Phengite Content Research Articles

Relationships Between Eclogite‐Facies Mineral Assemblages, Deformation Microstructures, and Seismic Properties in the Yuka Terrane, North Qaidam Ultrahigh‐Pressure Metamorphic Belt, NW China

AbstractTo understand the relationships between eclogite‐facies mineral assemblages, deformation microstructures, and the seismic properties of subducting oceanic crust, eclogites from the Yuka terrane, North Qaidam ultrahigh‐pressure metamorphic belt, NW China, were studied. Observations of mineral textures, deformation microstructures, and petrofabrics in the eclogites indicate that garnet, omphacite, and phengite were deformed by intracrystalline deformation (i.e., dislocation creep) during prograde metamorphism. In contrast, amphibole, which was formed by the topotactic replacement of omphacite at fluid‐present conditions, is considered to have been deformed by diffusional flow (dissolution‐precipitation creep) during amphibolite‐facies retrogression associated with exhumation. Based on the petrofabrics in the samples, the seismic properties of the eclogites were calculated depending on eclogite‐facies mineral assemblages such as garnet + omphacite, garnet + omphacite + phengite, garnet + omphacite + phengite + lawsonite, garnet + omphacite + phengite + amphibole, and garnet + omphacite + amphibole. We found that the seismic signatures of each of the eclogite‐facies mineral assemblages were different. In particular, phengite‐bearing eclogites (the garnet + omphacite + phengite/garnet + omphacite + phengite + amphibole assemblages), depending on phengite content, produced the strongest seismic anisotropy (AVp and AVs), with a strong polarization anisotropy, that was at least three times higher than bimineralic (phengite‐absent) eclogites (garnet + omphacite assemblage). Our results indicate that phengite, as a stable phase at high‐pressure and high‐temperature conditions, can play an important role in the creation of trench‐parallel seismic anisotropy in the eclogite‐facies mineral assemblages found in subduction zones.

Graphite‐schist blocks in the Franciscan Mélange, San Simeon, California: Evidence of high‐P metamorphism

AbstractThe seacliff exposure at San Simeon, California, contains graphite‐schist blocks in a shale‐matrix, an undocumented lithology within the Franciscan mélange. Thirty graphite‐schist blocks were studied to discover all the varieties in this classic locality of mélange. Based on their mineralogical assemblage and composition, and textural characteristics the graphite‐schists in San Simeon are subdivided into two main types (Type I and II) with two subdivisions each (A and B). Type IA and IIA blocks are the most abundant. Type IA graphite‐schists are siltstone/fine greywacke‐like, preserve sedimentary textures, and lack lawsonite. Type IB graphite‐schists are mineralogical and texturally similar to Type IA schists, but are finer grained siltstone and shale. Type IIA graphite‐schists are compositionally layered and contain quartz‐ and albite‐rich layers and dark graphite‐ and intergrown mica/chlorite‐rich layers. Nine out of the 15 Type IIA blocks contain lawsonite. Two Type IIA blocks also contain aragonite (+calcite) in veins. Type IIB graphite‐schists are mostly composed of quartz and minor graphite, intergrown chlorite and white mica, and white mica pseudomorphs after lawsonite. The phengite content of mica in Type IIA blocks is higher than that of mica in Type IA graphite‐schists, confirming they were metamorphosed under high‐P/low‐T conditions. Type IA blocks were recrystallized between 200 and 250 °C at <~3 kbar; whereas, Type IIA blocks were metamorphosed under higher pressure conditions, probably at 250–300 °C and 3–5 kbar. Most likely both types of graphite‐schists were derived from a similar layered siltstone/fine greywacke/shale protolith. Organic matter‐rich sediments deposited in the trench axis were subducted along with oceanic crust during Franciscan subduction. Type I graphite‐schists were subducted to depths <10 km, whereas Type II graphite‐schists were subducted to depths ~15 km where they were underplated under high‐P conditions. The graphitic metasedimentary rocks were juxtaposed with mafic lithologies from the subducted oceanic crust that were metamorphosed to blueschist facies and retrograded to greenstone as they returned to the surface in the subduction channel shear zone.

