Provenances of metaterrigenous sequences in the Sredinny and Ganalsky uplifts, Kamchatka in the light of New Sm-Nd isotopic data

Despite the long history of studying metamorphic rocks in the Sredinny and Ganalsky uplifts of Kamchatka, their tectonic setting and origin, as well as the time of sedimentation, magmatism, and metamorphism, remain a matter of debate and wide discussion. Our isotopic study shows that composite sections of metaterrigenous rocks of the Sredinny and Ganalsky ranges (Kolpakova, Kamchatka, Malka, Kikhchik, and Ganal groups) reveal no significant difference in the Nd isotopic composition, which is evidence for the geochemical similarity of their provenances in contrast to previous conjectures that these groups vary in age from Archean to Upper Cretaceous and were formed in regions distant from one another and distinct in geodynamic setting. New Sm-Nd isotopic data and recent U-Pb (SHRIMP II) timing of zircons allow us to state that the metaterrigenous rocks of the Sredinny and Ganalsky uplifts actually make up a single terrigenous sequence of a great thickness. This sequence accumulated in the Cretaceous shallow-water epicontinental basin, underwent contact metamorphism affected by intrusions varying in age and composition, was involved in zonal regional metamorphism in the early Eocene, and only in the Pliocene was it dismembered into the Sredinny and Ganalsky uplifts as a result of rifting.     

The Late Cretaceous Aarya flora of the northern Okhotsk region and phytostratigraphy of the lower part of the Okhotsk-Chukotka volcanogenic belt section

The Zarya flora comes from volcanogenic sedimentary rocks of the Zarya and Parnyi formations that correspond to the basal part of the section of the Okhotsk-Chukotka volcanogenic belt in the Omsukchan district (Magadan oblast, Russia). The revision of its taxonomic composition resulted in identifying approximately 25 species of horsetails, ferns, cycads, ginkgoalens, czekanowskians, conifers, and angiosperms. The Zarya flora is characterized by a combination of the Early Cretaceous relicts (Hausmannia, Birisia, Sphenobaiera, Phoenicopsis, Nilssonia, Podozamites) and typical Late Cretaceous taxa (Taxodium, Sequoia, Menispermites, Dalembia, Trochodendroides, Cissites, Terechovia, Platanaceae). Among all the paleofloral assemblages of the Okhotsk-Chukotka volcanogenic belt, the Zarya flora is the most similar to the Turonian-Coniacian Arman flora of the Magadan region, which indicates their synchronism and floral unity. The Chingandzha flora of the Omsukchan area, which comes from the same stratigraphic level as Zarya flora, differs substantially from the latter in its taxonomic composition. It is conceivable that the Chingandzha flora was confined to a large river valley which was connected to coastal lowlands. The plant remains of the Arman flora with many mountain relicts could be buried in sediments of intermountain troughs isolated from coastal lowlands. Araucarites ochotensis sp. nov. is described.     

Lateral structural variability in zone of eocene island-arc-continent collision,Kamchatka

The lateral variability of structural elements in the collision zone of the Cretaceous-Paleocene Achaivayam-Valagin island arc with the northeastern Asian margin is considered. The similarity and difference of Eocene collision structural elements in the north and the south of Kamchatka are shown. In northern Kamchatka, the continent-arc boundary is traced along the Lesnaya-Vatyn Thrust Fault, which completed its evolution about 45 Ma ago. The thin, near-horizontal allochthon of this thrust, composed of island-arc rocks, overlies the deformed but unmetamorphosed terrigeneous sequences of the Asian margin. The general structure of this suture in the Kamchatka Isthmus and southern Koryakia is comparable with the uppermost subduction zone, where a thin lithospheric wedge overlaps intensely deformed sediments detached from the plunging plate. In southern Kamchatka (Malka Uplift of the Sredinny Range), the arc-continent collision started 55–53 Ma ago with thrusting of island-arc complexes over terrigenous rocks of continental margin. However, the thickness of the allochthon was much greater than in the north. Immediately after this event, both the autochthon and lower part of allochthon were deformed and subsided to a significant depth. This subsidence gave rise to metamorphism of both the autochthon (Kolpakov and Kamchatka groups, Kheivan Formation) and lower allochthon (Andrianovka and Khimka formations). The anomalously fast heating of the crust was most likely related to the ascent of asthenospheric masses due to slab breakoff, when the Eurasian Plate was plunging beneath the Achaivayam-Valagin arc.     

