Ti and REE.LA-ICPMS U–Pb dating of metarodingite garnet Grt1 samples yielded overlapping ages between 43.6 ± 0.9 and 44.1 ± 1.3 Ma, which are consistent with previous estimates of peak metamorphic conditions. Our data indicate that the metarodingite Grt1 grew within <2 Myr, suggesting a rapid burial rate of the unit or fast and pulsed garnet growth aided by the presence of fluids in the intergranular medium. Titanite from the blackwall sample yields an age of 45.0 ± 1.7 Ma and zircon rim an age of 48.0 ± 1.1 Ma, indicating that fluid release and chloritization occurred at peak conditions. U–Pb ages of garnet in the blackwall samples (Grt2) are significantly younger, yielding ages of 40.1 ± 0.9 Ma and 38.4 ± 0.8 Ma, respectively. The presence of multiple generations of metasomatic garnet in the blackwall suggests that periods of increased rock permeability occurred repeatedly, plausibily aided by deformation, as supported by the consistency of our multi-mineral geochronology. We show how garnet from metarodingites and blackwalls records the metamorphic-metasomatic history and can be used to obtain information on (de)hydration reactions and fluid flow during subduction.

Variations in orientation relationships between rutile inclusions and garnet host relate to magmatic growth zoning

Rutile inclusions in almandine-spessartine garnet from a peraluminous pegmatoid from the Moldanubian zone (Bohemian Massif, AT) show distinct changes in aspect ratio, shape preferred orientations (SPO) and crystallographic orientation relationships (COR) along the transition between microstructurally different growth zones in the garnet core and rim. For identification of the COR characteristics we pool specific CORs based on their common axial relationship into three COR groups: Group 103R/111G, Group 001R/111G and Group 001R/100G. The rutile inclusions in the garnet core domains are elongated along the four Grt⟨\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle$$\\end{document}111⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rangle$$\\end{document} directions and are dominated by COR Group 103R/111G. The garnet rim zone additionally contains rutile needles elongated along Grt⟨\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle$$\\end{document}100⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rangle$$\\end{document}. Here, Group 001R/111G and 001R/100G are more abundant than in the garnet core. Needle-shaped rutile in the rim shows a systematic correlation between SPOs and CORs as needles elongated parallel to Grt⟨\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle$$\\end{document}111⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rangle$$\\end{document} are dominated by Group 103R/111G and 001R/111G, whereas those needles elongated parallel to Grt⟨\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle$$\\end{document}100⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rangle$$\\end{document} exclusively pertain to CORs of 001R/100G. Furthermore, the frequency of each particular SPO in the garnet rim clearly depends on the local growth direction of the particular Grt{112} sector. Facet-specific variations in rutile SPO frequencies in different sectors and growth zones of garnet were observed even between equivalent directions, indicating that the microstructures and textures of rutile inclusions reflect varying parameters of garnet growth. The characteristic differences in COR groups of different garnet growth zones are referred to compositional changes in the bulk melt or compositional boundary layer, associated with magmatic fractional crystallisation.

Detrital zircon REE and tectonic settings

Earth's long-term history is preserved in the rock archive, yet this record is incomplete, hindering our understanding of how tectonic processes have evolved and shaped our planet's evolution. The physio-chemical resilience of zircon and its ability to be rapidly analyzed for age, isotopic, and elemental data has made it a key phase in unraveling Earth's long-term evolution. The europium anomaly (Eu/Eu*) and the differentiation degree between light and heavy rare earth elements (LREEN¯/HREEN¯) of zircons positively correlate with the pressure of magma crystallization because high pressure inhibits plagioclase crystallization and promotes garnet growth. However, this correlation can be influenced by fluctuations in redox conditions, protolith composition, and water content of magma, whose stability varies in different tectonic settings. Therefore, the correlation coefficient between Eu/Eu* and LREEN¯/HREEN¯ of detrital zircons (rDz) provides a new proxy to help distinguish tectonic settings. We evaluate this new proxy using available global detrital zircon REE data from different tectonic settings in modern and deep times. The data from well-constrained simple tectonic settings suggest that rDz is higher in convergent settings (0.53–0.85) than in collisional settings (0.12–0.51). Convergent settings can also yield relatively low rDz (0.28–0.55) when there is a contribution of a mantle plume lying below the overriding plate of a subduction zone.When studying a detrital zircon archive derived from a large distributive province that includes sources from a variety of settings, known provenance information should be used to disentangle mixed source signals. Also, only considering detrital zircon grains that formed a short time (e.g., 50 or 100 Myr) prior to deposition can reduce the likelihood of multi-cycle distal mixing of contemporaneous but tectonically distinct zircons in a sample. Additionally, greater confidence in interpretations of changes in tectonic settings may be derived from evolution patterns of rDz instead of isolated values. A decreasing trend can likely be interpreted as a transition from convergent to collisional or mantle plume settings, whereas an increasing trend implies an opposite tectonic transition.

