T. A. Yasnygina

Holocene volcanism in central Mongolia and Northeast China: Asynchronous decompressional and fluid melting of the mantle

Volcanic eruptions in central Mongolia during the latest Pleistocene and Holocene time preceded an initial Holocene volcanic event of 8740 ± 400 years ago in Northeast China and terminated simultaneously with that event as inferred from 14C datings. Alkali basaltoid magmatic material from a partially melted (1.5–3%) mantle source was erupted in the Taryat Basin of central Mongolia, at first along a nearly east-west line of volcanoes, and afterwards material of higher melting (up to 5%) was discharged along the north-northeast line of Khorgo edifices. A material of similar composition was erupted in the Jingpohu area, Northeast China during the period from 5430–4400 BP. Initial liquids of ∼2% beneath the Frog Pool volcanic center and ∼5% beneath Crater Forest were expressed varying liquids beneath the latter area, yielding final melts of ∼5%. The action of the decompressional and the fluid mechanism was followed by in eruptions of, respectively, isotope-homogeneous magmas in central Mongolia and isotope-heterogeneous magmas depleted in high field strength elements (Nb, Ta, Ti) in Northeast China.

Tetrad effect in rare earth element distribution patterns: Evidence from the Paleozoic granitoids of the Oka zone, Eastern Sayan

The tetrad effect is a systematic change in REE properties, which is expressed as a split of a normalized REE pattern into four rounded segments (tetrads) and caused by the formation of complex compounds. It was identified in the granitoids of the Oka zone of the Eastern Sayan by ICP determination of REE contents. The granitoids have a high F content, a prominent negative Eu anomaly, and a low K/Rb ratio. An increase in the tetrad effect in the granitoids correlates with increasing Nb/Zr, Y/Ho, and Rb/Sr ratios and SiO2 contents and decreasing Zr/Hf, Ce/Pb, La/Sm, and La/Ta ratios and the contents of Fe2O3 and Na2O. In terms of correlation of the tetrad effect with the Y/Ho, Zr/Hf, and K/Rb ratios, the Li-F and alkali rare metal granitoids of the Oka zone are correlated with, respectively, the granitoids of the northern and northwestern part of the Bohemian Massif, eastern Germany, and the Baerzhe Massif, northeastern China.

Cretaceous complexes of the frontal zone of the Moneron-Samarga Island arc: Geochemical data on the basalts from the deep borehole on Moneron Island, the Sea of Japan

The variations of petrogenic oxides and trace elements have been studied in the Cretaceous volcanic rocks recovered by a deep borehole from the depth interval of 1253–4011 m on Moneron Island. The volcanic section is subdivided into two complexes: the Early Cretaceous and Late Cretaceous. The rocks of the Early Cretaceous Complex occur below 1500 m. Chemically, they belong to low-potassium island arc tholeiites, and their trace element distribution suggests their formation in a suprasubduction mantle wedge under the influence of water fluids that were subsequently released from subducted sediments and oceanic plate during the dehydration of subducted sedimentary rocks and oceanic basalts and, finally, mainly from basalts. The Early Cretaceous basalts from the borehole are interpreted as ascribing to the frontal part of the Moneron-Samarga island arc system. The volcanic rocks of the Late Cretaceous Complex are situated at depths above 1500 m. They also were formed in a suprasubduction setting, but already within the East Sikhote-Alin continental-margin volcanic belt that was initiated after the accretion of the Moneron-Samarga island arc system to the Asian continent. The island-arc section of the Moneron borehole contains basaltic andesite dikes, which are geochemically comparable with the Early-Middle Miocene volcanic rocks of Southwestern Sakhalin.

