Compositional evolution and cryptic variation in pyroxenes of the peralkaline Lovozero intrusion, Kola Peninsula, Russia

摘要

The Lovozero alkaline massif is the largest of the world's layered peralkaline intrusions (approx 650 km~2). We describe the evolution of clinopyroxene from the liquidus to the late residual stage throughout the whole vertical section (2.5 km thick) of the Lovozero Complex. Microprobe data (approx 990 analyses) of the clinopyroxenes define a relatively continuous trend from diopside containing 15-20 percent hedenbergite and 10-12 percent aegirine components, to pure aegirine. The main substitutions during the evolution of the Lovozero pyroxenes are (Na,Fe~(3+),Ti) for (Ca,Mg,Fe~(2+)). The composition of the pyroxene changes systematically upwards through the intrusion with an increase in Na, Fe~(3+) and Ti and decrease in, Ca and Mg. The compositional evolution of the Lovozero pyroxene reflects primary fractionation processes in the alkaline magma that differentiated in situ from the bottom to the top of the magma chamber as a result of magmatic convection, coupled with the sedimentation of minerals with different settling velocities. The temperature interval of pyroxene crystallization is very wide and probably extends from 970 to 450 deg C. The redox conditions of pyroxene crystallization in the Lovozero intrusion were relatively low, approximating the QFM buffer.