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Geodynamic evolution in the late Paleozoic is significant for understanding the final amalgamation of the Central Asian Orogenic Belt (CAOB). No consensus has yet been reached regarding the late Paleozoic geodynamic evolution of the northern Great Xing'an Range (GXR) in northeastern China, the eastern CAOB. Furthermore, late Paleozoic syenogranitediabase dyke association is present in the Xiaokele area in northern GXR. It provides an important opportunity to understand the nature of magmatism and the geodynamic evolution during this period. This paper presents new zircon U-Pb ages, zircon Hf isotopic compositions, and geochemical data of whole rocks for Xiaokele syenogranite and diabase. Zircon U-Pb dating suggests that the Xiaokele syenogranite (292.5 [+ or -] 0.9 Ma) and diabase (298.3 [+ or -] 1.5 Ma) were emplaced during the early Permian. The Xiaokele syenogranites have high Si[O.sub.2] contents, low MgO contents, and enriched zircon [[epsilon].sub.Hf](t) values, suggesting that their primary magma was generated by the partial melting of the juvenile crustal material. The Xiaokele diabases have low Si[O.sub.2] contents, high MgO contents, are enriched in large-ion lithophile elements, depleted in high-field-strength elements, and exhibit enriched zircon [[epsilon].sub.Hf](t) values. They derived from a lithospheric mantle source that had previously been metasomatized by slab-derived fluids. Combined with previous research results, we believe that the continent-continent collision between the Xing'an and Songliao blocks occurred during the late early Carboniferous - early late Carboniferous (330-310 Ma), and the two blocks were transformed into a post-collisional extensional setting during the latest Carboniferous - early Permian. Key words: early Permian, syenogranite, diabase, post-collisional extensional setting, Xiaokele, northern Great Xing'an Range. L'evolution geodynamique au Paleozoique tardif est importante pour la comprehension de l'amalgamation finale de la ceinture orogenique d'Asie centrale (COAC). Aucun consensus n'a encore ete etabli quant a l'evolution geodynamique au Paleozoique tardif du Grand Xing'an (GX) septentrional de l'est de la COAC dans le nord-est de la Chine. En outre, une association d'age paleozoique tardif de syenogranite et de dykes de diabase est presente dans la region de Xiaokele du GX septentrional, qui offre une bonne occasion de mieux comprendre la nature du magmatisme et de l'evolution geodynamique durant cette periode. L'article presente de nouveaux ages U-Pb sur zircons, des donnees sur la composition des isotopes de Hf des zircons et des donnees geochimiques sur roche totale pour le syenogranite et la diabase de Xiaokele. La datation U-Pb sur zircons indiquerait que le syenogranite (292,5 [+ or -] 0,9 Ma) et la diabase (298,3 [+ or -] 1,5 Ma) de Xiaokele ont ete mis en place durant le Permien precoce. Les syenogranites de Xiaokele ont des teneurs elevees en Si[O.sub.2] et faibles MgO et presentent des valeurs enrichies de [[epsilon].sub.Hf](t) des zircons, ce qui indiquerait que leur magma primaire est un produit de la fusion partielle de materiau crustal juvenile. Les diabases de Xiaokele ont des teneurs en Si[O.sub.2] faibles et en MgO elevees et sont enrichies en elements lithophiles a grand rayon ionique, appauvries en elements a forte liaison atomique et presentent des valeurs enrichies de [[epsilon].sub.Hf](t) des zircons. Elles proviennent d'une source dans le manteau lithospherique ayant prealablement subi un metasomatisme cause par des fluides issus d'une plaque subductee. A la lumiere de ces resultats et de resultats de travaux anterieurs, nous croyons que la collision continent-continent des blocs de Xing'an et de Songliao s'est produite durant l'intervalle du Carbonifere precoce tardif au Carbonifere tardif precoce (330 Ma-310 Ma) et que les deux blocs se sont transformes en un milieu d'extension post-collision durant l'intervalle de la fin du Carbonifere au Permien precoce. [Traduit par la Redaction] Mots-cles: Permien precoce, syenogranite, diabase, milieu d'extension post-collision, Xiaokele, Grand Xing'an septentrional., Introduction The tectonic evolution of orogenic belts is characterized by changes in the associated magmatic composition (Harris et al. 1986). Post-collisional magmatism, as one of the common characteristics of many [...]

The structure and composition of granites provide clues to the nature of silicic volcanism, the formation of continents, and the rheological and thermal properties of the Earth's upper crust as far back as the Hadean eon during the nascent stages of the planet's formation.sup.1-4. The temperature of granite crystallization underpins our thinking about many of these phenomena, but evidence is emerging that this temperature may not be well constrained. The prevailing paradigm holds that granitic mineral assemblages crystallize entirely at or above about 650-700 degrees Celsius.sup.5-7. The granitoids of the Tuolumne Intrusive Suite in California tell a different story. Here we show that quartz crystals in Tuolumne samples record crystallization temperatures of 474-561 degrees Celsius. Titanium-in-quartz thermobarometry and diffusion modelling of titanium concentrations in quartz indicate that a sizeable proportion of the mineral assemblage of granitic rocks (for example, more than 80 per cent of the quartz) crystallizes about 100-200 degrees Celsius below the accepted solidus. This has widespread implications. Traditional models of magma formation require high-temperature magma bodies, but new data.sup.8,9 suggest that volcanic rocks spend most of their existence at low temperatures; because granites are the intrusive complements of volcanic rocks, our downward revision of granite crystallization temperatures supports the observations of cold magma storage. It also affects the link between volcanoes, ore deposits and granites: ore bodies are fed by the release of fluids from granites below them in the crustal column; thus, if granitic fluids are hundreds of degrees cooler than previously thought, this has implications for research on porphyry ore deposits. Geophysical interpretations of the thermal structure of the crust and the temperature of active magmatic systems will also be affected. Thermobarometry and diffusion modelling in quartz crystals show that some granites may crystallize at much lower temperatures than we had thought, possibly explaining observations of cold magma storage., Author(s): Michael R. Ackerson [sup.1] [sup.2] , B. O. Mysen [sup.1] , N. D. Tailby [sup.3] , E. B. Watson [sup.2] Author Affiliations: (1) Geophysical Laboratory, Carnegie Institution of Washington, [...]

The Coldbrook Group is a unique suite of Late Ediacaran volcanic and epiclastic rocks that together with related plutons was emplaced in the Avalonian Caledonia terrane of southern New Brunswick. [...]

