Parageneses and compositions of amphiboles from Franciscan jadeite–glaucophane type facies series metabasites at Cazadero, California

Abstract Sodic amphiboles are common in Franciscan type II and type III metabasites from Cazadero, California. They occur as (1) vein-fillings, (2) overgrowths on relict augites, (3) discrete tiny crystals in the groundmass, and (4) composite crystals with metamorphic Ca–Na pyroxenes in low-grade rocks. They become coarse-grained and show strong preferred orientation in schistose high-grade rocks. In the lowest grade, only riebeckite to crossite appears; with increasing grade, sodic amphibole becomes, first, enriched in glaucophane component, later coexists with actinolite, and finally, at even higher grade, becomes winchite. Actinolite first appears in foliated blueschists of the upper pumpellyite zone. It occurs (1) interlayered on a millimetre scale with glaucophane prisms and (2) as segments of composite amphibole crystals. Actinolite is considered to be in equilibrium with other high-pressure phases on the basis of its restricted occurrence in higher grade rocks, textural and compositional characteristics, and Fe/Mg distribution coefficient between actinolite and chlorite. Detailed analyses delineate a compositional gap for coexisting sodic and calcic amphiboles. At the highest grade, winchite appears at the expense of the actinolite–glaucophane pair. Compositional characteristics of Franciscan amphiboles from Ward Creek are compared with those of other high P/T facies series. The amphibole trend in terms of major components is very sensitive to the metamorphic field gradient. Na-amphibole appears at lower grade than actinolite along the higher P/T facies series (e.g. Franciscan and New Caledonia), whereas reverse relations occur in the lower P/T facies series (e.g. Sanbagawa and New Zealand). Available data also indicate that at low-temperature conditions, such as those of the blueschist and pumpellyite–actinolite facies, large compositional gaps exist between Ca- and Na-amphiboles, and between actinolite and hornblende, whereas at higher temperatures such as in the epidote–amphibolite, greenschist and eclogite facies, the gaps become very restricted. Common occurrence of both sodic and calcic amphiboles and Ca–Na pyroxene together with albite + quartz in the Ward Creek metabasites and their compositional trends are characteristic of the jadeite–glaucophane type facies series. In New Caledonia blueschists, Ca–Na pyroxenes are also common; Na-amphiboles do not appear alone at low grade in metabasites, instead, Na-amphiboles coexist with Ca-amphiboles throughout the progressive sequence. However, for metabasites of the intermediate pressure facies series, such as those of the Sanbagawa belt, Japan and South Island, New Zealand, Ca–Na pyroxene and glaucophane are not common; sodic amphiboles are restricted to crossite and riebeckite in composition and clinopyroxenes to acmite and sodic augite, and occur only in Fe₂O₃-rich metabasites. The glaucophane component of Na-amphibole systematically decreases from Ward Creek, New Caledonia, through Sanbagawa to New Zealand. This relation is consistent with estimated pressure decrease employing the geobarometer of Maruyama et al. (1986). Similarly, the decrease in tschermakite content and increase in NaM₄ of Ca-amphiboles from New Zealand, through Sanbagawa to New Caledonia is consistent with the geobarometry of Brown (1977b). Therefore, the difference in compositional trends of amphiboles can be used as a guide for P–T detail within the metamorphic facies series.     

Amphibole zonation in metabasites as a guide to the evolution of metamorphic conditions

In low grade metabasites the amphibole components tremolite, glaucophane, edenite and tschermakite have their activities controlled by interactions with the excess components albite, clinozoisite, chlorite, quartz and H₂O vapor. Three types of reaction are involved, (i) Those in which only components of condensed phases take part: isopleths of equilibrium constant are straight lines in the P-T plane. (ii) Dehydration reactions in which entropy change due to change in Al coordination is of the same sign as that due to dehydration: isopleths of constant K are positive at low pressure and negative at high pressure. (iii) Dehydration reactions in which entropy change due to Al coordination change is opposite in sign to that of dehydration: isopleths of constant K loop in the P-T plane with positive slopes at low and at high pressure. Zonation in naturally occurring amphiboles records the evolution of metamorphic conditions in particular rocks. In an example from the eastern Alps (Austria) early conditions calculated as 15 kb, 200 ° C evolve upgrade to 6 kb, 525 ° C implying concurrent heating and erosion. The record of evolving conditions may span some 30 Ma of geological history.     

