Report of Activities 2003 97 Age and Petrochemistry of Mafic Sills in Rocks of the Northwestern Margin of the Meguma Terrane, Bear River - Yarmouth Area of Southwestern Nova Scotia C. E. White and S. M. Barr1 Introduction overlying Halifax Formation), the Silurian White Rock Formation and the Early Devonian Torbrook Formation (Figs. 1, 2, 3). The Cambrian to Early Devonian Goldenville, Halifax, White Rock and Torbrook formations, The Goldenville Formation consists of grey exposed along the northwestern margin of the medium- to thick-bedded metasandstone, locally Meguma Terrane in southwestern Nova Scotia, are interlayered with green, cleaved metasiltstone and characterized by the presence of abundant mafic rare black slate (Horne et al., 2000; White et al., sills (Loring, 1954; Taylor, 1969; Smitheringale, 2001). A distinctive feature of the Goldenville 1973; Doyle, 1979; Trapasso, 1979; White et al., Formation near High Head (Figs. 1, 3) is a <1 km 1999). Sills are particularly abundant in the thick interval of mainly grey-green (High Head Halifax, White Rock and Torbrook formations in member) metasiltstone that contains abundant trace the Wolfville-Kentville, Nictaux-Torbrook and fossils. The Early Cambrian deep-water ichnofossil Bear River areas (Fig. 1), and are so abundant in Oldhamia was observed within this unit, which places that they could be termed swarms. Recent suggests that the lower part of the Goldenville fieldwork related to the Southwest Nova Scotia Formation (below this interval) may extend into the Mapping Project in the Bear River to Yarmouth Neoproterozoic. area has documented the presence of additional mafic sills not shown on previous maps, and some The Halifax Formation in the map area has of these sills are located well down in the been subdivided into three stratigraphic units stratigraphy of the Goldenville Formation. named, from oldest to youngest, the Bloomfield, Acacia Brook/Cunard and Bear River/Sissiboo The purpose of this paper is to provide River members (White et al., 1999, 2001; Horne et information about the field relations, age, and al., 2000). The Bloomfield member consists of petrochemistry of sills in the area between Bear distinctly banded maroon and green, thinly bedded River and Yarmouth. The data are used to metasiltstone and slate. Conformably overlying the characterize and compare these sills, and to Bloomfield member is black to rust-brown slate interpret their chemical affinity and tectonic setting with minor metasandstone layers of the Acacia through an emplacement history that appears to Brook/Cunard member. The Bear River/Sissiboo have extended from Early Cambrian to Middle River member is interpreted to conformably overlie Devonian. the Acacia Brook/Cunard member and consists of silty slate with minor metasandstone and slate. Geological Setting Early Tremadoc acritarch microfossils were reported by Doyle (1979) from the Halifax Stratigraphic units in the map area include the Formation exposed along Bear River near the Cambrian to Ordovician Meguma Group, contact with the White Rock Formation. Specimens (consisting of the Goldenville Formation and of the graptolite Rhabdinopora flabelliforme, an 1Department of Geology, Acadia University, Wolfville, Nova Scotia B4P 2R6 White, C. E. and Barr, S. M. 2004: in Mineral Resources Branch, Report of Activities 2003; Nova Scotia Department of Natural Resources, Report 2004-1, p. 97-117 98 Mineral Resources Branch Figure 1. Simplified geology map of the Bear River – Yarmouth area of southwestern Nova Scotia showing locations of the Bear River and Mavillette - High Head areas (in boxes). Inset map shows the location of the Wolfville-Kentville (W) and Nictaux-Torbrook (N) areas. Report 99of Activities 2003 Figure 2. Simplified geology map of the Bear River area showing approximate locations and orientations of type I and type II sills. Locations of the geochemical samples A to T and 1 to 32 (Table 1) are shown. 100 Mineral Resources Branch Figure 3. Simplified geology map of the Mavillette - High Head area showing approximate locations and orientations of Mavillette sill samples (B01-RJH numbers) and type I (32) and type II sills. index species marking the Cambrian-Ordovician have yielded Tremadoc acritarchs (W. A. M. boundary (Cooper et al., 2001), were reported from Jenkins, personal communication, 1977, in Schenk, the uppermost beds exposed in the Bear River 1995a). member (White et al., 1999). In addition, units that are interpreted to represent the uppermost Halifax Unconformably overlying the Halifax Formation elsewhere in the Meguma Group Formation in the Bear River and Weymouth areas (Rockville Notch and Moshers Island formations) are slate, metasiltstone and metasandstone of the Report of Activities 2003 101 White Rock Formation (White et al., 1999; Horne and Barr, 1983; MacDonald et al., 1992; Ham, et al., 2000). Fossils from the upper part of this 1994; White et al., 1999; Horne et al., 2001). formation in the Bear River area were assigned to the Late Silurian by Blaise et