The structure and cryptic layering of the Pados-Tundra ultramafic complex, Serpentinite belt, Kola Peninsula, Russia

Abstract

The Paleoproterozoic Pados-Tundra ultramafic complex, ~6 x 1.5–2.1 km in size and ~2.1 Ga in age, located in the Kola Peninsula of Russia, is the main representative of the Serpentinite belt in the northern Fennoscandian Shield. It is composed of fragmented or foliated bodies of dunite–harzburgite –orthopyroxenite; these have an elevated potential for Cr and the platinum-group elements (PGE). In general, the complex consists of the Dunite Zone (olivine cumulates) and Orthopyroxenite Zone (orthopyroxene + olivine cumulates, interlayered); its upper zones of more evolved rocks of mafic compositions appear to have been removed by erosion. The complex shows well-recognized patterns of cryptic layering, documented along cross-sections in grains of olivine and Ca-poor pyroxene. Narrow ranges of high-Mg compositions are observed in olivine [Fo85.5–90.6] and orthopyroxene [Wo<0.1–3.0En85.1–91.2Fs8.1–12.5]. Their trends of crystallization indicate that cumulate olivine, orthopyroxene, and olivine–orthopyroxene rocks (dunite and orthopyroxenite, with subordinate harzburgite and olivine-bearing orthopyroxenite) become, in general, more evolved toward the internal portions of the complex; stratigraphically lower and early-crystallizing cumulates are exposed closer to its outer contact. The compositions of early phases of cumulus origin, Fo91 olivine and En91, orthopyroxene are notably magnesian, implying an elevated Mg# in the parental magma. The anomalously Cr–Al-rich grains of serpentine (up to ~2.5 wt.% Cr2O3 and ~4.0wt.% Al2O3), hitherto unreported, are present in specimens of dunite near the northeastern margin of the complex. Supercooling and metastable crystallization likely affected the melt in the eastern portion of the complex near the Dunite block (i.e., host for segregations and stratiform-like layers of chromitite) and relatively close to the outer contact. An uncommon mineralization of the PGE is associated with the chromitite deposits at Pados-Tundra.