Complex geochronological and isotope-geochemical studies showed that the Late Quaternary Elbrus volcano (Greater Caucasus)
experienced long (approximately 200 ka) discrete evolution, with protracted periods of igneous quiescence (approximately 50
ka) between large-scale eruptions. The volcanic activity of Elbrus is subdivided into three phases: MiddleNeopleistocene (225–170
ka), Late Neopleistocene (110–70 ka), and Late Neopleistocene-Holocene (less than 35 ka).
Petrogeochemical and isotope (Sr-Nd-Pb) signatures of Elbrus lavas point to their mantle-crustal origin. It was shown that
hybrid parental magmas of the volcano were formed due to mixing and/or contamination of deep-seated mantle melts by Paleozoic
upper crustal material of the Greater Caucasus. Mantle reservoir that participated in the genesis of Elbrus lavas as well
as most other Neogene-Quaternary magmatic rocks of Caucasus was represented by the lower mantle “Caucasus” source. Primary
melts generated by this source in composition corresponded to K-Na subalkali basalts with the following isotopic characteristics:
87Sr/86Sr = 0.7041 ± 0.0001, ƒNd = +4.1 ± 0.2, 147Sm/144Nd = 0.105–0.114, 206Pb/204Pb = 18.72, 207Pb/204Pb = 15.62, and 208Pb/204Pb = 38.78. The temporal evolution of isotope characteristics for lavas of Elbrus volcano is well described by a Sr-Nd mixing
hyperbole between “Caucasus” source and estimated average composition of the Paleozoic upper crust of the Greater Caucasus.
It was shown that, with time, the proportions of mantle material in the parental magmas of Elbrus gently increased: from ∼60%
at the Middle-Neopleistocene phase of activity to ∼80% at the Late Neopleistocene-Holocene phase, which indicates an increase
of the activity of deep-seated source at decreasing input of crustal melts or contamination with time. Unraveled evolution
of the volcano with discrete eruption events, lacking signs of cessation of the Late Neopleistocene-Holocene phase, increasing
contribution of deep-seated mantle source in the genesis of Elbrus lavas with time as deduced from isotope-geochemical data,
as well as numerous geophysical and geological evidence indicate that Elbrus is a potentially active volcano and its eruptions
may be resumed. Possible scenarios were proposed for evolution of the volcano, if its eruptive activity were to continue. 相似文献
An isotopic geochronological study of Russia’s largest Strel’tsovka uranium district has been carried out. Polychronous granite
generation, which determined the structure of the pre-Mesozoic basement, had important implications for the formation of volcanotectonic
structural elements bearing economic uranium mineralization. The study of U-Pb, Rb-Sr, and Sm-Nd isotopic systems of whole-rock
samples and minerals of granitic rocks allowed us to estimate the deportment of these systems in spatially conjugated granite-forming
and hydrothermal processes differing in age and gave grounds for revising the age of granites pertaining to the Urulyungui
Complex and refining the age of the Unda Complex. 相似文献
New isotope-geochronological data (K-Ar, Rb-Sr) provide tight geochronological constraints on the history of Late Cenozoic magmatism on the southern slope of the Greater Caucasus. Several previously unknown, rhyodacite intrusive bodies with an emplacement age of 6.9 ± 0.3 Ma (Late Miocene) are reported from the Kakheti-Lechkhumi regional fault zone in the Kvemo Svaneti-Racha area. Therefore, a pulse of acid intrusive magmatism took place in a period previously considered amagmatic in the Greater Caucasus. The petrological, geochemical, and isotopic data suggest that these rhyodacites are produced by crystallization differentiation of mantle-derived magmas, which are similar in composition to Miocene mafic lavas that erupted a few hundred thousand years later in the adjacent Central Georgian neovolcanic area. The presented results allow the conclusion that the volcanic activity within the Central Georgian neovolcanic area occurred at 7.2–6.0 Ma in two discrete pulses: (1) the emplacement of acid intrusions and (2) the eruption of trachybasaltic lavas. The emplacement of rhyodacite intrusions in the Kvemo Svaneti-Racha area marked the first pulse of young magmatism on the southern slope of the Main Caucasus range and simultaneously represented the second magmatic pulse (after granitoid magmatism of the Caucasian Mineral Waters region) within the entire Greater Caucasus. 相似文献
