Elzevirian Orogeny Research Articles

Age and provenance of Grenville supergroup rocks, Trans-Adirondack Basin, constrained by detrital zircons

Research Article| February 01, 2015 Age and provenance of Grenville supergroup rocks, Trans-Adirondack Basin, constrained by detrital zircons Jeff Chiarenzelli; Jeff Chiarenzelli * 1Department of Geology, St. Lawrence University, Canton, New York 13617, USA *E-mails: jchiaren@stlawu.edu; dkratzmann@santarosa.edu; bselleck@colgate.edu; William.deLorraine@HudBayMinerals.com. Search for other works by this author on: GSW Google Scholar David Kratzmann; David Kratzmann * 2Earth and Space Science Department, Santa Rosa Junior College, PC 668, Call Hall, 680 Sonoma Mountain Parkway, Petaluma, California 94954-2522, USA *E-mails: jchiaren@stlawu.edu; dkratzmann@santarosa.edu; bselleck@colgate.edu; William.deLorraine@HudBayMinerals.com. Search for other works by this author on: GSW Google Scholar Bruce Selleck; Bruce Selleck * 3Department of Geology, 333 Ho Science Center, Colgate University, Hamilton, New York 13346, USA *E-mails: jchiaren@stlawu.edu; dkratzmann@santarosa.edu; bselleck@colgate.edu; William.deLorraine@HudBayMinerals.com. Search for other works by this author on: GSW Google Scholar William deLorraine William deLorraine * 4St. Lawrence Zinc Company, 408 Sylvia Lake Road, Gouverneur, New York 13642, USA *E-mails: jchiaren@stlawu.edu; dkratzmann@santarosa.edu; bselleck@colgate.edu; William.deLorraine@HudBayMinerals.com. Search for other works by this author on: GSW Google Scholar Author and Article Information Jeff Chiarenzelli * 1Department of Geology, St. Lawrence University, Canton, New York 13617, USA David Kratzmann * 2Earth and Space Science Department, Santa Rosa Junior College, PC 668, Call Hall, 680 Sonoma Mountain Parkway, Petaluma, California 94954-2522, USA Bruce Selleck * 3Department of Geology, 333 Ho Science Center, Colgate University, Hamilton, New York 13346, USA William deLorraine * 4St. Lawrence Zinc Company, 408 Sylvia Lake Road, Gouverneur, New York 13642, USA *E-mails: jchiaren@stlawu.edu; dkratzmann@santarosa.edu; bselleck@colgate.edu; William.deLorraine@HudBayMinerals.com. Publisher: Geological Society of America Received: 19 Oct 2014 Revision Received: 07 Dec 2014 Accepted: 08 Dec 2014 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2015 Geological Society of America Geology (2015) 43 (2): 183–186. https://doi.org/10.1130/G36372.1 Article history Received: 19 Oct 2014 Revision Received: 07 Dec 2014 Accepted: 08 Dec 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jeff Chiarenzelli, David Kratzmann, Bruce Selleck, William deLorraine; Age and provenance of Grenville supergroup rocks, Trans-Adirondack Basin, constrained by detrital zircons. Geology 2015;; 43 (2): 183–186. doi: https://doi.org/10.1130/G36372.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Detrital zircons from quartzites constrain the timing of deposition, provenance, and basin evolution of the Grenville supergroup in the Adirondack Lowlands. Three samples from the stratigraphic succession yield maximum depositional ages of 1284 ± 16 Ma to 1257.6 ± 16 Ma. Shifts in provenance from rifting (unimodal volcanic), carbonate platform (Laurentian interior), deep-water clastics (southern Adirondack arc), to final basin fill (Laurentian interior) are documented. The Trans-Adirondack backarc basin is one of several that opened at ca. 1.30 Ga, filled with thick carbonate-dominated sequences, and closed during the Elzevirian orogeny (ca. 1245–1220 Ma), terminating widespread extension across the leading edge of southeast Laurentia. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The extent of juvenile crust in the Grenville Province: Nd isotope evidence

