Pre-35 Ma Na-rich magmatic events in the Yardoi area, southern Tibet

The Yardoi gneiss dome, the easternmost one of the Northern Himalayan Gneiss Dome (NHGD), is composed of Cenozoic granites in the core rimed by high grade metapelites with subordinated metabasites. SHRIMP zircon U/Pb dating on granitic rocks shows that the leucogranite formed at 35.3±1.1 Ma [1], and the two-mica granite at 42.6±1.1 Ma. Both suites of granites are substantially older than those to the west. The two-mica granite has similar age of formation and geochemistry to those of the Dala pluton [2, 3]. Most of the leucogranites as well as the two-mica granites from the Yardoi area are of peraluminous, Na-rich (A/CNK>0.99, Na2O/K2O>1). High Sr/Y(>45.0) and La/Yb(>43.0), low Y(<10 ppm) and Yb(<1 ppm) indicate that they are adakitelike magmas. As compared to those in the garnet amphibolite, both suites of granites have similar Sr (Sr/Sr(t) = 0.7133~0.7193), but unradiogenic Nd (eNd(t) =-8.9~-14.9) isotope compositions. Metapelites have highly radiogenic Sr (Sr/Sr(t)= 0.8581~0.9591) and unradiogenic Nd (eNd(t) =11.3~-16.6) values. These data indicate that (1) the Yardoi area had experienced at least two episodes of Na-rich granitic magmatism; (2) high-pressure partial melting of a source consisting dominantly of amphibolite with subordinated metapelite was responsible for the generation of adakite-like melts; (3) amphibolite partial melting overwhelmed the melting of metapelite. Pre-35 Ma melting events that produced adakitic magmas suggest that melting of the overthickened continental crust might be a major factor that initiated the extension and formation of the Southern Tibet Detachment System (STDS) and the rapid extrusion of the High Himalayan metamorphic wedge.