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

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 have been aﬀected 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 orogenesis [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.

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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

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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. The rocks Laurentia and the accreted Avalon terrane. Moreover, we within and bounding the Blair River inlier were metamor- present a petrogenetic model that attempts to constrain phosed to amphibolite facies along shear zones. Deformation the formation of the anorthositic dykes and the Red River was accompanied by partial melting and mylonitization in anorthosite. some cases. In between these shear zones, anorthosites, monzodiorite-gabbro, and granitic and syenitic plugs are 2. Geological Background undeformed. A suite of eight samples was collected from the Blair River The Blair River inlier includes several anorthosite bodies, the inlier. Two samples from the main body of the Red River largest of which is the Red River anorthosite suite (Figure 2). anorthosite, three samples from an anorthositic dyke that This intrusion is about 11 km long and up to 1 km wide. It intrudes the Fox Back Ridge diorite/gabbro intrusion, and occurs within a NNE-trending belt of quartz-feldspathic three host metagabbroic rocks of the Fox Back Ridge intru- gneiss, which is about 30 km long and 2-8 km wide [15]. sion [15] were collected. The metagabbro unit was originally The belt is bounded by mylonite zones, and thus, any direct mapped together as orthogneiss and paragneiss with minor correlation with other units in the area is tenuous. The Red amphibolite- and kyanite-bearing horizons by Smith and River anorthosite is representative of the Cape Breton anor- MacDonald [23] and assigned to the Helikian (850 Ma to thosites [20]. It is composed of anorthosites passing in places 1600 Ma) Pollets Cover River Group by Barr et al. [24]. into interlayered anorthositic gabbro and pyroxene-rich However, Miller [22] reexamined the area and identiﬁed rocks [15, 21]. The anorthosite grades from massive and the region west of the Red River anorthosite as a distinct equigranular to foliated. Its texture ranges from protoclastic intrusive unit and named it the Fox Back Ridge intrusion of and porphyroclastic to recrystallized granoblastic. The dom- probable Paleozoic age [17]. The dated samples were col- inant mineral is plagioclase (An35-50) with grain size ranging lected along the Cabot Trail highway immediately west of

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60°42ʹW

N 500 meters

CBCH-1 BR-1-1 BR-2-1 BR-1-2 BR-2-4

CBGR-1 421 ± 3 Ma BR-3-1 BR-3-2

46°48ʹN

Fox Back Ridge diorite/gabbro Red River anorthosite complex Monzodiorite/gabbro Mylonite Orthogneiss and paragneiss Road Sample location

Figure 3: Detailed geological map of the Red River anorthosite and the sampling locations for this study. The boundaries between units showing nature of contacts, deﬁned, approximate, and assumed as mapped by Miller [22] and Smith and MacDonald [23].

the Red River anorthosite (Figure 3). Sample CBCH-1 is an retain the euhedral to subhedral texture and polysynthetic amphibole-rich metagabbro, and CBGR-1 is an anorthositic twinning. Similar to plagioclase, there is a minority of small, dyke that cross-cuts the Fox Back Ridge intrusion. subround, unaltered clinopyroxene crystals but most crystals are altered to uralite. The original rock texture is diﬃcult to 3. Petrography discern as there is a weak foliation of the uralite and saussurite, but it appears that the rock may have been ophitic. 3.1. Anorthosite. Sample CBGR-1 (46°48′18.17″N, 60°41′ Apatite is relatively abundant and occurs as either small 57.08″W) was collected from an undeformed anorthositic euhedral crystals or larger tabular crystals. The oxide min- dyke that intruded the host Fox Back Ridge intrusion. It is erals, including hematite, appear to be exclusively secondary comprised of 90-95 vol.% plagioclase with minor (~5 vol.%) as they are commonly found within or spatially associated pyroxene and accessory amounts (<<1 vol.%) of Fe-Ti oxide with uralite. Short and narrow quartz veins are present minerals and apatite. The plagioclase crystals tend to be but comprised <2 vol.%. blocky rather than tabular and display polysynthetic twinning under cross-polars. Generally, the crystals have similar 4. Methods grain size but some, based on the twinning angle, appear to be bent (Figures 4(a) and 4(b)). The pyroxene crystals 4.1. Zircon U-Pb Geochronology. Zircon separation from are small, anhedral, and interstitial to the plagioclase. samples CBCH-1 and CBGR-1 was processed using magnetic Although both orthopyroxene and clinopyroxene are iden- separation and heavy-liquid techniques at the Yu-Neng Rock tiﬁed in the anorthosite, most pyroxene crystals are and Mineral Separation Company (Lanfang, Hebei, China). altered to uralite suggesting that it was likely clinopyrox- Cathodoluminescence (CL) images were taken at the Insti- ene [22]. Secondary muscovite is replacing some plagio- tute of Earth Sciences, Academia Sinica, Taipei, for selecting clases, and a narrow carbonate vein cross-cuts a portion suitable positions for U-Pb analyses and to examine individ- of the thin section. ual crystal internal structures. Zircon U-Pb isotopic analyses were performed by laser ablation-inductively coupled plasma 3.2. Metagabbro. The metagabbro (CBCH-1; 46°48′31.57″N, mass spectrometry (LA-ICP-MS) at the Department of 60°41′44.53″W) from the Fox Back Ridge intrusion is coarse Geosciences, National Taiwan University, Taipei, using an to medium grained and is comprised primarily of replace- Agilent 7500s Q-ICP-MS and a Photon Machines Analyte ment minerals of plagioclase (45-50 vol.%) and clinopyrox- G2 193 nm laser ablation system following the method of ene (40-45 vol.%), with abundant apatite (~5 vol.%), and Chiu et al. [25]. A spot size of 35 μm with a laser repeti- minor amounts (≤5 vol.%) of secondary oxide minerals tion rate of 5 Hz was used, and the laser energy density (Figures 4(c) and 4(d)). The plagioclase is mostly altered to was 3.83 to 5.33 J/cm2. Calibration was performed by using saussurite, but there is a minority of unaltered crystals that the zircon standard GJ-1 (608:5±0:4 Ma; [26]) and 91500

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Cb Pl Pl

Px Ms Pl 0.5 mm 0.5 mm

(a) (b)

Ur Ur Pl Cpx Pl Qz Ur Ur Ur

0.5 mm 0.5 mm

Figure 4: Photomicrograph of the anorthositic rocks (CBCH-1) in (a) plane-polarized light and (b) cross-polars. Photomicrograph of the metagabbro (CBCH-1) in (c) plane-polarized light and (d) cross-polars. Ur: uralite; Pl: plagioclase; Cpx: clinoyroxene; Qz: quartz; Ms: muscovite; Cb: carbonate.

