The suture zone of the Tibetan Plateau is a hotspot for giant, high-steep rock landslides. The stability of these rock masses is controlled not only by diverse exogenic and endogenic processes but, more fundamentally, by ongoing mineral-geochemical processes within them. We investigated a typical schist/gneiss landslide body, enriched with an assemblage of albite (Ab), garnet (Grt), actinolite (Act), epidote (Ep), biotite (Bt), chlorite (Chl), and quartz (Qtz), through detailed field surveys and electron microscopy. This study reveals how the evolution of mineral parageneses governs slope stability and landslide initiation. Our findings indicate the following: (1) In schists and gneisses, primary high-strength minerals (e.g., garnet (Grt), hornblende (Hbl)) are commonly altered to sheet-silicates and fibrous minerals like chlorite (Chl), epidote (Ep), and muscovite (Ms) during retrograde metamorphism or hydrothermal alteration. These secondary minerals form foliated mechanical weak zones, significantly reducing the overall shear strength of the rock mass. (2) Key geochemical reactions, notably the chloritization of biotite (Bt) and sericitization of plagioclase (Pl), alter the geochemistry of local pore fluids, create preferential pathways for fluid migration, and intensify water-rock interactions. (3) Oriented parageneses of actinolite (Act), epidote (Ep), chlorite (Chl), and quartz (Qtz) constitute potential, bedding-parallel mechanical anisotropy planes. The rock mass becomes prone to instability along these weakness planes under tectonic stress or gravitational unloading. We conclude that for schist and gneiss slopes widespread in the suture zone, mineral-geochemical evolution is the intrinsic control on long-term strength and deformation behavior. Integrating paragenetic and alteration sequence analysis into stability assessments enables genetic identification of potential slip boundaries and prediction of rock mass degradation. This work provides a novel geochemical perspective on the deep-seated origins of large-scale landslides, showcasing a cutting-edge application of mineralogy to major engineering geological challenges.