Assemblage Albite Research Articles

BORALSILITE AND Li,Be-BEARING “BORON MULLITE” Al8B2Si2O19, BREAKDOWN PRODUCTS OF SPODUMENE FROM THE MANJAKA PEGMATITE, SAHATANY VALLEY, MADAGASCAR

Boralsilite Al16B6Si2O37 and Li,Be-bearing “boron mullite” Al8B2Si2O19 were discovered as breakdown products of spodumene from the elbaite-subtype Manjaka granitic pegmatite, Sahatany Valley, Madagascar. The pegmatite mineral assemblage is relatively simple (Ab + Qz > Kfs > Tur + Spd > Ap + several accessory B-rich phases). This pegmatite forms an E–W-trending elongated body emplaced in an Mg-rich calc-silicate rock; the pegmatite exocontact locally contains thin reaction zones (Tur+Phl±Brl). Boralsilite assemblages are closely associated with prismatic spodumene crystals, up to 10 cm long, mostly as narrow albite > K-feldspar zones (albite > K-feldspar > boralsilite > “boron mullite”), up to ∼0.5 mm thick, or rarely as inclusions in the central parts of spodumene crystals. Long prismatic crystals and fibers of boralsilite, up to 200 μm long, and rare elongated grains of “boron mullite” Al8B2Si2O19, up to 100 μm long, are mostly enclosed in albite and rarely in K-feldspar. The EMP and LA-ICP-MS data yielded relatively variable Al/Si ratios (∼5.4–9.8; ∼3.2–4.6) and concentrations of B (∼48500–59500; ∼24000–28000 ppm), Li (688–1433; 3339–4287 ppm), and Be (288–1082; 1017–2728 ppm), in boralsilite and “boron mullite”, respectively. Textural relations of the boralsilite assemblage assuming immobile Al suggest the hypothetical reaction: 46 LiAlSi2O6 + 30 Na+ (melt/aq.) + 6 H3BO3 = Al16B6Si2O37 + 30 NaAlSi3O8 + 46 Li+ (melt/aq.) + 9 H2O + 4 O2; part of the Li may have entered rare secondary clay minerals and partly also the “boron mullite”, but most of the Li was mobilized. Fluid/melt with very high a (B2O3), a (Na), and low a (H2O), along with high a (CO2) indicated by the absence of significant amount of hydrous phases, very minor extent of exocontact reactions along the pegmatite, and secondary rhodochrosite, are responsible for the origin of boralsilite. Very late solidus or early subsolidus conditions at temperature ∼350–450 °C and pressure ∼2–3 kbar, estimated for the crystallization of the assemblage albite + boralsilite, are significantly lower than those given for fibrous boralsilite at other localities of mostly anatectic granitic pegmatites (Larsemann Hills, Antarctica; Almgjotheii, Norway; and Horni Bory, Czech Republic).

Premelting polymerization of crustal and mantle fluids, as indicated by the solubility of albite + paragonite + quartz in H 2O at 1 GPa and 350–620 °C

The composition and structure of dissolved silicates in model crustal and upper-mantle fluids was assessed by measuring the solubility of the assemblage albite + paragonite + quartz in H 2O from 350 to 620 °C, at 1 GPa. Natural, low albite and quartz were equilibrated with H 2O; paragonite grew in all experiments due to incongruent dissolution of albite + quartz. Melting occurred at 635 ± 5 °C. Solute concentrations at subsolidus conditions were determined by analysis of quenched fluids or mineral weight-loss and mass balance. Bulk solubility of the mineral assemblage increased from ∼ 1 to ∼ 8 oxide wt.% with rising temperature. Si, Al, and Na all increase in concert. The solutions were slightly peralkaline, and possess Si/(Na + Al) ≫ 1.5 (molar) at all conditions studied. Extrapolated thermodynamic data were used to predict solubility at the conditions investigated experimentally. Calculated solubility agreed with that measured from 350 to ∼ 500 °C; however, above 500 °C, the calculations underpredict solubility to an increasing degree as the hydrothermal melting point is approached. The excess measured solubility points to increasing abundance of aqueous Si, Al–Si, and Na–Al–Si polymers. Polymerized solutes predominate in all near-solidus solutions, rising to > 80% of total dissolved solids at the melting point. The observations support a conceptual model in which, as the temperature of the system rises isobarically at 1 GPa, the silicate components dissolved in the aqueous phase begin to polymerize significantly within ∼ 100 °C of the melting point. The polymerized solutes may facilitate condensation of more polymerized hydrous silicate liquid at the hydrothermal melting point. From the perspective of isobaric cooling, the fluid crossing the solidus retains Na–Al–Si–O clusters or fragments that are less polymerized than those which comprised the melt, but more polymerized than has previously been inferred for the aqueous phase. The high concentrations of Na, Al, and Si in the form of polymeric clusters will yield substantial mass transfer by near-solidus metamorphic and magmatic fluids in the crust and mantle, and likely promote dissolution, transport and precipitation of nominally insoluble, refractory rock components.

