Lack of information regarding lithium (Li) crystal chemistry in numerous minerals, especially those containing trace amounts of Li (ranging from a few to tens of ppm), limits our understanding of Li isotopic fractionation in pegmatites. In this study, we examined the Li isotopic composition and Li content in various Li-poor (e.g., quartz or feldspar) together with Li-rich (sopdumene or lepidolite) mineral phases within granitic pegmatites. We compiled a comprehensive dataset, encompassing a broad spectrum of Li contents (ranging from a few to tens of thousands of ppm) and Li isotopic values (−8‰ to 41‰). The minerals exhibit distinct Li isotopic signatures. Specifically, elbaite and beryl show the highest values, while biotite displays a negative average. Compared to individual minerals, whole rocks demonstrate lower Li isotopic values, with pegmatites exhibiting the highest and non-granitic pegmatite wall rocks showing the lowest. Our study also uncovers a clear “V”. shape relationship between Li isotopic values and logarithm of Li contents, with different mineral groups occupying specific regions within this shape. Furthermore, a significant correlation was observed between average Li isotopic values and Li-O (OH, F) bond lengths in various minerals. These discoveries underscore the crucial role of crystal chemistry in shaping the Li isotopic behavior in pegmatites from a statistical perspective.
Based on a compilation of AFT, AHe ages and apatite MTLs from previous studies, the following conclusions can be made regarding the spatial and temporal distribution of exhumation in Altai-Sayan region.
(1) The oldest AFT ages, found in the Gobi Altai, suggest that this region has been tectonically stable since the Late Jurassic.
(2) Early Cretaceous rapid cooling is focused in northern Chinese Altai and western Gorny Altai, associated with the Mongol-Okhotsk orogeny. Late Cretaceous rapid cooling identified in most other areas of Altai-Saya region is best explained as being associated with fault reactivation due to the subsequent collapse of the Mongol-Okhotsk Orogen.
(3) Cenozoic reactivation is difficult to be record with low-temperature systems due to very limited exhumation over this time frame. As a result, the timing and mechanism of any Cenozoic reactivation in the Altai-Saya region remains unclear.
(4) At the orogenic scale, the AFT ages in the northern part of the Altai-Saya region are younger than those in the southern part, indicating that the basement of northern part, weakened by its earlier extensional tectonism, was more easily reactivated.