Growth Of Amphibole Research Articles

Chemical Trends In the Meech Lake, Québec, Carbonatites and Fenites

Abstract Near Meech Lake, Quebec, the edges of Mesoproterozoic carbonatite dikes are composed of calcite, dolomite, fluorapatite, phlogopite, amphibole, and pyrochlore. The carbonatite is separated from amphibole-fenite by a narrow, fine-grained reaction selvage of phlogopite pierced with long prisms of amphibole. The amphibole is mainly richterite, but it extends to magnesio-arfvedsonite (overgrowth, crystal rim). Uranium-rich pyrochlore is metamict and ranges from calciopyrochlore to kenopyrochlore with Ta-U enrichment in crystal rims. Chemical characteristics of the suite are: (1) F and Nb highest in the selvage, and (2) decline of Sr and Ce outwards from the carbonatite. A similar pattern (this research) is found at Fen, Norway. Rare earths are enriched in LREE with smooth downward-sloping patterns, in chondrite-normalized curves, to HREE. Two major surges of mineralization are suggested: (1) early, metasomatic-alkalic, creating fenites with enrichment in Mg, Na, and K; and (2) later igneous depositing carbonatites and introducing first F, P, and Nb, then Ca, Sr, and Ce. Thermochemical and geochronological data place carbonate equilibration at 700 °C and the emplacement at 1026 Ma b.p. Calciocarbonatites, in monzonitic orthogneiss, are enriched in Ba and Ce. They are composed of baryte, calcite, phlogopite, fluorapatite, magnesio-riebeckite, and non-metamict allanite-(Ce). A mica selvage is present, but amphibole fenite is almost completely lacking. Magnesiocarbonatite has a well-developed selvage against granite but lacks significant amphibole fenite. In breccia cement at nearby Fortune Lake, pyrochlore is associated with abundant fluorapatite but lacks carbonates. The Cambro-Proterozoic calciocarbonatite near Fen, Norway is particularly Nb-rich in breccia zones, and pyroxene fenite takes the place of amphibole fenite at Meech Lake. In contrast to a relatively anorogenic regime during carbonatite petrogenesis at Fen, metamorphism has obscured pyrochlore zonation and enhanced amphibole growth at Meech Lake.

U–Pb zircon and biostratigraphic data of high-pressure/low-temperature metamorphic rocks of the Talea Ori: tracking the Paleotethys suture in central Crete, Greece

Inherited deformation microfabrics of detrital quartz grains and U–Pb (Laser ablation (LA)-ICPMS and ID TIMS) ages of detrital zircons separated from the Phyllite–Quartzite Unit s.l. of the Talea Ori, central Crete, suggest strikingly different source rocks. Albite gneiss of the lower Rogdia Beds includes Cambrian and Neoproterozoic rounded zircons with main U–Pb age peaks at 628 and 988 Ma. These and minor Paleoproterozoic and Archean peaks, together with the lack of Variscan-aged and Mesoproterozoic zircons, are similar to the age spectra obtained from the Phyllite–Quartzite Unit s.str. of the Peloponnesus and eastern Crete and from the Taurides. All of these zircons should be derived from the northeastern passive margin of Gondwana (Cimmeria). Metatuffites of the uppermost Rogdia Beds and metasandstone of Bali beach, on the other hand, include euhedral detrital zircons displaying a Variscan U–Pb age spectra at ca. 300 Ma with concordia ages at 291 ± 3, 300 ± 1 Ma (Rogdia) and 286 ± 3, 300 ± 3, 313 ± 2 Ma (Bali). Both types of metasediments and their zircons are similar to those of the pre-Alpine basement and overlying Tyros Beds of eastern Crete, revealing a provenance at the southern active margin of Laurasia. Thus, in central Crete the Paleotethys suture should be situated inside the Rogdia Beds. Magmatic zircons separated from a rhyolite boulder of the lower Achlada Beds yielded a concordant U–Pb zircon age at 242 ± 2 Ma placing a maximum age for the deposition of the (meta)conglomerate from which the boulder was collected. This age is compatible with an Olenekian-early Anisian age of the underlying Vasilikon marble suggested by new findings of the foraminifera Meandrospira aff. pusilla. Both the Achlada Beds and the Vasilikon marble can be attributed to the lower Tyros Beds of eastern Crete. The Alpine deformation led to a pervasive mylonitic foliation, which is affecting most of the studied rocks. This foliation results from D2 top-to-the-north shearing, which post-dates the growth of blue amphiboles (crossite).

