P. A. Serov

Magmatic sources of dikes and veins in the Moncha Tundra Massif, Baltic Shield: Isotopic-geochronologic and geochemical evidence

The dike-vein complex of the Moncha Tundra Massif comprises dolerites, gabbro-pegmatites, and aplites. The dolerite dikes are classified into three groups: high-Ti ferrodolerites, ferrodolerites, low-Ti and low-Fe gabbro-dolerites. The U-Pb age of the ferrodolerites is 2505 ± 8 Ma, and the amphibole-plagioclase metagabbroids hosting a ferrodolerite dike are dated at 2516 ± 12 Ma. Data on the U-Pb isotopic system of zircon from the gabbro-pegmatites and titanite from the aplites indicate that the late magmatic evolution of the Moncha Tundra Massif proceeded at 2445 ± 1.7 Ma, and the youngest magmatic events in the massif related to the Svecofennian orogeny occurred at 1900 ± 9 Ma. The data obtained on the Sm-Nd and Rb-Sr isotopic systems and the distribution of trace elements and REE in rocks of the dike-vein complex of the massifs provide insight into the composition of the sources from which the parental magmas were derived. The high-Ti ferrodolerites were melted out of a deep-sitting plume source that contained an asthenospheric component. The ferrodolerites were derived from a mantle MORB-type source that contained a crustal component. The parental melts of the gabbro-dolerites were melted out of the lithospheric mantle depleted in incompatible elements after Archean crust-forming processes above an ascending mantle plume, with the participation of a crustal component. The gabbro-dolerites and the rocks of the layered complex of the Moncha Tundra Massif exhibit similar geochemical characteristics, which suggest that their parental melts could be derived from similar sources but with more clearly pronounced crustal contamination of the parental melts of the rocks of the massif itself. The geochemical traits of the gabbro-pegmatites are thought to be explained not only by the enrichment of the residual magmas in trace elements and a contribution of a crustal component but also by the uneven effect of sublithospheric mantle sources. The aplites were derived from a sialic crustal source.

Petrogeochemical and isotope peculiarities of supersubduction terrigenous deposits: The example of Predivinsk Terrane of the Yenisei Ridge

1039 This study is aimed at revealing petrogeochemical and isotope specifics of terrigenous rocks occurring in close paragenetic association with island arc magmatic complexes from the example of Neoproterozoic com plexes of the Predivinsk Terrane of the Yenisei Ridge. This terrane is located in the southwestern part of the ridge and composes the Sayany–Yenisei accretion– collisional belt. It consists of a number of tectonic plates overthrusted in the NE direction along the Priy enisei (Predivinsk) thrust (Fig. 1) [1–4]. Two large tectonic blocks with different sets and compositions of rock associations (Western and Eastern) are distin guished within the terrane.

Archean rock homologs in the Kola superdeep borehole section in the northern part of the White Sea mobile belt, Voche-Lambina test site

The Archean Complex homologs of the Kola superdeep borehole (SG-3) were identified in the northern part of the White Sea mobile belt. Tonalite-trondhjemite-granodiorite gneisses of the Voche-Lambina test site and metavolcanic dacite-rhyodacite rocks of the borehole SG-3 were formed at the stages of 2.97–2.82, ∼2.81, and 2.78–2.79 Ga. The Sm-Nd model ages of the studied rocks do not exceed 3.1 Ga, and their positive ɛNd(t) values vary from +0.5 to +3.34. They are characterized by Mg# = 0.20−0.44, similar concentrations (HFSE) of Zr, Nb, Y, and also Rb, Cr, and Ni, and sharply differentiated spectra of the REE distribution (Ce/Sm = 3.2−5.8; Gd/Yb = 2.6−7.1). Primary melts were formed in balance with garnetamphibole restite under P ≥ 15−16 kbar.

