Zircons were studied from high-pressure/low-temperature metamorphosed meta-igneous lithologies from Syros. These rocks carry several zircon generations related to each other by dissolution---reprecipitation processes. One generation is pristine zircon that shows growth zoning, relatively elevated contents of trivalent cations and high Th/U ratios. The other end-member is a skeletal zircon generation with negligible trivalent cation contents and low Th/U ratios ( 0 1). Texturally between these two, there is a range of zircon crystals with complex inclusion populations of Y---HREE---Th phases and fluid inclusions, showing variable progress of replacement--recrystallization. Both end-members yield distinct sensitive high-resolution ion microprobe (SHRIMP) U---Pb ages. The pristine generation has an age of 80 2 1 6Ma from a metagabbro, and 76 4 2 1Ma from a meta-plagiogranite dyke. The skeletal, low-Th/U zircon generation yields an age of 52 4 0 8Ma. The older, Late Cretaceous, zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust. The younger, Eocene, age, however, dates a zircon recrystallization event, which possibly coincides with high solubility and mobility of high field strength elements in a high-pressure aqueous fluid phase. Intergrowth relations between zircon and peak-metamorphic garnet, and excellent agreement of the U---Pb ages with white mica Ar---Ar ages for the same samples support the conclusion that Eocene is the true age of high-pressure metamorphism on Syros.

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Zircons from Syros, Cyclades, Greece—Recrystallization and Mobilization of Zircon During High-Pressure Metamorphism

A detailed isotopic study of the Manaslu leucogranite was carried out. A U-Pb age of 25 Ma and a whole rock Rb-Sr age isochron of 18 Ma were obtained, suggesting that the magmatic activity lasted at least 7 Ma. Initial Sr isotopic ratios are very high (0.740 to 0.760) and initial Nd isotopic ratios are low (ɛ

 Nd
 in
 : −13 to −16), and they show the existence of large isotopic variations even at the metre scale. These are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes (e.g. fusion, mixing by convection). These results also support the crustal origin of this leucogranite. The Tibetan slab paragneisses, whose Sr and Nd isotopic ratios are very similar to those of the granite at an age of 20 Ma, are the most probable parental material. Nd model ages for both the leucogranite and the gneisses are in the range 1.5–2 Ga. A model of formation of the Manaslu granite by coalescence of different batches of magma is in agreement with the present data.

Isotopic study of the Manaslu granite (Himalaya, Nepal): inferences on the age and source of Himalayan leucogranites

Large igneous provinces (LIPs) are high volume, short duration pulses of intraplate magmatism consisting mainly of flood basalts and their associated plumbing system, but also may include silicic c...

Large igneous provinces (LIPs), giant dyke swarms, and mantle plumes: significance for breakup events within Canada and adjacent regions from 2.5 Ga to the PresentThis article is one of a selection of papers published in this Special Issue on the the theme Lithoprobe—parameters, processes, and the evolution of a continent.Lithoprobe Contribution 1482. Geological Survey of Canada Contribution 20100072.

Summary This overview summarizes aspects of 150 years of research in Alpine tectonics and in particular introduces the tectonic setting for the more detailed papers in this volume. The Alpine Mesozoic ocean, Tethys, formed as a large elongate pull-apart basin in the Jurassic, as a consequence of the opening of the Atlantic and of the movement of Africa towards the east relative to a fixed Europe. The NNE trending Tethys was bounded by WNW trending transforms, by the European/Iberian margin in the W and by the Adriatic promontory of Africa in the E, and its shape determined the present day configuration of the arcs of the Alpine chain. The closing of this ocean and the collision tectonics began during the Cretaceous, as Africa moved to the NE relative to Europe and as the N Atlantic gradually opened, to drive Iberia and the southern part of the European plate to the E. Subduction of oceanic crust and adjacent continental crust led to high pressure metamorphism of Cretaceous age. Ophiolites were obducted over the southern continental margin, but after collision the shear sense reversed so that the Austro-Alpine nappes of the African Adriatic promontory overthrust Europe in a WNW direction. During the main Tertiary deformation the overall anticlockwise rotation of Africa led to a change-over from N to NW and WNW-directed collisional structures. The E-W striking sector of the Alps in Switzerland and Austria is therefore a diffuse transpressive dextral shear belt, approximately reworking the northern transform boundary of Tethys, modifying it by compression related to the rotation of the African Adriatic promontory. Approximately 250 km of European lithosphere were involved in the building of the western Alps. As Alpine nappes consist largely of rock material confined to the upper crust, a large amount of lower crust and lithospheric mantle of the two continental blocks must be duplicated and/or subducted during the Alpine collision history.

Alpine tectonics — an overview

1891–1883 Ma Southern Bastar–Cuddapah mafic igneous events, India: A newly recognized large igneous province

Crystallization and age of zircon from Corsican ophiolitic albitites: consequences for oceanic expansion in Jurassic times

The magmatic events occurring within the two main cycles in the south-central part of the Labrador Trough (New Quebec) have been dated. In cycle 1, a granophyre dike related to the Cramolet Lake gabbro sill, which intrudes the Seward subgroup, has a U–Pb zircon age of 2169 ± 2 Ma. In cycle 2, the tholeiitic basalts of the Willbob (Hellancourt) Formation and the related mafic–ultramafic sills are dated at ca. 1900 Ma by the Pb–Pb method. These data confirm the existence of at least two main magmatic cycles separated by about 270 Ma. The magma source was depleted upper mantle, and the magma did not experience any significant crustal contamination, as indicated by the μ1 ratio (7.9) and (+4) for the tholeiitic basalts. The μ1 value for the intrusive rocks (8.03) and the average value for the gabbroic rocks of cycle 2 (+2.8) and for the granophyre of cycle 1 (+1.05) could be the result of slight crustal contamination or of mantle heterogeneity. Whatever the cause of these values, the data indicate the prolong...

D. Ohnenstetter

Papers

Crystallization and age of zircon from Corsican ophiolitic albitites: consequences for oceanic expansion in Jurassic times

Aphebian mafic–ultramafic magmatism in the Labrador Trough (New Quebec): its age and the nature of its mantle source