D. V. Kovalenko

New Data on the Composition of Island-Arc Volcanic Rocks in the Eastern Mountain Range, Kamchatka

Neotectonic uplifts of the Vostochnyi (Eastern) mountain range of Kamchatka are composed of Cretaceous‐Paleogene complexes with fold-nappe and imbricate structures that represent fragments of the southern Ozernov‐Valagin segment of the Achaivayam‐Valagin paleoarc [1]. The composition of island-arc volcanic rocks developed within the uplifts has been studied to a variable extent. Among the best studied are the volcanic rocks of the Ozernoi Peninsula and Tumrok Ridge [2‐4]. We obtained new data on the composition of the least studied volcanic rocks of the Kumroch Ridge, as well as on the Tumrok and Valagin ridges, which allowed us to refine the geodynamic conditions of the volcanic sequences of this segment of the paleoarc. The sampling localities of the studied volcanic rocks are shown in Fig. 1 and Table 1. In the Kumroch Ridge, the Late Campanian‐Danian island-arc rocks compose the tectonic nappes of an autochthon (rocks of the Khapitsk sequence) overlain by a serpentinite melange nappe. The nappe includes blocks of Albian‐Cenomanian and Santonian‐Early Campanian siliceous rocks, basalts, diabases, gabbros, and serpentinized peridotites. Volcanic rocks of the Khapitsk sequence are mainly composed of clinopyroxene‐plagioclase porphyritic basalts and basaltic andesites. In the P 2 O 5 ‐TiO 2 diagram [5], these volcanic rocks are plotted in the island-arc rock field. In terms of REE distribution patterns (Fig. 2), the studied samples are ascribed to the calc-alkaline and tholeiitic series. The location of volcanic rocks in the section suggests the simultaneous formation of these two rock groups. In the REE patterns (Fig. 2), the samples of calc-alkaline series demonstrate LREE enrichment relative to HREE, while LREE and MREE contents in tholeiitic rocks show a similar (almost chondrite-type) pattern, with an insignificantly elevated LREE content. The spidergrams of the studied volcanics have island-arc signatures: deep negative anomalies of HFSE (Nb, Ta, Zr, Ti) and REE enrichment relative to LILE (Ba, U, Rb) (Fig. 3). The calc-alkaline rocks have higher contents of Nb, Ta, Ba, U, and Rb. The two rock groups also differ in bulk composition. Relative to the tholeiitic basalts, the calc-alkaline rocks have a more potassic composition. In general, they are depleted in Ti and enriched in P (Table 1).

Tectonomagmatic cycles and geodynamic conditions of formation of the ore-bearing systems in the Southern Argun’ Region

The evolution of the geological structure in the Southern Argun’ Region is studied in terms of changing geodynamic conditions of the Proterozoic, Caledonian, and Variscan Tectonomagmatic Cycles, which also under Mesozoic tectonomagmatic activation led to the formation of latite igneous rocks enriched in Au, Cu–Mo, Pb–Zn–Ag, volcanic and plutonic complexes of the caldera structures with Mo–U, Pb–Zn, and fluorite ores, and rare-metal granites with a Sn–W–Li–Ta spectrum.

Geodynamic formation conditions and magmatic sources of late cambrian sills and dikes in the daribi range (Western Mongolia)

The Daribi Range is located in the southwestern part of western Mongolia at the conjunction of two large lithotectonic zones. In the east, the wide Shargyn–Nura depression separates the range from the Dzabkhan zone with the basement composed of Precambrian metamorphic sequences, which are overlain by the Vendian–Cambrian carbonate–terrig enous cover. In the west, the range is bordered by the Ozernaya zone of Early Caledonides also Vendian– Cambrian in age.

Adakite Volcanism at the Continental Margin and Associated Problems. Part I. Adakites from the Upper Reaches of the Valovayam River: New Age, Mineral, and Chemical Data and Petrological Modeling

The paper presents new age and petrological–geochemical data on the rocks of the Valovayam River area (Kamchatka Isthmus) and some areas of the Sredinny Range of Kamchatka (Orlov Volcano, Polovinnaya, Stolbovaya, and Matera mountains). Analysis of the obtained data shows that the spatial and temporal association of the intraplate and island-arc volcanism in the Valovayam River area is not accidental, but resulted from melting of a common source related to the mantle diapir. Additional data on the composition of the mantle xenoliths and the traces of their melting in the form of residual glasses make it possible to relate the formation of different rock series into a single process. A two-stage petrogenetic model is proposed: (1) formation of “black pyroxenites” as a result of melting of mantle lherzolites and (2) formation of the whole rock assemblage through melting of the same lherzolites and newly formed pyroxenites. The concurrent study of the rocks of similar age from the Valovayam River area and the Sredinny Range allows us to distinguish two types of adakites: high-Si adakites (HSA) in the former area and low-Si adakites (LSA) in the latter. The formation of the first type is related to the fractionation, melting, and breakdown of amphibole, while the formation of the second type was controlled by the fractionation and melting of pyroxene.