Eocene (48.7 Ma) eclogite at Chepelare, Central Rhodopes, Bulgaria: A key to the tectonics of the Rhodopes

AbstractThe presence of an Eocene subduction event, in addition to Jurassic and Cretaceous ones, in the Rhodopes of Greece and Bulgaria is controversial. We report the first dating of an eclogite from the Lower Allochthon of the Rhodopes. The eclogite is a mafic lens enclosed in orthogneiss of the Arda‐2 Unit at Chepelare, Central Rhodopes. It was metamorphosed under conditions of ca. 25 kbar, 700–750°C. Garnet grains have outward‐increasing Mg contents and omphacite inclusions with the highest jadeite contents in their rims, indicating garnet growth under increasing pressure. Lu–Hf geochronology yielded an age of 48.7 ± 1.5 Ma. The age is linked to a late stage of garnet growth close to peak pressure, i.e. to subduction.

Combining traditional and quantitative multiscale structural analysis to reconstruct the tectono-metamorphic evolution of migmatitic basements: the case of the Valpelline Series, Dent-Blanche Tectonic System, Western Alps

Due to the ongoing development of new technologies, many instruments are available to assist geological investigations at different scales. These techniques, including 3D outcrop modelling from aerial photogrammetry and quantitative microstructural analysis are useful in crystalline basements studies. This contribution combines traditional and quantitative multiscale structural analysis techniques to the migmatitic rocks of the Valpelline Series (Dent-Blanche Tectonic System, Western Alps). Conventional structural analysis is integrated with the extraction of structural data from 3D models of representative smooth outcrops. Quantitative microstructural and mineral-chemical analyses are combined to link structural and metamorphic evolution. This approach allows identifying and correlating foliations that developed during three tectono-metamorphic stages. The first (D1) includes solid-state deformation associated with an early foliation (S1) preserved within metabasite boudins enclosed in migmatite gneiss. The second (D2) is related to the dominant foliation in migmatite gneiss (S2), coeval with the regional scale anatexis and growth of garnet and cordierite. The third (D3) is related to the late folding of S2 and the development of a sillimanite-rich axial plane foliation (S3) which wraps around garnet and cordierite. Finally, this work discusses pros and cons of each innovative methodology, still emphasising the importance of using manually acquired field data as ground control.

Retentiveness of rare earth elements in garnet with implications for garnet Lu‐Hf chronology

AbstractIncorporation of rare earth elements (REE) in garnet enables garnet chronology (Sm‐Nd, Lu‐Hf), and imparts a garnet‐stable signature on cogenetic phases, which allows petrochronology and general petrogenetic tracing of garnet stability in minerals and melts. Constraints on the uptake and redistribution mechanisms, as well as on the diffusive behaviour of REE in garnet are required for allowing accurate interpretation of REE signatures and ages. Garnet REE profiles are often measured to gain insight into the nature and cause of REE zoning. Interpretation of such profiles is nevertheless complicated by poor constraints on the extent of diffusive relaxation. This is especially relevant for Lu, which, according to experiments, has a relatively high diffusivity and thus may re‐equilibrate with possible consequences for Lu‐Hf chronology. To provide new insight into the REE systematics of garnet, we applied quantitative trace‐element mapping of garnet grains from metamorphic rocks that record peak temperatures above 750°C and cooling rates as low as 1.5°C Ma−1. Garnet in all samples preserves Rayleigh‐type or oscillatory growth zoning with sharply defined interfacial angles that match the garnet habit. Re‐equilibration of REE compositions appears restricted to domains with nebulous and patchy zoning, which likely form by interface‐coupled dissolution and re‐precipitation reactions mediated by fluids or melts, rather than REE volume diffusion. The possible effect of Lu diffusion in the analysed grains was investigated by comparing the observations to the results from 2D numerical modelling using Lu diffusivities from recent diffusion experiments. This test indicates that Lu diffuses significantly slower in natural garnet than experiments predict. The retentiveness of REE in garnet demonstrates the reliability of REE signatures in magmatic tracing and petrochronology and establishes Lu‐Hf chronology as a robust means of dating garnet growth and recrystallization in metamorphic rocks, including those that underwent high‐ or ultrahigh‐temperature conditions.

