Coexisting retrograde jadeite and omphacite in a jadeite-bearing lawsonite eclogite from the Motagua Fault Zone, Guatemala

Abstract

Coexisting jadeite and omphacite were found as retrograde minerals in a jadeite-bearing lawsonite-eclogite from the Motagua Fault Zone, Guatemala. The lawsonite-eclogite is characterized by the occurrence of garnet porphyroblasts up to 2.5 cm in size, and the eclogite-facies parageneses, almandine-rich garnet + impure jadeite + lawsonite + rutile + quartz; garnet contains inclusions of impure jadeite, phengite, ferroglaucophane, chlorite, lawsonite, rutile, ilmenite, and quartz. Textural relations and parageneses and compositions of minerals indicate that the lawsonite-eclogite experienced two stages of metamorphism: prograde eclogite-facies stage (M 1 ) and retrograde stage (M 2 ). The impure jadeite (Jd-I) of the M 1 eclogite-facies occurs in both the matrix and as inclusions in garnet, and contains considerable amounts of augite and aegirine components (Jd 6 1 - 7 5 Aug 1 6 - 2 4 Ae 0 - 1 8 ). It is partly recrystallized to retrograde M 2 jadeite (Jd-II) (Jd 7 4 - 8 7 Aug 9 - 1 6 Ae 0 - 1 1 ) and omphacite (Jd 4 2 - 5 0 Aug 3 6 - 4 6 Ae 7 - 1 6 ); some of these two sodic pyroxenes may have crystallized from fluids. Both M 2 jadeite and omphacite show textural equilibrium and are believed to have grown concurrently. Based on the observed compositions and the phase relations of sodic pyroxenes from Carpenter (1980), the M 1 impure jadeite (Jd-I) may have had a disordered C2/c symmetry at T=ca. 450 °C and P = ca. 1.8-2.4 GPa, and was subsequently crystallized into jadeite (Jd-II) plus ordered P2/n omphacite during retrogression with infiltration of fluids at T < ca. 300 °C and P = ca. 0.7 GPa (M 2 ). The extreme low-T conditions during retrogression may have prevented reaction between eclogitic jadeite and adjacent minerals. Instead, eclogitic impure jadeite (plus fluid) has recrystallized into the retrograde jadeite + omphacite pair with a wide compositional gap.