The prehistory of Suomussalmi, eastern Finland; the first billion years as revealed by isotopes and the composition of granitoid suites mallikuva; kuva-alan koko voi vaihdella GEOLOGICAL SURVEY OF FINLAND The prehisTory of suomussalmi, easTern finland; Th

• Main Authors: Perttu Mikkola, Docent Ulf B Andersson, Professor Hervé Martin
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Fennoscandian; karelia*; karelian
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1059.9028; http://tupa.gtk.fi/julkaisu/erikoisjulkaisu/ej_079_synopsis.pdf

Cover: Migmatized TTG suite granitoid in a road cut near the Suomussalmi church village. Light grey leucosome and dark grey mesosome form a banded structure that is folded and then cross cut by younger pink leucogranite dykes, some of which have also been deformed. The length of the hammer is 60 cm. Photo: Perttu Mikkola, GTK. ABSTRACT The four papers summarized in this thesis deal with the Archean and earliest Paleoproterozoic granitoid suites observed in the Suomussalmi district, eastern Finland. Geologically, the area belongs to the Kianta Complex of the Western Karelian Terrane in the Karelian Province of the Fennoscandian shield. The inherited zircons up to 3440 Ma old together with Sm-Nd and Pb-Pb data confirm the existence of previously anticipated Paleoarchean 'protocrust' in Suomussalmi. The general timeline of granitoid magmatism is similar to that of the surrounding areas. TTG magmatism occurred in three distinct phases: ca 2.95 Ga, 2.83-2.78 Ga and 2.76-2.74 Ga. In Suomussalmi the TTGs sensu stricto (K 2 O/Na 2 O < 0.5) belong to the low-HREE type and are interpreted as partial melts of garnet amphibolites, which did not significantly interact with mantle peridotites. Transitional TTGs (K 2 O/Na 2 O > 0.5), present in Suomussalmi and absent from surrounding areas, display higher LILE concentrations, but otherwise closely resemble the TTGs sensu stricto and indicate that recycling of felsic crust commenced in Suomussalmi 200 Ma earlier than in surrounding areas. The youngest TTG phase was coeval with the intrusion of the Likamännik-kö quartz alkali feldspar syenite (2741 ± 2 Ma) complex. The complex contains angular fragments of ultrabasic rock, which display considerable compositional heterogeneity and are interpreted as cumulates containing clinopyroxene (generally altered to actinolite), apatite, allanite, epidote, and albite. The quartz alkali feldspar syenite cannot be regarded as alkaline sensu stricto, despite clear alkaline affinities. Within Likamännikkö there are also calcite ...