Omphacite Inclusions Research Articles

In situ U–Pb dating and trace element analysis of zircons in thin sections of eclogite: Refining constraints on the ultra high-pressure metamorphism of the Sulu terrane, China

In situ U–Pb dating and trace element analysis of zircons, combined with a textural relationship investigation in thin section, is a powerful tool to constrain the ultra high-pressure stage of high-grade metamorphism. Two types of zircon grains have been identified in thin sections of a retrograde eclogite from the main hole of the Chinese Continental Scientific Drill project in the Sulu UHP terrane. Type 1 zircon grains occur as inclusions in fresh garnet and omphacite, and Type 2 zircon grains were found in symplectite around omphacite. The fresh rims of Type 1 zircons and mantles of a few Type 2 zircons exhibit remarkably lower REE, Y, Nb and Ta contents than the inherited zircon cores, suggesting coeval growth with garnet, rutile and apatite during UHP metamorphism. These may have formed in the UHP metamorphism and survived retrograde metamorphism. The weighted average 206Pb/ 238U age of these zircon domains (230 ± 4 Ma, 2 σ) agrees well with the published age of coesite-bearing zircon separates (230 ± 1 Ma, 2 σ), suggesting that the peak UHP metamorphism in the Sulu terrane may have occurred at ~ 230 Ma. Zircon domains surrounded or cut across by symplectite could have been altered by retrograde metamorphism. Together, they provide a younger weighted average 206Pb/ 238U age of 209 ± 4 Ma (2 σ). These retrograde zircon domains have similar REE compositions to the ~ 230 Ma UHP zircon domains. These observations imply that the ~ 209 Ma zircon domains could have formed by fluid activity-associated alterations in the amphibolite-facies metamorphism, which could have resulted in the complete loss of Pb but not REEs in these domains.

Inclusion study in zircon from ultrahigh-pressure metamorphic rocks in the Kokchetav massif: an excellent tracer of metamorphic history

Abstract: Zircon is an excellent material to preserve the complex history of ultrahigh-pressure (UHP) metamorphic rocks, whereas mineralogical evidence of UHP conditions is mostly obliterated in matrix assemblages as a result of extensive retrograde overprinting during exhumation. Zircons from the Kokchetav UHP–HP massif contain numerous inclusions of graphite, quartz, garnet, omphacite, jadeite, phengite, phlogopite, rutile, albite, K-feldspar, amphibole, zoisite, kyanite, calcite, dolomite, apatite and monazite, as well as the diagnostic UHP minerals, such as microdiamond and coesite, which were identified by laser Raman spectroscopy. The internal structure of zircon displays a distinct zonation, which comprises an inherited core, a wide mantle and an outer rim, each with distinctive inclusion micro-assemblages. The low-pressure mineral inclusions, such as graphite, quartz and albite, are common in the inherited core and thin outer rim, whereas diamond, coesite and jadeite occupy the mantle domain. The zircon core and outer rim are of detrital and relatively low-grade metamorphic origin, whereas the mantle domain is of HP to UHP metamorphic origin. The mineral assemblages and chemistry of inclusions preserved in zircon have been used to constrain the metamorphic P – T path of the Kokchetav UHP–HP rocks, and indicate peak metamorphism at 60–80 kbar and 970–1100 °C followed by nearly isothermal decompression at 10 kbar and c . 800 °C. Sensitive high-resolution ion microprobe U–Pb spot analyses of the zoned zircon indicate four discrete ages of the Kokchetav metamorphic evolution: (1) a Middle Proterozoic protolith age; (2) 537 ± 9 Ma for UHP metamorphism; (3) 507 ± 8 Ma for the late-stage amphibolite-facies overprint; (4) 456–461 Ma for post-orogenic thermal events. This indicates that Middle Proterozoic supracrustal protoliths of the Kokchetav UHP–HP rocks were subducted to mantle depths in the Middle Cambrian, and exhumed to mid-crustal levels in the Late Cambrian. The zonal arrangement of inclusions and the presence of coesite and diamond without back reaction imply that aqueous fluids were low to absent within zircon, and that zircon is capable of retaining minerals of each metamorphic stage. We suggest that the study of inclusions in zircon is a powerful method to clarify the multiple stages and timing of metamorphic evolution of UHP–HP rocks, the evidence for which has been more or less obliterated in the host rock.

