Interrelated P-T-t-d paths in the Variscan Erzgebirge dome (Saxony, Germany): Constraints on the rapid exhumation of high-pressure rocks from the root zone of a collisional orogen

The Erzgebirge dome consists of several superimposed composite tec tectonom etamorphic units of medium- to high-grade metamorphic rocks from different crustal depths. These exhibit high pressure-high temperature and even ultra high-pressure imprints inherited from the root zone of a Variscan orogen an d were exhumed almost immediately after attainment of maximum pressures at similar to 341 Ma. At present, the entire stack of tectonometamorphic units lieu underneath an upper-crustal sequence of Paleozoic metasediments and t ectonic slivers of pre-Carboniferous metamorphic rocks. Shear zones active at different times and at different depths are preserved , mainly recording two successive stages of the exhumation history between 340 and 330 Ma. Tectonic transport during exhumation was remarkably constan t in an E-W direction, swinging to NW-SE in the eastern part of the Erzgebi rge parallel to a ductile transtensional zone (Elbe zone) that was concomit antly active. The various tectonometamorphic units have characteristically correlated, convergent P-T-t-d paths (both "cooling during decompression" a nd ''heating during decompression") that can be deduced from the dominant q uartzofeldspathic rocks. These paths indicate successive exhumation of hott er rocks from increasingly deeper structural positions and juxtaposition ag ainst cooler rocks in higher positions. concomitant with the excision of in termediate crustal levels, ai interpret this type of successive vertical te lescoping of the metamorphic profile to be the result of extension of the t hickened tectonometamorphic stack. Extensional unroofing in the middle and upper crust was contemporaneous wit h and outlasted underthrusting and hence prograde metamorphism and deformat ion at deeper levels of the tectonometamorphic pile. Underthrusting is docu mented bq a major inversion of the maximum pressure conditions in the lower most units. However. structures related to compressional stacking now gener ally occur only as relies transposed by extensional deformation at lower pr essure, or are restricted to rare small slivers with preserved prograde str uctures. Sedimentation of Lower Dinantian turbidites occurred along the fla nks of the Erzgebirge dome during the exhumation process. The extrusion of high-pressure rocks is interpreted to have been driven mai nly LS a major regional buoyancy instability caused by the delamination of the lithospheric mantle underneath the neighboring Bohemian Massif, which r epresented overthickened crust at least from the Devonian to the early Vise an. Major controlling factors ere boundary forces exerted Ly the thickened crustal bulge on the neighboring thin crustal segments in the north and eas t, effecting lateral extension of this orogenic wedge and extrusion-i.e., c onvective upward now of gravitationally unstable crustal material.