Glaucophanites and eclogites from Val Chiusella,Sesia-Lanzo zone (Italian Alps)

In the metabasites of Val Chiusella, metamorphic assemblages are present, corresponding to the glaucophane schist facies, i.e. garnet glaucophanites to omphacite-garnet glaucophanites, as well as to the eclogite facies, i.e., glaucophane eclogites, eclogites, and omphacite felses. Both groups of assemblages are divided by the critical reaction 1 zoisite +1 glaucophane 1.2 omphacite+0.8 garnet+0.7 paragonite +1.4 quartz+0.8 H₂O. From textural evidence it is clear that in the investigated area this reaction proceeded to the right according to a prograde metamorphism. Correspondingly, K ^garn-cpx _D(Fe/Mg) values of coexisting garnet-omphacite pairs in the glaucophane schist facies assemblages are higher than in the eclogite facies assemblages and reflect a temperature increase from about 450 ° C to about 550 ° C at minimum water vapour pressures of 12 to 16 kb.     

Evolution of a tourmaline-bearing lawsonite eclogite from the Elekdağ area (Central Pontides,N Turkey): evidence for infiltration of slab-derived B-rich fluids during exhumation

An undated high-pressure low-temperature tectonic mélange in the Elekda area (central Pontides, N Turkey) comprises blocks of MORB-derived lawsonite eclogite within a sheared serpentinite matrix. In their outer shells, some of the eclogite blocks contain large (up to 6 cm) tourmaline crystals. Prograde inclusions in poikiloblastic garnet from a well-preserved eclogite block are lawsonite, epidote/clinozoisite, omphacite, rutile, glaucophane, chlorite, Ba-bearing phengite, minor actinolite, winchite and quartz. In addition, glaucophane, lawsonite and rutile occur as inclusions in omphacite. These inclusion assemblages document the transition from a garnet-lawsonite-epidote-bearing blueschist to a lawsonite eclogite with the peak assemblage garnet + omphacite I + lawsonite + rutile. Peak metamorphic conditions are not well-constrained but are estimated approximately 400–430°C and >1.35 GPa, based on Fe–Mg exchange between garnet and omphacite and the coexistence of lawsonite + omphacite + rutile. During exhumation of the eclogite–serpentinite mélange in the hanging wall of a subduction system, infiltration of B-rich aqueous fluids into the rims of eclogite blocks caused retrogressive formation of abundant chlorite, titanite and albite, followed by growth of tourmaline at the expense of chlorite. At the same time, omphacite I (X_Jd=0.24–0.44) became unstable and partially replaced by omphacite II characterized by higher X_Jd (0.35–0.48), suggesting a relatively low silica activity in the infiltrating fluid. Apart from Fe-rich rims developed at the contact to chlorite, tourmaline crystals are nearly homogeneous. Their compositions correspond to Na-rich dravite, perhaps with a small amount of excess (tetrahedral) boron (~5.90 Si and 3.10 B cations per 31 anions). ¹¹ B values range from –2.2 to +1.7. The infiltrating fluids were most probably derived from subducting altered oceanic crust and sediments.     

The Blosseville Coast basalts of East Greenland: Their occurrence,composition and temporal variations

In the system CaO-MgO-A1₂O₃-SiO₂ the tie lines connecting anorthite with other phases are sequentially broken down with increasing pressure according to the following univariant reactions: anorthite+ enstatite_ss+sillimanite pyrope-grossular_ss+quartz (3), anorthite+enstatite_ss pyrope-grossular_ss+diopside_ss+quartz (2), anorthite+pyrope-grossular_ss+ quartz diopside_ss+kyanite (4) and anorthite+diopside_ss grossular-pyrope_ss +kyanite+quartz (8). At 1,200 ° C these reactions occur at 14.5± 0.5, 15.5±0.5, 19.5±0.5 and 26.4±1 kilobar and have positive slopes (dP/dT) of 1±0.5, 2.8±0.5, 13.3±0.5 and 24±2bars/°C respectively. An invariant point involving kyanite rather than sillimanite, occurs at 850 °C±25 °C and 14.5±0.5kbar at the intersection of reactions (3), (2) and (4). Reaction(4) exhibits significant curvature with an increase in dP/dT from 13.3±0.5 to 18.5± 0.5 bars/°C between 1,050° and 850° C. The pressure at which the complete grossular-pyrope join is stable with quartz is estimated at 41 ± 1 kbar at 1,200 ° C. The pressure at which garnet appears according to reaction (2) is lowered by 5 kbar for a composition with anorthite and orthopyroxene (En_0.5Fs_0.5). Enstatite and plagioclase (An_0.5Ab_0.5) first produce garnet at 2 kbar higher pressure than enstatite and pure anorthite (reaction (2)). The calcium content of garnet in various divariant assemblages is relatively insensitive to temperature but very sensitive to pressure, it is therefore a useful geobarometer. At metamorphic temperatures of 700–850 °C pressures of 8–10 kbar are required for the formation of quartz-bearing garnet granulites containing calcic plagioclase and with (Mg/Mg+Fe) bulk = 0.5.     