Avalonia, the largest accreted crustal block in the Appalachian orogen, consists of Neoproterozoic magmatic arc sequences that represent protracted and episodic subduction-related magmatism before deposition of an Ediacaran -Ordovician cover sequence including siliciclastic rocks. Zircon crystals were obtained from arc-related magmatic rocks and from clastic sedimentary sequences and analyzed in situ for their Hf-isotope composition. The majority of magmatic and detrital zircons are dominated by initial [sup.176]Hf /[sup.177]Hf values that are more radiogenic than chondritic uniform reservoir (CHUR) with calculated crust formation Hf-[T.sub.DM] model ages that range from 0.84 to 1.30 Ga. These results suggest formation by partial melting of juvenile mantle in a Neoproterozoic continental arc. Some zircons have Hf-[T.sub.DM] model ages ca. 1.39-3.09 Ga with ζHf values of 33.9 to -0.5 and more clearly indicate involvement of older lithosphere in their petrogenesis. Whole-rock Sm-Nd isotopic compositions from felsic volcanic rocks are characterized by positive initial ζNd values with Mesoproterozoic depleted mantle model ages consistent with juvenile extraction. Results suggest a dominant mantle component with long-term light rare earth element (LREE) depletion mixed with an older crustal component with long-term LREE enrichment. The pattern of [T.sub.DM] model ages and variations in Lu-Hf and Sm-Nd isotopic character are compatible with a ca. 1.0-1.2 Ga igneous tectonomagmatic event that formed basement to Neoproterozoic magmatic arcs in Avalonia. The presence of evolved isotopic signatures, however, indicates that significant older Proterozoic crust is present locally beneath Avalonia, suggesting that Avalonia formed in a single Neoproterozoic arc system that generated juvenile mantle-derived crust, coupled with lesser anatectic reworking of significantly older crust. Le plus grand bloc crustal accrete de l'orogene des Appalaches, Avalonia, est compose de sequences d'arcs magmatiques datant du Neoproterozoique qui representent un magmatisme prolonge et episodique relie a la subduction avant la deposition d'une sequence couverture a l'Ediacarien-Ordovicien, laquelle comprend des roches siliclastiques. Des cristaux de zircon ont ete obtenus de roches magmatiques, reliees a des arcs, et de sequences sedimentaires clastiques; ils ont ete analyses in situ pour leur composition en isotopes Hf. La plupart des zircons magmatiques et detritiques sont domines par des valeurs [sup.176]Hf/[sup.177]Hf initiales qui sont plus radiogeniques que celles du modele CHUR (chondritic uniform reservoir) avec des ages de formation de la croute, calcules selon le modele d'un manteau appauvri (Hf-[T.sub.DM]), qui varient de 0,84 a 1,30 Ga. Ces resultats suggerent une formation par fusion partielle du manteau juvenile dans un arc continental au Neoproterozoique. Quelques zircons ont des ages selon le modele Hf-[T.sub.DM] d'environ 1,39-3,09 Gaavec des valeurs ζHf de-33,9 a-0,5 et indiquent ainsi plus clairement l'implication d'une lithosphere plus ancienne dans leur petrogenese. Les compositions isotopiques Sm-Nd, sur la roche entiere, de roches volcaniques felsiques sont caracterisees par des valeurs initiales ζNd positives avec des ages de modele de l'appauvrissement du manteau au Mesoproterozoique qui concordent avec une extraction juvenile. Selon les resultats, une composante dominante du manteau avec un appauvrissement a long terme en elements de terres rares legers (LREE) s'est melangee avec une composante plus ancienne de la croute qui avait un enrichissement a long terme en elements de terres rares. Le patron des ages du modele [T.sub.DM] et les variations des caracteres isotopiques Lu-Hf et Sm-Nd sont compatibles avec un evenement tectono-magmatique (~ 1,0-1,2 Ga) qui a forme des arcs magmatiques du socle au Neoproterozoique en Avalonia. La presence de signatures isotopiques evoluees indique toutefois qu'une quantite importante d'ancienne croute proterozoique est presente par endroits sous Avalonia, suggerant la formation d'Avalonia dans un systeme d'arc unique au Neoproterozoique qui a genere la croute juvenile derivee du manteau, ainsi qu'un remaniement anatectique moindre d'une croute beaucoup plus ancienne. [Traduit par le Redaction], Introduction Crust formation and the growth of continents occur principally in convergent margin tectonic settings in which large aqueous fluid fluxes and (or) melt expelled from subducting oceanic lithosphere cause [...]

