Formation of syn-plutonic dike in the Cretaceous Ryoke granitic rocks, SW Japan:

A geochronological approach to magma chamber process

Abstract

Syn-plutonic dikes show the intermingling process of partially molten silicic and mafic magmas. We found some syn-plutonic dikes with different ages between their silicic and mafic rocks from the Chubu and Kinki districts. The zircon U-Pb ages of the silicic rocks are 90-94 Ma while those of the mafic rocks are 70-75 Ma. It is unlikely that the granitic magma of 90-94 Ma survived in partially molten state until the injection of the mafic magma at 70-75 Ma because the time discrepancy of 14-20 Ma seems too long for a lifetime of a single magma chamber.

In a silicic rock of the syn-plutonic dike we found a zircon grain which show Cathodoluminescence image of an irregular zoning with partly dissolved and re-grown texture. The U-Pb age of the re-grown part of it is ca. 70 Ma and the that of the undissolved core is ca. 90 Ma. Considering that the mafic rocks nearby is of 70 Ma and the host rock of the syn-plutonic dike is Older Ryoke granitoid of 90-94 Ma, it could be reasonably interpreted that the syn-plutonic dike was formed by injection of mafic magma at ca. 70 Ma into the already solidified 90-94 Ma granitoids and partially melted them, leading to the physical interaction of the local silicic crystal mush and the mafic magma, unlike the case where mafic magma was injected into pre-existing mushy granitic magma chamber.