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.     

Lawsonite-bearing eclogitic metabasites in the Cazadero area, northern California

Summary In the Cazadero area, northern California, Lawsonite-bearing eclogitic metabasites occur in association with glaucophane schists. Lawsonite-bearing eclogitic metabasites are coarse-grained, and characteristically lack albite. Representative mineral assemblages are; (1) garnet + omphacite + Lawsonite + epidote + glaucophane + chlorite + quartz, (2) garnet -F- omphacite + Lawsonite + pumpellyite + glaucophane + actinolite + quartz, (3) garnet + omphacite + Lawsonite + pumpellyite + epidote + glaucophane + quartz. They can be represented on an A1₂O₃-Fe₂O₃-MgO-Na₂O diagram in which all minerals are projected from quartz, Lawsonite, almandine garnet, and H₂O-predominant fluid. On the basis of the garnet-clinopyroxene geothermometry and phase relations, the metamorphic conditions for the formation of Lawsonite-bearing eclogitic metabasites are estimated at 360-445 °C and more than 9 ± 1 kbar. Lawsonite-bearing eclogitic metabasites formed near the univariant curve albite = jadeite +quartz. A petrogenetic grid constructed by Schreinemakers' method shows that the Lawsonite-bearing eclogitic metabasites in the Cazadero area formed under transitional P-T conditions between those of the garnet-bearing glaucophane schists in New Caledonia and lawsonitebearing eclogitic metabasites in Corsica.Zusammenfassung Im Gebiet um Cazadero, Nordkalifornien, kommen Lawsonit-führende eklogitische Metabasite in Assoziation mit Glaukophanschiefern vor. Den grobkörnigen Lawsonitführenden Metabasiten fehlt charakteristischerweise Albit. Repräsentative Mineralparagenesen sind: (1) Granat + Omphacit + Lawsonit + Epidot + Glaukophan + Chlorit + Quarz, (2) Granat + Omphacit + Lawsonit + Pumpellyit + Glaukophan + Aktinolith + Quarz, (3) Granat + Omphacit +Lawsonit + Pumpellyit + Epidot + Glaukophan + Quarz. Sie lassen sich in einem A1₂O₃-Fe₂O₃-MgO-Na₂O Diagramm, in dem alle Minerale von Quarz, Lawsonit, Almandin-reichem Granat und einem H₂O-dominierten Fluid projiziert werden, darstellen. Die Metamorphosebedingungen der Lawsonitführenden eklogitischen Metabasite werden auf Grund von Granat-Klinopyroxenthermometrie und der Phasenbeziehungen mit 360-445°C und mehr als 9 ± 1 kbar abgeschätzt. Die Lawsonit-führenden eklogitischen Metabasite bildeten sich nahe der univarianten Reaktion Albit = Jadeit +Quarz. Ein petrogenetisches Netz, konstruiert nach der Schreinemakers Methode, zeigt, daß die P-T Bedingungen der Lawsonitführenden eklogitischen Metabasite im Gebiet von Cazadero im übergangsbereich zwischen jenen von Granat-führenden Glaukophanschiefern in Neukaledonien und Lawsonit-führenden eklogitischen Metabasiten in Korsika liegen.

