LATE MESOPROTEROZOIC (CA. 1.0 GA) DEPOSITION OF PROTOLITHS OF THE CARVERS GAP AND CLOUDLAND GNEISSES, MARS HILL TERRANE, NC-TN: NEW SHRIMP U-PB AGES OF DETRITAL ZIRCON AND MONAZITE

The Carvers Gap granulite gneiss (CGgg), part of the Mars Hill terrane of the Appalachian Blue Ridge province, was previously interpreted as a 1.8-Ga metaigneous rock. However, external morphology, and a wide variety of color and CL zoning patterns of zircon from CGgg suggest that these grains are detrital. Rounded detrital zircon grains also were obtained from quartzitic and pelitic layers of the adjacent metasedimentary Cloudland gneiss (CLg). Both of these Mars Hill terrane units have been metamorphosed to granulite facies.

A Relative Probability curve for SHRIMP U-Pb ages of zircon from CGgg has peaks at about 1.69, 1.33, 1.20, and 1.02 Ga; the youngest age group (n=4) is 1023 ± 17 Ma, is interpreted as the maximum age of deposition of CGgg protolith. Zircon from CLg yields different age peaks, at 1.17, 1.08, and 1.05 Ga; the youngest age groups are 1037 ± 12 Ma (quartzite, n=6) and 1029 ± 9 Ma (pelite, n=5). Rounded, oscillatory-zoned detrital cores of monazite from CLg pelite are ca. 1.07-1.01 Ga; the youngest age group is 1022 ± 12 Ma (n=8). Because these granulite facies metasedimentary rocks were derived from provenances composed of high-grade rocks, metamorphic zircon rims can be pre- or post-depositional, complicating interpretations of age data. After careful consideration of CL images, morphology, and ages, and using the Unmix routine of Isoplot, we conclude that CGgg and CLg were metamorphosed twice, at about 1.01 and 0.98 Ga. We suggest that the previous Paleoproterozoic age assignment for CGgg reflects the ages of provenances, not the time of deposition of the protoliths of CGgg and Clg.

Palinspastic reconstructions of Paleozoic displacement place the Mars Hill terrane about 350 km to the southeast of its present location. Based on previous whole-rock Pb and Nd isotopic data, and detrital zircon ages, the sedimentary protoliths of CGgg and CLg probably were derived from basement rocks that are similar in age to rocks in the northern Appalachians (Hudson Highlands) and possibly Amazonia. The location of the provenance(s) is unknown; they may be buried in the Paleozoic hinterland to the southeast.