MAGMATIC EVIDENCE FOR LOCALIZED LITHOSPHERIC DELAMINATION RESULTING FROM CONTINENTAL COLLISION IN THE PAMIR-TIBET OROGEN

The Pamir-Tibet orogen comprises several terranes (e.g., Qiangtang, South Pamir) that were accreted to southern Asia during the Mesozoic. The terranes are younger and weaker than the para-autochthonous to cratonic blocks that anchor the northern Pamir-Tibetan Plateau. The boundary between the terranes and stable lithospheric blocks, the Tanymas-Jinsha suture zone, is an important rheological discontinuity within the lithosphere. A pulse (36-42 Ma; zircon U-Pb ages) of geographically restricted (~150 km diameter ellipsoidal area) magmatism, referred to as the Vanj magmatic complex in the Pamir, is interpreted to be related to a mantle drip or small delamination event that developed along this rheological discontinuity, 10–20 Myr after subduction of oceanic lithosphere terminated and India first collided with Asia. The age, geochemistry (SiO₂ = 54–75 wt. %; high-K calc-alkaline to shoshonitic; metaluminous; La/Yb_n = 13–57; Mg# = 35–41), isotopic composition (εNd_t = -7 to -3; ⁸⁷Sr/⁸⁶Sr_i, = 0.706–0.710; zircon εHf_t = -3 to +1; zircon δ¹⁸O_VSMOW = 6.0–7.6 ‰), outcrop pattern, and tectonic position of the Vanj complex suggest that it is related to a series of magmatic complexes that occur in the northern Qiangtang terrane in Tibet, adjacent the Jinsha suture. The Eocene Vanj and Qiangtang magmatic complexes are collectively interpreted as nearly synchronous, discrete drips of mantle lithosphere or small delamination events that occurred for >2,500 km along strike in the Pamir-Tibet orogen. Syn-collisional magmatic products like the Vanj complex may be characteristic of continental collisional orogenesis. The results of this study support recent numerical models that suggest deformation and crustal thickening may be concentrated along rheological boundaries during continental collision and that discontinuities may help nucleate delamination events. In the case of the Vanj and Qiangtang complexes, lithospheric deformation and subsequent delamination occurred 300-600 km inboard (North) of the India-Asia suture zone.