Sources of Archean sanukitoids (High-Mg subalkaline granitoids) in the Karelian craton: Sm-Nd and Rb-Sr isotopic-geochemical evidence

Abstract

The Sm-Nd systematics of sanukitoids with an age of 2715–2740 Ma in the Western, Eastern, and Central domains of the Karelian craton with various crustal evolutionary histories indicates that the mafic and acid rocks of the sanukitoid series were derived from two contrasting sources: enriched lithospheric mantle and lower crust. The basic sanukitoids of the Western domain were derived from the mantle enriched long before its melting [ɛNd(2715) = −0.48 ± 0.22]. The source of the acid magmas was the young juvenile crust of TTG composition [ɛNd(2715) increases to +1.2]. The mantle source of mafic sanukitoids in the Eastern domain was enriched shortly before melting [ɛNd(2740) = +1.58 ± 0.01], whereas the acid melts came from an ancient crustal source [ɛNd(2740) decreases to −3.0]. For sanukitoids in the Central domain, the time span between the enrichment of the mantle source and its melting was the shortest [ɛNd(2725) = +2.05 ± 0.15], and the contribution of the juvenile TTG crust was insignificant [ɛNd(2725) deceases to +1.7]. The variations in the isotope characteristics of the acid members of the sanukitoid series are consistent with the known age heterogeneity of the crust of the domains. The lateral isotopic-geochemical heterogeneity of the lithospheric mantle source of the sanukitoids is thought to have been related to its two-stage reworking (at 3.2 and 2.8–2.9 Ga) under the effect of TTG granitoids, which are regarded as the melting products of the subducted oceanic crust. The sanukitoids provide information on the geochemical structure of the Archean lithosphere, which is reflected in Archean crust-building processes. The Rb-Sr isotope system of the Neoarchean sanukitoids underwent transformations on the mineralogical scale and within small massifs in the course of at least two Paleoproterozoic tectono-thermal events. A trace of the event at ∼2.1 Ga is left in the Rb-Sr system of monomineralic fractions from a weakly deformed syenite of the sanukitoid series in the Central Domain. Later event (∼1.7 Ga) was recorded in the minerals of the Teloveis sanukitoid massif, which hosts a gold mesothermal deposit in the Western domain. Monomineralic fractions of muscovite and biotite from the wall-rock metasomatites and of plagioclase, microcline, and biotite from metasomatites away from the orebodies yield isochron ages of 1719 ± 60 and 1717 ± 27 Ma. This age of the metasomatic alterations of the Neoarchean sanukitoids is able to explain the broad and unsystematic variations in the Rb-Sr isotope-geochemical characteristics of these rocks. Our data on the Paleoproterozoic age of the mesothermal gold ore mineralization at the Teloveis deposit provide additional lines of evidence for the complex tectonic and metallogenic evolution of the Karelian GGT in the Early Precambrian.