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Lawsonit-führende eklogitische Metabasite im Gebiet um Cazadero, Nordkalifornien

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With 9 Figures     

Phase Equilibria of Hornblende‐Bearing Eclogite in the Western Dabie Mountain,Central China

The high-pressure (HP) eclogite in the western Dabie Mountain encloses numerous hornblendes, mostly barroisite. Opinions on the peak metamorphic P-T condition, PT path and mineral paragenesis of it are still in dispute. Generally, HP eclogite involves garnet, omphacite, hornblendes and quartz, with or without glaucophane, zoisite and phengite. The garnet has compositional zoning with XMg increase, XCa and XMn decrease from core to rim, which indicates a progressive metamorphism. The phase equilibria of the HP eclogite modeled by the P-T pseudosection method developed recently showed the following: (1) the growth zonation of garnet records a progressive metamorphic PT path from pre-peak condition of 1.9–2.1 GPa at 508°C–514°C to a peak one of 2.3–2.5 GPa at 528°C–531°C for the HP eclogite; (2) the peak mineral assemblage is garnet+omphacite+glaucophane+quartz±phengite, likely paragenetic with lawsonite; (3) the extensive hornblendes derive mainly from glaucophane, partial omphacite and even a little garnet due to the decompression with some heating during the post-peak stage, mostly representing the conditions of about 1.4–1.6 GPa and 580°C–640°C, and their growth is favored by the dehydration of lawsonite into zoisite or epidote, but most of the garnet, omphacite or phengite in the HP eclogite still preserve their compositions at peak condition, and they are not obviously equilibrious with the hornblendes.     

Experiments on the phase transition calcite+wollastonite+ +epidote=grossular-andraditess+CO2+H2O

The reaction 2 epidote+2 calcite+3 wollastonite3 grossular-andradite_ss+ 2 CO₂+1 H₂O has been explored by hydrothermal experiments at a total fluid pressure of 1000 bars. For a grossular-andradite_ss of andradite ₂₅ composition, the isobaric univariant curve passes through the points 458°C: X_CO2=0.00; 521°C: X_CO2=0.026; 523°C: X_CO2=0.052; 526°C: 0.088; 528°C: X_CO2=0.104. This curve intersects the isobaric univariant curve of the reaction calcite+quartz+[H₂O] wollastonite+CO₂+[H₂O] at the isobaric invariant point around 528°C and X_CO2=0.12. At higher values of X_CO2, this reaction is replaced by another one, namely: 2 epidote+5 calcite+3 quartz3 grossular-andradite_ss+5 CO₂+ 1 H₂O. It is demonstrated that both the reactions do actually take place during the metamorphism of calcareous rocks. The petrologic significance of contrasted sequence of reactions within this system observed by various workers is also discussed.     

Paragenesis of sodic pyroxene-bearing quartz schists: implications for the P-T history of the Sanbagawa belt

Sodic pyroxene (jadeite content X _jd=0.1–0.3) occurs locally as small inclusions within, albite porphyroblasts and in the matrix of hematite-bearing quartz schists in the Sanbagawa (Sambagawa) metamorphic belt, central Shikoku, Japan. The sodic, pyroxene-bearing samples are characteristically free from chlorite and their typical mineral assemblage is sodic pyroxene+subcalcic (or sodic) amphibole+phengitic mica+albite+quartz+hematite+titanite±epidote. Spessartine-rich garnet occurs in Mn-rich samples. Sodic pyroxene in epidote-bearing samples tends to be poorer in acmite content (average X _Acm=0.26–0.50) than that in the epidote-free samples (X _Acm=0.45–0.47). X _Jd shows no systematic relationship to metamorphic grade, and is different among the three sampling regions [Saruta-gawa, Asemi-gawa and Bessi (Besshi)]. The average X _Jd of the Saruta-gawa samples (0.21–0.29) is higher than that of the Asemi-gawa (0.13–0.17) and Bessi (0.14–0.23). The P-T conditions of the Asemi-gawa and Bessi regions are estimated at 5.5–6.5 kbar, >360°C in the chlorite zone, 7–8.5 kbar, 440±15°C in the garnet zone and 8–9.5 kbar, 520±25°C in the albite-biotite zone. Metamorphic pressure of the Saruta-gawa region is systematically 1–1.5 kbar higher than that of the Asemi-gawa and Bessi regions, and materials of the Saruta-gawa region have been subducted to a level 3–5 km deeper than materials that underwent metamorphism at equivalent temperatures and are now exposed in the Asemi-gawa and Bessi regions. Pressure slightly increases toward the north (structurally high levels) through the Sanbagawa belt of central shikoku. Two types of zonal structure were observed in relatively coarse-grained sodic pyroxenes in the matrix. One type is characterized by increasing X _Jd from core to rim, the other type by decreasing X _Jd from core to rim. Both types of zoned pyroxenes show an increase in X _{Fe 2+}[=Fe²⁺/(Fe²⁺+Mg)] from core to rim. The first type of zoning was observed in a sample from the chlorite zone of lowest grade, whereas the latter occurs in the garnet and albite-biotite zones of higher grade. The contrast in zonal structure implies that dP/dT during prograde metamorphism decreased with increasing metamorphic grade and may have been negative in some samples from the higher-grade zones. The estimated dP/dT of the prograde stage of the chlorite zone is 3.2 kbar/100°C, and that of the garnet and albite-biotite zones is -1.8 to 0.9 kbar/100°C. The variation of dP/dT at shallow and deep levels of a subduction system probably reflects the difference of heating duration and/or change in thermal gradient of the subduction zone by continuous cooling of the surrounding mantle.     

