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

Abstract

The Erzgebirge dome consists of several superimposed composite tectonometamorphic units of medium-to high-grade metamorphic rocks from different crustal depths. These exhibit high pres sure-high temperature and even ultrahigh-pressure imprints inherited from the root zone of a Variscan orogen and were exhumed almost immediately after attainment of maximum pressures at ~341 Ma. At present, the entire stack of tectonometamorphic units lies underneath an upper-crustal sequence of Paleozoic metasediments and tectonic 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 constant in an E-W direction, swinging to NW-SE in the eastern part of the Erzgebirge parallel to a ductile transtensional zone (Elbe zone) that was concomitantly active. The various tectonometamorphic units have characteristically correlated, convergent P-T-t-d paths (both "cooling during decompression" and "heating during decompression") that can be deduced from the dominant quartzofeldspathic rocks. These paths indicate successive exhumation of hotter rocks from increasingly deeper structural positions and juxtaposition against cooler rocks in higher positions, concomitant with the excision of intermediate crustal levels. We interpret this type of successive vertical telescoping of the metamorphic profile to be the result of extension of the thick ened tectonometamorphic stack.Extensional unroofing in the middle and upper crust was contemporaneous with and outlasted underthrusting and hence prograde metamorphism and deformation at deeper levels of the tectono metamorphic pile. Underthrusting is documented by a major inversion of the maximum pressure conditions in the lowermost units. However, structures related to compressional stacking now gener ally occur only as relics transposed by extensional deformation at lower pressure, or are restricted to rare small slivers with preserved prograde structures. Sedimentation of Lower Dinantian turbidites occurred along the flanks of the Erzgebirge dome during the exhumation process.The extrusion of high-pressure rocks is interpreted to have been driven mainly by a major regional buoyancy instability caused by the delamination of the lithospheric mantle underneath the neighbor ing Bohemian Massif, which represented overthickened crust at least from the Devonian to the early Visean. Major controlling factors were boundary forces exerted by the thickened crustal bulge on the neighboring thin crustal segments in the north and east, effecting lateral extension of this orogenic wedge and extrusion-i.e., convective upward flow of gravitationally unstable crustal material.

Devonian deep-crustal metamorphism and exhumation in the Variscan Orogen: evidence from SHRIMP zircon ages from the HT-HP granulites and migmatites of the Góry Sowie (Polish Sudetes)

“…However, if the zircon ages of c. 340 Ma in these granulites do correspond to the peak granulite facies conditions [7], and the 40 Ar/ 39 Ar mica cooling cooling ages are within the range of 330-340 Ma [60], the exhumation rate of the Śnieżnik granulites would have to be by an order of magnitude higher. A similar scenario of extremely fast exhumation (10-15 mm/year) is known from the 340 Ma granulites of the Granulitgebirge [12] and Erzgebirge [2,3].…”

“…Originally, the older granulites were interpreted as related to the subduction of continental crust, whereas the younger ones were regarded as products of deep crustal processes connected with collision during the Variscan orogeny [1]. However, an alternative scenario is proposed [2][3][4] in which the younger granulites were exhumed from a deeply subducted orogenic root in subduction-exhumation channels activated after delamination of the lithospheric mantle that caused a major isostatic instability. Before the exhumation, excess crustal thickness prevailed for about 50 Ma.…”

“…The age bimodality of the HP-HT rocks in the Variscan Belt is of key importance for orogenic models and for indicating that the very high-grade rocks of deep-crustal derivation of the older group were already at shallow depths (as inferred also from sedimentary records, [13]) at the time when their younger counterparts still resided at deep crustal levels [2][3][4]. This HP-HT bimodality seemingly occurs in a relatively small area of the Sudetes, at the NE margin of the Bohemian Massif (SW Poland), where the older HP-HT granulites are found in the Góry Sowie Block (GSB), and the apparently younger eclogites and omphacite-granulites are known from the Orlica-Śnieżnik Dome (OSD; Fig.…”

“…Timing of particular tectonothermal events in the granulites and enclosing gneisses/migmatites provide important constraints on tectonic evolution models and paths of exhumation of deep crustal segments (represented by granulite compexes) in orogenic belts. Typical examples can be found, e.g., in the Variscan Belt of Europe [1][2][3][4][5][6][7][8][9].…”

Discrimination of TiO2 polymorphs in sedimentary and metamorphic rocks

“…The Erzgebirge is characterized by a large-scale antiformal structure, intruded by posttectonic granitoids of late-Variscan age. It represents a stack of tectonic units resulting from continent-continent collision processes (deformation, metamorphism, and exhumation) during the Variscan orogeny (e.g., Willner et al 1997Willner et al , 2000Rötzler et al 1998;Mingram 1998).…”