Table of Contents for Journal of Geosciences. List of articles from the latest print issue.http://www.jgeosci.orgen-US http://www.jgeosci.org/jgeosci_cover.jpghttp://www.jgeosci.org/ Hanžl P; Vol. 53, issue 2, pages 201 - 234
]]> http://www.jgeosci.org/rss.php?ID=jgeosci.022 Editorial http://www.jgeosci.org/rss.php?ID=jgeosci.022 Valtr V, Hanžl P; Vol. 53, issue 2, pages 119 - 126
The internal structure of the Bogd fault, which is one of the largest intra-continental faults in Asia, was studied using the geophysical methods of very low frequencies, vertical electrical sounding and induced polarization vertical sounding. The geophysical profile was situated on the eastern foothill of the Chandman Khayrkhan Uul (SW Mongolia) near the branching of the Chandman rupture, recently seismically active western part of the Bogd fault zone. Geophysical measurements confirmed several sub-parallel faults that are steeply dipping to the south. An imbricated structure inside the fault zone and anastomosing trends of mylonitic zones were revealed by the orientation of the low resistivity zones. The most distinct anomalies indicating the fault zone were localised in mylonitized graphitic limestones, which were preferentially involved in the structure of the fault. The orientations of geophysical anomalies together with geological and structural data from the surface indicate steep thrusting of the southern block over the northern one, accompanying the dominating strike-slip component of the movement along the Chandman rupture. Geometry of faults delimiting the Chandman Khayrkhan Uul, strong asymmetrical morphology of mountain and relics of the Quaternary gravels in its upper part indicate rapid uplift of the range during the Cenozoic in the restraining bend. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.023 Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.023 Aichler J, Malec J, Večeřa J, Hanžl P, Buriánek D, Sidorinová T, Táborský Z , Bolormaa K, Byambasuren D; Vol. 53, issue 2, pages 123 - 138
Geological mapping and regional geochemical prospection on a scale of 1:50,000, covered an area of about 4,000 km² in the Zamtyn Nuruu region of the eastern Mongolian Altay. Northern part of the region is formed by the Lake Zone Terrane, the southern by the Gobi Altay Terrane; these two units are separated by the Bogd fault. The panned-concentrate heavy mineral survey was combined with the dry stream-sediment geochemistry, both being complemented by lithogeochemical sampling.
The dry stream-sediment geochemistry proved to be a suitable method for regional prospection in semiarid to arid areas of Mongolia. The morphology and composition of placer gold grains were studied in order to estimate the distance from the primary source, and to constrain the nature of prospective primary mineral deposits. Based on the results of the regional geochemical survey and geological mapping, follow-up works were focused on prospective areas including the reconnaissance field works, detailed geochemical sampling, and geophysical measurements.
New occurrences and signs of Au, Cu, Zn and (Ba)-As-Sb-Hg mineralizations were discovered. The ascertained gold-bearing mineralizations are represented namely by Au and Au-(Ag) quartz veins in basic metavolcanic rocks of the Neoproterozoic Khan Taishir and volcanic rocks of the Permian Delger Khangay formations. Gold locally accompanies vein–disseminated base metal mineralization in some Proterozoic and Lower Palaeozoic formations. A prospective occurrence of clastic gold in Mesozoic conglomerates and sandstones was found at the northern edge of the Zamtyn Nuruu around the Samoandamba Uul. The geochemical survey has revealed a contrast in occurrence of ore indices between the two terranes formig the studied area. While the Lake Terrane and especially the Neoproterozoic Khan Taishir Formation is rich in gold and base metal showings the Gobi Altay Terrane is barren. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.025 Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.025 Rukavičková L, Hanžl P; Vol. 53, issue 2, pages 181 - 191
A group of mud volcanoes was discovered in the NW part of the Gobi Altay on a northern foothill of the Khar Argalantyn Nuruu Mts. Several mud cones and mud mounds with elevation up to 0.8 m, as well as pools with muddy water and mud outflows were encountered. Jurassic to Quaternary sedimentary successions provide the source for the mud volcanoes. Jurassic sediments cover transgressively Permian volcanics forming several confined aquifers within a subsided, triangle shaped block. The aquifers are saturated with groundwater derived from the Khar Argalantyn Nuruu Mts. To the northwest and on the east this block is bordered by sealed faults. The significant recent seismic activity occurring in the area is related to Cenozoic faults as a response to the collision of the Indian and Eurasian continental plates. The mud volcanoes originated probably as a result of the Gobi-Altay Earthquake in 1957. The earthquake of magnitude 8.3 generated new fractures or reactivated existing fractures. The impulse of the earthquake exceeding the thixotropy of fine-grained sediments initiated the rise of the mud volcanoes. The recent equilibrium of pressure condition in the area is labile. Earthquakes or increase in piezometric level of groundwater caused by heavy rainfalls can trigger activity of the mud volcanoes. A high content of smectites in sediments and loss of binding of clay particles due to soluble salts support mud outflows during such starting events. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.024 Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.024 Hrdličková K, Bolormaa K, Buriánek D, Hanžl P, Gerdes A, Janoušek V; Vol. 53, issue 2, pages 139 - 165
