Petrogenesis of the Late Cenozoic collision volcanism in the central part of the Lesser Caucasus (Azerbaijan)

The Late Cenozoic volcanics of the Lesser Caucasus have similar trace-element and REE patterns with negative anomalies of Nb, Ta, Hf, and Zr. They are highly enriched in Rb, Ba, Th, and La and depleted in Ti, Yb, and Y with respect to N-MORB, which indicates their formation from the subduction-metasomatized lithospheric mantle. Partial melting of the subcontinental mantle lithosphere and crustal assimilation and fractional crystallization controlled the magma evolution in the collisional magmatic belts.     

Late Cenozoic volcanic province in Central and East Asia

The paper presents materials on the inner structure of the Late Cenozoic within-plate volcanic province in Central and East Asia, in which two subprovinces are distinguished: Central Asian and Far Eastern, which comprise a number of autonomously evolving volcanic areas. Some of the volcanic areas are proved to have evolved for a long time, starting in the Late Mesozoic. In spite of differences in their age and structural setting, the volcanic areas evolved according to similar scenarios in the Late Cenozoic. Magmatism in the province was related to a mantle source of the within-plate type. The magmatic associations are dominated by mafic alkaline high-K rocks. The rocks are geochemically close to basalts of the OIB type, and their isotopic composition corresponds to a combination of mantle sources of the PREMA, EMI, and EMII types at the predominance of PREMA. Geological, geochemical, and isotopic lines of evidence suggest that magmatism in the province was related to mantle plumes. This is consistent with geophysical data, which testify that the volcanic areas are underlain by upwellings of the asthenospheric mantle or plumes. Seismic tomography data indicate that the “stems” of the plumes can be traced down to the upper and lower mantle. The province is thought to have been produced when the eastern margin of the Asian plate overlapped one of the branches of the Pacific superplume at approximately 160 Ma. This branch of the superplume is pronounced in the modern mantle structure as a cluster of mantle plumes that control (according to seismic tomography data) the interaction zone of the Pacific and Asian lithospheric plates.     

Magmatic evolution of Late Cenozoic volcanic rocks of the Lau Ridge,Fiji

Three main groups of lavas are exposed on islands of the Lau Ridge: the Lau Volcanic Group (LVG), 14.0–5.4 Ma, are predominantly andesite; Korobasaga Volcanic Group (KVG), 4.4–2.4 Ma, are predominantly basalt and Mago Volcanic Group (MVG), 2.0–0.3 Ma, are basalt-hawaiite. LVG and KVG lavas are mostly medium-K tholeiitic rocks with high LILE/HFSE ratios characteristic of islands ares, while MVG lavas are ne-normative alkalic rocks with high LILE and HFSE, characteristic of ocean island basalts. LVG lavas have high ?_Nd (+8.0–+8.4) and low ⁸⁷Sr/⁸⁶Sr (0.70273–0.70349) similar to N-MORB, whereas KVG lavas have slightly more radiogenic values (?_Nd=+7.5?+8.4; ⁸⁷Sr/⁸⁶Sr=0.70323-0.70397). MVG lavas form an isotopically distinct group having lower ?_Nd (+4.6–+4.9) and (⁸⁷Sr/⁸⁶Sr ranging from 0.70347–0.70375). LVG lavas were erupted in a primary oceanic island arc (Vitiaz arc) during the Miocene. Basaltic lavas were derived by approximately 19% partial melting of mantle wedge peridotite with only minor subduction component. Andesites and dacites were produced by low-pressure plagioclase-pyroxene-titanomagnetite dominated crystal fractionation. KVG lavas were erupted during the period immediately prior to or during the initial stages of rifting in the Lau Basin, and, like LVG lavas, show significant chemical differences at the northern and southern ends of the Lau Ridge. Lavas at the northern end (type (ii)) appear to be derived from a more depleted source than LVG but with a greater amount of subduction component. Those at the southern end (type (i)) probably came from a slightly more enriched source with less subduction component. MVG basalts and hawaiites were derived from an enriched mantle with little or no subduction input. The hawaiites (type (i)) could not have been derived from the basalts (type (ii)), and the two magma types must have come from different sources, indicating mantle heterogeneity. The lack of subduction influence indicates the MVG lavas are tectonically unrelated to the present-day Tonga arc, and the lack of depletion indicators suggests they have tapped a different (new?) part of the mantle wedge. This may reflect introduction of sub-Pacific mantle through the present Tonga-Lau subduction system.     

