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

Abstract

An integrated study of zircon UPb dating and Hf isotopes, 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 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 ∼ +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 ∼ −7.1, consistent with derivation from the hydrous-melting of orthogneiss. The lower Zhibai formation experienced higher-pressure melting during the Late Oligocene-Early Miocene, while the upper Pai formation experienced moderate-pressures 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.