G. T. R. Droop

Fluids, P–T paths and the fates of anatectic melts in the Earth's crust

This paper presents a model for the longevity and physical behaviour of anatectic melts in the Earths crust. It is assumed that a thermal anomaly has caused partial fusion of biotite–plagioclase–quartz rocks, to produce a granitic melt and restitic solids. The focus is on the potential chemical and physical outcomes, as a function of the various P–T-fluid conditions. Rocks undergoing fluid-present melting will sustain an increase in bulk density and will become more gravitationally stable. Diapiric up-welling of such material should not occur. In fluid-absent melting, the bulk density will decrease and foster gravitational instability. Stresses in the surrounding rocks could, however, lead to fracture and rapid evacuation of the melt. With excess fluid and without melt segregation, isobaric cooling will cause immediate freezing. If T rises above the biotite–quartz melting reaction, freezing will still occur at the original reaction T. Diapiric up-welling should not occur. Biotite-bearing diatexites might be formed by large degrees of fusion, but magma ascent is unlikely. In isothermal decompression, immediate freezing will ensue, and the migmatite should contain orthopyroxene rather than biotite. With excess fluid and melt segregation, cooling will induce freezing at the fluid-saturated granite solidus. Magma ascent could, however, occur, even for XFlH2O=1. In isothermal decompression, melt will persist to the granite solidus for the imposed XFlH2O. Limited magma ascent could occur. All granites should contain primary igneous biotite. For fluid-deficient rocks without melt segregation, freezing will occur at the T of the original melting. If the reaction is overstepped, solidification will occur at the original XFlH2O. Magma formation is unlikely. Migmatites should contain either pyroxene retrogressed to biotite, or biotite alone. On decompression, immediate freezing is likely if T remains at the biotite–quartz reaction. For higher T, melt will survive decompression and freeze at the fluid-saturated solidus for some increased XFlH2O. Magma ascent is possible, and the melt will freeze very close to the wet granite solidus. In fluid-deficient rocks with melt segregation, cooling and crystallization will cause fluid saturation. In situ segregated melt will freeze at the original XFlH2O. Melt ascent will, however, cause exsolution of a CO2-rich fluid. With fluid escape, melt will freeze close to the wet granite solidus. An increase in melt proportion may accompany magma ascent or source up-welling. Any low-P granite intrusion should be charnockitic. For fluid-absent rocks without melt segregation, freezing will occur on cooling to the T of the original reaction. Only transitory migmatites could be formed, but melt will survive decompression almost to the H2O-saturated granite solidus. The rocks will reveal former melt presence only in the existence of veins of coarse retrograde biotite. With isothermal decompression, further melting may occur, forming an orthopyroxene-bearing granulite-facies migmatite. In fluid-absent situations with melt segregation, magma could be efficiently transported to higher crustal levels. If the melt proportion were insufficient for magma formation, granulite-facies migmatites would result. Ascending melt would progressively dissolve any entrained restite. The granites produced should be charnockitic. To aid in interpretation of the modelling results, we suggest that it may be slightly more common for the highest-grade metamorphic terranes to exhibit isobaric cooling paths, that fluid-present anatexis is normally limited to the shallower parts of the crust, and that fluid-absent reactions are more likely at greater depths. We conclude that the best models for natural fluid-present anatexis are those dealing with essentially pure H2O fluid and that fluid-deficient conditions will dominate over fluid-excess. We also believe that it is common for melts to be physically segregated on scales larger than the diffusion paths of many elements.

A TEM study of disequilibrium plagioclase breakdown at high pressure: the role of infiltrating fluid

AbstractHigh-pressure metamorphism (∼600° C, ∼20 kbar) of the Allalin Gabbro (Western Alps) resulted in the breakdown of plagioclase (∼An63) to fine-grained zoisite, jadeite, kyanite and quartz. In rare cases this reaction failed to reach completion. The resulting textures of partial reaction have been studied by transmission and analytical electron microscopy. In localised regions of a plagioclase crystal where the extent of reaction is <10%, only zoisite developed and the orientation relationship 1

Amphibolite facies metamorphism in the Schapenburg schist belt: A record of the mid-crustal response to ~3.23 Ga terrane accretion in the Barberton greenstone belt

\left( {100} \right)_{{\text{Zo}}} //\left( {101} \right)_{{\text{P1}}}

Evidence for a genetic granite–migmatite link in the Dalradian of NE Scotland

and 1

Hydrous cordierite in granulites and crustal magma production

\left( {012} \right)_{{\text{Zo}}} //\left( {010} \right)_{{\text{P1}}}

Contact metamorphism and partial melting of Dalradian pelites and semipelites in the southern sector of the Etive aureole

is frequently present. In regions where 10–50% of plagioclase has transformed, the reaction plagioclase+H2O→zoisite+kyanite+quartz +(NaSiCa−1Al−1)pl has occurred. The systematic orientation relationship between plagioclase and zoisite is absent at ≥50% transformation. Complete breakdown of plagioclase occurred in localized micron-scale domains by the reaction plagioclase+H2O→zoisite+jadeite+kyanite+quartz and the reaction products are variably orientated with respect to each other. Incomplete reaction, together with the concentration of reaction products around cracks in original plagioclase grains, suggests that extent of reaction was controlled primarily by the availability of H2O. The textural observations are interpreted in terms of two possible disequilibrium reaction models. (1) Plagioclase persists metastably with its original igneous composition to a pressure > 17 kbar at 600° C. Reaction to the equilibrium assemblage then develops adjacent to cracks in response to the presence of aqueous fluid. At intracrystalline sites, only partial reaction occurs because Jadeite, and sometimes kyanite and quartz, fail to nucleate for kinetic reasons. (2) Localized regions of a plagioclase crystal partially equilibrate at several stages during the increase of pressure (∼9–17 kbar at 600° C), possibly due to discrete episodes of fluid infiltration. In both these models, the extent of reaction may be limited by NaSi-CaAl interdiffusion in plagioclase.