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Exsolution and alteration microtextures in alkali feldspar phenocrysts from the Shap granite

Published online by Cambridge University Press:  05 July 2018

Martin R. Lee ,
Kim A. Waldron  and
Ian Parsons
Martin R. Lee
Affiliation:
Department of Geology and Geophysics, The University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, Scotland
Kim A. Waldron
Affiliation:
Department of Geology and Geophysics, The University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, Scotland
Ian Parsons
Affiliation:
Department of Geology and Geophysics, The University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, Scotland
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Abstract

Alkali feldspar phenocrysts (bulk composition Or75.0Ab24.6An0.4) in the subsolvus Shap granite comprise a fine-scale mixture of subregular pristine crypto- and micro-perthites with altered, micropore-rich feldspar with irregular microstructures. The regular perthites are strain-controlled intergrowths of Albite and/or Periclinetwinned albite exsolution lamellae within tweed orthoclase. The microperthites formed at ⩽ 590°C by heterogeneous nucleation of thin albite films which coarsened to > 1 µm length. Cryptoperthites developed at < 400°C by homogeneous nucleation of sub-µm long platelets between films. Platelets are coherent, but the coarser microperthite lamellae are semi-coherent, with pairs of misfit dislocations sub-regularly spaced along the albite-orthoclase interface. As much as 30% of any one feldspar crystal is turbid, a result of the formation of numerous µm to sub-µm sized micropores during deuteric alteration. In some areas, deuteric fluids gained access to the interior of feldspar crystals by exploiting semi-coherent film lamellae. Albite was selectively dissolved and micropore-rich irregular microcline was reprecipitated in its place. In other parts of the feldspars deuteric recrystallization completely cross-cuts the pristine microtextures and patch perthites have formed. These are coarse, incoherent to semi-coherent intergrowths of irregular microcline (replacing tweed orthoclase) and Albite-twinned albite. The deuteric reactions occurred at < 400°C; the main driving force for dissolution and reprecipitation was decrease in the elastic strain energy at the coherent interfaces of crypto-and micro-perthite lamellae, and the recrystallization of tweed orthoclase to irregular microcline.

Keywords

alkali feldsparargon systematicsdeuteric alterationdislocationselectron microscopyetchingexsolutiongraniteorthoclasemicroclineperthite
Type
Mineralogy
Information
Mineralogical Magazine , Volume 59 , Issue 394 , March 1995 , pp. 63 - 78
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1995

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Footnotes

*

Present address: Department of Geology, Colgate University, Hamilton, NY 13346, USA