Age and origin of rocks of the Daldyn Group of the Anabar Shield: Evidence from U-Pb zircon dating,Sm-Nd and Lu-Hf isotope systematics

The Hf-Nd isotope systematics was used to determine the genesis of zircons from granulites of the Daldyn Group of the Anabar Shield. Obtained age of magmatic crystallization for biotite–hypersthene crystalline schists and garnet amphibolites agree with position of zircons within terrestrial array. Magmatic genesis of plagiogranite neosome under granulite conditions was established for leucocratic plagiogneisses.     

Oceanic and riftogenic metavolcanic associations of greenstone belts in the northwestern part of the Sharyzhalgai Uplift, Baikal region

The Onot and Bulun terranes are confined to the Sharyzhalgai Uplift in the southwestern margin of the Siberian craton. They consist of alternating blocks and nappes of Paleoarchean tonalite-trondhjemite-granodiorite complex and supracrustal metasedimentary-volcanogenic rocks of greenstone belts (GSB). The lower part of the Onot GSB is made up of a bimodal association of aporhyolite microgneisses with subordinate amphibolites, while the upper part consists of amphibolites associated with banded iron formation, metapelites, dolomitic marbles, and magnesites. The Urik GSB in the Bulun block comprises three rock associations: (1) garnet amphibolites and amphibolites alternating with kyanite-bearing mica schists and quartzite schists; (2) garnet-bearing biotite and amphibole crystalline schists with tectonic lenses of garnet amphibolites; (3) biotite and amphibole-biotite orthogneisses and biotite plagiogneisses. The microgneisses (metarhyolitoids) of the Onot belt are correlated with within-plate volcanic rocks and A-type granites. The composition of the amphibolites corresponds to high-Mg low-Ti tholeiitic basalts. The formation of metavolcanic rocks of the Onot GSB was related to the rifting of the Paleoarchean continental crust, which is supported by the formation of felsic metavolcanic rocks from an ancient tonalite source and by the geochemical signatures of crustal contamination of metabasalts. The amphibolites of the Urik GSB are subdivided into three petrogeochemical types. The first and second types correspond to high-and low-Mg tholeiitic basalts and have practically flat multielement patterns. The amphibolites of the third type correspond to subalkaline leucobasalts. Two types of orthogneisses are comparable with intermediate-acid volcanic rocks of the andesite-dacite and adakite series. In terms of geochemistry, the metamafic volcanic rocks of the Urik GSB represent the rocks of the oceanic crust. Oceanic settings of their formation are confirmed by an association of metavolcanic rocks with abyssal distal siliceous-argillaceous deposits. The formation of two types of intermediate-acid metavolcanic rocks of andesite-dacite and adakite associations, as well as garnet-bearing paraschists, was presumably related to subduction settings.     

Prospects of large-scale noble metal mineralization in the Upper Selemdzha minerogenic zone (by the example of the Malomyr and Sagur-Semertak ore clusters)

The Upper Selemdzha minerogenic zone, which has the potential for noble metal mineralization in the Upper Amur, has been studied. It is confined to the thick strata of Paleozoic terrigeneous-sedimentary rocks (sandstones, siltstones, clay schists, packages of their thin rhythmical interlayering, sedimentary breccias, and conglomerates) in the Tokura subzone of the Selemdzha-Kerbinsk zone in the Amur-Okhotsk area of the Mongol-Okhotsk system, overlain by Early Cretaceous volcanogenic sedimentary rocks and intruded by Early Cretaceous quartz diorites, granodiorites, and dacites. In the Malomyr and Sagur-Semertak ore clusters, there are a few known potential deposits of noble metals and ore occurrences; when studied and prospected, they can exhibit vein-veinlet noble metal mineralization with rich contents of gold and platinoids in ore columns and metasomatic deposits.     