Research Article| May 01, 2010 The extent of juvenile crust in the Grenville Province: Nd isotope evidence A.P. Dickin; A.P. Dickin † School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada †E-mail: dickin@mcmaster.ca Search for other works by this author on: GSW Google Scholar R.H. McNutt; R.H. McNutt School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada Search for other works by this author on: GSW Google Scholar C. Martin; C. Martin School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada Search for other works by this author on: GSW Google Scholar A. Guo A. Guo * School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada *Present address: State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xian 710069, China Search for other works by this author on: GSW Google Scholar Author and Article Information A.P. Dickin † School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada R.H. McNutt School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada C. Martin School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada A. Guo * School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario L8S 4M1, Canada †E-mail: dickin@mcmaster.ca *Present address: State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xian 710069, China Publisher: Geological Society of America Received: 02 Dec 2007 Revision Received: 04 May 2009 Accepted: 28 May 2009 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2010 Geological Society of America GSA Bulletin (2010) 122 (5-6): 870–883. https://doi.org/10.1130/B26381.1 Article history Received: 02 Dec 2007 Revision Received: 04 May 2009 Accepted: 28 May 2009 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A.P. Dickin, R.H. McNutt, C. Martin, A. Guo; The extent of juvenile crust in the Grenville Province: Nd isotope evidence. GSA Bulletin 2010;; 122 (5-6): 870–883. doi: https://doi.org/10.1130/B26381.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A large Nd isotope data set is presented to generate a crustal formation age map for the southwest Grenville Province, placing it in the context of the province as a whole. Previous studies have shown that most crust in the central and eastern parts of the province is pre-Grenvillian, composed largely of juvenile 1.5, 1.7, 1.9, and 2.7 Ga arc terranes, partly reworked by later ensialic arcs and collision zones. In contrast, new Nd data reported here show that juvenile monocyclic crust is restricted to the SW Grenville Province, in part of the Central Metasedimentary Belt interpreted as a failed backarc rift zone. There is a clear isotopic distinction between this rift zone and the pre-Grenvillian crust on the flanks. On the flanks of the rift zone, there is evidence for two crust-forming events, comprising an earlier (1.5 Ga) juvenile Pinwarian event, and a later 1.4–1.3 Ga continental margin arc (the Elzevirian Orogeny). Based on the geographical distribution of Nd model ages in the SW Grenville Province, a revised tectonic model for the Grenville Orogeny is developed. In this model, backarc spreading created a rift zone behind a (pre-Grenvillian) Elzevirian subduction zone that obliquely straddled the edge of a Mesoproterozoic continental margin. As a result of this geometry, the rift zone died out to the north as it propagated into cold Archean crust. Elzevirian subduction was probably halted by the collision of the continental margin with an offshore arc, and later with a major continental block, giving rise respectively to the Shawinigan and the Ottawan orogenic events. These collisional events can be grouped as parts of the Grenville Orogenic Cycle. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Grenville orogenic cycle of southern Laurentia: Unraveling sutures, rifts, and shear zones as potential piercing points for Amazonia

