Otto van Breemen

Tectonic regimes and terrane boundaries in the high-grade Sveconorwegian belt of SW Norway, inferred from U-Pb zircon geochronology and geochemical signature of augen gneiss suites

Six units from three suites of gneissic megacrystic granitoids in the Sveconorwegian Province of SW Norway were dated by the U–Pb zircon method. The Gjerstad suite crystallized between 1.19 and 1.15 Ga in the Bamble, Telemark and Rogaland–Vest Agder terranes; it has an A-type geochemical signature suggesting intrusion in a tensional setting. The Vennesla unit of this suite yields an age of 1166+61−21 Ma. The Feda suite intruded in Rogaland–Vest Agder during a short period at 1.05 Ga (four units between 1051+2−8 and 1049+2−8 Ma) and has a high-K calc-alkaline trend suggesting a subduction- related setting. The Fennefoss augen gneiss of Telemark (1035+2−3 Ma) possesses a geochemical signature transitional between high-K calc-alkaline and A-type. The spatial association of tensional plutonism with the Kristiansand–Porsgrunn shear zone between Bamble and Telemark suggests that crustal thinning occurred along this axis between 1.19 and 1.13 Ga, preceding Sveconorwegian compressive shearing. The boundary between Rogaland–Vest Agder and Telemark, the Mandal line, probably separated, at 1.05 Ga, a mobile belt (Rogaland–Vest Agder) and a colder, more rigid terrane (Telemark). Major strike-slip shearing along the southern section of the Mandal line was probably associated with intrusion of an elongate pluton of the Feda suite and ductile amphibolite-facies deformation along a banded gneiss unit.

Great Slave Lake shear zone, NW Canada : mylonitic record of Early Proterozoic continental convergence, collision and indentation

Abstract Great Slave Lake shear zone, in the NW Canadian Shield, is an excellent example of the kinematic, magmatic and thermal evolution of a crustal-scale shear zone associated with a collisional continental boundary. It is a 25 km wide corridor of granulite to lower greenschist facies mylonites and cataclastic fault rocks developed in the deep-seated parts of an Early Proterozoic (2.0–1.9 Ga) magmatic arc, constructed on the upper (Rae) plate at the contact between the Archean Slave and Rae continents. The rocks of Great Slave Lake shear zone, the Thelon magmatic arc and the Taltson magmatic zone are all components of the same magmatic arc, but their geological histories reflect different aspects of the continental interaction.

1.1 Ga K-rich alkaline plutonism in the SW Grenville Province

U−Pb zircon and baddeleyite dating of six syenitic stocks establishes that the ultrapotassic, potassic alkaline and shoshonitic magmatism with island-arc affinities in the Central Metasedimentary Belt (CMB) of the southwestern Grenville Province, Canada took place between 1089 and 1076 Ma, along a 400-km-long, northeast-trending plutonic belt. These ages indicate that ultrapotassic rocks with arc affinities are not unique to the Phanerozoic. West to east emplacement ages along a northern and southern cross-section of this belt range from 1083±2 Ma (Kensington), through 1081±2 Ma (Lac Rouge) to 1076−1+3Ma (Loranger) in the north, and from 1089−3+4Ma (loon Lake) and 1088±2 Ma (Calabogie), to 1076±2 Ma (Westport) in the south. Although closely spaced in time, in detail these ages suggest a slight younging of this magmatic activity to the southeast. Integration of the geochronological data with the spatial extent and potassic character of the plutons shows that the K-rich alkaline suite is distinct from the nepheline-syenite belt of the Bancroft terrane and from the syenite-monzonite suite of the Frontenac terrane of the CMB, and it is considered to be a magmatic episode unique to the Elzevir terrane and its Gatineau segment. The timing and the postmetamorphic emplacement of these plutons indicate that the regional greenschist to granulite-facies metamorphism of the country rock (precise age unknown) is older than 1089 Ma throughout the entire Elzevir terrane. The potassic magmatism is interpreted as the initiation of the 1090–1050 Ma Ottawan Orogeny in the Elzevir terrane; thus, the regional metamorphism in this terrane, previously assigned to the Ottawan Orogeny, is an earlier event. The contemporaneous emplacement of this postmetamorphic plutonic belt with Keweenawan volcanism is at variance with current tectonic models which consider the Keweenawan rift to be formed at the same time as regional metamorphism in the CMB.

