GEOLOGY AND GEOCHEMISTRY OF THE JONESTOWN VOLCANICS, PENNSYLVANIA

The Jonestown Volcanic Complex is a 30 km² area of igneous rocks located in the Ordovician Hamburg Klippe, a Taconic allochthon. Detailed field mapping of these igneous rocks shows that hypabyssal rocks (diabase) comprise about eighty percent of the areal extent, while basaltic volcanic rocks, including pillows and pillow breccia, comprise only twenty percent, and are restricted to one of the four structural belts of igneous rocks. Field observations show that it is doubtful that there are intact sedimentary or igneous contacts between the igneous rocks and the flysch of the Hamburg Klippe, adjacent to them. The volcanic rocks, however, are locally in original association with massive limestone, the closest regional lithologic analogues of which are Laurentian platform carbonates. The volcanics and associated limestone may be a structural equivalent of the Lebanon Valley sequence to the south, and the limestone resembles the Annville Formation of that sequence. We suggest that the quartzose sandstone capping the Bunker Hills is an outlier of the Silurian Bloomsburg/Tuscarora Formation. Whole rock analysis of trace and rare earth elements shows that the Jonestown igneous rocks are basaltic, and that they are mildly enriched relative to MORB and have sub-alkaline tendencies. Trace element patterns of the hypabyssal rocks can be interpreted to contain hints of continental contamination. Uncertainties in the age of these rocks allow for alternative interpretations of the igneous activity, including a seamount in the Taconic ocean before collision, or on the Laurentian foreland either well before, or during, the arrival of the Taconic subduction system. All possibilities require the igneous rocks to be transported significant distances relative to the flysch. The chemical evidence for crustal involvement and the association of volcanic rocks exclusively with carbonate rocks lacking clastic input suggest a Laurentian foreland interpretation is more likely. The preferred hypothesis, which requires subsequent out of sequence thrusting, is that the igneous activity occurred on the Laurentian foreland just before Taconic collision.