Resorption Of Garnet Research Articles

Symplectite and kelyphite formation during decompression of mafic granulite from Gjelsvikfjella, central Dronning Maud Land, Antarctica

Abstract. Central Dronning Maud Land (cDML) is part of the late Mesoproterozoic Maud Belt in East Antarctica, which was metamorphosed and deformed during the Ediacaran–Cambrian Gondwana assembly. Here we study high-pressure (HP) mafic rocks in Gjelsvikfjella, cDML, which occur as lenses and pods transposed in highly strained, upper amphibolite-facies gneisses. We present a P–T–t history for the HP rocks based on mineral assemblages, reaction textures and new U–Pb zircon data. Relics that indicate an early HP granulite-facies stage have been identified in anhydrous garnet–clinopyroxene rocks. The peak-pressure assemblage was plagioclase-free and contained garnet, titanite, clinopyroxene and quartz. The HP assemblage has been extensively overprinted by lower-pressure phases and exhibits a variety of symplectite and corona textures that record the post-peak-pressure evolution of the rocks. Decompression and heating in the granulite-facies field resulted in the replacement of titanite by ilmenite–clinopyroxene symplectite, formation of clinopyroxene–plagioclase intergrowths and resorption of garnet by plagioclase–clinopyroxene kelyphite. Formation of late orthopyroxene in symplectites and kelyphites demonstrates that the P–T evolution entered the medium-pressure granulite-facies field. The peak metamorphic stage was followed by retrograde cooling into the amphibolite facies. In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating of zircons indicate Mesoproterozoic protolith ages (1150–1000 Ma) and Ediacaran–Cambrian metamorphic reworking at ca. 568 and ca. 514 Ma. The HP granulites were formed and exhumed during a clockwise P–T evolution related to continental collision during Gondwana amalgamation, followed by post-collisional extension and orogenic collapse.

Zircon and monazite reveal late Cambrian/early Ordovician partial melting of the Central Seve Nappe Complex, Scandinavian Caledonides

The Seve Nappe Complex (SNC) comprises continental rocks of Baltica that were subducted and exhumed during the Caledonian orogeny prior to collision with Laurentia. The tectonic history of the central SNC is investigated by applying in-situ zircon and monazite (Th-)U–Pb geochronology and trace element analysis to (ultra-)high pressure (UHP) paragneisses in the Avardo and Marsfjället gneisses. Zircons in the Avardo Gneiss exposed at Sippmikk creek exhibit xenocrystic cores with metamorphic rims. Cores show typical igneous REE profiles and were affected by partial Pb-loss. The rims have flat HREE profiles and are interpreted to have crystallized at 482.5 ± 3.7 Ma during biotite-dehydration melting and peritectic garnet growth. Monazites in the paragneiss are chemically homogeneous and record metamorphism at 420.6 ± 2.0 Ma. In the Marsfjället Gneiss exposed near Kittelfjäll, monazites exhibit complex zoning with cores enveloped by mantles and rims. The cores are interpreted to have crystallized at 481.6 ± 2.1 Ma, possibly during garnet resorption. The mantles and rims provide a dispersion of dates and are interpreted to have formed by melt-driven dissolution-reprecipitation of pre-existing monazites until 463.1 ± 1.8 Ma. Depletion of Y, HREE, and U in the mantles and rims compared to the cores record peritectic garnet and zircon growth. Altogether, the Avardo and Marsfjället gneisses show evidence of late Cambrian/early Ordovician partial melting (possibly in (U)HP conditions), Middle Ordovician (U)HP metamorphism, and late Silurian tectonism. These results indicate that the SNC underwent south-to-north oblique subduction in late Cambrian time, followed by progressive north-to-south exhumation to crustal levels prior to late Silurian continental collision.

Pronounced and rapid exhumation of the Connecticut Valley Trough revealed through quartz in garnet Raman barometry and diffusion modelling of garnet dissolution–reprecipitation reactions

AbstractMetamorphic rocks from the Connecticut Valley Trough (CVT), Vermont, and Massachusetts, have been examined using quartz‐in‐garnet (QuiG) and conventional thermobarometry, thermodynamic reaction modelling, diffusion modelling, and 40Ar/39Ar thermochronology to constrain their P–T–t paths during Acadian metamorphism and subsequent exhumation. Numerous samples, collected in the vicinity of the Acadian domes, contain garnet porphyroblasts that display cloudy zones characterized by numerous fluid inclusions and modified garnet compositions associated with the replacement of the original garnet by biotite±muscovite±plagioclase±quartz±low Xgrs/enriched Xsps. QuiG and conventional thermobarometry constrain both the conditions of garnet nucleation and peak P–T conditions to have occurred at ~0.85–1.05 GPa, ~550–600°C. Most notably, QuiG barometry was performed on inclusions adjacent to these reaction zones in conjunction with Gibbs method reaction modelling to reveal that these dissolution–reprecipitation reactions occurred during nearly isothermal decompression from the peak P–T conditions to around ~0.3 GPa, 550°C. Diffusion modelling reveals that the Mn zoning profiles created during garnet resorption that accompanied decompression formed in less than c. 3 Ma, which constrains the tectonic exhumation to have occurred at 8–10 mm/year. Subsequent cooling to 500°C occurred rapidly at a rate of 100°C/Ma, followed by slower cooling reaching 1.7°C /Ma by the mid Carboniferous. This is the first reported example of QuiG barometry revealing a multi‐stage metamorphic history and highlights the utility of this method for unravelling complex metamorphic terranes.

