Granulite

About: Granulite is a research topic. Over the lifetime, 6763 publications have been published within this topic receiving 268925 citations.

Orogeny, migmatites and leucogranites: A review

Abstract: The type ofP-T-t path and availability of fluid (H2O-rich metamorphic volatile phase or melt) are important variables in metamorphism. Collisional orogens are characterized by clockwiseP-T evolution, which means that in the core, where temperatures exceed the wet solidus for common crustal rocks, melt may be present throughout a significant portion of the evolution. Field observations of eroded orogens show that lower crust is migmatitic, and geophysical observations have been interpreted to suggest the presence of melt in active orogens. A consequence of these results is that orogenic collapse in mature orogens may be controlled by a partially-molten layer that decouples weak crust from subducting lithosphere, and such a weak layer may enable exhumation of deeply buried crust. Migmatites provide a record of melt segregation in partially molten crustal materials and syn-anatectic deformation under natural conditions. Grain boundary flow and intra-and inter-grain fracture flow are the principal grain scale melt flow mechanisms. Field observations of migmatites in ancient orogens show that leucosomes occur oriented in the metamorphic fabrics or are located in dilational sites. These observations are interpreted to suggest that melt segregation and extraction are syntectonic processes, and that melt migration pathways commonly relate to rock fabrics and structures. Thus, leucosomes in depleted migmatites record the remnant permeability network, but evolution of permeability networks and amplification of anomalies are poorly understood. Deformation of partially molten rocks is accommodated by melt-enhanced granular flow, and volumetric strain is accommodated by melt loss. Melt segregation and extraction may be cyclic or continuous, depending on the level of applied differential stress and rate of melt pressure buildup. During clockwiseP-T evolution, H2O is transferred from protolith to melt as rocks cross dehydration melting reactions, and H2O may be evolved above the solidus at lowP by crossing supra-solidus decompression-dehydration reactions if micas are still present in the depleted protolith. H2O dissolved in melt is transported through the crust to be exsolved on crystallization. This recycled H2O may promote wet melting at supra-solidus conditions and retrogression at subsolidus conditions. The common growth of ‘late’ muscovite over sillimanite in migmatite may be the result of this process, and influx of exogenous H2O may not be necessary. However, in general, metasomatism in the evolution of the crust remains a contentious issue. Processes in the lower-most crust may be inferred from studies of xenolith suites brought to the surface in lavas. Based on geochemical data, we can use statistical methods and modeling to evaluate whether migmatites are sources or feeder zones for granites, or simply segregated melt that was stagnant in residue, and to compare xenoliths of inferred lower crust with exposed deep crust. Upper-crustal granites are a necessary complement to melt-depleted granulites common in the lower crust, but the role of mafic magma in crustal melting remains uncertain. Plutons occur at various depths above and below the brittle-to-viscous transition in the crust and have a variety of 3-D shapes that may vary systematically with depth. The switch from ascent to emplacement may be caused by amplification of instabilities within (permeability, magma flow rate) or surrounding (strength or state of stress) the ascent column, or by the ascending magma intersecting some discontinuity in the crust that enables horizontal magma emplacement followed by thickening during pluton inflation. Feedback relations between rates of pluton filling, magma ascent and melt extraction maintain compatibility among these processes.

Calculated phase equilibria in K2O-FeO-MgO-Al2O3-SiO2-H2O for sapphirine-quartz-bearing mineral assemblages

Abstract: Sapphirine, coexisting with quartz, is an indicator mineral for ultrahigh-temperature metamorphism in aluminous rock compositions. Here a new activity-composition model for sapphirine is combined with the internally consistent thermodynamic dataset used by THERMOCALC, for calculations primarily in K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH). A discrepancy between published experimentally derived FMAS grids and our calculations is understood with reference to H2O. Published FMAS grids effectively represent constant aH2O sections, thereby limiting their detailed use for the interpretation of mineral reaction textures in compositions with differing H2O. For the calculated KFMASH univariant reaction grid, sapphirine + quartz assemblages occur at P–T in excess of 6–7 kbar and 1005 °C. Sapphirine compositions and composition ranges are consistent with natural examples. However, as many univariant equilibria are typically not ‘seen’ by a specific bulk composition, the univariant reaction grid may reveal little about the detailed topology of multi-variant equilibria, and therefore is of limited use for interpreting the P–T evolution of mineral assemblages and reaction sequences. Calculated pseudosections, which quantify bulk composition and multi-variant equilibria, predict experimentally determined KFMASH mineral assemblages with consistent topology, and also indicate that sapphirine stabilizes at increasingly higher pressure and temperature as XMg increases. Although coexisting sapphirine and quartz can occur in relatively iron-rich rocks if the bulk chemistry is sufficiently aluminous, the P–T window of stability shrinks with decreasing XMg. An array of mineral assemblages and mineral reaction sequences from natural sapphirine + quartz and other rocks from Enderby Land, Antarctica, are reproducible with calculated pseudosections. That consistent phase diagram calculations involving sapphirine can be performed allows for a more thorough assessment of the metamorphic evolution of high-temperature granulite facies terranes than was previously possible. The establishment of a a-x model for sapphirine provides the basis for expansion to larger, more geologically realistic chemical systems (e.g. involving Fe3+).

