New Light on the Oughterard Granite: Connemara’s S-Type Granite, Ireland

Abstract

Most of the Connemara granites, including the Galway Granite batholith, have a clearly defined circular or ellipsoidal outcrop shape and are I-Type granites often with K-feldspar phenocrysts, whereas the Oughterard Granite (OG) forms numerous small intrusions scattered east-west over 40km with two larger linked bodies near Oughterard and is S-Type and aphyric. The only comprehensive mapping, mineralogical and chemical study of the OG was published over 50 years ago before the important 1974 recognition of the distinction of I- and S-Type granites was first made. New mapping of the southern Oughterard area with small OG intrusions is presented and many scattered published chemical and Rb, Sr, S and Pb isotopic studies integrated to confirm overall their individual conclusions that the peraluminous OG is of S-Type, being formed from melted Dalradian rocks as are the fluid-carried associated uneconomic mineral deposits. The OG is confined to the high sillimanite zone, mostly south of, and mainly later than, the 465-464Ma D4 Connemara Antiform, where a wide (>15km N-S) >70km E-W band of 469Ma gabbros and 467Ma quartz diorite gneisses intruded into already hot Dalradian rocks undergoing regional metamorphism. This generated the OG magmas by partial melting of pelites and semi-pelites. The upward intrusion of the OG magma was significantly delayed, as has been shown by precise dating in other granites elsewhere, to ~464-461Ma during the late stages and mostly after D4 but before the country rocks had completely cooled. The OG intrusions as a whole probably form the second largest area of S-Type granite in Ireland, after the Leinster Granite.