Geochemical Aspects of the Frictional Melting of Metapsammites during Seismic Slips (with Reference to Pseudotachylytes of the Ladoga Region)

Abstract

Based on analytical data on the geochemistry of tectonic pseudotachylytes and their host rocks, the specific trends of redistribution of major, trace, and rare earth elements in the course of seismogenic frictional melting of the arkosic metaterrigenous rocks from three zones of regional metamorphism with different temperatures (greenschist, amphibolite, and granulite) in the Northern Ladoga region are discussed. Oppositely directed trends of changes in the contents of the oxides of major elements in the protolith–blastocataclasite–pseudotachylyte triad were revealed, and a unidirectional increase in the basicity of the frictional melt as compared with the protolith was established at the same time. Geochemical evidence for partial selective melting of the protoliths is discussed. The specific features of the fractionation of trace and rare-earth elements during the transition of the protolith into the melt, as well as during its subsequent partial recrystallization, are shown. The emergence of a peak of elevated europium contents relative to the protolith in the melt matrix of all three sampling points is noted. Based on changes in the concentrations of these elements in the zones of pseudotachylyte substrate generation and in the areas of its moving and injection, the estimates of their differential mobility during frictional melting in a dynamic slip zone are given.