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Journal of the Geological Society; 1996; v. 153; issue.4; p. 625-635;
DOI: 10.1144/gsjgs.153.4.0625
© 1996 Geological Society of London

Article

Crystallization of the Shap Granite: evidence from zoned K-feldspar megacrysts

R. A. COX1,3, T. J. DEMPSTER1, B. R. BELL1 & G. ROGERS2

1 Department of Geology & Applied Geology, University of Glasgow, Glasgow G12 8QQ, UK
2 Isotope Geosciences Unit, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow C75 0QU, UK

Textural, chemical and Sr isotopic studies of feldspars from the subsolvus Shap granite, northern England, demonstrate that a number of magma mixing events have dominated the evolution of this pluton. K-feldspar megacrysts are phenocrysts formed in the magma chamber. They contain a number of Ba-rich zones that developed during periods of slight dissolution and regrowth linked to the hybridization of the granite by the intrusion of basic magmas. Diorite enclaves represent the relicts of these magmas and these also contain K-feldspar megacrysts, which show evidence of major dissolution. They are xenocrysts picked up from the host granite and incorporated in the basic magma. Increasing H2O contents during fractional crystallization caused a late switch from growth of megacrysts to finer-grained K-feldspars in the matrix. The chemically and isotopically zoned K-feldspar megacrysts preserve an exceptional record of the evolution of the magma, and the zones also had a significant influence on the development of exsolution microtextures during cooling.


Keywords: K-feldspar, granites, 87Sr/86Sr, barium, hybridization.




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