(NTS 11K/07, 11K/10, 11K/11):

Transcription

1 Report of Activities U-Pb Age Data for Belle Côte Road Orthogneiss, Taylors Barren Pluton and Bothan Brook Pluton, Southern Cape Breton Highlands (NTS 11K/07, 11K/10, 11K/11): Igneous Ages and Constraints on the Age of Host Units, and Deformational History R. J. Horne, G. Dunning 1 and R. Jamieson 2 Introduction The geology of the Cape Breton Highlands is complex and has stimulated considerable debate, primarily over the regional-scale tectonic architecture of the area. Barr and Raeside (1989) divided Cape Breton Island into four pre- Carboniferous tectono-stratigraphic terranes: Mira, Bras d Or, Aspy and Blair River (Fig. 1). Keppie (1990) and Keppie et al. (1990) included the whole of Cape Breton Island in the Avalon Terrane, inferring correlation of units across the terranes of Barr and Raeside (1989), including Precambrian metasedimentary rocks (George River Group and equivalents) and a Cambrian-Ordovician overstep sequence. Lynch (1996) proposed that the distribution of units reflects a folded nappe consisting of higher grade units overlying lower grade units. It is apparent that at the heart of much of the debate is the correlation (or not) of rock units based on lithology and age, and the interpreted relationship between rock units. It is equally apparent that, despite the amount of documented information, the range of interpretations for the region is based not on what is known, but rather what is unknown. The objective of this report is to publish the results of U-Pb dating of three samples from plutonic units in the south-central Cape Breton Highlands, all in the Aspy Terrane of Barr and Raeside (1989). In addition to providing ages for plutonic activity, these data help constrain the age of meta-stratified units intruded by the plutonic units and the age of deformational events for which relations with the dated units are known. This report is essentially an appendix to the map report on the geology of the south-central Cape Breton Highlands by Horne (1995), to which the reader is referred for more detailed information and discussion on the geology of the area. Coupled with established field relations, the age data reported here provide further understanding of the geological history of the area. In addition, the data have some consequences for regional correlations, and therefore tectonostratigraphic interpretations. Results of the dating reported here have been referred to in Horne (1995) and, more recently, in Price et al. (1999), where the significance of these ages has been discussed. A summary of the more important implications is presented below. Setting The study area is situated in the Aspy Terrane of Barr and Raeside (1989), which consists of Ordovician or older metamorphic (sedimentary and igneous) rocks of variable grade intruded by Ordovician, Silurian, and Devonian plutons (Fig. 1). Contacts between metamorphic units within the Aspy Terrane are commonly obscured by deformation and metamorphism, and postmetamorphic faults, thus clouding the relationships between units. This has led to various interpretations, particularly regarding the relationship between the higher- and lower-grade units. Currie (1987) considered the high-grade gneissic rocks (e.g. Pleasant Bay complex) to 1 Department of Earth Sciences, Memorial University of Newfoundland, St. John s, Newfoundland, A1B 3X5 2 Department of Earth Sciences, Dalhousie University, Halifax Nova Scotia B3H 3J5 Horne, R. J., Dunning, G. and Jamieson, R. 2003: in Mineral Resources Branch, Report of Activities 2002; Nova Scotia Department of Natural Resources, Report , p

2 58 Mineral Resources Branch Figure 1. Simplified geology map of western Cape Breton Highlands (modified from Price et al., 1999) showing the location of the study area.

