Geophysics combined with Petrography – a promising approach for 3D crystalline basement characterization?

High-grade metamorphic and plutonic rocks in crystalline complexes are of increasing interest as host rock for geothermal plays and for understanding plate tectonics. In this regard the Pretzsch-Prettin Crystalline Complex (PPCC, Saxony-Anhalt) as a part of the Mid German Crystalline Rise is one prominent example of a late-orogenic intrusion in the former Rheic Suture Zone. Former 2D geophysical investigations focussing on the setting and outline of the PPCC remained ambiguous and suggested a combined interpretation with other geoscientific disciplines.

Recently, we applied a 3D geophysical and geological modelling to the PPCC incorporating well data, seismic depth maps and gravity data. 3D gravity modelling revealed important information on the general geological setting of the overburden and basement depth, but did not allow detailed interpretation in areas of similar basement density. Limiting the generally ambiguous gravity modelling results required incorporation of petrographic data like rock composition from thin sections and density measurements at rock samples, which were linked to the modelled density zones of the basement.

This approach revealed first evidence of a northward extension of the comparable low density monzogranites to granodiorites of the PPCC into the North German Basin. Quartz monzonites and monzodiorites with up to 25 % biotite and amphibole agree with up to 0.15 g/cm³ higher basement densities in the surrounding of the PPCC. Generally, incorporating petrographic data into the gravity modelling process enabled us to better confine zones of different crystalline compositions, which might be applicable to other areas.