ONSET OF GRAMPIAN OROGENESIS CONSTRAINED BY HIGH PRECISION SM-ND GARNET AGE OF THE BALLANTRAE OPHIOLITE

The Grampian phase of the Caledonian Orogeny in Scotland is characterized by relatively short-lived, high-temperature medium-pressure metamorphism initiated by the closure of a marginal ocean basin. The Ballantrae Ophiolite represents the remnant of this marginal sea obducted onto continental crust, thus it is believed that the age of obduction of this ophiolite complex gives the oldest age of orogenesis in the region (Oliver et al.2000; Oliver 2001; Hamilton et al.1984). While many studies have focused on the duration and intensity of peak conditions (e.g. Oliver et al. 2000; Viete et al. 2013; Baxter et al. 2002) the age of this first collision remains poorly constrained. In fact, it is suggested that the entire orogeny lasted only ~15 m.y. (Oliver et al. 2000) but its onset is reported as 478 ± 8 Ma. based on a K-Ar age from Bluck et al. (1980). Hamilton et al. (1984) give a Sm-Nd age of a metapyroxenite from the Ballantrae Ophiolite of 505 m.a. ±11 m.y., though this age is believed to record internal thrusting within the unit prior to obduction. In order to better constrain the timing of obduction of the Ballantrae Ophiolite, and thus the start and duration of Grampian metamorphism and orogenesis, we report results of new ¹⁴⁷Sm-¹⁴³Nd dating of a garnet amphibolite from the metamorphic aureole at the base of the ophiolite complex. The sample was collected from the Knocklaugh region of the Ballantrae Ophiolite. The garnet separate underwent partial dissolution cleansing to reveal a ¹⁴⁷Sm/¹⁴⁴Nd of at least 1.16817 ± 0.00043. This cleansed garnet was paired with the whole rock for isochron analysis. Our results indicate a garnet age of 477.6 ± 1.9 Ma (MSWD=2.0; n=7). In conjunction with a biostratigraphic constraint on deposition of detrital garnet (Tucker and Mckerrow, 1995), this new age constrains the duration of the Grampian episode to 12.6 ± 3.1 Ma and ultimately provides the basis for further study of the timescales of metamorphism in the region.