Aluminous Metapelites Research Articles

Implications of garnet nucleation overstepping for the P–T evolution of the Lesser Himalayan Sequence of central Nepal

AbstractRecent studies have demonstrated that kinetic factors can significantly influence garnet nucleation, delaying its appearance with respect to the equilibrium predictions. Overstepping of garnet nucleation occurs in both contact and regional metamorphic settings, with extremely variable degrees: The factors controlling such highly variable degrees of overstepping are still unclear. This study focuses on garnet nucleation and growth in aluminous metapelites from the Barrovian inverted metamorphic pile of the upper portion of the Lesser Himalayan Sequence (Upper‐LHS; central Nepal), with the aim of (i) investigating if (and how) the bulk‐rock composition can influence overstepping of garnet nucleation and (ii) which are the implications of garnet nucleation overstepping for the P–T evolution of a Barrovian metamorphic sequence. Detailed petrographic, microstructural and compositional data are presented for six phyllitic schists, containing porphyroblasts of garnet, staurolite and/or kyanite. Their P–T evolution is constrained through thermodynamic forward modelling (i.e., isochemical phase diagrams combined with isopleth thermobarometry), assuming that equilibrium was attained at every stage of their metamorphic evolution. Comparison between the P–T conditions inferred for the growth of garnet core, the assemblages predicted to be stable at these P–T conditions and the modelled garnet‐in reaction boundary suggests that the studied samples have experienced different degrees of apparent thermal (ΔT) and/or baric (ΔP) overstepping of garnet nucleation. We suggest that the bulk‐rock MnO and CaO amounts might have influenced the apparent ΔT and ΔP overstepping of the garnet‐in reaction: more specifically, the higher the bulk‐rock MnO content, the more pronounced the apparent ΔT overstepping, whereas the lower the bulk‐rock CaO content, the greater the ΔP overstepping. However, rather than an effective delay of garnet appearance with respect to equilibrium predictions, the apparent ΔT overstepping of garnet nucleation could reflect the attainment of the critical 0.5% threshold of garnet abundance, below which garnet is not readily detected in thin section. Kinetic factors seem much less critical in controlling the growth of the garnet rim at peak P–T conditions, confirming that peak metamorphic conditions constrained through equilibrium approaches based on the composition of garnet rim and of the matrix assemblage can be considered as reliable. Overall, the P–T paths of the studied samples are characterized by prograde heating coupled with tectonic overload (peak‐P conditions of 9.5–10.5 kbar, 580–590°C), followed by heating during exhumation (peak‐T conditions of 8.2–8.9 kbar, 610–630°C), supporting those thermo‐mechanical models that predict a period of slowdown (or quiescence) of the Main Central Thrust activity.

Timing and significance of pre-Olarian metamorphism in the Willyama Supergroup, Broken Hill, Australia

In the Willyama Supergroup of the Curnamona Province, the intensity of granulite facies Olarian metamorphism at c. 1600 Ma has hampered efforts to elucidate the timing and nature of preceding lower-grade metamorphic events. Previous dating of pre-peak monazite inclusions in aluminous metapelites has been complicated by potentially incomplete isotopic ‘shielding’ by their host porphyroblasts, owing to pervasive deformation and recrystallisation of these rocks during the c. 1600 Ma event. Near the Broken Hill Pb-Zn orebody, quartz-feldspar-biotite-garnet (‘Potosi-type’) gneiss proved more resistant to recrystallization during granulite facies metamorphism: new Sensitive High Resolution Ion MicroProbe (SHRIMP) U-Pb analyses of monazite from this rock delineated a population of relatively Y-rich (median 0.96 wt%) cores at 1636.8 ± 4.4 Ma (95% confidence), overprinted by Y-poor (median 0.10 wt%) rims and domains at 1597.3 ± 3.3 Ma. Garnet is a sink for Y in metamorphic rocks, which suggests that Y-rich c. 1637 Ma monazite growth predated garnet formation (and possibly occurred at relatively low metamorphic grade), whereas the Y-poor nature of the c. 1597 Ma rims and domains implies that dissolution and reprecipitation during Olarian metamorphism postdated porphyroblastic garnet growth. Coexisting low-Th/U zircon rims are dominated by a 1600.7 ± 3.6 Ma population, emphasising that the event responsible for c. 1637 Ma monazite growth was largely invisible to zircon. Interlayered mafic granulite 70 m away yielded partially recrystallised zircons with dark-CL internal domains which establish a minimum age of 1639 ± 8 Ma for the igneous precursor. Recrystallisation was accompanied by actinide depletion, with bright-CL, U-poor and low-Th/U domains dated at 1596 ± 8 Ma. Our new 1640–1635 Ma dates constitute robust isotopic evidence for pre-Olarian mafic magmatism and metamorphism within the lower Willyama Supergroup. We infer an extensional environment in the upper mid-crust for this tectonothermal activity, coeval with ongoing sedimentation and deposition of the upper Paragon Group at the surface. Extension of the eastern Curnamona Province at c. 1640 Ma is coeval with sediment-hosted base-metal mineralisation in the Mount Isa Inlier and the McArthur Basin, and is consistent with tectonic regimes prevailing throughout Proterozoic southern, central and eastern Australia at that time.

