Record of Early-Stage Rodingitization from the Purang Ophi-olite Complex, Western Tibet

Rodingitization, commonly coupled with serpentinization of ultramafic rocks, bears significant information for fluid-rock interactions and element transfer from sea-floor to subduction zone envi-ronments. Numerous outcrops of rodingites are exposed along the Yarlung Zangbo suture zone (YZSZ) of southern Tibet, providing us an excellent opportunity to probe the petrogenetic processes, and unravel their implications for regional tectonic evolution. Several studies have been performed on rodingites from the eastern to central portions of the YZSZ, whereas limited work has ever been conducted on rodingitized rocks from the western segment of the YZSZ, precluding a comprehensive understanding of this lithological type. In this paper, we present detailed studies of petrology, mineral, whole-rock geochemistry and phase equilibrium modeling on a suite of newly recognized rodingites within the Purang ophiolite massif in the southwestern part of the YZSZ. The rodingites have a major metasomatic mineral association of chlorite, clinozoisite, amphibole and minor amounts of plagioclase, representing products of an early-stage rodingitization. They generally present compositions of low SiO₂ (48.89 wt.%-53.57 wt.%), Fe₂O₃^T (3.77 wt.%-5.56 wt.%), Na₂O (1.31 wt.%-1.93 wt.%), Al₂O₃ (4.78 wt.%-8.84 wt.%), moderate CaO (9.69 wt.%-11.23 wt.%), and high MgO (24.11 wt.%-26.08 wt.%) concentrations with extremely high Mg^# values[Mg^#=100×Mg/(Mg+Fe²⁺) molar] of 89-92. Bulk-rock recalculation reveals that the rodingites have a protolith of mantle-derived olivine gabbro or gabbronorite. They have low rare earth element compositions (∑REE=2.4 ppm-6.5 ppm) and are characterized by flat LREE and slightly enriched HREE patterns with positive Eu anomalies; they also exhibit positive anomalies in Sr, U and Pb and negative anomalies in high-field strength elements, including Nb, P and Ti, suggesting for a subduction-zone imprinting. Phase equilibrium modeling shows that the rodingitization did take place at P < 2 kbar and T=~350-400℃, consistent with low greenschist facies conditions. Taking into account of all these petrological and geochemical features, we propose that the rodingites record evidence of early-stage fluid-rock interactions between olivine gabbroic rocks and Ca-rich fluids, which may have derived from weakly serpentinized ultramafic country rocks. Although this process may initially have occurred in a mid-ocean ridge setting, an obvious overprinting by supra-subduction zone fluids in a fore-arc environment is recognized.