3 Report of Activities represent basement to lower grade rocks (e.g. Jumping Brook metamorphic suite) in the Cheticamp area. Plint and Jamieson (1989), Jamieson et al. (1987) and Barr and Jamieson (1991) considered the higher grade rocks to be high-grade equivalents of the lower grade units. Lynch (1996) suggested that the high-grade rocks represent the erosional remnant of a folded nappe emplaced over the lower grade rocks. As mentioned above, the dated samples are from units within the map area of Horne (1995) and will be discussed with respect to the field relationships established there (Fig. 2). The sample of Belle Côte Road orthogneiss was collected north of the map area of Horne (1995); however, this unit has been correlated with the Belle Côte Road orthogneiss in this map area by Price et al. (1999). The dated rock units belong to a coherent assemblage that has shared a long geological history, which predated intrusion of the oldest dated unit, the Belle Côte Road orthogneiss (Horne, 1995). The Sarach Brook metamorphic suite and Kathy Road diorite (Bras d Or terrane) are separated from this assemblage within the study area by faults (Fig. 2). General field relationships constrain the following geological history for the area (Fig. 2; see Horne, 1995; Price et al. 1999; MacDonald, 1996). (1) Deposition of protoliths of the meta-stratified units, including the First Forks Brook gneiss, Jumping Brook metamorphic suite and Middle River Metamorphic suite. (2) Regional metamorphism and deformation recorded by an S 1 foliation preserved as inclusion trails within garnet porproblasts. (3) Syn-tectonic intrusion of the Belle Côte Road orthogneiss, with formation of the principal gneissic (metamorphic layering) fabric (S 2 ). (4) Syn-tectonic intrusion of the Taylors Barren pluton with development of a weak to welldeveloped foliation (S 2 ) (augen fabric) parallel to the gneissic fabric in the Belle Côte Road orthogneiss and older units. (5) Regional-scale folding of the S 2 foliation to produce a map-scale fold. (6) Intrusion of unfoliated plutons (Bothan Brook, West Branch North River, Peters Brook). (7) Development of regional-scale, mylonitic shear zones (Southern Highlands and Coinneach Brook shear zones) which deform unfoliated plutons. (8) Possible folding of shear zones. (9) Brittle faulting, including deformation of shear zones. Sample Locations, Description and Age Data Samples were selected to provide maximum information over the range of geologic history of the area, with the three units including undeformed (Bothan Brook pluton), moderately deformed (Taylors Barren pluton) and strongly deformed (Belle Côte Road orthogneiss) plutonic units. Relative age relations had been established between units (Horne, 1995; Price et al., 1999; MacDonald 1996) as well as deformational events (Horne, 1995). Therefore, in addition to providing the crystallization age of the plutons, the ages constrain the time of deformational events (e.g. Table 3, Horne, 1995). The U-Pb dating reported was carried out at the geochronology laboratories of Memorial University, using laboratory procedures similar to those described in Dubé et al. (1996). The three dated samples represent igneous units, and the objective was to establish crystallization ages for these units. Belle Côte Road Orthogneiss The analysed sample of the Belle Côte Road orthogneiss is the same sample analyzed by Jamieson et al. (1986; i.e., their Belle Côte Road orthogneiss sample). The data of Jamieson et al. (1986) showed evidence of an older component of radiogenic lead and the best estimate for the crystallization age was given as /-10 Ma (Jamieson et al., 1986). The sample was reanalysed in hope of establishing a more precise age of crystallization. The Belle Côte Road orthogneiss consists of well foliated granodioritic to tonalitic orthogneiss; however, minor amphibolite and paragneiss are common (Horne, 1995; Price et al. 1999; Marcotte 1987; Jamieson et al. 1987). A description of the sample, which was collected along the Belle Côte Road (Fig. 3), is provided in Jamieson et al. (1986), with a summary given here. The sample consists of pink- to white-weathering gneisic tonalite to granodiorite with diffuse compositional layering defined by alternate biotite-rich and feldspar-rich layers. Mineralogy includes oligoclase, quartz,