Mesoarchean (ultra)-high temperature and high-pressure metamorphism along a microblock suture: Evidence from Earth's oldest khondalites in southern India

Early continent-building processes on Earth are challenging to investigate, particularly since juvenile felsic crust formed during the Early Archean (4.0–3.2 Ga) is rarely preserved. Thus, associated sedimentary records are of fundamental importance, although in many cases these have been metamorphosed and reworked to various degrees since deposition. Here we present new petrological and zircon and monazite U–Pb age data from one of the Earth's oldest ‘khondalite’ (granulite-facies aluminous metapelite) belt, which we define as the Mercara khondalite belt, and associated charnockite and mafic granulite from the Mercara suture, the collision zone welding the Coorg and Western Dharwar Blocks in southern India. Petrologic analyses and phase equilibria modelling of the khondalites and associated charnockite and mafic granulite reveal a clockwise pressure–temperature (P–T) path with a peak temperature of above ca. 900 °C, and pressures up to 12 kbar. Detrital zircon grains in the metasedimentary rocks have magmatic cores with oscillatory zoning and ages up to ca. 3.5 Ga, and metamorphic overgrowths with ages of 3.1–3.0 Ga. Monazite in the khondalites yield identical metamorphic ages in the range of 3.1 to 3.0 Ga. Some of these rocks are overprinted by a younger thermal event at ca. 2.8–2.6 Ga. We correlate the high P–T metamorphism with the subduction-collision history between the northern margin of the Mercara block and the Western Dharwar Craton during the Mesoarchean, which indicates that plate tectonics had been established on Earth by at least ca. 3.1 Ga, in agreement with many independent lines of evidence. The Mesoarchean Mercara khondalite belt signals emergence of continents on the early Earth with active drainage systems leading to the deposition of voluminous detritus. The ca. 3.1 to 3.0 Ga (ultra) high temperature and high pressure metamorphism also coincides with the timing of assembly of the Earth's oldest supercontinent Ur

Seemingly disparate temperatures recorded in coexisting granulite facies lithologies

AbstractPhase equilibrium modelling of a conformable sequence of supracristal lithologies from the Bushmanland Subprovince of the Namaqua–Natal Metamorphic Complex (South Africa) reveals a disparity of some 60–70°C in estimated peak metamorphic temperature. Aluminous metapelites were equilibrated at ~770–790°C, whereas two‐pyroxene granulite and garnet–orthopyroxene–biotite gneiss record distinctly higher conditions of ~830–850°C. Semi‐pelite and Mg–Al‐rich gneisses yield poorly constrained estimates that span the range derived from other lithologies. All samples record peak pressure of ~5–6 kbar, and followed a roughly isobaric heating path from andalusite‐bearing greenschist/lower amphibolite facies conditions through a tight clockwise loop at near‐peak conditions, followed by near‐isobaric cooling. The disparity in peak temperatures appears to be robust, as the low‐variance assemblages in all samples reflect well‐known melting reactions that only occur over narrow temperature intervals. The stable coexistence of both products and reactants of these melting reactions indicates that they did not go to completion before metamorphism waned. Calculated pressure–enthalpy diagrams show that the melting reactions are strongly endothermic and therefore buffer temperature while heat is consumed by melting. Because the respective reactions occur at distinct P–T conditions and have different reactant assemblages, individual lithologies are thermally buffered at different temperatures and to different degrees, depending on the occurrence and abundance of reactant minerals. Our calculations show that all lithologies received essentially the same suprasolidus heat budget of 19 ± 1 kJ/mol, which led to the manifestation of lower peak temperatures in the more fertile and strongly buffered aluminous metapelites compared with more refractory rock types. If little to no thermal communication is assumed, this implies that lithology exerts a first‐order control over the heating path and the peak temperature that can be attained for a specific heat budget. Our results caution that the metamorphic conditions derived from pelitic granulites should not be assumed or extrapolated to larger sections of an orogenic crust that consist of other, more refractory lithologies.

