The relationship between adakitic, calc-alkaline volcanic rocks and TTGs: implications for the tectonic setting of the Karelian greenstone belts, Baltic Shield

Abstract

Two types of coeval acid-intermediate rocks with different petrological, geochemical and isotopic features have been discovered among volcanic rocks and surrounding synkinematic tonalite–trondhjemite–granodiorite (TTG) plutons of Late Archaean greenstone belts in the Karelian granite–greenstone terrane. Type-1 rocks comprise trondhjemites and sub-volcanic, occasionally volcanic dacite–rhyolite rocks. They are characterized by high Sr, low Y and HREE contents, high Sr/Y ratios, and strongly fractionated REE patterns with no significant positive or negative Eu anomaly. Initial ɛ Nd is positive, indicating a generation from juvenile source with little or no contribution of ancient continental crust. Type 2 is represented by diorite–granodiorites and calc-alkaline basalt–andesite–dacite–rhyolite (BADR) series. As compared to type 1, these rocks differ by their lower Sr, higher Y and HREE contents, lower Sr/Y ratios and less fractionated HREE patterns with negative Eu anomalies. Initial ɛ Nd varies from negative to positive values, thus indicating a variable contribution of sialic crust. Geochemistry of the two magmatic series suggests their formation in a convergent plate margin setting. The type-1 rocks resemble Phanerozoic adakites, which represent slab-derived melts contaminated by overlying mantle wedge. The type-2 rocks resemble BADR series, which were derived from a mantle wedge metasomatized by slab-derived fluids and melts, with subsequent variable crustal contamination. The spatial distribution of these two types of magmatic series defines the asymmetry of the studied granite–greenstone structures, which presumably reflects the primary lateral zoning of island arc formed under specific thermal conditions in the Archaean mantle. Adakite melts upraised to the surface in the frontal part of the island arc, where mantle wedge was thin, showing no or little interaction with metasomatized mantle, and formed adakite-type plutonic and sub-volcanic rocks. At greater depths, adakitic melts and fluids interacted with the overlying mantle wedge and caused its partial melting with generation of calc-alkaline BADR volcanic rocks and diorite–granodiorite plutons in the rear part of the island arc. Our data suggest that greenstone belt volcano-plutonic arcs were initiated on different types of crust, which presumably determined the petrogenetic and isotope variations of the studied BADR- and adakite-type island-arc complexes.