The development of pseudotachylyte in the Ivrea—Verbano Zone of the Italian Alps

Abstract

Pseudotachylyte was formed as part of the retrogressive tectonometamorphic evolution of the Ivrea-Verbano Zone during its post-Hercynian uplift from the lower crust during the Mesozoic and Cenozoic. The pseudotachylytes preferentially occur in mafic rocks and in paragneisses and were generated along brittle faults which locally cut older ductile shear zones. Fracturing and the selective melting of minerals resulted in an aphanitic matrix containing host rock fragments and newly formed minerals (e.g., biotite, quartz, sericite, feldspar). Friction melt-induced hydraulic fracturing of the adjacent rocks facilitated the formation of pseudotachylyte veins. The veins occur as networks or as single, 0.1 m to several metres thick injections with the geometry of pull-apart structures. In zones with a high density of contemporaneously-formed pseudotachylyte veins, the fractured wall rock lost cohesion and became intrusive. Masses of angular to rounded fragments became mobilized when the macroscopically homogeneous pseudotachylyte matrix exceeded about 30% of the total rock volume. Geochemical data show that the pseudotachylytes developed from the immediately adjacent host rocks. Scanning and transmission electron microscopy indicate that the enrichment or depletion of individual elements in the pseudotachylyte relative to the host rock depended on the amount of selectively melted minerals that were incorporated (amphiboles, plagioclase, quartz and/or sheet silicates) and on the degree of secondary alteration. The development of pseudotachylyte took place in the upper crust during retrogressive metamorphism, which commenced in the ductile deformation field with the formation of high-temperature mylonites, followed by the generation of low-temperature mylonites and finally by brittle deformation.