Based on chemical composition and mineralogy, three types of Triassic diabase are recognized in Pennsylvania. The probable oldest type (Quarryville type) occurs as an olivine tholeiite dike swarm. The York Haven type is a quartz tholeiite, forming sheets, dikes, and a few flows. The youngest Rossville type is also quartz tholeiite that occurs as sheets and dikes. Within samples of the same type, chemical composition is very uniform. In content of major elements, rare earths, and Ba, the Rossville type resembles island-arc tholeiites. The York Haven type is similar to continental tholeiites.

Based on calculated cooling rates and the homogeneity within each type of magma, plus paleomagnetic data, we conclude that each type was emplaced within a relatively short time period, and that all sheets, dikes, and flows of a single compositional type are essentially contemporaneous. The trend of Triassic diabase dikes in Pennsylvania parallels the trend of Precambrian and Paleozoic dikes, suggesting that trends of dikes may reflect pre-existing structural weaknesses in the basement rather than being an exact indicator of stress orientation during Triassic time.

The two quartz tholeiites can be formed by crystallization of 30 to 45 percent of the olivine tholeiite magma as olivine, minor clinopyroxene, and plagioclase or spinel, accompanied by assimilation of orthopyroxene, probably from the mantle. Rare-earth and Sr-isotope data suggest that the York Haven type probably assimilated as much as 20 percent crustal material, whereas the Rossville type assimilated little or none. These phenomena of multiple-stage fractionation and reaction of the magma with mantle and crust probably apply to most magmas.

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