F. P. Mitrofanov

Isotope data on the age and genesis of layered basic-ultrabasic intrusions in the Kola Peninsula and northern Karelia, northeastern Baltic Shield

Abstract New UPb zircon ages and SmNd data are presented for layered basic-ultrabasic intrusions of the Kola Peninsula and northern Karelia in the northeastern part of the Baltic Shield. All ages are in the range between 2.5 and 2.4 Ga, but there appear to exist three phases of intrusion. Thus the Monchegorsk Intrusion and the Fedorovy-Pansky Massif have ages between 2493 and 2470 Ma, the Kivakka and Lukkulaisvaara massifs of the Olanga group of intrusions between 2445 and 2435 Ma, while the Imandra Lopolith is 2396±7 Ma old. A narrow range of negative ϵNd(T) values between −2.3 and −1.2 suggests formation by partial melting of a semi-homogeneous enriched mantle source.

Timing and duration of Palaeoproterozoic events producing ore-bearing layered intrusions of the Baltic Shield: metallogenic, petrological and geodynamic implications

Abstract There are two 300–500 km long belts of Palaeoproterozoic layered intrusions in the Baltic (Fennoscandian) Shield; the Northern (Kola) Belt and the Southern (Fenno-Karelian) Belt. New U–Pb (TIMS) ages and radiogenic isotopic (Nd–Sr–He) data have been determined for mafic-ultramafic Cu–Ni–Ti–Cr and PGE-bearing layered intrusions of the Kola Belt. U–Pb zircon and baddeleyite data from gabbronorite and anorthosite bodies of the Fedorovo-Pansky, Monchepluton, Main Ridge (Monchetundra and Chunatundra) and Mt Generalskaya intrusions, and from gabbronorite intrusions and dykes associated with the Imandra lopolith, yield ages from c. 2.52–2.39 Ga. The age range of 130 Ma recorded in the Kola Belt samples, associated with at least four intrusive phases (three PGE-bearing and one barren), is significantly greater than that for intrusions of the Southern (Fenno-Karelian) Belt which clusters at 2.44 Ga. Nd isotopic values for the Kola Belt range from −1.1 to −2.4 and indicate an enriched mantle ‘EM-1 type’ reservoir for these layered intrusions. Initial Sr isotopic data for the Kola intrusions are radiogenic relative to bulk mantle, with ISr values from 0.703 to 0.704, but geochemical data and 4He/3He isotopic ratios of various minerals record a significant contribution from a mantle source rather than simply crustal melting. The geological and geochronological data indicate that in the eastern part of the Baltic Shield, mafic‐ultramafic intrusive magmatism was active over a protracted period and was related to plume magmatism associated with continental breakup that also involved the Superior and Wyoming provinces.

Late Archaean SmNd model ages from the Voche-Lambina area: implications for the age distribution of Archaean crust in the Kola Peninsula, Russia

Abstract Although an Early Archaean age for the “Saamian” gneisses has been generally accepted in the former Soviet Union, partly on the basis of their commonly granulite-facies metamorphic grade and partly on account of their structural complexity and complex intrusion history, these rocks have yielded no precise isotopic ages in excess of ∼ 2.9 Ga. SmNd isotopic analyses are used to test the possible presence of Early Archaean (>∼ 3.5 Ga) “Saamian” gneisses at Voche-Lambina in the Kola Peninsula. Here tonalitic, trondhjemitic and granodioritic gneisses, presumed to be Early Archaean (Saamian) in age, are overlain by a sequence of Late Archaean (Lopian) supracrustals. A sample of the structurally oldest trondhjemitic gneiss, with a minimum age of 2.64 Ga, has a depleted mantle model age (tDM) of 2875 Ma. Tonalitic gneiss, also assumed on structural grounds to be Early Archaean, has a tDM age of 2808 Ma identical to its UPb zircon age of 2807±9 Ma. This suggests that a depleted mantle model may be appropriate for at least this part of the Kola region in the Late Archaean. Lopian rhyodacite, with a UPb zircon age of 2800 Ma and a RbSr isochron age of 2540±83 Ma, has a tDM age of 2756 Ma. In each case the close agreement between the SmNd model age and the time of intrusion or eruption leaves little scope for a significantly older crustal history for these rocks. The data provide no evidence for a Saamian component in this part of the Kola Peninsula.

Two-phase mechanism of the formation of platinum-metal basites of the Fedorova Tundra intrusion on the Kola Peninsula: New data on geology and isotope geochronology

This report contains the results of the authors’ studies on geology and isotope geochronology, which allowed them to formulate the thesis of a two-phase mechanism of the formation of rock associations in the Fedorova Tundra massif. During the former phase (2526–2507 My), a laminated series of the massif was formed, with shows of platinum-metal mineralization of reef type. During the later phase (2493–2485 My), a taxitic zone of the massif appeared, with a highly developed basal platinum-metal mineralization.

