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 important to link seismic properties to the ongoing processes within lower crustal evolution. This includes high temperature, pre‐ and post‐migmatization solid state deformation as well as melt‐present deformation. 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 to assess the link between seismic properties and rock structure and rheology. Field observation along with microstructural features suggest that the characteristics of hornblende and plagioclase within the residuum of the metatexite can be used to derive information on the pre‐melt deformation. Residuum's plagioclase CPO (crystallographic preferred orientations) 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. The seismic properties (seismic velocity, anisotropy, Vp/Vs ratio, etc.) for pre‐ and post‐melt have similar values expected values for solid mafic rocks, whilst the syn‐melt seismic velocities are generally lower and Vp/Vs ratios and seismic anisotropies are higher.

Syn‐Shearing Deformation Mechanisms of Minerals in Partially Molten Metapelites

“…This observation is not in agreement with that of Shao et al. (2021), wherein the reported neosomes (melt‐derived phases) are weakly strained than the residuum (pre‐melt phases). Shao et al.…”

“…The GOS further reveals that all the melt-derived phases are more strained (higher RMS GOS value) than quartz (RMS GOS = 0.35°), which is one of the starting materials. This observation is not in agreement with that of Shao et al (2021), wherein the reported neosomes (melt-derived phases) are weakly strained than the residuum (pre-melt phases). Shao et al (2021) report that the deformation was partitioned into the melt and consequently the solid phases had weaker CPOs than the minerals strained during pre-melt conditions.…”

“…Shao et al. (2021) report that the deformation was partitioned into the melt and consequently the solid phases had weaker CPOs than the minerals strained during pre‐melt conditions.…”

Syn-shearing deformation mechanisms of minerals in partially molten metapelites

“…The GOS further reveals that all the melt-derived phases are more strained (higher RMSGOS value) than quartz (RMSGOS = 0.35°), which is one of the starting materials. This observation is not in agreement with that of Shao et al (2021), wherein the reported neosomes (melt-derived phases) 15 are weakly strained than the residuum (pre-melt phases). Shao et al (2021) report that the deformation was partitioned into the melt and consequently the solid phases had weaker CPOs than the minerals strained during pre-melt conditions.…”

“…The existing deformation models of partially molten rocks, mostly derived from studying migmatites, illustrate that (a) both melt and melt-crystallized phases are strained during the synmelting and late-stage solid state deformation, respectively (Prakash et al, 2018) 5 creep accommodates strain during both pre-and post-melt conditions, but dissolutionprecipitation and rigid body rotation dominate under melt-present deformation (Shao et al, 2021); (c) in-situ coarsening is followed by dislocation creep of grains nucleating from the melt (Lee et al, 2020); (d) fine grain size and water saturated conditions favor diffusion creep (Dell'Angelo & Olgaard, 1995;Kilian et al, 2011), whereas presence of melt favors grain boundary sliding (Lee et al, 2018;Stuart et al, 2018); and (e) the minerals crystallized from the melt may deform by dislocation creep (Miranda & Klepeis, 2016). Crystallographic preferred orientations (CPOs) are generally developed during dislocation creep and therefore provide constrains on the predominant deformation mechanisms.…”