A tale of two orogens: the contrating T-P-t history and geochemical evolution of mantle in high- and ultrahigh-pressure metamorphic terranes of the Norwegian Caledonides and the Czech Variscides

Garnet peridotite (GP) massifs within collisional orogenic belts provide windows into deeper levels of the mantle (100 to possibly >400 km) than do spinel peridotites. A geochemical, isotopic and petrological study of GP from high- and ultrahigh-pressure metamorphic terranes in the Western Gneiss Region, Norwegian Caledonides, and the Gfohl Nappe, Bohemian Massif, Czech Variscides, suggest that more than one type of mantle can be involved in continent-continent collision. GP from the Western Gneiss Region are chemically depleted, give consistent LT-HP estimates of garnet equilibration, and define old (Pre-Caledonian) Sm-Nd mineral crystallization and recrystallization ages. These ages match similarly old ages in the associated crust, implying that the mantle and crust were coupled. The Norwegian peridotites apparently sat in a cold, static mantle environment for a long time before they were incorporated into the Caledonian orogen and are excellent examples of cold, melt-depleted, LIL-poor, buoyant lithosphere of the type that would be expected beneath the Baltic or Laurentian Shield prior to Caledonian collision. GP from the Gfohl Nappe in the Bohemian Massif are more complex than the Norwegian peridotites. Relative to Norwegian GP, they are chemically and mineralogically more heterogeneous, some are much more depleted, others are more metasomatized, TP equilibration estimates are much more variable but all formed at significantly higher temperatures, and none give Sm-Nd mineral ages that are significantly pre-Variscan. They seem to represent hot, active mantle that was depleted, metasomatised, recrystallized and stirred by convection shortly before and during the Variscan Orogeny. Some GP (sensu stricto) are believed to represent the lithospheric mantle metasomatized by at least two stages of melt migration that presumably occurred above a subduction zone during the closure of the sea or seas that separated Gondwana and Laurussia. Other 'transitional' (i.e. spinel-bearing) GP are believed to represent hot asthenosphere that penetrated the lithosphere and rose to the base of the orogen during the collision of Laurussia and Gondwana, possibly as a result of the delamination of thickened lithosphere. This event provided a thermal pulse that set, or re-set, the ages of many garnet-bearing systems (garnet peridotites, eclogites, granulites) and resulted in the production of enormous amounts of granitoids that characterize the Variscan orogen.