Tectonic implications of the discovery of a Shawinigan ophiolite (Pyrites Complex) in the Adirondack Lowlands

During the assembly of Rodinia, large parts of the Grenville Province (Fig. 1; Rivers, 1997), including the Adirondack Mountains of New York, underwent three distinct tectonic events over more than 350 m.y. (ca. 1350–1000 Ma; Elzevirian, Shawinigan, and Grenvillian orogenies of Rivers, 2008). However, even within a relatively small portion of the orogen such as the Adirondacks, the extent and nature of these events varies significantly (Heumann et al., 2006; Bickford et al., 2008). Defining and understanding the tectonic context of regional metamorphism, the nature of the boundaries and relative motion along them, and the tectonic events that caused them, are major challenges in the Adirondacks, the Grenville Province, and deeply exhumed orogens worldwide. Furthermore, advances in our understanding of the Grenville Province are critical to facilitate our interpretation of the deep crustal architecture of orogenic belts worldwide (Rivers, 2008), the assembly of supercontinents, and temporal changes in tectonic processes.

One of the most fundamental boundaries in the Adirondacks is the Carthage-Colton shear zone (Fig. 2), which separates Grenvillian rocks of the Central Metasedimentary Belt (Adirondack Lowlands) from those of the Central Granulite terrane (Adirondack Highlands). In contrast to the Highlands, and despite many similarities, the Lowlands have a recognized stratigraphy and greater volume of supracrustal rocks (80%–85%; Carl and deLorraine, 1997), lower metamorphic grade, different structural style and grain, and lack widespread (any?) Ottawan deformation and thermal overprint and magmatism related to the Lyon Mountain Granite suite (however, see the monazite results of Hudson et al., 2004, Dahl et al., 2005, and Hudson et al., 2008). These differences have been attributed to variations in crustal level during tectonism, prior to orogenic collapse manifested by downdropping of the Lowlands along the Carthage-Colton shear zone (McLelland et al., 1996; Selleck et al., 2005). Selleck et al. (2005) have confirmed the Carthage-Colton shear zone's role in extensional orogenic collapse and channeling of late to post-tectonic leucogranites (Lyon Mountain Granite) during the late Ottawan phase of the Grenvillian orogeny (1090–1020 Ma; Rivers, 2008); however, its earlier history is less well constrained, although differences in metamorphic grade and geochronology have been documented (Mezger et al., 1992; Streepy et al., 2001).

Radiometric ages and isotopic data, underpinned by field and structural studies in the Lowlands and contiguous portions of the Grenville Province, have provided constraints by which possible tectonic models for the southern Grenville Province can now be more fully evaluated (McLelland et al., 1996; Hanmer et al., 2000; Rivers, 2008; McLelland et al., 2010). Given the general lack of pervasive Ottawan deformation and associated granulite-facies metamorphism, the Lowlands provide a rare window into the Shawinigan orogeny where original stratigraphic relations are at least partially preserved. Despite this, rocks of Adirondack Lowlands and the contiguous Canadian portion of Frontenac terrane are complexly deformed and metamorphosed at high grade, and the new data described herein provide additional constraints that can be incorporated into future discussions and models.

Southeast Laurentia is believed to have been a long-lived Andean-style margin developed above NW-directed subduction zones (Carr et al., 2000; Hanmer et al., 2000; Rivers and Corrigan, 2000). Proposed models include the development of a backarc basin in the Ontario-Quebec-Adirondack segment after 1300 Ma, with closure of the basin and associated tectonism at ca. 1200 Ma. Based on Nd isotopic evidence, Dickin and McNutt (2007) recently proposed that much of the Central Metasedimentary Belt in Ontario and Quebec is a failed backarc rift zone underlain by both ensialic and ensimatic crust. Chiarenzelli et al. (2010b, this volume) propose that a similar and contemporaneous backarc basin (Trans-Adirondack Basin) existed between the Frontenac terrane and the southern Adirondacks. The mafic and ultramafic rocks described here form part of an obducted ophiolite sequence that once floored the Trans-Adirondack Basin (Chiarenzelli et al., 2010a).

Herein, the first documented occurrence of ultramafic rocks in the Adirondack Lowlands, New York, is summarized (Chiarenzelli et al., 2007; Lupulescu et al., 2008). The tectonic constraints provided by their field relations, geochemical characteristics, U-Pb zircon geochronology, and Nd T_DM ages are discussed. A model for the evolution of supracrustal rocks in the Lowlands prior to the Shawinigan orogeny is proposed in light of evolving models for Grenville tectonism now incorporating the occurrence of ultramafic and mafic rocks of oceanic affinity (ophiolite) in the Adirondack Lowlands.

The Adirondack Lowlands consist of a diverse array of metaigneous and metasedimentary rocks (Fig. 3). The lower marble, currently the oldest recognized lithologic unit in the Lowlands, consists of a diverse assemblage of calcitic marble with interlayered calc-silicates, quartzites, biotite and pelitic gneisses, and arkosic metasandstones and tourmalinites (Brown, 1988, 1989). Sillimanite-garnet ± corundum ± spinel assemblages rim some of the Hyde School Gneiss phaccoliths and have yielded metamorphic temperatures of 830–860 °C, substantially above reported regional metamorphic temperatures (McLelland et al., 1991; Hudson and Dahl, 1998). These rocks, once considered a base to the Lowlands metasedimentary sequence, have been reinterpreted as a contact metamorphic zone associated with the intrusion of the Hyde School Gneiss at ca. 1172 Ma (McLelland et al., 1991). Marbles, volumetrically significant in the Lowlands, are also found widely throughout Central Metasedimentary Belt and Adirondack Highlands (McLelland and Isachsen, 1986; Easton, 1992; deLorraine and Sangster, 1997), but correlation across this wide area is unlikely due to structural complexities and distances.

The Popple Hill Gneiss overlies the lower marble and consists of a wide variety of lithologies including pelitic and pssamitic gneisses containing primarily quartz-biotite-plagioclase–K-feldspar. Locally, the Popple Hill Gneiss contains large proportions of garnet and/or sillimanite and anatectic melt. Its overall composition is highly variable and has been described as dacitic by Carl (1988); and it represents a partially melted and highly metamorphosed shale-sand sequence with possible volcaniclastic input. In some areas it is extensively migmatized and in others contains numerous (up to 100-m-thick) concordant amphibolitic layers interpreted as sills and/or dikes of basaltic composition (Carl, 2000). Zircons separated from the Popple Hill Gneiss from locations throughout the Adirondack Lowlands and Highlands yield zircon populations indicating anatexis at 1180–1160 Ma in the Lowlands and Highlands (Heumann et al., 2006), as well as 1080–1040 Ma in the eastern Highlands (Bickford et al., 2008). Zircons, interpreted as detrital cores, yield ages as young as ca. 1220 Ma, constraining the maximum age of at least part of the Popple Hill Gneiss and overlying upper marble.

