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doi:10.2204/iodp.proc.335.201.2015 Data report: microanalytical and geothermometric investigations on granoblastic dikes from the gabbro–dike transition from Hole 1256D (IODP Expedition 335, East Pacific Rise)1Jürgen Koepke2 and Niko Goetze2AbstractIn this data report we present detailed microanalytical results obtained with electron microprobe on 6 granoblastic dikes recovered during Integrated Ocean Discovery Program Expedition 335 from the gabbro–dike transition in Hole 1256D (Eastern Equatorial Pacific). The data set includes 79 compositions of plagioclases, clinopyroxenes, orthopyroxenes, amphiboles, and Fe-Ti oxides based on 1150 single analyses. All samples except one display phenocrysts of plagioclase in a granoblastic matrix equilibrated in the pyroxene hornfels facies. The analytical results reveal that the plagioclase phenocrysts are much higher in anorthite component than those of the granoblastic matrix, with compositions typical of plagioclases from fresh pillow basalts and dikes from the upper part of the drill core, implying that these phenocrysts correspond to relics inherited from the primary magmatic stage. Four samples display domains where clinopyroxene and orthopyroxene are arranged in vein- or bandlike structures, or in coarser grained monomineralic clusters. Clinopyroxenes and orthopyroxene from these domains show compositions overlapping widely with those of the matrix pyroxenes. This supports the hypothesis that these domains are derived from precursor structures generated by pervasive hydrothermal alteration, like hydrothermal veins and patches in the basalts. They were subsequently transformed by the contact metamorphic overprint into parageneses typical for the pyroxene hornfels facies, showing textural domains like bands and clusters inherited from the hydrothermal alteration stage, but with mineral compositions identical to those of the granoblastic matrix. Two-pyroxene geothermometry reveals equilibrium temperatures for the peak metamorphism of the granoblastic overprint ranging from 930°C to 990°C, characteristic of an equilibration within the pyroxene hornfels facies. The estimated temperatures are very similar to equilibrium temperatures estimated by the same method for granoblastic dikes drilled during Expedition 312. These temperatures, together with the geological setting of the granoblastic dikes within the gabbro–dike transition in fast-spread crust, imply that the granoblastic hornfels can be regarded as preserved fragments of a conductive thermal boundary layer. |
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