Mechanisms and durations of metamorphic garnet crystallization in the lower nappes of the Caledonian Kalak Nappe Complex, Arctic Norway

AbstractThe 3D microstructure and compositional zoning of garnet populations in micaschists from the Kolvik and Bekkarfjord nappes indicate the quasi‐equilibration of their major components across the rock matrices during interface‐controlled, size‐independent garnet growth. There is microstructural evidence for foliation‐parallel, small‐scale resorption of garnet rims in the Kolvik Nappe, influencing the metamorphic peak conditions obtained from thermodynamic modelling. The local chemical compositions of rims less affected by resorption indicate a peak temperature of ~630°C, which is ~40°C higher than the temperature obtained from resorbed rims of the largest garnet crystal. Using a diffusion geospeedometry approach that considers the geometry of the compositional zoning of the garnet population, as well as the higher, more realistic peak temperature, a duration of 1 to 4.9 Myr is obtained for garnet growth in the Kolvik Nappe. This is approximately 1 order of magnitude faster than duration estimates obtained when using the apparent, lower temperature estimated from the resorbed garnet rims. Concomitantly to garnet growth in the Kolvik Nappe, garnet overgrowths formed in the Bekkarfjord Nappe for circa 2.5 Myr at metamorphic peak temperatures of ~560°C. The garnet growth durations obtained here are comparable with the uncertainty on the Lu–Hf garnet–whole rock isochron ages of 419.9 ± 2.4 Ma and 423.0 ± 1.9 Ma, previously obtained for these rocks. These results provide new insight into the timescales of repeated Barrovian‐type metamorphic events experienced by the lower nappes of the Kalak Nappe Complex during the Caledonian Orogeny in Arctic Norway. This study emphasizes the importance of microstructural and chemical characterization of garnet populations in assessing metamorphic crystallization mechanisms and the extent of equilibration of garnet‐forming components during prograde metamorphism. Moreover, our results provide means for reducing the uncertainty on metamorphic durations obtained via diffusion geospeedometry and, so, contributing to our understanding of geological timescales and processes.

The P–T–X conditions of staurolite-garnet metabasites, NW Iran: Implications for metamorphism during Arabia-Eurasia collision

The staurolite-bearing Avajiq metabasites (AMB) crop out near the NW border of Iran with Turkey and contain the peak assemblage garnet + Ca-amphibole + staurolite + biotite + chlorite1 + ilmenite. The AMB record peak pressure–temperature (P-T) conditions of >1 GPa at ∼650 °C; the moderately high pressure is consistent with the presence of staurolite in metabasalt. Garnets exhibit a weak prograde zoning, with relatively homogenous cores and a slight increase in Mg and decrease in Ca, Fe, and Mn toward the rim, consistent with garnet growth during a clockwise P-T path. Texturally late margarite + biotite + muscovite correspond to retrograde P-T conditions of ∼0.5 GPa at ∼500 °C. This stage was followed by further retrogression under greenschist facies conditions recorded by late pumpellyite + paragonite ± calcite. Based on the MORB-like composition of the AMB, it has been unclear whether the metabasites are associated with Neo-Tethyan ophiolites in the area or formed within a continental setting. However, the petrologic association of metabasites with micaschist, gneiss and marble and upper amphibolite facies conditions of metamorphism indicate that the basaltic rocks likely metamorphosed during Arabia-Eurasia continental collision, following consumption of the intervening Neo-Tethys Ocean and emplacement of the ophiolitic rocks in the region.