内蒙古达里诺尔晚新生代火山群喷发特征研究

达里诺尔火山群有近百座晚新生代单成因火山,其地质地貌形态各异,喷发形式多样。这些火山既有爆破式喷发,如:夏威夷式喷发,斯通博利式喷发,强斯通博利式喷发和射汽岩浆喷发;也有溢流式喷发,如盾状火山;还有岩浆缓慢侵出,如大黑山。火山群内典型火山机构表明,不同的喷发方式穿插于火山喷发过程:早期火山活动多以侵出和溢流为主,逐渐转变为岩浆爆破式喷发(强斯通博利式,斯通博利式),晚期又过渡为溢流式喷发,喷发过程大体经历一个爆破强度弱-强-弱的转变。射汽岩浆型的火山则是以剧烈的射汽岩浆爆炸开始,后期逐渐转弱为岩浆爆破喷发和溢流喷发。火山喷发过程中火山产物出现牛顿流体,宾汉流体,层流,颗粒流,涌流,空降等不同类型的运动形式,自火口向远源运动,形成差异化的火山产物。岩浆的输送速率、上升速度,以及围岩的类型,可能是造成达里诺尔火山群多样化喷发的主要因素。     

Geochemistry of the Lesser Antilles volcanic island arc

New analyses of 1518 rocks for major and certain trace elements are used to examine chemical variations between the 15 larger volcanic islands of the Lesser Antilles island arc. The depth to the top of the subduction zone dipping westward at about 40° lies about 100km below all the volcanoes of the arc. Most of the sampled eruptions are post-Miocene (5-1 m.y.) although south of Martinique, the Oligocene-Miocene and the younger arc are superimposed.There is a chemical variation along the arc axis, from alkalic (southern) through calc-alkalic (central) to tholeiitic (northern) volcanic suites. Three islands are examined in detail as type examples of this variation, i.e. Grenada (south), Dominica (centre), and St. Kitts (north). The Grenada suite includes basanites, alkalic basalts, and subalkalic basalts, andesites and dacites. The subalkalic basalts, andesites and dacites each fall into three chemical groupings along the axis of the arc, distinguished especially by K, Zr, Ni and Cr abundances. The whole Lesser Antilles assemblage is characterised by low K abundances and low K/Rb ratios, compared with other island arcs.The magmas are believed to have evolved through processes of partial melting and crystal fractionation. Partial melting of garnet Iherzolite at about 100km depth in a relatively ‘fertile’ zone of upper mantle in the southern sector, above the subducted slab of basaltic ocean crust, could have produced the undersaturated alkalic magmas. In the central and northern sectors, where the crustal structures are more complex, partial melting may have occurred within more ‘barren’ upper mantle, to produce tholeiitic and calc-alkalic magmas depleted in certain trace elements. In either case, water was probably added to the melted zone from the subducted and hydrated oceanic crust, since the whole arc assemblage was erupted explosively and the rocks are rich in A1₂O₃, plagioclase is very calcic, and amphibole is an important phase. The second process was crystal fractionation at low pressure, as evidenced by the abundance of cumulate xenoliths. Separating phases for the southern volcanoes were olivine, calcic augite and Cr-spinel, followed by hornblende, anorthite and Ti-magnetite at lower temperatures. There is little evidence of the higher-temperature fractionation controls for the central and northern volcanoes.     

