3 Williams (1978) originally included the St. Croix terrane within the Avalon zone based on correlation with strata in the Saint John area of coastal New Brunswick. Subsequently, informed by a field trip in the Penobscot Bay region led by D.B. Stewart in 1999, van Staal et al. (2004) assigned the St. Croix terrane to Ganderia (Fig. 1). In particular, the Megunticook Formation was found to closely resemble the Crocker Hill Formation of the Lower Cookson Group in southwestern New Brunswick. This re-interpretation allows for the existence of an Avalon Seaway that accounts for the distribution of arc rocks, high-pressure metamorphism, and seismic reflections related to the latest Silurian–Middle Devonian Acadian collision between Avalonia and Ganderia (van Staal et al. 2009). Additionally, van Staal et al. (1998, 2012) presented extensive arguments for Ganderia having originated near the Columbian margin of South America and for a Middle to Late Cambrian departure from Gondwana. Johnson et al. (2012) documented pre-Middle Ordovician (Penobscottian) deformation in the Annidale terrane of New Brunswick; Reusch and van Staal (2012) speculated that a quartz-pebble conglomerate in southwestern Penobscot Bay might record a nearby pre-Middle Ordovician Penobscottian event.

4 Smith et al. (1907) defined the Islesboro Formation (Fig. 2) including carbonate members and the uppermost Coombs Limestone member, and correlated overlying rocks with the Battie Quartzite of the St. Croix terrane. Stewart and Lux (1988) reported Neoproterozoic ages for the amphibolite-facies Seven Hundred Acre Island Formation, exposed beneath the Islesboro Formation along an antiform. Berry and Osberg (1989) described a third sequence, which we refer to here informally as the Turtle Head Cove unit, interpreted to overlie both the Islesboro and Seven Hundred Acre Island formations with profound unconformity; of note, Stewart (1998) did not separate this third sequence from the Islesboro Formation.

5 In general, the identities of peri-Gondwanan elements (e.g., Landing 1996; Fyffe et al. 2011; Pothier et al. 2015), their provenance (e.g., Nance et al. 2008; Barr et al. 2012, 2014), and mechanisms of transfer to the Appalachian margin of Laurentia (e.g., Neuman and Max 1989; van Staal et al. 2012; Waldron et al. 2014b) remain matters of ongoing debate. Fundamental problems of both local and broader significance include (1) provenance of the Islesboro basement; (2) relationship to the rifting of Ganderia from Gondwana and subsequent opening of the Rheic Ocean (Schulz et al. 2008; Nance et al. 2012; van Staal et al. 2012); (3) mechanism of juxtaposition of the St. Croix-Ellsworth terranes; (4) timing of the juxtaposition; and (5) whether, during closure of the Iapetus Ocean (between Ganderia and Laurentia), subduction was initiated internally (van Staal et al. 2012) or not (Waldron et al. 2014).

6 The purpose of our reconnaissance investigation of Islesboro is to expand the geochronological database by U–Pb dating of detrital and igneous zircon in samples of metasedimentary and volcaniclastic rock from this critical area. We collected samples from a quartzite that Stewart (1998) showed at the top of the Islesboro Formation (Fig. 3); greywacke from the base of the Turtle Head Cove unit (Fig. 4); and a volcaniclastic rock from the Kedears Hill sulphide prospect (Fig. 4). The geochronological results require a reevaluation of both local field relationships and, in light of recent detrital zircon studies in West Africa (Abati et al. 2010; Bradley et al. 2015) and offshore Massachusetts (Kuiper et al. 2017), global paleogeography.

7 West of the Turtle Head Fault (Fig. 2), the St. Croix terrane (Tucker et al. 2001) comprises, in ascending stratigraphic order, the Rockport, Cookson, and Benner Hill groups. The Proterozoic Rockport Group (Berry in press) is capped by the Coombs Limestone and Rockport Quartzite, which are in turn unconformably overlain by conglomerate and sandstone of the Simonton Corners Formation. The latter basal unit of the Cookson Group is correlated across the West Rockport Fault with the Megunticook Formation that is locally underlain conformably by the Battie Quartzite. The Megunticook Formation comprises a sequence of quartzite and quartz-mica schist (Bickel 1971; Berry and Osberg 2000). The structurally overlying Penobscot Formation, also part of the Cookson Group, consists of rusty black schist with rare quartzose sandstone (Bickel 1971). The Penobscot Formation is correlated with the Calais Formation, which near the Maine-New Brunswick border contains Tremadocian graptolites (Fyffe et al. 2011). The latest Cambrian Gushee Member of the Penobscot displays a clear island-arc geochemical signature (Burke et al. 2016). Highest in the sequence, the Benner Hill Group is notable for containing the only pre-Silurian fossils in the region, which are highly stretched brachiopods of Caradocian (~Sandbian) age (Berry et al. 2000).

