Mesoscale and microscopic structures record compaction and crystal-melt segregation in silicic mushes

Gravitationally-driven compaction has been proposed as an important mechanism for crystal-melt segregation in silicic mushes. However, there is no microstructural evidence of crystal deformation consistent with compaction in silicic plutonic rocks. We have investigated the Caijiang pluton in South China that provides compelling evidence for compaction and melt segregation preserved within rock records. This pluton consists of porphyritic syenite (59-62 wt% SiO₂), quartz monzonite (67-70 wt% SiO₂) and granite (70-79 wt% SiO₂) from bottom to top. These rocks contain K-feldspar megacrysts (~ 55.0% in syenite, ~ 38.6% in quartz monzonite and ~ 33.8% in granite) that exhibit a subvertical alignment. A compositional gap from 62 to 67 wt% SiO₂ and a gradual increase downwards in the proportion of K-feldspar megacrysts suggest a silicic mush system, where porphyritic quartz monzonite and granite represents fractionated melts from a silicic parental magma while porphyritic syenite represents complementary cumulates. K-feldspar megacrysts, whether touching or suspended, display evidence of internal deformation such as bending, melt-filled fractures and contact melting. Phenocrysts among K-feldspar megacrysts also exhibit signs of internal deformation including bend twins and melt-filled fractures in feldspars and undulose extinction in quartz and biotite; however, no deformations are evident in groundmass with no undulose extinction in quartz. Collectively, these observations indicate that compaction accompanied by crystal deformation occurred after the formation of solid framework by K-feldspar megacrysts, and continued until final melt solidified as groundmass. Our findings are crucial for enhancing understanding regarding mechanisms driving melt segregation and differentiation in silicic mushes.