(32) Deciphering the Mechanisms and Timing of Reaction Rind Formation on Metabasaltic Pods (Sivrihisar Massif, Turkey)

In the Sivrihisar Massif (Turkey), blueschist and eclogite facies metabasaltic rocks commonly occur as cm-to- m scale pods or lenses hosted in blueschist, marble, calc- silicate, and quartzite. Although most of these pods are... [ view full abstract ]

In the Sivrihisar Massif (Turkey), blueschist and eclogite facies metabasaltic rocks commonly occur as cm-to- m scale pods or lenses hosted in blueschist, marble, calc- silicate, and quartzite. Although most of these pods are lithologically homogeneous (i.e., the same lithology from core to margin), some of the pods are lithologically heterogeneous and have reaction zones or rinds developed at the contact between the metabasaltic rocks and their host rocks. To better understand the mechanisms and timing of reaction rind formation, we conducted a petrographic and geochemical study of a lithologically heterogeneous pod and its associated host chlorite schist. The pod core consists of omphacite + glaucophane + lawsonite, with minor amounts of chlorite, whereas the pod rind consists of chlorite + lawsonite + titanite, with minor amounts of omphacite. The host schist has a similar mineralogy to the pod rind (chlorite + talc + titanite), but lacks lawsonite. Instead, the major rare earth element (REE) carrier in the host schist is allanite. Compared to the pod core, which has a basaltic protolith, the alteration rind and host schist have higher concentrations of MgO (23 – 26 wt.%), Cr (211 - 1527 ppm), and Ni (980 - 1137 ppm), and lower concentrations of SiO 2 (~ 42 wt.%) and CaO (3 – 4 wt.%). As high concentrations of MgO, Cr, and Ni are associated with ultramafic rocks, this suggests that the formation of the reaction rind and host schist involved interactions (via mechanical mixing and/or fluids) with serpentinites. Comparing the compositions and zoning patterns of mineral phases (chlorite, lawsonite, and omphacite) that are found in both the metabasaltic core and the alteration rind will allow us to decipher the timing of these chemical changes relative to the timing of mineral growth.