The petrology and geochemistry of Caledonian Newry Granodiorite from the Tertiary Igneous Complex of Slieve Gullion, North East Ireland

The central complex of Slieve Gullion, northeast Ireland, exposes the eroded core
of a large Tertiary shield volcano in which relics of an older Caledonian granodiorite,
the Newry Granodiorite, appear as screens separating successive intrusions of Tertiary
gabbro and granophyre. Thermal metamorphism and extensive partial melting of this
granodiorite have produced marked petrographic and textural changes which are not
obvious at grain boundaries between quartz and feldspar. Here the coarse granitoid
fabric of the unmodified granodiorite gives way to micrographic intergrowths of quartz
and feldspar in the thermally metamorphosed rocks. The textural changes are most
obvious in granodiorite adjacent to the Tertiary basic intrusions. Here, in addition
to the textural changes in the felsic minerals, Ca-rich and Ca-poor pyroxene replace
the amphibole and biotite which are typical of the unmodified granodiorite. Tem-
peratures calculated from coexisting clinopyroxene and orthopyroxene in hybrid
granodiorite adjacent to Tertiary dolerite are between 8000C and 8300C.
The chemistry of the modified granodiorite remains unchanged suggesting that the
metamorphic process was isochemical. Localised hybrids formed at contacts with the
Tertiary basaltic and granitic rocks. These hybrid rocks display chemical evidence for
interaction between the Caledonian Newry Granodiorite and the Tertiary magmas of
the central complex. Variation diagrams suggest that the hybrid rocks formed by the
mixing (assimilation) of increments of fused or partially fused Newry Granodiorite
with Tertiary granitic or basaltic mag