TECTONIC SYNTHESIS OF RECENT GEOCHRONOLOGICAL AND GEOCHEMICAL DATA IN THE CENTRAL APPALACHIAN PIEDMONT OF SE PA AND N DEL

Recent geochronological and geochemical data on the Wilmington Complex (WC) and adjacent Wissahickon Formation (WISS) prompt a new understanding of Central Appalachian tectonic evolution. Early Ordovician arc magmatism was followed by significant crustal thickening, possibly due to arc-continent collision. In the Silurian, a high-T, low-to-moderate-P thermal regime and younger mafic-silicic magmatism suggest thinning of arc lithosphere, possibly backarc extension behind a west-dipping subduction zone. The timing of arc magmatism is constrained by U-Pb zircon ages of about 480 Ma from WC orthogneisses with arc geochemical signatures. Similar ages of monazite cores in WISS meta-pelites, and the presence of amphibolite dikes with boninitic affinity in both WC and WISS, suggest that the original contact between these units is intrusive and that early metamorphism in the WISS occurred in response to heating from arc magmas. Ordovician volcanic and plutonic rocks in the WC were buried to depths corresponding to at least 500 MPa before intrusion of the Arden and Bringhurst plutons at about 430 Ma, indicating significant tectonic thickening. Pre-Silurian tectonic burial is consistent with structural and biostratigraphic evidence for convergence in the foreland, and is interpreted to have resulted from the collision of the WC arc and the Laurentian margin.

High-T low-P metamorphism in the WISS and moderate-P granulite facies metamorphism in the WC, are approximately the same age as the Arden pluton and therefore are interpreted to reflect the same Silurian thermal regime. Significant mafic magmatism in the Arden and Bringhurst plutons suggests that advection of mantle heat was important in establishing the steep geothermal gradient. This thermal regime is interpreted to result from post-collisional thinning of the lithosphere resulting from either delamination of the down going slab, possibly marking a shift in subduction polarity, or backarc extension above a younger, west-dipping subduction zone. The backarc setting is the preferred interpretation based on the geochemical similarity of the Silurian mafic rocks to E-MORB or backarc basin basalts. Late Ordovician and Silurian arc-related rocks indicative of this subduction geometry are present to the north and south of the study area.