Crystallographic preferred orientations of exhumed subduction channel rocks from the Eclogite Zone of the Tauern Window (Eastern Alps, Austria), and implications on rock elastic anisotropies at great depths

Crystallographic preferred orientations (CPO) of rocks from an exhumed subduction channel of the Alpine orogen were determined using time-of-flight neutron diffraction. This method allows the investigation of large polymineralic samples and, more importantly, the application of full pattern fit methods to constrain CPOs of mineralogically complex rocks. Samples studied include intensely deformed fresh and retrogressed eclogites, as well as metasediments, which are interleaved with the eclogites in the subduction channel. From the CPO, seismic properties of the samples were calculated. P-wave anisotropies of the eclogite samples are fairly low, with an average of about 1.5%, and mainly constrained by pronounced omphacite CPO. Growth and deformation of retrograde amphibole in the eclogites also led to a pronounced CPO, which has a large impact on seismic anisotropies by raising them to up to 3.7% and changing the orientations of velocity maxima. Elastic anisotropies of the subducted metasediments are higher (up to 7.4%) and constrained by quartz and mica CPO in clastics and by calcite CPO in marble. VP/VS ratios may help to distinguish fresh eclogites from retrogressed ones, and both rock types from mantle peridotites of downgoing lithospheric slabs in seismic imaging. Our data also indicate that subducted terrigenous sediments not only are strongly anisotropic, but also have low VP/VS ratios. This way there may be the potential to image them by seismic tomography at depth in active subduction channels.

The Cimmerian accretionary wedge of Anarak, Central Iran

The occurrence in Iran of several ophiolite belts dating between Late Palaeozoic to Triassic poses several questions on the possible existence of various sutures marking the closure of the Palaeotethys ocean between Eurasia and this Gondwana-derived microplate. In this scenario, the Anarak region in Central Iran still represents a conundrum. Contrasting geochronological, paleontological, paleomagnetic data and reported field evidence suggest different origins for the Anarak Metamorphic Complex (AMC). The AMC is either interpreted, as: (1) relict of an accretionary wedge developed at the Eurasia margin during the Palaeotethys subduction as part of the Cimmerian suture zone of NE Iran, displaced to Central Iran by a large counter-clockwise rotation of the central Iranian blocks; (2) autochthonous unit forming a secondary branch of the main suture zone.Our structural, petrographic and geochemical data indicate that the AMC consists of several metamorphic units also including dismembered “ophiolites” which display different tectono-metamorphic evolutions. Three main ductile deformational events can be distinguished in the AMC. The Morghab and Chah Gorbeh complexes preserve a different M1 metamorphism, characterized by blueschist relics in the S1 foliation of the former unit, and greenschist assemblages in the latter. They share a subsequent similar D2 deformational and M2 metamorphic history, showing a prograde metamorphism with syn- to post-deformation growth of blueschist facies mineral assemblages on pre-existing greenschist facies associations. High pressure, low temperature (HP/LT) metamorphism responsible for the growth of sodic amphibole has been recognized also within marble lenses at the contact between the Chah Gorbeh Complex and serpentinites. Evidence of HP/LT metamorphism also occurs in glaucophane-bearing meta-pillow lavas and serpentinites, which contain antigorite and form most of the “ophiolites” within the AMC. Structural relationships show that the Chah Gorbeh and Morghab units and the “ophiolites” were tectonically coupled within an accretionary wedge before the D2 folding stage. The other units of the AMC lack evidence of HP metamorphism in the area around Anarak, especially the Lakh Marble, a large thrust sheet that occupies the uppermost structural position in the AMC. Available radiometric ages of trondhjemite dikes and stocks that intruded the accretionary wedge, as well as our new data, constrain the subduction event at the end of the Carboniferous, before 290Ma. These data suggest that the AMC is part of an allochthonous crustal fragment belonging to the Variscan belt developed along the southern Eurasian margin before the Cimmerian collision of Iran. Subsequent deformational events that occurred during the Mesozoic and the Cenozoic, up to the Miocene and possibly later, resulted in folding, thrusting and faulting that dismembered the original structure of the wedge accompanying its displacement to the present day position.

Phase equilibria modelling of retrograde amphibole and clinozoisite in mafic eclogite from the Tso Morari massif, northwest India: constraining the P–T–M(H2O) conditions of exhumation

AbstractPhase equilibria modelling of post‐peak metamorphic mineral assemblages in (ultra)high‐P mafic eclogite from the Tso Morari massif, Ladakh Himalaya, northwest India, has provided new insights into the potential behaviour and source of metamorphic fluid during exhumation, and constrained the P–T conditions of hydration. A series of P–M(H2O) pseudosections constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO) system show that a number of petrographically distinct hydration episodes occurred during exhumation from peak P–T conditions (~640 °C, 27–28 kbar), resulting in the formation of abundant compositionally zoned amphibole and minor clinozoisite poikiloblasts at the expense of a peak assemblage dominated by garnet and omphacite. Initial hydration is interpreted to have occurred as a result of the destabilization of talc following isothermal decompression to ~23 kbar, which led to the formation of barroisite–winchite amphibole core domains. An episode of fluid infiltration from an external source at ~19 kbar, with or without syn‐decompressional cooling to ~560 °C, resulted in further barroisitic–winchitic amphibole growth, followed by the formation of clinozoisite poikiloblasts. Continued buoyancy‐driven exhumation to the base of the lower crust is constrained to have taken place with no additional fluid input. A final hydration event is characterized by the formation of magnesiohornblende rims on the barroisite–winchite cores, with the former interpreted to have formed during later prograde overprinting in the middle crust associated with the final stages of exhumation. Notably, the vast majority of externally sourced H2O, comprising just over half of the current bulk rock fluid content, was added during this later hydration event. In a middle crustal setting, this is interpreted as the result of devolatilization reactions occurring in migmatitic host orthogneiss and/or metasedimentary units, or following the crystallization of partial melt.

