THE FORMATION OF SKARN AND BENDING OF ROOF SEDIMENTS INTO THE YORK HAVEN DIABASE PLUTON (YHDP), BED OF SUSQUEHANNA RIVER, FALMOUTH, LANCASTER COUNTY, PENNSYLVANIA

Visible only during low water levels is a coarse grained skarn, formed at the upper contact of the (YHDP) with the Triassic Gettysburg Formation. This rock is both an endo and exoskarn. These Skarns are in contact with diabase or separated from it by beds of cordierite hornfels. Bedding is poorly preserved in the skarns.

The exoskarn is most commonly a biotite skarn along with a hornblende skarn. The biotite skarn has a light blue gray color. When broken this rock reveals large biotite cleavages up to 3cm long. Biotite crystals pass through the bedding in the skarn. Other minerals seen are calcium plagioclase, hornblende, diopside, minor scapolite, epidote, sphene, magnetite and garnet. A darker variety of this skarn contains olivine and biotite, occurring as large porphyroblasts in a plagioclase diopside groundmass.

Beside hornblende other minerals found in the hornblende skarn are plagioclase, biotite, sphene, epidote, minor apatite, scapolite, magnetite, quartz and garnet. This gray rock is not as coarse as the biotite skarn. In some samples bedding is disrupted by formation of metasomatic minerals. Late forming cordierite rims and replaces hornblende and other minerals in the skarn. This skarn may also contain augite.

The endoskarn exhibits an altered diabase chill margin with large biotite crystals intergrown with augite and plagioclase. The glassy chill is not present where these skarns are formed. Glassy chill diabase is present near by. The endoskarn is less common than the exoskarn at this locality.

Further into the river the hornfels beds are bent downward into the diabase pluton, surrounded on three sides by diabase. Hornfels parallels the diabase contact forming ridges in the river bed until reaching this point and bending at a sharp angle into the igneous rock

The upper contact of the intrusion is in sharp contrast to the lower contact exposed downstream. Hornfels under the pluton is extremely fine grained to aphanitic in texture. Even a limestone bed has been converted to cryptocrystalline marble. There is no evidence of metasomatic activity at the lower contact of the (YHDP) with the New Oxford Formation at this locality.