Retrogressed eclogite (20 kbar, 1020 °C) from the Neoproterozoic Palghat–Cauvery suture zone, southern India

The Palghat–Cauvery suture zone in southern India separates Archaean crustal blocks to the north and the Proterozoic Madurai block to the south. Here we present the first detailed study of a partially retrogressed eclogite (from within the Sittampundi anorthositic complex in the suture zone) that occurs as a 20-cm wide layer in a garnet gabbro layer in anorthosite. The eclogite largely consists of an assemblage of coexisting porphyroblasts of almandine–pyrope garnet and augitic clinopyroxene. However, a few garnets contain inclusions of omphacite. Rims and symplectites composed of Na–Ca amphibole and plagioclase form a retrograde assemblage. Petrographic analysis and calculated phase equilibria indicate that garnet–omphacite–rutile–melt was the peak metamorphic assemblage and that it formed at ca. 20kbar and above 1000°C. The eclogite was exhumed on a very tight hairpin-type, anticlockwise P–T path, which we relate to subduction and exhumation in the Palghat–Cauvery suture zone. The REE composition of the minerals suggests a basaltic oceanic crustal protolith metamorphosed in a subduction regime. Geological–structural relations combined with geophysical data from the Palghat–Cauvery suture zone suggest that the eclogite facies metamorphism was related to formation of the suture zone. Closure of the Mozambique Ocean led to development of the suture zone and to its western extension in the Betsimisaraka suture of Madagascar.

Age and genesis of the Myanmar jadeite: Constraints from U-Pb ages and Hf isotopes of zircon inclusions

Myanmar jadeite (jadeitite) is well known for its economical value and distinctive tectonic locality within the collisional belt between India and Eurasian plates. However, it is less studied for its genesis and geodynamic implications due to precipitous topography, adverse weather and local military conflicts in the area. By means of combined ICP-MS and LA-MC-ICPMS techniques, we have carried out in-situ trace elements, U-Pb and Lu-Hf isotopes for zircon inclusions in a piece of jadeite gem sample. CL imaging suggests that the zircons are metasomatic in origin, and contain mineral inclusions of jadeite and omphacite. Seventy-five analyses on 16 grains of the zircons yield a U-Pb age of 158 ± 2 Ma. The Myanmar zircons differ from other types in that they have no significant Eu anomalies despite high HREE concentrations. Measured 176Hf/177Hf ratios range from 0.282976 to 0.283122, with an average value of 0.283066 ± 7; eHf(t) value of 13.8 ± 0.3 (n=75). These results indicate that the Myanmar jadeite was formed in the Late Jurassic, probably by interaction of fluid released from subducted oceanic slab with mantle wedge. Therefore, its formation has no genetic relationship to the continental collision between Indian and Euroasian plates.

Conditions of formation of the Atbashi Ridge eclogites ( South Tien Shan)

New data on the conditions and time of formation of eclogites in the Atbashi Ridge (South Tien Shan) were obtained. The PT-conditions of crystallization of high-pressure minerals from eclogites have been established: 23–25 kbar, 510–570 °C. Analysis of fluid inclusions showed that the metamorphic processes of the eclogite formation involved salt-aqueous fluids (up to 6–12 wt.% NaCl). Primary fluid inclusions were trapped by omphacite and garnet on their high-pressure (23–25 kbar) crystallization. The pressures determined from thermodynamic data on the inclusions (6–7.2 kbar) reflect the adaptation of the host-mineral matrix to the decrease in ambient parameters, which led to the fluid re-equilibration in the inclusions. The pseudosecondary inclusions in omphacite, quartz, and garnet reflect the conditions of regressive processes running on the ascent of eclogites at pressures of up to 4.5–6.2 kbar. The stepwise 40Ar/ 39Ar dating showed that the high-pressure minerals from the Atbashi Ridge eclogites crystallized at 324–327 Ma. Data on trace and rare-earth elements indicate that the protoliths of two established types of eclogites (with characteristics of N-MORB and plateau basalts) formed synchronously as a result of the interaction of MOR magmatic systems with plateau-basaltic plume magmatism. These protoliths were basic paleo-oceanic complexes, whose fragments are localized as ophiolites in the Atbashi Ridge structures. The maximum time gap between the formation of ancient oceanic crust and the eclogite metamorphism in the subduction zones of the studied associations is ∼120 Myr.

Oriented quartz + calcic amphibole inclusions in omphacite from the Saualpe and Pohorje Mountain eclogites, Eastern Alps—An assessment of possible formation mechanisms based on IR- and mineral chemical data and water storage in Eastern Alpine eclogites

