Geochronology of the Suomenniemi rapakivi granite complex revisited: Implications of point-specific errors on zircon U-Pb and refined λ₈₇ on whole-rock Rb-Sr

Abstract

Multi-grain isotope dilution and secondary ion microprobe zircon U-Pb as well as wholerock Rb-Sr isotope dilution data on the late Paleoproterozoic Suomenniemi rapakivi granite complex (exposed on the northern flank of the Wiborg batholith in southeastern Finland) are discussed in the light of point-specific errors on Pb/U and proposed new values of the decay constant of 87 Rb, λ 87 . U-Pb zircon data on hornblende granite and biotite granite of the main metaluminous-marginally peraluminous granite fractionation series of the Suomenniemi batholith indicate crystallization in the 1644-1640 Ma range, with a preferred age at 1644±4 Ma. A cross-cutting hornblende-clinopyroxene-fayalite granite is probably slightly younger, as are quartz-feldspar porphyry dikes (1634±4 Ma) that cut both the main granite series and the metamorphic Svecofennian country rocks of the Suomenniemi batholith. Recalculation of whole-rock Rb-Sr data published on the main granite series of the batholith by Rämö (1999) Sr i of 0.7062±0.0024. This relatively high initial ratio is indicative of a major Proterozoic crustal source component in the granites of the batholith. The main granite series of the batholith probably cooled relatively rapidly to and below the closure temperature of the Rb-Sr isotope system, with little subsequent subsolidus adjustment. The three discrete silicic magmatic phases of the batholith (the main granite series, the hornblende-clinopyroxene-fayalite granite, and the quartz-feldspar porphyry dikes) were all probably emplaced before the main volume of rapakivi granite (the Wiborg batholith proper) in southeastern Finland. The Suomenniemi batholith thus represents an early magmatic precursor to the classic Wiborg batholith and was emplaced clearly before the massive rise of isotherms associated with the ascent and crystallization of the magmas that formed the bulk of the Wiborg batholith system. Keywords (GeoRef Thesaurus, AGI): granites, rapakivi, batholiths, absolute age, U/Pb, zircon, geochemistry, isotopes, Rb/Sr, thermal ionization mass spectroscopy, secondary ion mass spectroscopy, Proterozoic, Suomenniemi, Finland *Corresponding author e-mail: tapani.ramo@helsinki.fi Editorial handling: Jussi S. Heinonen (jussi.s.heinonen@helsinki.fi)Geochronology of the Suomenniemi rapakivi granite complex revisited: Implications of point-specific errors on zircon U-Pb and refined λ 87 on whole-rock Rb-Sr 26 IntroductionThe classic Wiborg rapakivi granite batholith of southeastern Finland and its satellite intrusions were emplaced into the Paleoproterozoic Svecofennian metamorphic crust as a sequence of discordant, epizonal complexes that are overwhelmingly silicic but also include members of basic and rare intermediate rocks (Haapala & Rämö, 1990;Rämö & Haapala, 2005). The main igneous body -the Wiborg batholith proper -is ~ 150 km in its current exposed diameter, with one-third concealed under the Gulf of Finland and underneath Neoproterozoic-Paleozoic sedimentary rocks in the south. The Wiborg batholith probably co...

Mesoproterozoic geomagnetic reversal asymmetry in light of new paleomagnetic and geochronological data for the Häme dyke swarm, Finland: Implications for the Nuna supercontinent

“…The new age narrows down the emplacement time of Häme dyke swarm with slightly different trend of strikes and now the petrologically different dyke sets show coeval ages . See as well Rämö et al (2014) and Rämö and Mänttäri (2015).…”

Clinopyroxene episyenites in a Proterozoic rapakivi granite, SE Finland — recrystallization textures, mass transfer and implications for the petrology of A-type granite complexes

“…It is possible that episyenitization and recrystallization of the drill core episyenites occurred intermittently over a prolonged time window (Cuney et al 2012). The thermal history of the 1.644 Ga Suomenniemi complex is quite convoluted because of spatially connected~1.635 Ga dike magmatism (Rämö 1991;Rämö and Mänttäri 2015) and subsequent,~1.63 Ga (Heinonen et al 2016) emplacement of the large Wiborg rapakivi granite batholith just south of the Suomenniemi complex (Fig. 1).…”

“…Irrespective of the extent of recrystallization, preservation of igneous zircon is suggested by single grain U-Pb ages (ca. 1.645 Ga; Suikkanen and Rämö 2017; see also Rämö and Mänttäri 2015) and δ 18 O VSMOW values (ca. 8‰; Suikkanen and Rämö 2017;Elliott et al 2005).…”

“…The epizonal 1.644 Ga Suomenniemi rapakivi granite complex in southeastern Finland comprises a series of subalkaline amphibole, biotite-amphibole, biotite, and topaz granites (Rämö 1991;Rämö and Mänttäri 2015; Fig. 1).…”

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“…For generation and localization of the brittle structures, diverse pieces of evidence suggest significant structural inheritances from older structures: i) the repeated reactivations of the ductile shear zones (e.g. Heeremans & Wijbrans 1999;Väisänen & Skyttä 2007;Mattila & Viola 2014), ii) ductile shear zones and fault zones acting as intrusion pathways allowing the emplacement of the 1.65-1.47 Ga rapakivi granites and diabase dykes during the Mesoproterozoic extension (Heere mans & Wijbrans 1999; Korja et al 2001;Rämö & Haapala 2005;Pajunen et al 2008;Mattila & Viola 2014;Rämö & Mänttäri 2015), and iii) structural and lithological control over the orientation distribution of joints (Wennerström et al 2008), including the control of early faulting over the density and orientation of the subsequently developed joints (Fig. 1; Skyttä et al 2021).…”