Red Indian Line Research Articles

Geochronology of the Oliverian Plutonic Suite and the Ammonoosuc Volcanics in the Bronson Hill arc: Western New Hampshire, USA

Abstract U-Pb zircon geochronology by sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) on 11 plutonic rocks and two volcanic rocks from the Bronson Hill arc in western New Hampshire yielded Early to Late Ordovician ages ranging from 475 to 445 Ma. Ages from Oliverian Plutonic Suite rocks that intrude a largely mafic lower section of the Ammonoosuc Volcanics ranged from 474.8 ± 5.2 to 460.2 ± 3.4 Ma. Metamorphosed felsic volcanic rocks from within the Ammonoosuc Volcanics yielded ages of 460.1 ± 2.4 and 455.0 ± 11 Ma. Younger Oliverian Plutonic Suite rocks that either intrude both the upper and lower Ammonoosuc Volcanics or Partridge Formation ranged in age from 456.1 ± 6.7 Ma to 445.2 ± 6.7 Ma. These new data and previously published results document extended magmatism for >30 m.y. The ages, along with the lack of mappable structural discontinuities between the plutons and their volcanic cover, suggest that the Bronson Hill arc was part of a relatively long-lived composite arc. The Early to Late Ordovician ages presented here overlap with previously determined igneous U-Pb zircon ages in the Shelburne Falls arc to the west, suggesting that the Bronson Hill arc and the Shelburne Falls arc could be part of one, long-lived composite arc system, in agreement with the interpretation that the Iapetus suture (Red Indian Line) lies to the west of the Shelburne Falls–Bronson Hill arc system.

Tectonometamorphic evolution along the Iapetus suture zone in Newfoundland: Evidence for polyphase Salinic, Acadian and Neoacadian very low- to medium-grade metamorphism and deformation

The Red Indian Line (RIL) in central Newfoundland is the suture, where the main tract of the Iapetus Ocean was closed at ~452 Ma during accretion of the peri-Gondwanan Victoria arc with the composite active Laurentian margin. The protracted deformation history of this soft collision started at ~471 Ma with accretion of oceanic terranes to the active composite Laurentian margin. After Iapetus closure both colliding active margins were progressively deformed and metamorphosed during Silurian and Devonian (Salinic, Acadian and Neoacadian orogenic cycles).Peak conditions of the very low- to medium-grade, heterogeneously distributed metamorphism were determined by pseudosection techniques within the range of 2–7 kbar, 230–450 °C during increase of the metamorphic field gradient from ~12 °C/km to ~32 °C/km over time. Multiple metamorphic crystallisation stages were dated by white mica 40Ar/39Ar spot and plateau ages, additional Rb-Sr mineral isochrons involving white mica and one U/Pb age of titanite. All resulting ages between 439 ± 4 Ma and 356 ± 16 Ma postdate the closure of Iapetus.Results differ along two transects: The oldest ages of 443–421 Ma (Salinic orogenic cycle) were observed along the northern transect through the RIL zone with minimal younger overprint. Hence low temperature, intermediate to high pressure conditions (4.0–7.0 kbar, 230–340 °C) achieved during Taconic-Salinic underthrusting are well preserved. During Acadian dextral transpression the Taconic-Salinic structural wedge was tilted subvertically.In contrast, rocks along the southern transect through the RIL zone mainly show Acadian ages of 408–390 Ma with local preservation of older ages. Acadian deformation occurred under low temperature/low pressure conditions (~250–450 °C, 2.5–4.6 kbar). Also Silurian terrestrial cover rocks were buried under these conditions. Acadian-Neoacadian deformation (393–340 Ma) becomes younger towards the northwest and progressively localized in transcurrent fault zones. This final foreland deformation at shallow crustal level established the Acadian/Neoacadian orogenic front in central Newfoundland slightly northwest of the RIL.

Upper crustal investigation of the Gulf of Saint Lawrence region, eastern Canada using ambient noise tomography

AbstractWe studied the 3‐D shear wave velocity (Vs) structure in the Gulf of St. Lawrence (GSL) and adjacent onshore areas to 20 km depth by inverting Rayleigh wave dispersion extracted from the vertical components of continuous ambient seismic noise waveforms. The region is divided into three broad zones based on their Vs characteristics. In the northwest, the Grenville Province (i.e., the exposed edge of predominantly Middle‐Proterozoic Laurentian crust) is dominated by high Vs, except for well‐known anorthosite sites, which are characterized by relatively lower Vs. In contrast, the central segment of the GSL region corresponds to a belt with generally low Vs at upper crustal levels. In the southeastern part of the GSL, prominent low Vs in the uppermost crust are found to coincide with locations of subsidiary basins of the Canadian Maritime Basin, while higher Vs characterize the accreted Appalachian terranes where they are exposed on land. The Grenville Province is wedged out at depth by the Red Indian Line, which is the suture between composite Laurentia and peri‐Godwanan Ganderia in the Canadian Appalachians. The geometry and Vs characteristics of the south‐easternmost peri‐Gondwanan terranes of Avalonia and Meguma suggest that they may be fully or partially structurally overlying a basement with distinct seismic characteristics, which could be a vestige of the West African craton that was underthrust beneath composite Laurentia during the terminal Alleghenian continent‐continent collision. In the middle of the GSL, the 3‐D geometry of the Canadian Maritimes sedimentary basins overlying the Appalachian terranes shows that the depth to the top of basement is in excess of 8 km.

