Disequilibrium in the Ross of Mull Contact Metamorphic Aureole, Scotland: a Consequence of Polymetamorphism

The Ross of Mull pluton consists of granites and granodioritesand intrudes sediments previously metamorphosed at amphibolitefacies. The high grade and coarse grain size of the protolithis responsible for a high degree of disequilibrium in many partsof the aureole and for some unusual textures. A band of metapelitecontained coarse garnet, biotite and kyanite prior to intrusion,and developed a sequence of textures towards the pluton. InZone I, garnet is rimmed by cordierite and new biotite. In ZoneII, coarse kyanite grains are partly replaced by andalusite,indicating incomplete reaction. Coronas of cordierite + muscovitearound kyanite are due to reaction with biotite. In the higher-gradeparts of this zone there is complete replacement of kyaniteand/or andalusite by muscovite and cordierite. Cordierite chemistryindicates that in Zone II the stable AFM assemblage (not attained)would have been cordierite + biotite + muscovite, without andalusite.The observed andalusite is therefore metastable. Garnet is unstablein Zone II, with regional garnets breaking down to cordierite,new biotite and plagioclase. In Zone III this breakdown is welladvanced, and this zone marks the appearance of fibrolite andK-feldspar in the groundmass as a result of muscovite breakdown.Zone IV shows garnet with cordierite, biotite, sillimanite,K-feldspar and quartz. Some garnets are armoured by cordieriteand are inferred to be relics. Others are euhedral with Mn-richcores. For these, the reaction biotite + sillimanite + quartz garnet + cordierite + K-feldspar + melt is inferred. Usinga petrogenetic grid based on the work of Pattison and Harte,pressure is estimated at 3·2 kbar, and temperature atthe Zone II–III boundary at 650°C and in Zone IV asat least 750°C. KEY WORDS: contact metamorphism; disequilibrium     

Experimental investigation of the formation of cordierite-orthopyroxene parageneses in pelitic rocks

Cordierite and orthopyroxene (or orthoamphibole) are widespread in migmatitic terranes, and partial melting of pelitic rocks may be important in their production. In particular, the reaction quartz +albite+biotite+garnet+water vapor = cordierite +orthopyroxene or orthoamphibole+melt was among reactions discussed by Grant (1973) but poorly constrained in pressure-temperature space.This reaction involves too many phases to be readily studied experimentally. Therefore simpler melting and dehydration reactions involving quartzalbite-biotite-cordierite-orthopyroxene were investigated.In conjunction with the work of Hoffer (1976, 1978) these experiments place useful constraints on the above reaction and on the reaction quartz+albite+aluminosilicate+biotite+vapor = cordierite+garnet+melt. In pelitic rocks near the second illimanite isograd, cordierite and garnet may coexist with melt as low as 660° C and cordierite and orthopyroxene may coexist with melt at temperatures less than 675° C. In the absence of significant Mn or Ca, in pelitic rocks within the realm of melting, biotite+garnet assemblages are probably limited to pressures greater than 2kb and aluminosilicate+biotite assemblages to pressures greater than 3kb.     

Dehydration melting and devolatilization during exhumation of high-grade metapelites: the Tatra Mountains, Western Carpathians

Partial melting and retrogression related to Variscan tectonic exhumation have been recognized in the high-grade metapelites of the Tatra Mountains, Western Carpathians. Staurolite and kyanite relics document an early stage of the prograde metamorphism at c. 600 °C and 9–10 kbar. An increase in temperature to >730 °C at 11–12 kbar resulted in partial melting and incipient migmatization in the stability field of kyanite. Further heating at decreasing pressure during the earliest stage of exhumation led to the dehydration-melting of muscovite and biotite at >750–800 °C and 6–10 kbar, producing garnet-bearing granite as leucosomes in migmatite. Subsequent cooling is documented by garnet resorption by biotite and sillimanite (a reversal of the prograde biotite dehydration-melting reaction). This was followed by nearly isothermal decompression to c. 4–5 kbar producing cordierite and some melt due to biotite decomposition. Later nearly isobaric cooling led to cordierite pinitization and formation of orthoamphibole, chlorite and carbonates. Densities of primary, monophase CO₂–N₂ inclusions (0.69–1.06 g cm^?3) from the migmatite leucosome are consistent with the near-peak and retrograde conditions. Highly varying N₂ contents (5–30 mol%) are thought to result from the nitrogen uptake in retrograde K-bearing minerals, or dilution by CO₂ liberated during interaction of melt-derived water with metapelite graphite. The relatively high nitrogen content, not observed until now in migmatites, could have been inherited from the high-pressure metamorphism stage. It is assumed that the water-absent composition of fluid inclusions is not representative of the bulk water content (X_H2O≤0.7), which was masked by mechanical separation of the CO₂- and H₂O-dominated immiscible phases, and/or by post-entrapment modifications of the fluid inclusions. Decompression and the final stage of exhumation were accomplished by top-to-the-south thrusting as well as west–east (orogen-parallel) extension. They were most probably related to regional uplift and gravitational collapse of thermally weakened Variscan crust.     

