P. Kepezhinskas

Summary: The Kamchatka Peninsula of northeastern Russia is located along the northwestern margin of theBering Seaandconsists of zones of complexly deformed accreted terranes, Along thenorthern portion of the peninsula, pro-gressing from thenorthwestem Bering ...

Abstract High-potassium magmas were erupted in the western and eastern volcanic zones of the northern segment of the Kamchatka arc (Russia) during late Eocene to Pliocene time. Shoshonite suites of similar age show across-arc trends in major and trace element chemistry suggesting their derivation from different mantle sources. Low-Ti shoshonites from the western (rear-arc) volcanic zone are thought to have been generated in a refractory mantle wedge affected by the addition of an LILE- and LREE-enriched component possibly identified as the slab-derived hydrous fluid. High-Ti shoshonites from the eastern (frontal arc) volcanic zone were probably derived through partial melting of a more fertile source modified by the addition of a high-Na component as suggested by major and trace element systematics and radiogenic isotope relationships. This component is compositionally similar to a felsic trondhjemitic melt generated during partial melting of the hot downgoing slab in the amphibolite facies. The volcanism in the northeastern zone of the Kamchatka arc is associated with the subduction of hot oceanic lithosphere of the Komandor Basin which was generated during two stages of extension (late Eocene-Oligocene and late Miocene-Pliocene). Slab-derived melts (adakites) were erupted along with Neogene high-Ti shoshonites in the eastern volcanic zone of the northern Kamchatka arc. Two types of shoshonites can be found in volcanic arcs. Low-Ti shoshonites are related to the temporal evolution of the subduction zone and occur at the latest stages of its development following predicted depth-K relationships. High-Ti shoshonites are associated with melting of a slab-modified mantle during the earliest stages of intra-arc rifting followed by the eruption of high-Na, Nb-enriched arc basalts. Nb-enriched arc basalts and high-Ti shoshonites are spatially and temporally associated with slab melts (adakites) in the northern segment of the Kamchatka arc.

Small-scale variations in composition of mantle-derived peridotites have been investigated in the 0°-15°N portion of the Mid-Atlantic Ridge (MAR), thanks to a relatively close-spaced peridotite sample coverage achieved by combining samples collected by Russian and U.S. expeditions. Areal variations in the composition of mantle-equilibrated minerals olivine, orthopyroxene, clinopyroxene, and spinel have been interpreted as due primarily to regional variations in the initial composition, degree of partial melting, and thermal structure of the upper mantle. Mantle rocks from the eastern part of the Romanche transform frequently contain a trapped fraction of basaltic melt, while undepleted mantle prevails in the western part of the Romanche, suggesting a ``cold'' upper mantle thermal regime in this region, which prevented significant melting. Immediately to the north, the St. Paul Fracture Zone (FZ) upper mantle shows intermediate degrees of melting, except for St. Peter-Paul Island which exposes metasomatized mantle rocks chemically and isotopically different from other oceanic peridotites. Between St. Paul FZ and 4°N (Strakhov FZ) we have an area of strongly depleted upper mantle. Farther north the Doldrums FZ area (~8°N) appears to be underlain by moderately depleted upper mantle with some melt entrapment. The Vema FZ (11°N) is underlain by relatively homogeneous upper mantle which has undergone a rather low degree of melting. The Mercurius and Marathon transforms (between 12° and 13°N) expose moderately depleted peridotites.Finally, the 15°20' FZ area shows relatively undepleted upper mantle on the northern side of the transform and at sites distant from the MAR axis and strongly depleted mantle south of the transform. The strongly depleted mantle from the 2°-3°N and 14°-15°N regions is associated spatially with light rare earth element enriched mid-ocean ridge basalt showing a ``hot spot''-type geochemical signature. The areal association of refractory peridotites with enriched basalt and with zero-age topographic highs in the 2°-3°N and 14°-15°N regions can be explained either by the influence of mantle thermal plumes or by the presence in the mantle of metasomatized, H2O-rich domains which would cause enhanced melting and provide a source for basalt enrichment. These mantle domains might be relicts of an originally subcontinental mantle.

ABSTRACT: The prospect of partial melting of the subducted oceanic crust to produce arc magmatism has been debated for over 30 years. Debate has centred on the physical conditions of slab melting and the lack of a definitive, unambiguous geochemical signature and petrogenetic process. Experimental partial melting data for basalt over a wide range of pressures (l-32kbar) and temperatures (700-1150°C) have shown that melt compositions are primarily trondhjemite-tonalite-dacite (TTD). High-Al (> 15 % A12O3 at the 70 % SiO2 level) TTD melts are produced by high-pressure (>15kbar) partial melting of basalt, leaving a restite assemblage of garnet + clinopyroxene + hornblende. A specific Cenozoic high-Al TTD (adakite) contains lower Y, Yb and Sc and higher Sr, Sr/Y, La/Yb and Zr/Sm relative to other TTD types and is interpreted to represent a slab melt under garnet amphibolite to eclogite conditions. High-Al TTD with an adakite-like geochemical character is prevalent in the Archean a...

