tectonothermal evolution and provenance of the Tyrone Central Inlier, Ireland: Grampian imbrication of an outboard Laurentian microcontinent?, The

Journal of the Geological Society, May 2008 by Chew, D M, Flowerdew, M J, Page, L M, Crowley, Q G, Daly, J S, Cooper, M, Whitehouse, M J

^sup 40^Ar-^sup 39^Ar and Rb-Sr dating

Three samples were selected for ^sup 40^Ar-^sup 39^Ar dating. These samples include two muscovite-bearing pegmatites (samples TCI-2 [H71868128] and TCI-10 [H71378376]) and one biotite-bearing petite (sample TCI-8 [H73178423]). Samples were irradiated at the CLICIT facility at the University of Oregon and were analysed at the ^sup 40^Ar-^sup 39^Ar geochronology laboratory at the University of Lund. The analytical technique are presented in the Supplementary Publication, and data are presented in Table 2 and Figure 4. For the two pegmatite samples, the cores of coarse (>10 mm in diameter) muscovite crystals were analysed. Sample TCI-2 yielded a ^sup 40^Ar-^sup 39^Ar plateau age of 465.8 �1.1 Ma whereas sample TCI-10 gave a ^sup 40^Ar-^sup 39^Ar plateau age of 467.8 � 0.8 Ma (Fig. 4, Table 2). This same sample (TCI-10) yielded one concordant U-Pb zircon analysis at 477 � 12 Ma (Fig. 3, Table 1). A ^sup 40^Ar-^sup 39^Ar plateau age of 468.5 � 1.4 Ma (Fig. 4, Table 2) was obtained from biotite defining the main fabric in a garnet-bearing pelite (sample TCI-8). All three samples are characterized by consistently low Ca/K ratios (Table 2), consistent with the presence of a single K-rich phase. However, the presence of extraneous argon (Kelley 2002) in these samples cannot be ruled out. The samples contain large quantities of radiogenic ^sup 40^Ar, so the data cluster close to the ^sup 39^A/^sup 40^Ar axis on an inverse isochron correlation diagram, thus yielding a poorly constrained intercept with the ^sup 36^Ar/^sup 40^ Ar axis.

The same two muscovite-bearing pegmatite samples selected for ^sup 40^Ar-^sup 39^Ar dating were also dated by Rb-Sr at University College Dublin. The analytical technique is described in the Supplementary Publication and data are presented in Table 3. Both samples are characterized by high Rb/Sr ratios (20.5 and 155 respectively, Table 3). No inclusions of any other high Rb/Sr phase such as biotite were detected under the petrological microscope. They yielded virtually identical muscovite-feldspar ages and initial ^sup 87^Sr/^sup 86^Sr ratios (TCI-2: 457.2 � 6.7 Ma, ^sup 87^Sr/ ^sup 86^Sr^sub i^ = 0.729361; TCI-10: 458.0 � 6.8 Ma, ^sup 87^Sr/^sup 86^Sr^sub i^ = 0.72930, combined isochron with all four analyses: 457.4 � 3.9 Ma, ^sup 87^Sr/^sup 86^Sr^sub i^ = 0.729361, Table 3), although the low Rb/Sr phase is different in the two samples (TCI-2: plagioclase; TCI-10: K-feldspar).

The discrepancy between the c. 458 Ma Rb-Sr muscovite ages from the pegmatite suite and the c. 468 Ma ^sup 40^Ar-^sup 39^Ar muscovite ages from the same samples is puzzling. The closure temperature of moscovite in the ^sup 40^Ar-^sup 39^Ar system is conventionally assumed to be lower than that of Rb-Sr (e.g. Cliff 1985), but, as mentioned above, the possibility of extraneous argon cannot be ruled out. Both systems are likely to recording rapid cooling from a c. 470 Ma pegmatite intrusion event (one concordant point from sample TCI-10 yields a ^sup 206^Pb/^sup 238^U age of 477 � 12 Ma). Rapid cooling at 470-460 Ma is recorded along the Laurentian margin in Ireland in the Slishwood Division (Flowerdew et al. 2000) and in the Dalradian rocks of Achill Island (Chew et al. 2003) and Connemara (Friedrich et al. 1999b).

P-T estimates of peak metamorphic assemblages

The P-T conditions of metamorphism of the Tyrone Central Inlier have been constrained by three separate approaches: (1) 'conventional' geothermobarometry, using experimentally determined equilibria of mineral reactions in P-T space; (2) a refinement of the above technique, which involves seeking additional reactions that are not experimentally determined in conjunction with an internally consistent thermodynamic dataset (e.g. Powell & Holland 1988) and using computer software (such as the average P-T function of THERMOCALC (Powell & Holland 1988)) to find the optimum P-T in such over-determined systems; (3) construction of phase diagrams that summarize the entire pressure-temperature-composition information of metamorphic rocks in an appropriate model system for a specified bulk composition (i.e. a P- T pseudosection, e.g. Powell et al. 1998; Will 1998).