Provenance of the Meguma terrane, Nova Scotia: rifted margin of early Paleozoic Gondwana John W.F,Waldron, Chris E. White, SandraM. Barr, Antonio Simonetti,and Larry M. Heaman Abstract: Detrital zircon ages from the lower part of the Late Proterozoic(?)to Middle Cambrian Goldenville Group in the Meguma te(ane of Nova Scotia suggestderivation from local sourcesin the Avalonian and Pan-African orogenson the margins of Early Cambrian Gondwana. Samplesfrom near the top of the group show a brcader distribution, including ages back to Archean. The eNddata show a correspondingtrend, from slightly positive in the lower Coldenville Group to highly negative in the upper Goldenville Group and overlying Upper Cambrian to Lower Ordovician Halifax Group. The trends are consistentwith deposition of the lower part of the Meguma successionin a rift, in which uplifted rift-flanks werc the main source of the early basin fill, whereassubsequent themal subsidenceof rift margins allowed for more wide- spread sedimentsourcing in younger units. The dft was possibly located between Gondwana and Avalonia, and may have been the locus for separationof Avalonia from Gondwana to form pafi of the Rheic Ocean. R6sum6: Les 6gesdes zircons d6tritiques du Groupe de Goldenville (Protdrozoiquetardif(?) i Cambrien moyen) dans le terrane de Meguma de la Nouvelle-Ecossesuggdrent une provenancede sourceslocales dans les orogEnesavalonien et pan-atricain sur les bordures du continent de Gondwana au Cambrien pr6coce.Des 6chantillonspr6levds prds du sommet du groupe montrent une distribution plus vaste, incluant des dges arch6ens.Les donn6eseNd montrent une tendancecorre- spondante,de l6garernentpositive dans le Grcupe de Goldenville infdrieur d hautementn6gative dans le Grcupe de Gold- enville sup6deur et dans le Groupe de Halifax susjacent (Cambrien sup6rieure Ordovicien inf6rieur). Les tendances concordent avec la ddposition de la partie inf6rieure de la successionde Meguma dans une fosse d'effondrement; les flancs soulev6sde cette fosse auraient constitu6 la source principale de mat6del de remplissagedu bassin pdmitif alors qu'une subsidencethermique subs6quentedes borduresde la fosse aurait conduit i une source de s6dimentsplus 6tenduepour les unit6s plus jeunes. La fosse d'effondrement 6tait possiblementsitu6e entre le Gondwana et I'Avalonie et elle constitue pos- siblement le lieu de la s6parationde I'Avalonie du Gondwana pour la formation d'une partie de I'oc€an Rh6ique. [Traduit par la R6daction] Introduction adiacent to northwest Africa and was transferTed to Lauren- tia during the Acadian orogeny, whereas others have pro- Meguma, the most outboard terrane of the Canadian Ap- posed an origin juxtaposed with westem Amazonia (e.g., palachians (Fig. la), has no clear conelative elsewhere in Keppie 1977) and (or) that Meguma travelled with Avalonia the Appalachian-Caledonideorogen. Although generally re- during the early Paleozoic (Muryhy et al. 2OO4a). garded as a peri-Gondwanan terane, Meguma shows marked contrasts with adjaqent Avalonia, and its origin has We report here the results of U-Pb dating of detrital zircon been controversial. Some authors (e.g., Schenk 1997 and and Sm-Nd isotopic analyses from Cambrian-Ordovician referencestherein) have proposed that Meguma odginated metasedimentaryunits in Meguma that are relevant to this conboversy. The data demonstxatethat sedimentprovenance changedrapidly from a restrictedand juvenile sourceto more Received 6 October 2008. Accepted 27 Jan!aty 2009. Published diverse and isotopically evolved sourceswith abundantPale- on the NRC ResearchPress Web site at cies.nrc.caon 2 March 2009. oproterozoic zircon, consistentwith formation on the Gond- wanan margin in an evolving dft that subsequentlybecame Paper handled by Associate Editor B. Muryhy. inactive and underwentthermal subsidence. J.W.F, Waldronr and L.M. Heaman. Departmentof Eafth and Atmospheric Sciences,University of Alberta, Edmonton, AB Stratigraphic context T6G 2E3. Canada. C.E. White. NS Departmentof Natural Resources,PO Box 698, Meguma is charactedzedby unique stratigraphy, includ- Halifax. NS B3J 2T9. Canada. ing a thick (>10 km) Cambrian (and possibly older) to Early S.M, Barr. Department of Earth and Environmental Science, Ordovician turbiditic clastic succession,historically assigned Acadia Unive$ity, Wolfvi e, NS B4P 2R6, Canada. to the Meguma Group, and divided into a lower, coarser A. Simonetti. Department of Earth and Atmospheric Sciences, grained Goldenville Formation and an upper, dominantly Univenity of Alberta, Edmonton, AB T6G 2E3, Canada;Present fine-grained Halifax Formation. Shatigraphic work (O'Brien address:Department of Civil Engineering & Geological 1988; Waldron 1992) and recent mapping (e.g., White 2007) Sciences,156 