Ca Amphibole Research Articles

Mafic veins in the mantle section of the Oman ophiolite: Petrological and geochronological insights from the Oman Drilling Project cores

The magmatic and tectonic evolution of the Oman ophiolite was investigated using the core samples obtained in the Oman Drilling Project. The core samples were recovered from three drilling sites (BA1B, BA3A, and BA4A) in the mantle section composed mainly of harzburgite and discordant dunite and wehrlite. A large number of mafic veins crosscut the structures of both mantle lithologies, indicating that they intruded after the formation of discordant dunites. The wehrlites include Fe3+-rich and TiO2-poor chromian spinel, suggesting that the discordant dunites and wehrlites were involved in hydrous, Ti-poor arc magmatism. The composition of plagioclase and clinopyroxene and rare-earth element (REE) patterns of clinopyroxene for the mafic veins suggest that the mafic veins were produced from mid-ocean ridge basalt (MORB)-like parental melts. On the other hand, early crystallization of clinopyroxene and Fe3+-rich and TiO2-poor chromian spinel in the mafic veins imply that the parental melts were also hydrous. Zircon UPb dating of the mafic veins yields the igneous age of about 91 Ma, which is younger than the V1 and V2 volcanic sequences. Depending on the distance from the mafic veins, the various REE patterns of clinopyroxene and Ca amphibole in the Oman harzburgites indicate local peridotite metasomatism during intrusion and percolation of late-stage melts that formed the mafic veins. Ca amphibole in the harzburgites and mafic veins lack significant enrichment in fluid-mobile elements, indicating that the hydrous phases were formed by fluids associated with hydrothermal activities in a spreading environment instead of slab-derived fluids. The mafic veins were likely produced in a spreading environment on a supra-subduction zone. As their formation took place after the arc magmatism that formed the Oman V2 lava sequence and discordant dunites, as a result of slab roll-back. This seafloor re-spreading on a supra-subduction zone after the V2 magmatism recorded in the mantle section might have been involved in the formation of the plume-related V3 lava sequence overlying the V2 lavas.

Eclogite with biotite porphyroblasts—Which conditions are responsible for their formation? An example from the northern Fleur‐de‐Lys Supergroup, Newfoundland, Canada

AbstractAn eclogite from the Early Palaeozoic Fleur‐de‐Lys Supergroup in Newfoundland was studied because of its biotite porphyroblasts, which very rarely occur in this rock type. Thermodynamic modelling suggests that eclogitic biotite in common metabasite (former basalt–gabbro) is limited to (1) bulk‐rock compositions, which are relatively rich in Fe2+ and K and poor in Fe3+, and (2) the low‐pressure range of the eclogite facies. The latter reason is supported by the determination of the pressure–temperature (P–T) path of the Newfoundland eclogite. Chemical zonation of garnet, presence of phengite with Si contents of ~3.4 per formula unit, Zr contents in rutile and petrographic observations resulted in a P–T trajectory starting at medium‐pressure conditions. Nearly isothermal burial led to a peak pressure of 18–19 kbar at ~575°C, followed by exhumation and slight heating. Deformation occurred at or close to the peak pressure. Subsequent introduction of hydrous fluids caused the formation of porphyroblasts of biotite and Ca–amphibole in the pressure range of 12–17 kbar at peak temperatures of 625–640°C. Retrogression led to very fine‐grained symplectites around omphacite and phengite and marginal replacement of biotite porphyroblasts by plagioclase and titanite. Geodynamic scenarios invoking either a flat subduction of oceanic crust followed by continent–continent collision or intracontinental subduction along a transpressional fault system might best explain the formation of eclogite with biotite porphyroblasts in general. For the Newfoundland eclogite, the latter scenario is preferred.

Metamorphic history of mafic granulite and garnet‐bearing amphibolite from the Oki‐Dogo Island, Japan

