The Gophu La and Western Lunana granites: Miocene muscovite leucogranites of the Bhutan Himalaya

Abstract

Petrographical, mineralogical and geochemical data are given for the Gophu La leucogranite, the second largest leucogranite body of the Bhutan Himalaya and for the smaller Western Lunana leucogranite. Both leucogranites were emplaced in the amphibolite-facies gneisses of the High Himalaya basement nappes during the Miocene. The Gophu La leucogranite is about 300 km 2 in outcrop surface and is homogeneous, showing only moderate variations in chemical and modal composition. The Western Lunana leucogranite has a far smaller outcrop surface (about 40 km 2) and is less homogeneous than the Gophu La leucogranite. Both are fine- to medium-grained, hololeucocratic granites with white mica, biotite and minor tourmaline. Significant accessories are andalusite and sillimanite. The muscovites are phengitic, being notably high in Fe and Mg, and the biotites are Fe-rich. The average chemical compositions are similar to that of other Himalayan leucogranites and are typical for minimum melts. Both granites preserve geochemical features recording the heterogeneity of the parent material and were probably produced during the uplift of tectonically thickened continental crust.