Retrograde Reaction Textures Research Articles

Formation of low-pressure reaction textures during near-isothermal exhumation of hot orogenic crust (Bohemian Massif, Austria).

Two types of aluminous paragneiss from the Loosdorf complex (Bohemian Massif, NE Austria) contain coarse-grained granulite assemblages and retrograde reaction textures that are investigated to constrain the post-peak history of the Gföhl unit in the southern Bohemian Massif. Both types have a peak assemblage garnet-biotite-sillimanite-plagioclase-K-feldspar-quartz-granitic melt ±kyanite ±ilmenite ±rutile, recording peak metamorphic conditions of 0.9-1.1 GPa and 780-820°Cestimated by isochemical phase equilibrium modelling. The first sample type (Ysper paragneiss) developed (i) cordierite coronae around garnet and (ii) cordierite-spinel and cordierite-quartz reaction textures at former garnet-sillimanite interfaces. Calculated chemical potential relationships indicate that the textures formed in the course of a post-peak near-isothermal decompression path reaching 0.4 GPa. Texture formation follows a two-step process. Initially, cordierite coronae grow between garnet and sillimanite. As these coronae thicken, they facilitate the development of local compositional domains, leading to the formation of cordierite-spinel and cordierite-quartz symplectites. The second sample type (Pielach paragneiss) exhibits only discontinuous cordierite coronae around garnet porphyroblasts but lacks symplectites. The formation of cordierite there also indicates near-isothermal decompression to 0.4-0.5 GPa and 750-800°C. This relatively hot decompression path is explained by the contemporaneous exhumation of a large HP-UHT granulite body now underlying the Loosdorf complex. The timing of regional metamorphism in the granulites and the southern Bohemian Massif in general is well constrained and has its peak at 340Ma. Monazite from Loosdorf paragneiss samples yield a slightly younger age of 335Ma. Although the ages overlap within error, they are interpreted to reflect near-isothermal decompression and exhumation resulting in the formation of the observed reaction textures.

Petrology and geochronology of ‘muscovite age standard’ B4M

Abstract Muscovite B4M, distributed in 1961 as an age standard, was ground under ethanol. Five grain size fractions were obtained and characterized by X-ray diffraction. They display a mixing trend between a phengitic (enriched in the fraction <0.2 µm) and a muscovitic component (predominant in the fraction >20 µm). High-pressure phengite is preserved as a relict in retrograde muscovite. Electron microprobe analyses of the distributed mineral separate reveal at least four white mica populations based on Si, Al, Mg, Na, Fe and F. Rb/K ratios vary by one order of magnitude. Rb–Sr analyses link the mineralogical heterogeneity to variable Rb/Sr and 87 Sr/ 86 Sr ratios. The grain size fractions define no internal isochron. Relict fine-grained phengite gives older ages than coarse-grained retrograde greenschist facies muscovite. The inverse grain size–age relationship also characterizes 39 Ar/ 40 Ar analyses. Cl/K anticorrelates with step ages: Cl-rich coarse muscovite is younger than Cl-poor fine relict phengite. Sr and Ar preserve a similar isotopic inheritance despite peak metamorphism reaching 635±20 °C. A suitable mineral standard requires that its petrological equilibrium first be demonstrated. Relicts and retrograde reaction textures are a guarantee of isotopic disequilibrium and heterogeneous ages within single crystal at the micrometre scale. Supplementary material: Electron microprobe results on two grain mounts of the unprocessed B4M separate as distributed and on a whole-rock thin section of Brione gneiss from the teaching collection of the Universität Bern are available at: www.geolsoc.org.uk/SUP18590 .

Geochemical signatures of metasedimentary rocks of high-pressure granulite facies and their relation with partial melting: Carvalhos Klippe, Southern Brasília Belt, Brazil