Melt inclusions at MT. Edixon (Antarctica): Chemistry, petrology and implications for the evolution of the Lanterman range

The recovery of melt inclusions (MI) in peritectic phases of metamorphic rocks demonstrates their anatectic origin even in the absence of other textural or field evidences, and gives the opportunity to retrieve the chemical composition of primary melts and infer melt entrapment conditions. Here we report the first recovery of such inclusions in a medium-grade schist from the Edixon Metamorphic Complex (EMC) of the Lanterman Range in northern Victoria Land, Antarctica. This is the first study of melt inclusions from an Al2SiO5-free, graywacke-type protolith. Inclusions are hosted in garnet. They are both polycrystalline nanogranitoids, i.e., crystallized former MI, and fluid inclusions (FI) coexisting in the same clusters and displaying frequent negative crystal shape and textures indicative of a primary origin. The MI contain albite, K-feldspar, quartz, muscovite, biotite ± calcite ± siderite ± H2O, while the FI are composed by CO2 + H2O + N2 + CH4 ± H2S and contain step-daughter calcite.The schist hosting the garnets with melt inclusions contains muscovite, biotite, garnet, plagioclase, K-feldspar, and quartz. Extensive resorption of garnet and replacement by biotite, along with composition of micas that suggest medium-temperature, subsolidus conditions, indicate that after anatexis the schist was thoroughly recrystallized under retrograde conditions. The MI were remelted in a piston cylinder apparatus at conditions of 740–900 °C at 0.8–1.0 GPa with complete remelting taking place at 760–780 °C. The remelted glass is sub-alkaline, peraluminous (ASI ≈ 1.3–1.5) with CaO ≈ 1.2–2.1 wt% and K2O ≈ 3.4–4.1 wt% and contains ≈ 4.3–5.3 wt% of H2O and 400–1150 ppm of CO2. The P-T conditions of melt entrapment were retrieved combining results of experimental remelting with calculated phase equilibria in the MnCNKFMASHT model system, and are between 740 and 780 °C and 0.7 to 0.9 GPa. The new and different P-T conditions for a high grade rock of the EMC with respect to other published data, confirm that this metamorphic complex is not homogeneous, and that anatexis may have taken place at different crustal levels during the evolution of the Cambro-Ordovician Ross Orogen.

Polyphase Zircon Growth during Slow Cooling from Ultrahigh Temperature: an Example from the Archean Pikwitonei Granulite Domain

AbstractQuantifying the timescales of Archean ultrahigh temperature (UHT) metamorphism is essential for constraining the style of plate tectonics on the early Earth. However, such timescales can be difficult to quantify, due to the antiquity of Archean rocks and the extreme thermal conditions of UHT metamorphism. We constrain the timescales of Archean UHT metamorphic processes recorded by a single rock sample from the Pikwitonei granulite domain (northwestern Superior Province), through the integration of two U–Pb zircon petrochronologic techniques. In this study we combine: (1) high-spatial resolution laser ablation split-stream inductively coupled mass spectrometry (LASS) on in situ zircon (in thin section) and hand-picked zircon; and (2) high-precision isotope dilution thermal ionization mass spectrometry (ID-TIMS) analyses on microsampled fragments from the same hand-picked zircon analysed by LASS. Phase equilibria modelling and Zr-in-rutile thermometry suggest the rock followed a P–T path characterized by decompression at > 960 °C, followed by near-isobaric cooling at ∼0·8 GPa. In situ LASS zircon analyses could be interpreted to record zircon growth at broadly ∼2665 Ma, though the large uncertainties on isotopic dates make potentially distinct growth episodes difficult to distinguish. ID-TIMS U–Pb dates of zircon fragments reveal a polyphase zircon growth history over a 24 Ma duration, from 2673 to 2649 Ma. Zircon trace element compositions, textures, and microstructural relationships, as well as evaluation of zircon-garnet equilibrium, suggest zircon grew during melt crystallization, after UHT decompression and garnet resorption. Variable Ti concentrations within zircon domains indicate: (1) zircon crystallized through the temperature interval of ∼875 °C to ∼730 °C, potentially in isolated rock domains with variable zircon saturation temperature; and/or (2) zircon crystallized over a narrower temperature interval in isolated rock domains with variable aTiO2 and/or aSiO2. Collectively, the data suggest the west-central Pikwitonei granulite domain reached peak UHT conditions prior to 2673 Ma, after which suprasolidus conditions in the lower crust persisted for at least 24 Ma. Such an interpretation would be impossible if based on either the LASS or ID-TIMS zircon data alone, which highlights the utility of applying both techniques in tandem to constrain metamorphic timescales in ancient UHT terranes.

