Late Mesozoic subduction-accretion in the southern Qiangtang: Insights from the Sumxi igneous complex of west-central Tibet

Abstract

Knowledge of the evolution of the Bangong-Nujiang Tethyan Ocean is crucial for reconstructing the paleography of the Tethyan Realm, given its significance as a key component of the eastern Tethys. Nonetheless, there has been uncertainty regarding both the timing and the processes involved in the closure of this ocean. This study focused on a 110−106 Ma igneous complex comprising basalts−basaltic andesites, trachyandesites, and granodiorites from the Sumxi area in the western part of the Qiangtang terrane of west-central Tibet. The basalts−basaltic andesites have SiO2 contents of 52.5−58.7 wt% and MgO contents of 2.89−4.63 wt%, and exhibit some arc-like geochemical signatures. However, these rocks also have elevated Nb contents (>10 ppm) and Nb/La ratios (>0.5), as well as enriched Sr-Nd isotopic composition [εNd(t) = −7.40 to −6.00], implying that they are products of a mantle source metasomatized by adakitic melts. The trachyandesites are characterized by intermediate compositions (SiO2 = 63.6−65.2 wt%), high Mg number (40−60), and more enriched εNd(t) values (−8.37 to −7.49). Comparing their geochemical composition to that of mélange rocks, it is postulated that these trachyandesites were formed through the partial melting of a mantle source including mélange matrix rocks within a subduction zone. The granodiorites exhibit adakitic geochemical features (Sr = 830.14−1032.70 ppm, Y = 14.86−15.37 ppm, Sr/Y = 54−68), indicating that they originated from the partial melting of a thickened lower crust in a continental arc setting. Our results, in combination with a synthesis of tectonomagmatism along the Bangong-Nujiang suture zone, provide convincing evidence for subduction of an oceanic plateau and subsequent slab roll-back. The Sumxi igneous complex, with its clear arc affinity, suggests that the Bangong-Nujiang Tethyan Ocean, or at least its western part, remained open until the late Early Cretaceous (ca. 106 Ma).