The U–Pb age and Lu–Hf isotope composition of detrital zircon from metasedimentary rocks of the Onot greenstone belt (Sharyzhalgay uplift,southern Siberian craton)

We present data on the composition of metasedimentary rocks from the greenstone belt of the Onot terrane (Sharyzhalgay uplift) and results of U–Pb dating (SHRIMP II) and Lu–Hf isotope study of detrital zircon from garnet–staurolite schists. The metasedimentary rocks of the Onot greenstone belt are dominated by garnet- and staurolite-bearing schists alternating with amphibolites (metabasalts) in the upper part of the section. Compositionally the protoliths of garnet–staurolite schists correspond to sedimentary rocks, ranging from siltstone to pelitic mudstone. The trace-element characteristics of the garnet–staurolite schists indicate that the terrigenous material was derived from three different rock types, such as tonalite–trondhjemite plagiogneisses (elevated Gd/Yb ratios), mafic rocks (elevated Cr/Th ratios and reduced Th/Sc ratios), and felsic igneous rocks formed by crustal melting (the presence of a Eu minimum), which agrees with the set of potential source rocks from the Onot terrane. The age of predominant detrital zircon reflects the erosion of mainly Neoarchean igneous rocks; this fact, combined with the poor rounding of zircon and tectonically active sedimentation conditions accompanied by mafic volcanism, suggests that the probably depositional age is ca. 2.7 Ga. Older source rocks (2.80–3.35 Ga) contributed to the sediment deposition along with the Neoarchean ones. According to the Hf isotope composition of detrital zircon from the garnet–staurolite schists, the source provenances had different crustal prehistories. The source provenances include Paleoarchean and juvenile Neoarchean crust and rocks formed by the mixing of melts from ancient and juvenile crustal sources.     

Petrogeochemistry of rocks of the Ulitarechka Sequence,a new upper Archean lithostratigraphic unit in the central part of the Kola Peninsula

Fine-grained garnet amphibolites, amphibole plagioschists, biotite-amphibole, and amphibole-biotite, epidote-biotite and biotite plagiogneisses were identified during geological mapping in the Ulita River basin in the central part of the Kola Peninsula. These rocks bear evidence of volcanogenic origin and were combined into a single lithostratigraphic unit named the Ulitarechka Sequence. On the basis of the composition of rock-forming minerals, the metamorphism of amphibolites of the Ulitarechka Formation was determined as transitional between the amphibolite and granulite facies. In terms of petrography, chemistry, and mineralogy, the rocks of the Ulitarechka Formation are well correlated with the amphibolites of the upper Archean Voche-Lambina Sequence composing the base of the Lopian Tersk-Allarechensk greenstone belt. The REE distribution patterns of the Ulitarechka Sequence are similar to those of amphibolites of the Purnach and Kuksha formations of the lower part of the Paleoproterozoic Imandra-Varzuga rift structure, suggesting their derivation from a similar magmatic source. Since the boundary between the Kola and Belomorian blocks is marked by the Lopian rocks of the Tersk-Allarechensk belt, it can be shifted to the western contact of the recognized stratified rocks of the Ulitarechka Sequence.     