Formation of spinel-cordierite-feldspar-glass coronas after garnet in metapelitic xenoliths: reaction modelling and geodynamic implications

Spinel + cordierite + K‐feldspar + plagioclase + glass form coronas around garnet in metapelitic xenoliths at El Hoyazo and Mazarrón, two localities of the Neogene Volcanic Province (NVP) of SE Spain. The presence of fresh glass (quenched melt) in all phases shows that corona development occurred under partial melting conditions. Algebraic analysis of mass balance in the NCKFMASH system suggests the reaction Grt + Sil + Bt + Pl = Spl + Crd + Kfs + melt as the most plausible model for the development of coronas in the El Hoyazo sample, and indicates that biotite was required as reactant for the formation of cordierite. The P–T conditions for the formation of coronas are estimated at ～820 ± 50 °C, 4.5 ± 0.6 kbar at El Hoyazo, and ～820 ± 50 °C, 4.0 ± 0.4 kbar at Mazarrón. The El Hoyazo xenoliths record a complex P–T history, characterized by early melt production during heating and additional melting during decompression. A local cooling event characterized by minor retrograde reaction and melt crystallization preceded ascent and eruption. This study shows that detailed xenolith analysis may be used to track magma evolution in a chamber.     

Aluminous reaction textures in orthoamphibole-bearing rocks: the pressure–temperature evolution of the high-grade Proterozoic of the Bamble sector, south Norway

Aluminous reaction textures in orthoamphibole-bearing rocks from the Froland area, Bamble, south Norway, record the prograde pressure–temperature path of the high-grade Kongsbergian Orogeny (c. 1600–1500 Ma) and the low–mid amphibolite facies overprint during the Sveconorwegian Orogeny (c. 1100–1000 Ma). The rocks contain anthophyllite/gedrite, garnet, cordierite, biotite, quartz, andalusite, kyanite, Cr-rich staurolite, tourmaline, ilmenite, rutile and corundum in a variety of parageneses. The P–T path is deduced from petrographic observations, mineral chemistry and zoning, geothermometry and (N)FMASH equilibria. The results indicate the sequence of metamorphic stages outlined below. (a) An M₁ phase characterized by the presence of strongly deformed andalusite, gedrite and tourmaline. (b) An M₂ phase with the development of kyanite after andalusite and the growth of staurolite associated with strong Na–Al–Mg zoning in orthoamphibole, indicating an increase in pressure (4 8 kbar) and temperature (500° 650°C). (c) Pressure decrease at high P (6–7 kbar) and high T (600–700 °C) during M_3a with the production of cordierite ° Corundum between kyanite, staurolite and orthoamphibole and cordierite growth between corundum and orthoamphibole. (d) Temperature increase to 740 ± 60 °C and 7 kbar; static growth of garnet (M_3b) at the metamorphic climax (peak T). The heat supply necessary to explain the temperature increase between the M_3a and M_3b phases is correlated with synkinematic enderbitic–charnockitic and basic intrusions in the Arendal granulite facies terrain. (e) M_3b metamorphic conditions were followed by an initial isobaric cooling path (early M₄) and late-stage pressure decrease (late M₄). Early M₄ conditions of 6–7 kbar and 550–600 °C, assuming P_H2O < P_total are indicated by a retrograde talc–kyanite–quartz assemblage in late quartz–cordierite veins. Late M₄ conditions of 3–4 kbar and 420–530 °C are inferred from a kyanite–andalusite–chlorite–quartz assemblage in vein-cordierite. The M₁–M₃ stages are interpreted as being the result of the same metamorphic P–T path, which was caused by both tectonic and magmatic thickening. A prolonged crustal residence time is proposed for the Bamble sector before uplift during the later stages of M₄ occurred.     

Separating metamorphic events in the Fosdick migmatite–granite complex,West Antarctica