Petrology of Franciscan Metabasites Along the Jadeite-Glaucophane Type Facies Series, Cazadero, California

The Cazadero blueschist allochthon lies within the Central MelangeBelt of the Franciscan assemblage in the northern Coast Rangeof California. Mineral compositions and assemblages of morethan 200 blueschists from Ward Creek were investigated. Theresults delineate lawsonite-, pumpellyite-, and epidote-zones.The lawsonite and pumpellyite zones are equivalent to the TypeII metabasites of Coleman & Lee (1963) and are characterizedby well-preserved igneous textures, relict augite, and pillowstructures, whereas epidote zone rocks are equivalent to theType III strongly deformed and schistose metabasites. Chlorite,phengite, aragonite, sphene, and minor quartz and albite areubiquitous. The lawsonite zone metabasites contain lawsonite ( < 3 wt.per cent Fe₂O₃), riebeckite-crossite, chlorite, and Ca-Na-pyroxene;some rocks have two distinct clinopyroxenes separated by a compositionalgap. The clinopyroxene of the lowest grade metabasites containsvery low X_jd. In pumpellyite zone metabasites, the most commonassemblages contain Pm + Cpx + Gl + Chl and some samples withhigher Al₂O₃ and/or Fe₂O₃ have Pm + Lw + Cpx + Chl, Actinolitejoins the above assemblage in the upper pumpellyite zone wherethe actinolite-glaucophane compositional gap is well defined.The epidote zone metabasites are characterized by the assemblagesEp + Cpx + two amphiboles + Chl, Lw + Pm + Act + Chl, and Ep+ Pm + two amphiboles + Chl depending on the Fe₂O₃ content ofthe rock. In the upper epidote zone, winchite appears, Fe-freelawsonite is stable, pumpellyite disappears and omphacite containsvery low Ac component. Therefore, the common assemblages areEp + winchite + Lw, and Lw + Omp + winchite. With further increasein metamorphic grade, epidote becomes Al-rich and lawsoniteis no longer stable. Hence Ep + winchite + omphacite ? garnetis characteristic. Mineral assemblages and paragenetic sequences delineate threediscontinuous reactions: (1) pumpellyite-in; (2) actinolite-in;and (3) epidote-in reactions. Using the temperatures estimatedby Taylor & Coleman (1968) and phase equilibria for Ca-Na-pyroxenes,the P–T positions of these reactions and the metamorphicgradient are located. All three metabasite zones occur withinthe aragonite stability field and are bounded by the maximumpressure curve of Ab = Jd + Qz and the maximum stabilities ofpumpellyite and lawsonite. The lawsonite zone appears to bestable at T below 200?C with a pressure range of 4–6?5kb; the pumpellyite zone between 200 and 290?C and the epidotezone above 290?C with pressure variation between 6?5 and 9 kb.The metamorphic field gradient appears to have a convex naturetowards higher pressure. A speculative model of underplatingseamounts is used to explain such feature.     

Chromiferous spinels of the elephant's head dike

Analysis by optical, X-ray diffraction and microprobe methods, of essentially unzoned, disseminated spinels within cumulus picrites and olivine gabbros reveals an unbroken range of compositions from aluminian chromite (34% Cr₂O₃) to chromian magnetite (4% Cr₂O₃). TiO₂ contents vary between 0.5 and 7.7%. Exsolution of ilmenite indicates originally higher TiO₂ contents. Quenched contact-facies rocks with abundant olivine phenocrysts contain strongly zoned spinels in which a titaniferous chromian magnetite rim (16% Cr₂O₃, 10% TiO₂) encloses cores of weakly titanian chromite (40% Cr₂O₃). Platy dendrites of exsolved spinel occur in abundance within cumulus olivines. The evidence suggests that crystallization of the disseminated spinels occurred under the influence of an increase in oxygen fugacity towards the interior of the intrusion, and that the compositional diversity has stemmed from the homogenization of originally zoned grains mantled to varying degrees by high-Ti, low-Cr rims.     

Relative geobarometry from high-pressure rocks of quartzofeldspathic composition from the Sesia Zone,Western Alps,Italy

In the Sesia Zone of the Western Alps, Italy, early Alpine blueschist to eclogite facies metamorphism of rocks of quartzofeldspathic composition has produced the same high-pressure assemblage of; quartz, Na-pyroxene, Na-amphibole, paragonite, phengite, zoisite, garnet, magnetite, sphene and Fe-sulphide (=the QFS assemblage) over an area (> 150 km². Relative gradients in pressure and temperature over this region are reflected in the variations in mineral chemistries of the individual phases of the quartzofeldspathic assemblage through continuous reactions. Mineralogical discontinuities do not occur in the QFS assemblage of this region. Increases in the Jadeite content of the pyroxenes (X _{Jd 0.48} to X _{Jd 0.93}) and in the glaucophane content of the amphiboles (X _{Gl 0.89} to X _{Gl 0.96}) occur from the southwest to the northeast of the region studied. Analysis of coexisting garnets and pyroxenes indicate that the compositional variation of amphiboles and pyroxenes is associated with a decrease in the grossular component of the coexisting garnet. Zoned pyroxenes and garnets, together with the regional trends in mineral chemistries suggest that the evolution of the QFS assemblage with increasing pressure may be modelled by pressure-sensitive continuous reactions in which amphibole, zoisite and the more jadeitic pyroxene constitute the high-pressure assemblage. Chemographic constraints permit the positioning in pressure/temperature space of the compositional isopleths of those model continuous reactions involving these phases which meet the textural and chemical criteria observed in the natural assemblages. The low dP/dT slope (–20 bars/° C) of these isopleths causes the continuous reactions to be useful for geobarometric calculations at pressures above the absolute breakdown of albite to jadeite plus quartz. In addition the pseudobinary loops for the other continuous reactions which are potentially useful geobarometers and involve either the NaAlCa_–1Mg_–1 exchange or the MgCa_–1 exchange are calculated. Comparison of mineral chemistries with the isopleths yields a relative barometric scheme for the localities studied. With these barometric observations, it is possible to show that the P-T path which the Sesia body travelled towards the final recorded state was one of increasing pressure. Other blueschist and eclogite occurrences from Syros and Sifnos which contain rocks of quartzofeldspathic composition are also examined.     

