Rocks are polymineralic crystalline aggregates. Quantifying and understanding their mechanical behavior remains a challenge because multiple feedbacks are at play in crystalline aggregates between parameters such as temperature (T), pressure (P) and characteristics such as grain sizes and microstructure. In addition, natural observations and many works emphasize that in polymineralic rocks, heterogeneities in minerals’ mechanical properties induce stress and strain partitioning with dramatic consequences for the global mechanical behaviour. A common approach in rheological studies under high pressures is to investigate monomineralic aggregates as a proxy, and the mineralogical diversity of deep subduction zones aggregates seldom has been considered. Here, I will present how HP deformation experiments with in-situ x-ray measurements and mean field models of polymineralic aggregates can help us with this problem. Questions that can be answered include sorting out which phases are susceptible to control the aggregate behavior and for which mineralogy. To that extent, we investigate the strain (rate) or stress ranges in the rock that can exist due to different elastic and plastic properties of minerals. The microstructural characteristics, part of which can be monitored in-situ, remain crucial when considering these questions. I will illustrate the presentation with high-pressure experiments using in-situ X-Ray measurements, mean field models of aggregates, and our current effort using HP tomography on deformed subduction zones rocks.