Abstract
The high values of La/Yb (>15–20) obtained from late Eocene to Pliocene volcanic rocks of central Tibet indicate that source regions were in a thickened (>35 km) crust. The higher Neogene La/Yb values compared to lower Paleogene La/Yb values required continuous crustal thickening, as La/Yb values are proportional to crustal thickness. Although the Paleogene high La/Yb values can be explained by coeval thrusting, this mechanism is unable explain the Neogene higher La/Yb values because central Tibet is covered by undeformed flat-lying early Miocene strata. Here we correlate the temporal variation of Cenozoic volcanic geochemistry with the regional geology of central Tibet, which was part of a larger Eocene–Oligocene thrust belt and the related foreland basin. The Paleogene foreland basin was partitioned into the southern (Hoh Xil) and northern (Qaidam) subbasins by the Kunlun fault ca. 25–20 Ma. The two subbasins were at similar elevations at the time of their formation, but the southern basin, at ∼5 km, is >2 km higher than the northern basin at ∼2.8 km. Because the southern basin and the Paleogene thrust belt have been associated with Neogene volcanism and high La/Yb values, we suggest that the elevation difference between the two basins was induced by >15 km of magmatic inflation sourced from mantle melting. The magnitude of magmatic inflation is comparable to that of tectonic thickening in central Tibet, highlighting the important and often neglected role of syncollisional mantle melting and its induced magmatic inflation in constructing the thickened Tibetan crust.