Manganiferous schists and their origin,Hidaka Mountains,Hokkaido, Japan

Manganiferous quartz-mica schists (4 m in stratigraphic thickness) overlie epidote amphibolite in the Chiroro River area, Hidaka Mountains, Hokkaido. The schist layers have a considerable range of A/F ratios and bulk oxidation ratios which vary from 21.5 to 100. Manganese contents are from 4 to 30 times higher than that of the average shale with 0.09% MnO. The schists are essentially quartz-white mica-biotite-Mn garnet-tourmaline-±epidote-magnetite assemblages. A highly oxidized layer (5–8 cm thick) 95 cm above the epidote amphibolite contact is characterized by viridine-piemontite-spessartine-Mn white mica-Mn tourmaline-Ti-Mn haematite indicative of both high initial manganese content and very high f _O2 conditions of recrystallization.Viridine contains up to 17 mol% Mn³⁺SiO₅ and coexists with piemontite with between 13.6 and 15.4 wt% Mn₂O₃. Mn-poor-Fe-rich (Ps₃₂) epidote occurs in the less oxidized schist enclosing the viridine-piemontite bearing seam. Garnets vary widely in composition with end member variations (mol%) of Spess_22.9–80.5; And_0.2–11.7; Alm_1.1–57.1; Pyr_2.0–12.2; Gross_7.0–49.0. The more manganiferous garnets occur in rocks with higher oxidation ratios while almandiferous varieties occur in schists with low oxidation ratios. Biotite ranges from green to red-brown varieties (increasing Ti and Fe) with Mg/ (Mg+Fe) ratios varying from 56 to 48. Ten to fifteen percent octahedral R²⁺ is replaced by Al indicating a trend towards eastonite-siderophyllite. The white micas deviate only slightly from dioctahedral stoichiometry but have up to 25% of octahedral sites occupied by Fe, Mg and to a lesser extent Mn and Ti as R²⁺ Si⁴⁺2Al³⁺ and in highly oxidized rocks as (Fe,Mn)³⁺Al³⁺. The white mica in the highly oxidized viridine-piemontite schist is pale pinkishorange, exhibits reverse pleochroism, and has between 0.30 and 0.43 wt% Mn₂O₃.There is a close comparison, both in terms of stratigraphic thickness and Fe-Mn variation, between the Chiroro schist sequence and many oceanic cores so that the bulk chemistry and mineralogy of the pelitic schists is largely an extension of the original Eh-pH conditions of hemipelagic sedimentation and post-depositional adjustments during diagenesis. The thin viridine-piemontite bearing schist is correlated with an oxidized, Fe-Mn rich layer commonly found in present day oceanic cores. The viridine presumably formed by reaction of original ferro-manganese microgranules and clay minerals. Halmrolytic alteration of the underlying metabasalt resulted in leaching of Mn and Fe (in particular) into the overlying sediments and the formation of concentrations of haematite — manganese oxide — Mn garnet along the schist-epidote amphibolite contact.Estimation of the P-T conditions of metamorphism from the phase relations and compositions in the epidote amphibolite associated with the manganiferous schist gives T °C = 530560 and a minimum P _fluid > 3 kb which corresponds to the epidote amphibolite facies of Barrovian-type terrains.This paper is dedicated to Professor Kenzo Yagi on the occasion of his retirement from the Chair of Mineralogy, Department of Geology and Mineralogy, Hokkaido University, Sapporo, Japan     

Jadeite–garnet glaucophane schists in the Bizan area,Sambagawa metamorphic belt,eastern Shikoku,Japan: significance and extent of eclogite facies metamorphism