Low-Sulfide PGE ores in paleoproterozoic Monchegorsk pluton and massifs of its southern framing, Kola Peninsula, Russia: Geological characteristic and isotopic geochronological evidence of polychronous ore–magmatic systems

New U–Pb and Sm–Nd isotopic geochronological data are reported for rocks of the Monchegorsk pluton and massifs of its southern framing, which contain low-sulfide PGE ores. U–Pb zircon ages have been determined for orthopyroxenite (2506 ± 3 Ma) and mineralized norite (2503 ± 8 Ma) from critical units of Monchepluton at the Nyud-II deposit, metaplagioclasite (2496 ± 4 Ma) from PGE-bearing reef at the Vurechuaivench deposit, and host metagabbronorite (2504.3 ± 2.2. Ma); the latter is the youngest in Monchepluton. In the southern framing of Monchepluton, the following new datings are now available: U–Pb zircon ages of mineralized metanorite from the lower marginal zone (2504 ± 1 Ma) and metagabbro from the upper zone (2478 ± 20 Ma) of the South Sopcha PGE deposit, as well as metanorite from the Lake Moroshkovoe massif (2463.1 ± 2.7 Ma). The Sm–Nd isochron (rock-forming minerals, sulfides, whole-rock samples) age of orthopyroxenite from the Nyud-II deposit (2497 ± 36 Ma) is close to results obtained using the U–Pb method. The age of harzburgite from PGE-bearing 330 horizon reef of the Sopcha massif related to Monchepluton is 2451 ± 64 Ma at initial εNd =–6.0. The latter value agrees with geological data indicating that this reef was formed due to the injection of an additional portion of high-temperature ultramafic magma, which experienced significant crustal contamination. The results of Sm–Nd isotopic geochronological study of ore-bearing metaplagioclasite from PGE reef of the Vurechuaivench deposit (2410 ± 58 Ma at εNd =–2.4) provide evidence for the appreciable effect of metamorphic and hydrothermal metasomatic alterations on PGE ore formation. The Sm–Nd age of mineralized norite from the Nyud-II deposit is 1940 ± 32 Ma at initial εNd =–7.8. This estimate reflects the influence of the Svecofennian metamorphism on the Monchepluton ore–magmatic system, which resulted in the rearrangement of the Sm–Nd system and its incomplete closure. Thus, the new isotopic geochronological data record the polychronous development of the Monchegorsk ore–magmatic systems and the massifs in its southern framing.

Autonomous anorthosites of the Anabar Shield: Age, geochemistry, and formation mechanism

The new high-accuracy data on U–Pb zircon geochronology, Sm–Nd systematics, and geochemistry of anorthosites of the Anabar Shield are discussed. It is established that anorthosite massifs are composed of gabbro–anorthosites (1.96 Ga old) and oligoclasites (1.93 Ga old) in association with monzodiorites (1.84–1.90 Ga old) and porphyroblastic granites. These rocks were generated in the Archean (3.2–2.7 Ga ago) in the lower crust from quartz–diorite melts under the plume tectonics regime in line with the filterpressing mechanism. The rocks were successively exhumed to upper levels of the crust owing to the Paleoproterozoic impact-triggered process to form a tectonically juxtaposed complementary magmatic complex.

Ocellar-porphyroblastic granitoids of the western part of the Aldan Shield: Geochemistry, age, and mechanism of formation

462 The western part of the Aldan Shield occupies the whole area of the Olekma–Vitim Highland. In this area the Charskaya ring structure with a diameter of 260–280 km [1, 2] includes almost all structural– material complexes of the Early Precambrian: from crystalline infra and supracrustal rocks of Pale oarchean and Mesoarchean metavolcanogenic–sedi mentary formations of greenstone suture zones (troughs) to sedimentary rocks of the Udokan proto platform trough. Different magmatic and palingene– metasomatic rocks of the Archean and Paleoprotero zoic are widely abundant in the region (Fig. 1). Because of good outcropping, we may clearly observe the geological relationships between these polychro nous formations, the age of which in most cases was determined by U–Pb and, rarely, Sm–Nd methods (see [1] for references).