AbstractOki metamorphic rocks have long been considered as a constituent of the Hida Belt based on their geographic proximity, lithology, and Permo‐Triassic metamorphism. However, recent geochronological studies have demonstrated that both para‐ and ortho‐gneisses in the Oki‐Dogo Island display Paleoproterozoic protolith formation and two separate phases of metamorphism at 1.85 Ga and 250–230 Ma. Consequently, the Oki metamorphic rocks are closely connected to the Paleoproterozoic massifs in the Korean Peninsula, although little is known about their pressure (P)–temperature (T) history. Here, we provide petrological data on mafic metamorphic rocks in the Oki‐Dogo Island. The mafic lithologies are classified into mafic granulite, amphibolitized granulite and amphibolite. In addition, we first discover a garnet‐bearing variety of amphibolite from the Oki‐Dogo Island. The texture and composition of Ca amphibole suggest these rock types share a common P–T history but the dominant mineral assemblage in each rock type records different stages of metamorphism. The inferred P–T history includes two distinct events. The first event includes a low‐P granulite facies stage (~900°C, 0.7–0.8 GPa) and subsequent amphibolite facies retrogression. This event is linked to the continuous compositional change of Ca amphibole from Ti‐rich pargasite to hornblende/actinolite. The second event is prograde amphibolite facies metamorphism, which is associated with the formation of tschermakitic hornblende and calcic plagioclase. In high Fe/(Mg + Fe) rocks, garnet was formed at ~550–580°C, 0.45–0.50 GPa in this stage. Depending on the age of the first event, the low‐P granulite facies metamorphism is likely to have occurred in a similar tectonic setting as the Paleoproterozoic crustal metamorphism in the Yeongnam Massif or the Permo‐Triassic ultrahigh‐T metamorphism in the northern Gyeonggi Massif.

Mineralogy and paragenesis of the Meso-Proterozoic Rohil uranium deposit, North Delhi Fold Belt, Rajasthan, India

The Rohil uranium deposit, located about 70 km south of the Khetri copper deposit in the North Delhi Fold Belt, Rajasthan, India, is albitite-hosted. This deposit is associated with Na-metasomatism, mainly affecting the metasedimentary wall rocks. Quartzite, quartz biotite schist, quartz amphibole schist, and graphite schist underwent variable degrees of albitization, as documented by a wide range of altered wall rocks, including weakly altered metasediments to almost pure albitites. These host rocks consist of metamorphic mineral assemblages such as Ca plagioclase – quartz – biotite – Ca amphibole – graphite – garnet – sillimanite – corundum – ilmenite. The hydrothermal alteration assemblages contain variable amounts of albite, chlorite, biotite, sericite, orthoclase, sphene, rutile, allanite, actinolite, and calcite. The textural relationships between primary host rock minerals and hydrothermal alteration products suggest four main stages of metasomatism: (1) an early pre-albitization stage, (2) a Na-metasomatic stage, that are followed by (3) a Na‐Ca-Mg- and (4) K- metasomatic stage. Albites are of two types: albite-1 is subhedral to euhedral and equidimensional, formed by the alteration of metamorphic plagioclase during the Na-metasomatic stage. Albite-2 is feathery and formed in fractures/cavities by precipitation of alkali-rich fluid during Na-Mg-Ca metasomatic stages. Albitization is pervasive on a regional scale. The hydrothermal metasomatic fluids utilized structures such as shears and breccias in the metasediments as conduits and as the loci of uranium mineralization. Major uranium minerals are uraninite, U-silicate, and brannerite. Uranium-bearing minerals occur as disseminations, in veins and as minute inclusions in biotite and titanite within albitized metasediments. The petrographic studies suggest that the metasomatic alteration of host rocks is characterized by albitization, actinolitization, carbonatization, chloritization, sericitization, biotitization, sulfidation, and silicification. The close association of U, REE-bearing minerals, and albite-2 indicates that the main U-REE mineralization stage was coeval with the Na-Ca-Mg and K-metasomatic stages.

Temporal and spatial mineralogical changes in clasts from Mariana serpentinite mud volcanoes: Cooling of the hot forearc-mantle at subduction initiation

