The Kostomuksha greenstone belt is located in the northwestern part of the Karelian granite-greenstone terrane (GGT). The geological structure of the area is defined by dome-like tonalite-trondhjemite domains surrounded by Late Archaean supracrustal sequences that constitute a number of conjugate synforms. The NNE-trending Kostomuksha synform (KS) is the largest of these and is over 20 km long and 10 km wide. It is clearly asymmetric: the thickness of the supracrustal sequence ranges from 3.5 km on the western limb to several hundred metres on the eastern limb. The metamorphic grade increases from greenschist in the central part to amphibolite facies along the margins.
New geological data imply that the KS consists of at least two distinct lithotectonic terranes. A mafic terrane occupies the SW part of the synform and includes a sequence of pillowed, variolitic and massive tholeiites alternating with massive and differentiated komatiite lava flows and finely bedded, ultramafic, ash flow tuffs. Numerous sill-like gabbro and peridotite bodies form an integral part of the sequence. The textural features and provenance of the volcanic rocks indicate that they were deposited at shallow-water depths. The komatiite-basalt sequence is intruded and overlain by felsic subvolcanic, volcanic and volcaniclastic lithologies. The second terrane is developed in the NE part of the synform. It consists largely of shelf-type sedimentary rocks. The base is marked by pebbly conglomerates, grits and psammites. These grade upwards into a 2 km thick banded iron formation (BIF), which is host to Kostomuksha iron deposit. Field data indicate that the two terranes are separated by a major tectonic detachment surface consisting of a shear zone several meters thick defined by mylonites developed in rocks from both terranes. We argue that the two domains moved and were spatially juxtaposed along this zone.
Field mapping and close examination of six 300 to 600 m
deep drill holes with excellent core recovery allowed a detailed study of the mafic terrane. The komatiites and basalts have
very uniform chemical and Nd-isotope compositions throughout. The basalts exhibit flat HREE distributions ((Gd/Yb)N = 1.00-1.04) and are moderately depleted in LREE ((La/Sm)N = 0.62-0.88).
The komatiites show extremely fractionated LREE patterns ((La/Sm)N = 0.38-0.51) and are depleted in Al and HREE (Al/Ti = 17, (Gd/Yb)N = 1.2). Using several independent approaches we infer that the erupted komatiite liquid contained 27±0.6% MgO. The parental komatiite magmas are interpreted to have formed in equilibrium with residual majorite garnet in a deep mantle plume (Campbell and Grifiths, 1992). The komatiites and basalts define a well constrained Sm-Nd isochron (MSWD = 0.8) with a slope corresponding to an age of 2843±39 Ma, slightly older than a U-Pb zircon age of 2795±29 Ma from the overlying rhyolites. In addition to LREE, the komatiites and basalts are depleted in Zr and Th and show positive Nb-anomalies. This evidence together with high eNd values of +2.9 to +3.4 strongly suggest that no crustal rocks were involved in petrogenesis of the mafic-ultramafic magmas. In contrast, the felsic volcanics are characterized by low eNd values of 0 to -2.4 and were derived from LREE-enriched material that records a long crustal residence time. Both domains were intruded by late granitoids dated at 2720±15 Ma.
Based on the assumption that the Fe/Mg ratio in a primitive magma is directly related to its liquidus temperature (Longhi et al., 1978) and using a published technique (Abbott et al., 1994; Nisbet et al., 1993) we have estimated average liquidus temperatures of erupted komatiite liquid at 1547±6°C. By further assuming that the liquidus temperature is directly related to the potential mantle temperature (Abbott et al., 1994) we calculated the latter to be 1767±8°C. Moreover, McKenzie and Bickle (1988) have shown that the crustal thickness is controlled by the mantle temperature such that hotter mantle produces thicker oceanic crust. For the mafic terrane our calculations give 54±1 km for crustal thickness. The buoyancy of the oceanic lithosphere is a function of its crustal thickness, and Abbott et al. (1995) established that oceanic plates with crust over 25 km thick are unsubductable. The estimated crustal thickness for the mafic terrane is therefore at least two times in excess of the critical value. We conclude that the mafic terrane represents a remnant of an upper crustal part of a once areally extensive Late Archean oceanic plateau, which was accreted to and obducted onto an older continental margin represented by the sedimentary terrane. The felsic rocks within the mafic terrane are thought to represent collision-related volcanism that erupted through the accreted plateau material. The Kostomuksha greenstone belt is a potential candidate for one of the typical tectonic settings for Archean GGT.
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