Publication

The Archean continental crusts account for ca. 20% of the present volume, but the thermal history of the Earths' mantle suggests much more continental crusts were formed in the early Archean. Because the Archean continental crust underwent severe metamorphism, it is important to avoid influence by the later thermal events. We carried out a comprehensive geochronological work of Cathodoluminescence (CL) observation and U-Pb dating of zircons from orthogneisses and supracrustal rocks over the Saglek Block to obtain their protolith ages. The zircons were classified into three domains of core, mantle and rims, and the cores were further classified into three groups of inherited, altered and zoned cores based on the zonation on the CL images. We estimated the protolith ages from Pb-Pb ages of the zoned-cores of zircons with low U contents. We made a detailed sketch of a small outcrop in St. John's Harbour South (SJHS) area, and classified the orthogneisses and mafic enclaves into seven generations based on the geologic occurrence. The first and second generations comprise mafic rocks and lack magmatic zircons. We conducted CL imaging and U-Pb dating of zircons from the third, sixth and seventh generation of the orthogneisses to estimate the protolith ages at 3902 ± 25, 3892 ± 33 and 3897 ± 33 Ma for each, supporting the presence of the over 3.9 Ga Iqaluk Gneiss. The geological occurrence that the mafic rocks occur as enclaves within the 3.9 Ga Iqaluk Gneiss indicates that they are the oldest supracrustal rocks in the world. Our geochronological and geological studies show the Uivak Gneiss is quite varied in lithology and age from 3.6 to >3.9 Ga, and tentatively classified into six groups based on their ages. The oldest Uivak Gneiss components including the Iqaluk Gneiss are present around the SJHS area, and the orthogneisses become young as it is away. The lines of evidence of overprinting of younger granitoid on older granitoid in small outcrops and geological-map scale as well as presence of inherited zircons even in the oldest suite suggests that crustal reworking played an important role on erasing the ancient crusts.