Mafic and ultramafic clasts (mostly ~1–5 cm in size) were recovered from three different serpentinite mud volcanos in the Mariana forearc during Integrated Ocean Drilling Program (IODP) Expedition 366. Mafic clasts from drill sites distant from the trench bear lawsonite, Al-rich riebeckite, jadeitic pyroxene (~80 mol% jadeite), and aragonite as metamorphic minerals. In contrast, mafic clasts from drill sites closer to the trench are characterized by prehnite–pumpellyite-facies mineral associations and/or the presence of analcime and natrolite. An occurrence of antigorite-bearing ultramafic clasts becomes progressively more frequent with distance from the trench. One amphibolite clast from a mud volcano near the trench also has prehnite filling veins, and it also occurs as pseudomorphs after plagioclase. Amphibolite clasts at other mud volcanoes distant from the trench are partially overprinted by blueschist-facies minerals. The apparent metamorphic grades increase with distance from the trench; these metamorphic conditions represent the increasing depth from zeolite- to lawsonite–blueschist-facies conditions in a subduction zone. Considering the consistency of the low-temperature metamorphic grade of mafic and ultramafic clast mineralogy in each mud volcano, they likely reflect the thermal structure of the slab-mantle interface before the ascent. As a result, these clasts were brought up to the seafloor en masse by the serpentinite mudflow. The polymetamorphosed amphibolite clasts suggest cooling of the hot forearc-mantle at the initiation of Mariana subduction in the Eocene. The ultramafic clasts in the mud volcanoes distant from the trench frequently contain Ca amphibole and talc, which indicates hot mantle hydration by metasomatic fluids released from the slab at subduction initiation.

Mantle–Crustal Nature of Early Paleozoic Alkaline Intrusions in Central Sangilen, Tuva (from Nd, Sr, Pb, C, and O Isotope Data)

Abstract —The Paleozoic foidolite–foyaite plutons of the Sangilen upland (Bayan-Kol, Dakhu-Nur, Chik, and Kharly ones) might have formed in the Late Cambrian–Early Ordovician (~490–500 Ma, Sm–Nd and U–Pb); they are the result of the oldest alkaline magmatism in southeastern Tuva. The intrusion was accompanied by the formation of high-temperature (up to ~600–900 ºC) endogenous carbonate rocks containing calcite, alkali pyroxene, Na–Ca amphibole, biotite, fluorapatite, microcline, and nepheline. Silicate and carbonate derivates were produced, most likely, from genetically related heterogeneous sources with εNd(T) varying from 3.0 to 6.3 and from –0.5 to 6.5, respectively, which might be due to the mixing of the depleted (PREMA) and enriched (EM) mantle materials. Initial ratios 207Pb/206Pb ≈ 0.89 and 208Pb/206Pb ≈ 2.15 in K-feldspar from calcitic rocks are close to those of EM 1. The correlation between the stable-isotope ratios (δ18O ~ 7.2–19.5, δ13C from –6.0 to –1.4‰) and the high 87Sr/86Sr(T) ratio (0.7057–0.7076) indicates a significant crustal contamination of magma in the upper horizons of the lithosphere and a minor impact of a meteoric fluid. The assumed synchronous formation of the studied plutons and other alkaline rock complexes of the Early Paleozoic Large Igneous Province in the west of the Central Asian Orogenic Belt as well as their isotope similarity do not rule out that the intrusion took place in the plume–lithosphere interaction setting.

Blueschist facies overprint of late Triassic Tethyan oceanic crust in a subduction–accretion complex in north-central Anatolia, Turkey

The late Cretaceous accretionary complex along the İzmir–Ankara–Erzincan suture zone, northern Turkey, includes various types of metamorphic rock together with radiolarite, sandstone, mudstone, serpentinite, basalt and limestone. Meta-plagiogranite blocks (up to 5 m in diameter) and a meta-gabbro slice (800 × 500 m) cross-cut by meta-plagiogranite are observed in a matrix of serpentinite, mudstone and radiolarian chert. These meta-gabbros and meta-plagiogranites show subduction-related geochemical characteristics. Laser ablation inductively coupled plasma mass spectrometry U–Pb dating of zircons from two meta-plagiogranites yielded 222.3 ± 1.5 and 227.2 ± 1.6 Ma crystallization ages. This study shows that the meta-gabbro and meta-plagiogranite from the accretionary complex are remnants of the metamorphosed equivalents of the late Triassic lower crustal rocks of the subducted Tethyan oceanic crust. They are the oldest lower crustal rocks observed in the İzmir–Ankara–Erzincan suture zone. Both the meta-gabbros and meta-plagiogranites have Na–Ca amphiboles, indicating blueschist facies metamorphism. However, phengites from a blueschist facies rock yielded an 40 Ar/ 39 Ar age of 104.3 ± 0.7 Ma, indicating that blueschist metamorphism occurred during the Albian. All these data suggest that the oceanic crust formed during the Norian, metamorphosed during the Jurassic to early Cretaceous and then dismembered during accretionary complex formation in the late Cretaceous. Supplementary material: A complete description of the analytical methods ( electron microprobe, LA-ICP-MS zircon U–Pb and 40 Ar/ 39 Ar age analyses, pseudo-section modelling and THERMOCALC) are available at https://doi.org/10.6084/m9.figshare.6509984 Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Polyphase Inclusions in Chromium Spinels from Upper Triassic Gravelites from the Northeastern Part of the Siberian Platform