Reply to: Comment on “Solubility of the Assemblage Albite+K-Feldspar+Andalusite+Quartz in Supercritical Aqueous Chloride Solutions at 650 °C and 2 kbar” by D.J. Wesolowski

Reply to: Comment on “Solubility of the Assemblage Albite+K-Feldspar+Andalusite+Quartz in Supercritical Aqueous Chloride Solutions at 650 °C and 2 kbar” by D.J. Wesolowski

Comment on “Solubility of the assemblage albite+K-feldspar+andalusite+quartz in supercritical aqueous chloride solutions at 650 °C and 2 kbar” by T.M. Pak et al., Chemical Geology, vol. 200, pp. 377–393

Comment on “Solubility of the assemblage albite+K-feldspar+andalusite+quartz in supercritical aqueous chloride solutions at 650 °C and 2 kbar” by T.M. Pak et al., Chemical Geology, vol. 200, pp. 377–393

Paragonite stability at 700°C in the presence of H2O–NaCl fluids: constraints on H2O activity and implications for high pressure metamorphism

We carried out reversed piston-cylinder experiments on the equilibrium paragonite = jadeite + kyanite + H2O at 700°C, 1.5–2.5 GPa, in the presence of H2O-NaCl fluids. Synthetic paragonite and jadeite and natural kyanite were used as starting materials. The experiments were performed on four different nominal starting compositions: X(H2O)=1.0, 0.90, 0.75 and 0.62. Reaction direction and extent were determined from the weight change in H2O in the capsule, as well as by optical and scanning electron microscopy (SEM). At X(H2O)=1.0, the equilibrium lies between 2.25 and 2.30 GPa, in good agreement with the 2.30–2.45 GPa reversal of Holland (Contrib Miner Petrol 68:293–301, 1979). Lowering X(H2O) decreases the pressure of paragonite breakdown to 2.10–2.20 GPa at X(H2O)=0.90 and 1.85–1.90 GPa at X(H2O)=0.75. The experiments at X(H2O) = 0.62 yielded the assemblage albite + corundum at ≤1.60 GPa, and jadeite + kyanite at ≥1.70 GPa. This constrains the position of the isothermal paragonite–jadeite–kyanite–albite–corundum–H2O invariant point in the system Na2O–Al2O3–SiO2–H2O to be at 1.6–1.7 GPa and X(H2O)~0.65±0.05. The data indicate that H2O activity, a(H2O), is 0.75–0.86, 0.55–0.58, and <0.42 at X(H2O)=0.90, 0.75, and 0.62, respectively. These values approach X(H2O)2, and agree well with the a(H2O) model of Aranovich and Newton (Contrib Miner Petrol 125:200–212, 1996). Our results demonstrate that the presence or absence of paragonite can be used to place limits on a(H2O) in high-pressure metamorphic environments. For example, nearly pure jadeite and kyanite from a metapelite from the Sesia Lanzo Zone formed during the Eo-Alpine metamorphic event at 1.7–2.0 GPa, 550–650°C. The absence of paragonite requires a fluid with low a(H2O) of 0.3–0.6, which could be due to the presence of saline brines.

