Behavior of trace elements in the contact zones of agpaitic intrusions of the Kola region: The role of fluids

Abstract

The contact zones of large intrusions are objects ofspecial interest to researchers, because the boundariesbetween materials with strongly different compositionand physical properties demonstrate a processes, thestudy of which allows us to reveal the mechanisms ofmetasomatic transformations of rocks, first of all,peculiarities of redistribution of major and trace elements. The study of the contact influence of alkalinemagmas, the geochemical specifics of which is controlled by high concentrations of incoherent elements,is of special importance, because thick fenite zoneswith unique mineralization with sizes comparable withthose of massifs are formed at the contacts of alkalineintrusions. Currently fenitization zones of alkaline–ultrabasic massifs with carbonatites are the mainobjects of investigations [1–5]; geochemical study offenitization zones of large agpaitic intrusions withrelated industrial mineralization in fact was not performed.Our study was aimed at determination of the character of trace element distribution, the composition offluids, and estimation of their influence on elementmigration during metasomatism of Precambriangneiss in the contact zone between the Khibiny andLovozero massifs, the largest alkaline intrusions of theKola province. Northern contacts of agpaitic syenitewith Archean gneiss were the objects of investigation.The contact between nepheline syenite of theperipheral part of the massif (khibinite) and Archeangranite gneiss of the basement represented by pyroxene–amphibole and biotite varieties was revealed inthe Northern Lyavuchorr area of the Khibiny Massif,in the section with a length of 1.2 km. The thickness ofthe fenitization zone does not exceed 10 m. Nearcontact gneiss contains veins of finegrained alkalinesyenite (“umptekite”) with a thickness of 1–10 mobserved at a distance of 0.5 km from the contact. Theendocontact zone with a thickness of 20–50 m is represented by pegmatoid nepheline syenite with largesegregations of K–Na feldspar, clinopyroxene,nepheline, and eudialyte.The endocontact zone of the northeastern part ofthe Lovozero Massif (Vavnbed Mountain) is composed of massive nepheline syenite being graduallyreplaced by aegirine–amphibole–nepheline–Kfeldspar pegmatoid with large sc hlierenlike eudialyte segregations towards the contact. Transformations ofbasement rocks are mostly controlled by the influenceof aegirine–microcline–albite veins with zircon andilmenite mineralization on them. Gneiss alterationswere revealed in replacement of the initial mineralparagenesis by newly formed potassic feldspar, albite,nepheline, aegirine, arfvedsonite, and others.