Formation of Amphibole and Clinozoisite-Epidote in Eclogite owing to Fluid Infiltration during Exhumation in a Subduction Channel

Two eclogites, one from the southern Dabie Shan, China, and the other from the western Erzgebirge, Germany, were studied because of amphibole and clinozoisite^epidote formation either as typical porphyroblasts or as oriented fine-grained matrix minerals. A nearly isobaric prograde path was derived for the eclogite from the Erzgebirge, mainly on the basis of garnet zonation and the reconstruction of P^T pseudosections in the system SiO2^TiO2^ Al2O3^MgO^MnO^FeO^CaO^Na2O^K2O^H2O^O2. Peak P^T conditions were 2·8 GPa and 6708C. The retrograde path shows significant cooling during pressure release. In contrast, the Dabie Shan eclogite contains sparse Mg-rich garnet cores pointing to a granulite stage. Subsequently, peak-pressure conditions of 3·5^4·0 GPa were reached at 6208C, followed by a retrograde path that was initially nearly isothermal and then similar to that of the Erzgebirge eclogite. Most of the amphibole and clinozoisite^ epidote grew during early retrogression in the pressure interval 1·8^2·3 GPa (Erzgebirge) or 2·4^2·7 GPa (Dabie Shan) owing to infiltration of hydrous fluids amounting to 1wt % and 0·5 wt % H2O, respectively. As a result of this hydration omphacite was partially decomposed, but the remaining omphacite demonstrates that infiltration of H2O essentially ceased at 1·8 GPa (Erzgebirge) or 2·4 GPa (Dabie Shan). The fluid infiltration and hence the growth of amphibole and clinozoisite^epidote is proposed to have occurred in a deep-seated subduction channel during ascent of eclogitic material. Subsequently, the eclogites were captured between colliding continental plates and further exhumed in an exhumation channel.

The P-T-X(fluid) evolution of meta-anorthosites in the Eastern Granulites, Tanzania

Meta-anorthosite bodies are typical constituents of the Neoproterozoic Eastern Granulites in Tanzania. The mineral assemblage (and accessory components) is made up of clinopyroxene, garnet, amphibole; scapolite, epidote, biotite, rutile, titanite, ilmenite and quartz. Within the feldspar-rich matrix (70–90% plagioclase), mafic domains with metamorphic corona textures were used for P–T calculations. Central parts of these textures constitute high-Al clinopyroxene – which is a common magmatic mineral in anorthosites – and is therefore assumed to be a magmatic relict. The clinopyroxene rims have a diopsidic composition and are surrounded by a garnet corona. Locally the pyroxene is surrounded by amphibole and scapolite suggesting that a mixed CO2–H2O fluid was present during their formation. Thermobarometric calculations give the following conditions for the metamorphic peak of the individual meta-anorthosite bodies: Mwega: 11–13 kbar, 850–900 °C; Pare Mountains: 12–14 kbar, 850–900 °C; Uluguru Mountains: 12–14 kbar, 850–900 °C. The P–T evolution of these bodies was modelled using pseudosections. The amount and composition of the metamorphic fluid and <0.5 mol.% fluid in the bulk composition is sufficient to produce fluid-saturated assemblages at 10 kbar and 800 °C. Pseudosection analysis shows that the corona textures most likely formed under fluid undersaturated conditions or close to the boundary of fluid saturation. The stabilities of garnet and amphibole are dependent on the amount of fluid present during their formation. Mode isopleths of these minerals change their geometry drastically between fluid-saturated and fluid-undersaturated assemblages. The garnet coronae developed during isobaric cooling following the metamorphic peak. The cooling segment is followed by decompression as indicated by the growth of amphibole and plagioclase. The estimated of the metamorphic fluid is ∼0.3–0.5. Although the meta-anorthosites have different formation ages (Archean and Proterozoic) they experienced the same Pan-African metamorphic overprint with a retrograde isobaric cooling path. Similar P–T evolutionary paths are known from the hosting granulites. The presented data are best explained by a tectonic model of hot fold nappes that brought the different aged anorthosites and surrounding rocks together in the deep crust followed by an isobaric cooling history.