Volcanic rocks of the Jurassic Iskut River Formation (IRF) in northwestern British Columbia (Canada) host several volcanogenic massive sulfide (VMS) deposits, including the exceptionally high-grade Eskay Creek Ag-Au-Cu-Pb-Zn deposit. The IRF comprises voluminous pillow basalt (>90%), minor rhyolite, and sedimentary rock of late Early to early Middle Jurassic age, filling a series of sub-basins along a 300 by 50 km north-trending belt. Two geochemically distinct types of tholeiitic basalts interfinger; both resemble back-arc basin basalts formed from the melting of asthenospheric and sub-arc mantle sources. Group 2 basalts are more enriched in light rare-earth elements, Ba, K, Sr, Th, and U, and have lower positive [ζ.sub.Nd] values than group 1 basalts (+3.2 to +6.3 versus +6.9 to +8.4, respectively). The compositional differences between group 1 and group 2 basalts are interpreted to result from crustal contamination in group 2. Group 1 basalts are most common in the southern part of the IRF belt where they are closely associated with the Eskay Creek, Bonanza, and Hidden Creek (Anyox) VMS deposits. Group 2 basalts are most abundant in the northern half of the belt and are not associated with exploited mineral deposits. The lack of crustal contamination in group 1 basalts indicates that they formed from rapidly ascending magma in an advanced rift setting and were associated with high heat flow that drove hydrothermal circulation. Group 1 and group 2 basalts are reliably discriminated by Ta/Th 2.5 in the latter. This geochemical criteria can therefore be used as an exploration tool to identify VMS permissive sub-basins and (or) stratigraphy in the IRF. Les roches volcaniques de la Formation d'Iskut River (Jurassique) dans le nord-ouest de la Colombie- Britannique, Canada, contiennent plusieurs gisements de sulfures massifs volcanogenes (VMS), incluant le gisement d'Eskay Creek, a teneur exceptionnellement elevee en Ag-Au-Cu-Pb-Zn. La Formation d'Iskut River comprend beaucoup de basaltes a coussins (>90 %), un peu de rhyolite et de roches sedimentaires datant du Jurassique precoce tardif a moyen precoce, remplissant une serie de sous-bassins le long d'une ceinture de 300 sur 50 km a tendance nord. Deux types de basaltes tholeiitiques a geochimie distincte sont interdigites; les deux ressemblent a des basaltes de bassins d'arriere arc formes durant la fusion de sources asthenospheriques et de manteau sous des arcs. Les basaltes de groupe 2 sont plus enrichis en elements de terres rares Ba, K, Sr, Th et U et ont des valeurs [ζ.sub.Nd] positives inferieures aux basaltes de groupe 1 (respectivement +3,2 a +6,3 comparativement a +6,9 a +8,4). Les differences de composition entre les basaltes de groupe 1 et de groupe 2 sont interpretees comme etant le resultat d'une contamination de la croute dans le cas du groupe 2. Les basaltes de groupe 1 se trouvent plus frequemment dans la partie sud de la ceinture de la Formation d'Iskut River ou ils sont etroitement associes aux gisements VMS d'Eskay Creek, de Bonanza et de Hidden Creek (region d'Anyox). Les basaltes de groupe 2 sont plus abondants dans la moitie nord de la ceinture et ils ne sont pas associes a des gisements mineraux exploites. Le manque de contamination par la croute dans les basaltes de groupe 1 indique qu'ils ont ete formes a partir d'un magma qui montait rapidement dans un environnement de distension avancee et qu'ils etaient associes a un important flux thermique qui poussait la circulation hydrothermale. Les basaltes de groupes 1 et 2 peuvent etre distingues de maniere fiable par le rapport Ta/Th qui est 2,5 pour le groupe 2. Ce critere geochimique peut donc etre utilise comme un outil d'exploration pour identifier les sous-bassins et (ou) une stratigraphie admettant des VMS dans la Formation d'Iskut River. [Traduit par le Redaction], Introduction Volcanogenic massive sulphide (VMS) deposits are important sources of copper, zinc, lead, gold, and silver (e.g., Franklin et al. 2005). They form at or near the sea floor as [...]

The Great Bear magmatic zone, located in Wopmay orogen, is a 1.875-1.84 Ga belt, 450 km long by 100 km wide of volcanic and allied plutonic rocks interpreted as a Paleoproterozoic magmatic arc. The belt, which contains economically important mineralization, was folded and subsequently cut by a swarm of northeast-striking transcurrent faults, which are part of a regional conjugate fault system interpreted to result from terminal collision of the Nahanni--Fort Simpson terrane. Fault reconstructions based on the interpretation of aeromagnetic data and geological maps provide first-order models of deformation mechanisms associated with, and the configuration of the Great Bear magmatic zone prior to, its dissection by northeast-striking transcurrent faults. The models show that vertical axis block rotation (plane strain) of ~4.5° can explain fault offsets in the south, but that greater rotation is required to explain many of the displacements in the north. However, offsets on transcurrent faults that border the Camsell River district are greater than can be explained by vertical axis block rotation model alone and may include a component of Mesoproterozoic contractional deformation associated with the Racklan-Forward orogeny. Following reconstruction, iron oxide alkali alteration and associated mineralization, which pre-date transcurrent faulting, form a pair of northerly trending zones on the east and west margins of the belt. We suggest that these zones, whose exposure is related to broad synclinal folding of some of the oldest rocks in the Great Bear magmatic zone, are where iron oxide copper-gold (IOCG)-targeted exploration efforts should be focused on these areas in both outcrop and subcrop. La zone magmatique de Great Bear, situee dans l'orogene Wopmay, est une ceinture agee de 1,875 a 1,84 Ga, d'une longueur de 450 km et d'une largeur de 100 km; elle est composee de roches volcaniques et de roches plutoniques associees, lesquelles sont interpretees comme etant un arc magmatique datant du Paleoproterozoique. La ceinture, qui contient une mineralisation d'importance economique, a ete plissee et par la suite recoupee par un essaim de failles de decrochement faisant partie d'un systeme regional de failles conjuguees, interpretees comme le resultat de la collision terminale du terrane Nahanni--Fort Simpson. Les reconstructions des failles, basees sur l'interpretation de donnees aeromagnetiques et de cartes geologiques, fournissent des modeles de premier ordre des mecanismes de deformation associes a sa dissection par des failles de decrochement a direction nord-est; les donnees permettent aussi la configuration de la zone magmatique de Great Bear avant les deformations. Selon les modeles, la rotation du bloc autour d'un axe vertical (deformation plane) d'environ 4,5° peut expliquer les rejets des failles au sud, mais il faut une plus grande rotation pour expliquer plusieurs des deplacements au nord. Toutefois, les rejets sur les failles decrochantes qui longent le district de la riviere Camsell sont superieurs a ce qui peut etre explique par le seul modele de rotation d'un bloc autour d'un axe vertical et ils pourraient comprendre une composante de deformation de contraction, au Mesoproterozoique, associee a l'orogene Racklan-Forward. Apres la reconstruction, une alteration alcaline d'oxyde de fer et une mineralisation associee, precedant les failles de decrochement, forment une paire de zones a tendance nord sur les bordures est et ouest de la ceinture. Selon nous, ces zones, dont l'affleurement est relie a un vaste plissement synclinal de quelques-unes des plus vieilles roches de la zone magmatique de Great Bear, sont des endroits cibles pour l'exploration de gisements oxyde de fer, cuivre, or (IOCG) a la fois dans les affleurements et les sous-affleurements. [Traduit par le Redaction], Introduction The Great Bear magmatic zone (Fig. 1a) is interpreted to be a continental magmatic arc, within the Paleoproterozoic Wopmay orogen of the Northwest Territories, Canada (Hoffman and McGlynn 1977; [...]

