Late Cretaceous Gangdese intrusions of adakitic geochemical characteristics, SE Tibet: Petrogenesis and tectonic implications

The Gangdese batholith emplaced from the Cretaceous to Eocene in southern Tibet has been widely regarded as the major constituent of an Andean-type convergent margin resulting from northward subduction of the Neo-Tethyan oceanic lithosphere under Asia. While the Gangdese batholith consists predominantly of calc-alkaline rocks, we identify from the eastern part of the batholith a suite of epidote-bearing granodiorites that shows adakitic geochemical characteristics, marked with apparently higher La/Yb and Sr/Y, and lower Y and HREE, than other Gangdese rocks and common arc magmas. SHRIMP zircon U–Pb analyses of two of the samples yielded 206Pb/238U dates of 80.4 ± 1.1 and 82.7 ± 1.6 (2σ) Ma, which constrain the emplacement ages of the adakitic rocks. Trace element modeling suggests that these rocks originated from partial melting of a garnet amphibolite source that, on the basis of the Sr and Nd isotope data [ISr = 0.7044–0.7048; εNd(T) = + 3.2 to + 0.9], we interpret to be a newly underplated, mafic lower crust, rather than the subducted Neo-Tethyan oceanic crust. This juvenile crust was produced by Cretaceous basaltic underplating above the mantle wedge and then thickened by the tectonic contraction owing to flattening of the Neo-Tethyan subduction, a process that also led to the adakitic magmatism. Our interpretation involving a Late Cretaceous flat-slab subduction and related orogenesis in southern Tibet is consistent with petrographic data, such as the occurrence of magmatic epidote and muscovite rimmed with resorption texture in the granodiorites, which indicate deep-seated emplacement followed by rapid tectonic exhumation.