Protoliths of the 3.8-3.7 Ga Isua greenstone belt, West Greenland

The Isua greenstone belt (Fig. 1) contains the oldest known, relatively wel l preserved, metavolcanic and metasedimentary rocks on Earth. The rocks are all deformed and many were substantially altered by metasomatism, but both the deformation and metasomatism were heterogeneous. Transitional stages c an be seen from relatively well preserved primary volcanic and sedimentary structures to schists in which all primary features have been obliterated. Likewise different kinds, and different episodes, of metasomatic alteration can be seen that produced a diversity of different compositions and metamo rphic mineral assemblages from similar protoliths. New geological mapping h as traced out gradations between the best preserved protoliths and their di verse deformed and metasomatised equivalents. By this means, the primary na ture of the schists that make up most of the Isua greenstone belt was reint erpreted, and a new map that better portrays the primary nature of the rock s has been produced. The previously mapped stratigraphy was found to be of little value in understanding the geology. Stratigraphic units were defined by different and diverse criteria, such as current composition. structure, metamorphic texture, and inferred protoliths. Much of this stratigraphy re presents a misinterpretation of the primary nature of the rocks. The new wo rk indicates that most of the Isua greenstone belt consists of fault-bounde d rock packages, mainly derived from basaltic and high-Mg basaltic pillow l ava and pillow lava breccia, chert-BIF, and a minor component of elastic se dimentary rocks derived from chert and basaltic volcanic rocks. A previousl y mapped, extensive, unit of felsic volcanic rocks was found to be derived from metasomatised basaltic pillow lava and pillow breccia intruded by nume rous sheets of tonalite. (C) 2001 Elsevier Science B.V. All rights reserved .