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The Tertiary Skaergaard intrusion, East Greenland, intruded at the shallow crustal unconformity between Precambrian amphibolite-facies gneisses and overlying Tertiary Plateau Basalts. Maximum contact metamorphic temperatures in quartzo-feldspathic gneisses were determined in two sample traverses across the aureole on the western contact of the intrusion using a combination of microstructural observations (both optical and cathodoluminescence) and the titanium-in-quartz (TitaniQ) thermometer. The onset of recrystallization of the quartz in the gneisses occurred between 390 and 340 m from the contact whereas H2O-fluxed melting occurred in gneisses closer than 130 m from the contact (where T > ∼ 675°C). The maximum temperature recorded by quartz at the contact is ∼865 ± 70°C. Melt fractions reach 50–60 vol. % in some samples although the melt is heterogeneously distributed on all scales. Minor bands of amphibolite-facies mafic gneiss are extensively reacted to an anhydrous pyroxene-bearing hornfels close to the contact, whereas those further than ∼130 m are overprinted by a greenschist-facies assemblage. Discrepancies between the expected temperature for the amphibolite- to greenschist-facies reaction and temperatures obtained from adjacent quartzo-feldspathic gneisses are consistent with the formation of the anhydrous pyroxene hornfels directly from the mafic gneiss, with the lower-grade greenschist-facies assemblage forming on the retrograde path after the establishment of limited hydrothermal activity. It is unlikely that devolatilization reactions in the gneiss produced sufficient H2O to account for the pegmatitic features formed in the Marginal Border Series in the intrusion. A simple one-dimensional thermal model, neglecting any advection of heat by hydrothermal circulation, was fitted to the profile of maximum temperature through the aureole. The generally lower temperatures seen in the gneiss compared with those previously reported for the contact metamorphosed basalts higher up the walls of the intrusion are consistent with a heterogeneous release of latent heat of crystallization. [ABSTRACT FROM AUTHOR]

The mineralogy of thermometamorphic granites is relatively simple, making it possible to track the spatial distribution of chemical and mineralogical variations in these rocks and investigate the processes that underpin these metamorphic reactions. We have undertaken a detailed investigation of metagranites from the contact aureole that fringes a quartz diorite intrusion of Late Permian age, emplaced into Carboniferous peraluminous granites of the Gennargentu Igneous Complex (Sardinia, Italy). New data are presented including the petrography of metagranites within a 500 m zone adjacent to the quartz diorite intrusion, the compositions of minerals and bulk-rocks, and the oxygen isotope compositions of separated minerals. We have used these data to assess the mobility of elements, expressed as Δoxide, in the aureole, and the physical conditions of fluid-assisted thermometamorphism. Modal variations and the oscillatory zoning of plagioclase demonstrate that the shallow (P ≤200 MPa) quartz diorite intrusion was emplaced through a number of magmatic injections. The border zone of the quartz diorite intrusion presents evidence of two main processes: hybridization between andesite and rhyolite magmas and volatile saturation of the mingled magma. Modal differences in the contact zone with respect to the protolith (i.e. peraluminous granite), variations in mineral composition, temperature constraints and ΔK2O, ΔNa2O, ΔSiO2 and ΔAl2O3 indicate that a relatively large volume of the host granite (up to 400 m from the contact) was metasomatized by high-temperature (650–350°C) fluids derived from the mingled zone of the quartz diorite intrusion. In detail, the metasomatic K2O-rich fluid reacted with albite to form K-feldspar, and triggered the recrystallization of quartz and plagioclase to higher calcium concentrations. The progressive increase in the MgO/(MgO + FeO) of chlorite closer to the contact indicates that this phase also recrystallized. The iron released during chlorite recrystallization was buffered by hematite formation in the pores of metasomatic K-feldspar. The Gennargentu metagranites provide evidence that metasomatic fluids can play a major role in driving metamorphic reactions in contact aureoles. For instance, the expected increase of Ca in plagioclase owing to thermal equilibration was not achieved in the high-T zone of the aureole because of fluid-assisted removal of cations. We conclude that caution should be taken when interpreting the processes that underpin contact metamorphism in terms of thermally driven, ionic diffusion alone, because the role of fluids may be significant, if not overwhelming, in the domains closest to the magmatic source. [ABSTRACT FROM AUTHOR]