Mineralization processes of the Mount Garnet Zinc deposit, NE Queensland (Australia): Insights from garnet geochemistry and U-Pb geochronology

The Mount Garnet Zinc deposit in NE Queensland, Australia, is located in the region of voluminous magmatism associated with the formation of Palaeozoic continental arcs of Gondwana. Ore-forming events of this deposit are related to the intrusion of the Kennedy Igneous Association (KIA) – a large suite of igneous rocks spanning Late Carboniferous to Early Permian in age – into the Silurian limestone (Chillagoe Formation) in the Hodgkinson Province (Mossman Orogen). Petrographic observations and in situ major and trace element analysis reveal the sequence of garnet growth and fluid-rock interaction during skarn formation. Three types of garnets are identified, Grt I (including IA and IB) and Grt II. Grt IB is related to Zn-Pb mineralization. All garnets have elevated Sn and W and the highest levels of W and Sn are observed in Grt II. Garnet U-Pb ages of Grt IB provide the first direct geochronological result of hydrothermal minerals in the Mount Garnet region. New data argues that the base metal mineralization may take place during early Permian period and be sourced from the Claret Creek Supersuite. Sn-W mineralization may be associated with the O’Briens Creek Supersuite.

Formation of multistage garnet grains by fragmentation and overgrowth constrained by microchemical and microstructural mapping

AbstractGarnet is an exceptionally useful mineral for reconstructing the evolution of metamorphic rocks that have experienced multiple tectonic or thermal events. Understanding how garnet crystallizes and its mechanical behaviour is important for establishing a petrological and temporal record of metamorphism and deformation and for recognizing multiple geologic stages within the growth history of an individual crystal. Here, we integrate fine‐scale microstructural (electron backscatter diffraction [EBSD]) and microchemical (Laser Ablation Inductively Coupled Plasma Mass Spectrometry [LA‐ICP‐MS] mapping) data obtained on a polycyclic garnet‐bearing micaschist from the Alpine belt. Results suggest that fragmentation of pre‐Alpine garnet porphyroblasts occurred during the late pre‐Alpine exhumation and/or the onset of the Alpine burial, such that the older pre‐Alpine garnet fragments were transported/redistributed during Alpine deformation and acted as nucleation sites for Alpine garnet growth. These processes produced a bimodal garnet size distribution (millimetre‐ and micrometre‐sized grains). Thermodynamic modelling indicates that Alpine garnet grew during the final stage of burial (from 1.9 GPa 480°C to 2.0 GPa 520°C) and early exhumation (down to 1.6 GPa 540°C) forming continuous idioblastic rims on and sealing fractures in pre‐Alpine garnet grains. We propose that fragmentation–overgrowth processes in polycyclic rocks, coupled with ductile deformation, may produce a bimodal garnet size distribution in response to fragmentation and re‐distribution of pre‐existing grains; these clasts can act as new nucleation sites during a subsequent orogenic cycle.

Rapid development of spiral garnets during subduction zone metamorphism revealed from high-resolution Sm-Nd garnet geochronology

Abstract Multiple studies have applied zoned garnet geochronology to place temporal constraints on the rates of metamorphism and deformation during orogenesis. We report new high-resolution isotope dilution–thermal ionization mass spectrometry Sm-Nd isochron ages on concentric growth zones from microstructurally and thermodynamically characterized garnets from the Betic Cordillera, southern Spain. Our ages for the garnet core (13.64 ± 0.31 Ma), mantle (13.41 ± 0.37 Ma), and rim (13.34 ± 0.45 Ma) indicate rapid garnet growth and are consistent with published garnet ages interpreted to reflect high-pressure metamorphism in the region. Thermodynamic analysis indicates garnets grew during subduction at ~1.5–2.0 GPa and 570–600 °C. The core to rim duration of spiral garnet growth was just a few hundred thousand years. While other zoned garnet studies have shown similar rapid growth in subduction zone settings, this is the first documentation of such rapid growth of a spiral garnet. Combining this garnet growth duration with the magnitude of spiral inclusion trail curvature, we compute a strain rate of ~10−13 s−1, an order of magnitude faster than all previous spiral garnet studies. We interpret that these spiral garnets recorded a rapid pulse of deformation and strain during the final stages of subduction and incipient exhumation.

