IS THE AVONDALE MASSIF PALEOZOIC? NEW INSIGHTS FROM SHRIMP ZIRCON AND U-TH-TOTAL PB MONAZITE AGES

The Avondale Massif is one of several bodies of basement gneiss in the central Appalachian Piedmont of Pennsylvania which historically has been thought to be Mesoproterozoic in age and metamorphosed during the Grenville orogeny. New SHRIMP U-Pb zircon and EMP U-Th-total Pb monazite data indicate that a portion of the Avondale massif is lower Paleozoic and likely related to the Wilmington Complex magmatic arc. The Springton Gneiss is a body of tonalitic gneiss, which preserves primary igneous textures, that occurs along the eastern margin of the Avondale massif. U-Pb SHRIMP zircon analysis yields an igneous crystallization age of 476 ± 4 Ma, identical to zircon ages of the Wilmington Complex arc. The Sycamore Mills formation is amphibolite facies pelitic gneiss and schist which crops out just to the west of the Springton Gneiss, within the Avondale massif. EMP analysis of monazite reveals six compositional domains, defined using Th/U vs. Ce/Y as a compositional discriminator, which yield domain ages ranging from 399 ± 14 Ma (weighted average ± 1 standard error of the mean) to 485 ± 8 Ma, with most monazite growth occurring between 470 and 485 Ma. Additional U-Pb SHRIMP zircon analysis of felsic gneiss from the Avondale massif yields a Silurian age, 421 ± 4 Ma, similar to the Arden pluton, which intrudes the Wilmington Complex.

These results suggest that metamorphism in the Sycamore Mills Formation was contemporaneous with Wilmington Complex magmatism and demonstrate that at least some portion of the Avondale massif is not Mesoproterozoic. Instead, the Avondale massif may be basement upon which the Wilmington Complex arc is built. However, all samples analyzed to date are from the eastern portion of the Avondale massif. Farther west, Avondale massif gneiss is in contact with Glenarm Group metasedimentary rocks, the Cockeysville Marble and Setters Formation, which also overlie known Mesoproterozoic gneiss in the core of the Woodville nappe. Thus, either the Wilmington Complex is built upon Mesoproterzoic gneiss, similar to that of the Woodville nappe or the boundary between arc rocks and Mesoproterozoic gneiss is an unrecognized cryptic suture.