44 research outputs found
Accretion, Soft and Hard Collision: Similarities, Differences and an Application from the Newfoundland Appalachian Orogen
We argue there is no distinction between accretion and collision as a process, except when accretion is used in the sense of incorporating small bodies of sedimentary and/or volcanic rocks into an accretionary wedge by off-scraping or underplating. There is also a distinction when these terms are used in classifying mountain belts into accretionary and collisional orogens, although such classifications are commonly based on a qualitative assessment of the scale and nature of the accreted terranes and continents involved in formation of mountain belts. Soft collisions occur when contractional deformation and associated metamorphism are principally concentrated in rocks of the leading edge of the partially pulled-down buoyant plate and the upper plate forearc terrane. Several young arc-continent collisions show evidence for partial or wholesale subduction of the forearc such that the arc is structurally juxtaposed directly against lower plate rocks. This process may explain the poor preservation of forearcs in the geological record. Soft collisions generally change into hard collisions over time, except if the collision is rapidly followed by formation of a new subduction zone due to step-back or polarity reversal. Thickening and metamorphism of the arc's suprastructure and retro-arc part of upper plate due to contractional deformation and burial are the characteristics of a hard collision or an advancing Andean-type margin. Strong rheological coupling of the converging plates and lower and upper crust in the down-going continental margin promotes a hard collision. Application of the softâhard terminology supports a structural juxtaposition of the Taconic soft collision recorded in the Humber margin of western Newfoundland with a hard collision recorded in the adjacent Dashwoods block. It is postulated that Dashwoods was translated dextrally along the Cabot-Baie Verte fault system from a position to the north of Newfoundland where the Notre Dame arc collided ca. 10 m.y. earlier with a wide promontory in a hyperextended segment of the Laurentian margin.Nous soutenons qu'il n'y a pas de distinction entre l'accrĂ©tion et la collision en tant que processus, sauf lorsque l'accrĂ©tion est utilisĂ©e dans le sens d'incorporer de petits corps de roches sĂ©dimentaires et / ou volcaniques dans un prisme d'accrĂ©tion par raclage ou sous-placage. Il y a Ă©galement une distinction lorsque ces termes sont utilisĂ©s pour classer les chaĂźnes de montagne en orogĂšnes d'accrĂ©tion et de collision, bien que ces classifications soient gĂ©nĂ©ralement basĂ©es sur une Ă©valuation qualitative de l'Ă©chelle et de la nature des terranes accrĂ©tĂ©s et des continents impliquĂ©s dans la formation des chaĂźnes de montagnes.Des collisions molles se produisent lorsque la dĂ©formation par contraction et le mĂ©tamorphisme associĂ© sont principalement concentrĂ©s dans les roches du front de la plaque chevauchante partiellement abaissĂ©e et du terrane dâavant-arc de la plaque supĂ©rieure. Plusieurs jeunes collisions arc-continent montrent des preuves d'une subduction partielle ou totale de l'avant-arc de telle sorte que l'arc est directement structurellement juxtaposĂ© contre les roches de la plaque infĂ©rieure. Ce processus peut expliquer la mauvaise prĂ©servation des avant-arcs dans les archives gĂ©ologiques. Les collisions molles se transforment gĂ©nĂ©ralement en collisions dures au fil du temps, sauf si la collision est rapidement suivie de la formation d'une nouvelle zone de subduction en raison d'un recul ou d'une inversion de polaritĂ©. L'Ă©paississement et le mĂ©tamorphisme de la suprastructure de l'arc et de la partie rĂ©tro-arc de la plaque supĂ©rieure dus Ă la dĂ©formation par contraction et Ă l'enfouissement sont les caractĂ©ristiques d'une collision dure ou d'une marge de type andin en progression. Un fort couplage rhĂ©ologique des plaques convergentes et de la croĂ»te infĂ©rieure et supĂ©rieure dans la marge continentale descendante favorise une collision dure.L'application de la terminologie molle-dure corrobore une juxtaposition structurelle de la collision molle taconique enregistrĂ©e dans la marge de Humber de l'ouest de Terre-Neuve avec une collision dure enregistrĂ©e dans le bloc de Dashwoods adjacent. Il est postulĂ© que le bloc de Dashwoods a Ă©tĂ© dĂ©placĂ© de maniĂšre dextre le long du systĂšme de failles Cabot-Baie Verte Ă partir d'une position au nord de Terre-Neuve oĂč l'arc Notre Dame est entrĂ© en collision environ 10 m.a. plus tĂŽt avec un large promontoire dans un segment en hyper-extension de la marge laurentienne.
