Microstructural and petrological data from the Jumping Brook metamorphic suite, western Cape Breton Highlands, suggest that a single episode of syntectonic prograde metamorphism, followed by uplift, cooling and associated retrogression, affected these rocks during mid-Palaeozoic times. Microstructures indicative of progressive crenulation foliation development can be traced from low-grade (chlorite zone) through high-grade (kyanite zone) rocks, allowing a clear sequenm of porphyroblast growth to be established. Metamorphic reactions and P-T calculations suggest L. ' * * --'metamorphic conditions of 700-7WC at 8-10 kbar were achieved in kyanite zone rocks. Although a complete P-T-r path was not defined, combined petrological and geochronological data can be used to constrain computed P-T-t models. These models suggest that a component of post-metamorphic tectonic exhumation is required to explain the observed times of cooling and uplift. The microstructural and petrological data to not support the interpretation that the high-grade rocks represent preexisting crystalline basement. Indeed, the metamorphic history, geochronology and computed tectonic models all point to a single, short-lived episode of Silurian-Devonian volcanism, intrusion, convergence, regional metamorphism and uplift, probably resulting from collision tectonics at an irregular continental margin.Key w d Appalachian geology; crenulation foliation; microstructures; porphyroblast growth; prograde metamorphism; P-T-r models; tectonic exhumation. REGIONAL SETTINGThe metamorphic rocks in question form part of the Jumping Brook metamorphic suite (Fig. l), which includes tholeiitic metabasalts with island-arc affinities (Connors, 4e7 U# H. E. PLINT & R. A . J A M I E S O N 1986) overlain by a thick sequence of silicic, pelitic, semi-pelitic and psammitic schists and gneisses (Jamieson er ul ., 1987). Metamorphic grade increases from west to east, roughly parallel to lithological boundaries, towards a granitoid gneiss complex that extends through much of the central Cape Breton Highlands. The study concentrated on metasedimentary rocks northcast of Cheticamp (Fig. 1). where porphyroblasts are well developed at all metamorphic grades and where later structural disruption is minimal. Throughout this paper, reference is made to 'low-grade' rocks, comprising chlorite through garnet-zone phyllites and schists, 'medium-grade' rocks, comprising staurolite-bearing phyllites and schists, and 'high-grade' rocks, comprising kyanite-bearing schists and gneisses. The critical question is whether the medium-grade, staurolitebearing rocks are unconformable on, or gradational into, the kyanite-zone gneisses.The regional structural framework provides a context in which to view the microstructural observations. Early tight to isoclinal folds produced the main foliation, S1, which is generally parallel to transposed compositional layering. Regional dips are moderate to steep to the west and are folded around a major north-plunging D3 antiform ( Fig. 1; Craw, 1984;Plint, 198...

Mapping of metavolcanic, metasedimentary, and metaplutonic rocks in the vicinity of the Northeast Margaree River, west-central Cape Breton Highlands, has been carried out on 1:10 000 and 1:25 000 scales. The Jumping Brook metamorphic suite (redefined) consists of a lower unit of metabasalts and associated pyroclastic and sedimentary rocks (Faribault Brook metavolcanics), overlain by quartz-rich sedimentary rocks with local conglomerate and tuff (Barren Brook schist), and semi-pelitic to psammitic schists (Dauphinee Brook schist), which grade into porphyroblastic schists ranging up to staurolite-kyanite grade (Corney Brook schist). Medium grained amphibolite (George Brook amphibolite) with relict dioritic texture is common in the sequence and probably represents synvolcanic intrusions. The Belle Cote Road gneiss, an Ordovician-Silurian tonalitic to granodioritic orthogneiss considered part of the Pleasant Bay Complex, forms an elongated belt through the central highlands. These units can be traced from the Cheticamp River into the central highlands in the area bounded by Highland and Fielding roads. They are intruded by a range of foliated to undeformed Silurian to Devonian granitoid rocks. Massive sulphide deposits, common in the vicinity of Faribault Brook, are localized near the transition from metavolcanic to metasedimentary rocks. However, shearing has remobilized sulphides in some deposits, while others are associated with unrelated, later shear zones. The age of the volcanic and sedimentary protoliths of the Jumping Brook metamorphic suite is not known, but a Lower Paleozoic age, previously considered unlikely, now appears possible.

New 40Ar/39Ar analyses of hornblende, muscovite, biotite, and K-feldspar constrain the timing of deformation and cooling of the southern Taltson magmatic zone, which underwent lower granulite to upper amphibolite grade deformation, in part synchronous with voluminous 1.99–1.92 Ga magmatism. New data are combined with existing K–Ar dates into a regional cooling framework to provide thermotemporal constraints on the deformational history. 40Ar/39Ar hornblende ages of ca. 1900 Ma are interpreted to record relatively rapid cooling following ductile thrusting on the Andrew Lake shear zone, and younger anatectic magmatism. These data, with published K–Ar and U–Pb data, support relatively rapid cooling of the Taltson magmatic zone from monazite closure temperature of 725 °C at ca. 1930 Ma to 525 °C at ca. 1900 Ma. Cooling rate estimates are about 7 °C/Ma, which suggests moderate exhumation rates during the high-grade part of the deformational history. A muscovite 40Ar/39Ar plateau age of 1803 ± 11 Ma is consistent with the mean muscovite K–Ar age of 1792 Ma, indicating regional cooling through about 350 °C at ca. 1800 Ma. 40Ar/39Ar ages from magmatic biotite of 1856 and 1799 Ma also suggest slow cooling during greenschist grade deformation, which can be no older than ca. 1860 Ma. A K-feldspar 40Ar/39Ar age of 1681 Ma provides a lower limit for the time of greenschist grade deformation. Cooling rate estimates during amphibolite to greenschist grade deformation are 1.75–2.25 °C/Ma.

No abstract