Magmatic unmixing in spinel from late Precambrian concentrically-zoned mafic–ultramafic intrusions, Eastern Desert, Egypt

Spinel is widespread in the ultramafic core rocks of zoned late Precambrian mafic–ultramafic complexes from the Eastern Desert of Egypt. These complexes; Gabbro Akarem, Genina Gharbia and Abu Hamamid are Precambrian analogues of Alaskan-type complexes, they are not metamorphosed although weakly altered. Each intrusion is composed of a predotite core enveloped by pyroxenites and gabbros at the margin. Silicate mineralogy and chemistry suggest formation by crystal fractionation from a hydrous magma. Relatively high Cr₂O₃ contents are recorded in pyroxenes (up to 1.1 wt.%) and amphiboles (up to 1.4 wt.%) from the three plutons. The chrome spinel crystallized at different stages of melt evolution; as early cumulus inclusions in olivine, inclusions in pyroxenes and amphiboles and late-magmatic intercumulus phase. The intercumulus chrome spinel is homogenous with narrow-range of chemical composition, mainly Fe³⁺-rich spinel. Spinel inclusions in clinopyroxene and amphibole reveal a wide range of Al (27–44 wt.% Al₂O₃) and Mg (6–13 wt.% MgO) contents and are commonly zoned. The different chemistries of those spinels reflect various stages of melt evolution and re-equilibration with the host minerals. The early cumulus chrome spinel reveals a complex unmixing structures and compositions. Three types of unmixed spinels are recognized; crystallographically oriented, irregular and complete separation. Unmixing products are Al-rich (Type I) and Fe³⁺-rich (Type II) spinels with an extensive solid solution between the two end members. The compositions of the unmixed spinels define a miscibility gap with respect to Cr–Al–Fe³⁺, extending from the Fe³⁺–Al join towards the Cr corner. Spinel unmixing occurs in response to cooling and the increase in oxidation state. The chemistry and grain size of the initial spinel and the cooling rate control the type of unmixing and the chemistry of the final products. Causes of spinel unmixing during late-magmatic stage are analogous to those in metamorphosed complexes. The chemistry of the unmixed spinels is completely different from the initial spinel composition and is not useful in petrogenetic interpretations. Spinels from oxidized magmas are likely to re-equilibrate during cooling and are not good tools for genetic considerations.     

A model for garnet and plagioclase growth in pelitic schists: implications for thermobarometry and P-T path determinations

Numerical models of the progressive evolution of pelitic schists in the NCMnKFMASH system with the assemblage garnet + biotite + chlorite ± staurolite + plagioclase + muscovite + quartz + H₂O are presented with the goal of predicting compositional changes in garnet and plagioclase along different P-T paths. The numerical models support several conclusions that should prove useful for interpreting the P-T paths of natural parageneses: (i) Garnet may grow along P-T vectors ranging from heating with decompression to cooling with compression. P-T paths deduced from garnet zoning that are inconsistent with these growth vectors are self-contradictory. (ii) There is a systematic relation between garnet and plagioclase composition and growth such that for most P-T paths, garnet growth requires plagioclase consumption. Furthermore, mass balance in a closed system requires that as plagioclase is consumed the remaining plagioclase becomes increasingly albitic. Inclusions of plagioclase in the core of garnet should be more anorthitic than those near the rim and zoned matrix plagioclase should have rims that are more albitic than the cores. Complex plagioclase textures may arise from the local variability of growth and precipitation kinetics. (iii) A decrease of Fe/(Fe + Mg) in a garnet zoning profile is a reliable indicator of increasing temperature for the assemblage modelled. However, there is no single reliable ΔP monitor and inferences about ΔP can only be made by considering plagioclase and garnet together. (iv) Consumption of garnet during the production of staurolite removes material from the outer shell of a garnet and may make recovery of peak metamorphic compositions and P-T conditions impossible. Low ‘peak’temperatures typically recorded by staurolite-bearing assemblages may reflect this phenomenon. (v) Diffusional homogenization of garnet affects the computed P-T path and results in a clockwise rotation of the computed P-T vector relative to the true P-T path.     

