Using isotopic and chronologic data to fingerprint strata: Challenges and benefits of variable sources to tectonic interpretations, the Paro Formation, Bhutan Himalaya

We combine detrital zircons (DZ) and epsilon neodymium (ε _Nd) signatures with field mapping in the Paro Formation in western Bhutan. DZ age spectra are strongly variable and display signatures that have been used to uniquely identify both Greater Himalayan (GH) and Lesser Himalayan (LH) strata. DZ age peaks from six quartzite samples require sources for ∼0.5, 0.8, 1.2, 1.4, 1.7, 1.8, and 2.5 Ga zircons in the Paro Formation. The youngest (∼0.5 Ga) zircons argue for a Cambrian maximum deposition age. Two samples have a youngest 1.8 Ga peak typically attributed to Paleoproterozoic LH rocks. A ∼450 Ma crystallization age from two granite samples constrains the minimum deposition age as Ordovician. New ε_Nd signatures from six detrital samples from the Paro Formation show significant variation with lithology. Schists have ε_Nd(0) values between -12.0 and -16.9, while quartzite values vary between -18.8 and -24.5. These data imply that the Paro Formation was derived from both young and old sources, with DZ and ε_Nd values obtained from the same quartzite samples requiring old detritus while the ε_Nd values obtained from interbedded schist require younger detritus. Using published isotopic and chronologic definitions of Himalayan strata, schist-rich layers would be considered GH, while the interbedded quartzite would be LH. Thus, the Paro Formation refutes the generally accepted notion that different Himalayan tectonostratigraphic zones have unique DZ and ε_Nd signatures. Our data recommend caution in the use of DZ and ε_Nd signatures for tectonic interpretation, especially when making correlations with studies that extend 1000s of km along strike.