Mineral chemistry and geochemical behavior of hydrothermal alterations associated with mafic intrusive-related Au deposits at the Atud area, Central Eastern Desert, Egypt

Vein-type gold deposits in the Atud area are related to the metagabbro–diorite complex that occurred in Gabal Atud in the Central Eastern Desert of Egypt. This gold mineralization is located within quartz veins and intense hydrothermal alteration haloes along the NW–SE brittle–ductile shear zone, as well as along the contacts between them. By using the mass balance calculations, this work is to determine the mass/volume gains and losses of the chemical components during the hydrothermal alteration processes in the studied deposits. In addition, we report new data on the mineral chemistry of the alteration minerals to define the condition of the gold deposition and the mineralizing fluid based on the convenient geothermometers. Two generations of quartz veins include the mineralized grayish-to-white old vein (trending NW–SE), and the younger, non-mineralized milky white vein (trending NE–SW). The ore minerals associated with gold are essentially arsenopyrite and pyrite, with chalcopyrite, sphalerite, enargite, and goethite forming during three phases of mineralization; first, second (main ore), and third (supergene) phases. Three main hydrothermal alteration zones of mineral assemblages were identified (zones 1–3), placed around mineralized and non-mineralized quartz veins in the underground levels. The concentrations of Au, Ag, and Cu are different from zone to zone having 25–790ppb, 0.7–69.6ppm, and 6–93.8ppm; 48.6–176.1ppb, 0.9–12.3ppm, and 39.6–118.2ppm; and 53.9–155.4ppb, 0.7–3.4ppm, and 0.2–79ppm for zones 1, 2, and 3, respectively.The mass balance calculations and isocon diagrams (calculated using the GEOISO-Windows program) revealed the gold to be highly associated with the main mineralized zone as well as sericitization/kaolinitization and muscovitization in zone 1 more than in zones 2 and 3. The sericite had a higher muscovite component in all analyzed flakes (average XMs=0.89), with 0.10%–0.55% phengite content in wall rocks and 0.13%–0.29% phengite content in mineralized quartz veins. Wall rocks had higher calcite (CaCO3) contents and lower MgCO3 and FeCO3 contents than the quartz veins. The chlorite flakes in the altered wall rocks were composed of pycnochlorite and ripidolite, with estimated formation temperatures of 289–295°C and 301–312°C, respectively. Albite has higher albite content (95.08%–99.20%) which occurs with chlorite in zone 3.

Fluid-driven low-grade metamorphism in polydeformed rocks of Avalonia (Arisaig Group, Nova Scotia, Canada)

The Lower Silurian—Lower Devonian Arisaig Group (Antigonish Highlands) in the Canadian Appalachians is a sequence of shallow marine strata deposited after the accretion of Avalonia to Baltica during the closure of the Iapetus Ocean. Deformation of the strata is widely attributed to the Devonian Acadian orogeny and produced shallowly plunging regional folds and a cleavage of varying penetrativity. Phyllosilicate minerals from the finest-grained rocks exhibit very low-grade (diagenetic-anchizone) metamorphic conditions. X-ray diffraction study reveals that the sampled rocks contain quartz, K-white mica, chlorite, and feldspars; illite–smectite and chlorite–smectite mixed-layers are common but Na–K mica and kaolinite occur only in some samples. The identification of illite–smectite mixed-layers in diagenetic samples, with Kubler Index >0.50 Δ°2θ and the highly heterogeneous b-cell dimension of the K-white micas are in agreement with the variable chemical composition of dioctahedral micas, which present low illitic substitution and variable phengitic content. The spatial variation in the above crystal-chemical parameters was plotted along a NW–SE composite cross section across the regional folds. No correlation was found between the metamorphic conditions and either the stratigraphic depth or the strain values measured by phyllosilicates orientation analyses, as a function of the penetrativity of the cleavage. However, the metamorphic grade generally increases towards the Hollow Fault, and is highest in samples located within a 1 km corridor from the fault surface. Incipient cleavage is observed in the anchizonal samples located in the vicinity of the Hollow Fault and in some of the diagenetic samples, indicating cleavage development under low temperatures (<200 oC). These relationships, together with regional syntheses, suggest low-grade metamorphism post-dated regional folding and was coeval with Late Carboniferous dextral movement along the Hollow Fault. Fluid circulation associated with movement along this major fault may be the driving mechanism for the increasing metamorphism towards it.

