Contrasting metamorphic terranes near Chéticamp, Cape Breton Highlands, Nova Scotia

Abstract

Two contrasting metamorphic terranes can be recognized in northwestern Cape Breton Island. One terrane (Pleasant Bay complex) consists of biotite gneiss and quartzite with minor calc-silicate lenses that were metamorphosed in Late Precambrian time (about 550 Ma) and were subsequently intruded by Silurian salic and mafic plutons that were, in turn, deformed and intruded by granite in Devonian time. The other terrane (Jumping Brook complex) consists of volcanogenic and sedimentary schists of probable Silurian age that were metamorphosed in Devonian time. P-T estimates indicate that the older parts of the Pleasant Bay complex were metamorphosed at about 790°C and 7 kbar (1 kbar = 100 MPa) at low to moderate water fugacities during a major intrusive episode. The Jumping Brook complex exhibits a single progressive metamorphic sequence now disrupted by faulting. P-T conditions during this Devonian (370 -390 Ma) metamorphism varied from greenschist (300°C at < 3 kbar) to amphibolite (650°C at 4 kbar) facies. Metamorphism probably occurred in a thermal dome. The data suggest a moderately deformed basement-cover relation between the Pleasant Bay and Jumping Brook complexes. Deux terranes mCtamorphiques differents sont reconnus dans le nord-ouest de l'ile du Cap-Breton. Un terrane (complexe de Pleasant Bay) est form6 de gneiss i biotite et de quartzite accompagnCs d'une faible quantitC de lentilles calco-silicatCes qui furent mCtamorphisCes au temps du Pkcambrien tardif (environ 550 Ma) et subsCquemment recoupCes par des plutons mafiques et saliques siluriens qui i leur tour furent dCformCs et recoupCs par un granite au Dtvonien. L'autre terrane (complexe de Jumping Brook) inclut des schistes ~Cdimentaires et volcanogCniques, possiblement d'bge silurien, qui furent mCtamorphisCs au DCvonien. Les P-T estimCes indiquent que les zones les plus bgCes du complexe de Pleasant Bay furent mCtamorphisCes i environ 790°C et h 7 kbar (1 kbar = 100 MPa), dans des conditions de fugacitCs aqueuses faibles i modCrCes durant la plus grande pCriode de l'intrusion. Le complexe de Jumping Brook est form6 d'une unique stquence i mCtamorphisme progressif et maintenant disloquCe par des failles. Les conditions de P-Tdurant 1'CvCnement mktamorphique au DCvonien (370-380 Ma) variaient du facibs des schistes verts (300°C i < 3 kbar) au facibs des amphibolites (650°C et 4 kbar). Les transformations metamorphiques ont probablement en lieu dans un d6me thermique. Les rksultats indiquent une relation substratum-couverture modCrCment dCformCs entre les complexes de Pleasant Bay et de Jumping Brook.[Traduit par la revue]Can. J. Earth Sci. 24, 2422Sci. 24, -2435Sci. 24, (1987 Introduction Distinction between basement and cover rocks in high-grade polymetamorphic terranes can rarely be made according to unambiguous and noncontroversial criteria. Structural complexity and metamorphic overprinting obscure field criteria for distinguishing basement and cover, and thermal and hydrothermal events may partially or completely reset isotopic systems. The weste...

The significance of Mn‐rich ilmenite and the determination of P–T paths from zoned garnet in metasedimentary rocks from the western Cape Breton Highlands, Nova Scotia

“…Interestingly, the P–T estimates of Currie () and Currie and Lynch () suggest lower pressures in the JBMS as compared with this study and that of Plint and Jamieson () (Figure d). For example, Currie () reported cordierite and pressures of ~4 kbar using chlorite–muscovite–biotite barometry for a sample near JB78 (Figure ).…”

“…Interestingly, the P–T estimates of Currie () and Currie and Lynch () suggest lower pressures in the JBMS as compared with this study and that of Plint and Jamieson () (Figure d). For example, Currie () reported cordierite and pressures of ~4 kbar using chlorite–muscovite–biotite barometry for a sample near JB78 (Figure ). Additionally, Currie () reported a 2 km section within the JBMS typified by the progressive appearances of regional staurolite, cordierite, and andalusite, which was never recognized in other petrological and/or mapping studies of the CBH (e.g.…”

“…The approximate boundaries between the low‐, medium‐, and high‐grade belts of Plint and Jamieson () are indicated. Similarly, the approximate locations of samples reported on by Currie () are indicated by the white stars…”

Microstructure, metamorphism and tectonics of the western Cape Breton Highlands, Nova Scotia

“…Figs 5 and 6) rules out the suggestion that the low-to-medium grade rocks represent a low-pressure regional assemblage, distinct from kyanite-bearing highgrade rocks (cf. Currie, 1987a). Although the presence of older enclaves within the @ekes cannot be ruled out entirely, basement rocks do not appear to be important at the present level of exposure.…”

“…These minerals formed locally as the result of contact metamorphism; no regionally developed andalusite or cordierite has been observed in this study (cf. Currie, 1987a). In the same area, post-tectonic retrogression of garnet, staurolite and kyanite to muscovite and chlorite probably resulted from fluid release during cooling of the Salmon Pool pluton.…”