Anatexis at different crustal levels in the Eastern Himalayan Syntaxis: Implications for crustal exhumation and uplift

Liang, Chenhua; Zhang, Hongfei; Luo, Biji; Harris, Nigel and Guo, Liang (2025). Anatexis at different crustal levels in the Eastern Himalayan Syntaxis: Implications for crustal exhumation and uplift. Lithos, 506-507, article no. 108026.

DOI: https://doi.org/10.1016/j.lithos.2025.108026

Abstract

An integrated study of zircons, U-Pb dating and Hf isotope, and whole rock major and trace element analyses was conducted on Zhibai, Pai, and Duoxiongla formation leucosomes and leucogranites from the Namche Barwa, Eastern Himalayan Syntaxis to constrain crustal anatexis across granulite amphibolite transitions and its linkage to orogenic dynamics. Anatectic samples exhibit systematic variations in age clusters, mineral assemblages, and geochemical characteristics. Two anatectic episodes are identified: Late Oligocene Early Miocene (~23-17 Ma) and Late Miocene (~10 5 Ma). The ~23-17 Ma anatectic samples from the Zhibai and Pai formations, composed of peritectic K feldspar, pyroxene, and garnet, show high FeO^T+MgO contents, lower Al₂O₃ and Na₂O contents, negative Eu anomalies (Eu/Eu* = 0.02 to 0.32), and negative ε_Hf(t) values of -23.8 to -1.53, with abundant inherited zircons (ε_Hf(t) = -18.9 to +9.7), indicating that they could form by dehydration melting of metagreywackes. Some ~23-21 Ma and ~10 Ma samples from the Zhibai formation are comprised of fine-grained quartz and plagioclase with minor clinopyroxene and hornblende. They show high Al₂O₃, Na₂O, CaO, low FeO^T+MgO and K₂O/Na₂O values, without cumulate textures. These 23-21 Ma samples show negative ε_Hf(t) values (-19.8 to -0.6) with T_DM2(Hf) of 2.4-1.1 Ga, consistent with formation from hydrous melting of old heterogeneous basement rocks. In contrast, the 10 Ma samples show positive Eu anomalies (Eu/Eu* = 1.21) and ε_Hf(t) values of +10.9 to +16.7, suggesting that they were derived from hydrous melting of juvenile crustal rocks. The Pai and Duoxiongla formation’s samples (~10-5 Ma) have plagioclase and hornblende, with high Na₂O, flat to weakly negative Eu anomalies (Eu/Eu* = 0.49-0.87), and negative zircon ε_Hf(t) values of -17.5 to -7.1, consistent with derivation from the hydrous melting of orthogneiss. The lower Zhibai formation experienced higher pressure melting during Late Oligocene-Early Miocene, while the upper Pai formation experienced moderate pressure melting. Early Miocene anatexis and exhumation in the Zhibai formation correlate with Himalayan crustal uplift. Late Miocene exhumation and decompression-driven hydrous melting across all formations reflect rapid crustal uplift linked to local crustal-scale mechanical-thermal coupling. The study highlights how pressure regimes, source heterogeneity, and melt mechanisms govern multi-stage anatexis at varying depths, tightly linked to the Himalayan orogen’s evolution.

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