Mantle sources of the Cenozoic volcanic rocks of the Lake Kizi region in the eastern Sikhote Alin

The Middle Cenozoic lava sequence of the Lake Kizi region was studied. It characterizes the activity of sources in the Northern zone of the eastern Sikhote Alin: a Middle Eocene pulse of slab-related magmatism and prolonged injection of magmas from the sublithospheric convecting mantle in the Late Oligocene. Low contents of high field strength elements (Nb and Ta) with low Nb/Ta, Ce/Pb, and Nb/La and high K/Nb ratios and a low (87Sr/86Sr)0 of 0.703399 were determined in a Middle Eocene dacite with an age of ∼43.5 Ma. Three phases of Late Oligocene volcanic eruptions were distinguished: (1) basaltic andesites (29–27 Ma), (2) basaltic trachyandesites and trachyandesites (27–24 Ma), and (3) andesites (∼23 Ma). The lavas of the first and third phases showed low Ce/Pb, Nb/La, and Ba/La and high K/Nb ratios, which are also characteristic of supraslab processes. The lavas of the second phase are shifted with respect to these ratios toward ocean island basalt compositions. The entire Late Oligocene volcanic sequence falls within a narrow range of the initial strontium isotope ratios, (87Sr/86Sr)0, from 0.703661 to 0.703853. Such ratios are characteristic of volcanic and subvolcanic rocks with ages of ∼37, 31–23, and ∼16 Ma over the whole region of the Tatar Strait coast.

Trace elements and Sr isotopes in the crude oils from the Sakhalin offshore fields

Data on the concentrations of rare-earth elements and other metals in oils from Sakhalin fields were obtained by inductively coupled plasma mass spectrometry. The chemical preparation of the samples included the stages of acid digestion and ashing. In terms of the total metal contents; V, Ni, and Fe ratios; and Sr isotope ratios, the oil samples are divided into three groups. The crude oils from the extreme northeastern part of the island shelf, which are assigned to group I, differ from the other oils in the high contents of V and other metals, as well as the high V/Ni and V/Fe ratios. The crude oils of the group II fields located southward have elevated contents of REE and some other lithophile elements and moderate V/Ni and V/Fe ratios. Group III fields from the same area and from the Tatar Strait have low metal concentrations and low V/Ni and V/Fe ratios. The 87Sr/86Sr ratio is comparatively low in the group II oil (0.70838–0.70858) and elevated in the group III oil (0.70854–0.70911). The Sr isotope ratio of the crude oils of the Kaigan-Vasyukan deposit (group I) originated from marine organic matter (0.70867–0.70884) is correlated with 87Sr/86Sr ratios in sea-water with an age of 18.0–10.5 Ma. Based on the REE patterns, the V/Ni ratios, and the results of the multielement factor analysis, the crude oils of group III and, to a lesser extent, of group II are similar to the West Siberian crude oils.

Mantle sources of the Cenozoic volcanic rocks of East Asia: Derivatives of slabs, the sublithospheric convection, and the lithosphere

The spatial-temporal variations of the trace element and Sr isotope composition were analyzed in the Middle-Late Cenozoic volcanic rocks of the East Asian continental margin. The heterogeneity of the sublithospheric mantle was characterized, the active sources of the supraslab mantle were distinguished, and the Southern and Northern sublithospheric convective subdomains of the Transbaikalian low-velocity domain at a depth of 410–200 km were distinguished. The supply of isotopically homogenous mantle from the convective subdomains was replaced in space and time by their mixtures. Isotopically highly depleted material was derived from the mantle above the Kula-Izanagi and Pacific slabs ∼43.5, 23–17, and <15 Ma, while moderately depleted mantle was derived from the Northern and Southern subdomains ∼37, 31–23, and ∼16 Ma and 19–12 Ma, respectively. Similar convective but shallower isotopically enriched material was determined in mixtures from the Southern convective subdomain in the Northeast Japanese arc within 30–9 Ma and in mixtures with isotopically enriched lithospheric mantle in the central Heilongjiang Province <9.6 Ma. Lithospheric melts erupted during drastic changes in the dynamics of the sublithospheric convective subdomains.

