Mg-Ca-Fe isotopes of post-collisional magmatic rocks record the crust-mantle interaction processes beneath southern Tibet

The non-traditional Mg, Ca and Fe isotopes have proven to be useful tools for investigating the petrogenesis of the mantle- and crust-derived rocks. Individual non-traditional isotopes in continental basalts are well-studied, particularly in subduction settings. In contrast, there is a significant lack of comprehensive research on various non-traditional stable isotopes related to magmatic processes within continental collision zones, especially during post-collisional stage. To address this knowledge gap, this study presents stable Mg-Ca-Fe isotopic data for the Miocene post-collisional magmatic rocks including ultrapotassic, potassic, and adakitic rocks from the Lhasa Terrane in southern Tibet. The ultrapotassic rocks exhibit slightly lower delta 26Mg (-0.28 +/- 0.07%0, 2SD, n = 18) and significantly lower delta 44/40Ca (0.69 +/- 0.03%0, 2SD, n = 18) values than the depleted mantle. The high CaO/Al2O3 and CaO/TiO2 ratios of the ultrapotassic rocks are typical characteristics of carbonate metasomatism, suggesting that the light Mg and Ca isotopic composition may be attributed to the involvement of recycled Carich sedimentary carbonate in the mantle source. Additionally, these ultrapotassic rocks display delta 56Fe values (0.026-0.163%0, 2SD, n = 18) ranging from the normal peridotite mantle to much higher values, suggesting a likely origin from a pyroxenite-bearing lithospheric mantle source. These findings imply that subducted oceanic crust and overlying Ca-rich sediments plausibly account for the formation of a mantle source characterized by distinctive Mg-Ca-Fe isotope compositions. The adakitic rocks exhibit the highest delta 26Mg (-0.13 +/- 0.10%0, 2SD, n = 15) and delta 56Fe (up to 0.304%0) values, along with the lowest delta 44/40Ca values (0.60 +/- 0.07%0, 2SE, n = 15) among these post-collisional magmatic rocks. They also demonstrate strong correlations between delta 56Fe and (Gd/ Lu)N ratios, indicating a significant role of garnet in their source, consistent with their origin from the thickened lower crust associated with the collision between India and Asia continents. Given the consistent delta 56Fe values (0.102 +/- 0.067%0, 2SD, n = 5) observed in the adakitic rocks from the western Lhasa Terrane, we posit that it reflects the Fe isotopic composition of the thickened lower crust in southern Tibet. The potassic rocks display lighter Ca isotopes (delta 44/40Ca = 0.67 +/- 0.03%0, 2SE, n = 5) than the mantle, consistent with a carbonate metasomatized source. They exhibit varying delta 26Mg (-0.31 +/- 0.07%0 to -0.04 +/- 0.07%0, 2SD) and relatively uniform delta 56Fe (0.186 +/- 0.051%0, 2SE, n = 6) values, falling within the range of nearly contemporaneous ultrapotassic and adakitic rocks. This suggests that potassic rocks may be formed through the mixing of mantle materials resembling ultrapotassic rocks and thickened crustal components resembling adakitic rocks in the lower crust. Building upon these findings, we propose that the distinctive geochemical characteristics, as well as the spatial and temporal distribution of post-collisional magmatic rocks in southern Tibet, are the outcome of partial melting within a carbonated pyroxenite-bearing lithospheric mantle, coupled with the involvement of thickened crust and their subsequent interactions.