Magma Sources in Cordilleran Settings

Abstract

The three distinct magma sources in tectonic regions of convergent plate boundaries with associated cordillera are: (1) subducted oceanic crust of basalt (modified by sea-water) and serpentinite, with an uncertain quantity of sediments, (2) mantle peridotite, and (3) continental crust (heterogeneous, dominantly tonalitic gneiss). The compositions of melts derived from each of these sources by partial melting at various depths can be determined. The effects of fractionation during uprise on the liquid compositions can also be estimated if the phase relationships are known from magma source to emplacement or eruption site. Experimental data provide constraints for testing petrological and geophysical models. The degree of constraint and power of discrimination among competing hypotheses are increased by consideration of geochemical data in conjunction with experimental data. Our knowledge of the geophysics and thermal structure remains uncertain. Physical aspects, such as the behavior of partially molten rocks, control magma compositions. Dehydration of subducted oceanic crust provides aqueous fluids for metasomatism or partial melting of oceanic crust or overlying mantle. For warm crust, major melting of amphibolite may yield progenitors of andesite magmas. Otherwise, partial melting of oceanic crust yields hydrous siliceous magmas that leak into mantle and react with it. Partial melting of mantle peridotite modified by fluids or magmas yields H_2O-under saturated basic magma. Phlogopite-peridotite produced by reaction with magma could yield alkalic magmas when transported to deeper levels. Andesitic magmas can be generated from mantle peridotite only under restricted conditions probably not attained in cordilleran settings. The normal product of partial melting of the continental crust is H_2O-undersaturated rhyolitic melt. Andesitic magmas could be produced from continental crust only by extreme heating. Underplating by basalts gives the prospect of such heating, and also for the mixing of basalt and rhyolite magmas. The batholiths and eruptive rocks of cordillera are products of complex, multi-stage processes.

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