Mineralogical, IR-spectral and geochemical monitoring of hydrothermal alteration in a deformed and metamorphosed Jurassic VMS deposit at Arroyo Rojo, Tierra del Fuego, Argentina

Fil: Biel, C.. Universidad de Zaragoza; Espana. Productos Minerales para la Industria S.A.; Espana

The metamorphic history of rocks buried, accreted and exhumed in an accretionary prism: an example from the Otago Schist, New Zealand

AbstractAlthough subgreenschist facies metamorphic rocks are widespread in the upper crust, mineralogical processes affecting these rocks are poorly understood. Subgreenschist mineralogical transitions have been invoked as critical controls on the mechanical behaviour of rocks within the crustal seismogenic zone, calling for further study of very low‐grade metamorphic assemblages. In this study a multi‐technique thermobarometric study of the Chrystalls Beach Complex mélange, which is located within the Otago Schist accretion‐collision assemblage of the South Island of New Zealand, is presented. The Chrystalls Beach Complex comprises highly sheared trench‐fill sedimentary rocks and scattered pillow basalts, and is inferred to have formed during Jurassic subduction under the paleo‐Pacific Gondwana margin. Equilibrium mineral assemblages indicate peak P–T conditions in the range 400–550 MPa and 250–300 °C, which is supported by chlorite thermometry. Relatively high pressures of burial and accretion during foliation development are inferred from phengite content and b0 spacing analyses of white mica. Rare lawsonite occurs in a post‐foliation vein, and illite ‘crystallinity’ measurements indicate a thermal overprint during exhumation. These P–T estimates and their relative chronology indicate that the mineral assemblages developed along a clockwise P–T path. Based on variability in P–T estimates from different techniques, mineral assemblages developed during burial are largely overprinted during exhumation at similar or higher‐T than experienced along the prograde path. Observed subduction‐related subgreenschist assemblages are therefore likely to indicate lower‐P than experienced during subduction, as higher‐P mineral compositions re‐equilibrate during exhumation. The P–T path inferred in this study is similar in shape to P–T paths for higher grade parts of the Otago Schist, and other exhumed accretionary prisms around the world, and is therefore probably common for rocks buried, accreted and exhumed in accretionary prisms.

The b-spacing values of white mica from low-grade metapelites of central Nepal Lesser Himalaya and their tectono-metamorphic implications

) vi Si iv = Al vi Al iv ) between the ideal muscovite and celadonite end-members, which controls the chemistry of phengites, is thought to be particularly sensitive to pressure at low-temperature conditions and serves as a qualitative geobarometer (Guidotti and Sassi 1986). Although, the b-spacing values of white micas have been widely used to estimate the palaeopressure in low-grade terrains all over the world, its application in the Himalaya is relatively rare. In the present study, we present the b-spacing values of white mica across Pokhara-Butwal road and Kali Gandaki valley sections of the Lesser Himalaya in central Nepal, and discuss their tectono-metamorphic implications. Geological setting The Lesser Himalaya in the Pokhara area is divided into four tectonic units from south to north: Parautochthon with Palpa Klippe lying between the Main Boundary Thrust (MBT) and the Bari Gad-Kali Gandaki Fault (BKF), Thrust Sheet I (TS I) situated between the BKF and the Phalebas Thrust, Thrust Sheet II (TS II) bounded by the Phalebas Thrust and the Lower MCT, and the MCT zone bounded by the Lower MCT and the Upper MCT, respectively. The Lesser Himalaya mostly comprises low-grade metamorphic rocks such as slates, phyllites, quartzites, and dolomites (Nawakot Complex) of the Precambrian age. The Nawakot Complex rocks are unconformably overlain by the Gondwana and post- Gondwana sedimentary rocks (Tansen Group) in the Parautochthon (Sakai 1983). Metamorphic grade gradually changes from diagenesis to lower anchizone in the Parautochthon to upper anchizone in the TS I, epizone in the TS II and garnet zone in the MCT zone (Paudel 2000). Sampling and analytical techniques A total of 320 pelitic rocks with unique assemblage Mica+Chl+Ab+Qtz (Chl-zone) and lacking detrital white mica were used to measure b-spacing values. A few samples from the northern part of the TS II are from Bt-zone and samples from the MCT zone belong to Grt-zone. Measurement was done on <2µm powder fraction of each sample. Diffractometer setting was constant for all the samples (Rigaku Geigerflex diffractometer, Cu cathode, Ni filter, 35 kv tube voltage, 20 mA current, time constant=2 sec, scatter slit=1° , receiving slit=0.3 mm, divergence slit=1° ). The 63-59.5° 2θ range was scanned at 0.25 2e/min, and b was determined from (060) peak (approx. 61.5° 2θ) using the (211) quartz reflection (approx. 60° 2θ) as an internal standard. Mean b- spacing values were calculated from five repeated measurements for each sample. Results and implications The relationship between the b-spacing values with the composition of white mica was accessed by compositional analysis of white mica in the same samples. A fairly good positive linear correlation was found between the two (Figure 1). This indicates that the b-spacing values serve as indirect measures of phengite content of white mica in the Lesser Himalaya and hence the pressure condition. The plots of b-spacing values versus the whole rock

