Garnet Breakdown Research Articles

Silurian high-pressure granulites from Central Qiangtang, Tibet: Constraints on early Paleozoic collision along the northeastern margin of Gondwana

High-pressure (HP) granulites are commonly regarded as indicators of plate convergence and collision following the subduction of oceanic or continental crust. In this study we report the discovery of Silurian HP basic granulites from Central Qiangtang on the Tibetan Plateau. Detailed petrology and geochronology reveal a three-stage metamorphic history based on inclusions, reaction textures, and garnet zoning patterns. Peak metamorphism at 830–860 °C and 1.15–1.45 GPa (M1) is defined by high-Ca garnet cores, high-Al clinopyroxene, and high-Na plagioclase. Symplectites or coronas of orthopyroxene + plagioclase ± magnetite around garnet porphyroblasts indicate garnet breakdown reactions at ca. 810–830 °C and 0.65–0.85 GPa (M2). Kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 590–650 °C and 0.62–0.82 GPa (M3). These results help define a sequential P–T path containing near-isothermal decompression (ITD) and near-isobaric cooling (IBC) stages. Identification of mineral inclusion assemblages in zircons dated by U–Pb SHRIMP and LA–ICP–MS reveals peak HP metamorphism at ca. 427–422 Ma, subsequent near-isothermal decompression with associated retrograde reactions at ca. 392–389 Ma, and continued cooling at ca. 360 Ma. The P–T–t path of HP basic granulites reflects collision followed by extensional exhumation during early Paleozoic orogenesis. The present results indicate the occurrence of a collisional event along the northern margin of Indo-Australian Gondwana during the Silurian. Renewed Gondwana-directed subduction and subsequent collision probably led to the opening of the Paleo-Tethys Ocean.

Lawsonite geochemistry and stability – implication for trace element and water cycles in subduction zones

AbstractThis contribution reviews the existing data on lawsonite stability and trace element geochemistry, and provides new data for metabasaltic and metasedimentary (quartzite) rocks from New Caledonia, Turkey and California. Lawsonite is a major host of REE, Sr, U, Th and Pb in basaltic compositions. Trace element‐rich lawsonite also occurs in metasedimentary rocks, in which comparatively fewer phases compete for trace elements than in metabasaltic rocks. Trace element patterns in lawsonite are influenced by the coexistence or breakdown of allanite, titanite, apatite and garnet that compete for these elements in high‐P metamorphic rocks. Lawsonite is restricted to cool geotherms and therefore is an indicator mineral for subduction‐zone metamorphism. The lawsonite stability field shows a strong dependence on temperature and composition and it is largest in rocks with a high normative anorthite content and, in basaltic systems, carbon content. Along cold geotherms, lawsonite can transport water and trace elements to great depths, providing a source for these elements in the deep mantle. Along warmer geotherms, lawsonite disappears on a continuous reaction, gradually releasing water over a temperature interval of several tens of degrees. During lawsonite breakdown in complex systems, Th and LREE remain trapped in newly formed accessory allanite. However, owing to extreme LREE content, allanite has lower Pb/Ce and Sr/Nd than lawsonite, resulting in a relative enrichment of Sr and Pb compared with Ce and Nd in the fluids produced during lawsonite breakdown. Existing experimental data on the solidus of altered oceanic crust suggest that the lawsonite‐breakdown reaction is within 50 °C of the solidus at sub‐arc pressures of 3–4 GPa.

Experimental synthesis of isochemical kelyphite — a preliminary report

We observed the breakdown of natural garnet in a high–pressure experiment (0.85 GPa and 1100 °C for 2 days) using a piston–cylinder apparatus. Two different types of kelyphites were formed: (1) isochemical and (2) metasomatic. The isochemical kelyphite was composed of orthopyroxene + spinel + plagioclase, whereas the metasomatic one, having a composition altered significantly from the original garnet, was composed of olivine + spinel + plagioclase. These synthetic kelyphites were studied and characterized using EPMA, SEM, EBSD, and TEM and were shown to have many microstructural features as well as their mineral assemblages in common with observed natural kelyphites. The kelyphitization was very heterogeneous and local and was apparently controlled by grain surfaces and intra–grain cracks in garnet. The metasomatic kelyphite apparently formed by a reaction between garnet and aqueous fluids that may have been introduced from outside the platinum capsule through holes accidentally made in the capsule.

