Journal of Petrology, Volume 40, Issue 4: April 1999.

The Petrology of Mullite-bearing Peraluminous Xenoliths: Implications for Contamination Processes in Basaltic Magmas

R. JEREMY PRESTON¹, TIM J. DEMPSTER², BRIAN R. BELL² AND GRAEME ROGERS³

¹DEPARTMENT OF GEOLOGY AND PETROLEUM GEOLOGY, MESTON BUILDING, KING'S COLLEGE, UNIVERSITY OF ABERDEEN, OLD ABERDEEN AB24 3UE, UK
²DIVISION OF EARTH SCIENCES, DEPARTMENT OF GEOGRAPHY, UNIVERSITY OF GLASGOW, GLASGOW G12 8QQ, UK
³ISOTOPE GEOSCIENCES UNIT, SCOTTISH UNIVERSITIES RESEARCH AND REACTOR CENTRE, EAST KILBRIDE, GLASGOW G75 0QF, UK

A suite of high-level inclined sheets ranging in composition from basalt through to rhyolite is intruded into the Palaeogene lava field and underlying Moine Supergroup basement rocks around Loch Scridain, Isle of Mull, Scotland. Many of the sheets are highly xenolithic, containing a wide variety of crustal xenolith types derived from the Moine metasedimentary rocks, along with various gabbroic cumulate xenoliths. The most common xenolith types are almost pure quartzites and a variety of mullite-bearing aluminous buchites, many of the latter having thick crystalline selvages of plagioclase, corundum and aluminous spinel. The plagioclase is highly calcic (up to An₈₇), and adjacent to the host basalt is commonly oscillatory zoned, implying crystallization from a melt. Trapped between plagioclase crystals are pockets of quenched, contaminated basic melt, which contain skeletal plagioclase and clinopyroxene, and preserve evidence of local mixing between the host basalt and the aluminous crustal melts. Sr and Nd isotope values of the buchite cores [e.g. (⁸⁷Sr/⁸⁶Sr)₅₅ = 0·7074-0·7115], plagioclase selvages [e.g. (⁸⁷Sr/⁸⁶Sr)₅₅ = 0·7137-0·7148], and associated trapped melts [e.g. (⁸⁷Sr/⁸⁶Sr)₅₅ = 0·7126-0·7128], imply a complex series of magma-xenolith interactions. The textural characteristics, mineral chemistry and isotope geochemistry of these rims suggest that they have crystallized from a hybrid liquid formed by the complex interaction of the aluminous liquids with basic magmas. Such interaction proceeded via liquid-liquid diffusion, physical mixing of melts and a variety of reactions between the crystallization products of the buchites and the basaltic liquids. These crustal xenoliths preserve a detailed record of mineral-melt reactions within a suite of basaltic sheets, dominated by both the production of granitic melts and the `bulk' melting of Al-rich micaceous lithologies.

Keywords: xenolith;metasedimentary;mullite buchite; plagioclase; isotope geochemistry; crustal contamination; basalt

Pages 549-573