Rb sr method of dating
During fractional crystallization, Sr tends to be come concentrated in plagioclase, leaving Rb in the liquid phase.
Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation. If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and the Sr ratio.
These lava flows are Cenozoic in age, and some of them spill into the canyon.One particular Precambrian layer known as the Cardenas Basalt has been dated by radiometric methods to about 1.1 billion years in age.