Metamorphic P–T path and tectonic implications of pelitic granulites from the Daqingshan Complex of the Khondalite Belt, North China Craton

Pelitic granulites are cropped out in the Daqingshan Complex of the Khondalite Belt, a Paleoproterozoic tectonic belt in the Western Block of the North China Craton. Petrological studies show that these granulites contain four distinct metamorphic assemblages. The core of a garnet porphyroblast, along with fine-grained inclusions of biotite+plagioclase+K-feldspar+quartz±sillimanite±ilmenite, defines the prograde metamorphic (M1) stage. The peak (M2) assemblage consists of garnet (mantle)+biotite+K-feldspar+plagioclase+quartz±sillimanite±orthopyroxene±ilmenite±magnetite±rutile. Peak metamorphism was followed by a near-isothermal decompression (M3) and the development of coronae of garnet+biotite+cordierite+plagioclase+quartz±K-feldspar±sillimanite±spinel±ilmenite±magnetite (M3) in the Crd–Grt–Bt gneisses during the following garnet breakdown reactions: garnet+sillimanite+melt→cordierite+biotite+Fe-oxide and garnet+melt→biotite+quartz+plagioclase. Retrograde cooling (M4) assemblages are represented by biotite+muscovite+sillimanite+quartz+plagioclase+K-feldspar±ilmenite. Quantitative phase equilibria modeling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 was applied to obtain P–T conditions of <780°C and <9kbar for M1, 840–880°C and 9–11kbar for M2, 800–870°C and 5.0–7.5kbar for M3, and <660°C and 4.1–6.9kbar for M4. The combination of the mineral assemblages, mineral compositions, and metamorphic reaction histories in the Daqingshan pelitic granulites defines a clockwise P–T path that involves periods of near-isothermal decompression and late cooling that followed the peak granulite-facies metamorphism. This result is consistent with the tectonic history of the Daqingshan Complex in the Khondalite Belt, which involved continent–continent subduction and collision followed by exhumation and cooling. This suggests a continent–continent collisional event between the Yinshan and Ordos blocks, which became amalgamated to form the Paleoproterozoic Khondalite Belt in the Western Block of the North China Craton.

A P–T–t–D discontinuity in east-central Nepal: Implications for the evolution of the Himalayan mid-crust

Rock specimens collected from lower, middle, and upper portions of the exhumed Himalayan mid-crust in the upper Tama Kosi region of east-central Nepal yield pressure–temperature–time–deformation (P–T–t–D) paths that demonstrate the presence of a structural, metamorphic, and geochronologic discontinuity. A P–T–t–D path from below the discontinuity defines a loading path with deformation at staurolite-grade metamorphic conditions occurring at ~10–8Ma. P–T–t–D paths from above the discontinuity, in contrast, record an earlier and protracted metamorphic history that includes decompressional heating. Monazite ages from specimens collected above the discontinuity constrain prograde garnet growth to be >21Ma with decompression-related garnet breakdown initiating at c.19Ma and likely continuing until c.15Ma. These contrasting P–T–t–D histories separated by a tectonometamorphic discontinuity are consistent with an evolutionary model for the Himalaya in which rocks above the discontinuity were metamorphosed in the deep hinterland of the orogen and deformed during lateral extrusion from beneath Tibet while rocks below the discontinuity were deformed and metamorphosed later within the shallower foreland of the orogen. These results demonstrate the spatial and temporal relationships and compatibility between lateral midcrustal extrusive flow and wedge taper processes within an evolving orogen.

Discrimination of protolithic versus metamorphic zircon ages in eclogites: Constraints from the Erzgebirge metamorphic core complex (Germany)

Protolithic ages of meta-basites are crucial for tectonic reconstructions as they constrain the lifetime of oceans. However, in high-grade metamorphic regions most isotope systems record metamorphic ages due to the metamorphic overprint. The mineral zircon is the most suitable geochronometer for the determination of protolithic ages in such rocks due to its high closure temperature. Zircon from eclogites from the Eastern Erzgebirge metamorphic complex yielded both protolithic (~540Ma; 5 samples) and metamorphic (~340Ma; 4 samples) 207Pb/206Pb ages. Detailed investigations of zircon grains within thin rock sections show that the metamorphic ages are related to new zircon growth caused by water infiltration that triggered decomposition of omphacite into an amphibole–plagioclase symplectite. Rutile and apatite frequently grew together with metamorphic zircons indicating local mobilization of Zr, Nb, LREE and P. Eclogites with omphacite breakdown are very dark in colour. In difference, lighter coloured eclogites contain K-feldspar and quartz indicating minor melt infiltration that caused breakdown of garnet and transformation of amphibole into biotite. This supports additional new growth of zircon grains. In some samples idiomorphic zircon grains with a lot of micro-inclusions indicate a coupled dissolution–reprecipitation process. Additional rounded-resorbed and highly fractured zircon grains together with internal zircon textures imply multiple melt infiltration triggering multiple phases of zircon growth. Therefore, scattering zircon ages may reflect multiple growth stages or inheritance rather than prolonged periods of metamorphic growth. In contrast, protolithic zircon ages were found in eclogites where no water or melt infiltration occurred. This emphasizes the importance of water and/or melt infiltration for the new growth of zircon grains in eclogites as was already shown for granulites in various studies. In the Eastern Erzgebirge, most eclogites and amphibolites were derived from MORB-type or arc-related basalts while the plagioclase-rich amphibolites formed mainly from crustal sources. The protolith ages of meta-basites from the Eastern Erzgebirge (~540Ma) document a bimodal magmatic activity in a Cadomian back-arc basin.