Oxygen isotopes are a valuable tool for determining the role of supracrustal rocks in the genesis of magmas and help constrain the source regions of plutonism in the northern Appalachian mountains of North America. Zircons from 35 igneous or metaigneous rocks mainly from southern New England, USA, were analyzed for [sup.18]O/[sup.16]O to locate and investigate the origin of anomalous [δ.sup.18]O magmas and to evaluate the regional meteoric water-rock interaction. [δ.sup.18]O values of the zircons obtained using the laser fluorination technique vary between 2.3 [per thousand] and 11.0‰, relative to Vienna Standard Mean Ocean Water (VSMOW). Zircons from two plutons in the Avalon terrane reveal the presence of anomalously low- [δ.sup.18]O ( Les isotopes de l'oxygene sontun outil important pour determiner le role des roches supracrustales dans lagenese des magmas et ils aident a delimiter les regions sources du plutonisme dans le nord des montagnes appalachiennes de l'Amerique du Nord. Des zircons provenant de 35 roches ignees ou meta-ignees, principalement du sud de la NouvelleAngleterre, ont ete analyses pour [sup.18]O/[sup.16]O afin de localiser et d'etudier l'origine des magmas a anomalie [δ.sup.18]O et d'evaluer les interactions regionales entre l'eau meteorique et les roches. Les valeurs [δ.sup.18]O des zircons, obtenues au moyen de la technique de fluoration au laser, varient entre 2,3 ‰ et 11,0 ‰ VSMOW (eau oceanique moyenne normalisee de Vienne). Les zircons provenant de deux plutons du terrane d'Avalon revelent la presence de zircons a teneur [δ.sup.18]O anormalement basse (> (Ordovicien tardif a Silurien precoce) du nord du Connecticut (zircon [δ.sup.18]O en vrac = 7,56 ‰) indiquent l'incorporation de quantites importantes de composantes sedimentaires a teneur elevee [δ.sup.18]Oa partir des regions sources plus anciennes dans ces magmas. Les terranes d'Avalon et de Gander du sud-est de la Nouvelle-Angleterre et la bordure est de Laurentia ont subi de longs historiques d'enfouissement ou de subduction, de fusion et d'assimilation de roches provenant de la surface au cours du Neoproterozoique et du milieu du Paleozoique. [Traduit par lar Redaction], Introduction Compared with other rock-forming minerals such as quartz and feldspar, zircon is highly resistant to oxygen isotopic exchange during post-crystallization hydrothermal alteration (e.g., Valley 2003) and thus can be [...]

The Cantung tungsten-copper (W-Cu) skarn orebodies are hosted by Proterozoic and Lower Cambrian metasedimentary rocks next to the Cretaceous 'Mine Stock' monzogranite. Paleomagnetic analyses of 283 specimens from the Open Pit scheelitechalcopyrite orebody (17 sites) and from adjacent host rocks including the aplite dikes (11 sites) isolated a stable characteristic remanent magnetization (ChRM), mostly by alternating field and then thermal step demagnetization. The step demagnetization results along with rock magnetic analyses of the W concentrate show that the main remanence carriers are single- or pseudosingle-domain pyrrhotite, titanomagnetite, and (or) magnetite. There is no statistically significant difference at 95% confidence between the site mean ChRM directions for the W-Cu ore, the host rock, or the aplite dikes populations. This result indicates that the intrusion of the Mine Stock is coeval with the genesis of the scheelite skarn ore and with dike emplacement to give an overall mean ChRM direction of declination 342.9°, inclination 82.0° (N =22 sites, radius of cone of 95% confidence [α.sub.95] = 4.2°, precision parameter k = 54.7) that defines a paleopole at 76.2°N latitude, 212.2°E longitude (radius of cone of 95% confidence [A.sub.95] = 8.1°). This paleopole is concordant with the coeval 98 Ma North American paleopole at 92% confidence, which provides strong evidence that the eastern Selwyn Basin has been an autochthonous part of North America since the mid- Cretaceous. Les gisements de skarns a tungstene-cuivre (W-Cu) de Cantung sont encaisses dans des roches metasedimentaires datant du Proterozoique et du Cambrien inferieur, a proximite du stock intrusif de monzogranite << Mine Stock >> datant du Cretace. Des analyses paleomagnetiques sur 283 echantillons provenant du gisement nomme << Open Pit >> de scheelitechalcopyrite (17 sites) et des roches encaissantes adjacentes, incluant des dykes d'aplite (11 sites), ont permis d'isoler une aimantation remanente caracteristique stable (ChRM), principalement par alternance des champs puis par desaimantation thermique par etapes. Les resultats de la desaimantation par etapes, jumeles aux analyses magnetiques du concentre W, montrent que les principaux porteurs de la remanence sont une pyrrhotite, une titano-magnetite et/ou une magnetite, a domaine unique ou pseudo-unique. Il n'y a pas de difference statistique significative, au niveau de confiance de 95 %, entre les directions moyennes de la ChRM pour le minerai W-Cu, la roche encaissante ou les populations de dykes d'aplite. Selon ce resultat, l'intrusion du<>est contemporaine de la genese du minerai de skarns de scheelite et de la mise en place des dykes pour donner une direction moyenne generale de declinaison de la ChRM de 342,9 °, d'inclinaison de 82,0 ° (N = 22 sites, rayon du cone de confiance a 95 % [α.sub.95] = 4,2 °, parametre de precision k = 54,7) definissant un paleopole a une latitude de 76,2 °N et a une longitude de 212,2 °E (rayon du cone de confiance a 95 % [A.sub.95] = 8,1 °). Ce paleopole concorde avec le paleopole nord-americain contemporain de 98 Ma avec un niveau de confiance de 92 %, ce qui fournit de fortes indications que le bassin Selwyn oriental forme une partie autochtone de l'Amerique du Nord depuis le Cretace moyen. [Traduit par la Redaction], Introduction The 98-95 Ma Tungsten plutonic suite in the eastern Selwyn Basin forms an ~ 300 km long northwest-trending belt of plutons in the western Northwest Territories (NWT) and eastern [...]