Two‐stage Variscan metamorphism in the Canigou massif: Evidence for crustal thickening in the Pyrenees

AbstractThe Variscan metamorphism in the Pyrenees is dominantly of the low‐pressure–high‐temperature (LP‐HT) type. The relics of an earlier, Barrovian‐type metamorphism that could be related to orogenic crustal thickening are unclear and insufficiently constrained. A microstructural and petrological study of micaschists underlying an Ordovician augen orthogneiss in the core of the Canigou massif (Eastern Pyrenees, France) reveals the presence of two syntectonic metamorphic stages characterized by the crystallization of staurolite (M1) and andalusite (M2), respectively. Garnet is stable during the two metamorphic stages with a period of resorption between M1 and M2. The metamorphic assemblages M1 and M2 record similar peak temperatures of 580°C at different pressure conditions of 5.5 and 3 kbar, respectively. Using chemical zoning of garnet and calculated P–T pseudosections, a prograde P–T path is constrained with a peak pressure at ~6.5 kbar and 550°C. This P–T path, syntectonic with respect to the first foliation S1, corresponds to a cold gradient (of ~9°C/km), suggestive of crustal thickening. Resorption of garnet between M1 and M2 can be interpreted either in terms of a simple clockwise P–T path or a polymetamorphic two‐stage evolution. We argue in favour of the latter, where the medium‐pressure (Barrovian) metamorphism is followed by a period of significant erosion and crustal thinning leading to decompression and cooling. Subsequent advection of heat, probably from the mantle, leads to a new increase in temperature, coeval with the development of the main regional fabric S2. LA‐ICP‐MS U–Th–Pb dating of monazite yields a well‐defined date at c. 300 Ma. Petrological evidence indicates that monazite crystallization took place close to the M1 peak pressure conditions. However, the similarity between this age and that of the extensive magmatic event well documented in the eastern Pyrenees suggests that it probably corresponds to the age of monazite recrystallization during the M2 LP‐HT event.

Monazite Behaviour and Time-scale of Metamorphic Processes along a Low-pressure/High-temperature Field Gradient (Ryoke Belt, SW Japan)

Low-pressure/high-temperature metamorphic rocks exposed in the western part of the Ryoke belt (Iwakuni–Yanai area, SW Japan) include a section with increasing temperature conditions from ∼425 to 880°C. We use this setting to explore the evolution of monazite grain size, texture and composition, and variations in the whole-rock composition of 11 metapelite, metapsammite or metachert samples collected along the metamorphic field gradient. Monazite grain size increases with rising metamorphic grade, regardless of the whole-rock composition. From low- to high-grade conditions we infer: (1) the initial nucleation of monazite aggregates after allanite (∼425°C); (2) monazite coarsening and coalescence driven by incipient monazite recycling; that is, dissolution of small grains to grow larger ones by Ostwald ripening (500–600°C); (3) a first major recycling stage enhanced by fluid liberation owing to muscovite breakdown (600–630°C); (4) a second recycling stage assisted by an increase in the proportion of anatectic melt owing to biotite breakdown (> 850°C). A succession of four compositional domains is recognized in monazite. We emphasize the usefulness of comparing their Ce/ThMnz, Ce/YMnz and Th/UMnz molar ratios with those derived from whole-rock analyses to constrain the origin of each domain. Domain I, with variable ratios, reflects the progressive transfer of Th ± U from allanite to monazite at low-grade conditions. Domain II, with Ce/ThMnz matching the whole-rock values, indicates growth under rock(decimetre)-scale equilibrium conditions. Domains II and III, with Th/UMnz and Ce/YMnz departing from the whole-rock values, record the competition with zircon (for U) and garnet (for Y) during growth at peak P–T conditions. Domain IV points to Y supply by garnet resorption during retrograde chloritization (< 550°C). In the highest-grade sample, zircon grains included in garnet or cordierite show metamorphic rims with sillimanite and Si-rich inclusions. These rims formed at suprasolidus conditions (650–880°C) and yield 206Pb/238U ages of 103–97 Ma (± 5 Ma), which bracket the timing of high-temperature metamorphism. Monazite dating by electron microprobe and laser ablation inductively coupled plasma mass spectrometry reveals two age groups. For domains I–III, some relatively old 206Pb/238U ages (99–95 ± 3–5 Ma) represent minimum estimates for the timing of prograde to peak metamorphism, whereas the similar oldest 206Pb/238U age for domain IV (93 ± 7 Ma) points to rapid cooling after the temperature peak. A more dominant population of younger Th–U–Pb and 206Pb/238U dates is ascribed to age resetting by heterogeneous annealing of the monazite crystal lattice. The youngest concordant 206Pb/238U age of 86 ± 4 Ma is correlated with the end of intermediate-temperature conditions (< 200–300°C) below which monazite, in metamorphic rocks as well as in the surrounding granitoids, eventually behaved as a closed isotopic system. This contribution is dedicated to the memory of Professor Kazuhiro Suzuki, for his contribution to geochronology and Japanese geology.

