V. G. Sakhno

First SHRIMP U-Pb zircon dating of magmatic complexes in the southwestern Primor’e region

Recent thorough studies of this complex in the southwestern Primor’e region revealed massifs of Late Permian–Early Jurassic granitoids in the western part of the Grodekovo batholith correlative with the Rya zanovka and Gvozdevo complexes developed in the southern part of this region (Fig. 1) [3]. It was estab lished that the Ryazanovka Complex contains gold mineralization, which put forward the problem of esti mating the gold resource potential of this area. Of interest is also the activity scale of Silurian granitoid magmatism in the entire region. In this connection, its granitoid complexes have been studied using the SHRIMP U–Pb chronological method.

Monzonitoid magmatism of the copper-porphyritic Lazurnoe deposit (South Primor’e): U-Pb and K-Ar geochronology and peculiarities of ore-bearing magma genesis by the data of isotopic-geochemical studies

Magmatic rocks from the copper-porphyritic Lazurnoe deposit (Central Primor’e) have been studied. It has been found that rocks from the Lazurnyi massif are referred to gabbro-monzodiorites, monzodiorites, and monzo-granodiorites formed during two magmatic phases of different ages. The earlier phase is represented by gabbro-monzodiorites and diorites of the North Stock, and the later one, by gabbro-monzodiorites and monzo-grano-diorites of the South Stock. On the basis of isotopic dating by the U-Pb (SHRIMP) method for zircon and by the K-Ar method for hornblendes and biotites, the age of magmatic rocks is determined at 110 ± 4 for the earlier phase and at 103.5 ± 1.5 for the later one. Examination of the isotopic composition for Nd, Sr, Pb, Hf, δ18O, and REE spectra has shown that melts of the first phase are contaminated with crustal rocks and they are typical for a high degree of secondary alterations. Potassiumfeldspar, biotite, propylitic alterations, and sulfidization are manifested in these rocks. The rocks of the later stage of magmatism are characteristic for a primitive composition of isotopes and the absence of secondary alterations. They carry the features of adakite specifics that allows us to consider them derivatives of mantle generation under high fluid pressure. The intrusion of fluid-saturated melts of the second phase into the magmatic source of the first phase caused both an alteration pattern of rocks and copper-porphyritic mineralization. Isotopes of sulfur and oxygen allow us to consider the ore component to be of magmatic origin.

Granitoid magmatism of the Lermontovskaya and Vostok-2 tungsten ore-magmatic systems: U-Pb (SHRIMP) dating and isotope characteristics (3He/4He) of ores

U-Pb (SHRIMP) isotope dating of magmatic complexes of tungsten ore-magmatic systems (OMSs) resulted in a conclusion about the close age (103–105 Ma) of their formation in the magmatic rock masses from the Tatibinsk plutonic series whose location is controlled by the Sikhote-Alin’ strike-slip fault system. The OMSs are formed within the zones of strike-slip fault extensions (“windows”) dipping into the mantle, which establishes the conditions for the participation of deep fluids in the generation of melts at the level of crustal-mantle interaction. The new data from the study of the isotope composition of helium in the fluid inclusions showed that the content of mantle helium in ores of tungsten OMSs is approximately 10%.

Early Archean sialic crust of the Siberian craton: Its composition and origin of magmatic protoliths

This study demonstrates that the base of the Archean deep-seated granulite complexes within the Siberian craton consists of a metabasite-enderbite association. The major and trace element distribution patterns revealed that the protoliths of this association are represented by calc-alkaline andesites and dacites, containing several minor sequences of komatiitic-tholeiitic volcanic rocks. The origin of the primary volcanic rocks of the metabasite-enderbite association is inferred on the basis of a model of mantle plume magmatism, which postulates that both andesitic and dacitic melts were derived from the primary basitic crust at the expense of heat generated by ascending mantle plumes. The formation of the protoliths of the Archen metabasite-enderbite association of the Siberian craton began at 3.4 Ga and continued until the late Archean.

The Malinovka GoldBearing Ore-Magmatic System of Central Sikhote Alin (Primor'e Region, Russia): Geochronology, Petrogeochemistry, and Isotopic Signatures of Igneous Complexes

The central part of Sikhote Alin is a domainmarked by wide development of Mesozoic sedimentary–volcanogenic and terrigenous rocks intruded bya system of composition and agevariable intrusions.Two series are definable among them: (1) the earlier(135–125 Ma) Khungari Complex represented bygabbro, pyroxenites, and granites; (2) the later (115–105) Tatibi Complex largely composed of gabbroids,adamelites, and granites [1].These complexes are confined to the CentralSikhote Alin strikeslip fault system, which wasformed in response to the convergence of the AsianContinent and Pacific Plate. Compression and subsequent leftlateral movements of blocks stimulatedevelopment of pullapart extension structures thatrepresent peculiar through

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.

