U–Pb Zircon geochronology of the Cambro-Ordovician metagranites and metavolcanic rocks of central and NW Iberia

New U–Pb zircon data from metagranites and metavolcanic rocks of the Schist-Graywacke Complex Domain and the Schistose Domain of Galicia Tras-os-Montes Zone from central and NW Iberia contribute to constrain the timing of the Cambro-Ordovician magmatism from Central Iberian and Galicia Tras-os-Montes Zones which occurred between 498 and 462 Ma. The crystallization ages of the metagranites and metavolcanic rocks from the northern Schist-Graywacke Complex Domain are as follows: (a) in west Salamanca, 489 ± 5 Ma for Vitigudino, 486 ± 6 Ma for Fermoselle and 471 ± 7 Ma for Ledesma; (b) in northern Gredos, 498 ± 4 Ma for Castellanos, 492 ± 4 Ma for San Pelayo and 488 ± 3 Ma for Bercimuelle; (c) in Guadarrama, 490 ± 5 Ma for La Estación I, 489 ± 9 Ma for La Cañada, 484 ± 6 Ma for Vegas de Matute (leucocratic), 483 ± 6 Ma for El Cardoso, 482 ± 8 Ma for La Morcuera, 481 ± 9 Ma for Buitrago de Lozoya, 478 ± 7 Ma for La Hoya, 476 ± 5 Ma for Vegas de Matute (melanocratic), 475 ± 5 Ma for Riaza, 473 ± 8 Ma for La Estación II and 462 ± 11 Ma for La Berzosa; and (d) in Toledo, 489 ± 7 Ma for Mohares and 480 ± 8 Ma for Polán. The crystallization ages of the metagranites from the Schistose Domain of Galicia Tras-os-Montes Zone are 497 ± 6 Ma for Laxe, 486 ± 8 Ma for San Mamede, 482 ± 7 Ma for Bangueses, 481 ± 5 Ma for Noia, 480 ± 10 for Rial de Sabucedo, 476 ± 9 Ma for Vilanova, 475 ± 6 Ma for Pontevedra, 470 ± 6 Ma for Cherpa and 462 ± 8 Ma for Bande. This magmatism is characterized by an average isotopic composition of (⁸⁷Sr/⁸⁶Sr)_485Ma ≈ 0.712, (ε_Nd)_485Ma ≈ ?4.1 and (T_DM) ≈ 1.62 Ga, and a high zircon inheritance, composed of Ediacaran–Early Cambrian (65 %) and, to a lesser extent, Cryogenian, Tonian, Mesoproterozoic, Orosirian and Archean pre-magmatic cores. Combining our geochronological and isotopic data with others of similar rocks from the European Variscan Belt, it may be deduced that Cambro-Ordovician magmas from this belt were mainly generated by partial melting of Ediacaran–Early Cambrian igneous rocks.     

U–Pb Dating,Oxygen and Hafnium Isotope Ratios of Zircon from Rocks of Oceanic Core Complexes at the Mid-Atlantic Ridge: Evidence for the Interaction of Contemporary and Ancient Crusts in the Spreading Center of the Ocean Floor

Doklady Earth Sciences - The U–Pb age and oxygen and hafnium isotope ratios of zircon from rocks of oceanic core complexes along the Mid-Atlantic Ridge have been studied using SHRIMP and...     

Palaeogeography and crustal evolution of the Ossa–Morena Zone,southwest Iberia,and the North Gondwana margin during the Cambro-Ordovician: a review of isotopic evidence