The Ocean â Continent Transition Zones Along the Appalachian â Caledonian Margin of Laurentia: Examples of Large-Scale Hyperextension During the Opening of the Iapetus Ocean
A combination of deep seismic imaging and drilling has demonstrated that the ocean-continent transition (OCT) of present-day, magma-poor, rifted continental margins is a zone of hyperextension characterized by extreme thinning of the continental crust that exhumed the lowermost crust and/or serpentinized continental mantle onto the seafloor. The OCT on present-day margins is difficult to sample, and so much of our knowledge on the detailed nature of OCT sequences comes from obducted, magma-poor OCT ophiolites such as those preserved in the upper portions of the Alpine fold-and-thrust belt. Allochthonous, lens-shaped bodies of ultramafic rock are common in many other ancient orogenic belts, such as the Caledonian â Appalachian orogen, yet their origin and tectonic significance remains uncertain. We summarize the occurrences of potential ancient OCTs within this orogen, commencing with Laurentian margin sequences where an OCT has previously been inferred (the Dalradian Supergroup of Scotland and Ireland and the Birchy Complex of Newfoundland). We then speculate on the origin of isolated occurrences of Alpine-type peridotite within Laurentian margin sequences in Quebec â Vermont and Virginia â North Carolina, focusing on rift-related units of Late Neoproterozoic age (so as to eliminate a Taconic ophiolite origin). A combination of poor exposure and pervasive Taconic deformation means that origin and emplacement of many ultramafic bodies in the Appalachians will remain uncertain. Nevertheless, the common occurrence of OCT-like rocks along the whole length of the Appalachian â Caledonian margin of Laurentia suggests that the opening of the Iapetus Ocean may have been accompanied by hyperextension and the formation of magma-poor margins along many segments.SOMMAIREDes travaux dâimagerie sismique et des forages profonds ont montrĂ© que la transition ocĂ©an-continent (OCT) de marges continentales de divergence pauvre en magma exposĂ©e de nos jours, correspond Ă une zone dâhyper-Ă©tirement tectonique caractĂ©risĂ©e par un amincissement extrĂȘme de la croĂ»te continentale, qui a exhumĂ© sur le fond marin, jusquâĂ la tranche la plus profonde de la croĂ»te continentale, voire du manteau continental serpentinisĂ©. Parce quâon peut difficilement Ă©chantillonner lâOCT sur les marges actuelles, une grande partie de notre comprĂ©hension des dĂ©tails de la nature de lâOCT provient dâophiolites pauvres en magma dâune OCT obduite, comme celles prĂ©servĂ©es dans les portions supĂ©rieures de la bande plissĂ©e alpine. Des masses lenticulaires de roches ultramafiques allochtones sont communes dans de nombreuses autres bandes orogĂ©niques anciennes, comme lâorogĂšne CalĂ©donienne-Appalaches, mais leur origine et signification tectonique reste incertaine. Nous prĂ©sentons un sommaire des occurrences dâOCT potentielles anciennes de cet orogĂšne, en commençant par des sĂ©quences de la marge laurentienne, oĂč la prĂ©sence dâOCT a dĂ©jĂ Ă©tĂ© dĂ©duites (le Supergroupe Dalradien dâĂcosse et d'Irlande, et le complexe de Birchy de Terre-Neuve). Nous spĂ©culons ensuite sur l'origine de cas isolĂ©s de pĂ©ridotite de type alpin dans des sĂ©quences de marge des Laurentides du QuĂ©bec-Vermont et de la Virginie-Caroline du Nord, en nous concentrant sur les unitĂ©s de rift d'Ăąge nĂ©oprotĂ©rozoĂŻque tardif (pour Ă©viter les ophiolites du Taconique). La conjonction dâaffleurements de piĂštre qualitĂ© et de la dĂ©formation taconique omniprĂ©sente, signifie que l'origine et la mise en place de nombreuses masses ultramafiques dans les Appalaches demeureront incertaines. NĂ©anmoins, la prĂ©sence frĂ©quente de roches de type OCT tout le long de la marge CalĂ©donnienne-Appalaches de Laurentia suggĂšre que l'ouverture de l'ocĂ©an Iapetus peut avoir Ă©tĂ© accompagnĂ©e dâhyper-Ă©tirement et de la formation de marges pauvres en magma le long de nombreux segments
Time-Transgressive Salinic and Acadian Orogenesis, Magmatism and Old Red Sandstone Sedimentation in Newfoundland
We propose an intimate relationship between Silurian terrestrial red bed sedimentation (Old Red Sandstone), slab breakoff-related magmatism and deformation in the Newfoundland Appalachians. Red bed sedimentation started during the Early Silurian, and records the progressive rise of the Salinic mountains in the tectonic hinterland of the orogen. The red beds were mainly deposited in molasse-style foreland basins in front of an east-propagating terminal Salinic deformation front. New UâPb zircon dating of volcanic rocks interlayered with the Silurian red beds in key structural locations yielded ages ranging between 425 and 418 Ma, which, combined with the existing geochronological database, suggests that the sedimentary rocks are progressively younger from west to east and overstep the accreted Gondwana-derived terranes. We propose that deposition of the red beds is a good proxy