al. (1991) and Bouyx Mafic Sills et al. (1997). In contrast, the White Rock Formation in the Yarmouth area is composed mainly of metavolcanic rocks (Taylor, 1967; Introduction Hwang, 1985; MacDonald, 2000; White et al., 2001; MacDonald et al., 2002). Contacts with the Two prominent sets of mafic sills are present in the underlying Halifax Formation are shear zones map area (Fig. 1). Type I sills are restricted to the (Culshaw, 1994; Culshaw and Liesa, 1997; White Meguma Group and are inferred to be et al., 2001; Moynihan, 2003) that have yielded ca. penecontemporaneous with their host rocks and, 320 Ma muscovite ages, indicating a lower therefore, latest Neoproterozoic to Early Pennsylvanian age for development (Culshaw and Ordovician in age. Type II sills intrude the Reynolds, 1997; Moynihan, 2003). A felsic tuff Meguma Group as well as the White Rock and from the upper part of the White Rock Formation Torbrook formations, but predate the South in the Yarmouth area yielded a U-Pb zircon age of Mountain Batholith, Ellison Lake Pluton and ca. 438 Ma (MacDonald et al., 2002), whereas Clayton Hill Pluton; hence, they are early to middle felsic crystal tuff at the base of the section in the Devonian in age. A third set of mafic sills was Torbrook area yielded a similar U-Pb (zircon) age recognized, which are texturally and of 442 ± 4 Ma (Keppie and Krogh, 2000). Recently mineralogically similar to Mesozoic basalt of the discovered fossils from near the base of the North Mountain Formation. These probable formation in the Yarmouth area suggest that the Mesozoic sills are not common and will not be formation in that area may extend down into the discussed further here. Based on limited data, type Ordovician, but is dominantly Silurian (A. Boucot, I and II sills were interpreted by Barr et al. (1983) written communication, 2004). to be tholeiitic transitional to alkalic, with the older sills more alkalic in character, and speculated to Gradationally overlying the White Rock have been emplaced in a continental, within-plate Formation in the Bear River, Torbrook and environment. Weymouth areas are metasiltstone, slate, metasandstone and marble of the Early Devonian Type I Sills Torbrook Formation (Smitheringale, 1973; White Type I sills are typically light grey and fine-grained et al., 1999; Horne et al., 2000; Bouyx et al., with an average thickness of 2 m, rarely exceeding 1997). Numerous mafic sills and rare dykes intrude 3 m (Fig. 4). They are concordant with bedding but the Meguma Group and White Rock and Torbrook are not laterally extensive, typically pinching out formations but not the South Mountain Batholith. along strike. Dykes similar to type I sills are rare; These sills are described in more detail below. however, locally some sills bifurcate and cut across bedding or cut up-section to another bedding The Meguma Group, White Rock Formation surface. Typically the sills display fine-grained to and Torbrook Formation (and mafic sills) were glassy (now altered) chill margins at the top and deformed during the Devonian Acadian Orogeny. bottom of the sill. They are also commonly Deformation resulted in regional-scale vesicular and locally show multiple intrusions one northeastward-trending folds with an axial planar into another (e.g. Doyle 1979, plate 3-3, p. 54). cleavage and regional greenschist facies One sill in the Bloomfield member had a metamorphism (Taylor, 1969; Smitheringale, 1973; pronounced amygdaloidal margin towards the White et al., 1999; Horne et al., 2000). All of these inferred top of the sill and in the High Head units were intruded by late syntectonic, medium- to member along the coast (Fig. 3) a thin sill displays coarse-grained monzogranite and granodiorite of peperite-like structures along its lower contact with the Late Devonian South Mountain Batholith and the country rocks (Fig. 5a). Some sills in the Bear the Ellison Lake and Clayton Hill plutons (Allen River member along the Bear River Estuary display 102 Mineral Resources Branch irregular contacts or interfinger with the host a result of deuteric and autometasomatic processes sedimentary rocks (Doyle, 1979; Barr et al., 1983), such as those described by Poage et al. (2000) or and laminations in the slate and sill contacts are in later regional/retrograde metamorphism. places highly contorted (Fig. 5b). These relationships suggest that sill emplacement was Type I sills, together with the Meguma Group, penecontemporaneous with deposition of the have been deformed into F1 folds (Fig. 5c). The Meguma Group. sills are characterized by upright, subhorizontal to shallowly northeast-plunging, northeast-trending Type I sills are highly altered with only relict folds with a steep axial planar cleavage (Fig. 7a). igneous textures preserved (e.g. flow alignment These structures mimic those in the folded country defined by pesudomorphed euhedral plagioclase, rocks (Fig. 7b) and indicate that the sills were porphyritic and glomeroporphyritic textures) and originally emplaced horizontally along bedding no igneous minerals.