Three generations of anorthosite-mangerite-charnockite-granite (AMCG) magmatism, contact metamorphism and tectonism in the Saguenay-Lac-Saint-Jean region of the Grenville Province, Canada

Four A-type granitoid plutons from the Saguenay-Lac-Saint-Jean region of the Grenville Province, lacking regionally imposed solid-state deformation, give UPb zircon igneous crystallisation ages of 1146 ± 3 Ma, 1082 ± 3 Ma, 1067 ± 3 Ma, and 1020+4−3 Ma. These ages indicate that the region has not been affected by the Ottawan period of the Grenville orogeny. A compilation of these and existing UPb age data reveals three periods of Mesoproterozoic magmatism in this region: 1160-1140 Ma, 1082-1050 Ma and 1020-1010 Ma. Each of these periods appears to have had some or all of the components of the well-known anorthosite-mangerite-charnockite-granite suite, and the first two periods can be correlated with magmatism elsewhere in the Grenville Province. Sparse UPb metamorphic ages also fall within these periods and are interpreted to reflect regional-scale contact metamorphism produced by the large plutons. In addition, the few ages of strike-slip movements on major shear zones also fall within these time periods. It is difficult to interpret these data as reflecting separate orogenic events, as there is little definitive evidence of calc-alkaline magmatism, thrusting or regional metamorphism in this region. Perhaps during the interval 1160 to 1010 Ma magmatism was driven by repeated upwelling of hot mantle, either in a plume or above a sinking lithospheric slab, but access to the mid-levels of the crust currently exposed was controlled by strike-slip movements on major vertical shear zones that resulted from plate-tectonic activity.

Rb-Sr whole rock isotopic studies of Lewisian metasediments and gneisses in the Loch Maree region, Ross-shire

Fifty-three whole rock Rb–Sr isotopic analyses are reported on two metasedimentary schist and four gneiss units in the Lewisian of the Loch Maree region. Isochron dates from the Gairloch metasediments–Ard gneisses (1980 ± 50 m.y.) and the Loch Maree gneisses (1965 ± 100 m.y.) date the main phase of metamorphism of the Laxfordian episode at 1975 ± 75 m.y. A 1745 ± 160m.y. whole rock isochron date on the Carnmore kyanite–biotite gneisses can be related to late tectonic pegmatite intrusions. Whole rock data points from the quartzofeldspathic gneisses from the Carnmore district do not define an isochron. A 1500 ± 95 m.y. isochron date for metasediments of the Loch Maree Group is interpreted in terms of final closure, during epeirogenic uplift, after a 200–300 m.y. period of isotopic exchange below an impervious cap of tectonically overthrusted hornblende schist and quartzofeldspathic gneiss. Initial 87Sr/86Sr considerations indicate both a crustal history for the gneisses of the Loch Maree and Carnmore districts going back to 2.7–2.8 b.y. and deposition of the Gairloch metasediments and Loch Maree Group after 2.2 b.y. ago, presumably unconformably on the older metamorphic complex. The Laxfordian orogenic cycle is defined, in the classical stratigraphic sense, to consist of a depositional episode 2.2–2.0 b.y. ago, an orogenic episode 2.0–1.7 b.y. ago and an epeirogenic episode 1.7–1.5 b.y. ago. There are marked similarities in the Laxfordian and Svecokarelian orogenic cycles, and the Lewisian chronology for the Loch Maree region shows striking parallelism with the Precambrian chronology of NW. Europe.

PLATE TECTONICS BEFORE 2.0 Ga: EVIDENCE FROM PALEOMAGNETISM OF CRATONS WITHIN SUPERCONTINENT NUNA

Laurentia, the core of Paleo- to Mesoproterozoic supercontinent Nuna, has remained largely intact since assembly 2.0 to 1.8 billion years ago [Ga]. For earlier times, previous paleomagnetic data on poorly dated Paleoproterozoic mafic intrusions yielded ambiguous estimates of the amount of separation between key cratons within Nuna such as the Slave and Superior. Recent developments in paleomagnetism and U-Pb baddeleyite geochronology, including new results reported herein, yield sufficiently precise data to generate partial apparent polar wander paths for both the Slave and Superior craton from 2.2 to 2.0 Ga. Our new apparent polar wander comparison confirms earlier speculations that processes similar to plate tectonics, with relative motion between the Slave and Superior cratons, were operative leading up to the final assembly of supercontinent Nuna.

