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

Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, U.S.A.

Correspondence: ^* E-mail: tatsukix{at}pangea.stanford.edu

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 (M₁) and retrograde stage (M₂). The impure jadeite (Jd-I) of the M₁ eclogite-facies occurs in both the matrix and as inclusions in garnet, and contains considerable amounts of augite and aegirine components (Jd_61–75Aug_16–24Ae_0–18). It is partly recrystallized to retrograde M₂ jadeite (Jd-II) (Jd_74–87Aug_9–16Ae_0–11) and omphacite (Jd_42–50Aug_36–46Ae_7–16); some of these two sodic pyroxenes may have crystallized from fluids. Both M₂ 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 M₁ 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 (M₂). 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.

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