Investigating low dispersion isotope dissolution Lu-Hf garnet dates via in situ Lu-Hf geochronology, Kanchenjunga Himal

Re-examination of three specimens from the Kanchenjunga Himal of Nepal via in situ Lu-Hf garnet geochronology yields evidence of multiple garnet growth events. Spot analyses from grain cores in two specimens define Paleozoic regressions whereas analyses from grain rims in the same specimens define low-precision regressions consistent with the timing of Himalayan orogenesis. These dates contrast with previously published low dispersion, ca. 290 Ma isotope dissolution (ID) Lu-Hf garnet dates for the same rocks. Modelling of Lu and spot age distribution in representative grains from the specimens examined yields calculated dates that approximate the Permian-age regressions through the original ID data. These findings demonstrate that it is possible to generate low dispersion ID Lu-Hf data from multi-generational garnet with significantly different-age growth events when approximately equal proportions of the different age reservoirs are included in multi-component aliquots.

Garnet chemical zoning: a clue for the tectono-metamorphic history of the Proterozoic Mayombe chain (West Congo Belt), Congo-Brazzaville

Abstract The Mayombe chain of Congo is part of the West Congo Belt, which belonged to the western Gondwana supercontinent. It consists of Paleoproterozoic gneisses and schists that are tectonically stacked and overthrust Neoproterozoic low-grade metamorphic rocks. Although Neoproterozoic context of the chain is relatively well established, the tectono-metamorphic evolution of its Paleoproterozoic basement still under discussion. Petrography, garnet chemistry and phase equilibria modelling were used to constrain tectono-metamorphic evolution of meta-plutonic and meta-sedimentary rocks from the Western Domain of the Mayombe chain. Microprobe analysis reveals three garnet types: (i) 2-stage garnets with distinct cores (Grt1) and rims (Grt2), (ii) unzoned garnet showing narrow diffusion zones along cracks and rims and (iii) syn-kinematic garnet with normal growth zoning. These complex and simple features of garnet growth are, respectively, related to a polycyclic evolution linked in this area to: (i) the superposition of Eburnean (c. 2000 Ma) and Pan-African (c. 600 Ma) orogenies and (ii) a monocyclic evolution related to a single Pan-African event taking into account ages of the protoliths. The oldest metamorphic assemblage (Eburnean) is preserved in amphibolite facies conditions marked by the first generation of garnet, whereas the younger (Pan-African) event varies from amphibolite facies in the southwest (4–6 kbar, 550°C–600°C) to greenschist facies in the northeast (4–6 kbar at 450°C–550°C) confirming the westward increase in metamorphic grade during the Pan-African event. Mineral equilibria modelling shows also a relatively HP episode culminated at 11.5–12.5 kbar and 525°C–550°C which tectonic environment stills less understood.

Disruption of a high‐pressure unit during exhumation: Example of the Cycladic Blueschist unit (Thera, Ios and Naxos islands, Greece)