Contrasting mafic to felsic HP-HT granulites of the Blanský les Massif (Moldanubian Zone of southern Bohemia): complexity of mineral assemblages and metamorphic reactions

*Corr esponding author A detailed petrological and geochemical study of the Blanský les Granulite Massif (BLGM; Moldanubian Zone of the Bohemian Massif) provides evidence for processes and mineral assemblages previously unrecognized, or not fully appreciated, in the South Bohemian HP–HT Granulite Complex. It also underlines the “patchwork”-like nature of the BLGM, caused by distinct protolith compositions as well as the differences in occurrence, or preservation, of mineral assemblages, reaction textures, and/or variable changes in chemical compositions. Such a complexity reflects super position of several processes operating from the eclogite-facies metamorphic peak to the final emplacement, as well as variable resistance of individual rock types with distinct metamorphic fabrics. Even on a thin section scale, domains preserving evidence for early phases of eclogite-facies metamorphism, including original textural relations and largely intact HP–HT mineral assemblages, occur in close proximity to domains dominated by phases produced during later decompression. Consistent high-pressure estimates, 2.2–2.3 GPa (GASP), are newly obtained for S 1 /S 2 fabrics in the prevailing (type 1) felsic granulites, and correlate well with the recent finds of rare omphacite relics in the mafic granulites. It indicates that much of the HP history of the felsic and mafic rock types was shared and took place under eclogite-facies conditions. Three important metamorphic processes leading to chemical changes in felsic granulites are described. (1) The small bodies of Kfs-dominated hyperpotassic granulites with minor Grt (or, rarely, Di) are fairly common and provide direct evidence for operation of non-eutectic HP melting. These occurrences are accompanied by compositionally complementary residual (type 2) felsic granulites, rich in Qtz and Ky and partly depleted in alkalis with Zr. Owing to their Ca-poor composition, type 2 granulites were not involved in some of the decompression reactions, such as formation of plagioclase rims around kyanite and garnet breakdown, and thus their mineral assemblages remained effectively “frozen” during decompression. They further imply that the partial melting and genesis of the hyperpotassic melts took place under eclogite-facies conditions. (2) Another case of syn-metamorphic modification is represented by a local limited base-cation loss during superimposed amphibolite-facies de-alkalization, accompanying fluid flux along S 3 shear domains (formation of feldspar-free Qtz–Sill bands). (3) On a microscale, the retrogression reactions, often incomplete, were clearly associated with transfer of ions. The felsic Moldanubian HP–HT granulites were assumed by several authors to have undergone only limited modification during peak metamorphism, most notably loss of Cs, Th and U. Clearly, some of them may have suffered more significant changes, such as perceptible loss of alkalis or even relatively immobile elements (e.g., zirconium), compared to the likely primary protolith(s) composition.

A window into the Early to mid‐Cretaceous infrastructure of the Yukon‐Tanana terrane recorded in multi‐stage garnet of west‐central Yukon, Canada

