Kyanite Quartzite Research Articles

Tourmaline composition and boron isotopes record lateritic weathering during the Great Oxidation Event

AbstractIdentifying evidence of oxidative weathering in the geological record is essential to trace the evolution of Earth's atmosphere oxygenation. Metamorphosed residues of lateritic weathering have been identified as two rock types in the 2.1‐Ga‐old Cercadinho Formation, Piracicaba Group, Quadrilátero Ferrífero of Minas Gerais. One is tourmaline–hematite–sillimanite–kyanite quartzite; the other is rutile–tourmaline–hematite–muscovite phyllite. Both rocks have abundant tourmaline with δ11B values between about −17‰ and −13‰. The Cercadinho tourmaline is roughly parallel to the povondraite–“oxy‐dravite” join of meta‐evaporitic tourmaline, in its more aluminous segment, offset to higher contents of iron. These compositional and isotopic characteristics of the Cercadinho tourmaline indicate that continental evaporitic brines interacted with aluminium‐ and iron‐rich residues of lateritic weathering. The abundance of disseminated tourmaline, a mineral poorly reported from palaeosols worldwide, implies a boron‐rich brine overprint on the lateritic profile before the onset of metamorphism, reflecting a climatic change from humid to arid conditions in a continental setting. The recognition of lateritic weathering in the Cercadinho Formation contributes to the amount of evidence for increased levels of atmospheric oxygen between 2.22 and 2.06 Ga ago.

QUARTZ RAW MATERIALS OF THE KARELIAN-KOLA REGION: MAJOR RESEARCH RESULTS

The article draws attention to the fact that the crystal lattice of quartz allows for very little isomorphism, most elements encountered in quartz (Na, K, Ca, Mg, Ba, Sr, Rb, REE, Mn, U, Th) are associated with fluid and micromineral inclusions. Quartz is regarded as an industrial mineral with the main focus on its use as high-purity quartz. The scheme of the industrial application of quartz and the products of its technological processing is provided. The chronological sequence of stages in research on quartz raw materials in the Karelian-Kola quartz province is described, and the industrial significance of various genetic typesof quartz deposits (vein, pegmatite, silexite, high-purity quartzites, muscovite and kyanite quartzites, quartz conglomerates, silicites) is demonstrated. The various characteristics of the main deposits of quartz raw materials at present (Perchatka, Fen’kina-Lampi, Melomais, and Stepanovo Ozero) are reported. The role of the Institute of Geology KarRC RAS in producing recent results allowing new approaches to the assessment of quartz raw material in the Karelian-Kola region to be designed is highlighted. The work of IG KarRC RAS staff in collaboration with other organizations in the country (VIMS, IGEM RAS, IG Komi SC Ural Branch RAS, GI KSC RAS, TsNIIgeolnerud, Tsentrkvarz) has contributed significantly to the science-based assessment of quartz raw material in the Karelian-Kola quartziferous province of Russia. The proposed solutions have been tested in application to the methods for evaluating the quality of raw materials and enrichment technology using natural quartz from Perchatka, Melomais, Rukhnavolok, etc.

The kyanite quartzite of Bosland (Suriname): evidence for a Precambrian metamorphosed alteration system

AbstractThis article investigates the origin of a rare occurrence of kyanite quartzites in the Palaeoproterozoic greenstone belt of Suriname. The rocks form elongated hills in the Bosland area, Brokopondo district, where they are associated with meta-sedimentary, meta-volcanic and granitic lithologies. Their mineral content and unusual Si- and Al-rich chemical composition are inferred to be the result of advanced argillic alteration of felsic volcanic tuffs and a later overprint by regional metamorphism up to lower amphibolite facies during the Trans-Amazonian orogeny. Structurally, the Bosland area seems centred within a contractional strike-slip duplex of a major dextral fault system. The alteration was probably associated with a high-sulphidation environment and involved significant to almost complete removal of alkali and alkaline earth elements. Pseudosection modelling and textures suggest that the precipitation–temperature (P–T) history of the kyanite quartzites started with shallow (<2kbar) hydrothermal alteration of the acidic tuffaceous volcanics, possibly in the andalusite stability field (T>350°C), and ended in peak metamorphic conditions in the kyanite–staurolite stability field (P>4kbar andT=500–650°C). Alteration events that preceded the peak of Trans-Amazonian metamorphism may be more common in the rock record of Suriname's greenstone belt, which lends support to the hypothesis that gold mineralisations in the region can be pre-orogenic.