A newly identified Gondwanan terrane in the northern Appalachian Mountains: Implications for the Taconic orogeny and closure of the Iapetus Ocean: COMMENT

On the basis of detrital zircon U-Pb ages, MacDonald et al. (2014) propose a new Gondwanan terrane, the Moretown terrane, which would have been accreted to Laurentia during the Taconic orogeny in the New England Appalachians. This terrane, solely represented by the Moretown Formation, would be bound to the west by the Red Indian Line, the Iapetan suture between Laurentia and Gondwana. We present an alternative interpretation that takes better account of the Neoproterozoic to Ordovician geological history of the Vermont and Quebec Appalachians, where there is no evidence for such a suture. In northern Vermont, the Moretown Formation is overlain by the Cram Hill Formation, which correlates with the Saint-Daniel Melange of Quebec (Doolan et al., 1982). The Saint-Daniel Melange (ca. 465–460 Ma) forms the base of an Ordovician peri-Laurentian synorogenic forearc basin interpreted to be the result of the synto late-Taconian exhumation of the orogenic wedge (Tremblay et al., 2011). The Cram Hill Formation also overlies the Umbrella Hill Conglomerate, which unconformably overlies Cambrian Laurentian margin metasandstone of the Stowe Formation (Doolan et al., 1982). It has also been proposed that the Umbrella Hill Conglomerate locally grades upward into the Moretown Formation (Doolan et al., 1982). Based on stratigraphic relationships and petrography, Stanley and Ratcliffe (1985) argued that the Moretown Formation derived in part from the erosion of metamorphosed rift clastics and volcanics exposed in the Green Mountains anticlinorium and was deposited in an Ordovician forearc setting. The maximum age of the Moretown Formation can be constrained by the youngest dated detrital zircon at 513.8 ± 0.5 Ma (MacDonald et al.), but attributing a minimum age yields ambiguous results due to superposed deformation and metamorphism, especially where the Moretown Formation shares complex relationships with 496 to 462 Ma intrusions and fault-bounded slivers (Ratcliffe et al., 1998). MacDonald et al. propose to use the 502 ± 4 Ma U-Pb age of the Newfane tonalite (Aleinikoff et al., 2011), that they cite as crosscutting the Moretown Formation. However, the dated sample of this tonalite was collected from the Cram Hill Formation, not the Moretown. Nevertheless, the 475.0 ± 0.1 Ma Hallockville Pond Gneiss in Massachusetts seems to clearly intrude the Moretown Formation (MacDonald et al.). On these bases, we prefer to use maximum and minimum ages of ca. 514 and 475 Ma for the deposition of the Moretown Formation, the latter age being possibly as young as ca. 465 Ma if one considers the stratigraphic relationships of northern Vermont. Regarding that the oldest preserved arc-related rocks are dated at ca. 496 Ma in the area (i.e., the Barnard Gneiss; Ratcliffe et al., 1998), the formation of a forearc basin represented in part by the Moretown Formation, could have been initiated as early as late Cambrian–earliest Ordovician. In Vermont and adjacent Quebec, Taconian orogenesis on the Laurentian margin has been attributed to ophiolite obduction, accretion of volcanic arc rocks and nappe emplacement that were initiated at ca. 475–470 Ma (Tremblay et al., 2011; Castonguay et al., 2012). MacDonald et al. demonstrate that the Moretown Formation shows a dominant Neoproterozoic detrital zircon population between ca. 650 and 520 Ma, with minor peaks at ca. 1050 and 1200 Ma. As a result, they propose an easterly Gondwanan source and that the Moretown Formation must have a different provenance relative to peri-Laurentian clastic rocks, which are dominated by ca. 1000 and 1500 Ma detrital zircons attributed to Grenvillian sources. However, rift-related ca. 615 to 550 Ma volcanic rocks, dikes, and clastic sedimentary rocks are locally prominent along the Laurentian margin, especially along the axis of the OttawaBonnechere graben and the Sutton–Green Mountains anticlinorium (Hodych and Cox, 2007). These Neoproterozoic rocks thus represent alternative sources of detritus that could have been eroded from uplifted Taconian forearc highs and deposited within a forearc basin, in part represented by the Moretown Formation. Also, MacDonald et al. cite Tucker and Robinson (1990) as attributing to Gondwana the 613 ± 3 Ma gneisses of the Pelham dome, and use this as an argument to better delineate the Red Indian Line. However, Tucker and Robinson (1990) state that distinguishing between a Gondwanan and Laurentian affinity for these rocks is not possible based simply on the presence of Neoproterozoic rocks. Relocating the Red Indian line to the west of the Moretown Formation is not reconcilable with data from northern Vermont and Quebec, where there is no evidence for such a suture or a subduction polarity reversal during the Late Ordovician, as suggested by MacDonald et al. The detrital zircon record of the Moretown Formation can be alternatively explained by the erosion of Ediacaran Iapetan rift facies during the Taconic orogeny and formation of a peri-Laurentian forearc basin, without requiring Gondwanan contributions.

