The Oligocene to the Miocene was a critical period for the growth of the Tibetan Plateau. This growth is commonly considered to have been controlled by deep geodynamic processes. The ultrapotassic and adakite-like igneous rocks that developed during this period offer constraints on these deep-seated processes in southern Tibet. Whole-rock geochemistry, U-Pb zircon geochronology, and in situ zircon Hf isotopes have been determined for the mafic enclaves and host adakite-like granitoids in the Oligocene Chongmuda-Mingze intrusive complex of southern Tibet. The host rocks, including granodiorite and monzogranite, are mainly high-K and calc-alkaline in composition. Their whole-rock geochemistry (low MgO, Ni, and Cr contents; negative ε_Nd(t) values [−2.5 to −3.4]; and low ⁸⁷Sr/⁸⁶Sr_(i) values [0.7061–0.7066]) and in situ zircon ε_Hf(t) values (0.6–6.1) indicate that they were derived by partial melting of a juvenile lower crust, implying that the southern Tibetan crust was already thickened to up to 50 km before ∼30 Ma. Mafic enclaves show typical igneous textures, acicular apatites, backveining structures, quenched margins, and crystallization ages identical to those of the host granites, indicating that they are of magmatic origin. The mafic enclaves have high-K to shoshonitic metaluminous compositions and are strongly enriched in large-ion lithophile elements and light rare earth elements, are depleted in high-field-strength elements, have negative ε_Nd(t) values (−2.6 to −4.9), have relatively high ⁸⁷Sr/⁸⁶Sr_(i) values (0.7060–0.7072), and have low zircon ε_Hf(t) values (2.3–5.5), indicating that they were derived from a relatively enriched lithospheric mantle source. Our new data, together with previously published work, lead us to suggest that deep geodynamic processes below the southern Tibetan region during the Oligocene to the Miocene were characterized by convective removal of the lower lithosphere. Upwelling of the asthenosphere induced by the delamination of the southern Tibetan lithospheric root could have supplied heat to induce anatexis of the residual lithosphere of southern Tibet, generating the adakite-like rocks and related mafic enclaves there.

Many intrusions with adakite-like affinities in collisional zones have obviously higher K₂O contents and K₂O/Na₂O ratios compared with 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, ε_Nd(t) = −3·2 to −2·8], and zircon ε_Hf(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 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 elements (e.g. Ni = 9·5–36·2 ppm) and total REE, and lower Mg# values than the lamprophyres and mafic microgranular enclaves. The adakite-like intrusions have positive large ion lithophile element anomalies, especially potassium, negative high field strength element anomalies, negative ε_Nd(t) (−5·5 to −3·3), and high (⁸⁷Sr/⁸⁶Sr)_i (0·7064–0·7070) and zircon ε_Hf(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 c. 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, ε_Nd(t) = −6·6 to −4·1, and variable zircon ε_Hf(t) values (−0·6 to +3·2). Petrological and geochemical evidence suggests 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 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, and 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 that 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.

Magmatic fluid degassing within shallow magma chambers underneath the ore bodies is critical to the formation of porphyry Cu-Au deposits (PCDs). Yet, it remains unclear how the fluid degassing influences the development of PCDs. Here, geochemical data of apatite, amphibole, and plagioclase from ore-bearing and coeval barren porphyries have been analyzed in Sanjiang metallogenic belt, China. The ore-bearing porphyries normally exhibit high and wide XF/XCl (31.76–548.12) and XF/XOH (0.779–7.370) ratios of apatites, which are evidently higher than those of the barren porphyries (XF/XCl of 1.03–26.58; XF/XOH of 0.686–3.602). Combined with the continuous variation features of Cl/OH ratios and H2O contents of melts calculated by amphiboles, as well as fluid migration models, we constrained the mechanisms of fluid degassing within shallow magma chambers underneath PCDs. There are three different ways of fluid degassing, while only fluid degassing via fluid channel stage can migrate and focus the metal-rich fluids effectively, conducive to the development of PCDs. The mechanisms of magmatic fluid degassing processes are further controlled by the storage depths of magma chambers and initial H2O contents of the magmas revealed by the compositions of amphibole, plagioclase, and thermodynamic modeling. Magmas with shallower storage depths and higher initial H2O contents are more likely to experience extensive and focused fluid degassing, leading to the generation of PCDs. This study demonstrates the potential utility of integrated mineral analyses and thermodynamic modeling for investigating the mechanisms of magmatic fluid degassing in porphyry systems, as well as for identifying prospective buried PCDs.