A comparative study of the chemical compositions of chromium spinel and polyphase inclusions has been carried out. Chromium spinels were separated from the concentrate of the Carnian Stage (T3) rock sample collected at the northeastern fringe of the Siberian platform. The chromium spinels show a wide variety of compositions, most of which are of the “Kurung” type, which is considered to be a deceptive indicator of a kimberlite. They contain inclusions in the size range of 10–100 μm and contain several mineral phases. The most frequent inclusions are olivine and clinopyroxene; additional phases are nepheline, K–Fsp, Ti–Mg–Ca–amphibole, Ti–biotite (phlogopite), apatite and perovskite. Ilmenite and iron sulfides are less common. The data obtained on the composition of these inclusions indicate differences in the conditions of their formation and possibly different types of parental sources. A possible type of provenance source could be potassium alkali basites.

浅所貫入深成岩のCa角閃石の累帯構造( 角閃石研究の現状と課題-地団研第37回総会造岩鉱物シンポジウム-)

浅所貫入深成岩のCa角閃石の累帯構造( 角閃石研究の現状と課題-地団研第37回総会造岩鉱物シンポジウム-)

Metamorphic evolution of blueschists, greenschists, and metagreywackes in the Cretaceous Mt. Hibernia Complex (SE Jamaica)

A suite of samples collected mainly from river cobbles in the Mt. Hibernia Complex in SE Jamaica includes metamorphosed basaltic and clastic sedimentary rocks, which are interpreted here as part of a Cretaceous accretionary prism. The metabasic rocks occur as blueschists and greenschists, whereas the metasedimentary rocks are former greywackes s.l ., or psammopelites, and can now be characterized as phyllites. Eight characteristic mineral assemblages can be defined within the metabasic sample suite, involving amphibole (Na amphibole and/or actinolite), pumpellyite, lawsonite, epidote, chlorite, titanite, phengite, albite and quartz. Augite is an important relict phase of the magmatic source rocks. White mica, chlorite and quartz predominate in the phyllites in variable proportions, but Na amphibole, epidote, albite and titanite may also be important. Calculated P–T pseudosections show that the assemblage glaucophane–pumpellyite–chlorite occurs only in a restricted field along the lower P–T limit of the blueschist facies. Pseudosections also indicate that peak P–T conditions for the blueschist samples studied here lie in the range 6.5–7.2 kbar and 310–340°C, that is at the upper stability limit of pumpellyite and lawsonite, and in the range 6.4–7.8 kbar and 340–380°C for greenschist samples. The P–T range for phyllites overlaps with both. Thus, blueschists and greenschists represent comparable pressure but different temperature conditions of formation. Literature data on the mapped distribution of metabasic rock types suggests an isobaric metamorphic field gradient in the Mt. Hibernia Complex, with temperatures increasing slightly but significantly from blueschist to greenschist zones. We interpret this gradient as reflecting either increasing temperatures in the accretionary complex away from the subducting slab or thermal re-equilibration in a cooling subduction zone. Sodium-Ca amphiboles, predominantly winchite, are commonly observed as rims around Na amphibole and formed together with albite porphyroblasts during decompression. Comparison with published information on the miscibility gap between Na and Ca amphiboles shows that many of these compositions fall within this gap for temperatures around 350°C. Thus, these amphiboles either grew metastably below the critical temperature of the gap, or some as yet undefined parameter must be responsible for drastically altering the shape of the gap. Published descriptions of deformation in the Mt. Hibernia Complex show that coaxial pure shear initially predominates, as is common in accretionary complexes; however, a late shearing event, postulated here to be caused by the collision between the Great Arc of the Caribbean and the continental margin of North America at 75–70 Ma, is held responsible for the rapid exhumation of the combined blueschist–greenschist sequence.

