U-Pb Zircon Geochronology Using LA-ICP-MS in the North Pole Dome, Pilbara Craton, Western Australia: A New Tectonic Growth Model for the Archean Chert/Greenstone Succession

Abstract

In situ, spot U-Pb ages of zircons from rhyolite, tuffaceous chert, spherule-bearing chert, and granite in the North Pole Dome obtained by laser ablation ICP-MS (LA-ICP-MS) combined with cathode luminescence imaging (CLI) reveal zircon growth patterns that reflect their igneous origins. Felsic lava forming the structural top of the succession, previously dated as 3458 + 9.1/-4.2 Ma by TIMS, has been dated as 3660 ± 52 Ma. These 3.6 Ga zircons from felsic lava, initially considered to be xenocrysts, are interpreted as euhedral igneous crystals as their CL images show no overgrowth rims and/or evidence of melting. Progressive downward in the section, zircons from tuffaceous cherts yield ages of 3463 ± 34 Ma and 3454 ± 46 Ma. This Archean volcaniclastic sequence was intruded by the North Pole Monzogranite at 3391 ± 33 Ma. Our reconstructed stratigraphy shows that the chert-greenstone secession, previously thought to be a single intact stratigraphic unit, actually consists of five units; most are typified by a base of mafic-ultramafic lavas and a capping chert sequence. The zircon ages demonstrate that these five units decrease in age progressively downward; thus the North Pole Dome chert/greenstone succession was formed by layer-parallel thrusting and horizontal shortening, and grew downward by imbricate stacking.