RECORDING TECTONIC PROCESSES IN THE ARCHEAN BY DATING DETRITAL ZIRCONS IN LATE ARCHEAN SEDIMENTARY ROCKS: AN EXAMPLE FROM THE ISLAND LAKE GREENSTONE BELT, SUPERIOR PROVINCE, MANITOBA, CANADA

Recent advances in ‘in-situ' U-Pb dating techniques allow geologists to obtain large and robust data sets of the ages of detrital zircons in sedimentary rocks of contrasting ages and origins. We employed a technique involving laser ablation coupled to a multi-collector ICP-MS that permitted rapid dating of ~400 detrital zircons. The main objectives are to investigate the distribution of ages within the Island Lake group, a “Timiskaming type” sedimentary group located in the Island Lake greenstone belt, and to determine what tectonic processes were responsible for opening the basin in which these sediments were deposited. “Timiskaming type” groups are clastic sedimentary sequences that are characteristic of many Archean greenstone belts. They are the stratigraphically youngest supracrustal group and are spatially associated with late shear zones. Traditionally these groups are interpreted to have been deposited in strike-slip basins opened by horizontal plate tectonic processes. More recent studies have suggested that these sediments were deposited in inter-diapiric basins formed by vertical tectonic processes. Understanding the formation and development of these basins is critical in deciphering the type of tectonic regimes that operated during the Archean.

The ²⁰⁷Pb/²⁰⁶Pb ages of detrital zircons in the Island Lake group match known ages of volcanism and plutonism in the belt. Near the base of the sequence there is a strong influence of ages between 2.95 Ga - 2.80 Ga, which correlates to ages of volcanism in the belt. At the top of the sequence there is a peak of ages at 2.75 Ga - 2.72 Ga, which correlates with the ages of plutonic events in the belt. This data is consistent with a model that involves erosion down through a supracrustal pile in the early stage of basin formation and sedimentation followed by unroofing of plutons in the latter stages. The model presented here involves sagduction and diapirism processes that produce an inter-diapiric basin as a result of vertical tectonics.