A TECTONICS TEST OF THE MOST COMMONLY USED GEOCHEMICAL DISCRIMINANT DIAGRAMS AND PATTERNS

During the last two decades many tectonomagmatic geochemical discriminant diagrams were created and are now being widely used in paleotectonic reconstructions. The sources of original data and discriminating quality of fourteen of the most commonly used diagrams are examined herein. Over 200 Jurassic rift basalts, diabases and chilled diabase margins from eastern North America are plotted on these diagrams. The inconsistent results from other regions with known tectonic settings are also discussed. It is found that most diagrams fail to identify continental basalts (CB), the majority of which plot in every field except within-plate basalts (WPB). Because continental basalts do plot in other fields, such as MORB and arc basalts, the discriminating ability of these diagrams is insufficient without additional criteria. Large amounts of chemical data (especially trace- and rare-earth element data) and intensive studies of continental basalts were not available until the late 1980's. Lack of representative data at the time these diagrams were created is a major reason for their failure to identify continental basalts. Moreover, the genesis of continental basalt magma involves a series of complicated processes that may vary from place to place even within the same tectonic province. As a result, the geochemical patterns of continental basalts cover a wide range in compositions of isotopes, trace elements, REE, as well as major and minor elements. It is, therefore, difficult to create a universal chemical diagram that can discriminate continental basalts from all others. Our conclusion is that the reconstruction of paleotectonic settings should not be done by using geochemical data alone. Although this may seem obvious to many, perusal of the literature shows that tectonic setting assignments frequently are based almost entirely on geochemistry. Tectono-stratigraphic analysis based on knowledge of field relations, structure and petrology of epiclastic, volcaniclastic and igneous protoliths is an essential adjunct to geochemistry in determining ancient tectonic environments.