Fitzpatrick Hall, University Dame, of Notre Notre resulted in subdivision of thesethick formations and their el- Dame. IN 46556. USA. evation to group status. The entire package now is termed lConesponding author (e-mail: [email protected]). the Meguma Supergroup(White 2008). Can.J. EarthSci.46: 1-8 (2009) doi:10.1139/E09004 Publishedby NRC Researchkess Can.J. EadhSci. Vol.46,2009 Fig. 1. (a) Meguna Terrane showing units and sample locations. Paleocunent rose diagrams rcprcsentflutes and cunent ipples, corrected for tectonic distortion and rotated to horizontal (Waldron and Jensen 1985; waldron 1988). Inset: Location in Appalachial orogen, afler Hibbard et al. (2006). Fm., Formation; Gp., Group; L.. Lower; U., Upper. (r) Schematic stratigraphiccolumns for Meguma, showing sample bcations. (.) Values of ex6 at 540 Ma, against straligraphicposition, west and easl of ChebogucPoint shear zone (CPSZ). Red line on both plots shows the trend west of the CPSZ. for compffison. Data fiom this study and Clarke and Halliday (1985). Clarke et al. (1988), and Currieet ai. (1998). b Yarmouth-Digby 12 .#,^,; Easlof CheboouePoinl Halifax-Canso shearione 29 18 ^^25 CunadFm r'zla 23 -o 2215 JU,C\ t-- )^ o- "church x PoinlFm. B 38 , F$t tNd at 540t a Publishedby NRC ResearchPress Waldronet al. The Goldenville Group is dominated by psammite (meta- Results sandstone), with subordinate pelitic rocks (metasiltstone, slate, argillite). A locality close to tre top of the group U-Pb detrital zircon dating (Figs. la, lb) yielded a Middle Cambrian trilobite faunule Sample 1, from the lowest point in the exposedstuatigraphy (Pratt and Waldron 1991). Nearer the base of the exposed west of the CPSZ (Fig. lb), shows a strongly clustered age Goldenville Group (Fig. 1b), the trace fossil Oklhamia indt- distribution; 7l of 76 grains lie between 750 and 540 Ma. cates an Early Cambrian or possibly late Neoproterozoicage The density distribution has a strong peak at -640 Ma, with (White et al. 2005). The overlying Halifax Group is domi- subsidiary peaks at 560,585, and possibly 700 Ma. The nated by pelite, associatedwith fine- and very fine-grained youngestgrain, at 544 t 18 Ma, representsthe maximum dep- metasandstone.Rare graptolites high in the Halifax Group ositional age of the sampleand henceof the exposedGolden- (Fig. lb) indicate an Early Ordovician age (e.g., Cumming ville Group. Of the remaining grains,two are Neoproterozoic, r985). and three are Paleoproterozoic. -4 Northwest of the Chebogue Point shear zone (CPSZ; Sample 2, from a metasandstonebed km higher in the section, Fig. la), the Halifax Group is overlain unconformably by an Oldhamia-beaing interval of fine-grained meta- sedimentaryrocks Silurian volcanic and sedimentary rocks assigned to the known as the High Head member (White et al- 2005), shows White Rock Formation and equivalent units, deposited in a an even more restricted detrital aee range:all 124 grains give shallow-madne rift environment (MacDonald et al. 2002, ages berween740 and 540 Mi, with distinct peaks at and referencestherein). The overlying Early Devonian Tor- 560,580, and 620 Ma. A grain at 537 t 15 Ma provides brook Formation records shelf conditions (Jensen 1975). In the most reliable ma"ximumdepositio, nal age. addition to the contrast in stratigraphy (Fig. 1b), this part of Meguma is characterizedby abundantmafic sills (White and Sample 5, from the statigraphically highest metasand- Barr 2004; White et al. 2006). Furthermore,our limited pa- stone unit in the Goldenville Group northwest of the CPSZ (Figs. leocurrentdara from rhe area indicatesou(hward paleoflow 1a, 1b), shows a much more diverse age spectrum.A (Fig. 1a), in contrast with northwestward to northeastward large Neoproterozoiccluster (78 of 125 grains) has peats at -550, flow reported elsewhere in the terrane (Schenk 1970; Wal- 590, and possibly 620 Ma. A concordant grain at dron and Jensen1985; Waldron 1988). 529 i 19 Ma representsthe best constraint on depositional age. A second large peak occurs at 2060 Ma. Scattered The basementof Meguma is poorly known. Sm-Nd data grains show Mesoproterozoic and older Paleoproterozoic from mafic and felsic volcanic rocks in the White Rock For- ages. Seven Archean grains range from 2620 to 3036 Ma. mation are mainly positive, suggestingcrustal contamination Sample 13 is from an equivalent stratigraphiclevel east of of mainly mantle-derived magmas (MacDonald et al. 2002) the CPSZ (Fig. lb), adjacentto a fossiliferous Middle Cam- but also consistent with sources in Avalonian basement brian bed (Pratt and Waldron 1991). Of 47 grains (Keppie et al. 1991). Greenoughet a1. (1999) recorded con- with <10% discordance,12 arc late Neoproterozoic,grouped cordant and slightly discordant zircons with Avalonian and at -560,