Ilímaussaq ‘en miniature’: closed-system fractionation in an agpaitic dyke rock from the Gardar Province, South Greenland (contribution to the mineralogy of Ilímaussaq no. 117)

AbstractBased on petrography, mineral chemistry, and petrology, the physico-chemical evolution of an agpaitic dyke was found to be very similar to that of the neighbouring Ilímaussaq complex. Various mineral assemblages were used to reconstruct the crystallization conditions of the dyke rock for different stages during cooling. The early magmatic phenocryst assemblage is alkali feldspar + nepheline + augite + olivine + magnetite and indicates liquidus temperatures of ∼850ºC, silica activities of ∼0.5, and oxygen fugacities of FMQ –1.5 to –3. The groundmass assemblage albite + microcline + nepheline + sodalite + arfvedsonite + aegirine + aenigmatite + astrophyllite indicates lower temperatures of between 600 and 450ºC, at silica activities of 0.25, and oxygen fugacities around the FMQ buffer. Amphibole composition strongly responds to fluorite saturation and proves crystallization occurred in a system closed to oxygen. Late-stage hydrothermal conditions are indicated by the conversion of nepheline and sodalite to analcime and the growth of aegirine on arfvedsonite. These late-stage reactions are constrained to temperatures of &lt;300°C, water activities of between 0.5 and unity, and oxygen fugacities above MH. The dyke has to be regarded as a small equivalent of the larger Ilímaussaq complex, in which identical differentiation processes proceeded at a scale very different in terms of magma volume and cooling times.

Solubility of the assemblage albite+K-feldspar+andalusite+quartz in supercritical aqueous chloride solutions at 650 °C and 2 kbar

The solubility of the high grade pelite assemblage albite+K-feldspar+andalusite+quartz at 650 °C and 2 kbar was determined in aqueous solutions over a total chloride range of 0.01–3 m Cl tot using rapid-quench hydrothermal technique. The concentration of Na, K, Si, and Al was determined in the fluid phase after quench. The K/Na ratio was determined by approaching the equilibrium from below and above. It is 0.34 at low chloride concentrations and decreases slightly to 0.31 with increasing total chloride. Silica and aluminum concentrations were determined only from undersaturation. The silica solubility is found to be independent of chloride concentration and is ∼0.13 molal. Aluminum is nearly independent of chloride concentration decreasing only slightly from ∼0.0015 to ∼0.0007 molal. Comparison of the experimental data with thermodynamic model calculations demonstrates that the silica concentrations are well predicted, while significant differences exist between individual databases for Al speciation and its total concentration. Al concentrations are underestimated by up to 10 to 15 orders of magnitude using the SUPCRT92 database. Predicted K/Na ratios are underestimated by up to 30%. The best predictions achieved for this simplified high-grade pelite assemblage are those using the SUPCRT92 database with revised thermodynamic data for feldspars and K- and Na-species (J. Phys. Chem. Ref. Data 24 (1995) 1401) and additional Al-species (Am. J. Sci. 295 (1995) 1255; Geochim. Cosmochim. Acta 61 (1997) 2175). The use of ideal mixing for neutral complexes in combination with the extended Debye–Hückel activity model for the charged species yields the most compatible speciation model.

Thrusting and sinistral wrenching in a pre-Eocene HP-LT Caribbean accretionary wedge (Samaná Peninsula, Dominican Republic)

