Northern Boundary of Gondwana Inferred From Geochemical Characteristics of Granites East of the Eastern Syntaxis

Abstract

Abstract The Bangong–Nujiang Meso-Tethyan Suture in eastern Tibet is still difficult to trace, mainly because Cenozoic shortening has juxtaposed narrow terranes, basins, and suture-related mélanges. We present field observations, zircon U-Pb ages, Lu-Hf isotopes, and whole-rock geochemistry for three granitic samples from the Lhasa–Tengchong and South Qiangtang–Baoshan blocks east of the eastern syntaxis. The work addresses three related questions: whether the Bangong–Nujiang and Bitu/Longmu Co–Shuanghu sutures merge in this region, whether the Cretaceous granites record the eastern continuation of Bangong–Nujiang subduction, and whether a ca. 455 Ma granite block can be linked to the South Qiangtang–Baoshan basement. Samples WX-14 and WX-17 yield zircon U-Pb ages of 94 ± 0.7 and 109 ± 2.0 Ma, respectively, and are interpreted as high-K calc-alkaline I-type to mixed-source granitoids formed in an arc-related, post-collisional setting. Sample WX-15 yields a crystallization age of 455 ± 5 Ma and negative zircon εHf(t) values, consistent with derivation from ancient crust comparable to Early Paleozoic basement magmatism along the northern Gondwanan margin. Because WX-15 occurs as a fault-bounded block within Carboniferous mélange, for this reason, it is taken as evidence for South Qiangtang–Baoshan affinity only when considered together with regional stratigraphic continuity, neighboring Chongshan Group basement, and published mapping. Taken together, these data support, but do not by themselves prove, a tectonic framework in which the Bangong–Nujiang and Bitu/Longmu Co–Shuanghu sutures remain separate and subparallel across the narrowed Zuogong Basin in eastern Tibet.