Rehydration of eclogites and garnet-replacement processes during exhumation in the amphibolite facies

Abstract EPMA and LA-ICP-MS trace-element maps have been acquired from amphibolitized eclogites from the Diego de Almagro Metamorphic Complex (Chile). Several garnet growth pulses and garnet resorption stages are revealed by major elements chemical zoning and by heterogeneous Y and rare earth element (REE) behaviour, associated with subduction and exhumation of these rocks. Distribution of REE in prograde garnet is texturally and chemically coupled with the breakdown of REE-bearing minerals while formation of epidote and titanite generations during amphibolitization is recorded by complex textures involving new garnet generation and overprinting phases. The latest overprint stage is characterized by fine-grained intergrowth between garnet and epidote micro-veins, phengite, hornblende, albite and titanite. Garnet cracks have been gradually re-equilibrated during this event witnessing short-scale dissolution–transport–precipitation. Pseudosection modelling shows that local variability in water content during amphibolitization controls garnet stability at the expense of epidote. Overprinting microstructures are explained by the effect of locally-derived aqueous fluids that trigger the ‘unlocking’ of elements from the reacting eclogite-facies paragenesis. These findings highlight the microscopic characteristics of amphibolitization processes documented in exhumed eclogite-facies terranes and shed light on the importance of thorough micro-chemical investigations while undertaking pressure–temperature (PT) estimates on rocks with strong textural disequilibrium.

An inverse modeling approach to obtain P–T conditions of metamorphic stages involving garnet growth and resorption

This contribution presents an approach and a computer program (GrtMod) for numerical simulation of garnet evolution based on compositions of successive growth zones in natural samples. For each garnet growth stage, a new local effective bulk composition is optimized, allowing for resorption and/or fractionation of previously crystallized garnet. The successive minimizations are performed using the Nelder–Mead algorithm; a heuristic search method. An automated strategy including two optimization stages and one refinement stage is described and tested. This program is used to calculate pressure–temperature ( P–T ) conditions of crystal growth as archived in garnet from the Sesia Zone (Western Alps). The compositional variability of successive growth zones is characterized using standardized X-ray maps and the program XMapTools. The model suggests that Permian garnet cores crystallized under granulite-facies conditions at T > 800 °C and P = 6 kbar. During Alpine times, a first garnet rim grew at eclogite-facies conditions (650 °C, 16 kbar) at the expense of the garnet core. A second rim was added at lower P (∼11 kbar) and 630 °C. In total, garnet resorption is modeled to amount to ∼9 vol% during the Alpine evolution; this value is supported by our observations in X-ray compositional maps.

Transient allanite replacement and prograde to retrograde monazite (re)crystallization in medium-grade metasedimentary rocks from the Orlica-Śnieżnik Dome (Czech Republic/Poland): Textural and geochronological arguments

We use textural and geochronological arguments to constrain monazite petrogenesis and the age of metamorphic events in medium-grade metasedimentary rocks from the European Variscan Belt (Orlica-Śnieżnik Dome, Czech Republic/Poland). The relationships between monazite, allanite and other major and accessory phases are characterized for staurolite-, sillimanite- and kyanite-grade, garnet-bearing mica schist samples. Garnet porphyroblasts with prograde compositional zoning include allanite in the core and monazite at the rim. Allanite is virtually absent from the matrix; it shows replacement textures involving a cluster of Th±U±Y-rich silicates or phosphates. Monazite, with frequent micro-cracks or trimmed grain boundaries, abounds in the mica-rich matrix; it occurs as lone grains close to partially resorbed garnet, apatite or staurolite, and as subhedral and compositionally zoned aggregates associated with allanite and xenotime. Electron microprobe analyses of monazite define Pb vs. Th* isochrons (ages=363–343Ma) with positive intercepts (Pb0=40–120ppm) which suggest the presence of initial Pb and age overestimation. Isotopic analyses obtained by laser ablation coupled with mass spectrometry confirm the minor presence of initial Pb which leads to variable discordance in the 206Pb/238U–207Pb/235U system. Conversely, the 206Pb/238U–208Pb/232Th system is not visibly affected; analyses are concordant and 208Pb/232Th ages range from 358 to 299Ma (±7–8Ma) regardless of monazite texture or composition. They define a main group at 330–310Ma, with few older outliers. All observations suggest that a first monazite generation (MnzI) formed via allanite breakdown at prograde to peak P–T conditions (5–7kbar/575–640°C). A comparison with previous geochronological results reveals that the age of this prograde reaction is, however, preserved by only few domains yielding 360–340Ma. Instead, our results emphasize the importance of retrograde processes which built the dominant age population of 330–310Ma. The latter ages are ascribed to a combination of (1) transient monazite growth after allanite and controlled by P availability, (2) variable recrystallization/replacement of older monazite grains, and (3) minor monazite neoformation due to the resorption of garnet or apatite. The young monazite grains or domains (MnzII) mostly formed during fluid influx and chloritization; they constrain the end of the orogenic history when mid-crustal rocks were exhumed and re-hydrated at temperatures below ~300°C.