8 The Ellsworth terrane comprises a structurally complex assemblage of low-grade, but highly deformed, chloritequartz-feldspar schist, Middle Cambrian marine bimodal volcanic rock (e.g., North Haven Greenstone, Castine Volcanics), rare pelagic sedimentary rock, and rare serpentinized peridotite (Fig. 2). The thickness of this terrane, based on seismic studies, is on the order of several kilometers (Stewart 1998). The presumed steeply dipping Turtle Head Fault juxtaposes the Ellsworth and St. Croix terranes both to the northeast and southwest of Islesboro. However, in several locations (Fig. 2), Ellsworth rocks directly overlie black shale of the Penobscot Formation in an inferred thrust fault relationship. East of Castine, the Penobscot Formation is exposed in the Lord’s Cove window (Osberg et al. 1985; Reusch 2002a, 2002b). On the northwest shore of North Haven Island, Penobscot Formation black shale is injected into overlying fractured North Haven Greenstone (Reusch et al. 2003); east of Rockland, pillow lavas of the North Haven Greenstone are thrust over Penobscot Formation (Osberg et al. 1985). The maximum age of this thrusting event is Early Ordovician. The Turtle Head and related steep faults, according to Stewart et al. (2001), show dextral offsets of Late Silurian isograds that pre-date Late Devonian plutons. On North Islesboro, Stewart (1998) showed a narrow belt of Middle Cambrian Castine Volcanics, one of the marine volcanic units of the Ellsworth terrane, bounded by splays of the Turtle Head Fault (Fig. 2).

9 Strata within the Islesboro fault block (Stewart 1998) include, in ascending order, the amphibolite-facies Seven Hundred Acre Island Formation of Proterozoic age (Stew-art et al. 2001), the greenschist-facies Islesboro Formation of undefined age (Smith et al. 1907), and the third sequence of Berry and Osberg (1989) here referred to as the Turtle Head Cove unit.

10 The Seven Hundred Acre Island Formation (Brookins 1976; Stewart 1998) comprises a platform sequence of marble (Fig. 5E), quartzite, quartz-rich schist, and sparse amphibolite. It was metamorphosed to amphibolite facies during the Neoproterozoic based on ⁴⁰Ar/³⁹Ar ages of ca. 670 to 650 Ma for hornblende from amphibolite, and on a U–Pb age of 646.7 ± 2.7 Ma for zircon from a cross-cutting granitic pegmatite (Stewart et al. 2001).

11 The Islesboro Formation, which surrounds fault-bounded inliers of the Seven Hundred Acre Island Formation (Stewart 1998), consists of heterogeneous siliciclastic rock (Fig. 5D) and lesser carbonate rock. Distinctive brown-weathering carbonate members (Figs. 2 and 5A), shown separately on previous maps, constrain the internal structure of this unit.

12 The Turtle Head Cove unit consists of low-grade sedimentary rock present on the west side of Seven Hundred Acre Island and northwest side of North Islesboro. On Seven Hundred Acre Island, a basal conglomerate and sandstone grade upward into predominant phyllite and lesser sandstone (Fig. 5B and 5C). This unit was interpreted by Berry and Osberg to overlie the Islesboro and Seven Hundred Acre Island formations with angular unconformity.

14 In the eastern part of North Islesboro (Fig. 3), the Coombs Limestone (ZXc) and overlying Hutchins Island Quartzite (ZXh) constitute a newly recognized area of high-grade Proterozoic basement. Our field observations generally support the quartzite-cored syncline shown on previous geologic maps (Smith et al. 1907; Osberg et al. 1985; Stewart 1998) that extends southward from east of Coombs Point through Hutchins Island to the central small headland on the south shore of Coombs Cove. Specifically, the quartzite unit overlies the carbonate unit at Point Comfort, in Coombs Cove on the western limb of the syncline close to its axial trace, at the eastern headland of Hutchins Island on the eastern limb, and at the north end of the small ledge to the northeast (Fig. 3). Not shown on previous maps are small lenses of amphibolite present in the southwestern part of Hutchins Island (sample locations IL-15-3 and -4).

