Petrography and Chemical Composition of a Suite of Ultramafic Xenoliths from Lashaine, Tanzania

• Published in: The Journal of geology Vol. 88; no. 3; pp. 343 - 352
• Main Authors: Pike, Jane E. N., Meyer, C. E., Wilshire, H. G.
• Format: Journal Article
• Language: English
• Published: University of Chicago Press 01-05-1980
• Subjects: Chemical composition; Dunite; Garnets; Geological Notes; Minerals; Olivine; Peridotite; Rock textures; Rocks; Spinel; Volcanoes
• ISSN: 0022-1376; 1537-5269
• DOI: 10.1086/628512

Mafic and ultramafic xenoliths from the Lashaine volcano, Tanzania are composed of garnet and spinel peridotites and pyroxenites and rocks enriched in hydrous phases. Composite xenoliths, composed of two or more lithologies, include peridotite with thin phlogopite bands, and peridotite-pyroxenite assemblages. The dominant metamorphic textures of all lithologies are overprinted by several generations of partial melt textures; the youngest is interstitial glass quenched upon eruption. Earlier partial melting took place under conditions that permitted relatively coarse crystallization of the melts that now are represented by the pyroxenites and hydrous mineral veins and dikes, emplaced in the peridotite long before eruption of the Lashaine volcano. Metasomatic reaction between peridotite and a pyroxenite dike yielded compositional changes in the peridotite that are like the chemical trends separating garnetiferous from nongarnetiferous peridotites in the suite as a whole. We infer that the chemical heterogeneity of the suite resulted from episodic partial melting, concentration of fluids in veins, and metasomatic reactions between veins and garnet-peridotite wall rock. Evidence from Lashaine and other localities indicates that local melting-vein formation-metasomatism has occurred repeatedly in the mantle, probably in diapiric masses. Alkalic components become concentrated by these events until ultimately enough melt is produced to erupt as alkalic basalt.