Investigation of zircon by CL (Cathodoluminescence) and Raman Spectroscopy

Puturge metamorphites consists of schist, gneisse, metagranite gneisse, amphibolite, kyanite quartzite and marble type rocks. Mineralogical studies, geochemical analysis (LA- ICPMS), Raman spectroscopy and cathodoluminescence (CL) imaging that it representing amphibolite facies and greenschiste facies. Zircon imaging called as a metamict from the cathodoluminescence images of zircon minerals. The partially radiated zircon particles is higher radiogenetic mineral ratio in comparison with other zircon particles. The ratio of the radiogenetic elements (U, Pb and Th) arises from chemical difference between the core and rims of zircons. The solubility of zircon effects environmental conditions such as high pH, Zr with hydroxyl ions. Especially alkaline fluids in environment can dissolve zircon. The results show that radiogenetic elements loss in zircons can be generated from metamict zircon through volume diffusion at low temperatures or by an external fluid (H2O). The loss of lead in zircon signifies that the fluids inserting the crystal lattice causes radiation damage processes.

Mineralogical and geochemical investigations of silicate-rich mine waste from a kyanite mine in central Virginia: implications for mine waste recycling

A kyanite mine in central Virginia produces a silicate-rich waste stream which accumulates at a rate of 450,000–600,000 tons per year. An estimated 27 million tons of this waste stream has accumulated over the past 60 years. Grain size distribution varies between 1.000 and 0.053 mm, and is commonly bimodal with modes typically being 0.425 and 0.250 mm and uniformity coefficients vary from 2.000 to 2.333. Hydraulic conductivity values vary from 0.017 to 0.047 cm/s. Mineralogy of the waste stream consists of quartz, muscovite, kyanite and hematite. Muscovite grains have distinct chemical compositions with significant Na2O content (1.12–2.66 wt%), TiO2 content (0.63–1.68 wt% TiO2) and Fe content, expressed as Fe2O3 (up to 1.37 wt%). Major element compositions of samples were dominated by SiO2 (87.894–90.997 wt%), Al2O3 (6.759–7.741 wt%), Fe2O3 (1.136–1.283 wt%), and K2O (0.369–0.606 wt%) with other components being <1.000 wt%. Elements of environmental concern (V, Cr, Ni, Cu, Zn, As, Ag, Sn, Sb, Ba, Hg, Tl, and Pb) were detected; however, the concentrations of all elements except Ni were below that of the kyanite quartzites in the region from which the waste is derived. Both major and trace element compositions indicate minimal variation in composition. The waste stream has potential for recycling. Muscovite is suitable for recycling as a paint pigment or other industrial applications. Muscovite and hematite are commonly intergrown and are interpreted to be material where much of the elements of environmental concern are concentrated. Reprocessing of the waste stream to separate muscovite from other components may enable the waste stream to be used as constructed wetland media for Virginia and nearby states. Recycling of this mine waste may have a positive impact on the local economy of Buckingham County and aid in mitigation of wetland loss.

Kyanite Quartzites in the Piedmont Province of Virginia: Evidence for a Possible High-Sulfidation System