The composition of mineral phases and their modal proportions have been determined for three representative Eoalpine eclogites from the Saualpe type locality/Eastern Austria (sample SKP31) and the Pohorje Massif/Slovenia (CM31/03 and CM15/01) using electron microprobe, laser ICP-MS, IR spectroscopy and modal analysis to evaluate possible mechanisms for the formation of composite oriented calcic amphibole + quartz inclusions (COIs) in omphacite and to assess the relative importance of hydrous and nominally anhydrous phases as H 2O carriers in these eclogites. For omphacites in CM31/03 with a zonal distribution of COIs, a comparison of water and trace element concentrations of areas containing COIs and those free of COIs and a comparison with the trace element concentration of calcic amphibole indicate that COIs have formed through an open-system alteration of clinopyroxene and not through a closed system exsolution process. In sample SKP31, both textural and mineral chemical evidence suggests that COIs did not form by exsolution involving a Ca-Eskola component in clinopyroxene but formed by progressive growth under eclogite-facies P-T conditions and prior to the onset of retrogressive symplectite formation analogous to the formation of poikiloblastic quartz–calcic amphibole grains in the matrix. Bulk H 2O contents of the eclogites are between ca. 750 and 2150 ppm with 6–25% of the total water contributed by nominally anhydrous minerals (NAMs). Because of high modal amounts of 37–65%, omphacite is the major nominally anhydrous water carrier, containing 145–580 ppm H 2O with significant concentration variations on a thin section scale. Due to their very low H 2O concentrations of < 5–10 ppm (garnet, kyanite) or insignificant modal amounts ≤ 3% (rutile) the remaining NAMs contribute less than 1.5% to the bulk eclogite H 2O content. Calcic amphibole forming part of COIs may be a major carrier of H 2O as evidenced by CM31/03 containing both COIs and texturally primary calcic amphibole. In this sample calcic amphibole of the COIs contributes 63% whereas primary calcic amphibole only accounts for 13% of the bulk water. The relative order of H 2O concentrations in NAMs is H 2O rutile > H 2O omphacite ≫ H 2O garnet for CM31/03 and CM15/01 and H 2O rutile ≈ H 2O omphacite ≫ H 2O garnet in SKP31.

Constraining ultrahigh-pressure (UHP) metamorphism and titanium ore formation from an infrared microthermometric study of fluid inclusions in rutile from Donghai UHP eclogites, eastern China

Eclogites in the Donghai area of the Sulu ultrahigh-pressure (UHP) metamorphic belt in eastern China contain both disseminated rutile and rutile deposits. The rutile occurs as small inclusions in garnet, omphacite, phengite, and quartz, as larger euhedral to subhedral grains in equilibrium with garnet, omphacite, phengite, and kyanite, and as discrete veinlets, aggregates, and megacrysts in cracks and along grain boundaries. To investigate the processes of rutile mineralization and its relationship to the evolution of the Sulu UHP metamorphic belt, we sampled the exposed Maobei deposit at Donghai and drill core between 100 and 2000 m in the nearby Chinese Continental Scientific Drilling Project (CCSD) main hole to carry out a detailed infrared microthermometric fluid inclusion study. Three types of fluid inclusions were identified: aqueous (Type I), CO 2 -H 2 O (Type II), and CH 4 (Type III). Type I and Type II inclusions are the most common. Primary and pseudosecondary Type I inclusions, which were trapped during recrystallization of the eclogite or early amphibolite facies retrograde metamorphism, have a narrow range of homogenization temperatures but a wide range of salinities. In contrast, secondary Type I inclusions, which formed entirely during retrograde amphibolite-facies metamorphism associated with exhumation of the UHP rocks, have higher and more varied homogenization temperatures but medium to low salinities. Type II inclusions formed contemporaneously with the Type I variety and are characterized by having small amounts of CO 2 and N 2 in addition to H 2 O. In addition, some of the rutile grains contain sparse inclusions composed of CH 4 (Type III). Our results combined with those of previous studies, suggest a great diversity of fluid composition in the peak metamorphic to early retrograde stages, implying very limited fluid-rock interaction during syn- to post-peak high-pressure (HP) and UHP metamorphism. This study shows that infrared microthermometric investigation of fluid inclusions can provide information not only on the composition of titanium ore-forming fluids but also on the relationship between mineralization and the evolution of UHP metamorphic belt.

Lawsonite eclogites from the Pinchi Lake area, British Columbia—new P– T estimates and interpretation

Lawsonite eclogites from Pinchi Lake, British Columbia, occur as tectonic blocks associated with lawsonite, aragonite, and jadeite-bearing blueschists. One tectonic block from Tezzeron Lake shows extensive lower-temperature retrogression. The block from Beaver Lake Road (BLR) shows less retrogression and the garnet crystals contain abundant inclusions of lawsonite and omphacite suggesting these minerals were stable throughout most of the crystallization history. Compositional profiles and element maps of garnets show: (1) Mn enrichment in cores with decrease toward rims; (2) little change in Ca from core to rim; (3) general decrease in Fe/Mg from core to rim. Phase diagram sections for sample BLR-3 and garnet-core compositions suggest garnet-core growth at 505 °C and 24.3 kbar (1 kbar = 0.1 GPa). Temperature estimates based upon garnet–clinopyroxene Fe–Mg exchange geothermometry are, for inclusions in garnet at 25 kbar, 450 ± 30 °C (4 spots); 435 ± 15 (10); and 445 ± 25 (6). For garnet rims and matrix clinopyroxene the results are: 515 ± 50 °C (2); 440 ± 25 (5); 475 ± 20 (2). These results suggest very little if any statistically significant changes in temperature during crystal growth. Estimates of pressure are based upon several different calibrations of the garnet–clinopyroxene–phengite equilibria. Using the calibration of Krogh-Ravna we obtain pressures of: 24.2–26.8 kbar, compatible with that estimated for garnet-core growth in BLR-3. For the glaucophane = talc + jadeite geobarometer, the activity relations for glaucophane are not well constrained. At 450 °C a pressure of 26 kbar is calculated (one sample). A pressure of 25 kbar at 450 °C suggests depths of ~ 90 km, leading to a P– T gradient of ~ 5 °C km − 1 . This gradient passes through the inferred stability field of lawsonite eclogites and the calculated stability fields of lawsonite for Pinchi Lake samples. This is very near the P– T stability field of coesite, which is characteristic of Ultra-High-Pressure metamorphism.