Sapphirine bearing granulites from the Sipiwesk Lake area of the late Archean Pikwitonei granulite terrain,Manitoba, Canada

Sapphirine occurs in the orthopyroxene-cordierite and feldspar-sillimanite granulites in the Sipiwesk Lake area of the Pikwitonei granulite terrain, Manitoba (97°40W, 55°05N). The orthopyroxene-cordierite granulites have extremely high Al₂O₃ (24.5 wt%) and MgO (24.6 wt%) contents and contain sapphirine (up to 69.2 wt% Al₂O₃), aluminous orthopyroxene (up to 8.93 wt% Al₂O₃), cordierite, spinel, phlogopite, and corundum. Sapphirine forms coronas mantling spinel and corundum. Corona sapphirine is zoned and its composition varies through the substitution (Mg, Fe, Mn) Si=2 Al as a function of the phases with which it is in contact. Textural and chemical relationships of sapphirine with coexisting phases indicate that spinel + cordierite reacted to form orthopyroxene + sapphirine under conditions of increasing pressure. Moreover, decreasing core to rim variation of Al₂O₃ in orthopyroxene porphyroblasts suggests decreasing temperature during sapphirine formation. On the basis of experimentally determined P-T stability of the assemblage enstatite + sapphirine + cordierite, and the Al content of hypothetical Fe²⁺-free orthopyroxene associated with sapphirine and cordierite, metamorphic temperatures and pressures are estimated to be 860–890° C and 3.0–11.2 kbar.In the feldspar-sillimanite granulites, sapphirine occurs as a relict phase mantled by sillimanite and/or by successive coronas of sillimanite and garnet. These textural relations suggest the reaction sapphirine + garnet + quartz = orthopyroxene + sillimanite with decreasing temperature. Compositions of minerals in the assemblage garnet-orthopyroxene-sillimanite-plagioclase-quartz, indicate metamorphic P-T conditions of 780–880° C and 9±1 kb.The metamorphic conditions estimated in this study suggest that the sapphirine bearing granulites in the Sipiwesk Lake area represent Archean lower crustal rocks. Their formation might be related to the crustal thickening processes in this area as suggested by Hubregtse (1980) and Weber (1983).     

Origin of zoned spinel by coupled dissolution–precipitation and inter-crystalline diffusion: evidence from serpentinized wehrlite, Bangriposi, Eastern India

Textural and mineral–chemical characteristics in the Bangriposi wehrlites (Eastern India) provide insight into metamorphic processes that morphologically and chemically modified magmatic spinel during serpentinization of wehrlite. Aluminous chromite included in unaltered magmatic olivine is chemically homogenous. In sub-cm to 10s-of-micron-wide veins, magnetite associated with antigorite and clinochlore comprising the serpentine matrix is near-stoichiometric. But Al–Cr–Fe³⁺ spinels in the chlorite–magnetite veins are invariably zoned, e.g., chemically homogenous Al-rich chromite interior successively mantled by ferritchromite/Cr-rich magnetite zone and magnetite continuous with vein magnetite in the serpentine matrix. In aluminous chromite, ferritchromite/Cr-rich magnetite zones are symmetrically disposed adjacent to fracture-controlled magnetite veins that are physically continuous with magnetite rim. The morphology of ferritchromite–Cr-rich magnetite mimics the morphology of aluminous chromite interior but is incongruous with the exterior margin of magnetite mantle. Micropores are abundant in magnetite veins, but are fewer in and do not appear to be integral to the adjacent ferritchromite–Cr-rich magnetite zones. Sandwiched between chemically homogenous aluminous chromite interior and magnetite mantle, ferritchromite–Cr-rich magnetite zones show rim-ward decrease in Cr₂O₃, Al₂O₃ and MgO and complementary increase in Fe₂O₃ at constant FeO. In diffusion profiles, Fe₂O₃–Cr₂O₃ crossover coincides with Al₂O₃ decrease to values <0.5 wt% in ferritchromite zone, with Cr₂O₃ continuing to decrease within magnetite mantle. Following fluid-mediated (hydrous) dissolution of magmatic olivine and olivine + Al–chromite aggregates, antigorite + magnetite and chlorite + magnetite were transported in 10s-of-microns to sub-cm-wide veins and precipitated along porosity networks during serpentinization (T: 550–600 °C, f(O₂): ?19 to ?22 log units). These veins acted as conduits for precipitation of magnetite as mantles and veins apophytic in chemically/morphologically modified magmatic Al-rich chromite. Inter-crystalline diffusion induced by chemical gradient at interfaces separating aluminous chromite interiors and magnetite mantles/veins led to the growth of ferritchromite/Cr-rich magnetite zones, mimicking the morphology of chemically modified Al–Cr–Fe–Mg spinel interiors. Inter-crystalline diffusion outlasted fluid-mediated aluminous chromite dissolution, mass transfer and magnetite precipitation.     

New Compositional and Structural Constraints on the Smithsonian Microanalytical Reference Materials: Amphiboles from Kakanui and Arenal

The amphiboles from Kakanui and Arenal are two natural minerals that have been used worldwide as microanalytical reference materials, but their compositions and crystal structures are still poorly constrained. In this paper, we report new data on H₂O and trace element mass fractions and single-crystal structural refinement of these two amphiboles. H₂O mass fractions of the Kakanui and Arenal amphiboles determined via Karl-Fischer titration are 0.92 ± 0.18 (2s) and 1.56 ± 0.22% m/m (2s), respectively; these values estimated based on crystal-structure refinement are 0.86 and 1.46% m/m, respectively. Trace element mass fractions measured via LA-ICP-MS in two laboratories are in good agreement, and spots from five fragments for both Kakanui and Arenal amphiboles are generally consistent within reproducibility precision (2s). Our measurements indicate a better homogeneity for the amphiboles from Kakanui than that from Arenal. According to the latest scheme for amphibole classification and nomenclature (Hawthorne et al. 2012), the sample from Arenal is a (partially dehydrogenated) pargasite, and that from Kakanui is a kaersutite. The significant amount of oxo-component and that ^CTi⁴⁺ content is strongly ordered at the M(1) site for both amphiboles indicate crystallisation under high fO₂ conditions.     

