Slow, dense replenishments of a basic magma chamber: the layered series of the Newark Island layered intrusion, Nain, Labrador

Abstract

The Newark Island layered intrusion is a composite layered intrusion within the Nain anorthosite complex, Labrador. The intrusion comprises a lower layered series (LS) dominated by troctolites, olivine gabbros and oxide-rich cumulates and an upper hybrid series (HS) characterized by a wide range of mafic, granitic and hybrid cumulates and discontinuous layers of chilled mafic rocks (Wiebe 1988). The HS crystallized from a series of replenishments of both silicic and basic magmas. The LS crystallized from periodically replenished basic magmas. The LS has a lower zone that consists mainly of olivine-plagioclase cumulates and contains minor cryptic reversals in mineral compositions that resulted from replenishments of relatively primitive magma. An upper zone is dominated by olivine-plagioclaseaugite-ilmenite cumulates. Cumulus titanomagnetite and pyrrhotite occur within some oxide-rich cumulates, and the stratigraphically highest layers contain cumulus apatite. At intermediate levels in the sequence, cumulus inverted pigeonite occurs in place of olivine. Several prominent regressions in the stratigraphy of the upper zone are marked by fine-grained troctolitic layers with much higher Mg no. [100 MgO/(MgO+FeO)] and anorthite than underlying cumulates. These layers coarsen upward and grade back to oxide-bearing olivine gabbros within thicknesses ranging from 10 cm to 15 m. Dikes that cut the LS have major- and trace-element compositions that strongly suggest that they are feeders for the replenishments. In the lower zone when olivine and plagioclase were the only cumulus phases, replenishments were less dense than the resident magma and rose as plumes and mixed with it. Precipitation of cumulus oxides in the upper zone lowered the density of resident magma so that subsequent replenishments were more dense than resident magma. Replenishments that occurred after oxides began to precipitate had small injection velocities. These post-oxide injections flowed along the interface between resident magma and the cumulate pile and precipitated flow-banded, fine-grained troctolites.