Petrology of the upper layered horizon of the West-Pansky Tundra intrusion (Kola Peninsula, Russia)

The 250-300 m thick upper layered horizon (ULH) of the Early Proterozoic West-Pansky Tundra intrusion is marked in the gabbronorite section of the latter by occurrence of olivine (f#(Ol) = 19-22%), higher alkalinity of plagioclase (75-80% against 65-70% An), and lower f# of pyroxenes (f#(Opx) = 17-19% against 22-28%). It involves three layers of finely laminated rocks separated by zones of massive gabbronorites. The layers are made up of anorthosites and gabbronorites (layer I), olivine norites, olivine leucogabbronorites, anorthosites, and gabbronorites (layer II), and troctolites, anorthosites, and gabbronorites (layer III). Olivine-bearing rocks, gabbronorites, and anorthosites often show intrusive contacts. The horizon contains a few levels of PGE-rich low-sulfide mineralization. High heterogeneity and intricate rhythmic, often broken, layering of rocks, abrupt compositional changes of minerals toward high-temperature members of their solid solutions, appearance of olivine (a new liquidus phase), and the presence of PGE-rich low-sulfide mineralization allows us to classify the ULH as a critical zone of the West-Pansky Tundra intrusion. The origin of the critical zone is related to mixing of saturated tholeiitic melt with fresh Ol-rich tholeiitic magma. Crystallization trends of the hybrid melts in the Ol-Cpx-Pl-Q-H2O phase diagram provide a satisfactory account for the observed succession of cumulates in the three layers of the ULH. Some discrepancies between the theoretically expected fractionation trends of hybrid melts and the observed vertical succession of cumulates are accounted for by variations of the following factors: i) either continuous or discontinuous volume crystallization, ii) in situ crystallization either with or without accumulation of low-temperature cotectic components at the front of crystallization, iii) the presence of roughly concordant layers formed either by intrusion of fresh batches of magma into the partially consolidated portions of the rock massif or by postcumulus redistribution of poorly consolidated cumulates. The UHL rocks crystallized at 900 to 1000 degrees C and P-H2O of 1 to 1.5 kbar. Crystallization of the intrusion occurred in hypabyssal conditions in a chamber with the top no deeper than 2-3 km.