This study investigates the provenance and tectonic background of the lower Silurian Longmaxi Formation black shale of the Yanyuan Basin in the western Kangdian ancient land and provides guidance for shale gas exploration and development in the area. The mineral petrological and geochemical characteristics of the Longmaxi Formation black shale in this area have been studied in detail. The study area is mainly a passive continental margin environment, but also has the attributes of an active continental margin and island arc environment due to the collision between the western oceanic crust and the Yangtze continental crust. The source rocks are mainly felsic igneous rocks, with a small contribution of intermediate–basic rocks. It is inferred that the Kangdian ancient land in the eastern part of the area could be the main provenance area, but with the contribution of sediments derived from oceanic island arc located in the west. During the whole Longmaxi period, the tectonic movement was intense, the climate was cold, the degree of chemical weathering was low, and it was a highly volatile geological sedimentary basin. Therefore, the highly siliceous organic-rich mudstone, which was different from the Sichuan Basin, was deposited.

The TuguanZhai rare earth deposit in Tengchong, along with the Longan and Yingpanshan deposits in Longchuan, is a significant ion-adsorption type rare earth (iREE) deposit in Yunnan, China. Previous studies mainly focused on the geochemistry of residual regolith or the migration and enrichment mechanism of rare earth elements (REEs), but lacked systematic analysis of the protoliths. To constrain this deposit and its protolith rock, called Tuguanzhai granite, we systematically integrate petrology features, petrogeochemistry, zircon U-Pb date, and artificial heavy mineral separation (AHMS). Specifically, iREE-host granites include two main periods in this area: the Early Cretaceous (112.13 ± 0.75 Ma) and the Paleocene–Eocene (52.78 ± 0.28 Ma, 48.56 ± 0.19 Ma). The former includes three types of biotite monzogranite with different grain sizes, and the latter is mainly medium-grained biotite monzogranite with local mylonitization. Geochemical features show that these granites generally share high alkalinity compositions (w(K₂O + Na₂O): 7.15 to 12.75 wt%) and potassium contents (w(K₂O): 3.89 to 8.36 wt%). The mineralized granites exhibit significantly higher concentrations of the total REEs than non-mineralized granites, along with a strong enrichment of light REEs. Moreover, the results of AHMS reveal that the REE contents of apatite, allanite, and titanite in mineralized granites are 4.98, 1.29, and 1.90 times more abundant than in non-mineralized granites, respectively. Due to REEs being released from these REE-rich minerals in humid environments, there exists significant potential for iREE formation and exploration in the Early Cretaceous granites in western Yunnan. We innovatively propose the “abundance of easily leachable minerals” as a key indicator for iREE mineralization and exploration, having found it to be better than the traditional total REE contents.