Volcanism and tectonic style dominate geothermal activity in the Tengchong block

Volcanism and tectonic processes play a decisive role in the formation of high-temperature geothermal systems in the Tengchong region. This study integrates field mapping, structural analysis, mineralogy and whole-rock geochemistry of the volcanic rocks in the Xinhua-Puchuan area (Tengchong block) to decipher the geothermal activity. Two types of tectonics have been recognized: the ductile shear deformation and the brittle fracture. The shear zones are the near N-S trending right-lateral strike-slip shear zones that have transformed the host rocks into mylonitic rocks. The brittle fractures consist of the near N-S trending and near NWW-SEE trending faults. The near N-S trending faults were developed on the pre-exist ductile shear deformation zone, acting as the conducting heat channels for the geothermal field. The near NWW-SEE trending secondary faults are strike-slip normal faults that facilitate the release of heat and geothermal fluids to form the hot springs. The volcanic rocks, including Pleistocene basalt, andesite and dacite, were widely erupted along the near N-S trending faults. The basalts are the continental within-plate alkaline olivine-basalts formed in a continental rift or extensional tectonic setting. The mantle-derived magma underwent a fractional crystallization (FC) process. From olivine-basalt to dacite, the dominated dark mineral phenocrysts track progressive differentiation: olivine→clinopyroxene(Cpx)/ orthopyroxene(Opx) → hornblende ± biotite, with plagioclase evolving fromlabradorite (An 64) to andesine (An 37). Furthermore, the major and trace elements also change systematically with the SiO₂ contents from the basalt to dacite. The pyroxene phenocrysts in the dacites are diopside and enstatite. The two-pyroxene thermometer suggests that the pyroxene crystallization temperatures range from 1161 to 994 °C. Tschermakite hornblende in dacitic magma records crystallization conditions at 5.7–6.0 kbar and 849–902 °C, consistent with a residual magma chamber at mid-crustal depth. These results establish a genetic linkage between the regional tectonic framework and geothermal manifestation: ductile shear zones act as primary conduits transferring heat from the mid-crustal magma reservoir, while brittle fault networks enable efficient heat extraction and surface expression.