The Centre 3 layered gabbro intrusion, Ardnamurchan, NW Scotland

Detailed remapping of the Palaeogene Ardnamurchan Centre 3 gabbros, NW Scotland, suggests that this classic sequence of ring-intrusions forms a composite layered lopolith. The area mapped by previous studies as the Great Eucrite gabbro intrusion comprises 70% by area of Centre 3. Field observations suggest that most of the other smaller ring-intrusions of Centre 3 (interior to the Great Eucrite) constitute either distinct petrological facies of the same intrusion, or included country-rock or peridotite blocks. These observations, together with syn-magmatically deformed inward-dipping modal layering, are used here to support the interpretation that significant central sagging occurred in the intrusion at a late stage in its crystallization history.

Tectono-thermal evolution of deep crust in a Mesoproterozoic continental collision setting: the Manicouagan example

High-pressure metamorphic rocks in continental collision belts provide important information on the orogenic evolution of deep crust. In the Grenville Province, a prime example is the Manicouagan Imbricate Zone (MIZ), a Mesoproterozoic crustal section that was metamorphosed up to 1800 MPa and 850-950°C at ~1050 Ma. High temperatures and the presence of synmetamorphic within-plate tholeiitic gabbro intrusion in the MIZ attest to thermal perturbation during burial and emplacement of mantle-derived melts, consistent with thinning of lithospheric mantle. Subsequently, the MIZ was extruded by a combination of northwest-directed thrusting over a crustal-scale ramp and extension at higher crustal levels. In the middle MIZ, extension was coeval with thrusting, but perpendicular to the thrust direction, and was apparently controlled by ductility contrasts between slices. In contrast, the highest levels show evidence of southeast-directed extension, i.e., of opposite sense to the thrust direction, that postdated thrusting within the MIZ. This episode was followed by renewed northwest-directed thrusting of medium-pressure Mesoproterozoic units over the MIZ, and then by final southeast-directed extension. Overall configuration of the MIZ was achieved between ~1040-990 Ma and is consistent with outward propagation of the orogen. Alternating periods of thrusting and extension displacement are a likely result of adjustments permitting maintenance of dynamic equilibrium within the advancing stack. The tectonothermal characteristics of the MIZ allow comparison between the Grenville Province and younger, thermally perturbed orogens (i.e., Variscan Belt) and may be the final product of a Tibetan Plateau style evolution.