The Geochemistry of Thermal Aureoles at Cashel, Co. Galway and Comrie, Perthshire

[摘要] Detailed investigations of the Cashel-Lough Wheelaun and Comrie thermal aureoles show that similar intensive mineralogical changes have taken place in both aureoles irrespective of the intrusive and country rock types and the regional metamorphism experienced by the country rocks. The chemistry in the two aureoles also generally shows similar trends although original sedimentary variations across the Aberfoyle slates severely affected the behaviour of most elements. In the Cashel thermal aureole, it is shown that the elements which are commonly enriched in acidic rocks viz : Si, K, Na, P, U, Rb, Ba, Th and to some extent Zr were removed from the envelope hornfelses, those commonly enriched in mafic rocks viz: Mg, Fe, Cr, Ni, and Co were enriched whereas those elements which are intermediate in character viz: Al, Ti, Ca, and Sr were both enriched and removed. Eu was removed much less significantly compared to the other REE thus giving increasing positive Eu anomalies as the degree of hornfelsing and partial melting rise. In the Comrie thermal aureole, the sedimentary variations mention ed above strongly affected the variation of elements during hornfelsing and partial melting of the Aberfoyle Slates, although the available evidence suggests similar trends to the Cashel thermal aureole. Mineralogical, chemical and Sr isotope evidence show that the Cashel trondhjemite migmatite leucosomes were produced at degrees of partial melting of between 15 and 30% and their chemical compositions compare reasonably well with the melts derived from the metasediments as calculated using the strongly compatible elements Cr, Ni and Co. This conclusively indicates they were derived from the metasediments and so does the Cashel microgranite sill. The Comrie leucosomes are dominantly late stage magmatic injections although the melts, which were probably near eutectic in composition in part, derive from the partially melted metasediments. The elements as deduced from the Cashel, and probably also the Comrie and Cortlandt, New York aureoles, were fractionated into the melts in the following order :Si>K>Na>Ca>Mn>AI>Fe>Mg and Rb>Ba>Sr>Ga>Cr, Ni, Co. This calculated order of fractionation of elements, which is the opposite to magmatic crystallization, provides a unique picture on the mode of interaction of mantle derived magmas with the earth's crust and therefore gives clear explanation on how S-type granites and other crustally contaminated rock types are produced, especially as the melts derived from both the Cashel and Comrie hornfelses are S-type melts as defined by Chappell and White (1974).

[发布机构] University:University of Glasgow

[关键词] Geology, Geochemistry