Stable H–C–O isotope and trace element geochemistry of the Cummins Range Carbonatite Complex, Kimberley region, Western Australia: implications for hydrothermal REE mineralization, carbonatite evolution and mantle source regions

The Neoproterozoic Cummins Range Carbonatite Complex (CRCC) is situated in the southern Halls Creek Orogen adjacent to the Kimberley Craton in northern Western Australia. The CRCC is a composite, subvertical to vertical stock ∼2 km across with a rim of phlogopite–diopside clinopyroxenite surrounding a plug of calcite carbonatite and dolomite carbonatite dykes and veins that contain variable proportions of apatite–phlogopite–magnetite ± pyrochlore ± metasomatic Na–Ca amphiboles ± zircon. Early high-Sr calcite carbonatites (4,800–6,060 ppm Sr; La/YbCN = 31.6–41.5; δ13C = −4.2 to −4.0 ‰) possibly were derived from a carbonated silicate parental magma by fractional crystallization. Associated high-Sr dolomite carbonatites (4,090–6,310 ppm Sr; La/YbCN = 96.5–352) and a late-stage, narrow, high rare earth element (REE) dolomite carbonatite dyke (La/YbCN = 2756) define a shift in the C–O stable isotope data (δ18O = 7.5 to 12.6 ‰; δ13C = −4.2 to −2.2 ‰) from the primary carbonatite field that may have been produced by Rayleigh fractionation with magma crystallization and cooling or through crustal contamination via fluid infiltration. Past exploration has focussed primarily on the secondary monazite-(Ce)-rich REE and U mineralization in the oxidized zone overlying the carbonatite. However, high-grade primary hydrothermal REE mineralization also occurs in narrow (<1 m wide) shear-zone hosted lenses of apatite–monazite-(Ce) and foliated monazite-(Ce)–talc rocks (≤∼25.8 wt% total rare earth oxide (TREO); La/YbCN = 30,085), as well as in high-REE dolomite carbonatite dykes (3.43 wt% TREO), where calcite, parisite-(Ce) and synchysite-(Ce) replace monazite-(Ce) after apatite. Primary magmatic carbonatites were widely hydrothermally dolomitized to produce low-Sr dolomite carbonatite (38.5–282 ppm Sr; La/YbCN = 38.4–158.4; δ18O = 20.8 to 21.9 ‰; δ13C = −4.3 to −3.6 ‰) that contains weak REE mineralization in replacement textures, veins and coating vugs. The relatively high δD values (−54 to −34 ‰) of H2O derived from carbonatites from the CRCC indicate that the fluids associated with carbonate formation contained a significant amount of crustal component in accordance with the elevated δ13C values (∼−4 ‰). The high δD and δ13C signature of the carbonatites may have been produced by CO2–H2O metasomatism of the mantle source during Paleoproterozoic subduction beneath the eastern margin of the Kimberley Craton.

High-pressure polymetamorphic garnet growth in eclogites from the Mariánské Lázně Complex (Bohemian Massif)

The Marianske-Lazně Complex is a Cambro-Ordovician terrane with metabasites of oceanic-crust affinity and tectonically emplaced between the Saxothuringian Zone and the Tepla-Barrandian Unit (western part of the Bohemian Massif). It is formed by amphibolite (retrogressed after eclogite), serpentinized peridotite, coronitic metagabbro with paragneiss, and felsic orthogneiss. Available geochronological data support Variscan ages for the eclogite-facies metamorphism and subsequent amphibolite-facies reequilibration. Garnet crystals with multi-stage growth features in eclogite were studied to analyse the relation of their chemical zoning with pressure–temperature ( P – T ) changes and/or reactions among coexisting phases during metamorphism. The core garnet has highly contrasted compositions with high Mg and low Ca compared to the surrounding host-garnet crystal. Compositional zoning and crystallographic orientation of the garnet showed that the cores and rims were formed during two different metamorphic events. In addition to conventional geothermobarometry, pseudosection and garnet isopleths were used to estimate P T conditions for both metamorphic events. The older core (garnet I) indicates high-pressure amphibolite facies and the younger rim (garnet II) indicates eclogite-facies conditions. Garnet (II) contains inclusions of Na–Ca amphibole and omphacite, suggesting a prograde metamorphism from blueschist- to eclogite-facies conditions. Full multicomponent-diffusion modelling of compositional zoning at the interface of amphibolite-/eclogite-facies garnets in conjunction with the retrieved P – T paths were used to evaluate the average heating/cooling rates during the eclogite-facies event. These two metamorphic events deciphered from different garnet generations are consistent with available geochronological data and bring new insight into the subduction history of the Saxothoringian and/or Rehic oceanic basins during the Variscan orogeny in the Bohemian Massif.

