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

Abstract

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 δ26Mg (−0.28 ± 0.07‰, 2SD, n = 18) and significantly lower δ44/40Ca (0.69 ± 0.03‰, 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 Ca-rich sedimentary carbonate in the mantle source. Additionally, these ultrapotassic rocks display δ56Fe values (0.026–0.163‰, 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 δ26Mg (−0.13 ± 0.10‰, 2SD, n = 15) and δ56Fe (up to 0.304‰) values, along with the lowest δ44/40Ca values (0.60 ± 0.07‰, 2SE, n = 15) among these post-collisional magmatic rocks. They also demonstrate strong correlations between δ56Fe 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 δ56Fe values (0.102 ± 0.067‰, 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 (δ44/40Ca = 0.67 ± 0.03‰, 2SE, n = 5) than the mantle, consistent with a carbonate metasomatized source. They exhibit varying δ26Mg (−0.31 ± 0.07‰ to −0.04 ± 0.07‰, 2SD) and relatively uniform δ56Fe (0.186 ± 0.051‰, 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.