Deformation behavior and inferred seismic properties of tonalitic
migmatites at the time of pre-melting, partial melting and
post-solidification
Abstract
As seismic data from the lower crust becomes more readily available, it
is import ant to link seismic properties to the ongoing processes within
lower crustal evolution. It includes high temperature, pre- and
post-migmatisation solid state deformation as well as melt-present
deformation condition. We selected two tonalitic migmatites with
variable former melt content (one metatexite and one diatexite) from the
lower crustal Daqingshan area, northern North China Craton (NCC) to
assess the link between seismic properties and rock structure and
rheology. Field observation along with microstructural features suggest
that the character of hornblende and plagioclase within the residuum of
the metatexite can be used to derive information regarding the
pre-migmatisation deformation. Residuum’s plagioclase CPO
(crystallographic preferred orientation) is consistent with high
temperature dislocation creep as the main deformation mechanism;
similarly, hornblende shows a strong CPO related to dislocation creep.
During syn-melt (melt present) conditions, phenocrysts of plagioclase in
the metatexite’s neosome and K-feldspar and peritectic hornblende in the
diatexite’s neosome are present. The rheology of the rock was dominated
by melt; hence is inferred to follow Newtonian flow. After melt
crystallization deformation is minor but again dominated by dislocation
creep. For seismic properties (seismic velocity, anisotropy, Vp/Vs
ratio, etc.), in pre- and post-melt conditions, migmatites have normal
values. While in syn-melt condition, seismic velocities have a greater
decrease, Vp/Vs ratios have a great increase, seismic anisotropies are
unusually high.