Late Jurassic (151–147 Ma) Dike Magmatism of the Northeastern Margin of the Siberian Craton

Doklady Earth Sciences - Dikes of intermediate and felsic composition from the area of the Vyun deposit and the Shumnyi occurrence, both of which belong to the Yano–Kolyma gold belt...     

Yurchik Massifs in Kamchatka: Age and affiliation to magmatic associations

Petrographic and isotopic-geochemical data obtained on basic and ultrabasic rocks from the Yurchik Massif in the Ganal block of crystalline rocks in Kamchatka indicate that the distribution of major and trace elements in these rocks are analogous to those in the fractionation products of high-Al tholeiites occurring in island arcs in the eastern continental margin of Eurasia. Allivalites and dunites found as nodules in gabbronorites and gabbro of the massif are thought to be early cumulates of arc basalts. Petrographic and geochemical characteristics of the Yurchik Massif make it different from Ni-bearing Paleocene-Eocene (approximately 50 Ma) norite-cortlandite intrusions in the Sredinnyi Range of Kamchatka. U-Pb zircon and ⁴⁰Ar/³⁹Ar dates for rocks from the massif definitely testify to its younger, Early Miocene (approximately 22 Ma) age.     

Magnetostratigraphy of the Middle–Upper Cambrian Verkhnyaya Lena Group and Lower Ordovician Ust-Kut formation in the southern Siberian Platform

The available paleomagnetic data on the Verkhnyaya Lena Group from different areas of the southern Siberian Platform are revised. The group rests unconformably upon the Lower Cambrian strata and is overlain by Lower Ordovician rocks, which determines conditionally the age of its red-colored deposits. Paleomagnetic correlation of composite sections through the region using defined zones of normal and reversed magnetic polarity serves as a basis for development of the magnetostratigraphic scale for the Verkhnyaya Lena Group. The scale includes nine magnetic zones, which play the role of markers; seven of them are traceable in all the examined sections of the southern Siberian Platform. By the distribution of zones with normal (N) and reversed (R) polarity, the magnetostratigraphic scale is subdivided into three parts. Its lower part is represented by reversed polarity, which is characteristic of the second half of the Lower Cambrian. The middle part is characterized by frequently alternating zones with normal and reversed polarity corresponding to the Middle Cambrian. The upper part of the scale corresponds to the interval of reversed polarity characteristic of the Upper Cambrian and Lower Ordovician. The Middle–Upper Cambrian boundary is located near the last N–R reversal of the geomagnetic field in the Cambrian. The magnetostratigraphic scale includes nine orthozones united into three superzones, which are attributed to two hyperzones of magnetic polarity.     

Enigmatic sedimentary–volcanic successions in the central European Variscides: a Cambrian/Early Ordovician age for the Wojcieszów Limestone (Kaczawa Mountains,SW Poland) indicated by SHRIMP dating of volcanic zircons

Metamorphosed volcanic and sedimentary successions in the central European Variscides are, in many areas, poorly biostratigraphically constrained, making palaeotectonic interpretations uncertain. In such instances, geochronological data are crucial. Sensitive high resolution ion microprobe (SHRIMP) dating of volcanic zircons from a quartz–white mica schist (interpreted as deformed metavolcaniclastic/epiclastic rock) within the stratigraphically controversial Wojcieszów Limestone of the Kaczawa Mountains (Sudetes, SW Poland), near to the eastern termination of the European Variscides, has yielded an age of 498 ± 5 Ma (2σ error), corresponding to late Cambrian to early Ordovician magmatism in that area and constraining the depositional age of the limestones. The new SHRIMP data are not consistent with the recent revision of the age of the Wojcieszów Limestone based on Foraminifera findings that ascribed them to a Late Ordovician—Silurian or even younger interval. They are though, consistent with sparse macrofossil data and strongly support earlier interpretations of the lower part of the Kaczawa Mountains succession as a Cambrian–Early Ordovician extensional basin‐fill with associated initial rift volcanic rocks, likely emplaced during the breakup of Gondwana. Copyright © 2008 John Wiley & Sons, Ltd.     

U-Pb dating of the baddeleytte-zircon system from Pt-bearing dunite of the Konder massif, Aldan shield: New data

The U-Pb age was obtained for the coexisting baddeleytte-zircon system from dunites of the Konder massif, Aldan shield. Four groups of zircons are heterogeneous by morphology, habit, age, and geochemistry in contrast to homogeneous baddeleytte. The studied zircon groups are characterized by several U-Pb age clusters in the range of 1895 ± 50 to 125.8 ± 3.8 Ma, which indicates their long evolution in Pt-bearing dunites. The young assemblage of baddeleytte and zircon (124.9 ± 1.9 and 125.8 ± 3.8 Ma, respectively) also differs from ancient zircons in the morphology and geochemistry and probably dates to a much later event of diapir evolution, which may be referred to the formation of apatite-phlogopite ore pyroxenites inside the dunite core and tectono-magmatic activation of the Aldan shield.     

Geological Position,Age, and Sources of the Tallai Gabbro–Diorite–Plagiogranite Pluton (Middle Vitim Highland)

Doklady Earth Sciences - The results of geochronological study (U–Pb SHRIMP-II) of zircons from granodiorites and plagiogranites of the main phase of the Tallai pluton of the...