The 1650^1540 Ma rapakivi granites and associated basic rocks (e.g. gabbro^anorthosites) in southern Finland intrude the local juvenile Paleoproterozoic crust with little isotopic contrast to the contemporary mantle. Conventional isotopic studies have not been able to provide a satisfactory answer to the long-running controversy over the sources involved in their petrogenesis. In situ zircon laser ablation inductively coupled plasma mass spectrometry analysis of LuĤ f isotopes and trace elements has been used to resolve the source and magmatic evolution of the rapakivi granites and associated massif-type anorthositic rocks in southern Finland. Initial Hf isotope compositions of zircon extracted from the rapakivi granites are relatively homogeneous and overlap with the modeled evolution of the Paleoproterozoic crust of southern Finland. Within-sample variation in zircon of the basic rocks is considerably greater than that of the granites and the initial Hf isotope compositions vary from crustal values (e Hf $0) to depleted mantle (e Hf $þ9) values. Furthermore, Ti abundances that correlate with Hf isotope compositions record significant crustal assimilation during crystallization of the mafic magmas. Our in situ Hf isotope results enhance the resolution of isotopic methods in pursuit of the sources of the Precambrian rapakivi suites.The new data argue for the truly bimodal nature of rapakivi magmatism, involving at least two distinct magma sourcesça relatively homogeneous Paleoproterozoic crustal component for the granites and a depleted, mid-ocean ridge basalt source-like mantle component for the basic rocks.

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The 1.64 Ga Ahvenisto complex, southeastern Finland, is an anorthosite-mangeritecharnokite-granite (AMCG) suite in which diverse interaction styles of coeval mafic and felsic magmas are observed. Commingling, resulting in mafic pillows and net-veined granite dykes, and chemical mixing producing hybrid rocks, are the most common interaction types. Detailed description of the factors that controlled the interaction styles and relationships between involved rock types are provided using targeted mapping, petrography, and geochemical analyses complemented by chemical mixing and melt viscosity modeling. Interaction occurred at intermediate stages in the magmatic evolution of the complex: when the last fractions of mafic (monzodioritic) melts and the earliest fractions of felsic (hornblende granitic) melts existed simultaneously. Differentiation of mafic magma has produced three monzodioritic rock types: 1) olivine monzodiorite (most mafic, Mg# 49-40), 2) ferrodiorite (Mg# 42-33), and 3) massive monzodiorite (most evolved, Mg# 28-27). The types form an evolutionary trend, and each exhibits different style of interaction with coeval hbl-granite resulting from contrasting conditions and properties (temperature, viscosity, composition). The variation in these properties due to magma evolution and relative proportions of interacting magmas dictated the interaction style: interaction between olivine monzodiorites and granite was almost negligible; ferrodiorites intermingled forming pillows with granitic veins intruding them; and chemical mixing of massive monzodiorite and hbl-granite produced hybrid rocks.

Sample ID & spot # Sequence in Drift corrected Samples Standards Stage position Field aperture centering session 18 O/ 16 O*  18 O (‰)  18 O (‰) x y X Y Session 1 915ox_857_@25 Notes:Samples are labeled according to the NORDSIM labfile system, n3602 corresponds to sample A1271, n3603 to A1933, n3604 to A1360, n3605 to A1306 and n3606 to A118.Analyses were performed at the same time with samples from other projects, which accounts for the missing sample numbers in the end of session 1 and at the beginning of session 2 sequence.

The mid-Proterozoic, locus classicus Wiborg rapakivi granite batholith of southeastern Finland and adjacent Russia comprises a varying, bimodal (silicic-basic) sequence of plutonic, subvolcanic, and volcanic rocks. At the current level of erosion silicic rocks are dominant, the most prominent of which are wiborgites and dark wiborgites (that have been considered to mark the main build-up stage of the batholith) and pyterlites. New observations and optical microscopy data from the dark wiborgite-dominated Ristisaari Island in the southern, off-shore part of the Wiborg batholith show that dark plagioclase megacrysts in dark wiborgite are calcic xenocrysts. They were probably incorporated into wiborgite magma from consanguineous massiftype anorthosite magmas in the course of the evolution of the bimodal magmatic system. Our new ID-TIMS U-Pb zircon age of the Ristisaari Island dark wiborgite, 1627±3 Ma, is the youngest isotopic age so far determined for the plutonic rocks of the Wiborg batholith. This, combined with preexisting U-Pb zircon data, implies a minimum duration of 12 m.y. (1642-1630 Ma) for the emplacement of the plutonic rocks of the batholith. Combined with data on highlevel dike rocks, a window of at least 20 m.y. (1642-1622 Ma) is implied. Furthermore, as the batholith grew, the overall locus of magmatism may have shifted southwards. New whole-rock Nd isotope data on the dark wiborgite of the Ristisaari Island and three further granites of the batholith, as well as Nd (whole-rock) and Sr (whole-rock, plagioclase) isotope data on a spectrolite massif-type anorthosite from the east-central part of the batholith, are also presented. These data suggest that the lithosphere across the Wiborg batholith area in the southeastern part of the Svecofennian orogen may vary slightly in overall mantle separation age.