The world’s largest kyanite mine is hosted within kyanite quartzite at Willis Mountain in the Piedmont Province of Virginia, and numerous smaller bodies occur throughout this region. Most occur in the Ordovician Chopawamsic or Milton terranes, which consist primarily of arc-derived metavolcanic rocks, volcanogenic metasedimentary rocks, and felsic plutonic rocks. Earlier investigators viewed these rocks as metamorphosed aluminous sandstones, but we report mineralogical and geochemical evidence that the protoliths were hydrothermally altered igneous rocks. All rocks contain accessory rutile, white mica (locally green Cr muscovite), and locally abundant pyrite. Topaz, lazulite, and other rare minerals occur in some samples. Whole-rock analyses for 22 samples from nine separate localities show very low to negligible concentrations of K, Na, Ca, Mg, and many trace elements; and a strong depletion in Ga relative to Al. Analyses of the rare earth elements (REE) in four samples show that all are light REE enriched, with ladlelike chondrite-normalized patterns (relative depletions from Gd through Ho). Reconnaissance oxygen isotope data on quartz separates show a range in δ18O values from 3.5 to 11.6 per mil and probably reflect hydrothermal fluids with variable meteoric and magmatic signatures. We suggest that these rocks originated by severe leaching in a high-sulfidation (advanced argillic) alteration system, resulting in protoliths dominated by quartz and kaolinite. Such an interpretation is consistent with mineralogical and compositional features and with the regional geologic setting, which includes numerous small but historically productive deposits of Fe, Au, Cu, Zn, and S. In addition, thermodynamic calculations suggest that sulfuric acid-bearing fluids are the only type capable of significantly fractionating Ga from Al.

High-purity quartz mineralisation in kyanite quartzites, Norway

This study presents an evaluation of Norwegian kyanite quartzites from Gullsteinberget, Knosberget, Kjeks- berget, Sormbrua, Tverradalen, Juovvacorru and Nasafjellet as potential deposits of high-purity quartz (HPQ) for use as raw material for special applications in high-technology industries. Fine-grained quartz, which forms 70 to 85 vol.% of these rocks, generally contains less than 50 μ gg −1 (total sum) of the structurally incorporated trace elements B, Li, Al, Ge, Ti, Fe, Mn, K and P. The concentrations are in the same range as those found in HPQ products, which are being mined and produced in Norway and elsewhere. Quartz analyses were performed using laser ablation- inductively coupled plasma mass spectrometry. Compli- mentary whole-rock analyses and cathodoluminescence studies of quartz were carried out to reveal processes, which have led to the low trace-element concentrations in quartz. This discovery, together with a better knowledge of the processes leading to the formation of HPQ in kyanite quartzites, could lead to the recognition of a new global type of HPQ resource applicable for industrial use. However, the processing technology necessary to separate HPQ from kyanite quartzite economically has not been developed so far.

Multiphase solid inclusions in UHP rocks (Su-Lu, China): Remnants of supercritical silicate-rich aqueous fluids released during continental subduction

Primary multiphase solid (MS) inclusions without preserved fluid are found within peak minerals in kyanite quartzite ± topaz and kyanite–phengite–epidote eclogite from Donghai area (Su-Lu terrane). Typical mineral association in inclusions is: paragonite + muscovite + anhydrite ± corundum ± “alunite-type” sulphate ± zircon ± calcite ± chlorite ± SiO 2 ± barite ± pyrite ± apatite in quartzites, and paragonite + rutile + apatite ± amphibole ± Zn-staurolite ± magnetite ± plagioclase ± zircon ± pyrite ± “alunite-type” sulphate ± Zn–Mg–Fe–Al–Ti spinel in eclogites. On the basis of the fluid inclusion textures and of the daughter-phase assemblage, calculated fluid composition is as follows: in quartzites 24 wt.% SiO 2, 30 wt.% Al 2O 3, 9 wt.% CaO, 5 wt.% K 2O, 3 wt.% Na 2O, 11 wt.% SO 3, 18 wt.% H 2O, with traces of TiO 2, Fe 2O 3, FeO, MgO, BaO, P 2O 5, Cl −, F, and (CO 3) 2−, and in eclogites 26 wt.% SiO 2, 21 wt.% TiO 2, 20 wt.% Al 2O 3, 2 wt.% MgO, 4 wt.% FeO, 6 wt.% Fe 2O 3, 7 wt.% CaO, 3 wt.% Na 2O, 3 wt.% P 2O 5, 7 wt.% H 2O, 1 wt.% Cl, and traces of ZnO, MnO, K 2O, SO 3, and F. Originally trapped chloride-poor aqueous fluids contained very high amounts – in the order of tens of wt.% – of Si 4+, Al 3+, and Ti 4+. Solute species reflect the chemical composition of the host rocks: Mg 2+, Fe 2+, Ti 4+, P 5+, and Na +, are abundant in the fluids present in eclogites, while fluids are enriched in Al 3+, K +, Na +, (SO 4) 2−, (CO 3) 2− in quartzites. We propose that multiphase solid inclusions represent remnants of high-density supercritical silicate-rich aqueous fluids that were in equilibrium with peak minerals at UHP conditions. These fluids show characters which are transitional between aqueous fluids and silicate melts, and were probably produced by dehydration reactions of the host rocks during the latest stages of subduction.

