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Journal of Petrology | Volume 40 | Number 6 | Pages 935-956 | 1999
© Oxford University Press 1999

Megacrysts and Associated Xenoliths: Evidence for Migration of Geochemically Enriched Melts in the Upper Mantle beneath Scotland

B. G. J. Upton1,*, R. W. Hinton1, P. Aspen1, A. Finch2 and J. W. Valley3

1 Department of Geology and Geophysics, Edinburgh University Edinburgh EH9 3JW, UK
2 Faculty of Science and Computing, Luton University Luton LU1 3JU, UK
3 Department of Geology and Geophysics, University of Wisconsin Madison, WI 53706, USA

Received January 22, 1997; Revised typescript accepted December 8, 1998


  Abstract

Megacrysts, principally anorthoclase, Fe-rich biotite, clinopyroxene, magnetite, zircon and apatite, occur in alkali basaltic hosts at a number of Scottish localities. These minerals occur, not only as discrete, and composite megacrysts, but also as polycrystalline syenite (anorthoclasite) xenoliths. Composite xenoliths provide evidence that the anorthoclasites may occur as (pegmatitic) veins traversing pyroxenitic wall-rocks which may themselves be localized metasomatized peridotites within the shallow mantle. The anorthoclasites crystallized from highly trace element enriched melts which, in the case of the most geochemically extreme samples, were also peraluminous. Ion microprobe analyses show that the peraluminous (corundum-bearing) anorthoclasites comprise light rare earth element (LREE)-enriched alkali feldspars together with corundum and Nb-rich oxides (ilmenorutile, samarskite, yttro-niobate and columbite). The high contents of incompatible elements, together with oxygen isotope data, indicate crystallization of these syenitic facies from felsic melts, possibly originating through partial melting of metasomatized mantle lithologies. The aluminous character may be explained in terms of preferential loss of alkalis in fugitive carbonatitic fractions separated from the felsic melts.

KEY WORDS: anorthoclase; corundum; mantle; megacrysts; Scotland

* Corresponding author.


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