AbstractAmphibolite facies metasedimentary schists within the Yukon‐Tanana terrane in the northern Canadian Cordillera reveal a two‐stage, polymetamorphic garnet growth history. In situ U‐Th‐Pb Sensitive High Resolution Ion Microprobe dating of monazite provide timing constraints for the late stages of garnet growth, deformation and subsequent decompression. Distinct textural and chemical growth zoning domains, separated by a large chemical discontinuity, reveal two stages of garnet growth characterized in part by: (i) a syn‐kinematic, inclusion‐rich stage‐1 garnet core; and (ii) an inclusion‐poor, stage‐2 garnet rim that crystallized with syn‐ to post‐kinematic staurolite and kyanite. Phase equilibria modelling of garnet molar and compositional isopleths suggest stage‐1 garnet growth initiated at ~600 °C, 8 kbar along a clockwise P–T path. Growth of the compositionally distinct, grossular‐rich, pyrope‐poor inner portion of the stage‐2 overgrowth is interpreted to have initiated at higher pressure and/or lower temperature than the stage‐1 core along a separate P–T loop, culminating at peak P–T conditions of ~650–680 °C and 9 kbar. Stage‐2 metamorphism and the waning development of a composite transposition foliation (ST) are dated at c. 118 Ma from monazite aligned parallel to ST, and inclusions in syn‐ to post‐ST staurolite and kyanite. Slightly younger ages (c. 112 Ma) are obtained from Y‐rich monazite that occurs within resorbed areas of both stage‐1 and stage‐2 garnet, together with retrograde staurolite and plagioclase. The younger ages obtained from these texturally and chemically distinct grains are interpreted, with the aid of phase equilibria calculations, to date the growth of monazite from the breakdown of garnet during decompression at c. 112 Ma. Evidence for continued near‐isothermal decompression is provided by the presence of retrograde sillimanite, and cordierite after staurolite, which indicates decompression below ~4–5 kbar prior to cooling below ~550 °C. As most other parts of the Yukon‐Tanana terrane were exhumed to upper crustal levels in the Early Jurassic, these data suggest this domain represents a tectonic window revealing a much younger, high‐grade tectono‐metamorphic core (infrastructure) within the northern Cordilleran orogen. This window may be akin to extensional core complexes identified in east‐central Alaska and in the southeastern Canadian Cordillera.

Garnet Breakdown, Symplectite Formation and Melting in Basanite-hosted Peridotite Xenoliths from Zinst (Bavaria, Bohemian Massif)

Complex, symplectite-bearing pseudomorphs after garnet recently found in unique basanite-hosted peridotite xenoliths from Zinst, Bavaria, allow the study of the interaction between garnet peridotite and melts or fluids both prior to entrainment of the xenoliths and during their ascent. Based on microstructures and crystallographic fabric, and major and trace element mineral chemistry, four distinct concentric zones were defined in various types of pseudomorph: Zone I, coarse-grained (1 mm) aggregate of orthopyroxene + clinopyroxene + spinel with a granular structure; Zone II, fine- to medium-grained (order of 10-100 mu m) orthopyroxene + spinel symplectite; Zone III, fine-grained (5-300 mu m), radially fibrous orthopyroxene + spinel symplectite with interstitial anorthite; Zone IV, ultrafine-grained (1 mu m) orthopyroxene + spinel + anorthite symplectite with an internal domain substructure. Zones III and IV have bulk compositions of pyrope-rich garnet. All zones exhibit perfect inter-sample correlation and document the discontinuous evolution of peridotite under changing conditions with successively increasing rates of garnet breakdown. Based on thermometry and microstructural relations, a sequence of three pre- and syn-volcanic events is discerned. The first traceable event corresponds to regional heating in the uppermost mantle probably related to the early stages of Tertiary rifting, which triggered the reaction between garnet and olivine (Zone I) leading to a partial re-equilibration of the rock at 1040-1080 degrees C within the spinel peridotite stability field. Subsequently a short period of heating by similar to 100-250 degrees C led to largely isochemical, fluid-mediated in situ melting of garnet and to the formation of kelyphite by crystallization from the melt (Zone III). The subsequent metasomatic alteration by external, Na-rich, K-poor, carbonate-bearing melts or fluids suggests that this phase of garnet breakdown occurred largely prior to formation of the xenolith, preceding the emplacement of the basanite magma. Finally, after xenolith formation, and associated with rapid, isochemical, decompression during exhumation, the garnet relics were transformed into microsymplectite (Zone IV). The positive volume change associated with this reaction caused fracturing, producing radial cracks that emanate from Zone IV and extend into the adjacent peridotite, allowing infiltration of basanite-derived melt components. The well-developed and clearly separated symplectite zones indicating the isochemical breakdown of garnet are uncommon in garnet peridotites worldwide. Their existence at Zinst is explained by an extremely short time span between the formation of the kelyphite, metasomatism by Na- and carbonate-rich agents and the final garnet breakdown during the host basanite eruption, allowing for rapid quenching of the multiple advancing reaction fronts.

Ultra-high temperature granulite-facies metamorphic rocks from the Mozambique belt of SW Tanzania

