Melting or Assimilation Processes in Mafic Rocks of Deap-Crust Origins and Generation of Granitic Magma

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Gabbroic to trondhemitic veins form network strctures in amphibolites in the contact anreole of a dioritic pluton. The field relations, texture, petrography, major and trace elements chemistry of the gabbro-quartz diorite-tonalite-trondhjemite veins and the host depleted amphibolites domonstrate that metaluminous calc-alkaline magmas were generated by the partial melting of source amphibolites of high-alkali tholeiite composition. Partial melting of the amphibolites took place under hydrous conditions of over 800ﾟC and 3 kbar, and produced tonalitic and trondhjemitic liquids, leaving the depleted amphibolites consist of hornblende, clinopyroxene, ilmenite, and calcic plagioclase (An_<70-83> as residual minerals. The gabbro and quartz diorite are cummulates fractionated from the tonalitic liquid. The tonalitic liquid differentiated to form more silicic tonalite, but did not come to trondhjemite. Theoretical calculation of REE patterns demonstrates disequilibrium melting process dominated the formation of the tonalitic and trondhjemitic magama. Metaluminous tonalitic to trondhjemitic magmas, characterized by positive Eu anomalies, and low Y and Yb contents, can be produced by the hydrous partial melting of an alkali basalt in the mid-crust (about 10 km), and not necessarily by melting of basalts transformed to eclogite in the deep crust or mantle.