Prograde pressure‐temperature path of jadeite‐bearing eclogites and associated high‐pressure/low‐temperature rocks from western Tianshan, northwest China

Abstract Jadeite‐bearing eclogites and associated blueschists locally crop out in a greenschist facies area at Kuldkourla, near the Akeyazhi River in the western Chinese Tianshan region, northwestern China. Garnet in these metamorphic rocks shows prograde zoning with increasing Mg and decreasing Mn from the crystal center towards the rim, and is divided into Ca‐poor/Fe‐rich core and Ca‐rich/Fe‐poor mantle parts. The garnet cores include the assemblages of (i) jadeite/omphacite (Xjd = 0.34–0.96) + barroisite/taramite; and (ii) omphacite + barroisite/pargasite, with paragonite, epidote, rutile and quartz as major phases with rare albite. The garnet mantles rarely contain inclusions of omphacite, glaucophane, epidote, rutile and quartz. Major matrix phases of the pre‐exhumation stage are omphacite, glaucophane, paragonite, rutile and quartz. These mineral parageneses give pressure (P)‐temperature (T) conditions of 0.9 GPa/390°C−1.4 GPa/560°C for the stage of the garnet core formation, 1.8 GPa/520°C for the stage of the garnet mantle formation, and 2.2 GPa/495°C‐2.4 GPa/535°C for the peak eclogite facies assemblage in the matrix. The estimated P‐T conditions and continuous changes of mineral parageneses imply a counterclockwise P‐T path which is a combination of (i) an early prograde stage of high‐pressure/low‐temperature (HP/LT) blueschist facies and/or LP/LT eclogite facies; (ii) a later prograde stage involving compression with minimal heating; and (iii) a climax‐of‐subduction stage characterized by a slight decrease of temperature with increasing pressure. The negative dP/dT of the latest subduction stage is possibly a record of the following events after a continuous subduction and ridge approach: (i) material migration within the upper part of the subducting slab, which has an inverse thermal gradient caused by ductile flow and/or slab break during subduction; and/or (ii) temporary cooling of the wedge mantle–slab interface by continuous subduction of a relatively cold slab following subduction of a hotter ridge.

Age and duration of eclogite-facies metamorphism, North Qaidam HP/UHP terrane, Western China

Amphibolite-facies para-and orthogneisses near Dulan, at the southeast end of the North Qaidam terrane, enclose minor eclogite and peridotite which record ultra-high pressure (UHP) metamorphism associated with the Early Paleozoic continental collision of the Qilian and Qaidam microplates. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. SHRIMP-RG U-Pb and REE analyses of zircons from four eclogites yield weighted mean ages of 449 to 422 Ma, and REE patterns (flat HREE, no Eu anomaly) and inclusions of garnet, omphacite, and rutile indicate these ages record eclogite-facies metamorphism. The coherent field relations of these samples, and the similar range of individual ages in each sample suggests that the ∼25 m.y. age range reflects the duration of eclogite-facies conditions in the studied samples. Analyses from zircon cores in one sample yield scattered 433 to 474 Ma ages, reflecting partial overlap on rims, and constrain the minimum age of eclogite protolith crystallization. Inclusions of Th + REE-rich epidote, and zircon REE patterns are consistent with prograde metamorphic growth. In the Luliang Shan, approximately 350 km northwest in the North Qaidam terrane, ages interpreted to record eclogite-facies metamorphism of eclogite and garnet peridotite are as old as 495 Ma and as young as 414 Ma, which suggests that processes responsible for extended high-pressure residence are not restricted to the Dulan region. Evidence of prolonged eclogite-facies metamorphism in HP/UHP localities in the Northeast Greenland eclogite province, the Western Gneiss Region of Norway, and the western Alps suggests that long eclogite-facies residence may be globally significant in continental subduction/collision zones.

