David Moynihan

Lower Palaeozoic rocks of the Meguma terrane were extensively deformed and metamorphosed during the Devonian Acadian orogeny, which resulted from oblique collision with the Neoproterozoic Avalon terrane. In the Yarmouth area of southwest Nova Scotia, the Cambrian-Ordovician Goldenville and Halifax (HF) Formations are overlain by the Silurian White Rock Formation (WRF), which forms the core of an axially-stretched, SW-plunging Acadian (D1) syncline. Acadian structures are well-preserved in the core of the syncline, which is dominated by mafic volcanogenic rocks. On the steep limbs, D1 structures in dominantly metasedimentary outer parts of the WRF and underlying HF are overprinted in broad shear zones, previously recognised as manifestations of the Carboniferous Alleghanian orogeny (D2).

D2 shear zones are wide (3-5 km), strike-parallel regions of pervasive, greenschist-facies, polyphase ductile strain, with foliation (S2) sub-parallel to their boundaries. The Cranberry Point Shear Zone (CPSZ), on the SE-dipping, NW limb of the Yarmouth syncline is characterised by pitching lineations and flattening strain, compatible with triclinic transpression. Kinematic indicators are largely absent from central regions of the CPSZ, but indicate a component of (shear zone) centre-side-up on each of its margins. The Chebogue Point Shear Zone, on the opposite limb of the syncline, is similar to the CPSZ in many respects, but is poorly exposed. A component of NW-side-up is indicated on its SE margin. Although D2 deformation was concentrated on steep limbs, pelitic rocks in the core of the syncline were also affected.

Metamorphic grade ranges from biotite zone in the Goldenville Formation and central parts of the WRF to staurolite-andalusite zone within each of the D2 shear zones. P-T estimates obtained using conventional and multi-equilibrium thermobarometric techniques, together with constraints from petrogenetic grids, suggest peak M1 conditions of 550-600 0C and approximately 4 kb in outer parts of the WRF and HF. Peak metamorphism was contemporaneous with the latter stages of D1 deformation, ca. 380 Ma. M2 retrograde metamorphism accompanied D2 deformation at a temperature of approximately 400 0C. Ubiquitous ca. 325 Ma laserprobe and step-heating ⁴⁰Ar/³⁹Ar muscovite ages record syn/post-M2 cooling. Older ages obtained from the central parts of pegmatitic muscovite record fractional ⁴⁰Ar loss and place a minimum age of 360 Ma on post-M1 cooling below approximately 350 ^oC.

The presence of higher-grade rocks within Alleghanian shear zones is interpreted to reflect differential exhumation of M1 assemblages, possibly as a result of variable flattening during D2 (325 Ma) transpression. This suggestion is supported by kinematic indicators on shear zone margins.

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Supervisor: Rebecca A. Jamieson