Sr, Nd, and Hf Isotope Composition of Rocks of the Reft Gabbro–Diorite–Tonalite Complex (Eastern Slope of the Middle Urals): Petrological and Geological Implications

This paper reports Rb–Sr and Sm–Nd isotope data on the gabbro–diorite–tonalite rock association of the Reft massif (eastern margin of the Middle Urals) and Lu–Hf isotope data on zircon populations from these rocks. In terms of Nd and Hf isotope composition, the rocks of the studied association are subdivided into two distinctly different groups. The first group consists of gabbros and diorites, as well as plagioclase granites from thin dikes and veins cutting across the gabbros. In terms of 43Nd/144Ndi = 0.512518–0.512573 (εNd(T) = +8.6...+9.7) and 176Hf/177Hfi = 0.282961–0.283019 (εHf(T) = +15.9...+17.9), these rocks are practically identical to depleted mantle. Their Nd and Hf model ages show wide variations, but in general are close to their crystallization time. The second group is represented by tonalites and quartz diorites, which compose a large body occupying over half of the massif area. These rocks are characterized by the lower values of 143Nd/144Ndi = 0.512265–0.512388 (εNd(T) = +3.7...+6.0) and 176Hf/177Hfi = 0.282826–0.282870 (εHf(T) = +11.1...+12.7). The TDM values of the second group are much (two–three times) higher than their geological age (crystallization time), which indicates sufficiently long crustal residence time of their source. The initial 87Sr/86Sr in the rocks of both the groups varies from 0.70348 to 0.70495. This is likely explained by the different saturation of melts with fluid enriched in radiogenic Sr. The source of this fluid could be seawater that was buried in a subduction zone with oceanic sediments and released during slab dehydration. Obtained data make it possible to conclude that the formation of the studied gabbro–diorite–tonalite association is a result of spatially and temporally close magma formation processes in the crust and mantle, with insignificant contribution of differentiation of mantle basite magma.

The Paleoproterozoic Kolvitsa Anorthosite Massif: New Data on the U–Pb Age (ID TIMS) and Geochemical Features of Zircon

The rocks and minerals of the Kolvitsa massif are studied by complex isotopic–geochronological and geochemical (U–Pb, Sm–Nd, REE contents of zircons) methods. The isotopic U–Pb age (2448 ± 5 Ma) of single zircon grains, which were extracted from metagabbro of the block, is consistent with previous data on the rocks of the Kandalaksha anorthosite block. According to Sm–Nd studies of minerals of the metamorphic complex (apatite, sulfide minerals, garnet) and rocks of the block, the age of their metamorphic transformations is 1985 ± 17 Ma. The calculated temperature of the closure of the U–Pb system and crystallization of zircon of metagabbro of the Kolvitsa massif is 778°C. The REE patterns of single zircon grains indicate their magmatic origin.

The paleoproterozoic Kandalaksha Anorthosite Massif: New U–Pb (ID–TIMS) data and geochemical features of zircon

New U–Pb and Sm–Nd isotope data have been obtained in the Kandalaksha–Kolvitsa zone, Baltic Shield, on accessory zircon and rutile, along with whole-rock and secondary metamorphic minerals. Isotope U–Pb age of single zircon grains from metagabbro of the Kandalaksha Anorthosite Massif is 2453.5 ± 4.8 Ma, which is close to the U–Pb age of zircon from the Kolvitsa Massif metagabbro (2448 ± 5 Ma). For the first time, REEs in zircon grains of the Kandalaksha metagabbro have been analyzed and the results have been plotted. Ti-in-zircon thermometry has been applied using LA–ICP–MS: it yielded an average temperature of zircon crystallization of 844°C. The isotope and geochemical new data obtained indicate a magmatic genesis of the zircon crystals studied.

The Paleoproterozoic Fedorov–Pana Layered PGE complex of the northeastern Baltic Shield, Arctic Region: New U–Pb (baddeleyite) and Sm–Nd (sulfide) data

For the first time zircons have been extracted from gabbro–norite of a lower layered horizon of the West Pana Massif in the Pt–Pd Kievei deposit of the Fedorov–Pana Layered Complex. Those zircons have been used for U–Pb dating along with Sm–Nd age determination on sulfide minerals. The obtained new isotopic data are a U–Pb zircon age of 2500 ± 4 Ma, while the Sm–Nd (mineral and whole-rock) isochron yielded 2483 ± 86 Ma. These results correspond to the first phase of the Pt–Pd reef complex formation in the Layered Complex. The Pt–Pd reef formation has been dated by U–Pb baddeleyite and zircon analyses in the East Pana Massif to 2464 ± 12 Ma. The 2485–2464 Ma time span corresponds to the second phase of the Pt–Pd reef formation in the Fedorov–Pana ore cluster.