Evidence for oceanic subduction at the NE Gondwana margin during Permo‐Triassic times

AbstractBlueschist was recently recognized within the Lhasa terrane, which is one of the NE Gondwana blocks. In this rock, the Mn and Mg contents of garnet enclosing aegirine‐rich clinopyroxene, rutile and quartz decrease and increase, respectively, from core to rim. Amphibole changes from glaucophane through Na–Ca amphibole to Ca amphibole. The Si contents of phengite are high in the centre but low along the rim. The P–T path, starting above 2.5 GPa–450 °C and showing subsequently first a temperature increase to 500 °C and then a pressure release via blueschist conditions to 0.6 GPa, was reconstructed using a P–T pseudosection calculated for the P–T range 0.4–2.8 GPa and 250–650 °C. This path points to deep subduction of a cold oceanic crust probably beneath the NE Gondwana margin during Permo‐Triassic times. This finding contributes to a better understanding of the pre‐Cenozoic history of major terranes of NE Gondwana.

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.

Mineral assemblages and genesis of hornfelses in the outer contact zone of the Khibina Massif, Kola Peninsula, Russia

The paper presents materials on the fabric of the western, southwestern, and southern exocontact zones of the Khibina alkaline pluton and metavolcanic rocks of the Il’mozerskaya Formation of the Paleoproterozoic Imandra-Varzuga riftogenic structure. The volcanics of the Imandra-Varzuga structure were originally metamorphosed to the greenschist facies (at temperatures of ≥300°C and pressures of ≥2.0–2.5 kbar) and were afterward metamorphosed to the pyroxene-hornfels facies under the thermal effect the Khibina pluton with the development of a hornfels zone 150–400 m thick. According to their composition, the hornfelses are subdivided into three zones: inner, intermediate, and outer. The inner zone is up to 30 m thick and consists of hornfelses of clinopyroxene-plagioclase composition with olivine as a typomorphic mineral and with variable amounts of amphibole. The intermediate zone occurs at a distance of 30–200 m from the pluton, is separated from the inner zone by the olivine isograde, and consists of amphibole-clinopyroxene-plagioclase hornfelses. The outer zone, 200–400 m away from the contact of the pluton, is made of fine-grained melanocratic hornblende hornfelses. The thermal transformations of the metavolcanics involved the gradual replacement of their low-temperature mineral assemblage (actinolite + albite) by a higher temperature one (clinopyroxene + amphibole + andesinebytownite ± olivine). Our data on the chemical composition of the rock-forming minerals of the hornfelses indicate that the olivine is ferrohortonolite-fayalite, the clinopyroxene belongs to the augite-ferroaugite series, and occasional orthopyroxene grains (which were found only in the intermediate zone) are ferrohypersthene. The amphibole in the hornfelses of the intermediate zone and the outermost (farthest from the contact) part of the inner zone is edenite, a Ca amphibole. The amphibole in hornfelses near the contact is kataphorite of the Na-Ca amphibole group. The plagioclase composition generally corresponds to andesine and bytownite and is albite-oligoclase near the contact with the pluton. The hornfelses adjacent to the contact bear rare sanidine grains. The mineral thermo-and barometry of the hornfelses yielded temperatures of 700–640°C and pressures of 1–1.5 kbar. The temperature determined for the zone exactly at the contact was approximately 700°C, which corresponds to the initial temperature of the rocks in contact with the magma and is close to the crystallization temperature of the nepheline syenites of the Khibina pluton.

Mineral isograds and metamorphic zones of the Anglesey blueschist belt, UK: implications for the metamorphic development of a Neoproterozoic subduction–accretion complex

AbstractThe 560–550 Ma blueschists and associated rocks in Anglesey, UK were derived from a subduction–accretion complex. The blueschist unit is divided into three mineral zones by two newly mapped metamorphic isograds; zone I sub‐greenschist facies, (crossite isograd), zone II blueschist facies, (barroisite isograd), zone III epidote‐amphibolite facies. The zones and isograds dip gently to the east, and decrease in metamorphic grade from the central high‐pressure zone III to lower grade zones II and I to the west and east. The P–T conditions estimated from zoned amphibole indicate an anticlockwise P–T path following adjustment to a cold geotherm. This path is well preserved in the compositional zoning of Na–Ca amphibole that have a core of barroisite surrounded by a rim of crossite, although this is only locally developed. The sense of subduction was to the east and exhumation to the west, as indicated by the metamorphic isograds. The symmetrical arrangement of the metamorphic zones with the deepest high‐pressure rocks in the middle suggests an isoclinal antiformal structure that formed by wedge extrusion during exhumation in the subduction zone.