Prograde Metamorphic History of UHP Granulites from the Moldanubian Zone (Bohemian Massif) Revealed by Major Element and Y + REE Zoning in Garnets

Major and trace element distribution in garnet crystals from felsic granulites of the Kutná Hora Complex provides evidence of two metamorphic events, the first related to continental subduction at ultrahigh-pressure (UHP) conditions and the second to a granulite-facies overprint that occurred at mid- to lower crustal levels. To reconstruct P–T paths of both metamorphic events, pseudosection modeling was combined with major and trace element zoning in garnet. The granulites contain lenses and boudins of mantle-derived garnet peridotite and eclogite. UHP conditions are confirmed by the presence of inclusions of micro-diamond and coesite in garnet and zircon in the granulites. Compositional zoning in garnet provides evidence for a pre-granulite-facies high-pressure (HP)–UHP metamorphic history of the rocks. The two distinct metamorphic events are documented by high Y + HREE concentrations in the garnet cores and annuli in the mantle part of the garnet grains. The cores of large garnets with relatively high Ca contents and bell-shaped Mn profiles suggest formation during a prograde low- to medium-temperature metamorphic event that was coeval with HP–UHP metamorphism. Decompression and cooling during exhumation of the rocks led to partial resorption of garnet and release of trace elements into the matrix. The new garnet with high Y + HREE in the annuli was formed during the granulite-facies event at crustal levels.

LASS U–Th–Pb monazite and rutile geochronology of felsic high-pressure granulites (Rhodope, N Greece): Effects of fluid, deformation and metamorphic reactions in local subsystems

The specific chemical composition of monazite in shear zones is controlled by the syndeformation dissolution–precipitation reactions of the rock-forming minerals. This relation can be used for dating deformation, even when microfabric characteristics like shape preferred orientation or intracrystalline deformation of monazite itself are missing. Monazite contemporaneously formed in and around the shear zones may have different compositions. These depend on the local chemical context rather than reflecting successive crystallization episodes of monazite.This is demonstrated in polymetamorphic, mylonitic high-pressure (HP) garnet–kyanite granulites of the Alpine Sidironero Complex (Rhodope UHP terrain, Northern Greece). The studied mylonitic rocks escaped from regional migmatization at 40–36Ma and from subsequent shearing through cooling until 36Ma. In-situ laser-ablation split-stream inductively-coupled plasma mass spectrometry (LASS) analyses have been carried out on monazite from micro-scale shear zones, from pre-mylonitic microlithons as well as of monazite inclusions in relictic minerals complimented by U–Pb data on rutile and Rb–Sr data of biotite.Two major metamorphic episodes, Mesozoic and Cenozoic, are constrained. Chemical compositions, isotopic characteristics and apparent ages systematically vary among monazite of four different microfabric domains (I–IV). Within three pre-mylonitic domains (inclusions in (I) pre-mylonitic kyanite and (II) garnet porphyroclasts, and (III) in pre-mylonitic microlithons) monazite yields ages of ca. 130–150Ma for HP-granulite metamorphism, in line with previous geochronological results in the area. Patchy alteration of the pre-mylonitic monazite by intra-grain dissolution–precipitation processes variably increased negative Eu anomaly and reduced the HREE contents. The apparent age of this altered monazite is reduced.Monazite in the syn-mylonitic shear bands (IV) differs in chemical composition from unaltered and altered monazite of the three pre-mylonitic domains by having a significantly more pronounced negative Eu anomaly, a flatter HREE pattern, and high Th content. These compositional characteristics are linked with syn-mylonitic formation of plagioclase and resorption of garnet in the shear bands under amphibolite facies conditions. The absence of pre-mylonitic monazite in the shear zones, in contrast to the other domains, suggests complete dissolution of old and formation of new monazite. This probably results from an increased alkalinity and reactivity of the fluid that again is controlled by syn-mylonitic interaction with feldspar and apatite in the shear zones. There, the deformation was accommodated by dissolution precipitation creep at ca. 690±50°C and 6–7.5kbar. Growth of monazite at 55±1Ma dates this deformation, which precedes the regional migmatization of the Sidironero Complex, whereas rutile and biotite ages reflect these later stages. This new pressure–temperature–time constraint for a relictic deformation structure provides insight into the still missing parts of the overall metamorphic, deformation and exhumation processes of the UHP units in the Rhodope.