Abstract High-grade metasedimentary rocks can preserve geochemical signatures of their sedimentary protolith if significant melt extraction did not occur. Retrograde reaction textures provide the main evidence for trapped melt in the rock fabrics. Carvalhos Klippe rocks in Southern Brasilia Orogen, Brazil, present a typical high-pressure granulite assemblage with evidence of mica breakdown partial melting (Ky + Grt + Kfs ± Bt ± Rt). The metamorphic peak temperatures obtained by Zr-in-Rt and ternary feldspar geothermometers are between 850 °C and 900 °C. The GASP baric peak pressure obtained using grossular rich garnet core is 16 kbar. Retrograde reaction textures in which the garnet crystals are partially to totally replaced by Bt + Qtz ± Fsp intergrowths are very common in the Carvalhos Klippe rocks. These reactions are interpreted as a result of interactions between residual phases and trapped melt during the retrograde path. In the present study the geochemical signatures of three groups of Carvalhos Klippe metasedimentary rocks are analysed. Despite the high metamorphic grade these three groups show well-defined geochemical features and their REE patterns are similar to average compositions of post-Archean sedimentary rocks (PAAS, NASC). The high-pressure granulite facies Grt-Bt-Pl gneisses with immature arenite (wacke, arkose or lithic-arenite) geochemical signatures present in the Carvalhos Klippe are compared to similar rocks in amphibolite facies from the same tectonic framework (Andrelândia Nappe System). The similar geochemical signatures between Grt-Bt-Pl gneisses metamorphosed in high-pressure granulite facies and Grt-Bt-Pl-Qtz schists from the Andrelândia and Liberdade Nappes, with minimal to absent melting conditions, are suggestive of low rates of melt extraction in these high-grade rocks. The rocks with pelitic compositions most likely had higher melt extraction and even under such circumstances nevertheless tend to show REE patterns similar to average compositions of post-Archean sedimentary rocks (PAAS, NASC).

Trace element composition of rutile and the application of Zr-in-rutile thermometry to UHT metamorphism (Epupa Complex, NW Namibia)

The trace element composition of rutile and Zr-in-rutile temperatures were determined for ultra-high temperature (UHT) metamorphic rocks from the Epembe Unit of the Epupa Complex in NW-Namibia. Ortho- and paragneisses record Mesoproterozoic peak metamorphic UHT conditions of 970 ± 40 °C at 9.5 ± 2 kbar, as estimated from conventional thermobarometry and constraints from pseudosection modeling. Rutile exhibits superchondritic concentrations of V, Zr, Nb, Hf, Ta, Th and U while rare earth elements (REE) are far less enriched. Zr and Hf correlate positively with two distinct trends. Nb and Ta as well as Cr and V show a positive correlation although with less clear trends. Only Hf correlates with Zr, suggesting a decoupling of Zr and Hf from the other high field-strength elements (HFSE) probably during retrogression. In general, the non-homogeneous HFSE distribution in rutile indicates that equilibrium trace element distribution achieved during UHT peak metamorphic conditions was either almost completely erased or had never been achieved as a common feature of all rutile grains. The retrograde metamorphic evolution of the UHT rocks is interpreted to be responsible for trace element redistribution under equilibrium conditions restricted to small domains. This has affected the trace element composition of the rutile grains investigated here thereby disturbing their UHT signature, which may cause problems for provenance studies involving such ‘disturbed’ grains. A systematic comparison of all available Zr-in-rutile thermometer calibrations shows that beside of one, all give similar temperature estimates for the studied samples. No systematic differences regarding the Zr content were observed between rutile grains in different textural positions (i.e. matrix grains, those shielded by host minerals or post-peak grains). However, calculations revealed a broad range of temperatures between < 400 °C and > 1000 °C. The large spread of calculated temperatures is interpreted to result from intergrain diffusion and trace element exchange by fluid-mediated recrystallization during the retrograde metamorphic evolution. This interpretation is supported by the presence of extensively formed retrograde reaction textures involving hydrous phases such as cordierite and biotite in the studied samples. In addition, tiny (≤ 5 μm) Zr-silica-rich phase separations which occur either homogeneously or heterogeneously distributed in single rutile grains may cause intergrain Zr variations.