The metamorphic rocks in the Neoproterozoic (Pan-African) Mozambique belt of southwestern Tanzania, around the town of Songea, can be subdivided into one- and two pyroxene bearing charnockitic gneisses, migmatitic granitoid gneisses and amphibolite-facies metapelites. Lower-grade amphibolite-facies rocks are rare and can be classified as sillimanite- and/or garnet-bearing metapelites. Most of the studied charnockitic gneisses show excellent corona textures with large orthopyroxene grains rimmed by clinopyroxene, followed by quartz and well developed garnet rims due to the reaction Opx+Pl=Grt+Cpx+Qtz that formed during isobaric cooling. These and other charnockitic gneisses show symplectites of orthopyroxene and An-rich plagioclase that resulted from the breakdown of garnet during isothermal decompression due to the reaction Grt+Cpx+Qtz=Opx+Pl. Geothermobarometric calculations yield up to ~1050°C and up to ~12kbar for peak metamorphic conditions. These are higher temperature and slightly lower pressure conditions than reported for other granulite-facies terrains in the Mozambique belt of Tanzania. Single zircon Pb–Pb evaporation and U-Pb SHRIMP ages for magmatic zircons extracted from two charnockitic and two granitic gneisses cluster in two groups, one at ~750Ma and one at ~1150Ma with the older reflecting the time of emplacement of the igneous precursors, and the younger approximating the time of charnockitization. These protolith ages are similar to those farther east in the Masasi area of southern Tanzania, as well as in northern Mozambique and in southern Malawi, and suggest that the Mozambique belt consists of chronologically heterogeneous assemblages whose pre-metamorphic tectonic setting remains obscure.

Geochemistry and petrology of pyroxenite xenoliths from Cenozoic alkaline basalts, Bohemian Massif

Pyroxenites occur as rare, but important, mantle xenoliths within Cenozoic volcanic rocks of the Central European Volcanic Province (CEVP). We report the petrography, geothermobarometry, mineral chemistry as well as clinopyroxene trace-element and Sr-Nd isotopic compositions of six pyroxenite xenoliths hosted by Tertiary-Quaternary volcanic rocks from Kozakov (NE Bohemia), Dobkovicky and Kuzov (Ohre/Eger Rift), and Lutynia (SW Poland). Three Kozakov xenoliths record a complex nature and evolution. As suggested by contrasting estimated temperatures (596-663 degrees C and 1008-1067 degrees C) and textures, olivine clinopyroxenite and one websterite were likely derived from different depths and may represent crystallization products of transient melts. Another websterite contains symplectite pseudomorphs after garnet which originated by exsolution from highly aluminous clinopyroxene, and the garnet breakdown was associated with melt introduction. This websterite P-T conditions (17.3 to 21.4 kbar and 1080-1200 degrees C) correspond to derivation from depths of 55-69 km. The composition of websterite from Dobkovicky suggests its origin as a cumulate from melt derived from a highly depleted mantle source with a composition similar to Depleted MORB Mantle (DMM). The position and petrological significance of websterite from Kuzov remains unclear because of a lack of data from associated peridotites. At Lutynia, cryptic metasomatism of peridotites by a CO2-bearing alkaline melt was described previously, and the composition of melt calculated to be in equilibrium with Lutynia websterite clinopyroxene is very similar to that in equilibrium with metasomatized amphibole-bearing peridotite. Therefore, the Lutynia websterite may represent a cumulate from percolating melt that metasomatized the lithospheric mantle in this area.

Isochemical breakdown of garnet in orogenic garnet peridotite and its implication to reaction kinetics

An isochemical kelyphite (orthopyroxene+spinel+plagioclase) that has nearly the same bulk chemical composition as the precursor garnet was found within a matrix of ordinary kelyphites (orthopyroxene+clinopyroxene+spinel±amphibole) in garnet peridotites from the Czech part of the Moldanubian Zone. It was shown that the kelyphitization of garnet took place in three stages: (1) the garnet-olivine reaction, accompanied by a long-range material transfer across the reaction zone, and (2) the isochemical breakdown of garnet, essentially in a chemically-closed system, and finally, (3) an open-system hydration reaction producing a thin hydrous zone (amphibole+spinel+plagioclase), which is located between the isochemical kelyphite and relict garnet. The presence of relict garnet suggests that this breakdown reaction of the second stage did not proceed to a completion probably being hindered by the formation of the hydrous zone at the reaction front. It was found by electron back-scattered diffraction method that orthopyroxene and spinel do not show any topotaxic relationship in the first type of kelyphite; whereas they show locally topotaxic relationship in the isochemical kelyphite. The transition from the first type to the second type of kelyphite is discussed on the basis of the detailed observations in the transition zone between the two kelyphites. More widespread occurrence of isochemical kelyphite is expected to occur in orogenic peridotites as well as from xenoliths brought by volcanics.

