Gravimetric data on structure of the crust in Central and Western Caucasus

Analysis of gravimetric data in the Central and Western Caucasus shows three zones within the main range. These zones coincide with regions of linear, transitional, and domed folding which occur in sequence in the Central Caucasus, Western Caucasus, and western orogenic end of the main range. The Central Caucasus is an area of maximum depth of the M surface. The 'thickness of the granite layer decreases westward and increases eastward. The,position of the deep structures leaves no doubt that the strike of the Caucasus is the dominant direction in this area, though this is most clearly seen in the granite layer. The Caucasus as a folded area arose in relation to the production of the granite layer, the basalt masses being much less active in producing the mountain system of the Caucasus. Maps showing depths of the granite, basalt, Conrad surface, and M surface as well as structural sections and gravity profiles are included. — C. E. Sears.     

Geochronology and genesis of subalkaline basaltic lava rivers at the Dzhavakheti highland,lesser Caucasus: K-Ar and Sr-Nd isotopic data

The paper reports newly obtained K-Ar isotopic-geochronological data on the age of three lava flows (Khrami, Mashavera, and Kura), which begin at the Dzhavakheti volcanic highland in southern Georgia. All of the dated rocks, including those from the Kura Flow, which was previously considered as the Pleistocene, are demonstrated to have a Pliocene age. The lavas of the longest Khrami Flow were erupted at 3.25–3.10 Ma, and those of the Kura and Mashavera Flows at 2.20–2.05 Ma, a fact testifying to two pulses of volcanic activity at the Dzhavakheti Highland. The petrogeochemical and isotopic characteristics of the rocks (⁸⁷Sr/⁸⁶Sr = 0.7039–0.7042; ∈_Nd = 3.4–5.1) indicate that they are subalkaline within-plate basalts formed by the fractional crystallization of a basic mantle melt with the usually discontinuous selective or rarely continuous contamination with material that was not in geochemical equilibrium with the melt. The volcanics of the Khrami Flow are characterized by the less radiogenic Sr isotopic composition and the highest ∈_Nd values, while the younger rocks of the Mashavera and Kura Flows have similar and more “crustal” isotopic signatures. The ⁸⁷Sr/⁸⁶Sr ratios of the Dzhavakheti subalkaline basalts are close to the initial Sr isotopic ratios of the Quaternary and Middle Pliocene dacite lavas from the same territory. Considered together with petrogeochemical and geological data, this suggests that all young rocks in Southern Georgia were produced in similar tectonic and geodynamic environments.     

New data on the structure and composition of stromatolite buildups (Northern Anabar Region)

Lamination, a characteristic feature of stromatolites, is related to thin alternation of mineral and organogenic layers. The present paper is devoted to structures of Lower Riphean stromatolites in the Fomich River area (northern Anabar Uplift) that were described for the first time and studied with an electron microscope. Two Colonnella forms with different microstructures were examined. It has been established that a banded texture is observed if an organogenic layer is mainly composed of tightclusters of filiform (theread like) particles. However, a clotted texture is recorded if the layer is represented by isolated coccomorphic (rounded) particles. The chemical composition of these tiny structures, which can accumulate several elements, is discussed and analyzed. In the past, the organogenic layer represented a biofilm that comprised a complex-function cyanobacterial community.     

Deep structure of the western part of the Central Caucasus from geophysical data

The paper presents new data on seismotectonic studies along the Adygei profile in the western part of the Central Caucasus and provides an overview of deep geophysical studies of the Greater Caucasus. For the first time, comprehensive geophysical characteristics of a crustal section of the Greater Caucasus across an orogenic structure (along the Adygei profile) have been obtained with a uniform step of observations. Based on factual data obtained by such methods as converted waves from distant earthquakes, magnetotelluric sounding, and gravimagnetic surveys, sinking of the marginal part of the southern microplate into the mantle is verified. It is noted that the contemporary Alpine structure of the Greater Caucasus formed during gentle thrusting of the Earth’s crust (Scythian Plate) from the north on the consolidated crust of the southern microplate.     

Lead Isotope composition and origin of the quaternary lavas of Elbrus Volcano, the Greater Caucasus: High-precision MC-ICP-MS data

A systematic study of Pb isotope composition was carried out for Elbrus Volcano, one of the Europe’s largest volcanoes, using high-precision method of multi-collector inductively coupled plasma mass spectrometry. The measurement error of Pb isotope ratios was estimated from the results of replicate analyses of international BCR-1 and AGV-1 standards as ±0.03% (±2SD). The study of a representative collection showed that dacites of all three phases of the Elbrus eruptive activity are characterized by relatively small-scale variations of Pb isotope composition: ²⁰⁶Pb/²⁰⁴Pb 18.621–18.670, ²⁰⁷Pb/²⁰⁴Pb 15.636–15.659, and ²⁰⁸Pb/²⁰⁴Pb 38.762–38.845. New Pb isotope geochemical characteristics in combination with existing Sr-Nd data indicate that the parental magmas of Elbrus are of mixed mantle-crust origin. They were formed by interaction of mantle-derived melts with continental crust of the Greater Caucasus during continental collision between the Eurasian, Arabian, Turkish, and Iranian plates.     