Abstract The magnetic anomaly map of North America serves as a useful base from which to attempt palinspastic reconstruction of terranes accreted during the Elzevirian orogeny (1250–1200 Ma); the Shawinigan (1200–1150 Ma), Ottawan (1080–1020 Ma), and Rigolet (1020–1000 Ma) phases of the Grenvillian orogeny; and post-Grenvillian magmatism (760–600 Ma) and deformation prior to Iapetan rifting at 565 Ma. Accreted terranes had unique histories prior to amalgamation and share common tectonic events afterwards. Comparisons with magnetic signatures of the Paleozoic craton–craton suture, sutures of accreted terranes, and the Jurassic rifted-margin for the southern-central Appalachians provide a basis for discriminating among alternative Grenvillian sutures beneath the Appalachian orogen. The Elzevirian suture is partially preserved beneath the Appalachians where it separates the Reading Prong terrane from Laurentia (i.e., Adirondacks and composite-arc terrane and Canadian Grenville Province). The Shawinigan suture is partially preserved in the Llano area (Texas), but separated the now-fragmented and allochthonous Amazonian (as indicated from Pb-isotope data) blocks of the outboard Blue Ridge terrane from the Reading Prong terrane in the Appalachians. Isolated blocks of the Sauratown Mountains terrane are interpreted as outboard of the Blue Ridge terrane, but were also accreted during the Shawinigan phase. Within present-day Laurentia, the only fragment of a terrane believed to have been accreted during the main Ottawan phase is the Mars Hill terrane (North Carolina–Tennessee). This suggests that the outboard Ottawan suture may have served as the locus of Iapetan rifting along much of Laurentia. The Rigolet phase (1020–1000 Ma) is characterized by widespread “Basin and Range” type extension (NW–SE) associated with sinistral or dextral movement on the NY-AL lineament, mobilization of core-complexes (Adirondack Highlands), and AMCG magmatism along the outboard flank of the extensional region. Following the Rigolet phase, the Appalachian region continued to be characterized by NW–SE extension during the passage of a possible hotspot along a NE-track (760–600 Ma) across the Blue Ridge and other terranes, and during initial Iapetan rifting (565 Ma). The palinspastic rifted-margin of Laurentia crosses many of these terranes and sutures as well as the possible region of Rigolet extension and the possible hotspot track, thus providing many potential piercing points within the Grenville orogen for comparison with Paleozoic terranes like the Precordillera in South America.

Early History of the Carthage‐Colton Shear Zone, Grenville Province, Northwest Adirondacks, New York (U.S.A.)

Abstract The Adirondack Mountains expose two distinct tectonic elements of the Proterozoic Grenville Province of northeastern North America: the Adirondack Lowlands and Highlands. The Lowlands are located along the eastern edge of the Metasedimentary Belt, and the Highlands form the western portion of the Granulite Terrane. The two are separated by the Carthage‐Colton Shear Zone (CCSZ). U/Pb titanite and 40Ar‐39Ar hornblende ages in the Lowlands are ca. 100 m.yr. older than in the Highlands, across the CCSZ. While both the Lowlands and the Highlands record a history of metamorphism during the Elzevirian Orogeny (ca. 1150 Ma), only the Highlands record evidence of a major phase of Grenvillian thermotectonic activity (Ottawan Orogeny) at ca. 1090–1030 Ma. Proposed tectonic models require that the Lowlands were either laterally separated from the Highlands or were at structurally higher levels during this metamorphism. New U/Pb and 40Ar‐39Ar ages from titanite and hornblende‐bearing mylonites in the Dana Hil...

Structure of the Lac Nominingue – Mont-Laurier region, Central Metasedimentary Belt, Quebec Grenville Province

The Lac Nominingue – Mont-Laurier region of the Central Metasedimentary Belt, Grenville Province of Quebec, comprises the granulite-facies Bondy gneiss complex (core of the Bondy gneiss dome) and overlying Sourd group metasedimentary rocks. A metamorphic foliation – transposed compositional layering (S1; host to peak-pressure parageneses) has been folded by isoclinal folds (F2 and F3) crosscut by leucosomes that host peak-temperature assemblages. The orthopyroxene isograd cuts obliquely across F3 folds, indicating that 1.20–1.18 Ga granulite-facies metamorphism post-dated D3. D3 structures are cut by ductile shear zones and boudinaged in D4 and are folded by regional-scale, open, upright north–south folds (F5). Folds with shallowly dipping axial surfaces (F6) are subsequently developed in the Sourd group. F5 (and probably F6) developed prior to intrusion of the ca. 1165 Ma Chevreuil suite. In the Nominingue–Chénéville deformation zone (NCDZ) east of the Bondy gneiss dome, Chevreuil intrusions contain north-striking magmatic and tectonic foliations. These, along with host gneisses and metasedimentary rocks, are displaced by conjugate ductile shear zones (northeast dextral and south-southeast sinistral) and north-northeast-striking thrusts. Late open folds (F8) with east-northeast-striking axial surfaces produce dome and basin interference patterns. F2 to F5 folds may have formed during either subhorizontal, east–west contraction or east–west extension resulting from orogenic collapse or convective lithospheric thinning following crustal thickening during terrane assembly in the Elzevirian orogeny. Structures in the NCDZ imply ESE–WNW contraction and NNE–SSW (orogen-parallel) extension in D7 syn- to post-intrusion of the Chevreuil suite. F8 folds imply a late, Grenvillian SSE–NNW contraction.