The Fosdick migmatite–granite complex in West Antarctica records evidence for two high‐temperature metamorphic events, the first during the Devonian–Carboniferous and the second during the Cretaceous. The conditions of each high‐temperature metamorphic event, both of which involved melting and multiple melt‐loss events, are investigated using phase equilibria modelling during successive melt‐loss events, microstructural observations and mineral chemistry. In situ SHRIMP monazite and TIMS Sm–Nd garnet ages are integrated with these results to constrain the timing of the two events. In areas that preferentially preserve the Devonian–Carboniferous (M₁) event, monazite grains in leucosomes and core domains of monazite inclusions in Cretaceous cordierite yield an age of c. 346 Ma, which is interpreted to record the timing of monazite growth during peak M₁ metamorphism (～820–870 °C, 7.5–11.5 kbar) and the formation of garnet–sillimanite–biotite–melt‐bearing assemblages. Slightly younger monazite spot ages between c. 331 and 314 Ma are identified from grains located in fractured garnet porphyroblasts, and from inclusions in plagioclase that surround relict garnet and in matrix biotite. These ages record the growth of monazite during garnet breakdown associated with cooling from peak M₁ conditions. The Cretaceous (M₂) overprint is recorded in compositionally homogeneous monazite grains and rim domains in zoned monazite grains. This monazite yields a protracted range of spot ages with a dominant population between c. 111 and 96 Ma. Rim domains of monazite inclusions in cordierite surrounding garnet and in coarse‐grained poikiloblasts of cordierite yield a weighted mean age of c. 102 Ma, interpreted to constrain the age of cordierite growth. TIMS Sm–Nd ages for garnet are similar at 102–99 Ma. Mineral equilibria modelling of the residual protolith composition after Carboniferous melt loss and removal of inert M₁ garnet constrains M₂ conditions to ～830–870 °C and ～6–7.5 kbar. The modelling results suggest that there was growth and resorption of garnet during the M₂ event, which would facilitate overprinting of M₁ compositions during the M₂ prograde metamorphism. Measured garnet compositions and Sm–Nd diffusion modelling of garnet in the migmatitic gneisses suggest resetting of major elements and the Sm–Nd system during the Cretaceous M₁ overprint. The c. 102–99 Ma garnet Sm–Nd ‘closure’ ages correspond to cooling below 700 °C during the rapid exhumation of the Fosdick migmatite–granite complex.     

Constraints from monazite and xenotime growth modelling in the MnCKFMASH‐PYCe system on the P–T path of a metapelite from Shillong‐Meghalaya Plateau: implications for the Indian shield assembly

The Palaeo‐Mesoproterozoic metapelite granulites from northern Garo Hills, western Shillong‐Meghalaya Gneissic Complex (SMGC), northeast India, consist of resorbed garnet, cordierite and K‐feldspar porphyroblasts in a matrix comprising shape‐preferred aggregates of biotite±sillimanite+quartz that define the penetrative gneissic fabric. An earlier assemblage including biotite and sillimanite occurs as inclusions within the garnet and cordierite porphyroblasts. Staurolite within cordierite in samples without matrix sillimanite is interpreted to have formed by a reaction between the sillimanite inclusion and the host cordierite during retrogression. Accessory monazite occurs as inclusions within garnet as well as in the matrix, whereas accessory xenotime occurs only in the matrix. The monazite inclusions in garnet contain higher Ca, and lower Y and Th/U than the matrix monazite outside resorbed garnet rims. On the other hand, matrix monazite away from garnet contains low Ca and Y, and shows very high Th/U ratios. The low Th/U ratios (<10) of the Y‐poor garnet‐hosted monazite indicate subsolidus formation during an early stage of prograde metamorphism. A calculated P–T pseudosection in the MnCKFMASH‐PYCe system indicates that the garnet‐hosted monazite formed at <3 kbar/600 °C (Stage A). These P–T estimates extend backward the previously inferred prograde P–T path from peak anatectic conditions of 7–8 kbar/850 °C based on major mineral equilibria. Furthermore, the calculated P–T pseudosections indicate that cordierite–staurolite equilibrated at ~5.5 kbar/630 °C during retrograde metamorphism. Thus, the P–T path was counterclockwise. The Y‐rich matrix monazite outside garnet rims formed between ~3.2 kbar/650 °C and ~5 kbar/775 °C (Stage B) during prograde metamorphism. If the effect of bulk composition change due to open system behaviour during anatexis is considered, the P–T conditions may be lower for Stage A (<2 kbar/525 °C) and Stage B (~3 kbar/600 °C to ~3.5 kbar/660 °C). Prograde garnet growth occurred over the entire temperature range (550–850 °C), and Stage‐B monazite was perhaps initially entrapped in garnet. During post‐peak cooling, the Stage‐B monazite grains were released in the matrix by garnet dissolution. Furthermore, new matrix monazite (low Y and very high Th/U ≤80, ~8 kbar/850–800 °C, Stage C), some monazite outside garnet rims (high Y and intermediate Th/U ≤30, ~8 kbar/800–785 °C, Stage D), and matrix xenotime (<785 °C) formed through post‐peak crystallization of melt. Regardless of textural setting, all monazite populations show identical chemical ages (1630–1578 Ma, ±43 Ma). The lithological association (metapelite and mafic granulites), and metamorphic age and P–T path of the northern Garo Hills metapelites and those from the southern domain of the Central Indian Tectonic Zone (CITZ) are similar. The SMGC was initially aligned with the southern parts of CITZ and Chotanagpur Gneissic Complex of central/eastern India in an ENE direction, but was displaced ~350 km northward by sinistral movement along the north‐trending Eastern Indian Tectonic Zone in Neoproterozoic. The southern CITZ metapelites supposedly originated in a back‐arc associated with subducting oceanic lithosphere below the Southern Indian Block at c. 1.6 Ga during the initial stage of Indian shield assembly. It is inferred that the SMGC metapelites may also have originated contemporaneously with the southern CITZ metapelites in a similar back‐arc setting.     