We present new major element and isotopic (Nd-Sr-Hf-Pb) data and modelling from alkaline rocks of the Monteregian Igneous Province of southern Quebec (Canada) that constrain the mantle source and the magmatic origin of these rocks. The whole-rock chemical composition of the intrusions is consistent with fractional crystallization of an assemblage of olivine [+ or -] clinopyroxene (± plagioclase) derived from ocean island basalts (OIB)-like magmas, and variations in the Sr and Nd isotope compositions suggest as much as 20% crustal contamination. The bulk of the Nd-Sr-Hf and Pb isotopic data form a tight cluster between a depleted mantle end-member (HIMU, high-U/Pb mantle) and an enriched mantle (EMI) end-member and are thought to reflect a sub-continental lithospheric mantle that was metasomatized by a convecting asthenospheric plume. Variations in these isotopic compositions along the west-east axis of the Monteregian Province (from the Oka carbonatite to the Mount Shefford intrusion) may reflect various degrees of mixing between HIMU and EMI enriched mantle reservoirs. Anomalously low [sup.207]Pb/[sup.204]Pb and [sup.208]Pb/[sup.204]Pb isotopic ratios from some of the intrusions likely indicate incorporation of an Archean component within the lithospheric mantle. We propose a model in which Monteregian magmatism formed from melting of a predominantly Proterozoic metasomatized lithospheric mantle in response to lithospheric extension during the opening of the North Atlantic Ocean at ca. 124 Ma. L'origine de la Province Ignee des Monteregiennes, Sud Quebec (Canada), est fortement debattue, principalement pour savoir lequel du modele du point chaud ou du modele du rift continental prevaut. Dans cette etude, nous presentons de nouvelles donnees geochimiques et isotopiques (Nd-Sr-Hf-Pb) des roches de la Province Ignee des Monteregiennes afin de contraindre la source mantellique et l'origine de ce magmatisme. La composition chimique des intrusions montre la presence de cristallisation fractionnee d'un assemblage olivine ± clinopyroxene (± plagioclase) derive de magmas de type OIB. Les variations de compositions isotopiques en Sr et Nd suggerent un maximum de 20% de contamination bien que les donnees isotopiques du Sr, Nd, Hf et Pb montrent un regroupement de points entre le pole mantellique type HIMU (appauvri) et le pole type EMI (enrichi). Ces compositions isotopiques semblent ainsi refleter un manteau lithospherique sub-continental metasomatise par la remontee d'un manteau asthenospherique convectif. Une evolution Ouest-Est de la composition isotopique (Nd, Sr, Hf, Pb) est observee, des carbonatites d'Oka vers l'intrusion de Shefford. Cette tendance parait refleter la variation du taux de melange entre les reservoirs HIMU et EMI; la signature isotopique HIMU est dominante a l'Ouest tandis que la signature isotopique EMI est dominante vers l'Est. De faibles rapports isotopiques en [sup.207]Pb/[sup.204]Pb et [sup.208]Pb/[sup.204]Pb sont observes dans certains echantillons et indiquent l'incorporation d'un materiel Archeen dans le manteau lithospherique Proterozoique. Ces resultats indiquent qu'un manteau lithospherique metasomatise est a l'origine du magmatisme des Monteregiennes, possiblement en reponse a une extension lithospherique durant l'ouverture de l'Ocean Atlantique Nord vers 124 Ma., Introduction Magmas of continental igneous provinces range from large volume tholeiitic flood basalts such as the Deccan Province and Columbia River basalts (e.g., Hawkesworth and Gallagher 1993, and references therein) [...]

The Horse Cove Complex (HCC) is a recently recognized late Ediacaran swarm of mafic-to-felsic dykes that occurs in the Newfoundland Avalon Zone. It is focused along the extrapolated trace of the Topsail Fault along the east coast of Conception Bay and is hosted by mafic submarine volcanic rocks, diorite, and, locally, granodiorite. The HCC has been divided into rock units that represent dykes of mafic-to-felsic composition, hosted by feldspar porphyry, gabbro, and diorite. The age of magmatism in the complex has been bracketed by U-Pb zircon ages, determined by chemical abrasion--thermal ionization mass spectrometry (CA-TIMS), of feldspar porphyry (581 ±2.0 Ma) and an andesitic dyke (578 ±2.3 Ma). Magmatism occurred over a period of 6.5 Ma (or less) and overlaps in age with felsic and mafic volcanic centres and related intrusions on the northeastern Avalon Peninsula. Feldspar porphyry and rhyolitic dykes show arc-related chemistry and may be co-magmatic. The mafic-to-intermediate rocks in the HCC are calc-alkaline to tholeiitic and exhibit a range of compositions, from those that have the characteristics of enriched mid-ocean ridge basalt (E-MORB-like) to those that show subduction-related light rare-earth element (LREE) enrichment and negative Nb anomalies. This range of compositions has ζNd values (at 580 Ma) of +6.4 to +4.1. The HCC likely formed in a back-arc basin environment, in which rocks with LREE-enriched mantle sources and subduction-contaminated sources were emplaced side by side and closely in time. The complex may represent the last phase of subduction-related magmatism in the eastern Avalon Zone in Newfoundland prior to deep marine, deltaic, and alluvial fan sedimentation during the late Neoproterozoic. Le complexe de Horse Cove a ete recemment reconnu comme un essaim de dykes, mafiques a felsiques, datant de l'Ediacarien, situe dans la zone Avalon de Terre-Neuve. Il est localise le long de la trace extrapolee de la faille Topsail le long de la cote est de la baie Conception et il est encaisse dans des roches meres volcaniques sous-marines mafiques, des diorites et, par endroits, des granodiorites. Le complexe de Horse Cove a ete subdivise en unites rocheuses representant des dykes de composition mafique a felsique, encaisses dans des feldspaths porphyriques, des gabbros et des diorites. L'age du magmatisme dans le complexe a ete delimite par des ages determines sur des zircons par U-Pb, CA-TIMS [abrasion chimique--spectrometrie demasse a thermoionisation] dans des feldspaths porphyriques (581 ± 2,0Ma) et un dyke d'andesite (578 ± 2,3Ma). Lemagmatisme a eu lieu sur une periode de 6,5 Ma (ou moins) et chevauche, dans le temps, des centres volcaniques felsiques et mafiques ainsi que des intrusions connexes du nord- est de la peninsule d'Avalon. Les dykes de feldspath porphyrique et de rhyolite presentent une chimie reliee aux arcs; ils pourraient etre comagmatiques. Les roches, mafiques a intermediaires, dans le complexe de Horse Cove sont calcoalcalines a tholeiitiques et presentent une gamme de compositions, depuis celles du type E-MORB (basaltes de rides medio-oceaniques enrichis) a celles qui presentent un enrichissement en elements de terres rares legeres reliees a une subduction ainsi que des anomalies Nb negatives. Cette gamme de compositions a des valeurs 3Nd (a 580 Ma) de +6,4 a +4,1. Le complexe de Horse Cove s'est probablement forme dans un environnement de bassin d'arriere-arc, dans lequel des roches ayant des sourcesmanteliques enrichies en elements de terres rares legeres et de sources contaminees par la subduction ont etemises en place cote a cote presque en meme temps. Le complexe pourrait representer la derniere phase d'un magmatisme relie a la subduction dans l'est de la zone d'Avalon a Terre-Neuve avant la sedimentation enmer profonde, deltaique et de cone alluvial au cours du Neoproterozoique tardif. [Traduit par la Redaction], Introduction The newly recognized and informally named Horse Cove Complex (HCC; Sparkes 2006) is a map unit in the Avalon Zone of Newfoundland. It occurs over a distance of approximately [...]