Petrology, geochemistry and Re–Os isotopes of peridotite xenoliths from Yantai, Shandong Province: Evidence for Phanerozoic lithospheric mantle beneath eastern North China Craton

Petrology, geochemistry and Re–Os isotopes of peridotite xenoliths from Yantai (Shandong Province) are reported in this paper with aims of constraining the age and evolution of the lithospheric mantle beneath eastern North China Craton. The Yantai xenoliths contain predominant harzburgites and subordinate lherzolites. Although their highly incompatible element compositions have been modified by metasomatism, the heavy rare earth element (REE) and Y contents in the Yantai peridotites are primarily governed by partial melting, which started in garnet stability field then continued in spinel stability field after breakdown of garnet to two pyroxenes and spinel. Such a polybaric melting produced a residual mantle in which degree of depletion decreases with depth. Os isotopic ratios of the most refractory peridotites (Al2O3<1.2wt.%) range from 0.117 to 0.126, yielding TRD model ages between 0.5 and 1.7Ga. This suggests co-existence of Phanerozoic and Proterozoic mantle beneath Shandong Province. Alternatively, the whole lithospheric mantle beneath Yantai was likely formed during the Phanerozoic, given the resemblance of their Os isotopic ratios with those of abyssal peridotites. The latter interpretation is consistent with the fact that all the studied samples plot along the oceanic trend in a plot of forsterite content in olivine versus olivine mode. It also gains further support from the contrasting εNd between the late Mesozoic lithospheric mantle and Cenozoic mantle beneath the region. The data presented in this study therefore argue for a complete replacement of the cratonic mantle by upwelling asthenosphere.

The tectonometamorphic evolution of Southampton Island, Nunavut: Insight from petrologic modeling and in situ SHRIMP geochronology of multiple episodes of monazite growth

Petrologic modeling and in situ SHRIMP monazite geochronology reveal a complex, polycyclic tectonometamorphic history for Southampton Island, Nunavut, that provides further insights into the assembly of the Nuna supercontinent. Although early, cryptic metamorphic events at ca. 2.6–2.5 and 2.3Ga are indicated by monazite ages in an Archean metasedimentary sample, Paleoproterozoic tectonometamorphism is widespread and most profound. Monazite inclusions in garnet from two samples define a lower amphibolite- to upper amphibolite-facies M1–D1 event between 1883±9 and 1879±7Ma. These sampled rocks, as well as pre-1900Ma magmatic rocks, occur on the west side of the island's exposed Precambrian bedrock, outboard of the Hearne craton but on strike with its geophysically defined eastern edge beneath Hudson Bay. This timing and geographic distribution, as well as the orientation of S1 foliations, are consistent with the model of a ca. 1900Ma Rae–Hearne collision, with M1–D1 on Southampton Island reflecting reworking of the Rae upper plate following northeast-directed subduction.Textural variations among monazite dated in two samples tightly bracket the initiation of M2–D2 between 1868±5Ma and 1861±4Ma. M2–D2 monazite crystallized at 1851±5Ma in one sample with a more calcic composition interpreted to have shifted the allanite to monazite reaction to higher temperatures relative to less calcic rocks. Clockwise P–T paths in these recumbently folded high-grade gneisses support a collisional setting, with the timing of M2–D2 consistent with age constraints on Baffin Island for the accretion of Meta Incognita microcontinent to the southeast flank of the Rae craton. The Rae crust of Southampton Island is interpreted as the upper plate during this collision on the basis of the ∼10Myr earlier deformation on Southampton Island compared to the interpreted lower plate (Meta Incognita microcontinent), as well as textural observations in some samples that M2 metamorphism predated D2 deformation. Advective heat transfer would account for the relatively short time inferred between ca. 1870Ma collision and ca. 1868–1861Ma monazite growth during prograde metamorphism.Equant matrix monazite that overgrew the S2 foliation in one sample documents post-D2 metamorphism in the northern part of exposed basement at 1841±4Ma. Four samples from the south record post-D2 monazite growth between 1826±9 and 1815±7Ma. These ages highlight a thermal culmination associated with extensive crustal melting and ca. 1830–1820Ma plutonism attributed to a convergent setting culminating with ca. 1820Ma arrival of the Superior craton. Exhumation and cooling is dated by ca. 1790–1780Ma monazite associated with garnet breakdown.

Timing of eclogite-facies metamorphism of the Chuacús complex, Central Guatemala: Record of Late Cretaceous continental subduction of North America's sialic basement

