TWO-STAGE EXHUMATION OF THE HIMALAYAN METAMORPHIC CORE: INSIGHTS FROM GREATER HIMALAYAN ROCKS IN CENTRAL AND EASTERN BHUTAN (Invited Presentation)

The metamorphic core of orogenic belts can preserve a detailed record of deep-crustal processes involved in crustal thickening and thrust belt development, including how different units are constructed and exhumed. Within the Himalayan orogen, there has been extensive debate over the exhumation history of mid-crustal Greater Himalayan (GH) rocks, with some models arguing for the importance of channel flow and others for wedge-type extrusion. To further investigate the construction and exhumation history of the GH unit, we performed monazite and rutile petrochronology and ⁴⁰Ar/³⁹Ar muscovite thermochronology on samples from two transects in Bhutan that span a >9 km structural thickness of the GH unit. Low HREE and Y metamorphic monazite crystallized under garnet-stable conditions from ca. 48–18 Ma in structurally higher GH samples in comparison to ca. 20–16 Ma from structurally lower samples. The same trend is observed in high HREE and Y monazite that record garnet breakdown and decompression: ca. 44–15 Ma in higher GH rocks and ca. 18–13 Ma in lower GH rocks. In comparison, U-Pb rutile dates and ⁴⁰Ar/³⁹Ar muscovite cooling ages are consistent across the GH section, clustering between ca. 11–8 Ma and ca. 10–8 Ma, respectively. The monazite data record southward-migrating, in-sequence, ductile underplating of progressively deeper GH rocks along intra-GH shear zones. Movement along these shear zones constructed a composite GH section, but also resulted in decompression and Early Eocene–Early Miocene exhumation. Following its construction, the GH unit remained at >500°C until ca. 11 Ma based on Ti-in-zircon thermometry and rutile petrochronology. After construction of the composite GH unit, duplexing of structurally underlying Lesser Himalayan rocks caused passive uplift and associated erosion of the entire GH section. The rutile and muscovite ages reflect associated cooling and exhumation of the GH from ca. 11–8 Ma. In summary, combined geo- and thermo- petrochronology from GH rocks in central and eastern Bhutan define middle Eocene to Early Miocene construction and initial decompression facilitated by ductile underplating along intra-GH shear zones and the basal Main Central thrust, followed by middle Miocene and younger final cooling and exhumation of the GH above growing Lesser Himalayan duplexes.