Eclogite facies metamorphic rocks have been discovered from the Bizan area of eastern Shikoku, Sambagawa metamorphic belt. The eclogitic jadeite–garnet glaucophane schists occur as lenticular or sheet‐like bodies in the pelitic schist matrix, with the peak mineral assemblage of garnet + glaucophane + jadeite + phengite + quartz. The jadeitic clinopyroxene (X_Jd 0.46–0.75) is found exclusively as inclusions in porphyroblastic garnet. The eclogite metamorphism is characterized by prograde development from epidote–blueschist to eclogite facies. Metamorphic P–T conditions estimated using pseudosection modelling are 580–600 °C and 18–20 kbar for eclogite facies. Compared with common mafic eclogites, the jadeite–garnet glaucophane schists have low CaO (4.4–4.5 wt%) and MgO (2.1–2.3 wt%) bulk‐rock compositions. The P–T– pseudosections show that low X_Ca bulk‐rock compositions favour the appearance of jadeite instead of omphacite under eclogite facies conditions. This is a unique example of low X_Ca bulk‐rock composition triggered to form jadeite at eclogite facies conditions. Two significant types of eclogitic metamorphism have been distinguished in the Sambagawa metamorphic belt, that is, a low‐T type and subsequent high‐T type eclogitic metamorphic events. The jadeite–garnet glaucophane schists experienced low‐T type eclogite facies metamorphism, and the P–T path is similar to lawsonite‐bearing eclogites recently reported from the Kotsu area in eastern Shikoku. During subduction of the oceanic plate (Izanagi plate), the hangingwall cooled gradually, and the geothermal gradient along the subduction zone progressively decreased and formed low‐T type eclogitic metamorphic rocks. A subsequent warm subduction event associated with an approaching spreading ridge caused the high‐T type eclogitic metamorphism within a single subduction zone.     

Pyroxmangite: Stability in H2O-CO2 mixtures at a total pressure of 2000 bars

The curve of the reaction: 1 Rhodochrosite + 1 Quartz Pyroxmangite was determined experimentally using gas mixtures with different CO₂H₂O ratios at a total pressure of 2 kb. In pure CO₂, the equilibrium temperature is 508±2° C. By plotting the equilibrium data in terms of InX_CO2 versus reciprocal temperature, a H of reaction of 51.7 kcal was obtained. This plot shows a straight-line relationship, indicating internal consistency of the data, and is in good agreement with previous findings that CO₂ and H₂O mix ideally within the investigated temperature range (400–600° C). When applied to pyroxmangites formed during moderate greenschist facies metamorphism, these data indicate a high acitivity of H₂O.     

Coalification and graphitization in high-pressure schists in New Caledonia

The northern portion of the Tertiary high-pressure schist belt in New Caledonia contains, from west to east, a metamorphic progression from lawsonite-albite facies through glaucophanitic greenschists to eclogitic albite-epidote amphibolites. This belt is flanked to the west by Upper Cretaceous-Eocene metasediments, of prehnite-pumpellyite grade. Paraschists throughout this whole sequence contain abundant carbonaceous material which shows a progressive metamorphism from coal to graphite. Structural analysis of lithostatic load and oxygen isotope data have provided a PT profile for the carbon metamorphism. In the prehnite-pumpellyite metasediments, phytoclasts were progressively coalified to anthracite rank under PT conditions which extended up to 3 kb/255 ° C at the lawsonite isograd where graphite first appears. On the high grade side of the lawsonite isograd a transitional mixed zone of continued coalification and graphitization occurred within the PT range 3 kb/255 ° C to 5.5 kb/335 ° C which included the ferroglaucophane isograd. Immediately beyond this zone all phytoclasts were completely graphitized before the epidote isograd was reached at 6.3 kb/ 390 ° C. The prevailing metamorphic environment retarded coalification, but accelerated graphitization, under conditions of high pressure and a low temperature gradient (7 ° C/km) that had been generated within the sedimentary pile by rapid tectonic thickening and consequent deep burial.     