Amphibole zoning in the Garland Peak Syenite,Red Hill complex,New Hampshire: camptonitic parental magmas and differentiation to silica-oversaturated syenites

The Garland Peak Syenite (GPS) of the Red Hill complex, New Hampshire, consists predominantly of amphibole, oligoclase, perthite, and quartz; amphiboles have homogeneous kaersutite cores with strongly zoned rims ranging in composition from pargasite to hastingsite to hornblende. The thin section scale association of kaersutite, an amphibole that typically crystallizes in silica-undersaturated magmas, with quartz suggests that the GPS magma experienced substanital changes in magmatic composition, including silica activity, during its crystallization history. Kaersutite-bearing camptonites are also associated with the Red Hill complex. The camptonite amphiboles are very similar in composition to the core kaersutites in the GPS, suggesting that the earliest GPS liquid may have had camptonitic affinities. In order to elucidate the process where-by silica-undersaturated magmas differentiate to saturation, amphiboles in these rocks were analyzed by electron and ion microprobe techniques. Amphiboles show a progressive increase in REE abundances from the camptonites to the GPS kaersutite cores to the GPS pargasite/hastingsite/hornblende rims. The systematic change in REE concentrations, and the variations in V, Ti, Sr versus Zr, Eu/Eu⁰ and La/Yb versus Ce, suggest a possible differentiation relationship for the amphiboles and imply that the GPS was derived from magmas similar to camptonites. Rimward depletions in Sr, Ti, V, and Eu/Eu⁰, and the increase in La/Yb values suggest that parental camptonites fractionated plagioclase, magnetite, and amphibole to produce the silica-oversaturated GPS. Bulk-rock modelling agrees with the trace element record preserved in the amphiboles, that plagioclase, magnetite, and amphibole fractionation caused silica saturation. Minor pegmatitic patches occur in the GPS. Ferrohornblendes in the pegmatites have REE abundances distinct from the other GPS amphiboles, and this difference may be due to open system processes.     

Eclogites and polyphase P–T cycling in the Caledonian Uppermost Allochthon in Troms, northern Norway

Eclogites in the Tromsø area, northern Norway, are intimately associated with meta-supracrustals within the Uppermost Allochthon of the Scandinavian Caledonides (the Tromsø Nappe Complex). The whole sequence, which includes pelitic to semipelitic schists and gneisses, marbles and calc-silicate rocks, quartzofeldspathic gneisses, metabasites and ultramafites, has undergone three main deformational/metamorphic events (D₁/M₁, D₂/M₂ and D₃/M₃). Detailed structural, microtextural and mineral chemical studies have made it possible to construct separate P–T paths for these three events. Chemically zoned late syn- to post-D₁ garnets with inclusions of Bt, Pl and Qtz in Ky-bearing metapelites indicate a prograde evolution from 636°C, 12.48 kbar to c. 720°C, 14–15 kbar. This latter result is in agreement with Grt–Cpx geothermometry and Grt–Cpx–Pl–Qtz geobarometry on eclogites and trondhjemitic to dioritic gneisses. Maximum pressures at c. 675°C probably reached 17–18 kbar based on Cpx–Pl–Qtz inclusions in eclogitic garnets, and Grt–Ky–Pl–Qtz and Jd–Ab–Qtz in trondhjemitic gneisses. Post-D₁/pre-D₂ decompressional breakdown of the high-P assemblages indicates a substantial drop in pressure at this stage. Inclusions and chemical zoning in syn- to post-D₂ garnets from metapelites record a second episode of prograde metamorphism, from 552°C, 7.95 kbar, passing through a maximum pressure of 10.64 kbar at 644°C, with final equilibration at c. 665°C, 9–10 kbar. The corresponding apparently co-facial paragenesis Grt + Cpx + Pl + Qtz in metabasites yields c. 635°C, 8–10 kbar. In the metapelites post-D₃, Grt in apparent equilibrium with Bt, Phe and Pl yield c. 630°C, 9 kbar. The D₁/M₁ and D₂/M₂ episodes are exclusively recorded in the Tromsø Nappe Complex and must thus pre-date the emplacement of this allochthonous unit on top of the underlying Lyngen Nappe, while the D₃/M₃ episode is common for the two units. A previously published Sm–Nd mineral isochron (Grt–Cpx–Am) on a partly retrograded and recrystallized ecologite of 598 ± 107 Ma represents either the timing of formation of the eclogites or the post-eclogite/pre-D₂ decompression stage, while a Rb–Sr whole rock isochron of an apparently post-D₁/pre-D₂ granite of 433 ± 11 Ma is consistent with a K–Ar age of post-D₁/pre-D₂ amphiboles from a retrograded eclogite of 437 ± 16 Ma which most likely record cooling below the 475–500°C isotherm after the M₃ metamorphism.     

