Eocene–Oligocene granitoids in southern Tibet: Constraints on crustal anatexis and tectonic evolution of the Himalayan orogen

Tectonic models for the evolution of the Himalayan orogen interpret the Greater Himalayan crystalline complex (GHC) to be the result of either thick-skinned thrusting involved Indian basement, thin-skinned thrusting involving exotic terranes, middle-crustal ductile flow, or wedge extrusion of the Indian crust during India–Asia collision. Two key pieces of information needed to test the validity of these models is the temporal–spatial distribution of, and the identification of the dynamic mechanisms involved in, regional melting under southern Tibet. Here, we document an Eocene–Oligocene melting event in southern Tibet, which forms a 150-km-long, NW–SE-trending granitoid belt along the Zedong–Lhunze traverse between the Indus–Yarlung suture (IYS) and the south Tibetan detachment (STD). U–Pb dating of magmatic zircons indicates that this granitoid belt youngs northward from ∼46 Ma (in Lhunze) to ∼30 Ma (in Zedong). 40Ar/39Ar dating of deformed biotite within 42–46 Ma granitoids constrains the timing of shearing to ∼39–41 Ma.The granitoid belt of southern Tibet is dominated by Eocene two-mica granites in the Tethyan Himalaya, with minor ∼30 Ma granodiorites along the IYS and ∼35 Ma granites in the Yelaxiangbo dome, where Indian mid-crustal rocks are exposed. The ∼35 Ma granites are characterized by variable Na2O/K2O ratios (1.03–4.44), relatively high Sr concentrations, and high Sr/Y (14.0–126.3) and La/Yb (11.1–42.8) ratios, which distinguish these granitoids from Miocene leucogranites in the Himalaya. Comparison of the Sr–Nd isotopic compositions of these granites with mid-crustal amphibolites exposed in the Yelaxiangbo dome suggests that the granites were derived from melting of the amphibolites at ∼880 °C and ∼10 kbar. The ∼30 Ma granodiorites and ∼42–46 Ma two-mica granites are Na-rich and peraluminous, and are adakitic. They contain inherited Proterozoic zircons, and have a much wider range in εNd(t) of –14.9 to –2.5 and (87Sr/86Sr)i of 0.7062–0.7188, and have a Nd isotopic model age of 1486–1978 Ma, indicating that these magmas were derived from a thickened Indian lower crust and were subsequently mixed with amphibolite-derived granite melts or were contaminated by the middle crust under southern Tibet. An apparent northward-younging age trend and shearing of the Eocene–Oligocene granitoids requires the southward migration of slices of middle crustal material, in which the Eocene granitoid magmas were emplaced and stored. Our data, along with structural, metamorphic, and intrusive histories of the Himalaya, lead us to propose a model for crustal anatexis and tectonic evolution of the Himalayan orogen, controlled by a number of large-scale events, such as slab break-off, buoyancy-driven uplift, lateral movement, and subsequent exhumation of slices of the subducted Indian crust during Indo-Asia collision at 55–40 Ma.

North–south sketch section through southern Tibet and Himalaya during late Eocene-early Oligocene, showing initial geometry of exhumation of the subducted Indian crustal mass.Figure optionsDownload high-quality image (316 K)Download as PowerPoint slideHighlights
► The Eocene granites in the eastern Tethyan Himalayan show affinity with adakite.
► Zircon U–Pb data indicate the adakitic rocks formed in the period from 30 to 46 Ma.
► These granitoids define a magmatic trail southward becoming older from 30 to 46 Ma.
► Exhumation of the subducted Indian crustal mass was initiated at ∼46 Ma.