Mineral and fluid inclusions in zircon of UHP metamorphic rocks from the CCSD-main drill hole: A record of metamorphism and fluid activity

The Chinese Continental Scientific Drilling (CCSD) main drill hole (0–3000 m) in Donghai, southern Sulu orogen, consists of eclogite, paragneiss, orthogneiss, schist and garnet peridotite. Detailed investigations of Raman, cathodoluminescence, and microprobe analyses show that zircons from most eclogites, gneisses and schists have oscillatory zoned magmatic cores with low-pressure mineral inclusions of Qtz, Pl, Kf and Ap, and a metamorphic rim with relatively uniform luminescence and eclogite-facies mineral inclusions of Grt, Omp, Phn, Coe and Rt. The chemical compositions of the UHP metamorphic mineral inclusions in zircon are similar to those from the matrix of the host rocks. Similar UHP metamorphic P– T conditions of about 770 °C and 32 kbar were estimated from coexisting minerals in zircon and in the matrix. These observations suggest that all investigated lithologies experienced a joint in situ UHP metamorphism during continental deep subduction. In rare cases, magmatic cores of zircon contain coesite and omphacite inclusions and show patchy and irregular luminescence, implying that the cores have been largely altered possibly by fluid–mineral interaction during UHP metamorphism. Abundant H 2O–CO 2, H 2O- or CO 2-dominated fluid inclusions with low to medium salinities occur isolated or clustered in the magmatic cores of some zircons, coexisting with low- P mineral inclusions. These fluid inclusions should have been trapped during magmatic crystallization and thus as primary. Only few H 2O- and/or CO 2-dominated fluid inclusions were found to occur together with UHP mineral inclusions in zircons of metamorphic origin, indicating that UHP metamorphism occurred under relatively dry conditions. The diversity in fluid inclusion populations in UHP rocks from different depths suggests a closed fluid system, without large-scale fluid migration during subduction and exhumation.

Ultrahigh-pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: II Oxygen isotope and fluid inclusion distributions through vertical sections

In order to reconstruct the formation and exhumation mechanisms of UHP metamorphic terrains, the Chinese Continental Scientific Drilling Program (CCSD) has been carried out in Donghai of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt, East China. Eclogite, gneiss, amphibolite (retrograded from eclogite), ultramafic rocks, and minor schist and quartzite have been drilled. Aiming to reveal the fluid behaviour in a vertical sequence of an UHP slab, we investigated fluid inclusion and oxygen isotope characteristics of selected drillcores from the main hole and the pilot-holes PP2 and ZK 703 of the CCSD. More than 540 laser-ablation oxygen isotope analyses on garnet, omphacite, quartz, kyanite, amphibole, phengite, rutile, epidote, amphibole, plagioclase, and biotite from various rocks in the depth range of 0–3,000 m (mainly eclogite and gneiss) show that the investigated rocks can be divided into two groups: 18O-depleted rocks (as low as δ18O = −7.4‰ for garnet) indicate interaction with cold climate meteoric waters, whereas 18O-normal rocks (with bulk δ18O > +5.6‰) have preserved the O-isotopic compositions of their protoliths. Meteoric water/rock interaction has reached depths of at least 2,700 m. Oxygen isotope equilibrium has generally been achieved. Isotopic compositions of mineral phases are homogeneous on a mm to cm scale regardless of lithology, but heterogeneous on the scale of a few metres. Oxygen isotope distributions in the vertical sections favour an “in situ” origin of the UHP metamorphic rocks. The very negative δ18O eclogites usually have higher hydroxyl-mineral contents than the normal δ18O rocks, indicating higher water content during UHP metamorphism. Fluid inclusion data suggest that rocks with depleted 18O compositions have had different fluid histories compared to those with normal δ18O values. Rocks with depleted 18O mainly have primary medium-to-high salinity inclusions in omphacite, kyanite and quartz, and abundant secondary low-salinity or pure water inclusions in quartz, indicating a high-salinity-brine-dominated fluid system during peak UHP metamorphism; no carbonic inclusions have been identified in these rocks. By contrast, primary very high-density CO2 inclusions are commonly found in the rocks with normal δ18O values. These observations suggest that fluid and oxygen isotope composition of minerals are related and reflect variable degrees of alterations of the Dabie-Sulu UHP metamorphic rocks.

Enigmatic diamonds in Archean calc-alkaline lamprophyres of Wawa, southern Ontario, Canada

