The 90 Ma island-arc type plutonism in the subduction-accretionary complex in Langxian County area, Tibet

The pre-collision magmatism took place in the accretionary wedges of many orogens. This paper presents the geological features of Langxian tonalite stock, zircon U-Pb data, and geochemical and Sr-Nd isotopic data for the tonalite to discuss whether island-arc type magmatism took place in the accretionary wedges of Yarlung Zangbo belt. The results indicate that the deformation degree of stock is distinctively lower than that of its wall rocks which comprises ophiolite mélange (145.7±2.5 Ma), and the age of the stock is (92.58±0.72)Ma (MSWD=1.8). The diorite is characterized by calc-alkaline series, and high Mg, Na, abundances of compatible elements (Cr=71.48×10^-6-86.74×10^-6, Co=22.32×10^-6-23.52×10^-6, Ni=33.51×10^-6-36.31×10^-6). The rock is enriched in Rb, Ba, Th, Pb, U, K, and depleted in Nb, Ta, Zr, Hf(Ti). REE patterns show strong enrichment of LREE and depletion of HREE (La_N/Yb_N=6.95-7.44), with indistinct Eu anomaly (δEu=0.82-0.91). Meanwhile, the values of ε_Nd(t) vary from 2.68 to 3.23. Based on these results in combination with previous data, the authors suggest that Langxian diotite intrusion was formed in an accretionary wedge island arc setting, related to southward migration of the Yarlung Zangbo subduction, and derived from the magma mixture of depleted mantle and subduction accretionary complex in the ocean ridge subducting at 90 Ma or so.