Abstract

Mesozoic and Cenozoic granites are widely distributed in the South Lhasa terrane of southern Tibet and record information about subduction of the Tethys oceanic plate and the India–Eurasia collision. This paper presents petrological, geochemical, zircon U–Pb and Hf isotopic characteristics of Cenozoic intrusive rocks in the Luocang area, South Lhasa terrane. Our data show these granites have ages of 54.4 ± 0.5 Ma and 47.2 ± 0.3 Ma (early Eocene), and are high-K, calc-alkaline, peraluminous, differentiated, I-type granites. The εHf(t) values of two samples are −9.59 to −4.15 and −7.85 to −5.36. These results suggest that the granites were derived from melting of the crust with no input of mantle source material. A review of data from Cenozoic granites of the South Lhasa terrane shows that they formed in a tectonic environment characterised by a gradual west-to-east transition to a post-collisional setting and are associated with breakoff of the Neo-Tethys oceanic plate. During the early Eocene continental collision, breakoff of the Neo-Tethys oceanic lithosphere caused the Indian plate to fold back resulting in a major asthenospheric disturbance. This was accompanied by an input of heat to the overlying continental crust, which caused melting of the mid to lower crust of the Lhasa terrane. The ascending magma underwent fractional crystallisation and was intruded to form the Eocene granites in the Luocang area.

1. KEY POINTS

2. Two granitoids in the Luocang area of the South Lhasa terrane (southern Tibet) are high-K, calc-alkaline, peraluminous, differentiated, I-type granites and have negative εHf(t) values (−10 to −4).

3. The U–Pb zircon ages of these two granitoids are 54.4 ± 0.5 Ma and 47.2 ± 0.3 Ma (early Eocene).

4. The Luocang granites were formed by mantle heat input into the lower to middle crust of the Lhasa terrane; these crustal melts underwent fractional crystallisation during emplacement.