Oxygen and hydrogen isotopic studies of ore deposition and metamorphism at the Raul mine,Peru

The volcano-sedimentary sequence at the Raul mine, central Peru, consists of andesitic volcanics, graywackes, and siltstones, and has been metamorphosed to the upper greenschist-lower amphibolite facies at temperatures of 400–500°C. Isotopic data (O and H) have been collected from: (a) quartz and magnetite from stratiform ores, (b) amphiboles from amphibolite units that host stratiform ores, (c) calcite from late veins, (d) detrital quartz from graywackes, and (e) whole rocks.Interunit differences in quartz and magnetite δ¹⁸O values suggest that these minerals have resisted isotopic exchange during metamorphism, and that quartz-magnetite isotopic temperatures (380–414°C) represent primary formational temperatures. Calculated δ¹⁸O values of water in equilibrium with quartz and magnetite range from 9.1 to 12.6%..Amphibole δ¹⁸O and δD values show no interunit differences and suggest that the amphiboles have exchanged isotopes with a large metamorphic fluid reservoir. Calculated δ¹⁸O_H2O and δD_H2O values range from 8 to 12%. and ?3 to +42%., respectively.δ¹⁸O_H2O values calculated from δ¹⁸O calcite and fluid inclusion filling temperatures range from 7.5 to 10%.. Water extracted from fluid inclusions in calcite has a δD value of ?20%..δ¹⁸O values of metamorphosed graywackes and volcanic sediments are not atypical, but andesitic lavas are ¹⁸O-rich (8–10%.) compared to normal andesites.Waters involved in ore deposition, metamorphism, and late vein formation at Raul are all thought to have a common source, principally seawater. The δ¹⁸O_H2O and δD_H2O values could be produced by evaporation of seawater, shale ultrafiltration, and isotopic exchange with host rocks during deep circulation through the volcano-sedimentary pile.A model is proposed whereby coastal ocean water is restricted from the open sea by volcanic island arcs, and subsequently undergoes evaporation. Circulation of this water is initiated by heat associated with seafloor volcanism. ¹⁸O-enrichment in andesites may be produced by isotopic exchange with high ¹⁸O waters at elevated temperatures and sufficiently high water/rock ratios.     

The P-T-fO 2 stability of deerite,Fe 12 2+ Fe 6 3+ [Si12O40](OH)10

New equilibrium experiments have been performed in the 20–27 kbar range to determine the upper thermal stability limit of endmember deerite, Fe ₁₂ ²⁺ Fe ₆ ³⁺ [Si₁₂O₄₀](OH)₁₀. In this pressure range, the maximum thermal stability limit is represented by the oxygen-conserving reaction: deerite(De)=9 ferrosilite(Fs)+3 magnetite(Mag)+3 quartz(Qtz)+5 H₂O(W) (1). Under the oxygen fugacities of the Ni-NiO buffer the breakdown-reduction reaction: De=12 Fs+2 Mag+5 W+1/2 O₂ (10) takes place at lower temperatures (e.g. T=63° at 27 kbar). The experimental brackets can be fitted using thermodynamic data for ferrosilite, magnetite and quartz from Berman (1988) and the following 1 bar, 298 K data for deerite (per gfw): V^o=55.74 J.bar^-1, S^o=1670 J.K^-1, H _f ^o =-18334 kJ, =2.5x10^-5K^-1, =-0.18x10^-5 bar^-1. Using these data in conjunction with literature data on coesite, grunerite, minnesotaite, and greenalite, the P-T stability field of endmember deerite has been calculated for P _s=P _{H 2}O. This field is limited by 6 univariant oxygenconserving dehydration curves, from which three have positive dP/dT slopes, the other three negative slopes. The lower pressure end of the stability field of endmember deerite is thus located at an invariant point at 250±70°C and 10+-1.5 kbar. Deerite rich in the endmember can thus appear only in environments with geothermal gradients lower than 10°C/km and at pressures higher than about 10 kbar, which is in agreement with 4 out of 5 independent P-T estimates for known occurrences. The presence of such deerite places good constraints on minimum pressure and maximum temperature conditions. From log f _{O 2}-T diagrams constructed with the same data base at different pressures, it appears that endmember deerite is, at temperatures near those of its upper stability limit, stable only over a narrow range of oxygen fugacities within the magnetite field. With decreasing temperatures, deerite becomes stable towards slightly higher oxygen fugacities but reaches the hematite field only at temperatures more than 200°C lower than the upper stability limit. This practically precludes the coexistence deerite-hematite with near-endmember deerite in natural environments.     

