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Watersheds are dynamic systems, marked by constant interaction of river waters with groundwaters and other environmental compartments (atmosphere, biosphere, lithosphere, and pedosphere). The composition of natural waters results from these interactions, but that of rocks dominates. Among the trace constituents of surface waters, the concentrations of rare earth elements (REE) depend on chemical weathering of the rocks and, secondarily, on hydrogeochemical processes and reactions. The results of this study of mineralogy and geochemistry of the main rock types found in the sub-basins of the Atibaia (SP) and Jaguari (SP/MG) rivers will help to understand hydrogeochemical processes in these watercourses. The rocks found and sampled in the study area are, mostly, granitoids but some samples show metamorphism. The mineral composition of these rocks includes accessories such as allanite, rutile, garnet, zircon and apatite. The rock chondrite REE normalized patterns show enrichment in light REEs and depletion in heavy REEs, which is in agreement with the mineralogical composition of the samples.