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[SCG52-06] How to make 'syn-plutonic dike' --- Another story from the Ryoke granitic province, Southwest Japan
Keywords:syn-plutonic dike, U-Pb ages, Ryoke granitic rocks, magma chamber process, Southwest Japan, Cretaceous
Syn-plutonic dike is interpreted to express a magma chamber process with intermingling silicic and mafic magmas in fluidal state.
In the Ryoke granitoids in Southwest Japan, mafic rocks often occur in so-called syn-plutonic dikes. They are pillow-like balls in the silicic matrix (pillow-type), and dikes intruding silicic matrix with silicic back-veins into them (dike-type). We found the 14-20Ma time discrepancies between mafic and silicic rocks in above both two types of syn-plutonic dikes in the Ryoke granitoids, 90-94Ma of silicic rocks and 70-75Ma of mafic rocks in zircon U-Pb ages. The time duration of 14-20Ma seems too long to imagine that these two components of the syn-plutonic dike was generated during a lifetime of a single magma chamber.
In a silicic rock of those syn-plutonic dike, we found a partly dissolved and re-grown intrastructure of a zircon grain in the CL image, with the ages of cc.70Ma in the re-grown part and cc.90Ma in the undissolved core. The mafic rocks of the syn-plutonic dike nearby gives cc.70Ma and the host Ryoke granitoids around them give 90-96Ma. So it could be interpreted that the syn-plutonic dike was formed with injection of the mafic magma at cc.70Ma into the already solidified 90-96Ma granitoids and partially melted them, which led the physical interaction of local silicic mushy melt and mafic magma. The partly dissolved and re-grown zircon described above could tell that story of forming syn-plutonic dike.
As this type of the “syn-plutonic dikes” occur widespread in a km-sized area in the Ryoke granitoids, that process may be regarded as local and partial revival of granitic magma chamber.
In the Ryoke granitoids in Southwest Japan, mafic rocks often occur in so-called syn-plutonic dikes. They are pillow-like balls in the silicic matrix (pillow-type), and dikes intruding silicic matrix with silicic back-veins into them (dike-type). We found the 14-20Ma time discrepancies between mafic and silicic rocks in above both two types of syn-plutonic dikes in the Ryoke granitoids, 90-94Ma of silicic rocks and 70-75Ma of mafic rocks in zircon U-Pb ages. The time duration of 14-20Ma seems too long to imagine that these two components of the syn-plutonic dike was generated during a lifetime of a single magma chamber.
In a silicic rock of those syn-plutonic dike, we found a partly dissolved and re-grown intrastructure of a zircon grain in the CL image, with the ages of cc.70Ma in the re-grown part and cc.90Ma in the undissolved core. The mafic rocks of the syn-plutonic dike nearby gives cc.70Ma and the host Ryoke granitoids around them give 90-96Ma. So it could be interpreted that the syn-plutonic dike was formed with injection of the mafic magma at cc.70Ma into the already solidified 90-96Ma granitoids and partially melted them, which led the physical interaction of local silicic mushy melt and mafic magma. The partly dissolved and re-grown zircon described above could tell that story of forming syn-plutonic dike.
As this type of the “syn-plutonic dikes” occur widespread in a km-sized area in the Ryoke granitoids, that process may be regarded as local and partial revival of granitic magma chamber.