Silica-undersaturated trachytic rocks of central Scotland

The paper presents a review of silica-undersaturated salic intrusions (nepheline-normative trachytes and phonolites) from the Carboniferous-Permian magmatic province in central Scotland. New whole-rock major and trace element analyses, together with Sr and Nd isotopic data, are presented for six intrusions (Bass Rock, North Berwick Law, Traprain Law, Hairy Craig, Fintry and Loudoun Hill) and two syenite autoliths within the Loudoun Hill intrusion. On the basis of the conventional total alkalis versus silica diagram, all six intrusions are trachytes except for Traprain Law, which is a phonolite. Traprain Law is the most highly evolved and the most undersaturated in silica. The Loudoun autoliths are the most basic (Mg numbers of 21.8-27.3) and are inferred to be fragments of cumulates entrained at shallow crustal depths. Whilst all eight samples have approximately constant initial ɛNd (+4.3-+1.99), initial 87Sr/86Sr data are more variable (0.725929-0.709817). Although four of the samples have isotopic characteristics on, or close to, the Sr-Nd mantle array, those from Fintry, Bass Rock, Loudoun Hill and North Berwick Law lie at higher 87Sr/86Sr<INF>i</INF> values, probably as a result of secondary alteration. From their petrographic and geochemical affinity, the six intrusions are considered to have had similar petrogenesis as late-stage residues from mildly silica-deficient basalt magmas, produced in deep crustal chambers and subsequently intruded with minimal crustal interaction.