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Henrikki Rutanen, Åbo Akademi University (Finland)
Ulf B. Andersson, Swedish Museum of Natural History (Sweden)
Markku Väisänen, University of Turku (Finland)
Åke Johansson, Swedish Museum of Natural History (Sweden)
Olav Eklund, University of Turku (Finland)
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Voluminous magmas were emplaced at c. 1.8 Ga in the Fennoscandian Shield, mostly within the >1500 km long Transscandinavian Igneous Belt (TIB) in the west, the Central Lapland Granitoid Complex in the north, and in a belt ranging from Russian Karelia, through south Finland to south-central Sweden. In southern Finland and Russian Karelia a number of minor, post-tectonic plutons of variable composition have intruded the 1.95-1.86 Ga Svecofennian crust. We have studied six of these: the Turku, Renko, Parkkila, Luonteri, Pirilä and Petravaara intrusions, and measured the U-Pb zircon SIMS age of the Petravaara intrusion (1811±6 Ma; MSWD=1.5).
Rock types are mainly alkali-calcic, from I-type syeno/monzogabbros to syenogranites, incl. two S-type granites (SiO2 47-74 wt%, Mg# 46-18). The geochemical signatures are predominantly shoshonitic, LILE-LREE-enriched and HFSE-depleted, with continental arc affinity.
Most of the isotope data fall within the narrow ranges of εNd(t) 0 to +1 and 87Sr/86Sr(t) 0.7027-0.7033, i.e. are mildly depleted to chondritic, and overlap with data from other c. 1.8 Ga post-collisional rocks in southern Finland. One monzodiorite from Renko yielded slightly more depleted composition (εNd(t) +1.4, 87Sr/86Sr(t) 0.7019). The south Finland data overlaps the least depleted compositions of coeval mafic TIB rocks, but are generally more enriched than the 'mildly depleted mantle' identified for TIB (εNd(t) +1 to +2 and 87Sr/86Sr(t) 0.7022-0.7028). Two S-type granites, associated with the monzodiorites, yielded elevated 87Sr/86Sr(t) values (0.7048 and 0.7062), at similar εNd(t) values (-0.1 and +0.6). The Petravaara granodiorite in the east (εNd(t) = -2.8), shows involvement of an Archaean crustal component, probably of lower crustal origin according to the low 87Sr/86Sr(t).
The monzogabbroic-monzodioritic rocks are inferred to ultimately be derived from depleted mantle wedge material that was subjected to sediment-derived, CO2-rich, fluid/melt enrichment, while sources for the coeval mafic TIB rocks in the west were enriched predominantly by H2O-rich fluids. The enrichment process took place mainly during the Svecofennian subduction-related orogeny, not earlier than 2.1 Ga, as indicated by the TDM ages. However, due to the high Nd and Sr contents of the mafic magmas, contamination by Svecofennian crust will pass virtually unnoticed by the isotopic systems. The relatively high εNd(t) and elevated 87Sr/86Sr(t) values of the S-type granites indicate that they formed from evolved Rb-rich early Svecofennian metasediments as by-products of the post-collisional I-type magmatism. Apart from the Petravaara intrusion in the east, no crustal source components older than Svecofennian have been identified in these intrusions.
This c. 1.8 Ga magmatism may represent minor Svecofennian activity related to the simultaneous collision with Sarmatia in the southeast and continental-arc magmatism in the west, perhaps utilising deep shear zones.
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