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Geoscience Canada

Meguma Terrane Revisited: Stratigraphy, Metamorphism, Paleontology, and Provenance Chris E. White and Sandra M. Barr

Volume 39, Number 1, 2012

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Cite this document White, C. E. & Barr, S. M. (2012). Meguma Terrane Revisited:: Stratigraphy, Metamorphism, Paleontology, and Provenance. Geoscience Canada, 39(1), 8–12.

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GAC–MAC 2012: FIELD GUIDE SUMMARY

Meguma Terrane Revisited: phosed (greenschist to amphibolite stratigraphic differences. The forma- facies) during the Early to Middle tions of the Rockville Notch Group, as Stratigraphy, Devonian Neoacadian orogeny (ca. well as two distinct suites of mafic sills Metamorphism, 405–365 Ma), and intruded by numer- and dykes (White and Barr 2004), ous, late syntectonic to post-tectonic, occur only northwest of the shear Paleontology, and mainly Middle to Late Devonian, pera- zone (Figs. 1, 2). Provenance luminous, granitic plutons (Figs. 1, 2). Rocks underlying the Gold- This orogenic event is not yet well enville Group are not exposed and GAC–MAC 2012 St. John’s post- understood but was related, at least in metamorphic rocks in the Trafalgar meeting field trip part, to dextral transpressive accretion area in the northern part of the terrane of Meguma terrane against adjacent (Fig. 1), interpreted previously to rep- Chris E. White1 and Sandra M. Barr2 Avalonia along the Cobequid–Ched- resent possible basement units (e.g. 1 Nova Scotia Department of Natural abucto fault system. Subsequent Car- Dostal et al. 2006), have been demon- Resources, P.O. Box 698, Halifax, Nova boniferous motion on the fault system strated to be the result of contact Scotia B3J 2T9 and renewed transpression throughout metamorphism of the Goldenville and E-mail: [email protected] the Meguma terrane was related to Halifax groups around Devonian gran- docking of Gondwana (Africa) out- itoid plutons (White et al. 2009; Scal- 2 Department of Earth and Environmental board of Meguma terrane (Fig. 3). lion et al. 2011). Gneissic xenoliths in Science, Acadia University, Wolfville, Nova Although long considered a rare mafic dykes in the Sheet Harbour Scotia B4P 2R6 monotonous stratigraphic succession, area (Fig. 1) also have been interpreted detailed mapping during the past 15 to represent Meguma basement (Owen SUMMARY years (e.g. Horne and Pelley 2007; et al. 1988; Greenough et al. 1999). White 2010a, b; White et al. 2009) Based on age and isotopic signatures, This field trip highlights our current revealed that the Goldenville and Hali- the gneissic xenoliths were inferred to understanding of the unique geology fax groups can be divided into region- have been derived from underlying of the Meguma terrane of southern ally extensive and distinctive forma- Avalonian crust, and have been used as Nova Scotia, the most easterly (out- tions and members (Fig. 2). An exam- evidence for the argument that Megu- board) component of the northern ple is the High Head member in the ma has Avalonian basement (Murphy Appalachian orogen (Fig. 1, inset). lower part of the Goldenville Group et al. 2004). Seismic data show that Recent work has changed previous (Church Point formation) north of the leading edge of Meguma has been interpretations of the terrane and pro- Yarmouth (Fig. 1), which contains a thrust over Avalonia (e.g. Keen et al. vided new insights into its provenance previously undiscovered assemblage of 1991a, b; Loncarevic et al. 1989), pre- and relationship to other peri-Gond- diverse and well preserved Early Cam- sumably during the Neoacadian oroge- wanan terranes, as well as its history of brian deep water trace fossils (Gingras ny, but their original relationship accretion to composite Laurentia in the et al. 2011). A major breakthrough in remains unknown. mid- to late Paleozoic. understanding the stratigraphy was the Meguma is generally accepted The Meguma terrane is char- recognition of a distinctive marker to be a peri-Gondwanan terrane, acterized by a thick package of Early unit, in many places manganese-rich although the specific location along the Cambrian to Early Ordovician tur- and coticule-bearing, which forms the margin of Gondwana where it origi- biditic metasandstone and slate (Gold- uppermost unit (100–300 m in thick- nated remains speculative. Traditionally, enville and Halifax groups), locally ness) of the Goldenville Group the Goldenville and Halifax groups overlain unconformably by a thin throughout the Meguma terrane were interpreted to have been deposit- sequence of slate, quartz ite and vol- (White and Barr 2010). Another ed on the continental rise and/or slope canic rocks (Rockville Notch Group) important advance was the recognition to outer shelf of a passive margin that that ranges from Early Silurian to Early that the Chebogue Point shear zone was part of, or adjacent to, the North Devonian (Figs. 1, 2). These rocks (Fig. 1) divides the terrane into north- African craton during the Cambrian, were de formed and variably metamor- western and southeastern parts with perhaps in close association with GEOSCIENCE CANADA Volume 39 2012 9

