METASEDIMENTARY ROCKS OF THE ROSEMARKIE INLIER, GREAT GLEN FAULT ZONE, SCOTLAND: MONAZITE-XENOTIME PETROCHRONOLOGY AND SHEAR SENSE/DEFORMATION TEMPERATURES INDICATED BY QUARTZ C-AXIS FABRICS

The Rosemarkie Inlier on the NW side of the GGFZ is composed of foliated and lineated amphibolite facies Moine metasedimentary rocks in which foliation strikes NE-SW (parallel to the GGFZ) and dips steeply to the SE, and lineation plunges gently-moderately to the ENE. Sheared leucogranite sheets intruded sub-parallel to foliation carry a similar foliation and lineation and have monazite and zircon crystallization ages of 397.6±2.2 and 400.8±2.6 Ma, respectively (Mendum & Noble 2010).

New LA-ICPMS monazite and xenotime analyses from a metasedimentary unit (a Fsp-Bt-Qz-Grt gneiss) in the northern part of the Inlier yield a weighted average 206Pb/238U age for >95% concordant monazite analyses of 405±5 Ma (n = 43). Trace element compositions imply two populations of monazite, one with more pronounced Eu anomalies and one with less significant Eu anomalies associated with core domains. Concordant analyses from these populations yield concordia ages of 403±4 Ma (n = 23, MSWD = 1.16) and 419±5 Ma (n = 10, MSWD = 0.21), respectively. A concordia age for xenotime is 406±6 Ma (n = 10). All quoted age uncertainties include an added 1% external uncertainty. The average X_Y+HREE content in the younger, deeper Eu anomaly, monazite population is 0.0746, which yields a temperature of 622 °C using the Heinrich et al. (1997) calibration.

A deformation temperature of 625 °C is indicated by the opening angle (Faleiros et al. 2016) of an asymmetric cross-girdle quartz c-axis fabric in a Fsp-Qz-Ms-Grt gneiss from the same outcrop in which both Fsp and Qz have recrystallized by grain boundary migration. The quartz fabric demonstrates that the moderately plunging lineation (52° to 065°) parallels the maximum principal stretch direction and is associated with an oblique sinistral (NW side up to SW) shear sense which is confirmed by asymmetric mica fish.

Close similarity of temperatures indicated by monazite-xenotime thermometry and the quartz fabric suggests that the accessory phases crystallized during oblique sinistral shearing in Lower Devonian times – potentially indicating slightly earlier motion on the GGFZ than recorded by the leucogranites. The likely timing of high-temperature deformation in the Rosemarkie Inlier is one component in a complex regional-scale unroofing history during a transition from thrusting to strike-slip deformation.