15 The type locality of the Coombs Limestone (Smith et al. 1907) is Coombs Point. This carbonate unit crops out between Coombs Point and Point Comfort, along the southern shore of Coombs Cove, the eastern headland of Hutchins Island, and on a small ledge to the northeast. It consists of thin- to thick-bedded, grey calcitic and buff to brown-weathering dolomitic marble (Fig. 5E), minor quartzite, both white and pink, and sparse calc-silicate rock.

16 The Hutchins Island Quartzite crops out at Coombs Point, Point Comfort, Hutchins Island, and on the hill south of Coombs Cove. An occurrence of cross beds at Point Comfort (H.N. Berry IV, written communication, 1999) suggests that the quartzite conformably overlies the Coombs Limestone. On the south shore of Hutchins Island, thick-bedded white quartzite was sampled for detrital zircon (Fig. 3; Sample IL-14-6; 44°21.042ʹN, 68°52.244ʹW). Thin sections of the quartzite reveal locally abundant chlorite and feldspar, and a strong penetrative fabric (Fig. 6A). Nearby outcrops and those in the southwest part of the island are quartzo-feldspathic gneiss and schist, and rare, discontinuous, thin (<1 m) lenses of amphibolite. The amphibolite is medium grained, contains abundant chloritized hornblende and sericitized plagioclase, and displays a strong fabric (Fig. 6D).

17 At least two generations of folds are present. A presumably older generation consists of open to isoclinal, variably northwest- to southeast-plunging folds with a steeply dipping axial-planar foliation. These folds are nearly perpendicular to the contacts with adjacent Islesboro Formation, suggesting a structural discordance. Other observed structures include common boudinaged beds of quartzite and dolomitic marble, southwest-plunging folds, and late faults.

18 Several observations suggest that the contact relationship of the Proterozoic basement with the adjacent Islesboro Formation (CZi) may be a fault. On the western shore of Coombs Cove, in rocks mapped as Islesboro Formation, graded beds indicate tops to the northwest, opposite the inferred east-younging direction of the western limb of the syncline. On the east shore of Decker Point, rocks mapped as Islesboro Formation (Stewart 1998) are lithologically distinct compared to those on the western shore of Coombs Cove. Finally, the presence of amphibolite on Hutchins Island indicates a metamorphic discordance across the contact, in addition to the structural discordance described above. We are unable to determine whether the Precambrian basement inliers are horsts or klippes.

19 Based on lithologies and field relationships, the Proterozoic basement of Coombs Limestone and Hutchins Island Quartzite on North Islesboro correlates with the Seven Hundred Acre Island Formation of Stewart (1998) exposed in nearby inliers. In addition, these rocks may also be equivalent to the upper part of the Rockport Group of Osberg et al. (1985) exposed on the western shore of Penobscot Bay.

20 Smith et al. (1907) did not designate a specific type locality for the Islesboro Formation. However, the shoreline outcrops of interbedded siliciclastic and minor carbonate rock at Grindel Point (Figs. 2 and 5D) have been taken as representative (e.g., Berry 2007). The presence there of interstratified and discordant greenstone distinguishes the Islesboro Formation from the amphibolite-bearing Seven Hundred Acre Island Formation, and also from the weakly metamorphosed and less-deformed (Fig. 5B) Turtle Head Cove unit in which greenstone is evidently absent. At Grindel Point, thin-bedded siliciclastic strata and distinctive orange-weathering carbonate are inter-bedded (Fig. 5D). Rare sandstone lenses several millimeters thick and several centimeters long, oriented obliquely to bedding, are possible worm burrows (e.g., Berry 2007, photo 10, near center). Beds dip generally north, display very tight horizontal folds (Berry 2007), and are strongly crenulated on moderately north-plunging folds. Conformable greenstone beds, likely mafic tuff, occur in places. Discordant greenstone dykes up to 5 m thick cut the foliation. The thinly bedded, crenulated aspect of these outcrops and presence of abundant greenstone are features of the Ellsworth Schist (Reusch 2003), a potential correlative.

