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 mélange. 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 mélange in the Beila area, central Tibet, crystallized at ∼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 (⁸⁷Sr/⁸⁶Sr)_i values (0.7085–0.7147); low Ba/Th, U/Th, and Sr/Y ratios; and negative values of ε_Nd(t) (−8.7 to −9.8) and zircon ε_Hf(t) (−7.4 to −9.9). The ε_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.

The Bangong‐Nujiang Suture Zone (BNSZ) in central Tibet represents relics of the Meso‐Tethys Ocean that once separated the Lhasa and Qiangtang terranes. When and how this ocean closed has been a matter of debate, and the cause of the uncertainty stems primarily from a lack of consensus as to whether Early Cretaceous ophiolite exists within the BNSZ. New geochronological and geochemical data presented here identify ∼138–134 Ma MORB‐like and OIB‐type mafic rocks in the Kangqiong area of the western BNSZ. These data, together with the spatially associated peridotite and marine sediment, provide the first firm evidence for the presence of Early Cretaceous ophiolite within the BNSZ. Our integrated study of geological, mineralogical, and geochemical data suggests that the Kangqiong Ophiolite formed in a forearc setting related to the subduction re‐initiation of the Meso‐Tethys Ocean. By combining the present results with a synthesis of tectonomagmatism in central Tibet, including the existence of an Early Jurassic oceanic plateau within the Meso‐Tethys Ocean, the magmatic lull (∼145–130 Ma) in the Qiangtang Terrane, an angular unconformity between Lower Jurassic oceanic rocks and Upper Jurassic shallow‐marine strata, and the compositional differences and migration of magmatism before and after the magmatic lull, we propose a new model of the evolution of the Meso‐Tethys Ocean. This model proposes that the collision of an oceanic plateau with the Qiangtang continental margin caused the termination of Meso‐Tethyan subduction during the Late Jurassic and subsequent re‐initiation behind the oceanic plateau during the Early Cretaceous.

Orthopyroxene-bearing granitic rock (e.g., charnockite) is relatively rare but provides an excellent opportunity to probe the thermal and tectonic evolution of deep orogenic crust because of its distinct mineral assemblage. Here we present petrological, mineralogical, elemental, and Sr–Nd–Hf–O isotopic data for late Eocene (ca. 36 Ma; zircon U–Pb ages) volcanic rocks exposed in the Ejiu region in the southern Qiangtang Terrane to investigate how the central Tibetan crust evolved to its modern thickness and thermal state. The Ejiu volcanic rocks (EVRs) are trachydacites with anhydrous mineral assemblages (i.e., two pyroxenes, sanidine, plagioclase, and ilmenite, without amphibole and biotite) and geochemical characteristics (e.g., high P₂O₅ and TiO₂) that resemble those of charnockite-type magmatic rocks. Mineral and whole-rock thermometry and hygrometry suggests that the parent magma crystallized under hot (~1000 °C) and dry (H₂O < 2 wt.%) condition. Besides, the EVRs display adakitic affinities according to their high SiO₂ and Al₂O₃ contents, high Sr/Y, La/Yb, and Gd/Yb ratios, and low Y and Yb contents, without marked negative Eu anomalies. The calculated melts in equilibrium with pyroxenes also display adakitic compositions (e.g., high Sr/Y and La/Yb ratios), indicating that the adakitic compositions of the EVRs did not result from late-stage magmatic evolution. In addition, the melts of the EVRs were saturated in TiO₂, as inferred from the high TiO₂ contents of these rocks and the presence of ilmenite. An integrated analysis of the geochemical, petrological, and mineralogical data suggests that the EVRs were neither evolutional products nor partial melts of hydrous mafic materials at normal crustal pressures, but were formed by fusion of an eclogitized mafic protolith with residue containing garnet and rutile but lacking amphibole and plagioclase. The whole-rock Sr–Nd and zircon Hf isotope compositions of the EVRs [(⁸⁷Sr/⁸⁶Sr)_i = 0.7053 to 0.7066; ε_Nd(t) = −1.40 to −0.99; zircon ε_Hf(t) = +1.08 to +5.31] indicate that the parental protolith was relatively juvenile in nature, but also contained some supracrustal materials given the high zircon δ¹⁸O values [zircon δ¹⁸O = +8.21‰ to +11.00‰]. The above arguments lead us to propose that of partial melting of a previously dehydrated—but chemically undepleted—mafic lower continental crust at high pressure (>1.5 GPa) and high temperature (>1000 °C) generated the EVRs. Based on a synthesis of independent geological and geophysical data, we further suggest that the southern Qiangtang Terrane crust of the central Tibetan Plateau was thick, dry, and elevated during the Late Cretaceous to early Eocene time, and that it became abnormally hot owing to the ascending asthenosphere after lithospheric foundering during the middle Eocene.