Evidence for seamount accretion to a peri-Laurentian arc during closure of Iapetus 1This article is one of a series of papers published in CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.2 Geological Survey of Canada Contribution 20100465.

The Red Indian Line is the fundamental Iapetus suture zone in the Newfoundland Appalchians along which the main tract of the Iapetus Ocean was consumed. Despite being the site of the closure of a wide ocean, few vestiges of the Iapetus plate have been accreted along Red Indian Line. Ordovician rocks in the Notre Dame Bay area preserve the only evidence for accretion of a seamount in Newfoundland. The seamount is characterized by alkali basalt and hypabyssal rocks that are juxtaposed with Darriwilian peri-Laurentian volcanic arc rocks (466 ± 4 and 467 ± 4 Ma) along a major mylonite zone. The mylonite zone lacks sedimentary rocks suggesting that the seamount was accreted to the arc along a sediment-starved interface and that significant subduction erosion took place along the Laurentian margin. Identification of subduction erosion indicates that an accretionary prism did not exist outboard of Laurentia in Newfoundland, in contrast to the well developed accretionary prisms of the Caledonides.

Tectono-stratigraphic setting of the Moreton’s Harbour Group and its implications for the evolution of the Laurentian margin: Notre Dame Bay, Newfoundland1This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

The Moreton’s Harbour Group lies along the Red Indian Line, the fundamental Iapetus suture that separates rocks of peri-Laurentian affinity with rocks of peri-Gondwanan affinity in the Newfoundland Appalachians. Characterization of age and environment of formation of the Moreton’s Harbour Group is an important constraint on evolution of the Laurentian margin during Ordovician closure of Iapetus Ocean and associated marginal basins. The Moreton’s Harbour Group comprises a fault-bounded ophiolitic sequence of layered gabbro, sheeted diabase, pillow basalt, and felsic intrusive rocks. It is offset by high-angle shear zones that were contemporaneous with a 477.4 ± 0.4 Ma syn-tectonic and syn-magmatic suite of trondhjemite and tonalite. Trace element data from the felsic suite indicate formation in a supra-subduction zone setting, although isotopic data from the felsic intrusive rocks (εNd (–5.02) to (–10.53), Tdm 1200–1800 Ma) indicate a significant amount of contamination from Mesoproterozoic or older continental crust. The age and tectonic setting of the Moreton’s Harbour Group suggest that it is the northernmost extent of the ca. 480 Ma Annieopsquotch Ophiolite Belt. We present a model in which the Moreton’s Harbour Group formed in response to propagation of the Annieopsquotch Ophiolite Belt spreading centre into the Dashwoods microcontinent. This ridge propagation model supports the formation of the Annieopsquotch Ophiolite Belt immediately outboard of Dashwoods and explains its rapid accretion to the composite Laurentian margin.

Where is the Iapetus suture in northern New England? A study of the Ammonoosuc Volcanics, Bronson Hill terrane, New Hampshire1This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

The ∼470 Ma Ammonoosuc Volcanics of the Bronson Hill terrane of New Hampshire have back-arc basin basalt compositions. Major and trace element compositions compare favorably to coeval volcanic rocks in the Miramichi Highlands of New Brunswick and the Munsangan and Casco Bay volcanics of Maine, back-arc basin basalts of known peri-Gondwanan origins. Additionally, the Ammonoosuc Volcanics have Nd and Pb isotopic compositions indicative of peri-Gondwanan provenance. Thus, the Ammonoosuc Volcanics correlate with Middle Ordovician, peri-Gondwanan, Tetagouche–Exploits back-arc rocks of eastern New England and Maritime Canada. This correlation indicates that the Red Indian Line, the principle Iapetus suture, lies along the western margin of the Bronson Hill terrane. However, the younger (∼450 Ma) Oliverian Plutonic Suite rocks that intruded the Ammonoosuc Volcanics, forming domes along the core of the Bronson Hill anticlinorium, have Laurentian isotopic signatures. This suggests that the Ammonoosuc Volcanics were thrust westwardly over the Laurentian margin, and that Laurentian basement rocks are present under the Bronson Hill terrane. A plausible explanation for these relationships is that an easterly dipping subduction zone formed the Ammonoosuc Volcanics in the Tetagoughe–Exploits oceanic tract, just east of the coeval Popelogan arc. With the closure of the Iapetus Ocean, this terrane was thrust over the Laurentian margin. Subsequent to obduction of the Ammonoosuc Volcanics, subduction polarity flipped to the west, with the Oliverian arc resulting from a westerly dipping subduction zone that formed under the Taconic Orogeny-modified Laurentian margin.

Laurentia-derived detritus in the Badger Group of central Newfoundland: deposition during closing of the Iapetus Ocean1This article is one of a series of papers published in CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.2Geological Survey of Canada Contribution 20110273.

