The mechanisms responsible for the initial uplift of the Tibetan Plateau remain poorly constrained, given that most existing models emphasize its later episodic rise and outward expansion. At the heart of this uncertainty lies the Qiangtang block, which formed the core of the proto-plateau during the Eocene. Its crustal thickening history holds critical clues to understanding early uplift, yet the lack of direct lower-crustal material has long hindered insights into the deep processes involved. Here we report newly discovered Eocene (37.3−36.5 Ma) low-Mg adakitic rocks from the northeastern Qiangtang block that originated from partial melting of newly underplated juvenile lower crust. Their depleted isotopic signatures (87Sr/86Sr)i = 0.7051−0.7054; εNd(t) = −0.23 to +0.28; εHf(t) = +5.3 to +9.5; δ18O = 4.8‰−6.0‰ provide geochemical evidence for a juvenile lower-crustal source. These rocks offer a rare opportunity to quantify the vertical contribution of magmatic underplating and crustal melting to surface uplift. By integrating petrology, geophysical imaging, paleoaltimetry, and density-based isostatic modeling, we estimate that magmatic underplating and crustal melting contributed up to ∼32% of the early plateau uplift. These findings challenge conventional interpretations that attribute early plateau growth solely to crustal shortening and instead underscore the key role of deep crustal magmatism in shaping the proto-Tibetan landscape.
Wei et al. (Thu,) studied this question.