Variations in fluid activity across the Etive thermal aureole, Scotland: evidence from cordierite volatile contents

Rigby, Martin J.; Droop, G. T. R.; Bromiley, Geoffrey

doi:10.1111/j.1525-1314.2007.00752.x

Cited by 23 publications

(19 citation statements)

References 37 publications

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“…Studies of the volatile contents of cordierite by SIMS (Moazzen et al. , 2001) and FTIR (Rigby et al. , 2008) showed that hornfelses from below the melt‐in isograd were fluid‐saturated at the peak of metamorphism whilst most sillimanite‐zone hornfelses were significantly fluid‐undersaturated, a conclusion supported by low water activity values calculated using phase equilibria (Droop & Moazzen, 2007).…”

Section: Geological Backgroundmentioning

confidence: 99%

“…The work harnesses the capabilities of THERMOCALC (Powell & Holland, 1988; Holland & Powell, 1998; White et al. , 2001, 2007) of modelling the phase assemblages of partially molten rocks, and applies them together with field studies of leucosome volumes to explore melt fertility of pelites in the well characterized 2.2 kbar thermal aureole of the Etive Complex, Scotland (Droop & Moazzen, 2007; Rigby et al. , 2008).…”

Section: Introductionmentioning

confidence: 99%

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

Droop¹,

Brodie²

2012

Journal Metamorphic Geology

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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 ...

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Section: Geological Backgroundmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Droop¹,

Brodie²

2012

Journal Metamorphic Geology

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“…Prior to retrogression all available free-H 2 O will be partitioned into the melt (Powell, 1983;Stevens and Clemens, 1993;Clemens and Droop, 1998;Johannes and Holtz, 1996;Harley and Carrington, 2001;Rigby et al, 2008d), only when the melt crystallizes and expels H 2 O as free-fluid phase will subsequent retrogression of the peak-metamorphic assemblage take place (Stevens, 1997;. The formation of muscovite adjacent to the leucosomes and the wide-spread retrogression of K-feldspar indicate that H 2 O was released from the melt, which in turn fuelled the retrogression.…”

Section: Resultsmentioning

confidence: 99%

“…In the absence of a melt phase, below the onset of partial melting, mineral assemblages are saturated with respect H 2 O (e.g. Powell, 1983;Harley et al, 2002;Rigby et al, 2008d;Rigby and Droop, 2008). At suprasolidus conditions, the metamorphic fluid is lost via partitioning into the melt phase (e.g.…”

Section: Estimated Fementioning

confidence: 99%

“…Conversely, the pseudosection approach can be applied to a wide range of rocks, irrespective of the variance. Consider the following mineral assemblage developed in a pelitic rock (MM184) from the Etive thermal aureole, Scotland (Droop and Moazzen, 2007;Rigby et al, 2008d): and + cd + ksp + bi. How can conventional thermobarometry determine the P-T conditions of equilibration for this assemblage?…”

Section: Estimated Fementioning

confidence: 99%

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Conflicting P–T paths within the Central Zone of the Limpopo Belt: A consequence of different thermobarometric methods?

Rigby

2009

Journal of African Earth Sciences

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a b s t r a c tA single metapelitic sample from the Verbaard locality, near Messina was investigated in order to construct a P-T path and moreover, highlight pertinent contradictions in the current P-T database. Interpretations based on P-T pseudosections, garnet isopleth thermobarometry and mineral mode/isopleth modelling indicate that the mineral assemblages, textures and zonations developed in the metapelite formed along a single clockwise P-T path. The metamorphic evolution is characterized by an early high-pressure phase at 10-11 kbar/800°C, followed by a simultaneous pressure decrease and temperature increase to $8/850°C and subsequent retrogression via decompression-cooling to 4-5 kbar at T < 650°C. Growth zoning in garnet provides evidence for an earlier, prograde history, however, as potential melt-loss was not accounted for this must be deemed speculative. The results of this study agree entirely with that of [Zeh, A., Klemd, R., Buhlmann, S., Barton, J.M. 2004. Pro-and retrograde P-T evolution of granulites of the Beit Bridge Complex (Limpopo Belt, South Africa); constraints from quantitative phase diagrams and geotectonic implications. Journal of Metamorphic Geology 22, 79-95], who adopted a similar approach to thermobarometry i.e. pseudosections. The results are, however, inconsistent with recent publications that argue for a twofold, metamorphic history defined by two decompression-cooling paths (DC1 $2.6 Ga and DC2 $2.0 Ga) that are separated by an isobaric heating path ($2.0 Ga). The disparity in the results obtained from different workers can be explained by an examination of the thermobarometric methods employed. The methodology employed to derive the twofold, polymetamorphic P-T path appears to be erroneous. At present, the most reliable and robust method for determining P-T paths is the pseudosection approach to thermobarometry. Future modelling of Limpopo Belt granulites should adopt this strategy and ensure potential melt-loss is taken into account. Alternatively, this potential problem can be avoided altogether by investigating rocks of mafic composition.

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Quantitative analysis of H2O and CO2 in cordierite using polarized FTIR spectroscopy

Ventura

Radica

Bellatreccia

et al. 2012

Contrib Mineral Petrol

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Variations in fluid activity across the Etive thermal aureole, Scotland: evidence from cordierite volatile contents

Cited by 23 publications

References 37 publications

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

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

Conflicting P–T paths within the Central Zone of the Limpopo Belt: A consequence of different thermobarometric methods?

Quantitative analysis of H2O and CO2 in cordierite using polarized FTIR spectroscopy

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