Melting of the continental crust: fluid regimes, melting reactions, and source-rock fertility

Clemens, J.D. (2006) Melting of the continental crust: fluid regimes, melting reactions, and source-rock fertility. In: Brown, M. and Rushmer, T., (eds.) Evolution and differentiation of the continental crust. Cambridge, UK : Cambridge University Press. pp. 297-331. ISBN 0521782376


This chapter presents a synthesis of partial melting in Earth’s continental crust. However, rather than simply attempting to synthesize all the data from previous experimental and theoretical studies, the chapter takes a definite scientific position that reflects present understanding of crustal melting. Unless otherwise stated, the term "granitic" is used in the broad sense to denote magma or melt compositions ranging from syenogranite through to tonalite. The word "fertility" is used to denote the relative capacity of a given protolith material to produce granitic partial melt, under the specified physicochemical conditions (mainly P, T, and fO2). The high temperatures that are required to partially melt crustal rocks and form granitic magmas equate with the conditions of upper amphibolite-to granulite-facies metamorphism. This is one important reason for the inferred intimate connection between the production of granulite-facies rocks, the production and withdrawal of partial melts, and the differentiation of the continental crust (see, for example, Fyfe, 1973; Clemens, 1990; Thompson, 1990). Partial melting may occur in response to various intracrustal processes that occur during tectonic thickening and orogenic collapse of the crust (e.g., Patiño Douce et al., 1990; Harris & Massey, 1994). However, the thermal requirements of granulite-facies processes generally demand that additional, extra-crustal heat sources be available to drive the reactions. Thermal modeling has demonstrated that thickened crust (with a normal thermal profile) does not reach the temperatures necessary to partially melt (on time scales of up to 100 Myr) unless large amounts of aqueous fluid are also introduced to depths of 20–40km (e.g., England & Thompson, 1984). From the analysis of England and Thompson (1984) it appears that the only exceptions to this general rule would occur where the crust has unusually low thermal conductivity (< 2 W/m/K) combined with very high surface heat flow (> 65 mW/m2). Thus, except in some migmatite terranes, production of voluminous, mobile, granitic magma commonly involves advection of mantle heat to the continental crust. The most likely vectors of this heat would be underplated or intraplated mafic magmas. Such under-accretion probably represents the major means by which Earth’s continents have grown in volume since the Archean (e.g., Rudnick, 1990). The only alternative is to postulate that the crust was unusually enriched in heat-producing elements (e.g., Sandiford et al., 1998; McLaren et al., 1999; Chapter 11). This does seem to be the case in a few specific terranes, but is unlikely to be a general feature. In any case, the withdrawal of large volumes of granitic magmas from the deep crust, and their emplacement at higher crustal levels, has two major consequences. The first consequence is that the deep crust will be left in a mafitized, partially dehydrated, residual condition (e.g., Brown & Fyfe, 1970; Fyfe, 1973; Clemens, 1990; Thompson, 1990). To become exposed at the surface, such dense rocks would need to undergo a second major tectonic episode, perhaps temporally unrelated to the original partial-melting event, (e.g., the residual metapelites ("stronalites") of the Ivrea Zone). This necessity may contribute to the relative scarcity of such rocks, in comparison with the abundance of their mobile magmatic counterparts. Delamination and foundering into the mantle may also be the fate of some residual lower crust (e.g., Bohlen, 1991). The second consequence is that the upper crust will become enriched in felsic minerals and heat-producing elements. This is probably the major mechanism for post-Archean and ongoing, large-scale crustal differentiation (Vielzeuf et al., 1990).

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