Abstract

Trondhjemitic and tonalitic liquids may form either by igneous differentiation of less silicic, more mafic liquids or by partial melting of rocks of basaltic composition. Low-Al2 O3 trondhjemitic-tonalitic liquids (defined as containing less than 15 percent Al2 O3) have formed in modern plate-tectonic environments by crystal fractionation of low-potassium ande-sitic liquid and in Precambrian environments by the partial melting of amphibolite and hornblende-bearing gabbro, in which process plagioclase is a residual phase and garnet and (or) hornblende are not. High-Al2 O3 trondhjemitic-tonalitic liquids (containing 15 percent or more of Al2 O3) are generated in both old and modern convergent and tensional tectonic environments, either by hornblende-controlled fractionation of hydrous basaltic liquid or by partial melting of metabasaltic rocks, in which process garnet and (or) hornblende are residual.

A model for the origin of the andesite-free bimodal trondhjemite-basalt suites that are found in lower Archean gray gneiss complexes is based on a 1968 model of Green and Ringwood; it proposes (1) mantle upwelling and basaltic vol-canism to form a thick pile, (2) metamorphism of the lower parts of this pile to amphibolite, (3) partial melting of the amphibolite to yield trondhjemitic-tonalitic liquids, (4) ascent and extrusion or intrusion of these liquids into the upper crust before the fraction of melting of the parental amphibolite exceeds about 40 percent, (5) transformation of the residue of partial melting to anhydrous, refractory assemblages, and (6) continuation of mantle upwelling and basaltic volcanism as trondhjemitic-tonalitic liquids are being extruded.

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