General fractional crystallization model for shoshonitic magmatic rocks in southern Finland

We have characterized the geochemistry, age and whole-rock Sr-Nd isotope composition of the Tistronskär LREE-
rich monzodiorite, and the geochemistry and age of the Loukee high Ba-Sr granite dykes in western and southern
Finland. In this study, we aim to: (i) investigate the petrogenesis of the post-orogenic magmatism (1.81-1.76 Ga) in
the southern Finland; (ii) show the distribution of post-orogenic magmatism in the Svecofennian province and (iii)
evaluate, which processes control the geochemical variations in the shoshonitic magma series.
The zircon U-Pb geochronology defines an age of 1806 ± 3 Ma for the Tistronskär monzodiorite and an age
of 1794 ± 12 Ma for the Loukee granite, which are similar to other post-orogenic shoshonitic intrusions in western
and southern Finland (Rutanen et al., 2011). The obtained near-chondritic initial εNd value of -0.3 ± 0.4 at 1806 Ma
suggest a subcontinental lithospheric mantle source for the Tistronskär monzodiorite. Overall geochemistry and age
indicate that the parental magma for the Tistronskär monzodiorite as well as other shoshonitic intrusions was formed
in a post-orogenic setting in a mature continental arc environment, predated by voluminous anatectic leucogranite
magmatism at 1.86-1.79 Ga. Slab retreat due to roll-back, lower crust delamination, or the decay of radioactive
isotopes, is suggested to be the cause of magmatism in a mature continental arc environment. Moreover, the
composition, timing and isotope data of the shoshonitic magmatism are broadly compatible with melts generated in
an enriched mantle source, where earlier subduction enriched the mantle wedge in Ba, Sr and LREE from subducting
sediments.
Shoshonitic rocks in southern Finland range from monzogabbro through monzonites to granites and
geochemical characteristics imply that fractional crystallization processes controlled the geochemistry of the whole
magmatic series. To examine this assumption, we have modelled the fractional crystallization of basaltic trachy-
andesitic magma (stemmed from an enriched mantle wedge sources) intruding the upper crustal level using the
Magma Chamber Simulator (Bohrson et al., 2020). Modelling results are consistent with fractional crystallization
along the liquid lines of descent of spinel, apatite, hematite/ilmenite and clinopyroxene, followed by orthopyroxene,
plagioclase, biotite, and K-feldspar at lower temperatures (< 900 °C). Melt composition changes from basaltic trachy-
andesite to rhyolite following the shoshonitic series. Suggested major element modelling acts as a framework for the
more precise trace element modelling. As the transition from major- to trace element modelling is at some point
mandatory because, for example, classifying granites according to major elements is difficult due to granite forming
melts following the eutectic minimum trends, and thus clustering the major element composition (Moyen et al.,
2021). Our fractional crystallization model is a generalization meaning that geochemical patterns seen in the
shoshonitic rock series stemmed from individual intrusion. This kind of generalization is mandatory in order to adapt
magmatic models to the entire orogeny and not just locally. This kind of over-generalizing approach also creates
concerns, for example in the case of possible false correlations in non-homogeneous data (Rollinson & Pease, 2021).