A suite of 80 macrodiamonds recovered from volcaniclastic breccia of Wawa (southern Ontario) was characterized on the basis of morphology, nitrogen content and aggregation, cathodoluminescence (CL), and mineral inclusions. The host calc-alkaline lamprophyric breccias were emplaced at 2.68–2.74 Ga, contemporaneously with voluminous bimodal volcanism of the Michipicoten greenstone belt. The studied suite of diamonds differs from the vast majority of diamond suites found worldwide. First, the suite is hosted by calc-alkaline lamprophyric volcanics rather than by kimberlite or lamproite. Second, the host volcanic rock is amongst the oldest known diamondiferous rocks on Earth, and has experienced regional metamorphism and deformation. Finally, most diamonds show yellow-orange-red CL and contain mineral inclusions not in equilibrium with each other or their host diamond. The majority of the diamonds in the Wawa suite are colorless, weakly resorbed, octahedral single crystals and aggregates. The diamonds contain 0–740 ppm N and show two modes of N aggregation at 0–30 and 60–95% B-centers suggesting mantle storage at 1,100–1,170°C. Cathodoluminescence and FTIR spectroscopy shows that emission peaks present in orange CL stones do not likely result from irradiation or single substitutional N, in contrast to other diamonds with red CL. The diamonds contain primary inclusions of olivine (Fo92 and Fo89), omphacite, orthopyroxene (En93), pentlandite, albite, and An-rich plagioclase. These peridotitic and eclogitic minerals are commonly found within single diamonds in a mixed paragenesis which also combines shallow and deep phases. This apparent disequilibrium can be explained by effective small-scale mixing of subducted oceanic crust and mantle rocks in fast “cold” plumes ascending from the top of the slabs in convergent margins. Alternatively, the diamonds could have formed in the pre-2.7–2.9 Ga cratonic mantle and experienced subsequent alteration of syngenetic inclusions related to host magmatism and ensuing metamorphism. Neither orogenic nor cratonic model of the diamond origin fully explains all of the observed characteristics of the diamonds and their host rocks.

Age and duration of eclogite-facies metamorphism, North Qaidam HP/UHP terrane, Western China

Amphibolite-facies para-and orthogneisses near Dulan, at the southeast end of the North Qaidam terrane, enclose minor eclogite and peridotite which record ultra-high pressure (UHP) metamorphism associated with the Early Paleozoic continental collision of the Qilian and Qaidam microplates. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. SHRIMP-RG U-Pb and REE analyses of zircons from four eclogites yield weighted mean ages of 449 to 422 Ma, and REE patterns (flat HREE, no Eu anomaly) and inclusions of garnet, omphacite, and rutile indicate these ages record eclogite-facies metamorphism. The coherent field relations of these samples, and the similar range of individual ages in each sample suggests that the ∼25 m.y. age range reflects the duration of eclogite-facies conditions in the studied samples. Analyses from zircon cores in one sample yield scattered 433 to 474 Ma ages, reflecting partial overlap on rims, and constrain the minimum age of eclogite protolith crystallization. Inclusions of Th + REE-rich epidote, and zircon REE patterns are consistent with prograde metamorphic growth. In the Luliang Shan, approximately 350 km northwest in the North Qaidam terrane, ages interpreted to record eclogite-facies metamorphism of eclogite and garnet peridotite are as old as 495 Ma and as young as 414 Ma, which suggests that processes responsible for extended high-pressure residence are not restricted to the Dulan region. Evidence of prolonged eclogite-facies metamorphism in HP/UHP localities in the Northeast Greenland eclogite province, the Western Gneiss Region of Norway, and the western Alps suggests that long eclogite-facies residence may be globally significant in continental subduction/collision zones.

The Xiaojiao high-grade eclogite-type rutile deposit in Jiangsu, China: Geology, geochemistry and metallogenesis

The Xiaojiao rutile deposit with stratiform, banded and lens-shaped rutile-bearing eclogite in the Sulu ultra-high pressure metamorphic zone represents the largest and richest primary rutile mineralization in the zone. Four kinds of rutile can be recognized. They are: (1) inclusions in garnet and omphacite; (2) intergranular infilling between garnet and omphacite; (3) altered remnant; and (4) hydrothermal infilling in veinlets. Electron microprobe data show that sodic metasomatism probably from metamorphic fluid could play an important role in rutile mineralization during high and ultra-high pressure meta-morphism. The present paper concludes that according to geochemical characteristics, the eclogite originated from mafic and ultramafic rocks in the upper mantle and crust-mantle boundary and the Xiaojiao eclogite-type rutile deposit is genetically closely related to ultra-high pressure metamorphism.

UHP metamorphism and exhumation of the Dabie Orogen, China: Evidence from SHRIMP dating of zircon and monazite from a UHP granitic gneiss cobble from the Hefei Basin

