Petrogenesis and Geological Implications of the Oligocene Chongmuda-Mingze Adakite-Like Intrusions and Their Mafic Enclaves, Southern Tibet

Abstract

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 εNd(t) values [−2.5 to −3.4]; and low 87Sr/86Sr(i) values [0.7061–0.7066]) and in situ zircon εHf(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 εNd(t) values (−2.6 to −4.9), have relatively high 87Sr/86Sr(i) values (0.7060–0.7072), and have low zircon εHf(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.