Because it hosts the Balmat sphalerite and talc deposits, the stratigraphy of the upper marble is well characterized by hundreds of drill cores that have penetrated the Sylvia Lake syncline. It is the uppermost unit in the Lowlands and has been subdivided into 16 units consisting primarily of pure and silicated dolomitic marbles (deLorraine and Sangster, 1997). Other units include anhydrite, talc-tremolite-cummingtonite schist, and pyritic pelitic schists. Unit 4 is noteworthy for the presence of possible overturned stromatolites on the recumbent limb of the Sylvia Lake syncline. Deposition in shallow, marine conditions with abundant algal life is supported by the presence of natural gas discovered during drilling related to initial sphalerite exploration efforts (Brown, 1932) and the occurrence of widespread “fetid” marbles in both the upper and lower marbles.

In addition to the aforementioned lithologies, a wide variety of amphibolitic and metagabbroic units of uncertain origin are found throughout the Lowlands (Carl, 2000). Some are clearly intrusive and display igneous textures and plutonic contacts, and are best considered metagabbros or metadiorites. Many of these may be mafic equivalents of the Antwerp-Rossie plutonic suite. Others are strongly deformed and highly disrupted and intruded by granitic to tonalitic members of the Antwerp-Rossie suite and thus have a minimum age of ca. 1200 Ma (Wasteneys et al., 1999; Chiarenzelli et al., 2010b). Still others are intimately associated with thin, pyritic gneiss units that can be traced for over 50 km or more in NE-trending belts in the Lowlands that were mined for sulfur in the late 1800s and early 1900s (Prucha, 1957). In the Colton-Pierrepont area (Fig. 3), amphibolites appear to extensively intrude rocks of the Popple Hill Gneiss and lower marble (Tyler, 1979) in accord with our own observations.

Several intrusive suites are known throughout the area and intrude the metasedimentary sequence noted above (Carl and deLorraine, 1997). These include the Antwerp-Rossie suite intruded at ca. 1200 Ma (Wasteneys et al., 1999; Chiarenzelli et al., 2010b), the Hermon granitic gneiss (1182 ± 7 Ma; Heumann et al., 2006), and the Hyde School Gneiss (ca. 1172; Wasteneys et al., 1999). The Hyde School Gneiss has a variable composition (granitic to tonalitic) and is primarily exposed in about a dozen large, elliptical domes. It and the Rockport Granite, exposed in the Frontenac terrane, may represent transitional magmatism following intrusion of the Antwerp-Rossie suite and predating initiation of the anorthosite-mangerite-charnockite-granite (AMCG) magmatism (Chiarenzelli et al., 2010b). Rocks of the AMCG suite include the Edwardsville syenite (1164 ± 4 Ma; McLelland et al., 1993). These ages and dating of zircon from leucosomes in the Popple Hill Gneiss (Heumann et al., 2006) indicate high-grade metamorphic conditions and deformation occurred during the peak of Shawinigan orogenesis from 1180 to 1160 Ma). Shortly thereafter, by ca. 1155 Ma, regional deformation ceased and cooling of the area began (Mezger et al., 1991; McLelland et al., 2010).

Inspection of a map of the Adirondack Lowlands (Fig. 3; Rickard et al., 1970) reveals a strong NE-trending structural grain defined by linear lithologic belts. This is particularly true for the supracrustal units but also in part for the Antwerp-Rossie suite and Hermon granitic gneiss suites. The Hyde School Gneiss, however, appear to form foliation deflecting composite plutons or structural domes originally termed phaccoliths by Buddington (1939). In addition, a progressive change in metasedimentary rock type is apparent across strike. For example, larger bodies of quartzite consisting of quartzose, feldspathic, or calc-silicate–bearing metamorphosed sandstones are generally restricted to the area NW of the Black Lake shear zone. Farther to the SE they give way to the lower marble, then the Popple Hill Gneiss, and then the upper marble. Near the Carthage-Colton shear zone the lower marble reappears, suggesting that it extends underneath the Popple Hill Gneiss and upper marble. Plutons of the Antwerp-Rossie suite are located primarily within the northwest portion of the lower marble belt, while the Hermon granitic gneiss extensively intrudes the Popple Hill Gneiss belt, also showing a strongly linear outcrop pattern, consistent with emplacement as intrusive sheets (Fig. 3). Rocks of the Hyde School Gneiss suite are found across the entire region, and correlative felsic plutons of the Rockport Granite are found west of the Black Lake shear zone and across the Frontenac terrane (Marcantonio et al., 1990).

The rocks in the Lowlands have undergone extensive deformation and metamorphism to upper amphibolite facies resulting in partial melting and zircon growth in pelitic gneisses (Heumann et al., 2006). DeLorraine and Sangster (1997) note that rocks in the Balmat Mine have undergone four phases of deformation. Early structures include intrafolial, isoclinal folds that deform compositional layering interpreted as bedding where it is concordant to major lithologic units. These isoclines, and the prominent axial planar foliation formed in schistose lithologies, are refolded about isoclines whose axial traces trend NE and form complex recumbent folds. Later upright, tight to open, NNE and NW folds refold earlier structures resulting in fold interference patterns.

Structural cross sections suggest steepening and tightening of large-scale Phase 2a antiforms and synforms to the southeast and continuing deformation (Phase 2b) resulting in porpoising hinges and sheath folds (deLorraine and Sangster, 1997). The overall structure is consistent with a SE-verging fold-and-thrust belt. Some authors have suggested the occurrence of NE-trending structural panels bounded by shear zones and major lineaments such as the Black Lake shear zone. However, deLorraine and Sangster (1997) conclude that major stratigraphic units cross such boundaries arguing against terrane boundaries or considerable displacement. The presence of southeast-vergent thrust sheets has been established based on the structural discontinuities, the presence of mylonitic shear zones, and the excision of stratigraphic units (deLorraine and Sangster, 1997; Hudson and Dahl, 1998). For example, the upper and lower marbles are in structural contact near Edwards, and the entire Popple Hill Gneiss section has been excised by the Elm Creek thrust (Elm Creek slide of deLorraine and Sangster, 1997). While the regional stratigraphy has been firmly established, it is disrupted and crosscut by major structures and plutonic rocks in many areas and best considered a tectonostratigraphy on the local level, unless evidence to the contrary is provided, such as in the Sylvia Lake syncline (deLorraine and Sangster, 1997).