Petrology and geochemistry of the Cretaceous granitoid magmatism of Central Kamchatka,exemplified by the Krutogorova and Kol’ intrusive complexes

The problem of the geochemical classification of granitoid magmatism in the zone of interaction of oceanic and continental plates is considered in this paper by the example of Mesozoic granitoids of the Krutogorova and Kol’ intrusive complexes of the Sredinny Range, Kamchatka. Based on new geological, petrological, and geochemical data (including the Sr, Nd, and Pb isotope systematics of rocks), it was shown that the protoliths of the granitoids were volcanic-terrigenous sequences accumulated within a Cretaceous marginal basin in the eastern Asian continent. The granitoids crystallized at ～80 Ma (SHRIMP U-Pb age) under the conditions of the andalusite-sillimanite depth facies corresponding to a pressure of approximately 2 kbar and induced contact metamorphism in the host sequences, which are made up of sediments with sheetlike bodies of mafic and ultramafic volcanics (Kikhchik Group and its metamorphic analogues of the Kolpakova, Kamchatka, and Malki groups). The lower age boundary of sedimentation of the host sequences and the time of basic volcanism coincide with the beginning of the formation of the Okhotsk-Chukotka volcanic belt. Such a correlation is not accidental and reflects a genetic connection between the processes of magmatic activation in the continental-margin sedimentary basin and the formation of the continental margin volcanic belt in eastern Asia. The development of basic volcanism in the sedimentary basin accompanied by the ascent of deep fluids resulted in the entrainment of crustal materials into magmatic processes and the formation of crustal magma chambers, the activity of which was manifested by the eruption of intermediate and silicic lavas and emplacement of shallow granitoid intrusions of considerable areal extent. These intrusions induced contact metamorphism in the enclosing volcanosedimentary complexes. The subsequent Eocene (60-50 Ma) collision processes related to the obduction of the oceanic segment of the crust of the transitional zone onto the Asian continental margin resulted in the tectonic piling of the rocks of Central Kamchatka and strong crustal thickening, which was favorable for its metamorphic alteration reaching the kyanite-sillimanite depth level of the amphibolite facies under the influence of a thermal front and deep fluids affecting lower crustal zones. The Eocene regional metamorphism caused not only metamorphic transformations, migmatization, and granitization in the sequences of the Sredinny Range, which underwent only contact hornfels formation during the first stage, but also metamorphism, migmatization, and extensive foliation in the igneous rocks of the Kol’ and Krutogorova complexes, which were transformed into gneissic metagranites.     

Two stages of granite formation in the Sredinny Range, Kamchatka: Tectonic and geodynamic setting of granitic rocks

The newly formed continental crust in southern Kamchatka was created as a result of the Eocene collision of the Cretaceous-Paleocene Achaivayam-Valagin island arc and the northeastern Asian margin. Widespread migmatization and granite formation accompanied this process in the Sredinny Range of Kamchatka. The tectonic setting and composition of granitic rocks in the Malka Uplift of the Sredinny Range are characterized in detail, and the U-Pb (SHRIMP) zircon ages are discussed. Two main stages of granite formation—Campanian (80–78 Ma ago) and Eocene (52 ± 2 Ma ago) have been established. It may be suggested that granite formation in the Campanian was related to the partial melting of the accretionary wedge due to its under-plating by mafic material or to plunging of the oceanic ridge beneath the accretionary wedge. The Eocene granitic rocks were formed owing to the collision of the Achaivayam-Valagin ensimatic island arc with the Kamchatka margin of Eurasia. In southern Kamchatka (Malka Uplift of the Sredinny Range), the arc-continent collision started 55–53 Ma ago. As a result, the island-arc complexes were thrust over terrigenous sequences of the continental margin. The thickness of the allochthon was sufficient to plunge the autochthon to a considerable depth. The autochthon and the lower portion of the allochthon underwent high-grade metamorphism followed by partial melting and emplacement of granitic magma 52 ± 2 Ma ago. The anomalously rapid heating of the crust was probably caused by the ascent of asthenospheric magma initiated by slab breakoff, while the Eurasian Plate plunged beneath the Achaivayam-Valagin arc.     