Neotectonic transformation of Cenozoic fold structures in the northwestern Caucasus

The performed morphotectonic regionalization of the northwestern Caucasus shows that the fold structures directly expressed in the topography of the territory and continuing to evolve under the settings of contemporary lateral shortening predominate in the northwestern Caucasus. A map of fold structures expressed in the topography of the northwestern Caucasus is presented. The districts distinguished therein correspond to the largest regional tectonic units, the fold topography of which occurs at various stages of tectonic evolution from primary brachyanticlinal ridges of the Taman and Sochi districts to the complex fold–thrust and inversion fold ridges of the axial zone. Data on active newly formed fold and inversion structures are given. These inherited structures develop under the combined action of selective denudation, beddingplane upthrow faulting, and thrusting.     

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.

     

History of development of late Cenozoic volcanism in the Central Caucasus

Violent volcanism developed in the central part of the Caucasus during the last stage of the Alpine orogenic cycle. Three main epochs of volcanic development are here established: the first -late Miocene-early Pliocene; the second-late Pliocene; the third — Quaternary. These epochs of volcanic activity can be subdivided into a series of phases and subphases. The total volume of volcanic products is in the order of 2000 km³. The acidic volcanic rocks are mostly rhyolitic ignimbrites and have a volume larger than 800–820 km³.

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Zusammenfassung Im Zentralteil des Großen Kaukasus tritt im Spätstadium des alpinen orogenen Zyklus ein starker Vulkanismus auf. Drei Hauptzeiten der vulkanischen Entwicklung werden verzeichnet: zunächst im späten Miozän bis frühen Pliozän; zweitens im späten Pliozän; drittens im Quartär. Die Zeiten der Vulkantätigkeit können in Phasen und Subphasen unterteilt werden. Das Gesamtvolumen der Vulkanprodukte beträgta etwa 2000 km³, wobei saure Gesteine, vorwiegend rhyolithische Ignimbrite, nicht weniger als 800–820 km³ umfassen.

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Résumé Dans la partie centrale du Grand Caucase, au stade postérieur du cycle orogénique se manifeste un volcanisme très fort. On distingue trois époques principales d'activité volcanique: 1re - miocène postérieur-pliocène antérieur;2me -pliocene postérieur; 3 me - quaternaire. Les époques de l'activité volcanique sont subdivisées en phases et sous-phases. Le volume commun des produits volcaniques est environ 2000 km³; les roches acides généralement sont les brêches de nuées ardents à liparite, dont le volume est non moins 800–820 km³.

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Dedicated to Professor Dr. A.Rittmann on the occasion of his 75. birthday     

Continental accretion and incremental deformation in the thermochronologic evolution of the Lesser Caucasus

Apatite fission-track analysis and thermochronologic statistical modeling of Precambrian-Oligocenc plutonic and metamorphic rocks from the Lesser Caucasus resolve two discrete cooling episodes.Cooling occurred during incremental crustal shortening due to obduction and continental accretion along the margins of the northern branch of the Neotethys.(1) The thermochronometric record of a Late Cretaceous(Turonian-Maastrichtian) cooling/exhumation event,coeval to widespread ophiolite obduction,is still present only in a relatively small area of the upper plate of the Amasia-Sevan-Akera(ASA) suture zone,i.e.the suture marking the final closure of the northern Neotethys during the Paleogene.Such area has not been affected by significant later exhumation.(2) Rapid cooling/exhumation occurred in the Early-Middle Miocene in both the lower and upper plates of the ASA suture zone,obscuring previous thermochronologic signatures over most of the study area.Miocene contractional reactivation of the ASA suture zone occurred contemporaneously with the main phase of shortening and exhumation along the Bitlis suture zone marking the closure of the southern branch of the Neotethys and the ensuing ArabiaEurasia collision.Miocene collisional stress from the Bitlis suture zone was transmitted northward across the Anatolian hinterland,which was left relatively undeformed,and focused along preexisting structural discontinuities such as the eastern Pontides and the ASA suture zone.     