Corundum formation by metasomatic reactions in Archean metapelite, SW Greenland: Exploration vectors for ruby deposits within high-grade greenstone belts

Corundum (ruby-sapphire) is known to have formed in situ within Archean metamorphic rocks at several localities in the North Atlantic Craton of Greenland. Here we present two case studies for such occurrences: (1) Maniitsoq region (Kangerdluarssuk), where kyanite paragneiss hosts ruby corundum, and (2) Nuuk region (Storø), where sillimanite gneiss hosts ruby corundum. At both occurrences, ultramafic rocks (amphibole-peridotite) are in direct contact with the ruby-bearing zones, which have been transformed to mica schist by metasomatic reactions. The bulk-rock geochemistry of the ruby-bearing rocks is consistent with significant depletion of SiO2 in combination with addition of Al2O3, MgO, K2O, Th and Sr relative to an assumed aluminous precursor metapelite. Phase equilibria modelling supports ruby genesis from the breakdown of sillimanite and kyanite at elevated temperatures due to the removal of SiO2. The juxtaposition of relatively silica- and aluminum-rich metasedimentary rocks with low silica ultramafic rocks established a chemical potential gradient that leached/mobilized SiO2 allowing corundum to stabilize in the former rocks. Furthermore, addition of Al2O3 via a metasomatic reaction is required, because Al/Ti is fractionated between the aluminous precursor metapelites and the resulting corundum-bearing mica schist. We propose that Al was mobilized either by complexation with hydroxide at alkaline conditions, or that Al was transported as K-Al-Si-O polymers at deep crustal levels. The three main exploration vectors for corundum within Archean greenstone belts are: (1) amphibolite- to granulite-facies metamorphic conditions, (2) the juxtaposition of ultramafic rocks and aluminous metapelite, and (3) mica-rich reactions zones at their interface.

Proveniência dos arenitos da Bacia de Campos (Andar Alagoas) por meio da composição da granada

Neste trabalho, realizou-se um estudo da composição química da granada para a identificação das principais rochas-fonte envolvidas na proveniência dos arenitos da Bacia de Campos, Andar Alagoas. Em análises obtidas por microssonda eletrônica, dos 17 poços do segmento offshore do estudo, verificou-se a contribuição de diferentes rochas-fonte portadoras de granadas, identificadas como dos tipos A, B e C. Entre elas, predominaram as granadas do tipo B. Um refinamento dessa classificação permitiu categorizar tais granadas em granadas do tipo Bi, derivadas de rochas ígneas ácidas e intermediárias, notadamente granitoides. A ausência ou pequena incidência de zonações composicionais e de inclusões, embora não excluam totalmente as outras categorias, apontam para essa classificação. Granadas do tipo D e E não aparecem em nenhuma das amostras analisadas. A interpretação dos resultados de aproximadamente 4.000 análises das 65 amostras indicou duas contribuições principais para a proveniência desses arenitos: no setor sul da bacia, material derivado do terreno Cabo Frio (rochas metamórficas da fácies anfibolito superior a granulitos, granitos, metapelitos aluminosos e secundariamente metabasitos) e, na porção norte da bacia, derivação do terreno oriental do Orógeno Ribeira (terreno de alto grau e granitoides associados). A direção de suplemento sedimentar mais importante é de sudoeste para nordeste, interpretada como resultado da identificação dessa maior contribuição do terreno-fonte. Os arenitos localizados no setor norte, com granadas derivadas do tipo B e Bi, sobretudo, indicam a presença dominante de rochas granitoides na área-fonte. Essas rochas-fonte geram areias de composição mais quartzosas e/ou quartzo-feldspáticas, com maior potencial para produzirem arenitos-reservatórios de melhor qualidade.

