Geology of the Goat Hill serpentine barrens, Baltimore mafic complex, Pennsylvania

The Goat Hill Serpentine Barrens tract is located in the southwestern corner of Chester County, Pennsylvania, just north of the Maryland border. It is underlain largely by serpentinite bedrock of the Baltimore Mafic Complex (BMC). This complex of ultramafic and associated gabbroic rocks is believed to be a remnant from the roots of an island arc complex. Evidence supporting this origin includes the oxide composition of chromite in ores, the relative abundances of platinum group elements in chromite ore composites, and the association with Bald Friar Metabasalt backarc and "Conowingo Creek metabasalt" forearc basalts nearby. Various portions formed at 490 +/- 20 Ma above a southeast-dipping subduction zone. This zone incorporated detritus from the margin of the Laurentian continent and nearby microcontinents, as well as ultramafic oceanic material from the floor of the Iapetus Ocean isolated from the mantle during the older, 735-Ma period of rifting. Both the backarc and forearc basalts are found as masses along with ultramafic fragments from the BMC proper in an ophiolite mélange that formed in the trench that developed on the northwestern side of the arc complex as the Octoraro basin was technically closed. Traditionally, the Iapetus Ocean is considered to have closed during the Taconic Orogeny, circa 450 Ma, by obduction of the island arc and mélange onto the margin of Laurentia. The obduction of the BMC was likely aided by the presence of zones of low competence developed within steatized ultramafic fragments within the mélange-bearing trench. The BMC was subjected to a high temperature, low pressure metamorphism circa 440 Ma. This metamorphism and associated igneous activity appears to have been mainly the result of crustal thinning following the Taconic orogeny and, to a lesser extent, loading from the orogeny. The serpentinites within the Goat Hill tract have widely varying compositions as a result of the sequence of minerals that crystallized from the BMC when it was still part of the ocean floor (forsterite to orthopyroxene to clinopyroxene to pyroxenes plus plagioclase), later metamorphic events after it had been obducted onto the continental margin, and variable leaching by recent meteoric water.