Geochemical characteristics of igneous rocks as petrogenetic indicators / Геохимические характеристики магматических пород как петрогенетические индикаторы

In Chapter 1 we noted that magmas with distinctive major element characteristics are associated with specific tectonic settings. For example, calc-alkaline series magmas are apparently associated uniquely with subduction, while low-K tholeiitic basalts are the typical products of magma generation at constructive plate margins. However, in general, the major element characteristics of primary mantle derived magmas are * not particularly sensitive indicators of tectonic setting. Thus tholeiitic basalts are generated at mid-oceanic ridges but also in back-arc basins, oceanic islands, island arcs, active continental margins and continental flood basalt provinces. Fortunately, it is now well established that distinctive trace element and Sr—Nd—Pb isotopic signatures are associated withdifferent magma generation environments, although their petrogenetic interpretation in some instances remains ambiguous (Chs. 5—12).
On the basis of the major element chemical variation diagrams discussed in Chapter 1, we established the existence of three dominant magma series, tholeiitic, calc-alkaline and alkaline. Within each of these series there is a continuous spectrum of rock types ranging from basic to acid, which appear to be genetically related. In terms of Figure 1.2 we can consider the existence of a primary magma spectrum, generated by partial melting processes within the upper mantle (Ch. 3), and a range of more differentiated (more Si02-rich) magmas related to the primary magmas by processes of fractional cystallization, magma mixing and crustal contamination (Ch. 4), which often occur in high-level magma chambers. <...>