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 We st-Pansky Tundra intrusion is marked in the gabbronorite section of the lat ter by occurrence of olivine (f#(Ol) = 19-22%), higher alkalinity of plagio clase (75-80% against 65-70% An), and lower f# of pyroxenes (f#(Opx) = 17-1 9% against 22-28%). It involves three layers of finely laminated rocks sepa rated by zones of massive gabbronorites. The layers are made up of anorthos ites and gabbronorites (layer I), olivine norites, olivine leucogabbronorit es, anorthosites, and gabbronorites (layer II), and troctolites, anorthosit es, and gabbronorites (layer III). Olivine-bearing rocks, gabbronorites, an d anorthosites often show intrusive contacts. The horizon contains a few le vels 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 o f their solid solutions, appearance of olivine (a new liquidus phase), and the presence of PGE-rich low-sulfide mineralization allows us to classify t he ULH as a critical zone of the West-Pansky Tundra intrusion. The origin o f 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 disc repancies 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 discontinuo us volume crystallization, ii) in situ crystallization either with or witho ut accumulation of low-temperature cotectic components at the front of crys tallization, iii) the presence of roughly concordant layers formed either b y intrusion of fresh batches of magma into the partially consolidated porti ons of the rock massif or by postcumulus redistribution of poorly consolida ted cumulates. The UHL rocks crystallized at 900 to 1000 degrees C and P-H2 O of 1 to 1.5 kbar. Crystallization of the intrusion occurred in hypabyssal conditions in a cha mber with the top no deeper than 2-3 km.