On the interpretation of retrograde reaction textures in granulite facies rocks

AbstractRetrograde reaction textures, such as partial pseudomorphs, coronae and symplectic coronae are a relatively common feature of granulite facies rocks and represent evidence of change of mineral modes or assemblages. Thus, such reaction textures represent one of the more potentially useful indicators of retrograde P–T evolutions. However, the interpretation of such textures requires the simultaneous consideration of the formation of the texture itself in terms of the spatial relationships in a rock, and of the changes in P–T conditions that drove the development of the reaction texture. Calculated phase diagrams provide a robust framework in which to consider reaction textures, especially via the combined use of pseudosections to understand the P–T changes and μ–μ diagrams to understand the spatial arrangement of the minerals. Consideration of the chemical potential gradients that may exist across a given texture means that elucidating P–T conditions and P–T paths must be done with care. The evolution of a given reaction texture may be complex with different layers or individual minerals in a texture forming at different times. Thus, the approach of writing a reaction to represent the inferred local reactant and product phases and simply ascribing such reaction textures to crossing equivalent univariant reactions from petrogenetic grids or traversing through fields in pseudosections without due consideration of chemical potential gradients may result in an incorrect interpretation. Furthermore, the potential for a non‐continuous metamorphic evolution of a rock by polymetamorphism or some other hiatus in the textural/mineral‐assemblage evolution creates further uncertainty in the interpretation of a texture in terms of its reaction history and P–T path. A common interpretation of coronae and symplectic coronae, on the basis of the preservation of the apparent reactants, is that they represent reactions that failed to go to completion for various kinetic reasons. While this may be true in some cases, many other textures may actually represent reaction having gone to completion with, for example, silicate melt being the phase completely consumed. Furthermore, even where a reaction has failed to go to completion, the possible reasons for this may be different for different rocks.

Geodynamic evolution of Mt. Riiser-Larsen, Napier Complex, East Antarctica, with reference to the UHT mineral associations and their reaction relations

Abstract Mt. Riiser-Larsen is the largest outcrop in the Archaean–early Proterozoic Napier Complex, East Antarctica. The area is structurally divided into the Main and the Western Blocks by the subvertical Riiser-Larsen Main Shear Zone (RLMSZ) of about 200 m width composed of mylonite and pseudotachylite. Mineral parageneses including sapphirine+quartz and osumilite, diagnostic of ultrahigh-temperature (UHT) metamorphism, are found in Mg-rich aluminous, siliceous and quartzo-feldspathic gneiss layers in both the Main and the Western Blocks of the Mt. Riiser-Larsen area. Some of the sapphirine–quartz associations are accompanied by retrograde reaction textures, which include growth of cordierite and/or garnet between sapphirine and quartz in the Main Block, and of orthopyroxene+sillimanite in the Western Block. These textures indicate the reaction (1) sapphirine + quartz ( + orthopyroxene ) = cordierite and (2) sapphirine + quartz ( + orthopyroxene ) = garnet in the Main Block and (3) sapphirine + quartz = orthopyroxene + sillimanite in the Western Block. Phase equilibria and P – T pseudosections for sapphirine+quartz-bearing associations suggest that these three reactions took place during a temperature drop from 1100 °C to 1000 °C at pressures of 0.6–0.8 GPa in the Main Block and 0.8–0.9 GPa in the Western Block. The geological structure and distribution of the UHT rocks provide an insight into the vertical extent of the&gt;1000 °C UHT metamorphic zone: a minimum thickness of 4–5 km of the UHT-metamorphosed layers, which become deeper towards the west in the Main Block. The Western Block represents a c. 0.1–0.3 GPa ( c. 3–10 km) deeper structural level than the Main Block. In addition to the extent of the horizontal distribution of UHT metamorphism in the Napier Complex, our results on the vertical component provide new constraints for modelling the heat source and tectonic process of the unusually high-temperature regional metamorphism in the late Archaean–early Proterozoic. Electron microprobe monazite U–Th–Pb dating for hydrated and mylonitized sapphirine–quartz gneiss gave a wide spectrum of monazite age distribution between 2300 and 800 Ma, suggesting the tectonic uplift and juxtaposition of the two blocks in the Mt. Riiser-Larsen area later than the mid–late Proterozoic.

Mesoproterozoic Reworking of Palaeoproterozoic Ultrahigh-temperature Granulites in the Central Indian Tectonic Zone and its Implications

