Sediment melting during subduction initiation: Geochronological and geochemical evidence from the Darutso high-Mg andesites within ophiolite melange, central Tibet

In addition to fluids, the concept of sediment-derived melts infiltrating the fore-arc mantle during subduction initiation has been proposed based on studies of modern subduction zones and ophiolite melange. However, outcrops that contain the products of such melts are rare, especially in conjunction with boninite. New U-Pb zircon dating reveals that the Darutso volcanic rocks (DVRs) within ophiolitic melange in the Beila area, central Tibet, crystallized at similar to 164-162 Ma. This is the first recognition of Jurassic volcanic rocks in the middle section of the Bangong-Nujiang Suture Zone. Geochemically, the DVRs are high-Mg andesites with moderate SiO2 (59.03-63.62 wt %) and high MgO (3.74-6.53 wt %), Cr (up to 395 ppm), and Mg-# (50.3-67.9). They also have high Th contents, (La/Sm)(N) ratios, and (Sr-87/Sr-86)(i) values (0.7085-0.7147); low Ba/Th, U/Th, and Sr/Y ratios; and negative values of epsilon(Nd)(t) (-8.7 to -9.8) and zircon epsilon(Hf)(t) (-7.4 to -9.9). The epsilon(Nd)(t) values of the DVRs overlap those of regional sediments. Detailed analyses of these geochemical characteristics indicate that the DVRs were derived from partial melting of subducted sediments and subsequent interaction with overlying mantle peridotite in a shallow and hot setting. In combination with the regional geology, in particular adjacent ophiolites that contain MORB-like and boninite mafic lavas, these rocks collectively recorded the evolution of a fore-arc setting during the initiation of the northward subduction of the south branch of the Bangong-Nujiang Ocean. Therefore, the results provide direct evidence for sediment melting during subduction initiation and constrain the Jurassic tectonic evolution of the Lhasa terrane.