Lower Proterozoic orthorocks in the Svecofennides of the Savo belt (Western Ladoga Region): Geochemical properties

On the basis of petrochemical data, orthorocks are defined among highly metamorphosed sequences of the Landenpokh’ya Group attributed to the Kalevian and Svecofennian systems in the Ladoga region. The contents of major and trace elements in orthorocks, which constitute some volcanogenic sedimentary complexes in the western Ladoga region, are discussed. The volcanics of the first complex (Kukhka Island area) belong to the calc-alkaline series and are characterized by the presence of OIB-type basaltic andesites. The basites of the second complex (Kil’pola Island area) are attributed to the tuffaceous rocks of the WPB-type tholeiitic series. The volcanics of the third complex (Kuznechnoe-Khiitola area) are largely represented by dacites referred to the Svecofennian mature island arc. The presented geological and geochemical data imply that the first and second complexes, which include intraplate volcanics, could be formed on the basite Jatulian-Ludikovian basement (protolith). These Kalevian rocks of the western Ladoga region are correlated with the volcanogenic sedimentary complexes in the southern part of the Savo belt in Finland and belong to the Karelian province.     

New age data on the magmatic rocks from the western sector of the Okhotsk-Chukotka volcanogenic belt

New age data are reported for the magmatic rocks from the western flank of the Okhotsk-Chukotka volcanogenic belt consisting of the Ul’ya and Kuidusun volcanic zones. Four U-Pb SHRIMP zircon dates within 106-90 Ma were obtained for the upper part of the Emanrin Formation and the lower part of the Ul’ya Group of the Ul’ya volcanic zone. Large Verkhneallakh and Atarbai subvolcanic intrusions from the Kuidusun volcanic zone were dated using the Rb-Sr method at 115–107 Ma and 85 Ma, respectively. U-Pb dating of zircon microsamples from adamellites and diorites of the Sizindzha Massif yielded, respectively, ages of 91 and 90 Ma. New geochronological data indicate that the Selitkan-Sungari and Okhotsk-Chukotka volcanogenic belts are coeval and can be united in a common Okhotsk-Sungari system of volcanic belts and zones (megabelt).     

Petrological-geochemical features of the Cretaceous and Cenozoic intrusive magmatism of Kamchatka,the melt sources,and the geodynamic settings

A comparative analysis of the geological setting and composition was carried out for the Cretaceous, Eocene, and Miocene-Pliocene granitoids of Kamchatka. New petrochemical, geochemical, and isotope data are reported. The alkaline granitoids and granites of the Sredinny Range in Kamchatka have an enriched isotope composition and elevated contents of Rb, Th, U, and LREE as compared to their analogs in the eastern part of the region. The largest scale Cretaceous crustal magmatism was formed in a setting of intense tectonic motions and metamorphism. The smaller scale Eocene magmatism produced crustal granitoid melts in the Sredinny Range of Kamchatka and mantle initially basaltic melts that evolved to granites in the southeastern Kamchatka and Ganalsky Range. These processes were accompanied by the rejuvenated of the older crust and the local formation of a new crust. The low-volume crustal-mantle Miocene-Pliocene magmatism of variable composition was developed in volcanic belts, forming the upper crustal horizons on the existing crystalline basement.     

Granulites of the Kolpakovskaya Series in the sredinny range,Kamchatka: A myth or reality?

Geological, mineralogical, and geothermobarometric data testify that the regional metamorphism of the terrigenous protolith of the Kolpakovskaya Series, which composes the stratigraphic basement of the Kamchatka Median Crystalline Massif, corresponded to the kyanite mineral subfacies of the amphibolite facies at temperatures of 560–660°C and pressures of 5.9–6.9 kbar. This metamorphism predetermined wide kyanite development in high-Al garnet-biotite plagiogneisses. The younger granitization and migmatization of the plagiogneisses took place at a decrease in the pressure (depth), as follows from the textures of kyanite reaction replacement by andalusite in both the metamorphic rocks and the vein synmetamorphic granitoids and pegmatites. The temperature of the granitization and migmatization processes in the plagiogneisses was estimated at 620–650°C, and the pressure was evaluated at 1.9–3.0 kbar. Acid leaching that accompanied the granitization and migmatization processes resulted in the intense replacement of biotite by sillimanite (fibrolite) and, to a lesser degree, muscovite in the metamorphic and vein magmatic rocks. The highest temperature orthopyroxene-cordierite-biotite-orthoclase-plagioclase-quartz mineral assemblages were determined to have been formed in the Kolpakovskaya Series at a temperature of 830–840°C not by the regional metamorphism but in contact aureoles around gabbro-granitoid intrusions of the Lavkinskii intrusive complex of Oligocene-Miocene age in garnet-biotite and kyanite-garnet-biotite plagiogneisses of the amphibolite facies and cannot thus be regarded as evidence of an early granulite stage in the metamorphism of these rocks.     