Progressive metamorphism of pelitic rocks from protolith to granulite facies, Dutchess County, New York, USA: constraints on the timing of fluid infiltration during regional metamorphism

A complete Barrovian sequence ranging from unmetamorphosed shales to sillimanite–K-feldspar zone metapelitic gneisses crops out in a region extending from the Hudson River in south-eastern New York state, USA, to the high-grade core of the Taconic range in western Connecticut. NNE-trending subparallel biotite, garnet, staurolite, kyanite, sillimanite and sillimanite–K-feldspar isograds have been identified, although the assignment of Barrovian zones in the high-grade rocks is complicated by the appearance of fibrolitic sillimanite at the kyanite isograd. Thermobarometric results and reaction textures are used to characterize the metamorphic history of the sequence. Pressure–temperature estimates indicate maximum metamorphic conditions of 475 °C, c. 3–4 kbar in the garnet zone to >720 °C, c. 5–6 kbar in the highest grade rocks exposed. Some samples in the kyanite zone record anomalous (low) peak conditions because garnet composition has been modified by fluid-assisted reactions. There is abundant petrographic and mineral chemical information indicating that the sequence (with the possible exception of the granulite facies zone) was infiltrated by a water-rich fluid after garnet growth was nearly completed. The truncation of fluid inclusion trails in garnet by rim growth or recrystallization, however, indicates that metamorphic reactions involving garnet continued subsequent to initial infiltration. The presence of these textures in some zones of a well-constrained Barrovian sequence allows determination of the timing of fluid infiltration relative to the P–T paths. Thermobarometric results obtained using garnet compositions at the boundary between fluid–inclusion-rich and inclusion-free regions of the garnet are interpreted to represent peak metamorphic conditions, whereas rim compositions record slightly lower pressures and temperatures. Assuming that garnet grew during a single metamorphic event, infiltration must have occurred at or slightly after the peak of metamorphism, i.e. 4–5 kbar and a temperature of c. 525–550 °C for staurolite and kyanite zone rocks.     

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.     

Tectonic controls on metamorphism, partial melting, and intrusion: timing and duration of regional metamorphism and magmatism in the Ni de Massif, Turkey

Mafic high-pressure granulite, eclogite and pyroxenite xenoliths have been collected from a Mesozoic volcaniclastic diatreme in Xinyang, near south margin of the Sino-Korean Craton (SKC). The high-pressure granulite xenoliths are mainly composed of fine-grained granoblasts of Grt+Cpx+Pl+Hbl±Kfs±Q±Ilm with relict porphyritic mineral assemblage of Grt+Cpx±Pl±Rt. P–T estimation indicates that the granoblastic assemblage crystallized at 765–890 °C and 1.25–1.59 GPa, corresponding to crustal depths of ca. 41–52 km with a geotherm of 75–80 mW/m². Calculated seismic velocities (V_p) of high-pressure granulites range from 7.04 to 7.56 km/s and densities (D) from 3.05 to 3.30 g/cm³. These high-pressure granulite xenoliths have different petrographic and geochemical features from the Archean mafic granulites. Elevated geotherm and petrographic evidence imply that the lithosphere of this craton was thermally disturbed in the Mesozoic prior to eruption of the host diatreme. These samples have sub-alkaline basaltic compositions, equivalent to olivine– and quartz–tholeiite. REE patterns are flat to variably LREE-enriched (La_N/Yb_N=0.98–9.47) without Eu anomaly (Eu/Eu*=0.95–1.11). They possess 48–127 ppm Ni and 2–20 ppm Nb with Nb/U and La/Nb ratios of 13–54 and 0.93–4.75, respectively, suggesting that these high-pressure granulites may be products of mantle-derived magma underplated and contaminated at the base of the lower crust. This study also implies that up to 10 km Mesozoic lowermost crust was delaminated prior to eruption of the Cenozoic basalts on the craton.     

Low-pressure regional metamorphism in the Omeo Metamorphic Complex, Victoria, Australia

The Omeo Metamorphic Complex forms the southern end of the Wagga Metamorphic Belt, which is the main locus of Palaeozoic low-pressure metamorphism in the Lachlan Fold Belt, south-eastern Australia. It comprises metamorphosed Ordovician quartz-rich turbidites originally derived from Precambrian cratonic rocks. Prograde regional metamorphism occurred in the early Silurian, very soon after sedimentation had ceased. The sequence of metamorphic zones, with increasing grade, is: chlorite, biotite, cordierite, andalusite–K-feldspar and sillimanite–K-feldspar. Migmatites occur in the sillimanite–K-feldspar zone, but large bodies of S-type granite were derived from rocks underlying the exposed Ordovician sequence. P and T estimates for the highest grade rocks are T = 700°C and P = 3.5 kbar, indicating a very high P–T gradient of 65°C/km.
The high heat flow during prograde metamorphism probably resulted from a combination of a thermal anomaly persisting from a pre-metamorphic back-arc basin environment, and intrusion of hot, mantle-derived magmas into the lower and middle crust.
Regional retrograde metamorphism coincided with a general reheating of the crust in the Siluro-Devonian, accompanied by intrusion of many I-type plutons and resetting of the K–Ar dates of some earlier plutons. The Omeo Metamorphic Complex was exposed to erosion at this time.     

