Insights on crustal growth from detrital zircons in ancient glacial deposits

Abstract:

Continental ice sheets can erode vast areas and carry unsorted sediment great distances before being deposited en masse. The resulting glacial diamictites can provide important information on the integrated geologic history of continents. Detrital mineral analysis, such as U-Pb geochronological and Hf isotope geochemical characterization of zircon, is one way of interrogating this sedimentary archive, and here we apply this approach to glacial diamictites of Mesoarchean and Neoproterozoic age in southern Africa and Paleoproterozoic age in North America.

Several major magmatic events between 3.3 and 2.9 Ga, most notably at 3.08 Ga are recorded in detrital zircons from several of the Mesoarchean diamictites. Hf isotopes in these zircons are almost exclusively superchondritic, indicating that these are major juvenile crust forming events. In contrast, Neoproterozoic Namibian diamictites are almost devoid of Archean zircons and record a major 1.9 Ga population with subchondritic epsilon Hf, consistent with the reworking of Neoarchean (2.5-2.7 Ga) crust. Paleoproterozoic diamictites from North America (Ontario and Wyoming) are all dominated by a common age peak at 2.7 Ga with a narrow superchondritic Hf isotopic range, consistent with a major crustal growth event at this time in North America.

Major globally recognized crust forming events that occurred at 2.7 Ga and orogenic events that occurred at 1.9 Ga are recorded by these diamictites. Lack of pre-Mesoarchean U-Pb ages and the superchondritic epsilon Hf of the Neoarchean and Mesoarchean detrital zircons are suggestive of minimal emergent continental crust prior to ~3.6 Ga or reflect significant preservation and/or recycling issues for the early Earth that are not well understood.