Deciphering the protracted geological history of the Okhaldunga region, east-central Nepal Himalaya through mica RbSr and KCa geochronology

The exhumed metamorphic core of the Himalayan orogen comprises meta-sedimentary and meta-igneous rocks that are pervasively deformed, with a tectonic foliation typically defined by the alignment of muscovite/biotite. Three specimens from different lithotectonic units were selected for in situ Rb Sr and K Ca geochronology of muscovite and biotite to compare the geological histories recorded therein. Geochronological data and chemistry from the same ablated volumes were integrated with detailed microstructural observations/analyses. In all cases, the two isotopic systems are decoupled, yielding different dates. From higher to lower structural levels, the K Ca dating of muscovite and biotite yields isochron dates of 26 ± 5 Ma, 33 ± 6 Ma, and 39 ± 10 Ma. In contrast, Rb Sr geochronology on biotite returned dates of 17 ± 0.2 Ma, ca. 15 Ma, and 6 ± 0.7 Ma, while muscovite Rb Sr analyses from the same specimens define dates of 46 ± 1 Ma, a range from 1630 to 5 Ma, and 40 ± 9 Ma, respectively. The late Miocene Rb Sr biotite dates are consistent with existing records of exhumation, whereas the muscovite Rb Sr dates record events ranging from early Proterozoic protolith crystallization to late prograde Eocene metamorphism. In contrast, the K Ca muscovite dates are more concise, interpreted to reflect late prograde to early retrograde Himalayan orogenesis. The decoupling of Rb Sr and K Ca systems enable a single mineral phase, and in some cases a single analysis, to provide multi-temporal record of orogenesis, opening new opportunities to resolve the timing and duration of crustal processes in metamorphic belts worldwide.