Syn-deformation fluid-assisted growth of monazite during renewed high-grade metamorphism in metapelites of the Central Rhodope (Bulgaria, Greece)

We present textural, chemical and U–Th–Pb-age data on monazites from garnet–kyanite (Grt–Ky) metapelites, from the Chepelare Shear Zone (Bulgaria) and the Nestos Shear Zone (Greece), in the Central Rhodope. Samples from both locations have experienced two stages of high temperature metamorphism during Alpine times. The first event involved mid-Mesozoic granulite facies dehydration melting. The second event involved mid-Cenozoic lower-grade fluid-assisted partial melting. The latter is well expressed in adjacent felsic rocks but had limited impact on the Grt–Ky metapelites. Most samples display evidence for strong ductile shearing in the presence of fluids. Monazite is present in the highly foliated matrix and as inclusions in garnet and kyanite. Unlike the inclusions, matrix monazites display features of fluid-assisted dissolution–recrystallization. Y-poor domains with U–Th–Pb ages of between ca. 115 and 165Ma represent the largest part of the grains. Y-rich domains with mid-Cenozoic ages occur as rims, or as small satellite grains surrounding the Mesozoic grains. The Cenozoic monazite domains crystallized at the expense of the Mesozoic ones and simultaneously incorporated Y provided by the fluid-assisted resorption of garnet. An age of ca. 36Ma is obtained for the samples of both shear zones, interpreted as dating the main episode of monazite growth during the Cenozoic. Similar ages exist for the crystallization of leucosomes and pegmatites in the adjacent migmatitic gneisses, indicating that the fluids responsible for the precipitation of the Cenozoic monazites were probably released during the crystallization of nearby anatectic melts. Together with associated rutile and biotite, many newly grown monazites show a preferred orientation paralleling the matrix foliation. This supports the hypothesis of dynamic dissolution–precipitation as an efficient mean to promote renewed monazite crystallization during ductile deformation of the host rock at ca. 36Ma.

Transformation of garnet megacrysts captured by alkali mafic magma

The results of research of symplectites from the Shavaryn-Tsaram (Hangaj plateau, Mongolia) and Bartoj (Dzhida basaltic field, Russia) alkali basaltic rocks are presented. The symplectite compositions and structures were studied, and the physical and chemical parameters at which primary megacrysts were transformed into secondary mineral assemblages were defined. It is established that both garnet megacrysts and garnet-clinopyroxene aggregates were formed at pressures of 10–13 kbar and temperatures over 1300°C. The transformation of garnet into minerals of the secondary assemblage is considered as solid state water assisted resorption of garnet at a depth corresponding to pressures of 4–8 kbar and temperatures ranging from 1000 to 1300°C. The kelyphitic rims on the garnet megacrysts resulted from melting of the megacrysts at the contact with the hosting alkali basaltic rock.

A Garnet–Zircon Oxygen Isotope Record of Subduction and Exhumation Fluids from the Franciscan Complex, California

Tectonic blocks of high-grade metamorphic rock hosted by the Franciscan me¤ lange commonly preserve textural and geochemical evidence for metasomatism and blueschist-facies overprinting within a subduction zone. These exhumation events can obscure information on the nature of subduction fluids at depth that might otherwise be preserved by these rocks in this tectonic setting. Cation and oxygen isotope zoning in garnets from eclogite and garnet^hornblende rocks with minimal textural evidence of retrogression reveals separate prograde and retrograde fluid histories. Garnet cores preserve a homogeneous prograde oxygen isotope history with both high (dO 11o) and low ( 4o) values in different samples probably recording altered ocean-crust protolith compositions. Multiple episodes of garnet resorption and regrowth record a dramatically changing bulk oxygen isotope ratio resulting in 6^7o neoformed garnet rims with sharp core^rim boundaries in both samples. Matrix omphacite and hornblende are in profound oxygen isotope disequilibrium with garnet cores but are in equilibrium with garnet rims despite the appearance of overall textural equilibrium in thin section.This cryptic metasomatism found in samples from different locations within the Franciscan Complex brings into question existing thermobarometry on non-inclusion phases for this complex, as well as interpretations based on whole-rock geochemical data. Zircon ages and oxygen isotope equilibrium with late-forming garnet are consistent with postmetasomatic formation of zircon.The ability of garnet and zircon to record multiple fluid events and tie them to P^T^t^fluid history is a powerful tool in gaining an improved understanding of the complex fluid environments within subduction zones.

Monazite ages and pressure–temperature–time paths from anatectites in the southern Ruby Range, Montana, USA: evidence for delamination, ultramafic magmatism, and rapid uplift at ca. 1780 Ma

