U-Pb zircon and monazite geochronology and hafnium isotopic geochemistry of neoacadian and early alleghanian plutonic rocks in the Alabama Eastern Blue Ridge, Southern Appalachian Mountains

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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, I report new whole rock geochemistry, U-Pb zircon SHRIMP-RG (Sensitive High Resolution Ion Micro Probe-Reverse Geometry) 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 ± 1.5 Ma) and the Bluff Springs Granite (363.8 ± 2.9 Ma). The Enitachopco trondhjemite dike also displays a Neoacadian age of 366.5 ± 3.5 Ma. Zircon Hf isotope data from the low Sr/Y suite range from -11.2 to +2.0. 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 334.6 ± 3.2 Ma and 343.4 ± 3.4 Ma, respectively. An additional peak at 324.4 ± 3.3 may represent a Pb-loss event. Another sample of Almond trondhjemite yielded complex ages with a peak at 349.1 ± 1.8 Ma 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 ± 3.1 Ma supporting a ca. 345 Ma crystallization 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 (-εHf) to a mixed signature (+εHf and -εHf), reflecting both mantle and lower crustal melting. This transition may be related to slab break off following Neoacadian deformation.

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Geology, Petrology