Subduction of Indian continental lithosphere constrained by Eocene-Oligocene magmatism in northern Myanmar

Abstract Subduction of the Indian continental lithosphere in the eastern Tibet-Himalaya orogenic belt remains unclear. Newly reported zircon U-Pb ages in this study indicate that the Shangalon intermediate-felsic magmatic rocks in northern Myanmar formed at Eocene-Oligocene (~ 40–32 Ma). They have calc-alkaline to shoshonitic characteristics, LREE-enriched patterns, enrichments in LILE (e.g., Rb, Cs), and depletions in HFSE (e.g., Nb). Obviously, they show the higher initial Sr and lower Nd-Hf isotopic compositions (87Sr/86Sri = 0.7054–0.7082, eNd(t) = −5.3 to −0.4, and eHf(t) = −3.4 to 10.8) than Cretaceous arc-related mafic-felsic rocks in the West Burma terrane. A positive correlation between Nd isotopic compositions and SiO2 contents indicates that the Eocene Shangalon diorite and andesite with the lowest eNd(t) values (−5.3 to −4.0) likely derived from a mantle source contaminated by the subducted Indian continental lithosphere. Coupled with regional coeval OIB-like mafic melts and high temperature metamorphism, the Eocene-Oligocene Shangalon magma might have formed in the Neo-Tethyan oceanic slab break-off setting after the India-Asia collision. In addition, Eocene granodioritic rocks have the adakitic features, which possibly resulted by fractional crystallization of hornblende. The hornblende-dominated crystallization in the Shangalon ore-bearing granodioritic rocks is well consistent with magma evolution of H2O-rich melts, which play an important role in the formation of the Eocene Shangalon porphyry Cu-Au deposit.