A Late Cretaceous collision of the southernmost portion of the North American continental margin with an undetermined southern block was first established based on the sedimentation history of the plate's supracrustal cover, which is overthrust by harzburgite-dominated nappes of the Guatemala Suture Complex. The collision is also well registered in the metamorphic evolution of continental eclogites of the Chuacús complex, a geologic unit that represents Mesoproterozoic–Triassic sialic basement of North America at the boundary with the Caribbean plate. Garnet–clinopyroxene–phengite thermobarometry of eclogites hosted in Chuacús gneisses indicates near ultra-high-pressure conditions to ~700°C and ~2.1–2.4GPa. SHRIMP-RG U–Pb dating of eclogite metamorphic zircon yielded a 75.5±2Ma age (95% confidence level). Chondrite-normalized rare-earth element patterns of zircon lack Eu anomalies and show depletions in heavy rare earths, consistent with zircon growing in a plagioclase-free, garnet-rich, eclogite-facies assemblage. Additionally, a Sm–Nd clinopyroxene-two garnet–whole rock isochron from an eclogite band yielded a less precise but consistent age of 77±13Ma. The above features imply subduction to >60km depth of at least a portion of the North American sialic basement during Late Cretaceous collision.The Chuacús complex was overprinted by an amphibolite-facies event. For instance, mafic high-pressure paragneiss contains symplectite, resorbed garnet, and amphibole+plagioclase poikiloblasts. Zircon rims from the paragneiss sample show rare-earth patterns consistent with plagioclase growth and garnet breakdown. Their 74.5±3.5Ma SHRIMP-RG U–Pb age is therefore interpreted as the time of retrogression, which is consistent with previously published results.Within error, the ages of the eclogite-facies event and the amphibolite-facies retrogression are equivalent. Thus exhumation of the Chuacús slab from mantle to mid-crustal depth was quick, taking few million years. During exhumation, partial melting of Chuacús gneisses generated ubiquitous pegmatites. One of the pegmatites intruded the North Motagua mélange, which is a serpentinite-rich subduction complex of the Guatemala Suture Complex containing Early Cretaceous oceanic eclogites. U–Pb, Rb–Sr, and K–Ar ages of the pegmatite range ~76–66Ma. Thus initial juxtaposition of continental and oceanic high-pressure belts of the Guatemala Suture Complex predates Tertiary–present strike-slip faulting between the North-American and Caribbean plates.

Evidence for a Caledonian amphibolite to eclogite facies pressure gradient in the Middle Allochthon Lindås Nappe, SW-Norway

The Proterozoic anorthosite–mangerite–charnockite complex dominating the Lindås Nappe in the Scandinavian Caledonides was locally eclogitized in the southwestern part of the nappe during the Caledonian orogeny, whereas only amphibolite facies assemblages are recorded in the rest of the nappe. Sveconorwegian granulites of anorthositic to jotunitic composition in the northernmost eclogite-free exposures of the nappe exhibit large garnet phenoblasts (ca. 900°C) that are fractured and partly replaced by a Caledonian symplectitic amphibolite facies assemblage (ca. 515°C). Metamorphic zircon attributed to this garnet breakdown is dated by ID-TIMS U–Pb at 430 ± 3 Ma, suggesting that the amphibolite facies overprint was coeval with the formation of eclogite 30 km further south, probably implying that the section across the nappe represents a Caledonian pressure gradient. The rocks also preserve a complex Sveconorwegian history including an age of 969 ± 6 Ma, which we interpret as dating magmatic emplacement of jotunitic–anorthositic portions of the complex, 936 ± 12 Ma reflecting the granulite facies metamorphism, and 908 ± 16 Ma, representing a late generation of zircon best explained as having formed by metasomatic processes. Caledonian shearing severely deformed zircon grains in an amphibolite facies shear zone, resetting their U–Pb systems, and forming new ones, hereby also demonstrating a case of resetting and recrystallization of low-U zircon. Our data, gained from diverse lithologies, illustrate several processes involved in making and resetting zircon as well as indicate the contemporaneous evolution and similar origin of the Lindås Nappe and the Jotun Nappe Complex.

Spinel + quartz-bearing ultrahigh-temperature granulites from Xumayao, Inner Mongolia Suture Zone, North China Craton: Petrology, phase equilibria and counterclockwise p-T path

The Khondalite Belt within the Inner Mongolia Suture Zone (IMSZ) in the North China Craton is a classic example for Paleoproterozoic ultrahigh-temperature (UHT) metamorphism. Here we report new spinel-bearing metapelitic granulites from a new locality at Xumayao within the southern domain of the IMSZ. Petrological studies and thermodynamic modeling of the spinel + quartz-bearing assemblage shows that these rocks experienced extreme metamorphism at UHT conditions. Spinel occurs in two textural settings: 1) high XZn(Zn/(Mg + FeII + Zn) = 0.071–0.232) spinel with perthitic K-feldspar, sillimanite and quartz in the rock matrix; and 2) low XZn (0.045–0.070) spinel as inclusions within garnet porphyroblasts in association with quartz and sillimanite.Our phase equilibria modeling indicates two main stages during the metamorphic evolution of these rocks: 1) near-isobaric cooling from 975 °C to 875 °C around 8 kbar, represented by the formation of garnet porphyroblasts from spinel and quartz; and 2) cooling and decompression from 850 °C, 8 kbar to below 750 °C, 6.5 kbar, represented by the break-down of garnet. The spinel + quartz assemblage is considered to have been stable at peak metamorphism, formed through the break-down of cordierite, indicating a near isothermal compression process. Our study confirms the regional extent of UHT metamorphism within the IMSZ associated with the Paleoproterozoic subduction-collision process.