References

Aberdam, D. (1965) Utilisation de la microscopie electronique pour l'etude des feldspaths. Observations sur des microperthites. Sci. de la Terre, 6, 76 pp.Google Scholar
Aveline, W. T., Hughes, T. McK. and Strahan, A. (1888) The geology of the country around Kendal, Sedbergh, Bowness and Tebay. Mem. Geol. Surv. U. K., 94 pp.Google Scholar
Bambauer, H. U., Krause, C. and Kroll, H. (1989) TEM-investigation of the sanidine/microcline transition across metamorphic zones. Eur. J. Mineral., 1, 47–58.CrossRefGoogle Scholar
Bott, M. H. P. (1974) The geological interpretation of a gravity survey of the English Lake District and the Vale of Eden. J. Geol. Soc. London, 130, 309–31.CrossRefGoogle Scholar
Brown, W. L. and Macaudiere, J. (1986) Mechanical twinning of plagioclase in a deformed meta-anorthosite. Contrib. Mineral. Petrol., 92, 44–56.CrossRefGoogle Scholar
Brown, W. L. and Parsons, I. (1983) Nucleation on perthite-perthite boundaries and exsolution mechanisms in alkali feldspars. Phys. Chem. Minerals, 10, 55–61.CrossRefGoogle Scholar
Brown, W. L. and Parsons, I. (1984) The nature of potassium feldspar, exsolution microtextures and development of dislocations as a function of composition in perthitic alkali feldspars. Contrib. Mineral Petrol, 86, 335–41.CrossRefGoogle Scholar
Brown, W. L. and Parsons, I. (1988a) Zoned ternary feldspars in the Klokken intrusion: exsolution microtextures and mechanisms. Contrib. Mineral. Petrol, 98, 444–54.CrossRefGoogle Scholar
Brown, W. L. and Parsons, I. (1988fo) Intra-and intergrain exchange and geothermometry in granu-lite-facies feldspars. Terra Cognita, 8, 263.Google Scholar
Brown, W. L. and Parsons, I. (1989) Alkali feldspars: ordering rates, phase transformations and behaviour diagrams for igneous rocks. Mineral Mag., S3, 25–42.CrossRefGoogle Scholar
Brown, W. L. and Parsons, I. (1993) Stored elastic strain energy: the driving force for low-temperature reactivity and alteration of alkali feldspar. In Defects and Processes in the Solid State: Geoscience Applications. The McLaren Volume, Boland, J. and Fitz Gerald, J. D., (eds.), 267-90.Google Scholar
Burgess, R., Kelley, S. P., Parsons, I., Walker, F. D. L. and Worden, R. H. (1992). 40Ar-39Ar analysis of perthite microtextures and fluid inclusions in alkali feldspars from the Klokken syenite, South Greenland. Earth Planet. Sci. Letters, 109, 147–67.CrossRefGoogle Scholar
Caunt, S. L. (1986) Igneous and metamorphic processes in the Shap granite and its aureole. Unpubl. Ph.D. thesis, University of Leeds.Google Scholar
Champness, P. E. and Lorimer, G. W. (1976) Exsolution in silicates. In Electron Microscopy in Mineralogy, Wenk, H. R. (ed.), 174-204.Google Scholar
Elkins, L. T. and Grove, T. L. (1990) Ternary feldspar experiments and thermodynamic models. Amer. Mineral, 75, 544–59.Google Scholar
Evangelakakis, C, Kroll, H., Voll, G., Wenk, H-R., Meisheng, H. and Kopcke, J. (1993) Low-temperature coherent exsolution in alkali feldspars from high-grade metamorphic rocks of Sri Lanka. Contrib. Mineral. Petrol, 114, 519–32.CrossRefGoogle Scholar
Firman, R. J. (1957) Fissure metasomatism in volcanic rocks adjacent to the Shap granite, Westmorland. Q. J. Geol. Soc. London, 113, 205–22.CrossRefGoogle Scholar
Firman, R. J. (1978) Intrusions. In The Geology of the Lake District, Moseley, F. (ed.), 146-67.Google Scholar
Fitz Gerald, J. D. and Harrison, T. M. (1993) Argon diffusion in K-feldspar I: microstructures in MH-10. Contrib. Mineral. Petrol, 113, 367–80.CrossRefGoogle Scholar
Fleet, M. E. (1982) Orientation of phase and domain boundaries in crystalline solids. Amer. Mineral, 67, 926–36.Google Scholar
Fuhrman, M. L. and Lindsley, D.H. (1988) Ternary-feldspar modelling and thermometry. Amer. Mineral, 73, 201–15.Google Scholar
Gandais, M., Guillemin, C. and Willaime, C. (1974) Study of boundaries in cryptoperthites. Eighth Int. Cong. Elec. Micros. Canberra, 508-9.Google Scholar