X-ray single-crystal structure refinement of an OH-rich topaz from Sulu UHP terrane (Eastern China) Structural foundation of the correlation between cell parameters and fluorine content

An OH-rich topaz, Al 2 SiO 4 F 1.4 OH 0.6 , occurring in a kyanite quartzite of the Sulu UHP terrane (eastern China) has been optically and chemically (electron microprobe) characterised and its crystal structure refined by single-crystal X-ray diffraction data. Anisotropic refinement converged to R = 0.024 in the Pbnm space group with cell parameters a = 4.6696(8) A, b = 8.8486(17) A, c = 8.3915(22) A. The position of the hydrogen atom has been detected; because the OH → F substitution is less than 50 %, the disorder of H does not lower the overall symmetry. The influence of the OH → F substitution on the lattice parameters is discussed in terms of crystal structure and shown to affect primarily the a and b cell edges; instead c is almost constant. A critical revision of earlier linear correlations of the F content ( w f ) vs the lattice parameters and the availability of new data recently published, allowed to obtain improved regression lines for w f vs a ( r 2 = 0.982) and b ( r 2 = 0.968) cell edges.

Genesis and transformations of monazite, florencite and rhabdophane during medium grade metamorphism: examples from the Sopron Hills, Eastern Alps

Electron microprobe studies on the age, mineral chemistry and alteration on accessory LREE-phosphate minerals have been carried out in medium-grade metamorphic rocks of the Sopron Hills belonging to the Lower Austroalpine tectonic unit. Monazite (and xenotime) is relatively common, whereas rhabdophane and florencite are restricted to certain rock types. A first generation of monazite was formed in mica schists during the pre-Alpine, Hercynian metamorphism at 575–700 °C and 1.8–3.8 kbar as evidenced by P–T data from the literature, their mineral paragenetic and textural characteristics and supported by Th–U–total Pb ages of ca. 300 Ma. In orthogneisses, monazite is rare and of igneous origin. Kyanite quartzites and leucophyllites that were formed by Mg metasomatism contain inherited monazite from the precursor rocks. A new generation of monazite was also formed during the Alpine metamorphism at ≤550 °C, 13 kbar according to the literature data, giving ages around 75 Ma. Pronounced negative Eu anomalies were found in the igneous monazites (Eu/Eu*<0.35), while most of the metamorphic monazites have moderately negative Eu anomalies (Eu/Eu*>0.4). Small differences have been observed in Y and HREE contents, whereas the LREE sections of the rare-earth element (REE) patterns nearly coincide. Th and Ca enter the monazite structure at the expense of REE, nearly according to the brabantitic replacement 2REE 3+↔Th 4++Ca 2+. In some mica schists, monazite is altered to rhabdophane. Rhabdophane, distinguished from monazite by quantitative electron microprobe analysis by low-oxide total, is found in many mica schists and orthogneisses. It forms fine-grained aggregates, often attached to apatite or monazite. It usually has higher Y and Ca contents and a less pronounced negative Eu anomaly than that of coexisting monazite. It may have been formed either by crystallization from REE-containing hydrous solutions or from monazite reacting with Y–Ca-containing solutions. Florencite appears only in some leuchtenbergite-bearing leucophyllites, kyanite quartzites and REE-rich clasts. It is often idioblastic and may be grown on apatite or monazite. It is chemically close to its ideal composition, but Ca, Sr and Th may replace REE in minor amounts. In some grains, ThO 2 may reach 10 wt.%. The data indicate that the charge balance is maintained by different mechanisms in low- and high-thorian florencite. No Y or HREE (above Gd) could be measured in florencite. No fractionation was observed between coexisting monazite and florencite; however, monazite inclusions in florencite are depleted in La–Ce and enriched in HREE.

