Comments on the age of the oughterard granite, Connemara, Ireland

✍ Scribed by Bernard E. Leake

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 151 KB
• Volume: 23
• Category: Article
• ISSN: 0072-1050
• DOI: 10.1002/gj.3350230307

✦ Synopsis

Kennan et al. (1987) suppose that the Oughterard Granite, Connemara, has an intrusive age of 407 k 23 Ma and thatdhe granite belongs to the main (-400 Ma) phase of late Caledonian granite intrusions of Britain and Ireland. This is however neither reliably established nor even probably correct for the following reasons.

( I ) Kennan et a1 (1987) have themselves demonstrated that the granite has 'initial isotopic heterogeneities on the whole-rock scale' (p. 273) and 'is [certainly] not a homogeneous Rb-Sr isotopic system on any large scale' (p. 277). This therefore unequivocally prohibits the obtaining of a Rb-Sr age.

Rb/% plot approximate to linear distributions could be a function of isotopic mixing, either partial dispersion of an originally homogeneous composition or partial homogenization of originally dispersed values but as nothing is known about the mixing compositions that might have been involved no reliable age can be obtained.

(3) The fact that sample from each of the four localities studied in the eastern granite yield 'ages' substantially and consistently older (488, 498, 487 and 532 Ma) than the postulated 407 f 23 Ma date alone casts severe doubt on the validity on the younger age but the older ages themselves are not reliable. (4) In view of the low Rb/Sr (< 0.5) and the initial isotopic inhomogeneity it would have been most valuable to determine a number (not one; see below) of K feldsparmica-whole rock lines using individual rock specimens in order to date the last closure of the mineral systems and hence determine a minimum age.

The only such date available is that given in Leggo el al. (1966) and this yields 459 k 7 Ma (A 1.42 x