Evolution and metallogenic settings of the Pados-Tundra chrome-bearing ultramafic complex, Kola Peninsula: isotope Sm-Nd and U-Pb evidence

The Pados-Tundra massif is located in the western Kola Peninsula and included in the Notozero ultrabasic rock complex (Vinogradov, 1971). The intrusion occurs as a body of ca. 13 km² stretched out to the north-east. Enclosing rocks are Archaean granite- and granodiorite-gneisses. There are three major areas in the massif structure (Mamontov, Dokuchaeva, 2005): endocontact area, rhythmically layered series, and upper area. The endocontact area with thickness of 10-20 m occurs as schistose amphibole rocks formed during the metamorphism of main rocks. The rhythmically layered series occurs as a number of rocks from dunites to orthopyroxenites and composes most of the massif. There are 7 rhythms in total, each of which starts with dunites and ends with orthopyroxenites. Dykes of mezo- and leucocratic gabbro, diorites, and hornblendites are developed in the series rocks. The upper gabbronorite area can be partially observed in the north-eastern massif. Presumably, its major volume has been overlapped by enclosing rocks as a result of the overthrust. In the massif, there are 4 horizons of disseminated stratiform chromite ores, which are confined to dunites and serpentinites, as well as to a number of lens- and column-like bodies (podiform type) of chromite ores (Mamontov, Dokuchaeva, 2005; Barkov et al., 2017). Previous isotope-geochronological studies have determined the massif rock age of 2.15 Ga (Shapkin et al., 2008). However, further geological field observations and analysis of the obtained data assume that the intrusive is much older.

New Sm-Nd geochronological data indicate that the massif rocks and its rhythmically layered series are of Paleoproterozoic age, which is similar to the age of the Cu-Ni-Co-Cr-PGE ore-magmatic system of the Fennoscandian Shield (Amelin et al., 1995; Bayanova et al., 2014, 2017, 2019; Hanski et al., 2001; Huhma et al., 1990, 1996; Layered intrusions ...; 2004; Maier, Hanski, 2017; Mitrofanov et al., 2019; Peltonen, Brugmann, 2006; Puchtel et al., 2001; Serov, 2008; Serov et al., 2014; Sharkov, 2006; Sharkov, Smolkin, 1997). Complex Sm-Nd and U-Pb isotope-geochronological studies have allowed determining the major formation and alteration stages of the Pados-Tundra complex rocks:

–  formation of the rhythmically layered series rocks of the intrusive 2485±77 Ma, harzburgites of the layered series – 2475±38 Ma;

– metamorphism of the massif rocks at the turn of 1.95 - 1.9 Ga;

– postmetamorphic cooling of the complex rocks tо 650°-600°С at the turn of 1872±76 Ma (Sm-Nd for metamorphic minerals) and then to 450°-400°С (U-Pb for rutile, 1804±10 Ma).

Therefore, the study results expand geography the East-Scandinavian large Palaeoproterozoic igneous province and are prospective for further study of analogous ultramafite-mafite complexes.

All investigations and were supported by the RFBR 18-05-70082, 18-35-00246, Presidium RAS Program #48 and are in frame of the Theme of Scientific Research 0226-2019-0053.