The Lapland-Kola orogen in the northern Baltic Shield consists of a number of Archean terrains which alternate with Proterozoic supracrustal belts. The Archean Inari Terrain (IT) is sandwiched between the SW thrusted Lapland Granulite Belt (LGB) - Tanaelv Belt (TB) to the south-west and the Polmak-Pasvik-Pechenga Belt (PPP) to the north-east. It is intruded by a number of calc-alkaline intrusions dated at 1.94 Ga (U/Pb zircon; Meriläinen, 1976).
Tectonic modelling of the Lapland-Kola orogen indicates closure of an oceanic basin and formation of an island-arc in the PPP during SW-subduction beneath the IT (Berthelsen and Marker,1986) accompanied by the emplacement of the calc-alkaline intrusives in the IT. However, Barbey et al. (1984) believe that these intrusives were related to NE subduction in the LGB. The present study indicates that destruction and SW thrusting of the LGB was related to north-eastward subduction even further to the south-west, as evidenced by the calc-alkaline suite in the TB, which may represent a trapped and sheared island-arc. By further dating and investigation of the geochemistry of the calc-alkaline intrusives in the IT we hope to be able to determine the polarity of the proposed subduction events and to identify the various crustal and mantle components involved in their formation. This study focusses on a series of intrusions at the northern end of the IT, sampled along the Tana River which cuts across the IT from the PPP in the east to the LGB in the west.
Three intrusions are identified in the field; collectively they fall on a normal calc-alkaline trend in terms of SiO2 and CaO/Na2O+K2O. The most easterly intrusion is the Spotted Meta-diorite. This includes both slightly olivine normative (normative ol < 2%;SiO2 = 53-54 wt. %) and quartz normative (normative Q <4%;SiO2 = 56 wt.%) compositions and, in terms of SiO2 and FeO*/MgO is predominantly tholeiitic. The most westerly intrusion, the Porphyritic Granodiorite is quartz normative (normative Q = 23-25%; SiO2 = 63-65 wt%) and also slightly peraluminous (normative c < 1.5%). In contrast the central intrusion (Mixed Intrusion) exhibits a bi-modal distribution of compositions. Samples from more easterly outcrops are gabbroic to dioritic and are exclusively quartz normative (normative Q = 2-6%; SiO2 = 51 - 56 wt.%) the two most mafic of these are tholeiitic in terms of SiO2 and FeO*/MgO. More westerly samples are granodioritic (normative Q = 16-18%; SiO2 = 63-64 wt.%). Broadly speaking the existing samples show a trend from east to west of decreasing alkalinity.
The most mafic compositions are characterised by an enrichment of the LILE, a negative Nb anomaly and a slight enrichment of the HFSE, all of which are similar to modern calc-alkaline island arc magmatism. The enrichment of HFSE demonstrates a moderately evolved arc setting, although there is no evidence to suggest the involvement of older crust in the magma genesis. The diorites and granodiorites all have I-type granite signatures and represent melt produced from the mixing of mantle derived melts and the existing early formed arc material. Further evidence for this mixing of sources comes from the diorites of the Mixed Intrusion.
Existing whole rock Pb-isotope data for the three intrusions define a trend in 206Pb/204Pb - 207Pb/204Pb corresponding to an age of (1.91 Ga). However, the three intrusions are perhaps better treated separately. The 206Pb/204Pb - 207Pb/204Pb data for the Spotted Meta-diorite give an age of 1.95 Ga, which agrees well with the existing zircon age of 1.94 Ga (Meriläinen, 1976). The 206Pb/204Pb - 207Pb/204Pb data for the Porphyritic Granodiorite also correlate but give an age of 1.53 Ga. This age does not correspond to any known geological event in the region, it may instead correspond to the evolution of a Pb-Pb mixing line with time. The Pb data for the Mixed Intrusion are scattered. The one granodioritic sample lies on the Spotted Diorite isochron, whereas the two dioritic samples fall on the Porphyritic Granodiorite correlation.
Barbey, et al., Bull. Geol. Soc. Finl. 56, 161-188 (1984).
Berthelsen, A. & Marker, M., Tectonophysics 126, 31-55 (1986).
Meriläinen, K., Geol. Survey Finland, Bulletin 281, (1976).