A. I. Yakushev

The quaternary volcanic rocks of the Geghama highland, Lesser Caucasus, Armenia: Geochronology, isotopic Sr-Nd characteristics, and origin

This paper reports on isotope-geochronologic, petrologic, and isotope-geochemical (Sr-Nd) studies of Quaternary magmatism in the Geghama neovolcanic area, Armenia, Lesser Caucasus. According to these studies, the period of youngest volcanic activity in the region lasted about 700 000 years, from 700 ka to a few tens of thousands of years ago. We found the time limits of five discrete phases of Quaternary volcanism: I (about 700 ka), II (550−480 ka), III (190−150 ka), IV (110−70 ka), and V (later than 50 ka). These phases seem to have lasted a few tens of thousands of years and were separated by quiescent periods of comparable durations.The petrologic and isotope-geochemical characteristics of Geghama effusive rocks show that they belong to the bimodal association rhyolite-trachyandesite and basaltic trachyandesite; this association was largely generated by fractional crystallization of primary basite melts, with the assimilation of crustal material by deep magmas being much less important. The isotopic parameters of the volcanic rocks studied here (0.70410–0.70437 for 87Sr/86Sr and +3.3 to +4.0 for ЄNd) are practically identical for intermediate to basic and acid varieties in this association and are similar to those for young basites in the other areas of the Lesser Caucasus; this circumstance indicates a common origin for all Quaternary magmatic formations in the region. The petrogenesis of these varieties probably involved a single mantle source of the OIB type with certain regional peculiarities in the composition of the melts it generated.An analysis of the locations of Quaternary volcanoes in central Armenia (Geghama and Aragats areas) and in the Kars plateau, which is adjacent to it in the west, provided evidence of an eastward lateral migration of youngest magmatic activity in the region over time. The latest episodes of this migration took place in the eastern Geghama area, which must be the first to produce eruptions in the future.

Sources and fluid regime of quartz-carbonate veins at the Sukhoi Log gold deposit, Baikal-Patom Highland

Complex (δ18O, δ13C, 87Sr/86Sr, 143Nd/144Nd, and REE composition) data were obtained on quartz-carbonate veins in metasedimentary rocks to elucidate the material sources and to evaluate fluid regime during low-sulfide gold-quartz ore mineralization at the Sukhoi Log deposit. In order to use an oxygen isotopic thermometry for quartz veins, we calibrated empirical dependence of fractionation factors between vein quartz and altered wall rocks. The temperature range of quartz equilibration with wall rocks was evaluated at 380–190°C. Independent temperatures obtained using this thermometer indicate that the vein ankerite can be both earlier and later than vein quartz. The isotopic systematics (δ13C and δ18O) of ankerite in the quartz-carbonate veins, carbonates in the ore-hosting shales of the Khomolkho and Imnyakh formations both within and outside mineralized zones at the deposit indicate that the ore-hosting rocks and veins in the mineralized zone contain incoming carbonate, which was most probably borrowed from the carbonate rocks of the Imnyakh Formation. REE composition of vein ankerite shows that these elements were transported by fluid as carbonate complexes. The behavior of the Eu/Eu* and (La/Yb)n ratios and Mn of the vein ankerite suggest that during carbonate crystallization the system was closed with respect to fluid. Sr-Nd isotope systematics indicates that the isotopic parameters of the vein ankerite were formed with the participation of metasedimentary host rocks of both the Imnyakh and Khomolkho formations, which are contrastingly different in Nd isotopic composition. A fluid/rock ratio during metasomatic processes in the wall rocks was calculated for two scenarios of their thermal history: with a continuously operating heat source beneath the Sukhoi Log structure and with a linear cooling of the structure. The effective integral W/R ratios calculated lie within the range of 0.046–0.226 and suggest that the veins were produced with the metamorphic fluid. Low W/R ratios are inconsistent with the mechanism of vein quartz crystallization due to fluid oversaturation with respect to SiO2 at decreasing temperature. We believe that the main mechanism responsible for the origin of these veins was variations of fluid oversaturation due to pressure variations (pressure solution mechanism). This hypothesis is consistent with the reported isotopic-geochemical characteristics of the wall rocks at the Sukhoi Log deposit.

Late Cenozoic Volcanic Activity in Western Georgia: Evidence from New Isotope Geochronological Data

The products of volcanic activity from the Kutaisi area and Guria (western Georgia) were studied in terms of isotope geochronology to determine the age of rocks and to confirm their attribution to Cenozoic formations. The results obtained show that the erupted rocks in the Kutaisi area were formed during the three pulses of Mesozoic volcanic activity: the Bajocian, Kimmeridgian-Tithonian, and Turonian-Santonian. It was shown that no displays of Late Cenozoic volcanism occurred in this region of the western Georgia. Because of this, its inclusion into the Central Georgian neovolcanic province, earlier supposed, seems to be improper. By the data of isotope geochronology, Guria is the only region of western Georgia where volcanic activity occurred in post-Paleogene period. Two pulses of young volcanism were revealed: of about 15.5 and 9–7.5 My. The former was related to the introduction of syenite intrusion, and the latter, to subaqueous exudation of subalkaline Neogene lavas. All the outcrops of Neogene rocks we found and dated in Guria fit within the well-pronounced sublatitudinal linear band which probably represents the occurrence in the Middle Miocene of a local zone of extension appearing under conditions of total compression during the collision of the Eurasian and Arabian lithospheric plates.

