G. V. Ledneva

High-pressure ultramafics in the lower crustal rocks of the Pekul’ney complex, central Chukchi Peninsula. 1. Petrography and mineralogy

Dunites, peridotites, olivine and spinel pyroxenites, and metagabbroids have been described in the tectonic blocks of the Pekul’ney complex of the central Chukchi Peninsula together with garnet-hornblende-clinopyroxene and zoisite (clinozoisite)-garnet-hornblende rocks, which are indicative of high-pressure complexes. However, the interpretations of previous researchers on the composition, structure, setting, and processes of formation of this rock association are highly controversial. The petrographic and mineralogical results reported in this paper indicate that the blocks of the complex host bodies of cumulate ultramafics among metamorphic rocks. These relationships were supported by the finding of xenoliths and xenocrysts of metamorphic rocks in the ultramafics. The metamorphic country rocks are lower crustal amphibolites and schists with peak metamorphic parameters corresponding to the high-pressure portion of the epidoteamphibolite facies (610–680°C and 9–14 kbar). All the varieties of ultramafic rocks studied in the blocks of the complex are assigned to a single cumulate series (from dunite to clinozoisite-garnet hornblendite), and the compositions of their primary minerals show regular correlations similar to crystallization differentiation trends. Specific features of the ultramafics of the Pekul’ney complex are the early crystallization of hornblende (which is present already in peridotites), wide range of garnet crystallization (associating with clinopyroxene, ceylonite, and hornblende), presence of magmatic clinozoisite in the most evolved assemblages (with garnet, hornblende, and clinopyroxene), and absence of evidence for plagioclase crystallization. Clinopyroxene from the most evolved ultramafic rocks contains more than 15 wt % Al2O3. The classification of the rocks of the complex provides a basis for the interpretation of geological relations between them and the elucidation of the characteristics of the internal structure of the blocks of the complex and bodies of cumulate ultramafic rocks in them.

U-Pb zircon age of gabbroids of the Ust’-Belaya mafic-ultramafic massif (Chukotka) and its interpretation

The Ust’-Belaya mafic-ultramafic massif is assigned to the Western Koryak fold belt and largely composed of residual spinel peridotites, layered spinel and plagioclase peridotites, and gabbros. These rocks are crosscut by occasional plagiogranite and diorite veins and exhibit locally a close spatial association with basalts and carbonate-sedimentary deposits of Late Devonian and Early Carboniferous age. Based on this evidence, the massif was ascribed to the pre-Late Devonian ophiolite association. Our study presents new U-Pb SHPIMP II zircon ages and petrographic and mineralogical data on samples of the layered amphibole gabbro and vein diorite from the Ust’-Belaya massif. The approximate concordant U-Pb age corresponding to a timing of of amphibole gabbro crystallization is 799 ± 15 Ma, and the concordant U-Pb age reflecting a timing of of vein diorite crystallization is 575 ± 10 Ma. These ages coupled with geological studies of the massif, petrological and mineralogical investigations of the dated samples, as well as literature data on the petrology of peridotites and the age of formed plagiogranites suggest that the peridotites and layered gabbros of the Ust’-Belaya massif were formed by the Late Riphean, whereas the vein diorite and plagiogranite were resulted from a later (Vendian-Cambrian) magmatic stage. The peridotites and gabbros of the massif display no genetic relationship with spatially associated basalts and sedimentary rocks and, thus, they cannot be considered as members the pre-Late Devonian ophiolitic association. The results of this study will inevitably lead to a significant revision of geological and geodynamic interpretations of the Ust’-Belaya mafic-ultramafic massif. However, uneven study of the Precambrian complexes of the Koryak and Chukchi areas, their evolution in different structures of the region cannot yet be described by a single geodynamic scenario.