The Wolf Creek ultramafic bodies of the southern Ruby Range, southwest Montana, intruded as high-temperature magma during the late stages of an Early Proterozoic orogeny affecting the Great Falls Tectonic Zone of the western Wyoming craton. Field and petrologic evidence indicate high-temperature metamorphism and deformation uniquely affected metapelitic anatectites adjacent to the ultramafic bodies. Leucosomes are mesoperthite and quartz with garnet. Restite contains sillimanite, garnet, quartz, microcline, and biotite that was partially resorbed by the melt-producing reaction. Late cordierite and sillimanite replace garnet in the migmatites and other pelitic gneiss in the area. Petrogenic grids for the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) system indicate that conditions during anatexis were 750–800 °C, 600–900 MPa. Anatexis occurred during deformation of metapelites and associated rocks adjacent to the larger ultramafic bodies, but crystallization and cooling occurred after deformation, as indicated by the preservation of igneous textures and mesoperthite with undeformed lamellae in the leucosome. Element mapping of monazite using electron-probe micro-analysis (EPMA) identifies four distinct chemical domains of monazite growth during the orogenic event in addition to relict, Archaean to earliest Proterozoic cores. U–Th–Pb dating of the domains establishes two periods of growth of matrix monazite in the paleosome that indicate fabric-forming prograde metamorphism at 1829 ± 17 and 1805 ± 4 Ma (2σ). Coarser, igneous monazites from the leucosome also contain two distinct chemical zones visible in yttrium maps, indicating growth with garnet and then during garnet resorption as decompression led to replacement of garnet by cordierite. The domain ages cannot be statistically distinguished. Together they yield an age of 1782 ± 9 Ma (2σ). The data imply that regional thickening leading to upper amphibolite facies metamorphism occurred prior to 1800 Ma. Thickening was followed after ∼25 Ma by the intrusion of ultramafic magmas, contact anatexis, and then decompression. This sequence is consistent with mantle delamination after orogenic thickening.

P–T history and geochemical characteristics of mafic granulites and charnockites from west of Periya, North Kerala, southern India

The study region forms the northern part of Kerala (south India) and constitutes part of granulite–facies rocks of the exhumed Proterozoic south Indian Granulite Terrain (SGT). The SGT consists of a large variety of rock types with a wide range of mineral parageneses and chemical compositions, namely charnockite, mafic granulite, gneiss, schist, anorthosite, granite and minor meta-sedimentary rocks. Garnet–bearing mafic granulites occur as small enclaves within charnockites. We report for the first time P–T constraints on the prograde path preceding peak metamorphism in the northernmost part of Kerala. An increase of the Mg, Fe and decrease of Ca and Mn contents from the core towards the rim of garnet in the mafic granulites suggest prograde garnet growth. The prograde path was followed by peak metamorphism at a temperature of c. 800°C and a pressure of c. 7.5kbar as computed by isopleths of XMg garnet, XCa garnet and XAn plagioclase. The resorption of garnet in various reaction textures and the development of spectacular orthopyroxene–plagioclase, biotite–quartz and hornblende–plagioclase symplectites characterize the subsequent stages of metamorphism. The PT path is characteristically T-convex suggesting an isothermal decompression path and reflects rapid uplift followed by cooling of a tectonically thickened crust. Diffusion modeling of Fe–Mg exchange between garnet and hornblende suggests a near-peak cooling rate of 10–70°C/myr. Such cooling rates are too high to be accounted for by normal isostatic uplift and erosion and suggest that the terrain was tectonically exhumed. Charnockites are richer in SiO2 and lower in MgO and CaO when compared to mafic granulites. Their REE patterns are relatively flat and show prominent negative europium anomalies. The mafic granulites are metamorphosed tholeiitic basalts as revealed by major- and trace-element geochemistry.

The behaviour of monazite from greenschist facies phyllites to anatectic gneisses: An example from the Chugach Metamorphic Complex, southern Alaska

Monazite is a common accessory mineral in various metamorphic and magmatic rocks, and is widely used for U–Pb geochronology. However, linking monazite U–Pb ages with the PT evolution of the rock is not always straightforward. We investigated the behaviour of monazite in a metasedimentary sequence ranging from greenschist facies phyllites into upper amphibolites facies anatectic gneisses, which is exposed in the Eocene Chugach Metamorphic Complex of southern Alaska. We investigated textures, chemical compositions and U–Pb dates of monazite grains in samples of differing bulk rock composition and metamorphic grade, with particular focus on the relationship between monazite and other REE-bearing minerals such as allanite and xenotime. In the greenschist facies phyllites, detrital and metamorphic allanite is present, whereas monazite is absent. In lower amphibolites facies schists (~550–650°C and ≥3.4kbar), small, medium-Y monazite is wide-spread (Mnz1), indicating monazite growth prior and/or simultaneous with growth of garnet and andalusite. In anatectic gneisses, new low-Y, high-Th monazite (Mnz2) crystallised from partial melts, and a third, high-Y, low-Th monazite generation (Mnz3) formed during initial cooling and garnet resorption. U–Pb SHRIMP analysis of the second and third monazite generations yields ages of ~55–50Ma. Monazite became unstable and was overgrown by allanite and/or allanite/epidote/apatite coronas within retrograde muscovite- and/or chlorite-bearing shear zones. This study documents polyphase, complex monazite growth and dissolution during a single, relatively short-lived metamorphic cycle.