The overall picture is one of a more or less coherent stratigraphic sequence from the Frontenac terrane across the Lowlands which has been folded and thrusted, and intruded by a diverse set of preorogenic, synorogenic, and postorogenic plutonic rocks. The belt-like, NE-trending occurrence of many of the lithologies is likely a function of their original distribution and subsequent shortening, largely perpendicular to their strike. Large-scale folds form a series of upright to inclined antiformal and synformal structures, such as the Sylvia Lake syncline. Interspersed shear zones and ductile thrusts disrupt the stratigraphy. The significance of orogen-parallel structures in the region has yet to be fully evaluated in the region, although oblique motion on the Carthage-Colton mylonite zone and other shear zones is likely (Johnson et al., 2004; Baird, 2006; Reitz and Valentino, 2006).

At Pyrites, New York (Fig. 3), metamorphosed ultramafic-mafic rocks (Pyrites Complex) are in structural contact with rusty, graphite-bearing, pyritic gneisses (Fig. 4). The ultramafic lithologies occur as a large, compositional layered xenolith or core within more extensive and deformed belt of metagabbroic and amphibolitic rocks. Primary compositional layering (decimeter to meter scale) in the peridotite is discordant to the regional and local foliation trends and lithologic contacts (Fig. 5). A large (2-m-wide), undeformed, and coarse-grained (2–3 cm) phlogopite lamprophyre cuts the peridotite and its compositional layering. Layers within the peridotite include coarse (1–3 cm), knobby weathered pyroxenite (Fig. 6) and altered, massive to weakly layered peridotite. In all, ∼1 km² of mafic and ultramafic rock is exposed (Fig. 7). While the aerial extent of the ultramafic complex is modest, the largest gravity values in St. Lawrence County extend from the outcrop of ultramafic rocks near Pyrites for ∼10 km to the southwest (Fig. 8), suggesting that a considerable volume of dense rock occurs at depth (Revetta and McDermott, 2003). In addition, narrow bands of highly tectonized metagabbro, amphibolite, and hornblende schist can be traced from this central block both NE and SW for several for ∼15 km. To the west, the amphibolites are associated with rusty pyritic gneisses mined extensively for sulfur at the Stellaville mine from 1883 to 1921 (Prucha, 1957). Together these rocks occur as a folded layer and/or thrust sheet(s) within the lower marble (Rickard et al., 1970) and therefore must predate Shawinigan deformation in the Lowlands (ca. 1160–1200 Ma).

Samples were collected from exposures of ultramafic and mafic rocks along the Grasse River in Pyrites ∼15 km northwest of the trace of the Carthage-Colton mylonite zone. The ultramafic rocks range in color from pale green to greenish black and display extensive development of magnesium-rich, hydrous, secondary minerals. For the peridotite and pyroxenite these include mixtures of tremolite, talc, serpentine, chlorite, and dolomite (Lupulescu et al., 2008) and are consistent with loss on ignition values that average ∼8 wt%. The lamprophyre contains large phenocrysts of phlogopite, apatite, and secondary(?) annite, in addition to other hydrous magnesium silicates. Of the pseudomorphically replaced primary minerals (CIPW-normative olivine, orthopyroxene and clinopyroxene, chromite, and calcic plagioclase) only chromite and limited amounts of augite are preserved. On the International Union of the Geological Sciences (IUGS) ultramafic plutonic rock classification diagram, using CIPW norms (Streckeisen, 1973), they plot mostly as lherzolite but show a range in composition, some of which may be due to alteration (Fig. 9). The large relic grain size, textures, and decimeter-scale layering confirm an original variation in composition. Accessory minerals include magnetite, ilmenite, chromite, apatite, and trace amounts of titanite, zircon, and rutile. Geochemically the rocks range from 33.74 (phlogopite lamprophyre) to 52.09 (metagabbro) wt% SiO₂, with ultramafic cumulate rocks containing 25–33 wt% MgO and up to 5440 ppm Cr and 1588 ppm Ni (Table 1). In addition, the rocks are characterized by low (<1.0 wt%) TiO₂.

Routine petrographic and scanning electron microscopy (SEM) investigation revealed the occurrence of small (50–200 micron) zircon crystals in some of the ultramafic rock (Figs. 10 and 11). Zircon grains were separated by standard techniques and purified by handpicking prior to mounting in epoxy for cathodoluminescence (CL), sensitive high-resolution ion microprobe (SHRIMP), and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analysis. Sensitive high-resolution ion microprobe analyses were carried out at the John De Laeter Center for Isotope Research in Perth, Australia, and procedures followed the methods of Nelson (1997). Laser ablation ICP-MS analyses were carried out at the Arizona Laserchron Center in Tucson, Arizona, and methods are published on the web (http://www.geo.arizona.edu/alc). Geochemical analyses were carried out by standard X-ray fluorescence (XRF), inductively coupled plasma–optical emission spectroscopy (ICP-OES) and ICP-MS techniques at GeoAnalytical Laboratory, Washington State University at Pullman and ACME Analytical Laboratories in Vancouver, British Columbia. Strontium and neodymium isotopes were measured at the Isotope Geology and Geochronology Research Facility at Carleton University, Ottawa (Cousens, 1996; Cousens et al., 2008). Carbon and oxygen isotopes from thin carbonate veins were analyzed at the Department of Atmospheric Sciences, State University of New York at Albany.

Small (50–200 μm) zircon crystals were separated from a peridotite sample containing 41.59 wt% SiO₂ and 32.07 wt% MgO. The zircon grains have anhedral to faceted shapes and display fine and sector zoning in CL (Fig. 11). No evidence of cores or metamorphic rims was visible. The grains averaged 408 ± 91 ppm U and have Th/U ratios of 0.27 ± 0.03 (Table 2). Sensitive high-resolution ion microprobe (SHRIMP) II analyses (Fig. 12; Table 2) yielded two age groups—1202 ± 20 Ma (2σ, n = 2) and 1140 ± 7 Ma (2σ, n = 20). Zircon of similar morphology and size discovered embedded in serpentine, replacing a knobby weathering pyroxenite (38.48 wt. % SiO₂ and 33.33 wt. % MgO), yielded a U-Pb LA-ICP-MS age of 1197 ± 5 Ma (2σ, n = 43; Fig. 13; Table 3). The zircons averaged 460 ± 296 ppm U and have Th/U ratios of 0.55 ± 0.33. Four grains were discordant and are not shown on Figure 13; a fifth grain yields a nearly concordant age of 558 ± 38 Ma.

Samples of mafic and ultramafic rocks, all collected along the banks of the Grasse River in Pyrites, were analyzed for Rb-Sr and Sm-Nd isotopic systematics at the Department of Earth Sciences, Carleton University (Fig. 14; Tables 4 and 5). Neodymium T_DM ages have been calculated from seven samples of mafic-ultramafic rocks and range between 1450 and 2620 Ma. Calcite samples from three thin (<5 mm) and undeformed carbonate veins cutting the peridotite and hosting small metamorphic zircons were analyzed for carbon and oxygen isotopic composition at the Department of Atmospheric Sciences, State University of New York at Albany. They yield δ¹³C^PDB values between –6.975 and –7.358 and δ¹⁸O^SMOW values between 15.629 and 15.668 (Table 6; ⁸⁷Sr/⁸⁶Sr = 0.708891–0.709748).

