SHRIMP U-PB ZIRCON GEOCHRONOLOGY FROM THE GLOBE MASSIF AND CONNECTIONS WITH THE FRENCH BROAD MASSIF

In northwest North Carolina, the Globe massif exposes Mesoproterozoic rocks in the Grandfather Mountain window and is separated from the French Broad massif by the Linville Falls fault and other faults. The Globe massif consists of the Blowing Rock Gneiss (BRG), and the Wilson Creek Gneiss (WCG), which are intruded by felsic dikes, mafic dikes of the Linville Metadiabase, and ~765 Ma Brown Mountain Granite. The massif is overlain by the Grandfather Mountain Formation and Chilhowee Group, and is variably overprinted by Paleozoic deformation. The WCG is an extensive map unit, exposed in two thrust sheets in the window, and includes a variety of rock types: granite gneiss, diorite gneiss, and layered gneiss. Zircons from coarse-grained WCG granite gneiss yield a preliminary ²⁰⁷Pb/²⁰⁶Pb date of ~1140 Ma; a sample near Jonas Ridge yields discordant data with an upper intercept date of 1143 ± 11 Ma. A third WCG granite sample from a structurally higher thrust sheet within the window exposed in a roadcut on US Hwy 321 at Kirby Creek yields a ²⁰⁷Pb/²⁰⁶Pb date of 1158 ± 7 Ma, and is cut by a porphyritic felsic dike that latter yields a ²⁰⁶Pb/²³⁸U date of 737 ± 10 Ma. Two samples of BRG, typical augen gneiss and less common leucocratic augen gneiss, exposed in a large roadcut on Hwy 321 yield ²⁰⁶Pb/²³⁸U dates of 1080 ± 10 and 1087 ± 14 Ma, respectively, and in agreement with a published U-Pb date. Layered WCG in the roadcut is cut by leucocratic BRG and yields a preliminary zircon ²⁰⁷Pb/²⁰⁶Pb date of ~1280 Ma, indicating that pre-Shawinigan components are present in the WCG. All samples include discordant data that define an imprecise Paleozoic lower Concordia intercept; however, no datable Paleozoic rims are present. These preliminary results reveal a similar tectonic history to the northern end of the French Broad Massif that record deformation and magmatism associated with 1.3–1.2 Ga pre-Grenville crust, ~1.15 Ga Shawinigan, and ~1.05 Ga Ottawan events. Neoproterozoic rifted-related cover sequences (Grandfather Mountain and Mount Rogers Fms.) overly both massifs and further support a palinspastic connection between Globe and French Broad massifs. Connections to the Suaratown Mountain massif, which lacks identified ~1.05 Ma magmatism, is speculative.