Large crustal input to high δ18O anorthosite massifs of the southern Grenville Province: new evidence from the Morin Complex, Quebec

Mid-Proterozoic anorthosite-suite magmatism is a major volumetric component of the southern Grenville Province, and provides an important probe of the compositions and types of lower crustal rocks. The ∼1.15 Ga Morin Complex (Quebec) consists of two anorthosite plutons with distinct compositions. Plagioclase from the western lobe of the anorthosite has δ18O values that average 9.6 ± 0.7‰, which is ∼3‰ higher than the values found in “normal” anorthosites and in mantle-derived mafic igneous rocks worldwide. Plagioclase from the eastern lobe of the massif (deformed by the Morin Shear Zone) has δ18O values that average 8.7 ± 0.6‰, also high compared to mantle-derived rocks. Numerous lines of evidence, including homogeneity of δ18O values within individual plutons, O–Sr–Nd mixing relations, and preservation of igneous δ18O in adjacent mangerite units argue against anorthosite interaction with high δ18O fluids as the cause of the high δ18O values seen in both anorthosite lobes. High δ18O values are best explained as primary magmatic compositions resulting from melting and assimilation of crustal materials by the anorthosite's parent magma. The Morin and Marcy massifs are located in the Allochthonous Monocyclic Belt of the Grenville Province, and have the highest known δ18O values for anorthosites in the Grenville. Although the Monocyclic Belt is juvenile in terms of radiogenic isotope systematics, the new oxygen isotope data indicate the presence high δ18O supracrustal materials at the base of the crust, probably buried during the ∼1.2 Ga Elzevirian orogeny in the Monocyclic Belt prior to anorthosite magmatism. This process is not recognized in other parts of the Grenville Province and points to differences in the pre-1.2-Ga continental margins.

Convergent margin on southeastern Laurentia during the Mesoproterozoic: tectonic implications

A continental-margin magmatic arc is inferred to have existed on the southeastern (present coordinates) margin of Laurentia from Labrador to Texas from ~1500-1230 Ma, with part of the arc subsequently being incorporated into the 1190-990 Ma collisional Grenville Orogen. Outside the Grenville Province, where the arc is known as the Granite-Rhyolite Belt, it is undeformed, whereas within the Grenville Province it is deformed and metamorphosed. The arc comprises two igneous suites, an inboard, principally quartz monzonitic to granodioritic suite, and an outboard tonalitic to granodioritic suite. The quartz monzonite-granodiorite suite was largely derived from continental crust, whereas the tonalitic-granodiorite suite is calc-alkaline and has a juvenile isotopic signature. Available evidence from the Grenville Province suggests that the arc oscillated between extensional and compressional settings several times during the Mesoproterozoic. Back-arc deposits of several ages, that formed during relatively brief periods of extension, include (1) mafic dyke swarms subparallel to the arc; (2) continental sediments, bimodal volcanics and plateau basalts; (3) marine sediments and volcanics formed on stretched continental crust; and (4) ocean crust in a marginal basin. Closure of the back-arc basins occurred during the accretionary Pinwarian (~1495-1445 Ma) and Elzevirian (~1250-1190 Ma) orogenies, as well as during three pulses of crustal shortening associated with the 1190-990 Ma collisional Grenvillian Orogeny. During the Elzevirian Orogeny, closure of the Central Metasedimentary Belt marginal basin in the southeastern Grenville Province was marked by subduction-related magmatism as well as by imbrication of back-arc deposits. The presence of a continental-margin magmatic arc on southeastern Laurentia during the Mesoproterozoic implies that other coeval magmatism inboard from the arc took place in a back-arc setting. Such magmatism was widespread and chemically diverse and included large volume "anorogenic" anorthosite-mangerite-charnockite-granite (AMCG) complexes as well as small volume alkaline, quartz-saturated and -undersaturated "within-plate" granitoids. Recognition of the ~300 million year duration of the Mesoproterozoic convergent margin of southeastern Laurentia suggests that there may be useful parallels with the evolution of the Andes, which has been a convergent margin since the early Paleozoic.