for the time of cratonization of the accreted terranes. Eastward migration of the Salinic deformation front was accompanied by eastward-widening of a slab-breakoff-related asthenospheric window. The latter is interpreted to have formed due to a combination of progressive steepening of the down-going plate following entrance of the leading edge of the Gander margin and its eduction. Gander margin eduction (reversed subduction) is proposed to have been instigated by the trench migration of the Acadian coastal arc built upon the trailing edge of the Gander margin, which developed contemporaneously with the Salinic collision. The resultant thinning of the lithosphere beneath the Salinic orogen, built upon the leading edge of the Gander margin immediately prior to the onset of the Early Devonian Acadian orogeny, set the stage for generation of the widespread bloom of Acadian magmatism.SOMMAIRENous proposons quâil y a eu une relation intime entre la sĂ©dimentation des couches rouges continentales au Silurien (vieux-grĂšs-rouges), un magmatisme liĂ© Ă une rupture de segments de croĂ»te, et la dĂ©formation appalachienne Ă Terre-Neuve. La sĂ©dimentation des couches rouges qui a dĂ©butĂ© au dĂ©but du Silurien tĂ©moigne du soulĂšvement progressif des monts saliniques de lâarriĂšre-pays tectonique de lâorogĂšne. Les couches rouges se sont dĂ©posĂ©es sous forme de molasses dans des bassins dâavant-pays, Ă lâavant du front de dĂ©formation salinique terminale qui se dĂ©ployait vers lâest. De nouvelles datations U-Pb sur zircon de roches volcaniques interstratifiĂ©es avec des couches rouges siluriennes en des lieux structurels stratĂ©giques montrent des Ăąges qui varient entre 425 Ma et 418 Ma, ce qui, combinĂ© aux bases de donnĂ©es gĂ©ochronologiques existantes permet de penser que les roches sĂ©dimentaires sont progressivement plus jeunes dâouest en est, et quâelles surplombent les terranes accrĂ©tĂ©s du Gondwana. Nous suggĂ©rons que les couches rouges sont de bons indicateurs temporels de la cratonisation des terranes accrĂ©tĂ©s. La migration vers lâest du front de la dĂ©formation salinique a Ă©tĂ© accompagnĂ©e par un Ă©largissement vers lâest dâune fenĂȘtre asthĂ©nosphĂ©rique liĂ©e Ă une rupture de la croĂ»te. Cette derniĂšre aurait Ă©tĂ© provoquĂ©e par la combinaison de lâenfoncement progressif de la plaque qui a suivi lâentrĂ©e du bord dâattaque de la marge de Gander, et son Ă©duction. Nous proposons que lâĂ©duction (lâinverse de la subduction) de la marge de Gander a Ă©tĂ© provoquĂ©e par la migration de la fosse tectonique cĂŽtiĂšre acadienne, induite par la migration du bord dâattaque de la marge de Gander, contemporaine de la collision salinique. Lâamincissement de la lithosphĂšre sous lâorogĂšne salinique qui en a rĂ©sultĂ©, et qui sâest dĂ©ployĂ© au bord dâattaque de la marge de Gander juste avant lâenclenchement de lâorogĂ©nie acadienne au dĂ©but du DĂ©vonien, a prĂ©parĂ© le terrain du dĂ©ploiement Ă grande Ă©chelle du magmatisme acadien
From Large Zones to Small Terranes to Detailed Reconstruction of an Early to Middle Ordovician ArcâBackarc System Preserved Along the Iapetus Suture Zone: A Legacy of Hank Williams
The Annieopsquotch accretionary tract (AAT) comprises a thrust stack of Lower to Middle Ordovician arc and backarc terranes that were accreted to the composite Laurentian margin of Iapetus during the Middle to Late Ordovician. Geological relationships suggest that the constituent terranes of the AAT initially formed outboard of the composite Laurentian margin in an extensional arc that underwent multiple rifting episodes prior to its accretion. The initiation of AAT magmatism led to the development of Tremadocian to Floian supra-subduction zone ophiolites (481 to 477 Ma) with organized ridges indicated by the presence of well-developed sheeted dyke complexes. This spreading centre propagated through a fragment of Laurentian crust and separated it from the composite Laurentian margin. This Laurentian crust fragment then formed the basement to subsequent Floian to Darriwilian AAT arc magmatism. The Floian arc (473 to 468 Ma) underwent extensive rifting indicated by organized spreading in the Lloyds River backarc basin, which was floored by juvenile backarc ophiolitic crust (472 Ma). The establishment of the Darriwilian arc (467 to 462 Ma) was in part coeval with yet another stage of rifting. Darriwilian magmatism is characterised by significant along-strike variability, ranging from continental to primitive calc-alkaline arc to tholeiitic backarc-like magmatism. The diversity of Darriwilian magmatism can be attributed to fragmentation and magmatic reworking of Laurenian-derived basement along strike in the same arc undergoing disorganized spreading. The development of the AAT is interpreted to be similar to that of the modern Izu â Bonin â Mariana arc in the western Pacific.SOMMAIRELa bande dâaccrĂ©tion dâAnnieopsquotch (AAT) est constituĂ©e dâun empilement de chevauchements de lâOrdovicien prĂ©coce Ă moyen, et de terranes dâarc et dâarriĂšre-arc qui se sont accrĂ©tĂ©s