The isotopic (δD, δ18O, δ13C, and 87Sr/86Sr) and geochemical characteristics of hydrothermal solutions from the Mid-Atlantic Ridge and the material of brucite-carbonate chimneys at the Lost City hydrothermal field at 30°N, MAR, were examined to assay the role of the major factors controlling the genesis of the fluid and hydrothermal chimneys of the Lost City field. The values of δD and δ18O in fluid samples indicates that solutions at the Lost City field were produced during the serpentinization of basement ultramafic rocks at temperatures higher than 200°C and at relatively low fluid/rock ratios (<1). The active role of serpentinization processes in the genesis of the Lost City fluid also follows from the results of the electron-microscopic studying of the material of hydrothermal chimneys at this field. The isotopic (δ18O, δ13C, and 87Sr/86Sr) and geochemical (Sr/Ca and REE) signatures indicate that, before its submarine discharging at the Lost City field, the fluid filtered through already cold altered outer zones of the Atlantis Massif and cooled via conductive heat loss. During this stage, the fluid could partly dissolve previously deposited carbonates in veins cutting serpentinite at the upper levels of the Atlantis Massif and the carbonate cement of sedimentary breccias underlying the hydrothermal chimneys. Because of this, the age of modern hydrothermal activity at the Lost City field can be much younger than 25 ka. 相似文献
This paper reports an integrated petrological, geochronological, and isotopic geochemical study of the Pliocene Dzhimara granitoid massif (Greater Caucasus) located in the immediate vicinity of Quaternary Kazbek Volcano. Based on the obtained results, it was suggested that the massif has a multiphase origin, and temporal variations in the chemical composition of its granitoids and their possible sources were determined. Two petrographic types of granitoids, biotite-amphibole and amphibole, were distinguished among the studied rocks of the Dzhimara Massif belonging to the calc-alkaline and K-Na subalkaline petrochemical series. The latter are granodiorites, and the biotite-amphibole granitoids are represented by calc-alkaline granodiorites and quartz diorites and subalkaline quartz diorites. Geochemically, the granitoids of the Dzhimara Massif are of a “mixed” type, showing signatures of S-, I-, A-, and even M-type rocks. Their chemical characteristics suggest a mantle-crustal origin, which is explained by the formation of their parental magmas in a complex geodynamic environment of continental collision associated with a mantle “hot field” regime.
The granitoids of the Dzhimara Massif show wide variations in Sr and Nd isotopic compositions. In the Sr-Nd isotope diagram, their compositions are approximated by a line approaching the mixing curve between the “Common” depleted mantle, which is considered as a potential source of intra-plate basalts, and crustal reservoirs. It was suggested that the mantle source (referred here as “Caucasus”) that contributed to the petrogenesis of the granitoids of the Dzhimara Massif and most other youngest magmatic complexes of the region showed the following isotopic characteristics: 87Sr/86Sr ? 0.7041 ± 0.0001 and
The Middle-Late Pliocene K-Ar ages (3.3–1.9 Ma) obtained for the Dzhimara Massif are close to the ages of granitoids from other Pliocene “neointrusions” of the Greater Caucasus. Based on the geochronological and petrological data, the Dzhimara Massif is formed during four intrusive phases: (1) amphibole granodiorites (3.75–3.65 Ma), (2) Middle Pliocene amphibole-biotite granodiorites and quartz diorites (～3.35 Ma), (3) Late Pliocene amphibole-biotite granodiorites and quartz diorites (～2.5 Ma), and (4) K-Na subalkaline biotite-amphibole quartz diorites (～2.0 Ma).The close spatial association of the Pliocene multiphase Dzhimara Massif and the Quaternary Kazbek volcanic center suggests the existence of a long-lived magmatic system developing in two stages: intrusive and volcanic. Approximately 1.5 Ma after the formation of the Dzhimara Massif (at ca. 400–500 ka), the activity of a deep magma chamber in this area of the Greater Caucasus resumed (possibly with some shift to the east). 相似文献
