Mesoproterozoic metaplutonic rocks from the southern French Broad massif in southwestern North Carolina preserve Grenville magmatic and metamorphic ages (1.28–1.0 Ga). New in-situ zircon U-Pb measurements from eight basement lithologies, obtained by laser ablation and ion-probe techniques, delineate Elzevirian (1283–1250 Ma), Shawinigan (1191–1126 Ma), and Ottawan (1034–1008 Ma) magmatism and metamorphism. Magmatism produced dominantly calc-alkaline plutons with normative compositions of quartz diorite, quartz monzodiorite, granodiorite, and monzogranite. The majority of magmatism in the field area occurred between ca. 1190 and 1125 Ma, a period correlative to the Shawinigan phase of the Grenville orogeny. Syn- to post-emplacement metamorphism of these units is evidenced by complex zircon U-Pb systematics in all Shawinigan age plutons. Ottawan magmatism, expressed by emplacement of the Max Patch granite and protoliths of the Earlies Gap granitoid gneiss, is less voluminous than Shawinigan magmatism; a difference that mirrors their relative proportions elsewhere in the orogen. Ottawan metamorphism is recorded by growth of new zircon rims around magmatic core domains in several samples and spans a period of ca. 25 m.y. from ca. 1035 to 1010 Ma. The timing of the older and younger magmatic suites from the southern French Broad massif correlate to well-established times of Shawinigan and Ottawan magmatism and metamorphism in other Grenville-age terranes within the U.S. (e.g., Appalachian inliers, Adirondacks).
Bibliographical noteFunding Information:
1. Pre-Elzevirian magmatism is supported by the 1283–1250 Ma Earlies Gap granitoid gneiss (map units Yepg and Yegpg) and by zircon xenocrysts within Earlies Gap granitoid gneiss (Yepg), Fines Creek gneiss (Ymg), and Max Patch granite (Ym).
This research was funded by NSF EAR 1447611 to Samson, NSF EAR- 1447605 and NSF I&F EAR-1551342 to Moecher, and a Geological Society of America (GSA) Graduate Research Grant to the lead author. The funding sources did not have any interference in the study design, collection of analyses, in writing of the report, or the interpretation of the data. Jay Thomas is acknowledged for his help with teaching the lead author how to make “the best” zircon grain mounts.
© 2021 Elsevier B.V.
- Blue Ridge
- French Broad Massif
- Southern Appalachians
- Zircon U-Pb
ASJC Scopus subject areas
- Geochemistry and Petrology