New data on the age and genesis of igneous rocks in the Kolyuchinskaya Guba (eastern Chukotka)

New Data on the Age and Genesis of Igneous Rocks in the Kolyuchinskaya Guba (Eastern Chukotka)

Calc-Alkaline Magmatism of the Omgon Range: Evidence for Early Paleogene Extension in the Western Kamchatka Segment of the Eurasian Continental Margin

The hypabyssal rocks of the Omgon Range, western Kamchatka, that intrude Upper Albian-Lower Campanian deposits of the Eurasian continental margin belong to three coeval (62.5–63.0 Ma) associations: (1) ilmenite gabbro-dolerites, (2) titanomagnetite gabbro-dolerites and quartz microdiorites, and (3) porphyritic biotite granites and granite-aplites. The Early Paleocene age of the ilmenite gabbro-dolerites and biotite granites was confirmed by zircon and apatite fission-track dating. The ilmenite and titanomagnetite gabbro-dolerites were produced by the multilevel fractional crystallization of basaltic melts with, respectively, moderate and high Fe-Ti contents and the contamination of these melts with rhyolitic melts of different compositions. The moderate-and high-Fe-Ti basaltic melts were derived from mantle spinel peridotite variably depleted and metasomatized by slab-derived fluid prior to melting. The melts were generated at variable depths and different degrees of melting. The biotite granites and granite aplites were produced by the combined fractional crystallization of a crustal rhyolitic melt and its contamination with terrigenous rocks of the Omgon Group. The rhyolitic melts were likely derived from metabasaltic rocks of suprasubduction nature. The Early Paleocene hypabyssal rocks of the Omgon Range were demonstrated to have been formed in an extensional environment, which dominated in the margin of the Eurasian continent from the Late Cretaceous throughout the Early Paleocene. Extension in the Western Kamchatka segment preceded the origin of the Western Koryakian-Kamchatka (Kinkil’) continental-margin volcanic belt in Eocene time. This research was conducted based on original geological, mineralogical, geochemical, and isotopic (Rb-Sr) data obtained by the authors for the rocks.

High-pressure ultramafics in the lower crustal rocks of the Pekul’ney complex, central Chukchi Peninsula. 2. Internal structure of blocks and ultramafic bodies, geologic and geodynamic setting of rock formation

The detailed mapping of the blocks of the Pekul’ney complex revealed that cumulate ultramafics occur as separate tabular bodies among metamorphic rocks and are only fragmentarily observed in some of the blocks. Within these bodies, different types of ultramafics are regularly and multiply intercalated, forming banded structures, which supports their assignment to a single cumulate series. The tabular ultramafic bodies investigated in different blocks of the Pekul’ney complex are from 350 to 1100 m thick, and their internal structure is made up of intercalated regular rhythms of dunites-peridotites and olivine pyroxenites-olivine-free ultramafics (garnet, ceylonite, and clinozoisite clinopyroxenites, websterites, and hornblendites) and units of irregularly interlayered dunites, peridotites, and olivine pyroxenites. The thickness of individual regular rhythms ranges from 50 to 410 m. The cumulate ultramafics of the Pekul’ney complex were derived from a water-rich highly magnesian primary melt, which was equilibrated with mantle harzburgites, within a wide temperature range at pressures of 11–13 kbar in the geodynamic setting of the base of an ensialic arc. The Pekul’ney complex can be considered as a reference object for the petrological and geochemical investigation of the evolution of suprasubduction mantle melts during their high-pressure fractionation.

Petrology and Chemistry of an Early Proterozoic Lherzolite-Gabbronorite-Anorthosite Pluton of the White Sea Complex, Northern Karelia, Russia

Two of the most well-preserved igneous bodies in the early Preeambrian White Sea complex— the Severnyy and Yuzhnyy massifs on Pezhostrov Island—have been studied in order to gain a better understanding of ultramafic-mafic magmatism in the Belomorian tectonic block. These massifs represent portions of a single, differentiated pluton, ranging in composition from lherzolite to gabbronorite to anorthosite. Mineral-chemical and trace-element compositions of chill margins from this pluton were used to model the differentiation in this ancient magma chamber. Major-element compositions of minerals suggest that plagioclase in these rocks is not in equilibrium with the mafic minerals. This possibly is the result of suspension of less dense, early-formed plagioclase in more dense, early residual liquids. Later, as the liquid density decreased because of precipitation of mafic phases, plagioclase began to precipitate. We speculate that the liquid density did not decrease to a point where plagioclase would settle, unt...

