Evolution in composition of granitic rocks controlled by time-dependent changes in petrogenetic processes: examples from the archaean of eastern finland

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The grey gneisses of Eastern Finland form the basement on which the Archaean greenstone belts were developed. They are composed of tonalites, trondhjemites and granodiorites (TTG) emplaced during two distinct magmatic episodes at 2.86 Ga (Kivijärvi gneisses) and 2.65 Ga (Naavala). Petrogenetic models based on trace element behaviour indicate that the TTG are ultimately derived from the mantle by a mechanism involving two stages: 1. (1) melting of the mantle to form a tholeiitic crust; and 2. (2) re-melting of these tholeiites transformed into garnet amphibolites to yield TTG rocks. The low Yb contents of Archaean TTG are explained by the influence of residual garnet and hornblende in the source rocks involved in magma genesis. After 2.5 Ga, the role of these minerals becomes progressively less important and this is interpreted in terms of a cooling Earth model. At present, the subducted oceanic crust is relatively ‘cold’ and the geothermal gradient at its interface with the overlying mantle wedge is low. In addition, the descending lithospheric slab undergoes dehydration before partial melting is able to occur and the liberated fluids pass upward into the mantle wedge or base of the continental crust where partial melting may begin. These magmas have calc-alkaline affinities, and have equilibrated with residual olivine and pyroxene in the source. Consequently, the liquids derived from partial melting have high Yb contents. During the Archaean, subducted oceanic crust was relatively ‘hot’ and the geothermal gradients in the interface zone with the mantle were high (c. 20–25°C km −1. Thus, partial melting of the subducted oceanic material was possible at temperatures between 650 and 700°C before dehydration would take place. Residual phases in the source would include garnet and hornblende, thus explaining the low Yb contents of the TTG association. It is concluded that there has been a change in the site of cale-alkaline magma genesis since Archaean times.