Abstract

Euhedral crystals of almandine-rich garnets occur in unmetamorphosed lavas, tuffs, and intrusive rocks of the Borrowdale volcanic series (Ordovician) over a wide area in the Lake District of northwest England. These have been known for many years and both pyrogenetic and metamorphic origins have been proposed by various workers. Eight complete garnet analyses (major and trace elements) are presented here, on the basis of which, together with field and microscopic data, a pyrogenetic origin is favored. It is suggested that garnets in slates in the walls of magma reservoirs and vents could be incorporated in the magma under conditions of excess gas pressures as might occur during the formation of welded tuff.

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