(1065 Ma; [27]). The Plešovice (337:1±0:4 Ma; [28]) zircon grains of the anorthosite sample (CBGR-1) range in zircon standard was also used for data quality control. size from ~25 μmto~100 μm in length/width and typically Measured U-Th-Pb isotope ratios were calculated using show oscillatory zonation (Fig. DR1). Ninety-five of 97 anal- the GLITTER 4.4.4 software [29], and the relative standard yses of zircons are within 10% discordance and yielded a total deviations of reference values for GJ-1 were set at 2%. The range of 206Pb/238U ages from 401 ± 9 Ma (1σ)to794 ± common lead was corrected using the common lead cor- 18 Ma (1σ). Four populations are identified based on the rection function of Andersen [30, 31], and the weighted clustering of the data (Table DR1). The oldest three zircons mean U-Pb ages and concordia plots were created using yielded ages of 501 ± 11 Ma (1σ), 657 ± 15 Ma (1σ), and Isoplot v. 4.1 [32]. Only results within 10% discordance 794 ± 18 Ma (1σ). The second oldest group consists of 13 ðð − ½206 238 207 235 Þ ∗ Þ 1 Pb/ Uuncorrected age/ Pb/ Uuncorrected age 100 middle to late Ordovician zircons. This group appears to were used for the data interpretations. cluster into two subgroups: (1) 463 ± 10 Ma (1σ)to449 ± 10 Ma (1σ) and (2) 442 ± 11 Ma (1σ)to443 ± 10 Ma (1σ). 4.2. Major and Trace Elemental Geochemistry. Whole-rock The largest group (76) of zircons range in age from 415 ± 9 major and trace elements were analyzed at the Activation Ma (1σ)to437 ± 10 Ma (1σ), whereas the youngest group Laboratories Ltd. in Ancaster, Ontario, Canada, using lith- is comprised of two Early Devonian zircons (401 ± 9 Ma ium metaborate-tetraborate fusion. Major elements were (1σ) and 406 ± 9 Ma (1σ)). The oldest zircons and the determined by an inductively coupled plasma-optical emis- Middle to Late Ordovician zircons are interpreted to repre- sion spectrometer, whereas trace element analyses were sent inheritance, whereas the youngest zircons may be related performed by an inductively coupled plasma-mass spec- to the effects of regional deformation during the Early Devo- trometer. Replicate analyses of the reference standard nian. Consequently, we interpret the crystallization age to be rocks indicate that 1 sigma error is between 2 and 8% of based on the Late Silurian zircons which yielded a weighted- the values cited. The detailed information on the major mean 206Pb/238U age of 425:1±2:2 Ma (2σ, Figure 5(a)). and trace element analyses is available at the Activation Individual zircon grains of the metagabbro (CBCH-1) Laboratories web site (https://www.actlabs.com). range in size from ~50 μmto~225 μm (Fig. DR2), typically show oscillatory zonation, and have fragmented, anhedral 5. Results to subhedral textures. The zircons from this sample tend to have uniform CL brightness compared to those in the anor- 5.1. Geochronology. The CL images show that some crystals thosite (CBGR-1). Seventy-seven of 82 analyses are within have bright cores with darker rims, whereas others are 10% discordance and yielded a total range of 206Pb/238U ages entirely bright or entirely dark. The crystals have a variety from 404 ± 10 Ma (1σ)to468 ± 12 Ma (1σ). Three distinct of habits that include euhedral prismatic, subhedral round zircon age populations are identified based on age gaps to subround, and anhedral fragmented (Fig. DR1). Individual (Table DR2). The largest (65 zircons) group ranges from

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0.076 470 5.2. Whole-Rock Geochemistry. Five anorthosites and three Weighted-mean = 425.1 ± 2.2 Ma CBGR-1 metagabbros from the Blair River inlier were analyzed for n = 76 MSWD = 0.40 whole-rock and trace elemental analyses (Table 1). The anorthosite samples are alkalic and can be divided into two 0.072 groups (Figure 6(a)). One group (BR-1-1, BR-1-2) from the Red River anorthosite is broadly similar to the mineral

U ≈ ≈ chemistry of labradorite (SiO2 53.7 wt%, Al2O3 26.2 wt%, 238 0.068 ≈ ≈ ≈ CaO 9 wt%, Na2O 4.8 wt%, and K2O 0.6 wt%), although Pb/ Fe O t, MgO, and TiO are higher and reﬂect the presence

206 2 3 2 of oxide and mafic silicate minerals in the mode 0.064 (Figure 6(b)). In contrast, the second group (BR-3-1, BR- 3-2; CBGR-1) from the anorthositic dykes are somewhat 390 ≈ ≈ Anorthosite similar to andesine (SiO2 56 wt%, Al2O3 24 wt%, � ≈ ≈ ≈ Data-point error ellipses are 2 CaO 5 wt%, Na2O 6.2 wt%, and K2O 3 wt%) but K2O, 0.060 TiO ,FeO t, and MgO are higher and reflect the presence 0.44 0.48 0.52 0.56 0.60 2 2 3 of oxide minerals, mafic silicates (biotite), K-feldspar, and 207Pb/235U mica (Figure 6(b)). Including data from Miller et al. (a) (2000), the anorthositic rocks have a relatively restricted SiO (wt%) content but the Fe/(Fe + Mg) ratio is variable Weighted-mean = 423.8 ± 2.5 Ma CBCH-1 2 0.076 n = 65 470 (~0.3 to ~0.8) (Figure 6(c)). Both types of anorthosite MSWD = 0.30 are peraluminous (Al/Ca+Na+K (mol.) SI > 1) and have relatively high volatile content as loss on ignition (LOI) 450 0.072 is between 1.4 and 2.3 wt%. The quantities of transition elements for the anorthosites are low (<30 ppm) across U