4 60 Mineral Resources Branch microcline and biotite with accessory oxide minerals, apatite, zircon, garnet and tourmaline, and secondary chlorite (Jamieson et al., 1986). This description is similar to the field and petrographic descriptions of Horne (1995), Price et al. (1999) and Marcotte (1986). Three fractions of zircon gave 207 Pb/ 206 Pb ages of 442 (Z1), 442 (Z2) and 447 (Z3) Ma (Table 1). Fraction Z2 is concordant and a line through 442 and 0 incorporates fraction Z1 (Fig. 4). Fraction Z3 is below this line and is interpreted to reflect a minor component of inherited Pb, which was evident in the previous analysis of this sample reported by Jamieson et al. (1986). A minor component of inherited zircon is not unexpected considering the common occurrence of paragneiss xenoliths within this unit, for example, an enclave of First Forks Brook east of the sample location (Fig. 3). From these data, the best estimate for a crystallization age is 442±3Ma. Taylors Barren Pluton The Taylors Barren pluton consists of a variably foliated, pink-red, augen monzogranitesyenogranite consisting of K-feldspar, plagioclase, quartz, biotite (mainly altered to chlorite) and accessory opaque minerals, apatite, titanite and zircon (Horne, 1995; MacDonald, 1996). The dated sample of Taylors Barren pluton was collected on a branch of Nile Brook just south of Fielding Road (Fig. 5). The sample consists of a weakly foliated augen granite (compare Figs. 10b of Horne, 1995), which was selected to limit any potential effect that deformation may have had on resetting or disturbing the crystallization age for the sample. The sample was free of xenoliths, as is generally the case with the Taylors Barren pluton. The sample yielded high quality zircon and two fractions of euhedral grains, which were strongly abraded, are concordant (Fig. 4, Table 1). The age of crystallization based on all Pb/Pb and U/Pb ages (429 to 431 Ma, Table 1) is 430±2Ma. Bothan Brook Pluton The Bothan Brook pluton consists of mediumgrained, equigranular to slightly megacrystic pink granite (eg. Fig. 13b of Horne, 1995), consisting of K-feldspar, plagioclase, chlorite (after biotite?) quartz and accessory muscovite, sphene, epidote and opaque minerals. This unit is relatively free of xenoliths. The sample analysed is from along Bothan Brook (Fig. 6). The sample yielded a large amount of zircon as euhedral prisms and needles. Three fractions of zircon analysed are 2.5-5% discordant, defining a line with an upper intercept of 376 Ma and a lower intercept of 0 Ma (Fig. 4; Table 1), for an interpreted crystallization age of 376±3 Ma. Discussion The age data are interpreted to provide crystallization ages for the respective plutons, recording a plutonic history from Late Ordovician-Early Silurian to Late Devonian. Established field relationships of the dated units place constraints on the ages of other units and Figure 2 (facing page): Simplified geology map of the study area with inset diagrams illustrating observed field relationships. Trend of map ornamentations parallel the principal tectonic fabric (S 2 ). Locations of Figures 3 and 4 are indicated. Inset diagrams: (a) Paragneiss of the First Forks Brook gneiss possibly representing enclaves of Jumping Brook metamorphic suite in the Belle Côte Road orthogneiss; (b) A satellite intrusion of Belle Côte Road orthogneiss in the Middle River metamorphic suite; (c) Contact between the Belle Côte Road orthogneiss and Taylors Barren pluton, with xenoliths of the former in the latter; (d) satellite intrusions of Taylors Barren pluton within the Pleasant Bay complex; (e) Dyke of granodiorite of the West Branch North River pluton cutting the Belle Côte Road orthogneiss; (f) Dyke of granodiorite of the West Branch North River pluton cutting the Taylors Barren pluton; (g) Dyke of Margaree pluton cutting the Belle Côte Road orthogneiss; (h) Dyke of Peters Brook pluton cutting the Taylors Barren pluton; (i) Dyke of Peters Brook pluton cutting the First Forks Brook gneiss;(j) Folded granite dyke cutting the First Forks Brook gneiss; (k) Folded dyke cutting the Middle River metamorphic suite; (l) Folded dyke of Taylors Barren pluton within the Belle Côte Road orthogneiss; (m) Mylonitic contact between the Bothan Brook pluton and Sarach Brok metamorphic suite; (n) mylonitic contact between the Jumping Brook metamorphic suite and the West Branch North River pluton (after Horne, 1995).