REDISTRIBUTION OF THE RARE-EARTH ELEMENTS AMONG COEXISTING MINERALS IN METAMAFIC ROCKS ACROSS THE EPIDOTE-OUT ISOGRAD: AN EXAMPLE FROM THE ST. ANTHONY COMPLEX, NORTHERN NEWFOUNDLAND, CANADA

Integrated results of in situ major-element and rare-earth-element analyses of coexisting Ca amphibole, plagioclase, epidote and titanite have been made from a suite of metamafic rocks from the St. Anthony Complex, Newfoundland, across the epidote-out reaction isograd to examine the redistribution of the REE as a consequence of the breakdown of epidote. Important major-element variations include a decrease in X Cum Amp and an increase in X An Pl across the reaction isograd, and an increase in X Czo Ep within the epidote-amphibolite facies. These major-element changes correlate with variations in ∑REE and the LREE:HREE ratio in several of the phases, and also with systematic changes in D *REE partitioning across the epidote-out isograd. Onuma-type diagrams for the partitioning of the REE between coexisting minerals confirm that the REE substitute at single elastic sites in titanite, epidote and plagioclase, whereas in Ca amphibole they occupy two sites, M 4 and M 4′, with the size of the M 4′ peak being positively correlated with X Cum Amp in Ca amphibole. A REE mass balance across the epidote-out isograd yields very good matches between the measured and reconstructed REE abundances in all cases, indicating that metamorphism was essentially isochemical with respect to the rare-earth elements, and that all the hosts for these elements were analyzed. The following major-element mass-balanced reaction, determined by the method of singular value decomposition from mineral compositions in representative samples of epidote amphibolite and plagioclase amphibolite from this ophiolitic complex, provides a realistic representation of the epidote-out isograd in the study area: 2.94 CaAmp1 + 5.00 Pl1 + 1.00 Ep + 5.21Ttn1 = 3.14 CaAmp2+ 5.65 Pl2 + 5.29 Ttn2. Integration of the calculated stoichiometric coefficients of this reaction with the measured REE contents in each phase provides a good match for the MREE and HREE abundances, but a poorer match for the LREE, which show a deficiency on the product side. However, the results are compatible with the measured increases in LREE in Ca amphibole and plagioclase on the product side of the reaction, as a result of the breakdown of epidote and a reduction in LREE abundance in titanite. We highlight the role of epidote and titanite as the principal carriers of the REE in epidote amphibolite, and that of Ca amphibole as the dominant carrier in plagioclase amphibolite.

Petrology of two continental alkaline intraplate series at Emi Koussi volcano, Tibesti, Chad

Tibesti volcanoes (Chad) are poorly documented, as much for volcanological evolution as for mineralogy and geochemistry. Emi Koussi volcano is a wide ignimbritic shield-like volcano that results from the succession of three eruptive sequences, each one related to a caldera collapse. Activity occurred from 2.4 Ma to the recent Quaternary. The continental alkaline volcanics of Emi Koussi exhibit two bimodal lava series, silica-saturated (up to 11% normative quartz) and silica-undersaturated (up to 29% normative nepheline). Basanite and tephrite, trachyte and phonolite are dominant while trachyandesite is very scarce. The basanites and tephrites contain olivine, diopside, plagioclase, nepheline, kaersutite and titanomagnetite. The trachytes contain alkali feldspar, plagioclase, Na–Ca amphibole (kataphorite, richterite), mica, augite, aegirine augite, titanomagnetite and ilmenite. In phonolites, alkali feldspar, andesine, nepheline or sodalite, hedenbergite, aegirine augite, amphibole (ferropargasite and kaersutite), titanite and titanomagnetite occur. Trachyandesite contains olivine, augite, edenite, phlogopite, andesine, alkali feldspar, titanomagnetite and ilmenite. The two suites exhibit a wide compositional gap of about 10% SiO 2. Trachytes are peralkaline and comenditic. Major and trace element data are compatible with differentiation dominated by fractional crystallisation. The fractional crystallisation has been modelled using major-element least-squares mass balance models. Crystal fractionation involving olivine, clinopyroxene, amphibole and plagioclase is the best way to explain the petrogenesis of the undersaturated suite. The silica-saturated suite recorded removal of olivine, pyroxene, amphibole, mica, plagioclase, alkali feldspar, magnetite and apatite. Trace element data, including rare earth elements, calculated using the Rayleigh equation, confirm the models for the undersaturated suite, because there is a close agreement between the observed and calculated data. But the petrogenesis of the silica-saturated suite is probably related to other processes, such as crustal contamination.