In situ U–Pb dating of zircon formed from retrograde garnet breakdown during decompression in Rogaland, SW Norway

Regionally metamorphosed metapelites from Rogaland, SW Norway, contain zircon formed during the decompression reaction garnet + sillimanite + quartz → cordierite. The zircon, which occurs as inclusions in cordierite coronas around garnet, is texturally, chemically and isotopically distinct from older zircon in other textural settings in the matrix. A SHRIMP U–Pb age of 955 ± 8 Ma based on analyses in thin section on the decompression zircon from the cordierite coronas, therefore dates a point on the retrograde path, estimated from garnet–cordierite equilibria to be 5.6 kbar, 710 °C. This population was under‐represented in conventional SHRIMP analyses of individual zircon in a mono‐mineralic grain mount and, in the absence of a textural context, its significance unknown. The dominant age identified from SHRIMP analyses of the grain mount, in combination with analyses from matrix zircon in thin section, was 1035 ± 9 Ma. Based on the lack of consistent textural relationships with any specific minerals in thin section, as well as rare earth element chemistry, the 1035‐Ma population is interpreted to represent zircon growth during incipient migmatization of the rocks at 6–8 kbar and c. 700 °C. This is consistent with previous estimates for the age of regional M₁ metamorphism during the Sveconorwegian Orogeny. The most important outcome of this study is the successful analysis of zircon grains in a specific, well‐constrained reaction texture. Not only does this allow a precise point on the regional P–T path to be dated, but it also emphasizes the possibility of zircon formation during the retrograde component of a typical metamorphic cycle.     

Cordierite-bearing schists and gneisses from Timor, eastern Indonesia: P-T conditions of metamorphism and tectonic implications

In the Boi Massif of Western Timor the Mutis Complex, which is equivalent to the Lolotoi Complex of East Timor, is composed of two lithostratigraphical components: various basement schists and gneisses; and the dismembered remnants of an ophiolite. Cordierite-bearing pelitic schists and gneisses carry an early mineral assemblage of biotite + garnet + plagioclase + Al-silicate, but contain no prograde muscovite; sillimanite occurs in a textural mode which suggests that it replaced and pseudomorphed kyanite at an early stage and some specimens of pelitic schist contain tiny kyanite relics in plagioclase. Textural relations between, and mineral chemistries of, ferro-magnesian phases in these pelitic chists and gneisses suggest that two discontinuous reactions and additional continuous compositional changes have been overstepped, possibly with concomitant anatexis, as a result of decrease in P_load during high temperature metamorphism. The simplified reactions are: garnet and/or biotite + sillimanite + quartz + cordierite + hercynite + ilmenite + excess components. P-T conditions during the development of the early mineral assemblage in the pelitic gneisses are estimated to have been P + 10 kbar and T > 750°C, based upon the plagioclase-garnet-Al-silicate-quartz geobarometer and the garnet-biotite geothermometer. P-T conditions during the subsequent development of cordierite-bearing mineral assemblages in the pelitic gneisses are estimated to have been P + 5 kbar and T + 700°C with X_H2O < 0.5, based upon the Fe content of cordierite occurring in the assemblage quartz + plagioclase + sillimanite + biotite + garnet + cordierite coexisting with melt. Final equilibration between some of the phases suggests that conditions dropped to P > 2.3 kbar and T > 600°C. A similar exhumation P-T path is suggested for the pelitic schists with early metamorphic conditions of P > 6.2 kbar and T > 745°C and subsequent development of cordierite under conditions in the range P = 3-4 kbar and T = 600-700°C. The tectonic implications of these P-T estimates are discussed and it is concluded that the P-T path followed by these rocks was caused by decompression during rifting and synmetamorphic ophiolite emplacement resulting from processes during the initiation and development of a convergent plate junction located in Southeast Asia during late Jurassic to Cretaceous time.     

A multi-stage pressure—temperature record in the Chilka Lake granulites: the epitome of the metamorphic evolution of Eastern Ghats,India?