21 The Islesboro Formation is lithologically heterogeneous. On the western shore of Coombs Cove, to the southwest of Hutchins Island, distinctively thin-bedded, fine-grained siliciclastic strata display graded beds; similar distinctively thin-bedded strata, including packages of “ribbon” carbonate, are present on the west shore of Pendleton Point (Fig. 2). To the east of Coombs Cove, in cliffs along the eastern shore of Decker Point (Fig. 3), steeply dipping, massive, light-weathering strata (feldspathic slate of Stewart 1998) strike south-southeast; similar massive, light-weathering rocks at Hewes Point (Fig. 2), a few kilometers to the southeast, display two cleavages but extremely sparse bedding. Decker Point itself is held up by a resistant siliceous rock of uncertain protolith and relationship to adjacent units.

22 A narrow belt of Castine Volcanics crops out along the western shore of North Islesboro (Stewart 1998). At the headland west of Meadow Pond (Fig. 3), this unit, although shown as quartzite and conglomerate by Stewart et al. (2001), consists of thick-bedded tuffaceous sedimentary rock, tuff, and lithic tuff. The outcrops are rusty due to minor disseminated pyrite. These strata display only a weak penetrative deformation. Possible correlatives include the felsic volcanic and volcaniclastic rocks at the Kedears Hill sulphide prospect (in which case this unit was incorrectly correlated with the Castine Volcanics), and also the undated volcaniclastic rocks at Perkins Point (Fig. 2) north of Castine (Smith et al. 1907).

23 Rocks northwest of the North Islesboro fault (Fig. 3) display a low grade of metamorphism relative to those southeast of the fault. Whereas locally two cleavages are present in these northwestern strata (e.g., Gregg 1979; Fig. 5C), in general the structure is characterized by a relatively open fold style and steeply dipping slaty cleavage of presumed pressure-solution origin (Gregg 1986).

24 The Kedears Hill area contains important outcrops used for geochronology in this study. Our mapping (Fig. 4) suggests that several unrelated units are juxtaposed along splays of the North Islesboro fault. In the excavated area on the northeast side of Kedears Hill, the four units we recognize are therefore described in geographic order.

25 The first unit, occurring in the most southeastern exposures, is orange-weathering calcareous phyllite that carries a well-developed, steeply dipping foliation and in places is highly sheared and cut by irregular quartz ± carbonate veins. A recent excavation exposes a thin tuff bed and southeast-dipping fault surface (Fig. 4).

26 Northwest of the calcareous phyllite and a thin strip of pyritic black shale is the second unit that comprises poorly exposed felsic volcanic rocks within the excavated area, including a previously undescribed sulphide prospect (Fig. 4). Massive white albite-rich rhyolite occurs along a ridge located approximately 40 m to the southwest of the prospect, in nearby sub-crop, and as several 2- to 3-m, iron-stained boulders that contain abundant, 1–3 cm clots of coarse pyrite. A small roadbed 25 m to the north of the excavated area exposes a rusty weathering, strongly foliated felsic rock that was sampled for zircon (IL-14-5; 44°22.454ʹN, 68°53.613ʹW). The wide range of ages obtained for this rock (see below) supports an origin as a volcaniclastic sediment, referred to here for simplicity as a tuff. Thin section study shows predominant quartz with lesser muscovite (Fig. 6C); textures, including the presence of doubly terminated euhedral zircon grains without evidence of sedimentary abrasion, suggest a significant ash component.

27 Approximately 40 to 50 m to the southwest of the prospect is the third unit in the excavated area, consisting of several outcrops of massive, quartz-rich greywacke (sample IL-14-4; 44°22.429ʹN, 68°53.652ʹW). We consider these strata to be the basal part of the Turtle Head Cove unit based on more complete exposures of a nearly identical sequence on the southwestern shore of Seven Hundred Acre Island. Minor pyrite occurs in disseminations and locally in thin (1–3 mm) laminae. In thin section (Fig. 6B), the sand grains are angular and poorly size-sorted, surrounded by a volumetrically minor matrix of chlorite and muscovite.

28 The fourth unit, considered to conformably overlie the greywacke, is extensively exposed in the larger, non-metallic quarry to the northwest (Fig. 4) and on both shores of Turtle Head Cove ca. 1 km to the northeast (Fig. 3; Gregg 1979). This unit consists of pervasively Mn-stained, grey-green mudstone and shale locally containing sparse, white, centimeter-sized possible lapilli and elongate, small (1–2 cm) lenses of quartz ± carbonate. Interbedded greywacke is volumetrically minor. These rocks are at most weakly metamorphosed and display a prominent, steeply dipping, northeast-striking cleavage; a second cleavage is much less prominent, more widely spaced, and dips moderately southeast.

