Regional isotopic patterns in granitic rocks of southern Tibet and magmatic contributions to crustal thickening during the Indo-Asian collision

Southern Tibet hosts one of the largest and youngest examples of a continental-margin composite granitic batholith, the Gangdese Batholith. This batholith is located immediately adjacent to the Indus-Tsangpo Suture (ITS) and is inferred to have formed largely during the pre-collision subduction period pre-50 Ma. However, crystallization ages extend from Cretaceous to Miocene, so granitic magmatism spanned the collision time and reaches almost to the present. Isotopic and trace element compositions of granitoids are known to reflect both mantle magma processes and crustal age and thickness, so regional variations in the Nd, Sr, Hf and Pb isotopic compositions can help to constrain the precollision Tibet crustal structure and the syn- and post-collisional modifications. We focus on magmatic contributions from the mantle that may have been responsible for some of the crustal thickening that occurred during collision. Granitoids in a N-S traverse near Lhasa and north of the ITS show a gradient in ɛNd, with mantle-like values adjacent to the ITS (+2 to +6) and much lower values (-5 to -14) at 80 to 120 km north of the ITS. This type of spatial gradient in ɛNd is found in other continental margin batholiths that are associated with subduction but not continental collision. Positive ɛNd values usually indicate granitoids emplaced into a continental margin consisting of oceanic crust with sedimentary cover. There is a 60 km-wide zone north of the ITS where this type of crust hosted the batholith even though the magmatism occurred slightly after the beginning of collision. Decreasing ɛNd northward indicates a decreasing ratio of mantle input/crustal assimilation due to progressively thickened crust. A discontinuity at about 29.8N latitude (just N. of Lhasa) may be the southern edge of pre-collision Tibetan basement. Miocene granites near the ITS differ from older granitoids in that they have lower ɛNd, suggesting crustal thickening and structural rearrangement by Miocene time. Mantle magmatic additions to the thin edge of southern Tibet could have increased the crustal thickness by as much as 20 to 30 km. At distances greater than about 100km north from the ITS, it appears that there were no significant magmatic additions to the crust, either before or after collision. Peraluminous (2-mica) granites north of 30.0N latitude have ɛNd of -7 to -14, corresponding to basement model ages of 1.2 to 1.8 Ga. The patterns in the Lhasa area have been generalized over a larger area by Zhu et al (2010) using Hf isotopes, and suggest that the Lhasa block had a keel-like shape prior to collision. A thin southern edge of Tibet could have affected the early stages of development of the collision, perhaps promoting subduction of India.