28 Petrology, Igneous (Posters)

Sunday, 9 October 2011
9:00 AM - 6:00 PM
Hall C

U-PB ZIRCON AND MONAZITE GEOCHRONOLOGY AND HAFNIUM ISOTOPIC GEOCHEMISTRY OF NEOACADIAN AND EARLY ALLEGHENIAN PLUTONIC ROCKS IN THE ALABAMA EASTERN BLUE RIDGE, SOUTHERN APPALACHIAN MOUNTAINS

Paper number: 28-1
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

The Alabama Eastern Blue Ridge (EBR) of the Southern Appalachian Mountains hosts a variety of felsic plutonic rocks, which intrude multiply deformed Neoproterozoic to Ordovician (?) metasedimentary rocks. Plutons consist of two distinct suites based on geochemical composition and degree of deformation: pre- to syn-kinematic Neoacadian, low Sr/Y plutons (ca. 380-360 Ma) and late- to post-kinematic, Early Alleghanian high Sr/Y plutons (ca. 350-330 Ma). Here, we report new whole rock geochemistry, U-Pb zircon SHRIMP-RG ages, and Hf isotope data for 6 plutons in the Alabama EBR. Low Sr/Y plutons are predominantly biotite-muscovite granites and granodiorites and include the Rockford Granite (376.6 ± 2.8 Ma) and the Bluff Springs Granite (363.8 ± 3.6 Ma). Zircon Hf isotope data from the low Sr/Y suite range from -11.2 to +1.8. These plutons are in general strongly deformed, and display geochemical characteristics consistent with mid crustal (<35 km) partial melting of pre-existing continental crust. By contrast, high Sr/Y plutons are deformed to undeformed, and consist of low-K tonalites and trondhjemites (e.g., Almond trondhjemites and Blakes Ferry pluton) with geochemical characteristics suggestive of deep-crustal partial melting of a garnet amphibole-bearing source. Two samples of the Almond trondhjemite (Wedowee pluton and Almond pluton) yielded ages of 334.3 ± 3.0 Ma and 345.0 ± 3.4 Ma, respectively. An additional peak at 324.4 ± 3.3 may represent a Pb-loss event. The undeformed Blakes Ferry pluton also yielded complex results with Grenville-age cores (ca. 1000-1080 Ma), and rim ages ranging from ca. 350 to 330 Ma with peaks at 343.1 ± 3.3 Ma and 331.1 ± 3.8. Igneous monazite yielded an age of 345.9 ± 4.4 Ma supporting a ca. 345 Ma age. Hf isotope data from the high Sr/Y suite range from -14.6 to +5.6.

We propose that the transition from Neoacadian, low Sr/Y, mid crustal partial melting to Early Alleghanian high Sr/Y deep crustal partial melting reflects thickening of the EBR during Neoacadian deformation. Hf isotope values also transition from crustal values (-eHf) to a mixed signature (+eHf and -eHf), reflecting both mantle and lower crustal melting. This transition may be related to either lithospheric delamination, and/or slab break off following Neoacadian deformation.