Geochemistry and Metamorphic Parameters of Rocks in the Batomga Granite-Greenstone Terrane, Aldan Shield

The protoliths of the Early Proterozoic metamorphic complex in the Batomga granite-greenstone terrane are proved to comprise two petrochemical series of volcanic rocks: calc-alkaline and komatiite-tholeiite. The metavolcanic rocks of the calc-alkaline series are metamorphosed basalts, andesites, dacites, and rhyolites. The topology of the trace-element patterns of the acid volcanics is similar to that of Archean gray gneisses in platform basements, and this suggests that the petrologic mechanisms that produced the protoliths could be similar. The metavolcanics of the komatiite-tholeiite series are determined to include komatiite and tholeiite basalts. Their chemical composition is consistent with the fractionation model of high-Mg basalts in intermediate chambers under low pressures. The Nb, Y, and Zr concentrations of the metatholeiites testify that their parental melts were derived from a plume source. The metamorphic culmination parameters of the rocks corresponded to the boundary between the amphibolite and granulite facies of elevated pressure.

Chemical composition and geochemical characteristics of the Koksharovka alkaline ultrabasic massif with carbonatites, primorye

New geochemical data are discussed on the magmatic complexes of the Koksharovka alkaline ultrabasic massif of Late Jurassic age obtained by the ICP-MS method. Based on the first results on rare earth geochemistry of carbonatites and associating pyroxenites and geological observations, the magmatic origin of the Koksharovka carbonatites was substantiated, and the problems of formation of accompanying igneous rocks were considered.

Geochemical Evolution of the Zadoi Alkaline-Ultramafic Massif, Cis-Sayan Area, Southern Siberia

The unaltered magmatic rocks of the Zadoi Massif were analyzed for Sr isotopic composition and concentrations of major oxides and trace elements by ICP MS. The evolution of the massif involved four phases: (i) perovskite and ilmenite clinopyroxenites, (ii) ijolites, (iii) nepheline syenites, and (iv) carbonatites. The perovskite clinopyroxenites have anomalously high Ce/Pb (223–1132) and Pr/Sr × 1000 (70–360) ratios at a low initial Sr isotopic ratio (87Sr/86Sr)0 = 0.70247–0.70285. The ilmenite clinopyroxenites have Ce/Pb and Pr/Sr × 1000 ratios approaching those in basalts of oceanic islands (OIB) (decreasing to 39 and 30, respectively) at a simultaneous increase in the (87Sr/86Sr)0 ratios (0.7030–0.7036). The ijolites and nepheline syenites have patterns of incompatible trace elements similar to those in OIB and the highest (87Sr/86Sr)0 ratios (0.70346–0.70414). The carbonatites are complementarily enriched in incompatible elements of the nepheline syenites and have (87Sr/86Sr)0 = 0.7029–0.7034, which is comparable with the range of analogous ratios for the ilmenite clinopyroxenites. Our geochemical data indicate that the carbonatites were formed as an immiscible liquid or fluid, which separated from the ijolite-nepheline syenite melt during its interaction with the source material of the perovskite and ilmenite clinopyroxenites.

The activation and cessation of Late Cenozoic extension in the lithosphere at the margin of the Baikal Rift Zone: Alternating sources of volcanism in the Vitim Upland

A comparative analysis of the concentrations of major oxides, trace elements, and the 143Nd/144Nd ratios in representative sequences of volcanic and subvolcanic rocks in the western and eastern Vitim Upland has revealed petrogenetic groups with different relationships among components from lithosphere and sublithosphere sources. It is hypothesized that the initial 16–14-Ma eruptions of picrobasalts and Mg basanites in the east of the upland resulted from high-temperature melting, hence, the melting of sublithospheric peridotite and lithospheric Mg-pyroxenite mantle material with mildly and strongly depleted isotope compositions of Nd relative to the value in the primitive mantle (0.512638). The broad range of varying lava compositions in the 14–9 Ma time span was caused by “passive” rifting in the west of the upland and by “active” rifting in the east. The “passive” rifting manifested itself in the melting of lithospheric material with some admixture of material from the underlying asthenosphere, while the “active” rifting lifted deep-lying mantle material. The structural rearrangement that has been occurring in the Baikal Rift System during the last 9 Ma resulted in stopping the rifting in the area of study. Relaxation, flattening and thinning of the lithosphere beneath the east part of the system during the 1.1–0.6 Ma time span caused magma effusion with values of 143Nd/144Nd that are typical of a moderately depleted asthenospheric source contaminated with deeper mildly depleted mantle material.