Hydroxyl-rich topaz in high-pressure and ultrahigh-pressure kyanite quartzites, with retrograde woodhouseite, from the Sulu terrane, eastern China

Hydroxyl-rich topaz was recognized in thick kyanite quartzites from both high-pressure (HP) and ultrahigh-pressure (UHP) belts of the Sulu terrane, China. These quartzites contain variable proportions of quartz, kyanite, and topaz, with minor phengite, pyrite, and rutile. Some topaz grains from the UHP belt contain abundant inclusions of oriented kyanite, whereas those from the HP belt are partially replaced by woodhouseite [CaAl 3 (PO 4 )(SO 4 )(OH) 6 ]. Most topaz crystals contain 9.5 to 13.5 wt% F [0.92-1.30 atoms per formula unit (apfu)], indicating 35-55% substitution of F by OH. Such naturally occurring, hydroxyl-rich topaz has not been previously reported. Some topaz grains from the HP belt show distinct zoning: (1) decreasing F content from narrow cores (13.3-16.5 wt%) to thick rims (9.5 wt%) or (2) oscillatory zoning (9.44-12.77 wt%). Unit-cell parameters of topaz show a positive linear correlation between the OH content and a and b as well as volume. Based on our petrologic data, the experimentally determined curve of Ky + H 2 O = topaz-OH at very high pressures, and calculated OH/(OH + F) isopleths of topaz at low pressures, the topaz with X OH ~ 0.35 from the UHP belt may have formed at P-T conditions within the coesite stability field. The P-T condition of HP topaz is less well constrained; its high X OH (0.40-0.55) may have been caused by lower metamorphic temperatures and higher initial X OH in comparison with UHP topaz. Hydroxylrich topaz together with other hydrous minerals in UHP rocks may be important carriers of H 2 O to mantle depths of 100-200 km during continental subduction.

Mineral inclusions in zircons of para- and orthogneiss from pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project

The pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project (CCSD), with depth of 432 m, is located in the Donghai area in the southwestern Sulu terrane. The core samples are mainly comprised of paragneiss, orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite-bearing kyanite quartzite. All analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism. Coesite and coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss, eclogite and phengite-bearing kyanite quartzite samples. In the paragneiss samples, UHP mineral inclusion assemblages mainly consist of Coe+Omp+Grt+Phe, Coe+Jd+Phe+Ap preserved in the mantles (M) and rims (R) of zircons. These UHP mineral inclusion assemblages yield temperatures of 814–852 °C and pressures of ≥28 kbar, presenting the P– T condition of UHP peak metamorphism of these country rocks. According to the mineral inclusions and cathodoluminescence images of zircons, the orthogneisses can be divided into two types: UHP (OG1) and non-UHP (OG2). In OG1 orthogneisses, low-pressure mineral inclusion assemblage, mainly consisting of Qtz+Phe+Ab+Ksp+Ap, were identified in zircon cores (C), while coesite or coesite-bearing UHP mineral inclusions were identified in the mantles (M) and rims (R) of the same zircons. These features suggest that the OG1 orthogneisses, together with the paragneisses, phengite-bearing kyanite quartzite and eclogite experienced widespread UHP metamorphism in the Sulu terrane. However, in the zircons of OG2 orthogneiss samples, no UHP mineral inclusions were found. Inclusions mainly comprised Qtz+Phe+Ap and were identified in cores (C), mantles (M) and rims (R) of OG2 zircons; the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims. These features indicate that the OG2 orthogneisses in pre-pilot drillhole CCSD-PP1 did not experience UHP metamorphism. Therefore, we should not rule out the possibility that some orthogneisses in Sulu terrane might represent relatively low-pressure granitic intrusives emplaced after the UHP event.

