Many intrusions with adakite-like affinities in collisional zones have evidently higher K2O contents and elevated K2O/Na2O ratios than that of the counterparts in subduction zones. A suite of Eocene post-collisional high K2O adakite-like intrusions, mafic microgranular enclaves, and potassic-ultrapotassic lamprophyres in the Machangqing complex are associated with the Indian-Asian collision within the western Yangtze Craton, southeastern Tibet. The potassic-ultrapotassic lamprophyres with a zircon U-Pb age of 34.1±0.2 Ma, have high K2O and MgO contents, are enriched in light rare earth elements and large ion lithophile elements, and display high Rb/Sr, and low Ba/Rb and Nb/U ratios. They show enriched isotopic compositions (i.e., (87Sr/86Sr)i = 0.7070-0.7082, εNd(t) = −3.2 to − 2.8), and zircon εHf(t) values (−1.6 to + 2.6). Their parental magmas are inferred to have been derived from partial melting of an enriched lithospheric mantle, metasomatized by subduction-related fluids. The adakite-like intrusions with zircon U-Pb ages of 35.4±0.4 Ma and 35.2±0.3 Ma, are characterized by high SiO2 (68.8-71.1 wt.%) and Al2O3 (14.0-15.3 wt.%) contents, high Sr/Y (41-118) ratios, and low Y (5.3-14.7 ppm) contents. They show low contents of compatible element (e.g., Ni = 9.5-36.2 ppm) and total REE contents, and lower Mg# values than the lamprophyres and mafic microgranular enclaves. The adakite-like intrusions have positive large ion lithophile elements anomalies, especially potassium, negative high field strength elements anomalies, negative εNd(t) (−5.5 to − 3.3), high (87Sr/86Sr)i (0.7064-0.7070), and zircon εHf(t) values (0.0 to + 2.7), indicating that they were formed by partial melting of the juvenile lower crust. Mafic microgranular enclaves hosted in the adakite-like intrusions, with U-Pb ages similar to the lamprophyre of ca. 34 Ma, exhibit disequilibrium textures, and some of them contain phlogopite. They exhibit potassic-ultrapotassic affinity, and relatively high compatible element contents. They are also characterized by enriched isotopic compositions with (87Sr/86Sr)i = 0.7063-0.7074, εNd(t) = −6.6 to − 4.1, and variable zircon εHf(t) values (−0.6 to + 3.2). Petrological and geochemical evidence suggest that the mafic microgranular enclaves were formed by magma mixing between potassic-ultrapotassic and pristine adakite-like melts. We propose a magma mixing model for the origin of the high K2O adakite-like intrusions from the Machangqing complex. In this model, the formation of high K2O adakite-like intrusions occurred in three stages: (1) partial melting of metasomatized lithospheric mantle that generated potassic-ultrapotassic mafic melts; (2) underplating of these mafic melts beneath thickened juvenile lower crust resulted in partial melting of juvenile mafic lower crust and the generation of adakite-like melts; (3) magma mixing involved 80% pristine adakite-like melts and 20% potassic-ultrapotassic melts. This leads to the enrichment of K2O in these adakite-like intrusions, while magma differentiation further promotes K2O enrichment. These results are applicable to compositionally similar adakite-like rocks produced in other collisional zones, such as the Tibet, Sulu-Dabie and Zagros orogenic belts. From which we conclude that in continental collision zones, the post-collisional mantle-derived magmas characterized by potassic-ultrapotassic affinities are spatially associated with coeval collision-related adakite-like intrusions originated from lower crustal melting. The emplacement of adakite-like and potassic-ultrapotassic rocks is controlled by the same fault systems, which increases the possibility of interaction between these two magma suites.
