Abstract
Detailed field and mineralogical studies of the associated porphyritic felsite and granophyre ring dykes of Slieve Gullion, N. Ireland, have been made to elucidate the genesis of the rock and of the alkali feldspars. Central, acentric subsidence of country rock within the ring fracture allowed the partially-crystalline magma to rise. Upon crystallization of one-third of the magma the roof shattered to form agglomerate, and the loss of volatiles led to rapid crystallization of the felsite. A subsequent subsidence without roof shattering led to formation of the granophyre.
The majority of the alkali feldspar phenocrysts fall between the sanidine and orthoclase series, consisting of intergrowths of monoclinic K-feldspar, anorthoclase and sodium-rich plagioclase. Two crystals from the granophyre consist of intergrowths of orthoclase, microcline and low-temperature plagioclase. It is thought that the majority of the phenocrysts are true phenocrysts formed slowly at considerable depth. During the cooling period they changed from homogeneous sanidine into their present assemblages through a sequence of unmixing and ordering reactions. It is thought that the local content of volatiles was probably the dominant factor in determining the extent of adjustment to the low-temperature assemblages. The occurrence of microcline is remarkable in hypabyssal rocks. It is thought probable that it results from fragments of Newry granodiorite caught up in the magma, although it might have arisen from plagioclase crystals by ionic exchange.
