Age, evolution and regional setting of the Palaeoproterozoic Umba igneous suite in the Kolvitsa-Umba zone, Kola Peninsula: constraints from new geological, geochemical and U-Pb zircon data

The Umba igneous complex consists of an enderbite-charnockite suite, includ ing porphyritic variety of charnockites, and a porphyritic granite. Both ar e intruded by irregular veins or minor bodies of later reddish granite. The porphyritic charnockites locally contains abundant xenoliths of country ro cks and its contamination by sedimentary material is expressed by a minor c ontent of garnet, that increases in amount in areas with sedimentary inclus ions. The Umba igneous complex and the Umba block metasediments were deform ed together during two episodes of deformation. The first one was a major e pisode of thrusting with the formation of a penetrative shear foliation (S- 1), which dips gently eastwards, and a gently SE-plunging lineation. Coeval with this thrusting, the boundary between the Umba block and the Poriya Gu ba series in the southeast developed as a strike-slip shear zone, that juxt aposed the two blocks along a tectonic melange zone. The S-1-shearing defor med the enderbite-charnockite suite, and probably also the porphyritic gran ite, into plate-like, eastward-dipping bodies. Predating the shearing. the metasediments underwent high-grade metamorphism and anatexis leading to a h igh degree of partial melting. This anatexis is also found in the enderbite -charnockite suite, but in a much smaller scale and mainly in the marginal parts of the bodies. The second episode of deformation formed narrow locali zed extensional shear zones (S-2), which are developed in all rock units. T he S-1-shearing in the tectonic melange zone occurred under high-pressure m etamorphism during cooling at constant pressure (T = 806-818 degreesC, P = 9.3-9.5 kbar) and then at decreasing pressure due to tectonic uplift. Both seem to have gone through the same deformation events as the metasediments. The S-2-extension occurred under decompression (P = 7.5-8.0 kbar, T = 860- 840 degreesC) caused by uplift or tectonic erosion of the thrust pile. Thou gh indistinguishable in the field the enderbite-charnockite suite form a di scontinuous suite with a trondhjemitic trend for the former, and a calc-alk aline trend for the latter. Geochemical study shows that the charnockite gr oup is more strongly differentiated than the enderbite group and that magma tic differentiation in the charnockites were controlled by K-feldspar fract ionation. The enderbites, on the other hand, lack differentiation and are c onsidered to have crystallized rapidly from its magma source. The charnocki tes came from a different source that, judging from the high K/Rb ratio. fo rmed at a deeper crustal level than the enderbites. Both members of the end erbite-charnockite suite formed due to subduction in an island are setting, and Sm-Nd model ages of 2.1-1.9 Ga indicate that the Palaeoproterozoic sui te has a juvenile character. Conventional U-Pb zircon dating of the porphyr itic charnockite has given discordant ages of 1912.5 +/- 7.7 Ma, 1949 +/- 7 Ma and 1966 +/- 9. Our preferred interpretation is that the 1912.5 +/- 7.7 Ma age represents the age of intrusion, or maximum intrusion age of the ch arnockites, and that the 1949 +/- 7 Ma and 1966 +/- 9 Ma ages for the abrad ed type represent ages or mixed ages of inherited zircons from the contamin ating Umba block metasediments. The youngest detrital zircons in these meta sediments have similar ages. Their source could have been early magmatic ar e intrusives. which were eroded shortly after their formation. If the Umba metasediments were deposited in a magmatic are setting their initial deform ation in an evolving are may have provided the necessary heat flow for anat exis and high-grade peak metamorphism of the metasediments. Therefore, the intrusions of the enderbite-charnokite suite during the late r evolution of the magmatic are could have post-dated the peak of metamorph ism, but still pre-date collision and thrusting leading to tectonic telesco ping of the units, and thus explain the lower metamorphic grade in the Umba igneous complex compared to the metasediments. If the 1912.5 +/- 7.7 Ma ag e represents the maximum time of intrusion, the true intrusion age might be slightly younger. (C) 2000 Elsevier Science B.V. All rights reserved.