Metamorphic reprocessing of a serpentinized carbonate-bearing peridotite after detachment from the mantle wedge: A P–T path constrained from textures and phase diagrams in the system CaO–MgO–Al 2O 3–SiO 2–CO 2–H 2O

In the central Rhodope mountains of Greece a carbonate-bearing metaperidotite lens ∼ 200 × 500 m in size crops out as part of the high- to ultrahigh-pressure metamorphic Upper Sidironero Complex ∼ 500 m SE of the Gorgona Village, north of Xanthi town. It is composed primarily of coarse grained (3–20 mm in size) olivine and orthopyroxene, medium grained clinohumite and medium to fine grained tremolite, chlorite, dolomite, magnesite, talc, antigorite and various spinel phases. Whole-rock chemistry, mineral textures and compositions, and phase diagram calculations show that the metaperidotite was subjected to a prograde HP metamorphism, isofacial with the surrounding migmatitic gneisses, metapelites and amphibolites. The prograde character of metamorphism is demonstrated by inclusions of talc, antigorite, chlorite, dolomite, magnesite and Ti-clinohumite in ferrit-chromite, olivine, and orthopyroxene, as well as of olivine in orthopyroxene, and by the typical change in composition of zoned spinel minerals from ferrit-chromite in the core to chromian spinel at the rim. The prograde path is characterized by successive growth of amphibole, Ti-clinohumite, olivine and orthopyroxene, followed by the breakdown of Ti-clinohumite to olivine + Mg-ilmenite and of chlorite to olivine + spinel, probably during exhumation. The construction of a partial petrogenetic P– T grid in the system CaO–MgO–Al 2O 3–SiO 2–CO 2–H 2O (CMASCH) for Ca-poor ultramafic bulk compositions has proven highly useful for the reconstruction of the metamorphic evolution and a P– T path, indicating that the use of univariant reactions in mixed volatile systems is highly warranted. The P– T path is clearly constrained to pressures below 1.5–1.7 GPa by the absence of clinopyroxene. These pressures are slightly lower than those recorded in the closely associated Jurassic eclogites and much lower than those recorded in the diamond-bearing gneisses 5 km to the south in the same tectonic unit. The carbonate-bearing metaperidotite from Gorgona probably represents a fragment of the hydrated mantle wedge. This is indicated by the REE compositions which differ from those of ophiolitic peridotites and resemble those of spinel or garnet peridotites of sub-continental origin. The ultramafic slice was incorporated tectonically into the subduction channel, most likely by tectonic erosion in the Early Jurassic, but did not experience ultrahigh-pressure metamorphism like the nearby metapelites that exhumed along the same subduction channel.

Amphibole crystallization from a water-saturated andesite melt: Experimental data at P = 2 kbar

This paper reports the results of an experimental investigation of the influence of iron oxidation state in the water-saturated andesite melt of Shiveluch Volcano (Kamchatka) on amphibole composition. Experiments with durations of 2–4 h were carried out at a pressure of 2 kbar and temperatures close to the amphibole liquidus (950°C). The degree of iron oxidation (NNO and NNO + 1) in the initial glass was determined by Mossbauer spectroscopy. The analyses of experimental amphiboles were recalculated to the distribution of elements among crystallographic sites with the estimation of the degree of iron oxidation on the basis of the IMA-97 method (Leake et al., 1997). A negative correlation was established between AlVI and the sum (Fe3+ + Ti). In the most oxidized amphiboles, (Fe3+ + Ti) reaches 1.8 atoms per formula unit (a.f.u.). The kinetics of growth affects the amphibole composition more significantly than pressure (according to the data of Schmidt, 1992). Quenching (supercooling by more than 100°C) produces amphibole crystals with AlVI = 1.3 a.f.u. and Fe3+ = Ti = 0. The obtained results allowed us to interpret the conditions of amphibole growth in the andesites of Shiveluch Volcano.

Multi-stage reaction history in different eclogite types from the Pakistan Himalaya and implications for exhumation processes

Metabasites were sampled from rock series of the subducted margin of the Indian Plate, the so-called Higher Himalayan Crystalline, in the Upper Kaghan Valley, Pakistan. These vary from corona dolerites, cropping out around Saif-ul-Muluk in the south, to coesite–eclogite close to the suture zone against rocks of the Kohistan arc in the north. Bulk rock major- and trace- element chemistry reveals essentially a single protolith as the source for five different eclogite types, which differ in fabric, modal mineralogy as well as in mineral chemistry. The study of newly-collected samples reveals coesite (confirmed by in situ Raman spectroscopy) in both garnet and omphacite. All eclogites show growth of amphiboles during exhumation. Within some coesite-bearing eclogites the presence of glaucophane cores to barroisite is noted whereas in most samples porphyroblastic sodic–calcic amphiboles are rimmed by more aluminous calcic amphibole (pargasite, tschermakite, and edenite). Eclogite facies rutile is replaced by ilmenite which itself is commonly surrounded by titanite. In addition, some eclogite bodies show leucocratic segregations containing phengite, quartz, zoisite and/or kyanite. The important implication is that the complex exhumation path shows stages of initial cooling during decompression (formation of glaucophane) followed by reheating: a very similar situation to that reported for the coesite-bearing eclogite series of the Tso Morari massif, India, 450 km to the south-east.

