Geochronology and excess Ar geochemistry of the Lhotse Nup leucogranite, Nepal Himalaya

Abstract

40Ar/ 39Ar dating experiments on several coexisting minerals from two close-by leucogranite outcrops near Lhotse Nup glacier (Nepal Himalaya) reveal a complex behaviour. Four biotite and muscovite ages cluster around 15.5 Ma, a lower value than literature Rb/Sr ages on splits of the same four micas, suggesting a discrepancy with the ideal cooling age sequence observed in the Alps. A strongly discordant Ar-Ar spectrum on tourmaline does not allow a chronological interpretation. A potassium feldspar shows a slow-cooling staircase spectrum with a superimposed saddle diagnostic of excess Ar. HF leaching removed excess Ar but caused great perturbations to the minimum step ages, isochron plots, and the release of reactor-produced Ar isotopes. The present data require that the currently fashionable interpretations of feldspar systematics be radically changed. The main chronological conclusions rely on the eight mica ages. Their decrease from 18.2 to 15.3 Ma dates the cooling of the Lhotse Nup leucogranite.