The Kyffhäuser Crystalline Complex, Central Germany, forms part of the Mid-German Crystalline Rise, which is assumed to represent the Variscan collision zone between the East Avalonian terrane and the Armorican terrane assemblage. High-precision U–Pb zircon and monazite dating indicates that sedimentary rocks of the Kyffhäuser Crystalline Complex are younger than c. 470 Ma and were intruded by gabbros and diorites between 345 ± 4 and 340 ± 1 Ma. These intrusions had magmatic temperatures between 850 and 900°C, and caused a contact metamorphic overprint of the sediments at P–T conditions of 690–750°C and 5–7 kbar, corresponding to an intrusion depth of 19–25 km. At 337 ± 1 Ma the magmatic–metamorphic suite was intruded by granites, syenites and diorites at a shallow crustal level of some 7–11 km. This is inferred from a diorite, and conforms to P–T paths obtained from the metasediments, indicating a nearly isothermal decompression from 5–7 to 2–4 kbar at 690–750°C. Subsequently, the metamorphic–magmatic sequence underwent accelerated cooling to below 400°C, as constrained by garnet geospeedometry and a previously published K–Ar muscovite age of 333 ± 7 Ma. With respect to P–T–D–t data from surrounding units, rapid exhumation of the KCC can be interpreted to result from NW-directed crustal shortening during the Viséan. [ABSTRACT FROM AUTHOR]

The northern margin of the Inland Branch of the Pan-African Damara Orogen in Namibia shows dramatic along-strike variation in metamorphic character during convergence between the Congo and Kalahari Cratons (M3 metamorphic cycle). Low-P contact metamorphism with anticlockwise P–T paths dominates in the western domains (Ugab Zone and western Northern Zone), and high-P Barrovian metamorphism with a clockwise P–T path is documented from the easternmost domain (eastern Northern Zone). The sequence of M3 mineral growth in contact aureoles shows early growth of cordierite porphyroblasts that were pseudomorphed to biotite–chlorite–muscovite at the same time as an andalusite–biotite–muscovite transposed foliation was developed in the matrix. The peak-T metamorphic assemblages and fabrics were overprinted by crenulations and retrograde chlorite–muscovite. The KFMASH P–T pseudosection for metapelites in the Ugab Zone and western Northern Zone contact aureoles indicates tight anticlockwise P–T loops through peak metamorphic conditions of 540–570°C and 2·5–3·2 kbar. These semi-quantitative P–T loops are consistent with average P–T calculations using THERMOCALC, which give a pooled mean of 556 ± 26°C and 3·2 ± 0·6 kbar, indicating a high average thermal gradient of 50°C/km. In contrast, the eastern Northern Zone experienced deep burial, high-P/moderate-T Barrovian M3 metamorphism with an average thermal gradient of 21°C/km and peak metamorphic conditions of c. 635°C and 8·7 kbar. The calculated P–T pseudosection and garnet compositional isopleths in KFMASH, appropriate for the metapelite sample from this region, document a clockwise P–T path. Early plagioclase–kyanite–biotite parageneses evolved by plagioclase consumption and the growth of garnet to increasing XFe, XMg and XCa and decreasing XMn compositions, indicating steep burial with heating. The developed kyanite–garnet–biotite peak metamorphic parageneses were followed by the resorption of garnet and formation of plagioclase moats, indicating decompression, which was followed by retrogressive cooling and chlorite–muscovite growth. The clockwise P–T loop is consistent with the foreland vergent fold–thrust belt geometry in this part of the northern margin. Earlier formed (580–570 Ma) pervasive matrix foliations (M2) were overprinted by contact metamorphic parageneses (M3) in the aureoles of 530 ± 3 Ma granites in the Ugab Zone and 553–514 Ma granites in the western Northern Zone. Available geochronological data suggest that convergence between the Congo and Kalahari Cratons was essentially coeval in all parts of the northern margin, with similar ages of 535–530 Ma for the main phase of deformation in the eastern Northern Zone and Northern Platform and 538–505 Ma high-grade metamorphism of the Central Zone immediately to the south. Consequently, NNE–SSW-directed convergent deformation and associated M3 metamorphism of contrasting styles are interpreted to be broadly contemporaneous along the length of the northern margin of the Inland Branch. In the west heat transfer was dominated by conduction and externally driven by granites, whereas in the east heat transfer was dominated by advection and internally driven radiogenic heat production. The ultimate cause was along-orogen variation in crustal architecture, including thickness of the passive margin lithosphere and thickness of the overlying sedimentary succession. [ABSTRACT FROM AUTHOR]