The North Caribbean margin is an example of an oblique convergence zone where the currently exposed HP–LT rocks are systematically localised close to strike-slip faults. The petrological and structural study of eclogite and blueschist facies rocks of the peninsula of Samaná (Hispaniola, Dominican Republic) confirms the presence of two different metamorphic units. The former diplays low metamorphic grade (Santa Barbara unit), characterized by the assemblage albite - lawsonite (7.5 ± 2 kbar and 320 ± 80 °C). The latter (Punta Balandra unit), thrust over the first unit towards the NW, and is characterized by the occurrence of blueschist and eclogite facies assemblages (13 ± 2 kbar and 450 ± 70 °C), within oceanic metasediments. The isothermal retrograde evolution occurred in epidote-blueschist facies conditions (9 ± 2 kbar and 440 ± 60 °C). The late greenschist facies evolution is contemporaneous with conjugate NW–SE extension and E–W strike-slip faulting. This late extension is for regional dome and basin structures. According to their lithotectonic, structural and metamorphic characteristics, the metamorphic nappe stack of Samaná may be interpreted as a fragment of an accretionary wedge thrust onto the North American continental shelf. Evolution of the wedge was associated with the active subduction of the North American plate, under the Greater Antilles arc, at the level of the Puerto Rico trench. During active Late Cretaceous convergence, the HP rocks were initially exhumed, within the accretionary prism, by thrusting parallel to the NE–SW direction of convergence. Subsequently, during the Eocene collision between the Caribbean plate and the North American margin, the installation of a transtensive regime of E–W direction supports the local development of conjugate extension of NW–SE direction that facilitated the final phase of exhumation of the HP rocks.

Thrusting and sinistral wrenching in a pre-Eocene HP-LT Caribbean accretionary wedge (Samaná Peninsula, Dominican Republic)

The North Caribbean margin is an example of an oblique convergence zone where the currently exposed HP-LT rocks are systematically localised close to strike-slip faults. The petrological and structural study of eclogite and blueschist facies rocks of the peninsula of Sarnaná (Hispaniola, Dominican Republic) confirms the presence of two different metamorphic units. The former diplays low metamorphic grade (Santa Barbara unit), characterized by the assemblage albite - lawsonite (7.5 ± 2 kbar and 320 ± 80 °C). The latter (Punta Balandra unit), thrust over the first unit towards the NW, and is characterized by the occurrence of blueschist and eclogite facies assemblages (13 ± 2 kbar and 450 ± 70 °C), within oceanic metasediments. The isothermal retrograde evolution occurred in epidote-blueschist facies conditions (9 ± 2 kbar and 440 ± 60 °C). The late greenschist facies evolution is contemporaneous with conjugate NW-SE extension and E-W strike-slip faulting. This late extension is for regional dome and basin structures. According to their lithotectonic, structural and metamorphic characteristics, the metamorphic nappe stack of Sarnaná may be interpreted as a fragment of an accretionary wedge thrust onto the North American continental shelf. Evolution of the wedge was associated with the active subduction of the North American plate, under the Greater Antilles arc, at the level of the Puerto Rico trench. During active Late Cretaceous convergence, the HP rocks were initially exhumed, within the accretionary prism, by thrusting parallel to the NE-SW direction of convergence. Subsequently, during the Eocene collision between the Caribbean plate and the North American margin, the installation of a transtensive regime of E-W direction supports the local development of conjugate extension of NW-SE direction that facilitated the final phase of exhumation of the HP rocks. © 2000 Éditions scientifiques et médicales Elsevier SAS

Mineralogical and fluid inclusion studies of low-sulfidation epithermal veins at Osilo (Sardinia), Italy

