The northernmost exposures of rocks formed during the Late Neoproterozoic Cadomian
orogeny in the Channel Islands–northern France region occur on Alderney. The island mainly comprises
foliated quartz diorite, once considered to be 2 Ga, pre-Cadomian basement, and an undeformed
basic to intermediate plutonic complex. A precise age of 610±2 Ma, based on U–Pb analyses
of single and small groups of zircons, for the foliated Fort Tourgis quartz diorite demonstrates that the
oldest rocks were emplaced and deformed during a Cadomian magmatic event. The age is virtually
identical to ages from similar, foliated syntectonic quartz diorite bodies on the islands of Guernsey
and Sark and at La Hague (north Normandy), indicating that this magmatic and deformational event
was regional in extent. Discordant zircon xenocrysts define an upper intercept age of c. 2 Ga indicating
the presence of Palaeoproterozoic basement at depth. Single zircons from the undeformed Bibette
Head granodiorite give a precise U–Pb age of 572±1 Ma. This age is closely similar to that for the
emplacement of the Northern Igneous Complex of Guernsey. The emerging data indicate that
Cadomian magmatism in the northern Channel Islands region was not a protracted continuum, but
occurred during two distinct, short-lived events separated by c. 30–40 my.