40Ar/39Ar geochronology studies of rocks in eastern Himalaya syntaxis

Conventional 40Ar/39Ar geochronology studies have been done on 20 separate minerals in 13 samples which are from the Yarlung-Tsangpo River gorge, eastern Himalaya syntaxis. The 40Ar/39Ar geochronology data show that: (1) the values of (40Ar/Ar-16), of all the minerals tested are close to 295. 5; (2) most plateau ages are accord with their respective inverse isochron ages within error range. Age statistics indicate that most 40Ar/39Ar ages are focused on 1.3Ma and 2.5Ma approximately, indicating that Namjagbarwa region has undergone two fast cooling-uplift events in middle Pliocene and early Pleistocene respectively. The samples are. collected from the northern to the southern parts of the gorge, they are from various altitude and belong to different tectonic units, and the petrogenesis and lithology of their protolith are different, but the 40Ar/39Ar ages of most minerals (including hornblende, biotite, muscovite and K-feldspar etc.) from various samples are similar, and the age of different minerals from a single sample are not distributed according to the value of the closure temperature to argon isotopic system. Therefore, it seems that the cooling rate of the rocks in Namjagbarwa region has being so fast since Pliocene that the cooling process to argon isotopic system of minerals from this area are apparently different from ones from slow cooling environment. The cooling rate of Namjagbarwa region has being 120 similar to 240 degrees C/Ma and so an uplift rate of 3. 4 similar to 6. 9 min/a since the latest 3Ma according to the data published in this paper.