The thermodynamic regime of metamorphism in the ancient subduction zones

Based on mineralogical themometry and baroraetry and computation of mineral reactions modelling metamorphic sequence, a geotherm for metamorphic belts of the subduction zones has been deduced. Relatively low PT-values (3 kbar/200° C) correspond to zeolite and prehnite-pumpellyite metasediments and at higher pressures and temperatures (10 kbar/400 °C) lawsonite-glaucophane assemblages become unstable. The PT-curve achieves maximum at 11 kbar and 470° C to drop down to normal geotherm (Perchuk 1977). High concentration of H₂O in the metamorphic fluid has been revealed, the difference between P_f1 and being less than 2 kbar. Consideration has also been given to specific thermodynamic regime of zeolite and prehnite-pumpellyite zones of the younger island arcs, where lawsoniteglaucophane zones are absent. Here the geotherm has been found to rise from 0.2kbar/120° C up to 4 kbar/350° C and -regime similar to that of glaucophane schists formations.     

Cold subduction of the Neotethys: the metamorphic record from finely banded lawsonite and epidote blueschists and associated metabasalts of the Nagaland Ophiolite Complex,India

In this study, we have deduced the thermal history of the subducting Neotethys from its eastern margin, using a suite of partially hydrated metabasalts from a segment of the Nagaland Ophiolite Complex (NOC), India. Located along the eastern extension of the Indus‐Tsangpo suture zone (ITSZ), the N–S‐trending NOC lies between the Indian and Burmese plates. The metabasalts, encased within a serpentinitic mélange, preserve a tectonically disturbed metamorphic sequence, which from west to east is greenschist (GS), pumpellyite–diopside (PD) and blueschist (BS) facies. Metabasalts in all the three metamorphic facies record prograde metamorphic overprints directly on primary igneous textures and igneous augite. In the BS facies unit, the metabasalts interbedded with marble show centimetre‐ to metre‐scale interlayering of lawsonite blueschist (LBS) and epidote blueschist (EBS). Prograde HP/LT metamorphism stabilized lawsonite + omphacite (X_Jd = 0.50–0.56 to 0.26–0.37) + jadeite (X_Jd = 0.67–0.79) + augite + ferroglaucophane + high‐Si phengite (Si = 3.6–3.65 atoms per formula unit, a.p.f.u.) + chlorite + titanite + quartz in LBS and lawsonite + glaucophane/ferroglaucophane ± epidote ± omphacite (X_Jd = 0.34) + chlorite + phengite (Si = 3.5 a.p.f.u.) + titanite + quartz in EBS at the metamorphic peak. Retrograde alteration, which was pervasive in the EBS, produced a sequence of mineral assemblages from omphacite and lawsonite‐absent, epidote + glaucophane/ferroglaucophane + chlorite + phengite + titanite + quartz through albite + chlorite + glaucophane to lawsonite + albite + high‐Si phengite (Si = 3.6–3.7 a.p.f.u.) + glaucophane + epidote + quartz. In the PD facies metabasalts, the peak mineral assemblage, pumpellyite + chlorite + titanite + phengitic white mica (Si = 3.4–3.5 a.p.f.u.) + diopside appeared in the basaltic groundmass from reacting titaniferous augite and low‐Si phengite, with prehnite additionally producing pumpellyite in early vein domains. In the GS facies metabasalts, incomplete hydration of augite produced albite + epidote + actinolite + chlorite + titanite + phengite + augite mineral assemblage. Based on calculated T–M(H₂O), T–M(O₂) (where M represents oxide mol.%) and P–T pseudosections, peak P–T conditions of LBS are estimated at ~11.5 kbar and ~340 °C, EBS at ~10 kbar, 325 °C and PD facies at ~6 kbar, 335 °C. Reconstructed metamorphic reaction pathways integrated with the results of P–T pseudosection modelling define a near‐complete, hairpin, clockwise P–T loop for the BS and a prograde P–T path with a steep dP/dT for the PD facies rocks. Apparent low thermal gradient of 8 °C km^?1 corresponding to a maximum burial depth of 40 km and the hairpin P–T trajectory together suggest a cold and mature stage of an intra‐oceanic subduction zone setting for the Nagaland blueschists. The metamorphic constraints established above when combined with petrological findings from the ophiolitic massifs along the whole ITSZ suggest that intra‐oceanic subduction systems within the Neotethys between India and the Lhasa terrane/the Karakoram microcontinent were also active towards east between Indian and Burmese plates.     

