The origin of early Archean banded iron formations and of continental crust, Isua, southern West Greenland

Abstract

A long-standing geological debate is whether the Fe component of Precambrian banded iron formations (BIFs) was derived from continental sources, or had its origin in hydrothermal alteration of the oceanic crust. Recent REE, and Nd and Fe isotope studies of iron-rich layers in BIFs have provided strong supporting evidence for the hydrothermal origin of the Fe component. Pillow basalts from the 3800–3700 Ma Isua greenstone belt, West Greenland, underwent significant enrichment of Fe (up to 21 wt.% Fe 2O 3) and some chalcophile elements (e.g., Pb = 5.4–7.2 ppm) prior to a late Archean tectonothermal metamorphic event recorded in the region. Published and new Pb-Pb isotopic data from the Isua pillow basalts and the spatially associated BIFs suggest that the enrichment of Fe and Pb and deposition of the BIFs took place between 3800 and 3600 Ma. Similar Pb isotopic compositions in pillow basalts and BIFs are consistent with a common source of Pb for both supracrustal rocks. Iron and Pb were probably scavenged from the oceanic crust and were re-deposited on the ocean floor by hydrothermal fluids, suggesting that high-temperature hydrothermal alteration of early Archean oceanic crust played an important role in the deposition of the Isua BIFs. Geochemical, lithological, and structural data from the Isua greenstone belt are all collectively consistent with a convergent margin geodynamic setting. The Isua BIFs are spatially and temporally associated with boninitic and island arc picritic pillow basalts. Given the observations that Cenozoic boninites and picrites tend to form in an intra-oceanic arc-forearc setting, we suggest that the Isua BIF-boninite and BIF-picrite associations were also deposited in a similar geodynamic setting. The original pillow basalt-BIF stratigraphy was dismembered and transposed throughout the belt during early to late Archean tectonothermal events. The high-temperature hydrothermal alteration of the oceanic crust produced significant hydrothermal discharge with large quantities of Fe necessary to contribute to the deposition of the Isua BIFs. The hydrothermal alteration may have resulted from the opening of an asthenospheric window developed as a consequence of ridge subduction beneath an early Archean arc-forearc region. The ridge subduction model can also explain the origin of the contemporaneous tonalite-trondhjemite-granodiorite (TTG) intrusions in the Isua region. Partial melting of laterally accreted and thickened oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of asthenospheric windows may have produced TTG melts. The diapirically upwelling TTG melts formed the early Archean continental crust and the complementary eclogitic residues contributed to the sub-continental lithospheric mantle.