Along-Strike Variations in the Timing of Melt Crystallization and Metamorphism Across Central and Eastern Bhutan: New Insights from LASS Monazite Geochronology and Trace-Element Abundances

Abstract:

Continent–continent collisional systems represent the largest orogens on Earth and provide locations to study processes that drive the transition from contraction and crustal thickening to extension and collapse. The Greater Himalayan Zone (GHZ) exposed along strike of the Himalayan orogen contains exhumed mid-crustal metasedimentary rocks. To better understand the history of burial, crustal flow, and partial melting during the early stages of Himalayan tectonics in the Eocene to ~40 Myr into its orogenic evolution, monazite was analyzed from five migmatitic gneisses and five host gneisses exposed across two transects within central and eastern Bhutan. Monazite was analyzed in situ by the split-stream laser-ablation (LASS) ICPMS technique, which allows simultaneous collection of U-Th-Pb isotopes and trace-element abundances. The migmatites from the eastern Bhutan transect yield monazite dates that record melt crystallization as young as ca. 15–13 Ma. The host gneisses yield similar to younger (down to ca. 11 Ma) dates, documenting coeval to continued metamorphism of the GHZ. In comparison, melt crystallization in the central Bhutan rocks ended by ca. 18 Ma, and metamorphic monazite from a metapelite record metamorphism until ca. 14 Ma. In the migmatite and host-rock samples from both transects, the trace-element data show an inverse correlation between date and the HREE concentration. This trend likely documents the breakdown of garnet, which probably coincides with the first stages of GHZ exhumation. Thus, the LASS data showed that garnet breakdown and GHZ exhumation occurred from ca. 18 to 14 Ma in eastern Bhutan and ca. 20 to 17 Ma in central Bhutan. The new monazite data suggest different histories for the melt crystallization, metamorphism, and exhumation of the GHZ rocks between central and eastern Bhutan, even though the present day rocks from the two transects are only exposed ~60 km apart. Moreover, in comparison to other parts of the eastern Himalaya, the new geochronometric data correlate with the previously-described trend in which melt crystallization and metamorphism occured at progressively younger times from west (ca. 26 Ma) to east (ca. 13 Ma).