Sediment-rich mélange diapirs have been suggested to transport key chemical slab signatures and volatiles to arc magma sources. Here, we assess the phase equilibria, buoyancy and implications for chemical geodynamics of a previously unexplored hydrous shaly-rich mélange (5-10 wt.% H2O) with minor ultramafic component (9:1 ratio) from deep forearc to subarc depths (2-3 GPa and 700-1150°C). Their solidi lie between <645 to ~700°C and upon partial melting, produce dacitic to rhyolitic melts (water-free basis) in coexistence with abundant biotite, pargasitic-amphibole, and quartz (low dense minerals), garnet, and enstatite (rutile/Ti-magnetite ± apatite) that favors the onset of diapirs in thinner mélange channels (<100m) with lower mélange and mantle viscosities in all slab geotherms. At >850°C the low dense mineral abundance decreases, and the mélange loses buoyancy, requiring thicker mélange channels (>100-800m) with higher mélange and mantle viscosities. Thinner and thicker mélanges form smaller (<1 km radius) and larger diapirs (>1 km radius), respectively. While smaller are more susceptible to mineralogical equilibration, lose buoyancy and stall, larger diapirs may sustain buoyancy and relaminate under the arc crust. High LILE/HFSE signatures in most arc lavas may be explained by aqueous fluids and low degree mélange partial melting (<850°C) within the channel or a diapir close to the channel where rutile/apatite/Ti-Fe-oxides minerals are stable, and the presence of garnet impart high LILE/HREE. Although high degree partial melts due to diapirism or slab tears explain dacitic to rhyolitic arc lavas, they would not fractionate HFSEs from LILEs to explain the high HFSE/LILE arc lava signature.