The Ross of Mull pluton consists of granites and granodiorites and intrudes sediments previously metamorphosed at amphibolite facies. The high grade and coarse grain size of the protolith is responsible for a high degree of disequilibrium in many parts of the aureole and for some unusual textures. A band of metapelite contained coarse garnet, biotite and kyanite prior to intrusion, and developed a sequence of textures towards the pluton. In Zone I, garnet is rimmed by cordierite and new biotite. In Zone II, coarse kyanite grains are partly replaced by andalusite, indicating incomplete reaction. Coronas of cordierite + muscovite around kyanite are due to reaction with biotite. In the higher-grade parts of this zone there is complete replacement of kyanite and/or andalusite by muscovite and cordierite. Cordierite chemistry indicates 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 unstable in Zone II, with regional garnets breaking down to cordierite, new biotite and plagioclase. In Zone III this breakdown is well advanced, and this zone marks the appearance of fibrolite and K-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 cordierite and are inferred to be relics. Others are euhedral with Mn-rich cores. For these, the reaction biotite + sillimanite + quartz → garnet + cordierite + K-feldspar + melt is inferred. Using a petrogenetic grid based on the work of Pattison and Harte, pressure is estimated at 3·2 kbar, and temperature at the Zone II–III boundary at 650°C and in Zone IV as at least 750°C. [ABSTRACT FROM AUTHOR]

A well-preserved contact aureole at the base of the sheeted dyke complex in the Troodos ophiolite, Cyprus, records the partial melting of the roof of a mid-ocean ridge magma chamber. Hydrothermally altered dykes within the lowermost 10–30 m of the sheeted dyke complex were recrystallized to pyroxene and hornblende hornfels and, closest to the underlying gabbros (<10 m), partially melted (up to >30%) at water-undersaturated conditions and temperatures ≥875°C. Geological considerations indicate that the duration of melting events was probably on the time scale of eruptive cycles (i.e. years to centuries). This resulted in disequilibrium partial melting and highly imperfect melt segregation within the contact aureole. We hypothesize that partial melting is a common process within the roof zones of axial magma chambers at fast-spreading ridges and that a component of geophysically imaged axial magma chambers may be partially melted upper crust rather than partially crystallized magma. Field relations suggest that where the highest degrees of melting occurred, a mixture of anhydrous hornfels and hydrous leucocratic melt disaggregrated, and was assimilated into the magma chamber. [ABSTRACT FROM PUBLISHER]