The Dunnage zone of central Newfoundland records the closing of an oceanic tract — the Iapetus Ocean. Within this tract, the Red Indian line separates arc terranes developed close to Laurentia (Notre Dame subzone) from those associated with the peri-Gondwanan terrane Ganderia (Exploits subzone). Sandstones and conglomerates of the Badger Group were deposited on the Exploits subzone, southeast of the Red Indian line, from Late Ordovician (Katian) to early Silurian (Llandovery) time. Two samples were collected from the base and top of the Badger Group for detrital zircon U/Pb dating. The lower sample yielded detrital zircon populations with a large statistical peak at ca. 1.0 Ga, and other features characteristic of derivation from Laurentian sources. Paleozoic zircons with ages from Late Cambrian to Late Ordovician were probably derived from the Notre Dame arc on the margin of Laurentia. The upper sample yielded a comparable age distribution, but with a much smaller proportion of Mesoproterozoic relative to Paleozoic detrital zircon. These results date the earliest arrival of Laurentia-derived detritus on the peri-Gondwanan Exploits subzone crust in the early Katian (ca. 455 Ma). However, the absence, in both samples, of Neoproterozoic grains suggests that there was still a seaway separating the Gander margin from arc terranes accreted to Laurentia until at least the middle Llandovery (ca. 436 Ma).

Preservation of terranes during the assembly of the Annieopsquotch Accretionary Tract: Inferences from the provenance of a Middle Ordovician ophiolite to arc transition, central Newfoundland Appalachians1Geological Survey of Canada Contribution 20100463.2This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

The Newfoundland Appalachians are a classic area for studying the record of arc development and terrane accretion processes with excellent exposure to different crustal levels that are minimally deformed and metamorphosed. This area also provides a link between the once continuous Appalachian and Caledonian orogens. The Annieopsquotch Accretionary Tract lies along the Red Indian Line, within the peri-Laurentian realm of the central Newfoundland Appalachians. The Darriwilian (468–461 Ma) tectonostratigraphic units of the Annieopsquotch Accretionary Tract are commonly characterized by polymictic volcanogenic conglomerate horizons. A conglomerate horizon at the interface between a suprasubduction zone ophiolite and its calc-alkaline volcanic arc cover sequence is herein investigated for zircon and geochemical provenance. Geochronology revealed a maximum age of deposition of 467 ± 4 Ma with zircon inheritance ranging from ca. 500 to 2800 Ma, consistent with a peri-Laurentian continental basement source. Four types of volcanogenic conglomerate clasts are noted on the basis of lithogeochemistry: arc andesite; calc-alkaline basalt; tholeiitic basalt; and non-arc rhyodacite. Tholeiitic basalt clasts are likely locally derived, perhaps from the underlying Skidder Formation. Other volcanic clasts do not have any known geochemical equivalents in the Annieopsquotch Accretionary Tract and hence appear to be exotic. The dominant zircon population suggests that the exotic clasts were derived from a ca. 467 Ma peri-Laurentian andesitic volcanic arc that once formed part of the Annieopsquotch Accretionary Tract.

The petrogenesis of the Highlandcroft and Oliverian Plutonic Suites, New Hampshire: Implications for the structure of the Bronson Hill terrane

Both Laurentian and peri-Gondwanan origins for the Oliverian and Highlandcroft Plutonic Suites of New Hampshire have been proposed. A peri-Gondwanan origin places the Red Indian Line, the principle Iapetus suture, to the west of the Bronson Hill Anticlinorium and requires the plutons to be allochthonous. In contrast, a Laurentian margin emplacement setting places the Red Indian Line farther to the east in New Hampshire. The combination of ε~Nd~ and ^207^Pb/^204^Pb values are most compatible with the suites representing mixtures of arc-derived magmas and Laurentian crust, indicating that both suites were derived to the west of the Red Indian Line. The tectonic setting for the arc probably was a westerly dipping subduction zone under the Laurentian margin, resulting from a polarity flip after the collision of the Shelburne Falls arc during the Taconic orogeny. Bulk-rock major and trace elements, as well as Nd, Sr, and Pb isotopic results, indicate that the Highlandcroft and Oliverian Plutonic Suites of New Hampshire are both members of the same arc complex.

Distinct Taconic, Salinic, and Acadian deformation along the Iapetus suture zone, Newfoundland Appalachians

Structural mapping in central Newfoundland has identified seven distinct phases of deformation (D1 to D7), the most significant of which are D1, D2, and D4. D1 involved the formation of a Middle and Late Ordovician south-southeast-directed thrust belt and concomitant development of mylonite and phyllonite. A Late Ordovician to Early Silurian D2 thrust and fold belt overprints D1 mylonitic deformation and is the most distinctive deformation event in the study area. Late Silurian to Devonian D4 is responsible for folds and north-northwest-directed dextral thrust and reverse faults that overprint D1 to D3 structures. D4 structures in central Newfoundland include the Exploits–Gander boundary. Subsequent deformation is generally of local significance only. The arc–back-arc complexes making up the various terranes in central Newfoundland are predominantly juxtaposed along D1 shear zones, which include the Red Indian Line. Our data indicate that terrane boundaries initiated during D1 may have protracted deformation histories spanning several deformation events. This has important implications for the interpretation of terrane boundaries in Newfoundland, as D1 terrane boundaries may be interpreted as D2 or D4 shear zones depending on the intensity of overprinting or reactivation. The deformation history proposed in this paper corresponds closely to that of established Appalachian orogenic cycles. D1 is correlated with the Ordovician Taconic orogeny and involved accretion of arc–back-arc complexes to the Laurentian margin. D2 and D4 are correlated with the Ordovician–Silurian Salinic and Silurian–Devonian Acadian orogenies, which involved the subsequent accretion of the Ganderia and Avalonia microcontinents to the Laurentian margin, respectively.