Ă la marge composite laurentienne japĂ©tienne Ă lâOrdovicien moyen Ă tardif. Les faits gĂ©ologiques relevĂ©s portent Ă penser que les terranes constitutifs de lâAAT se sont constituĂ©s Ă lâextĂ©rieur de la marge laurentienne dans un arc dâextension qui a subi de multiples Ă©pisodes de rifting avant son accrĂ©tion. Lâinitiation du magmatisme de lâAAT a menĂ© au dĂ©veloppement de zones dâophiolites de supra-subduction du TrĂ©madocien au Floien (481 Ma Ă 477 Ma), avec des crĂȘtes ordonnĂ©es mises en Ă©vidence par la prĂ©sence de complexes de tapis de dikes bien dĂ©veloppĂ©s. Ce centre dâextension sâest propagĂ© Ă travers un fragment de la croĂ»te laurentienne, et lâa ultimement sĂ©parĂ© de la marge composite laurentienne. Et, du Floien au Darriwilien, ce fragment de croĂ»te laurentienne a servi de substratum au magmatisme dâarc de lâAAT. Au Floien (473 Ma Ă 468 Ma), cette zone dâarc a subi un important rifting, comme lâindique la distension ordonnĂ©e du bassin dâarriĂšre-arc de Lloyds River, lequel a servi de semelle Ă une croĂ»te ophiolitique dâarriĂšre-arc (472 Ma). La mise en place de lâarc au Darriwilien (467 Ma Ă 462 Ma) a coexistĂ© pour un temps avec un autre Ă©pisode de rifting. Le magmatisme darriwilien est caractĂ©risĂ© par une variabilitĂ© de composition importante parallĂšlement Ă sa direction, passant dâune composition dâarc continental Ă celle dâarriĂšre-arc primitif calco-alcalin jusquâĂ une composition de magmatisme de type tholĂ©iitique dâarriĂšre-arc. La diversitĂ© du magmatisme darriwilien peut ĂȘtre attribuĂ©e Ă la fragmentation et au remaniement magmatique de la croĂ»te dâorigine laurentienne parallĂšlement Ă la direction dâun mĂȘme arc subissant une distension dĂ©sordonnĂ©e. Nous proposons que le dĂ©veloppement de lâAAT a Ă©tĂ© similaire Ă celui de lâarc moderne IzyâBoninâMarianne du Pacifique occidental
UâPb Age of the Stanley Brook Granite, Grand Manan Island, New Brunswick
The Stanley Brook Granite on Grand Manan Island has yielded an Early Cambrian U-Pb zircon age of 535 ± 2 Ma. The granite was emplaced into sedimentary rocks of the Flagg Cove Formation of the Castalia Group, proving that the latter is no younger than Early Cambrian. Previous dating of detrital zircons from the Flagg Cove Formation limits its maximum depositional age to be no older than 574 ± 7 Ma. The intrusive age of the Stanley Brook Granite is essentially identical to the 539 ± 3 Ma age obtained previously from dacitic tuïŹ of the Priest Cove Formation, suggesting a comagmatic relationship between the two. The contact between the Flagg Cove and Priest Cove formations is invariably faulted. However, the consanguineous relationship between the Priest Cove Formation and Stanley Brook Granite suggests that the Priest Cove Formation was deposited on the Flagg Cove Formation. This proposed stratigraphic relationship supports the interpretation that the tuïŹaceous and volcaniclastic sedimentary rocks of the Priest Cove Formation represent a distal facies of the mafic flows and fragmental volcanic rocks of the Ross Island Formation. The age of the Stanley Brook Granite is consistent with the interpretation that Mesoproterozoic (?) to Neoproterozoic platformal carbonate rocks of the Brookville terrane represent basement to Neoproterozoic volcanic rocks of the New River terrane.
RĂSUMĂ
LâĂąge U-Pb sur zircon du granite du ruisseau Stanley, sur lâĂźle Grand Manan, se situerait Ă environ 535 ± 2 Ma et remonterait au dĂ©but du Cambrien. Ce granite a Ă©tĂ© mis en place dans les roches sĂ©dimentaires de la Formation de Flagg Cove du groupe de Castalia, ce qui Ă©tablirait que cette derniĂšre formation date tout au plus du dĂ©but du Cam-brien. Les travaux de datation prĂ©cĂ©dents des zircons dĂ©tritiques de la Formation de Flagg Cove ont fixĂ© lâĂąge maximal de la sĂ©dimentation Ă 574 ± 7 Ma, tout au plus. Pour lâessentiel, lâĂąge de lâintrusion du granite du ruisseau Stanley correspond Ă lâĂąge de 539 ± 3 Ma dĂ©jĂ dĂ©terminĂ© pour le tuf dacitique de la Formation de Priest Cove, ce qui donnerait Ă penser quâil y a une relation comagmatique entre les deux. La zone de contact entre les formations de Flagg Cove et de Priest Cove est pratiquement toujours faillĂ©e. Toutefois, lâorigine commune de la Formation de Priest Cove et du granite du ruisseau Stanley porte Ă croire que la sĂ©dimentation de la Formation de Priest Cove sâest superposĂ©e Ă la Formation de Flagg Cove. Ce lien au plan stratigraphique viendrait corroborer lâinterprĂ©tation voulant que les roches sĂ©dimentaires tufacĂ©es et volcanoclastiques de la Formation de Priest Cove sont de fait un faciĂšs Ă©loignĂ© des coulĂ©es mafiques et des roches volcanoclastiques de la Formation de Ross Island. LâĂąge Ă©tabli pour le granite du ruisseau Stanley va dans le sens de lâinterprĂ©tation voulant que les roches carbonatĂ©es dans la succession de plateformes du MĂ©soprotĂ©rozoĂŻque (?) au NĂ©oprotĂ©rozoĂŻque du terrane de Brookville forment le socle des roches volcaniques du NĂ©oprotĂ©rozoĂŻque du terrane de New River. [Traduit par la redaction
