Testing the driving mechanisms for exhumation of the Greater Himalayan zone in eastern Bhutan: Implications of split-stream zircon and monazite geochronology

In order to test (1) among driving mechanisms for exhumation of the migmatitic Greater Himalayan zone (GHZ) in central and eastern Bhutan; and (2) the significance of the Kakhtang thrust (KT), an out of sequence thrust that divides the GHZ into upper (GHSu) and lower (GHZl) structural levels, zircon and monazite from twenty samples from the Lesser Himalayan zone (LHZ) and the GHZ were analyzed for U--Th--Pb isotopes and trace-element abundances using split-stream laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Cathodoluminescence images of the zircons typically revealed three generations of crystallization: (1) an inherited or prograde-to-peak metamorphic core; (2) an oscillatory-zoned, intermediate area in some leucocratic samples, interpreted as recording melt crystallization; and/or (3) an outermost, unzoned metamorphic rim around zone 2 in leucocratic samples or around the inherited cores of gneiss and pelitic samples. These three phases of zircon growth record the Eocene-Miocene burial and metamorphism of Neoproterozoic-Ordovician marine sedimentary rocks that originally lay off the margin of India prior to India-Asia collision. In eastern Bhutan, ca. 36--27 Ma prograde-to-peak metamorphism in the GHZu was followed by melt crystallization from ca. 27--21 Ma in most of the GHZu, with slightly younger dates of ca. 21--17 Ma near the KT. Coeval to later continued metamorphism until ca. 24--13 Ma was recorded in the outermost, unzoned zircon rims and in monazite crystals from the GHZu and GHZl. In comparison, in central Bhutan, similar to slightly younger, ca. 38--22 Ma prograde-to-peak metamorphism occurred in the GHZl. In addition, GHZu leucosomes recorded ca. 27--21 Ma melt crystallization and ca. 21--13 Ma metamorphism. Calculated zircon/garnet partition coefficients ( TEDzir/grt) showed garnet-present zircon growth for the prograde-to-peak metamorphic ages. However, the majority of trace-element patterns showed negative Eu anomalies (plagioclase-stable) and moderate to steep HREE slopes (garnet-unstable) for zircon and monazite that recorded the timing of melt crystallization and the latest metamorphism. Overall, the youngest metamorphic ages from all samples were within error of each other; thus, the ages did not vary across the KT, suggesting that the KT was not as significant of a structure as previously described. Moreover, a critical taper-type emplacement model predicts that melt crystallization and metamorphism will occur nearly homogeneously across the section. In contrast, a channel flow-type exhumation model predicts heat advection towards the surface, causing the dates to young towards the center of the channel. Based on the data presented above, the results favor more of a critical-taper style of exhumation. Thus, a thick (~25 km) ductile thrust sheet consisting of partially migmatitic GHZ rocks reached peak metamorphic conditions by ca. 27 Ma in the GHZu and ca. 22 Ma in the GHZl, based on the youngest prograde-to-peak metamorphic ages, and then continued to be metamorphosed until ca. 13 Ma. Subsequently, the GHZ rocks were rapidly exhumed and cooled by ca. 11 Ma, based on a 40Ar/39Ar muscovite cooling age.