Geochemical, Isotopic and Petrological Constraints on the Origin and Evolution of the Recent Silicic Magmatism of the Greater Caucasus

Significant volumes of rhyolites and granites of the Pliocene-Pleistocene age are exposed in the collision zone of the Greater Caucasus, Russia. The volcanic history of the region includes ignimbrites and lavas associated with the Chegem caldera (2.9 Ma) and Elbrus volcano (1.98 and 0.7 Ma) and rhyolitic necks and granites in Tyrnyauz (1.98 Ma). They are characterized by a similar bulk and mineral composition and close ratios of incompatible elements, which indicates their related origin. The 1.98 Ma Elbrus ignimbrites, compared to the 2.9 Ma Chegem ignimbrites, have elevated concentrations of both compatible (Cr, Sr, Ca, Ni) and incompatible elements (Cs, Rb, U). We argue that the Elbrus ignimbrites were produced from magma geochemically similar to Chegem rhyolites through fractionation crystallization coupled with the assimilation of crustal material. The 1.98 Ma Eldjuta granites of Tyrnyauz and early ignimbrites of the Elbrus region (1.98 Ma) are temporally coeval, similar mineralogically, and have comparable major and trace element composition, which indicates that the Elbrus ignimbrites probably erupted from the area of modern Tyrnyauz; the Eldjurta granite could represent a plutonic reservoir that fed this eruption. Late ignimbrites of Elbrus (0.7 Ma) and subsequent lavas demonstrate progressively more mafic mineral assemblage and bulk rock composition in comparison with rhyolites. This indicates their origin in response to the mixing of rhyolites with magmas of a more basic composition at the late stage of magma system development. The composition of these basic magmas may be close to the basaltic trachyandesite, the flows exposed along the periphery of the Elbrus volcano. All studied young volcanic rocks of the Greater Caucasus are characterized by depletion in HSFE and enrichment in LILE, Li, and Pb, which emphasizes the close relationship of young silicic magmatism with magmas of suprasubduction geochemical affinity. An important geochemical feature is the enrichment of U up to 8 ppm and Th up to 35 ppm. The trace element composition of the rocks indicates that the original rhyolitic magma of Chegem ignimbrites caldera was formed at >80%–90% fractionation of calc-alkaline arc basalts with increased alkalinity. This observation, in addition to published data for isotopic composition (O-Hf-Sr) of the same units, shows that the crustal isotopic signatures of silicic volcanics may arise due to the subduction-induced fertilization of peridotites producing parental basaltic magmas before a delamination episode reactivated the melting of the former mantle and the lower crust.

Possibilities of radiocarbon (14C) method for age determination of geological events in the Elbrus volcanic centre area

Радиоуглеродный метод (14С) позволяет датировать геологические события по детриту и древним углям из погребенных почв, начиная с 40-50 тыс. лет тому назад и до сотен лет. Однако для его широкого применения в пределах Эльбрусского вулканического центра (высота вулканической постройки 5 642 м), при датировании времени излияния отдельных лавовых потоков, имеются объективные ограничения. Они заключаются в том, что начиная с отметки от 3 300 м и выше в горах Северного Кавказа отсутствует травяно-кустарниковая растительность и почвенный слой с гумусом, и именно на этих высотах расположе- на значительная часть голоценовых лавовых потоков и туфовых горизонтов Эльбруса, а с 3 700 м и выше вулканическая постройка покрыта вековыми ледниками и снежниками. В результате проведенных исследо- ваний для голоцена были установлены: частоты повторяемости извержений вулкана Эльбрус и землетря- сений, проявившихся в Приэльбрусье. В итоге была выявлена важная закономерность, свидетельствующая о том, что к концу голоцена частоты повторяемости извержений Эльбруса и землетрясений резко воз- растают. Установлено совпадение времени проявления вулканической и сейсмической активности (причем вторая проявлялась несколько раньше первой). При этом периоды вулканической и сейсмической активности резко возрастают, а периоды покоя сокращаются. Эти данные имеют важное значение для оценки возмож- ности возобновления вулканической и сейсмической активностей в пределах Приэльбрусья в геологическом будущем. Radiocarbon (14C) method makes possible to dating of geological events on detritus and older coals from buried soil from 40-50 thousands years ago up to hundreds years. However, for its wide applica-tion within of Elbrus volcanic centre (altitude of Elbrus volcano is 5642 m) for dating time of separate lava fl ows eruption, there are an objective boundaries. They are implies that starting with altitude of 3300 m and higher in the Northern Caucasian mountains the grass and handicraft and soil with humus are absent, and just on this altitude the most part of Holocene lava fl ows and tuff horizons are located and with 3700 m and higher volcano is covered by glaciers and perpetual snow (fi rn). As a result of carried out investigation for Holocene the density of recurrence of Elbrus volcano eruptions and earthquakes manifestation in Elbrus volcanic area have been established. As a sum the important relationship, testifi es that to the end of Holocene the density of recurrence of Elbrus eruptions and earthquakes are sharply increase. Agreement in time between volcanic and seismic activity (but manifestation the second one was some early then fi rst one). In so doing, periods of volcanic and seismic activity are sharply increase, but periods of rest are shrink. This data have an important relationship for estimation possibility of restating of volcanic and seismic activities in Elbrus volcanic area in geological future.