Numerous amphibolitic to metagabbroic rocks occur throughout the Lowlands primarily in elongate NE-trending belts (Fig. 3). While amphibolitic lithologies dominate, hornblendic schists, hornblendite, and coarser-grained metagabbroic variants also occur. These belts are structurally complex, highly disrupted and intruded by granitic to tonalitic rocks of the Antwerp-Rossie, Hermon granitic gneiss, Hyde School Gneiss, and AMCG suites. These crosscutting relationships indicate they are at least 1200 Ma old, but because of their tectonized contacts with surrounding supracrustal units, there are few constraints on their maximum age. However, they may well be the oldest rocks in the Lowlands. A wide variety of other mafic rocks, primarily occurring as small plutons or dikes, has been discussed by Carl (2000). Many of these occur as concordant layers in the Popple Hill Gneiss (up to 100 m wide) and the Hyde School Gneiss. Some, such as the Balmat gabbro, intrude the sphalerite-galena–bearing rocks in upper marble sequence at Balmat. Thus field relations indicate mafic rocks are both older, and younger, than the supracrustal sequence in the Lowlands.

A dozen samples of mafic rocks in the Lowlands were sampled for geochemistry and Sm-Nd isotopic studies (Coffin, 2008). These included a variety of amphibolites and metagabbros. One metagabbro sample, the Dana Hill metagabbro (Johnson et al., 2004), was taken just east of the Carthage-Colton shear zone in the Highlands. Three samples were taken in the Balmat area, while two were taken from sequences that included pyritic gneisses at Stellaville and Antwerp. A sample was also collected from clinopyroxene-bearing, thinly banded, amphibolite at Seven Springs, near Colton, New York. Likely a metasedimentary or volcaniclastic rock, it is intruded by a coarse-grained, K-spar augen, Hermon-type, granitic gneiss, and it structurally underlies carbonate rocks of the upper marble. Two samples were collected from the Pierrepont sigmoidal structure, a large, thick, S-shaped belt of amphibolite near the Carthage-Colton shear zone (Fig. 3). The geochemistry and location of these samples are given in Table 7. These samples of amphibolitic-metagabbroic rocks from throughout the Adirondack Lowlands and the Dana Hill metagabbro yield Nd T_DM model ages that range between 1740 and 1170 Ma.

Ultramafic rocks are found in several different settings. We suggest that the ultramafic rocks described here are most likely to have been tectonically derived from the mantle beneath or adjacent to a collapsing backarc basin. Their geological association (metabasalts and gabbros, marine metasedimentary rocks, sulfide-rich ores, and metachert-pelitic gneisses), structural emplacement rather than intrusive relationships, extensive hydrothermal alteration, enriched geochemistry and Nd systematics, and association with crosscutting lamprophyre rule out an intrusive origin. The possibility exists that they were juxtaposed with high-grade gneisses after metamorphism because they retain their hydrous nature and low-medium grade mineral assemblages; however, it is also plausible that they represent a core region that was never dehydrated despite metamorphism.

The Pyrites Complex contains a variety of ultramafic rocks, presumably of mantle affinity (Fig. 9), as well as, lamprophyre dikes. However, spidergrams of rare-earth element (REE) patterns and incompatible elements from both ultramafic and mafic samples display strong enrichment in REE and large-ion lithophile elements (LILE), and relative depletion in high field strength elements (HSFE) such as Nb, Ta, Ti, and Zr when compared to chondrites and primitive mantle (Fig. 15). Patterns are strikingly similar to those of ultrapotassic rocks derived from metasomatized mantle spanning the Snowbird tectonic zone in the Churchill Province of subarctic Canada (Cousens et al.,2001; Chiarenzelli et al., 2010a). Disturbance of both Sm-Nd and Rb-Sr isotopic systems is shown by errorchrons that yield large errors, even when the most distributed and noncollinear samples are excluded (Fig. 14). Other indicators of isotopic alteration include an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.703226 and Nd T_DM model ages (Tables 4 and 5) more than a billion years older than the age of the rocks. These characteristics are consistent with derivation from hydrous, metasomatized mantle (Peck et al., 2004; Chiarenzelli et al., 2010a) and preclude derivation from typical depleted mantle reservoirs.

The amphibolites and metagabbro samples from across the Lowlands have similar geochemical trends as the Pyrites Complex but vary in detail (Fig. 15). In general, they have flatter and higher REE concentrations than the ultramafic rocks and lack a pronounced Ti anomaly. On the Nd evolution diagram (Fig. 16), they and the Pyrites ultramafic rocks display a relatively steep evolutionary path for mafic and ultramafic rocks, which typically yield poorly constrained model ages because of their low slope (Dickin and McNutt, 2007). However, several anomalies occur. In particular, samples of sheared metagabbroic and phlogopite lamprophyre from the Pyrites Complex have low slopes and yield Nd T_DM (1300 Ma) mantle separation ages of 2260 and 2620 Ma, unrealistically older than their possible age range and that of cogenetic samples. Both of these samples have abundant phlogopite, which may have preferentially incorporated Nd.

The Pyrites Complex and Lowlands amphibolite belts form boudins and disrupted belts largely within carbonate rocks of the lower marble. At Pyrites ultramafic rocks are in contact with graphite-bearing, pyritic gneisses interpreted as deep-water, anoxic, and in part, chemogenic muds (Chiarenzelli et al., 2007). They contain a variety of accessory minerals including chromite, sphalerite, pyrrhotite, chalcopyrite, molybdenite, galena, stannite, and V-bearing rutile (Tiedt and Kelson, 2008). The origin of the chromite in the pyritic gneisses may have been, in part, derived from the erosion or slumping of ultramafic rocks along fractures or transform faults or through structural interleaving.

Field investigation reveals that a wide range of metasedimentary lithologies occurs within the pyritic gneisses at Pyrites including pelitic schists and biotite ± garnet ± sillimanite quartzofeldspathic gneisses, pyritic breccias (ore zone), and thin (1–2 cm), fine-grained positively weathering siliceous layers interlayered within garnet-biotite-sillimanite gneisses. Geochemical analyses (Table 8) indicate the pelitic layers in the Popple Hill Gneiss and at Pyrites have compositions similar to that of typical shales, while resistant layers contain up to 80% or more SiO₂. The ore and some pelitic gneisses from Pyrites contain elevated levels of As, Cu, Ni, V, and Zn, among other metals. One sample contained 1.95 wt% MnO. These results suggest the pyritic gneiss sequence includes components of detrital, as well as chemogenic origin, including quartzite units which are interpreted as thin chert layers and metal-rich, hydrothermal, pyritic ore. The occurrence of abundant graphite suggests the depositional environment allowed the preservation of organic matter and was likely reducing. Collectively, the rocks at Pyrites and amphibolitic belts throughout the Lowlands represent a highly dismembered, metamorphosed, and incomplete ophiolitic sequence obducted during Shawinigan orogenesis.

