NEW U-PB ZIRCON CONSTRAINTS OF ALLEGHANIAN PLUTONISM IN THE SOUTHERN APPALACHIANS

The Alleghanian Orogeny in the Southern Appalachians records the last stage of the assembly of supercontinent Pangaea. The magmatic expression of this Paleozoic collision between Laurentia and Gondwana includes widespread, but not particularly voluminous granitic plutons. The age constraints from these putative Alleghanian plutons are crucial to understanding the kinematic and temporal history of the orogen. However, multi-grain U-Pb zircon analyses from these plutons yield statistically indistinguishable magmatic and xenocrystic U-Pb ages, and post-crystallization disturbances have limited the number of precise age determination of the plutons by Rb-Sr and K-Ar methods. Therefore, the exact timing extant temporal of Alleghanian plutonism is still unclear. We provide new U-Pb zircon ages of 16 Alleghanian plutons that intruded the Inner Piedmont, Carolina, Uchee, Pine Mountain Window, and Suwannee terranes in the Southern Appalachians. The U-Pb zircon ages of the plutons range from 337 to 288 Ma, which improves the chronology of plutonism in the southern Appalachian Orogen.

The end of the Neoacadian magmatism and the beginning of Alleghanian plutonism in the Southern Appalachians is also poorly defined due to the lack of reliable geochronologic data. In order to separate the Alleghanian and Neoacadian plutonism, we combine our results with the existing data, which yields a total of 42 U-Pb zircon ages of plutonic rocks ranging from 380 to 280 Ma. The results do not show a distinct age break, suggesting that Neoacadian and Alleghanian plutonism is at least semi-continuous, which further suggests that these two orogenies are not exclusive in time. A new comprehensive tectonic model is required to explain this relationship.