Docking of the Central Metasedimentary Belt to Laurentia in geon 12: evidence from the 1.17-1.16 Ga Chevreuil intrusive suite and host gneisses, Quebec

The Chevreuil intrusive suite (1.17-1.16 Ga) represents a chronological field marker of regional extent that intruded the Central Metasedimentary Belt in the western Grenville Province of Quebec after peak metamorphism. Style and site of magma emplacement, and extent of deformation of Chevreuil plutons and dykes permit unravelling of the early Grenvillian evolution of the belt with respect to cratonal North America. The suite comprises a series of vertically layered gabbro stocks and monzonite-diorite-gabbro sheet intrusions, and a swarm of microdiorite dykes that cut across gneisses. The dykes display systematic variations in extent of deformation across the belt. We targeted U-Pb geochronology on gneisses within the identified strain windows; they preserve the record of a ca. 1.20 Ga high pressure-temperature (P-T) metamorphic event. The sheet intrusions define magmatic corridors all along, and concordant with, the western, northern, and eastern tectonic boundaries of the belt. The concordant and elongate shape of these bodies results from emplacement, not deformation. Chevreuil magmas thus sealed the belt boundaries largely in their current positions, with the implication that docking of Elzevirian and pre-Elzevirian terranes with cratonal North America predates 1.17 Ga. We interpret the 1.20 Ga metamorphism as evidence for the initiation of Grenvillian continent-continent collision during the culmination of the Elzevirian orogeny at ca. 1.22 Ga. Emplacement-related fabrics indicate that the Chevreuil suite and the coeval Morin anorthosite suite intruded during renewed orogenesis. This orogenic pulse (Shawinigan) is not accretionary, but represents a strongly partitioned, compressive, intraplate reactivation event.

