ALTERATION OF KIMBERLITE AT THE CONTACT WITH DOLERITE, AS IN YAKUTIA

Abstract

In one area of Yakutia there is a kimberlite pipe that is truncated by a thick (about 100 m) Permo-Triassic dolerite sill and other, smaller bodies of the traprock association. At the contact with the dolerite sill, the pipe rocks are altered. The sill is symmetrical. Thus, the coarse-grained dolerite of its central zone grades into medium- and finely crystalline dolerite toward the periphery, with a chilled zones produced at the contact with the host rocks of the pipe. The kimberlite of the pipe is mainly serpentine and carbonate, with well preserved structure and texture of an autolytic kimberlite breccia. At the contact with the dolerite, the rocks filling the kimberlite pipe have been altered, more strongly in the hanging wall of the sill where several small apophyses extend out from it, and less so in the footwall. The presence of the chilled zone and the dolerite at the contact with the host rocks is fine-grained indicates that the latter rocks underwent very little alteration during intrusion of the sill. The alteration of the rocks filling the pipe prior to the intrusion of the mafic magma was affected by such factors as their inhomogeneity and their location with respect to the contact with the lower Paleozoic clastic-carbonate host rocks. Therefore the zoning produced by the intruding magma is not identical over large zones. The authors found that injection of mafic magma into the kimberlite body metamorphosed the carbonate-serpentine rock, producing new minerals at the contacts. The metamorphism of areas farthest from the sill consisted of recrystallization of the minerals of the pipe. In the direction of the sill, this was succeeded by chloritization, which was dominant in all parts of the pipe adjacent to the dolerite.