Petrogenetic constraints for the genesis of Archaean sanukitoid suites: geochemistry and isotopic evidence from Karelia, Baltic Shield

New Sm–Nd isotope data are presented for late Archaean sanukitoid suites of the Karelian granite-greenstone terrane of the Baltic Shield. Within the terrane, there are regional variations in the Nd isotope compositions of the sanukitoids. Sanukitoids in the youngest Central Karelian domain have Nd isotope characteristics similar to depleted mantle, whereas those from the older West Karelian and Vodlozero domains have lower initial eNd values. The isotopic heterogeneity of the sanukitoids is interpreted to result from variations in the time interval between mantle source enrichment and the partial melting that produced the sanukitoid magmas. We therefore suggest a two-stage model of the generation of sanukitoid magmas. During first stage, the mantle was metasomatised by fluids and/or melts generated during subduction or tectonic underplating. Later, between 2.74 and 2.70 Ga, a tectonothermal anomaly generated the sanukitoid magmas by melting of the previously metasomatised mantle. Most of the sanukitoid intrusions are cut by calc-alkaline lamprophyre dykes that have the similar geochemical signatures to their sanukitoid wall rocks. New Sm–Nd isotope data for these lamprophyres suggest a genetic kinship between the sanukitoids and the lamprophyres. Comparison between chemical signatures of sanukitoids and modern, subduction-related magmas shows that Archaean sanukitoids actually have no modern analogues. Sanukitoid melts can be modelled as partial melts of mantle that had been metasomatised a short time previously. The melts could have been in equilibrium with residues containing Ol, Cpx plus minor Phl and Grt, at a pressure of about 2.5 GPa and a temperature of 1150–1230 8C. The small percentage of residual phlogopite would retain Rb in the mantle and explain the peculiar signature of low-Rb and high Ba and Sr, characteristic of the sanukitoids. We envisage a temporal evolution of the composition of the mantle in this region. This pattern of evolution controlled production of the sanukitoid melts and explains why the sanukitoids have such a restricted range of ages. Sanukitoid magma production must have followed from a major, global mantle enrichment event. These magmas could not have been produced earlier because the Archaean mantle had not become sufficiently enriched before 2.7 Ga. We suggest that their scarcity after the major period of sanukitoid magmatism may be due to the increased thickness of overlying crust, which can inhibit mantle enrichment

Meso-Neoproterozoic Mafic Sills along the South-Eastern Margin of the Siberian Craton, SE Yakutia: Petrogenesis, Tectonic and Geochemical Features

We report major and trace element concentrations, along with Nd isotope compositions, for Late Mesoproterozoic to Early Neoproterozoic dolerite sills from the Sette-Daban ridge (southern Verkhoyansk, south-east Siberia). Based on their major element composition, all rocks correspond to low-Ti (<3 wt% TiO2) moderately alkaline basalts. The intrusions can be subdivided into two groups based on their trace element compositions. One group includes sills mainly distributed in the southern part of the study area (Yudoma group), with mid-ocean ridge basalt (MORB) trace element patterns enriched in aqueous fluid mobile incompatible (FMI) elements (Sr, Pb, Ba, U). The second group includes sills mostly distributed in the northern part of the study area, enriched in immobile incompatible (II) elements (Th, Nb, light rare earth elements (LREE)) and to a lesser extent, in aqueous fluid mobile elements. The Nd isotope signatures of the dolerites characterize a depleted mantle source, with a small enrichment from recycled continental crust. The geochemical characteristics of these igneous rocks are analogous to low-Ti basalts of large intraplate provinces (e.g., the Karoo and Siberian Traps). We propose that they formed by rifting-induced melting of the heterogeneous metasomatized shallow spinel-bearing mantle zone. We suggest that two different melting sources were involved in the generation of the two geochemically distinct sill groups, including the addition of two different subduction components. The southern sills were formed by melting of depleted lithospheric mantle enriched with FMI elements, corresponding to subduction-induced metasomatic alteration by fluids at shallow depths. The northern dolerites were formed by melting of depleted lithospheric mantle enriched with II elements, associated with the melting of subducted sediments at deeper depths