The structure and the phyllosilicates (chemistry, crystallinity and texture) of Talas Ala-Tau (Tien Shan, Kyrgyz Republic): comparison with more recent subduction complexes

Geological mapping and structural analysis of the Talas Ala Tau (Tien Shan, Kyrgyz Republic) have revealed a complex structure composed of folds with axial-plane cleavage and thrust faults verging towards the NE. The main structures of the range correspond to minor Tertiary and Carboniferous–Permian deformation superimposed on the main deformation event that took place during the Baikalian orogeny. The pervasive axial-plane cleavage diminishes in penetrativity from the hinterland to the foreland in both the Uzunakhmat and Karagoin sheets. The main thrusts developed phyllonitic shear-related rocks on the hangingwall immediately above the thrust planes. A crystal-chemical study of the phyllosilicates growth during the Baikalian deformation event along a cross-section revealed changes in the crystallinity, composition and lattice parameters of them. The phyllosilicates present in the Talas Ala Tau rocks were crystallized in very low-grade metamorphic conditions, that is below 300 °C, as indicated by their Kübler Index (KI), which decreases from SW towards the NE. Detailed TEM study of the phyllosilicates reveals a clear textural difference at the lattice level between samples with higher or lower KI parameters. There is also a clear difference in crystal-chemical parameters (KI and b) and composition between the phyllosilicates growth in relation to the axial-plane cleavage and the ones belonging to the thrust-related phyllonites. The first ones are more affected by the ferrimuscovitic vector than the phyllosilicates of phyllonites, closer to the theoretical phengitic component. Huge ranges of values of phengitic content of micas at sample level are interpreted as the result of a decompression path from at least 8 kbar. We propose a subduction geodynamic environment for the regional deformation and the origin of the phyllosilicates, as they are similar to those obtained in more recent accretionary complexes.

Hydrothermal Alteration and Volatile Element Halos for the Rosebery K Lens Volcanic-Hosted Massive Sulfide Deposit, Western Tasmania