Crustal Evolution of the Northeast Laurentian Margin and the Peri-Gondwanan Microcontinent Ganderia Prior to and During Closure of the Iapetus Ocean: Detrital Zircon UâPb and Hf Isotope Evidence from Newfoundland
Detrital zircon populations in sedimentary rocks from the Laurentian margin and the accreted microcontinent Ganderia on both sides of the main Iapetus suture (Red Indian Line) in central Newfoundland have been studied by combined UâPb and LuâHf isotope analyses. Variation in ΔHf(t) values with age of zircon populations of distal provenance (>900 Ma) reflect the crustal evolution within the source continents: in zircon derived from Laurentia, episodes of juvenile magma production in the source could be detected at 1.00 â 1.65 and 2.55 â 3.00 Ga, and mixing of juvenile and recycled crust in continental magmatic arcs occurred at 0.95 â 1.40, 1.45 â 1.60, 1.65 â 2.05 and 2.55 â 2.75 Ga. These ages are consistent with the crustal history of northeastern Laurentia. Similarly, zircon of distal provenance from Ganderia reveals times of juvenile magma production in the source at 0.70 â 0.90, 1.40 â 1.75, 1.85 â 2.40 and 2.7 â 3.5 Ga, and episodes of mixing juvenile and recycled crust at 0.95 â 1.35, 1.45 â 1.60, 1.70 â 2.15 and 2.6 â 2.8 Ga. These data reflect the crustal evolution in the present northern part of Amazonia, its likely source craton.     The evolution of magmatic arcs at the margins of both continents can be studied in a similar way using detrital zircon having a proximal provenance (<900 Ma). In contrast to the Laurentian margin, Ganderia is characterized by development of Neoproterozoic â Cambrian continental arcs (ca. 500 â 670 Ma) that were built on the margin of Gondwana. ΔHf(t) values indicate recycling of Neo- and Mesoproterozoic crust. During and following accretion of the various elements of Ganderia to Laurentia, the syn-tectonic Late Ordovician to Silurian sedimentary rocks deposited on the upper plate (composite Laurentia) continued showing only detritus derived from Laurentia. These sedimentary rocks contain detrital zircon from Iapetan juvenile, continental and successor arcs that were active between ca. 440 and 550 Ma, and from continuing magmatic activity until 423 Ma. Arrival of the first Laurentian detritus at the outermost part of Ganderia indicates that the Iapetus ocean was closed at ca. 452 Ma. The magmatic arcs along the former Laurentian margin in Newfoundland evolved differently. In the northwestern part, ΔHf(t) values point to recycling of Mesoproterozoic and Paleoproterozoic crust. In the southwest, ΔHf(t) values indicate addition of juvenile crust, recycling of Mesoproterozoic crust and mixing with juvenile magma. SOMMAIRELes populations de zircons dĂ©tritiques des roches sĂ©dimentaires issus de la marge laurentienne et du microcontinent dâaccrĂ©tion de Ganderia, des deux cĂŽtĂ©s de la principale suture Iapetus (linĂ©ation de Red Indian) dans le centre de Terre-Neuve, ont Ă©tĂ© Ă©tudiĂ©s par analyses combinĂ©es UâPb et LuâHf. Les variations des valeurs ΔHf(t) en fonction de lâĂąge des populations de zircons distaux (>900 Ma) reflĂštent lâĂ©volution de la croĂ»te des continents sources : les zircons de Laurentie ont permis de dĂ©tecter des Ă©pisodes magmatiques juvĂ©niles dans la source entre 1,00 - 1,5, et 2,55 - 3,00 Ga, ainsi que des Ă©pisodes de mĂ©lange de croĂ»te juvĂ©nile avec des croĂ»tes dâarcs magmatiques continentaux recyclĂ©s entre 0,95 â 1,40, 1,45 â 1,60, 1,65 â 2,05, et 2,55 â 2,75 Ga. Ces datations correspondent bien Ă lâhistoire de la croĂ»te de la portion nord-est de la Laurentie. De mĂȘme, le zircon distal de Ganderia rĂ©vĂšle des Ă©pisodes de production de magmas juvĂ©niles dans la source entre 0,70 - 0,90, 1,40 - 1,75, 1,85 - 2,40, et 2,7 - 3,5 Ga, ainsi que des Ă©pisodes de mĂ©langes de matĂ©riaux juvĂ©niles et de croĂ»tes recyclĂ©s entre 0,95 - 1,35, 1,45 - 1,60, 1,70 - 2,15, et 2,6 - 2,8 Ga. Ces donnĂ©es reflĂštent lâĂ©volution de la croĂ»te dans la portion nord actuelle de lâAmazonie, son craton source probable.    LâĂ©volution des arcs magmatiques Ă la marge de ces deux continents peuvent ĂȘtre Ă©tudiĂ©es de la mĂȘme maniĂšre en utilisant le zircon dĂ©tritique proximal (<900 Ma). Contrairement Ă la marge laurentienne, celle de Ganderia est caractĂ©risĂ©e par le dĂ©veloppement dâarcs continentaux NĂ©oprotĂ©ozoĂŻque-Cambrien (env. 500 â 670 Ma) qui se sont constituĂ©s Ă la marge du Gondvana. Les valeurs de ΔHf(t) indiquent un recyclage de la croĂ»te au NĂ©oprotĂ©rozoĂŻque et au MĂ©soprotĂ©rozoĂŻque. Durant et aprĂšs lâaccrĂ©tion des divers Ă©lĂ©ments de Ganderia et de la Laurentie, les roches sĂ©dimentaires syntectoniques de la fin de lâOrdovicien et du Silurien qui se sont dĂ©posĂ©es sur la portion supĂ©rieure de la plaque (Laurentie composite) ne montrent toujours que des dĂ©bris provenant de la Laurentie. Ces roches sĂ©dimentaires renferment des zircons dĂ©tritiques juvĂ©niles iapĂ©tiques, et dâarcs continentaux et dâarcs subsĂ©quents, qui ont Ă©tĂ© actifs entreentre (env. 440 et 550 Ma) et dâune activitĂ© magmatique continue jusquâĂ 423 Ma. Lâapport des premiers dĂ©bris Ă la marge extrĂȘme de Ganderia indique que lâocĂ©an sâest fermĂ©e il y a env. 452 Ma. Les arcs magmatiques le long de l'ancienne marge laurentienne Ă Terre-Neuve ont Ă©voluĂ© diffĂ©remment. Dans la portion nord-ouest, les valeurs de ΔHf(t) indiquent un recyclage de la croĂ»te au MĂ©soprotĂ©rozoĂŻque et au PalĂ©oprotĂ©rozoĂŻque. Dans la portion sud-ouest, les valeurs de ΔHf(t) indiquent lâajout dâune croĂ»te juvĂ©nile, un recyclage de la croĂ»te mĂ©soprotĂ©rozoĂŻque et un mĂ©lange avec un magma juvĂ©nile
