Notes on the inclusions in a greyish kyanite

The inner zone of the Sardinia Variscan segment consists of two metamorphic complexes: I) A polymetamorphic Migmatite Complex, with migmatites showing polyphase anatectic processes, in the presence of kyanite or sillimanite. The Migmatite complex preserved decametric lenses of eclogite relicts (eclogites A) affected by high T, high- to intermediate P recrystallization under granulite facies conditions The decompressional garnet + Ca-clinopyroxene + amphibole ± orthopyroxene-bearing assemblages developed in granoblastic textures generally in no stress conditions. In most cases, only symplectite textures provide evidence for the eclogitic event. II) A medium grade, mostly metapelitic complex consisting of Grt, Ky, Stau-bearing micaschists and paragneisses includes quartzites and garnet-bearing amphibolite boudins with N-MORB chemical affinity. Relicts of eclogite assemblages were locally found in the metabasite (eclogites B). In eclogites A, the geothermobarometric parameters yield temperatures in the range 690°-760°C for minimum pressure A1.3 GPa. Pyroxene compositions accord with temperatures in excess of 700°C. In eclogites B, the thermometric calibrations provide temperatures in the range 610°-700°C for pressures 1.3-1.5 GPa, based on the jadeite content. The temperatures are consistent with the biotite+muscovite+garnet+kyanite+staurolite assemblage in the host paragneisses, and with lack of anatectic processes. The age of 457±2 Ma, obtained by U/Pb dating on one sample of Type A eclogite is interpreted as a minimum estimate for the magmatism of the eclogite protolith. A second zircon population defined an age of 403±4 Ma interpreted as dating the zircon crystallization during the high-grade event. The relationships between Types A and B eclogites, and their bearing on the regional framework (Sardinia, Ligurian Alps) are discussed.

The rate at which North Carolina kyanite of various grain sizes decomposes into mullite and glass in the range 1350° to 1600 °C. is given. Kyanite from other sources is also discussed.

For the first time, an unusual assemblage of talc-phengite-chlorite-K-feldspar was found in quartz schists from the Sanandaj-Sirjan zone in the Nahavand area in western Iran. The talc-bearing quartz schists occur as small bodies or lenses within pelitic schist layers and contain talc, phengite, chlorite, K-feldspar and quartz as major mineral constituents with subordinate amounts of calcite and graphite. Textural analysis revealed that talc, phengite, chlorite and K-feldspar are in sharp contact and no reaction rims between them were observed. Constructed petrogenetic gird in the K 2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) model system containing talc, phengite, chlorite, K-feldspar, phlogopite and kyanite with excess quartz and H 2O shows that divariant assemblage of talcphengite- chlorite-K-feldspar is stable over a wide P-T range defined by the following two univariant reactions: phengite + talc + quartz = chlorite + K-feldspar + kyanite + H 2O and chlorite + phlogopite + quartz = talc + phengite + K-feldspar + H 2O. Constructed Al2O3-KAlO2-MgO+FeO (AKM) compatibility diagrams predict that phengite (X Ph = 0.280, YPh = 0.860), chlorite (XChl = 0.570, YChl = 0.640), talc (XTlc = 0.160, YTlc = 0.02) and Kfeldspar are stable at P = 11 kbar and T = 400°C. This relatively high-pressure assemblage could be formed during the subduction of the Neo-Tethys oceanic plate under Iranian microcontinent.

EPR spectra of Fe3+ in andalusite and kyanite (both having formula Al2SiO5) are remeasured at bands which included 60 GHz. Results, besides the unusually large b and other spin-Hamiltonian parameters, include also the (true) g-factors. The spectra due to Fe3+Fe3+ pairs are identified in andalusite and analysed. EPR-Spektren von Fe3+ in Andalusit and Kyanit (beide mit der Formel Al2SiO5) werden erneut bei Frequenzen gemessen, die 60 GHz einschliesen. Als Ergebnis werden, neben dem ungewohnlich grosen b-Wert und anderen Spin-Hamiltonparametern, die (wahren) g-Faktoren erhalten. Durch Fe3+Fe3+-Paare hervorgerufene Spektren werden in Andalusit identifiziert und analysiert.