The intrusion of a Tertiary gabbroic sill of 6 m thickness, in which magma flow was locally turbulent, into garnet-grade pelitic gneiss and psammite at 600 bars at Traigh Bhàn na Sgùrra on the Ross of Mull, Scotland, led to the development of a contact aureole of 3 m width. Observed reactions include muscovite breakdown, melting on quartz–feldspar grain boundaries, and isochemical breakdown of biotite and garnet. A simple, two-stage thermal model fitted to the profile of maximum temperature is consistent with a 5 month period of turbulent magma flow. Five generations of micro-cracks occur in the psammite. The oldest pre-dates contact metamorphism and is marked by sub-parallel fluid inclusion arrays. The next resulted from anisotropic thermal expansion caused by magma intrusion. Internally generated stresses related to an increase in volume associated with muscovite breakdown formed two further sets of cracks associated with release of H2O and melting. Melting on quartz–feldspar grain boundaries also resulted in crack formation. The final stage of cracking was the result of anisotropic thermal contraction. Despite the high crack density at the metamorphic peak, little or no melt segregation occurred, demonstrating that micro-cracking alone is not sufficient (at least on this time scale) for melt segregation in static anatectic environments. [ABSTRACT FROM PUBLISHER]

Quartz–calcite sandstones experienced the reaction calcite + quartz = wollastonite + CO2 during prograde contact metamorphism at P = 1500 bars and T = 560°C. Rocks were in equilibrium during reaction with a CO2–H2O fluid with XCO2 = 0·14. The transition from calcite-bearing, wollastonite-free to wollastonite-bearing, calcite-free rocks across the wollastonite isograd is only several millimeters wide. The wollastonite-forming reaction was driven by infiltration of quartz–calcite sandstone by chemically reactive H2O-rich fluids, and the distribution of wollastonite directly images the flow paths of reactive fluids during metamorphism. The mapped distribution of wollastonite and modeling of an O-isotope profile across a lithologic contact indicate that the principal direction of flow was layer-parallel, directed upward, with any cross-layer component of flow <0·1% of the layer-parallel component. Fluid flow was channeled at a scale of 1–100 m by pre-metamorphic dikes, thrust and strike-slip faults, fold hinges, bedding, and stratigraphic contacts. Limits on the amount of fluid, based on minimum and maximum estimates for the displacement of the wollastonite reaction front from the fluid source, are (0·7–1·9) × 105 cm3 fluid/cm2 rock. The sharpness of the wollastonite isograd, the consistency of mineral thermobarometry, the uniform measured 18O–16O fractionations between quartz and calcite, and model calculations all argue for a close approach to local mineral–fluid equilibrium during the wollastonite-forming reaction. [ABSTRACT FROM PUBLISHER]

At Cima Uzza, the tonalitic Adamello batholith and volumetrically minor gabbroic stocks and dikes were emplaced into a thick sequence of silica-poor dolostone. Stable isotopic data from dolomite in dolomitic marble in the contact aureole [[delta]18O = 29·7 ± 1·3%° (1[sgr]), [delta]13C = 2·2 ± 1·3%°] and from equivalent dolostone outside the aureole ([delta]18O = 28·8 ± 1·1%°, [delta]13C = 2·5 ± 0·2%°) are indistinguishable. These high and uniform [delta]18O values in the dominant lithology suggest metamorphism in a closed system, as does a closed-system reaction model that is consistent with observed isograd development. Localized fluid infiltration is reflected by low [delta]18O (13·2-18·5%°) and [delta]13C (-2·6 to -6·7%°) calcite values in siliceous marble xenoliths entrained in small gabbroic stocks at the tonalite margin. Such siliceous marble probably formed by Si and Al metasomatism of dolomitic marble during infiltration of magmatic fluids because no equivalent lithology occurs outside the aureole. Brucite marble, which occurs in xenoliths and as reaction rims around mafic dikes, displays a wide range of [delta]18O and [delta]13C values which are generally intermediate between those for siliceous marble and dolomitic marble. This range of isotopic values in brucite marble is consistent with a hydrothermal formation from a silica-poor dolostone protolith. In contrast, high [delta]18O values from centimeter-scale calc-silicate nodules and layers, which are irregularly distributed in the dolomitic contact aureole, indicate a sedimentary origin for such rocks. Hydrothermal alteration of dolomitic marble at Cima Uzza is limited to small areas near gabbroic stocks and dikes, whereas negligible country rock alteration is associated with the larger tonalite. The limited infiltration of magmatic fluids possibly reflects a low volatile content in the tonalitic magma. In addition, low permeability in the thick sequence of dolomitic marble could explain the small amount of fluid flow relative to other carbonate contact aureoles. [ABSTRACT FROM PUBLISHER]

