Komatiitic subvolcanic rocks in the Mount Khanlauta Massif, Serpentinite Belt (Kola Peninsula)

For the first time, exposures of ultramafic rocks of subvolcanic origin have been investigated at the foot of Mount Khanlauta (Kola Peninsula) located at a distance of approximately 2 km from the southwestern margin of the Pados-Tundra layered complex of dunite-harzburgite-orthopyroxenite composition, which hosts zones of chromitite and unconventional PGE mineralization. The ultramafic body is composed of micro- to fine-grained harzburgite and subordinate orthopyroxenite and has a cryptically zoned structure. The body is of E-W strike and has a small size with an apparent thickness of approximately 0.1 km; the exposed outcrops exhibit a blocky surface as a result of extensive cracking caused by degassing and rapid cooling of a parental komatiitic melt. Elements of columnar parting are recognized, which have a hexagonal shape in cross section and are consistent with the inferred subvolcanic origin. Two zones are identified. Zone I is formed by a more magnesian olivine with Mg# = 86.0-87.9 (0.15-0.21 wt.% MnO). In Zone II, olivine grains are notably less magnesian (Mg# = 81.8-84.1) and invariably have higher contents of manganese (0.19-0.30 wt.% MnO). The presence of this zoning is corroborated by the lateral distribution of accessory chromian spinel grains with maximum Mg# values (>20) in Zone I. The Khanlauta rocks also contain subordinate orthopyroxene (Mg# = 86.3-87.2), amphiboles of the tremolite-actinolite series, and anthophyllite of deuteric (autometasomatic) origin along with accessory minerals: ilmenite, hematite ( approximately 15 mol.% escolaite, Cr (sub 2) O (sub 3) , in solid solution), and mono- and diphase grains of sulfides in the form of intergrowths of Co-bearing pentlandite (Ni/Fe = 0.9-1.3; 1.00-16.74 wt.% Co; up to 1.7-6.8 wt.% Cu) and heazlewoodite (locally in intergrowth with hematite). The inferred front of crystallization moved in the western direction, causing the formation of Zone II from a more fractionated melt with a notably lower Mg# value. A sharp increase in oxygen fugacity locally caused the formation of anomalous parageneses of chromite and ilmenite, in which the observed Mg# values of ilmenite are considerably greater than those of the coexisting chromite. The geochemical whole-rock characteristics based on major, minor, and trace elements (including LILE, REE, and HFSE), as well as the compositions and trends of chromian spinels, are similar to those in differentiated (zoned) sills of the Chapesvara Complex, which are closely associated with the Pados-Tundra layered complex. The obtained data indicate comagmatic relationships among the Khanlauta Massif, zoned sills of the Chapesvara Complex, and the Pados-Tundra layered complex. All of them crystallized from a primitive highly magnesian Fe- and Cr-enriched komatiitic magma (Al-undepleted). Thus, they belong to a single subvolcano-plutonic association being part of the Serpentinite Belt-Tulppio Belt (SB-TB) megastructure of presumably Paleoproterozoic age.