Ar ar dating of basalts
The published geologic literature contains seven K-Ar analyses of Cardenas Basalt and two K-Ar analyses of Precambrian diabase.1115 Descriptions of these rocks are found in the appendix of this paper and the K-Ar data have been collected into Table 1. The new K-Ar data have been combined with the published data into table 1.Of the 13 new analyses, eight are from Cardenas Basalt and five are from Precambrian diabase.These mafic sills crop out in seven locations along a 70–80 km length of the Grand Canyon (fig. They are composed chiefly of medium-grained ophitic diabase.The dikes have a similar composition but are finer grained, as are the chilled margins of the sills.
A split from each sample was submitted for 66 element geochemical analysis, and a slab was retained for thin-section petrographic analysis.
Thirteen new K-Ar analyses of Proterozoic mafic rocks of Grand Canyon are added to nine published K-Ar analyses.
We report a new fourteen-point K-Ar isochron “age” of 516 ± 30 Ma which is strongly discordant with the published Rb-Sr isochron “age” of 1.07 ± 0.07 Ga for Cardenas Basalt.
In contrast, the 200-m-thick upper member of the Cardenas Basalt comprises four to six, aphyric, intersertal to intergranular flows that change sequentially from basaltic andesite to basalt and then back to basaltic andesite upward through the section.
The dramatic change to the resistant, finer-grained flows of the upper member coincides with the abrupt change in magma chemistry.
Early differentiation and crystal settling in the sills is evidenced by granophyre layers up to 10-m thick and felsite dikes, and by olivinerich layers.