Upper Cambrian to Upper Ordovician peri-Gondwanan Island arc activity in the Victoria Lake Supergroup, Central Newfoundland: Tectonic development of the northern Ganderian margin

The Exploits Subzone of the Newfoundland Appalachians comprises remnants of Cambro-Ordovician peri-Gondwanan arc and back-arc complexes that formed within the Iapetus Ocean. The Exploits Subzone experienced at least two accretionary events as a result of the rapid closure of the main portion of the Iapetus tract: the Penobscot orogeny (c. 480 Ma), which juxtaposed the Penobscot Arc (c. 513 –486 Ma) with the Gander margin, and c. 450 Ma collision of the Victoria Arc (c. 473 –454 Ma) with the Annieopsquotch Accretionary Tract that juxtaposed the peri-Laurentian and peri-Gondwanan elements along the Red Indian Line. The newly recognized Pats Pond Group forms a temporal equivalent to other Lower Ordovician intra-oceanic complexes of the Penobscot Arc. The Pats Pond Group (c. 487 Ma) has a geochemical stratigraphy that is consistent with rifting of a volcanic arc. An ensialic setting is indicated by low εNd values (εNd 0.3 to -0.5) near the stratigraphic base and its abundant zircon inheritance (c. 560 Ma and 0.9 –1.2 Ga). The spatial distribution of Tremadocian arc –back-arc complexes indicates that the Penobscot arc is best explained in terms of a single east-dipping subduction zone. This model is favored over west dipping models, in that it explains the distribution of the Penobscot arc elements, continental arc magmatism, and the obduction of back-arc Penobscot ophiolites without requiring subduction of the Gander margin or subduction reversal. The newly recognized Wigwam Brook Group (c. 454 Ma) disconformably overlies the Pats Pond Group and records the youngest known phase of ensialic arc volcanism (εNd –4.1) in the Victoria Arc, which is also related to east-dipping subduction. Thus the Penobscot and the overlying Victoria Arc are reinterpreted in terms of a single, relatively long-lived east-dipping subduction zone beneath the peri-Gondwanan microcontinent of Ganderia. The cessation of arc volcanism towards the top of the Wigwam Brook Group and the subsequent syn-tectonic sedimentation in the Badger Group constrain the arrival of the leading edge of Ganderia with the ensialic arc complexes to the Laurentian margin to c. 454 Ma.

Ages and origins of rocks of the Killingworth dome, south-central Connecticut: Implications for the tectonic evolution of southern New England

The Killingworth dome of south-central Connecticut occurs at the southern end of the Bronson Hill belt. It is composed of tonalitic and trondhjemitic orthogneisses (Killingworth complex) and bimodal metavolcanic rocks (Middletown complex) that display calc-alkaline affinities. Orthogneisses of the Killingworth complex (Boulder Lake gneiss, 456 ± 6 Ma; Pond Meadow gneiss, ∼460 Ma) were emplaced at about the same time as eruption and deposition of volcanic-sedimentary rocks of the Middletown complex (Middletown Formation, 449 ± 4 Ma; Higganum gneiss, 459 ± 4 Ma). Hidden Lake gneiss (339 ± 3 Ma) occurs as a pluton in the core of the Killingworth dome, and, on the basis of geochemical and isotopic data, is included in the Killingworth complex. Pb and Nd isotopic data suggest that the Pond Meadow, Boulder Lake, and Hidden Lake gneisses (Killingworth complex) resulted from mixing of Neoproterozoic Gander terrane sources (high ²⁰⁷Pb/²⁰⁴Pb and intermediate ε_Nd) and less radiogenic (low ²⁰⁷Pb/²⁰⁴Pb and low ε_Nd) components, whereas Middletown Formation and Higganum gneiss (Middletown complex) were derived from mixtures of Gander basement and primitive (low ²⁰⁷Pb/²⁰⁴Pb and high ε_Nd) sources. The less radiogenic component for the Killingworth complex is similar in isotopic composition to material from Laurentian (Grenville) crust. However, because published paleomagnetic and paleontologic data indicate that the Gander terrane is peri-Gondwanan in origin, the isotopic signature of Killingworth complex rocks probably was derived from Gander basement that contained detritus from non-Laurentian sources such as Amazonia, Baltica, or Oaxaquia. We suggest that the Killingworth complex formed above an east-dipping subduction zone on the west margin of the Gander terrane, whereas the Middletown complex formed to the east in a back-arc rift environment. Subsequent shortening, associated with the assembly of Pangea in the Carboniferous, resulted in Gander cover terranes over the Avalon terrane in the west; and in the Middletown complex over the Killingworth complex in the east. Despite similarities of emplacement age, structural setting, and geographic continuity of the Killingworth dome with Oliverian domes in central and northern New England, new and published isotopic data suggest that the Killingworth and Middletown complexes were derived from Gander crust, and are not part of the Bronson Hill arc that was derived from Laurentian crust. The trace of the Ordovician Iapetan suture (the Red Indian line) between rocks of Laurentian and Ganderian origin probably extends from Southwestern New Hampshire west of the Pelham dome of northcentral Massachusetts and is coverd by Mesozoic rocks of the Hartford basin.