A detailed study of alteration mineralogy, mineral chemistry, and lithogeochemistry in the host rocks surrounding the A-B and K lenses at the north end of the Rosebery mine has revealed a series of overlapping alteration halos with characteristic mineralogy and geochemistry. The study involved logging and sampling from nine drill holes spaced at varying distances from the A-B and K lenses. The stratiform Zn-Pb-Cu ore lenses have a sheetlike morphology and are hosted by a mixed sequence of rhyolitic to dacitic massive mediumgrained quartz-porphyritic pumice breccia, black mudstone, and crystal-rich volcaniclastic sandstone, overlying a thick homogeneous sequence of rhyolitic pumice breccia. The major alteration minerals at Rosebery are arranged in a complex series of zones passing away from the deposit-quartz-sericite zone, Mn carbonate zone, chlorite zone, and outer sericite zone. The chlorite zone is best developed in the immediate footwall below the copper-pyrite-rich ore lenses, whereas the strongest Mn carbonate alteration occurs in the immediate hanging-wall volcanics or lateral to the ore lenses. The outermost visible sericitic alteration extends about 60 to 100 m into the footwall, 10 to 20 m into the hanging wall, and over 500 m along the upper contact of the footwall pumice breccias. Thallium and antimony form the most extensive trace element halos related to the mineralization. Thallium forms a halo that extends 200 to 300 m into the overlying volcanics and 60 to 100 m into the footwall. Anomalously high thallium also occurs over 500 m along strike marking the contact between the footwall pumice breccias and the overlying volcaniclastic sandstones. Proximal to the ore lenses Tl and Sb values range from 10 to 100 ppm, compared to the halo zone where they vary from around 1 to 10 ppm. Studies of alteration mineral chemistry at Rosebery have revealed some important relationships that may assist exploration. The Mn content of alteration carbonate increases toward ore, both along strike and across strike. Close to ore, alteration carbonates contain >20 mole percent MnCO3 (kutnahorite, rhodochrosite, Mn siderite, and Mn ankerite), whereas at distances of 40 to 60 m across strike the mole percent MnCO3 in carbonate drops to below 10 percent. At greater than 80 m, the carbonates are Mn-poor calcites and are commonly located in synmetamorphic structures. White mica composition changes with stratigraphy and alteration assemblages and may be related to the mineralizing event, although this has not been convincingly demonstrated. Proximal white mica contains minor Ba substituting for octahedral Al. However, except for their Ba content, these phengetic white micas are similar to those found in nonmineralized areas of the Mount Read Volcanics. Sodic white mica with up to 0.35 Na/(Na + K) and a low phengite content (<0.5 Fe + Mg cations) occurs in a zone of volcanic sandstones and black slates overlying the ore deposit. This research has lead to the development of a series of proximal, medial, and distal vectors useful for both regional and mine-scale exploration. The most useful vectors, listed from proximal to distal, include Zn, Ba content of white mica, Na2O, K2O, Ishikawa alteration index (AI), S/Na2O, Ba/Sr, Mn content of carbonate, Tl, and Sb.

Transmission Electron Microscopy Study of Very Low-Grade Metamorphic Rocks in Cambrian Sandstones and Shales, Ossa-Morena Zone, Southwest Spain

AbstractThis study examines the evolution of the texture, structure, and chemical composition of rocks derived from clastic materials of the Ossa-Morena Zone (Hesperian Massif, Spain). Previous studies of phyllosilicates in these rocks (by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray analysis, and electron microprobe) indicated a temperature decrease from bottom (epizone conditions) to top (diagenetic conditions) of the rock section.At the nanometer scale, phyllosilicate packets form large angles where grains intersect with no preferred orientation. With metamorphic grade, packets are wide and defect free, compared to packets at lower grade. These packets are ∼ 15 layers under diagenetic conditions to &gt;80 layers in the epizone. Dioctahedral K-rich micas (muscovite, phengite, and illite) have coexisting 1Md, 1M, and 2M polytypes. Long-period polytypes of 4, 5, and 6 layers are reported for the first time in dioctahedral K-rich micas. The chemical compositions of the micas are nearly identical in the anchizone and the diagenetic zone, comprising an illitic (0.8 atoms per formula unit, a.f.u., of K) and a phengitic component (0.15 a.f.u, of Mg and 0.13 a.f.u, of Fe). Fe may correspond to a ferrimuscovitic substitution. Epizone samples have a high phengitic content (Mg = 0.24 a.f.u.) and almost no illite component. One diagenetic sample has coexisting berthierine, trioctahedral chlorite, sudoite, and corrensite. Berthierine and chlorite are identical in composition. Because of the clastic nature of the system, the composition of corrensite is not typical of other corrensites, with higher Al content, Fe/Mg ratio at ∼1, and K as the exchangeable cation.Textural differences between the diagenetic zone and the anchizone are the progressive increase in the size of dioctahedral K-rich mica grains, which involves an increasing illite crystallinity based on the Kübler index. The chemical compositions of these micas are illite (diagenesis and anchizone) and phengite in the epizone. There are no intermediate phases, suggesting a compositional gap between illite and phengite. The coexistence of different polytypes of dioctahedral K-rich micas and the absence of chemical homogeneity indicate disequilibrium in the Cambrian pelitic rocks studied.

