THE PRESERVATION OF ORIGINAL MINERALOGY AND TEXTURES IN THE SERPENTINIZED ULTRAMAFIC ROCKS OF THE BALTIMORE MAFIC COMPLEX, EXPOSED AT THE PENN-MD MATERIALS QUARRY, FULTON TOWNSHIP, LANCASTER COUNTY, PENNSYLVANIA

After examining thin sections of rocks I collected at the Penn-Md Materials Quarry in Fulton Twp, Lancaster County, it was readily apparent that upwards of 50% of the original Fosteritic olivine is still present in some of these serpentinized dunites and peridotites. Previous research has indicated only about 5% preservation of fosterite in the State Line Serpentinites of Pennsylvania; H. L. McKague (1964)

Hand samples of the least altered rocks are black, gray and greenish grey in color. Olivine can be seen as glassy reflective crystals in this serpentinite. The least altered rocks quickly develop an orange-rust weathering on exposed surfaces. Some large blocks at the quarry show visible igneous layering of dunite and peridotite. Dunite forms thin bands, while peridotite forms thicker bands in some of these rocks. Original igneous textures and olivine crystals can also be seen on sawn surfaces with the naked eye, even better with a hand lens. Partly altered olivine has a brown color in some of the more serpentinized rocks at this quarry.

When seen with a polarizing microscope, olivine stands out from serpentine. Pyroxene is much more altered than olivine within these rocks. Least altered dunite is medium to course- grained, granular- textured fosteritic rock, consisting of what was almost 100% olivine. Original grain boundaries are indicated by thin lines of magnetite. Crisscrossing lines of serpentine are present in these olivines. Serpentine replaces the outer portions of some of these crystals.

Very little of original pyroxene is unaltered in the serpentinized peridotites seen in thin section. These crystals are now mostly serpentine and magnetite. They appear black even in hand samples. I have determined that this pyroxene is clinopyroxene from extinction angles seen with a polarizing microscope. This indicates this peridotite was wehrlite before its partial alteration to serpentinite.

McKague (1964) sampled rocks exposed at the nearby Cedar Hill Quarry. Why there is a dramatic change in preservation of original mineralogy between these two nearby localities is an interesting question.