Thickness change of the Tibetan lithosphere since Early Cretaceous as revealed by Ce/Yb and Sm/Yb ratios of volcanic rocks

The growth of lithospheric thickness beneath the Tibetan Plateau since Early Cretaceous is demonstrated by changes in partial melting depth of the mantle as reflected in the Ce/Yb and Sm/Yb ratios of volcanic rocks. The ratios indicate that the lithospheric thickness of the Tibetan blocks ranged between 50–90 km in most areas and between 100–120 km in several areas during the Cretaceous. It increased from 60–110 km in Eocene to 170–200 km in Early Miocene in the Lhasa block, from 110–120 km in Early Oligocene to 170–180 km in Early Miocene in the Qiangtang block, and from ~130 km in Late Palaeogene to 160–190 km in Early Miocene for the Hohxil and Kunlun blocks. The thickness of the Lhasa block thus increased ~100 km at an average rate of 3.0 mm/a during Early Cenozoic. The indicated increase in Early Miocene for the Lhasa, Qiangtang, and Hohxil and Kunlun blocks to 160–195 km nearly matches the present 160–200 km plateau lithospheric thickness as inferred from seismic tomography. It is reasonable to assume that the crust, as well as the lithospheric mantle for all blocks thickened during the Palaeogene and reached its present thickness in Early Miocene.

基于火山岩的铈/镱（Ce/Yb）与钐/镱（Sm/Yb）比值所反映的地幔部分熔融深度变化，证实了早白垩世以来青藏高原下方岩石圈厚度（lithospheric thickness）的增长过程。上述比值数据显示，白垩纪时期，青藏高原各地块的岩石圈厚度多介于50~90 km之间，部分区域可达100~120 km。拉萨地块（Lhasa block）的岩石圈厚度从始新世的60~110 km增至早中新世的170~200 km；羌塘地块（Qiangtang block）的厚度则从早渐新世的110~120 km增至早中新世的170~180 km；可可西里与昆仑地块（Hohxil and Kunlun blocks）的厚度则从古近纪晚期的约130 km增至早中新世的160~190 km。早新生代期间，拉萨地块的岩石圈厚度累计增长约100 km，平均增厚速率为3.0 mm/a。上述研究显示的拉萨、羌塘、可可西里及昆仑地块在早中新世增厚至160~195 km的结果，与通过地震层析成像（seismic tomography）推断出的现今青藏高原岩石圈厚度（160~200 km）高度吻合。据此可合理推测：所有地块的地壳及岩石圈地幔均在古近纪期间发生增厚，并于早中新世达到现今的厚度。