Concordant pulse in Mn, Y and HREEs concentrations during UHP eclogitic garnet growth: Transient rock dynamics along a cold subduction plate interface

Abstract

We herein report strikingly concordant Mn, Y, and HREE garnet zoning in ultrahigh-pressure low-temperature oceanic eclogites found in the fossil Devonian-Carboniferous subduction plate interface in SW-Tianshan. Zr-in-rutile thermometry, thermodynamic modeling and diffusion-controlled garnet growth modeling show that this type of zoning can be explained by a short-lived transient, most likely a ∼80°C thermal pulse (from 490 to 570°C). This event could be as transient as ∼300 Ka, assuming a prograde to peak burial duration of 5 Ma. Such garnet zoning is observed for grains sampled meters apart (Fig. 1d) and was detected in other eclogites in the region. This transient event could reflect the short-lived movement of UHP oceanic slices, once detached, towards the hotter mantle wedge, and/or deformation-induced remobilization of fluids enhancing diffusion-controlled garnet growth.