Origin of atoll garnets in eclogites and implications for the redistribution of trace elements during slab exhumation in a continental subduction zone

• DOI: 10.2138/am.2007.2343
• Publication Date: 2007-07-05T18:12:33Z
• Authors: H. Cheng; E. Nakamura; K. Kobayashi; Z. Zhou
• Keywords: Atoll; Slab; Recrystallization (geology)

Detailed electron- and ion-microprobe analyses were carried out on atoll-shaped normal garnets in ultrahigh-pressure (UHP) metamorphic eclogite from Dabie, east-central China. Compositional profiles of both rings atoll show well-preserved growth zoning with a decrease Mn, Ca, heavy rare earth elements (HREEs), an increase Mg toward rims. Manganese middle element (MREE) enrichments are observed near garnet Island- peninsula-shaped inside atolls homogeneous major the same composition as rims, whereas HREE concentrations similar to those cores. Electron back-scatter diffraction (EBSD) that island- fractions have crystallographic orientations identical rings. These observations suggest formed by consumption earlier-formed cores fluid released hydroxyl exsolution nominally anhydrous minerals (NAMs) lawsonite decomposition at onset exhumation, (i.e., breakdown was re-growth outside inside). Though somewhat restricted, this study reveals because zircon act sinks for HREEs Zr, respectively, majority Zr likely re-incorporated into newly grown during Dabie UHP slab exhumation.