THE TRACE METAL CONTENT OF AMPHIBOLE AS A PROXIMITY INDICATOR FOR Cu-Ni-PGE MINERALIZATION IN THE FOOTWALL OF THE SUDBURY IGNEOUS COMPLEX, ONTARIO, CANADA

Alteration veins containing Cl-poor actinolite and actinolitic hornblende are a common feature throughout the Archean-aged gneisses and granitoids along the northern margin of the Sudbury Igneous Complex, Ontario, Canada. Crosscutting relations and structural measurements show that the veins postdate the Sudbury impact event but predate the emplacement of footwall-style Cu-Ni-PGE (platinum-group element) sulfide ores. The veins are four to five times more abundant in brecciated, mineralized outcrop than in unbrecciated, barren outcrop. Analyses of vein-hosted amphibole by electron microprobe (EMP) and laser ablation ICP-MS (LA-ICP-MS) show elevated concentrations of Ni (up to ~100 times background; increasing from ~100 ppm to ~1.0 wt %), Cu (up to ~4 times background) and Sn (up to ~2.5 times background) in proximity to sulfide veins. Copper and Sn contents in amphibole are only anomalous within 10 to 20 m of mineralization and require quantification by LA-ICP-MS. However, anomalous Ni concentrations in vein-hosted amphibole may be detected with a high level of confidence solely by EMP up to ~700 m from mineralization in rocks that contain no visible sulfides in hand sample or thin section and show no bulk rock Ni or Cu anomalies. The relationship between the Ni content in amphibole (in ppm) and mineralization proximity (distance “D” in meters) is given by the following equation: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[[Ni]_{amp} = 5100 (D)^{{-}0.45}.\] \end{document} The data indicate wide-scale Ni mobility in hydrothermal solutions in the Sudbury footwall. The formation of Ni-rich amphibole in the fracture networks, by either the growth of primary Ni-rich amphibole or metasomatic Ni enrichment of preexisting (Ni-poor) amphibole, was associated with circulation of a sodium- and halogen-rich fluid between ~390o and 550oC. Nickel may have been released into the footwall environment by extensive hydrothermal alteration of Ni-rich contact-style sulfide ores that lie in the hanging wall of the Cu-rich footwall-style ores. In the footwall rocks, pre-existing fracture networks (now amphibole-bearing veins) acted as common structural pathways, first for the circulation of early Ni-rich fluids associated with leaching in the contact region and the onset of footwall mineralization, and second for the emplacement of Cu-rich footwall-style sulfide veins. Other metal concentrations in amphibole (Pb, Zn, Co) show no relationship to mineralization proximity. These metals may have been derived locally from country rocks during alteration reactions that promoted amphibole growth in the fractures. Identification of fracture-hosted amphibole with anomalous Ni contents in thin section, or Ni-Cu-Sn anomalous amphibole grains liberated from surface-weathered rocks hosting sulfide veins may serve as useful proximity indicators for hidden footwall-style sulfide deposits at Sudbury.

Cretaceous and Triassic subduction-accretion, high-pressure-low-temperature metamorphism, and continental growth in the Central Pontides, Turkey

Biostratigraphic, isotopic, and petrologic data from the Central Pontides document major southward growth of the Eurasian continental crust by subduction-accretion during the Cretaceous and Triassic Periods. A major part of the accreted material is represented by a crustal slice, 75 km long and up to 11 km thick, consisting of metabasite, metaophiolite, and mica schist that represent underplated Tethyan oceanic crustal and mantle rocks. They were metamorphosed at 490 degrees C and 17 kbar in mid-Cretaceous time (ca. 105 Ma). The syn-subduction exhumation occurred in a thrust sheet bounded by a greenschist facies shear zone with a normal sense of movement at the top and a thrust fault at the base. A flexural Foreland basin developed in front of the south-vergent high-pressure-low-temperature (HP-LT) metamorphic thrust sheet; the biostratigraphy of the foreland basin constrains the exhumation of the HP-LT rocks to the lbronian-Coniacian, similar to 20 m.y. after the HP-LT metamorphism, and similar to 25 m.y. before the terminal Paleocene continental collision. The Cretaceous subduction-accretion complex is tectonically overlain in the north by oceanic crustal rocks accreted to the southern margin of Eurasia during the latest Triassic-earliest Jurassic. The Triassic subduction-accretion complex is made up of metavolcanic rocks of ensimatic arc origin and has undergone a high pressure, greenschist facies metamorphism with growth of sodic amphibole. Most of the Central Pontides consists of accreted Phanerozoic oceanic crustal material, and hence is comparable to regions such as the Klamath Mountains in the northwestern United States or to the Altaids in Central Asia.