On the origin and fluid content of some rare crustal xenoliths and their bearing on the structure and evolution of the crust beneath the Bakony–Balaton Highland Volcanic Field (W-Hungary)

Rarely occurring clinopyroxene-plagioclase bearing, felsic granulite and skarn xenoliths were studied from the mantle and crustal xenolith-bearing alkaline basaltic and pyroclastic localities of the Bakony–Balaton Highland Volcanic Field (W-Hungary). Geobarometry and geothermometry of the xenoliths made it possible to categorise them in three groups according to their depth of formation. The first group formed in the lower crust together with mafic and metasedimentary granulites. The second group represents magmatic intrusions of the middle crust, and the third one comprises contact metamorphic rocks of relatively shallow origin. The calculated pressure difference from the core and rim compositions of plagioclase and clinopyroxene as well as garnet breakdown reactions in some xenoliths show evidence for pressure decrease due to crustal thinning both in lower crustal and middle crustal xenoliths during formation of the Pannonian Basin. Fluid inclusion studies reveal the dominance of the CO2-rich fluids in the whole crustal section in contrast with fluids found in mafic garnet-bearing xenoliths. Crustal stratigraphy was constructed for the periods prior to the extension and after the extension on the basis of geobarometry and geophysical data. On the basis of mineral stabilities and geothermo-barometry, we estimated that the pre-extensional thickness of the lower and upper crust may have been 27–34 and 26–28 km, respectively. Comparison of pre-extensional and present-day thickness of the lower and upper crust indicate that thinning affected both the lower and the upper portion of the crust but on a different scale. The calculated thinning factors are between 2.25 and 3.4 for the lower crust and 1.3–1.56 for the upper crust.

ざくろ石かんらん岩中に見いだされたざくろ石の等化学組成分解によって形成したケリファイト─その２：形成過程と形成条件についての再検討

ざくろ石かんらん岩中に見いだされたざくろ石の等化学組成分解によって形成したケリファイト─その２：形成過程と形成条件についての再検討

Two-stage partial melting and contrasting cooling history within the Higher Himalayan Crystalline Sequence in the far-eastern Nepal Himalaya

The timing of partial melting and the pressure–temperature (P–T) paths in the High Himalayan Crystalline Sequence (HHCS) in far-eastern Nepal has been investigated using zircon chronology, rare earth element (REE) compositions, and P–T pseudosection analysis. Zircon from migmatites formed during Himalayan thermal events displays inherited magmatic core overgrown by two generations of metamorphic rims. The new rims are distinguished on the basis of their Tertiary ages, low MREE contents, and low Th/U ratios. The inner zircon rims from Sil+Grt+Bt+Kfs+Pl+Qtz and Ky+Sil+Grt+Bt+Ms+Pl+Qtz migmatites at different structural level of the HHCS display ages of c. 33–28Ma (Early Oligocene) and c. 21–18Ma (Early Miocene): these rims are characterized by flat MREE to HREE patterns and were overgrown by partial melt through muscovite dehydration melting under the stability of garnet, which occurred at P=c. 7–10kbar and T=c. 730–780°C, and at P=c. 8–14kbar and T=c. 720–770°C, respectively. The outer zircon rims are relatively enriched in HREE with respect to the inner rims and were overgrown at c. 27–23Ma (Late Oligocene) and at c. 18–16Ma (Early Miocene) during melt crystallization accompanying breakdown of garnet at P=c. 4–7kbar and T=c. 650–725°C. Early Miocene Ms–Bt leucogranites with two successively overgrown zircon rims at c. 18.3±0.3Ma and c. 16.3±0.2Ma were intruded into Early Oligocene migmatite hosts. Microstructural observations and the corresponding P–T conditions associated with the two generations of zircon rims indicate that the Early Oligocene and Early Miocene migmatites show relatively isobaric and nearly isothermal P–T paths during exhumation, respectively. The inferences are consistent with higher average cooling rates for the Early Miocene (c. 30–40°C/My) than the Early Oligocene (c. 15–25°C/My) migmatites, inferred from peak-T conditions and FT (c. 6Ma for both migmatites) and U–Pb zircon ages. The P–T–t paths of the two migmatites indicate that burial of the Early Miocene migmatites has been coeval with exhumation of the Early Oligocene migmatites, implying the formation of large-scale thrust within the HHCS.