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The Bay of Islands Complex of the Humber Arm allochthon, west Newfoundland, contains the best-exposed ophiolite in the Appalachian Orogen. Associated structural slices, the Little Port and Skinner Cove complexes, also contain rocks formed in an oceanic domain, although their relationship to the Bay of Islands Complex remains controversial.To constrain the origin of the complexes and obtain a better understanding of the geology of the Humber Arm allochthon, we have undertaken an integrated geochronological, geochemical, and isotopic study. A U/Pb zircon age of [Formula: see text] Ma for the Little Port Complex and a zircon and baddeleyite age of 484 ± 5 Ma for the Bay of Islands Complex have been obtained. Geochemical and isotopic data on trondhjemitic rocks from the two complexes indicate that petrogenetic models for these rocks must account for fundamental differences in source materials and mineralogy during differentiation. The Little Port Complex trondhjemites are characterized by initial εNd of −1 to +1, whereas those in the Bay of Islands have εNd of +6.5. Geochemical signatures in mafic and felsic volcanics of the complexes are diverse, and show a complete gradation between arc and non-arc.The Bay of Islands and Little Port complexes are not related by any form of a major mid-ocean-ridge – transform-fault model. An alternative model to explain the relationships between the two complexes interprets the Little Port as arc-related and the Bay of Islands as a suprasubduction-zone ophiolite.

The timing of appinitic and related granitic magmatism throughout the W Highlands of Scotland has been investigated using U-Pb geochronological techniques. An appinite from the Garabal Hill/Glen Fyne complex is dated at 429 ±2 Ma (zircon) and 422 ±3 Ma (titanite); appinites from Arrochar and Rubha Mór give titanite ages of 426 ± 3 Ma and 427 ± 3 Ma respectively. The Strontian granodiorite is 425 ±3 Ma (zircon) and 423 ±3 Ma (titanite), and a pyroxene-mica diorite from Rat again gives an age of 425 ± 3 Ma. The data show the value of using titanite to date basic rocks which may lack zircon, and of using inheritance-free phases in rocks which contain mostly inherited zircon. The narrow range of ages for this magmatism reported here contrasts with a broad smear of previously published ages, and indicates that the middle Silurian was a major period of transcurrent fault movement throughout the W Highlands. Voluminous magmatism of this age is also found in Newfoundland, highlighting the importance of this event in the wider context of the Caledonide orogen.

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