Formation stages and ore matter sources of the Devdoraki copper deposit,Kazbek volcanic center,the Greater Caucasus

Comprehensive petrological–mineralogical, geochronological, and isotope-geochemical studies have been carried out at the Devdoraki copper deposit situated in the Kazbek neovolcanic center, the frontier territory between Georgia and Russia. The formation history of this deposit has been deciphered on the basis of K–Ar isotopic geochronological data, and the multistage evolution of ore–magmatic system has been established. The subeconomic disseminated and less abundant stringer pyrite mineralization formed at the first stage in the Early Cretaceous back to 130–120 Ma at the retrograde stage of regional metamorphism. The second productive stage was related to intense Quaternary volcanism of the Kazbek center. The late stringer base-metal mineralization formed about 400 ka ago in connection with the activity of minor volcanoes in the eastern part of deposit. In its western part adjoining the Kazbek volcanic cone, ore formation apparently continued over the entire period of recent magmatic activity from 400 to 100 ka ago. It is quite probable that this process is currently proceeding at deep levels of the Devdoraki deposit. Pb–Pb isotope-geochemical data show that Jurassic metasedimentary rocks that host sulfide mineralization could have been a main source of matter for early pyrite. At the second stage of base-metal mineralization formation, the source of ore matter was earlier metamorphic pyrite combined with hydrothermal solutions related to Quaternary endogenic activity within the Kazbek volcanic center. Gangue mineral matter (quartz, carbonates) was supplied simultaneously from the postmagmatic hydrothermal solution and host shale.     

Late Cambrian deformation in the Lesser Himalaya

The Tons Valley, situated in the central-easternmost part of the Himachal Lesser Himalaya, adjoining the Garhwal Himalaya, shows geological features suggestive of a strong pre-Tertiary deformational episode. The Paleoproterozoic Dharagad Group, overlain by the Mesoproterozoic Deoban and Neoproterozoic Simla groups rest as a thrust sheet over the Middle Cambrian Chilar Formation, which occurs as windows and also as tectonic slivers within the thrust sheet designated as the Dharagad Thrust Sheet (DTS). The mineral lineation, inclination of tectonic slivers and overturned beds suggest that the DTS was translated from the NE. The westernmost and southwesternmost leading edges of the DTS are exposed at Subathu and Morni WNW and WSW respectively of the Tons Valley. The position of the leading edges of the DTS vis-à-vis the windows in the Tons Valley suggest a minimum translation of about 50 km for the DTS. The Simla Group at Subathu and the Deoban at Morni, forming parts of the DTS, constitute basement for the Thanetian–Lutetian Subathu Formation of the Himalayan Foreland Basin (HFB). This stratigraphic relationship unambiguously demonstrates that the Simla and the Deoban Groups, forming leading edges of the allochthonous DTS, were already translated and emplaced at Subathu and Morni before the creation of the HFB in which the deposition commenced with the Subathu Formation in Thanetian. It implies that the DTS was translated from the NE to the present position at Subathu and Morni in pre-Thanetian time. There is no direct evidence to constrain the age of the thrusting.In view of regional regression in Late Cambrian, a distinct angular unconformity between the Cambrian and the overlying Ordovician, Early Paleozoic metamorphism and extensive development of Early Paleozoic granites and their rapid exhumation, a Late Cambrian age is suggested for the DTS thrusting. Not only the direction of movement of the DTS is same as that of the Tertiary thrust sheets but also Cambrian folds are co-axial with the Tertiary folds. This strange coincidence shows that similar kinematic field existed during two tectonic events. A ridge, like the present Central Crystalline Axis, was elevated between the Tethyan and Lesser Himalayan basins, which contributed zircons of the Early Cambrian age to both basins.     

