Tectonic setting, magmatic evolution, and metallogeny of Paleogene potassic intrusions in western Yunnan, southeastern Tibetan Plateau

[Truncated abstract] In western Yunnan, southeastern Tibetan Plateau, porphyry Cu systems are spatially and temporally associated with Eocene-Oligocene potassic felsic intrusions in an intracontinental setting without contemporaneous oceanic subduction. The tectonic setting, magmatic evolution, and metallogeny of these porphyry systems remain unresolved. Specifically, the characteristics, tectonic setting, and origin of the bimodal potassic mafic and potassic felsic magmatic province, the functional relationships of potassic felsic intrusions to porphyry metallogenic variation, and the relationship between potassic magmatism and the Ailao Shan-Red River shear zone, or alternatively the Jinsha suture were all unconstrained. 22 SHRIMP zircon U-Pb ages from 19 potassic felsic intrusions located as far as 150 km east and 50 km west of the Ailao Shan-Red River shear zone show that the intrusions were emplaced between 36.9 ± 0.3 Ma and 32.5 ± 0.3 Ma. Ar-Ar dating reveals that the potassic mafic magmas were emplaced between 36.2 ± 0.2 Ma and 33.4 ± 0.5 Ma. Accordingly, the bimodal potassic province predates displacement on the Ailao Shan-Red River shear zone (ASRR) initiated at 32 Ma, suggesting they are genetically unrelated. Potassic mafic rocks, mainly lamprophyres, are compositionally shoshonitic to ultrapotassic. They are enriched in large-ion lithophile elements and light rare-earth elements, with strong positive Pb anomalies, depletion in high-field-strength elements, high initial 87Sr/86Sr (0.706 – 0.709) but negative εNd(t) (–10.5 to –0.9), and old Nd model ages of 1500-900 Ma. Both 207Pb/204Pb (15.568 – 15.702) and 208Pb/204Pb (38.755 – 39.362) are radiogenic. These features suggest that the source was likely continental lithospheric mantle metasomatized by Proterozoic subduction beneath the Yangtze Craton.