Zircon U-Pb chronology and Hf isotope of the Late Triassic andesitic magmatism in Dajiacuo, Tibet

The geodynamic setting of the Late Triassic-Early Jurassic magmatic activities in the Lhasa Terrane remains hotly debated. This paper reports zircon LA-ICP-MS U-Pb dating, Hf isotopic, and whole-rock geochemical data of the andesite porphyrites from Dajiacuo in the western segment of the southern Lhasa subterrane. The Dajiacuo andesite porphyrites consist mainly of hornblende, pyroxene, and altered fine-grained plagioclase. Two andesitic porphyrite samples have been dated at 204 +/- 4Ma and 203 +/- 2Ma, respectively, indicating the present of the Late Triassic andesitic magmatism in this region. In combination with the presence of pyroxene-bearing amphibole monzodiorite of 207.3 +/- 3.6Ma recently reported from Dajiacuo, it is suggesting that the Late Triassic magmatism was active in the western segment of the southern Lhasa subterrane. These two andesitic porphyrite samples mainly show positive zircon epsilon(Hf)(t) values (-2.2 similar to +8.8 and -2.0 similar to +10.7, respectively), comparable to those of the coeval magmatic records reported from the middle and eastern segments of the southern Lhasa subterrane (e.g., ca. 203 Ma syenogranite to the south of Gongbogyamda display zircon epsilon(Hf)(t)(t) of -4. 9 - + 2. 7, and ca. 205Ma monzogranite to the east of Xigaze exhibit zircon epsilon(Hf)(t)( t) of +11.9 +/- 15. 8). Such similarity indicates that the Late Triassic magmatic activity extended from Gongbogyamda in the east to Dajiacuo in the west, covering over 800km along the length of the southern Lhasa subterrane. Two andesitic porphyrite samples (SiO3 = 56. 1% 59. 0%) are characterized by low Me-# (45.8 similar to 48.7) and Al2O3 (16. 4% -17. 0%) contents and are shoshonitic or highK calc-alkaline. These new data, together with the data of volcanic rocks recently reported from the Lower Jurassic Sangri Group and the new identification of the Upper Triassic strata around the Dajiacuo, enable us to favor that the Late Triassic magmatic activity documented from Dajiacuo and other segments of the southern Lhasa subterrane is most likely associated with a back-arc setting in response to the southward subduction of the Bangong-Nujiang oceanic lithosphere.