Separating metamorphic events in the Fosdick migmatite–granite complex, West Antarctica

AbstractThe Fosdick migmatite–granite complex in West Antarctica records evidence for two high‐temperature metamorphic events, the first during the Devonian–Carboniferous and the second during the Cretaceous. The conditions of each high‐temperature metamorphic event, both of which involved melting and multiple melt‐loss events, are investigated using phase equilibria modelling during successive melt‐loss events, microstructural observations and mineral chemistry. In situ SHRIMP monazite and TIMS Sm–Nd garnet ages are integrated with these results to constrain the timing of the two events. In areas that preferentially preserve the Devonian–Carboniferous (M1) event, monazite grains in leucosomes and core domains of monazite inclusions in Cretaceous cordierite yield an age of c. 346 Ma, which is interpreted to record the timing of monazite growth during peak M1 metamorphism (∼820–870 °C, 7.5–11.5 kbar) and the formation of garnet–sillimanite–biotite–melt‐bearing assemblages. Slightly younger monazite spot ages between c. 331 and 314 Ma are identified from grains located in fractured garnet porphyroblasts, and from inclusions in plagioclase that surround relict garnet and in matrix biotite. These ages record the growth of monazite during garnet breakdown associated with cooling from peak M1 conditions. The Cretaceous (M2) overprint is recorded in compositionally homogeneous monazite grains and rim domains in zoned monazite grains. This monazite yields a protracted range of spot ages with a dominant population between c. 111 and 96 Ma. Rim domains of monazite inclusions in cordierite surrounding garnet and in coarse‐grained poikiloblasts of cordierite yield a weighted mean age of c. 102 Ma, interpreted to constrain the age of cordierite growth. TIMS Sm–Nd ages for garnet are similar at 102–99 Ma. Mineral equilibria modelling of the residual protolith composition after Carboniferous melt loss and removal of inert M1 garnet constrains M2 conditions to ∼830–870 °C and ∼6–7.5 kbar. The modelling results suggest that there was growth and resorption of garnet during the M2 event, which would facilitate overprinting of M1 compositions during the M2 prograde metamorphism. Measured garnet compositions and Sm–Nd diffusion modelling of garnet in the migmatitic gneisses suggest resetting of major elements and the Sm–Nd system during the Cretaceous M1 overprint. The c. 102–99 Ma garnet Sm–Nd ‘closure’ ages correspond to cooling below 700 °C during the rapid exhumation of the Fosdick migmatite–granite complex.

Implications of garnet resorption for the Lu-Hf garnet geochronometer: an example from the contact aureole of the Makhavinekh Lake Pluton, Labrador

In the contact aureole of the Makhavinekh Lake Pluton (MLP), Labrador, garnet resorption caused redistribution of Lu and loss of Hf, creating spuriously young Lu-Hf garnet ages. Garnet grew during granulite facies regional metamorphism at 1860-1850 Ma. At 1322 Ma, garnet rims were replaced by coronas of cordierite and orthopyroxene during contact metamorphism. Garnet-ilmenite Lu-Hf geochronology using bulk-garnet separates yields apparent ages that young from 1876 ± 21 Ma at 4025 m from the contact to 1396 ± 8 Ma at 450 m from the contact. Toward the contact, garnet crystals are progressively more resorbed. Concentrations of Lu measured by LA-ICP-MS along radial traverses on central sections through relict garnet decrease gently away from the cores but rise steeply within 50-200 lm of the edges of the relicts. Enrichments of Lu in rims of relict garnet demonstrates strong partitioning of Lu into garnet during resorption and modest intracrystalline diffusion. Hafnium distributions could not be measured, but considering the strong incompatibility of Hf with garnet, it is likely that nearly all Hf in resorbed portions of the garnet was lost from the crystals. Lu-Hf ages in the aureole are thus controlled predominantly by this retention of Lu and loss of Hf during garnet resorption. This deduction was tested with a simple numerical model in which the partial retention of Lu and loss of Hf is tracked as a population of garnet is resorbed. Assuming a spherical geometry for garnet porphyroblasts, Rayleigh fractionation is used to approximate initial Lu zoning profiles ranging from flat to steeply decreasing toward garnet rims. The model simulates: (i) Lu-Hf decay for a specified period before resorption; (ii) instantaneous resorption with retention of Lu and loss of Hf from the resorbed portion of the crystal and (iii) Lu-Hf decay during a specified period after resorption. Several parameters influence the modelled age, but garnet resorption and Lu retention are the primary factors. When all other parameters are held constant, larger amounts of resorption and higher degrees of Lu retention produce younger apparent ages (false ages). Similarly, flatter initial Lu profiles yield younger apparent ages as a consequence of the larger proportion of Lu and Hf that resides in the outer portions of the porphyroblast. The difference between the apparent and actual ages is greater if the duration of the pre-resorption decay period is large relative to the post-resorption decay period. Larger crystals in a Gaussian crystal-size distribution (CSD) generally dominate the Lu-Hf budget and produce an older apparent age relative to the age of the mean crystal size. Compared to a symmetrical Gaussian CSD, positively skewed CSDs result in reduced resorption of large crystals and produce an older apparent age. Application of the model to the MLP aureole, positing growth at 1850 Ma and resorption at 1320 Ma, yields model ages that young from 1850 to 1374 Ma toward the contact, in good agreement with the apparent ages determined from geochronology.