Volatiles in amphiboles from xenoliths,Vulcan's Throne,Grand Canyon,Arizona, USA

Analyses of major element and volatile components of amphiboles from Vulcan's Throne, a Recent volcano on the north rim of the Grand Canyon, Arizona, USA, have been performed by using the electron microprobe and high temperature mass spectrometry. The amphiboles occur as megacrysts, as oikocrysts in peridotite and pyroxenite xenoliths, in amphibole-rich selvages on lherzolite xenoliths, and as grains in hornblendite xenoliths. Total volatiles range from 1.27 to 1.75 wt.%. In all samples, H₂O is the principal volatile species. Lesser amounts of structurally bound fluorine, chlorine, and oxygen were also released. The amphiboles studied are hydroxyl-deficient. The O(3) site is probably partially occupied by O^2?, which was detected as O₂ during degassing of the amphibole. Ti shows a strong positive correlation with the amount of hydroxyl deficiency in the amphiboles except for one oxidized sample. Thus, Ti probably is significant in charge balancing the substitution of O^2? for OH^? and the substitution probably occurred during crystallization rather than by dehydrogenation. Small amounts of both oxidized and reduced carbon and sulfur-bearing volatile species (e.g., CO₂, CO, CH₄, SO₂, H₂S) were detected in all samples. The observation of reduced carbon species supports the hypothesis that the oxygen fugacity of at least portions of the upper mantle is probably less than the quartz-fayalite-magnetite buffer.     

Adaptation of the SELECTOR-C program package for solving petrogenetic problems of metamorphic rocks

The paper presents thermodynamic models for mineral solid solutions used in physicochemical simulations with the SELECTOR-C program package (PP) in application to metamorphic mineral-forming processes. It is demonstrated that the simulated FeO and MgO distribution in the mineral pairs garnetbiotite, garnet-orthopyroxene, orthopyroxene-biotite, orthopyroxene-olivine, garnet-cordierite, garnetclinopyroxene, and clinopyroxene-orthopyroxene in the model samples satisfactorily corresponds to available experimental and empirical data. Simulations of naturally occurring mineral associations are employed to demonstrate the capabilities of the new version of the SELECTOR-C PP as a tool for studying the evolution of mineral assemblages at varying P-T conditions and fluid regime, the perfectly mobile and inert behaviors of certain fluid components during the origin of mineral associations are demonstrated, the pseudosection method applied over a broad P-T range is used to trace systematic variations in the composition of mineral associations in granulite-facies metabasites and metapelites, and the upper limit of plagioclase stability is estimated for these rocks at pressures of 11–12 kbar. Principal differences are elucidated in the effect of rocks rich and poor in Fe³⁺ on the percolation of metamorphic fluid through them: Fe³⁺-rich rocks retain their own redox potential at a certain level by buffering reactions, whereas Fe³⁺-poor rocks rapidly exhaust their buffer capacity and acquire the redox potential of the inflowing external fluid. This allowed us to evaluate the logfO₂ at no higher than −17 (at T = 700°C and P = 6.8 kbar). Our simulation of the equilibrium of natural rock samples provides good reasons to believe that natural mineral assemblages can be formed at low fluid/rock ratios of no higher than 0.01–0.06.     

Glaucophane-bearing assemblage overprinted by greenschist-facies metamorphism in the Variscan Kaczawa complex, Sudetes, Poland

An Early Palaeozoic (Ordovician ?) metamudstone sequence near Wojcieszow, Kaczawa Mts, Western Sudetes, Poland, contains numerous metabasite sills, up to 50 m thick. These subvolcanic rocks are of within-plate alkali basalt type. Primary igneous phases in the metabasites, clinopyroxene (salite) and kaersutite, are veined and partly replaced by complex metamorphic mineral assemblages. Particularly, the kaersutite is corroded and rimmed by zoned sodic, sodic–calcic and calcic amphiboles. The matrix is composed of actinolite, pycnochlorite, albite (An ≤ 0.5%), epidote (Ps 27–33), titanite, calcite, opaques and, occasionally, biotite, phengite and stilpnomelane. The sodic amphiboles are glaucophane to crossite in composition with Na^B from 1.9 to 1.6. They are rimmed successively by sodic–calcic and calcic amphiboles with compositions ranging from magnesioferri-winchite to actinolite. No compositions between Na^B= 0.92 and Na^B= 1.56 have been ascertained. The textures may be interpreted as representing a greenschist facies overprint on an earlier blueschist (or blueschist–greenschist transitional) assemblage. The presence of glaucophane and no traces of a jadeitic pyroxene + quartz association indicate pressures between 6 and 12 kbar during the high-pressure episode. Temperature is difficult to assess in this metamorphic event. The replacement of glaucophane by actinolite + chlorite + albite, with associated epidote, allows restriction of the upper pressure limit of the greenschist recrystallization to <8 kbar, between 350 and 450°C. The mineral assemblage representing the greenschist episode suggests the P–T conditions of the high-pressure part of the chlorite or lower biotite zone. The latest metamorphic recrystallization, under the greenschist facies, may have taken place in the Viséan.     