Pumpellyites in two low grade metamorphic terranes North of Newcastle,NSW Australia

Microprobe analyses of pumpellyites from rocks of variable chemistry formed under similar metamorphic conditions in two Palaeozoic, low grade metamorphic terranes show that they have an extreme range in composition (FeO^*=0.9–22.96) and that Fe²⁺Mg²⁺ and Fe³⁺Al³⁺ are the dominant substitutions. A less extreme variation in composition of pumpellyites has been noted in samples taken from a metamorphosed differentiated metadolerite. On an A1-Fe^*-Mg diagram, these pumpellyites extend through the fields of high pressure to low pressure terranes, indicating that pumpellyite compositions should be used with caution when determining metamorphic conditions.Bulk chemical composition of the host rock does not appear to be a controlling factor in determining pumpellyite compositions. Rather, intensity of alteration, particularly of opaque mineral phases, fluid chemistry and variation in oxidation potential are considered to be more important variables. Coexisting epidote and composition of the precursor mineral also appear to be important in some rocks.     

The peristerite gap in low-grade metamorphic rocks

Coexisting Na-plagioclases from greenschists both in the thermal aureole of the Kasugamura Granite, Japan, and in the low-P metamorphic zone of Yap Island, western Pacific were analyzed in great detail; the peristerite solvus was determined for each suite. The asymmetric solvus has steep albite-rich and gentle oligoclase-rich limbs that are similar to those for higher pressure series. The present results together with those from Vermont, New Zealand, and the Sanbagawa belt indicate that the peristerite solvus shifts toward the albite component and higher temperature with increasing pressure. With increasing pressure, albite co-existing with oligoclase (An=100 Ca/Ca+ Na=20) varies in composition from An 8–9 (in Kasugamura), through An 3 (in Yap Island and Vermont), to An 1 (in New Zealand) and An less than 0.5 (in the Sanbagawa belt). The consolute temperatures for the peristerite solvus estimated from available geothermometry are 420° C in Kasugamura, 450–550° C in Vermont and 550°–600° C in the Sanbagawa belt. The variation of plagioclase composition in progressive metamorphic zones is explained by intersection of a plagioclase-forming reaction and the peristerite immiscibility gap in an isobaric T-X _An diagram. The greenschist zone is characterized by albite, the transition zone by occurrence of peristerite pairs and the amphibolite zone by plagioclase of An 20–50.     

Polyphase and anticlockwise P-T evolution for Franciscan eclogites and blueschists from Jenner, California, USA

High-grade exotic blocks in the Franciscan Complex at Jenner, California, show evidence for polydeformation/metamorphism, with eight distinct stages. Two parallel sets of mineral assemblages [(E) eclogite, and (BS) laminated blueschist] representing different bulk chemistry were identified. Stage 1, recorded by parallel aligned inclusions (S1) of crossite + omphacite + epidote + ilmenite + titanite + quartz (E), and glaucophane + actinolite + epidote + titanite (BS) in the central parts of zoned garnets, represents the epidote blueschist facies. The onset of a second stage (stage 2) is represented by a weak crenulation of S1 and growth of garnet. This stage develops a well-defined S2 foliation of orientated barroisite + epidote + titanite (E), or subcalcic actinolite + epidote + titanite (BS) at c. 90d? to S1, with syntectonic growth of garnet, defining the (albite-)epidote-amphibolite facies. A third stage, with aligned inclusions of glaucophane + (subcalcic) actinolite + phengite parallel to S2 in the outermost rims of large garnet grains, is assigned to the transitional (albite-)epidote-amphibolite/(garnet-bearing) epidote blueschist facies. The fourth stage represents the peak metamorphism, and was identified by unorientated matrix minerals in the least retrograded samples. In this stage the mineral assemblages garnet + omphacite + glaucophane + phengite (E) and garnet + winchite + phengite + epidote (BS) both represent the eclogite facies. Stage 5 is represented by the retrogression of eclogite facies assemblages to the epidote blueschist facies assemblages crossite/glaucophane + garnet + omphacite + epidote + phengite (E), and glaucophane + actinolite + epidote + phengite (BS), with the development of an S5 foliation subparallel to S2. Stage 6 represents a crenulation of S5, with the development of a well-defined S6 crenulation cleavage wrapping around relics of the eclogite facies assemblages. This crenulation cleavage is further weakly crenulated during a D7 event. Post-D7 (stage 8) is recorded by the growth of lawsonite + chlorite ± actinolite replacing garnet, and by veins of lawsonite + pumpellyite + aragonite and phengite + apatite. The different, yet coeval, mineral parageneses observed in rock types (E) and (BS) are probably due to differences in bulk chemistry. The metamorphic evolution from stage 1 to stage 8 seems to have been broadly continuous, following an anticlockwise P-Tpath: (1) epidote blueschist (garnet-free) to (2) (albite-)epidote-amphibolite to (3) transitional epidote blueschist (garnet-bearing)/(albite-)epidote-amphibolite to (4) eclogite to (5) epidote blueschist (garnet-bearing) to (6-7) epidote blueschist (garnet-free) facies to (8) lawsonite + pumpellyite + aragonite-bearing assemblages. This anticlockwise P-T path may have resulted from a decreasing geothermal gradient with time in the Mesozoic subduction zone of California at early or pre-Franciscan metamorphism.     