Geologic Setting of Eclogite-facies Assemblages in the St. Cyr Klippe, YukonâTanana Terrane, Yukon, Canada
The St. Cyr area near Quiet Lake hosts well preserved to variably retrogressed eclogite found as sub-metre to hundreds of metre-long lenses within quartzofeldspathic schist in south-central Yukon, Canada. The St. Cyr klippe consists of structurally imbricated, polydeformed and polymetamorphosed units of continental arc crust and ultramaficâmafic rocks. Eclogite-bearing quartzofeldspathic schist forms thrust slices in a 30 km long by 6 km wide, northwest-striking outcrop belt. The schist unit comprises metasedimentary and felsic intrusive rocks that are intercalated on the metre to tens of metres scale. Ultramafic rocks, serpentinite and associated greenschist-facies metagabbro form imbricated tectonic slices within the eclogite-bearing quartzofeldspathic unit, which led to a previously held hypothesis that eclogite was exhumed within a tectonic mĂ©lange. The presence of phengite and Permian zircon crystallized under eclogite-facies metamorphic conditions in the quartzofeldspathic host rocks indicate that the eclogite was metamorphosed in situ together with the schist as a coherent unit that was part of the continental arc crust of the YukonâTanana terrane, rather than a mĂ©lange associated with the subduction of oceanic crust of the Slide Mountain terrane. Petrological, geochemical, geochronological and structural similarities link St. Cyr eclogite to other high-pressure localities within Yukon, indicating the high-pressure assemblages form a larger lithotectonic unit within the YukonâTanana terrane.RĂSUMĂLa rĂ©gion de St-Cyr renferme des Ă©clogites bien conservĂ©es Ă lĂ©gĂšrement rĂ©trogradĂ©es qui se prĂ©sentent sous forme de lentilles allant de la fraction de mĂštre Ă quelques centaines de mĂštres de longueur, au sein dâun schiste quartzofeldspathique du centre-sud du Yukon au Canada. La klippe de St-Cyr est structurellement constituĂ©e dâunitĂ©s imbriquĂ©es, polydĂ©formĂ©es et polymĂ©tamorphisĂ©es de croĂ»te dâarc continental et de roches ultramafiques Ă mafiques. Les schistes quartzofeldspathiques Ă lentilles dâĂ©clogites forment des Ă©cailles de chevauchement dâune bande de 30 km de longueur par 6 km de largeur de direction nord-ouest. Les schistes sont constituĂ©s de roches mĂ©tasĂ©dimentaires et de roches intrusives felsiques intercalĂ©es Ă des intervalles qui vont du mĂštre Ă quelques dizaines de mĂštres. Les roches ultramafiques, serpentinites et mĂ©tagabbros au facies Ă schiste vert forment des Ă©cailles tectoniques imbriquĂ©es au sein de lâunitĂ© quartzofeldspathique Ă lentilles dâĂ©clogite, dâoĂč une prĂ©cĂ©dente hypothĂšse voulant que les Ă©clogites soient un produit dâexhumation Ă partir dâun mĂ©lange tectonique. La prĂ©sence de phengite et de zircon permien cristallisĂ© sous conditions mĂ©tamorphiques du faciĂšs Ă Ă©clogite au sein de la roche hĂŽte quartzofeldspathique indiquent que lâĂ©clogite a Ă©tĂ© mĂ©tamorphisĂ©e en place, avec le schiste comme unitĂ© cohĂ©rente du terrane de croĂ»te dâarc continental de YukonâTanana, plutĂŽt quâun mĂ©lange associĂ© Ă une subduction de croĂ»te ocĂ©anique du terrane de Slide Mountain. Des similaritĂ©s pĂ©trologiques, gĂ©ochimiques, gĂ©ochronologiques et structurales lient les Ă©clogites de St-Cyr Ă dâautres lieux de hautes pressions au Yukon, ce qui indique que les assemblages de hautes pressions forment une unitĂ© lithotectonique plus grande au sein du terrane de YukonâTanana
Influence of deformation and fluids on Ar retention in white mica: Dating the Dover Fault, Newfoundland Appalachians
White mica 40Ar/39Ar analyses may provide useful constraints on the timing of tectonic processes, but complex geological and thermal histories can perturb Ar systematics in a variety of ways. Ductile shear zones represent excellent case studies for exploring the link(s) between dynamic re-/neo-crystallization of white mica and coeval enhanced fluid flow, and their effect on 40Ar/39Ar dates. White mica 40Ar/39Ar dates were collected from compositionally similar granites that record different episodes of deformation with proximity to the Dover Fault, a terrane-bounding strike-slip shear zone in the Appalachian orogen, Newfoundland, Canada. 40Ar/39Ar data were collected in situ by laser ablation and by step heating single crystals. Results were compared to each other and against complementary U-Pb zircon and monazite, and K-Ar fault gouge analysis.