Petrological and Geochemical Characteristics of Ultrahigh-Pressure Metamorphic Rocks from the Dabie-Sulu Terrane, East-Central China

The Dabie-Sulu ultrahigh-pressure (UHP) terrane is the largest in the world. Mafic-ultramafic rocks occur as ubiquitous minor components in it, and preserve UHP index minerals such as coesite and microdiamond. Eclogites and garnet peridotites together with their country rocks including quartzofeldspathic, pelitic, psammatic, and carbonate rocks were subjected to coeval UHP meta-morphism during the Triassic collision between the Sino-Korean and Yangtze cratons. This review summarizes petrological and geochemical characteristics of eclogites, garnet peridotites, and UHP metasediments from published data and ongoing research in the Dabie-Sulu belt of east-central China. Parageneses of minerals from coesite-bearing eclogites encased in gneiss, garnet peridotite, and marble delineate clockwise P-T paths characterized by nearly isothermal decompression. Many eclogites have been overprinted by Barrovian-zone amphibolite- to granulite-facies assemblages, whereas others preserve primary igneous minerals and textures in the cores of coesite-bearing eclogitic blocks. In addition, eclogites contain UHP hydrous and carbonate phases including talc, epidote, zoisite, magnesite, and dolomite; these together with Ti-clinohumite, phlogopite, amphibole, chlorite, and possible talc in garnet peridotites and OH-topaz in kyanite quartzites document the role of hydrous mineral transport to the deep mantle in fluid-deficient UHP metamorphic regions. Both crustal- and mantle-derived garnet peridotites from the Dabie-Sulu region were recrystallized within the diamond stability field, with the Earth's lowest recorded geothermal gradient. <5°C/km, suggesting that UHP metamorphic recrystallization took place in a previously unrecognized, forbidden P-T region. Geochemical and isotopic data indicate that Dabie-Sulu mafic-ultramafic rocks have diverse origins; their compositions in some cases have been complicated by metamorphic recrystallization, crustal contamination, and fluid metasomatism. Nevertheless, REE geochemical and Nd isotope data clearly indicate that they have “continental” affinities and cannot represent a subducted Tethyan oceanic slab. Garnet peridotites and their enclosing eclogites display variable isotopic compositions; mantle-derived fragments preserve a mantle signature, whereas crust-hosted mafic-ultramafics display distinct crustal contamination and metasomatism. Among the many outstanding projects remaining to be investigated, geochemical and isotopic constraints of mantle-derived garnet peridotites and eclogites should provide an additional window to our understanding of mantle heterogeneity, metasomatism, slab/mantle interactions, and lithospheric evolution of the Sino-Korean craton.

Petrology of ultrahigh-pressure eclogites from the ZK703 drillhole in the Donghai, eastern China

The 558-m-deep ZK703 drillhole located near Donghai in the southern part of the Sulu ultrahigh-pressure metamorphic belt, eastern China, penetrates alternating layers of eclogites, gneisses, jadeite quartzites, garnet peridotites, phengite–quartz schists, and kyanite quartzites. The preservation of ultrahigh-pressure metamorphic minerals and their relics, together with the contact relationship and protolith types of the various rocks indicates that these are metamorphic supracrustal rocks and mafic-ultramafic rock assemblages that have experienced in-situ ultrahigh-pressure metamorphism. The eclogites can be divided into five types based on accessory minerals: rutile eclogite, phengite eclogite, kyanite–phengite eclogite, quartz eclogite, and common eclogite with rare minor minerals. Rutile eclogite forms a thick layer in the drillhole that contains sufficient rutile for potential mining. Two retrograde assemblages are observed in the eclogites: the first stage is characterized by the formation of sodic plagioclase+amphibole symplectite or symplectitic coronas after omphacite and garnet, plagioclase+biotite after garnet or phengite, and plagioclase coronas after kyanite; the second stage involved total replacement of omphacite and garnet by amphibole+albite+epidote+quartz. Peak metamorphic P– T conditions of the eclogites were around 32 to 40 kbar and 720°C to 880°C. The retrograde P– T path of the eclogites is characterized by rapidly decreasing pressure with slightly decreasing temperature. Micro-textures and compositional variations in symplectitic minerals suggest that the decompression breakdown of ultrahigh-pressure minerals is a domainal equilibrium reaction or disequilibrium reaction. The composition of the original minerals and the diffusion rate of elements involved in these reactions controlled the symplectitic mineral compositions.