Igneous zircon is an unusual primary phase in ultramafic rocks, and the lack of oscillatory zoning suggests the zircons are of metamorphic origin. Therefore the ages obtained (1140 ± 7 Ma; ca. 1200 Ma–1202 ± 20 and 1197 ± 5) are best interpreted as the timing of metamorphic zircon growth in rocks of the Pyrites Complex. The 1140 ± 7 Ma age postdates intrusion of rocks of the AMCG suite in the Lowlands, but is in good agreement with the U-Pb ages obtained on monazites separated from these rocks (McLelland et al., 1993), presumably cooling to blocking temperatures within tens of millions of years of the AMCG thermal pulse. Carbon and oxygen isotopes from the undeformed calcite veins that hosted some of the zircons recovered from the peridotite sample (1140 ± 7 Ma) plot within the Adirondack retrograde calcite field of Morrison and Valley (1988), suggesting the veins themselves are also post-Shawinigan and AMCG intrusion. The ca. 1200 Ma (1202 ± 20 and 1197 ± 5 Ma) age provides a minimum age for the ultramafic rocks and corresponds to the timing of calc-alkaline intrusion (Antwerp-Rossie suite; ca. 1200 Ma; Wasteneys et al., 1999; Chiarenzelli et al., 2010b) and initiation of Shawinigan dynamothermal events in the Adirondack Lowlands. The probable age of the associated ultramafic-mafic rocks is thought to be ca. 1300, in concert with estimates of the timing of rifting in the Central Metasedimentary Belt (Hanmer et al., 2000; Dickin and McNutt, 2007). The slope of these rocks on a Sm-Nd isochron falls between that of the juvenile crust in the Ontario Central Metasedimentary Belt (1280 Ma; Dickin and McNutt, 2007) and Quebecia (1530 Ma; Dickin and Higgins, 1992) and yields an imprecise age of 1440 ± 170 Ma (Chiarenzelli et al., 2010a).

A study of Nd isotopic systematics, geochemistry, and field relations of amphibolites and metagabbros suggests at least three distinct suites of mafic rocks occur in the Lowlands. They include: (1) highly enriched and metamorphosed ultramafic-mafic rocks of the Pyrites Complex, in structural contact with Lowlands supracrustals including pyritic gneisses, with Nd T_DM(1300) ages of 1445–1611 Ma and ε_Nd of 4.3–5.1 (below that of the depleted mantle curve of De Paolo, 1981); (2) amphibolitic rocks exposed in the Pierrepont amphibolite belt that intrude the Popple Hill Gneiss, have mid-ocean ridge basalt (MORB) to arc tholeiite chemistry, yield shallow Nd evolution pathways that have imprecise Nd model ages (shallow slope), and ε_Nd values of 5.4–6.8 that lie on the depleted mantle curve; and (3) amphibolites and metagabbros of calc-alkaline chemistry intrusive into the upper marble near Balmat, with restricted Nd T_DM(1210) ages of (1366–1393 Ma), relatively steep Nd evolution curves, and ε_Nd of 4.6–4.7 (Fig. 16).

As suggested above, the Pyrites Complex is believed to represent tectonic slivers of the upper mantle, oceanic crust, and overlying sediment that once floored the backarc basin developed between the Adirondack Lowlands and southern Highlands named the Trans-Adirondack backarc basin (Chiarenzelli et al., 2009, 2010b; McLelland et al., 2010). Lamprophyre dikes, which cut the ultramafic rocks, imply intrusion in an extensional tectonic regime. The amphibolitic rocks also have geochemistry characteristics compatible with melting of an enriched mantle wedge (Chiarenzelli et al., 2010a).

The amphibolitic rocks exposed at Pierrepont and intrusive sheets (sills) of amphibolite in the Popple Hill Gneiss are the least enriched in incompatible elements, a hallmark of a depleted mantle source (Figs. 15 and 16); however, they have a positive U and Pb anomaly and a large negative Nb anomaly, perhaps indicating a hybrid source. Likely they represent tapping of melts of asthenosphere centered under the Adirondack backarc during foredeep magmatism or the initial stages of subduction and convergence. Numerous amphibolitic dikes are also known from the Newcomb and Paradox Lake belt in the Adirondack Highlands where they extensively intrude marbles similar to those underlying much of the Lowlands and may be of the same age and origin. Metagabbro and metadiorites with calc-alkaline chemistry from the Balmat area are likely correlative with the Antwerp Rossie suite and related to subduction, closure, and collapse of the Trans-Adirondack backarc basin (Carl, 2000; Chiarenzelli et al., 2010b)

Three samples of amphibolites from the Pierrepont amphibolite belt yield nearly horizontal slopes and a wide range of Nd T_DM (1300 Ma) model ages (1160, 1470, and 1730 Ma) and ε_Nd values that fall on the depleted mantle evolution curve of De Paolo (1981). Given the low slope of each of the samples, the model ages are likely inaccurate. In particular, the 1160 Ma model age for the cm-scale, layered, clinopyroxene-bearing amphibolite sample from Seven Springs is not geologically reasonable, because it is crosscut by the Hermon granitic gneiss (ca. 1182 Ma). The difference in the slope of the three samples from Pierrepont and those of Pyrites Complex is in agreement with the observations of Tyler (1979), who found substantial variations in major element chemistry between the mafic rocks at Pyrites (Mg rich) and those in the Pierrepont amphibolite belt. The Pierrepont amphibolites are intrusive into the Lowlands supracrustal sequence, particularly garnetiferous pelitic gneisses of the Popple Hill Gneiss, whereas rocks of the Pyrites Complex are in structural contact with the supracrustal rocks and are clearly older than the Pierrepont amphibolites.

Aside from the aforementioned sample from the Pierrepont amphibolite belt with an unrealistically low model age, three samples from the Balmat area have Nd T_DM model ages that are very consistent and the youngest amongst the group of samples (1370, 1390, and 1390 Ma) and have similar ε_Nd values (4.5–4.7). On the Nd evolution diagram these samples lie along parallel pathways just slightly to the left of the main group containing the Pyrites Complex and bulk of the Lowlands mafic rocks. Because at least one of these samples (Balmat Gabbro) is known to intrude the metasedimentary sequence of the upper marble, they are considered to be mafic equivalents of the Antwerp-Rossie suite (Chiarenzelli et al., 2010b) and likely to be ca. 1210 Ma in age. Therefore, these model ages were calculated at 1210 Ma and indicate that an origin from typical depleted mantle reservoir is unlikely. The Antwerp-Rossie suite displays a wide range of silica concentrations, and recent work (Chiarenzelli et al., 2010b; Regan, 2010) has documented a bimodal composition, with early mafic plutons followed by those of granitic composition.