CRUSTAL STRUCTURE OF THE BALTIC SHIELD ALONG THE PECHENGA-KOSTOMUKSHA-LOVISA GEOTRAVERSE

Results of geologic and geophysical modeling are presented, based on detailed seismic studies along two profiles—Pechenga-Kostomuksha and Lieksa-Lovisa. Density, geothermal, magnetic, and geoelectric models were obtained from the interpretations of various geophysical fields and correlated with the reference seismic sections. All the models were combined in order to compile a geologic-geophysical crustal section. The crustal thickness along the Pechenga-Kostomuksha-Lovisa geotraverse varies from 38 to 65 km. Two anomalous structures have been observed that are referred to as the Belomorian-Karelian and Ladoga-Bothnian zones. These zones are characterized by enhanced values of magnetic fields, presence of seismic foci and wave attenuation, and variation of the depth and magnitude of modern crustal movements. These zones are distinguished by the discontinuity M reconstruction, an increase in transitional layer thickness (to 25 km) at the base of the crust, and an increase in depth down to the discontinuity ...

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.

Velocity heterogeneities in the lithosphere of the Fennoscandian (Baltic) shield

64 The velocity structure of the lithosphere in the Fennoscandian Shield is of great interest for two main reasons: classical problems in the study of the Earth’s deep structure and the problems in the definition of promising zones which display their mineralogenic features. To solve these problems, seismic tomography methods are commonly used to approach the inverse linearized 3D kinematic problem of seismic study. These methods have been developed since the 1960s, and they are now extensively employed for solving both the deep seismic sounding (DSS) and seismolog� ical problems. The Fennoscandian Shield has been extensively studied under international and regional seismic projects launched to better understand the structural characteristics of the Earth’s crust. Various metamor� phosed crystalline and igneous rock complexes were studied. Comparative analysis of wave fields in all the profiles is a reliable tool for selecting a general model of the Earth’s crust and using it as the basis for reveal� ing the general pattern of variations in the physical parameters of the crystalline crust of the study area. Such a model is an approximation of the real velocity structure of the crust; the sections of all profiles can be presented in a uniform fashion and a 3D velocity model can be constructed, based on this model [1–4].

New Massif of Archean Alkaline Syenites in the Murmansk Domain of the Kola Peninsula

Alkaline magmatism typical of a mature continental crust is among the most important geodynamic markers in the Early Precambrian. The Late Archean alkaline rocks represented by alkali granites, alkali gabbros, syenites, nepheline syenites, and carbonatites are known at the ancient shields [1]. In the Kola Peninsula, such magmatism (with an age of 2.75‐2.61 Ga) in the form of alkali granite and alkali gabbro‐nepheline syenite complexes occurs in the Archean Keivy and Ingozero structures [2, 3]. No Precambrian alkaline complexes have been found yet in the Archean Murmansk domain of the Kola peninsula. The Late Archean Iokan’gskii intrusive complex of porphyritic lepidomelane‐ferrohstingsite granites was previously ascribed by Batieva [2] to the alkali granite complex. However, later studies [4] showed that it belongs to the complex of ancient rapakivilike granites of normal alkalinity. During recent studies of the granitoids from the Barents Sea shore of the Kola Peninsula in the area of Gremikha settlement and Mt. Ostrovnaya, the small Late Archean alkaline massif called Panejavr Massif after an nearby lake was discovered and studied. The Panejavr Massif is located within the Iokan’gskii block of the Murmansk domain, and its position is controlled by the junction of two faults (Fig. 1). The massif is contained among Late Archean migmatite‐granites and metabasite dikes that fill the NE-trending faults. The Panejavr Massif is a complex stock having an autonomous internal structure discordant to the host rocks (Fig. 1) and consisting of two chemically and mineralogically contrasting intrusive phases.

New data on distribution of REEs in sulfide minerals and Sm-Nd dating of ore genesis of layered basic intrusions

This paper presents data on the distribution of REEs in sulfide minerals from ore-bearing gabbronorites in the Penikat layered intrusion and the results of their isotopic-geochronological Sm-Nd study. A new procedure for determination of REEs in the samples without preliminary separating and concentrating was tested on standard samples to be further used for analysis of sulfide minerals. Analysis of the spectra of the REE distribution in sulfides represents a distribution trend that is similar to the already studied bulk rock and allows deducing that the character of the REE distribution in sulfide minerals from gabbronorites in the Penikat layered intrusion was inherited from the parent magma melt; while the formation of sulfides took place at the stage of rock crystallization. The performed complex studies allow considering that sulfides can be successfully used together with the rock-forming minerals in Sm-Nd dating of ore-bearing mafite-ultramafite intrusions.