AN ISOLATED INTRUSION OF THE YORK HAVEN TYPE OLIVINE DIABASE, (YHTOD)TERRE HILL, LANCASTER COUNTY, PENNSYLVANIA

While investigating the small isolated diabase intrusion at the western end of Terre Hill, Lancaster County, Pennsylvania, I observed that this intrusion consisted of olivine diabase (OD). This intrusion’s north western portion ends at a fault. The Morgantown Diabase Pluton to the north is mostly devoid of olivine. There is one known occurrence of (OD) in this large intrusion. Kulp, Hughes; 1999

My field work has shown that this intrusion consists of a chill margin similar to that of the York Haven Type Diabase (YHTD). The interior consists only of (OD) which closely resembles that found in some of the (YHTD) intrusions of the Gettysburg and Newark Basins of Pennsylvania. Kulp, Hughes, 1999.

In thin section, this (OD) does not resemble the Quarryville Type Diabase, which forms only narrow dikes in Pennsylvania. (Large abundant plagioclase with alteration, no biotite, pervasive fracturing of olivine, ophitic texture pyroxene & plagioclase) Smith, 1973. Neither does it resemble the (OD) found within some of the Rossville Type Diabase intrusions of York County Pennsylvania. (Sparse large plagioclase phenocrysts, exsolution in clinopyroxene, blocky olivine crystals, dark olive green alteration of olivine) Kulp, Hughes, 2000.

Thin sections (TS) TR-876, TR-877 & TR-878 from this intrusion most resemble thin section OZ-64 from the Haycock Diabase Sheet of Bucks County, Pennsylvania. They also appear similar to other (TS) I have made from the (OD) found within the (YHTD) Intrusions of Pennsylvania. (abundant small round olivine, fewer large olivine, biotite surrounding opaque, small to medium fresh plagioclase, discrete orthopyroxene crystals.)

This is the first known intrusion of the (YHTOD) that is not enclosed within a larger diabase pluton. This intrusion indicates that the (YHTOD) is a separate pulse of basaltic magma into the Mesozoic Basins of Pennsylvania and is not a result of in situ differentiation of the (YHTD) magma that formed most of the diabase intrusions of Pennsylvania. There are 3 recognized diabase magma types in Pennsylvania. Smith, 1973. Is this (OD) a 4^th magma type, which should be added to the previous known diabase magmas?