238 430 all elements with Zn having the highest concentrations

Pb/ 0.068 (50 to 100 ppm). The primitive mantle-normalized incom-

206 patible element patterns show enrichment of the large ion 410 lithophile elements (Cs, Rb, Ba, and Sr), and Th and U 0.064 relative to the rare earth elements and the remaining fi 390 high- eld-strength elements (Figure 7(a)). The chondrite- Metagabbro normalized rare earth element patterns show enrichment Data-point error ellipses are 2� fl 0.060 of the light rare earth elements with relatively at heavy rare 0.44 0.48 0.52 0.56 0.60 0.64 earth elements. The prominent Eu-anomaly (Eu/Eu ∗ >2:5) 207Pb/235U is typical of anorthositic rocks and is a direct consequence (b) of their feldspar-rich nature (Figure 7(b)). The gabbroic rocks of this study (BR-2-1, BR-2-4, and Figure 5: (a) Concordia diagram of zircon U-Pb data from sample CBCH-1) are compositionally similar to alkali basalt as they CBGR-1. (b) Concordia diagram of zircon U-Pb data from sample have low SiO (<47 wt%), are alkalic (Na O+K O>4:5 CBCH-1. 2 2 2 wt%), and have moderate Mg# values (~45) and TiO2 (~2.0 wt%) contents (Figure 6(d)). The P2O5 is unusually high (~1.5 wt%), and the loss on ignition values are relatively 415 ± 10 Ma (1σ)to435 ± 11 Ma (1σ). The oldest group (6 high (LOI > 3:0 wt%). The transition metals (Sc ≈ 20 ppm, zircons) ranges from 440 ± 11 Ma (1σ)to468 ± 12 Ma (1σ), Co < 30 ppm, Ni < 10 ppm, Cu ≤ 30 ppm, and Zn ≤ 100 and the youngest (6 zircons) group ranges from 404 ± 10 ppm) are relatively low although V is high (240 ppm to Ma (1σ)to412 ± 10 Ma (1σ). The oldest age group con- 310 ppm). The primitive mantle-normalized incompatible sists of two subgroups where four of the zircons are element pattern of the gabbro is similar to ocean-island basalt between 440 ± 11 Ma (1σ) and 446 ± 11 Ma (1σ), and the (OIB), but there is enrichment in Cs, Ba, and a minor deple- remaining two are 464 ± 11 Ma (1σ) and 468 ± 11 Ma tion of Zr (Figure 7(a)). The chondrite-normalized rare earth (1σ). This group is considered to be inherited due to their element patterns of the gabbro are also similar to ocean- relatively minor abundance but also because middle to late island basalt as they have very high La/YbN (22 to 25) values Ordovician rocks are known in Cape Breton and without a significant Eu anomaly (Figure 7(b)). throughout Avalonia [33, 34]. We interpret the crystallization age to be indicative of the weighted-mean 6. Discussion 206Pb/238Uage(423:8±2:5 Ma, 2σ) of the zircons that comprise the middle group (Figure 5(b)). We note that 6.1. Regional Temporal Correlation. The Late Silurian the error ellipses of the zircons that comprise the emplacement ages of the metagabbro (423:8±2:5 Ma) and weighted-mean age are slightly discordant which may be anorthosite dyke (425:1±2:2 Ma) confirm that the two units related to Pb loss during the development of the foliation are contemporaneous. Consequently, it is possible that the within the metagabbro. intrusions may be members of the same igneous complex

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Table 1: Whole-rock geochemical data of the Blair River inlier rocks.

(a)

Sample BR-1-1∗ BR-1-2∗ BR-2-1 BR-2-4 CBCH-1 BR-3-1 BR-3-2 CBGR-1 Lat. 46°48′31.57″N46°48′18.17″N Lon. 60°41′44.53″W60°41′57.08″W Rock An An MG MG MG An An An

SiO2 (wt.%) 53.79 53.66 46.61 41.26 46.45 56.41 56.03 56.15

TiO2 0.20 0.14 1.95 1.97 1.79 0.14 0.13 0.22

Al2O3 26.22 26.33 17.19 18.26 18.05 22.62 24.33 23.57

Fe2O3t 2.30 2.16 11.56 11.75 11.14 2.85 1.84 1.80 MnO 0.03 0.03 0.16 0.13 0.15 0.06 0.03 0.03 MgO 1.63 0.93 4.85 4.75 4.72 1.23 0.56 0.60 CaO 8.76 9.74 7.68 8.27 6.92 5.46 4.61 5.46

Na2O 5.07 4.75 3.04 3.84 3.51 6.22 6.23 6.32

K2O 0.53 0.68 1.82 2.07 1.62 2.13 3.21 3.12

P2O5 0.03 0.05 1.58 1.60 1.28 0.07 0.32 0.10 LOI 1.82 1.41 3.44 6.40 4.01 2.27 1.86 3.04 Total 100.38 99.88 99.88 100.29 99.65 99.45 99.15 100.40 Mg# 58.4 46.0 45.4 44.5 45.6 46.1 37.6 39.8 Sc (ppm) 3 2 24 22 27 3 1 18 V 20 20 310 303 267 24 16 15 Cr b.d. b.d. b.d. b.d. b.d. 40 b.d. 18 Co 7 5 28 23 29 4 2 4 Ni b.d. b.d. b.d. b.d. 5 b.d. b.d. 6 Cu b.d. b.d. 30 30 22 b.d. b.d. 7 Zn b.d. b.d. 90 100 115 b.d. b.d. 73 Ga 17 19 22 23 24 18 18 19 Rb 14 14 31 38 33 42 64 72 Sr 970 963 1162 659 1113 869 848 963 Y 2 2.9 34.5 34.8 33.5 5.7 4.1 2.9 Zr 9 9 197 215 241 17 11 14 Nb b.d. 0.2 47.7 49.2 68.8 2.5 1.7 2.3 Cs 2.8 1.7 1.2 1.5 1.9 1.8 2.5 3.5 Ba 205 201 1441 1176 1482 987 1217 1602 La 2.5 3.2 96.1 91.7 99.9 7.7 7.9 6.6 Ce 5.00 6.19 173 172 181 14.2 14.4 11.6 Pr 0.61 0.73 18.3 18.5 20.1 1.64 1.53 1.4 Nd 2.54 2.96 67.4 69.2 69.8 6.34 5.69 5.0 Sm 0.48 0.66 11.1 11.4 11.2 1.25 1.01 0.89 Eu 0.64 0.71 3.25 3.35 3.51 1.01 1.04 1.45 Gd 0.51 0.50 8.56 8.83 10.5 1.12 0.91 0.83 Tb 0.05 0.09 1.14 1.18 1.28 0.16 0.11 0.10 Dy 0.35 0.52 6.29 6.42 6.58 0.98 0.68 0.51 Ho 0.06 0.09 1.18 1.18 1.26 0.20 0.13 0.09 Er 0.19 0.28 3.21 3.31 3.37 0.56 0.35 0.25 Tm 0.03 0.04 0.42 0.45 0.48 0.08 0.05 0.03 Yb 0.18 0.26 2.74 2.87 3.02 0.56 0.34 0.21 Lu 0.02 0.05 0.44 0.46 0.47 0.09 0.06 0.03 Hf 0.2 0.2 4.2 4.2 5.7 0.5 0.4 0.4

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Table 1: Continued.