A SHRIMP U-Pb study of zircons separated from an ultrahigh-pressure (UHP) granitic gneiss cobble (sample DS12) from the Upper Jurassic Fenghuangtai Formation of the Hefei Basin, north of the Dabie Orogen, has identified three different domains: (1) cores, some of which show straight boundaries and strong oscillatory zoning, with Th/U ratios of 0.12–0.70 and an imprecise upper intercept age of 777 ± 220 Ma (MSWD = 2.4); (2) mantles, variable in shape and cathodoluminescence (CL) intensity and containing many UHP mineral inclusions, including coesite and omphacite, and with Th/U ratios of 0.02–0.26 and a weighted mean 206Pb/ 238U age of 244 ± 5 Ma (MSWD = 4.7); and (3) rims, which are more homogeneous and luminescent in CL than the mantles and contain fewer UHP mineral inclusions, with Th/U ratios of 0.01–0.05 and a weighted mean 206Pb/ 238U age of 226 ± 2 Ma (MSWD = 0.65). Monazites from the same sample do not contain coesite and omphacite inclusions, but they do show sector zones and are composed of two contrasting domains. The light domains in backscattered electron (BSE) images have a Th/U ratio of 7.3–18.9 and a weighted mean 206Pb/ 238U age of 221 ± 2 Ma (MSWD = 0.94). The dark domains in BSE have a Th/U ratio of 2.8–7.2 and have a weighted mean 206Pb/ 238U age of 218 ± 2 Ma (MSWD = 0.96). Based on these data and a review of the literature, it can be concluded that (1) the protolith of gneiss sample DS12 is a granitoid of Neoproterozoic age; (2) the ages of 244 and 226 Ma for the mantle and rim domains of the zircons are time records of two discrete stages in the UHP evolution of Dabieshan; (3) the ages of 221 and 218 Ma for the light and dark gray domains, respectively, of the monazites record the times of two stages of retrogressive metamorphism during exhumation of the UHP granitic gneiss to a higher crustal level; (4) the exhumation rate of the gneiss is estimated to be >6 km/Ma; and (5) the UHP metamorphic blocks of the Dabie Orogen provided significant detritus to the Hefei Basin in the Jurassic.

High-pressure/ultrahigh-pressure eclogites from the Hong’an Block, East-Central China: geochemical characterization, isotope disequilibrium and geochronological controversy

The Hong’an Block (western Dabieshan) exposes a series of HP/UHP metamorphic rocks, with a S-to-N distribution from blueschist–greenschist, kyanite-free, to kyanite- and coesite-bearing eclogites. The available age data are inconclusive that hinder our understanding of the tectonic evolution of the Block. The metamorphic temperatures in the Hong’an Block (Tmeta ≈ 700 to 500°C) are lower by 50–150°C than that of the Dabie and Sulu terranes. In this work, we undertook new trace element and Sr–Nd–O isotopic analyses on minerals in order to gain more insight into the geochronological problems. The results are as follows: (1) Trace element distribution patterns suggest that garnet and omphacite in many cases are out of chemical equilibrium; and the presence of high-temperature LREE-rich mineral inclusions (e.g., epidote) in garnet and omphacite has contributed to isotope disequilibrium. (2) Sm–Nd isotope analyses yielded no isochron ages for the Hong’an eclogites. (3) Rb–Sr isotope analyses gave mixed results; in some cases, coexisting minerals are completely out of isotope equilibrium, and in others, isochron relationship is established, yielding ages from 210 Ma to 225 Ma. The pattern of Rb–Sr isotope disequilibrium appears to be independent of the petrological and O-isotope temperatures. (4) In contrast to the unequilibrated Sm–Nd isotopic systems, oxygen isotopes of the eclogite minerals seem to have attained isotope equilibrium or near-equilibrium. Oxygen isotope temperatures are comparable with petrological temperatures. However, this is an apparent feature due to mass balance constraints. (5) Whole-rock δ18O values show a large variation from +10‰ to −8‰, suggesting that their protoliths have undergone very different processes of water–rock interaction. In view of the overall geochronological information, we conclude that the HP/UHP metamorphism in the Hong’an Block took place in the Triassic at about 220–230 Ma, as observed in the Dabie and Sulu terranes. The significance of published Paleozoic dates (450–300 Ma) for the Xiongdian eclogite is not clear. However, any hypotheses advocating two periods of UHP metamorphic events for the same tectonic unit or in the same locality are not constrained by the geochronological data.

Reaction microtextures in jadeitites from the Nishisonogi metamorphic rocks, Kyushu, Japan

Jadeitite exhibiting jadeite with quartz inclusions has been newly found in the Nishisonogi metamorphic rocks, Kyushu, Japan. The jadeitites occur in boulders from a riverbed, together with boulders of albitites, clinozoisite-muscovite rocks, and serpentinites. The distribution of these boulders is confined to an area that is downstream from an outcrop of a serpentinite melange, suggesting that they were originally tectonic blocks in this serpentinite melange. The jadeitites consist mainly of jadeite with small amounts of muscovite, paragonite, phlogopite, albite, analcime, clinozoisite, and titanite. The jadeite consists of a core with abundant inclusions of quartz and omphacite, and a rim that is free from quartz inclusions. The quartz inclusions are in direct contact with the host jadeite, which has an almost pure NaAlSi2O6 composition (Jd100-Jd95). The volume fraction of the quartz inclusions (VQtz/(VJd + VQtz) = 0.20-0.28) in the jadeite core is close to the value (VQtz/(VJd + VQtz) = 0.27) of quartz produced by the reaction albite = jadeite + quartz. These findings suggest that the jadeite core was produced by an isochemical breakdown of albite at high-P/T conditions. In addition, the jadeite is locally replaced by albite and/or analcime at the rim and along microfractures. These microtextures provide information to deduce a retrograde P-T path during the exhumation of the jadeitite.

