Low total REE zircon formed in equilibrium with hornblende in granulitized eclogites: Implications for exhumation rates

Abstract

Exhumation rates of high-pressure rocks are paramount in determining plate tectonic processes, which requires absolute chronology of metamorphic stages. U-Pb geochronology of zircon and other accessory minerals has proven successful in dating different metamorphic stages, thus constraining geological rates. A common strategy to link U-Pb ages to metamorphic stages uses rare earth element (REE) patterns in the dated minerals. In this study, the changes in the REE composition of accessory and rock-forming minerals in response to changing assemblages have been investigated in granulitized eclogites and gneisses from the Ama Drime Massif, central Himalaya. Phase equilibrium modelling shows that the eclogite-facies assemblage formed at 660–720 °C and 1.6–1.9 GPa (M1), was overprinted at high-pressure granulite-facies (M2) and then ultra-high temperature conditions of >900 °C and 0.8–1.1 GPa (M3) and finally re-equilibrated at conditions of 780–810 °C and 0.8–1.0 GPa (M4). In the countryrock orthogneisses, monazite records partial resetting during granulite-facies overprinting at 26–19 Ma and melt crystallisation at 16–13 Ma, supported by textures, mineral inclusions and trace elements. In the associated granulitized eclogites, zircon records only granulite/amphibolite facies overprinting at ∼14 Ma, and titanite and rutile record cooling to 580–630 °C at 12.5–9 Ma. Granulite/amphibolite facies zircon has a low total REE relative to the protolith zircon, primarily due to the growth of REE-rich hornblende (total REE 80–260 μg/g), which removed 67–92% of the REE from the system. The low total REE of granulite/amphibolite facies zircon is comparable to the flat HREE reported for garnet-rich eclogite-facies zircon, and distinguishing these zircon types requires quantitative mineral volume estimates and other criteria. These findings may imply slower exhumation rates for some eclogite-facies terranes, such as the Tso Morari Himalaya and Papua New Guinea, than previously reported.