In the southern periphery of the Sausar Mobile Belt (SMB), the southern component of the Central Indian Tectonic Zone (CITZ), a suite of felsic and aluminous granulites, intruded by gabbro, noritic gabbro, norite and orthopyroxenite, records the polymetamorphic evolution of the CITZ. Using sequences of prograde, peak and retrograde reaction textures, mineral chemistry, geothermobarometric results and petrogenetic grid considerations from the felsic and the aluminous granulites and applying metamorphosed mafic dyke markers and geochronological constraints, two temporally unrelated granulite-facies tectonothermal events of Pre-Grenvillian age have been established. The first event caused Ultrahigh-temperature (UHT) metamorphism (M1) (T ∼950°C) at relatively deeper crustal levels (P ∼9 kbar) and a subsequent post-peak near-isobaric cooling P–T history (M2). M1 caused pervasive biotite-dehydration melting, producing garnet–orthopyroxene and garnet–rutile and sapphirine–spinel-bearing incongruent solid assemblages in felsic and aluminous granulites, respectively. During M2, garnet–corundum and later spinel–sillimanite–biotite assemblages were produced by reacting sapphirine–spinel–sillimanite and rehydration of garnet–corundum assemblages, respectively. Applying Electron Microprobe (EMP) dating techniques to monazites included in M1 garnet or occurring in low-strain domains in the felsic granulites, the UHT metamorphism is dated at 2040–2090 Ma. Based on the deep crustal heating–cooling P–T trajectory, the authors infer an overall counterclockwise P–T path for this UHT event. During the second granulite event, the Palaeoproterozoic granulites experienced crustal attenuation to ∼6•4 kbar at T ∼675°C during M3 and subsequent near-isothermal loading to ∼8 kbar during M4. In the felsic granulites, the former is marked by decomposition of M1 garnet to orthopyroxene–plagioclase symplectites. During M4, there was renewed growth of garnet–quartz symplectites in the felsic granulites, replacing the M3 mineral assemblage and also the appearance of coronal garnet–quartz–clinopyroxene assemblages in metamorphosed mafic dykes. Using monazites from metamorphic overgrowths and metamorphic recrystallization domains from the felsic granulite, the M4 metamorphism is dated at 1525–1450 Ma. Using geochronological and metamorphic constraints, the authors interpret the M3–M4 stages to be part of the same Mesoproterozoic tectonothermal event. The result provides the first documentation of UHT metamorphism and Palaeo- and Mesoproterozoic metamorphic processes in the CITZ. On a broader scale, the findings are also consistent with the current prediction that isobarically cooled granulites require a separate orogeny for their exhumation.

Eclogites of the Snowbird tectonic zone: petrological and U-Pb geochronological evidence for Paleoproterozoic high-pressure metamorphism in the western Canadian Shield

Eclogite occurs within the southern domain of the East Athabasca mylonite triangle in northern Saskatchewan. Situated at the boundary between the Archean Rae and Hearne Provinces of the western Canadian Shield, the East Athabasca mylonite triangle is a fundamental exposure of the ~3,000-km-long Snowbird tectonic zone. The eclogite occurs in association with a variety of lower crustal high-pressure granulites that record a complex metamorphic history from 2.6 to 1.9 Ga. Temperatures of the eclogite facies metamorphism are constrained by garnet-clinopyroxene exchange thermometry at 920–1,000 °C. Minimum pressure conditions are recorded by the jadeite+quartz=albite geobarometer at 1.8–2.0 GPa. A near-isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene-plagioclase and pargasite-plagioclase. U-Pb IDTIMS zircon geochronology of the eclogite yields a weighted mean 207Pb/206Pb date of 1,904.0±0.3 Ma, which we interpret as the time of peak eclogite facies metamorphism. SHRIMP in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207Pb/206Pb date of 1,905±19 Ma. Inclusion suites of high-pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism. Zircon from one eclogite sample has older cores that are 2.54 Ga, which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith. U-Pb rutile data indicate slow cooling at ~1°C/Ma below ~500 °C from 1.88 to 1.85 Ga. The formation and exhumation of the eclogites at ca.1.9 Ga has important implications for the tectonic significance of the Snowbird tectonic zone during the Paleoproterozoic. The eclogites described here are consistent with transport of continental crust to mantle depths during the Paleoproterozoic, followed by rapid buoyancy-driven exhumation to normal lower crustal depths.