Sample BR-1-1∗ BR-1-2∗ BR-2-1 BR-2-4 CBCH-1 BR-3-1 BR-3-2 CBGR-1 Ta 0.02 0.03 3.21 3.21 3.17 0.29 0.38 0.22 Th b.d. 0.24 5.43 6.85 8.33 1.33 3.07 1.53 U 0.02 0.17 1.93 1.88 2.12 0.42 0.89 0.70 Eu/Eu∗ 4.0 3.6 0.98 0.98 0.97 2.6 3.3 5.1

(La/Yb)N 9.8 8.9 25.2 22.9 23.7 9.8 16.7 22.6

(b)

Sample BIR-1 BIR-1 AGV-2 AGV-2 BCR-2 BCR-2 BHVO-2 BHVO-2 Rock r.v. m.v. r.v. m.v. r.v. m.v. r.v. m.v.

SiO2 (wt.%) 47.96 48.07 59.30 60.41 54.10 54.46

TiO2 0.96 0.96 1.05 1.05 2.26 2.27

Al2O3 15.50 15.59 16.91 17.31 13.50 13.62

Fe2O3t 11.30 11.16 6.69 6.83 13.80 13.80 MnO 0.175 0.17 0.10 0.10 0.20 0.20 MgO 9.70 9.65 1.79 1.79 3.59 3.63 CaO 13.30 13.30 5.20 5.22 7.12 7.16

Na2O 1.82 1.85 4.19 4.38 3.16 3.24

K2O 0.03 0.03 2.88 2.93 1.79 1.82

P2O5 0.02 0.02 0.48 0.48 0.35 0.36 LOI -0.24 0.00 0.38 Total Mg# Sc (ppm) 13.1 9.8 33.5 37.9 31.8 32.6 V 119 96.1 418 418 318 323 Cr 16.2 12.1 15.9 287 275 Co 15.5 13.4 37.3 37.4 44.9 43.6 Ni 18.9 183 120 105 Cu 51.5 49.5 19.7 28.4 129 127 Zn 86.7 73.1 130 143 104 110 Ga 20.4 18.5 22.1 23.1 21.4 22.4 Rb 67.8 61.3 46.0 49.8 9.3 10.4 Sr 660 560 337 339 394 390 Y 19.1 17.3 36.1 36.5 25.9 26.5 Zr 232 202 187 187 171 170 Nb 14.1 12.1 12.4 12.6 18.1 18.0 Cs 1.17 1.2 1.16 1.4 0.1 0.1 Ba 1134 1091 684 752 131 142 La 38.2 34.9 25.1 26.7 15.2 16.1 Ce 69.4 62.8 53.1 55.6 37.5 39.2 Pr 8.17 7.3 6.83 7.1 5.3 5.5 Nd 30.5 26.1 28.3 28.4 24.3 24.1 Sm 5.51 4.7 6.55 6.5 6.0 6.0 Eu 1.55 1.5 1.99 2.1 2.0 2.0 Gd 4.68 4.4 6.81 6.9 6.2 6.2 Tb 0.65 0.58 1.08 1.09 0.94 0.97 Dy 3.55 3.0 6.42 6.4 5.3 5.3 Ho 0.68 0.59 1.31 1.32 1.00 1.00 Er 1.83 1.59 3.67 3.67 2.51 2.53

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Table 1: Continued.

Sample BIR-1 BIR-1 AGV-2 AGV-2 BCR-2 BCR-2 BHVO-2 BHVO-2 Tm 0.26 0.23 0.53 0.54 0.33 0.34 Yb 1.65 1.42 3.39 3.41 1.99 2.02 Lu 0.25 0.22 0.51 0.52 0.28 0.28 Hf 5.14 4.66 4.97 5.10 4.47 4.55 Ta 0.87 0.75 0.79 0.81 1.15 1.18 Th 6.17 5.23 5.83 5.83 1.22 1.18 U 1.89 1.76 1.68 1.81 0.41 0.44 Eu/Eu∗

(La/Yb)N ∗ ∗ From Keppie et al. [16]. Lat.: latitude; Lon.: longitude; An: anorthosite; MG: meta-gabbro; t: total Fe expressed as Fe2O3. LOI: loss on ignition. Eu/Eu = ½ ∗ ð Þ ð 2+ ð 2+ 2+ÞÞ ∗ 2 EuN/ SmN +GdN normalized to C1 chondrite [96]. Mg# = Mg / Mg +Fe 100. N: normalized to the chondritic value [96]. b.d.: below detection; m.v.: measured value; r.v.: recommended value. The measured standard reference material trace element values are based on an average of three samples for AGV-2 and two samples each for BCR-2 and BHVO-2.