Fluid Regime in Southern Norway: The Record of Fluid Inclusions

Abstract: Fluid inclusions have been studied in representative rocks from Southern Norway, notably in the Bamble granulites. On the basis of the earliest fluid inclusions trapped in rock-forming minerals (mainly quartz), five major types of fluid distribution have been recognized: 2-phase aqueous (H20 dominant, without solid), carbonic (mostly pure CO2, possiDle occurrence of N2 and/or CH4), mixed 1 (aqueous and carbonic inclusions in comparable amounts, but in separate cavities), mixed 2 (aqueous and carbonic fluids in the same cavity, trapped in the miscible state of the H20-C02 system), brines (H20 + solids, NaC1 dominant). Only brines show a relation between a dominant inclusion type and a given protolith; these are especially abundant in 3 well-defined environments: Al-rich metasediments (meta-pelites), skarns and acid volcanics. The distribution of other types is more related to metamorphic grade: high- density carbonic inclusions are typical for the granulite- -facies domain, early 2-phase aqueous inclusions occur almost exclusively in the north-western part of the Bamble and in the Telemark gneiss-granites, mixed (1 and 2) inclusions characterize the complicated transition zone between the amphibolite- and granulite-facies domains north of the orthopyroxene-in isograd. P-T estimates from fluid inclusions are apparently very different for Bamble (maximum C02 density during peak metamorphism) and Rogaland (maximum CO2 density after the peak of metamorphism). Most of the C02 originates from the breakdown of carbonate melts (carbonatites) emplaced as immiscible droplets in deep-seated synmetamorphic intrusives. Southern Norway is a classical example of amphibolite — granulite facies transition in a Proterozoic terrain. Although the age of metamorphism remains controversial (see various entries in this volume, notably by R.H. Verschure, D. Demaiffe and J. Michot. D. Field et al.), the pressure and temperature conditions start to be relatively well understood (Jansen et al., this volume). All recent studies emphazise the key importance of fluids, notably H2O, the fugacity of which decreases suddenly at the granulite — facies boundary. Much information has been derived from the analysis, both experimental and theoretical, of characteristic mineral assemblages; however, since the first discovery of specific, high density CO2 inclusions in many granulites (Touret, 1971), it has become evident that the direct observation of fluids trapped in rock-forming minerals (notably quartz, plagioclase, pyroxene etc.) can provide a great deal of information. CO2-rich fluids, sometimes mixed with other species (N2, CH4) have later been observed in virtually all granulites in the world (Hollister and Crawford 1981, Roedder 1984), leading to the notion of “carbonic metamorphism”, which has provided a new insight into the geology of the lower continental crust (e.g Newton et al. 1980, Newton, this volume). Various studies dealing with Southern Norway have been issued in a number of publications (see e.g. review in Roedder 1984), but the rapid development of fluid-inclusion techniques, notably the possibility of in-situ, non-destructive analysis by micro-Raman spectroscopy, necessitates a critical reevaluation of earlier data. In this paper, a review of all studies performed in Soutern Norway during the last 15 years will be attempted. It will refine earlier syntheses (notably Touret 1981, 1984) and address three fundamental aspects of fluid investigations in high-grade metamorphic rocks: i) The distribution of fluid inclusions in amphibolite- and granulite-facies rocks and the relative importance of lithological composition (protolith) and metamorphic grade. ii) The pressure and temperature estimates derived from fluid inclusions and their comparison with data from solid mineral assemblages. iii) The origin of the CO2 fluids.