A zoned low P-high T complex at the level of anatexis—structural and plutonic patterns in metasediments of the Archean Yellowknife Supergroup, near Bathurst Inlet, N.W.T., Canada

Abstract Within metagreywackes and schists of the Archean Yellowknife Supergroup the development of an “anatectic stratigraphy” during the late stages of regional deformation has resulted in the formation of a complex with a metamorphicplutonic-structural zoning similar to that occurring in Phanerozoic low P-high T terrains. From bottom to top the “stratigraphy” consists of: (1) small-folded migmatite, grading with decreasing partial melting to (2) sillimanite schist, strongly strained by the emplacement of two-mica granites; (3) andalusite-cordierite schist, the site of large folds related to the emplacement of the complex. Each zone is characterized by a different structural style resulting from the interplay between metamorphic grade and deformation. For instance in the sillimanite zone, the structure was influenced by the extraction of syntectonic melt and its emplacement into fractures at low effective pressure. This zone was also the ductile envelope into which sills and plutons of two-mica granites derived from the lower migmatites migrated. The time taken for evolution of the complex has been constrained by U-Pb zircon and monazite dating of a member of the early tonalite-diorite suite and members of the two-mica granite suite. This shows the evolution of the complex was rapid, ca. 10 Ma, but possibly polyphase. The formation of a distinctively styled complex accompanied by syn-tectonic anatexis and plutonism, may be typical of lower crustal levels of the Yellowknife Supergroup and other low P-high T terrains.

The timing of gold mineralization in White Bay, western Newfoundland: Evidence from 40Ar/39Ar studies of mafic dykes that predate and postdate mineralization