The white schist assemblage in the kyanite quartzite from the ultrahigh-pressure metamorphic belt, eastern Dabie Mountains

The kyanite quartzite from the ultrahigh-pressure metamorphic belt in the eastern Dabie Mountains is composed mainly of quartz, kyanite, talc and epidote, which contains a typical mineral assemblage of white schist. It suffered an eclogite facies metamorphism ofP = (2.4 ±0.3) GPa andT = 630 ° C together with coexisting eclogite. The protolith may be an aplite replaced by high-pressure fluid of eclogite facies.

Mg-metasomatism and formation conditions of Mg-chlorite-muscovite-quartzphyllites (leucophyllites) of the Eastern Alps (W. Hungary) and their relations to Alpine whiteschists

The lower Austroalpine orthogneiss-micachist complex of the Sopron-Fertorakos area of W. Hungary contains Mg-chlorite-muscovite-quartzphyllites (leuco- phyllite) and Mg-chlorite-bearing kyanite quartzites whose chemical compositions differ greatly from their surrounding rocks. Formation of leucophyllites took place in shear zones and was associated with depletion in alkalies and iron and enrichment of magnesium and H2O. Mg-zonation of relict igneous muscovites of leucophyllites and changes in the whole rock chemical compositions suggest Mg-metasomatism. Material gains and losses have been assessed using the composition-volume relationship approach. Proceeding from metagranite through transition rocks to leucophyllites, MgO, H2O, FeO, and alkalies show continuously increasing dispersion in isocon plots with Mg-enrichment even in sheared gneiss not in contact with leucophyllite. The metasomatic processes that formed the Mg-rich rocks may be similar to those responsible for the formation of high pressure whiteschists in the Central and Western Alps. The geochemical characteristics of the Dora Maira whiteschists (Italy) and their country gneisses are very similar to those of the Sopron leucophyllites, supporting the theory that Mg-metasomatism produced the whiteschist chemistry. On the basis of oxygen isotope compositions of relict igneous muscovites, the precursor granitic rock had a δ18O value around 13‰ proving its crustal anatectic origin. The leucophyllites have whole rock oxygen isotope compositions around 8.5‰ which is in conflict with the theory of an Mg-rich sedimentary protolith. Rather, the low δ18O values reflect fluid/rock interaction with a low δ18O fluid. Quartz-mineral oxygen isotope fractionations yield a metamorphic temperature of 560 ± 30 °C which agrees with earlier estimates from mineral stabilities. Silicon contents of phengites correspond to a metamorphic pressure of ∼13 GPa at this temperature indicating eclogite facies metamorphism. The fluids in equilibrium with leucophyllites had oxygen isotope compositions around 7.9‰, similar to those calculated for the ultrahigh pressure Dora Maira whiteschists (7.6‰), further supporting the genetic link between the leucophyllites and whiteschists. Hydrogen isotope compositions of mixed white mica + chlorite samples from leucophyllites range from −40 to −35‰, correlating with chlorite contents. The calculated endmember chlorite and white mica have δD values of −30 and −40‰, respectively. The similar δD values of the white micas in leucophyllites, gneisses and metagranites suggest an overall equilibration with respect to H isotopes. The calculated δD value of the fluid is approximately 0‰, suggesting a seawater origin. This conclusion was also reached for the Dora Maira whiteschists. A possible fluid source that satisfies both metasomatic and isotopic data is dehydration of hydrothermally altered oceanic crust. The mafic–ultramafic complex of the Alpine Penninic unit underlying the Austroalpine nappes is a likely candidate. The subduction and subsequent dehydration of the ophiolite series would supply the Mg-rich fluids whose migration brought about the metasomatic alteration of the overlying gneiss-micaschist complexes.