Sources of Magmas and Conditions of Rock Formation in the Late Cenozoic Udokan Volcanic Plateau

Doklady Earth Sciences - The Udokan volcanic plateau differs from other volcanic regions of the Late Cenozoic volcanic province of East Asia in the high alkalinity of volcanic rocks, their...     

Late Mesozoic‐Cenozoic exhumation history of the Lesser Hinggan Mountains,NE China,revealed by fission track thermochronology

This study uses zircon and apatite fission‐track (FT) analyses to reveal the exhumation history of the granitoid samples collected from the Lesser Hinggan Mountains, northeast China. A southeast to northwest transect across the Lesser Hinggan Mountains yielded zircon FT ages between 89.8 ± 5.7 and 100.4 ± 8.6 Ma, and apatite FT ages between 50.6 ± 13.8 and 74.3 ± 4.5 Ma with mean track lengths between 11.7 ± 2.0 and 12.8 ± 1.7 µm. FT results and modelling identify three stages in sample cooling history spanning the late Mesozoic and Cenozoic eras. Stage one records rapid cooling from the closure temperature of zircon FT to the high temperature part of the apatite FT partial annealing zone (∼210–110 °C) during ca. 95 to 65 Ma. Stage two records a period of relative slow cooling (∼110–60 °C) taking place between ca. 65 and 20 Ma, suggesting that the granitoids had been exhumed to the depth of ∼1−2 km. Final stage cooling (60–20 °C) occurred since the Miocene at an accelerated rate bringing the sampled rocks to the Earth's surface. The maximum exhumation is more than 5 km under a steady‐state geothermal gradient of 35 °C/km. Integrated with the tectonic setting, this exhumation is possibly led by the Pacific Plate subduction combined with intracontinental orogeny associated with asthenospheric upwelling. Copyright © 2010 John Wiley & Sons, Ltd.     

New paleomagnetic and paleointensity data from Pliocene lava flows from the Lesser Caucasus

A paleomagnetic, rock-magnetic and paleointensity study has been carried out on 14 basaltic lava flows from two Pliocene (K–Ar age between 3.09 ± 0.10 Ma and 4.00 ± 0.15 Ma) sequences (Apnia and Korxi) from the eastern Djhavakheti Highland in southern Georgia (Caucasus).Measurement of strong-field magnetisation versus temperature curves yielded three types of thermomagnetic curves: (i) Reversible curves with magnetite as only remanence carrier (type H); (ii) irreversible curves with magnetite as only carrier of remanence (type H⁻) and (iii) irreversible curves showing a low Curie-temperature phase and magnetite (type L). Analysis of hysteresis curves showed that samples were characterised by a mixture of single-domain and multi-domain grains.Paleomagnetic experiments allowed determining characteristic components for all flows and normal polarities (6 flows), reversed polarities (7 flows) and intermediate polarities (1 flow) were observed.. Paleomagnetic poles were calculated using only those sites unequivocally showing normal or reversed polarities. The paleomagnetic pole obtained from flows of both combined sequences (latitude λ = 77.9°N, longitude ϕ = 152.1°E, n = 13, A₉₅ = 11.8°, k = 13.4) showed a good agreement with the 5 Ma window of the European synthetic apparent polar wander path of Besse and Courtillot (2002). The paleomagnetic direction of the combined Apnia-Korxi flows agrees well with the expected one, showing no significant tectonic rotation. The latter cannot be however, completely excluded in the Korxi section. In that section, analysis of the angular dispersion of virtual geomagnetic poles yields a much higher value than expected.Paleointensity experiments using the Coe method were performed on 31 specimens from 10 flows. After application of specific selection criteria, 19 samples from 8 flows were observed to provide successful determinations, with mean flow values showing a wide scatter. If only flows with more than one successful paleointensity determination are taken into account, virtual dipole moments (VDMs) vary between 3.5 × 10²² A m² and 8.3 × 10²² A m². In intermediate polarity site AP2 no weak transitional paleostrength values were observed.