Metamorphic imprint of accretion and ridge subduction in the Pan‐African Damara Belt, Namibia

AbstractRidge subduction is an inescapable plate tectonic process, but has only been documented in modern circum‐Pacific environments and not yet been recognized from suture zones associated with supercontinent assembly, likely because its imprint is obliterated by later collision. The formation of the Pan‐African Damara Belt of central Namibia involved northward subduction of the Khomas Sea underneath the Congo Craton, prior to final suturing of the Congo and Kalahari Cratons. The accretionary history of the Belt is preserved in the Southern and Southern Marginal Zones, which consist of turbiditic metasedimentary and intercalcated mafic rocks with MORB affinity. Two localities in the Kuiseb and Gaub canyons reveal that aluminous metapelites contain a fabric‐defining assemblage of fine‐grained muscovite, chlorite, biotite, quartz and graphite that is overprinted by randomly oriented porphyroblasts and poikiloblasts of garnet, staurolite, kyanite and biotite. Associated metamafic rocks consist of hornblende, chlorite, epidote, rutile and quartz, with actinolite cores preserved in amphibole porphyroblasts. Metamorphic conditions for the fabric‐defining assemblage are estimated at ∼10 kbar and 540–560 C, whereas peak metamorphism likely occurred at 10–10.5 kbar and 600 C. Consequently, these rocks preserve a two‐stage prograde metamorphic history, where initial tectonic burial was followed by relatively rapid, near‐isobaric heating without attendant deformation to peak metamorphic conditions. We propose that initial burial occurred through subduction and underplating to the accretionary prism, before ridge subduction and opening of a slab window heated the rocks to peak metamorphic conditions. The exceptional preservation of the tectono‐thermal imprint of the accretionary orogenic stage is due to the relatively soft, largely aborted collision that characterized the Damara orogeny, which can be attributed to the confined extent of the Khomas Sea.

Material transfer through the interface between peraluminous metapelite and gedrite-bearing gneiss at high temperatures and moderate pressures

Relations between gedrite-bearing gneisses and aluminous metapelites were studied in amphibolite- and granulite-facies rocks in the Sharyzhalgai Complex, Eastern Sayan. The rocks show reaction relations and provide evidence of simultaneous diffusion of components through the interface between these chemically unequilibrated rocks, with the displacement of the boundary between them. In the vicinity of the gedrite-bearing gneiss, garnet and sillimanite of the aluminous metapelite are replaced (up to complete disappearance) by cordierite. The XMg of the rock and its minerals simultaneously increase. The diffusion coefficients of major oxides were evaluated as follows (cm2/s): TiO2 = 6.98 × 10−15, Al2O3 = 1.45 × 10−14, FeO = 9.70 × 10−15, MgO = 3.27 × 10−14. The thicknesses 2h (cm) of the zones of diffusion-controlled transfer of various components were estimated as follows: TiO2 = 6.6, FeO = 7.8, K2O = 8.5, Al2O3 = 9.5, Fe2O3 = 14.1, MgO = 14.3. The average calculated velocity of the interface is V = 4.2 × 10−6 cm/year. The relative mobility of major oxides was determined as follows: MgO > Fe2O3 > Al2O3 > K2O > FeO > TiO2.