Several precious metal-bearing, low sulfidation epithermal veins occur in the rolling topography of the Osilo area, northern Sardinia. The Sa Pala de Sa Fae and the Sa Pedra Bianca veins were subject to intense diamond drilling exploration in the mid 1990 s. The veins extend for 1–3 km, dip steeply, and range from 1 to 10 m in width. High K-calc-alkaline volcanic deposits containing plagioclase phenocrysts (along with lesser pyroxene, amphibole, magnetite, olivine and sanidine) form the main host rocks. Gold grades in drill intersections range from 50 m from the vein, the assemblage albite + Fe-chlorite + illite + pyrite (± montmorillonite ± calcite ± K-feldspar) prevails regionally, and its formation is attributed to minor metasomatism of the country rock involving the addition of water, carbon dioxide and hydrogen sulfide. At distances <10 m from the vein, the assemblage quartz + K-feldspar + pyrite ± illite dominates, forming an alteration envelope that cross cuts regional alteration. Quartz and K-feldspar increase in abundance towards the vein. Quartz is the main vein mineral, and it displays a range of morphologies and textures including crustiform colloform banding, quartz pseudomorphs of platy calcite, breccias and coarse euhedral crystals. Electrum and argentite which are the main gold and silver minerals deposited during the early stages of vein mineralization with rhomb-shaped crystals of K-feldspar (adularia). Pyrite, plus lesser marcasite, arsenopyrite, stibnite and sphalerite, are the other sulfide phases in veins. Kaolinite ± halloysite ± jarosite form a late assemblage overprinting earlier hydrothermal alteration. It is mostly restricted to shallow depths of a few meters, except near veins. Most of this assemblage likely formed from weathering and oxidation of sulfides. Microthermometric measurements were made on quartz-hosted, two-phase (liquid + vapor) inclusions, containing ∼75% liquid; mean homogenization temperatures (∼750 measurements) range from 220 to 250 °C, and ice-melting temperatures (∼550 measurements) range from 0.0 to −2.3 °C. The presence of co-existing vapor-rich and liquid rich inclusions, with quartz pseudomorphs of platy calcite, indicate that boiling conditions existed. Slight vapor-bubble expansion of a few fluid inclusions subjected to crushing experiments indicates inclusion fluids contained variable but low concentrations of dissolved gas. This study shows that gold-silver mineralization formed in subvertical channels from ascending solutions at 250 °C at around 300 to 450 m below the paleo-water table in a typical low-sulfidation epithermal environment. Hydrothermal solutions that produced vein mineralization and related alteration were dilute (<4.1 equivalent wt.% NaCl and <4 wt.% CO2), near neutral pH, reduced and, at times, boiling.

Fluid fluxes during low-temperature alteration: implications of multi-style alteration assemblages from the Welsh Borderland, UK

Localized (domainal) low-temperature alteration may indicate variations in the chemistry of alteration fluids and/or discrete fluid flow paths during metamorphism. Occurrences of epidote- and pumpellyite-dominated domains are often used as evidence for large fluxes of Ca-rich fluids. However, comparative studies of two domainal alteration styles from basaltic to andesitic lavas and volcaniclastic rocks at Builth Wells, Wales, UK suggest that such interpretations cannot be applied universally. Here, only one set of domains can be attributed to large fluxes of Ca-rich fluids. In contrast, the second set of domains formed where the host rock supplied the necessary Ca, and fluid/rock ratios were relatively low. These domains are hosted by rocks which show a Caledonian regional metamorphic imprint, characterized by the alteration assemblage albite+chlorite+titanite±prehnite±pumpellyite±calcite±muscovite±quartz (considered to have formed at c. 200°C and ≤2.5 kbar). Type 1 domains lie along fluid channelways, such as veins. Pumpellyite is the dominant secondary mineral, but the alteration assemblage is variable with pumpellyite±prehnite±calcite±titanite±chlorite (±quartz). The domains formed at temperatures of 130–230°C during Ordovician hydrothermal activity, before the peak of regional metamorphism. In contrast, Type 2 domains are not associated with obvious fluid channelways. A central prehnite-dominated zone typically has a rim with the assemblage pumpellyite+calcite±prehnite±chlorite±titanite±K-rich phyllosilicate. These domains probably formed from pelitic xenoliths which contained a volcaniclastic component at temperatures of 130–230°C and at relatively low fluid/rock ratios. However, the timing of domain formation is uncertain. Pyrobitumen, in veins and disseminated throughout the rock, may have influenced the formation of both types of domain by stabilizing pumpellyite in preference to epidote and causing metamorphic calcite formation through the release of CO2 to the alteration fluids. © 1996 John Wiley & Sons, Ltd.

