Deformation behaviour of feldspar in greenschist facies granitoide shear zones
Mineralogy, Petrology and Geochemistry
Final Report Abstract
Our project provided valuable new insights into episodic deformation and mineral reactions of granitoid rocks at greenschist facies conditions, representing the depth-temperature area of the base of the seismogenic zone in the continental crust. The main conclusions can be summarized as follows: Whereas K-feldspar mainly deforms by fracturing at greenschist-facies conditions, albite deforms by a sequence of dislocation glide associated with fracturing (i.e., low-temperature plasticity) followed by strain-induced replacement, dissolution-precipitation creep and granular flow. Episodic mineral reactions as well as restricted transport within the fluid phase and/or restricted availability of fluid are indicated by incomplete replacement of monazite and xenotime forming apatite-allanite coronas and preserved REE gradients in single allanite/epidote grains, within the corona, and on the thin section scale. The oblate shape of the coronas aligned within the sample's foliation, and the deflection of the mylonitic foliation by the corona microstructure, suggests that the replacement reactions took place during deformation. Thus, the time of corona formation indicated to be at about 50-60 Ma was a time of active mineral reactions and deformation of the Austroalpine basement south of the western Tauern Window. Quartz microstructures from the DAV shear zone suggest a sequence of high-stress deformation during coseismic loading and dynamic recrystallization during postseismic creep (“kick-andcreep” history) as opposed to fast stress relaxation (“kick-and-cook” history) as indicated for example by quartz microstructures formed at greenschist facies conditions from the Silvretta basal thrust. From quartz-rich mylonitic pegmatites from the DAV shear zone several cycles of high-stress deformation and low-stress creep are indicated.
Publications
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2017: Recrystallization of quartz after low-temperature plasticity – The record of stress relaxation below the seismogenic zone, Journal of Structural Geology, 95, 77–92
Trepmann, C.A.; Hsu, C.; Hentschel, F.; Döhler, K.; Schneider, C.; Wichmann, V.
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2019: Deformation of feldspar at greenschist facies conditions - the record of mylonitic pegmatites from the Pfunderer Mountains, Eastern Alps, Solid Earth, 10(1), 95–116
Hentschel, F.; Trepmann, C. A.; Janots, E.