or magmatic system and either directly or indirectly related. Silurian 206Pb/238U ages of intrusive rocks (mafic to fel- There are significant local and regional lithotectonic implica- sic) are reported from the Avalon terrane of Newfoundland tions given that the anorthositic rocks are Late Silurian rather (Pass Island = 423 ± 4 Ma), Gander terrane of southern New Mesoproterozoic as previously interpreted. Brunswick (Welsford = 422 ± 1:0 Ma; Utopia = 428:3±1:0 Locally, the zircon U-Pb ages are similar to titanite, Ma, 425:5±2:1 Ma, 420:4±2:4 Ma, and 420 ± 3:5 Ma; and rutile, and hornblende ages reported for the dioritic rocks Bocabec = 422:1±1:3 Ma; Jake Lee Mountain = 418 ± 2 Ma), of the Fox Back Ridge intrusion (423 ± 3 Ma, U-Pb titanite; and coastal Maine (Moosehorn = 421:1±0:8 Ma; Penobscot 417 ± 6 Ma, 40Ar/39Ar hornblende) and Red Ravine syenite = 419 ± 2:0 Ma; Spruce Head = 421 ± 1:0 Ma; Cadillac (425 ± 2 Ma, U-Pb titanite) suggesting that they may be Mountain = 424 ± 2:0 Ma and 419 ± 2:0 Ma; Sedgwick = members of a larger igneous complex (Table 2; [22]). More- 419:5±1:0 Ma; and Cranberry Island = 424 ± 1:0 Ma). Early over, in western Cape Breton, there are a number of Silurian Ludlovian (427.4 Ma to 425.6 Ma) K-bentonite beds were plutonic rocks (MacLean Brook = 442:4±2:0 Ma; Lavis identified at Arisaig (Avalon terrane, Nova Scotia) and are Brook = 440:0±1:6 Ma; Gillanders Mountain = 436:7±2:6 thought to be regionally correlative with ash beds from Ma, 428:6±1:9Ma; Sammys Barren = 435 + 7/−3 Ma; Gills Gotland (Sweden) and those of the British Isles [47]. Similar Brook = 436:4±1:5 Ma; Lake Ainslie = 428:5±1:7 Ma; Grand aged rocks are found within the Meguma terrane of Western Falaise = 427:9±2:1Ma; Leonard MacLeod Brook = 420:9 Nova Scotia (Brenton pluton = 439 + 4/−3 Ma; White Rock ±2:8 Ma; and Glasgow Brook pluton = 416:0±1:9 Ma) that formation = 438 + 3/−2 Ma; and Mavillette gabbro = 426 ± have 206Pb/238Uagesfrom442:4±2:0 Ma to 416:0±1:9 Ma 2 Ma) as well [44–46, 48–58]. [17, 19, 33, 35]. In addition, middle to late Silurian Plutonism in the region continued from the Late Silurian (435 + 7/−3 Ma to 417 ± 6 Ma) postmetamorphic cooling to Early Devonian and may be related to either the early ages are reported for a variety of rocks in the Blair River stages of the Acadian orogeny (420 Ma to 410 Ma) or postcol- inlier [17]. lisional crustal relaxation. The plutons are found across the Regionally, middle to late Silurian magmatic rocks are trailing (eastern) edge regions of the Gander terrane and common in the Gander terrane of Newfoundland and New the boundary region between the Gander and Avalon ter- Brunswick (432 ± 2 Ma to 419:5±0:6 Ma), and Silurian ranes [38, 45, 46, 48, 53, 59–63]. Subsequent to the Avalon- (443 ± 4 Ma to 424 + 2/−1 Ma) magmatic rocks (Notre Dame Laurentia collision (435 Ma to 430 Ma), the Meguma terrane arc, Attean pluton, Quimby Formation) were also emplaced was thrust over Avalonia (~400 Ma) and both terranes were into the Laurentian margin crust [36–42]. Silurian magma- intruded by middle to late Devonian mafic and silicic plu- tism is attributed to the Salinic orogeny (445 Ma to 422 Ma) tonic rocks [33, 45, 60, 64–67]. Consequently, the precise which corresponds to the subduction and accretion of terrane correlation of the Blair River inlier rocks is difficult the leading (western) edge of Gander terrane to Laurentia. to discern based on just their ages as they are more likely to It is thought that the Salinic orogeny produced older be related to either the Avalon terrane or the Gander terrane subduction-related plutons and younger postcollisional than Laurentia. On the basis of whole-rock Nd isotope model (back-arc) plutons via slab detachment after the cessation ages and Pb isotopes of rocks from the Blair River inlier, of subduction [40–43]. The Salinic arc magmatic rocks are including ~580 Ma mafic dykes, Keppie et al. [16] suggests located in northern and central New Brunswick and western that they have more in common with Avalonian crust, specif- and central Newfoundland proximal to and within the ically Oaxacian crust (a proxy for Amazonia), than Laurentia Laurentian margin, but there are a number of postcollisional [9, 10, 15, 16, 33, 46, 48, 68]. Nevertheless, the new age dates plutons, batholiths, and intrusions that were emplaced closer indicate that they are contemporaneous with the period of to and along the boundary region of the Gander terrane and postcollisional magmatism that developed along the bound- the Avalon terrane [38, 42, 44–46]. ary between the Gander terrane and Avalon terrane.

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15 2

O) An-rich Alkalic 2 O (wt%) 2 O + K 1 2 O+K

2 Ab-rich

Na 5 Subalkalic CaO/(Na

0 0 20 30 40 50 60 70 80 15 20 25 30

SiO2 (wt%) Al2O3 (wt%) Anorthositic rocks is study Miller and Barr (2000) Metagabbro

(a) (b) 1.0 Harp Lake 1 anorthosite AR 0.8 P

R + D T

0.6 .1 TP TA Zr/Ti

Fe/(Fe+Mg) 0.4 A + BA

.01 F 0.2 B AB

0.0 20 30 40 50 60 70 80 .01 .1 1 10 100 SiO2 (wt%) Nb/Y

(c) (d) Figure 6: Geochemical diagrams of the Blair River inlier anorthositic rocks and the metagabbro. (a) Na2O+K2O (wt%) vs. SiO2 (wt%). (b) CaO/(Na2O+K2O) vs. Al2O3 (wt%) showing the calcic-sodic trend of the anorthositic rocks. (c) Molar Fet/(Fet+Mg) vs. SiO2 (wt%) including the range of the Mesoproterozoic Harp Lake (Nain Province, Labrador) anorthosite (from [77]). (d) Revised rock classiﬁcation diagram of Zr/Ti vs. Nb/Y [95]. R+D: rhyolite+dacite; AR: alkali rhyolite; P: phonolite; T: trachyte; TP: tephri-phonolite; TA: trachy andesite; F: foidite; AB: alkali basalt; B: basalt; A+BA: andesite+basaltic andesite. Additional Red River anorthosite data from Miller and Barr [15]. The open squares are samples BR-1-1 and BR-1-2. Closed squares are samples BR-3-1, BR-3-2, and CBGR-1.

An examination of the inherited zircons from the anor- ~790 Ma (Tables DR1 and DR2). Cambrian-Ordovician ages thositic rocks may provide additional support for the speciﬁc are common in Avalonian sedimentary and metasedimen- terrane correlation as they indicate the age of the country tary rocks but are also consistent with plutonic rocks in the rock through which the parental magma interacted with Avalon and Gander terranes [34, 67, 69, 70]. The oldest con- during emplacement. The majority of inherited zircons have cordant inherited zircon from the anorthosite dyke has a ages of ~440 Ma and~ 465 Ma with individual ages of 206Pb/238U age of 657 ± 15 Ma (1σ) which is more indicative ~500 Ma and~ 660 Ma and a slightly discordant age of of the Avalon terrane than the Gander terrane [59]. Zircons

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500 the zircon inheritance suggests that the rocks of the Blair River inlier are more likely correlative to the Avalon terrane.

100 6.2. Postcollisional Origin of the Fox Back Ridge Intrusion? The metagabbro is chemically similar to within-plate alkali basalt with LREE-enriched chondrite-normalized patterns 10 but dissimilar to gabbroic rocks of the Red River anorthosite suite (Figures 6(d), 7(b), and 8(a)–8(c)). It appears that the rocks were either unaffected or minimally affected by crustal contamination as they do not have negative primitive Sample/primitive mantle Sample/primitive 1 mantle-normalized Nb-Ta anomalies and their Nb/U ≤ (> 24) and Th/NbPM ( 1.1) ratios do not trend toward average crustal values (Figure 8(d)). Furthermore, it does not .1 appear that fractional crystallization of plagioclase or Fe-Ti Rb Ba Nb La Ce Sr Nd Zr Ti Y oxide minerals played a role in their formation as the (a) Eu/Eu∗ values are ~1 and they do not have negative anomalies 500 of Sr, Ba, or Ti (Figure 7(a)). This suggests, if early fractional crystallization occurred and produced the metagabbro, that only mafic silicate minerals (olivine, pyroxene) fractionated. 100 Therefore, it is likely that the chondrite-normalized rare earth element patterns are primarily the result of partial melting. The high Tb/YbN (1.8-1.9) and Sm/YbPM (4.2-4.5) ratios of the metagabbro indicate that garnet was likely a residual phase 10 in the mantle source suggesting that the minimum depth of melting was equivalent to the garnet-spinel transition

