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.

摘要

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.