MINERAL ASSEMBLAGES AND GENESIS OF HORNFELSES IN THE OUTER CONTACT ZONE OF THE KHIBINA MASSIF, KOLA PENINSULA, RUSSIA

Abstract

The paper presents materials on the fabric of the western, southwestern, and southern exocontact zones of the Khibina alkaline pluton and metavolcanic rocks of the Il'mozerskaya Formation of the Paleoproterozoic Imandra-Varzuga riftogenic structure. The volcanics of the Imandra-Varzuga structure were originally metamorphosed to the greenschist facies (at temperatures of ≥300°C and pressures of ≥2.0-2.5 kbar) and were afterward metamorphosed to the pyroxene-hornfels facies under the thermal effect the Khibina pluton with the development of a hornfels zone 150-400 m thick. According to their composition, the hornfelses are subdivided into three zones: inner, intermediate, and outer. The inner zone is up to 30 m thick and consists of hornfelses of clinopyroxene-plagioclase composition with olivine as a typomorphic mineral and with variable amounts of amphibole. The intermediate zone occurs at a distance of 30-200 m from the pluton, is separated from the inner zone by the olivine isograde, and consists of amphibole-clinopyroxene-plagioclase hornfelses. The outer zone, 200-400 m away from the contact of the pluton, is made of fine-grained melanocratic hornblende hornfelses. The thermal transformations of the metavolcanics involved the gradual replacement of their low-temperature mineral assemblage (actinolite + albite) by a higher temperature one (clinopyroxene + amphibole + andesinebytownite ± olivine). Our data on the chemical composition of the rock-forming minerals of the hornfelses indicate that the olivine is ferrohortonolite-fayalite, the clinopyroxene belongs to the augite-ferroaugite series, and occasional orthopyroxene grains (which were found only in the intermediate zone) are ferrohypersthene. The amphibole in the hornfelses of the intermediate zone and the outermost (farthest from the contact) part of the inner zone is edenite, a Ca amphibole. The amphibole in hornfelses near the contact is kataphorite of the Na-Ca amphibole group. The plagioclase composition generally corresponds to andesine and bytownite and is albite-oligoclase near the contact with the pluton. The hornfelses adjacent to the contact bear rare sanidine grains. The mineral thermo-and barometry of the hornfelses yielded temperatures of 700-640°C and pressures of 1-1.5 kbar. The temperature determined for the zone exactly at the contact was approximately 700°C, which corresponds to the initial temperature of the rocks in contact with the magma and is close to the crystallization temperature of the nepheline syenites of the Khibina pluton. © Pleiades Publishing, Ltd. 2007.