Vol 13, No 5 (2019)

Magnetotelluric soundings (MTS) are carried out in a broad frequency range from 10⁴ to 10^–3 Hz at a total of fifty-five sounding sites spaced about 5 km apart along the profile Dal’nerechensk–Roshchino–Plastun across the strike of the Sikhote Alin Orogenic Belt (SAOB). Based on the results of data interpretation, the crust and upper mantle geoelectric section is constructed down to a depth of 150 km. The section shows the crust of variable thickness between 20 and 40 km with a resistivity higher than 1000 Ohm m, which consists of different-resistivity blocks bordered by subvertical and inclined low-resistivity zones, from tens to a few hundreds of Ohm m. Three abnormal-resistivity zones are revealed in the crust and upper mantle. The upper mantle structure is nonuniform. Beneath the western and eastern parts of the SAOB, the lowest resistivity values are observed at 40–80 km depths. A high-resistivity zone of 300–500 Ohm m, stretching in the westerly direction from the coast beneath the Khanka superterrane, is identified in the 80–150 km depth range. This area is treated as remnants of the plate of an ancient subduction zone; two low-resistivity canals are distinguished here, which are associated with a slab-window. The results obtained for the northern and southern parts of the SAOB are compared and their similarities and dissimilarities are ascertained.

Based on a compilation of the vast literature, we typify the peculiar mantle-derived adakitic volcanic rocks in the Sea of Okhotsk, Kamchatka, and Bering Sea regions. The low-silica (LSA) and high-silica (HSA) types are distinguished in each region. The prevailing adakite type corresponds to the bimodal basalt–andesite series in the Kamchatka region and the basanite–dacite series in the Bering Sea region. In the Kamchatka region, the formation of LSA was related to the fractionation and melting of pyroxene, while the origin of HSA was related to the fractionation, melting, and breakdown of amphibole. Alkaline metasomatism plays a subordinate role in these processes. In the Bering Sea region, the formation of both types of adakites was mainly controlled by amphibole, with the assistance of alkaline metasomatism. The isotope–geochemical compositions of the adakites and within-plate volcanic rocks shows that both these rock types can be equally regarded as indicators of plume petrogenesis and geodynamics. Based on a comparison of the compositions of the adakites and glasses derived by xenolith melting, a two-stage genetic model is proposed: formation of black pyroxenites through melting of mantle lherzolites and generation of the entire rock complex owing to the repeated melting of the same lherzolites and newly formed pyroxenites. The formation of adakitic and within-plate signatures in the volcanic products beginning from the Eocene indicates the transition of the spacious territory of the NE continental margin from subduction-related compression to a setting of rift extension.

The analysis of features of the geological structure of the Adycha gold placer region shows that flat structures of the early collision stage control the setting of the ore fields. The ores are similar to host rocks in the main components, lithofile trace elements, and REEs. The features of the trace-element pattern and indicative ratios agree with the suggested metamorphic origin of the main volume of fluids, their weakly reduced and almost neutral conditions, and sorption rock–fluid interaction. A group of elements, the distribution of which is significantly distinct from those of the host rocks, is determined. The geochemical type of the ore region is caused by Mo, Au, Bi, and Sb, the contents of which are higher in all studied ore objects. Chromium, As, Pb, and Te are present in all ore fields, but not in all objects. The Co–Ni–W–Zn–Cd–Ag, Co–Ni–W–In–Mn–Cu–Sr, and Sn–In–Mn–Cu–Zn–Cd–Sr–Ag associations supplement the geochemical type of the Delyuvial’ny, Lazo, and Sentachan ore fields, respectively. Our studies along with thermobarogeochemical data refine the metamorphic–magmatic model of the formation of the gold–quartz deposits of the Kular–Nera belt and outline approaches for the practical application of these features.

The clay minerals in the Upper Pleistocene sediments from DSDP Hole 478 (a drilled depth of 464 m) located in the central area of the Guaymas Basin, Gulf of California, are studied. The composition of the clay minerals and their quantitative proportions in natural mixtures are determined by modeling of X-ray diffraction patterns for oriented specimens of finely dispersed particles. The terrigenous clay minerals are mainly dioctahedral (mixed-layer smectite–illite is dominant, the amount of illite is smaller) with admixtures of chlorite and kaolinite. They are distributed throughout the entire sedimentary section, except for thermally altered sediments from the contact zones with the top of the sills, where terrigenous dioctahedral clay minerals disappear, except for illite, and are substituted by trioctahedral mixed-layer chlorite–smectite and corrensite–chlorite; trioctahedral smectite also appears. Terrigenous illite is missing in the sediments at the base of the sedimentary cover exposed by the hole, on the top of the sill complex with a thickness of no less than 125 m.