EMPLACEMENT OF A LARGE IGNEOUS PROVINCE AS A POSSIBLE CAUSE OF BANDEDIRON-FORMATION 2.45 BILLION YEARS AGO

LATEST Archaean and earliest Palaeoproterozoic times (from 2.6 to 2.2- billion years ago) have generally been viewed as a largely quiescent p eriod of Earth history; the geological record indicates the very slow deposition of pelagic and chemical sediments(1,2), and bears only a li mited record of magmatic and tectonic activity(3-5). Such quiescence i s consistent with the contention that the Earth's main banded iron for mations (BIFs)-finely laminated chemical sedimentary rocks, rich in ir on oxide-formed slowly as oxygen abundances in the oceans gradually in creased, thus reducing the capacity of sea water to retain dissolved i ron(6-10). Here we show that a large igneous province, comprising >30, 000 km(3) of dolerite, basalt and rhyolite, accompanied deposition of a Hamersley Province BIF 2,449 +/- 3 million years ago. This observati on indicates that Hamersley BIFs formed during a major tectono-magmati c event and were deposited very much faster than previously thought, a t similar rates to (or faster than) modern pelagic sediments. Thus the largest Palaeoproterozoic BIFs, rather than simply reflecting a gradu al increase in the oxygen content of the oceans during a period of tec tonic quiescence, are more likely to have formed as a result of an inc reased supply of suboxic iron- and silica rich sea water upwelling ont o continental shelves during a pulse (or pulses) of increased submarin e magmatic and hydrothermal activity.