Although step-heat 40Ar/39Ar is a widely applied method in orogenic settings, this dataset shows that relatively flat step-heat 40Ar/39Ar spectra are in contradiction with wide spreads in in-situ 40Ar/39Ar dates from the same samples, and that plateau dates in some cases yielded mixed dates of equivocal geological significance. This result indicates that the step-wise release of Ar from white mica likely homogenizes and obscures spatially-controlled Ar isotope reservoirs in white mica from sheared rocks. In contrast, in situ laser ablation 40Ar/39Ar analysis preserves the spatial resolution of 40Ar reservoirs that have been variably reset by deformation and fluid interaction. This study therefore suggests that laser ablation is the best method for dating the timing of deformation recorded by white mica. Final interpretation of results should be guided by microstructural analysis, estimation of deformation temperature, chemical characterization of white mica, and complementary chronometers.
Overall the dataset shows that granitic protoliths were emplaced between 430-422 Ma (U-Pb zircon). High strain deformation along the Wing Pond Shear Zone occurred between ca. 422-405 Ma (U-Pb monazite and 40Ar/39Ar). Subsequent patchy Ar loss in white mica occurred locally during low T shear (40Ar/39Ar). K-Ar dating of authigenic illite in fault gouge from the broadly co-linear brittle Hermitage Bay Fault indicates that slip along the terrane boundary persisted until at least the Mississippian
Evidence of Late Ediacaran Hyperextension of the Laurentian Iapetan Margin in the Birchy Complex, Baie Verte Peninsula, Northwest Newfoundland: Implications for the Opening of Iapetus, Formation of PeriLaurentian Microcontinents and Taconic â Grampian Orogenesis
The Birchy Complex of the Baie Verte Peninsula, northwestern Newfoundland, comprises an assemblage of mafic schist, ultramafic rocks, and metasedimentary rocks that are structurally sandwiched between overlying ca. 490 Ma ophiolite massifs of the Baie Verte oceanic tract and underlying metasedimentary rocks of the Fleur de Lys Supergroup of the Appalachian Humber margin. Birchy Complex gabbro yielded a Late Ediacaran UâPb zircon IDâTIMS age of 558.3 ± 0.7 Ma, whereas gabbro and an intermediate tuffaceous schist yielded LAâICPMS concordia zircon ages of 564 ± 7.5 Ma and 556 ± 4 Ma, respectively. These ages overlap the last phase of rift-related magmatism observed along the Humber margin of the northern Appalachians (565â550 Ma). The associated ultramafic rocks were exhumed by the Late Ediacaran and shed detritus into the interleaved sedimentary rocks. Psammite in the overlying Flat Point Formation yielded a detrital zircon population typical of the Laurentian Humber margin in the northern Appalachians. Age relationships and characteristics of the Birchy Complex and adjacent Rattling Brook Group suggest that the ultramafic rocks represent slices of continental lithospheric mantle exhumed onto the seafloor shortly before or coeval with magmatic accretion of mid-ocean ridge basalt-like mafic rocks. Hence, they represent the remnants of an ocean â continent transition zone formed during hyperextension of the Humber margin prior to establishment of a mid-ocean ridge farther outboard in the Iapetus Ocean. We propose that microcontinents such as Dashwoods and the Rattling Brook Group formed as a hanging wall block and an extensional crustal allochthon, respectively, analogous to the isolation of the Briançonnais block during the opening of the Alpine LigurianâPiemonte and Valais oceanic seaways.Le complexe de Birchy de la pĂ©ninsule de Baie Verte, dans le nord-ouest de Terre-Neuve, est constituĂ© dâun assemblage de schistes mafiques, de roches ultramafiques et de mĂ©tasĂ©diments qui sont coincĂ©s entre des massifs ophiolitiques dâascendance ocĂ©anique de la Baie Verte au-dessus, et des mĂ©tasĂ©diments du Supergroupe de Fleur de Lys de la marge de Humber des Appalaches en-dessous. Le complexe de gabbro de Birchy a donnĂ© une datation U-Pb sur zircon ID-TIMS correspondant Ă la fin de lâĂdiacarien, soit 558,3 ± 0,7 Ma, alors quâun gabbro et un schiste tufacĂ© intermĂ©diaire montrent une datation LA-ICP-MS