Origin of adakitic intrusives generated during mid-Miocene east-west extension in southern Tibet

Adakite is an intermediate to felsic rock with low K, high Al, Na and Sr, and depleted in Y and HREE, usually occurring in arc settings related to subduction of an oceanic slab. Here we report the occurrence of potassic adakites from south Tibet in an orogenic belt produced by the Indo-Asian continent collision. These adakitic intrusives, as a product of Neogene east-west extension, occur in a Miocene Cu-bearing porphyry belt, which developed along the Gangdese arc paralleling the Yarlung-Zangbo suture, but is locally controlled by NS-striking normal faulting systems. Available age data define a duration of magmatism of 10-18 Ma for the adakitic intrusives and related extrusive analogues in south Tibet, which occur in a post-collisional extensional setting. Geochemical data indicate that these adakitic intrusives are shoshonitic and exhibit calc-alkaline composition with high K, and high Sr/Y and La/Y coupled with low Y and HREE, similar to adakites derived from slab melting. However, a wide range for epsilon(Nd(t)) (-6.18 to +5.52), initial Sr-87/Sr-86 (0.7049-0.7079), Pb-207/Pb-204 (15.502-15.626), and Pb-208/Pb-204 (38.389-38.960), as well as high K2O contents (2.6-8.6 wt%) and relatively high Mg# values (0.32-0.74) indicate that these adakitic magmas were formed by a complex mechanism involving partial melting of mafic materials in a thickened lower crust with input of enriched mantle and/or upper crust components. Absence of a negative Eu anomaly, extreme depletion in Y, Nb and Ti, and variable high Sr/Y and La/Yb ratios suggest that the lower crustal source is probably a hydrous amphibole eclogite or garnet amphibolite, as exhumed in the western and eastern Himalayan syntaxes on the Tibetan plateau. Partial melting of the lower crust was most likely triggered by mantle-derived ultra-potassic magmatism (17-25 Ma) formed by slab breakoff or mantle thinning. During the formation and migration of pristine adakitic melts, additional input of ultra-potassic magmas and upper crustal materials could account for the observed epsilon(Nd)-epsilon(Sr) signatures and high Rb/Sr, K and Mg# characteristics for most of the adakitic intrusives in south Tibet. (C) 2004 Published by Elsevier B.V.