The petrogenesis and tectonic significance of the latest Late Cretaceous arc igneous rocks in Sumdo area, Tibet

The tectonic evolution of the Neo-Tethys Ocean in the last phase of Late Cretaceous around 68Ma has been controversial. This paper firstly reports the last phase of Late Cretaceous arc igneous rocks in the Sumdo area of Tibet. They consist of granite porphyry and monzogranite. Zircon dating results show that the age of granite porphyry and monzogranite is 68Ma. The granite porphyry and monzogranite have the data SiO₂(68.5%~80.6%), K₂O (4.1%~6.5%), and P₂O₅ (0.011%~0.058%). The granite porphyry contains low Mg^# values (11.3~19.0), and the monzogranite has high Mg^# values (24.2~43.5). The granite porphyry and monzogranite show light REE enrichment, heavy REE depletion, and obvious negative Eu anomalies. They both are enriched in large-ion lithophile elements such as Rb, Th, U, K and Pb, and depleted in high-field-strength elements such as Nb, Ta and Ti. The granite porphyry has ε_Hf(t) values ranging between -0.9 and +2.9, corresponding to T_DM^C model ages 955~1196Ma. The ε_Hf(t) values of the monzogranite are -17.1~+7.9, with only one point being negative, corresponding to T_DM^C model ages 633~2219Ma. The authors have thus drawn the conclusion that the magma source of the granite porphyry and monzogranite in the Songdo area was new lower crust, but the granite porphyry was closer to the ancient lower crust. Combined with regional data, the authors hold that the Neo-Tethys Ocean belongs to the end stage of ocean ridge subduction in the last phase of Late Cretaceous around 68Ma.