Paleoproterozoic magmatism across the Archean-Proterozoic boundary in central Fennoscandia: Geochronology, geochemistry and isotopic data (Sm–Nd, Lu–Hf, O)

Abstract

The central Fennoscandia is characterized by the Archean-Proterozoic (AP) boundary and the Central Finland Granitoid Complex (CFGC), a roundish area of approximately 40,000km2 surrounded by supracrustal belts. Deep seismic reflection profile FIRE 3A runs across these units, and we have re-interpreted the profile and crustal evolution along the profile using 1.92–1.85Ga plutonic rocks as lithospheric probes. The surface part of the profile has been divided into five subareas: Archean continent (AC) in the east, AP, CFGC, boundary zone (BZ) and the Bothnian Belt (BB) in the west. There are 12 key samples from which zircons were studied for inclusions and analyzed (core-rim) by ion probe for U–Pb dating and oxygen isotopes, followed by analyzes for Lu–Hf by LA–MC–ICP–MS.The AC plutonic rocks (1.87–1.85Ga) form a bimodal suite, where the proposed mantle source for the mafic rocks is 2.1–2.0Ga metasomatized lower part of the Archean subcontinental lithospheric mantle (SCLM) and the source for the felsic melts is related plume-derived underplated mafic material in the lower crust. Variable degrees of contamination of the Archean lower crust have produced “subduction-like” Nb–Ta anomalies in spidergrams and negative εNd (T) values in the mafic-intermediate rocks. The felsic AC granitoids originate from a low degree melting of eclogitic or garnet-bearing amphibolites with titanite±rutile partly prevailing in the residue (Nb–Ta fractionation) followed by variable degree of assimilation/melting of the Archean lower crust. The AP plutonic rocks (ca. 1.88Ga) can be divided into I-type and A-type granitoids (AP/A), where the latter follow the sediment assimilation trend in ASI diagram, have high δ18O values (up to 8‰) in zircons and exhibit negative Ba anomalies (Rb–Ba–Th in spidergram), as found in sedimentary rocks. A mixing/assimilation of enriched mantle-derived melts with melts from already migmatized sedimentary rocks±amphibolites is proposed. The CFGC is characterized by both I-type and A-type (CFGC/A) intermediate and felsic granitoids. The I-type granitoids are divided into two groups at ≥1885Ma and ≤1882Ma, where the latter overlap in age with the CFGC/A granitoids. Both I-type CFGC and CFGC/A granitoids are interpreted to have formed from mixing of Paleoproterozoic SCLM-derived melts with crustal melts from hydrous and dry intermediate-felsic igneous sources, respectively. The geochemistry, dominantly δ18O values below 6.5‰ in zircons and TDM (2.11–2.42Ga) of the CFGC granitoids favor the occurrence of older crust (ca. 2.1–2.0Ga) in their genesis. The BZ granitoids are similar in age but more juvenile with TDM ages between 1.94Ga and 2.16Ga. The 1.92Ga granodiorite in the BB is correlated with juvenile gneissic tonalites and granodiorites found from the AP boundary.We suggest that the present high-velocity lower crust under the CFGC is composed of melt-extracted granulites (crustal source age≥2.0Ga) and mafic cumulates which both formed during 1.90–1.88Ga arc magmatism. The ≤1.88Ga stage represents the end of compression/transpression and is followed by 1.87–1.86Ga buckling, forming the Bothnian Oroclines.