Three Angolan carbonatites were selected to evaluate the change in composition of pyrochlores during magmatic evolution: the Tchivira carbonatites occur in a plutonic complex, the Bonga carbonatites represent hypabyssal carbonatites and the Catanda carbonatites are volcanic in origin. In Tchivira pyrochlore, zoning is poorly developed; fluorine is dominant at the Y site; chemical zoning may arise as a result of substitutions for Nb in the B site; and the rare earth element (REE), U, Th and large-ion lithophile element (LILE) contents are very low. Pyrochlores from Bonga show oscillatory zonation; the F and Na contents are lower than those in the pyrochlores from Tchivira; and as substitution of Na at the A site increases, the Th, U, REE contents and inferred vacancies also increase. Pyrochlores from Catanda display complex textures. They generally have a rounded corroded core, which is mantled by two or three later generations. The core composition is similar to the Bonga pyrochlores. The rims are enriched in Zr, Ta, Th, Ce and U, but depleted in F and Na. In pyrochlores from the Angolan carbonatites, the F and Na contents decrease from plutonic to volcanic settings and there is enrichment of Th, U and REE in the A site and Ta and Zr in the B site. Zoning may be explained by changes in the activity of F, due to the crystallization of fluorite or apatite in the plutonic and hypabyssal carbonatites, or to volatile exsolution in the volcanic carbonatites.

The Loch Shin Line is a geological–geophysical lineament associated with a zone of mantle-derived appinites, granites and strike-slip faulting that runs NW–SE across the Moine Nappe, northern Scotland. U–Pb zircon and Re–Os molybdenite dating of the Loch Shin and Grudie plutons, which lie immediately SW of the NW–SE Loch Shin–Strath Fleet fault system, yield c . 427–430 Ma ages that overlap within error. They also coincide with previously obtained U–Pb zircon ages for the Rogart pluton, which lies along-strike to the SE. Field and microstructural observations confirm the similarity and contemporaneous nature of the plutons and associated sulphide mineralization. Fluid inclusion analyses place further constraints on the P – T – X conditions during regional late Caledonian exhumation of the Moine Nappe. Synchronous to slightly younger brittle dextral strike-slip faulting along the WNW–ESE Loch Shin–Strath Fleet Fault System was probably antithetic to sinistral movements along the nearby Great Glen Fault Zone. Our findings support the hypothesis that the Loch Shin Line acted as a deep crustal channelway controlling the ascent and emplacement of Silurian magmas into the overlying Moine Nappe. We propose that this deep structure corresponds to the southeastern continuation of the Precambrian-age Laxford Front shear zone in the buried Lewisian autochthon. Supplementary Material: Field photographs, photomicrographs and fluid inclusion information are available at http://www.geolsoc.org.uk/SUP18859 .