Paper No. 9
Presentation Time: 10:30 AM
COMPLEX SOURCES OF ALLEGHANIAN GRANITES IN THE SOUTHERN APPALACHIAN OROGEN
The final stage of assembly of the supercontinent Pangaea included the collision between Peri-Gondwanan and Gondwanan terranes and Laurentia during the Alleghanian orogeny, which resulted in the emplacement of numerous granitic plutons in the southern Appalachian orogen. The origin of these Alleghanian granites is unclear and several proposals including subduction-related, crustal anatexis, and extensional delamination remain to be tested. We determined the geochemistry and the U-Pb and Hf compositions of zircons of eight granitoids emplaced in the Carolina, Inner Piedmont, and Uchee terranes to investigate their petrogenesis. The granitoids consist of granite and granodiorite and the U-Pb compositions of zircons from the granitoids indicate an Alleghanian age from 290 to 320 Ma. The wide range of alkali (alkali to calcic) and aluminous (metalumious to peraluminous) compositions indicate both S- and I- type origins for the granitoids. Most Alleghanian granitoids have rare earth patterns characterized by light rare earth element (LREE) enrichment with La/Yb ratio ranging from 10 to 66, Sr/Y from 4 to 39, and Eu anomaly from -0.6 to 0, not including the extreme ratio of the Panola granite from the Inner Piedmont. The Panola granite has extremely high Sr/Y (182) and moderate La/Yb ratios (18), suggesting a garnet-bearing source. The absence of an Eu anomaly, but a moderate La/Yb ratio (12) of the Motts granite (Uchee terrane) probably results from partial melting with a plagioclase-bearing residue. The wide range of initial εHf of zircons (-2.6 to +3.6) and whole rock initial εNd (-6.5 to +2.3) of granitoids support a mixing between mantle-derived and crustal components. In addition, a sample of the Sparta granite contains two zircon xenocrysts with U-Pb age of 749±7 Ma and 1286±13 Ma. The complex history of zircons in the granitoids also suggests multiple-age components in the sources of these Alleghanian plutons.