- GeoRef, Copyright 2004, American Geological Institute. Reference includes data from Bibliography and Index of Geology Exclusive of North America, Geological Society of America, Boulder, CO, United States
Some basaltic and andesitic rocks from the Tweed shield volcano are tholeiitic in character as evidenced by their chemistry and the nature of their pyroxenes inferred from optical data. A common mineral assemblage in the more basic members is olivine-clinopyroxene-plagioclase-opaques-(glass). Petrographic criteria that characterize tholeiites are often lacking, and some of the more basic lavas appear to link the more typical tholeiites with transitional olivine basalts. Many of the Tweed volcanic rocks are more potassic than similar rocks in other tholeiitic sequences. The likelihood of differing lineages in the shield area is noted.