ABSTRACT

Geological fractures commonly contain the minerals calcite and zeolite, which have crystallized in the fracture after its formation. The minimum age of formation of the fracture can be estimated from the age of the fracture-filling minerals. We tried to determine the age of fracturing by applying the Rb-Sr mineral isochron method to the fracture-filling paragenetic calcite and zeolite samples from fractures in the Kurihashi granodiorite and a skarn in the Kamaishi mine in northeastern Japan. The age of crystallization of fracture-filling minerals was estimated to be in the range 74 to 58 Ma in the Kurihashi granodiorite. The main causes of fracturing were considered to be igneous activities and the cooling of magma. We could not determine ages for some of the fractures we investigated, probably because of the subsequent low-temperature alteration after fracturing, or because of host wall rock contamination of the fracture-filling mineral samples. The ages determined for fractures in the skarn pre-dated the skarnization process. It is possible that these fractures formed before the skarnization. However, selective leaching of rubidium during skarnization reduces the Rb/Sr ratio. Therefore, skarnization can have the effect of producing erroneously old Rb-Sr ages. From the initial ⁸⁷Sr/⁸⁶Sr ratios, we determined changes in the strontium isotopic ratio in the local groundwater from the Late Cretaceous through the Paleocene, noting a significant increase at 62 Ma.