New paleomagnetic data from the hominin bearing Dmanisi paleo-anthropologic site (southern Georgia, Caucasus)

The Dmanisi site has yielded human remains and lithic industry associated with Late Pliocene-early Pleistocene fauna. The site is composed of volcanogenic sediments overlying basaltic lava flows. The lithostratigraphic sequence comprises two basic depositional units: Unit A, overlying the basalt flows, and Unit B on top. A paleomagnetic and rock-magnetic study has been carried out on 106 specimens from Units A and B and the uppermost basalt flow. The lava and Unit A provide normal polarities, while reversed polarities and anomalous directions are observed in Unit B, the latter probably due to overlapping of a secondary and a primary reversed polarity component. The lower part of the section shows a clear correlation with the Olduvai subchron, and the upper levels could be as young as 1.07 Ma. As human remains were found both in units with normal and reversed polarity, different non-contemporaneous human occupations might have been possible.     

Highly siderophile element composition of the Earth’s primitive upper mantle: Constraints from new data on peridotite massifs and xenoliths

Osmium, Ru, Ir, Pt, Pd and Re abundances and ¹⁸⁷Os/¹⁸⁸Os data on peridotites were determined using improved analytical techniques in order to precisely constrain the highly siderophile element (HSE) composition of fertile lherzolites and to provide an updated estimate of HSE composition of the primitive upper mantle (PUM). The new data are used to better constrain the origin of the HSE excess in Earth’s mantle. Samples include lherzolite and harzburgite xenoliths from Archean and post-Archean continental lithosphere, peridotites from ultramafic massifs, ophiolites and other samples of oceanic mantle such as abyssal peridotites. Osmium, Ru and Ir abundances in the peridotite data set do not correlate with moderately incompatible melt extraction indicators such as Al₂O₃. Os/Ir is chondritic in most samples, while Ru/Ir, with few exceptions, is ca. 30% higher than in chondrites. Both ratios are constant over a wide range of Al₂O₃ contents, but show stronger scatter in depleted harzburgites. Platinum, Pd and Re abundances, their ratios with Ir, Os and Ru, and the ¹⁸⁷Os/¹⁸⁸Os ratio (a proxy for Re/Os) show positive correlations with Al₂O₃, indicating incompatible behavior of Pt, Pd and Re during mantle melting. The empirical sequence of peridotite-melt partition coefficients of Re, Pd and Pt as derived from peridotites () is consistent with previous data on natural samples. Some harzburgites and depleted lherzolites have been affected by secondary igneous processes such as silicate melt percolation, as indicated by U-shaped patterns of incompatible HSE, high ¹⁸⁷Os/¹⁸⁸Os, and scatter off the correlations defined by incompatible HSE and Al₂O₃. The bulk rock HSE content, chondritic Os/Ir, and chondritic to subchondritic Pt/Ir, Re/Os, Pt/Re and Re/Pd of many lherzolites of the present study are consistent with depletion by melting, and possibly solid state mixing processes in the convecting mantle, involving recycled oceanic lithosphere. Based on fertile lherzolite compositions, we infer that PUM is characterized by a mean Ir abundance of 3.5 ± 0.4 ng/g (or 0.0080 ± 0.0009*CI chondrites), chondritic ratios involving Os, Ir, Pt and Re (Os/Ir_PUM of 1.12 ± 0.09, Pt/Ir_PUM = 2.21 ± 0.21, Re/Os_PUM = 0.090 ± 0.002) and suprachondritic ratios involving Ru and Pd (Ru/Ir_PUM = 2.03 ± 0.12, Pd/Ir_PUM = 2.06 ± 0.31, uncertainties 1σ). The combination of chondritic and modestly suprachondritic HSE ratios of PUM cannot be explained by any single planetary fractionation process. Comparison with HSE patterns of chondrites shows that no known chondrite group perfectly matches the PUM composition. Similar HSE patterns, however, were found in Apollo 17 impact melt rocks from the Serenitatis impact basin [Norman M.D., Bennett V.C., Ryder G., 2002. Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis. Earth Planet. Sci. Lett, 217-228.], which represent mixtures of chondritic material, and a component that may be either of meteoritic or indigenous origin. The similarities between the HSE composition of PUM and the bulk composition of lunar breccias establish a connection between the late accretion history of the lunar surface and the HSE composition of the Earth’s mantle. Although late accretion following core formation is still the most viable explanation for the HSE abundances in the Earth’s mantle, the “late veneer” hypothesis may require some modification in light of the unique PUM composition.