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The Lapland Granulite Belt of the northeastern Fennoscandian Shield is dominated by blastomylonitic migmatitic metagreywackes and a deformed, structurally conformable norite-enderbite series. The belt contains minor granodiorites and one major anorthosite massif and is crosscut by post-tectonic granite plutons and diabase dykes. The major and trace element composition of the enderbite series is similar to that of arc-related igneous rocks. Magmatically zoned zircon yield U-Pb secondary ion mass spectrometry ages of 1.91-1.92 Ga, while homogeneous metamorphic zircon domains are aged 1.88-1.90 Ga. Major enderbite bodies reveal only a few older inherited, xenocrystic zircon cores or grains, mainly Archaean in age, while a tonalite body and a narrow quartz norite vein have abundant inherited zircons comparable with the detrital zircon population in the metagreywackes. The intrusive series has slightly positive initial [ζ.sub.Nd] values, while the tonalite body has a slightly negative initial [ζ.sub.Nd], consistent with the occurrences of inherited zircon grains. The major intrusive series, interpreted to have been emplaced in an arc environment, has juvenile character and assimilated only a minor amount of older crustal material during ascent, intrusion, and cooling. Narrow veins and more evolved bodies had more interaction with the metasediments and other crustal rocks. The enderbite series of the Lapland Granulite Belt may be correlated with arc-related plutonism in the Nagssugtoqidian and Torngat orogens, but metamorphic and structural evolution took place considerably later in Greenland and the northeastern Canadian Shield, with the exception of the Inglefield Belt of northwestern Greenland. La ceinture des granulites de Laponie dans le nord-est du Bouclier fennoscandien est dominee par des metagrauwackes migmatitiques blastomylonitiques et une serie deformee norite-enderbite a structure concordante. La ceinture contient un peu de granodiorite et un grand massif d'anorthosite; elle est recoupee par des plutons granitiques et des dykes de diabase post-tectoniques. La composition des elements principaux et traces de la serie enderbite est semblable a celle de roches ignees reliees a des arcs. Un zircon zone par magmatisme a donne des ages U- Pb--sonde ionique (U-Pb-SIMS) de 1.91-1.92 Ga, comparativement a des ages de 1.88-1.90 Ga obtenus a partir de domaines homogenes de zircon metamorphique. Des amas importants d'enderbite ne presentent que quelques noyaux ou grains de xenocristaux de vieux zircons herites, datant surtout de l'Archeen, alors qu'un amas de tonalite et un filon etroit de quartz norite contiennent de nombreux zircons herites comparables a la population de zircons detritiques dans les metagrauwackes. La serie intrusive a des valeurs [ζ.sub.Nd] initiales legerement positives, alors que l'amas de tonalite a une valeur [ζ.sub.Nd] initiale legerement negative, ce qui concorde avec la presence de grains de zircon herites. La principale serie intrusive, interpretee comme ayant ete mise en place dans un environnement d'arc, a un caractere juvenile et elle n'a assimile qu'une faible quantite d'ancien materiel de la croute durant la remontee, l'intrusion et le refroidissement. Des filons etroits et des amas plus evolues ont eu plus d'interaction avec les metasediments et autres roches de la croute. La serie d'enderbite de la ceinture des granulites de Laponie peut etre correlee avec un plutonisme relie a des arcs dans les orogenes Nagssugtoqidian et Torngat, mais l'evolution metamorphique et structurale s'est produite considerablement plus tard au Groenland et dans le nord-est du Bouclier canadien, sauf pour la ceinture Inglefield du nord-ouest du Groenland. [Traduit par la Redaction], Introduction Granulite-facies mobile belts are formed in the internal parts of collisional orogens. Geochemical and geochronological data from high-grade metasedimentary and metaigneous complexes provide useful information about orogenic evolution and [...]

Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature (1,2). Radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time (3-5), with some estimates suggesting that over 70% of the present continental crustal reservoir was extracted by the end of the Archaean eon [3,5]. Patterns of crustal growth and reworking in rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics (6). In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago (7-9), suggesting that modern-day tectonic regimes operated during the formation of the earliest crustal rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9-3.5-Gyr-old rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens (10-12). These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes., Patterns of crustal growth and reworking reflect regional-scale geodynamics. In Phanerozoic to recent times, 'accretionary' orogens are important sites of crustal growth and reworking (10-13). These long-lived convergent plate boundaries [...]

The palaeoproterozoic Svecofennian orogen in southern Finland contains a number of orogenic gold occurrences. The Jokisivu gold deposit, comprising auriferous quartz veins, is hosted by syn-tectonic quartz diorites to gabbros. Mineralization occurs in approximately WNW--ESE- and WSW--ENE-trending shear zones, which probably branch from regional-scale NW--SE-trending shears. Ore zone fabrics post-date regional-scale folding and the metamorphic peak, and can be correlated with late Svecofennian regional shear tectonics ([D.sub.6]; 1.83-1.78 Ga), indicating that mineralization formed during the late stages of orogenic evolution. SIMS and TIMS U--Pb dating of three samples place tight constraints on the age of gold mineralization. Zircons from both unaltered and altered quartz diorites have ages of 1884 [+ or -] 4 Ma and 1881 [+ or -] 3 Ma, respectively. These are interpreted as the crystallization age of the rock and as providing the maximum age for mineralization. Zircon rims from an altered quartz diorite from the ore zone give ages of c. 1802 [+ or -] 15 Ma, which overlap with the 1801 [+ or -] 18 Ma titanite (mean Pb--Pb) age from the ore zone. The ages are similar to the age of the pegmatite dyke that cuts the ore zone and whose zircon age of 1807 [+ or -] 3 Ma is approximately the same as the 1791 [+ or -] 2 Ma monazite age (TIMS) giving the minimum age of the gold mineralization. The mineralization and its structural framework can be correlated with coeval late Svecofennian shear tectonics related to WNW--ESE-oriented shortening in southern Finland. Extensive c. 1.8 Ga granite magmatism, shear zone development and associated gold mineralization are of regional importance also in the northern and western Fennoscandian Shield (Finnish Lapand and Sweden). A Cordilleran-type setting can explain the widespread distribution of magmatism and gold mineralization associated with shortening, as well as the required heat source triggering hydrothermal fluid flow along shear zones. Keywords: U--Pb geochronology, orogenic gold, structural geology, Svecofennian orogeny, tectonic model, Finland. doi: 10.1017/S0016756809990628