Plutonic associations in orogenic belts are potent indicators of multi-stage fractionation of primitive arc magmas and growth of island-arc crust under evolving redox conditions in ancient subduction zones. Ildeus-Lucha ultramafic–mafic complex (ILC) was emplaced at 232–233 Ma within the Mesozoic Stanovoy convergent margin and subsequently underwent multi-stage hydrothermal alteration and greenschist to amphibolite facies metamorphism (~140 Ma), followed by adakite and K-lamprophyre magmatism (114–117 Ma). Dunite, wehrlite, pyroxenite and gabbro in the ILC are composed of olivine, clinopyroxene, orthopyroxene, plagioclase and late-magmatic amphibole. Mineral compositions in the ILC are typical of island-arc plutonic complexes characterized by early crystallization of orthopyroxene, plagioclase intercumulus and presence of magmatic amphibole. Ultramafic-mafic rocks display high-field strength (HFS) element depletions coupled with large-ion lithophile (LIL) element enrichments, indica...

The Urals are a long (3,000×200 to 400 km) Paleozoic belt separating Europe from Asia characterized by a narrow strip of low temperature/high pressure (LT/HP) metamorphic rocks outcropping along a major oceanic suture, the Main Uralian Fault. We have dated the HP metamorphism in the southwestem Urals by 39 Ar/ 40 Ar on phengites at 380 Ma. We propose that it was generated during eastward subduction of the East European Paleozoic passive margin below the oceanic mantle during the Devonian. The HP rocks were later exhumed (380 to 250 Ma) by the combination of westward thrusting and eastward normal faulting on the Main Uralian Fault

Fragments of Paleoproterozoic LIPs in the Eastern Fennoscandia: implications for the life span of the Lapland-Kola Ocean A.V. STEPANOVA*, A.V. SAMSONOV, E.B. SALNIKOVA, A.A. ARZAMASTSEV, I.S. PUCHTEL, P.K. KEPEZHINSKAS, S.V. EGOROVA, YU.O. LARIONOVA, K.G. EROFEEVA 1IGEM RAS, Staromonetny per., 35, Moscow, Russia, (*correspondence: stepanov@krc.karelia.ru) 2IPGG RAS, Makarova Emb., 2, St.-Petersburg, Russia 3University of Maryland, College Park, MD 20742, USA 4Kimberlitt AS, Oslo, Norway

Numerous silicate, Fe-oxide, and Cu–Au–Ag microspherules are found in Fe ores and pyroclastic rocks of the Kostenga deposit, Lesser Khingan (Far East of Russia). The silicate spherules contain immiscible Fe and Si glasses, vapor voids, and mineral inclusions. The Fe-oxide spherules host magnetite with a small amount of ilmenite and Fe-rich silicate glass. The Cu–Au–Ag spherules contain inclusions with dominant CuO in their composition. These microspherules are considered to form due to rapid uplift and degassing of liquation-differentiated ore-silicate melts. The possible volcanic formation of ores and precious metal mineralization is discussed for these types of the deposits.

Adakites were proposed over a decade ago to be products of the melting of young subducted oceanic crust. In fact, several new localities have been discovered since the original work documented approximately ten localities in modern arcs (e. g., southwestern Japan, Trans Mexican Volcanic Belt, etc.). But work over the past ten years has also shown that adakites can be generated by other processes during subduction (e. g., along the edge of tears in the subducting slab, remnant slabs left in the upper mantle, etc.). In addition, adakites appear to be associated with a suite of rocks including high-Mg andesites resulting from either adakite interaction with the mantle (Adak-type) or melting of the mantle during adakite interaction (Piip-type), niobium enriched arc basalts (NEAB) that are believed to be derived from the partial melting of a mantle metasomatized extensively by adakites, and possibly boninites (several researchers have found an adakite component in boninites). A new rock ...

The Kamchatka Peninsula of northeastern Russia is located along the northwestern margin of the Bering Sea and consists of zones of complexly deformed accreted terranes. Along the northern portion of the peninsula, progressing from the northwestern Bering Sea inland the Olyutorskiy, Ukelayat, and Koryak superterranes are accreted to the Okhotsk-Chukotsk volcanic-plutonic belt in northern-most Kamchatka. A sedimentary sequence of Albian to Maastrichtian age overlap terranes and units of the Koryak superterrane and constrains their accretion time with this region of the North America plate. Ophiolite complexes, widespread within the Koryak superterrane, are associated with serpentinite melanges and some of the ophiolite terranes include large portions of weakly serpentinized hyperbasites, layered gabbro, sheeted dikes, and pillow basalts outcropping as internally coherent blocks within a sheared melange matrix. Interpretation of magnetic anomalies allow the correlation of the Ukelayat ...

