Using the isotope dating of endocontact hybrid rocks for the age determination of mafic rocks (southern Siberian craton)

Geological observations and petrological and geochemical criteria are used to detect hybrid rocks at the endocontact of a dolerite dike. The hybrid rocks were produced when the material of a mafic intrusion mixed with a felsic melt. The latter was produced by the melting of the metamorphic rocks making up the Goloustnaya basement inlier of the Siberian craton, under the thermal effect of the intruded dike. Two age groups of zircon have been identified in the hybrid rock by SHRIMP analysis. The Paleoproterozoic age of inherited zircon (1902, 1864, 1859, and 1855 Ma) reflects the contribution of ancient sources to the hybrid-rock composition. The young, primary-magmatic, zircon grains, produced by melting at the endocontact of the mafic intrusion (494 ± 5 Ma), are coeval with the hybrid rocks, and their age indicates when the mafic rocks intruded the metamorphic framework. Dikes of close age, with similar geochemical characteristics, are present on the vast southern margin of the Siberian craton-from Goloustnaya to Biryusa salients. © 2013

Preliminary dating of the Viluy traps : eruption at the time of Late Devonian extinction events ?

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Preliminary dating of the Viluy traps (Eastern Siberia) : eruption at the time of Late Denovian extinction events ?

International audienceIn this short note, we report new age determinations from four samples of the Middle-Paleozoic Viluy Traps in Siberia, east of the more famous Permo-Triassic Siberian Traps. These samples, which were collected from three drill cores, have been analyzed in parallel and independently in the Orsay (France) and Berkeley Geochronology Center (BGC; USA) geochronology laboratories, using respectively the Cassignol-Gillot K-Ar and the Ar-40/Ar-39 techniques. Dating these samples as a concerted effort in two independent laboratories working jointly on their interpretation is a rather rare yet very valuable exercise. With the K-Ar technique, ages ranging from 338 to 367 Ma with uncertainties on the order of 5 Ma were obtained. With the Ar-40/Ar-39 technique, integrated ages range from 344 to 367 Ma, with uncertainties on the order of 1 Ma, and two samples yielded plateaus, i.e. the best determined ages, at 360.3 +/- 0.9 and 370.0 +/- 0.7 Ma. Three out of four ages yielded by the two separate methods are in agreement within uncertainties. One sample yields incompatible ages and could be from a later, altered dyke event. The Ar-40/Ar-39 plateau age of 370.0 0.7 Ma (conventional calibration) or 373.4 +/- 0.7 Ma (recalculated per Renne et al., 2010), the most reliable age obtained in this study, is compatible with recent determinations of the Late Devonian extinction events at the end-Frasnian (similar to 376 +/- 3 Ma). These results underscore a need for further work, in progress. (C) 2010 Elsevier B.V. All rights reserved

The oldest (~ 1.9 Ga) metadolerites of the southern Siberian craton: age, petrogenesis, and tectonic setting

Geological, geochronological, and isotope-geochemical studies of the metadolerites of the Angaul complex, widespread in the Urik-Iya graben of the southern Siberian craton, were carried out. The metadolerites forming separate conformal bodies (sills) among the metasandstones of the Ingash Formation were studied in detail. U-Pb zircon (SHRIMP) dating of metadolerites yielded an age of 1913 ± 24 Ma, and U-Pb baddeleyite (ID-TIMS) dating of these rocks yielded an age of 1914.0 ± 1.7 Ma. Thus, the date of 1914 ± 2 Ma can be taken as the most precise age estimate for the studied rocks. The metadolerites of the Angaul complex correspond in chemical composition to the normal-alkaline tholeiitic basalts. Metadolerites are differentiated rocks with mg# of 36 to 58. They show fractionated REE patterns: (La/Yb)n = 1.2-3.5. All metadolerites, independently of their mg# value, have low contents of Nb (1.6-10.2 ppm) and show well-pronounced negative Nb-Ta anomalies in multielement patterns (Nb/Nb* = 0.19-0.54). The metadolerites are characterized by positive eNd(T) values ranging from 0.4 to 5.2, which correlate well with their SiO2 content and mg# value. The isotope-geochemical parameters of the metadolerites of the Angaul complex indicate that fractional crystallization, along with the assimilation of the host rocks (AFC), might have been the main process during the formation of the most differentiated metadolerites. The geochemical characteristics of metadolerites with the maximum mg# values of 57-58 and eNd(T) = 5.2 suggest that the parental mantle source of the metadolerites resulted from mixing of predominant depleted mantle material with the subcontinental-lithosphere material. Intrusion of the dolerites of the Angaul complex, as well as the deposition of the sedimentary strata of the Ingash Group, took place at the Paleoproterozoic stage of intracontinental extension caused by the collapse of the orogen resulted from the collision of the Biryusa block with the Tunguska superterrane in the southern Siberian craton

The age and origin of volcanics in the Riphean section of the Siberian craton (western Baikal area)

In the western Baikal area, the structural position, composition, and age of volcanic rocks in the section of the Riphean margin of the Siberian craton were studied. The age of these rocks, earlier assigned to the Khotskaya Formation, is estimated at 274±3 Ma (concordia constructed over 11 zircon grains, SHRIMP-II). The geochemical and isotope compositions of volcanics evidence that they resulted from the melting of mantle source of EM-I type contaminated by crustal material. The intrusion of volcanics into the upper crustal horizons might have been caused by the evolution of the Permian active margin of the Siberian continent, which took place on the background of the closure of the Mongolo-Okhotsk ocean. Based on the results of studies, a new subvolcanic complex of Early Permian age has been recognized in the region, which includes the above-mentioned volcanics and earlier described porphyrite dikes of close age in the Sharyzhalgai uplift. The data obtained disprove the concept that the studied volcanics are of Riphean age; therefore, the available stratigraphic charts of the Siberian Precambrian must be revised

Early evolution of the Paleoasian ocean: LA-ICP-MS dating of detrital zircon from Late Precambrian sequences of the southern margin of the Siberian craton

We present U–Pb (LA-ICP-MS) data on detrital zircon from the Late Precambrian terrigenous rocks of the Baikal Group and Ushakovka Formation, western Cisbaikalia (southern flank of the Siberian craton). The sources of clastic material for the studied sediments are interpreted. The youngest group of detrital zircon grains from the upper Baikal Group and Ushakovka Formation permits assigning these sediments to the Vendian. The lack of Mesoproterozoic detrital zircon in most of the analyzed samples confirms the hypothesis of a global (~1 Gyr) break in endogenic activity within the southern flank of the Siberian craton through the Precambrian. The abundance of Neoproterozoic zircon in sandstones from the upper horizons of the Baikal Group and the Ushakovka Formation might be due to the shrinkage of the ocean basin as a result of the convergence of the craton with the microcontinents and island arcs within the Paleoasian ocean