Archean crustal evolution of the Suomussalmi district as part of the Kianta Complex, Karelia: Constraints from geochemistry and isotopes of granitoids

Abstract

This study presents new geochemical and isotopic data from granitoids belonging to the Kianta Complex of the Western Karelian terrane, collected from the Suomussalmi district in eastern Finland. Geochemically the Archean granitoids can be divided into five groups: TTGs (tonalite–trondhjemite–granodiorite, sensu stricto), transitional TTGs, sanukitoids (high-Mg granitoids), quartz diorites and anatectic leucogranitoids. This study includes chemical data for the first four and isotopic data for all the suites. The majority (78 out of 80 samples) of TTGs sensu stricto (K2O/Na2O<0.5) belongs to the low-HREE type (YbN<4) with highly variable (La/Yb)N (7–161) ratios and are interpreted as partial melts of garnet amphibolites. TTGs have low Mg numbers (average 38) and 90% of the samples have Cr and Ni contents below detection limits (30 and 20ppm respectively). This suggests absence of or limited interaction with mantle. Transitional TTGs (K2O/Na2O>0.5) display higher LILE concentrations, but otherwise closely resemble the TTGs sensu stricto. Sanukitoids and quartz diorites have moderate (La/Yb)N (6–50) and higher HREE, Cr and Ni than the TTGs. Sanukitoids can be differentiated from quartz diorites based on higher Ba (averages 1453 vs. 626ppm), K2O (averages 3.02 vs. 1.94%) and LREE (LaN averages 141 vs. 92) contents, although overlap between these two suites exists. TTG magmatism in Suomussalmi and the surrounding parts of Kianta Complex can be divided into three major phases: ca 2.95Ga, 2.83–2.78Ga and 2.76–2.74Ga. Sanukitoid magmatism (2.73–2.71Ga) partly overlaps in error limits with quartz diorites aged 2.70Ga and is coeval with one transitional TTG sample (2.72Ga). Migmatization of older rocks and intrusion of leucogranitoids took place at 2.70–2.69Ga. Geochemistry, inherited zircons (<3.44Ga) and the negative initial εNd values of most rocks indicate varying involvement of older crust in their genesis, but δ18O values of zircons show that source(s) with high δ18O value(s) were significant only in the genesis of the youngest TTG and sanukitoid samples.