Miocene N-MORB- and OIB-type diabase dikes in the western Yarlung Tsangpo suture zone, southern Tibet: Insights into the Indian slab tearing and asthenosphere–lithosphere interaction

Post-collisional magmatic rocks are widely distributed in southern Tibet, and their petrogenesis can provide critical information about lithospheric-scale thermal and compositional structures. However, the deep geodynamic processes responsible for post-collisional magmatism remain ambiguous. Here, Miocene (ca. 16 Ma) diabase dikes are identified in the Dare area within the western Yarlung-Tsangpo Suture Zone (YTSZ). The Dare diabase rocks can be divided into subalkaline and alkaline series. The subalkaline diabase rocks exhibit normal mid-ocean ridge basalt (MORB)-type multi-element distribution patterns and have high positive ε_Nd(t) values (+8.23 to +8.99) and low initial ⁸⁷Sr/⁸⁶Sr (0.7059–0.7074), La_N/Yb_N (0.50–0.72), and Y/Yb (8.94–9.91) values. By contrast, the alkaline diabase rocks display oceanic island basalt (OIB)-type multi-element distribution patterns, and have low positive ε_Nd(t) (+1.17 to +1.30) and initial ⁸⁷Sr/⁸⁶Sr (0.7064–0.7067) values and high La_N/Yb_N (16.57–20.45) and Y/Yb (12.69–13.44) values. The relatively depleted Sr-Nd isotope compositions and different rare earth element ratios reveal that the subalkaline and alkaline diabase rocks were most likely derived from asthenospheric mantle sources at depths of 55–63 km in the spinel-stability field and at depths of 85–91 km in the spinel-bearing garnet-stability field, respectively. The Sr-Nd isotope models, as well as several trace element ratios (e.g., Nb/Ta, Nb/U, and Ba/Nb), further demonstrate that these asthenosphere-derived melts were contaminated to varying degrees by the overlying Zhongba lithospheric mantle and crust during their ascent and emplacement, implying significant asthenosphere–lithosphere interaction and resultant thermal perturbation beneath the western YTSZ. Based on the spatiotemporal distribution of the post-collisional magmatic rocks, the bilateral tearing of the subducted Indian slab can best account for the E–W-trending zonal distribution patterns with an eastward younging trend in the 73–89°E segment and a westward younging trend in the 90–96°E segment. In addition, the derivative longitudinal tearing and subsequent slab advancement would lead to the locally southward and northward younging trends of post-collisional magmatic rocks, respectively.