Fluid fluxes during low‐temperature alteration: implications of multi‐style alteration assemblages from the Welsh Borderland, UK

Localized (domainal) low-temperature alteration may indicate variations in the chemistry of alteration fluids and/or discrete fluid flow paths during metamorphism. Occurrences of epidote- and pumpellyite-dominated domains are often used as evidence for large fluxes of Ca-rich fluids. However, comparative studies of two domainal alteration styles from basaltic to andesitic lavas and volcaniclastic rocks at Builth Wells, Wales, UK suggest that such interpretations cannot be applied universally. Here, only one set of domains can be attributed to large fluxes of Ca-rich fluids. In contrast, the second set of domains formed where the host rock supplied the necessary Ca, and fluid/rock ratios were relatively low. These domains are hosted by rocks which show a Caledonian regional metamorphic imprint, characterized by the alteration assemblage albite+chlorite+titanite±prehnite±pumpellyite±calcite±muscovite±quartz (considered to have formed at c. 200°C and ≤2.5 kbar). Type 1 domains lie along fluid channelways, such as veins. Pumpellyite is the dominant secondary mineral, but the alteration assemblage is variable with pumpellyite±prehnite±calcite±titanite±chlorite (±quartz). The domains formed at temperatures of 130–230°C during Ordovician hydrothermal activity, before the peak of regional metamorphism. In contrast, Type 2 domains are not associated with obvious fluid channelways. A central prehnite-dominated zone typically has a rim with the assemblage pumpellyite+calcite±prehnite±chlorite±titanite±K-rich phyllosilicate. These domains probably formed from pelitic xenoliths which contained a volcaniclastic component at temperatures of 130–230°C and at relatively low fluid/rock ratios. However, the timing of domain formation is uncertain. Pyrobitumen, in veins and disseminated throughout the rock, may have influenced the formation of both types of domain by stabilizing pumpellyite in preference to epidote and causing metamorphic calcite formation through the release of CO2 to the alteration fluids. © 1996 John Wiley & Sons, Ltd.

Very low-temperature metamorphism of shales from the Diablo Range, Franciscan Complex, California: New constraints on the exhumation path

We have determined illite crystallinity, coal rank, percent 2M 1 white K-mica polytype, and phase assemblages for shales and shale-matrix melange from various areas in the Diablo Range. Additional information concerning metamorphic conditions and fluid composition was derived from stable isotope work and fluid inclusion measurements. This study presents new constraints on the pressure-temperature evolution of the Franciscan units from the Diablo Range. Several shales indicate a lower pressure, possibly higher temperature overprint on the peak pressure assemblages. Peak pressure assemblages include jadeitic pyroxene, sodic amphibole, lawsonite, aragonite, and phengite. Reactions, which indicate a lower pressure, possibly higher temperature overprint, are the transition from aragonite to calcite, the formation of albite from jadeitic pyroxene + quartz, the compositional gradient from phengite to muscovite in white K-mica, and the transition from sodic to calcic amphibole. The assemblage albite, calcic amphibole, lawsonite, and chlorite found in a single shale from Pacheco Pass indicates the transition from blueschist to greenschist facies conditions. We suggest that this transition occurred at a pressure and temperature of 5–8 kilobars and 300–350°C. Fluid inclusion data from post-kinematic quartz veins from the Mount Hamilton area support the idea of a lower pressure, possibly higher temperature metamorphic event. Pressure-temperature conditions of trapping early H 2 O- and CH 4 -rich fluids were 4.5 kilobars and ≈230°C. Geothermal gradients for the transition from blueschist to greenschist facies and the formation of early fluid inclusions are estimated to be ≈14°C/km. Illite crystallinity for all areas indicates diagenetic to high-grade anchizonal conditions; coal ranks are between 1.6% and 2.5% mean Ro. These estimates are not consistent with metamorphic conditions deduced for two different shale assemblages from Pacheco Pass, suggesting that illite crystallinity and coal rank are retarded. Poor illite crystallinity is thought to result from small coherent scattering domain sizes along c* of white K-mica crystals rich in a phengite component (Dalla Torre et al., 1994a, in press). The exact cause of the retardation of coal rank is not clear. The high pressure in the Diablo Range may have affected the maturation of organic matter. Our results imply that coal rank data from high-pressure terranes must be interpreted with caution if used as an absolute paleothermometer.