Petrology of ultrahigh‐pressure rocks from the southern Su‐Lu region, eastern China

Abstract In the Su‐Lu ultrahigh‐P terrane, eastern China, many coesite‐bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet‐quartz‐jadeite rock and kyanite quartzite. In addition to garnet + omphacite + rutile + coesite, other peak‐stage minerals in some eclogites include kyanite, phengite, epidote, zoisite, talc, nyböite and high‐Al titanite. The garnet‐quartz‐jadeite rock and kyanite quartzite contain jadeite + quartz + garnet + rutile ± zoisite ± apatite and quartz + kyanite + garnet + epidote + phengite + rutile ± omphacite assemblages, respectively. Coesite and quartz pseudomorphs after coesite occur as inclusions in garnet, omphacite, jadeite, kyanite and epidote from both eclogites and metasediments. Study of major elements indicates that the protolith of the garnet‐quartz jadeite rock and the kyanite quartzite was supracrustal sediments. Most eclogites have basaltic composition; some have experienced variable ‘crustal’contamination or metasomatism, and others may have had a basaltic tuff or pyroclastic rock protolith.The Su‐Lu ultrahigh‐P rocks have been subjected to multi‐stage recrystallization and exhibit a clockwise P‐T path. Inclusion assemblages within garnet record a pre‐eclogite epidote amphibolite facies metamorphic event. Ultrahigh‐P peak metamorphism took place at 700–890° C and P&gt;28 kbar at c. 210–230 Ma. The symplectitic assemblage plagioclase + hornblende ± epidote ± biotite + titanite implies amphibolite facies retrogressive metamorphism during exhumation at c. 180–200 Ma. Metasedimentary and metamafic lithologies have similar P‐T paths. Several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in‐situ ultrahigh‐P metamorphism during the Triassic collision between the Sino‐Korean and Yangtze cratons.

Staurolite-quartzite bands in kyanite quartzite at Big Rock, Rio Arriba County, New Mexico ? A discussion

Schreyer and Chinner (1966) described staurolite-quartzite bands of unusual bulk chemical composition associated with the Big Rock kyanite deposit in northern New Mexico. They discussed two possible hypotheses-syngenetic or metasomatic-for the origin of the bands. This paper discusses additional evidence in support of the metasomatic hypothesis. Quartz-muscovite schist and quartz-kyanite rock are believed to be metarhyolite that was altered metasomatically by hydrothermal activity during metamorphism. Experimentally demonstrable reactions that involve ionic exchange equilibria between pore fluid and solid phases best explain the observed alteration. Staurolite-quartzite bands are postulated to be basalts or amphibolites that were hydrothermally altered in a manner analogous to alteration of the enclosing rocks. The process would deplete a basaltic parent rock in alkali and alkaline earth elements and produce a chemical composition similar to the bulk chemical composition of staurolite-quartzite. Schreyer and Chinner used textural evidence to prove that staurolite formed at the expense of chloritoid. Based on the chemical compositions of staurolite and relict chloritoid reported by them, several chemical equations were constructed, using the principle of ionic equilibria between pore fluid and solid phases, in an attempt to realistically portray the reaction. It is suggested that, under open-system metasomatic conditions, the ratio aMg+2/aH+ in the supercritical aqueous fluid is an important parameter that controls the stability of chloritoid vs. staurolite.

Staurolite-quartzite bands in kyanite quartzite at big rock, Rio Arriba County, New Mexico

Concordant “igneous-looking” bands of ferruginous bulk composition occur in a highly aluminous Precambrian metasedimentary series composed predominantly of kyanite quartzite. The bands consist of quartz, staurolite, and magnetite (partially martitized) with accessory amounts of muscovite, chlorite (pseudomorphous after biotite), chloritoid, apatite, and monazite. Quartz is found in three types (I–III) differing in appearance as well as in origin. Staurolite, in combination with quartz-II, shows peculiar radial sieve textures caused by mimetic crystallization after preexisting chloritoid rosettes. The chloritoid has been largely consumed, either by a reaction with hypothetical former kyanite to produce staurolite+ quartz with rock composition unchanged, or, possibly, by metasomatic introduction of oxygen (oxidation) to yield staurolite+quartz+magnetite; the remaining chloritoid, however, persisted in stable equilibrium with the other minerals of the rock. The staurolite quartzites are thus considered to represent original sedimentary bands which have undergone several stages of recrystallisation and (possibly) metasomatic modification during their metamorphic history. Their “igneous aspect” results from annealing crystallisation during a late static, i.e. postdeformational, thermal event of regional metamorphism.