Middle Devonian Paleomagnetism of Geological Complexes of Central Tuva

New paleomagnetic data are obtained for Middle Devonian rocks of Central Tuva. The rocks contain one-, two-, or three-component magnetization. The low-temperature (LT) components of magnetization are close to the directions of the present-day or Cenozoic magnetic field in Tuva. Based on the directions of the high-temperature (HT) components of magnetization, which were distinguished in the magnetite spectrum of blocking temperatures of up to 580оС, we revealed a prefolding magnetization of different polarity. The time when Middle Devonian rocks acquired the prefolding HT component of magnetization almost does not differ from the time of rock formation. Middle Devonian sequences were formed at low latitudes (19°–25° N). We calculated the Middle Devonian paleomagnetic pole (Φ =–13°, Λ = 106°, A95 = 7), which can be used to describe the movement of the Caledonian block in Central Asia, and probably Siberia, if these blocks had been tectonically coupled by the Devonian.

Tectono-Magmatic Cycles and Geodynamic Settings of Ore-Bearing System Formation in the Southern Cis-Argun Region

The ore-bearing geological structural units of the southern Cis-Argun region are considered in the context of varying geodynamic regimes related to the Proterozoic, Caledonian, and Hercynian tectono-magmatic cycles, as well as during the Late Mesozoic within-plate tectono-magmatic activity, which give rise to the formation of subalkaline igneous rocks of the Shakhtama Complex with Au, Cu–Mo, Pb–Zn–Ag metallogenic specialization; volcano-plutonic complexes of calderas with Mo–U, Pb–Zn, and fluorite ores; and rare-metal granite of the Kukulbei Complex with a Sn–W–Li–Ta spectrum of mineralization. The comparative geochemical characteristics inherent to Mesozoic ore-bearing felsic igneous rocks are considered, as well as geodynamic settings of ore-bearing fluido-magmatic systems, taking into consideration new data on geochemistry of bimodal trachybasalt–trachydacite series and rhyolite of the Turga Series, which fill the Strel’tsovka Caldera, whose trend of evolution is defined as a reference for geological history of the studied territory. The geodynamic conditions, phase composition, and geochemistry of rocks along with metallogenic specialization of Mesozoic volcano-plutonic complexes of southern Cis-Argun region are close to those of the Great Khingan Belt in northeastern China and eastern Mongolia.

Paleomagnetism of Mongolia

Most Lower Phanerozoic rocks of western Mongolia investigated were repeatedly remagnetized. They demonstrate a secondary magnetization component of normal and reversed polarity. The normal polarity components are related to Mesozoic rock remagnetization. The reversed polarity components were probably formed during the Carboniferous‒Permian Superchron of reversed polarity. The analysis of the distribution of the reversed polarity component in the geological structure of Mongolia allows some zoning to be outlined with the defining regions of Mongolia characterized by insignificant rock defamations with intricate post-Permian dislocations and a region marked by rotation of large blocks around the horizontal axis (Khan Khukhei Range). It is assumed that Ordovician rock of western Mongolia contains a magnetization component close to the primary one. If the assumption is valid, the presumably northern paleolatitude derived from this direction corresponds to the interval of 14°‒17°‒20° (minimum‒average‒maximum, respectively).

The beginning of volcanic activity within Sredinny metamorphic Massif (Sredinny Range, Kamchatka)

For the first time, the age of the beginning of the volcanic activity within Sredinny metamorphic Massif is determined (7–6 Ma). We suppose that this event was caused by the collision of Kamchatka with the Kronotsk arc that started about 7 Ma from accretion of Shipunsky peninsula. We demonstrate that at least two types of rocks were erupted within Sredinny Range of Kamchatka in late Miocene times: typical islandarc rocks were produced in the central and northern parts of the Range, and hybrid type rocks—in its southernmost part.

Geodynamic formation conditions of Early Cambrian lavas in the Ozernaya zone of Mongolia

Four types of pre-accretionary Early Cambrian lava sequences are distinguishable in the geological structure of the Ozernaya zone in western Mongolia: (I) close to N-MORB; (II) close to E-MORB; (III) enriched with trace elements and with HFSE minimums; (IV) depleted in trace elements and with HFSE minimums. All these lavas could have been formed in an island-arc‒back-arc basin system. N-MORB basalts were melted from depleted magma sources with с εNd(t) = 10.0–11.5. Plume melts originated from mantle sources with εNd(t) = 4.8–9.7. The sources of island arc lavas were characterized by εNd(t) = 7.3–9.9.

Isotopic and geochemical characteristics of Upper Cretaceous and Cenozoic flysch sediments of the Kamchatka Peninsula and southern Koryak region

The isotopic and geochemical studies of the Upper Cretaceous-Cenozoic flysch sequences of the Kamchatka Peninsula and southern Koryak region revealed that they were formed at least from two sources: one depleted (T) with low 87Sr/86Sr and high positive SrNd(T) values and one enriched (T) with high 87Sr/86Sr and negative SrNd(T) values. The enriched source was likely represented by complexes of ancient upper continental crust. The subduction-related rocks and, to a lesser extent, basalts of mid-oceanic ridges or back-arc basins could serve as a juvenile source for most of the flysch sediments. The Upper Cretaceous flysch sediments differ from their Cenozoic analogues in composition. The Upper Cretaceous rocks are dominated by enriched upper crustal material. The Cenozoic sequences of the Ukelayat Trough and Paleocene-Eocene sequences of the Kumroch Range contain a substantial amount of island-arc volcanoclastic material; the Eocene flysch of Karaginskii Island is compositionally similar to the Upper Cretaceous flysch sequences.