Survival of the Lhasa Terrane during its collision with Asia due to crust-mantle coupling revealed by ca. 114 Ma intrusive rocks in western Tibet

Survival of the Lhasa Terrane during its drift across the Tethyan Ocean and subsequent collision with Asia was likely maintained by mechanical coupling between its ancient lithospheric mantle and the overlying crust. Evidence for this coupling is provided by geochronological and geochemical data from high-Mg dioritic porphyrite dikes that intruded into granodiorites with dioritic enclaves within the Nixiong Batholith in the western segment of the central Lhasa subterrane, southern Tibet. Zircon LA-ICP-MS U-Pb dating indicates synchronous emplacement of dioritic porphyrite dikes (113.9 ± 2 Ma), dioritic enclaves (113.9 ± 1 Ma), and host granodiorites (113.1 ± 2 Ma). The hornblende-bearing granodiorites are metaluminous to weakly peraluminous (A/CNK = 0.95–1.05) and belong to high-K calc-alkaline I-type granite. These rocks are characterized by low Mg# (37–43), negative zircon ε_Hf(t) values (−6.8 to −1.2), and negative whole-rock ε_Nd(t) values (−8.1 to −5.4), suggestive of derivation through anatexis of ancient lower crust. The two least-mixed or contaminated dioritic porphyrite dike samples have high MgO (8.46−8.74 wt%), high Mg# (69−70), and high abundances of compatible elements (e.g., Cr = 673−646 ppm, Ni = 177−189 ppm), which are close to those of primitive magma. They are high-K calc-alkaline and show negative whole-rock ε_Nd(t) values (−1.9 to −1.2), indicating that these samples are most likely derived from the partial melting of ancient lithospheric mantle that was metasomatized by slab-derived fluids. The dioritic enclave samples are metaluminous high-K calc-alkaline and have varying negative whole-rock ε_Nd(t) values (−7.8 to −3.7), which are interpreted as the result of magma mixing between the ancient lower crust-derived melts and asthenospheric mantle- (rather than lithospheric mantle-) derived melts. The Nd isotope mantle model ages of the least-mixed or contaminated high-Mg dioritic porphyrite dike samples (1.1−1.4 Ga) are close to the Nd isotope two-stage model ages (1.3−1.6 Ga) and the zircon Hf isotope crustal model ages (1.1−1.5 Ga) of the ancient lower crust-derived granodiorites, indicating that the lithospheric mantle of the western segment of the central Lhasa subterrane is mechanically coupled to the overlying crust at ~114 Ma. In combination with the Proterozoic crustal rocks documented in the central and eastern segments of the central Lhasa subterrane, we propose that this coupling enabled it to resist subduction during accretion to Asia.