Lower to Middle Ordovician evolution of peri-Laurentian arc and backarc complexes in Iapetus: Constraints from the Annieopsquotch accretionary tract, central Newfoundland

The Annieopsquotch accretionary tract in Newfoundland is composed of a series of west-dipping structural panels, each containing remnants of ophiolitic and arc-backarc complexes of Laurentian affinity formed during the Ordovician closure of Iapetus. Panels were transferred from an upper-plate to a lower-plate setting during their Middle to Late Ordovician accretion to the Laurentian margin and become progressively younger eastward. Geochronological data indicate a complex and rapid history of generation and accretion of peri-Laurentian suprasubduction zone rocks. The rapid changes in tectonic environments and the complexity of the relationships are analogous to the complex arc-backarc relationships observed in the western Pacific today. The recognition of the peri-Laurentian provenance of these units based on stratigraphy, geochronology, isotopes, and geochemistry defines the position of the Red Indian Line, the fundamental suture zone in the northern Appalachians, but more importantly enables the development of a realistic tectonic model for the Annieopsquotch accretionary tract involving both thrust and sinistral transcurrent displacements. The oldest and most inboard unit in the Annieopsquotch accretionary tract is the Annieopsquotch ophiolite belt (ca. 480 Ma), which marks the initiation of subduction outboard of the Laurentian margin. The Lloyds River ophiolite complex (ca. 473 Ma) preserves a fragment of younger, more mid-ocean-ridge–like backarc-oceanic crust than the adjacent, structurally overlying Annieopsquotch ophiolite belt. The Lloyds River ophiolite complex originated as a backarc to the Buchans Group (ca. 473 Ma) ensialic bimodal calc-alkaline arc. The panels containing the Annieopsquotch ophiolite belt and Lloyds River ophiolite complex were stitched and overlain by ensialic arc rocks of the Otter Pond Complex (ca. 468 Ma) immediately after their accretion to composite Laurentia together with the structurally underlying Buchans Group. The youngest, structurally lowest two panels comprise the elements of the Red Indian Lake group (465–460 Ma), which record the opening of a backarc basin and the subsequent establishment of a bimodal ensialic calc-alkaline arc sequence. The observed relationships indicate that the Annieopsquotch accretionary tract was generated above a single west-dipping subduction zone outboard of the Laurentian margin over ∼20 m.y. Accretion mainly took place in two stages at ca. 468 and 450 Ma, which correspond with the collision between Laurentia and the Dashwoods ribbon continent and the collision with the peri-Gondwanan Victoria Arc along the Red Indian Line, respectively. Both collisions form part of the Taconic orogeny. The latter, Late Ordovician collision terminated the relatively rapid closure of the main Iapetan tract. The proposed model is similar to the correlative tracts in the British and Irish Caledonides, and may encourage a new look at the New England Appalachians.

A lost arc-back-arc terrane of the Dunnage oceanic tract recorded in clasts from the Garin Formation and McCrea mélange in the Gaspé Appalachians of Québec

Upper Ordovician Garin conglomerates (base of the Gaspe belt, Quebec Appalachians) contain three igneous clast populations. (1) Calc-alkaline intermediat-felsic rocks resemble Exploits (New Brunswick, Maine) and Notre Dame (Quebec, Newfoundland) subzone lavas. Clasts (monzonite-rhyodacite) give U-Pb zircon ages between 465 and 466 Ma, precluding correlation with Exploits rocks. We suggest that the suite represents peri-Laurentian continental arc magmas coeval with the Red Indian Lake Group of Newfoundland. Correlation of lavas from a Gaspe well with the Popelogan arc implies that the Red Indian Line passes through southern Gaspe. (2) Mafic tholeiitic-to-alkaline clasts resemble New Brunswick and Maine (Exploits subzone) alkaline lavas. We interpret them as magmas associated with initiation of a back-arc basin, possibly on the Laurentian margin. (3) Mafic-intermediate tholeiitic to calc-alkaline clasts have an oceanic subduction component and correlate with Notre Dame subzone lavas (Quebec, Newfoundland). They are interpreted as products of spreading and off-axis magmatism of a peri-Laurentian back-arc basin. We correlate McCrea melange lavas to the calc-alkaline intermediat-felsic and mafic-intermediate suites and interpret them as samples of a Notre Dame subzone terrane coeval with the Red Indian Lake Group of Newfoundland during initiation of a successor arc–back-arc, a terrane which is no longer preserved in New Brunswick and Maine but that provides some evidence for possible extensions of Newfoundland9s geology into the Gaspe Appalachians of Quebec. A mid-ocean-ridge basalt–like intrusion in the McCrea melange (Ar-Ar age: 471.2 ± 11.2 Ma) is coeval with peri-Laurentian ophiolites in Newfoundland and may record a back-arc opening event.

