At Safe Harbor, Pennsylvania, a vertical, north-easterly trending dike of Triassic diabase transects highly folded Antietam schist and Vintage dolomite. A deep quarry in the diabase affords a splendid opportunity to study the contact-metamorphic effects of this intrusion on the country rock, especially along the eastern boundary.
The unaltered Antietam schist is a fine-grained silvery-gray, well-foliated rock containing quartz, feldspar, biotite, and muscovite, but near the intrusion it has been greatly changed. The contact-metamorphic aureole may be divided roughly into two zones: (1) An inner zone, from 5 to 8 feet wide, of dark hornfels containing orthoclase, plagioclase, cordierite, quartz, and chlorite; and (2) an outer zone, of greater and more variable width, of coarser
and lighter-colored rock containing mainly plagioclase, quartz, muscovite, biotite, chlorite, and clinozoisite. Heat was the paramount factor in the metamorphism of the inner zone although hydrothermal solutions later played a role. In the outer zone, hydrothermal solutions were chiefly responsible for the alteration.
The unaltered Vintage dolomite is a foliated fine-grained, saccharoid rock composed of dolomite, feldspar, quartz, and phlogopite. The contact aureole, formed by intrusion of the diabase, consists of two parts: (1) an inner zone, some 20 feet wide, where the rock is a dense, black, nonfoliated limestone made up of carbonate (mostly calcite), phlogopite, antigorite (from forsterite), tremolite, diopside, talc, magnetite, and sphalerite; and (2) an outer zone, of variable width, in which the rock is nonfoliated and sugary-textured and contains carbonate (mostly dolomite), antigorite (from forsterite), phlogopite, and talc (?). The inner zone is the result of strong thermal metamorphism followed by strong hydrothermal alteration. The outer zone was produced by the same two processes operating less intensely.
From a consideration of the miner alassemblage of the hornfels close to the diabase, it is apparent that the Antietam schist was thermally metamorphosed under high temperature and moderate pressure of the pyroxene hornfels facies. Although the Vintage dolomite must have suffered similar thermal alteration, its mineral assemblage does not afford conclusive evidence. In the hydrothermal stage, water, iron, and magnesia, and possibly alumina and silica, were added to the schist, and water, magnesia, silica, iron, zinc, and sulphur were brought into the dolomite. However, the magnesia and silica were probably not magmatic in origin.
Judging from published accounts, the Palisades diabase of New Jersey and New York, like the Safe Harbor diabase, metamorphosed its wall rock under conditions of high temperature and moderate pressure characteristic of the pyroxene hornfels facies. The great Whin sill of northern England, however, was cooler and formed contact rocks belonging to the amphibolite facies.