28 The Grenville Orogen in Eastern North America

Saturday, 21 March 2020
8:00 AM - 12:00 PM
Regency Ballroom B

NEW SHRIMP U-PB GEOCHRONOLOGY FROM THE GLOBE MASSIF, GRANDFATHER MOUNTAIN WINDOW, NC

Paper number: 28-10
Program: Theme Sessions
Time: 11:00 AM – 11:20 AM

Authors

Abstract

Mesoproterozoic rocks are exposed in several massifs in the Blue Ridge from Maryland to Georgia. Although outliers from the Grenville Province, these massifs record evidence of multiple phases of Grenville orogenesis. In northwest North Carolina, the Globe Massif exposes Mesoproterozoic rocks in the Grandfather Mountain window and is separated from the larger French Broad Massif by the Linville Falls fault and other faults. The Globe Massif comprises the Blowing Rock Gneiss and various rock types of the Wilson Creek Gneiss (WCG), and is intruded by felsic dikes, mafic dikes of the Linville Diabase, and the Brown Mountain Granite. The rocks are overlain by the Grandfather Mountain Formation and Chilhowee Group, and are variably overprinted by Paleozoic deformation. The WCG is the most areally extensive unit, exposed in two thrust sheets in the window, and includes the greatest diversity of rock types: granite, diorite, and layered gneiss. A sample of coarse granite gneiss from the type area along Wilson Creek yielded a preliminary 207Pb/206Pb date of ~1140 Ma, and a second sample near Jonas Ridge yielded discordant data with an upper Concordia intercept date of 1143 ± 11 Ma. A third WCG granite sample from a structurally higher thrust sheet within the window exposed in a large roadcut on US Hwy 321 at Kirby Creek yielded a 207Pb/206Pb date of 1158 ± 7 Ma. The WCG is intruded by mafic and porphyritic felsic dikes, the latter yield a 206Pb/238U date of 737 ± 10 Ma. Two samples of Blowing Rock Gneiss, typical augen gneiss and leucocratic augen gneiss, exposed in a large roadcut on Hwy 321 yielded 206Pb/238U dates of 1080 ± 10 and 1087 ± 14 Ma, respectively, and in agreement with a published U-Pb date. Layered quartzo-feldspathic WCG in the roadcut is truncated at the contact with leucocratic Blowing Rock Gneiss, and yielded a preliminary zircon 207Pb/206Pb date of ~1280 Ma, indicating that pre-Shawinigan components are present in the WCG. Additionally, all samples included discordant dates that define imprecise Paleozoic lower intercept dates; however, no datable Paleozoic rims are present. These preliminary results reveal a similar tectonic history to rocks of the northern end of the French Broad Massif recording deformation and magmatism associated with 1.3–1.2 Ga pre-Grenville crust, ~1.15 Ga Shawinigan, and ~1.05 Ga Ottawan events.

Geological Society of America Abstracts with Programs. Vol. 52, No. 2

doi: 10.1130/abs/2020SE-344725

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