The importance of iron speciation (Fe+2/Fe+3) in determining mineral assemblages: an example from the high‐grade aluminous metapelites of southeastern Madagascar

AbstractMetapelitic granulites from the Anosyen domain of southeastern Madagascar are exposed in three intercalated formations: the Amparihy, Bakika and Ihosy formations. Although mineralogically distinct from each other, the rocks from these formations show very similar bulk‐rock compositions when measured on a FeT basis. The preserved mineral assemblages thus do not reflect differences in the ratios of the main rock‐forming oxides (i.e. Al2O3:FeT:MgO), but instead reflect variations in the pre‐metamorphic oxidation state of the protolith rocks. These differences in oxidation state are manifested via differences in iron speciation – either Fe+2 or Fe+3. The relatively reduced rocks of the Amparihy Formation preserve the assemblage bi–sp–sill–g–cd, which contrasts markedly with the mostly garnet and spinel‐absent bi–cd–sill–mt assemblages preserved in the strongly oxidized rocks of the Ihosy Formation. Compositionally intermediate rocks of the Bakika Formation are garnet bearing, but sillimanite‐absent, and contain the assemblage sp–g–cd–mag. Modelling of these rocks in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system suggests that they evolved along a heating and cooling P–T path with only limited decompression accompanying cooling on the retrograde path. Peak temperatures and pressures of ∼880–920 °C and 6–6.5 kbar are inferred for the majority of the Anosyen domain, with slightly lower peak temperatures (∼840 °C) estimated in the extreme northwest of the area. The high‐temperature and relatively low‐pressure nature of metamorphism suggests high geothermal gradients existed during orogenesis, which in southern Madagascar is related to the amalgamation of Gondwana (580–520 Ma). Although metamorphic temperatures may have been augmented via thermal advection from the emplacement of the syn‐ to post‐tectonic Ambalavao suite, the high geothermal gradients nevertheless suggest thin and consequently hot lithosphere existed prior to orogenesis.

A year in the life of an aluminous metapelite xenolith—The role of heating rates, reaction overstep, H2O retention and melt loss

Xenoliths of aluminous metapelite within the Platreef magmatic rocks of the Bushveld Complex, South Africa, are mineralogically and texturally zoned, with coarse-grained margins rich in acicular corundum, spinel and feldspar and cores rich in finer-grained aluminosilicate and cordierite. Xenoliths exhibiting remarkably similar features occur within other intrusions, suggesting a common origin. Using a single 3m wide xenolith as a case study, a model is proposed to explain their petrogenesis. Mineral equilibria calculations in the NCKFMASHTO system show that the thermal stability of the solid phases, in particular corundum, is highly sensitive to the quantity of H2O retained in the protolith. Simple thermal considerations suggest the outermost 10cm of the xenolith began to melt within a few hours following incorporation of the xenolith into the hot mafic/ultramafic magmas. Average heating rates of around 104°C/year were sufficiently fast that the stability of low-grade hydrous phases within the protolith was overstepped by several hundred degrees, leading to retention of some or all of the structurally bound H2O to suprasolidus conditions. As a result, marginal rocks developed peritectic corundum and spinel with H2O-saturated melt, now preserved as hornfels with a (micro) diatexitic morphology. In the core of the xenolith, temperatures increased much more slowly, enabling progressive metamorphism by continual prograde reaction and loss to the margins of H2O liberated by subsolidus dehydration reactions that consumed the low-grade hydrous phases before the rocks began to melt a week or more later. Thereafter, the preservation of fine-scale bedding in xenolith cores suggests that melt was lost efficiently as it was produced. Lower H2O contents extend the upper thermal stability of aluminosilicate and cordierite to much higher temperatures, an effect exacerbated by the effects of melt loss. Whereas corundum growth occurred at the margins of the xenolith at temperatures below 800°C, it is not predicted in the core until temperatures in excess of 1000°C. The mineralogy and textures within the xenolith are consistent with a single-stage process involving equilibrium metamorphism and partial melting.

Petrogenetic modelling of strongly residual metapelitic xenoliths within the southern Platreef, Bushveld Complex, South Africa

