Rheological inversion and mullion formation of a composite sill under bulk constriction (Odenwald, Germany).

We present new data from a composite sill, which intruded at deep structural levels (ca. 18 km depth) into quartzmonzodiorite of the southern Odenwald. Zircons and titanites of the quartzmonzodiorite yielded similar U-Pb ages at 344.3 ±0.6 and 343.2 ±2.1 Ma, respectively, reflecting fast cooling (≥76°C/m.y.) until the solidus was attained at ca. 680°C. Under these conditions, the quartzmonzodiorite was cut by a spessartite sill, which yielded a U-Pb titanite age at 342.0 ±1.0 Ma. Fast migration of contact melt into shrinkage cracks of the sill resulted in thin felsic veins.

Bulk constriction at T = ca. 660°C led to (1) subvertical prolate grain-shape fabrics, (2) increase of the sill’s dip, (3) doubling of the sill’s thickness, (4) mullions with cusps pointing into the host, and (5) boudinage of the felsic veins. The shape of the mullions, the boudinage of the felsic veins and the sigmoidal foliation/lineation indicate that the mafic sill was incompetent and the felsic material of host and veins was competent. This inversion in rheology can be explained by phase-boundary diffusion that was more effective in the fine grained mafic sill than in the coarse grained felsic host.

Funding by Deutsche Forschungsgemeinschaft (Zu73-35) is acknowledged.

References

Zulauf, G. et al. (2021). Development of a synorogenic composite sill at deep structural levels of a continental arc (Odenwald, Germany). Part 1: Sederholm-type emplacement portrayed by contact melt in shrinkage cracks. Tectonophysics, 805 and (2022). Part 2: Rheological inversion and mullion formation under bulk constriction. J. Struct. Geol., 155