P-T Evolution of a Variscan Lower-Crustal Segment: a Study of Granulites from the Schwarzwald, Germany

Pressure–temperature–time (P–T–t) paths of orogenic granulites provide important information on the thermal and chemical structure of the lower continental crust through time, and constraints on tectonic processes. We present the first detailed petrological investigation of granulites from the Variscan Schwarzwald. Pelitic granulites from the Central Schwarzwald Gneiss Complex (CSGC) are characterized by the peak assemblage garnet + rutile + kyanite + antiperthite ± quartz. Felsic to intermediate granulites from the Southern Schwarzwald Gneiss Complex (SSGC) exhibit different peak assemblages with clinopyroxene, orthopyroxene, ternary feldspar, garnet, quartz and sillimanite, and manifold retrograde reaction textures. Peak P–T conditions were calculated by two-feldspar thermometry, garnet–orthopyroxene thermometry and various geobarometers. Minimum estimates for peak conditions are 950–1010°C and 1·4–1·8 GPa for the granulites of the CSGC, which followed a clockwiseP–T path. The retrograde path is characterized by initial isothermal decompression, associated with partial melting, followed by isobaric cooling. Peak conditions for the SSGC are 1015°C and 1·5 GPa (minimum temperature, maximum pressure). No prograde relics are preserved, and isothermal decompression was less pronounced than in the CSGC. Other Variscan HP–HT granulites from Central Europe show similar lithologies, equilibration temperatures and ages (340–335 Ma). The heat for widespread high-temperature metamorphism in the Variscan lower crust could have been supplied by repeated intrusion of subduction-related basic magmas. Rapid, near-isothermal decompression of the granulites may have been facilitated by considerable volumes of partial melt and by orogenic extension.

Fluid‐rock history of granulite facies humite‐marbles from Ambasamudram, southern India.

AbstractAn extensive humite‐bearing marble horizon within a supracrustal sequence at Ambasamudram, southern India, was studied using petrological and stable isotopic techniques to define its metamorphic history and fluid characteristics. At peak metamorphic temperatures of 775±73°C, based on calcite‐graphite carbon isotope thermometry, the mineral assemblages suggest layer‐by‐layer control of fluid compositions. Clinohumite + calcite‐bearing assemblages suggest XCO2 &lt; 0.4 (at 700°C and 5 kbar), calcite + forsterite + K‐feldspar‐bearing assemblages suggest XCO2&gt;0.9 (at 790°C); and local wollastonite + scapolite + grossular‐bearing zones formed at XCO2 of c. 0.3. Retrograde reaction textures such as scapolite + quartz symplectites after feldspar and calcite and replacement of dolomite + diopside or tremolite+dolomite after calcite+forsterite or calcite+clinohumite are indicative of retrogression under high XCO2 conditions. Calcite preserves late Proterozoic carbon and oxygen isotopic signatures and the marble lacks evidence for extensive retrograde fluid infiltration, while during prograde metamorphism the possible infiltration of aqueous fluids did not produce significant isotopic resetting. Isotopic zonation of calcite and graphite grains was likely produced by localized CO2 fluid infiltration during retrogression. Contrary to the widespread occurrence of humite‐marbles related to retrograde aqueous fluid infiltration, the Ambasamudram humite‐marbles record a prograde‐to‐peak metamorphic humite formation and retrogression under conditions of low XH2O.

Metapelitic Migmatites From Pangidi Granulite Complex, Eastern Ghats Belt, India: Petrological Constraints on Crustal Melting and Tectonic Exhumation

Metapelitic migmatites in the Pangidi Granulite Complex, Eastern Ghats belt preserve a range of high-grade mineral assemblages that vary with bulk composition. Reaction textures preserved in the migmatites indicate that the crystallized granitic melt was formed in situ by biotite dehydration-melting reactions. Decompression occurred during an episode of partial melting and melt crystallization, and was synchronous with exhumation of migmatites along shear zones. Retrograde reaction textures and the calculated positions of the mineral reactions and discrete P-T points obtained by thermobarometry reflect ∼3 kbar (11 km) of exhumation from near peak P-T conditions of ∼9 kbar and 950°C. Decompression has been traditionally ascribed to be the cause of dehydration melting in many orogens, but the bulk of melting reactions in Pangidi were ultimately driven by elevated temperatures. Simple pressure release during exhumation did not force melting reactions, instead as the present study reveals, it is more likely that the presence of melt triggered deformation in the migmatites and facilitated exhumation.

