The Formation and Evolution of Uvarovite in UHP Serpentinite and Rodingite and its Constraints on Chromium Mobility in the Oceanic Subduction Zone

Abstract

The uvarovite-andradite and uvarovite-andradite-grossular solid-solution series are rare in nature. The discovery of uvarovite-andradite in serpentinite and rodingite from the ultra-high pressure (UHP) metamorphic belt in southwestern Tianshan provided an opportunity to investigate its behavior in the subduction zone. Uvarovite (defined as chromiumgarnet) from serpentinite is homogeneous in a single grain, covering compositions in the uvarovite-andradite solid solution series of Adr 58-66 Uv 33-41 , with few grossular components. Uvarovite from rodingites contain various Cr 2 O 3 contents (1.7-17.9 wt%) and mineral compositions being in the range of Adr 21-31 Uv 41-50 Grs 22-37 , Adr 52-90 Uv 5-25 Grs 0-21 and Adr 19-67 Uv 3-63 Grs 13-42 . Discontinuous chemical variation of uvarovite from core to rim indicates that uvarovite formed by consuming andradite and chromite, which could provide Ca, Cr, Al and Fe. Raman signals of water were identified for uvarovite from both serpentinite and rodingite, with high water content in uvarovite from serpentinite. The high pressure mineral assemblage, as well as the association with perovskite, indicated that the studied uvarovite from serpentinite and rodingite was formed through high pressure metamorphism, during the subduction zone serpentinization and rodingitization. High alkaline and highly reduced fluids released from serpentinization or rodingitization in the oceanic subduction zone promote the mobility of chromium and enable its long-distance migration.

Trace Elements of Multi‐stage Minerals and Titanite U‐Pb Dating for the Gneisses from Liansan Island, Sulu UHPM Belt

“…In addition, the partial melting processes in the continental subduction zones can modify major and trace element contents and isotopic compositions of the major rock-forming minerals (for example, garnet), as well as the typical accessory minerals (zircon, titanite, rutile, monazite etc.) in the ultra-high-pressure (UHP) rocks (Zhang et al, 2021;. Therefore, when combined with the results of petrological studies, the differential behaviors of such minerals during metamorphic-anatexis processes can be revealed and integral processes of deep continental subductioncollision-exhumations can be successfully reconstructed (Zheng, 2009;Zhang et al, 2022;Zhao et al, 2023).…”