It seems, the lower part of the crust predominantly consisted of juvenile mafic lithologies, which we argue resulted from intense basaltic magma underplating during Cambrian-Neoproterozoic, i.e. heat provided by advecting mantle-derived magmas underplating the crust or injected into the crust. It is interpreted as a hot zone, laterally extensive region of partial melting, at or near the Moho, provided intermediate-silicic magma bodies (e.g. Chmielowski et al. 1999). In this way the hot zone can be a major cause of geochemical variation in arc magmas, from acidic at low melt fraction, through acidic to intermediate at melt fractions of >0.35 (Annen et al. 2006). This implies that the ancient mafic lower continental crust of the UDMA might have been replaced by juvenile, mantle-derived material during northward subduction of the proto-Tethyan oceanic crust that provides new components to the continental crust.