Effective bulk composition changes due to cooling: a model predicting complexities in retrograde reaction textures

Diffusive processes are a strong function of temperature. Thus, during cooling of rocks, mineral grains may develop zoning profiles as successively larger parts of the grain “close” to the diffusive exchange with the rock. One of the consequences of this process is that, during cooling, successively larger parts of zoned minerals (depending on grain size) are effectively removed from the reacting part of the rock volume. Thus, the effective bulk composition of metamorphic rocks changes during cooling and the rate of its change will be a function of grain size. Because the sequence of metamorphic reactions seen by a given rock is a strong function of its bulk composition, this process may have the consequence that two rocks of identical overall bulk composition, but of different grain size, may experience a different sequence of reactions. Qualitatively identical peak paragenesis may therefore react to form qualitatively different retrograde reaction textures. The model is applied to examples in the pelitic system. There, garnet is usually the slowest diffusing phase developing zoning profiles during cooling and the effective removal of garnet from the reacting rock volume will cause changes of the effective bulk composition. It is shown that, during cooling of pelitic rocks from amphibolite facies conditions, typical aluminous peak parageneses of garnet-muscovite-kyanite ± biotite may react to form either staurolite, chlorite or muscovite (or different combinations thereof), depending on grain size. During cooling from the granulite facies, aluminous peak parageneses of garnet-cordierite-sillimanite may form biotite, either on the expense of cordierite or garnet, also depending on grain size. The two examples are illustrated with a series of reaction textures reported for amphibolite and granulite terrains in the literature.

Calc-silicate assemblages from the Kerala Khondalite Belt, southern India: implications for pressure-temperature-fluid histories

This paper reports several new localities of wollastonite- and scapolite-bearing calc-silicate assemblages from the granulite-facies supracrustal Kerala Khondalite Belt (KKB), southern India. Based on mineralogy, these calc-silicate rocks are classified into four types: Type I, lacking wollastonite and grossular; Type II, wollastonite-bearing but grossular-absent; Type III, wollastonite- and grossular-bearing; and Type IV, dolomitic marbles. Detailed petrographic studies reveal a variety of reaction textures overprinting the polygonal granoblastic peak metamorphic assemblages in these rocks. The Type II calc-silicate rocks preserve reaction textures, including meionite breaking down to anorthite-calcite-quartz, wollastonite breaking down to calcite-quartz and meionite-quartz symplectites after K-feldspar and wollastonite. Type III calc-silicate rocks have porphyroblastic and coronal grossular. Grossular-quartz coronas separating wollastonite and anorthite and the development of grossular within the anorthite-calcite-quartz pseudomorphs of meionite form important retrograde reaction textures in this type. In Type IV dolomitic marble assemblages, meionite forming in grain boundaries of calcite and feldspars, forsterite rimmed by diopside-dolomite and the formation of grossular in feldspar-rich zones are the important textures. Calculated partial petrogenetic grids in the CaOAl 2O 3SiO 2CO 2 system are used to deduce the pressure-temperature-fluid evolution of the calc-silicate rocks. The Type II assemblages provide CO 2 activity estimates of > 0.5, with a peak metamorphic temperature of about 790°C. Initial cooling followed by later CO 2 influx can be deduced from reaction modelling in these calc-silicate rocks. Type III assemblages are characterized by internal fluid buffering throughout their tectonic history. The formation of coronal grossular indicates an initial cooling from peak metamorphic temperatures of about 830°C deduced from vapour-absent meionite and grossular equilibria. Type IV marble assemblages also indicate internal fluid buffering followed by localized CO 2 influx. Overall, the calc-silicate rocks of the KKB define peak metamorphic temperatures in the range of 790–850°C, with an internally buffered fluid composition during the peak conditions. Initial cooling was followed by localized carbonic fluid influx that also post-dated decompression deduced from other rock types in the KKB.

Evidence for a 2 Ga subduction zone: Eclogites in the Usagaran belt of Tanzania

U-Pb geochronology on metamorphic minerals from a 35-km-long belt of eclogite-facies rocks in central Tanzania yields a Paleoproterozoic age of 2 Ga for the time of metamorphism. Peak metamorphic conditions found in eclogites (± kyanite) and metapelites reached about 750 °C and 18 kbar. A clockwise pressure-temperature path is deduced from mineral zonations, inclusion relations, and retrograde reaction textures. Near-isothermal decompression can be explained by erosion or tectonically controlled exhumation that followed tectonic thickening of the crust during subduction. Trace and rare earth element geochemistry indicates a mid-ocean ridge basaltlike mantle source for the precursors of the mafic members of the eclogite-facies rock suite. All the observations combined indicate that these high-pressure rocks are the oldest-known large-scale outcrops of eclogites formed during subduction of oceanic lithosphere. Linking eclogite formation to a Paleoproterozoic subduction event adds credibility to models of crust dynamics that advocate the operation of plate-tectonic processes early in Earth9s history. The paucity of Precambrian eclogites may then be addressed as a problem of preservation rather than lack of formation.

