Alleghanian Magmatism in the Southern Appalachians: Geochronology of the Multi-Phase Danburg-Sandy Hill Intrusion and Coeval Mafic Enclaves

Abstract:

Emplacement of numerous late syn- to post-tectonic Alleghanian granitoids in the southern Appalachian Orogen (SAO) are interpreted to mark final assembly of Pangea, though a comprehensive understanding of emplacement ages and formation mechanisms remains enigmatic. We obtained SHRIMP-RG ²⁰⁶Pb/²³⁸U ages of zircons from 7 intrusions in Georgia, USA, and are conducting a detailed study of the Danburg pluton (NE GA) to better understand the mechanisms leading to granite formation in the SAO. Magmatic ²⁰⁶Pb/²³⁸U zircon ages indicate a bimodal distribution of ages from ~342-321 in the Inner Piedmont and ~306-300 Ma in accreted Gondwanan terranes to the east, which is consistent with a growing database of U-Pb zircon ages. Ages of zircon xenocrysts, zircon δ¹⁸O values, and rare mafic enclaves support interpretations that these plutons derived from anatexis of older continental crust, with minor contributions from less-evolved sources. The Danburg Pluton and spatially-associated Sandy Hill ‘border facies’ intrude the Carolina terrane, and contain mafic enclaves that may represent a mantle contribution. The coarsely porphyritic, undeformed Danburg pluton is an I-type, magnesian, alkalic-calcic, metaluminous granodiorite to quartz syenite with ~63-73 wt. % SiO₂. The equigranular, undeformed Sandy Hill pluton is an I-type, magnesian to ferroan, alkalic-calcic, peraluminous granite with ~72-75 wt. % SiO₂. Whole rock samples from both intrusions show linearly-correlated major element trends, consistent with the two being related through crystal fractionation. The Danburg and Sandy Hill plutons yield indistinguishable ²⁰⁶Pb/²³⁸U zircon ages at the 2σ level (305.8±4.2 Ma and 301.1±3.6 Ma, respectively) with few xenocrysts. Premagmatic zircons with ages up to 335±6 Ma are inferred on the basis of age and zircon trace element chemistry. ²⁰⁶Pb/²³⁸U ages of titanite from three enclaves in the Danburg yield ages of 302.6±3.7 Ma, 311.7±6.1 Ma, and 312±11 Ma. Thus, all overlap the Danburg age (zircon) within 2σ uncertainty. Crystallization T’s calculated with Ti-in-zircon and Zr-in-titanite (at P = 0.3 GPa) thermometry are 717±69°C and 738±38°C (average±2 st.dev.). The age, temperatures, and textural evidence are interpreted to reflect quenching of mafic melt (enclaves) upon mixing with the coexisting felsic melt.