Whole-rock geochemistry of some tonalite and high Mg/Fe gabbro, diorite, and granodiorite plutons (sanukitoid suites) in the Kuhmo district, eastern Finland

Abstract

New whole-rock geochemical analyses (major and trace elements) of three tonalite plutons and high Mg/Fe granodiorites and diorites, termed sanukitoid suites, are presented from Neoarchaean Kuhmo district, eastern Finland. The 2.83 Ga and 2.78 Ga tonalites display only moderate fractionation of LREE/HREE thus demanding only minor or no residual garnet in their respective source regions. However, the more prominent HREE depletion in the 2.75 Ga tonalites requires more significant garnet residue. The relatively wide compo sitional variation in the tonalites implies that Archaean TTG (tonalite-trondhjemite-granodiorite) magmatism was derived in general from sources that exhibited considerable mineralogical heterogeneity. The ~2.70 – 2.74 Ga high-Mg/Fe granodiorites and diorites are characterised by strong fractionation of LREE/HREE, relatively high Ba and Sr, and elevated abundances of Cr and Ni as well systematic depletions in P and Ti, which are all typical features of Archaean sanukitoid suites. Some of the granodiorite plutons contain compositionally intermediate high-K enclaves, which share these geochemical characteristics, implying a contribution of mafic-intermediate LILE- and LREE- enriched source for the magma. The geochemical characteristics of the studied plutonic rocks demonstrate the role of contrasting sources for the tonalites and sanukitoid suites in the Kuhmo district.