Early-middle Triassic basic magmatism and metamorphism of ultramafic-mafic complexes of the Ust’-Belaya terrane (central Chukotka, NE Russia): 40Ar/39Ar ages, petrological and geochemical data, geodynamic interpretations

ABSTRACT The Ust’-Belaya terrane (Chukotka, NE Russia), belonging to the West Koryak fold belt, is made of mantle and crustal ultramafic-mafic complexes which originated during several discrete episodes of magmatic activity in the Neoproterozoic, the late Neoproterozoic-Cambrian and the early-middle Triassic and accreted to the Asian continental margin in the early Cretaceous. This paper focuses on the latest magmatic episode expressed in intrusion of microgabbro dikes and nearly coeval metamorphism superimposed on both ancient complexes and the dikes. We present original 40Ar/39Ar ages, mineral and bulk-rock chemistry of the microgabbro dikes, metamorphosed dike and vein cutting ultramafic-mafic complexes. Dike microgabbros resemble differentiated arc-tholeiitic magmas originated in a subduction setting. Differentiated magmas intruded into much older spinel peridotites and dunites located at mid-crustal levels. Intrusion and crystallization of these magmas was followed by down-going movement of spinel peridotites and rocks of the mantle-crust transition zone towards a mantle wedge where they were metamorphosed at high-P conditions. This metamorphism resulted in transformation of microgabbro to garnet amphibolite, diorite to albite-zoisite-paragonite-pargasite rock and spinel peridotites to metaperidotites. P-T parameters of this metamorphism reconstructed based on mineral assemblage of garnet amphibolite correspond to those of epidote amphibolite – amphibolite – amphibole eclogite facies transition. The peculiar zoisite (clinozoisite)-paragonite mineral assemblage typical of metamorphosed vein rock indicates high-P metamorphic conditions of epidote-amphibolite facies.

Ophiolitic Complex of the Matachingai River on Eastern Chukotka: Fragment of Lithosphere in Mesozoic Back-Arc Basin

The Matachingai River basin is known among the few ophiolitic complexes on eastern Chukotka as the southern boundary of the Chukotka Fold System (in terms of tectonics, the Chukotka microcontinent or a fragment of the Arctic Alaska–Chukotka microplate). This complex comprises tectonic blocks of residual spinel harzburgite with dunite bodies and pyroxenite, olivine gabbro, and leucogabbro veins; blocks of hornblende gabbro, diorite, and plagiogranite; and Upper Jurassic–Lower Cretaceous basaltic–cherty and cherty–carbonate rocks. The geological relationships of rocks within tectonic blocks, the compositions of primary minerals, the bulk geochemistry of rocks, as well as the strontium, neodymium, and lead isotopic compositions, make it possible to consider individual tectonic blocks of the complex as fragments of a disintegrated oceanic-type lithosphere that formed in a back-arc spreading center. The melts, crystallization products of which are represented by hornblende gabbro of blocks, olivine gabbro of veins, and basalts, separated from geochemically and isotopically heterogeneous mantle. Blocks composed of rocks with various modal composition are likely relicts of an oceanic lithosphere of different segments of a back-arc basin. The studied complex may be a lithosphere of one of the Middle–Late Jurassic back-arc basins. Fragments of these basins are retained in ophiolitic complexes on Great Lyakhovsky Island of the New Siberian Islands Archipelago, western Chukotka, and the Brooks Range in Alaska.

Pyroxenite veins in spinel peridotites of the Unnavayam sheet, Kuyul ophiolite terrane (Koryak Upland): Origin and setting of formation

Pyroxenite veins in mantle peridotites of the Unnavayam sheet of the Kuyul ophiolite terrane (Koryak—Kamchatka folded area) are composed of clinopyroxenite and websterite crystallized from a boninite-like melt. The host clinopyroxene-bearing spinel harzburgites are moderately depleted residues, whose mineral compositions and conditions of formation correspond to those of peridotites from mid-oceanic and back-arc spreading centers. Mantle peridotites of the Unnavayam sheet may have formed a part of the mantle wedge above a subduction zone and have been intruded by boninitic melts at a certain stage of their evolution.