Cognate Gabbroic Xenoliths from a Tholeiitic Subvolcanic Sill Complex: Implications for Fractional Crystallization and Crustal Contamination Processes

R. J. Preston1, 2 and B. R. Bell
Department of Geology and Applied Geology, University of Glasgow, Glasgow, G12 8QQ, UK
1Present address: Department of Geology and Petroleum Geology, Meston Building, King's College, University of Aberdeen, Old Aberdeen, AB24 3UE. Telephone: (01224) 273467 e:mail:
2SURRC contact: Dr G. Rogers - Isotope Geosciences Unit, Scottish Universities Research & Reactor Centre, East Kilbride, Glasgow, G75 0QF, UK.

Abstract: Intruded into the Palaeogene lava field and underlying Moine (Neoproterozoic) crystalline basement rocks around Loch Scridain, Isle of Mull, Scotland, is a suite of high-level, inclined, xenolithic sheets, ranging in composition from basalt, through andesite and dacite, to rhyolite. These sheets, associated with the Mull central volcano, were emplaced post 55 Ma. As well as numerous crustal xenoliths, the more basic members of the complex contain a diverse suite of ultrabasic and basic xenoliths. Xenolith types include feldspathic peridotite with cumulus olivine, pyroxenite, gabbro with cumulus plagioclase and cumulus clinopyroxene, and pure anorthosite. Mineralogical data, coupled with whole-rock major- and trace-element data from a small number of the xenoliths suggest that the xenoliths represent early-formed cumulates cognate with their host basalts. Sr and Nd isotope data from the xenoliths confirms the cognate origin, and also shows that the basic magmas suffered crustal contamination at an early stage.

Keywords: xenolith • feldspathic peridotite • pyroxenite • gabbro • anorthosite • fractional crystallization • crustal contamination

Mineralogical Magazine; June 1997 v. 61; no. 406; p. 329-349; DOI: 10.1180/minmag.1997.061.406.01
© 1997, The Mineralogical Society
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