Petrology of mesozoic monzonite-syenite magmatism of the Ketkap-Yuna magmatic province in the Aldan Shield

Genetic relationships between ore mineralization and associations of potassic basaltoids and comagmatic intrusions have been established in recent decades. The works of L.V. Tauson, O.A. Bogatikov, V.I. Kovalenko, and other researchers have shown that both alkaline basaltic and granitoid magmatism of the latite petrochemical type and related multiprofile mineralization were widespread in intracontinental consolidated areas during tectonomagmatic activation. Hence, problems of petrogenesis of latite magmatism and geodynamic regimes of its manifestation acquire great scientific and practical importance. In this context, we carried out petrological studies of the Ketkap trachydacite‐monzonite‐syenite complex of the Ketkap‐Yuna magmatogenic province (KYMP) on the Aldan Shield. A significant part of this rock complex can be derived from latite melts [8]. The formation of the Ketkap Complex is related to final stages of Mesozoic magmatism on the Aldan Shield [3‐8]. The complex exhibits four phases of evolution and is confined to the second cycle of tectonomagmatic reactivation (Table 1). Near the tectonomagmatic structures, the complex postdated intense phonotephrite‐phonolite‐ trachyte volcanism in the western KYMP (the Bokur Complex in the Bokur and Ulakhan calderas of the Ulakhan Massif). Its final stages are temporally and spatially close to alkaline syenite subintrusive magmatism (the Dariin Complex in the Usmun‐Dariin, Yuna‐Dan, and Ulakhan massifs [4, 5, 8]. The main phase of monzonite‐syenite magma intrusion is related to the fourth stage of Mesozoic magmatism on the Aldan Shield and is widely represented at the western flank of the Ketkap magmatogenic uplift of the KYMP. Moreover, small exposures of Ketkap monzonitoids are known in the KYMP in a series of other magmatogenic massifs in association with alkaline magmatites of the Dariin Complex. The fifth (final) stage of the Mesozoic magmatism was marked by the intrusion of a series of dikes of subalkaline potassic rocks, which also show spatiotemporal association with dikes of alkaline magmatites of the Dariin Complex. The geological position and petrographical features of the Ketkap Complex were described previously [3, 6, 8, and others]. Representative compositions of rocks and some of the results of their petrogeochemical and geodynamic typification are given in this work (Table 2). The rock complex includes feldspathoid-free rocks (shonkinites, syenites, monzonites, and quartz monzonites), monzodiorites, subvolcanic trachydacites,

First data on U-Pb SHRIMP-II isotopic dating based on zircons of ash sediments from Cenozoic depressions in Southwest of Primorie

U-Pb (SHRIMP-II) isotopic dating has been conducted for zircons of the ash sediments of the Ust’-Suifun Suite, which was the final stage of Late Cenozoic explosive volcanism in Southwest Primorskii krai. These pyroclastic units are widespread within sediments that fill in Cenozoic depressions including large coalfields. The concordant dates (23.7–24.6 Ma) are in line with the results of the K-Ar determinations for volcanic tephra (23.6–27.1 Ma) and correspond to the beginning of the active phase of spreading and taphrogenesis in the neighboring Trench of the Sea of Japan. These processes started as early as the Eocene and are reflected in the continental vicinity with the formation of riftogenic depressions and the occurrence of a peculiar gas volcanism.

Monzonitoid magmatism of the Glukhoe gold ore deposit (Primorye): U–Pb, SHRIMP dating, petrochemical and minor-element compositions, and peculiar features of noble metal mineralization

Monzogabbrodiorites and monzodiorites from the Tatibin Group of Central Sikhote Alin (Primorye), which hosts the Glukhoe gold ore deposit, are considered with discussion of the most important data on the geological structure and composition of magmatic complexes and the results of U–Pb and SHRIMP dating. It is first established that mineral associations of the gold ore deposit include native Pt, Cu, and other compounds and mineral associations. Their formation conditions of both scientific and practical significance are analyzed.

The first data on local U-Pb isotope dating of zircons from hypersthene plagiogneisses of the Dzhugdhur block, southeastern Aldan Shield

The Dzhugdzhur block is located in the basins of the Kun Man’e, Ayumkan, and Maya rivers (Fig. 1). The stratifiable nature of granulitic rocks within the Dzhugdzhur block was shown for the first time by V.M. Moshkin [2] who distinguished two strata among them: the lower was that of pyroxene–plagioclase– crystalline schists and the upper was that of biotite– garnet gneisses with marble interbeds. Later, the terri tory of the Dzhugdzhur block was studied by a geolog ical survey and mapped at 1 : 200 000 scale, under the leadership of Yu.N. Gamaleya [3]. During these works, a more detailed stratification of metamorphic units was proposed, with subdivision into four con formingly occurring formations (from bottom to top): Upper Sunnagin, Kyurikan, Sutama, and Khurdukan. However, based on analysis of the published data and our own studies, we decided to turn back to the two strata subdivision from [2]. The lower strata is com posed of hypersthene plagiogneisses (enderbites) interbedding with two pyroxene schists (metabasites). In the upper stratum, there are aluminiferous biotite– garnet gneisses with marble interbeds with another bed of enderbites and pyroxene biotite gneisses (Fig. 1). The lower stratum representing the initial stages of crustal evolution within the Dzhugdzhur block was distinguished in [1] as a metabasite–enderbite associ ation. Hypersthene plagiogneisses (enderbites), which make up most of this association, are composed by plagioclase (50–70%; 40–50% An) with antiperthite interpositions, quartz (5–10%), hypersthene (5–10%), hornblende (8–10%), and monocline pyroxene (up to 5%). Accessory minerals are apatite, zircon, magne tite, and ilmenite. Two pyroxene schists consist of pla gioclase (30–40%; 50–58% An), monocline pyroxene (20–40%), and rhombic pyroxene (5–10%). Some varieties of two pyroxene schists contain garnet or a biotite admixture. Accessory minerals are apatite, zir con, magnetite, and ilmenite. Ultrabasic crystalline schists (two pyroxene, two pyroxene–amphibole, olivine–two pyroxene–amphibole) are composed of orthopyroxene (10–35%), clinoorthopyroxene (10– 45%), olivine (0–19%), and amphibole (0–80%). Accessory minerals are magnetite, ilmenite, spinel, and apatite.