2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Evidence for the Mesozoic endogenous activity in the northeasern Fennoscandian Shield

 A.Arzamastsev^*, R.Veselovskiy^**

^*Geological institute of the Kola Science Center RAS, Apatity

 arzamas@geoksc.apatity.ru

^**Moscow State University, Moscow

ramzesu@ya.ru

The key problem for the determination of the conditions for the localization of the mantle melts which produced the Kola Alkaline Province is deciphering of the geodynamic conditions of the Phanerozoic period of the northeastern Fennoscandia and reconstruction of the motion trend of the Kola megablock in the Paleozoic time. The paleomagnetic method is often used, however, the paleomagnetic data for the time span of 400-340 Ma provide an uncertain Apparent Polar Wander (APW) path for the Fennoscandian Shield.

In order to obtain new data corresponding the modern criteria of reliability, we carried out reconnaissance paleomagnetic studies of dolerite and alkaline lamprophyric dyke swarms, whose ages fall within the interval of 390–370 Ma according to the geochronological methods (Rb–Sr, Sm–Nd, ⁴⁰Ar/³⁹Ar). During the field studies, we sampled dolerite dykes of the Barents Sea shore and the Pechenga area, 12 dykes composed of alkaline lamprophyres in Kandalaksha Bay (White Sea), and rocks from the Afrikanda and the Turii Mys intrusive massifs. Analysis of the magnetic cleaning results has shown that in the most of the examined dykes natural residual magnetization (NRM) can be represented by one, two, or three components (Fig. 1).

Fig. 1. Typical Zijderveld diagrams and stereograms for the samples where the Pz (a, b) of the Mz (c, d) component is identified; (e) distribution of Pz and Mz magnetization components in the studied dykes and their mean directions (filled squares) with confidence circles (gray domains). (1) projections of vectors to the horizontal plane (lower semisphere); (2) projections of vectors to the vertical plane (upper semisphere).

The Pz group is represented by NRM components, whose paleomagnetic pole, calculated at the site level for the mean direction of direct and inverted components of magnetization of the Pz group, is located in the immediate vicinity of the mid Devonian portion of the APWP curve for the East European Craton [Torsvik et al., 1996] (Fig. 2), which allows us to estimate the age of components of the Pz group as Devonian. The reasons for the primary character of magnetization components of the Pz group are their antipodality and difference of the calculated paleomagnetic pole from the earlier poles of the East European Craton.

The magnetization component, whose vectors compose the Mz group, was found in nearly all the studied dolerite dykes of the Barents Sea shore, in the northern framing of the Pechenga structure and alkali lamprophyres of the southern part of the region; the geochronological age of all these objects was also estimated to be as old as the Devonian. In a series of samples, this magnetization component is found jointly with the components of Devonian age (the Pz group), occupies the middle part of the spectrum of blocking temperatures, and has steep positive inclinations (Figs. 1c, 1d, 1e). The samples from Archean gneisses that host Devonian dykes, taken at distances up to several hundred meters from dykes to implement the baked contact test, usually carry only the Mz magnetization component. The paleomagnetic pole, calculated at the site level and corresponding to the average direction of the Mz component (Fig. 2), tends to the Mesozoic portion of the APW path for the East European Craton that can be considered as a direct sign about the time of formation of this magnetization component.

The paleomagnetic pole derived for the average direction of the Pz component reflects the direction of the geomagnetic field in the period when the studied dyke swarms were formed in the Devonian, and can be used (with substantial restrictions) for elaboration of the APW path for the East European Craton and for paleotectonic reconstructions. To increase the reliability and quality of determination for the Devonian paleomagnetic pole based on the dykes of the Kola Peninsula, a significant increase in the number of studied objects is required.

Interpretation of the Mesozoic magnetization component (Mz) in Devonian dykes and intrusive massifs seems to be more difficult. The secondary nature of this component is unlikely to be questionable, because it is found in a significant number of samples within the high_temperature interval and partially softens the Devonian primary mid temperature component. Bipolarity of the Mz component can indirectly evidence the duration of a thermal remagnetizing event being sufficient for polarity change (inversion) of the geomagnetic field to occur.

Fig. 2. Positions of the paleomagnetic poles for Pz and Mz components relative to the portion of the APWP curve for the East European Craton [Torsvik et al., 1996].

Paradoxically, analysis of the information shows an absence of any geological or geochronological evidence for Mesozoic thermal and/or other geological events that took place within the Kola Peninsula and the adjacent region of the Fennoscandian Shield that could be a reason for the appearance of magnetization related to the Mz component. Numerous isotopic datings of dykes and other objects from the Kola Peninsula made based on micas and amphiboles with the help of the ⁴⁰Ar/³⁹Ar method, which is the most sensitive to thermal effects, have not revealed any signs of disturbance of this isotopic system by post-Paleozoic processes. The data resulting from examination of singular zircon grains from Precambrian rocks of the Fennoscandian Shield via the SHRIMP method [Lokhov et al., 2004] signifies the presence of processes that led to partial lead losses from particular zones of zircons and to the appearance of new generations of zircons throughout the Phanerozoic. However, the calculated lower discordia crossings that yield age estimates in the interval of 700–250 Ma BP [Gol’tsin et al., 2006, Karyakin and Shipilov, 2009] apparently only show under the influence of the Devonian stage of magmatic activation [Larson and Tullborg, 1998]. The results of fission track dating [Hendriks et al, 2007] also add no certainty due to the uniqueness of available data for the studied territory; however, they do not exclude the presence of a certain overlain event of Mesozoic age.

Remagnetization throughout the Mesozoic–Paleozoic within the western part of the East European Craton was found in paleomagnetic studies of Ordovician and Devonian rocks in Leningrad oblast [Smethurst et al., 1997], of Proterozoic dykes in Karelia [Lubnina, 2009], and of Paleozoic sedimentary rocks in Estonia [Preeden et al., 2008]. The most significant events that could cause such an extensive remagnetization are probably related to activation of the Barents_Amerasian superplume and to formation of an extensive break of Jurassic_Cretaceous trap magmatism (“large magmatic province”) within the modern Arctic basin [Shipilov and Karyakin, 2010]. The ⁴⁰Ar/³⁹Ar isotopic dates obtained in recent years for flood basalts in Franz Josef Land (189.1 ± 11.4 Ma on Hooker Island and 191 ± 3Ma on Alexandra Land [Karyakin and Shipilov, 2009]) correspond to the initial phase of plume evolution manifested in breakup of the lithosphere and disintegration of the future Arctic Basin into block structures. The reconstructions [Shipilov and Karyakin, 2010] have shown that the center of magmatic activity covered Franz Josef Land and the Spitzbergen archipelagoes and, probably, the adjacent (in that time) northern part of the Fennoscandian Shield, while the fault zone related to the functioning apophyses of the plume were hampered by the paleomargin of the Barents Sea. The subsequent destruction and extension of the continental lithosphere in the Barents Sea region led to weakening of the thermal effect of the plume to the crystalline basement of the Arctic areas of Fennoscandia.

Thus, the results of paleomagnetic studies signify that, throughout the Phanerozoic, the eastern part of the Fennoscandian Shield both suffered the Devonian tectonic-magmatic activation and underwent the thermal effect of Mesozoic plume-lithospheric processes, which caused the appearance of marginal-continental polycyclic riftogenesis in the West Arctic.