Micropetrographic observations and mineral geochemistry data, along with previously reported whole-rock geochemistry, defined an A-type granite-affinity for the Tombstone and Deadman plutons of the 120-km-long, mid-Cretaceous Tombstone-Tungsten plutonic belt. This plutonic belt was intruded into the western Selwyn basin, the western-most edge of the ancestral North American craton, and is situated well inboard of any potential subduction-zone plutonism, which is found in the accreted terranes that were juxtaposed with the North American craton during the Jura-Cretaceous.
More than a dozen mineral specimens were petrographically observed and five were chemically analysed. Similarities found among the two plutons are their alkalic nature, minimal quartz content, presence of igneous andradite garnets and the presence of rare-earth-bearing minerals. Differences between the two plutons include the fact that the Tombstone pluton contains Th, Ce and La with magnetite and titanite, while the Deadman pluton contains Nd with Ce and La, as well as Ba-rich alkali feldspars. These observations are not commonly found in tectonic-related I- or S-type granitoids, and distinguish the Tombstone plutonic suite as being most similar to A-type granites.
