DETRITAL ZIRCON AGES FROM THE WISSAHICKON FORMATION, SOUTHEAST PENNSYLVANIA AND NORTHERN DELAWARE: REGIONAL TECTONIC IMPLICATIONS

The Wissahickon Formation has been provisionally subdivided into three units based on differences in lithology, metamorphic and deformation histories, contact relationships, and the tectonic affinity of interlayered amphibolite. From NW to SE these units are the Glenarm Wissahickon, the Mt. Cuba Wissahickon, and the type-Wissahickon, which occurs to the east of the other units across the steeply dipping Rosemont Shear Zone. Here we report the results of new LA-ICPMS U-Pb detrital zircon age analyses undertaken to further clarify the relationships between units.

The Glenarm and Mt. Cuba yield mainly Mesoproterozoic to earliest Neoproterozoic ages (1320 – 950 Ma). A smaller number (< 6 per sample) of younger (545 – 575 Ma) and older (1400 – 1800 Ma) grains are found in all samples. The Glenarm samples contain nine grains between 750 and 950 Ma and each yielded one Archean-aged zircon (2700 Ma); ages not found in the Mt. Cuba samples. The majority of ages correspond to Laurentian sources: pre-Grenville convergent-margin magmatism, the Grenville through Ricolet orogenies, and Catoctin rift-related magmatism. The early Neoproterozoic grains in the Glenarm samples could represent a Gondwanan source. The Glenarm and Mt. Cuba likely share a similar Laurentian source, but consistent differences suggest they could be different units.

The type-Wissahickon samples all contain Mesoproterozoic (1000 – 1200 Ma) zircon, but differ from each other. Ages from 750 to 900 Ma are present in W-1N (Wissahickon Ck. type locality) (n = 19) and CC46 (Chester Ck.) (n = 11), but none occur in W-2S (Fairmont Park). W-1N also has a late Cambrian peak (508 Ma, n = 9). W-2S has a small peak (n = 6) at 550 Ma and its largest peak (n = 39) from 1250 – 1500 Ma; the two other samples contain < 6 grains each in that range. The largest population (n = 29) in CC-46 spans 500 – 650 Ma. Both W-1N and CC-46 yield one 480 Ma grain. W-2S has a clear Laurentian affinity, with robust Grenville and Pinwarian peaks. While W-1N and CC-46 yield Grenville ages, the ages between 750 and 900 Ma indicate a non-Laurentian source. The 500 – 650 Ma peak in CC-46 could represent either Laurentian rifting or peri-Gondwanan arc-magmatism. These results suggest that W-1N and CC-46 were deposited in a basin receiving sediment from both Laurentian and Gondwanan sources during the early Ordovician.