FURTHER EVIDENCE FOR AN OLD CRUSTAL COMPONENT IN SOUTHEAST LAURENTIA BASEMENT FROM U-PB ZIRCON GEOCHRONOLOGY, EASTERN GREAT SMOKY MTNS

Original and subsequent mapping of the eastern Great Smoky Mtns. (ESGM) highlands basement complex in the Cherokee-Dellwood area by the USGS distinguished biotite (Bgn) and hornblende gneisses (Hgn) with metaplutonic orthogneiss. Zircon U-Pb SHRIMP geochronology by the USGS confirmed the largely Grenville affinity of the basement complex (magmatic rocks ranging from 1000 to 1180 Ma). Our recent work confirmed a considerably older magmatic component among rocks mapped as Hgn in the Dellwood area (zircon U-Pb SHRIMP age of 1330 Ma). The latter rocks are Hbl- or Cpx-granitic to tonalitic orthogneiss. The orthogneiss contains intermediate to mafic xenoliths, which suggest an even older crustal component is present in the ESGM basement complex. In an effort to further investigate the evolution of the EGSM basement complex we carried out LA-ICP-MS analysis of magmatic and metamorphic zircon from one sample of Hgn orthogneiss, zircon from two xenoliths within the previously analyzed 1330 Ma orthogneiss, and (nominally) detrital zircon in three samples of Bgn from the Dellwood and Clyde quadrangles. All zircon analyses were guided by CL/BSE images to avoid beam overlap of adjoining zircon growth zones and generation of meaningless ‘mixed’ ages.

All zircons exhibit complex growth zoning, suggesting multiple magmatic or metamorphic growth or recrystallization events. Zircon in Hgn contains xenocrystic cores with ages as old as 1300 Ma; thick magmatic overgrowths comprising the bulk of each grain yield ages of ~1180 Ma; variable thickness bright CL rims are 1000 Ma. The age of one xenolith, determined from slightly discordant analyses of magmatic zircon define an array with an upper intercept at 1381 +/- 6 Ma. A second xenolith with very complexly zoned zircons yielded fewer datable zircons that point to an age similar to the host. The age distributions of Bgn samples (n = 60 – 100 grains) all consist of variable contributions from Grenville (1050, 1150, 1250 Ma), ~1350 Ma, ~1600 Ma, and 1700-1800 sources. One sample is very similar to Snowbird Group detrital zircon age distributions, but two are distinctly different in having few Grenville zircon grains and being dominated by 1300-1800 Ma zircon grains. This, and the 1380 xenolith, support the presence of a significant, older crustal component in the southern Blue Ridge basement.