Scottish 'B' megacrysts and associated xenoliths: highly fractionated residues in the vicinity of the crust-mantle boundary

BGJ Upton, RW Hinton, P Aspen, Adrian Anthony Finch

Research output: Contribution to journalArticlepeer-review

99 Citations (Scopus)

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.

Original languageEnglish
Pages (from-to)935-956
Number of pages22
JournalJournal of Petrology
Volume40
Publication statusPublished - Jun 1999

Keywords

  • anorthoclase
  • corundum
  • mantle
  • megacrysts
  • Scotland
  • PARTITION-COEFFICIENTS
  • HIGH-PRESSURE
  • VOLCANIC-ROCKS
  • MAGMAS
  • ZIRCON
  • INCLUSIONS
  • ALKALINE
  • CORUNDUM
  • ELEMENTS
  • BASALTS

Fingerprint

Dive into the research topics of 'Scottish 'B' megacrysts and associated xenoliths: highly fractionated residues in the vicinity of the crust-mantle boundary'. Together they form a unique fingerprint.

Cite this