The [epsilon]Hf and [[delta].sup.18]O values in detrital zircons from the Slave craton, Canada, indicate three episodes of crust formation between ca. 4.5 and 2.8 Ga, namely at ca. 4.4-4.5 Ga, ca. 3.8 Ga, and ca. 3.4 Ga. Most of the juvenile crust appears to have been mafic in composition, and there is no clear evidence for initial granitic protocrust in the Hadean of the Slave craton. The range of initial [epsilon]Hf values in zircons increases from 3.9 to 2.8 Ga, indicating that both extraction of new material from mantle and reworking of the older crust are important for the secular evolution of the continental crust. A preliminary review of available Hf data in zircons suggests that the three episodes of crust generation may have been of global importance. The mafic crust formed in the Archean and the Hadean was then reworked for at least ~0.5-1.5 b.y., as indicated by data from the Slave craton, Gondwana, and the Limpopo Belt of Africa.

The Tyrone Central Inlier is a metamorphic terrane of uncertain affinity situated outboard of the main Dalradian outcrop (south of the Fair Head--Clew Bay Line) and could represent sub-arc basement to part of the enigmatic Midland Valley Terrane. Using a combination of isotopic, structural and petrographic evidence, the tectonothermal evolution of the Tyrone Central Inlier was investigated. Sillimanite-bearing metamorphic assemblages (c. 670 [degrees]C, 6.8 kbar) and leucosomes in paragneisses are cut by granite pegmatites, which post-date two deformation fabrics. The leucosomes yield a weighted average [sup.207][pb/.sup.206] pb zircon age of 467 [+ or -] 12 Ma whereas the main fabric yields a [sup.40][Ar.sup.-39] Ar biotite cooling age of 468 [+ or -] 1.4 Ma. The pegmatites yield 457 [+ or -] 7 Ma and 458 [+ or -] 7 Ma Rb-Sr muscovite feldspar ages and [sup.40][Ar.sup.-39] Ar step-heating plateaux of 466 [+ or -] 1 Ma and 468 [+ or -] 1 Ma, respectively. The metasedimentary rocks yield Palaeoproterozoic Sm--Nd model ages and laser ablation inductively coupled plasma mass spectrometry detrital zircon U--Pb analyses from a psammitic gneiss yield age populations at 1.05-1.2, 1.5, 1.8, 2.7 and 3.1 Ga. Combined, these data permit correlation of the Tyrone Central Inlier with either the Argyll or the Southern Highland Group of the Dalradian Supergroup. The inlier was thus part of Laurentia onto which the Tyrone ophiolite was obducted.

Dating the pre-Middle Ordovician metavolcanic rocks and metagranites of the Ollo de Sapo Domain has, historically, been difficult because of the small compositional variation, the effects of the Variscan orogeny and, as revealed in this paper, the unusually high fraction of inherited zircon components. The first reliable zircon data (U-Pb ion microprobe and Pb-Pb stepwise evaporation) indicate that the Ollo de Sapo volcanism spanned 495 [+ or -] 5 Ma to 483 [+ or -] 3 Ma, and was followed by the intrusion of high-level granites from 483 [+ or -] 3 Ma to 474 [+ or -] 4 Ma. In both metavolcanic rocks and metagranites, no less than 70-80% of zircon grains are either totally Precambrian or contain a Precambrian core overgrown by a Cambro-Ordovician rim. About 80-90% of inherited zircons are Early Ediacaran (602-614 Ma) and derived from calc-alkaline intermediate to felsic igneous rocks generated at the end of the Pan-African arc--continent collision. In the Villadepera region, located to the west, both the metagranites and metavolcanic rocks also contain Meso-Archaean zircons (3.0-3.2 Ga) which ultimately originated from the West African Craton. In the Hiendelaencina region, located to the east, both the metagranites and metavolcanic rocks lack Meso-Archaean zircons, but they have two different inherited zircon populations, one Cryogenian (650-700 Ma) and the other Tonian (850-900 Ma), which suggest older-than-Ediacaran additional island-arc components. The different proportion of source components and the marked variation of the [sup.87]Sr/[sup.86][Sr.sub.init.] suggest, at least tentatively, that the across-arc polarity of the remnants of the Pan-African arc of Iberia trended east-west (with respect to the current coordinates) during Cambro-Ordovician times, and that the passive margin was situated to the west. Keywords: magmatism, Ordovician, zircon, dating, U/Pb, Pb/Pb.