Experimental synthesis and phase relations of phengitic muscovite from 6.5 to 11 GPa in a calcareous metapelite from the Dabie Mountains, China

The stability and phase relations of phengitic muscovite in a metapelitic bulk composition containing a mixed H 2O+CO 2 fluid were investigated at 6.5–11 GPa, 750–1050°C in synthesis experiments performed in a multianvil apparatus. Starting material consisted of a natural calcareous metapelite from the coesite zone of the Dabie Mountains, China, ultrahigh-pressure metamorphic complex that had experienced peak metamorphic pressures greater than 3 GPa. The sample contains a total of 2.1 wt.% H 2O and 6.3 wt.% CO 2 bound in hydrous and carbonate minerals. No additional fluid was added to the starting material. Phengite is stable in this bulk composition from 6.5 to 9 GPa at 900°C and coexists with an eclogitic phase assemblage consisting of garnet, omphacite, coesite, rutile, and fluid. Phengite dehydrates to produce K-hollandite between 8 and 11 GPa, 750–900°C. Phengite melting/dissolution occurs between 900°C and 975°C at 6.5–8 GPa and is associated with the appearance of kyanite in the phase assemblage. The formation of K-hollandite is accompanied by the appearance of magnesite and topaz-OH in the phase assemblage as well as by significant increases in the grossular content of garnet (average X grs=0.52, X py=0.19) and the jadeite content of omphacite ( X jd=0.92). Mass balance indicates that the volatile content of the fluid phase changes markedly at the phengite/K-hollandite phase boundary. At P≤8 GPa, fluid coexisting with phengite appears to be relatively CO 2-rich (XCO 2/XH 2O=2.2), whereas fluid coexisting with K-hollandite and magnesite at 11 GPa is rich in H 2O (XCO 2/XH 2O=0.2). Analysis of quench material and mass balance calculations indicate that fluids at all pressures and temperatures examined contain an abundance of dissolved solutes (approximately 40 mol% at 8 GPa, 60 mol% at 11 GPa) that act to dilute the volatile content of the fluid phase. The average phengite content of muscovite is positively correlated with pressure and ranges from 3.62 Si per formula unit (pfu) at 6.5 GPa to 3.80 Si pfu at 9 GPa. The extent of the phengite substitution in muscovite in this bulk composition appears to be limited to a maximum of 3.80–3.85 Si pfu at P=9 GPa. These experiments show that phengite should be stable in metasediments in mature subduction zones to depths of up to 300 km even under conditions in which aH 2O≪1. Other high-pressure hydrous phases such as lawsonite, MgMgAl-pumpellyite, and topaz-OH that may form in subducted sediments do not occur within the phengite stability field in this system, and may require more H 2O-rich fluid compositions in order to form. The wide range of conditions under which phengite occurs and its participation in mixed volatile reactions that may buffer the composition of the fluid phase suggest that phengite may significantly influence the nature of metasomatic fluids released from deeply subducted sediments at depths of up to 300 km at convergent plate boundaries.

The stability and composition of phengitic muscovite and associated phases from 5.5 to 11 GPa: Implications for deeply subducted sediments

