Petrology of the Lower Layered Horizon of the Western Pansky Tundra intrusion, Kola Peninsula

The Lower Layered Horizon (LLH) was recognized in the gabbronoritic sequenc e of the Western Pansky Tundra intrusion based on the intercalation of comp ositionally contrasting leucocratic (anorthosite and leucogabbro) and melan ocratic (gabbronorite, norite, and pyroxenite) rocks. Leucogabbro and anort hosite layers, which often display intrusive relationships with the host ma fic rocks, are accompanied by oxide-sulfide mineralization with high PGE co ncentrations. Compared with the underlying gabbronorite, the LLH rocks are much lower in cumulus clinopyroxene or do not contain it at all, bear signi ficantly more calcic plagioclase, less ferrous orthopyroxene, and are chara cterized by an abrupt three- to fourfold increase in the Cr concentration o f the rocks. The diverse petrography of the rocks, their complicated relati onships, variations in the crystallization order of the minerals, a conspic uous shift in their chemistry toward the high-temperature end-members of th e solid solutions, and the occurrence of PGE mineralization provide evidenc e that LLH is a critical zone of the Western Pansky Tundra intrusion. Its o rigin is interpreted as brought about by unstable crystallization condition s because of magma replenishment in the chamber that contained the residual melt. The newly injected magma batches had a composition similar to that o f the parental melt of the intrusion. The mingling and mixing of the magmas supposedly resulted in a shift in the composition of the melt within the c rystallization zone of the chamber from the Opx + Cpx + Pl + L "eutectic" t o the Opx + Pl + L cotectic line and, hence, in the disappearance of liquid us clinopyroxene. The volumetric magma crystallization proceeded near the t emporary floor of the magmatic chamber and was discontinuous. Consequently, phases became separated by gravity within the chamber and gave rise to con trasting anorthosite-norite-pyroxenite layering. The PGE mineralization was presumably produced by an oxide-sulfide liquid, which separated from the t rapped melt of the anorthositic cumulates. During the postcumulus stage, te ctonic motions and/or the pressure of the overlying rocks caused an unstabl e state of the loose material of the anorthositic cumulates and its intrusi on into the surrounding mafiic rocks. The interval of the crystallization t emperatures of the LLH rocks was estimated at 900-1000 degrees C by the P-H 2O-T projection of the Ol(25-50)-CpX-Pl(<60)-Otz-H2O isopleth. The probable P-H2O interval was 1-1.5 kbar, judging from the absence of cumulative amph ibole in the rocks.