Abstract Metapelitic and charnockitic granulites exposed around Chilka Lake in the northern sector of the Eastern Ghats, India, preserve a multi-stage P—T record. A high-T decompression from above 10 kbar to 8 kbar around 1100°C has been determined from Mg-rich metapelites (X_Mg>0.60) with quartz-cordierite-orthopyroxene-sillimanite and cordierite—orthopyroxene—sapphirine—spinel assemblages. Between this and a second decompression to 6.0 kbar, isobaric cooling from 830 to 670°C at 8 kbar is evident. These changes are registered by the rim compositions of orthopyroxene and garnet in charnockites and metapelites with an orthopyroxene—quartz—garnet—plagioclase—cordierite assemblage, and are further supported by the garnet + quartz ± orthopyroxene + cordierite and biotite-producing reactions in sapphirine-bearing metapelites. Another indication of isobaric cooling from 800 to 650°C at 6.0 kbar is evident from rim compositions of orthopyroxene and garnet in patchy charnockites. Two sets of P—T values are obtained from metapelites with a quartz—plagioclase—garnet—sillimanite—cordierite assemblage: garnet and plagioclase cores yield 6.2 kbar, 700°C and the rims 5 kbar, 650°C, suggesting a third decompression. The earliest deformation (F1) structures are preserved in the larger charnockite bodies and the metapelites which retain the high P—T record. The effects of post-crystalline F2 deformation are observed in garnet megacrysts formed during or prior to F1 in some metapelites. Fold styles indicate a compressional regime during F1 and an extensional regime during F2. These lines of evidence and two phases of cooling at different pressures point to a discontinuity after the first cooling, and imply reworking. Two segments of the present P—T path replicate parts of the P—T paths suggested for four other granulite terranes in the Eastern Ghats, and the sense of all the paths is the same. This, plus the signature of three phases of deformation identified in the Eastern Ghats, suggests that the Chilka Lake granulites could epitomize the metamorphic evolution of the Eastern Ghats.     

Staurolite porphyroblast controls on local bulk compositional and microstructural changes during decompression of a St–Bt–Grt–Crd–And schist (Ancasti metamorphic complex,Sierras Pampeanas,W Argentina)

The staurolite–biotite–garnet–cordierite–andalusite–plagioclase–muscovite–quartz metapelitic mineral assemblage has been frequently interpreted in the literature as a result of superimposition of various metamorphic events, for example, in polymetamorphic sequences. The assemblage was identified in schists from the Ancasti metamorphic complex (Sierras Pampeanas of Argentina) where previous authors have favoured the polymetamorphic genetic interpretation. A pseudosection in the MnNCKFMASH system for the analysed XRF bulk composition predicts the stability of the sub‐assemblage staurolite–biotite–garnet–plagioclase–muscovite–quartz, and the compositional isopleths also agree with measured mineral compositions. Nevertheless, the XRF pseudosection does not predict any field with staurolite, andalusite and cordierite being stable together. As a result of more detailed modelling making use of the effective bulk composition concept, our interpretation is that the staurolite–biotite–garnet–plagioclase–muscovite–quartz sub‐assemblage was present at peak metamorphic conditions, 590 °C and 5.2 kbar, but that andalusite and cordierite grew later along a continuous P–T path. These minerals are not in mutual contact and are observed in separate microstructural domains with different proportions of staurolite. These domains are explained as a result of local reaction equilibrium subsystems developed during decompression and influenced by the previous peak crystal size and local modal distribution of staurolite porphyroblasts that have remained metastable. Thus, andalusite and cordierite grew synchronously, although in separate microdomains, and represent the decompression stage at 565 °C and 3.5 kbar.     

P–T–t evolution of spinel–cordierite–garnet gneisses from the Sauwald Zone (Southern Bohemian Massif, Upper Austria): is there evidence for two independent late-Variscan low-P/high-T events in the Moldanubian Unit?

The Sauwald Zone, located at the southern rim of the Bohemian Massif in Upper Austria, belongs to the Moldanubian Unit. It exposes uniform biotite + plagioclase ± cordierite paragneisses that formed during the post-collisional high-T/low-P stage of the Variscan orogeny. Rare metapelitic inlayers contain the mineral assemblage garnet + cordierite + green spinel + sillimanite + K-feldspar + plagioclase + biotite + quartz. Mineral chemical and textural data indicate four stages of mineral growth: (1) peak assemblage as inclusions in garnet (stage 1): garnet core + cordierite + green spinel + sillimanite + plagioclase (An_35–65); (2) post-peak assemblages in the matrix (stages 2, 3): cordierite + spinel (brown-green and brown) ± sillimanite ± garnet rim + plagioclase (An_10–45); and (3) late-stage growth of fibrolite, muscovite and albite (An_0–15) during stage 4. Calculation of the P–T conditions of the peak assemblage (stage 1) yields 750–840°C, 0.29–0.53 GPa and for the stage 2 matrix assemblage garnet + cordierite + green spinel + sillimanite + plagioclase 620–730°C, 0.27–0.36 GPa. The observed phase relations indicate a clockwise P–T path, which terminates below 0.38 GPa. The P–T evolution of the Sauwald Zone and the Monotonous Unit are very similar, however, monazite ages of the former are younger (321 ± 9 Ma vs. 334 ± 1 Ma). This indicates that high-T/low-P metamorphism in the Sauwald Zone was either of longer duration or there were two independent phases of late-Variscan low-P/high-T metamorphism in the Moldanubian Unit.     