AbstractReconstructing the original geometry of a high‐pressure tectonic unit is challenging but important to understand the mechanisms of mountain building. While a single nappe is subducted and exhumed, nappe‐internal thrusts may disrupt it into several subunits. The Middle‐CBU nappe of the Cycladic Blueschist Unit (Hellenide subduction orogen, Greece) shows evidence of such disruption along a Trans‐Cycladic‐Thrust (TCT), however, the timing of this thrusting is unknown. Here, we report multi‐petrological and geochronological data from the Middle‐CBU nappe from the Thera and Ios islands (Greece). Using Zr‐in‐rutile thermometry coupled with quartz‐in‐garnet elastic barometry, average P–T and phase equilibrium thermodynamic modelling, we show that garnet growth in Ios occurred during prograde metamorphism at 6.7 ± 1.4 kbar to 13.0 ± 1.6 kbar and 326 ± 20°C to 506 ± 13°C (2σ uncertainty) followed by early exhumation to 10.1 ± 0.6 kbar and 484 ± 14°C and a greenschist facies overprint at 5.7 ± 1.2 kbar and 416 ± 14°C. For Thera, we constrain peak HP conditions of 7.6 ± 1.8 kbar and 331 ± 18°C, followed by exhumation and equilibration at ~2 kbar and ~275°C using average P–T and phase equilibrium thermodynamic modelling. For Ios, Uranium‐Pb garnet geochronology provides ages of 55.7 ± 5.0 Ma (2σ uncertainties) for prograde and 40.1 ± 1.4 Ma for peak HP metamorphism. Combining our new P–T–t data from Thera and Ios islands with existing data from Naxos island, we conclude that the studied nappe segments represent remnants of a former coherent nappe. The P–T–t data define an Eocene subduction rate of 2.1 ± 1.0 km/Ma, which is distinctly slower than the current subduction rate of 40–45 km/Ma. After subduction, the exhumation of the Middle‐CBU nappe occurred during the Oligocene at different rates for different localities. The Middle‐CBU nappe of Naxos was exhumed at a rate of ~6 km/Ma, contrasting with the exhumation rate of ~3 km/Ma calculated for Ios. This result suggests that the Middle‐CBU nappe of Naxos rocks was thrust on the Ios one during the Oligocene. Using P–T–t data and assuming realistic subduction angles during the Eocene and the Oligocene, we present a 2D structural reconstruction of the Middle‐CBU nappe of these islands. This reconstruction helps to understand the mechanisms of subduction of a continental margin and its disruption during exhumation.

Geochemistry and geochronology of Beiya giant Au polymetallic deposit: Implications for multistage ore-fluid pulses in a skarn system

Garnet is one of the most typical minerals in skarn deposits, and its texture and composition is a proxy for the changing physicochemical conditions. Thus, mineralogical and geochemical characterization of different garnet types can provide important information on the nature and evolution of ore-forming fluids. The Beiya gold-polymetallic deposit is China’s largest skarn-type Au deposit with Au reserves of >380 t. Although several studies have addressed the skarn formation mechanism and nature of fluid evolution, a systematic study on the changes in the composition and texture of garnet from wall rocks to ore-related intrusion has not yet been attempted. In this work, we carry out a systematic investigation along a horizonal profile in the open pit of the Beiya deposit. Our results show that the garnet in the profile belongs to grossular-andradite solid solution series with all grains showing well-developed fine oscillatory zoning. The garnets are of two types: the first type (Grt I) formed within the endoskarn, is reddish to red with euhedral-subhedral megacryst structure (>10 mm) and has altered surfaces. The second type (Grt II) occur in the proximal exoskarn, is red to brown–red, euhedral-subhedral, with medium- to fine- grained structure (0.3–4 mm) and displaying oscillatory zoning. Unlike Grt I, hydrous minerals occur in the pores among Grt II garnets, such as calcite, quartz, epidote and chlorite. Also, calcite-chlorite vein, quartz vein and quartz-chalcopyrite-pyrite vein cuts the Grt II garnets. Grt I is dominantly grossular (Adr14.88-43.58Grs55.25-82.87), depleted in the large ion lithophile elements (LILE), relatively enriched in high field strength elements (HFSEs) and enriched in heavy rare earth elements (HREEs) with negative Eu anomalies. Grt II is mostly andradite (Adr62.18-98.91Grs0-36.22), depleted in the LILE and HFSE, enriched in light rare earth elements (LREEs) with slightly positive Eu anomalies. Here we selected Grt II for in-situ LA-ICP-MS U-Pb dating, since Grt I contains negligible uranium. The results yielded a lower intercept 206Pb/238U age of 35.3 ± 1.8 Ma (MSWD = 0.68, n = 23). The calculated oxygen isotopic compositions of hydrothermal fluids in equilibrium with two types of garnets range from 6.46 ‰ to 8.56 ‰ (average 7.89 ‰), which are within the range of values for magmatic fluids. We infer that the physicochemical conditions of the hydrothermal fluid changed from relatively reduced, nearly neutral, and low water/rock ratios to slightly oxidized, slightly acidic, and high water/rock ratios. The chlorine content in the fluid increased during garnet growth. The fluctuating compositions of garnets in the skarn suggest multistage ore-fluid activities and the fluid evolution provide insights into the ore-forming process.