Kyanite

Kyanite

Cyanite deposits of North Carolina

Kyanite

The Kwoiek Area of British Columbia contains a pendant or screen of metamorphosed sedimentary and volcanic rocks almost entirely surrounded by a portion of the Coast Range Batholith, and intruded by several dozen stocks. The major metamorphic effects were produced by the quartz diorite batholithic rocks, with minor and later effects by the quartz diorite stocks. The sequence of important metamorphic reactions in the metasedimentary and metavolcanic rocks, ranging in grade from chlorite to sillimanite, is: 1. chlorite + carbonate + muscovite → epidote + biotite 2. chlorite + carbonate → actinolite + epidote 3. chlorite + muscovite → garnet + biotite 4. chlorite + epidote → garnet + hornblende 5. chlorite + muscovite → garnet + staurolite + biotite 6. chlorite + muscovite → aluminum silicate + biotite 7. muscovite + staurolite → garnet + aluminum silicate + biotite 8. staurolite → garnet + aluminum silicate Continuous reactions, occurring between reactions 5 and 7, are: A. chlorite + (high Ti) biotite + Al2O3 (from plagioclase?)→ garnet + staurolite + (low Ti) biotite + O2 B. muscovite (phengitic) → garnet + staurolite +muscovite (less phengitic) + O2 (?) Detailed electron microprobe work on garnet, staurolite, biotite, and chlorite shows that: (1) The garnet porphyroblasts are zoned according to a depletion model, called the Rayleigh depletion model, which assumes equilibrium between the edge of a growing garnet and the minerals which are unzoned, notably biotite, chlorite, and muscovite, but which assumes disequilibrium within the garnet. (2) The staurolite porphyroblasts are also zoned, and from their zoning patterns reactions A, B, and 5 are documented. Progressive reduction of iron with increasing grade of metamorphism is also inferred from the staurolite zoning patterns. (3) During a late period of falling temperature garnet continued to grow and the biotite and chlorite reequilibrated. The biotite, chlorite, and garnet edge compositions can vary from point to point in a given thin section, indicating that the volume of equilibrium at the final stage of metamorphism was only a few cubic microns. (4) The horizon within the garnet that grew at maximum temperature can be identified. The Mg/Fe ratio of this horizon, if the garnet composition is a limiting composition in the Al2O3 - K2O - FeO - MgO tetrahedron, increases systematically with increasing metamorphic grade. Biotite and chlorite compositions also show a general increase in Mg/Fe ratio with increasing metamorphic grade, but staurolite appears to show the reverse effect. (5) The Mg/Fe ratio at the maximum temperature horizon of the garnet porphyroblasts is a function of its Mn content as evidenced from the study of five garnet-bearing rocks, collected from one outcrop area, with the same assemblage but with differing proportions of minerals. An important implication of zoned minerals is that the effective composition of a system in a phase lies on the join between the homogeneous minerals (if there are two) and not within three-or- four-phase fields when a zoned mineral, such as garnet or staurolite, is present in the assemblage. Study of the three aluminum silicates found in the Kwoiek Area showed that a constant pressure change in polymorphs from andalusite to kyanite to sillimanite took place with increasing temperature. This transition series is best explained by the metastable formation of andalusite. Photographic materials on pages 15, 121, 160, 162, and 164 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered.

Kyanite mine near Burnsville, Yancey County, North Carolina

cyanite schist

This paper investigates Al-silicate-bearing migmatite from NE Sardinia by using the P-T pseudosection approach with the aim to determine the P-T conditions of partial melting and those of melt crystallization. P-T pseudosections were calculated in the NCKFMASH system within the P-T range 500-800°C, 0.1-1.5 GPa by using the average compositions of metapelitic greywacke, average mesosome and average trondhjemitic leucosome, respectively. The P-T pseudosections calculated for the average metapelitic greywacke and for the average mesosome, contoured for melt volume %, Si/Al and Na/K molar ratios in melt point to P–T conditions  700-740°C, 1.1-1.3 GPa which are indicative of partial melting. The P-T pseudosection calculated for the average composition of trondhjemitic leucosomes, contoured for kyanite and biotite modal content and for XMg ratio in biotite indicates P-T conditions of 660-730°C, 0.75-0.90 GPa for the crystallization of the melt. The comparison between the Na/K and Si/Al ratios in leucosomes, and the same ratios modeled for the anatectic melt by an haplogranitic melt model is thus a powerful tool for the reconstruction of P-T conditions of partial melting also in pelitic rocks, provided that leucosomes represent pure melts and are not contaminated by restitic phases or feldspar cumulates.

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It is the purpose of the present paper to criticize the hypothesis of stress-minerals proposed by Alfred Harker, to make it clear that it is unneccessary to use such a concept, and to attempt the explanation of metamorphic rock characteristics by regarding temperature, pressuer (hydrostatic), and concentration of components as the only essential factors controlling metamorphism., Petrological and physico-chemical considerations do not show the necessity of Harker's hypothesis of stress-minerals, in the present writers' view., All the minerals which Harker ranked in the category of stress-minerals, seem to be capable of formed under some conditions without stress., They may be divided into two groups, according to the condition of generation., The first group including kyanite, staurolite, almandine, and chloritoids, may be formed only under very high pressures., They all have very high density., They occur sometimes in ignenous rocks, pegmatitic veins, hornfelses, etc., which do not show any special evidence of stress., The second group, containing chlorite, muscovite, epidote, etc., may be formed only at low temperatures., This is shown by the fact that they are hydrothermal or deuteric minerals, the temperature of whose formation is much lower than that of ordinary pyrogenetic or thermal-metamorphic minerals., Harker's stress-minerals seem to be such as are formed only under conditions of lower temperatures and perhaps higher pressures than in ordinary hornfelses., Each of various kinds of metamorphism, such as thermal, regional, etc., takes place under a certain definite condition of temperature and pressure., The kinds of metamorphic minerals developed are determined by the prevailing temperature and pressure as well as by the chemical composition of the metamorphosed rocks., Therefore, each kind of metamorphism is distinguished from the others by a characteristic set of metamorphic minerals.,

This paper deals with the metamorphic evolution of the Bacariza Fm that outcrops in the two uppermost structural units of the Cabo Ortegal Complex (NW Iberian Massif). This formation includes ultramafic and mafic granulites, garnet amphibolites and garnet trondhjemitic gneisses. Although mineral associations characteristic of high pressure granulites predominate in the least retrogressed of these rocks, the presence of relic kyanite along with the fact that plagioclase only appears in symplectitic textures resulting from de-jadeitization of pyroxenes point to an earlier eclogite facies metamorphism. Thermobarometric estimations indicate higher P-T conditions for the rocks in the uppermost structural unit.