Figure 15 shows the incompatible element chemistry of each of the ultramafic and mafic suites identified in this paper normalized to primitive mantle values of Sun and McDonough (1989). The range of values from ca. 1160 to 1150 Ma coronitic gabbros of the Adirondack Highlands has been overlain (Regan et al., 2011) for comparison purposes. Coronitic metagabbros in the Adirondacks are spatial and temporally associated with anorthosite massifs and have been considered by some workers as the unfractionated parental magma of the anorthosite massifs. In each case, the older rocks of the Lowlands have many incompatible elements with abundances an order or magnitude greater than those of the coronites. However, aside from enrichment in U and Pb, the mafic rocks from the Pierrepont amphibolite belt have lower concentrations, but similar incompatible element patterns, to the coronites. Samples from Balmat mafic rocks and the Adirondack Lowlands amphibolite belt show incompatible element patterns and Nd evolution pathways similar to the Pyrites Complex suggesting they were derived by partial melting of mantle material of a similar enriched nature.

The Adirondack Lowlands contain an assemblage of rocks that includes metamorphosed siliceous carbonates (in part stromatolitic) and evaporates (upper marble), pelitic, psammitic, and possible volcanoclastic rocks (Popple Hill Gneiss), shallow-water carbonates, metasandstones, and tourmaline-bearing sandstones (lower marble), chemogenic and metalliferous muds, oceanic crust (amphibolites), and ultramafic rocks (mantle). The most likely way to explain this diverse assemblage is tectonic interleaving and the obduction of oceanic crust and upper mantle along the soles of southeast-verging ductile thrusts during Shawinigan orogenesis. The interleaved metasedimentary rocks represent both rift-drift sedimentation (ca. 1300–1250 Ma) and the development and fill of a foreland basin formed during convergence (ca. 1250–1210 Ma). This tectonostratigraphic sequence was intruded by calc-alkaline to transitional mafic to felsic rocks of the Antwerp-Rossie (ca. 1200 Ma), Hermon granitic gneiss (ca. 1182 Ma), Rockport Granite and Hyde School Gneiss suites (ca. 1172 Ma), and members of the AMCG suite (ca. 1155 Ma). The accompanying deformation is largely responsible for the development of the structural grain and major structures observed in the Lowlands. Collectively the lithologies exposed represent the opening and closure of a backarc basin that existed between the Frontenac terrane and southern Adirondacks between ∼1300 and 1200 million years ago (Fig. 17).

Dickin and McNutt (2007) provide Nd isotopic evidence that a backarc rift basin developed underneath most of the Central Metasedimentary Belt at ∼1300 million years ago. We propose that a similar, and perhaps synchronous, basin developed between the Frontenac terrane and southern Adirondacks when 1350–1300 Ma tonalitic gneisses of the Dysart–Mount Holly arc rifted away from the margin of Laurentia (McLelland et al., 2010). This basin, named the Trans-Adirondack backarc basin (Chiarenzelli et al., 2009), was floored by oceanic crust, remnants of which are preserved in the Lowlands as disrupted amphibolite belts of the Pyrites Complex. In essence the entire southeast Grenville Province underwent mild extension resulting in a series of isolated rift basins in which much of the area was apparently at or near sea level for extended periods, allowing the accumulation of thick sequences of marbles and quartzites (Hanmer et al., 2000; Dickin and McNutt, 2007).

As no basement rocks have been found, supracrustal rocks in the Lowlands are allochthonous. The oldest U-Pb zircon ages come from calc-alkaline plutonic rocks of the Antwerp-Rossie suite (ca. 1200 Ma). Thin “quartzite” metasandstone units underlie much of the area to the north and west of the Black Lake shear zone in the Frontenac terrane and may well form part of the basal “rift” sequence. However, they are not thick, lack conglomeratic facies, and, in many cases, are more mature than typical rift arkoses. Tourmaline-bearing K-feldspar gneisses are known from the lower marble and may be rift-related, restricted basin arkoses (Brown and Ayuso, 1985). However, this indicates that relatively little of the original rift sequence that led to the development of the Trans-Adirondack Basin is apparently preserved in the Lowlands at the present erosion level. Alternatively, intensive tropical weathering and reworking may have enhanced the maturity of these rocks and/or relatively unique conditions and limited topographic relief occurred during rifting.

Detrital zircons from quartzites (metasandstones) in the Frontenac terrane contain zircons as young as 1301 ± 13 Ma (Sager-Kinsman and Parrish, 1993) and provide a maximum age for their deposition. Here we assume that they either stratigraphically underlie, or are time equivalents of, the lower marble. The lower marble in the Lowlands consists of a wide variety of lithologies including dolomitic marbles, calc-silicate rocks, quartzites, and tourmaline-bearing quartzofeldspathic gneisses and tourmalinites. Some of these lithologies may be evaporitic in origin and suggest restricted circulation during the initial development of the Trans-Adirondack Basin. As the basin expanded, oceanic crust developed and began to spread, and extensive carbonate deposition occurred on the flanks of the basin. The lower marble was recently dated by Lu-Hf techniques and yielded an age of 1274 ± 9 Ma (Barfod et al., 2005) and is consistent with deposition of the lower marble on the shelf during the drift phase of the Trans-Adirondack Basin.

The Elzevirian orogeny (1245–1220 Ma) resulted in deformation and metamorphism associated with the closure of the backarc basin in the Central Metasedimentary Belt (Dickin and McNutt, 2007; Rivers, 2008) and the likely emplacement of the Queensborough ophiolite (Smith and Harris, 1996). In addition, calc-alkaline plutonic rocks of this age are known from the Elzevir terrane and the southern Adirondacks, suggesting much, if not all, of the southern eastern Grenville Province was undergoing compression and subduction at this time. It is therefore likely that the Trans-Adirondack Basin began to collapse during, or shortly after this time.

The Popple Hill Gneiss has yielded detrital zircons as young as 1220 Ma and anatectic zircons from 1180 to 1160 million years in age (Heumann et al., 2006; Bickford et al., 2008) indicating that it was deposited after the Elzevirian orogeny but before Shawinigan orogenesis. In addition, it is cut by plutons of the Antwerp-Rossie suite (ca. 1200 Ma), Hermon granitic gneiss (ca. 1182 Ma), and Hyde School Gneiss (ca. 1172 Ma). These constraints require that it was likely deposited during closing phase of the Trans-Adirondack Basin. If so, it may represent flysch and/or molasse deposition within a foredeep during active tectonism.

