Journal of GEOsciences Table of Contents for the Journal of GEOsciences. List of articles from the latest print issue.http://www.jgeosci.orgen-US Journal of GEOsciences <![CDATA[ Stratigraphy, structure and geology of Late Miocene Verkhneavachinskaya caldera with basaltic-andesitic ignimbrites at Eastern Kamchatka ]]> Bergal-Kuvikas O, Leonov V, Rogozin A, Bindeman I, Kliapitskiy E, Churikova T; Vol. 64, issue 4, pages 229 - 250
We newly describe a paleovolcano in the Eastern Volcanic Belt of Kamchatka named Verkhneavachinskaya caldera (VC). According to geological mapping of the area covered by the deposits, mainly lava-like ignimbrites, the VC is interpreted as an eroded paleoshield volcano with a summit caldera. It is one of the largest (10 × 12 km diameter) and oldest (c. 5.78-5.58 Ma as newly dated by the Ar-Ar method) currently known paleovolcanoes in Kamchatka with such a good preservation. The lava-like welded ignimbrites correspond to basaltic andesite and andesite and are more mafic than pyroclastic rocks from the other post-Pliocene calderas in Kamchatka. The deposits of VC are interbedded layers of welded ignimbrites and volcaniclastic deposits. These deposits were formed during long-lived, continuous eruptions of hot pyroclastic flows and subsequent accumulations of volcaniclastic deposits (e.g. lahars). The VC provides new insights into the early stages of Eastern Volcanic Belt development after the Kronotsky Arc Terrane accretion. This potentially explains the origin of voluminous basaltic-andesitic ignimbrites formed on thin crust during the initial stage of the arc formation. ]]> Original paper
<![CDATA[ Radiogenic heat production of Variscan granites from the Western Bohemian Massif, Germany ]]> Scharfenberg L, Regelous A, De Wall H; Vol. 64, issue 4, pages 251 - 269
Much of the Mid-European basement has been consolidated during the Variscan Orogeny and includes large volumes of granitic intrusions. Gamma radiation spectroscopic measurements in three study areas along the western margin of the Bohemian Massif give a record of radiogenic element concentrations in the Variscan granites. Most intrusions of the Fichtelgebirge (except for the Tin Granite) and intrusive complexes in the Bavarian Forest show Th/U ratios exceeding unity, most likely related to abundance of monazite. In contrast, some of the Oberpfalz granites located near the Saxothuringian-Moldanubian boundary (Flossenbürg, Steinwald and Friedenfels types) are characterized by higher uranium concentrations and thus Th/U < 1. The low Th/U ratios here are in agreement with a possible U mobilisation along the Saxothuringian-Moldanubian contact zone observed in previous studies. Heat production rates of granites in the three study areas vary between 3.9 and 8.9 µW/m3, with a mean of 4.9 µW/m3. This classifies the intrusions as moderate- to high-heat-producing granites. Considering the huge volume of granitic bodies in the Variscan crust of the Bohemian Massif, the contribution of in situ radiogenic heat production had to have a major impact and should be considered in further thermal modeling. ]]> Original paper
<![CDATA[ Petrogenesis of fractionated nested granite intrusions: the Sedmihoří Composite Stock (Bohemian Massif) ]]> Trubač J, Janoušek V, Gerdes A; Vol. 64, issue 4, pages 271 - 294
The post-tectonic Sedmihoří Stock (SCS) is a circular body in plan-view, made up of three nested units: (i) ’Outer granite’ (OG), less fractionated Kfs-phyric Bt monzogranite (326.2 ± 1.2 Ma (2σ), LA ICP-MS Zrn), (ii) ’Inner granite’ (IG), more evolved Bt-Ms monzogranite (326.6 ± 1.2 Ma), and (iii) innermost leucogranite (’LG’), fine-grained Tur-Ms leucogranite.
All the varieties of this body are silica-rich (SiO2 > 71 wt. %) and subaluminous to strongly peraluminous (A/CNK = 1.01-1.25). Major-element contents do not vary greatly but the trace elements show more significant differences between the granite varieties. From OG to LG, the rocks show decreasing overall REE abundances and La/Yb ratios.
Significant differences in Th and U concentrations occur between the marginal OG (˜49 ppm Th; ˜9 ppm U) and the more central IG (˜8 ppm Th, ˜4 ppm U). Hydrothermal thorite (recorded in the OG), xenotime and monazite are the main accessories concentrating these radioactive elements. The final stage of the magma evolution is represented by the central LG pulse containing beryl, wolframite and high-Li muscovite.
All pulses of the SCS are characterized by crust-like Sr-Nd isotopic signatures. The inner IG shows more evolved Sr (87Sr/86Sr326 = 0.7098-0.7130) and less radiogenic Nd (εNd326 = -4.6 to -3.7) than the outer OG (87Sr/86Sr326 = 0.7067-0.7076; εNd326 = -2.5 to -2.7).
The granite batches were probably derived by anatexis of Neoproterozoic or, more likely, Cambrian metagreywackes of the Teplá-Barrandian Unit. Enriched-mantle derived magmas (redwitzites) played a very important role that not only provided the heat needed for the anatexis, but also variably contributed material via magma mixing processes. The hybridization was particularly significant for the origin of the outer unit, which is rich in mafic microgranular enclaves and whose Sr is the least and Nd most radiogenic within the SCS.
Taken together, field observations, structural relations, U-Pb zircon dating and geochemistry indicate that all pulses in the SCS were emplaced nearly simultaneously. However, each of them represented a single batch of magma with its own geochemical characteristics and petrogenetic features. ]]> Original paper