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

Sample JTP-210 exhibits prominent U-Pb age peaks at c. 1050 Ma, 1200 Ma and 1500 Ma, with more restricted peaks at c. 1800 Ma, 2500-2700 Ma and 3100 Ma (Fig. 6). Comparing this population distribution with published data from East Laurentia (e.g. Cawood et al. 2007) suggests that the Tyrone Central Inlier is of Laurentian affinity and is thus not exotic to the Laurentian margin. Comparing the Tyrone Central Inlier data with published detrital zircon ages from metasediments from this sector of the Laurentian margin (e.g. the Moine and Dalradian Supergroups) reveals that all these sequences exhibit prominent peaks in the 1000-1800 Ma age range (Cawood et al. 2007). It is the presence or absence of older (e.g. Archaean) detrital grains that is one of the most useful discriminators between these sequences. Although based on a small dataset (n = 33), the Tyrone Central Inlier exhibits strong similarities to the Argyll and Southern Highland Groups of the Dalradian, which are characterized by a significant population between 2.5 and 2.7 Ga (Cawood et al. 2003, 2007). This old population is missing from the Grampian Group of the Dalradian (Cawood et al. 2003, 2007; Banks et al. 2007), the Moine Supergroup (Friend et al. 2003; Cawood et al. 2004, 2007) and the Slishwood Division (Flowerdew et al. 2005, and unpublished data). The youngest detrital zircon, Z34_2, has a concordia age of 999 � 23 Ma, which shows that deposition of the Tyrone Central Inlier metasediments took place after this time.

Sm-Nd model ages (T^sub DM^)

Several studies have documented that the Sm-Nd isotopic signature of sedimentary rocks is usually unfractionated between the source area and the clastic sediment derived from it (e.g. Goldstein & Jacobsen 1988; Nelson & DePaolo 1988; McLennan et al. 1989; Mearns et al. 1989). The sediment produced may be of mixed provenance, and as such the Sm-Nd isotopic signature of a whole-rock sample is a weighted average of the Sm-Nd isotopic signature of its various protoliths.

Five psammitic samples and one pelitic sample from the Tyrone Central Inlier were analysed at University College Dublin. Analytical details are in the Supplementary Publication, and data are presented in Table 4. The ^sup 147^Sm/^sup 144^Nd ratios (0.099-0.1192, Table 4) of all six samples fall within the normal range for clastic sediments (Mearns et al. 1989). Depleted mantle model ages (TDM) ages range from 1.65 to 2.41 Ga (mean = 2.08 Ga, n = 6, Table 4). Comparing the Tyrone Central Inlier with Sm-Nd data from other metasedimentary rocks on the Laurentian margin and from the Midland Valley Terrane reveals similarities to Sm-Nd data from the Argyll and Southern Highland Groups of the Dalradian, consistent with the detrital zircon analysis from sample JTP-210. T^sub DM^ model ages for the Argyll Group range from 1.65 to 2.39 Ga (mean = 1.99Ga, n = 5, Daly & Menuge 1989; Flower-dew et al. 2000), and T^sub DM^ model ages for the Southern Highland Group are virtually identical and range from 1.79 to 2.36Ga (mean = 2.03, n = 5, Daly & Menuge 1989). In contrast, T^sub DM^ model ages from the Grampian and Appin Groups of the Dalradian are significantly younger, ranging from 1.64 to 1.7 Ga (mean = 1.67 Ga, n = 3) and from 1.73 to 2.02 Ga (mean = 1.91 Ga, n = 3), respectively (Daly & Menuge 1989). T^sub DM^ model ages from the Slishwood Division metasedimentary rocks (Fig. 1b) are also younger (one analysis at 1.59Ga, Flowerdew & Daly 2005; three analyses ranging from 1.54 to 1.58 Ga, mean = 1.55 Ga, Sanders et al. 1987). T^sub DM^ model ages for the felsic, predominantly metasedimentary, lower crustal xenoliths from the Midland Valley Terrane in Scotland have a mean T^sub DM^ age of 1.34 Ga (Halliday et al. 1993).