After deposition of the lower marble, the Trans-Adirondack Basin began to deepen and accumulate clastic detritus, some turbiditic, and perhaps including a metavolcanic component (Carl, 1988). The production of a linear foredeep is envisioned which was filled by the clastic detritus of the Popple Hill Gneiss, analogous the Ordovician Taconic foredeep of eastern New York and Vermont containing thick sequences of shale, sandstone, and thin ash layers, eventually giving way to the deposition of Silurian evaporates in a restricted basin. Given the time constraints (deposition of the Popple Hill Gneiss after ca. 1220 Ma), the foredeep may well have begun forming during Elzevirian orogenesis (1245–1220 Ma). Because the Popple Hill Gneiss lacks Archean detrital zircons, sediment was likely derived from the southeast and the basin separated from the Superior Province (Bickford et al., 2008). Eventual filling and/or contraction of the basin and restriction from open ocean circulation led to the deposition of the siliceous carbonate-evaporite sequence of the upper marble (deLorraine and Sangster, 1997). Deposition of the upper marble was punctuated by the pulses of sedimentary exhalative massive sulfide (SEDEX) deposition of zinc and lead sulfides derived from metalliferous hydrothermal fluids driven by convergence occurring external to, or beneath, the basin.

Supracrustal rocks of similar age, including highly aluminous metapelites and calc-silicates with minor quartzites and marble, are found in the Shawinigan domain of southwestern Quebec (Martignole, 1978; Corrigan and van Breeman, 1997). A sample of a quartzite layer from the St. Boniface metasedimentary unit contained a variety of detrital zircon populations, the youngest of which yielded a maximum age of 1176 ± 18 Ma. If these supracrustal rocks were deposited within the same backarc basin as supracrustal rocks of the Adirondacks, sedimentation in the northern extension of the basin and its closure outlasted that in the Adirondacks, ∼300 km to the S-SE, and attest to asynchronous tectonism across large areas.

Similar supracrustal rocks likely formed on opposing flanks of the basin and record the transition from continued spreading to eventual convergence. Supracrustal rocks similar to those of the metasedimentary sequence in the Adirondack Lowlands are found in Adirondack Highlands. In addition, the Irving Pond quartzite (Peck et al., 2010), with a maximum depositional age of 1300 Ma and overlying 1350–1300 Ma tonalitic gneisses of the southern Adirondacks, may represent coarse clastic deposition, fringing this terrane, which formed during the rift and drifted away from the remainder of Laurentia as the Trans-Adirondack Basin developed. Other rocks such as the Popple Hill Gneiss and upper marble have depositional ages (maximum age of ca. 1220 Ma; Heumann et al., 2006; Bickford et al., 2008) consistent with deposition during or just before Shawinigan contraction, perhaps in restricted basins whose boundaries were imposed by distal structural or tectonic controls of accommodation space.

Mafic dikes and sills, represented by the mafic rocks of the Pierrepont amphibolite belt, of mid-oceanic ridge chemistry and depleted mantle Nd signature were injected into the supracrustal sequence as subduction-derived, mafic, calc-alkaline melts were produced during the initial stages of convergence. These were followed closely in time by felsic calc-alkaline melts of the Antwerp-Rossie and later igneous suites with transitional geochemical characteristics including the Hermon, Hyde School–Rockport, and AMCG suites. This magmatism may have been channeled by temporal or spatial variations in deformation as the Antwerp-Rossie suite primarily intrudes the lower marble, the Hermon suite, the Popple Hill Gneiss, and the Hyde School–Rockport suite intrudes across the entire Lowlands-Frontenac region.

A fundamental question in the Adirondack segment of the Grenville orogen is the significance of the Carthage-Colton shear zone. While the late kinematic history of the shear zone is well constrained (Selleck et al., 2005), its early history is less well understood. Kinematic indicators have been used to suggest both strike-slip (Johnson et al., 2004) and reverse motion (Baird, 2006; Wiener et al., 1983). Similarities between supracrustal rocks on both sides of the Carthage-Colton shear zone have led many authors to suggest, or imply, they are part of the same or similar stratigraphic sequence (McLelland and Isashsen, 1986; Wiener, 1983; Heumann et al., 2006; Bickford et al., 2008). Geophysical data (seismic reflection and refraction) have found the upper crust to be homoeneous across the Carthage-Colton shear zone restricting its depth to the upper 2–3 km (Hughes and Luetgert, 1992). These studies confirm that the Carthage-Colton shear zone is not a suture (cf. Mezger et al., 1992), a conclusion also reached by Hanmer et al. (2000) using various lines of geological evidence. We contend that the Carthage-Colton shear zone is the sole of a major Shawinigan ductile fold-and-thrust system reactivated during orogenic collapse (Selleck et al., 2005) and that the differences across it stem primarily from its post-Shawinigan history and erosion level. The incorporation of mantle rocks and oceanic crust in the Adirondack Lowlands metasedimentary sequence requires opening of a basin, the generation of oceanic crust, and temporary separation of the Lowlands and Highlands between ca. 1300–1220 Ma, and eventual redocking of the outboard terranes in the southern Adirondack Highlands resulting in Shawinigan orogenesis. Note that the Dana Hill metagabbro analyzed in this study occurs on the Highlands side of the Carthage Colton mylonite zone and shows similarities to the Pyrites Complex. Other similar ultramafic-mafic bodies have not yet been identified in the Highlands.

Magmatic rocks of the Hermon granitic gneiss (ca. 1182 Ma), Rockport Granite, and Hyde School Gneiss (ca. 1172 Ma) have field relations that indicate intrusion during Shawinian orogenesis and may have, on a regional scale, led to anatectic melting of the Popple Hill Gneiss (1180–1160 Ma). In addition, their geochemical characteristics are transitional between those of the calc-alkaline Antwerp-Rossie suite and later A-type magmatism of the AMCG suite (1165–1150 Ma) as previously noted by Carl and deLorraine (1997). This suggests that the timing of melts generated by the subduction of oceanic crust of the Trans-Adirondack Basin were followed closely, and/or partially overlapped, by those of the AMCG. Rocks of the AMCG suite have been previously interpreted as the result of lithospheric delamination at end of Shawinigan orogenesis.

(1) The Pyrites Complex is part of an incomplete, highly deformed and metamorphosed ophiolite obducted during Shawinigan orogenesis between ca. 1200 and 1160 Ma. It contains two populations of small, metamorphic zircons that grew during the initiation of Shawinigan orogenesis (ca. 1200 Ma) and after intrusion of the AMCG suite in the Lowlands and during cooling following its thermal pulse (ca. 1140 Ma).

(2) The Pyrites Complex consists of altered and metamorphosed, upper mantle, ultramafic rocks, oceanic crust (amphibolites and metagabbros), and overlying, and in part chemogenic, metasedimentary rocks (pyritic pelitic gneisses and thin quartzite or chert layers). Evidence of sheeted dikes and pillow lavas is lacking because they were likely obscured during deformation and upper amphibolite facies metamorphism.

