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

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doi: 10.1016/S0024-4937(98)00020-6
Authors:Clemens, J. D.; Droop, G. T. R.
Author Affiliations:Primary:
Kingston University, School of Geological Sciences, Kingston-upon-Thames, United Kingdom
University of Birmingham, United Kingdom
University of Manchester, United Kingdom
Volume Title:Generation of granitic rocks and deep crustal processes
Volume Authors:Clemens, J. D., editor; Hutton, D. H.
Source:Lithos (Oslo), 44(1-2), p.21-36; 51st EUG symposium on Generation of granitic rocks and deep crustal processes, Strasbourg, France, Mar. 23-27, 1997, edited by J. D. Clemens and D. H. Hutton. Publisher: Elsevier, Amsterdam, International. ISSN: 0024-4937
Publication Date:1998
Note:In English. 34 refs.; illus.
Summary:This paper presents a model for the longevity and physical behaviour of anatectic melts in the Earth's 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 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. Abstract Copyright (1998) Elsevier, B.V.
Subsections:Igneous petrology
Subjects:Anatexis; Charnockite; Cooling; Crust; Decompression; Fluid dynamics; Granites; Igneous rocks; Intrusions; Magmas; Melting; Melts; Metamorphic rocks; Migmatites; Mineral assemblages; P-T conditions; Partial melting; Phase equilibria; Plutonic rocks; Segregation; Thermal anomalies; Water content
Abstract Numbers:99M/3136
Record ID:1999016317
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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