Directed growth of a sector-zoned garnet in a pegmatoid from the Bohemian Massif, Austria

Almandine-spessartine garnet from the Moldanubian Zone in the Bohemian Massif (Austria) records microstructural and compositional features of crystal growth imprinted during the transition from pegmatoid magmatic to subsolidus state. Directed garnet growth results in asymmetric compositional, morphological and microstructural zoning. In addition, the garnet core shows sector zoning reflected by the predominance of phosphate inclusions in {110} sectors, and rutile inclusions in {112} sectors of garnet. These sector-specific abundances of inclusions are also expressed in the electron probe microanalytical data of the non-equivalent garnet sectors. The contribution of the submicron and nano-inclusions of phosphates and rutile is of crucial importance for thermobarometric methods based on P, Na and Ti in garnet. The sector-specific abundance of inclusions is referred to the surface configuration of individual garnet facets. Contrastingly, the microstructural growth banding observed in the {112} garnet sectors supposedly results from compositional fluctuations of a boundary layer at the garnet-melt interface. These fluctuations control the details of the inclusion microstructures, which differ even in crystallographically equivalent sectors.

Continuous vs. discontinuous garnet growth in mylonitic micaschists from northeastern Sardinia, Italy: Evidence from LA-ICPMS trace element mapping

Garnet with complex, discontinuous zoning is a common occurrence in metamorphic terrains, and the relationship between major and trace element zoning can provide insight into the metamorphic evolution of the host rock. Mylonitic micaschists along the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain contain garnet porphyroblasts, enveloped by the S2 schistosity, with distinct core and rim domains. A large garnet porphyroblasts was investigated by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) mapping. The major element compositional variation follows a bell-shaped zoning, with Ca and Mn contents progressively decreasing, and Fe and Mg increasing, from the core to the outer rim. LA-ICPMS mapping revealed a thin and sharp annular enrichment zone in Y, Sc, Dy, Ho, Er, Tm at the mantle-rim boundary. The trace element (TE) compositional profiles show a central enrichment area for HREE (Tm, Yb, Lu). This enrichment decreases progressively, as a function of atomic number, for Er, Ho and Dy. Elements with even lower atomic number (Tb, Gd, Eu and Sm), are depleted in this central domain, but their content increases in broad shoulders towards the garnet rim. The position of these lateral shoulders migrates progressively rimwards with decreasing atomic number. The REE distribution, trend and behavior in the growth zones of the garnet is an example of TE control during a continuous growth ruled by diffusion-limited REE uptake. The Y + HREE annular enrichment zone, interpreted as resulting from a decrease in the garnet growth rate, reflects a short-lived episode in the garnet growth history.

Evidence for a pre-Himalayan metamorphism in the High Himalayan Crystalline of the Miyar Valley (NW India)

Most of the tectonic, metamorphic and geochronological data suggest that the Himalaya is essentially the consequence of a single orogenic cycle associated with the India-Asia collision during the Cenozoic era. Therefore, metamorphic assemblages and tectonic structures across the Himalayan range are systematically considered as post-collisional geological records. However, over the last decades, several observations arguing for geological events predating the continental collision have become increasingly recurrent in the literature. Nevertheless, although some of these arguments are thoroughly documented, they are unduly ignored in the construction of models drawing the tectono-metamorphic evolution of the Himalayan range. Yet, the occurrence of a pre-Himalayan history would have considerable consequences on the classical models for the building of the Himalaya. The recent discovery of inclusions of staurolite crystals in greenschist facies garnets from the Miyar Valley in Upper Lahul region (Himachal Pradesh; NW India) revives the debate on the existence of a pre-Himalayan metamorphism. Indeed, the occurrence of high-temperature staurolites included in greenschist facies garnets suggests that the High Himalayan Crystalline rocks experienced an amphibolite facies metamorphism prior the predominant Himalayan greenschist facies metamorphism observed in this part of the range. In this study, phase petrology, microtectonic investigations combined with preexisting geochronological data infer that the crystallization of the included staurolite predates the growth of Himalayan garnets. These original data bring new arguments to bear on the long lasting debate of the existence of a Pre-Himalayan orogenic cycle. They lead to the conclusion that the growth of staurolite predates the continental collision between India and Asia and reflects a metamorphic event that belongs to a pre-Himalaya orogenic cycle.