Magmatic activity within the Northern Caucasus in the Early Neopleistocene: Active volcanoes of the Elbrus center, chronology, and character of eruptions

Forecast of possible scenarios of eruptive activityfor volcanic centers and individual volcanoes as well asthe type and character of their future eruptions is animportant aspect of investigations of modern (Quaternary) magmatism of the Earth. Prognostic reconstructions of this type are impossible without analysis oftendencies in the development of volcanic centers anddetermination of the main time characteristics ofmagmatic activity occurred inside them. The Northern Caucasus is the only region in European Russiawhere modern magmatism was manifested, althoughonly to a small extent. Magmatic activity in the Anthropogene occurred there exclusively within the Elbrusneovolcanic center in the southern part of which theElbrus, one of the largest Quaternary volcanoes of theNorthern Eurasia, was formed lesser than 250 ka [1–3].Quaternary volcanism of the Elbrus center started~950–900 ka [4] with eruptions of three lava volcanoes (Tyzyl, Tashlysyrt, and Syltran) located withinthe linear submeridional zone at the Baksan andMalka interfluves. The regular decrease of SiO

Long-lived center of youngest volcanism in the Borzhomi region of Georgia: Isotopic-geochronological evidence

One of the well-known areas with youngest volcanism in the Lesser Caucasus is located in the vicinity of the Borzhomi City (central Georgia) famous by mineral water springs. This relatively small area hosts four volcanoes (Tsitelidabadzveli, Tsikhisdzhvari, Mukhera, and Sargavi), which yielded extended (from 2 to 13 km long) flows of andesite to, subordinate, basaltic andesite lavas (figure). In scientific works dedicated to different aspects of Neogene‐Quaternary magmatism of the Caucasus region, manifestations of basaltic andesite‐andesite volcanism in the examined part of the Lesser Caucasus mountainous system are usually united into the Bakuriani‐Borzhomi neovolcanic area [1 and others]. It is composed of Upper Cretaceous carbonate bands, Paleocene‐Lower Eocene terrigenous rocks of the Borzhomi series, and Middle Eocene‐Oligocene volcano-sedimentary sequences. Young lava flows a few tens of meters thick known in this area lie with stratigraphic and angular unconformities upon the Paleogene basement spreading over significant part of studied territory (Fig. 1). It should be noted that, according to the recent tectonic map of Georgia [2], magmatic activity in the Bakuriani‐Borhomi area was concentrated in the Adzharia‐Trialeti zone of the Lesser Caucasus fold system with volcanoes confined to the linear near-latitudinal Bakuriani abyssal fault zone at its intersection with the meridional Samsari Fault. Tsitelidabadzveli Volcano (2320 m) is located on the northern slope of the Trialeti Range 12 km south of Borzhomi. It represents an oval meridionally extended edifice up to 100 m high composed of pyroclastics (scoria, breccia, lapillis) with subordinate interbeds of dark gray and reddish lavas. Basaltic andesite lava flows erupted by this volcano fill a spacious depression 8 × 6 km in size and form the Dabadzveli Plateau, the surface of which is located at altitudes of 2100 to 1600 m and is slightly inclined in the northern direction toward the Kura River valley. Basaltic andesites of the Tsitelidabadzveli Volcano (samples YUG-156 from the volcano cone and YUG-157 from lavas of the Dabadzveli Plateau) represent massive to vesicular porphyric to subordinate aphyric rocks. Phenocrysts in porphyric varieties are dominated by plagioclase (labradorite) and clinopyroxene (augite) accompanied by subordinate orthopyroxene (hypersthene) and olivine. Tsikhisdzhvari Volcano (1818 m) is located 2.5 km

Age and origin of Miocene gabbroid intrusions in the northern part of the Lesser Caucasus

The results of isotope-geochronological and petrological-geochemical study are reported for Neogene mafic intrusive rocks distributed in the northern part of the Lesser Caucasus (Georgia). It is shown that the young plutonic bodies were formed here in two magmatic stages: in the Middle Miocene (around 15.5 Ma) and in the terminal Miocene (9-7.5 Ma). The first age group includes a microsyenitic massif in Guria (Western Georgia), which was formed in a setting of active continental margin related to the subduction of oceanic part of the Arabian plate beneath the Transcaucasus. The Late Miocene intrusive magmatism already records the incipient within-plate activity: small polyphase bodies of alkaline gabbroids and lamprophyres of Samtskhe (South Georgia) dated around 9-8.5 Ma and teschenite intrusions of Guria dated at 7.5Ma. Petrological-geochemical and isotope-geochemical data indicate that the parental melts of the rocks of all studied Neogene plutonic bodies of the Lesser Caucasus were derived from a single mantle source. Its characteristics are close to those of a Common hypothetical reservoir, which is usually regarded as a source of oceanic and continental hot spot basalts (OIB) but shows some regional peculiarity. The role of crustal assimilation and crystallization differentiation in the genesis of the Miocene rocks of Guria was limited, which is related to the rapid ascent of deep melts to the surface (in a setting of local extension) without intense interaction with host sequences under the absence of consolidated continental lithosphere beneath this part of the Transcaucasus. The parental mantle-derived magmas of the Neogene gabbroids of Samtskhe were strongly contributed by upper crustal material, which caused a change in their isotope (87Sr/86Sr up to 0.70465, ɛNd up to + 2.8) and geochemical characteristics relative to the regional mantle source. In addition, the crustal contamination of mantle basic melts during the late phases of the Samtskhe plutonic bodies formation led to their intense fractionation with precipitation of mainly olivine and pyroxene. The larger scale mantle-crustal interaction during formation of the Samtskhe intrusions was probably related to the fact that the upper lithosphere in this sector of the Transcaucasus contained large Paleozoic blocks, which were made up of granite-metamorphic complexes and prevented a rapid ascent of mantle melts to the surface. The rocks of these blocks were presumably assimilated by mantle magmas in the intermediate chambers at the upper crustal levels.

Manifestations of miocene acid intrusive magmatism on the southern slope of the Greater Caucasus: First evidence from isotope geochronology

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.