Origin and tectonic implications of Paleocene high-Mg dioritic plutons in the Lhasa terrane, Qulong area, Tibet

Zircon U-Pb ages, geochemical data, and Sr-Nd-Hf isotopic data are reported for the Paleocene Qulong high-Mg dioritic plutons (HMDPs) in southern Tibet of the India-Asia collision zone to investigate their source, petro- genesis, and tectonic setting. The Qulong HMDPs consist of diorite with a 206Pb/238U age of 63.45 +/- 0.87 Ma and gabbro-diorite with a 206Pb/238U age of 63.8 +/- 0.57 Ma. They are metaluminous calc-alkaline rocks and exhibit sanukitic HMD affinity with moderate SiO2 and TiO2 and high MgO (5.24-8.64 wt%) and Mg# (55.78-65.89). They exhibit large ion lithophile element and light rare earth element enrichment and Nb-Ta depletion, rela- tively low (Th/Nb)N ratios, and high (La/Sm)N , La/Ta, and La/Nb ratios, which are similar to oceanic island basalts derived from the subduction-related mantle source. In addition, relatively low initial 87Sr/86Sr ratios (0.7056-0.7062), positive epsilon Nd(t) (+2.23 to + 3.34), and epsilon Hf(t) (+4.55 to + 12.35) indicate the origin of the Qulong HMDPs to be the subduction-related lithospheric mantle which had been modified by the asthenosphere, and mixed with the sediment melt. This study suggests that the Qulong HMDPs are a product of the upwelling of the asthenosphere triggered by the slab roll-back of the subducting NeoTethys slab of the India-Asia collision at ca. 64 Ma.

Automated summary

Zircon U-Pb ages, geochemical data, and Sr-Nd-Hf isotopic data were analyzed to study the source, petrogenesis, and tectonic setting of the Paleocene Qulong high-Mg dioritic plutons in southern Tibet. The results indicate that these plutons are metaluminous calc-alkaline rocks with sanukitic HMD affinity. They show enrichment in ion lithophile and light rare earth elements, depletion in Nb-Ta, and low (Th/Nb)N ratios, similar to oceanic island basalts. The isotopic data suggest that the Qulong HMDPs originated from a subduction-related lithospheric mantle modified by the asthenosphere and mixed with sediment melt, triggered by the slab roll-back of the subducting NeoTethys slab of the India-Asia collision at ca. 64 Ma.