Study of the nature of high-magnesian xenoliths of the Gabbro-10 massif, Monchegorsk Complex, the Kola Region

Abstract

The results of studying geology, petrography and geochemistry of chlorite-amphibole rocks and fine-grained metagabbro in the Gabbro-10 massif have been presented. Chlorite-amphibole rocks are found as large fragments of irregular shape. They have a porphyraceous texture, a schistose structure and are enriched in magnesium and chromium (19 and 0.3 wt % respectively) concerning the metagabbro prevailing in the massif. According to the classification and variation diagrams of main elements, as well as to the diagrams of the trace-element composition normalized to primitive mantle, the chlorite-amphibole rocks are metamorphosed xenoliths of melanocratic norites of the Nyud-Poaz massif. Fine-grained metagabbros, on the other hand, occur in the form of numerous angular fragments concentrated near the lower contact of the intrusion. These massive rocks of the blastogabbroic texture are less enriched in magnesium and chromium (8 and 0.02 wt % respectively) and correspond to the composition of mesocratic gabbronorites. According to the spectra of trace elements normalized to the primitive mantle, fine-grained metagabbro, with pronounced negative Nb-Ta & Zr-Hf anomalies and positive Sr & Eu anomalies, corresponds to the rocks of the marginal zones of the NKT and Nude-Poaz massifs. However, the greatest similarity is established between these rocks and the schistose gabbroids of the marginal zone of the Gabbro-10 massif. Thus, the fragments of fine-grained metagabbro are possibly autoliths of the intrusion. Collectively, the obtained data have indicated the late intrusion of the Gabbro-10 massif following the crystallization of the Nude-Poaz rocks, and the presence of autoliths of its own marginal zone in the massif points at several magmatic impulses during its formation.

Keywords

• magmatic breccia
• contact zone
• layered intrusion
• Gabbro-10
• Monchegorsk Complex