AbstractXenoliths of quartz‐absent Fe‐rich aluminous metapelite are common within the platinum group element‐rich mafic/ultramafic magmatic rocks of the Platreef. Relative to well‐characterized protoliths, the xenoliths are strongly depleted in K2O and H2O, and have lost a substantial amount of melt (>50 vol.%). Mineral equilibria calculations in the NCKFMASHTO system yield results that are consistent with observations in natural samples. Lower‐grade rocks that lack staurolite constrain peak pressures to ∼2.5 kbar in the southern Platreef. Smaller xenoliths and the margins of larger xenoliths comprise micro‐diatexite rich in coarse acicular corundum and spinel, which record evidence for the metastable persistence of lower‐grade hydrous phases and rapid melting consequent on a temperature overstep of several hundred degrees following their incorporation in the mafic/ultramafic magmas. In the cores of larger xenoliths, temperatures increased more slowly enabling progressive metamorphism by continuous prograde equilibration and the loss of H2O by subsolidus dehydration; the H2O migrated to xenolith margins where it may have promoted increased melting. According to variations in the original compositional layering, layers became aluminosilicate‐ and/or cordierite‐rich, commonly with spinel but only rarely with corundum. The differing mineralogical and microstructural evolution of the xenoliths depends on heating rates (governed by their size and, therefore, proximity to the Platreef magmas) and the pre‐intrusive metamorphic grade of the protoliths. The presence or absence of certain phases, particularly corundum, is strongly influenced by the degree of metastable retention of lower‐grade hydrates in otherwise identical protolith bulk compositions. The preservation of fine‐scale compositional layering that is inferred to be relict bedding in xenolith cores implies that melt loss by compaction was extremely efficient.

Evolution of the REE mineralogy in HP–LT metapelites of the Sebtide complex, Rif, Morocco: Monazite stability and geochronology

REE mineralogy was characterized at the micrometer scale (using scanning electron microscopy) in the four tectono-metamorphic units of the Beni Mzala window, Sebtide Complex, Internal Rif, Morocco, which sample a HP-LT metamorphic gradient from subgreenschist to blueschist/eclogite facies. These tectonic units are composed of garnet-free metapelites (and associated synmetamorphic veins) from the same homogeneous aluminium-rich and calcium-poor protolith. In that context, mineralogical differences between units are assumed to result from contrasted pressure and temperature histories. Light REE-bearing Al-phosphates of the crandallite group with florencite-rich composition and variable goyazite content are the dominant LREE minerals in the lowest-grade units, Tizgarine (300 °C, 3 kbar as peak conditions) and Boquete de Anjera (350 °C, 10 kbar). In the latter unit, metamorphic florencite is associated with MREE-rich monazite. In the schist of the highest-grade units, Beni Mzala 2 (420–450 °C, 12 kbar) and Beni Mzala 1 (550 °C, 16 kbar), associations of allanite-rich epidote and synchisite (a LREE-fluorocarbonate) are found whereas florencite and monazite are no longer observed. At the wall of the quartz–kyanite veins, retrograde monazite-(Ce) with grain size of 20 to 50 μm is the only LREE-bearing mineral. Th–Pb and U–Pb SIMS data on some of these grains yield ages of 21.3 ± 1.7 Ma and 20.9 ± 2.1 Ma, respectively, consistent with previous K–Ar ages obtained on retrograde clay–mica mixtures in rocks from the same locality. The identification of a stability field for monazite in high-pressure aluminous metapelites with an upper thermal-limit below 450 °C as well as the derivation of meaningful U–Pb and Th–Pb SIMS ages demonstrates the monazite potential for dating HP-LT metapelites. However, since, under these conditions, monazite growth occurs below its admitted closure-temperature, these ages must be interpreted on the basis of monazite textural relationships.

Spatially‐focussed melt formation in aluminous metapelites from Broken Hill, Australia

AbstractLarge garnet poikiloblasts hosted by leucosome in metapelitic gneiss from Broken Hill reflect complex mineral–melt relationships. The spatial relationship between the leucosomes and the garnet poikiloblasts implies that the growth of garnet was strongly linked to the production of melt. The apparent difficulty of garnet to nucleate a large number of grains during the prograde breakdown of coexisting biotite and sillimanite led to the spatial focussing of melting reactions around the few garnet nuclei that formed. Continued reaction of biotite and sillimanite required diffusion of elements from where minerals were reacting to sites of garnet growth. This diffusion was driven by chemical potential gradients between garnet‐bearing and garnet‐absent parts of the rock. As a consequence, melt and peritectic K‐feldspar also preferentially formed around the garnet. The diffusion of elements led to the chemical partitioning of the rock within an overall context in which equilibrium may have been approached. Thus, the garnet‐bearing leucosomes record in situ melt formation around garnet porphyroblasts rather than centimetre‐scale physical melt migration and segregation. The near complete preservation of the high‐grade assemblages in the mesosome and leucosome is consistent with substantial melt loss. Interconnected networks between garnet‐rich leucosomes provide the most likely pathway for melt migration. Decimetre‐scale, coarse‐grained, garnet‐poor leucosomes may represent areas of melt flux through a large‐scale melt transfer network.

