HORNBLENDE BAROMETRY OF THE GALWAY BATHOLITH, IRELAND - AN EMPIRICAL-TEST

The application of four hornblende geobarometers, two empirical and tw o experimental, to the 400 Ma Galway Granite, Ireland gives a pressure of crystallization of the zoned hornblende cores in the western parts of the granite of 2.62 +/- 1.2 kb falling to < 1.53 +/- 1.02 kb at th e hornblende rims whereas in the more eastern part of the batholith th e value of 4.30 +/- 0.70 kb is obtained from unzoned hornblendes. Thes e results are consistent with field and petrographic evidence which in dicates a much deeper level of early crystallization of the granite in the central and eastern area with larger K-feldspar phenocrysts (up t o 6 cm). Although some of the uplift is related to late upward faultin g, the main uplift of the centre of the granite in the east was due to late magmatic differential slip against the marginal granite which be came vertically foliated. In the west the crystallization of hornblend e started and completed at lower pressures than in the east with final hornblende crystallization at the limit of the field of igneous hornb lende and at depth of < 5 km. Hornblende geobarometry reveals: (1) tha t different parts of some batholiths crystallized at very different pr essures (and therefore depths) and have been juxtaposed at the present level of erosion (2) that at least the early part of the crystallizat ion of some granites took place at significantly greater depths than t he final crystallization and emplacement position; (3) in zoned hornbl endes crystallization occurred during magma movement.