New constraints on the tectonometamorphic evolution of the Erzgebirge orogenic wedge (Saxothuringian Domain, Bohemian Massif)
Abstrakt
In contrast to well-documented (U)HP rocks of the Erzgebirge crystalline complex (Saxothuringian Domain, Bohemian Massif), the surrounding medium- to low-grade metasediments are scarcely studied, although they provide an important link between deep subduction and mid-crustal processes. To constrain the Variscan evolution from continental subduction to orogenic wedge, the transition from the low-grade phyllites to the footwall medium-grade micaschists was investigated.
We used thermodynamic modelling to constrain the P-T conditions of four deformation fabrics (D1-D4) identified by the structural analysis. The studied area was first affected by the M1-D1 event characterized by HP-LT minerals (garnet, chloritoid, phengite, paragonite, and rutile) defining the S1 foliation.
The calculated peak P-T conditions for M1 increase from 13 kbar and 520°C in phyllites to 25 kbar and 560°C in micaschists. The corresponding apparent geothermal gradient 6-11°C/km is typical for a subduction environment.
The M2-D2 event corresponds to deformation and metamorphic overprint of the previous fabric during partial decompression. The M3-D3 event is mainly developed in micaschists, and the M3 overprint intensifies towards the footwall.
It is accompanied by a subhorizontal S3 cleavage characterized by MP-MT assemblage containing biotite, staurolite and ilmenite. The inferred peak P-T conditions for M3 are 5-9 kbar and 595°C representing a barrovian-type geothermal gradient from 17-30°C/km.
Finally, all metamorphic fabrics were heterogeneously affected by the low-grade M4-D4 upright folding. The D1-D2 events are interpreted to record the growth and evolution of the Saxothuringian orogenic wedge while its present-day architecture resulted from significant vertical shortening D3 associated with barrovian type M3-metamorphism.
This stage corresponds to the ductile thinning of the wedge. A new model is proposed, in which the Erzgebirge part of the Saxothuringian Domain reveals a spectacular example of active margin evolution characterized by (i) formation of accretionary prism, (ii) building of the orogenic wedge by accretion of subducted continental crust, and (iii) its extensional collapse.