The role of water in the synthesis of glaucophane

Syntheses have been done at 2.5 GPa and 750 °C to examine the effects of varying the water content and composition of the solid starting mixture on the formation of glaucophane [Na 2 Mg 3 Al 2 Si 8 O 22 (OH) 2 ]. Amphiboles of apparently ideal glaucophane composition, confirmed by electron microprobe analyses, were obtained by treating oxide or oxide-hydroxide mixtures with bulk water contents of 4.5 wt% for durations of about 400 h in multiple treatments with intermittent grindings. This water content is necessary to permit growth of amphibole while preventing the intervention of a sheet silicate, which was identified in this study as vermiculite. To maximize amphibole yields, multiple treatments with intermittent grinding are needed on account of the low water contents that reduce the reaction rate of the synthesis process. Calculation of the solubility curves of talc, jadeite, and glaucophane in log[a Mg(OH) + /a H + ] vs. log[(a Na + )(a AlSiO₄ - )] space at 2.5 GPa and 750 °C shows that glaucophane has a very narrow field of stability without coexisting jadeite or talc. This same diagram can be used to illustrate that the growth of vermiculite relative to glaucophane is not an issue of metastability but of the activity of water.

Calcic amphibole growth and compositions in calc-alkaline magmas: Evidence from the Motru Dike Swarm (Southern Carpathians, Romania)

A Late Pan-African calc-alkaline dike swarm (basalt-andesite-dacite-rhyolite) has been investigated in a region of over 2000 km 2 in the Alpine Danubian window, South Carpathians (Romania). Amphibole phenocrysts and microphenocrysts have been investigated by wavelength-dispersive microprobe analysis and BSE imaging. The Ca-amphibole population, represented in all the lithologies, displays a large compositional range, interpreted as the result of two processes: (1) magmatic evolution (kaersutite → Ti-pargasite → pargasite → Ti-magnesiohastingsite → magnesiohastingsite → edenite → tschermakite → magnesiohornblende) linked to magmatic differentiation from andesitic basalt to rhyolite; and (2) deuteritic alteration of the primary amphibole related to late-emplacement hydrothermal activity (yielding numerous varieties comprising those cited above). In all rock types, amphibole phenocrysts equilibrated at a nearly constant pressure of about 0.6 ± 0.1 GPa, but their temperatures of crystallization ranged from 1000–900 °C for basaltic andesites to 700–600 °C for dacites. In rhyolites, edenite to magnesiohornblende crystals res ect a continuous range of P-T conditions from 700 °C/0.6 GPa to 600 °C/0.1 GPa, in agreement with their change of habit from euhedral to subhedral. Complex zonations in pargasite-magnesiohastingsite (including resorption) are interpreted in term of self-organization of

Subduction‐related metasomatism in the thinning lithosphere: Evidence from a composite dunite‐orthopyroxenite xenolith entrained in Mesozoic Laiwu high‐Mg diorite, North China Craton

The North China Craton (NCC) lost its Archean keels in the Phanerozoic. Prevalent and intensive magmatism, mineralization, and development of extensional basins in the late Mesozoic NCC imply that the late Mesozoic could be the key stage for this transformation. Ultramafic xenoliths in the Early Cretaceous high‐Mg diorites of Shandong province might provide key information about the transformation of subcontinental lithospheric mantle (SCLM) beneath the NCC. Here we present a unique composite dunite‐orthopyroxenite xenolith from Tietonggou, one of the high‐Mg diorite‐dominated plutons in Laiwu, Shandong province. The petrography and mineral chemistry of the xenolith suggest complicated metasomatic processes, which occurred before its entrainment in the host magma. Early stage metasomatism includes the growth of intergranular phlogopite and clinopyroxene and the development of a phlogopite‐ and amphibole‐bearing clinopyroxenite veinlet. Late‐stage metasomatism (termed Si (Na) metasomatism) is characterized by the growth of secondary orthopyroxene, Na‐rich plagioclase and amphibole with resorption of olivine and clinopyroxene, and the decomposition of phlogopite. The xenolith has exceptionally high concentrations of Na2O and Al2O3 and shows enrichments in Cs, Rb, Th, U, K, and the light rare earth elements. It also shows positive Pb and Sr anomalies and negative Nb, Ta, P, and Ti anomalies in a primitive mantle normalized spider gram. The geochemistry, as well as the elevated δ18O, suggests that this Si (Na) metasomatism is associated with subduction. The secondary orthopyroxene in the orthopyroxenite portion of the xenolith has exceptionally low Mg# values, which may be the result of reaction between silica‐rich melts and olivine with high melt:rock ratios. Mg‐Fe disequilibrium of the minerals in the orthopyroxenite indicates that Si (Na) metasomatism may have been introduced shortly before entrainment of the xenolith in the host magma. Correlation of the Sr‐Nd‐Pb isotopic compositions of ultramafic xenoliths, high‐Mg diorites, and adakitic granites of the Tietonggou indicates there may be a genetic relationship between these rocks. Thus we propose that the SCLM beneath Shandong may have been metasomatized by a slab‐derived melt. Since Si (Na) metasomatism occurred at the key stage of lithospheric thinning, oceanic subduction might have been involved in the thinning process.

