Symplectic intergrowths are important markers of incomplete retrograde metamorphism. Because they represent arrested reactions that were limited by diffusion, these textures provide constraints on how pressure and temperature varied following peak-grade metamorphism. For example, symplectic intergrowths of muscovite + quartz that formed in upper amphibolite facies sillimanite + K-feldspar wall rocks permit assessment of the thermal history of a magmatic arc post-intrusion. Wall rocks surrounding the last plutons to be emplaced within the Peninsular Ranges batholith, (Southern California, USA, to Baja California, Mexico), present an ideal setting to investigate retrograde textures that elucidate arc evolution. During intrusion, wall rocks reached 625 ± 25 °C and exceeded muscovite + quartz stability. Post-intrusion, muscovite + quartz symplectites grew at the expense of K-feldspar in response to falling temperature. Ti-in-quartz measurements reveal that symplectic quartz grew at ≥375 °C, which reflects temperatures as much as 250 °C lower than peak-grade metamorphism. This result has significant implications for the mechanical and thermal evolution of Peninsular Ranges batholith arc crust.