Research Article Formation of Anorthositic Rocks Within the Blair River Inlier of Northern Cape Breton Island, Nova Scotia (Canada)

GeoScienceWorld Lithosphere Volume 2020, Article ID 8825465, 21 pages https://doi.org/10.2113/2020/8825465 Research Article Formation of Anorthositic Rocks within the Blair River Inlier of Northern Cape Breton Island, Nova Scotia (Canada) J. Gregory Shellnutt ,1 Jaroslav Dostal,2 J. Duncan Keppie,3 and D. Fraser Keppie4 1Department of Earth Sciences, National Taiwan Normal University, 88 Tingzhou Road Section 4, Taipei 11677, Taiwan 2Department of Geology, Saint Mary’s University, 923 Robie Street, Halifax, Nova Scotia, B3H 3C3, Canada 3Departamento de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, 04510 Mexico DF, Mexico 4Department of Energy, Government of Nova Scotia, Joseph Howe Building, 12th Floor, 1690 Hollis Street, Halifax, Nova Scotia, B3J 3J9, Canada Correspondence should be addressed to J. Gregory Shellnutt; [email protected] Received 4 June 2019; Accepted 2 April 2020; Published 0 June 2020 Academic Editor: Sarah M. Roeske Copyright © 2020 J. Gregory Shellnutt et al. Exclusive Licensee GeoScienceWorld. Distributed under a Creative Commons Attribution License (CC BY 4.0). Rocks from the Blair River inlier of Northern Cape Breton Island (Nova Scotia, Canada) have been correlated with either the Grenville basement of eastern Laurentia or the accreted Avalon terrane. Additional zircon U-Pb dates of spatially associated anorthositic dykes (425:1±2:2 Ma) and a metagabbro (423:8±2:5 Ma) from the Fox Back Ridge intrusion of the Blair River inlier reveal Late Silurian emplacement ages. Their contemporaneity suggests that they may be members of a larger intrusive ∗ complex. The anorthositic rocks have high Eu/Eu values (>2.5), and bulk compositions are similar to the mineral compositions of labradorite (An50-70) and andesine (An30-50). The metagabbro is compositionally similar to alkali basalt and does not seem to ff ≤ : have been a ected by crustal contamination (Nb/U > 24; Th/Nb 1 1) although it was metamorphosed. The high Tb/Yb PM N (1.8-1.9) ratios suggest that the parental magma of the metagabbro was derived from a garnet-bearing peridotite. Fractional crystallization and mass balance calculations indicate that the anorthositic rocks can be derived by mineral accumulation from a fi ma c parental magma similar in composition to the metagabbro of this study. The Late Silurian ages suggest that the rocks were emplaced into the Avalon terrane after the closure of the Iapetus Ocean but before Early Devonian (415-410 Ma) sinistral transpression. 1. Introduction zoic units that accreted to Laurentia during Paleozoic oro- genesis [6, 7]. Laurentian basement inliers in the Appalachian The eastern margin of North America records a complex orogen include distinctive lithologies such as anorthosites, middle Proterozoic to middle Paleozoic geological history. charnockites, and granulites. These rock types are common The most complete record is preserved in a belt of basement in the Grenville Province but are mostly missing from the inliers with overlying cover sedimentary sequences that Appalachian outboard terranes. extend discontinuously from Alabama to Newfoundland Although earlier studies proposed a lithologic correlation along the western flank of the Appalachian orogen [1–3]. between rocks in northern Cape Breton Island (defined as the The inliers are composed of middle to late Proterozoic rocks Blair River inlier) and basement rocks of western Newfound- that were either formed or deformed during the Grenvillian land (e.g., [8, 9]), subsequent tectonic division models con- orogeny and dated at 1200-980 Ma [4]. These inliers include sidered Cape Breton Island to be part of the exotic Avalon rocks of easternmost Laurentia that are elsewhere typically terrane [10–12]. On the other hand, Barr and Raeside [13] included in the Appalachian orogen [5]. They are distinct revived the correlation of the Blair River inlier with the from the outboard Appalachian terranes (i.e., Avalonia, Humber Zone (Laurentian margin) as part of their redefini- Meguma, Carolinia) composed of Neoproterozoic and Paleo- tion of tectonic terranes in the northern Appalachian orogen. Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/8825465/5257667/8825465.pdf by guest on 25 September 2021 2 Lithosphere 60°00ʹW Appalachian Late Silurian plutons N Grenville Front Province Humber Zone 50°00ʹN Gulf of St. Lawrence Avalonia Notre Dame Exploits and Québec Reentrant New Brunswick Keppie et. al (2019 GanderNewfoundland zones Notre Dame Fig. 2 Magdalen Blair River Basin Lin et. al (2017) Exploits and inlier Aspy Gander zones 10-14 Barr et. al (2014) terrane 15 16 Cape 7 K-bentonite Breton 1 Bras 18 22 17 Island 2 d’Or 8 terrane M.F.Z. Nova Scotia Avalonia 3,4 5 6 20,21 Atlantic Ocean 19 23 26 Meguma Mira 200km 25 terrane 24 terrane 60°00ʹW Figure 1: Lithotectonic terranes, promontories, and reentrents of the northern Appalachians. The set map shows the terrane divisions of Cape Breton Island. The solid black line of Keppie et al. [16] marks the northwest boundary of Avalonia. The northwest (dashed line) and southeast (two-dots-dash) line boundaries of the Exploits and Gander subzones (Gander terrane) of Lin et al. [19] and Barr et al. [94]. M.F.Z.: Minas fault zone. Locations of Silurian plutons near the Avalon-Gander boundary are shown as white circles and listed in Table 2 under the No. column. At present, the basement rocks of the Blair River inlier in rocks include gabbro, syenite, anorthosite, granite, and Northern Cape Breton Island have different interpretations. granodiorite [15, 17]. Single-grain analyses of abraded zircon Brown [14], Barr and Raeside [13], and Miller and Barr from the anorthosite of the Blair River inlier (Red River anor- [15] correlate the inlier with Grenvillian (Laurentian) base- thosite) indicate that the minimum crystallization U-Pb age ment of the Humber Zone, whereas Neale and Kennedy [9] was 1095:3±1:5 Ma although there is evidence suggesting correlated them with the Gander terrane (peri-Gondwana) that the zircons experienced multiple stages of lead loss and Keppie [10] and Keppie et al. [16] interpreted them as [15]. The uniqueness of the Blair River inlier relative to the the basement rocks of the Avalon terrane (peri-Gondwana), peri-Gondwanan terranes (i.e., Avalon, Gander, and Meguma) thereby providing a source for the common 1-1.3 Ga detrital in the northern Appalachians suggests that it may represent a zircons in Avalonia (Figure 1). A critical factor in the corre- fragment of Laurentian basement and that the Red River anor- lation with Grenvillian basement is the presence of anortho- thosite is interpreted to be correlative with Proterozoic massif- site bodies, which are restricted to the area of the Blair River type anorthosite found in the Grenville Province [15, 17, 18]. inlier. In order to provide a better database for the compari- However, recent zircon U-Pb geochronology on the Red River son with anorthosite of the Grenville Province, we have anorthosite indicates that it was emplaced during the Late investigated these Cape Breton rocks. Silurian (421 ± 3 Ma) and contains Meso- to Neoproterozoic The Blair River inlier is located at the northernmost tip of (865 ± 18 Ma to 1044 ± 20 Ma)-inherited zircons [16]. An Cape Breton Island, Nova Scotia, and is comprised of evaluation of the whole-rock Pb isotopes of the Blair River Mesoproterozoic to Carboniferous rocks. It is bounded by inlier rocks, including the Red River anorthosite, indicates that the Gulf of St. Lawrence to the west and north and separated it has greater affinity to peri-Gondwana terranes than to Laur- from the adjacent Aspy terrane by the Wilkie Brook fault to entia [16]. Moreover, the absence of ophiolites between the the east and the Red River fault to the south. The Mesopro- Blair River inlier and the neighbouring Aspy terrane, which terozoic (~1.2 Ga to ~1.0 Ga) rocks consist of composite are present in southern Newfoundland, argues against a orthogneiss complexes (i.e., Sailor Brook gneiss, Otter Brook Laurentia-Avalon suture zone [16, 19]. gneiss, and Polletts Cove River gneiss) that are intruded by In this paper, we present new zircon U-Pb geochronology comparatively less deformed plutonic rocks. The plutonic and whole-rock geochemistry of metagabbro from the Fox Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/8825465/5257667/8825465.pdf by guest on 25 September 2021 Lithosphere 3 1080 +5/-3 Ma 60°30ʹ 1035 +12/-10 Ma 428 ± 7 Ma 47°00ʹ N 424 ± 3 Ma Gulf of St. Lawrence 978 +6/-5 Ma, 423 ±6 Ma Wilkie Brook fault 427 ± 2 Ma 435 +7/-3 Ma 421 ± 6 Ma Carboniferous rocks Devonian granite 425 ± 2 Ma Blair River inlier 424 +4/-3 Ma Silurian 996 +6/-5 Ma Sammys Barren granite 1095.3 ± 1.5 Ma & syenite Red Fox Back Ridge diorite 423 ± 3 Ma fault River 417 ± 6 Ma 421 ± 3 Ma Mesoproterozoic Fig. 3 Otter Brook gneiss Lowland Brook syenite Aspy fault Charnockite Red River anorthosite and Shear zone relate rocks Fault Sailor Brook gneiss 05 Poletts Cove River gneiss km Mylonite Figure 2: Simplified geological map of the Blair River inlier with locations of samples that were dated by Miller et al. [17], Miller and Barr [15], and Keppie et al. [16]. The map is modified from Miller and Barr [15]. Back Ridge diorite/gabbro intrusion and an anorthositic dyke from 1 to 5 mm [22]. The anorthositic rocks typically contain from the Blair River inlier. Our results provide new limits on minor amphibole with rare pyroxene relicts in the core. the emplacement age of anorthositic rocks within the Blair Both the anorthosite and the surrounding gneisses of the River inlier, which, in turn, have direct implications for the belt have experienced polyphase deformation, which has location of the boundary between the cratonic margin of transposed the contacts into parallel features.

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