Cambro-Ordovician palaeogeography and fragmentation of the North Gondwana margin is still not very well understood. Here we address this question using isotopic data to consider the crustal evolution and palaeogeographic position of the, North Gondwana, Iberian Massif Ossa–Morena Zone (OMZ). The OMZ preserves a complex tectonomagmatic history: late Neoproterozoic Cadomian orogenesis (ca. 650–550 Ma); Cambro-Ordovician rifting (ca. 540–450 Ma); and Variscan orogenesis (ca. 390–305 Ma). We place this evolution in the context of recent North Gondwana Cambro-Ordovician palaeogeographic reconstructions that suggest more easterly positions, adjacent to the Sahara Metacraton, for other Iberian Massif zones. To do this we compiled an extensive new database of published late Proterozoic–Palaeozoic Nd model ages and detrital and magmatic zircon age data for (i) the Iberian Massif and (ii) North Gondwana Anti-Atlas West African Craton, Tuareg Shield, and Sahara Metacraton. The Nd model ages of OMZ Cambro-Ordovician crustal-derived magmatism and Ediacaran-Ordovician sedimentary rocks range from ca. 1.9 to 1.6 Ga, with a mode ca. 1.7 Ga. They show the greatest affinity with the Tuareg Shield, with limited contribution of more juvenile material from the Anti-Atlas West African Craton. This association is supported by detrital zircons that have Archaean, Palaeoproterozic, and Neoproterozoic radiometric ages similar to the aforementioned Iberian Massif zones. However, an OMZ Mesoproterozoic gap, with no ca. 1.0 Ga cluster, is different from other zones but, once more, similar to the westerly Tuareg Shield distribution. This places the OMZ in a more easterly position than previously thought but still further west than other Iberian zones. It has been proposed that in the Cambro-Ordovician the North Gondwana margin rifted as the Rheic Ocean opened diachronously from west to east. Thus, the more extensive rift-related magmatism in the westerly OMZ than in other, more easterly, Iberian Massif zones fits our new proposed palaeogeographic reconstruction.     

Liquid content of the lower clouds of venus as determined from mariner 10 radio occultation

S-band (13.06-cm) and X-band (3.56-cm) radio occulation data obtained during the flyby of Venus by Mariner 10 on February 5, 1974 were analyzed to obtain the effects of dispersive microwave absorption by the clouds of Venus. The received power profiles were first corrected for the effects of refraction in the atmosphere of Venus, programmed changes in the pointing direction of the high-gain antenna, and limit-cycle motion of the spacecraft attitude control system. The resulting excess attenuation profiles presumbaly due to cloud absorption have been inverted discretely to obtain profiles of absorption coefficient at the two wavelenghts. The ratios of the absorptivities are consistent with a sulfuric acid-water mixture as the constituent of the absorbing clouds, having a sulfuric acid concentration of 75 ± 25%. Three absorption peaks are evident in the profiles at altitudes of 68, 60, and 48 km. With a sulfuric acid concentration of 75%, the upper cloud has a peak liquid content of 0.08 g/m³, and an integrated content of 0.024 g/cm², which corresponds roughly to terrestrial stratus or altostratus clouds. The major absorption layer has a peak of 1.1 g/m³ at an altitude of 48 km, with an integrated content of 0.5 g/cm², similar to that of terrestrial cumulus and cumulonimbus clouds. The absorption ratios for the middle cloud at 60 km are not consistent with a sulfuric acid-water mixture.     

Intrusive magmatism during early evolutionary stages of the Ural epioceanic orogen: U-Pb geochronology (LA ICP MS,NORDSIM, and SHRIMP II), geochemistry,and evolutionary tendencies

In recent years extensive data have been obtained on all geologically important intrusive complexes in the Central and Southern Urals by U-Pb zircon geochronologic high spatial resolution techniques (LA ICP MS, NORDSIM, and SHRIMP II). This made it possible to revise the current concepts for the magmatic activity of the Ural Paleozoic orogen.Intrusive magmatism that occurred early in the evolution of the Ural orogen was focused mostly in the Tagil megazone, was characterized by several common features, and took place nearly simultaneously within both of its zones: the Platinum Belt and the Tagil volcanic zone.The composition of the parental magmas of all complexes of this age corresponded to an ultramafic or mafic source; i.e., the magma was derived from a mantle source. The gabbroids most closely approximating the composition of the parental magmatic melts show geochemical features of suprasubduction melts, such as negative HFSE (Nb, Ti, and Zr) and positive Ba and Sr anomalies. The REE patterns of these rocks display variable La/Lu ratios, which are usually higher than 1. These geochemical features suggest that this magmatic source was a metasomatized mantle wedge, above which (at a depth of 40–25 km) a block of the pre-Ural basement occurred in Ordovician-Silurian time. The Tagil megazone started to develop on this block. By the Devonian, i.e., by the time when the Magnitogorsk zone began to evolve (～400 Ma) and continental-margin gabbro-tonalite-granodiorite magmatism was initiated (360 Ma), this basement had been destroyed by orogenesis. The major phases of Paleozoic magmatism in the Urals likely corresponded to global epochs of tectono-magmatic activity, because they correlate well with known data on the evolution of the ⁸⁷Sr/⁸⁶Sr ratio in Paleozoic seawater.     

