P.A. Serov

Early Paleozoic batholiths in the northern part of the Kuznetsk Alatau: Composition, age, and sources

The paper reports geological, chemical, and geochronological data on the Early Paleozoic granitoid and gabbro-granite associations, which compose the Kozhukhovskii and Dudetskii batholiths in the northern part of the Kuznetsk Alatau. The Kozhukhovskii batholith located in the Alatau volcanoplutonic belt is made up of tholeiitic, calc-alkaline, and subalkaline rocks that were formed in two stages. The first stage corresponded to the formation of granitoids of the Tylinskii quartz diorite-tonalite-plagiogranite complex (∼530 Ma, Tylinskii Massif, tholeiitic type) in an island arc setting. The second stage (∼500 Ma) produced the Martaiga quartz diorite-tonalite-plagiogranite complex (Kozhukhovskii Massif, calc-alkaline type) and the Krasnokamenskii monzodiorite-syenite-granosyenite complex (Krasnokamenskii Massif, subalkaline type) in an accretionary-collisional setting. The Dudetskii batholith is situated in the Altai-Kuznetsk volcanoplutonic belt and contains widespread subalkaline intrusive rocks (Malodudetskii monzogabbro-monzodiorite-syenite and Karnayul’skii granosyenite-leucogranite complexes) and less abundant alkaline rocks (Verkhnepetropavlovskii carbonatite-bearing alkaline-gabbroid complex), which were formed within the age range of 500–485 Ma. Our Nd isotopic studies suggest mainly a subduction source of the rocks of the Kozhukhovskii batholith (εNd from + 4.8 to + 4.2). Subalkaline rocks of the Dudetskii batholith exhibit wide isotopic variations. The Nd isotopic composition of monzodiorites and monzogabbro of the Malodudetskii Complex (εNd = + 6.6), in association with the elevated alkalinity and high Nb and Ta contents of these rocks, testifies to the predominant contribution of an enriched mantle source at the participation of a depleted mantle source. The lower εNd (from + 3.2 to + 1.9) in its syenites possibly indicates their generation through melting of metabasic rocks derived from enriched mantle protolith. The rocks of the Karnayul’skii Complex have lower Nb and Ta contents at similar εNd (+3.6), which suggests some crustal contribution to their formation.

Early cenozoic magmatism in the continental margin of Kamchatka

The paper presents isotopic-geochemical features of magmatic rocks that were produced at the continental margin of Kamchatka during its various evolutionary stages. Continental-margin magmatism in Kamchatka is demonstrated to have evolved from the Paleocene until the present time. The Paleocene and Middle-Late Eocene magmatic complexes show features of suprasubduction magmatism. The magmatic melts were derived from isotopically heterogeneous (depleted and variably enriched, perhaps, as a consequence of mixing with within-plate melts) mantle sources and were likely contaminated with quartz-feldspathic sialic sediments. The Miocene preaccretion stage differs from the Paleogene-Eocene one in having a different geochemical and isotopic composition of the mantle magma sources: the magmatic sources of the Miocene suprasubduction magmas contained no compositions depleted in radiogenic Nd isotopes, whereas the sources of the within-plate magmas were enriched in HFSE. The Late Pliocene-Quaternary postaccretion magmas of the Eastern Kamchatka Belt are noted for the absence of a within-plate OIB-like component.

The Volch’etundrovsky Massif of the autonomous anorthosite complex of the Main Range, the Kola Peninsula: Geological, petrogeochemical, and isotope-geochronological studies

The Volch’etundrovsky Massif occupies the middle part of the autonomous anorthosite complex of the Main Range, has a sheet morphology and marks the tectonic suture between the Kola block and the Belomorian mobile belt. The massif is characterized by homogenous structure and consists of the volumetrically dominant Main Zone including leucogabbro, leucogabbronorites, and anorthosites, and Marginal Zone made up of leuconorites and gabbronorites with subordinate plagioclasites and orthopyroxenites. Chemically, the rocks of the Volch’etundrovsky Massif are ascribed to the normal (tholeiitic and calc-alkaline) petrochemical series with typomorphic high Al2O3 contents (11.71–29.32 wt %). With Al2O3 increase in the leuconorite-anorthosite series, the SiO2 and TiO2 contents show weak variations, CaO and alkalis insignificantly increase, whereas the MgO and FeO contents sharply decrease. The rocks of the Volch’etundrovsky Massif reveal significant REE fractionation and increase in total REE content in the leuconorite-anorthosite series, most approximating the Paleoproterozoic (Sumian) anorthosites of the Kola region. The anorthosites and leucogabbro are characterized by flat HREE, while the leuconorites is strongly depleted in HREE due to garnet fractionation. All rocks of the massif have significant positive Eu anomalies caused by the plagioclase accumulation. Zircons are characterized by LREE depletion and enrichment in HREE. This defines the steep positive slope of the plots complicated by the negative Eu and positive Ce (in zircons from leucogabbro) anomalies, which is typical of the REE distribution patterns in the unaltered zircons from igneous rocks. In zircons from anorthosites, the Ce anomaly is weak to absent. The trace-element distribution in the rocks of the Volch’etundrovsky Massif show positive Ba, Ta, Pb, Sr, Sc, and V anomalies, being controlled by the mineral specifics of the massif and the presence of definite accessory minerals. New U-Pb zircon data on the rocks of the Volch’etundrovsky Massif indicate that the leuconorites from the Marginal Zone were formed 2473 ± 7 Ma and 2463 ± 2.4 Ma ago, and the leucogabbro from the Main Zone, 2467 ± 8 Ma. These rocks have negative ɛNd(T) from -1.54 up to -3.10, which indicates their derivation from enriched mantle reservoir variably contaminated by crustal material. The anorthosites of the Main Zone define an U-Pb age of 2407 ± 3 Ma and ɛNd(T) = −3.78, which presumably reflect the timing of hydrothermal-metasomatic alterations in the upper part of the magmatic chamber accompanied by significant crustal contamination.

