A study of the stratigraphy, structure and metamorphism of the Tarna area in the northern Swedish Caledonides.

A 250 sq km area near Tamaby in the Northern Swedish Caledonides has been mapped in detail predominantly at a scale of 1:20000. The rocks have been divided into three tectono-stratigraphic units; the lowest is the Seve-Koli unit, the highest the Laxfjallet unit which shows an affinity to the rocks of both north and south Storfjallet; these are separated in part by the Basement unit, which is tectonically sandwiched in an apparently intermediate position. Detailed stratigraphic, structural and metamorphic studies have been applied to each of these units with particular reference to the geometrical relationship they each bear to one another and also to the nature of the contact between the low- (Koli) and high-grade (Seve) rocks within the Seve-Koli unit. The recognition of a partially preserved ophielite complex in the lower poxt of the low-grade Koli sequence and certain lithofacies considerations have led to the adoption of a sedimentation model controlled predominantly by the development of a marginal back-arc basin in Lower Ordovician times similar to the plate tectonic model proposed by Gale and Roberts (1972) in the Trondheim region. An important early (D1, pre-main schistosity) synclinal repetition of the stratigraphy in the low-grade rocks is followed by a polyphase sequence of deformation (D2-D4) which is similar to that in the higher grade (Seve) rocks and also that in the Laxfjallet unit. The question of the nature of Seve-Koli contact is considered in the light of the metamorphism-deformation relationships throughout the unit, the siting of the different metamorphic isograds, the chemical composition of various garnets and amphiboles and the stratigraphy within the critical transition zone between the two rock suites. Kieft's (1952) interpretation of the petrology of the Laxfjallet rocks is revised and the relationship of these rocks to the tectonically higher Storfjail Nappe is indicated. The lithostratigraphy and tectonic position of the Basement unit, which forms the southernmost part of the Bangfjallet Inlier, is critically reviewed. In particular, the important cataclastic breakdown of the igneous component of the complex from quartz-syenite to flaser gneiss to mylonite is emphasized, whilst its earlier interpretation as a simple window structure is questioned. A detailed study of the geometry of certain F2-F4 minor folds and deformed lineation loci has allowed a qualitative interpretation of the mechanisms of folding to be made. This is combined with the finite strain analysis of a deformed conglomerate marker horizon which has elucidated the relationship between the finite strain ellipsoid and the different deformation phases, particularly the D2 phase of relatively homogeneous strain and the Dif phase of folding. The problem of the pronounced development of F2 fold axes parallel to the major axis of the D2 strain ellipsoid (WI to WNW orientation),and the wider question of the important development of fold axes oblique to the Caledonoid trend are discussed in the light of these results. Finally, preliminary basic v/hole-rock analyses are presented for different metavolcanic rocks within the Seve-Koli unit, and the various gabbro intrusions within the Laxfjallet unit. It is shown, in particular, that the greenstones associated with the serpentinites in the lower part of the Koli sequence are quartz-tholeiitic in composition with subordinate amounts of alkaline-basalt, and possibly show overall affinity to oceanic basalt. Calc-alkaline volcanics are only found slightly higher up the succession,and are pyroclastic and conspicuously more siliceous in character.