Abstract The Flatraket Complex, in the ultra-high-pressure (UHP) domains of the Western Gneiss Region (WGR) of Norway, preserves granulite facies assemblages, which were locally overprinted by eclogite and amphibolite facies metamorphism. Zircon and monazite indicate magmatic crystallization of the rocks at 1680–1640 Ma and constrain the timing of the granulite facies overprint at 1100 Ma. This age is older than previously reported ages of 1000–950 Ma for regional metamorphism reaching anatexis and locally granulite facies in the WGR. The granulites at Flatraket may have developed as a consequence of local metasomatism, perhaps linked to metasomatism occurring at the same time in the nearby Sandvik peridotite. Granitic rocks from neighbouring Kråkeneset indicate magmatic emplacement at ≥1650 Ma, during the event that formed the Flatraket Complex and the bulk of the WGR. A gabbro body at Kråkeneset is dated at 1255±8 Ma by baddeleyite, which was not affected by the granulite event, implying that the rock remained impermeable to fluids, reacting instead to some degree during the Caledonian UHP event.

Heterogeneous strain commonly serves as an important natural instrument for unraveling complex tectonic histories in polyphase metamorphic terranes. We present key examples of multi-scale heterogeneous deformation from two classic deep-crustal granulite terranes, the Athabasca Granulite Terrane in western Canada and the Limpopo Complex in southern Africa. These examples are chosen to illustrate how localized strain and attendant metamorphism played a key role in the development and preservation of important records of deep-crustal processes. In addition, several common characteristics of these terranes are identified through this analysis and include heterogeneous deep-crustal flow, regional-scale tectonic heterogeneity, and multistage exhumation with high-resolution records developed in locally hydrated shear zones. Better recognition of the fundamental spatial and temporal heterogeneity in these and other similar polymetamorphic terranes may help to reconcile apparently conflicting interpretations and tectonic models.

The Bondy gneiss complex preserves evidence of an early 1.20–1.18 Ga granulite-facies tectonometamorphic event within the Central Metasedimentary Belt of Québec, western Grenville Province. Peak metamorphic conditions, estimated with garnet-biotite, garnet-orthopyroxene, garnet-aluminosilicate-quartz-plagioclase, and Al-in-orthopyroxene thermobarometers, reached 950 °C and 10 kilobars, compatible with the presence of sillimanite with orthopyroxene in aluminous gneiss. Peak pressure occurred at 1.20 Ga and was followed by crystallization of peak temperature anatec-tic melt at 1.18 Ga and isothermal decompression as recorded by garnet-cordierite-orthopyroxene-bearing leucosomes and garnet zonation profiles and textures. These conditions are 150–200 °C and 2 kilobars higher than those previously documented in the region. Peak assemblages were overprinted in the Nominingue-Chénéville deformation zone, to the east of the Bondy gneiss complex, during deformation at amphi-bolite facies and intrusion of monzonite-diorite-gabbro sheets at 1.17–1.16 Ga. High concentrations of ferromagnesian inclusions in large garnet grains reduced the effective diffusional domain size, which, when combined with retrograde cation diffusion, locally obliterated evidence of peak metamorphic conditions in the gneiss complex. Retrograde cooling paths in the gneiss complex and the deformation zone are distinct, that of the gneiss complex being at least 0.5 kilobar lower at any given temperature.

Abstract A significant proportion of gold production from the Archean Yilgarn craton in Western Australia has come from lode- and vein-style deposits hosted in amphibolite and granulite facies metamorphic rocks rather than in greenschist facies terranes, which are the normal hosts of such deposits in greenstone belts. The deposits in higher metamorphic-grade terranes have many similarities to those in lower-grade terranes, particularly the structural controls on deposit siting and form, and metal inventories. Mineral assemblages in ore zones and surrounding alteration halos of deposits in high-temperature terranes are interpreted as belonging to either a high-temperature or a retrograde paragenesis. In metamorphosed igneous host rocks, quartz, biotite, Ca amphibole, plagioclase, and clinopyroxene, with generally only minor carbonate, are characteristic of the high-temperature paragenesis. Chlorite, sericite, and carbonate minerals, most commonly as patchy and partial replacement of higher-temperature minerals, belong to the retrograde paragenesis. Equilibration of the high-temperature assemblages was at temperatures and pressures similar to those of the local peak of metamorphism, or at slightly lower temperatures. Different high-temperature assemblages are related systematically to host-rock metamorphic grade. The retrograde minerals grew at greenschist or prehnite-pumpellyite facies conditions. Ore mineral assemblages are pyrrhotite dominated in almost all deposits with, in some cases, minor pyrite and chalcopyrite. In other assemblages, there is an important arsenopyrite component, with loellingite forming commonly in the cores of arsenopyrite grains. Compositions of ore minerals indicate variable equilibration at high temperatures or under greenschist facies conditions. Stable isotope compositions of vein minerals generally indicate equilibration at near-peak metamorphic conditions. Combined textural, structural, and geochemical data show that mineralization most likely occurred at near-peak metamorphic conditions in the majority of these gold deposits. Gold introduction at greenschist conditions during the retrograde evolution of the terranes is ruled out from some of the common mineralogical settings of gold. It is generally in equilibrium with high-temperature gangue minerals and can be as invisible gold in loellingite that was overprinted by arsenopyrite prior to cooling below amphibolite facies conditions. Additionally, there is generally a close correlation of gold grade and specific high-temperature gangue assemblages. The retrograde paragenesis in the deposits is considered to be unrelated to gold introduction, although retrograde processes may have caused some ore remobilization. Differentiation between synpeak metamorphic mineralization and mineralization at lower-grade conditions during the prograde history is not clear in every deposit, in part because of textural equilibration of minerals in the alteration halos, such that replacement relations and paragenetic sequences can rarely be determined. Synpeak metamorphic mineralization is, however, indicated by the structurally late timing of many ore-controlling structures, the relatively low deformation state of many veins compared to that of wall rock, and the lack of a C and O isotope signature indicative of decarbonation, despite the low carbonate content of the ores in higher-grade terranes. Subtle differences in element enrichment and depletion patterns in the alteration halos of deposits in different grade host rocks may reflect stability of different minerals at the time of mineralization.