Late Cretaceous Hydrous Melting and Reworking of Juvenile Lower Crust of the Eastern Gangdese Magmatic Arc, Southern Tibet

The Gangdese arc in southern Tibet formed during subduction of the Neo-Tethyan ocean and is characterized by mantle-derived magma accretion and juvenile crustal growth. A main period of magmatism and juvenile crustal growth occurred during the Cretaceous, but little is known about the petrological component, nature, and evolution of the lower arc crust at that time. Here, we report systematic petrologic, geochemical and geochronologic data for amphibole-bearing anorthosite and diorite from the eastern Gangdese arc. The studied rocks occur within early Late Cretaceous gabbro or meta-gabbro, and consist mainly of plagioclase and amphibole with other hydrous phases, including minor muscovite, biotite and epidote. Zircon U–Pb dating gives Late Cretaceous crystallization ages of ca. 78 Ma. The rocks have varying SiO₂ (47.44–57.35 wt%), Al₂O₃ (19.76–25.15 wt%), MgO (0.55–3.43 wt%) and FeO_T (0.89–8.31 wt%) contents that can be explained as resulting from a primary melt with either plagioclase addition or amphibole accumulation. Trace element compositions show unusually high Sr/Y (59.3 to 948, average 278), but moderate (La/Yb)_N (3.2 to 16.6, average 6.4). Isotopic compositions of Sr, Nd, and Hf (initial ⁸⁷Sr/⁸⁶Sr = 0.704186 – 0.704552, εNd(t) = +2.8 to + 4.6 and zircon εHf(t) = +10.2 to + 15.8) are comparable to the early Late Cretaceous gabbros that form the juvenile lower crust. These geochemical characteristics imply that the studied rocks were likely derived from hydrous anatexis of early Late Cretaceous juvenile lower crust. We suggest that shallow-angle subduction and dehydration of the Neo-Tethyan oceanic plate resulted in thickening and hydrous partial melting of juvenile lower crust to form water-rich anorthosite, diorite, and previously reported adakitic rocks during the Late Cretaceous. The juvenile lower crust of the Gangdese arc consisted of hydrous mafic granulites, and experienced distinct thickening and reworking during the Late Cretaceous.