5 Report of Activities

6 62 Mineral Resources Branch Figure 3. Map showing the location and setting of the Belle Côte Road othogneiss sample used for U-Pb dating; area of map indicated on Fig. 1 (map modified from Price et al. 1999). structural features, providing for a well constrained understanding of the geological history of the area. The reader is referred to Horne (1995) for a full discussion; however, an overview of the significance of the ages and a summary of the geologic history is given here. In addition, the ages provide new constraints on the ages of older units (First Forks Brook gneiss, Middle River and Jumping Brook metamorphic suites) and constrain the time of shearing along the SHSZ, which have consequences with respect to regional correlation of units and thus the regional architecture. Geological History Belle Côte Road Orthogneiss The Belle Côte Road orthogneiss forms part of the Pleasant Bay Complex, which in the study area includes the First Forks Brook gneiss. Within the study area, the Belle Côte Road orthogneiss is interpreted to intrude the First Forks Brook gneiss, Middle River metamorphic suite and Jumping Brook metamorphic suite (Horne, 1995; Price et al., 1999; Jamieson et al., 1987), and therefore constrains the age of these units as pre 442±3 Ma (Late Ordovician-Early Silurian). The metastratified units intruded by the Belle Côte Road orthogneiss preserve an earlier tectonic fabric (S 1 ) within garnet porphroblasts which predated the principal foliation (S 2 ) affecting the Belle Côte Road orthogneiss and the units which it intruded (Horne, 1995). Therefore, an episode of regional metamorphism and deformation predating intrusion of the Belle Côte Road orthogneiss must be accounted for when constraining the depositional ages of these units; these units could be considerably older and of various ages. Taylors Barren Pluton The 430±2 Ma age for the Taylors Barren pluton establishes the existence of Early Silurian plutonism within the Aspy Terrane. The foliation within Taylors Barren pluton parallels the principal

7 Report of Activities fabric in the Belle Côte Road orthogneiss, best demonstrated by folded dykes of Taylors Barren pluton cutting the Belle Côte Road orthogneiss (Fig 2, insets c, l; Fig. 11b, Horne 1995), where the principal foliation is axial planar to the folds. This observation indicates a protracted history for the development of the principal fabric (S 2 ) which overlaps emplacement of the Taylors Barren pluton, implying syn-tectonic emplacement of this pluton. The principal fabric in the Taylors Barren pluton and older units is folded into a regionalscale fold (Fig. 2; Horne, 1995, Price et al., 1999). Unfoliated plutons, including the West Branch North River and Peters Brook plutons, cut the Taylors Barren pluton and arguably postdate regional folding of the principal foliation, thus constraining the age of regional folding to between 430 Ma (emplacement of the Taylors Barren pluton) and emplacement of the West Branch North River and Peters Brook plutons (Devonian; see below). Bothan Brook Pluton Figure 4. U-Pb concordia diagram for zircon from (a) Belle Côte Road orthogneiss, (b) Taylors Barren pluton and (c) the Bothan Brook pluton; see Table 1 for isotopic data. The Bothan Book pluton was previously mapped as a separate pluton (e.g. O Beirne-Ryan and Jamieson, 1986), but generally included as part of the western lobe of the West Branch North River pluton, with which it is contiguous (Fig. 1), on compilation maps (Barr et al., 1992; Lynch et al., 1995; Keppie, 2000). Clear field relationships between the Bothan Brook pluton and West Branch North River plutons have not been documented. The 376 Ma age determined for the Bothan Brook pluton suggests that it is not part of the West Branch North River pluton, which is likely significantly older, based on an Rb/Sr whole rock isochron (399±5 Ma, O Beirne-Ryan and Jamieson, 1986) and 40 Ar/ 39 Ar biotite (?) (ca. 385; Reynolds et al., 1989) age data. The Bothan Brook pluton is deformed along its eastern margin by the Southern Highlands shear zone (SHSZ; Fig. 13c of Horne, 1995), thereby constraining the shearing to post- 376 Ma.