Finite Strain Analysis of the Wadi Fatima Shear Zone in Western Arabia,Saudi Arabia

Neoproterozoic rocks, Oligocene to Neogene sediments and Tertiary Red Sea rift-related volcanics (Harrat) are three dominant major groups exposed in the Jeddah tectonic terrane in Western Arabia. The basement complex comprises amphibolites, schists, and older and younger granites unconformably overlain by a post-amalgamation volcanosedimentary sequence (Fatima Group) exhibiting post-accretionary thrusting and thrust-related structures. The older granites and/or the amphibolites and schists display mylonitization and shearing in some outcrops, and the observed kinematic indicators indicate dextral monoclinic symmetry along the impressive Wadi Fatima Shear Zone. Finite strain analysis of the mylonitized lithologies is used to interpret the deformation history of the Wadi Fatima Shear Zone. The measured finite strain data demonstrate that the amphibolites, schists, and older granites are mildly to moderately deformed, where XZ (axial ratios in XZ direction) vary from 2.76 to 4.22 and from 2.04 to 3.90 for the R_f/φ and Fry method respectively. The shortening axes (Z) have subvertical attitude and are associated with subhorizontal foliation. The data show oblate strain ellipsoids in the different rocks in the studied area and indication bulk flattening strain. We assume that the different rock types have similar deformation behavior. In the deformed granite, the strain data are identical in magnitude with those obtained in the Fatima Group volcanosedimentary sequence. Finite strain accumulated without any significant volume change contemporaneously with syn-accretionary transpressive structures. It is concluded that a simple-shear deformation with constant-volume plane strain exists, where displacement is strictly parallel to the shear plane. Furthermore, the contacts between various lithological units in the Wadi Fatima Shear Zone were formed under brittle to semi-ductile deformation conditions.     

Late Jurassic-Neocomian volcanism of the northern Okhotsk region: Geology,tectonic settings,and mineralization

The paper deals with new data on the geology, geochronometry, and petrogeochemistry of the Upper Jurassic-Neocomian basaltic andesites constituting the base of the Cretaceous volcanogenic section in the northern Okhotsk region. The geochronometric data confirm the Late Jurassic-Neocomian age of these rocks (147–138 Ma, K-Ar method). Their characteristic features provide grounds for considering these rocks as fragments of the Uda volcanic belt. The comparison of the petrogeochemical characteristics with similar parameters of other different-age tectonic structures of the region under consideration that were formed in different geodynamic environments (intracontiental and continental marginal volcanogenic belts, island arcs) revealed similarity between the conditions of the formation of the Late Jurassic-Neocomian (Uda belt) and Albian-Cenomanian (Okhotsk-Chukotka belt) volcanics: both characterize continental marginal environments. It is assumed that the Late Jurassic-Neocomian volcanism associates with Au-Ag mineralization, which is also expected in other similar although poorly known volcanic structures.     

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.     

