SOME RELATIONSHIPS BETWEEN COEXISTING GRANITIC AND BASALTIC MAGMAS AND THE GENESIS OF HYBRID ROCKS IN THE TERTIARY CENTRAL COMPLEX OF SLIEVE-GULLION, NORTHEAST IRELAND

The Central Complex of Slieve Gullion, northeast Ireland, represents the dissected hearth of a Tertiary volcano in which granitic and basaltic magmas intermingled. Mixing between these magmas was influenced by marked physical and chemical differences between the opposing melts and by the degree of explosive brecciation and comminution accompanying degassing of the magmas. Chemical, thermal and viscosity contrasts between the contiguous magmas constitute the principal barriers to complete miscibility. Both superheating and volatile transfer between the opposing magmas can lead to a partial breakdown of this barrier by assisting diffusion. Hybrid rocks, rich in hornblende and granophyric mesostasis, show a progressive increase towards acid-basic contacts suggesting that hybrid magmas represent the ultimate product of reaction between opposing granitic and basaltic magmas. Geochemical variations within the hybrid suite indicate different degrees of mixing and the restriction of hybrids to isolated pockets, pipes, net-veins and zones of intrusion breccia emphasises the importance of explosive degassing and flowage in promoting magma mixing. © 1979.