Prograde Metamorphic Assemblage Evolution during Partial Melting of Metasedimentary Rocks at Low Pressures: Migmatites from Mt Stafford, Central Australia

The Mt Stafford area in central Australia preserves a lowpressure greenschistto granulite-facies regional aureole. The metasedimentary sequence has been divided into five zones from greenschist (Zone 1) to granulite facies (Zone 4) and a zone of hybrid diatexite formed from the introduction of granitic magma into the high-grade migmatites (Zone 5). Melt production was dominated by a series of multivariant biotite breakdown reactions, not the univariant reactions suggested by previous studies. Although the three main metasedimentary rock types produced similar amounts of melt at the highest grades, their melt production histories differed markedly as a function of temperature. Aluminous metapelites produced more melt at lower temperatures (Zones 2 and 3), whereas metapsammite and cordierite granofels experienced an additional major melt-producing step at higher temperatures (upper Zone 3 and Zone 4). This melting step involved the breakdown of biotite to produce garnet, K-feldspar and melt, and in some rocks the production of orthopyroxene. Melt production in Zone 4 exceeded 25 mol %, resulting in the formation of in situ diatexites. Complex relationships involving aluminosilicate porphyroblasts resulted in the breakdown of biotite and aluminosilicate being drawn out over a wide temperature range, from subsolidus conditions to temperatures close to 750 C. Initially, much of the melting developed around the aluminosilicate porphyroblasts during the breakdown of coexisting biotite, aluminosilicate and quartz. However, much of the rock was chemically isolated from the porphyroblasts and could not react to produce melt. As temperatures rose, the presence of the large isolated aluminosilicate porphyroblasts controlled the spatial development of quartzabsent, spinel-present compositional domains, the formation of spinel being governed by the silica-undersaturated breakdown of coexisting biotite and aluminosilicate.

Forthcoming papers in the Journal of Metamorphic Geology

The loss of a metamorphic fluid via the partitioning of H2O into silicate melt at higher metamorphic grade implies that, in the absence of open system behaviour of melt, the amount of H2O contained within rocks remains constant at temperatures above the solidus. Thus, granulite facies rocks, composed of predominantly anhydrous minerals and a hydrous silicate melt should undergo considerable retrogression to hydrous upper amphibolite facies assemblages on cooling as the melt crystallizes and releases its H2O. The common occurrence of weakly retrogressed granulite facies assemblages is consistent with substantial melt loss from the majority of granulite facies rocks. Phase diagram modelling of the effects of melt loss in hypothetical aluminous and subaluminous metapelitic compositions shows that the amount of melt that has to be removed from a rock to preserve a granulite facies assemblage varies markedly with rock composition, the number of partial melt loss events and the P–T conditions at which melt loss occurs. In an aluminous metapelite, the removal of nearly all of the melt at temperatures above the breakdown of biotite is required for the preservation of the peak mineral assemblage. In contrast, the proportion of melt loss required to preserve peak assemblages in a subaluminous metapelite is close to half that required for the aluminous metapelite. Thus, if a given proportion of melt is removed from a sequence of metapelitic granulites of varying composition, the degree of preservation of the peak metamorphic assemblage may vary widely.

