Sedimentary and volcanic rocks ranging in age from Precambrian to Lower Cretaceous underlie the Tombstone area (116 A/4). The conditions of metamorphism of the sedimentary sequence underlying the study area are reflected by the stability field of the univariant assemblage tremolite-calcite-quartz-diopside. Assuming a pressure of 2 kb, the temperature of metamorphism lies between 475° and 525° C.
The mineralogical zoning displayed in the host rocks away from the syenite is interpreted as being essentially the consequence of a decreasing amount of dolomite in the original sediments.