Evolution of a tourmaline-bearing lawsonite eclogite from the Elekdağ area (Central Pontides, N Turkey): evidence for infiltration of slab-derived B-rich fluids during exhumation

An undated high-pressure low-temperature tectonic melange in the Elekdag area (central Pontides, N Turkey) comprises blocks of MORB-derived lawsonite eclogite within a sheared serpentinite matrix. In their outer shells, some of the eclogite blocks contain large (up to 6 cm) tourmaline crystals. ‘Prograde’ inclusions in poikiloblastic garnet from a well-preserved eclogite block are lawsonite, epidote/clinozoisite, omphacite, rutile, glaucophane, chlorite, Ba-bearing phengite, minor actinolite, winchite and quartz. In addition, glaucophane, lawsonite and rutile occur as inclusions in omphacite. These inclusion assemblages document the transition from a garnet-lawsonite-epidote-bearing blueschist to a lawsonite eclogite with the peak assemblage garnet + omphacite I + lawsonite + rutile. Peak metamorphic conditions are not well-constrained but are estimated approximately 400–430°C and >1.35 GPa, based on Fe–Mg exchange between garnet and omphacite and the coexistence of lawsonite + omphacite + rutile. During exhumation of the eclogite–serpentinite melange in the hanging wall of a subduction system, infiltration of B-rich aqueous fluids into the rims of eclogite blocks caused retrogressive formation of abundant chlorite, titanite and albite, followed by growth of tourmaline at the expense of chlorite. At the same time, omphacite I (XJd=0.24–0.44) became unstable and partially replaced by omphacite II characterized by higher XJd (0.35–0.48), suggesting a relatively low silica activity in the infiltrating fluid. Apart from Fe-rich rims developed at the contact to chlorite, tourmaline crystals are nearly homogeneous. Their compositions correspond to Na-rich dravite, perhaps with a small amount of excess (tetrahedral) boron (~5.90 Si and 3.10 B cations per 31 anions). δ11 B values range from −2.2 to +1.7‰. The infiltrating fluids were most probably derived from subducting altered oceanic crust and sediments.

Devonian to Carboniferous collision in the Greenland Caledonides: U-Pb zircon and Sm-Nd ages of high-pressure and ultrahigh-pressure metamorphism

A variety of eclogites from an east-west transect across the North-East Greenland eclogite province have been studied to establish the timing of high pressure (HP) and ultrahigh-pressure (UHP) metamorphism in this northern segment of the Laurentian margin. Garnet + omphacite ± amphibole + whole rock Sm-Nd isochrons from a quartz eclogite, a garnet + omphacite + rutile eclogite and a partially melted zoisite eclogite in the western HP belt are 401±2, 402±9 and 414±18 Ma, respectively. Corresponding sensitive high-resolution ion microprobe (SHRIMP) 206Pb/238U ages of metamorphic zircon in the same samples are 401±7, 414±13, and 393 ±10 Ma. Metamorphic zircon domains were identified using morphology, cathodoluminescence (CL) imaging, U, Th, Th/U and trace element contents. Zircon from the quartz eclogite and the garnet + omphacite + rutile eclogite are typical of eclogite facies zircon with rounded to subhedral shapes, patchy to homogenous CL domains, low U, and very low Th and Th/U. The partially melted eclogite contains euhedral zircons with dark, sector-zoned, higher U, Th and Th/U inherited cores. Three cores give a Paleoproterozoic 207Pb/206Pb age of 1,962±27 Ma, interpreted as the age of the leucogabbroic protolith. CL images of the bright overgrowths show faint oscillatory zoning next to homogenous areas that indicate zircon growth in the presence of a HP melt and later recrystallization. Additional evidence that zircon grew during eclogite facies conditions is the lack of a Eu anomaly in the trace element data for all the samples. These results, combined with additional less precise Sm-Nd ages and our earlier work, point to a Devonian age of HP metamorphism in the western and central portions of the eclogite province. An UHP kyanite eclogite from the eastern part of the transect contains equant metamorphic zircon with homogeneous to patchy zoning in CL and HP inclusions of garnet, omphacite and kyanite. These zircons have slightly higher U, Th and Th/U values than the HP ones, no Eu anomaly, and are thus comparable to UHP zircons in the literature. The 206Pb/238U age of these zircons is 360±5 Ma, much younger than the HP eclogites. The same sample gives a Sm-Nd age of 342±6 Ma. Unlike the HP eclogites, the Sm-Nd age of the UHP rock is ca. 20 Ma younger than the U-Pb zircon age and most likely records slow cooling through the closure temperature, since peak temperatures were in excess of 900°C. Widespread HP metamorphism of both the Laurentian and Baltica continental margins marks the culmination of this continent–continent collision in the Devonian. Carboniferous UHP conditions, though localized in the east, suggest a prolonged collisional history rather than a short-lived Scandian orogeny. The traditional Silurian Scandian orogeny should thus be extended through the Devonian.