Late Middle Ordovician olistostrome formation and magmatism along the Red Indian Line, the Laurentian arc – Gondwanan arc boundary, at Sops Head, Newfoundland

The Sops Head Complex of Badger Bay, central Newfoundland, includes olistostromal and tectonized mélanges and marks the Red Indian Line, the boundary between the peri-Gondwanan Exploits and peri-Laurentian Notre Dame subzones of the oceanic Dunnage Zone. Basalts in the olistostromal mélange preserve peperitic contacts with mudstone and limestone within slumped sedimentary units, demonstrating that magmatism was coeval with olistostrome formation. Conodonts from the limestones date these events as late Darriwilian ("Llandeilian," Middle Ordovician) in age. The conodont fauna consists entirely of North Atlantic Faunal Realm elements, suggesting a peri-Gondwanan or mid-Iapetan provenance for limestone of the Sops Head Complex. Basalt geochemistry suggests magmatism in a mature volcanic-arc to back-arc setting. The Sops Head Complex shows similarities in age, provenance, and stratigraphic setting to the Dunnage Mélange, suggesting that the two units are correlative.

Stratigraphy, geochronology, and accretionary terrane settings of two Bronson Hill arc sequences, northern New England

The Ammonoosuc Volcanics, Partridge Formation, and the Oliverian and Highlandcroft Plutonic Suites of the Bronson Hill anticlinorium (BHA) in axial New England are widely accepted as a single Middle to Late Ordovician magmatic arc that was active during closure of Iapetus. Mapping and U–Pb dating indicate, however, that the BHA contains two volcano-sedimentary-intrusive sequences of probable opposite subduction polarity, here termed the Ammonoosuc and Quimby sequences. The Ammonoosuc sequence is defined by the Middle Ordovician Ammonoosuc Volcanics near Littleton, N.H., the type area, northeast to Milan, N.H., and Oquossoc, Me.; it also includes black slate of the Partidge Formation ( C. bicornis zone graptolites, ∼457 Ma). Related metamorphosed intrusive are the tonalitic Joslin Turn pluton (469±2 Ma), the Cambridge Black granitic pluton (468±3 Ma), and gabbro, tonalite (467±4 Ma), and sheeted diabase of the Chickwolnepy instructions. These intrusives cut lowermost Ammonoosuc (therefore >469 Ma). Probable uppermost Ammonoosuc is dated at 465±6 and 461±8 Ma. Successively below the Ammonoosuc are the Dead River and Hurricane Mountain Formations (flysch and melange), and the Jim Pond Formation (484±5 Ma) and Boil Mountain Complex (both ophiolite), which are structurally underlain by the Neoproterozoic(?) Chain Lakes massif. The Quimby sequence is defined by the Lower Silurian(?) to Upper Ordovician Quimby Formation, composed of bimodal volcanics (443±4 Ma) and sulfidic shale and graywacke that lie conformably to unconformably above the Ammonoosuc Volcanics and Partridge Formation. Also in the Quimby sequence are several granitic to sparsely gabbroic plutons of the Highlandcroft (441–452 Ma) and Oliverian (435–456 Ma) Plutonic Suites, which intrude the Dead River, Ammonoosuc and Partridge, but not the Quimby Formation. Based on faunal, paleolatitude, and isotropic data, the Ammonoosuc sequence and its correlative and underlying sequences formed off the southern Laurentian margin, but northwest of the principal Iapetan suture, or Red Indian line (RIL). The Boil Mountain–Jim Pond–Hurricane Mountain sequence was ramped northwestward over the Chain Lakes massif at ∼475 Ma, on the basal Boil Mountain surface. This obduction probably occurred slightly before obduction on the Baie Verte–Brompton surface (BBL), farther NW, over the Laurentian margin, and was followed by Dead River flysch sedimentation, which ended with the abrupt onset of Ammonoosuc-sequence arc magmatism at ∼470 Ma. Ammonoosuc eruptions probably ended at ∼460 Ma, when Iapetus closed along the Red Indian line. During a following magmatic hiatus of ∼3–5 m.y., now represented by portions of the Partridge Formation that overlie the Ammonoosuc Volcanics, subduction polarity reversed, and subduction resumed below the northwest-dipping Brunswick subduction complex (BSC) of New Brunswick, Canada. Quimby-sequence magmatism (∼456–435 Ma) on the the newly accreted Laurentian margin occurred above the BSC, whose footwall is now buried to the southeast by mainly Silurian clastic sediments of the Merrimack–Fredericton trough, deposited in the “Fredericton Sea”. In Silurian to Early Devonian time, the NW-dipping BSC footwall was paired with a SE-dipping subduction zone that produced arc magmas of the Coastal Volcanic belt, built on the composite Avalon and adjacent peri-Avalonian terranes. Orogen-normal extension produced by rapid rollback of both subduction zones narrowed the Fredericton Sea, produced the Central Maine and Connecticut Valley–Gaspé basins, and culminated in the Acadian orogeny when the sea completely closed in Early Devonian time.