Temperate-alkaline andesibasalts and andesites of the Elbrus volcanic region (Northern Caucasus): questions of petrogenesis, geodynamic typisation and geochemical specialization

В Центральной части Большого Кавказа в верховьях рек Баксана, Малки, Кубани расположены плейстоценголоценовые вулканические постройки Эльбрусского вулканического района, сложенные игнимбритами, лавами и туфами разнообразного состава от трахиандезитов, андезитов, до дацитов и риолитов. Преобладающей разновидностью являются лавы дацитового состава. Проведено изучение наиболее основных по составу вулканитов, представленных трахиандезибазальтами и трахиандезитами. Эти вулканиты образуют самостоятельные постройки в северной части вулканического района, а в дацитовых лавовых потоках вулкана Эльбрус они встречаются в виде ксенолитов. В статье дано краткое петрографическое описание этих разновидностей и приведены результаты RFA, ICPMS, 87Sr/86Sr анализов. Рассмотрены петрохимические и геохимические характеристики этих пород. Показано, что на дискриминационных диаграммах фигуративные точки группируются в полях характерных для внутриплитных и постколлизионных образований, а также островных дуг и зон активных континентальных окраин. Предполагается, что возникновение исходного расплава было обусловлено декомпрессионными явлениями в связи с синколлизионным рифтингом, проявившимся в пределах Транскавказской поперечной зоны. Спектры распределения редкоземельных элементов, нормированные к хондриту, у трахиандезибазальтов и трахиандезитов существенно не различаются. Они имеют вид наклонной линии, близкий к профилю ОIB, с заметной негативной европиевой аномалией. Отмечается обогащение пород LILe, LREE ,а также высокозарядными (Zr, Hf, U, Th) элементами. На фоне профиля OIB, в изученных породах заметен существенный трог по Nb, Ta и негативные аномалии Ti и Yb. На основе вариаций La/Sm и Sm/Yb отношений, с использованием модели равновесного плавления гранатовоо и шпинелевого перидотита, предполагается, что исходный расплав образовался при частичном ( 1) плавлении метасоматизированных пород литосферной мантии. Эволюционные преобразования первичного расплава связаны с процессом фракционирования, при участии плагиоклаза, оливина, титан содержащего минерала и его контаминации нижнекоровым материалом. При формировании дацитов имело место смешение этого эволюционировавшего мантийного расплава с более кислым коровым расплавом. Показано, что для трахиандезибазальтов и трахиандезитов, как и для всех пород Эльбрусского вулканического района, характерна халькофильнолитофильная (полиметальноредкометальная) геохимическая специализация. Это подтверждается тем, что в вулканитах установлены участки с аномально повыщенными концентрациями свинца, цинка, сурьмы, молибдена, ассоциирующими с зонами разрывной тектоники и маломощными зонами аргиллизации In the central part of the Greater Caucasus, in the upper reaches of the Baksan, Malka and Kuban rivers, there are Pleistocene Holocene volcanic structures of the Elbrus volcanic region, composed of ignimbrites, lavas and tuffs of various composition from trachiandesites, andesites to dacites and rhyolites. The predominant type is lava of dacite composition. The most basic in composition volcanic rocks, represented by trachyandesibasalts and trachyandesites were investigated. These volcanic rocks form independent structures in the northern part of the volcanic region, and in the dacitic lava flows of Elbrus volcano they are found in the form of xenoliths. The article provides a brief petrographic description of these facies and the results of RFA, ICPMS, 87Sr/86Sr analysis. Petrochemical and geochemical characteristics of these rocks are considered. It is shown that in discriminatory diagrams, figurative points are grouped in fields characteristic of intraplate and postcollisional formations, as well as island arcs and zones of active continental margins. It is assumed that the occurrence of the primitive liquid was conditioned by decompression phenomena in connection with the syncollisional rifting, which appeared within the Transcaucasian transverse zone. The distribution spectra of rareearth elements normalized to chondrite are not significantly different for trachyandesibasalts and trachyandesites. They have the form of an inclined line close to the OIB profile, with a noticeable negative europium anomaly. The enrichment of LILe, LREE rocks, as well as highlycharged (Zr, Hf, U, Th) elements is noted. Against the background of the OIB profile, a significant Nb, Ta trot and negative anomalies of Ti and Yb are noticeable in the investigated rocks. Based on variations of the La/Sm and Sm/Yb ratios, using the model of equilibrium melting of garnet and spinel peridotite, it is assumed that the primitive liquid was formed during partial ( 1) melting of metasomatized lithospheric mantle rocks. The evolutionary transformations of the primitive liquid are associated with the process of fractionation, the participation of plagioclase, olivine, titaniumcontaining mineral and its contamination by lower crust material. During the formation of dacites, this evolved mantle melt was mixed with a more acidic crust melt. It is shown that chalcophiliclithophilic (polymetalraremetallic) geochemical specialization is characteristic for trachiandesibasalts and trachyandesites, as well as for all rocks of the Elbrus volcanic region. This is confirmed by the fact that areas with abnormally elevated concentrations of lead, zinc, antimony, and molybdenum, associated with zones of fault tectonics and lowpower zones of argilization, have been established in volcanics