29 The east shore of Turtle Head contains nearly continuous outcrops of moderately northwest-dipping, interbedded sandstone and grey to black shale. Many of these outcrops have a white-weathering rind, thus suggesting a tuffaceous component. Carbonate-rich sandstone is locally present, including several beds that contain angular clasts of pyrite up to ~1 cm in diameter. The northernmost headland—Turtle Head—displays open folds that plunge gently northeast around a near-vertical cleavage (Fig. 3).

32 Whole-rock analyses of metasedimentary and meta-igneous rocks are presented in Table 1. On the south side of Hutchins Island, quartzite sample IL-14-6 is distinguished by high SiO₂ (80.92 wt %). Nearby amphibolite samples IL-15-3 and IL-15-4, within the same sequence, contain relatively high Fe₂O₃^T (13.90 and 13.78 wt %, respectively) and TiO₂ (2.05 and 1.78 wt %, respectively). Plagioclase-chlorite schist sample IL-15-5 from Grindel Point is broadly similar to the amphibolite samples in terms of major-element contents, but differs in some trace elements such as having much higher Cr (463 ppm vs ≤93 ppm) and Ni (130 ppm vs ≤30 ppm). Greywacke sample IL-14-4, from the north side of Kedears Hill, has relatively high contents of Al₂O₃ (11.38 wt %) and TiO₂ (1.07 wt %). Nearby sample IL-14-5 of felsic tuff contains appreciable K₂O (3.57 wt %) but sparse Na₂O (0.15 wt %), whereas pyritic rhyolite IL-15-2A, from the same area, has low K₂O (0.05 wt %) but high Na₂O (10.06 wt %).

33 Figure 7 shows geochemical plots for the mafic meta-igneous rocks based on whole-rock compositions. Data for the amphibolite samples from Hutchins Island and Seven Hundred Acre Island, and the plagioclase-chlorite schist from Grindel Point, all plot within the basalt field (Fig. 7A) and the field for Enriched-Mid-Ocean Ridge Basalt (E-MORB) (Fig. 7B). REE data show abundances of ~10× to ~100× chondrite and slightly elevated light rare earth elements (LREE), with amphibolite sample IL-15-4 from Hutchins Island having a positive Eu anomaly (Eu/Eu* = 1.52; Table 1); one amphibolite from Seven Hundred Acre Island is distinct in displaying a higher abundance of LREE and a higher La/Yb ratio (Fig. 7C). Excluding scatter shown by mobile elements (e.g., U, K, and Sr), most samples of the mafic meta-igneous rocks display broadly similar patterns based on Primitive Mantle normalization, including a lack of negative Ta anomalies and large positive Pb anomalies (Fig. 7D).

34 Sample IL-14-6 of quartzite, collected from the south shore of Hutchins Island, yielded 120 concordant U–Pb zircon ages (Fig. 8). Most are centered on ca. 2008 Ma, with minor peaks at ca. 2070, 1965, and 1895 Ma. No ages are younger than ca. 1800 Ma; five fall in the range of ca. 2300 to 2100 Ma. A gap is present between ca. 2400 and 2300 Ma. Nineteen ages are fairly evenly distributed between ca. 2750 and 2400 Ma.

35 Sample IL-14-4, a greywacke collected from near Kedears Hill, yielded 109 concordant U–Pb zircon ages (Fig. 8). A strong late Neoproterozoic maximum at ca. 624 Ma is flanked by lesser peaks at ca. 678, 577, and 515 Ma. The weighted average age of the three youngest grains is 515 ± 5 Ma and defines the maximum depositional age. Over half of the ages are Mesoproterozoic and Paleoproterozoic (ca. 2000–1000 Ma), with a peak at ca. 1204 Ma and 26 ages between ca. 2100 Ma and 1800 Ma. A gap is present between ca. 2550 Ma and 2100 Ma. The seven oldest ages, all Archean, are in the range of ca. 2750 Ma to 2550 Ma.

36 Sample IL-14-5 was collected from a felsic volcaniclastic rock. The sample had a relatively low yield of zircon (45 grains). Cathodoluminescence imaging of the zircon grains revealed several core-rim relationships as well as various morphologies including euhedral to anhedral; several of the zircon grains are fragments (Fig. 9). Because of the complex internal textural relationships, several grains were analyzed multiple times to try to date both cores and rims. Inherited cores (and several grains without core-rim relationships) are older than rims. The youngest coherent six grains analyzed define a proposed igneous age of 371.8 ± 5.7 Ma (Fig. 9B). In addition to these youngest Late Devonian ages, described below, the sample includes grains as old as Archean (Fig. 8). Within this older set of ages, all but six are late Paleoproterozoic to early Neoproterozoic (ca. 1900–800 Ma); two are latest Neoproterozoic. The oldest ages include one at ca. 2100 Ma (Paleoproterozoic), one at ca. 2600 Ma (Neoarchean), and two at ca. 3000 Ma (Mesoarchean).