Sample/chondrite (70 ± 20 km) zone [74]. Farther to the west, two diorite rocks 1 from the Fox Back Ridge intrusion reported by Miller [22] are interpreted to be within-plate basalt but also indicate that the intrusion likely differentiated from mafic to intermediate. The chemistry of the metagabbro is typical of mafic rocks 0.1 from within-plate (Figure 8(b)) settings but the regional tec- La Pr Eu Tb Ho Tm Lu tonic setting just prior (>430 Ma) to emplacement was likely Ce Nd Sm Gd Dy Er Yb one of compression (i.e., Salinic orogeny), and thus the gab- Anorthosite bros should be similar to volcanic-arc basalt or more likely this study demonstrate subduction-related geochemical signatures Miller and Barr (2000) [43, 62]. The tectonomagmatic dichotomy is difficult to Meta-gabbro OIB reconcile. However, compositionally similar rocks to the (b) metagabbro are known to occur in postcollisional settings. The Late Ediacaran (~580 Ma) Kekem (Cameroon) and Figure 7: (a) Primitive mantle normalized incompatible trace Guéra Massif (Chad) gabbros have low SiO2 (45.7 to element diagrams of the Red River anorthosite and metagabbro : 51.0 wt%) and high TiO2 and P2O5 (TiO2 =16 to 3.6 wt%; from the Blair River inlier. (b) Chondrite normalized rare earth : P2O5 =08 to 2.0 wt%) contents (Figures 8(b) and 8(c)). They element diagram of the rocks from the Blair River inlier. were emplaced 15-25 million years after the terminal colli- Additional data from Miller and Barr [15]. Normalizing values sion (600-590 Ma) between the Saharan Metacraton and the from Sun and McDonough [96]. Congo-Saõ Francisco Craton. Both the Kekam and Guéra gabbros are considered to be derived from a subduction- modified lithospheric mantle source from the spinel-garnet with ~650 Ma ages are identified in the Arisaig Group of the transition zone [75, 76]. Antigonish Highlands in the Avalon terrane, metasedimen- The Late Ediacaran tectonomagmatic setting of Central tary granulites from the structural basement of Meguma, Africa is broadly similar to that of the Blair River inlier before sedimentary rocks from eastern Newfoundland (Avalonia), and during emplacement of the metagabbro. The collision and metavolcanic schists from Creignish Hills (Avalonia) of between the Avalon terrane and Laurentia was likely pre- southwestern Cape Breton [67, 69, 71, 72]. Sedimentary ceded by westward subduction toward Laurentian and the rocks of southwest New Brunswick and coastal Maine have formation of Late Silurian arc and later back-arc (post-colli- a few detrital zircons with ages of ~650 Ma (12 zircons) but sional) magmatic rocks throughout Newfoundland, New four of the six units are south of the Turtle Head Fault [73]. Brunswick, and Nova Scotia [42, 43]. Collision and crustal The Turtle Head Fault was considered to be the boundary thickening occurred before 428 Ma which could account for between Gander and Avalon terrane although this interpre- subsequent mantle melting within the garnet stability field tation seems to have fallen out of favour [6, 57]. Presently, to produce the metagabbro. Keppie et al. [16] suggests that

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Pluton No. Location Terrane Age (Ma) Mineral Reference Red River sheet Cape Breton Island Blair River inlier 425:1±2:2 Zircon This study Meta-gabbro Blair River inlier 423:8±2:5 Zircon This study Red River anorthosite Blair River inlier 421 ± 3 Zircon Keppie et al. [16] Fox Back ridge Blair River inlier 423 ± 3 Titanite Miller et al. [17] Fox Back ridge Blair River inlier 417 ± 6 Hornblende Miller et al. [17] Red River syenite Blair River inlier 425 ± 2 Titanite Miller et al. [17] Lowland Brook syenite Blair River inlier 428 ± 7 Muscovite Miller et al. [17] Lowland Brook syenite Blair River inlier 424 ± 3 Titanite Miller et al. [17] Otter Brook gneiss Blair River inlier 423 ± 6 Titanite Miller et al. [17] Gneissic anorthosite Blair River inlier 427 ± 2 Titanite Miller et al. [17] Otter Brook gneiss Blair River inlier 421 ± 6 Phlogopite Miller et al. [17] Red River anorthosite Blair River inlier 424 + 4/−3 Titanite Miller et al. [17] Sammys Barren Blair River inlier 435 ± +7/−3 Zircon Miller et al. [17] MacLean Brook 1 Aspy 442:4±2 Zircon Slaman et al. [35] Lavis Brook 2 Aspy 440 ± 1:6 Zircon Slaman et al. [35] Gillanders Mountain 3, 4 Aspy 436:7±2:6, 428:6±1:9 Zircon Barr et al. [33], Lin et al. [19] Gills Brook 5 Aspy 436:4±1:5 Zircon Barr et al. [33] Lake Ainslie 6 Aspy 428:5±1:7 Zircon Barr et al. [33] Grand Falaise 7 Aspy 427:9±2:1 Zircon Slaman et al. [35] Leonard MacLeod Brook 8 Aspy 420:9±2:8 Zircon Barr et al. [33] Pass Island 9 Newfoundland Avalonia 423 ± 4 Zircon Kellet et al. (2014) Welsford 10 New Brunswick Ganderia 422 ± 1 Zircon Bevier [49] 428:3±1, 425:5±2:1, 420:4±2:4, Utopia 11–14 Ganderia Zircon, monazite Barr et al. [48], Mohammadi et al. [46] 420 ± 3:5 Ma Bocabec gabbro 15 Ganderia 422:1±1:3 Ma Zircon Clarke et al. [44] Jake Lee Mountain 16 Ganderia 418 ± 2 Zircon Mohammadi et al. [46] Moosehorn 17 Maine Coastal Maine 421:1±0:8 Zircon McLaughlin et al. [53] Penobscot 18 Coastal Maine 419 ± 2 Zircon Stewart et al. [56] Spruce Head 19 Coastal Maine 421 ± 1 Zircon Tucker et al. [57]

Cadillac Mountain 20, 21 Coastal Maine 424 ± 2, 419 ± 2 Zircon Seaman et al. [55] Lithosphere on 25 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/8825465/5257667/8825465.pdf Lithosphere

Table 2: Continued.

