Trace Element Geochemistry and Petrogenesis of Finnish Greenstone Belts

Abstract

Archean metavolcanic rocks from three greenstone belts (Suomussalmi, Kuhmo and Tipasjärvi) of eastern Finland have been subject to a detailed geochemical study which leads to a discussion of their petrogenesis and the problem of compositional heterogeneity in the Archean mantle. Lithostratigraphically, the greenstone belts are roughly divided into a lower and an upper volcanic sequence. Rocks of komatiitic and tholeiitic compositions are restricted to the lower sequence, while andesitic tuffs, dacite-rhyodacite lavas and minor basalts of alkaline affinity occur in the upper sequence. All rocks from the greenstone belts have been subject to regional metamorphism of the upper greenschist facies to the lower garnet amphibolite facies. Consequently, the geochemical distinction of original magma types and the discussion of petrogenesis have relied heavily on the abundances of less mobile elements, such as TiO2, rare earth elements (REE), and some transition metals (e.g. Ni and Cr). Using all the possible discriminants of major element compositions, we have concluded that two general magmatic series that exist in the lower volcanic sequence might be distinguished by the parameter of TiO2 content: the komatiitic series is characterized by having TiO2 ≤ 1.0 per cent and the tholeiitic series by ≥1.0 per cent. The general series do not imply that a cogenetic relationship linked only by fractional crystallization exists in each series. Several magmatic types could be distinguished by their characteristic REE distribution patterns. In general, the komatiitic rocks show flat HREE (heavy REE) and flat or depleted LREE (light REE) patterns; the tholeiitic rocks show fractionated patterns with some degree of LREE enrichment, whilst the acidic rocks demonstrate highly fractionated patterns with significant HREE depletion. Model calculations indicate that: (1) the komatiitic and the tholeiitic series have no clear genetic relationship; (2) some basaltic komatiites (MgO < 12 per cent) could have been derived by crystal fractionation from a melt of peridotitic komatiite composition (MgO ≈ 30 per cent), but others require various degrees of partial melting from the same or different source regions to account for their trace element abundances; (3) both partial melting and fractional crystallization have interplayed for the production of various rocks within the tholeiitic series; (4) three different types of source materials are proposed for all magmas from the lower volcanic sequence. All three types have the same initial HREE (about 2× chondrites) but different LREE (from very depleted to 2×, flat) abundances; (5) volcanic rocks of the upper volcanic sequence must have originated at great depths where garnet remains in the residue after partial melting and melt segregation. The recognition of the strongly LREE-depleted mantle sources, deduced from the REE patterns of peridotitic komatiites from Finland, Canada and Rhodesia, may suggest that this depletion is a worldwide phenomenon, and that the Archean upper mantle is as heterogeneous in composition as the modern upper mantle. The causal effect of the depletion might be related to the generation of some contemporaneous LREE-enriched tholeiitic rocks, or more likely, to contemporaneous or previous continental crust forming events.