A petrologic and fluid inclusion study of charnockites from the Lützow-Holm Bay region, East Antarctica: Evidence for fluid-rich metamorphism in the lower crust

The Proterozoic terrain of Lützow-Holm Bay in East Antarctica comprises an amphibolite- to granulite-facies progression from north to south, exposing an oblique section of the lower crust. The granulite terrain is represented dominantly by orthopyroxene-bearing anhydrous granulites (charnockites), garnet- and biotite-bearing gneisses (leptynites), and garnet, sillimanite and graphite bearing aluminous metapelites (khondalites). Thus it is petrographically identical with its Gondwanian counterparts in Sri Lanka and southern India. Charnockite formation is characterized by the consumption of garnet, biotite and quartz to produce orthopyroxene, alkali feldspar and plagioclase under isothermal conditions and at reduced water activities. A number of mineral-reaction textures, including orthopyroxene-plagioclase symplectites around clinopyroxene relics in intermediate charnockites and spinel rims between adjacent garnet and sillimanite in khondalites, suggest near-isothermal conditions of equilibration. Mineral phase equilibria barometry traverse across the granulite terrain indicates an increase in pressure from c. 5 kbar in the north to c. 8–9 kbar in the south. The temperature gradient of c. 100°C is less pronounced. Fluid inclusion studies indicate abundant CO 2 trapped within the charnockite minerals. The systematic variation in fluid densities with a garnet→feldspar→quartz succession within individual samples argues against post-metamorphic fluid infiltration and fits a scenario of mineral equilibration during near-isothermal uplift following a clockwise pressure-temperature-time path. Along the pressure gradient, the fluid densities show an increase from c. 0.98 to c. 1.10 g/cm 3. This strengthens other evidence of a synmetamorphic entrapment of CO 2. Preliminary carbon isotope measurements of CO 2 extracted from inclusions yield δ 13C values of c. −5 to −6‰, suggesting a sub-continental mantle source. The Gondwanan Lützow-Holm Bay terrain had undergone substantial extension during the late Proterozoic, with displacement along megafaults and emplacement of several alkaline-subalkaline granite and syenite plutons, and also K-basalts. It is therefore envisaged that the abundant CO 2 in the charnockites could have been derived from an attenuated continental lithosphere, and transferred through magmatic conduits.

Metamorphic evolution of aluminous granulites from Labwor Hills, Uganda

Sapphirine-cordierite-quartz and spinel-cordierite-quartz form relic assemblages of probable Archaean age in Fe-rich aluminous metapelites from Labwor Hills, Uganda, and reflect an unusually high temperature metamorphism (∼1,000° C) at pressures in the vicinity of 7–9 kbars and a(O2) near the magnetite-hematite buffer. Subsequent reaction textures include the replacement of spinel and cordierite by sillimanite and hypersthene and formation of sapphirine-hypersthene-K-feldspar-quartz symplectites which are interpreted as pseudomorphs after osumilite. A petrogenetic grid appropriate to these assemblages suggests these reaction textures may be due to cooling at constant or increasing pressure and constant a(O2), or decreasing a(O2) at constant temperature and pressure. The former interpretation is supported by the coexistence of ilmenohematite and magnetite during the development of the reaction textures, and by the comparatively low Al2O3-contents of secondary hypersthene. This pressure-temperature path implies that: (1) metamorphism occurred at deep levels within normal thickness crust, probably less than 40–45 km thick, due to an extreme thermal perturbation induced either by emplacement of mantle-derived magmas or by thinning of the subcontinental lithosphere in an extensional tectonic regime, (2) the excavation and surface exposure of the granulites is due to a subsequent, postgranulite facies metamorphism, crustal thickening most probably involving their incorporation into an allochthonous upper crustal thrust sheet during the formation of the Mozambique foldbelt.

Loss of metals from pelites during regional metamorphism

In aluminous metapelites the ratio H2O+/K2O decreases with increasing metamorphic grade and degree of reaction. This ratio is a very practical indicator for the progress of the mineral reconstitution during progressive metamorphism. With decreasing values of the ratio H2O+/ K2O the Cu concentration and the following element ratios also decrease either continuously or in stepwise fashion: Tl/K2O, Ba/K2O, Pb/K2O, Bi/K2O, Hg/K2O, Sr/Na2O, Zn/(Fe2++Mg), Cd/(Fe2++Mg); Rb/K2O remains approximately constant. In the aluminous metapelites of the Damara Orogen in Namibia the following losses occur between the biotite isograd and anatexis: 61% Cu, 20% Tl, 34% Ba, 59% Pb, 86% Bi, 46% Hg, 30% Sr, 25% Zn, 31% Cd. Thus the potential of regional metamorphism to form hydrothermal deposits in the low grade environment should not be neglected.