Abstract We report in this paper an unusual occurrence of platinum-group minerals in evolved explosive breccia associated with the Poperechny iron-manganese deposit (Lesser Khingan Range, Far East Russia). PGMs in andesite breccia are represented by Fe-Pt solid solutions (85%) and PGM (mostly Os-Ir-Ru) solid solutions, sulfides and sulfarsenides (15%). Textural and compositional variations in PGM assemblages suggest that Pt-Fe and Os-Ir-Ru solid solutions, as well as erlichmanite-laurite series sulfides were formed during high-temperature fractionation of mantle-derived mafic parental melt (similar to Alaskan-type complexes) and were entrained in the evolved andesitic melt during its emplacement in the crust. Pd-Pt plumbostannide and copper-gold solid solutions reflect late magmatic re-crystallization and metasomatism. Early Cretaceous (~125 Ma) age of ferroplatinum in the explosive breccia suggests that PGM-bearing ultramafic material could have been sampled during regional slab-window tectonics related to the Late Mesozoic subduction of Izanagi plate along southern margin of the North Asian continent.

Geology of the Norwegian Lapland is dominated by diverse Archean crystalline basement complexes superimposed with Proterozoic greenstone belts. Isotopic dating of detrital zircons from basement gneisses in the Kirkenes area establishes presence of Early Archean (3.69 Ga) crustal component as well as three major episodes of crustal growth at 3.2 Ga, 2.7-2.9 Ga and 2.5 Ga. Precambrian terranes are intruded by ultramafic-mafic dikes and sills that range in composition from komatiites and ultramafic-mafic lamprophyres to high-Mg basalts and low-Ti subalkaline basalts. Geochemical characteristics of these rocks fall into three principal groups: 1) enriched compositions with high Nd, Nb, Hf, Zr and Th concentrations and elevated La/Th and Nb/Th coupled with low La/Nb, Ba/Nb and U/Nb ratios; 2) compositions depleted in Th, Hf and Nb together with low LREE/HFSE (such as La/Nb) and LILE/HFSE (such as Ba/Nb and U/Nb) ratios; 3) transitional group clearly identified by marked depletions in Ti,...

A suite of adakites, Nb-enriched arc basalts (NEABs), and mantle pyroxenites from Kamchatka have been analyzed for Os and Sr isotopes in order to investigate the relationship between the lavas and their respective veined-mantle source regions, and the extent to which crustal contamination has contributed to the enriched isotopic signatures in the lavas. The mantle pyroxenites range in 187Os/188Os from 0.1280 to 0.4138, and in 87Sr/86Sr from 0.70305 to 0.70339. The adakites and NEABs for which both Sr and Os data are available fall within the compositional range of the pyroxenites for 187Os/188Os, varying from 0.1384 to 0.2983, and have 87Sr/86Sr ranging from 0.70294 to 0.70320. Os abundances in these lavas range from 12 to 112 ppt. Two distinct positive Os-Sr isotope correlations are observed amongst these samples, including a shallower trend that comprises pyroxenites and lavas from the northern arc front, and a steeper trend that comprises pyroxenites and lavas from behind the arc...

The prospect of partial melting of the subducted oceanic crust to produce arc magmatism has been debated for over 30 years. Debate has centred on the physical conditions of slab melting and the lack of a definitive, unambiguous geochemical signature and petrogenetic ...

Summary The Upper Cretaceous oceanic and island-arc terranes of Northeastern Kamchatka (Russia) are intruded by numerous ultramafic and mafic to intermediate plutons. Ultramafic plutons north of Epilchik Lake show a concentric structure: dunites in the core pass progressively outwards into wehrlites and pyroxenites and into gabbros along the rim which are occasionally phlogopite- and amphibole-bearing. Compositional layering and mineral orientations marked by spinel clusters in the dunites and by acicular amphiboles in the marginal gabbros are parallel to the contacts. The mafic to intermediate plutons studied in the Machevna area show similar concentric structure with a dioritic core and a gabbroic rim. Magmatic flow lines are near-vertical in the central part of the plutons. These internal structures and general geometry of the plutons suggest vertical magma intrusion into mega-tension gashes developed initially at a high angle to the subduction zone.

The Kamchatka arc (Russia) is located in the northwestern Pacific Ocean and is divided into three segments by major sub-latitudinal fault zones (crustal discontinuities). The southern (SS) and central (CS) segments are associated with the subduction of old Pacific ...