Composition, Sources, and Genesis of Granitoids in the Irtysh Complex, Eastern Kazakhstan

Intrusions of the Irtysh Complex are spatially restricted to the regional Irtysh Shear Zone (ISZ) and are hosted in blocks of high-grade metamorphic rocks (Kurchum, Predgornenskii, Sogra, and others) in the greenschist matrix of the ISZ. The massifs consist of contrasting rock series from gabbro to plagiogranite and granite at strongly subordinate amounts of diorite and the practical absence of rocks of intermediate composition (tonalite and granodiorite). The complex was produced in the Early Carboniferous, simultaneously with the onset of the origin of the ISZ itself. The granitoids composing the complex affiliate with diverse petrochemical series (from subaluminous plagiogranite of the andesite series to granite of the calc-alkaline series) and contain similar REE and HFSE concentrations [total REE = 103–163 ppm (La/Yb)n = 3.59–5.44, Zr (200–273 ppm), Nb (7.6–10.6 ppm), Hf (6.1–7.6 ppm), and Ta (0.68–1.19 ppm)] but are different in concentrations in LILE [Rb (3–9 and 121–221 ppm), Sr (213–375 and 77–148 ppm), and Ba (67–140 and 240–369 ppm)] and isotopic composition of Nd (ɛNd(T) from +5.3 in the plagiogranite to −1.2 in the granite) and O (δ18O from +9.4 in the plagiogranite to +14.5 in the granite). Data on the geochemistry and isotopic composition of metamorphic rocks of the Kurchum block and numerical geochemical simulations indicate that the granitoids were generated via the melting of a heterogeneous crustal source, which consisted of upper crustal metapelites and metabasites of the oceanic basement of the blocks of high-grade metamorphic rocks. The differences in the chemical and isotopic compositions of the granitoids were predetermined by the mixing of variable proportions of granitoid magmas derived from metapelite and metabasite sources.

Ospe–Luvtuaivench massif of metabasic rocks, Kola Peninsula, Russia: geologic structure and petrogeochemical and isotope geochemical evidence for its relation to the Imandra complex of layered intrusions

This paper reports the results of a combined study of the geology, petrochemistry, geochemistry, and isotope geochronology of the Ospe–Luvtuaivench massif of metabasic rocks in the Kola region consisting of three blocks (from northwest to southeast): Luvtuaivench, Ospe, and Ekostrov. The massif is approximately 17 km long and 6 km wide and is dominated by meso-and melanocratic orthoamphibolites (metagab-broids) with 5–15-m-thick layers of leucocratic epidote orthoamphibolite (metagabbro-anorthosite), which are changed upsection by metaferrogabbro with a horizon of ilmenite–magnetite ore at least 1 km in lateral extent and approximately 50 m thick. The petrogeochemical characteristics of metabasic rocks from the three blocks of the massif are identical and similar to those of the rocks of the Imandra complex of layered intrusions. In addition, metaultrabasic bodies (chlorite–actinolite, actinolite–chlorite–serpentine, and actinolite–serpentine rocks) of varying thickness were found in the Luvtuaivench and Ekostrov blocks. Based on their structural features differing from those of similar rocks of Imandra layered intrusions and petrogeochemical characteristics, these rocks were interpreted as xenolithic bodies. New U–Pb zircon ages were obtained for the metagabbroids of all blocks of the Ospe–Luvtuaivench massif: 2445 ± 5 Ma for the Luv-tuaivench block, 2455.8 ± 3.5 Ma for the Ospe block, and 2447 ± 4 Ma for the Ekostrov block; these estimates characterize the time of massif crystallization and are similar to the age of formation of typical intrusions of the Imandra layered complex. A U–Pb zircon age of 2444 ± 3 Ma was obtained for the crystallization of the Mount Yagel’naya gabbronorites of the Imandra complex. The following εNd(2450) values were obtained for the rocks of the massif: +0.61 for the Luvtuaivench block,–1.58 for the Ospe block, and –1.27 for the Eko-strov block; the gabbronorites of the Mount Yagel’naya massif yielded εNd(2450) =–0.99. These values are similar to those of the Paleoproterozoic layered intrusions. The similarity of the Ospe–Luvtuaivench massif to typical massifs of the Imandra complex in terms of internal structure, petrochemical and isotope geochemical characteristics, and age of rock formation provides compelling evidence for the interpretation of the massif as part of the Imandra complex.