The Rattling Brook deposit is a low-grade, disseminated to stockwork-style gold deposit hosted by Precambrian granodiorite and adjacent Cambrian sedimentary rocks. Alteration and gold mineralization also occur in foliated and metamorphosed mafic dykes, likely of late Precambrian age. The auriferous granodiorite is in turn cut by relatively fresh, unaltered, and locally chilled diabase dykes, interpreted as Paleozoic post-mineralization intrusions. A fresh post-mineralization diabase gave a 40Ar/39Ar amphibole plateau age of 412.9 ± 4.3 Ma, which is interpreted as the time of its crystallization, and which provides a younger limit for the timing of gold mineralization. An altered, metamorphosed dyke of pre-mineralization timing gave an identical 40Ar/39Ar biotite plateau age of 412.6 ± 2.3 Ma, which is more difficult to interpret. It could represent post-metamorphic cooling, or alternatively, resetting of metamorphic biotite during alteration related to gold mineralization. In the first case, the age provides a reasonable upper limit for the timing of gold mineralization, provided that the ambient temperature during mineralization was not significantly above the closure temperature for Ar in biotite (~ 300 oC). On this basis, gold mineralization at Rattling Brook occurred during the latest Silurian or earliest Devonian, between 415 and 409 Ma. The possibility that mineralization occurred at temperatures above 300 oC, prior to 415 Ma, cannot be completely excluded, but it must be younger than ca. 430 Ma, the time of peak metamorphism in adjacent areas. In conjunction with sparse data on the ages of gold deposits elsewhere in Newfoundland, the results support two discrete episodes of mineralization corresponding to the Silurian-Devonian boundary (420–410 Ma) and middle to late Devonian (380–370 Ma). These age groupings resemble those defined by recent Re-Os isotopic studies of sulphides from vein-style gold deposits in the Meguma terrane of Nova Scotia and may in part correspond to the timing of intrusion-related gold in New Brunswick. Given the small amount of data from all of these areas, further interpretation is speculative. However, such hints of discrete orogen-scale episodes of gold mineralization, perhaps correlative with regional tectonothermal events, provide a powerful incentive for further geochronological studies of gold mineralization in the Appalachian orogen. Resume Le gite du ruisseau Rattling est un gite aurifere a faible teneur qui, de dissemine, devient un stockwerk inclus dans de la granodiorite precambrienne et des roches sedimentaires cambriennes adjacentes. Une alteration et une mineralisation aurifere se manifestent egalement dans des dykes mafiques metamorphises et feuilletes, remontant probablement au Precambrien tardif. La granodiorite aurifere est a son tour recoupee par des dykes de diabase figes par endroits, non alteres et relativement sains, interpretes en tant qu’intrusions paleozoiques ulterieures a la mineralisation. Une diabase post-mineralisation inalteree a accuse un âge plateau sur amphibole 40Ar/39Ar de 412,9 ± 4,3 Ma, ce qui est interprete comme le moment de sa cristallisation et rapproche la limite du moment de la mineralisation aurifere. Un dyke metamorphise altere, anterieur a la mineralisation, a livre un âge plateau sur biotite 40Ar/39Ar identique de 412,6 ± 2,3 Ma, qui est plus difficile a interpreter. Celui-ci pourrait representer un refroidissement post-metamorphique ou, subsidiairement, une remise en place de la biotite metamorphique au cours de l’alteration apparentee a la mineralisation aurifere. Dans le premier cas, la datation fournit une limite superieure raisonnable quant au moment de la mineralisation aurifere, a condition que la temperature ambiante pendant la mineralisation n’ait pas ete substantiellement superieure a la temperature de convergence de l’AR dans la biotite (environ 300 oC). Le cas echeant, la mineralisation d’or du ruisseau Rattling est survenue au cours du Silurien tardif ou du Devonien precoce, entre 415 et 409 Ma. La possibilite que la mineralisation se soit produite a des temperatures superieures a 300 oC, avant 415 Ma, ne peut pas etre entierement exclue, mais la mineralisation doit etre anterieure a 430 Ma environ, point culminant du metamorphisme dans les secteurs voisins. Outre les rares donnees sur les âges des gites d’or ailleurs a Terre-Neuve, les resultats appuient la manifestation de deux episodes distincts de mineralisation correspondant a la limite siluro-devonienne (420–410 Ma) et a la periode du Devonien moyen a tardif (380–370 Ma). Ces groupes d’âges ressemblent a ceux definis lors de recentes etudes isotopiques Re-Os de sulfures provenant de gites auriferes filoniens dans le terrane de Meguma en Nouvelle-Ecosse et ils pourraient en partie correspondre au moment de l’apparition de l’or lie a une intrusion au Nouveau-Brunswick. Vu la somme restreinte de donnees tirees de l’ensemble de ces secteurs, effectuer une interpretation plus detaillee serait se livrer a des conjectures. Les indices comme les episodes distincts de mineralisation aurifere a l’echelle orogenique, qui pourraient etre correlatifs avec des evenements tectonothermiques regionaux, fournissent toutefois un puissant incitatif pour realiser des etudes chronologiques plus poussees de la mineralisation aurifere dans l’orogene des Appalaches.

Geochronology of the Baie Verte Peninsula, Newfoundland: Implications for the Tectonic Evolution of the Humber and Dunnage Zones of the Appalachian Orogen: A Discussion

In a recent paper on the geochronology of the Baie Verte Peninsula, Newfoundland, by Dallmeyer and Hibbard (1984), we note that the magnitude of errors on U-Pb zircon determinations cited in the captions to figures 3 and 6 and in the text are inconsistent with those shown in the actual figures. The errors used in the text are, furthermore, inconsistent with a proper method of linear regression which takes into account the excess scatter of points outside of analytical uncertainty. We provided Dr. Dallmeyer with what we considered to be realistic error estimates, and they are essentially those shown in the figures. For these sets of data we chose a York II approach, described below. The two sets of data, involving the Burlington and Dunamagon granitic intrusions, yield results which have MSWD values of 2.2 and 8.9, respectively, when regressed using the lo estimates of U-Pb errors (0.75%) and correlation coefficient (0.95) stated on p. 496 of their paper (Dallmeyer and Hibbard quoted 2a = 1.5%). When the MSWD exceeds 1, the lr error envelopes of the data points do not all overlap the best fit line, implying either uncertainty outside analytical uncertainty alone, or underestimation of the analytical uncertainty. Statistical constraints lead Brooks et al. (1972) to recommend that an MSWD greater than 2.5 indicates significant scatter beyond estimated analytical error. The program of York (1969) does not allow for excess scatter, but it may be taken into account by multiplying the errors in slope and intercept by a factor of /S/(n 2) (York