Amphibole pyroxenite xenoliths: Cumulate or replacement phenomena from the upper mantle,Nunivak Island,Alaska

Interstitial to poikilitic amphibole found in garnet pyroxenite xenoliths has been interpreted, in the past, to represent a critically silica undersaturated, residual intercumulus melt trapped by its cumulate assemblage of anhydrous phases. The textural features of such amphibole in pyroxenite xenoliths from Nunivak Island, Alaska, however, are more compatible with an origin by replacement of the anhydrous phases of the pyroxenite, following a period of cooling and sub-solidus recrystallization in the upper mantle. The reaction of amphibole and olivine to give orthopyroxene, observed in two specimens, requires that the associated fluid phase was not critically silica undersaturated. The amphibole is therefore thought to reflect the interaction of an alkali-bearing, migratory, aqueous fluid and an upper mantle consisting of spinel lherzolite cut by veins of spinel and garnet pyroxenite.     

Oxygen Fugacity and Water Activity during Thermal Peak and Retrogression of Granulites around the Sarvapuram Area,Karimnagar Granulite Terrane,Andhra Pradesh,India

The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnockite. It is largely made up of garnet, orthopyroxene, cordierite, biotite, plagioclase, K–feldspar, sillimanite and quartz. The peak metamorphic stage is represented by the equilibrium mineral assemblage i.e. garnet, orthopyroxene, cordierite, biotite, plagioclase, sillimanite and quartz. Breakdown of the garnet as well as preservation of the orthopyroxene–cordierite symplectite, formation of cordierite with the consumption of the garnet + sillimanite + quartz represents the decompressional event. The thermobarometric calculations suggest a retrograde P–T path with a substantial decompression of c. 3.0 kbar. The water activity(XH2 O) conditions obtained with the win TWQ program for core and symplectite compositions from garnet–bearing gneiss are 0.07–0.14 and 0.11–0.16 respectively. The quantitative estimation of oxygen fugacity in garnet–bearing gneiss reveal log f O2 values ranging from-11.38 to-14.05. This high oxidation state could be one of the reasons that account for the absence of graphite in these rocks.     

Petrology and P–T evolution of garnet peridotites from central Sulawesi, Indonesia

Alpine‐type orogenic garnet‐bearing peridotites, associated with quartzo‐feldspathic gneisses of a 140–115 Ma high‐pressure/ultra‐high‐pressure metamorphic (HP‐UHPM) terrane, occur in two regions of the Indonesian island of Sulawesi. Both exposures are located within NW–SE‐trending strike–slip fault zones. Garnet lherzolite occurs as <10 m wide fault slices juxtaposed against Miocene granite in the left‐lateral Palu‐Koro (P‐K) fault valley, and as 10–30 m wide, fault‐bounded outcrops juxtaposed against gabbros and peridotites of the East Sulawesi ophiolite within the right‐lateral Ampana fault in the Bongka river (BR) valley. Six evolutionary stages of recrystallization can be recognized in the peridotites from both localities. Stage I, the precursor spinel lherzolite assemblage, is characterized by Ol+Cpx+Opx±Prg‐Amp ± Spl±Rt±Phl, as inclusions within garnet cores. Stage II, the main garnet lherzolite assemblage, consists of coarse‐grained Ol+Opx+Cpx+Grt; whereas finer‐grained, neoblastic Ol+Opx+Grt+Cpx±Spl±Prg‐Amp±Phl constitutes stage III. Stages IV and V are manifest as kelyphites of fibrous Opx+Cpx+Spl in inner coronas, and Opx+Spl+Prg‐Amp±Ep in outer coronas around garnet, respectively. The final (greenschist facies) retrogressive stage VI is accompanied by recrystallization of Serp+Chl±Mag±Tr±Ni sulphides±Tlc±Cal. P–T conditions of the hydrated precursor spinel lherzolite stage I were probably about 750 °C at 15–20 kbar. P–T determinations of the peak stage IIc (from core compositions) display considerable variation for samples derived from different outcrops, with clustering at 26–38 kbar, 1025–1210 °C (P‐K & BR); 19–21 kbar, 1070–1090 °C (P‐K), and 40–48 kbar, 1205–1290 °C (BR). Stage IIr (derived from rim compositions) generally records decompression of around 4–12 kbar accompanied by cooling of 50–240 °C from the IIc peak stage. Stage III, which post‐dates a phase of ductile deformation, yielded 22±2 kbar at 750±25 °C (P‐K) and 16±2 kbar at 730±40 °C (BR). The granulite–amphibolite–greenschist decompression sequence reflects uplift to upper crustal levels from conditions of 647–862 °C at P=15 kbar (stage IV), through 580–635 °C at P=10–12 kbar (stage V) to 350–400 °C at P=4–7 kbar (stage VI), respectively, and is identical to the sequence recorded in associated granulite, gneiss and eclogite. Sulawesi garnet peridotites are interpreted to represent minor components of the extensive HP‐UHP (peak P >28 kbar, peak T of c. 760 °C) metamorphic basement terrane, which was recrystallized and uplifted in a N‐dipping continental collision zone at the southern Sundaland margin in the mid‐Cretaceous. The low‐T , low‐P and metasomatized spinel lherzolite precursor to the garnet lherzolite probably represents mantle wedge rocks that were dragged down parallel to the slab–wedge interface in a subduction/collision zone by induced corner flow. Ductile tectonic incorporation into the underthrust continental crust from various depths along the interface probably occurred during the exhumation stage, and the garnet peridotites were subsequently uplifted within the HP‐UHPM nappe, suffering a similar decompression history to that experienced by the regional schists and gneisses. Final exhumation from upper crustal levels was clearly facilitated by entrainment in Neogene granitic plutons, and/or Oligocene trans‐tension in deep‐seated strike–slip fault zones.     

