A 3714 MA PLUTON INTRUDING THE ISUA GREENSTONE BELT, SOUTHWEST GREENLAND: THE WORLD'S OLDEST VOLCANO-SEDIMENTARY SECTION (OR, AT LEAST PART OF IT) GETS OLDER

We document a rare cross-cutting relationship with the Isua greenstone belt (IGB), which changes the understanding of field relationships. The IGB, long thought to be deposited at 3.8-3.7 Ga and thus the oldest volcano-sedimentary section on Earth, has resisted efforts to be more precisely dated. This is partly due to the rarity of cross-cutting relationships within the strongly deformed belt. Nutman et al. (1997, 2002) delineated two shear-zone bounded sections, dated at ~ 3710 (north side of the belt) and ³ 3790 Ma (south side), based upon zircons from putative felsic volcanics and volcanoclastics. Given strong metasomatic effects in the belt, though, others have suggested these rocks are altered tonalite sills. The relationships with the enclosing gneiss units are also uncertain. Most workers agree the southern ~ 3.8 Ga tonalite has a sharp intrusive contact with the IGB, but the contact with the northern ~3.7 Ga tonalite has been interpreted as intrusive or tectonic.

We mapped a 5 x 1 km coarsely foliated pluton, of porphyritic diorite to tonalite, along the western edge of the northern gneiss. This pluton clearly cross-cuts strongly foliated amphibolite-facies greenstone at the edge of the IGB (a part of the belt shown by Nutman et al. 2002 as of unknown age). The "gray gneiss" that is more typical of the ~3.7 Ga northern gneiss has an irregular intrusive contact with the pluton. Using TIMS, we dated four single-grain fractions of gem-quality zircon from three samples of the pluton, from outcrops more than 2 km apart. These gave concordant, coincident analyses at 3714 ± 2 Ma. Zircons from a septum of the gray gneiss are concordant at 3699 ± 4 Ma (preliminary data). All zircons show simple oscillatory zoning.

These data require that at least this particular part of the IGB was deposited, deformed and metamorphosed before 3714 Ma, and that the ~3.7 Ga northern tonalite gneiss intrudes the IGB. Thus, at least the older part of the IGB had to be sandwiched between both 3.8 and 3.7 Ga tonalites at 3.7 Ga. We think it unlikely that a ~3710 Ma section, infolded with older rocks, would only appear tucked in with the supracrustal section. Thus it appears that the ~3710 Ma dates of Nutman et al. (1997) actually are from altered, intruding tonalite, suggesting the same is true of many of the ³ 3790 Ma dates. 3790 Ma then becomes the minimum age of the IGB.