Geological and tectonic setting

and are easier to interpret than Archean deposits, because actualistic plate tectonic models can be applied with fewer assumptions, and because fos- sils provide better constraints on the paleogeo- graphic context i.e. water depth, salinity, etc.. Nevertheless, there are considerable similarities between Ordovician and Archean volcaniclastic deposits. Plants and animals had not yet colo- nized the land in the Ordovician, so that erosion rates were more similar to the Archean than to the modern period. Recent work implies that sub- duction of pelagic sediments plays an important role in the geochemistry of arc-related volcanic rocks e.g. Davidson, 1987; White and Dupre´, 1989; Brenan et al., 1995; Elliott et al., 1997; Turner et al., 1997; Plank and Langmuir, 1998; Be´dard, 1999. During the Ordovician, the deep- sea floor had begun to be covered by pelagic biogenic siliceous ooze radiolarians, whereas or- ganisms producing present-day carbonate ooze had not yet evolved. Thus, the similarity of Or- dovician and Archean sedimentation patterns could lead to a similarity in the geochemistry of arc-related rocks. This study examines a volcaniclastic-rich for- mation of Ordovician age from north-central Newfoundland. Distinguishing between primary deposits of pyroclastic origin, and secondary de- posits of epiclastic origin is difficult, but crucial, for the paleogeographic reconstruction of ancient deposits. The distinction between primary and secondary deposits may be subtle because both tend to be emplaced by mass-flows with high concentrations of debris and limited turbulence Fisher and Schmincke, 1984; Cas and Wright, 1987; Stix, 1991; McPhie et al., 1993. Freshly deposited pyroclastic debris is readily remobilized by rain, wind and ice in subaerial environments, and by density currents of various origin in marine environments. The distinction between primary and secondary deposits in subaqueous settings is generally based on petrography and sedimentary structures Walker and Croasdale, 1972; Dimroth and Ya- magishi, 1987; Stix, 1991. Both provide informa- tion on the nature of constituents, on the mode of fragmentation, on mechanisms of transport, and about sedimentation processes. It is possible to identify the composition and origin of individual pyroclasts, and clast populations are preserved in both primary and rapidly resedimented deposits. However, the brittle nature and metastable miner- alogy of pyroclasts do not favor their preservation in deposits resulting from epiclastic weathering and erosion of previously sedimented pyroclastic deposits. Our understanding of subaqueous erup- tions and their deposits is currently limited, as a result of imperfections in theoretical modeling, and a paucity of well documented examples. A combined geochemical and stratigraphic approach using facies analysis is employed to distinguish eruption-related facies and secondary resedi- mented deposits. The Bobby Cove Formation is part of a se- quence of alternating flow-dominated and sedi- ment-dominated successions. Basaltic volcanism is characterized by the accumulation of thick 400 – 500 m sequences of pillow lavas and sheet flows Be´dard et al., 1999b. These basalt-dominated events alternate with periods of relative quies- cence and erosion marked by tilting of fault blocks, erosion, andor deposition of thick epi- clastic and debris flow deposits Be´dard et al., 1999c; Kessler and Be´dard, this volume. There are also periodic influxes of pyroclastic and epi- clastic debris and lavas from a nearby calc-alka- line volcano Be´dard et al., 1999b. Calc-alkaline volcanism may have occurred during basaltic qui- escence, leading to alternating sequences of basaltic flows and calc-alkaline deposits; or to mixed sequences of calc-alkaline tuffs and vol- canogenic epiclastic deposits derived from erosion of the tholeiitic flows Be´dard et al., 1999a. Alter- natively, calc-alkaline activity may have occurred synchronously with basaltic volcanism, leading to interfingering between tholeiitic basalts and calc- alkaline tuffs. The more explosive style of calc-al- kaline sequences is probably related to differences in lava chemistry and volatile content Gill, 1981; Arculus, 1994.

