Figure 7.

Schematic sketch of (a) Lower crustal drips with partial melting of submerged crust, giving rise to TTG. Drips occur randomly and are driven by convection of the underlying mantle; mantle plumes erode the lithosphere and inject layered intrusions and komatiitic melts. Older TTG form at shallower levels in the presence of plag, younger TTG in the presence of crustal grt. (b) Transitional TTG will form within drips as blends of melts derived from existing TTG and meta-basalt. Asymmetric drip or downwelling are stationary drips that develop over prolonged periods of time at the interface between thicker and thinner crustal planes. Through intra-crustal flow, asymmetric drips can develop through differences in crustal stiffness. The tilt will force rising TTG melt to pass through the overlying mantle, forming sanukitoids and predominantly accumulate at the base of the crust. Excess heat introduced through this underplating gives rise to partial melting of existing TTG to form ACP granites. Thicker plate regions represent proto-cratons, thinner plate regions proto-oceanic crust. At their interface, subduction eventually starts. Within stages, no temporal evolution is given. (Online version in colour.)