The Grenville Orogenic Cycle (ca. 1350-1000 Ma): an Adirondack perspective

The Adirondack Mountains are characterized by three major events that took place during the interval ca. 1350-1000 Ma. The earliest of these is the arc-related Elzevirian Orogeny (ca. 1350-1185 Ma) during which substantial volumes of juvenile calc-alkaline crust were added to the Adirondacks as well as to the northwest segment of the Central Metasedimentary Belt. Data from the southwestern United States as well as from Ireland and Baltica indicate that Elzevirian magmatism and orogeny were of global dimensions. Within the southwestern sector of the Grenville Province, the Elzevirian Orogeny culminated at ca. 1185 Ma when accretion of all outboard terranes was completed. Compressional orogeny related to this convergence resulted in overthickened crust and lithosphere which subsequently delaminated giving rise to orogen collapse and AMCG magmatism that swept southeastward from the Frontenac Terrane into the Adirondack Highlands during the interval ca. 1180-1130 Ma. Localized compressional events within neighboring parts of the Grenville Province emphasize the continued existence of contraction during this interval, although crustal extension caused local in sedimentary basins in which were deposited the Flinton and the St. Boniface Groups. The Adirondacks have not yet provided any record of events within the interval ca. 1125-1100 Ma, although there is evidence of contraction elsewhere in the southwestern Grenville Province at that time. At 1100-1090 Ma the northern Adirondack Highlands were invaded by mildly A-type hornblende granites (Hawkeye suite) that are interpreted to be the result of local crustal thinning contemporaneous with rifting and mafic magmatism taking place in the Midcontinent rift. Immediately following, at ca. 1090 Ma, the global-scale continental collision of the Ottawan Orogeny was initiated. Strong convergence, deformation, and metamorphism continued to at least ca. 1070 Ma, and rocks older than this are profoundly affected by this event. During the waning stages of the Ottawan Orogeny, overthickened crust and lithosphere delaminated and the orogen underwent collapse. Large extensional faults such as the Carthage-Colton-Labelle shear zone developed and rapidly exhumed granulite facies rocks in the mobile core of the orogen which centers on the Adirondack-Morin terranes and extends southeastward into the New York-New Jersey Highlands. Extensional faulting along the Carthage-Colton mylonite zone dropped the amphibolite facies Lowlands down to the west and into juxtaposition with granulite facies rocks of the Highlands. UPb cooling ages from garnet, monazite, and titanite exhibit a sufficiently broad spectrum to accommodate an initial rapid rate of rebound-related cooling followed by a slower, erosion-controlled cooling history. During delamination, late- to post-tectonic granites of the Lyon Mt. Gneiss (ca. 1070-1045 Ma) were emplaced. The youngest member of this suite is an undeformed fayalite granite dated at ca. 1045 Ma which crosscuts all older rocks and fabric. High-potassium, post-tectonic granites of similar age are common in other parts of the southwestern Grenville Province. Renewed contraction and metamorphism at ca. 1030 Ma demonstrate that the Ottawan Orogen was still experiencing convergence well after the peak of orogeny. However, most of the manifestations involve reactivation of older thrust faults, including the Grenville Front Tectonic Zone. The intrusion of small bodies of anorthosite at ca. 1015 Ma (i.e., Labrieville) provide further evidence for the emplacement of these rocks within collisional orogens, albeit in their collapsing phase.

Accretion, rifting, rotation and collision in the North Atlantic supercontinent, 1700-950 Ma

Abstract Events in the North Atlantic region, from c. 1700 to 950 Ma, are unified in a model compatible with palaeomagnetic results and Neoproterozoic supercontinent reconstructions. Following Labradorian-Gothian accretion, largely between 1700 and 1550 Ma, post-collisional effects continued until c. 1500 Ma. Continental extension from c. 1500 Ma onwards, related to ocean opening, developed granite-rhyolite provinces: the Central Metasedimentary Belt (Grenville Province) started to form at c. 1425 Ma, with arcs outboard at c. 1350–1300 Ma. Baltica started to separate from Laurentia and rotate at c. 1240 Ma, causing crustal delamination of its leading edge, in South Norway. Here, subduction-related gabbroids intruded at c. 1230–1225 Ma, as rotation proceeded. The Elzevirian Orogeny occurred at c. 1190 Ma in the Southwest Grenville Province, marking the start of convergence of Baltica, in which backarc extension was accompanied by granites and bimodal volcanics. Collision at c. 1080 Ma caused the Ottawan Orogeny which thrust accreted terranes northwestwards and closed the Mid-Continental Rift. In southern Baltica, complex deformation occurred. Post-collisional convergence and sinistral slip of Baltica against Laurentia (possibly related to convergence of Amazonia) caused thrusting, sinistral strike-slip faulting, folding and granitic intrusion until c. 950 Ma. Post-tectonic granites intruded until c. 900 Ma.

