Relict Unusually Low Iron Pyrope-Grossular Garnets in UHPM Calc-silicate Rocks of the Kokchetav Massif, Kazakhstan

Abstract

Knowledge of the compositional ranges of rock-forming minerals in ultrahigh-pressure (UHP) rocks occurring both as deep mantle xenoliths in kimberlites and subducted crustal metasediments is critical for estimating the conditions of their formation. Garnet is one of the most important minerals of UHP rocks. This study deals with unusually low iron (less than 2 wt% FeO) pyrope-grossular garnets with Mg# more than 90 and Ca# of ~60-65, which have been detected for the first time in nature as relics within some garnet grains of layered calc-silicate rocks from the Kokchetav massif. These relics represent the earliest generation of garnet formed under peak metamorphic conditions. Using a cathodoluminescence (CL) method, relics are visible in a number of samples as irregular, light, luminescent portions occurring unevenly distributed within garnet grains. Similar features are revealed by electron microprobe X-ray elemental maps of single mineral grains within a thin section, but not as fast as the CL method that takes only seconds. New results on calc-silicate rocks demonstrate that fluids must have penetrated carbonate-rich layers, whereas adjacent garnet-pyroxene rocks acted as impermeable barriers protected against retrograde attack. Oriented arrays of exsolution-like needles (rutile?, ilmenite?) were found for the first time within Ti-clinohumite, possibly formed during pressure release after peak metamorphic conditions.