Transition from the Zeolite to Prehnite-Pumpellyite Facies in the Karmutsen Metabasites, Vancouver Island, British Columbia

The upper Triassic Karmutsen metabasites from northeast VancouverIsland, B.C., are thermally metamorphosed by the intrusion ofthe Coast Range Batholith. The amygdaloidal metabasites developedin the outer portion of the contact aureole show a progressivemetamorphism from zeolite to prehnite-pumpellyite facies. Thesize of an equilibrium domain is extremely small for these metabasites,and the individual amygdule assemblages are assumed to be inequilibrium. Two major calcite-free assemblages (+chlorite+quartz)are characteristic: (i) laumontite+pumpellyite+epidote in thezeolite facies and (ii) prehnite+pumpellyite+epidote in theprehnite-pumpellyite facies. The assemblages and compositionsof Ca-Al silicates are chemographically and theoretically interpretedon the basis of the predicted P-T grid for the model basalticsystem, CaO-MgO-A1₂O₃-Fe₂O₃-SiO₂-H₂O. The results indicate:(1) local equilibrium has been approached in mineral assemblagesand compositions; (2) the X_Fe³⁺ values in the coexisting Ca-Alsilicates decrease from epidote, through pumpellyite to prehnite;(3) with increasing metamorphic grade, the Fe³⁺ contents ofepidotes in reaction assemblages decrease in the zeolite facies,then increase in the prehnite-pumpellyite facies rocks. Suchvariations in the assemblages and mineral compositions are controlledby a sequence of continuous and discontinuous reactions, andallow delineation of T-X_Fe³⁺ relations at constant pressure.The transition from the zeolite to prehnite-pumpellyite faciesof the Karmutsen metabasites is defined by a discontinuous reaction:0·18 laumontite+pumpellyite+0·15 quartz = 1·31prehnite+ 0·78 epidote+0·2 chlorite+ 1·72H₂O, where the X_Fe³⁺ values of prehnite, pumpellyite and epidoteare 0·03, 0·10 and 0·18, respectively.These values together with available thermodynamic data andour preliminary experimental data are used to calculate theP-T condition for the discontinuous reaction as P = 1·1±0·5 kb and T = 190±30°C. The effectsof pressure on the upper stability of the zeolite facies assemblagesare discussed utilizing T-X_Fe³⁺ diagrams. The stability of thelaumontite-bearing assemblages for the zeolite facies metamorphismof basaltic rocks may be defined by either continuous or discontinuousreactions depending on the imposed metamorphic field gradient.Hence, the zeolite and prehnite-pumpellyite facies transitionboundary is multivariant.     

The relation between Li, V, P2O5, and Al2O3 contents in marls and mudstones as indicators of environment of deposition

The contents of Li, V, P₂O₅, and Al₂O₃ in marls and mudstones collected from three different formations, cropping out in northern Iraq, showed that the ratios of Li/Al₂O₃, V/Al₂O₃, and P₂O₅/Al₂O₃ in these rocks can be used to discriminate environments of deposition of these formations. Relatively high values of the above ratios characterize the deep marine Shiranish environment while lowest values characterize freshwater environment of deposition of the Injana rocks. Despite that marls of the Fatha formation were deposited in marine lagoonal environment, the ratios P₂O₅/Al₂O₃ and V/Al₂O₃ could not be discriminated from the freshwater Injana mudstones. The well-known phenomenon that marine surface water and down to a depth of 50 m almost lacks soluble P explain this behavior. In such water, almost all the soluble P is consumed by surface organism rendering it unavailable for adsorption by the sediments. The high salinity of the lagoonal environment prevented adsorption of soluble V on clay minerals because of its inability to compete with other species of much higher concentrations.     

Fe-Ti-oxides in metamorphic basites from the Eastern Alps,Austria: a contribution to the formation of solid solutions of natural Fe-Ti-oxide assemblages

The development of Fe-Ti oxide assemblages in basic rocks from the Penninic series of the southern Venediger rea, Austria, during polyphase Alpine metamorphism has been studied. Textural and compositional relations indicate thorough reequilibration of the opaque mineral assemblages during late Barrovian metamorphism at essentially static conditions of lower amphibolite to greenschist facies. In contrast, silicate mineralogy of the preceeding blueschist to eclogite facies metamorphism might still be preserved to a large extent. Chemical adjustment of the Fe-Ti oxide minerals to decreasing P-T conditions is characterized by (1) formation of complex intergrowths of ilmenite and hematite solid solutions (<550° C), (2) the decomposition of hemo-ilmenite ₁ to ferrianilmenite₂+magnetite+rutile and of ilmeno-hematite₁ to titanhematite₂+rutile±magnetite (<450° C), and (3) low-grade oxidation of ferrianilmenite₂ to magnetite+hematite-rutile intergrowths or hematite +rutile and of titanhematite₂ to hematite-rutile intergrowths (≦400° C). Chemical equilibrium is suggested by the regular partitioning of Cr, V, Mg and Mn between coexisting hemo-ilmenite, ilmeno-hematite, and magnetite. The hematite-ilmenite miscibility gap has been delimited on the basis of the bulk compositions of the exsolved phases and the temperature estimates obtained from Fe-Ti oxide thermometry.