Distribution of pre-1400 Ma crust in the Grenville province: Implications for rifting in Laurentia-Baltica during geon 14

Recent geochronological data show that geon 14 and older rocks are found at least 700 km south of the Grenville front in the eastern Grenville province. In the southwestern Grenville province, comparable rocks extend only 200 km south; farther south, in the Central metasedimentary belt and the Adirondack Mountains, the crust mostly postdates 1350 Ma. We explain this contrast by postulating a major rifting event, commencing at ∼1430 Ma, during which large sections of older crust were detached from that part of southern Laurentia now occupied by the Central metasedimentary belt. Crust was also removed from the area between northwest Britain and Scandinavia. Failed arms extended (1) from Missouri to where the Belt Supergroup accumulated, (2) from the Central metasedimentary belt eastward into Labrador, and (3) between Baltica and east of Greenland. The geologic history following late geon 14 rifting was dominated by a southward-subducting magmatic arc that eventually collided with southern Laurentia-Baltica, resulting in the Elzevirian orogeny at ∼1200 Ma.

Trondhjemitic, 1.35–1.31 Ga gneisses of the Mount Holly Complex of Vermont: evidence for an Elzevirian event in the Grenville Basement of the United States Appalachians

A newly recognized suite of trondhjemite–tonalite and dacitic gneiss forms a 10 km wide belt of rocks within the Mount Holly Complex in the central part of the Green Mountain massif of Vermont. Field relationships and chemistry indicate that these gneisses are calc-alkaline, volcanic, and hypabyssal plutonic rocks older than the Middle Proterozoic regional deformation that affected the Mount Holly Complex. U–Pb zircon dates indicate ages as great as 1.35 Ga for crystallization of the volcanic protoliths and for intrusion of crosscutting trondhjemite. Tonalitic plutonism continued until 1.31 Ga.Map-scale contacts between the trondhjemitic–tonalitic–dacitic gneisses and the paragneiss sequence of the Mount Holly Complex are sharp, suggesting that the volcanic rocks of the trondhjemite–tonalite suite underlie the paragneiss units and do not intrude them. These relationships suggest that the trondhjemite–tonalite suite is either considerably older than, and unconformable beneath, the paragneiss cover rocks or represents a volcanic edifice slightly older than the deposition of the sedimentary precursor to the paragneiss units. The paragneiss and tonalite–trondhjemite gneisses are both intruded by younger granitoids that were intruded at about 1.25 Ga during strong dynamothermal metamorphism.The trondhjemitic gneisses of the Mount Holly Complex of Vermont have high Al2O3 and low Yb contents and light rare-earth element enrichment patterns that are more characteristic of continental than oceanic volcanic arcs. The Mount Holly intrusives and volcanics may have formed during 1.35–1.31 Ga ensialic volcanic-arc activity, contemporaneous with ensimatic arc activity during the early part of the Elzevirian phase of the Grenville orogeny. In Vermont, later deformation and granite intrusion at about 1.25 Ga coincide with the major pulse of the Elzevirian orogeny and associated trondhjemitic plutonism in the Central Metasedimentary Belt of eastern Canada.

The Flinton Group: a late Precambrian metasedimentary succession in the Grenville Province of eastern Ontario

Clastic and carbonate metasediments, preserved in narrow synclines, have been correlated over an area of 2000 km2. These strata, the Flinton Group, lie unconformably on metamorphosed volcanic, clastic, and carbonate rocks, and on large granitic intrusions. The group, which comprises six formations, has undergone at least two major folding episodes and one main regional metamorphism of varying grade. The only post-Flinton intrusions are pegmatites at high grade and one tectonically emplaced ultramafic slice.Depositional environment ranged from fluvial to moderate-depth marine. Rapid facies changes, coupled with persistence of some units along strike and close relationships between facies and underlying lithology, point to local sources and local tectonic control of deposition basins. At the onset of sedimentation, a deeply weathered source terrain yielded mature basal redbeds, which were succeeded by less mature clastics as block faulting caused increase of relief between sources and basins. These facies passed offshore into finer, more reduced sediments. Deposition took place between 1050 and 1080 (±25) Ma ago, after arc volcanism, plutonism, uplift, and erosion, and before major regional metamorphism. All these events can be grouped within the Grenvillian orogenic cycle, spanning at least the interval 1300–1000 Ma and including, in eastern Ontario, the pre-Flinton Elzevirian Orogeny and post-Flinton Ottawan Orogeny.