Caldera-forming volcanic eruptions are low-frequency, high-impact events capable of discharging tens to thousands of cubic kilometres of magma explosively on timescales of hours to days, with devastating effects on local and global scales (1). Because no such eruption has been monitored during its long build-up phase, the precursor phenomena are not well understood. Geophysical signals obtained during recent episodes of unrest at calderas such as Yellowstone, USA, and Campi Flegrei, Italy, are difficult to interpret, and the conditions necessary for large eruptions are poorly constrained (2,3). Here we present a study of pre-eruptive magmatic processes and their timescales using chemically zoned crystals from the 'Minoan' caldera-forming eruption of Santorini volcano, Greece (4), which occurred in the late 1600s bc. The results provide insights into how rapidly large silicic systems may pass from a quiescent state to one on the edge of eruption (5,6). Despite the large volume of erupted magma (4) (40-60 cubic kilometres), and the 18,000-year gestation period between the Minoan eruption and the previous major eruption, most crystals in the Minoan magma record processes that occurred less than about 100 years before the eruption. Recharge of the magma reservoir by large volumes of silicic magma (and some mafic magma) occurred during the century before eruption, and mixing between different silicic magma batches was still taking place during the final months. Final assembly of large silicic magma reservoirs may occur on timescales that are geologically very short by comparison with the preceding repose period, with major growth phases immediately before eruption. These observations have implications for the monitoring of long-dormant, but potentially active, caldera systems., Crystals in volcanic rocks provide records of magma reservoir processes and timescales before eruptions (7). A crystal growing from a magmatic melt incorporates trace elements in quantities governed by thermodynamic [...]

Chemical evolution of planetary bodies, ranging from asteroids to the large rocky planets, is thought to begin with differentiation through solidification of magma oceans many hundreds of kilometres in depth (1-3). The Earth's Moon is the archetypical example of this type of differentiation. Evidence for a lunar magma ocean is derived largely from the widespread distribution, compositional and mineralogical characteristics, and ancient ages inferred for the ferroan anorthosite (FAN) suite of lunar crustal rocks. The FANs are considered to be primary lunar flotation-cumulate crust that crystallized in the latter stages of magma ocean solidification. According to this theory, FANs represent the oldest lunar crustal rock type (2-4). Attempts to date this rock suite have yielded ambiguous results, however, because individual isochron measurements are typically incompatible with the geochemical make-up of the samples, and have not been confirmed by additional isotopic systems (5-9). By making improvements to the standard isotopic techniques, we report here the age of crystallization of FAN 60025 using the [sup.207]Pb-[sup.206]Pb, [sup.147]Sm-[sup.143]Nd and [sup.146]Sm-[sup.142]Nd isotopic systems to be 4,360 ± 3 million years. This extraordinarily young age requires that either the Moon solidified significantly later than most previous estimates or the long-held assumption that FANs are flotation cumulates of a primordial magma ocean is incorrect. If the latter is correct, then much of the lunar crust may have been produced by non-magma-ocean processes, such as serial magmatism (10)., To constrain the age of a FAN using multiple chronometers we (1) obtained a unique mafic-mineral-rich sample of FAN 60025 found by examining the lunar collection at the Johnson Space [...]

The U-Pb geochronology of three granitoid plutons and three granitic pegmatite dykes, largely from the Thompson Nickel Belt located along the northwestern Superior craton margin, was investigated to place constraints on the timing of felsic magmatism associated with closure of the Manikewan Ocean and final continent-continent collision to form the Trans-Hudson Orogen. These data indicate that 1840-1820 Ma granite magmatism along the Superior margin was more active than previously thought and that some magmatism extended beyond the Thompson Nickel Belt sensu stricto, including the 1836 ± 3 Ma Mystery Lake granodiorite, 1822 ± 5 Ma Wintering Lake granodiorite, and the 1825 ± 8 Ma Fox Lake granite located in the Split Lake Block. Granitic pegmatites within the Thompson Nickel Belt were emplaced late in the collisional history in the period 1.79-1.75 Ga and include a 1770 ± 2 Ma dyke exposed at the Thompson pit, a 1767 ± 6 Ma dyke at the Pipe Pit, and a 1786 ± 2 Ma dyke located at Paint Lake. The final stage of crustal amalgamation in the eastern Trans-Hudson Orogen involved Superior Province crustal thickening and partial melting forming 1.84-1.82 Ga granite magmas and then final collision at ~1.8 Ga between the Superior Province and a continental block to the west consisting of the previously amalgamated Sask and Hearne cratons. Heating of the Superior craton margin and granitic magmatism continued past peak metamorphism (1790-1750 Ma); this thermal event is represented by the emplacement of numerous late pegmatite dykes and evidenced by cooling dates recorded by metamorphic minerals (e.g., titanite) in reworked Archean gneisses and Proterozoic intrusions. La geeochronologie U-Pb de trois plutons granitoldes et de trois dykes de pegmatite granitique, provenant en grande partie de la Ceinture de nickel Thompson situee le long de la bordure nord-ouest du craton du Superieur, a ete etudieee afin de delimiter le moment du magmatisme felsique associe a la fermeture de l'ocean Manikewan et la collision finale continent a continent pour former l'orogene trans-hudsonien. Ces donnees indiquent que le magmatisme granitique (18401820 Ma) le long de la bordure du Superieur est plus commun que cru auparavant et qu'il s'etend au-dela de la Ceinture de nickel Thompson proprement dite, incluant le granodiorite de Mystery Lake (1836 ± 3 Ma), le granodiorite de Wintering Lake (1822 [+ o -] 5 Ma) et le granite de Fox Lake (1825 [+ o -] 8 Ma) situee dans le bloc Split Lake. Les pegmatites granitiques a l'interieur de la Ceinture de nickel Thompson ont ete mises en place tard dans l'historique de collision dans la periode 1,791,75 Ga et elles comprennent un dyke (1770 [+ o -] 2 Ma) affleurant dans la fosse Thompson, un dyke (1767 [+ o -] 6 Ma) dans la fosse Pipe et un dyke (1786 [+ o -] 2 Ma) situee au lac Paint. L'eetape finale de l'amalgamation de la croflte dans l'orogene transhudsonien oriental impliquait un eepaississement de la coflte de la province du Superieur et une fusion partielle formant des magmas granitiques (1,84-1,82 Ga) et la collision finale a ~1,8 Ga entre la Province du Superieur et un bloc continental a l'ouest qui comprenait les cratons amalgamees de Sask et de Hearne. L'eechauffement de la bordure du craton du Superieur et le magmatisme granitique ont continue aprees la crete du metamorphisme (1790-1750 Ma) et ils sont representees par la mise en place de nombreux dykes de pegmatites tardifs et des dates de refroidissement telles qu'enregistreees par des mineeraux meetamorphiques (p. ex. la titanite) dans des gneiss archeens et des intrusions proterozoiques retravaille;s. [Traduit par la Redaction], Introduction The Trans-Hudson Orogen in northern Manitoba and Saskatchewan is one of several Paleoproterozoic accretionary and collisional orogens in North America that formed during the breakup of one or more [...]