Figure 1. Simplified geological map of the Meguma terrane, southern Nova Scotia (after White 2010a). Inset map shows distri- bution of major tectonic elements of the northern Appalachian orogen after Hibbard et al. (2006). Abbreviations in inset map: CBI, Cape Breton Island; NB, New Brunswick; NL, Newfoundland; NS, Nova Scotia; QUE, Québec.

Armorica (Schenk, 1971, 1981, 1991, Cambrian to Tremadocian successions ing the Goldenville and Halifax groups 1997), but Amazonian connections of the Meguma terrane to those of the and metavolcanic and fossiliferous now appear more likely (e.g. Keppie Harlech Dome of North Wales, and metasedimentary rocks of the Silurian 1977; Waldron et al. 2009). Meguma proposed that both areas were part of White Rock Formation of the lower- also has been inferred to be part of a ‘Megumia domain’ that occupied a most Rockville Notch Group. We will and/or to have travelled with Avalonia rift in the margin of Gondwana. view the oldest exposed beds of the (e.g. Landing 2004; Murphy et al. 2004; Goldenville Group and the trace fossils Nance et al. 2008). FIELD TRIP ITINERARY of the High Head member as docu- Waldron et al. (2009) conclud- mented by Gingras et al. (2011), and ed that the stratigraphic differences Day 1 will focus on the stratigraphy the enigmatic contact between the Hal- across the Chebogue Point shear zone, and metamorphism of the Goldenville ifax Group and White Rock Forma- combined with similarities to Avalon- and Halifax groups along the ‘south tion, variously described in the past as ian detrital zircon populations, juvenile shore’ of Nova Scotia, east of the conformable, unconformable, faulted, Nd isotopic compositions, and sparse Chebogue Point shear zone, as well as and tectonically imbricated. faunal evidence indicating Avalonian some of the granitoid plutons that During Day 3 we will focus affinity (Pratt and Waldron 1991), sug- intruded those units. Stops will on stratigraphy in the upper part of gest that the Meguma terrane formed include the pyrite-rich Cunard Forma- the Goldenville Group and the overly- in a rift between Gondwana (Amazo- tion of the Halifax Group and under- ing Halifax Group in the Bear River nia) and Avalonia. Based on detrital lying manganiferous Moshers Island, and Wolfville areas, looking at typical zircon signatures, Barr et al. (in press) Government Point, and Green Har- localities for new formations defined in demonstrated similarity in the Early bour formations of the Goldenville the Halifax Group in that area, ages of Cambrian but diverging subsequent Group at both low and high metamor- which are well constrained by acritarch histories after that. Rifting at ca. phic grades, the latter including the fossils (White et al. 2011). 520–500 Ma may have moved Avalonia spectacular low-pressure/high-temper- For those interested, an away from Gondwana but Meguma ature andalusite–cordierite–staurolite optional Day 4, not included in the moved later (Late Ordovician–Early metapelite in the Shelburne metamor- formal part of the field trip, will con- Silurian) as evidenced by bimodal vol- phic culmination (Fig. 4). sist of stops in the Trafalgar area to canism in the White Rock Formation view the manganiferous garnet devel- (MacDonald et al. 2002). Waldron et Day 2 will look at rocks northwest of oped in granitoid rocks of the Trafal- al. (2011) emphasized similarities in the the Chebogue Point shear zone, includ- gar plutonic suite as a result of interac- 10

