Adakitic plutonic rocks in the Finnish Precambrian: Evolution, and areal, chemical, physical and age variations

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http://urn.fi/URN:ISBN:978-952-217-399-7
Title: Adakitic plutonic rocks in the Finnish Precambrian: Evolution, and areal, chemical, physical and age variations
Author: Ruotoistenmäki, Tapio
Contributor: University of Helsinki, Faculty of Science, Department of Geosciences and Geography
Doctoral Programme in Geosciences
Geological Survey of Finland
Publisher: Geologian tutkimuskeskus
Date: 2019-02-20
URI: http://urn.fi/URN:ISBN:978-952-217-399-7
http://hdl.handle.net/10138/298882
Thesis level: Doctoral dissertation (monograph)
Abstract: Adakitic plutonic rocks, or ‘adakitoids’, representing melts fractionated below plagioclase stability depths in the lower crust to upper mantle, cover a substantial fraction of the Finnish bedrock. Only some supracrustal sub-areas lack adakitic plutonic rock outcrops. In this study, I provide a summary of adakitoids in the Finnish Precambrian: their areal, chemical and physical variations and their evolution. The samples are considered on a regional to local area scale in fve separate sections. It is shown that adakitoids constitute a distinct chemically defned rock group, which overlaps groups defned by ‘traditional’ mineralogical and chemical methods. A very effective high-resolution tool for studying adakitoid chemistry is found to be the incompatible–compatible diagram (‘Pearce–Peate spectrum’) combined with normalization by geometric means of all Finnish plutonic rocks samples (AFP) from the Rock Geochemical Database of Finland (RGDB) maintained by the Geological Survey of Finland (GTK). The characteristics of adakitoids are considered by comparing their chemical spectra with those of granite and gabbro samples from the same database. The most distinct features of adakitoids are their relatively high Sr, Eu and ratios of LREE/HREE (light/heavy rare earth elements) and compatible elements/HREE. Moreover, their AFP-normalized spectra have relative maxima at Ba, K, Na2O, Ti, Li, CaO, V, Mn, Fe, Co and Mg. The adakitic plutonic rocks vary from granodiorites to tonalites and gabbros. The Proterozoic adakitoids are denser than Archaean adakitoids, reflecting their more mafc (sanukitoid) composition. However, their magnetic properties are alike. In Proterozoic sub-areas, the main characteristics of adakitoids and the average of all-plutonic rocks clearly differ, with the exception of two ‘oceanic’ sub-areas, whose spectral peaks coincide with those of the adakitoids, although with somewhat different trends. However, in Archaean sub-areas, the averages of all-plutonic rocks and adakitoids signifcantly correlate, which refers to similar evolutionary conditions and processes for both adakitoids and crust as a whole. In this study, the Proterozoic adakitoids are generally connected with ‘modern type’ plate tectonic processes, while Archaean crust and adakitoids (TTGs) are primarily connected with collision, stacking and over-/ underthrusting of inferred pre-existing microplates. However, in the Archaean Ilomantsi sub-area in eastern Finland, evolution related to ‘modern’, plate tectonic subduction is preferred. The chemical and mineralogical differences between adakitoids, sanukitoids and TTGs are obscure and overlapping. Therefore, straightforward association of these rock types with certain tectonothermal processes using tectonomagmatic geochemical discriminant diagrams must be carried out with care. The relative contents of pyroxenes, garnet and amphiboles in the restite of adakitic melts are evaluated by using the content ratios of trace elements, whose partition coeffcients for presumed basaltic source rocksdiffer substantially. The dominant restite minerals are interpreted to be clinopyroxene and orthopyroxene for both Proterozoic and Archaean adakitoids, while garnet and amphiboles also characterize the restites of Archaean adakitoids. The rapakivi granites correlate strongly negatively with adakitoids, thus giving indications of the characteristics of the complementary restitic material of the adakitoids. However, due to their complex uplift history, the rapakivis cannot be directly considered as adakitoid restites. Proterozoic adakiTutukimuksessa tarkastellaan syvällä maankuoren alaosissa - vaipan yläosissa syntyneiden magmakivien kehitystä sekä alueellisia, kemiallisia, fysikaalisia ja ikäjakaumia.
Subject: Geologia
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