The Composite Dikes at Mount Desert Island, Maine: An Example of Coexisting Acidic and Basic Magmas

Abstract

Composite dikes of Mount Desert Island are composed of pillow-like masses of basalt surrounded by granite and represent coexisting acidic and basic liquids. The contacts between basalt and granite ate sharp on both a megascopic and microscopic scale. Grain-size reduction at basalt pillow margins is generally present and is due to chilling. Evidence of commingling consists of quartz and microcline xenocrysts in the basalt as well as hybrid rocks formed by incomplete mixing of basalt and granite. Rare earth elements in experimental immiscible systems have been shown to be highly partitioned towards the basic liquid ($$D_{b/g} \sim 10$$). In the Mount Desert Island complex, however, the light REE's are enriched in the granite with $$D_{b/g}$$ ranging from 0.48 to 0.67. The REE and petrographie data are incompatible with in situ immiscibility for the origin of these composite dikes.