Pluton No. Location Terrane Age (Ma) Mineral Reference Sedwick 22 Coastal Maine 419:5±1 Zircon Stewart et al. [56] Cranberry Island 23 Coastal Maine 424 ± 1 Zircon Seaman et al. [55] Brenton pluton 24 Nova Scotia Meguma 439 + 4/−3 Zircon Keppie and Krogh [50] White Rock formation 25 Meguma 438 ± +3/−2 Zircon MacDonald et al. [52] Mavillette gabbro 26 Meguma 426 ± 2 Baddeleyite Warsame [58] 13 14 Lithosphere

10 B

Within-plate 10

1 OIB

E-MORB E Zr/Y /Yb MORB

N-MORB

Volcanic-arc .01 1 .1 1 10 100 10 100 1000 Nb/Yb Zr (ppm) (a) (b) 0.4 50 N-MORB OIB

40 0.3

30 Lower 0.2 crust La/Ba Nb/U 20 OIB 0.1 10 Upper crust 0.0 0 012345678910 0246810

Sm/YbPM /NbPM Kekem Metagabbro Guéra RRAS gabbro

Figure 8: (a) Th/Yb vs. Nb/Yb classiﬁcation of basaltic rocks [97]. (b) Zr/Y vs. Zr (ppm) classiﬁcation of basaltic rocks by Pearce and Norry [98]. E: mid-ocean ridge basalt (MORB) and within-plate basalt. (c) La/Ba vs. Sm/YbPM of the metagabbro and postcollisional gabbros from the Central African Orogenic Belt. (d) Nb/U vs. Th/NbPM of the metagabbro and post-collisional gabbros from the Central African Orogenic Belt, lower crust, and upper crust. Additional data from Kwékam et al. [75] and Shellnutt et al. [76] for the Kekam and Guéra gabbros and Dupuy et al. [21] for gabbroic rocks from the Red River anorthosite suite (RRAS). PM: normalized to primitive mantle values of Sun and McDonough [96]. OIB: ocean-island basalt; N-MORB: normal mid-ocean ridge basalt; E-MORB: enriched mid-ocean ridge basalt [96]. Lower and upper crust values from Rudnick and Gao [99].

the fault and shear zone pattern of NW Cape Breton is indic- ern New Brunswick. The Saint George Batholith (Welsford, ative of a positive ﬂower structure and that the Blair River Utopia, and Jake Lee Mountain plutons and Bocabec gabbro) inlier was rapidly exhumed during the Early Devonian intruded rocks of the southern Gander terrane and is very (415-410 Ma) probably due to sinistral reverse motion along close to the boundary with Avalonia [46]. In other words, it the Cape Ray suture that is related to sinistral transpression is possible that the gabbro was emplaced during a period of along the Dover fault in Newfoundland. Furthermore, the postcollisional crustal relaxation after the Avalon-Gander metagabbro is contemporaneous with the emplacement of collision but prior to Early Devonian sinistral fault motion the ‘A-type’ granites of the Saint George Batholith of south- in Newfoundland.

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Table 3: Starting compositions used for MELTS modeling. with collisional settings tend to be +1 to +4 log units (ΔFMQ +1 to +4) above that of mid-ocean ridge basalt although our Sample BR-2-1 Model choice of the NNO (ΔFMQ +0.7) buﬀer is not anomalously SiO2 (wt%) 46.61 47.36 high [86, 87]. The pressure (0.2 GPa) of the model is equal

TiO2 1.95 1.98 to a depth associated with the lower upper crust or upper middle crust. Al2O3 17.19 17.47 Fe O t 11.56 11.75 The results of the fractionation modeling are summa- 2 3 rized in Table DR3 and shows that the system is dominated MnO 0.16 0.16 by feldspar (55.6%), pyroxene (10.9%), and spinel (11.9%) MgO 4.85 4.93 fractionation with ~10% residual liquid remaining. The CaO 7.68 7.80 residual liquid will not match the bulk composition of the Na2O 3.04 3.09 anorthositic rocks given that they are ‘cumulate’ rocks, but

K2O 1.82 1.85 we selected the mineral compositions generated by the model that are within the compositional range of the minerals of the P2O5 1.58 1.61 H O 2.0 Red River anorthosites presented by Miller [22] in order to 2 ‘reconstruct’ the rocks by mass balance. Tables 4 and 5 sum- Pressure 0.2 GPa marize the results of mass balance and show that the modeled ƒO Δ 2 FMQ 0.7 mineral compositions derived from the metagabbro can yield FMQ: fayalite-magnetite-quartz buffer. compositions that are very similar to the anorthosite dykes and main body anorthosite. Miller [22] observed that the 6.3. Petrogenetic Model of the Anorthositic Rocks. Anorthosite anorthositic rocks are 90% to 97% plagioclase with a compo- is an uncommon plutonic rock that is almost entirely sitional range from An43-50 to An50-56 in the least altered (≥ 90%) comprised of plagioclase feldspar [77, 78]. There samples. The amount of plagioclase required in both mass are at least six different varieties of anorthosite identified by balance calculations are 94.2% and 90.2%, respectively. The Ashwal [77, 79] that include: Archean anorthosite, Protero- compositions of the feldspars used in the calculations are zoic massif anorthosite, layered intrusion anorthosite, oce- An49 for calculation 1 (Table 4) and An29 for calculation 2 fl anic anorthosite, anorthosite inclusions, and extraterrestrial (Table 5). The lower An content re ects the higher SiO2 ‘ ’ anorthosite. The most abundant type of terrestrial anortho- and Na2O contents and lower Al2O3 content of the sodic site is the Proterozoic massif-type anorthosite [80, 81]. anorthosite but also the lowest measured An component fi Proterozoic massif-type anorthosite is associated with large (An29) observed by Miller [22]. The ma c minerals observed (up to ~18 000 km2) composite batholiths that range in age in the anorthosite include biotite, chlorite, epidote, Fe-Ti from ~2.6 Ga to ~0.5 Ga. The batholiths also include silicic oxide minerals with recrystallized orthopyroxene, and altered rocks (mangerite, charnockite, and granitoids) that are coeval clinopyroxene [22]. In addition to 94.2% plagioclase (An49) but not petrogenetically related to the anorthosite [81, 82]. for calculation 1, 1.7% titanomagnetite, 4.0% orthopyroxene The plagioclase compositions are commonly between An70 (Mg# 70), and 0.1% apatite can reproduce the main body and An30 although some anorthosite massifs may have a anorthosite composition. In contrast, calculation 2 requires restricted range (An45±3) whereas others have a wide range 0.4% titanomagnetite, 0.17% apatite, and 9.0% biotite to [81]. The formation of massif-type anorthosite is not entirely reproduce the bulk composition of the anorthositic dykes. understood but is ultimately related to efficient segregation The modeled mineral compositions we used in our calcu- and accumulation of plagioclase from a mafic parental lation did not necessarily crystallize at the same time. For magma [77, 80–83]. example, the modeled orthopyroxene composition used in In order to test the possible petrological relationship the first mass balance calculation (Table 4) crystallized at a between the metagabbro and anorthositic rocks, we apply temperature of 950°C, whereas the modeled plagioclase fractional crystallization modeling using MELTS [84, 85]. compositions crystallized at 920°C and the Ti-magnetite For the model, we use a parental magma composition equal at 820°C. Conceptually, this suggests that some minerals to the metagabbro. The precise magmatic conditions of the (e.g., orthopyroxene) within the anorthosite were entrapped system at the time of emplacement are unknown; however, within a plagioclase mush whereas others (e.g., Ti-magnetite) given the likely collisional/post-collisional tectonic setting may have crystallized from an interstitial liquid within the [62], we based the modeling conditions (H2O content and plagioclase mush. ƒ O2) on such settings (Table 3). We use a relatively moderate water content (~2.0 wt%) for the parental magma and a rela- 6.4. Emplacement Model. Undoubtedly, there are a variety of tive oxidation state equal to the Ni-NiO buffer (ΔFMQ +0.7). processes that contribute to the formation of anorthosite The moderate water content is selected for two reasons: (1) massifs, but the principal feature is that plagioclase must fi fi the rock has high P2O5 content that suggests it was derived separate, probably by density strati cation, from a ma c from a volatile-rich system and (2) the rock composition parental magma and accumulate [77, 80, 81]. Our modeling indicates it already experienced fractionation of mafic silicate results suggest that a parental magma similar to the metagab- minerals (olivine, pyroxene) that likely increased the bulk bro of this study can produce the requisite minerals and their water content of the residual magma. The choice of the redox compositions that comprise the anorthositic dykes and the conditions is based on the fact that mafic magmas associated Red River anorthosite. Although we cannot quantify the