Three crystalline complexes were newly described in the area of the junction between the Mongolian Altay and Gobi Altay in the surroundings of Chandman Sum, SW Mongolia. Khan Khayrkhan, Chandman Khayrkhan and Unegt Uul crystalline complexes were distinguished on the basis of their geological position, distinct petrography, metamorphic style and contrasting geochronological data. These units are situated along the northern margin of the Gobi-Altay Terrane and at first sight seem to have a very simple and uniform history. However, detailed studies reveal a more complex and different evolution. The Unegt Uul Crystalline Complex is a tectonic mélange of leucogranites, amphibolites and mica schists exposed between the branch of the Bogd fault in the north and Lower Palaeozoic sediments in the south. Metamorphic rocks suffered a prograde metamorphic event reaching peak temperatures of c. 800 ± 70 °C and pressures of 9.6 ± 1 kbar estimated from amphibolites. This Complex represents the oldest of the three units; the age of leucogranite formation corresponds to Cambrian (518 ± 5 Ma). Chandman Khayrkhan Crystalline Complex is also restricted by Bogd fault from the north but southern boundary is limited by an intrusive contact with the Chandman Massif. It is composed of orthogneisses and migmatites with amphibolite and calc-silicate lenses. The Complex was affected by a HT, likely periplutonic, metamorphic event with subsequent retrogression. While the rocks of the Chandman Khayrkhan Crystalline Complex remain undated, the age of the granitic rocks in the Chandman Massif itself are Early Carboniferous (345 ± 2 Ma). The Khan Khayrkhan Crystalline Complex has tectonic contacts with the surrounding Palaeozoic volcanosedimentary units. It is built by orthogneisses, amphibolites, paragneisses and mica schists. Metamorphic style corresponds to a prograde event with peak at c. 670 °C and 7–10 kbar. The crystallization age for the granitic protolith to the orthogneisses was the latest Devonian (363 ± 3 Ma). ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.027 Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.027 Buriánek D, Hanžl P, Erban V, Gilíková H, Bolormaa K; Vol. 53, issue 2, pages 167 - 180
The Cretaceous continental sedimentation in SW Mongolia was commonly accompanied by volcanic activity. Bimodal volcanic association of the Cretaceous Khulsan Gol Formation is represented mostly by calc-alkaline and rare alkaline volcanic rocks (basaltic andesite, trachybasalt to trachyandesite lavas or tuffs with lahars). The lavas were extruded mainly subaerially, although locally there is a subaqueous component present corresponding to final phases of Mesozoic sedimentation in the Gobi-Altay rift zone. Variations in chemical and isotopic composition reflect crustal assimilation coupled with fractional crystallization of parental magma generated by melting of lower lithosphere and/or uppermost asthenospheric mantle. These within-plate volcanic rocks can be explained as initial stages of Cenozoic magmatic events in the Mongolia. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.026 Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.026 Hanžl P, Bat-Ulzii D, Rejchrt M, Košler J, Bolormaa K, Hrdličková K; Vol. 53, issue 2, page: 201 - 234
Three geological domains (Gobi Tien Shan, Shargyn Gobi and Trans-Altay) have been distinguished in the Trans Altay Gobi (SW Mongolia) consolidated during the Carboniferous and intruded by numerous plutons of the Devonian, Carboniferous and Permian ages. According to relationship to the Variscan orogenic cycle, pre-orogenic, syn-orogenic and post-orogenic intrusive bodies have been recognized. A pre-orogenic phase is represented by the Gurvan Khar Massif composed of geochemically primitive granodiorites that are geologically related to the relics of the Silurian–Devonian oceanic crust in the Zoolen Terrane (Shargyn Gobi Domain). Calc-alkaline intrusive rocks of I-type in the Gobi Tien Shan and Naran Sevest intrusive complexes exposed in the Gobi Tien Shan Domain are syn-orogenic to post-orogenic. Laser ablation U-Pb ICP-MS dating on zircon yielded an Early Carboniferous age for the Zamyn Belgekh Pluton, representing the largest plutonic body in this domain. Other plutons (Bayan Ayrag, Trans-Altay and Ikh Bayan) are of high-K chemistry, have Late Carboniferous to Permian age and are clearly post-orogenic. Minor gabbro massifs and small oval bodies of granites of the Ikh Bayan Massif in the Baytag Terrane (Shargyn Gobi Domain) are spatially related to the Trans-Altay shear zone of Late Carboniferous to Permian age. This zone accommodated dextral lateral movements between the Shargyn Gobi and Trans-Altay domains. ]]> http://www.jgeosci.org/rss.php?ID=jgeosci.028 Original Paper http://www.jgeosci.org/rss.php?ID=jgeosci.028