Basement evolution in the rhenohercynian segment: Discontinuous exhumation history of the Eckergneis complex (Harz mountains, Germany)

The Eckergneis complex in the Hart mountains (Germany) represents a fragment of the pre-Variscan basement of the East. Avalonian plate. During the Variscan orogeny, the complex was raised to a shallow crustal level by two intrusions. The Eckergneis metamorphites are mainly of pelitic and psammitic origin while orthoamphibolites are subordinate. The deposition of the sedimentary precursors probably occurred during Proterozoic times. The oldest deformational and metamorphic overprint still preserved (Mi) took place under granulite facies conditions (T: 720-780 degrees C, P: 6.7-8.3 kbar) and led to the formation of orthopyroxene-, orthoamphibole-, sillimanite- and cordierte-bearing mineral assemblages. During this event an S1 foliation and isoclinal Fl folds with northward plunging fold axes developed. Subsequent to this, large areas of the Eckergneis complex were overprinted under amphibolite-facies conditions (T: 560-650 degrees C, P > 5 kbar) by the tectono-metamorphic M2 event which induced the formation of the parageneses kyanite-staurolite-biotite-muscovite in metapelites and hornblende-plagioclase (An(38-51)) in amphibolites. The M2 event was accompanied by the S2 foliation and the development of open to closed F2 folds with E-W-striking fold axes. Different models for the tectono-metamorphic evolution and the exhumation of the Eckergneis complex are discussed. U-Pb dating and Rb-Sr whole rock dating suggest a Cadomian age of 560 Ma for the M1 event and a Lower Devonian age of 392 Ma for the M2 event. In Upper Carboniferous times, the Eckergneis was uplifted by coeval mafic (Harzburg gabbronorite) and granitic intrusions (Brocken granite). This was accompanied by a phase of strong thermo-metamorphism (M3). P-T conditions at the contact with the intrusions were beyond the solidus of metapelites as evidenced by anatectic mobilisates. In the central part of the complex, the temperatures of thermo-metamorphism were distinctly lower (500-550 degrees C). Metamorphic pressures during M3 did not exceed 2 kbar as indicated by petrological constraints, P-T estimates in adjacent contact metamorphic Palaeozoic rocks and the estimate for the overlying strata in Upper Palaeozoic. The final event was a strong hydrothermal overprint (M4) of the Eckergneis racks at temperatures of 340-360 degrees C leading to the decomposition of the regional- and thermo-metamorphic mineral assemblages.