Prolonged Neo-Tethyan magmatic arc in Myanmar: evidence from geochemistry and Sr-Nd-Hf isotopes of Cretaceous mafic-felsic intrusions in the Banmauk-Kawlin area

Cretaceous mafic-felsic intrusions are extensively distributed in the West Burma terrane (Myanmar), but their petrogenesis and tectonic setting still remain unclear. In this study, whole-rock geochemical and Sr-Nd as well as zircon U-Pb and Hf isotopic data for the Cretaceous mafic-felsic intrusions from the Banmauk-Kawlin area (northern Myanmar) are presented. Precise zircon U-Pb dating results indicate that they emplaced at Cretaceous ( 110-90 Ma) and likely belong to an eastward extension of the coeval Gangdese magmatic belt in the southern Lhasa terrane (Tibet). The studied Cretaceous intrusions (SiO2 = 41.97 to 74.54 wt%) mostly have calc-alkaline and Na-rich characteristics, and strong enrichments in large ion lithophile elements (e.g., Cs, Rb, and K), depletions in Nb and Ta on primitive mantle-normalized diagrams, consistent with geochemical characteristics of arc-type magmas. Most of Cretaceous gabbroic and dioritic intrusions with relatively depleted mantle Sr-Nd-Hf isotopic compositions (Sr-87/Sr-86(i) = 0.7041-0.7048, epsilon Nd(t) = 0.2-6.8, and epsilon Hf(t) = 4.3-15.1) and high Ba/La ratios were possibly derived from partial melting of mantle wedge metasomatized by slab-derived fluids and underwent a certain degree of fractional crystallization. Whereas Cretaceous granodiorite and granite probably formed by partial melting of juvenile arc lower crust on the basis of positive epsilon Nd(t)-epsilon Hf(t) values (- 2.4 to 6.6 and - 2.8 to 15.3) and low initial Sr isotopic ratios (Sr-87/Sr-86(i) = 0.7045-0.7063). A wide range of Th/Nb ratios and Sr-Nd-Hf isotopic values in these intrusions suggests that their juvenile arc lower crust possibly formed by melting of slab-derived fluids and/or sediment melts metasomatized mantle. Overall, Cretaceous ( 110-90 Ma) mafic-felsic intrusions in the West Burma terrane may have formed in an arc setting during subduction of the Neo-Tethyan oceanic lithosphere, suggesting a prolonged Neo-Tethyan magmatic arc system from southern Tibet to Southeast Asia.