TY - JOUR
T1 - Significance of the Ca-Na pyroxene-lawsonite-chlorite assemblage in blueschist-facies metabasalts
T2 - An example from the renge metamorphic rocks, Southwest Japan
AU - Tsujimori, T.
AU - Liou, J. G.
N1 - Funding Information:
This research was supported by the Japanese Society for the Promotion of Science, through a Research Fellowship for Research Abroad for the first author (Tsujimori) and the National Science Foundation through NSF grant EAR-0510325 (Liou). We thank A. Ishiwatari, H. Ishida, H. Shukuno, and M. Motoya for helping with the XRF and INNA analyses and T. Itaya for providing the facility for the EMP analyses. We thank C. G. Mattinson and W. G. Ernst for critically reviewing this manuscript.
PY - 2007/5
Y1 - 2007/5
N2 - Paleozoic lawsonite-bearing low-grade metabasalts with rare flattened pillow structures occur in the Chugoku Mountains, Southwest Japan. The meta-mafic rocks are divided into meta-pillow basalt core (PBC), meta-pillow basalt rim (PBR), and metabasaltic breccia (BB). The PBC have MORB like major and trace element concentrations, contain 5 wt% water, and a Na-amphibole-free mineral assemblage: Ca-Na pyroxene (maximum 29% jadeite component) + lawsonite + chlorite + quartz. In contrast, the PBR and BB contain a lawsonite-blueschist-facies mineral assemblage: Na-amphibole ± lawsonite + pumpellyite ± chlorite + albite + quartz. Parageneses and compositions of minerals yield equilibrium P-T conditions of T< 280°C and P = 0.6-0.7 GPa for all three rock types. Phase relations in the 2A1-Ca-2Fe3+-(Fe+Mg) tetrahedron suggest that the Na-amphibole-free mineral assemblage in PBC occurs only under relatively oxidized conditions. This mineral assemblage has been described from low-grade metabasalts in several high-P-low-T terranes, and might be a low-T equivalent of the lawsonite eclogite mineral assemblage. We propose that interaction of fluids with subducting old oceanic crust first produces the Ca-Na pyroxene + lawsonite + chlorite assemblage by hydration reactions at shallow depths in a cold subduction zone. With subduction to greater depths, such intensely hydrated metabasalts with the Na-amphibole-free mineral assemblage transforms to lawsonite eclogite below 300°C.
AB - Paleozoic lawsonite-bearing low-grade metabasalts with rare flattened pillow structures occur in the Chugoku Mountains, Southwest Japan. The meta-mafic rocks are divided into meta-pillow basalt core (PBC), meta-pillow basalt rim (PBR), and metabasaltic breccia (BB). The PBC have MORB like major and trace element concentrations, contain 5 wt% water, and a Na-amphibole-free mineral assemblage: Ca-Na pyroxene (maximum 29% jadeite component) + lawsonite + chlorite + quartz. In contrast, the PBR and BB contain a lawsonite-blueschist-facies mineral assemblage: Na-amphibole ± lawsonite + pumpellyite ± chlorite + albite + quartz. Parageneses and compositions of minerals yield equilibrium P-T conditions of T< 280°C and P = 0.6-0.7 GPa for all three rock types. Phase relations in the 2A1-Ca-2Fe3+-(Fe+Mg) tetrahedron suggest that the Na-amphibole-free mineral assemblage in PBC occurs only under relatively oxidized conditions. This mineral assemblage has been described from low-grade metabasalts in several high-P-low-T terranes, and might be a low-T equivalent of the lawsonite eclogite mineral assemblage. We propose that interaction of fluids with subducting old oceanic crust first produces the Ca-Na pyroxene + lawsonite + chlorite assemblage by hydration reactions at shallow depths in a cold subduction zone. With subduction to greater depths, such intensely hydrated metabasalts with the Na-amphibole-free mineral assemblage transforms to lawsonite eclogite below 300°C.
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U2 - 10.2747/0020-6814.49.5.416
DO - 10.2747/0020-6814.49.5.416
M3 - Article
AN - SCOPUS:34249806708
VL - 49
SP - 416
EP - 430
JO - International Geology Review
JF - International Geology Review
SN - 0020-6814
IS - 5
ER -