Precambrian sulphide deposits: R.W. Hutchinson,C.D. Spence and J.M. Franklin (Editors). Geological Association of Canada,Special Paper 25, H.S. Robinson Memorial Volume 1982, VII + 791 pp., US$ 47.00 plus US$ 2.50 postage and handling for members,and US$ 57.00 plus US$ 2.50 postage and handling for others,hardcover

The Precambrian of Madagascar appears to be a part of the Mozambiquian mobile belt. Some authors consider the southern part of the island to be constituted, from east to west, by more and more recent formations, i.e., the ultrametamorphic ‘Androyan’ sequence with an Archean reworked crust as the oldest one, first followed by the ‘Graphite’ sequence, then by the ‘Vohibory’ sequence. The cordierite and garnet banded gneisses and leptynites are widely distributed in the Androyan sequence. These gneisses result from an incipient partial melting of paragneisses. Garnet—plagioclase, garnet—cordierite and garnet—biotite geothermo- and geobarometers allow the estimation of the conditions of this migmatisation (T > 700°C, P_T = 5?5.5 kbar, PH₂O = 0.3?0.4 P_T). Some cordierite and garnet leptynites represent the leucosomes of this anatectic event. Since previous experiments showed that the seven-phase assemblage (quartz + plagioclase + K feldspar + garnet + biotite + sillimanite + cordierite) only occurs in a limited range of pressure and temperature (T = 690?710°C, P_T = 4?5.7 kbar, PH₂O = 0.2?0.5 P_T), we can consider these gneisses as very precise thermobarometric reference levels in southern Madagascar. This migmatic event may be contemporary with the Pan-African orogenesis.     

Mantle metasomatism and magma formation in continental lithosphere: Data on xenoliths in alkali basalts from the Makhtesh Ramon,Negev desert,Israel

Mantle xenoliths (lherzolites, clinopyroxene dunites, wehrlites, and clinopyroxenites) in the Early Cretaceous volcanic rocks of Makhtesh Ramon (alkali olivine basalts, basanites, and nephelinites) represent metasomatized mantle, which served as a source of basaltic melts. The xenoliths bear signs of partial melting and previous metasomatic transformations. The latter include the replacement of orthopyroxene by clinopyroxene in the lherzolites and, respectively, the wide development of wehrlites and olivine clinopyoroxenites. Metasomatic alteration of the peridotites is accompanied by a sharp decrease in Mg, Cr, and Ni, and increase of Ti, Al, Ca contents and ³⁺Fe/²⁺Fe ratio, as well as the growth of trace V, Sc, Zr, Nb, and Y contents. The compositional features of the rocks such as the growth of ³⁺Fe/²⁺Fe and the wide development of Ti-magnetite in combination with the complete absence of sulfides indicate the high oxygen fugacity during metasomatism and the low sulfur concentration, which is a distinctive signature of fluid mode during formation of the Makhtesh Ramon alkali basaltic magma. Partial melting of peridotites and clinopyroxenites is accompanied by the formation of basanite or alkali basaltic melt. Clino- and orthopyroxenes are subjected to melting. The crystallization products of melt preserved in the mantle rock are localized in the interstices and consist mainly of fine-grained clinopyroxene, which together with Ti-magnetite, ilmenite, amphibole, rhenite, feldspar, and nepheline, is cemented by glass corresponding to quartz–orthopyroxene, olivine–orthopyroxene, quartz–feldspar, or nepheline–feldspar mixtures of the corresponding normative minerals. The mineral assemblages of xenoliths correspond to high temperatures. The high-Al and high-Ti clinopyroxene, calcium olivine, feldspar, and feldspathoids, amphibole, Ti-magnetite, and ilmenite are formed at 900–1000°. The study of melt and fluid inclusions in minerals from xenoliths indicate liquidus temperatures of 1200–1250°C, solidus temperatures of 1000–1100°C, and pressure of 5.9–9.5 kbar. Based on the amphibole–plagioclase barometer, amphibole and coexisting plagioclase were crystallized in clinopyroxenites at 6.5–7.0 kbar.     

