The South-Western End of the Newry Igneous Complex. A Contribution towards the Petrogenesis of the Granodiorites

Abstract

Summary The south-western end of the Newry Complex is shown to be composed of hornblende-, biotite-, and porphyritic granodiorite, and the evidence as to the origin of these three varieties is presented. Biotite-granodiorite is found to have developed from biotite-hornfels, sometimes with trondhjemite as an intermediate stage, by the introduction of Na, Ca and Si, with the concomitant removal of Al, Fe, Mg, Na, K, H, Ti, P and Mn. This outgoing material has been fixed in adjacent bands of biotite- and diopside-hornfels, both these rock types becoming basified. As a result of this basification, the complement of granitization, the biotite-hornfels is enriched in biotite and cordierite, whilst the diopside-hornfels is converted first to biotite-hornblende-plagioclase-diabrochite (equivalent to quartz-biotite-diorite) and subsequently to oligoclase porphyroblast biotite-hornblende-plagioclase-diabrochite (equivalent to porphyritic quartz-biotite-diorite). Finally, the granitization of oligoclase porphyroblast diabrochite, by introduction of K and Si, and loss of Al, Fe, Mg, Ca, Na, H, Ti, and P, leads to the development of hornblende-granodiorite. The minimum introductions from an external source necessary for the granitization of the country rocks of the region were Na, Ca and Si, and after granitization was completed migratory material remained which was rich in Al, Fe, Mg and Ca, and included, in addition, Si, Na, K, H, Ti, P and Mn. By analogy with the small-scale complementary changes of granitization and basification, it is suggested that the basic material which migrated from the region of granitization as a whole was fixed (a) in the biotite-enriched contact aureole, and (b) in basic and ultrabasic roof rocks such as those which cap the granodiorites at the north-eastern end of the complex, some of which have already been shown to have been evolved from sediments. The evidence suggests that the processes involved in the evolution of the granodiorites and their complementary basic and ultrabasic types culminate in magma formation.