Most of the applicable thermometers involving cordierite require additional Fe–Mg phases and knowledge of pressure and fluid phase composition. These conditions are not necessarily met with natural cordierite-bearing metamorphic rocks. A potential pressure-independent geothermometer for cordierite- and plagioclase-bearing rocks is the experimentally determined inverse linear relationship between temperature and Na contents of Mg-cordierite coexisting with NaOH or albite and NaOH. In this paper, the Na-in-cordierite thermometer is empirically tested on cordierite-bearing metapelites from the island of Kos, contact metamorphosed by a quartz monzonite intrusion. The variety of phase assemblages in the metapelites allows for tight constraints to be placed on temperatures (508–762°C) and pressure (0.2 GPa) during contact metamorphism by calculated and experimentally determined phase equilibria and by Al-in-hornblende barometry. Fluid compositions are crudely estimated from the presence and absence of graphite and calcite and from 2Vx in cordierite. Applying the Na-in-cordierite thermometer to 19 rock samples taken at increasing distances from the intrusion yields realistic peak temperatures. The Na-in-cordierite temperatures are consistent with constraints from petrogenetic grids, with experimentally determined phase equilibria and with temperatures based on Fe–Mg exchange equilibria (garnet–cordierite and garnet–biotite), although the latter tend to give lower temperatures. The Na-in-cordierite temperatures show no systematic variations with Mg values and Al contents of cordierites, Na contents of coexisting plagioclase (in the range An10–An40) and biotite, and H2O/CO2 ratios in coexisting fluids, though the last are only poorly known. Cordierites have either high Na and significant Li contents or display low levels of both elements. Li is probably introduced into cordierite by a coupled substitution involving tetrahedral and octahedral sites, whereas Na may not be charge-balanced by lattice oxygen. The results obtained suggest that the experimentally established relationship between Na contents of cordierite and temperature is a useful thermometer for cordierite- and plagioclase-bearing metapelitic rocks. The nature of Na incorporation in cordierite and its relation to fluid composition, however, must be constrained by further experiments. [ABSTRACT FROM AUTHOR]

The mineralogy and petrology of the Penge Iron Formation of the Transvaal Supergroup, situated in the contact metamorphic aureole of the Bushveld Complex, was studied in detail to investigate the petrogenesis of crocidolite- and amosite-bearing rocks at Penge and Mafefe, in eastern Transvaal, South Africa. The rocks are laterally equivalent to the diagenetic to very low-grade metamorphic Asbesheuwels iron-formation succession in Griqualand West. Metamorphism at Penge apparently took place at 420–460°C and 2.6 ± 0.8 kbar as the result of the intrusion of the Bushveld Complex. The maximum metamorphic temperature at Mafefe was some 40–80°C lower than at Penge. Bulk-rock composition was a major factor in controlling the distribution of crocidolite and amosite in the succession. Riebeckite and crocidolite are absent from Al-rich rock units containing biotite with Si/Al>3, but are commonly associated with ferri-annite with Si/Al ratios of the order of three. In contrast, amosite is well developed in carbonaceous Al-rich biotite–grunerite hornfels. Couplets of amosite–crocidolite seams, developed in magnetite–grunerite banded iron-formation, may have formed from alternating mesobands of iron-silicate–riebeckite known from the diagenetic to very low-grade metamorphic Kuruman Iron Formation of the Asbesheuwels Subgroup. There is also evidence, especially in the case of one specific asbestos reef, that riebeckite–crocidolite have been replaced by grunerite–amosite with increasing grade of metamorphism from Mafefe towards Penge. In contrast, small amounts of grunerite appear to have been replaced by riebeckite during retrograde metamorphism. [ABSTRACT FROM AUTHOR]