Relationships between hornblende K-Ar ages, chemical composition and hydrogen isotopes, Connemara, western Ireland: evidence for a massive extinct hydrothermal system

Major element compositional analyses, K-Ar ages, deltaD parts per thousand and delta O-18 parts per thousand values for 30 zoned and unzoned hornblend es from Dalradian amphibolites and metagabbros. mostly in north Connemara a re reported. Although the cooling ages are expected from previous U-Pb zirc on studies to be c. 475-450 Ma the results obtained are from 556 +/- 6 to 4 10 +/- 9 Ma with an average of 470 Ma. Fluid movements. probably at 275 +/- 15 degreesC, i.e. much below Ar closure temperature for hornblende. errati cally reset the ages, as is shown by a negative correlation of hornblende d eltaD and age and a wide scatter of ages even within 2 m. The changes were implemented by deltaD exchange between fluid and hornblendes in which ionic porosity. Z, influenced the loss of Ar and possibly its gain from the flui d to give the excess Ar found in some samples. Z is controlled by hornblend e chemical composition. High Mg, Si and Mg/Fe and low Fe, Al, Ti, Na and pa rticularly low K, amphiboles giving low Z values retained Ar more firmly an d gained Ar more readily than compositions which had higher Z values, which gave younger ages. These range down to c, 400 Ma. being the age of the int rusion of the Galway Granite suite that initiated the fluid circulation. Th e scatter of ages is a consequence of incomplete equilibration, mainly beca use of the slow deltaD exchange rate below 350 degreesC and partly because the fluid permeated erratically in different areas and down cracks of all k inds, promoting irregular Ar movement. The meteoric fluid circulated throug h Connemara. the Galway Granite and at least some of the contiguous Siluria n sediments of the South Mayo trough. These overlying sediments may have co ntributed to the water circulated in north Connemara which was slightly les s negative deltaD than in central Connemara. For hornblende K-Ar ages to be a reliable measure of times of uplift and cooling, they need to be demonst rated to be free from the influence of hot fluids by showing no correlation of age with deltaD.