Petrology of metabasites from the Terekta Complex as a constituent of ancient accretionary prism of Gorny Altai

The blueschist/greenshist Terekta Complex is the only blueschist locality known in the Russian Altai. The Terekta metabasites contain Na and Na–Ca amphibole, actinolite, phengite, epidote, albite, quartz, calcite, magnetite (or hematite). Depending on the amphibole composition, these rocks were subdivided into blueschist, transitional blueschist/greenschist and greenschist. Both blueschists and transitional blueschist/greenschists (glaucophane-bearing and winchite–actinolite schists) have compositions similar to alkaline basalts of oceanic islands, whereas the greenschists correspond to ocean-floor tholeiitic basalts, or MORB. Available geothermobarometry yielded the following estimates of metamorphic conditions: T=350–400 °C and P=6–8 kbar. The different mineral assemblages of the metabasites are believed to be a result of their different lithologies. The presence of matabasalts with ocean island basalt and MORB affinity, as well as the occurrence of layered metachert, marble, metagraywacke, and plates of serpentinized dunites, pyroxenites indicate that the complex was very likely a subduction-accretionary complex. The complex contains rocks of accretionary wedge, and fragments of oceanic crust which are regarded to be a remnant of an Early Paleozoic subduction zone in the Russian Altai.

OXYGEN AND CARBON ISOTOPIC COMPOSITION OF GRENVILLE MARBLE, AND AN APPRAISAL OF EQUILIBRIUM IN THE DISTRIBUTION OF ISOTOPES BETWEEN CALCITE AND ASSOCIATED MINERALS, OTTER LAKE AREA, QUEBEC, CANADA

Marble occurs abundantly in a portion of the Grenville Province of the Canadian Shield extending from the Adirondack Mountains, through Ontario, and into the Laurentian Highlands of Quebec, a distance of 300 km. In the Otter Lake area of western Quebec, major marble units, which are obviously metamorphosed limestone, consist of combinations of calcite, dolomite, olivine, humite-group minerals, Ca pyroxene, Ca amphibole, biotite, graphite, and retrograde serpentine and brucite, whereas minor marble units, which are evidently metasomatic rocks, consist of combinations of (pink) calcite, Ca pyroxene, Ca amphibole, biotite, garnet, K-feldspar, scapolite, and titanite. The oxygen and carbon isotopic composition of calcite in the major marble units (δ18OVSMOW in the range 28 to 17‰; δ13CPDB–1 in the range +6 to −2‰) is the same as in the Adirondacks and in unmetamorphosed Proterozoic limestone; only slight shifts (~1‰) can be attributed directly to metamorphic reactions. In bodies of minor marble, mean concentrations of 18O and 13C are slightly lower (+14.5 and +0.23‰, respectively). Mean fractionation-factors (α, expressed as 1000 ln α, are as follows: 18O, calcite/dolomite, +0.295 ( n = 11); 13C, calcite/dolomite, −0.346 ( n = 12); 18O, calcite/biotite, +3.49 ( n = 5); 13C, calcite/graphite, +3.74 ( n = 5). Additional (exploratory) results ( n = 1) on the distribution of 18O are as follows: calcite/olivine, +3.5; calcite/clinohumite, +5.1; calcite/garnet, +2.4; calcite/titanite, +3.1; calcite/Ca pyroxene, +2.0; calcite/Ca amphibole, +2.4, calcite/K-feldspar +1.8; calcite/scapolite, +2.0; calcite/serpentine, +13.9; calcite/brucite, +19.7; a trend of increasing α is related to increasing OH/(O + OH) in the non-carbonates and the preference of OH for 16O. For the first group of mineral pairs ( n ≥ a small variation in α across the study area (relative standard deviation ~0.05%) is viewed as an expression of a close approach to isotopic exchange equilibrium, and for certain pairs from the second group ( n = 1), a temperature estimate close to 700° (the biotite–garnet temperature of associated gneisses) is also viewed as evidence for equilibrium. Attainment of isotopic exchange equilibrium was facilitated by dynamic recrystallization, which occurred during the peak of metamorphism.