45 Whole-rock geochemical data for relatively immobile trace elements indicate that protoliths of the mafic meta-igneous rocks (amphibolites) from Hutchins Island are basalt (Fig. 7A) having an E-MORB signature (Fig. 7B). Slightly elevated LREE/HREE ratios (Fig. 7C) are consistent with this classification. Importantly, the primitive mantle-normalized plot (Fig. 7D) lacks negative Ta anomalies, which would be evidence of an arc or subduction-related source for the parent basalts (e.g., Pearce et al. 2005). Very similar data reported by Stewart et al. (2001) for amphibolite from the Seven Hundred Acre Island Formation—including large positive Pb anomalies—suggest a common petrogenetic setting. One sample from Stewart et al. (2001) has a high Nb/Y ratio (Fig. 7A), which may be due to analytical error.

46 Interstratified siliciclastic and carbonate strata and greenstone in the Islesboro Formation are suggestive of a rift setting. Specifically, sample IL-15-5 is a metabasalt with an E-MORB (non-arc) affinity (Figs. 7B and 7D). A mechanism to explain the interstratified siliciclastic and carbonate metasedimentary rocks is by the erosion of horsts of Proterozoic basement composed of mixed siliciclastic and carbonate strata.

47 The age of the Islesboro Formation is bracketed only between ca. 647 Ma and 420 Ma. However, the present study adds two new constraints. This formation apparently underlies the ca. 515 Ma to 420 Ma Turtle Head Cove unit (Berry and Osberg 1989), discussed below. Newly recognized burrow-like structures, if indeed of this origin, may imply a post-Neoproterozoic age (e.g., Bromley 1996). Although Smith et al. (1907) considered the Islesboro Formation to unconformably overlie the Ellsworth Schist of Middle Cambrian age (Schulz et al. 2008), the two formations share several features, notably the presence of abundant greenstone and a similar deformational style, thus it is possible they are equivalent or nearly so. The Ellsworth terrane, which includes substantial bimodal tuff, has been argued to record Middle Cambrian separation of Ganderia from Gondwana as the Rheic Ocean began to open (Schulz et al. 2008). In this rifting context, inliers of Proterozoic basement and the Deer Isle serpentinite (Fig. 2), viewed as hyperextended basement by Reusch and Rust (2001), may be compared to Grenvillian inliers in the western Appalachians that originated as horsts later decapitated by thrust faults (Karabinos et al. 2017). Hence, the Islesboro Formation may record deposition within adjacent grabens, and the Rockland klippe (Fig. 2) of Proterozoic hanging wall on Ordovician (~Sandbian) footwall may represent an example of a thrust fault-decapitated horst.

48 Van Staal et al. (2012) incorporated the Middle Cambrian rift interpretation of the Ellsworth into a larger framework in which Ganderia departed Gondwana from a location on the Amazonian margin near present-day Columbia. These workers provided extensive arguments for an Amazonian provenance, but a cornerstone of the interpretation is the absence of a Mesoproterozoic sediment source in West Africa. The recent discovery of Mesoproterozoic detrital zircon ages in the foreland of the Mauritanide orogen (Bradley et al. 2015) permits a West African provenance for Ganderia as well. While it is appropriate here to resurrect West Africa as the potential birthplace of Ganderia and to suggest that a West African provenance might imply a simpler tectonic evolution, it is beyond the scope of this paper to fully evaluate all of the arguments for an Amazonian provenance of Ganderia.

49 Greywacke at the base of the Turtle Head Cove unit in the Kedears Hill area displays an immature texture and composition. This rock (Sample IL-14-4) has an intermediate Th/Sc ratio of 0.52, implying a mix of mafic and felsic detritus, and Zr/Sc ratio of 12.9, both of which are lower than in the mature Hutchins Island Quartzite (respectively 0.78 and 37.5). Together, these features argue against a passive- margin setting for the greywacke, the only setting in which maximum depositional age and depositional age are known to differ significantly; we therefore suggest that the sediment age may be close to the maximum depositional age.