The stability and composition of phengitic muscovite was investigated from 5.5–11 GPa, 700–1150°C in synthesis experiments performed in a multianvil apparatus. Starting materials consisted of natural minerals with a bulk composition similar to that of a K-deficient, intermediate dioctahedral-trioctahedral mica. Phengitic muscovite was found to be stable from 5.5–11 GPa at 900°C. Phengite melting occurs between 1075–1150°C at 7–8 GPa and 1000–1050°C at 10 GPa. At 10–11 GPa, 800°C octahedral cation deficient (OCD) muscovite and K-hollandite are observed rather than phengite.The average phengite content of muscovite is positively correlated with pressure ranging from 3.65 Si pfu at 5.5 GPa to 3.81 Si pfu at 11 GPa. The maximum phengite content of stable muscovite appears to be 3.80–3.85 Si pfu. Most phengite examined exhibits at least minor solid solution towards phlogopite, averaging 2.04 ± 0.06 (2σ) octahedral cations pfu.The hydrous phases phengite, lawsonite, topaz-OH, and Mg-pumpellyite occur between 6–8 GPa, 700–900°C. With increasing pressure lawsonite and Mg-pumpellyite dehydrate to form garnet between 8–9 GPa. With increasing temperature Mg-pumpellyite, lawsonite, and topaz-OH devolatilize to form garnet and kyanite between 900–1000°C at 7–8 GPa.Phengitic muscovite and topaz-OH would be stable in hydrous sediments in cool mature subduction zones to depths exceeding 360 km while lawsonite and Mg-pumpellyite would be stable to 240–300 km allowing the transport of H2O contained in these phases deep into the upper mantle. In warmer subduction zones the dehydration of Mg-pumpellyite, lawsonite, and topaz-OH; as well as the melting of phengite would result in fluid release at depths of 180–240 km.The presence of phengite at far greater depths than the zone of melt generation beneath arcs (100–150 km) requires a mechanism such as the partitioning of K, Be, B, Ba, and Rb into migrating fluids rather than the simple dehydration of phengite beneath arcs in order to provide for the transfer of these slab signature elements from phengite into arc magmas.

The effects of ferromagnesian components on the paragonite‐muscovite solvus: a semiquantitative analysis based on chemical data for natural paragonite‐muscovite pairs

ABSTRACTChemical data for 139 natural paragonite‐muscovite (Pg‐Ms) pairs illustrate the effects of ferromagnesian components on the P‐T‐X topology of the Pg‐Ms solvus. The pairs were selected on the basis of: reasonably accurate knowledge of the P‐T conditions of formation; evidence for close approach to equilibrium at peak metamorphic conditions; exclusion of pairs in which paragonite contains more than 5 mol% margarite; and exclusion of pairs from polymetamorphic rocks that contain more than one set of cogenetic Pg‐Ms pairs. Graphical analysis reveals considerable scatter in the data; nevertheless, it is evident that the muscovite limb of the solvus shifts markedly toward end‐member muscovite with increasing pressure from approximately 7 kbar to 21 kbar. This shift is attributed to a pressure‐induced increase of the ferromagnesian content of muscovite, which increases the size of the XII alkali site ‐ to the effect that K is more readily accommodated than Na. The data also suggest that the paragonite limb of the solvus migrates slightly toward end‐member paragonite with increasing pressure. Broadening of the Pg‐Ms solvus with increasing pressure reflects increasingly nonideal Na‐K mixing as the phengite content of muscovite increases. Due to the wide scatter of data for Pg‐phengitic‐Ms pairs, it is concluded that, at the present time, Pg‐Ms solvus thermometry is only viable for quasibinary Pg‐Ms pairs.

The composition of phyllosilicates in Precambrian, low-grademetamorphic, clastic rocks from the Southern Hesperian Massif (Spain) used as an indicator to metamorphic conditions

In order to obtain an indication of metamorphic conditions we have studied the mineralogy of the clastic rocks, in particular the phyllosilicates, from three Precambrian series in the Central-Iberian and Ossa-Morena zones of the Southern Hesperian Massif: the Esquisto Grauváquico Complex; the so-called Serie Negra; and the Azuaga Formation. For this purpose we have used the crystal-chemical parameters obtained via X-ray diffraction ( d 001, b 0, ratios of the basal intensity of the white micas and chlorites, and the crystallinity index of the mica) together with electron microprobe data from selected areas, obtained via backscattered electron images. The mineral assemblages, commonly quartz+muscovite +albite +chlorite ±biotite, and the values obtained for the illite crystallinity index (between 0.18° and 0.20° 2Θ) in rocks with non-diagnostic mineral assemblages point to at least greenschist-facies metamorphic conditons. The phengite content of the mica indicates an overall pressure regime near the limits between low and intermediate pressure, but some significant differences exist between the series studied. The Azuaga Formation shows the lowest pressure (about 4 kbar), while the Serie Negra indicates a pressure of at least 1 kbar more. In the Esquisto Grauváquico Complex, when the samples are considered overall, the results are similar to those of the Serie Negra, but there is inhomogeneity in the rocks from the two sections studied. The various types of chlorite, both textural and optical, are homogeneous in composition in each rock specimen and their chemical composition depends primarily on the composition of the host-rock.