Metamorphic P‐T paths from calcic pelitic schists from the Strafford Dome, Vermont, USA

ABSTRACTMetamorphism of the Gile Mountain Formation and Waits River Formation in the Strafford Dome and Townshend‐Brownington Syncline in east‐central Vermont records two nappe‐style events, D1 and D2, followed by doming. D1 formed a muscovite + biotite ± ilmenite schistosity subparallel to compositional layering, SO, and was followed by heating to garnet grade. The temperature and pressure at the end of D1 are estimated to be c. 450d̀ C and 6‐8 kbar. D2 variably crenulated and folded S1 during a nearly isothermal pressure increase of 1‐2 kbar, calculated from compositions of garnet, which have inclusions trails with progressive crenulation and rotation of the S1 fabric. Similar P‐T paths are computed for most of the area, suggesting that the later schistosity developed during emplacement of a regional nappe 3‐6 km thick. There is a general lack of D3 (dome‐stage) microstructures.Near the Strafford‐Willoughby Arch, staurolite and kyanite overgrew S2 in pelites, and plagioclase with increasing XAn overgrew S2 in calcic pelites, reflecting post‐D2 heating to a maximum of 550‐600d̀ C. Metamorphic pressures at the end of D2 are fairly constant on the west side of the dome, indicating minor dome‐stage uplift. In contrast, pressures at the thermal peak of metamorphism decrease by more than 4 kbar east of the dome. The observed pattern of isotherms and isobars is mainly the result of post‐metamorphic, differential uplift and unroofing.Finally, a minor, retrograde metamorphism produced the assemblage albite + epidote + K‐feldspar + muscovite + chlorite, with grade increasing east toward the Connecticut River.

Deformation of granitoids at low metamorphic grade. I: Reactions and grain size reduction

Deformation and chemical reaction in granitoids at greenschist faciès conditions were investigated in samples from three shear zones: two from the Alps (Corvatsch Granodiorite and Aiguilles Rouges Granite, Switzerland) and one from Eastern Australia (Wyangala Granite). When examined by light and electron microscopy, all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite: quartz deformed plastically, while both K-feldspar and plagioclase fractured and recrystallized in conjunction with chemical change, which had, as its end point, an assemblage albite + quartz ± white mica ± CaAl-silicates. K-feldspar and calcic plagioclase (depending on fluid chemistry) from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid. Since Ca-bearing plagioclase was not stable in these environments, myrmekite did not replace K-feldspar in any of the rocks examined. Recrystallization of feldspar, which should perhaps be termed neocrystallization, was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets. In old feldspar grains, particularly in plagioclases, dislocations exist in wall-like structures that are commonly associated with voids, suggesting origins in microfractures. These dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization. We conclude that recrystallization mainly occurred via a classical nucleation mechanism, with minor contributions from twin boundary migration. We argue that the 450–550°C lower limit for recrystallization of feldspar, referenced throughout the literature, is not applicable to the rock systems under investigation and should be discarded as a universal limit. Microfracturing, microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades (see part II of this paper). Neocrystallization, fluid access and nucleation appear to have accompanied, or immediately followed, the first deformation increments and were primarily responsible for continued grain size reduction. Transgranular fracture was comparatively important only at the onset of deformation. Since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade, in the presence of aqueous fluids in the Earth's crust, breakdown reactions and nucleation-recrystallization in feldspars may be important influences upon localization of deformation in granitoids.

Stability of the assemblage albite plus forsterite at high temperatures and pressures with petrologic implications

The maximum limits of the assemblage albiteforsterite have been determined experimentally at high pressures and temperatures. At subsolidus temperatures, albite plus forsterite is replaced at high pressures by jadeitic clinopyroxene and enstatitic orthopyroxene. The boundary for this reaction lies within experimental uncertainity of that for jadeite=albite+nepheline. Melting of albite+forsterite at high pressures produces enstatite+liquid, which is different from the low-pressure eutectic behavior. Melting rates are very slow and several hundred hours are required to establish equilibrium near the solidus. The subsolidus boundary for albite plus forsterite lies near that for sanidine plus forsterite, but with a shallower slope which more closely matches that of anorthite plus forsterite. Both albite plus forsterite and anorthite plus forsterite are replaced at high pressures by an assemblage containing clinopyroxene plus orthopyroxene, unlike sanidine plus forsterite, which is replaced by a feldspathoid plus orthopyroxene. The presence of sodium enlarges the depth region over which plagioclase lherzolite can stably exist; it may also stabilize alkali feldspar plus olivine in crustal rocks.

