Mechanics and mechanisms of magnetic underplating: inferences from mafic veins in deep crustal mylonite

摘要

Dioritic to gabbro-dioritic veins with extreme length to width ratios (> 1000 : 1) are localized along an amphibolite facies shear zone (the CBM Line) between exposed segments of originally middle and lower continental crust (Strona-Ceneri and Ivrea-Verbano Zones, northern Italy). The geometry of these veins and their mutual cross-cutting relationships with the mylonitic foliation indicate that veining was coeval with noncoaxial flattening of the lower crust in Early Permian time. The veins formed as closely spaced extensional shear fractures and propagated parallel to the originally gently to moderately dipping (<= 30 deg) mylonitic foliation. Vein opening at high angles (60-90 deg) to the inferred #sigma#_1 direction and subparallel to the pre-existing planar fabric requires that melt pressure slightly exceeded the lithostatic pressure and that differential stress was low (<= 10-20 MPa) in the vicinity of the veins. The interaction of regions of tensile stress concentration at vein tips caused the concordant veins to curve and link up across the mylonitic foliation. Once interconnected, the veins served as conduits for the rapid movement of mafic melt along the shear zone. Thermal modelling constrains the mafic melt in the narrowest, 1 mm wide veins to have crystallized almost instantaneously. Such veins extend no more than a meter from host veins into the country rock, indicating that the minimum rate of vein tip propagation and melt flow was at least several m/s. Maximum crystallization times of only hundreds to thousands of years for even the thickest mafic veins (10-100 m) in the IVZ are short compared to the 15-20 Ma duration of Early Permian crustal attenuation and magmatism in the southern Alps. This suggests that veining in the lower crust occurred episodically during extended periods of mylonitic creep. Concordant vein networks within deep crustal shear zones that are inclined (as the CMB Line may have been) can also channel overpressurized mafic melt from deeper sources, e.g. lower crustal magma chambers, into cooler, intermediate crustal rock. This locally widens the depth interval of combined viscous and brittle deformation within the crust and can trigger partial melting of the country rock.