Late Cretaceous Kholokhovchan Flora of Northeastern Asia: Composition,age and fossil plant descriptions

The Kholokhovchan Flora comes from tuffaceous – terrigenous deposits of the Vetvinskaya Member (Chalbugchan Group) in the Penzhina and Oklan rivers interfluve, Northeastern Russia. The depositional environment of the plant-bearing deposits is interpreted to have been a freshwater lake. The Kholokhovchan Flora hosts 42 fossil plant species belonging to Marchantiopsida, Polypodiopsida, Ginkgoales, Leptostrobales, Bennettitales, Pinales and Magnoliopsida. It is characterised by diverse angiosperms, less diverse conifers and ferns, by the presence of relatively ancient Sphenobaiera, Phoenicopsis and Pterophyllum together with advanced Late Cretaceous Taxodium, Glyptostrobus and angiosperms, among which platanoids are quite diverse. The Kholokhovchan Flora is most similar to Penzhina and Kaivayam floras of the Anadyr-Koryak Subregion and Arman Flora of the Okhotsk-Chukotka volcanogenic belt (Northeastern Russia) and should be dated as Turonian–Coniacian. The Kholokhovchan Flora, that populated volcanic plateaus and intermontane valleys, are characterised by a mixture of ancient “Mesophytic” plants with typical Late Cretaceous “Cenophytic” taxa. This peculiar composition probably reflects a gradual penetration of new angiosperm-dominated plant assemblages into older floras: during the Late Cretaceous, “Cenophytic” assemblages migrated along river valleys and other disturbed habitats into the interior of Asia, eventually occupying volcanogenic uplands, and in places replacing the “Mesophytic” fern-gymnospermous communities that existed there. Two new angiosperm species, as well as four the most characteristic conifers of the Kholokhovchan Flora, are described: Cupressaceae gen. et sp. indet. cf. Widdringtonites sp., Taxodium cf. olrikii, Taxodium sp., Glyptostrobus sp., Ettingshausenia vetviensis sp. nov. and Parvileguminophyllum penzhinense sp. nov.     

Proterozoic metavolcanics from western Sierras Pampeanas terrane, Argentina

A series of medium grade metamorphic rocks of the western sector of the Sierras Pampeanas Terrane in central western Argentina are represented by amphibolites, gneisses and schists derived from sedimentary as well as from igneous rocks. The metavolcanics consist of amphibolites, quartz-K-feldspar-muscovite schists, and hornblende-biotite and biotite-epidote-plagioclase schists. Based on petrographic and geochemical data they are interpreted as originating as basaltic tholeiites, rhyolites and mesosilicic volcanics. The distribution and geochemical behavior are similar to present day western Pacific lavas, mainly those developed on island arcs or heavily attenuated continental crust. Based on these characteristics, an accretionary tectonic model involving a series of island-arc collisions is proposed for the Proterozoic. The complex Proterozoic tectonic history of the western Sierras Pampeanas has been partially obliterated by the emplacement of the Early Paleozoic magmatic arc rocks.     

Volcanism of the southern part of the Sredinny Range of Kamchatka in the Neogene-Quaternary

Based on the geochemical characteristics of the Miocene-Quaternary volcanic rocks of the Sredinny Range of Kamchatka, we divide it into northern and southern provinces; the latter comprises the “eastern”, “western”, and “central” flanks. We present new data on the composition of Neogene-Quaternary volcanic rocks in the southern part of the Sredinny Range of Kamchatka: Khangar and Icha volcanic massifs and Mt. Yurtinaya on the “western” flank, Bystrinsky and Kozyrevsky Ridges on the “eastern” flank, and Anaunsky Dol and Uksichan massif located in between. We show systematic differences in the composition of rocks from the “western” and “eastern” flanks. During the Neogene, a typical island-arc volcanism took place within the “eastern” flank. Quaternary volcanic rocks of this area have both island-arc and within-plate geochemical features. We propose to call rocks of this type hybrid rocks. Within the “western” flank, hybrid volcanism has been manifested since the Neogene, while typical island-arc rocks are not found. Magma generation processes on the “western” flank of the Sredinny Ridge are influenced by an enriched mantle source; the effect of fluid is less pronounced here as compared to the rocks of the “eastern” flank, where it is clearly traced.