Petrology and geochronology of metabasites with eclogite facies relics from NE Sardinia: constraints for the Palaeozoic evolution of Southern Europe

In the Golfo Aranci high-grade metamorphic basement of NE-Sardinia (Italy), a fragment of the southern part of the Variscan chain, several metabasic rocks with relic eclogitic parageneses are interlayered with a dominant sequence of migmatitic paragneisses and felsic orthogneisses. Petrologic, geochemical and geochronological data on selected samples permitted the reconstruction of a long history starting from pre-Variscan magmatic activity and progressing with a polyphase metamorphism during the Variscan orogenic event. U–Pb LAM-ICPMS dating of zircon domains from two eclogites allowed us to constrain the emplacement of the mafic protolith in Ordovician times (460±5 Ma). The basic rocks were buried in a subduction-related environment with formation of Ky-bearing eclogite parageneses (undated event, ∼650 °C and ∼1.9 GPa). Subsequently, the eclogites underwent a strong reequilibration first under the granulite facies (sapphirine-bearing parageneses: 700–800 °C, ∼1.0 GPa) and then under the amphibolite facies with pervasive growth of brown amphibole and resetting of the U–Pb system in zircon (352±3 Ma). This high-pressure metamorphic basement and its strong analogies with the basement slices of Corsica, Provence, of the Ligurian Alps and the intra-Alpine massifs indicate the presence of a HP migmatite–eclogite belt extending for several hundreds of kilometers at the southern margin of the Variscan realm.

CHARACTERISATION OF THE LITHOSPHERE BENEATH THE NORTH-WESTERN ETHIOPIAN PLATEAU: SPINEL LHERZOLITES FROM INJIBARA (LAKE TANA PROVINCE)

We have investigated xenoliths of spinel lherzolites in a Quaternary basanitic flow near Injibara (Tana Lake Province) to characterise the mantle beneath the north-western Ethiopian Plateau. Previous studies of mantle rocks from the Main Rift (e.g. Mega, South Ethiopia; Bedini et al., 1997), have indicated that peridotites suffered modal (apatite) or cryptic (enrichment/depletion in LILE) metasomatism. The metasomatic agents were identified as silicate melts migrating through the lithospheric mantle. At Injibara, lherzolites [Ol (46-69), Opx (19-31), Cpx (9-22), and Spl (2-7) in vol.%] show porphyroclastic to equigranular textures. These rocks suffered a modal metasomatic event, testified by growth of Ti-pargasite, and enrichment in major (Mg,Na,Ti) and trace elements (LREE,Rb,Ba,Th,U,Pb). Equilibration temperatures are 907-1015 °C (median values 960-970°C), in agreement with previous data obtained from Injibara mantle xenoliths (954-985°C, Conticelli et al., 1999). Fluid inclusions (<1-40 _m) occur only in the most preserved rocks, along intragranular trails within porphyroclasts, and as rare primary inclusions in Opx neoblasts. At room temperature, monophase (L) and biphase (L+V) inclusions are observed with rare birefringent solids, recognised as magnesite by Raman spectroscopy. Microthermometric data reveal the presence of CO2 that melts (TmCO2) instantaneously at @ -56.6°C. Most of inclusions homogenise to the liquid (ThLCO2) between –39.2 and 31°C, and only rare homogenise to the vapour (ThVCO2; 28.3 - 30.9°C). For a temperature of 970°C, a minimum pressure of 0.95 GPa is obtained by the highest density (1.11 g/cm3) fluid isochore. In conclusion, our data indicate that the mantle under Injibara (North-Western Ethiopian Plateau) underwent a different evolution with respect to the mantle beneath the Rift. In mantle rocks beneath Injibara, growth of amphibole and the LILE enrichment are strictly associated to the migration of a CO2-rich fluid, whereas in rift peridotites, apatite growth and variable LILE contents are related to interaction with silicate melts.

