MONAZITE-XENOTIME THERMOMETRY CONSTRAINTS ON THE METAMORPHIC EVOLUTION OF THE SCANDIAN NAPPE STACK, NW HIGHLANDS TERRANE, SCOTLAND

The metamorphic nappe stack of northernmost Scotland formed during the Scandian phase of the Caledonian orogeny. Several major ductile thrusts define the structure of this area, known as the Moine (structurally lowest), Ben Hope, Naver and Skinsdale (structurally highest) thrusts. The goal of this work is to place tighter temperature-time constraints on the metamorphic evolution of the Scandian orogen and study the interaction between thrust-mediated advection of heat and metamorphism. The monazite-xenotime thermometer of Gratz and Heinrich (1997) has been used to place thermo-temporal constrains on the evolution of these thrust sheets. Monazite and xenotime have been analyzed by laser ablation split stream ICP-MS at the Univ. of California Santa Barbara and electron microprobe at the University of Massachusetts. A sample from the structurally highest, most hinterlandward, part of the Naver nappe has a population of monazite ages at 426 ± 3 Ma and temperatures of ~750^oC (X_YPO4 = 0.125). A sample just above the Naver thrust, in the most foreland part of the nappe, has xenotime dated at 424 ± 5 Ma that are thought to be in equilibrium with small high Yttrium monazite rims. These domains give temperatures of ~680^oC (X_YPO4 = 0.104). Monazite in the Ben Hope thrust sheet give ages of 413 ± 2 Ma associated with temperatures of ~500^oC (X_YPO4 = 0.062). Below the Ben Hope thrust, in the Moine thrust sheet, monazite is unstable and allanite is the dominant Ce-bearing phase. Xenotime is stable in these lower grade rocks and commonly overgrows chlorite that is forming from garnet breakdown. Chlorite in these samples shows a clear transition from syn-kinematic (forming tails on garnet porphyroblasts) to post-kinematic (isotropic mats close to garnet porphyroblasts). Thus, xenotime that can be related to these textural domains may constrain the age of final deformation in this orogenic system. Our data imply that a relatively low syn-thrusting thermal gradient is recorded across the Naver thrust sheet, with a foreland decreasing changing in temperature of 70^oC over a horizontal distance of 25 km.