2. Geological and tectonic setting

The Bobby Cove Formation is part of the Snooks Arm Group in the Notre Dame Bay area of Newfoundland Fig. 1; Hibbard, 1983; Be´dard et al., 1999b. Rocks of this group overlie the obducted oceanic crust of the Middle Ordovician Betts Cove Ophiolite sub-conformably, which probably formed in a marginal basin adjacent to the Laurentian margin Tremblay et al., 1997. The oceanic crust at Betts Cove is dominated by rocks of boninitic affinity Coish et al., 1982; Be´dard et al., 1998, 1999b; Be´dard, 1999, and so formed in a supra-subduction zone environment, either in response to arc splitting, or to extreme fore-arc extension leading to seafloor-spreading Stern and Bloomer, 1992; Tremblay et al., 1997; Be´dard et al., 1998. Recent mapping has revealed the presence throughout the Snooks Arm Group of numerous high- angle cross-cutting normal faults Be´dard et al., 1999a,b. Rapid changes in thickness and facies on either sides of these faults, an up-section de- crease in the magnitude of lithological offsets along individual faults, and the local presence of basalt within fault breccias and talus breccias, together suggest synvolcanic and synsedimentary faults. Extension was synchronous with eruption and sedimentation. Basaltic eruptions in the Snooks Arm Group were subaqueous, as indicated by the dominance of pillow lavas and pillow breccias. Interbedded black mudstones locally contain grap- tolites Snelgrove, 1931; Williams, 1992 and indi- cate deep-water. The Snooks Arm Group consists of three repeated cycles of basaltic lavas and volcanogenic sediments whose geochemistry docu- ment basin changes of tectonic regime Fig. 2. The repeated alternation of these distinct volcanic suites collectively imply a long-lived, extensional, subsid- ing back-arc setting with a simultaneous develop- ment of nearby, mature arc volcanoes. Fig. 1. Geological map of part of the Betts Cove ophiolite and its cover rocks, showing the overall stratigraphy of the Snooks Arm Group. Adapted from Be´dard et al. 1999b. The lower Snooks Arm Group Mount Misery, Scrape Point and Bobby Cove Formations forms the lowermost lava-sedimentary cycle. The Mount Misery Formation is dominated by basaltic pil- lowed and sheet flows of arc tholeiite composi- tion, geochemically transitional between the boninitic lavas of the ophiolite and the evolved, undepleted, ovelying tholeiites. This reflects a transition from the arc-dominated system of the ophiolite to a more mature back-arc system Be´- dard et al., 1999b. The Scrape Point Formation consists of interfingering volcanic and sedimen- tary rocks. The flows have only a weak arc signa- ture and are interpreted to represent mature back-arc basin basalts. The sedimentary rocks are very similar in mineralogy, texture and geochemi- cal signature to those of the overlying Bobby Cove Formation and are interpreted as precursors or distal equivalents of the conformably overlying Bobby Cove Formation. They show a dominant calc-alkaline affinity and, thus, the interfingering of tholeiitic and calc-alkaline rocks suggests stabi- lization and maturation of an arc system syn- chronous with back-arc spreading. The Bobby Cove Formation is the first map- pable unit of formational rank dominated by sedimentary rocks, and was emplaced during a hiatus in tholeiitic volcanism. The formation is dominated by a lower member that is about 400 m thick and contains a diverse suite of mafic to felsic calc-alkaline volcaniclastic rocks whose chemistry implies that they represent a mature arc volcano. Analyses of flows, dikes, and of individ- ual clasts suggest that there are two magmatic lineages: a High-Ti and a Low-Ti series