8 64 Mineral Resources Branch Figure 5. Map showing the location and setting of the sample of Taylors Barren Pluton sample (K ) used for U- Pb dating. Location of map indicated on Fig. 2 (map after Horne, 1995).

9 Report of Activities Table 1. Table of U-Pb isotopic data for zircon fractions from the Belle Cote Road orthogneiss, Taylors Barren pluton and the Bothan Brook pluton. Abbreviations: radradiogenic; pg-picograms; N-nonmagnetic, M-magnetic (number following N and M is refers to the degree of tilt of the Franz separator); Clr-clear; euh-euhedral; abrabraded; lg-large) Concentration (ppm) Measured Corrected Atomic Ratios Age (Ma) Fraction Weight (mg) U Pb rad Total Common Pb (pg) 206 Pb 204 Pb 208Pb 206Pb 206Pb 238U ± 207Pb 235U ± 207Pb 206Pb ± 206Pb 238U 207Pb 235U 207Pb 206Pb Belle Côte Road Orthogneiss Z1 clr euh prisms Z2 mo clr euh abr Z3 clr euh abr - crack K Taylor s Barrens Z1 nmo clr lg prism abr Z2 nmo clr prisms abr K Bothan Brook Z1 MO clr euh abr Z2 mo clr abr - small Z3 clr needles Notes: All data produced using standard procedures, ion exchange chemistry, thermal ionization mass spectrometry, accepted decay constants for U from Jaffey et al

10 66 Mineral Resources Branch Figure 6. Map showing the location and setting of the sample of Bothan Brook pluton (K ) used for U-Pb dating. Location of map indicated on Fig. 2 (map after Horne, 1995). Summary The above discussions illustrate the significance of the new age data when coupled with wellestablished field relationships between units and deformational features. Following is a summary of the geological history of the study area resulting from the new age data and established field relations. Ordovician or older (a) Deposition of the First Forks Brook, Jumping Brook and Middle River units. (b) Regional metamorphism with development of S 1 foliation.

11 Report of Activities Late Ordovician-Early Silurian (a) Syn-tectonic intrusion of the Belle Côte Road orthogneiss (442±3Ma), development of regional S 2 foliation. Early Silurian (a) Syn-tectonic intrusion of Taylors Barren pluton (430±Ma), continued development of regional S 2 foliation. (b) regional folding of regional S 2 foliation. Mid-Late Devonian (a) intrusion of unfoliated plutons, including West Branch North River pluton, Peters Brook pluton, Bothan Brook pluton (376±3 Ma), Boundary Line intrusive suite. (b) Ductile shearing - Southern Highlands Shear Zone. (c) Development of brittle faults. Regional Significance As discussed by Price et al. (1999), the interpreted Ordovician or older age for the First Forks Brook gneiss, Middle River metamorphic suite and Jumping Brook metamorphic suite, which are intruded by the Belle Côte Road orthogneiss, demonstrates that these units are not correlative with Silurian units such as the Sarach Brook metamorphic suite (433+7/-4 Ma U-Pb; Dunning et al., 1990) and the Money Point metamorphic suite (427.5±4 Ma; Keppie et al., 1991), as earlier thought (e.g., Barr and Jamieson, 1991). The age of the Belle Côte Road orthogneiss and its intrusive relationship with these older units would, however, not support the idea that the Pleasant Bay complex represents basement to units such as the Jumping Brook metamorphic suite as suggested by Currie (1987). Importantly, the higher grade Ordovician or older units (First Forks Brook gneiss, Middle River metamorphic suite and Jumping Brook metamorphic suite) are separated from the younger units (Sarach Brook metamorphic suite and Money Point metamorphic suite) by significant shear zones (eg. SHSZ, Fig. 2), thus concealing the original relationship between these rock units. The age of the Bothan Brook pluton constrains a post 376 Ma age for the Southern Highlands shear zone (SHSZ), which clearly deforms this pluton (Fig. 13c of Horne, 1995). The SHSZ is an important part of the postulated thrust system responsible for emplacement of the Cabot nappe of Lynch (1996), and the Late Devonian or younger age of deformation constained by the Bothan Brook pluton is somewhat younger the age of thrusting ( Ma) proposed by Lynch (1996). Acknowledgments Steve King assisted RJH with collection of samples of the Bothan Brook and Taylors Barren pluton. Many thanks to Dan Kontak who provided RJH with discussion and insight into the U-Pb data. We thank Sandra Barr for providing digital versions of Figures 1 and 3 (originally published in Price et al., 1999), and for helpful comments. References Barr, S. M., Jamieson, R. A. and Raeside, R. P. 1992: Geology, Northern Cape Breton Island, Nova Scotia; Geological Survey of Canada, Map 1752A, scale 1: Barr, S. M. and Rayside, R. P. 1989: Tectonostratigraphic terranes in Cape Breton Island, Nova Scotia: implications for the configuration of the northern Appalachian Orogen; Geology, v. 17, p Dubé, B., Dunning, G. R., LauziPre, K. and Roddick, J. C. 1996: New insights into the Appalachian Orogen from geology and geochronology along the Cape Ray fault zone, southwest Newfoundland; Geological Society of America Bulletin, v. 108, p Dunning, G. R., Barr, S. M., Raeside, R. P. and Jamieson, R. A. 1990: U-Pb zircon, titanite monazite ages in the Bras d Or and Aspy terranes of Cape Breton Island, Nova Scotia: implications for magmatic and metamorphic history; Geological Society of America Bulletin, v. 102, p Horne, R. J. 1995: Geology of the south-central Cape Breton Highlands (Parts of NTS sheets 11K/07 and 11K/10), Inverness and Victoria Counties, Nova Scotia; Nova Scotia Department of