Ultramafic and mafic xenoliths from Hierro,Canary Islands: evidence for melt infiltration in the upper mantle

Three groups of ultramafix xenoliths were collected from alkali basalt in the island of Hierro, Canary Islands: (1) Cr-diopside series (spinel harzbugite, lherzolite, dunite); (2) Al-augite series xenoliths (spinel wherlite, olivine clinopyroxenite, dunite, olivine websterite); (3) gabbroic xenoliths. The main textures are granoblastic, porphyroclastic and granular, but poikilitic textures, and symplectitic intergrowths of clinopyroxene (cpx) + spinel (sp)±orthopyroxene (opx)±olivine (ol) (in rare cases cpx+opx), occur locally. Textural relations and large inter- and intra-sample mineral chemical variations testify to a complex history of evolution of the mantle source region, involving repeated heating, partial melting, and enrichment associated with infiltration by basaltic melts. The oldest assemblage in the ultramafic xenoliths (porphyroclasts of ol+opx±sp±cpx) represents depleted abyssal mantle formed within the stability field of spinel lherzolite. The neoblast assemblage [ol+cpx+ sp±opx±plagioclase (plag)±ilmenite (il)±phlogopite (phlog)] reflect enrichment in CaO+Al₂O₃+Na₂O+ FeO±TiO₂±K₂O±H₂O through crystal/liquid separation processes and metasomatism. The Al-augite-series xenoliths represent parts of the mantle where magma infiltration was much more extensive than in the source region of the Cr-diopside series rocks. Geothermometry indicates temperature fluctuations between about 900–1000 and 1200°C. Between each heating event the mantle appears to have readjusted to regional geothermal gradient passing 950°C at about 12 kbar. The gabbroic xenoliths represent low-pressure cumulates.     

Petrology and retrograde P-T path in granulites of the Kanskaya formation, Yenisey range, Eastern Siberia

The Kanskaya formation in the Yenisey range, Eastern Siberia is a newly studied example of retrogression of granulite facies rocks. The formation consists of two stratigraphical units: the lower Kuzeevskaya group and the upper Atamanovskaya group. Rocks from both of these units show rare reaction textures such as replacement of cordierite by garnet, sillimanite and quartz, silimanite coronas around spinel and corundum, and garnet rims around plagioclase in metabasites, while plagioclase rims around garnet can be seen in associated metapelites. The paragenesis quartz + orthopyroxene + sillimanite is a feature of the Kuzeevskaya group. In many samples, chemical zoning of garnet and cordierite shows an increase in Mg from core to rim as well as the reverse.
Biotite-garnet-cordierite-sillimanite-quartz as well as spinel±biotite-garnet°Cordierite±sillimanite-quartz assemblages were studied using geothermometers and geobarometers based on both exchange and net-transfer reactions (Perchuk & Lavrent'eva, 1983; Aranovich & Podlesskii, 1983; Gerya & Perchuk, 1989). Detailed investigation of 10 samples including 1000 microprobe analyses revealed decompression (first stage) followed by the near isobaric cooling of the granulites. From geological studies, the 7 km total thickness of the sequence closely corresponds to the pressure difference (∼ 2.2kbar) measured by geobarometers in the samples taken from different levels in the sequence. Individual samples yield P-T paths ranging from 100°C/kbar to 140°C/kbar depending on their locations with respect to the large Tarakskiy granite pluton. In places the 100°C/kbar path changed to the 140°C/kbar due to the influence of the intrusion. In a P-T diagram these trajectories are subparallel lines, whose P-T maxima define the Archaean geotherm between 3.1 and 2.7 Ga, determined isotopically. A petrological model for P-T evolution of the Kanskaya formation is proposed.     

Stability of sapphirine-bearing mineral assemblages in the system FeO-MgO-Al2O3-SiO2 and metamorphic P-T parameters of aluminous granulites

Consistent thermodynamic data on the properties of pure mineral end members and the mixing properties of solid solutions in the system FeO-MgO-Al₂O₃-SiO₂ were employed to simulate phase relations of sapphirine, garnet, spinel, orthopyroxene, cordierite, quartz, Al silicates, and corundum. Compositional variations of the solid solutions with temperature notably modify the topology of the P-T diagrams, which differ from the petrogenetic grids widely used in the literature. It is worth noting that the evaluation of P-T metamorphic conditions based on reaction relations in sapphirine-bearing assemblages cannot be so far considered reliable enough. The lower stability limit of the sapphirine + quartz assemblage in the system in question is possibly located at much lower P-T parameters: at least 835°C and ∼6 kbar. The sapphirine + kyanite assemblage can be stable at temperatures below 860°C and a pressure of ∼11 kbar, and the stability field of the sapphirine + olivine assemblage is narrow and constrained to temperatures no higher than ∼800°C.