Fig. 2; Table 1. The High-Ti series is essentially indistin- guishable from underlying Mt. Misery tholeiites, suggesting a genetic link with these magmas. The Low-Ti series exhibits a characteristic calc-alka- line trend of depletion of FeO and TiO 2 , and SiO 2 -enrichment; a trend that cannot result from anhydrous crystallization of these magmas. Felsic volcaniclastic rocks become proportionally more significant upwards. The felsic rocks show consid- erable scatter, but most fall along the extension of the Low-Ti series trend. The lower member of the Bobby Cove Forma- tion records the principal calc-alkaline eruptive Fig. 2. Variation diagrams illustrating the range of magmatic and sedimentary compositions in the Snooks Arm Group. Field designated ‘SP, VB, RH’ encloses lavas from the Scrape Point, Venam’s Bight and Round Harbour Formations, as well as subvolcanic feeder sills. Calc-alkalineTholeiitic dis- criminant from Miyashiro, 1973. Curves labeled ‘B’ and ‘SP’ show pattern of low-pressure anhydrous fractional crystalliza- tion 40 of Bobby Cove and Scrape Point lavas respec- tively calculated with the program of Weaver and Langmuir 1980. Data from Be´dard et al. 1999a and references listed therein. P .A . Cousineau , J .H . Be ´dard Precambrian Research 101 2000 111 – 134 115 Table 1 Average analyses normalized to 100 from different lithofacies a Unit facies BCLM BCUM Clast Clast Clast Mafic tuff Mafic tuff Mafic tuff Dacite class Dacites Felsic tuff High-Ti lavas Low-Ti lavas Dyke SiO 2 53.7 50.3 47.3 47.8 51.5 62.9 61.5 73.4 52.1 49.6 48.3 55.6 0.80 0.52 0.88 0.95 0.31 0.65 0.85 0.30 1.28 TiO 2 0.88 1.14 0.86 15.7 17.2 14.4 315.0 14.0 16.7 16.4 17.1 12.6 15.9 15.7 14.0 Al 2 O 3 Fe 2 O 3 9.38 12.5 8.98 10.2 9.64 4.60 6.03 3.41 10.0 125 10.2 6.55 0.17 0.14 0.22 0.19 0.07 0.10 0.11 0.09 0.19 0.15 MnO 0.21 0.17 4.90 5.51 7.62 10.0 8.27 6.30 1.26 3.07 1.15 7.40 5.02 9.96 MgO 5.92 6.38 6.11 6.58 2.04 5.39 CaO 4.18 8.64 9.00 11.3 11.0 9.49 5.25 0.27 3.83 4.96 6.22 3.99 2.12 1.49 3.95 Na 2 O 1.88 1.90 3.55 2.08 0.26 0.36 1.81 3.12 0.72 3.38 1.78 1.43 0.91 0.08 0.92 K 2 O 0.10 P 2 O 5 0.06 0.20 B dl 0.07 0.10 0.16 B dl 0.08 0.27 0.10 0.11 2.68 10.3 3.34 2.87 1.01 2.43 3.21 2.11 3.87 2.76 LOI 3.44 3.50 100.7 100.0 100.4 100.8 97.78 100.4 98.17 100.0 100.1 100.0 100.4 100.9 Total 78 361 266 418 389 Cr 110 99 33 166 107 398 84 46 162 47 32 20 55 26 B dl 21 Ni 69 15 69 261 327 260 269 260 266 52 89 33 288 440 373 V 100 74 87 52 12 Cu 14 75 17 76 88 64 53 57 74 99 69 38 72 61 58 90 77 Zn 116 66 22 35 34 30 41 45 4 10 13 40 38 55 Sc 30 50 24 27 B dl 16 Co B dl 40 30 66 62 30 4.8 49 70 10.4 37 41 31 31 5.7 Rb 13 6 15 127 37 89 96 343 134 335 219 388 93 B 50 B 50 Ba Sr 111 169 147 121 203 82 285 440 176 373 121 149 1.3 B 3 1.4 2.8 3.6 10.4 B 3 4.3 B 3 B 3 Nb 2.1 3.0 B 0.3 0.18 0.08 B 0.3 0.11 0.16 0.26 0.50 – 0.10 B 0.3 B 0.3 Ta 53 46 38 36 108 149 Zr 149 66 56 55 58 59 17 12 16 20 13 25 23 28 27 Y 24 21 19 1.47 1.65 1.72 1.02 1.31 1.22 2.91 3.30 4.10 1.31 1.12 1.43 Hf 5.66 3.26 4.86 5.71 11.1 18.4 La 13.1 4.20 4.27 5.58 3.97 4.24 13.8 7.45 13.5 14.2 26.0 41.4 10.3 31.6 12.4 9.45 Ce 11.5 10.4 – – 1.84 – 1.94 1.86 3.11 – 3.99 – – – Pr 8.33 B 5 8.45 8.29 12.5 18.4 Nd 16.5 8.17 7.41 7.13 7.22 7.33 2.40 1.16 2.37 2.42 2.83 3.95 2.37 4.03 2.69 Sm 2.19 2.41 2.46 0.80 1.01 0.73 0.49 0.86 0.80 0.81 1.13 0.89 0.89 1.02 0.80 Eu – – 2.99 2.80 2.09 2.83 3.74 Gd 3.68 2.59 2.63 – – 0.48 0.220 0.52 0.52 0.39 0.43 0.59 0.69 0.61 Tb 0.54 0.45 0.50 – 4.52 – – 3.35 3.31 2.19 2.04 4.32 3.20 3.02 – Dy 0.63 B 0.5 0.65 0.67 0.45 0.32 Ho 0.91 0.96 0.64 B 0.5 0.59 0.65 – – 1.98 2.08 1.48 0.98 – 3.06 2.93 Er – 1.98 2.02 0.308 0.41 0.302 B 0.2 0.293 0.282 0.22 0.13 