Relative thermobarometry and metamorphic P-T, paths

Summary Relative thermobarometry involves the determination of changes in P and T experienced by a rock through analysis of mineral zoning, inclusion suites and reaction textures. For this purpose, the differential forms of the thermodynamic equations and mass balance constraints are particularly useful because the equations are linear, and direct computation of changes in dependent variables may be made by matrix inversion of the system of equations defining the thermodynamic constraints and multiplication by changes in independent variables. The number of independent variables equals the phase rule variance if only intensive variables are considered and equals two if mass balance constraints are imposed. Isopleth diagrams may be readily constructed for mineral composition, mineral moles or equilibrium constants provided that entropies, volumes and solution models for all phases as well as a reference assemblage equilibrated at a known P and T are available. An example is presented demonstrating the use of such diagrams in the interpretation of diffusion zoning and retrograde reaction textures in a high-grade gneiss from western New Hampshire, USA. The P-T paths computed from relative thermometry may be sensitive monitors of the tectonic history because errors of ± 5–10% propagate to small errors in the P-T path for small changes in P and T . The P-T paths from the lower plates of thrust sheets may reveal isothermal compression culminating at pressures representative of the maximum depth of burial. Subsequent heating or cooling of such rocks reveals how the thermal structure evolves after thrusting and may permit speculation about the pre-thrusting thermal structure.

The metamorphic evolution of granulites at Fyfe Hills; implications for Archaean crustal thickness in Enderby Land, Antarctica

Abstract Granulites at Fyfe Hills in Enderby Land, Antarctica, crystallized at temperatures in excess of 850°C, and possibly as high as 1000°C, and at pressures of 8‐10kbar during the mid to late Archaean. A number of features, including repeated retrograde metamorphism at 5.5‐8kbar, retrograde reaction textures, and rimward zoning in pressure sensitive systems, suggest that following peak metamorphism the granulites stabilized at a depth of 18‐26 km. After stabilization, the granulites cooled near‐isobarically to temperatures of 600‐700°C. Assuming a total crustal thickness of 35‐40 km during this late Archaean interval of isobaric cooling, the peak metamorphic crustal thickness is estimated at 35‐56 km. This estimate is significantly less than the 60‐70 km obtained by summing the depths of the present levels of exposure (26‐34 km) and the thickness of the crust presently beneath Fyfe Hills (approxi‐mately 35km) and is, therefore, consistent with independent evidence for extensive post‐Archaean thickening of the Enderby Land crust.

Petrogenesis of migmatites in the Huntly-Portsoy area, north-east Scotland

SummaryMigmatites are described from the Sillimanite-potash-feldspar Zone of the aureole around the Newer Basic suite of synorogenic intrusions. The lowest-grade migmatites are trondhjemitoid (characterized by the assemblage quartz-plagioclase-biotite) or muscovite-granitoid (quartz-plagioclase potash-feldspar-muscovite-sillimanite-biotite). With increasing grade, a transition occurs to cordierite-granitoid assemblages (quartz-plagioclase-potash-feldspar-cordierite-garnet-sillimanite-biotite), which persist to the highest grades observed, where there are also noritoid migmatites (quartz-plagioclase-orthopyroxene-cordierite-biotite). The trondhjemitoids are texturally simple because the minerals did not undergo dehydration reactions. Textural immaturity and consistently cotectic modal compositions indicate that their leucosomes originated as melts. Scatter of plagioclase compositions suggests that the partial melting occurred in small closed systems. The other migmatites have more fusible compositions, so it is deduced that they also underwent partial melting. Retrograde reaction textures are used to infer the sequence of reactions, involving muscovite and biotite, by which melting proceeded during prograde evolution. Whereas the fugacity of water probably varied among spatially associated trondhjemitoid leucosomes, in the muscovite-granitoids it was constrained to an approximately constant value, at given pressure and temperature, by the buffering effect of the mineral assemblage.