The lujavrite and khibinite bodies in the Poços de Caldas alkaline massif, southeastern Brazil: a structural and petrographic study

Abstract

Two small lujavrite-khibinite bodies, an eastern and a western one, are exposed at the northern edge of the alkaline Pocos de Caldas massif, southeastern Brazil. Detailed mapping reveals at the center of the better exposed western body a coarse-grained, mesocratic, gneissic-looking eudialyte nepheline syenite (lujavrite) with a strong subhorizontal foliation (Lu I), showing at the contacts a finer-grained border fades (Lu II). Two trachytoid nepheline syenites (NeS I and II) occur as an envelope to the central lujavrites, followed by an outer shell of coarsegrained eudialyte NeS (NeS III or khibinite). The internal structure of both bodies is that of a saucer, with successive foliated shells (in part absent in the eastern body) constituted by Lu I - Lu II- NeS I-NeS II, with rather steep dips at the contacts between the different fades; all are surrounded by the outer NeS III. Country rocks are tinguaites and a grey NeS (NeS IV). Tinguaites were the earliest rocks (intruded some 76-80 Ma ago), followed by the coarser rocks, with NeS IV emplaced before the lujavrites and associated types. The emplacement of the lujavrite bodies was a continuous process, initiated with forceful subhorizontal intrusion of strongly agpaitie magmas. Magma upwelling compressed the initial magma batch, generating the lujavrite caps by compaction and liquid extraction, with a foliated subhorizontal structure (Lu I) and an outer shell (Lu II). The intrusion forced at the same time a lateral expansion of the magma chamber. In this massif, agpaitie magmas appear always as late intrusions. A compareson with other occurrences (Ilimaussaq, Lovozero, Pilansberg, the Texan Trans-Pecos province) suggests that emplacement under forceful conditions may be a controlling factor in shaping the final fabric of lujavrites.