Fractional crystallization of hydrous basaltic ?arc-type? magmas and the formation of amphibole-bearing gabbroic cumulates

The Rymmen and Eriksberg gabbros preserve the crystallization products of hydrous basaltic liquids, ranging from anorthosite and troctolite to highly fractionated oxide gabbro, diorite, and leucotonalite. The high initial water content of the liquids that filled the magma chambers (1.5–3 wt%) allowed amphibole to saturate as an interstitial mineral even in the earliest formed cumulates. The increasing water content of the system helped to produce a thick sequence of cumulates with anorthitic plagioclase (An87–92). The growth of interstitial amphibole caused much more interstitial liquid to be incorporated into the cumulates than might be expected for dryer basaltic systems, producing a very imperfect fractional crystallization process. Comparisons of models of the trace-element evolution of the two intrusions reveal that the overall enrichments of some elements are adequately predicted by a perfect fractional crystallization model but all agree better with an imperfect fractional crystallization model. This reflects the change in the bulk distribution coefficients of the solid assemblage upon amphibole saturation, with the retention of elements compatible in amphibole in the crystal pile and the loss of elements incompatible in amphibole from it. These results stress the importance of understanding the physical, as well as the chemical processes involved in fractional crystallization and in particular the role of postcumulus processes.

Pyribole evolution during tremolite synthesis from oxides

The formation mechanisms of tremolite in a time series of hydrothermal experiments at 850 °C and 6.1 kbar were studied using transmission electron microscopy (TEM), analytical TEM, and Xray diffraction. The starting materials were unseeded mixtures of reagent-grade oxides. After the first 8 min of reaction, the synthesis products were diopside, enstatite, clinoenstatite, quartz, and minor C-centered and primitive monoclinic amphibole (13 wt% total). The compositions of the firstformed amphiboles vary considerably, from essentially pure cummingtonite (Ca/Mg = 0) to nearly ideal tremolite (Ca/Mg = 0.4). Three groupings of amphiboles with successively higher Ca/Mg ratios were identified, respectively 0.1, 0.22, and 0.35. Over a period of 97 h, the proportions of Mg-rich amphibole gradually decrease, and the average Ca/Mg ratios of all amphiboles steadily increase from about 0.22 to 0.35. Other changes include: (1) a decrease in the aspect ratio of amphibole crystals with increasing time; (2) a decrease in the number of non-amphibole chain multiplicity faults (CMFs), from early A′(2) values as low as 0.81 to a final value of 0.94; and (3) a distinct change in the rate of amphibole growth after 1 h of synthesis. These observations suggest that two mechanisms of amphibole formation are dominant: (1) formation of pyroxene modules with subsequent hydration and arrangement into the double-chain amphibole structure and (2) direct nucleation and growth of amphibole from hydrothermal solution. A third mechanism involving formation of talc-like modules, followed by their breakdown into double-chain modules, incorporation of Ca, and ordering into the amphibole structure, may also occur but probably only at the lamellar or unit-cell scale, because these syntheses were performed outside the stability field of talc.

Rejuvenation of the Fish Canyon magma body: A window into the evolution of large-volume silicic magma systems

Voluminous, unzoned, phenocryst-rich pyroclastic deposits, considered as erupted batholiths, provide a unique opportunity to investigate magmatic processes in silicic magmas. The Fish Canyon Tuff, a well-documented example of these monotonous ignimbrites, displays evidence for simultaneous dissolution of feldspars 1 quartz and crystallization of hydrous phases during gradual near-isobaric reheating from ;720 to 760 8C. These observations, along with a high crystallinity (45%) and near-solidus mineral assemblage, suggest that the Fish Canyon magma cooled to a rigid crystal mush before being partly remelted prior to eruption. Rejuvenation was triggered by intrusion of water-rich mafic magmas at the base of the Fish Canyon mush, but the mechanisms of heat transfer remain poorly understood. The growth of amphibole during reheating requires addition of mafic components, but the absence of any measurable gradients and the paucity of mafic enclaves in the Fish Canyon magma rule out a reheating event dominated by convective mixing with a mafic magma. Closed-system processes, such as heat conduction and convective self-mixing, could not account for the transport of externally derived mafic components. We performed numerical simulations of upward percolation of a hot, low-density H2O-CO2 fluid phase (gas sparging) through a crystalline framework saturated with rhyolitic melt to assess the efficiency of such a process in rejuvenating silicic mushes in open systems. Sparging by ;20‐40 km 3 of gas extracted from ;3000 km 3 of mafic magma is capable of reheating 7500 km 3 of silicic crystal mush by .40 8C in 150‐200 k.y. Moreover, the vertical thermal gradient after 150 k.y. in most of the mush is small (;25 8C in the upper 65%). Gas sparging also produces an increase in the internal pressure of silicic crystal mushes and may lead to the formation of crystal-poor rhyolites by expelling interstitial melt. However, our simulations predict that filter pressing driven by sparging of externally derived gas could not solely account for the generation of the most voluminous rhyolites.