Geothermal monitoring as a way to predict volcanic eruptions and estimate geothermal energy resources

Geothermal monitoring is an effective tool for predicting volcanic eruptions, as well as for assessing the geothermal energy potential of geothermal areas. Increased magmatic activity, an indicator of which is the penetration of hot volcanic gases through faults, has been observed in recent years on the Elbrus volcano. Since Elbrus is a year-round resort of world importance, in order to control volcanic and seismic activity, forecast and reduce the risks of eruption and earthquakes, it is recommended to drill a observation well on the slope of Elbrus with the installation of an underground fiber-optic system for temperature and pressure monitoring. In combination with microseismic, gravimetric and inclinometric observations, satellite IR imaging and geochemical gas testing, the continuously obtained information on the thermodynamic conditions of the subsoil will provide a reliable complex for the operational forecast of natural geophysical disasters. Utilization of the geothermal energy of the magma chamber in the artificial circulation systems of small GeoPPs, water injection from the surface and obtaining superheated water and steam from producing wells will reduce the risks of eruption and at the same time provide the resort with environmentally friendly thermal and electric power. Technological justification for the construction of a GeoPP will also require exploratory drilling to the area of ​​hot rocks, therefore information on the distribution of temperature and pressure along the wellbore is doubly valuable. In geothermal fields that are under development, to assess the spatial heterogeneity of the filtration characteristics can be a useful method of “thermal interference testing” – as a complement or alternative to hydrodynamic interference testing. It is recommended to conduct such an experiment at the North Mutnovsky geothermal field.

GEOLOGICAL FEATURE OF THE WESTERN SLOPE OF ELBRUS VOLCANO AND PRIELBRUSYE

Подробно рассматривается геологическое строение Западного Приэльбрусья и западный, наиболее сложно построенный склон вулкана Эльбрус. Выделены древнейшие толщи, прорывающая их огромная экструзия Кюкюртли, облекаемая более молодыми лавовыми потоками. Описаны самостоятельные центры извержений флюидолитов риолитового состава и их соотношение с вулканитами Эльбруса. Показано отсутствие кальдеры на Эльбрусе. Возраст вулканитов обоснован геологическими и геоморфологическими методами и неотектоническими построениями с учетом данных по изотопным методам In detail the geological feature of the Western Prielbrusye and the western, most difficult constructed slope of Elbrus volcano is considered. The most ancient thicknesses, the huge Kyukyurtli extrusive massif which is breaking through it’s and invested by younger lava streams are marked out. The independent eruptions centers of rhyolitic fluidolites and its ratio from Elbrus volcanic rocks are described. An absence of a caldera on Elbrus is shown. The age of volcanic rocks is proved by geological and geomorphological methods and neotectonic constructions taking into account data on isotope methods