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Late-orogenic Svecofennian deformation in SW-Finland ; implications from pegmatite emplacement and U-Pb columbite-tantalite ages

Collisional orogenic thickening of the Svecofennian crust in southern Finland resulted in anatectic melting of subemerged crust and formation of migmatites and granites.The activity culminated at ca 1830-1840 Ma, but continued with decreasing intensity until ca 1800 Ma.Migmatites and microline granites from the main anatectic phase are exposed along a 100 km wide and som 500 km long zone of intense transpressional deformation transecting southern Finland.During the last stages of the orogeny numerous granite pegmatites and minor microline granites were emplaced contemporaneously with postogenic granites.Precise U-Pb columbite-tantalite ages from rare-element pegmatites combined with structural studies on the emplacement mechanisms of these pegmatites indicate that a major pegmatite phase occured only a few million years before most of the postorogenic granites.The pegmatites were emplaced when the rheologic conditions changed from ductile to semi-ductile and brittle.Tantalite U-Pb ages from different segments of a pegmatite emplaced under semi-ductile deformation of a gabbro host-rock range between 1807.0+-2.9 Ma (2ó) and 1803.1+2.9/-2.0 Ma.Tantalite from a second pegmatite emplaced under brittle deformation of the same gabbro host are 1802.9+-1.3 Ma old.All ages are obtained from subconcordant to concordant data.Together with 1840-1830 Ma microcline granite sheets, that are related with early phases of transpressional deformation in southwestern Finland, the U-Pb tantalite ages from structurally late pegmatites indicate that the transpressional late Svecofennian tectonic regime in southwestern Finland persisted for at least 30-40 Ma.Ductile deformation, which affected the whole zone of transpressional deformation in southwestern Finland, was concentrated to wide east-westerly zones of dextral shearing.Later semi-ductile deformation was confined to narrower shear zones along E-W and NNE-SSW striking margins of rhomboidal blocks.Granite pegmatites are dominant associated with the different shearzones.They are usually concordant or oblique to the shear zones.The structurally youngest pegmatites are undeformed and are either flat-lying sheetlike bodies or steep northsoutherly veins.

Publication(s)

16.2.1996 / 17.3.1998