Hydrothermal porosity reduction of sandstone: An example from the Miocene Kawai Formation, Southwest Japan.

Hydrothermal porosity reduction of sandstone can be induced by intrusion of igneous rocks under proper conditions, even in the early stage of diagenesis. This paper presents one of the examples from Takuno, Nima, Shimane Prefecture, in which lith-arenite of the Kawai Formation is intruded by a small body of rhyolite measuring about 300m across and is hydrothermally reduced in its porosity from about 25 to 7% progressively toward the contact. In the alteration process, hydrothermal solution supersaturated with respect to quartz flowed into the contact and further flowed outward from the contact presumably, so that the nearer the contact, the more rapidly quartz was precipitated in the pores, and the more porosity was reduced in proportion to the volume of quartz precipitation. On the other hand, most of chemical components in the sandstone were conserved to from the mineral assemblage albite (Ab95-99), K-feldspar (Or94-99), muscovite and quartz. The mineral assemblage and chemical compositions of newly formed feldspars confine the alteration temperature, presumably in a range from 120 to 400°C.

Alpine metamorphism of Hercynian hornblende granodiorite beneath the blueschist facies schistes lustrés nappe of NE Corsica

Abstract The Hercynian granitic basement which forms the Tenda Massif in NE Corsica represents part of the leading edge of the European Plate during middle‐to‐late Cretaceous (Eoalpine) high P metamorphism. The metamorphism of this basement, induced by the overthrusting of a blueschist facies (schistes lustrés) nappe, was confined to a major ductile shear zone (c. 1000m thick) within which deformation increases upwards towards the overlying nappe. Metamorphism within the basement mostly records lower blueschist facies conditions (crossite + epidote) except near the base of the shear zone where the greenschist facies assemblage albite + actinolitic amphibole has developed instead of crossite. Study of the primary mafic phase breakdown reactions within hornblende granodiorite reveals the following metamorphic zonation. Zone 1: biotite to chlorite. Towards zone 2: biotite to phengite. Zone 2: Hornblende to actinolitic Ca‐amphibole + albite + sphene, and biotite to actinolitic Ca‐amphibole + albite + phengite + Ti‐ore + epidote. Zone 3: Hornblende to crossite + low Ti‐biotite + phengite + sphene, and biotite to crossite + low Ti‐biotite + phengite + Ti‐ore + sphene ± epidote. P‐T conditions at the base of the shear zone are estimated to have been 390‐490°C at 600‐900 M Pa (6‐9kbar) and the Corsican basement is therefore deduced to have been buried to 20‐30 km during metamorphism. This relatively shallow metamorphism contrasts with some other areas in the Western Alps where the Eoalpine event apparently buried the European continental crust to depths of 80 km or more. As there is no evidence for a long history of blueschist facies metamorphism prior to the involvement of the European continent, it is deduced that the Eoalpine blueschists were produced during the collision of the Insubric plate with Europe, rather than during Tethyan intraoceanic subduction. Coherent blueschist terrains such as the schistes lustres probably record buovant feature collision and obduction tectonics rather than any preceding oceanic subduction.

Geochronology of high-pressure rocks on Sifnos (Cyclades, Greece)

Polymetamorphic rocks of Sifnos (Greece) have been investigated by Rb-Sr, K-Ar, and fission track methods. Critical mineral assemblages from the northern and southernmost parts of Sifnos include jadeite+quartz+3T phengite, and omphacite+garnet +3T phengite, whereas the central part is characterized by the assemblage albite+chlorite+epidote+2M1 phengite.

The upper stability limit of the assemblage paragonite + quartz and its natural occurrences

The assemblage paragonite + quartz is encountered frequently in low- to medium-grade metamorphic rocks. With rising grade of metamorphism they react mutually to yield the condensed assemblage albite + Al2SiO5.