12 68 Mineral Resources Branch Natural Resources, Mineral and Energy Branch Paper 95-2, 61 p. Jamieson, R. A., van Breemen, O., Sullivan, R. W. and Currie, K. L. 1986: The age of igneous and metamorphic events in the western Cape Breton Highlands, Nova Scotia; Canadian Journal of Earth Sciences, v. 23, p Jamieson, R. A., Tallman, P., Marcotte, J. A., Plint, H. E. and Connors, K. A. 1987: Geology of the west-central Cape Breton Highlands, Nova Scotia; Geological Survey of Canada Paper Keppie, J. D. 2000: Geological Map of the Province of Nova Scotia; Nova Scotia Department of Natural Resources, Minerals and Energy Branch, Map ME , scale 1: Lynch, G Tectonic burial, thrust emplacement, and extensional exhumation of the Cabot nappe in the Appalachian hinterland of Cape Breton Island, Canada; Tectonics, v. 15, p Lynch, G., Barr, S. M., Houlahan, T. and Giles, P. 1995: Geological compilation, Cape Breton Island, Nova Scotia; Geological Survey of Canada, Open File Map 3159, scale 1: MacDonald, C. A. 1996: Petrography, geochemistry, and structure of the Taylors Barren pluton, Cape Breton Island, Nova Scotia; Unpublished BSc thesis, Acadia University, Wolfville, Nova Scotia. Marcotte, J-A. 1987: Regional mapping and petrographic study of a Silurian gneissic complex, Belle Côte Road, Cape Breton, Nova Scotia; Unpublished BSc thesis, Dalhousie University, Halifax, Nova Scotia. Price, J. R., Barr, S. M., Raeside, R. P. and Reynolds, P. H. 1999: Petrology, tectonic setting, and 40 Ar/ 39 Ar (hornblende) dating of the Late Ordovician - Early Sillurian Belle Côte Road orthogneiss, western Cape Breton Highlands, Nova Scotia; Atlantic Geology, v. 35, p Reynolds, P. H., Jamieson, R. A., Barr, S. M. and Raeside, R. P. 1989: An 40 Ar/ 39 Ar study of the Cape Breton Highlands, Nova Scotia: thermal histories and tectonic implications; Canadian Journal of Earth Sciences, v. 26, p