Anatectic melt volumes in the thermal aureole of the Etive Complex, Scotland: the roles of fluid‐present and fluid‐absent melting

Abstract

Pelitic hornfelses within the inner thermal aureole of the Etive igneous complex underwent limited partial melting, generating agmatic micro‐stromatic migmatites. In this study, observed volume proportions of vein leucosomes in the migmatites are compared with modelled melt volumes in an attempt to constrain the controls on melting processes. Petrogenetic modelling in the MnNCKFMASHT system was performed on the compositions of 15 analysed Etive pelite samples using THERMOCALC. Melt modes were calculated at 2.2 kbar (the estimated pressure in the southern Etive aureole) from solidus temperatures to 800 °C for both fluid‐absent and fluid‐present conditions. Volume changes accompanying fluid‐absent melting at 2.2 kbar were also calculated. P–T pseudosections reproduce the zonal sequence of the southern Etive aureole fairly well. The modelled solidus temperatures of silica‐rich pelitic compositions are close to 680 °C at 2.2 kbar and, in the absence of free fluid, melt modes in such compositions rise to between 12 and 29% at 800 °C, half of which is typically produced over the narrow reaction interval in which orthopyroxene first appears. Silica‐poor compositions have solidus temperatures of up to ∼770 °C and yield <11.4% melt at 800 °C under fluid‐absent conditions. For conditions of excess H2O, modelled melt modes increase dramatically within ∼13 °C of the solidus, in some cases to >60%; by 800 °C they range from 61 to 88% and from 29 to 74% in silica‐rich and silica‐poor compositions, respectively. Calculated volume changes for fluid‐absent melting are positive for all modelled compositions and reach 4.5% in some silica‐rich compositions by 800 °C. Orthopyroxene formation is accompanied by a volume increase of up to 1.48% over a temperature increase of as little as 2.7 °C, supporting the arguments for melt‐induced ‘hydrofracturing’ as a viable melt‐escape mechanism in low‐P metamorphism. Mineral assemblages in the innermost aureole support previous conclusions that partial melting took place predominantly under fluid‐absent conditions. However, vein leucosome proportions, estimated by image analysis, do not show the expected correlation with grade, and are locally greatly in excess of melt modes predicted by fluid‐absent models, particularly close to the melt‐in isograd. Melting of interlayered psammites, addition of H2O from interlayered melt‐free rocks, and metastable persistence of muscovite are ruled out as major causes of the excess melt anomaly. The most likely cause, we believe, is that local variations existed in the amount of fluid available at the onset of melting, promoted by focussing of fluid released by dehydration in the middle and outer aureole; however, some redistribution of melt by compaction‐driven flow through the vein channel network cannot be ruled out. The formation of melt‐filled fractures in the inner Etive aureole was assisted by stresses that caused extension at high angles to the igneous contact. The fractures were probably caused either by transient pressure reduction in the ...

The mechanism of magma ascent through the solid lithosphere and relation between mantle and crustal diapirism: numerical modeling and natural examples

“…The second threshold value of a melt fraction accepted in the models could be attained during diapiric ascent of granite magma. Experimental data (Droop and Brodie, 2012) on melting of H 2 O-rich pelites above the wet solidus temperature of 680 °C at 2.2 kbar support the argument for reaching melt fractions of up to 50-60%.…”

Local partial melting of the lower crust triggered by hydration through melt–rock interaction: an example from Fiordland, New Zealand

“…Pattison & Harte, 1988;Finger & Clemens, 1995;Symmes & Ferry, 1995;Johnson et al, 2003;Collins et al, 2016) and rocks undergoing prograde metamorphism release H 2 O into the surrounding rocks (e.g. White et al, 2005;Droop & Brodie, 2012). Thrusting of hot rocks over cooler, water-rich rocks results in the release of H 2 O into overlying rocks (e.g.…”

Crystallization of Heterogeneous Pelitic Migmatites: Insights from Thermodynamic Modelling

“…The quantities of H 2 O estimated to have infiltrated supra-solidus pelites range from 0Á8 to 5Á5 wt % (Johnson et al, 2001;Otamendi & Patiño Douce, 2001;White et al, 2005;Genier et al, 2008;Ward et al, 2008;Braga & Massonne, 2011). Jung et al (2000) and Droop & Brodie (2012) concluded that an aqueous fluid infiltrated at temperatures slightly above the solidus. Although many fluid behaviours intermediate between dehydration melting and fluid-saturated melting are possible, here we concern ourselves with what we term excess-water melting.…”

“…Much emphasis has been placed on prograde melting reactions in migmatites (White et al, 2001(White et al, , 2003(White et al, , 2004(White et al, , 2007Droop & Brodie, 2012;Redler et al, 2013). However, forming a migmatite also requires consideration of the crystallization reactions and processes (Brown, 2002;White et al, 2004;White & Powell, 2010), which take place after the melt has segregated from its source and accumulated in melt-dominated sites (e.g.…”