Iron-magnesium minerals from differentiated rocks of

Abstract. The compositions of the main rock-forming iron-magnesium minerals (clinopyroxenes, alkaline amphiboles, biotites) from the rocks of the Lovozersky alkaline massif have been studied by microprobe analysis. Clinopyroxenes of the studied rocks represent aegirine-diopside-hedenbergite solid solutions. They form trends on the triangular diagram Aeg-Di-Hd, testifying for the higher alkalinity of the clinopyroxenes from juvitic rocks and eudialite-lujavrites. Amphibole alkalinity increases in the sequence sodalite-syenite→lujavrite→eudialite-lujavrite→juvitic rocks. Sometimes biotite occurs in sodalite-syenites and corresponds to phlogopite-annite series. The compositions of the iron-magnesium minerals are connected to the exchange equilibria and depend on mineral forming T-P conditions formation. This fact allows the application of mineral geothermometers in order to estimate the forming conditions of the corresponding assemblages. Using Bt-Cpx and Amph-Cpx geothermometers we estimated the forming temperatures of clinopyroxene-bearing parageneses of the investigated sodalite-syenites. They vary in the range 990 to 595-630°C showing different stages of crystallization with the separation of several generations of certain minerals (clinopyroxenes, in particular)