The Paleoproterozoic (2.5-1.7 Ga) Midcontinent rift system of the northeastern Fennoscandian Shield versus The early Proterozoic Pechenga-Varzuga Belt: a case of back-arc spreading. Discussion
Two papers have appeared simultaneously: “The Paleoproterozoic (2.5–1.7 Ga) Midcontinent rift system of the northeastern Fennoscandian Shield,” in the Canadian Journal of Earth Sciences (CJES) (Smolkin 1997), and “The early Proterozoic Pechenga–Varuga Belt: a case of back-arc spreading,” in Precambrian Research (Sharkov and Smolkin 1997). It is of interest that although the two papers were published simultaneously they were submitted and accepted at different times: Sharkov and Smolkin, submitted 6 March 1995, accepted 14 August 1996; Smolkin, submitted 19 April 1996, accepted 28 November 1996. It is difficult to believe that Smolkin was either unaware of or had quickly forgotten the Sharkov and Smolkin article! There is, however, no reference to either article in the other. Both papers have been devoted to the same topic, namely, the palaeotectonic significance of the Palaeoproterozoic (ca. 2.5–1.8 Ga) Pechenga – Imandra/Varzuga and the Granulite belts on the Fennoscandian Shield. It is apparent already, from the titles, that these articles are in conflict with each other, though using essentially the same data. This is not totally unknown in science, but in this case, as the same author, that is, Smolkin, is the sole author of the one and the second author of the other, it is at best highly unusual! In the one paper (Sharkov and Smolkin 1997), the importance of subduction and islandarc construction in the Pechenga–Varzuga belt is emphasized, though in the other paper, it is ignored (Smolkin 1997). Subduction and island-arc construction is an integral part of the geotectonic modelling of this region, as shown by Mints (1993), Mints et al. (1996), and indeed our own work (Melezhik et al. 1994a; Melezhik and Sturt 1994, 1995). In fact, the idea of subduction and collisional tectonics as being important processes, in the later stages of development in this belt, was already present in the literature during the 1980’s (e.g., Berthelsen 1984; Marker 1985; Berthelsen and Marker 1986). There are a number of other disagreements between these two papers. One of these is a palaeotectonic interpretation of tholeiitic basalts of the Pilgujarvi Volcanic Formation (named Matert by Smolkin and Sharkov). These tholeiitic basalts have been considered analogous to normal mid-oceanic-ridge basalt (NMORB) since Kremenetsky and Ovchinnikov (1986) published their first results on rare earth elements (see also Melezhik and Sturt 1994). Although they ignore the previous work, Sharkov and Smolkin (1997) have, however, taken the NMORB signature of these rocks as a key element in arguing for the presence of ca. 2.0 Ga oceanic crust beneath the Pechega Belt. These tholeiites are also known to have enrichment in Fe, Ti, K, Rb, Ba (e.g., Predovsky et al. 1974; Hanski 1992), and in some cases light rare earth elements (LREE) (Melezhik et al. 1994a). They are also known by their spatial and temporal associations with ferropicrites (Hanski and Smolkin 1989) that are enriched in Fe, Ti, first-series transition metal incompatible elements, and LREE (Hanski and Smolkin 1989). These geochemical features have been assigned by Smolkin (1997) to contamination of both tholeiitic basalts and ferropicrites by continental crustal material and interpreted as the presence of continental crust beneath the Pechenga Belt, and will be discussed later. However, before discussing other apparent inconsistencies between these two articles, we would like to address a number of questions to Dr. Smolkin and make some comments concerning the article published in the Canadian Journal of Earth Sciences (Smolkin 1997).