(3) The Pyrites Complex originated as part of a backarc basin (Trans-Adirondack backarc basin), which once separated the Frontenac terrane from the southern Adirondacks and opened synchronously with the backarc basin–failed rift in the Central Metasedimentary Belt delineated by Nd isotopes (Dickin and McNutt, 2007). Broad extension and deposition of carbonate sediments across the Central Metasedimentary Belt at this time is indicated.

(4) The Pyrites Complex is interleaved with metamorphosed sedimentary rocks that formed on the flanks of this basin including the relatively well-preserved metasedimentary sequence in the Adirondack Lowlands. Similar, but less well-preserved metasedimentary rocks, are extensively intruded and disrupted in the Highlands (Chiarenzelli et al., 2011).

(5) Based on available geochronological constraints, the metasedimentary sequence can be subdivided into rocks deposited during the rift-drift (1300–1250 Ma) and basin closure (1250–1210 Ma) phase of the Trans-Adirondack Basin. This was followed by subduction, arc to transitional magmatism, and Shawinigan collisional orogenesis from 1200 to 1160 Ma, ophiolite obduction, and eventual lithospheric delamination and intrusion of the AMCG suite.

(6) The rift sequence, consisting primarily of thin metasandstones (quartzites, maximum age of 1301 Ma), is either poorly preserved, erosionally excised, poorly exposed, or atypical in its lack of coarse clastic and chemically immature, arkosic detritus. Tourmaline-bearing, quartzofeldspathic gneisses are known from the Lowlands, may well be metamorphosed arkosic sandstones, and are candidates for further investigation. The lack of basement rocks in the Lowlands suggests the supracrustal rocks may well be allochthonous. The drift sequence is dominated by thick carbonates of lower marble (ca. 1274 Ma).

(7) The basin closure sequence consists of clastic detritus of the Popple Hill Gneiss (1240–1220 Ma). The Popple Hill Gneiss represents the formation and initial molassoid and flysch fill of a foredeep basin. Filling of the basin and/or continued convergence led to the cessation of clastic deposition and renewed deposition of siliceous marbles and evaporates of the upper marble (ca. 1220–1210 Ma). These rocks, particularly the Popple Hill Gneiss, are extensively intruded mafic sills of MORB to arc tholeiitic chemistry (Carl, 2000). The overall sequence shows many similarities to the Ordovician to Silurian Taconic foreland basin exposed in central New York.

(8) Carbonate deposition of the upper marble was punctuated by the episodic deposition of metalliferous brines resulting in the stratiform sphalerite-pyrite-galena SEDEX deposits. Deposition of the SEDEX horizons was preceded by the deposition of evaporitic rocks. The sequence reflects basin compression, uplift, driving of hydrothermal fluids, and episodic restriction from open ocean circulation. In contrast to the widespread occurrence of rocks likely correlative to the lower marble and Popple Hill Gneiss throughout the Adirondack Region, rocks of the upper marble and associated Pb-Zn deposits are known only in the Lowlands and suggest deposition in a restricted and aerially limited basin or erosional removal.

(9) The Pyrites Complex and metasedimentary rocks of the Adirondack Lowlands are intruded by a wide variety of metaigneous rocks that record the transition from arc to AMCG magmatism. Magmatic rocks of the Antwerp-Rossie suite include early mafic, calc-alkaline tholeiites (ca. 1210 Ma), as well as, slightly younger (ca. 1200 Ma) calc-alkaline tonalites and granites. Subsequent suites including the Hermon granitic gneiss (ca. 1182 Ma), Rockport Granite, and Hyde School Gneiss (ca. 1172 Ma) have transitional geochemical features suggesting the mixing of subduction melts and AMCG magmatism (ca. 1155 Ma), triggered by slab breakoff and lithospheric delamination.

(10) Field relations, geochemistry, and neodymium isotopes can be used to distinguish three suites of mafic rocks in the Lowlands including the Pyrites Complex, MORB-like basalts of the Pierrepont amphibolite belt, and mafic members of the Antwerp-Rossie suite, including coarse-grained gabbros and diorites. They represent different stages in the tectonic history of the Trans-Adirondack Basin.

While many questions and details remain, taken together these observations suggest that this portion of the SE margin of Laurentia evolved from a rifted margin to a backarc basin to a subduction complex, and accretionary orogen between ca. 1300 and 1160 Ma. The rifted margin was located along, or near the Black Lake shear zone, the boundary between the Frontenac terrane and Adirondack Lowlands (Peck et al., 2004; Chiarenzelli et al., 2010b). The Trans-Adirondack backarc basin formed ca. 1300 Ma as tonalitic arc rocks of the southern Adirondacks rifted away from Laurentia. Sedimentary rocks, including quartzites and marbles were deposited along both flanks of the Trans-Adirondack Basin during drift. Initial docking of the Composite Arc Belt occurred during the earlier Elzevirian orogeny (ca. 1245–1220 Ma) to the NW and deformation and subduction stepped outward into the Trans-Adirondack Basin. Eventually, NW-directed subduction and convergence led to collision of the southern Adirondacks and the Dysart–Mount Holly complex (Adirondis of Gower, 1996) with the Laurentian margin between 1200 and 1160 Ma. Sedimentation, dominated by carbonate deposition during the drift phase (lower marble), gave way to the development of a foredeep basin whose molassoid fill was dominated by clastic detritus of the Popple Hill Gneiss whose detrital zircons suggest a maximum depositional age of ca. 1220 Ma. With further convergence and episodic restricted circulation from the open ocean, siliceous carbonates, evaporates, and SEDEX deposits of the upper marble were deposited in the collapsing basin. The entire sequence was deformed and intruded by calc-alkaline to transitional magmas of the Antwerp-Rossie (ca. 1210–1200 Ma), the Hermon granitic gneiss (ca. 1182), the Rockport Granite, and Hyde School Gneiss (ca. 1172), and eventually by late synorogenic to postorogenic AMCG plutons (ca. 1155 Ma). The Pyrites Complex is an example of the oceanic crust and upper mantle of the Trans-Adirondack Basin and was obducted and interleaved with the supracrustal rocks of the Lowlands during Shawinigan orogenesis.

The authors would like to acknowledge funding from St. Lawrence University and the New York State Museum to carry out the analyses presented in this manuscript. The authors are indebted to the critical review efforts of Drs. Toby Rivers and Michael Hudson, and one unidentified reviewer. We would like to thank our colleagues Drs. Jim McLelland, Bruce Selleck, and David Valentino for many stimulating discussions on Adirondack geology. National Science Foundation grant EAR-0732436 is acknowledged for support of the Arizona LaserChron Center. The editorial staff of Geosphere, particularly Dr. Carol Frost, is thanked for their time and effort.