Chen and Grapes propose that granites form in situ through the melting of mid-upper crustal rocks to produce a convecting magma layer. This is in contrast to the more widely accepted intrusion model where fusion occurs in the lower to midcrust (with or without the involvement of mantle material), followed by emplacement and crystallization (and local assimilation) in the mid- to upper crust. In the proposed in situ model, the mid-upper crust becomes a closed system requiring only heat input mainly supplied through plate convergence. The implication is that granites represent no net addition of material to the crust, but...

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