Isotopic-geochemical features and age of zircons in dunites of the platinum-bearing type Uralian massifs: Petrogenetic implications

This paper presents results of isotopic (Cameca IMS1270 NORDSIM and SHRIMP-II ion microprobes) and geochemical (LA-ICP MS) study of zircons in three dunite samples of the Uralian-Alaskan-type massifs of the Urals: Kosva, Sakharin, and Eastern Khabarny. The zircons in the dunites share common features. Each sample contains the following genetic and age groups of zircons: (1) xenogenic zircons of the Archean and Proterozoic age; (2) zircons of magmatic appearance, which in age and geochemistry are close to the zircons from associated gabbroids; (3) postmagmatic zircons that presumably crystallized from hydrothermal solutions. The xenogenic zircons of the Archean age in each of three samples comprise transparent fragments, which are depleted in U and other trace elements and presumably have mantle origin. Xenogenic zircons of the Proterozoic age (1500–2000 Ma) occur as oval grains with surface abrasion, the traces of their redeposition. The geochemical features of the xenogenic zircons unequivocally demonstrate their affiliation to the continental crust—the basement of the Uralian orogen. The zircons of magmatic habit in all the dunite samples are close in age to the associated gabbroids: 435–432 Ma in the Kosva Massif, 378–374 in the Sakharin Massif, and 407–402 Ma in the Eastern Khabarny Massif, and mark the age of dunite formation. In addition, the magmatic zircons from dunites and associated gabbroids share similar geochemical features. These data could serve as additional argument in support of cumulate origin of dunites in the Uralian-Alaskan-type complexes. The postmagmatic zircons are most enriched in trace elements and were presumably formed from a fluid phase, which was responsible for the recrystallization of dunites and redistribution of Cr-spinel and PGE mineralization.     

Age of zircon from apoharzburgite serpentinite representing mantle of the Uralian paleoocean

Over 60 zircon grains from apoharzburgite serpentinite were dated using SHRIMP–IIe/mc at the Laboratory IBERSIMS of the Granada University (Spain). The apoharzburgite serpentinite represents an oceanic mantle of the Uralian paleoocean, which was exhumed in the crustal structures of the Paleozoic Ural Mobile Belt during obduction. Individual grains span a huge ²⁰⁶Pb/²³⁸U age range from 2740 to 250 Ma and are clustered into six discrete age groups (in Ma): (I) > 2500, (II) 2500–1950, (III) 1260–1210, (IV) 480–400, (V) 370–330, and (VI) < 280. Two last groups were formed under the effect of granitoids on serpentinites. The traces of this effect were studied in outcrops and confirmed by age of zircon from contact talc–carbonate rock. The morphologies of zircon crystals from serpentinite bear signs typical of both magmatic and metamorphic varieties, which indicate their polygenetic–polychronous nature. No striking morphological features and peculiar U and Th contents were found in the studied zircons to discriminate unambiguously between different age groups. Pre-Paleozoic events with ages of groups I–III were found in zircons from many oceanic mantle rocks. The similarity of age groups of zircons from Paleozoic and modern oceanic lithosphere is caused by global mantle reworkings, which provoke magma generation and metasomatism probably accompanied by zircon crystallization.