The Neoarchean Pukaskwa batholith consists of pre-, syn-, and post-tectonic phases emplaced over an interval of 50 million years. Pre-tectonic phases are broadly synvolcanic and have a high-Al tonalite-trondhjemite-granodiorite (TTG) affinity interpreted to reflect derivation by partial melting of basaltic crust at lower crustal or upper mantle depths. Minor syn-tectonic phases slightly post-date volcanism and have geochemical characteristics suggesting some involvement or interaction with an ultramafic (mantle) source component. Magmatic emplacement of pre- and syn-tectonic phases occurred in the midcrust at paleopressures of 550-600 MPa and these components of the batholith are thought to be representative of the midcrust underlying greenstone belts during their development. Subsequent to emplacement of the syntectonic phases, and likely at approximately 2680 Ma, the Pukaskwa batholith was uplifted as a structural dome relative to flanking greenstone belts synchronously with ongoing regional sinistral transpressive deformation. The driving force for vertical tectonism is interpreted to be density inversion (Rayleigh-Taylor-type instabilities) involving denser greenstone belts and underlying felsic plutonic crust. The trigger for initiation of this process is interpreted to be an abrupt change in the rheology of the midcrust attributed to introduction of heat from the mantle attendant with slab breakoff or lithospheric delamination following the cessation of subduction. This process also led to partial melting of the intermediate to felsic midcrust generating post-tectonic granitic phases at approximately 2667 Ma. We propose that late density inversion-driven vertical tectonics is an inevitable consequence of horizontal (plate) tectonic processes associated with greenstone belt development within the Superior Province. Le batholite neeoarcheeen de Pukaskwa comprend des phases pre-, syn- et post-tectoniques mises en place durant une peeriode de 50 Ma. Les phases pre-tectoniques sont largement synvolcaniques et elles ont une affinite Al et TTG (<>) elevee qui est interpretee comme etant le reflet de la derivation par fusion partielle d'une croflte basaltique a des profondeurs de croflte inferieure ou de manteau superieur. Des phases syntectoniques mineures sont leegerement plus tardives que le volcanisme et elles ont des caracteristiques geochimiques qui suggerent une certaine implication ou interaction avec une composante de source ultramafique (manteau). La mise en place magmatique des phases pre;- et syntectoniques a eu lieu dans le milieu de la croflte a des paleeopressions de 550-600 MPa et ces composantes du batholite seraient representatives des ceintures de roches vertes sous-jacentes, au centre de la croflte durant leur deeveloppement. Apres la mise en place des phases syntectoniques, et probablement vers 2680 Ma, le batholite de Pukaskwa a ete souleve en dome structural par rapport aux ceintures de roches vertes de part et d'autre, et ce, de maniere synchrone avec la deformation transpressive regionale senestre en cours. La force motrice derriere le tectonisme vertical serait une inversion de densite (instabilitees de type Rayleigh-Taylor) impliquant les ceintures de roches vertes plus denses et la croflte felsique plutonique sous-jacente. Le declencheur pour l'initiation de ce processus est interprete comme etant un changement abrupt dans la rheologie de la croflte moyenne attribue a l'introduction de chaleur du manteau auxiliaire a la rupture de la dalle ou a la delamination lithospherique suivant la cessation de la subduction. Ce processus a aussi mene a une fusion partielle de la croflte moyenne intermediaire a felsique generant des phases granitiques post-tectoniques vers 2667 Ma. Nous proposons que la tectonique verticale poussee par l'inversion de densite tardive soit une consequence inevitable des processus tectoniques horizontaux (plaques) associesau developpement de ceintures vertes dans la Province du Superieur. [Traduit par la Redaction], Introduction Large, intermediate to felsic plutonic complexes of batholithic dimensions are a major component of the Superior Province and Archean cratons in general, but are poorly understood compared to the [...]

The Thompson Nickel Belt marks the boundary between the Archean Superior Province and the Trans-Hudson Orogen in Canada. It comprises Archean gneisses, and Paleoproterozoic rocks with metasediments and metavolcanites (Ospwagan Group) and intrusions. The gneisses are frequently migmatitic and host numerous pegmatites. The western belt boundary is a fault contact with the Kisseynew Domain of the Reindeer Zone. In the south, the transition zone between the belt and the Kisseynew Domain comprises granitoids and a detrital sequence (Grass River Group), part of which grades into turbidites in the Kisseynew Domain. The eastern belt boundary is a diffuse zone where the Archean east-west (E-W) structural trend changes into the north-northeast (NNE) trend of the belt. This paper presents U-Pb ages for granitoids and [sup.207]Pb/[sup.206]Pb detrital zircon ages from the Ospwagan and Grass River groups. Ages and a comparison of events in the belt and in the eastern Reindeer Zone have major implications. The change from stable platform deposits to syn-tec-tonic filling and emplacement of mafic intrusions in the Ospwagan Group are attributed to the convergence between the Reindeer Zone and the Superior Province at 1891-1885 Ma. At ca. 1850 Ma, continuing convergence led to drowning of marginal basins of the Superior craton and to the development of a transpressive regime in the belt, the onset of which could be as old as ca. 1885 Ma. Metamorphic ages of 1818-1785 record closure of the Kisseynew basin and crustal thickening. Collision of the new continental block with the Superior Province was accommodated by transpression until 1750-1720 Ma. La ceinture de Thompson est situee entre le Craton archeen du Superieur et l'orogene paleoproterozoique du Trans-Hudson, au Canada. Elle comprend des gneiss archeeens souvent migmatitiques, et des roches paleeoproteeroiques, metasediments et metavolcanites (Groupe Ospwagan), et intrusions. A l'ouest, une zone de faille marque le contact entre la ceinture et le domaine Kisseynew de la Zone de Reindeer. Vers le sud, cette zone de transition entre la ceinture et le domaine Kisseynew comprends des granitoldes et une sequence deetritique (Groupe Grass River), qui passe graduellement aux turbidites du domaine Kisseynew. Sur la bordure orientale de la ceinture, le grain structural E-W du Craton archeen se reoriente dans le grain NNE de la ceinture. Cet article presente des ages U-Pb pour differents granitoides et des ages [sup.207]Pb/[sup.206]Pb pour les zircons deetritiques des groupes Ospwagan et Grass River. Les ages et une comparaison des eveenements affectant la ceinture et l'est de la zone Reindeer ont des implications importantes. Dans le groupe Ospwagan, le passage de depots de type plate-forme a des depots syn-tectoniques, et l'intrusion de roches basiques, sont attribueees a la convergence entre la zone Reindeer et le Craton du Superieur vers 1891-1885 Ma. Vers 1850 Ma, la convergence conduit a l'enfouissement des bassins de la marge du Craton et a un regime transpressif pouvant avoir debute des ca. 1885 Ma. La fermeture du bassin de Kisseynew et l'epaississement crustal sont enregistres par des ages de meetamorphisme entre 1818 et 1785 Ma. Le regime transpressif se poursuit jusqu'a ca. 1750-1720 Ma. [Traduit par la Redaction], Introduction The structure and evolution of Paleoproterozoic orogens is of considerable interest for understanding early continental tectonic processes because they involve convergence of Archean cratons and accretion of juvenile crust. [...]