Concordia sur zircon de 564 ± 7,5 Ma et 556 ± 4 Ma, respectivement. Ces datations chevauchent la derniĂšre phase de magmatisme de rift observĂ©e le long de la marge Humber des Appalaches du Nord (565-550 Ma). Les roches ultramafiques associĂ©es ont Ă©tĂ© exhumĂ©es vers la fin de lâĂdiacarien et leurs dĂ©bris ont Ă©tĂ© imbriquĂ©s dans des roches sĂ©dimentaires. Les psammites de la Formation de Flat Point susjacente ont donnĂ© une population de zircons dĂ©tritiques typique de la marge laurentienne de Humber des Appalaches du Nord. Les relations chronologiques et les caractĂ©ristiques du complexe de Birchy et du groupe de Rattling Brook adjacent, permettent de penser que ces roches ultramafiques pourraient ĂȘtre des Ă©cailles de manteau lithosphĂ©rique continental qui auraient Ă©tĂ© exhumĂ©es sur le plancher ocĂ©anique peu avant ou en mĂȘme temps que lâaccrĂ©tion magmatique de roches mafiques basaltiques de type dorsale mĂ©dio-ocĂ©anique. Par consĂ©quent, elles seraient des vestiges dâune zone de transition ocĂ©an-continent formĂ©e au cours de lâhyper-extension de la marge de Humber avant lâapparition dâune dorsale mĂ©dio-ocĂ©anique plus loin au large dans lâocĂ©an IapĂ©tus. Nous proposons que des microcontinents comme de Dashwoods et du groupe de Rattling Brook ont constituĂ©s respectivement un bloc de toit et un allochtone crustal dâextension, de la mĂȘme maniĂšre que le bloc Briançonnais a Ă©tĂ© isolĂ© lors de lâouverture des bras ocĂ©aniques alpins de Ligurie-PiĂ©mont et de Valais.Fil: Van Staal, Cees R.. Geological Survey of Canada; CanadĂĄFil: Chew, Dave M.. Trinity College Dublin; IrlandaFil: Zagorevski, Alexandre. Geological Survey of Canada; CanadĂĄFil: Mcnicoll, Vicki. Geological Survey of Canada; CanadĂĄFil: Hibbard, James. North Carolina State University; Estados UnidosFil: Skulski, Tom. Geological Survey of Canada; CanadĂĄFil: Castonguay, SĂ©bastien. Geological Survey of Canada; CanadĂĄFil: Escayola, Monica Patricia. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de Estudios Andinos; ArgentinaFil: Sylvester, Paul J.. Memorial University Of Newfoundland; Canad
Assembly of the Annieopsquotch Accretionary Tract, Newfoundland Appalachians: Age and Geodynamic Constraints from SynâKinematic Intrusions
The Annieopsquotch Accretionary Tract (AAT) comprises several ophiolites and arcâbackâarc igneous complexes that were accreted to the Dashwoods microcontinent during the Ordovician Taconic orogeny. The Lloyds River Fault Zone, which separates the AAT from the Dashwoods microcontinent, yielded 40Ar/39Ar hornblende ages of ca. 470 Ma. The fault zone was intruded synâkinematically by the shoshonitic Portage Lake monzogabbro and the Pierreâs Pond suite, which gave U/Pb zircon ages of Ma plus Ma and Ma, respectively. The Otter Pond granodiorite intruded synâkinematically into the Otter Brook Shear Zone, which separates the Annieopsquotch ophiolite belt from the structurally underlying ophiolitic Lloyds River Complex. It yielded a U/Pb zircon age of Ma. The Buchans arc and its continental basement were accreted to the Lloyds River Complex prior to 468 Ma. Synâkinematic plutons have arc affinity, with ΔNd ranging between â0.9 and â6.8, and are coeval with the adjacent Notre Dame Arc. Our data thus suggest the majority of the AAT was accreted to the Dashwoods microcontinent by 468 Ma, when consanguineous, dominantly arclike plutons intruded within the AAT and adjacent Notre Dame Arc. The Portage Lake monzogabbro and Otter Pond mafic suite are more mafic than Notre Dame Arc plutons of similar age because of their intrusion into the thin, mafic crust of the AAT and ascent along shear zones. Our data indicate the formation and subsequent accretion of ophiolites and arcâbackâarc complexes occurred within a very short time span (5â10 Ma). The sources of AAT synâorogenic magmatism are diverse and include melting of subarc mantle during slab breakoff, lithospheric mantle, and lower crust. The Ordovician Appalachian margin of Laurentia grew by the accretion of oceanic terranes and intrusion of mantleâderived magma. Recycling of continental crust by rifting and subsequent collision played an important part of the tectonic evolution of the AAT