map of the Appalachian orogen (north), Canada-United States of America: Geological Survey of Cana- da Map 2041A, scale 1:1,500,000, 1 sheet. Horne, R. J. and Pelley, D. 2007: in Mineral Resources Branch, Report of Activi- ties 2006; Nova Scotia Department of Natural Resources, Report ME 2007- 1, p. 71–89. Keen, C.E., Kay, W.A., Keppie, J.D., Maril- lier, F., Pe-Piper, G., and Waldron, J.W.F. 1991a. Deep seismic reflection data from the Bay of Fundy and Gulf of Maine: tectonic implications for the northern Appalachians. Canadian Journal of Earth Sciences, v. 28, p. 1096–1111. Keen, C.E., MacLean, B.C., and Kay, W.A. 1991b. A Deep Seismic Reflection Profile across the Nova Scotia Conti- nental Margin, Offshore Eastern Canada. Canadian Journal of Earth Sciences, v. 28, p. 1112–1120. Keppie, J.D. 1977. Plate Tectonic Interpre- tation of Palaeozoic World Maps (with Emphasis on Circum-Atlantic Orogens and Southern Nova Scotia). Nova Scotia Department of Mines Paper, 77-3. Landing, E. 2004. Precambrian-Cambrian boundary interval deposition and the marginal platform of the Avalon microcontinent. Journal of Geody- namics, v. 37, p. 411–435. Loncarevic, B.D., Barr, S.M., Raeside, R.P., Keen, C.E., and Marillier, F. 1989. Northeastern extension and crustal expression of terranes from Cape Bre- ton Island, Nova Scotia, based on geophysical data. Canadian Journal of Earth Sciences, v. 26, p. 2255–2267. MacDonald, L.A., Barr, S.M., White, C.E., and Ketchum, J.W.F. 2002. Petrology, Figure 2. Schematic summary of geological events in the Meguma terrane after age, and tectonic setting of the White White (2010a) and references therein. Rock Formation, Meguma terrane, Nova Scotia: evidence for Silurian tion with manganiferous rocks of the Liscomb Complex, Nova Scotia, continental rifting. Canadian Journal Beaverbank formation. Canada. Lithos, v. 86, p. 77–90. of Earth Sciences, v. 39, p. 259–277. Gingras, M.K., Waldron, J.W.F., White, Moran, P.C., Barr, S.M., White, C.E., and REFERENCES C.E., and Barr, S.M. 2011. The evolu- Hamilton, M.A. 2007. Petrology, age, Barr, S.M., Hamilton, M.A., Samson, S.D., tionary significance of a lower Cam- and tectonic setting of the Seal Island Satkoski, A., and White, C.E. in press. brian trace fossil assemblage from the Pluton, offshore southwestern Nova Provenance variations in northern Meguma terrane, Nova Scotia. Canadi- Scotia. Canadian Journal of Earth Sci- Appalachian Avalonia based on detri- an Journal of Earth Sciences, v. 48, p. ences, v. 44, p. 1467–1478. tal zircon age patterns in Ediacaran 71–85. Murphy, J.B., Fernandez-Suarez, J., Keppie, and Cambrian sedimentary rocks, New Greenough, J.D., Krogh, T.E., Kamo, S.L., J.D., and Jeffries, T.E. 2004. Contigu- Brunswick and Nova Scotia, Canada. Owen, J.V., and Ruffman, A. 1999. ous rather than discrete Paleozoic his- Canadian Journal of Earth Sciences. Precise U–Pb dating of Meguma base- tories for the Avalon and Meguma Dostal, J., Keppie, D.J. Jutras, P., Miller, ment xenoliths: new evidence for Terranes based on detrital zircon data. B.V., and Murphy, B.J. 2006. Evidence Avalonian underthrusting. Canadian Geology, v. 32, p. 585–588. for the granulite-granite connection: Journal of Earth Sciences, v. 36, p. Nance, R.D., Murphy, J.B., Strachan, R.A., penecontemporaneous high-grade 15–22. Keppie, J.D., Gutierrez-Alonso, G., metamorphism, granitic magmatism Hibbard, J.P., van Staal, C.R., Rankin, D., Fernandez-Suarez, J., Quesada, C., and core complex development in the and Williams, H. 2006. Lithotectonic Linnemann, U., D’lemos, R,., and Pis- GEOSCIENCE CANADA Volume 39 2012 11

Figure 3. A tectonic model for the Neoacadian orogeny and related pluton emplacement (after Moran et al. 2007). Pluton emplacement in Meguma terrane may have been related to subduction of the Rheic Ocean lithosphere outboard of Meguma, likely combined with slab-breakoff which also caused high-temperature/low-pressure (HT-LP) metamorphism.

Figure 4. Regional metamorphic isograd map for the Meguma terrane compiled from data and sources in Raeside and Jamieson (1992), White (2003), and unpublished data. 12

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