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Table 4: Mass balance calculation of the Red River anorthosite.

Sample SiO2 TiO2 Al2O3 Fe2O3t MnO MgO CaO Na2OK2OP2O5 Total BR-1-1 54.58 0.20 26.60 2.33 0.03 1.65 8.89 5.14 0.54 0.03 100.00 BR-1-2 54.49 0.14 26.74 2.19 0.03 0.94 9.89 4.82 0.69 0.05 100.00 Mass balance∗ 54.65 0.15 26.26 2.38 1.00 9.66 5.15 0.56 0.04 99.99 Mineral Plag Ti-Mag Apatite Opx Element An49

SiO2 55.77 52.98

TiO2 8.38 0.08

Al2O3 28.05 2.72 2.28 FeO 36.52 17.94

Fe2O3 50.57 0.83 MnO MgO 1.80 24.34 CaO 10.13 55.84 1.52

Na2O 5.47 0.03

K2O 0.60

P2O5 42.05 Mineral Pl% Ti-Mag% Ap% Opx% Total Percentage∗ 94.2 0.17 0.1 4.0 100.0 Mineral symbols: An: anorthite component; Pl: plagioclase; Ti-Mag: titanomagnetite; Ap: apatite; Bt: biotite. Apatite composition taken from Deer et al. [100].

Table 5: Mass balance calculation of the anothorsitic sheet.

Sample SiO2 TiO2 Al2O3 Fe2O3t MnO MgO CaO Na2OK2OP2O5 Total BR-3-1 58.05 0.14 23.28 2.93 0.06 1.27 5.62 6.40 2.19 0.07 100.00 BR-3-2 57.59 0.14 25.01 1.89 0.03 0.58 4.74 6.40 3.30 0.33 100.00 Mass balance∗ 58.08 0.16 23.56 1.87 1.35 5.60 6.61 2.10 0.07 99.40 Mineral Plag Ti-Mag Apatite Biotite Element An29

SiO2 60.68 37.13

TiO2 8.38 1.37

Al2O3 24.56 2.72 15.37 FeO 36.52 16.78

Fe2O3 50.57 MnO MgO 1.80 14.89 CaO 6.10 55.84

Na2O 7.29 0.38

K2O 1.36 9.66

P2O5 42.05 Mineral Pl% Ti-Mag% Ap% Bt% Total Percentage∗ 90.2 0.4 0.17 9.0 99.77 Mineral symbols: An: anorthite component; Pl: plagioclase; Ti-Mag: titanomagnetite; Ap: apatite; Bt: biotite. Apatite composition taken from Deer et al. [100]. Biotite data from Shellnutt and MacRae [101].

processes that led to the concentration of the plagioclase, it the Late Silurian [43, 61, 88]. The emplacement of older was likely due to a combination of density stratiﬁcation and (>428 Ma) silicic plutons in Cape Breton Island and the south- residual melt segregation (c.f., [77]). Regarding the likely tec- ern margin of the Gander terrane prior to collision between the tonic setting of this system, as mentioned previously, the accreted margin of Laurentia and Avalonia are similar to I-type Avalon terrane collided with the margin of Laurentia during (cordilleran) granites, whereas younger (≤ 428 Ma) plutons are

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similar to A-type (postcollision) granites and within-plate gab- References bros [33, 35, 46, 55, 89–91]. Given the emplacement age and the timing of A-type granite plutonism in southern New [1] J. P. Hibbard, C. R. van Staal, and D. W. Rankin, “A compar- ative analysis of pre–Silurian crustal building blocks of the Brunswick (Saint George Batholith), it is likely that the anor- ” thositic rocks of the Blair River inlier were emplaced at the northern and the southern Appalachian orogen, American Journal of Science, vol. 307, no. 1, pp. 23–45, 2007. same postcollisional setting, which may be an important envi- ronment for the creation of some Proterozoic and Phanerozoic [2] J. B. Murphy, J. D. Keppie, J. Dostal, and R. D. Nance, “Neoproterozoic – early Paleozoic evolution of Avalonia,” massif-type anorthosites [81, 92, 93]. in Laurentia–Gondwana Connections Before Pangea,V. A. Ramos and J. D. Keppie, Eds., vol. 336, pp. 253–266, 7. Conclusions Geological Society of America Special Papers, 1999. “ The anorthositic and mafic rocks collected from the southern [3] C. R. van Staal and S. M. Barr, Lithospheric architecture and tectonic evolution of the Canadian Appalachians,” in Blair River inlier of Cape Breton are contemporaneous and Tectonic Styles in Canada Revisited, J. A. Percival, F. A. were emplaced during a period of Late Silurian crustal relax- Cook, and R. M. Clowes, Eds., vol. 49, pp. 41–95, The ff ation. The anorthositic rocks of this study are di erent LITHOPROBE perspective: Geological Association of whereas one is compositionally similar to labradorite, the Canada Special Paper, 2012. other is more similar to andesine although it has an [4] J. P. Hibbard, C. R. van Staal, D. Rankin, and H. Williams, appreciable amount of K2O. 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