Discussion on a reassessment of the early Ordovician 'Celtic' brachiopod province

R. B. Neuman, D. A. T. Harper & C. R. van Staal write: The results of several recent geological studies relevant to early Ordovician palaeogeography are not mentioned in this recent paper by Cocks & McKerrow (1993). Contributions of particular significance include the recognition that the Dunnage Zone of Newfoundland consists of two distinct parts, the Notre Dame and Exploits subzones of contrasting Ordovician stratigraphy, lithology, structure, metamorphism, plutonism, lead isotope signatures, mineralization, geophysics, and faunas ( Williams et al. 1988 ). The boundary between these subzones, named the Red Indian Line in Newfoundland, is probably a suture. Fossiliferous sedimentary rocks are rare in the Notre Dame Subzone, but Arenig-Llanvirn graptolites occur in shales at three places and at one place conodonts have been extracted from carbonate clasts in volcanic breccias. All these fossils are considered to have Laurentian affinities (Nowlan & Thurlow 1984; Williams 1992 ; Williams et al. 1992 ). Fossiliferous Lower Ordovician rocks are more abundant in the Exploits subzone. In addition to the several occurrences of Celtic assemblage brachiopods (Cl-C6 of Neuman & Harper 1992 ; Harper 1992 ; Cocks & McKerrow 1993), Gondwanan trilobites and graptolites (Cyclopyge and Undulograptus) are known in the Exploits subzone of central Newfoundland ( Williams et al. 1992 ) from shale that overlies a Lower Ordovician ophiolite fragment near an Occurrence of Celtic brachiopods (C5 of Neuman & Harper 1992 ). Palaeomagnetic studies of Newfoundland rocks near the Red Indian Line showed that these two subzones were separated by about 30° of latitude (van der Pluijm et al. 1990;

Early Silurian palaeolatitude of the Springdale Group redbeds of central Newfoundland: a palaeomagnetic determination with a remanence anisotropy test for inclination error

We studied the palaeomagnetism of red fine-grained sandstones and coarse siltstones of the early Silurian Springdale Group of central Newfoundland. At 10 sites, a high blocking temperature characteristic remanence carried by haematite was isolated. This remanence is shown to predate probable early Devonian folding. Anti-parallel north- and south-directed remanences through a 100 m section of redbeds and a positive conglomerate test on haematite-bearing volcanic clasts suggest absence of remagnetization. Inverting the south-directed sites and unfolding yields a characteristic remanence with a mean declination of 23.6 and a mean inclination of - 14.2 (myg5= 7.3, k = 45.4). The inclination corresponds to a probable early Silurian palaeolatitude of 7s f 4. We find no significant difference between early Silurian palaeolatitudes for central Newfoundland north and south of the Red Indian Line suture, and conclude that the part of the Iapetus Ocean across the suture had narrowed to less than about 5 by the early Silurian. This is consistent with palaeomagnetic results from Britain and Ireland that suggest no more than a narrow Iapetus at low palaeolatitude by the early Silurian. We also tested whether we have underestimated palaeolatitude because of sediment compaction reducing remanence inclination from that of the early Silurian field. We measured anisotropy of the isothermal remanence (IRM) acquisition for one specimen from each stable site, finding that a field of 200 to 800 rnT applied at 45 to bedding produced an isothermal remanence oriented on average at 42 to bedding. Theory then predicts that sediment compaction caused less than 2 average inclination shallowing in the Springdale Group redbeds, and less than a 1 underestimation of palaeolatitude.

Geological interpretation of a high resolution reflection seismic survey at the Buchans mine, Newfoundland

Sixteen kilometres of high resolution Vibroseis reflection seismic data have been acquired in the vicinity of the former Buchans mine. Direct identification of the cause of several reflectors is possible because the geology is tightly constrained by underground workings and drill holes both of which locally exceed 1 km depth. Many of the mine-scale thrust faults are imaged as reflectors but conformable and intrusive contacts generally responded poorly. A significant shallow-dipping thrust, the Powerline Fault, is recognized below the orebodies and traced throughout the Buchans area, primarily as a result of the seismic survey. It truncates ore stratigraphy and forms the floor thrust of a large duplex–stack, which hosts all the orebodies. Its presence has negative implications for exploration in the immediate mine area. Several lines of evidence suggest that this fault has a significant component of out-of-sequence movement. A strong reflector 4.5 km below Buchans is correlated with the surface expression of the Victoria River Delta Fault, an important regional structure, newly recognized southeast of Red Indian Lake. This shallow, north-dipping sole thrust forms the structural base of the Buchans Group and brings it above a younger fossiliferous Llanvirn volcanic sequence. This fault is not itself the Red Indian Line but is one of a series of faults that collectively effect substantial geological contrasts in central Newfoundland. The seismic survey was a cost-efficient means of gaining knowledge of Buchans structure, which might otherwise have been acquired at much higher cost and over a longer period of time.