Gittos, M. F., Lorimer, G. W. and Champness, P. E. (1976) The phase distributions in some exsolved amphiboles. In Electron Microscopy in Mineralogy, Wenk, H. R. (ed.), 238-47.Google Scholar
Grantham, D. R. (1928) The petrology of the Shap granite. Proc. Geol. Ass., 39, 299–331.CrossRefGoogle Scholar
Guthrie, G. D. and Veblen, D. R. (1991) Turbid alkali feldspars from the Isle of Skye, northwest Scotland. Contrib. Mineral. Petrol, 108, 298–304.CrossRefGoogle Scholar
Harker, A. and Marr, J. E. (1891) The Shap granite and associated igneous and metamorphic rocks. Q. J. Geol. Soc. London, 47, 266–327.CrossRefGoogle Scholar
Harker, A. and Marr, J. E. (1893) Supplementary notes on the metamorphic rocks around the Shap granite. Q. J. Geol Soc. London, 49, 359–71.CrossRefGoogle Scholar
Lee, M. K. (1986) A new gravity survey of the Lake District and three-dimensional model of the granite batholith. J. Geol. Soc. London, 143, 425–35.CrossRefGoogle Scholar
Locke, C. A. and Brown, G. C. (1978) Geophysical constraints on structure and emplacement of Shap granite. Nature, 272, 526–8.CrossRefGoogle Scholar
Nicholson, H. A. (1868) On the granite of Shap in Westmorland. Trans. Edinb. Geol. Soc, 1, 133–7.CrossRefGoogle Scholar
Parsons, I. (1978) Feldspars and fluids in cooling plutons. Mineral. Mag., 42, 1–17.CrossRefGoogle Scholar
Parsons, I., Rex, D. C, Guise, P. and Halliday, A. N. (1988) Argon-loss by alkali feldspars. Geochim. Cosmochim. Acta, 52, 1097–112.CrossRefGoogle Scholar
Smith, J. V. and Brown, W. L. (1988) Feldspar minerals. Vol. 1, Crystal structures, chemical and microtextural properties. 2nd edition. Springer-Verlag, Berlin.Google Scholar
Smith, P. and Parsons, I. (1974) The alkali feldspar solvus at 1 kilobar water vapour pressure. Mineral. Mag., 39, 747-67.CrossRefGoogle Scholar
Soper, N. J. and Kneller, B. C. (1990) Cleaved microgranite dykes of the Shap swarm in the Silurian of NW England. Geol. J., 25, 161–70.CrossRefGoogle Scholar
Wadge, A. J., Gale, N. H., Beckinsale, R. D. and Rundle, C. C. (1978) A Rb-Sr isochron for the Shap granite. Proc. Yorks. Geol. Soc, 42, 297–305.CrossRefGoogle Scholar
Waldron, K. and Parsons, I. (1992) Feldspar micro-textures and multistage thermal history of syenites from the Coldwell Complex, Ontario. Contrib. Mineral. Petrol, 111, 222–34.CrossRefGoogle Scholar
Waldron, K., Parsons, I. and Brown, W. L. (1993) Solution-redeposition and the orthoclase-microcline transformation: evidence from granulites and relevance to 18O exchange. Mineral. Mag., 57, 687–95.CrossRefGoogle Scholar
Waldron, K., Lee, M. R. and Parsons, I. (1994) The microstructures of perthitic alkali feldspars revealed by hydrofluoric acid etching. Contrib. Mineral. Petrol., 116, 360–4.CrossRefGoogle Scholar
Walker, F. D. L. (1990) Ion microprobe study of intragrain micropermeability in alkali feldspar. Contrib. Mineral. Petrol., 106, 124–8.Google Scholar
Walker, F. D. L. (1991) Micropores in alkali feldspars. Unpubl. Ph.D. thesis. University of Edinburgh.Google Scholar
Willaime, C. and Brown, W. L. (1974) A coherent elastic model for the determination of the orientation of exsolution boundaries: application to feldspars. Acta Crystallogr., A30, 316–31.CrossRefGoogle Scholar
Willaime, C, Brown, W. L. and Gandais, M. (1976) Physical aspects of exsolution in natural alkali feldspars. In Electron Microscopy in Mineralogy, Wenk, H. R., (ed.), 248-57.CrossRefGoogle Scholar
Wilson, M. J. and McHardy, W. J. (1980) Experimental etching of a microcline perthite and implications regarding natural weathering. J. Microsc, 120, 291–302.CrossRefGoogle Scholar
Worden, R., Walker, F. D. L., Parsons, I. and Brown, W. L. (1990) Development of microporosity, diffusion channels and deuteric coarsening in perthitic alkali feldspars. Contrib. Mineral. Petrol., 104, 507–14.CrossRefGoogle Scholar
Zeitler, P. K. and Fitz Gerald, J. D. (1986) Saddle-shaped mPitP9Ax age spectra from young, micro-structurally complex potassium feldspars. Geochim. Cosmochim. Acta, 50, 1185–99.CrossRefGoogle Scholar