U-Pb zircon ages for Yarlung Tsangpo suture zone ophiolites, southwestern Tibet and their tectonic implications

Ophiolite complexes preserved along the Yarlung Tsangpo suture zone (YTSZ) and obducted onto the northern continental margin of India in southern Tibet represent the remnants of the once extensive Permian-Mesozoic Neo-Tethyan Ocean that separated India from Asia. Complete ophiolite successions are preserved near Xigaze, whereas the rest of the belt is essentially represented by mantle rocks with subordinate disrupted lower crustal rocks. U-Pb zircon LA-MC-ICP-MS geochronology on two gabbro samples from the Luobusa ophiolite yielded concordant data with mean 206Pb/238U ages of 149.9 ± 1.4 (2σ) Ma and 150.0 ± 5.0 Ma. These ages are in contrast to a younger age of 131.8 ± 1.0 Ma obtained from a pegmatitic gabbro in Xigaze. Five U-Pb zircon TIMS ages from gabbroic samples in the western portion of the ophiolite belt reveal that the Dangxiong ophiolite formed between 126.7 ± 0.4 Ma and 123.4 ± 0.8 Ma. Zircons from the Jungbwa ophiolite have similar ages of 123.4 ± 0.8 Ma and 123.9 ± 0.9 Ma. A single zircon analysed from a gabbro in Kiogar has an age of 159.7 ± 0.5 Ma. Geochronological data reported here show YTSZ ophiolites formed in association with intra-oceanic subduction zone systems and are related a significant tectonic episode within the Tethyan Ocean during Late Jurassic to Early to mid Cretaceous times. © 2013 International Association for Gondwana Research

Discovery of an albite gneiss from the Ile de Groix (Armorican Massif, France): Geochemistry and LA-ICP-MS U-Pb geochronology of its Ordovician protolith

For the first time, an albite orthogneiss has been recognised and dated within the HP-LT blueschist facies metabasites and metapelites of the Ile de Groix. It is characterised by a peraluminous composition, high LILE, Th and U contents, MORB-like HREE abundances and moderate Nb and Y values. A U-Pb age of 480.8 +/- A 4.8 Ma was obtained by LA-ICP-MS dating of zircon and titanite. It is interpreted as the age of the magmatic emplacement during the Early Ordovician. Morphologically different zircon grains yield late Neoproterozoic ages of 546.6-647.4 Ma. Zircon and titanite U-Pb ages indicate that the felsic magmatism from the Ile de Groix is contemporaneous with the acid, pre-orogenic magmatism widely recognised in the internal zones of the Variscan belt, related to the Cambro-Ordovician continental rifting. The magmatic protolith probably inherited a specific chemical composition from a combination of orogenic, back-arc and anorogenic signatures because of partial melting of the Cadomian basement during magma emplacement. Besides, the late Devonian U-Pb age of 366 +/- A 33 Ma obtained for titanite from a blueschist facies metapelite corresponds to the age of the HP-LT peak metamorphism

Detrital zircon geochronology of the Cretaceous succession from the Iberian Atlantic Margin: palaeogeographic implications

Detrital zircon U–Pb data performed on eight Cretaceous sandstone samples (819 age isotopic results) from the Lusitanian basin (west Portugal) constrain the history of uplift and palaeodrainage of western Iberia following break-up of Pangaea and opening of the North Atlantic Ocean. We examined the links between shifts in provenance and known basinwide unconformities dated to the late Berriasian, Barremian, late Aptian and Cenomanian–Turonian. The detrital zircon record of sedimentary rocks with wider supplying areas is relatively homogenous, being characterized by a clear predominance of late Palaeozoic ages (c. 375–275 Ma) together with variable proportions of ages in the range c. 800–460 Ma. These two groups of ages are diagnostic of sources within the Variscan Iberian Massif. A few samples also reveal significant amounts of middle Palaeozoic (c. 420–385 Ma) and late Mesoproterozoic to early Neoproterozoic (c. 1.2–0.9 Ga) zircon, which are almost absent in the basement to the east of the Lusitanian basin, but are common in terranes with a Laurussia affinity found in NW Iberia and the conjugate margin (Newfoundland). The Barremian unconformity marks a sudden rise in the proportion of c. 375–275 Ma zircon ages accompanied by a decrease in the abundance of the c. 420–385 Ma and c. 1.2–0.9 Ga ages. This shift in the zircon signature, which is contemporaneous with the separation of the Galicia Bank from Flemish Cap, reflects increased denudation of Variscan crystalline rocks and a reduction in source material from NW Iberia and adjoining areas. The late Aptian unconformity, which represents the largest hiatus in the sedimentary record, is reflected by a shift in late Palaeozoic peak ages from c. 330–310 Ma (widespread in Iberia) to c. 310–290 Ma (more frequent in N Iberia). It is considered that this shift in the age spectra resulted from a westward migration of catchment areas following major uplift in northern Iberia and some transport southward from the Bay of Biscay under the influence of a well-established Atlantic circulation