This study examines detrital garnet compositions from five samples spanning a Late Miocene–Pliocene section of Himalayan sedimentary rocks (Surai Khola, Siwalik Group and central Nepal) to assess provenance and tectonic implications. A total of 100 detrital garnets were analysed for edge-to-edge compositional zoning, revealing distinct groups linked to specific hinterland regions. Manual classification identified garnet Groups 1 and 2 as grossular, Group 3 as spessartine, and Groups 4 through 7 as almandine, varying in XCa, XMg and XMn. Most garnets exhibit low XMg and flat zoning, with Groups 6* and 7* containing slightly higher XMg. Statistical clustering aligns broadly with manual groupings, which strengthens provenance interpretations. Comparisons with hinterland garnet compositions expand provenance options to magmatic garnets and rocks outside the Himalayan core units. Eight Siwalik Group garnets were modelled for pressure–temperature conditions and paths. Group 4 and 6 garnets, potentially linked to blueschist/eclogites or metamorphosed arc/Himalayan core rocks, record conditions of 510–538°C and 4.6–6.8 kbar, with isothermal burial over 0.5–2 kbar. Group 2 garnet, resembling compositions from North Himalayan granitic enclaves, yields core conditions of 480°C and 6 kbar and an N-shaped pressure–temperature path. Two Group 5 garnets with zoning like those in the High Himalayan leucogranites yield 520–528°C at 3.2–3.6 kbar. These findings provide insights into Himalayan erosion dynamics, hinterland exhumation and sediment transport pathways. Integrating garnet compositional zoning with statistical clustering and thermodynamic modelling is valuable for provenance studies of garnet-bearing sedimentary sections.