Cratonic crust, composition

Of all the estimates in Table 9, that of Taylor and McLennan (1985, 1995) stands out as being the most mahc overall (Figures 15 and 16). This mahc composition stems from their model for Archean crust, which constimtes 75% of their crust and is composed of a 2 1 mixture of mafic-to felsic-igneous rocks. This relatively mafic crust composition was necessitated by their inferred low heat production in Archean crust and the inferred large proportion of the Archean-aged crust. However, such a high proportion of mafic rocks in the Archean crust is at odds with seismic data (summarized in Section 3.01.3), which show that the crust of most Archean cratons is dominated by low velocities, implying the presence of felsic (not mafic) compositions, even in the lower crust. In addition, some of the... 

In contrast, deep crustal rocks found in metamorphic belts, granulites, are much more varied in composition and include both felsic and mafic varieties (Rollinson St Blenkinsop, 1995). This is consistent with a recent study in the North China Craton which showed that there the lower crust can be divided into two layers - an upper lower crust with velocities between 6.7 and 6.8 km/sec and a lowermost crust, with velocities between 7.0 and... 

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