PRECAMBRIAN GREENSTONE SEQUENCES REPRESENT DIFFERENT OPHIOLITE TYPES

We present a global geochemical dataset from one hundred-and-five greenstone sequences, ranging in age from the Eoarchean through the Archean and Proterozoic Aeons that we have examined to identify different ophiolite types (c.f. Dilek and Furnes, 2011) with distinct tectonic origins in the Precambrian rock record. We apply well-established discrimination systematics (built on immobile elements) of basaltic components of the greenstone sequences as our geochemical proxies. The basaltic rocks are classified under two major groups, subduction-related and subduction-unrelated. This analysis suggests that ca. 85% of the greenstone sequences can be classified as subduction-related ophiolites, generated in backarc to forearc tectonic environments. The chemical imprint of subduction processes on different greenstone sequences is highly variable, but particularly strong for the Archean occurrences, such as the 3.8 Ga Isua (Greenland) and the 3.8-4.3 Ga Nuvvuagittuq (Canada) greenstone belts. Subduction-unrelated greenstone sequences appear to have developed in all phases of ocean basin evolution, through continental rifting, rift-drift tectonics, seafloor spreading, and/or plume magmatism. For the time interval of ca. 3500 million years in the record of Precambrian greenstone evolution, a secular geochemical signature emerges from the oldest to the youngest, in which there is a gradual increase and decrease in the concentrations of incompatible (e.g. Zr) and compatible (e.g. Ni) elements, respectively. The compiled Precambrian greenstone data and our interpretations are consistent with the existence of interactive mantle-lithosphere dynamics, and plate-tectonic-like processes extending back to the Hadean-Archean transition.