The origin of the Paleoproterozoic belt (northeastern Fennoscandian Shield). Reply

Abstract

I thank Dr. V. Melezhik and Dr. B. Sturt for their comments on my paper published in the Canadian Journal of Earth Sciences (Smolkin 1997). I am ready to continue constructive discussion on the questions of the genesis of Paleoproterozoic magmatic complexes of the Kola region (northwestern Russia) that host sulfide, chromite, and titanomagnetite ore deposits, and on the problems of reconstructing Precambrian geodynamic processes. Dr. E. Sharkov and I have been working on these questions since 1984, based mainly on the results of petrological studies of magmatic sequences of basic–ultrabasic rocks of the Kola region and the adjacent areas of Karelia and Finland (Sharkov 1984; Smolkin 1985, 1991, 1992, 1993, 1997; Sharkov and Smolkin 1988, 1989, 1990, 1997). Over these years we have proposed geodynamic models, which were refined as new data on the geochemistry and isotope composition of the magmatic rocks, as well as the deep structure of the region, were obtained. Dr. Sharkov and I share a common point of view on the earliest stages of evolution of the largest Paleoproterozoic belts of the Kola region: the Pechenga–Varzuga and Lapland Granulite; however, our interpretations of the final (orogenic) stage of the history of development of these zones are different. In Dr. Sharkov’s opinion (Priyatkina and Sharkov 1979; Sharkov and Smolkin 1990, 1997), the final stage resulted in the formation of the Lapland deep fault as a collision suture zone between the Belomorian and Kola crustal megablocks and as an island arc, the root of which is now preserved as the Lapland granulite belt. In this scenario, the Pechenga– Varzuga belt located to the north could be a back-arc basin area. In my opinion, the Pechenga–Varzuga belt was a continental-rifting system that had not gone through an oceanization stage, and during the final stage the system was transformed into an orogenic zone (Smolkin 1991, 1992, 1993, 1997; Mitrofanov et al. 1995). My original data on formational analysis, the petrology of intrusive and volcanic basic–ultrabasic rock complexes of Archean and Proterozoic ages, and the chemical, mineralogical, and isotope composition of the rocks are presented and discussed in detail in my doctoral thesis (Smolkin 1991). Working on the model, I benefited from participation in major projects aimed at formational analysis of magmatic complexes of the Kola region (Bel’kov 1985), isotope dating of ore formations (Pushkaryov et al. 1988), reconstruction of endogenic regimes prevailing in the Precambrian (Batieva and Vinogradov 1991), the study of magmatism and metallogeny in paleorift systems of Karelia and the Kola Peninsula (Smolkin et al. 1993), and in Russian–Finnish research programs (“Types and epochs of development of greenstone belts and their metallogeny” and “Basic– ultrabasic magmatism”) (Alapieti et al. 1990; Smolkin 1992). In addition, from 1965 to 1970 the author was engaged in compilation of geological 1 : 50 000 maps in southern and northern parts of the Baikal province. To construct the model, I used a data base of isotope (U– Pb, Sm–Nd, Rb–Sr) analyses for the Kola Peninsula and Karelia (more than 350 analyses in total), some of which are presented in the Appendix to Smolkin (1997). An analysis of geophysical (vibroseismic) data on the deep structure of the Pechenga region was performed recently (Pozhilenko et al. 1997). The results of my work on classification and formational analysis of intrusive basic–ultrabasic complexes were employed when the recent geological 1 : 500 000 maps of the Kola Peninsula and Karelia–Kola region were compiled (Radchenko et al. 1994; St. Petersburg State Company (“Mineral”) 1996; Mitrofanov 1996). Dr. Melezhik and Dr. Sturt (Geological Survey of Norway), who published their first paper dedicated to the interpretation of the geodynamic settings of the Pechenga– Varzuga belt in 1994, have offered comments on my publications, specifically the Smolkin (1997) paper. In this Reply, I will try to explain my position with regard to the Precambrian geodynamic history of the region. Before starting the discussion, it must be noted that Dr. Melezhik examined the geochemical features and conditions of the formation of Proterozoic sedimentary rocks in the region, and compiled paleogeographic reconstruction and correlation of the stratigraphic sections of first the Imandra/Varzuga and then, since 1985, the Pechenga zones. The results of his studies were published in detail in Melezhik (1992).