Mineral chemistry study of progressive metamorphism in calcareous schists from Ankarvattnet, Swedish Caledonides

The composition of muscovite, chlorite and biotite has been studied in a low-grade calcareous phyllite-metagreywacke sequence (Blåsjö Phyllite) in the Seve-Köli Nappe Complex at Ankarvattnet, Swedish Caledonides. The b 0 lattice constant of muscovite is shown to be a good measure of the MgFe 2+ content, i.e. the phengite content in muscovite, according to the following equation: b 0 = 8.992 + 0.358RM or d 060, 331 = 1.499 + 0.060 RM where RM = mole prop. (2Fe 2O 3 + FeO + MgO). In the chlorite zone the b 0-values are mostly controlled by the rock composition, thus obliterating the effect of metamorphic conditions. In the limiting assemblages of the biotite zone muscovite is continuously de-phengitizated and Na-enriched with increasing grade. Furthermore the element distribution of Mg and Fe among coexisting phengite and chlorite is controlled by metamorphic grade (temperature). The appearance of biotite is found to be due to the following continuous reaction: phengitic muscovite + chlorite → less phengitic, more paragonitic muscovite + Al-enriched chlorite + biotite + quartz + water biotite + quartz + water

Regional distribution of white K-mica polymorphs and their phengite content in the Central Alps

Some 150 white K-micas from the Central Alps were analysed for their polymorph and phengite content.

Phengite compositions and post-nappe high-pressure metamorphism in the pennine zone of the French Alps

A systematic study of relict highly substituted phengite micas (Si 3.6–3.7) formed in the less deformed and less recrystallized rocks of the internal zone of the French Alps shows that silica content “isograds” for phengites cut across major thrust boundaries. This indicates that a high-pressure metamorphism was post-nappe in the orogenic sequence. Little substituted, secondary phengites and those found in highly recrystallized rocks reflect metamorphic conditions which occurred during the later mountain building phases of the orogeny. Variations of mica phengite content within mineral facies zones indicate rather important differences in pressure along the Pennine zone in going from low- to high-temperature mineral facies.

Compositional variation of muscovite as a function of metamorphic grade and assemblage in metapelites from N.W. Maine

Chemical data are presented for 49 muscovites from high and low Al specimens collected form N.W. Maine at metamorphic grades ranging from the upper staurolite to the upper sillimanite zone. Data also are presented for two muscovite from St. Paul Island, two muscovites+three paragonites from Gassetts, Vermont, and one muscovite from an adamellite in N. W. Maine. These data given further information on the effects of P, T, and bulk composition on muscovite composition. Specifically, temperature clearly influences the Na/Na+K ratio of muscovite in limiting assemblages but may not have much effect on the phengite content. Increase in pressure clearly does cause an increase in phengite content. Bulk composition (assemblage) has a very great effect on both Na/Na+K ratio and phengite content so that attempts to use either of these factors to monitor metamorphic grade should generally be done in the context of a limiting assemblage.

The petrological and geological significance of theb 0 values of potassic white micas in low-grade metamorphic rocks. An application to the Eastern Alps

In the Eastern Alps theb0 values of potassic white micas of Hercynian low-grade schists differ sharply from those of Alpine isograde and isochemical rocks. These differences, which indicate variations in the phengite content present as solid solution in the white micas, are to be referred to differences in pressure. In this way it is possible to recognize, in the low-grade metamorphic rocks of different tectonic units of the Eastern Alps, the lowpressure character of the Hercynian metamorphism, and the higher-pressure character of the Alpidic thermic event; this recognition is not possible using traditional methods, owing to the lack of characteristic minerals in the lowgrade metamorphic rocks. The results of the present report allow us to state that generally, when studying low-grade metamorphic complexes, theb0 measurements can supply valuable data which are not obtainable in any other way at the present time. These data are useful both for estimating recrystallization pressure and for recognizing the age of metamorphism.