Stability of pyrope-quartz in the system MgO-Al2O3-SiO2

Pyrope and quartz are stable with respect to aluminous enstatite and sillimanite at 1400 °C, 20 kb and at 1100 °C, 16 kb. The phase boundary limiting the coexistence of pyrope and quartz towards lower pressures is probably slightly curved. A slope of 15 bars/°C at 1400° and of 10 bars/°C at 1000 °C has been estimated from the experimental data. Between 1050 and 1100 °C the curve is intersected by the kyanite-sillimanite phase boundary. The calculated slope of the reaction aluminous enstatite + kyanite pyrope + quartz is negative (ca. 18–25 bars/°C). The existence of a negative slope has been demonstrated experimentally. Experimental evidence indicates that the assemblage aluminous enstatite and sillimanite is metastable with respect to sapphirine + quartz at high temperature. The invariant point involving the phases pyrope-sapphirine-aluminous enstatite-sillimanite-quartz is estimated to occur at 1125°±25 °C and 16±1 kb. A model phase diagram for the silicasaturated part of the system MgO-Al₂O₃-SiO₂ has been constructed. The position of three invariant points in this system has been estimated on the basis of presently available data.     

Geobarometry of low-temperature eclogites: applications of isothermal pressure-activity calculations

Estimation of metamorphic pressures in low temperature eclogite (Type C) is difficult because of the high variance mineral assemblages and problems in geothermometry, solution properties of low-temperature omphacite, and the thermodynamic properties of clinozoisite. We have considered equilibria in the CaO–FeO–MgO–TiO₂–Al₂O₃–SiO₂–H₂O (CFMTASH) system involving the phase components, quartz, rutile, kyanite, ilmenite, almandine, pyrope, grossular, clinozoisite, sphene, diopside, and H₂O-fluid There are four linearly independent equilibria involving the phase components in this system. Because kyanite can crystallize as a nearly pure phase, the lack of kyanite in a rock indicates that a _Al2SiO5 is<1.0. If we can estimate temperature independently, we can solve for a _Al2SiO5 and pressure by using two of the equilibria in isothermal pressure-activity diagrams. We have applied this approach to eclogites from New Caledonia and from southwestern Oregon. For the New Caledonia eclogites, calculated pressures range from 11.2 to 13.6 kbar at 500°C, and are consistent with the minimum pressures based upon the presence of jadeitic pyroxene+quartz and the lack of stable albite. Oregon eclogites come from different tectonic blocks and calculated minimum pressures of 11–12 kbar are based upon the presence of jadeitic pyroxene+rutile+garnet and lack of stable albite and ilmenite at reduced values of a _SiO2 (0.7–0.9).     

Pitfalls in geothermobarometry of eclogites: Fe<Superscript>3+</Superscript> and changes in the mineral chemistry of omphacite at ultrahigh pressures

The relatively low-variance mineral assemblage of a talc-kyanite eclogite from Dabie Shan enabled application of both conventional geothermobarometers (garnet-clinopyroxene geothermometer and the garnet-omphacite-phengite geobarometer) and a multi-equilibrium method to determine peak P-T conditions (THERMOCALC, average PT). The results were highly discrepant: 840 °C / 31.1 kbar vs. 590 °C / 29.8 kbar. Mössbauer spectroscopy showed that Fe³⁺/Fe_total in omphacite was significantly higher than the value obtained from standard formula recalculation. When the activities were corrected for Fe endmembers, geothermobarometry gave consistent results (606 °C / 31.3 kbar vs. 585 °C / 30.8 kbar). These are close to those obtained earlier by average PT, confirming the robustness of the multi-equilibrium approach. The high Fe³⁺ concentration in omphacite is best explained using a Ca-eskola endmember Ca_0.5[]_0.5AlSi₂O₆ and allowing corresponding vacancies in the omphacite structure.Editorial responsibility: W. Schreyer