0.46 0.29 B 0.2 0.268 Tm 1.83 1.23 1.76 1.74 1.49 0.87 3.17 Yb 2.39 1.97 1.86 2.52 2.20 0.29 0.166 0.270 0.247 0.25 0.14 0.33 0.50 0.29 0.32 Lu 0.37 0.28 – 14.0 – – 6.9 3.74 4.56 8.2 15 6.0 4.62 – Pb 1.37 1.10 1.08 1.22 3.01 5.15 4.37 Tb 0.66 0.91 0.88 0.94 0.88 0.56 0.69 0.45 0.51 1.36 2.36 0.55 1.79 0.59 U 0.31 0.46 B 05 Cs 0.69 0.079 1.26 3.47 0.49 1.86 1.53 0.61 0.60 B 0.5 B 0.5 0.70 a ‘BCLM’ and ‘BCUM’, Bobby Cove lower and upper members, respectively. episode in the Snooks Arm Group. Sharply con- trasting patterns of geochemical evolution Fig. 2, and distinct geochemical signatures imply that the Bobby Cove magmas were not cogenetic with the undepleted back-arc basin tholeiites Scrape Point or Venam’s Bight formations, though they may be cogenetic with the arc tholeiites Mt. Misery Formation. Consequently, the Snooks Arm Group appears to record two distinct mag- matic suites. There probably were two distinct volcanic plumbing systems fed by two different types of basalt. The upper member of the Bobby Cove Forma- tion is about 200 m thick and consists principally of volcanogenic turbidites, with rare rhyolitic tuffs Be´dard et al., 1999b. The contact between the upper and lower members is presumed to be conformable, although the contact is commonly occupied by thick up to 100 m tholeiitic ferro- gabbro sills comagmatic with the tholeiitic rocks of the overlying cycle Be´dard et al., 1999a,b. Presumably the magmas exploited the mechanical weakness represented by this contact. The turbid- ites themselves have bulk compositions reflecting derivation from calc-alkaline rocks similar to those of the lower Bobby Cove Table 1; Fig. 2, suggesting erosion and redeposition of unconsoli- dated, possibly originally subaerial tuffs. Notwith- standing, the absence of interbedded tholeiitic basalt implies that deposition of the Bobby Cove Formation took place during a period of quies- cence in the back-arc tholeiitic basaltic system. The entire upper Snooks Arm Group consists of alternating or simultaneous deposition of: 1 back-arc basin tholeiites; 2 arc tholeiites and calc-alkaline basalts to rhyolites; and 3 the ero- sion products of these two volcanic sequences during periods of volcanic quiescence. The dis- tinction between the first and second volcano-sed- imentary cycles lies mainly in the nature of volcaniclastic rocks found at their summits. The volcaniclastic rocks capping the first cycle the Bobby Cove Formation are a mixture of epiclas- tic and pyroclastic rocks, and in part represent the direct products of calc-alkaline volcanism. Those capping the second cycle Balsam Bud Cove For- mation are principally epiclastic rocks emplaced by mass-flow currents following slope failure. These deposits probably reflect a gradual waning of basaltic volcanic activity and incipient erosion of the basaltic volcanic edifice, though interbed- ded felsic tuffs in the basal part probably repre- sent a brief recrudescence of activity from the calc-alkaline volcano responsible for the Bobby Cove deposits. Clast compositions in the large- scale volcaniclastic mass-flow deposits debrites; Kessler and Be´dard, this volume also suggest that erosion was not limited to the freshly erupted tholeiitic lavas and felsic pyroclastic debris, but also included deeper parts of the volcanic stratig- raphy. The third cycle is incomplete, and only consists of tholeiites Round Harbour Formation essentially identical to some underlying tholeiites Venam’s Bight Formation, though cross-cutting felsic dikes of calc-alkaline affinity similar to the Bobby Cove felsic magmas suggest a subsequent pulse of calc-alkaline magmatism.

3. Classification and petrography of pyroclasts