High-precision isotope dilution--thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon and baddeleyite ages from the PX1 vertically layered mafic intrusion Fuerteventura, Canary Islands, indicate initiation of magma crystallization at 22.10 ± 0.07 Ma. The magmatic activity lasted a minimum of 0.52 Ma. [sup.40]Ar/[sup.39]Ar amphibole dating yielded ages from 21.9 ± 0.6 to 21.8 ± 0.3, identical within errors to the U-Pb ages, despite the expected 1% theoretical bias between [sup.40]Ar/[sup.39]Ar and U-Pb dates. This overlap could result from (i) rapid cooling of the intrusion (i.e., less than the 0.3 to 0.6 Ma [sup.40Ar][sup.39Ar] age uncertainties) from closure temperatures ([T.sub.c]) of zircon (699-988°C) to amphibole (500-600 °C); (ii) lead loss affecting the youngest zircons; or (iii) excess argon shifting the plateau ages towards older values. The combination of the [sup.40Ar][sup.39Ar] and U/Pb datasets implies that the maximum amount of time PX1 intrusion took to cool below amphibole [T.sub.c] is 0.8 Ma, suggesting PX1 lifetime of 520 000 to 800000 Ma. Age disparities among coexisting baddeleyite and zircon (22.10 ± 0.07/0.08/0.15 Ma and 21.58 ± 0.15/0.16/0.31 Ma) in a gabbro sample from the pluton margin suggest complex genetic relationships between phases. Baddeleyite is found preserved in plagioclase cores and crystallized early from low silica activity magma. Zircon crystallized later in a higher silica activity environment and is found in secondary scapolite and is found close to calcite veins, in secondary scapolite that recrystallised from plagioclase. close to calcite veins. Oxygen isotope δ [sup.18]O values of altered plagioclase are high (+7.7), indicating interaction with fluids derived from host-rock carbonatites. The coexistence of baddeleyite and zircon is ascribed to interaction of the PX1 gabbro with [CO.sub.2]-rich carbonatite-derived fluids released during contact metamorphism. Resume : Des datations U-Pb de haute precision ont ete obtenues par TIMS sur des cristaux de zircon et de baddeleyite extraits de l'intrusion mafique rubaneee PX1 (Fuerteventura, Canaries). Elles revelent que l'initiation de la cristallisation du magma a eu lieu il y a 22,10 ± 0,07 Ma. L'activite magmatique a dure; un minimum de 0,52 Ma. Des ages [sup.40Ar][sup.39Ar] obtenus sur amphibole sont de 21,9 [+ or -] 0,6 a 21,8 ± 0,3 Ma et sont identiques aux ages U/Pb, en deepit du biais theorique de 1 % attendu entre les deux systemes de datation utilises. Ceci peu etre cause par : (i) le refroidissement rapide de l'intrusion (inferieur aux incertitudes de 0,3 a 0,6 Ma des ages [sup.40Ar][sup.39Ar]) entre la temperature de fermeture du zircon (699-988 °C) et celle de l'amphibole (500-600 °C); (ii) une perte de plomb affectant les zircons les plus jeunes; ou (iii)un exces d'argon donnant des ages-plateau plus vieux. La combinaison des methodes de datations U-Pb et [sup.40Ar][sup.39Ar] suggere que le temps maximum necessaire a PX1 pour se refroidir en dessous de la temperature de fermeture de l'amphibole est de 0,8 Ma. Ceci signifie que la duree de vie de PX1 est de 520 000 a 800 000 ans. Les differences d'age observees entre baddeleyites et zircons (22,10 ± 0,07 et 21,58 ± 0,15 Ma) issus d'un eechantillon de gabbro localise; a la bordure ouest du pluton suggerent qu'il existe des relations geneetiques complexes entre ces deux phases minerales. La baddeleyite est associeee aux coeurs preserves de plagioclase et cristallise a partir d'un magma ayant une faible activite de silice. Les zircons, quant a eux, ont cristallise; dans un milieu a activitee de silice plus eeleveee et sont associesa des grains secondaires de scapolite issus de la recristallisation du plagioclase a proximitee de veines micromeetriques de calcite. Les valeurs de 818O de ces grains de plagioclase recristallisees sont eeleveees (+7,7) suggerant une interaction avec des fluides issus des carbonatites encaissantes. La coexistence de la baddeleyite et du zircon est donc attribueea l'interaction entre le gabbro de l'intrusion PX1 et un fluide riche en [CO.sub.2] relaiche; par les carbonatites encaissantes lors du me;tamorphisme de contact ge;ne;re; par la mise en place de PX1. [Traduit par la Redaction], Introduction A number of studies of intrusive bodies (Coleman et al. 2004; Glazner et al. 2004; de Saint-Blanquat et al. 2006; Bacon et al. 2007; Charlier et al. 2008; Michel [...]