Crustal evolution by arc magmatism

Abstract

Arc magmas generated at depths near 100 km by dehydration of subducting slabs are olivine-rich melabasalts, but the magmas that reach the surface in mature continental magmatic arcs have an average composition near that of rhyodacite. Depth-varying fractionations and equilibrations profoundly modify the initial magmas. Plate tectonics has operated throughout Proterozoic and Phanerozoic time much as it does now, and so many deeply eroded terrains must expose products of arc magmatism. Most exposed middle and deep continental crust consists of igneous rocks plus older rocks equilibrated at magmatic temperatures, variably deformed and metamorphosed subsequently; each crustal level displays a typical assemblage of magmatic rock types, which are here deduced to be products mostly of arc magmatism. Pre-arc mantle consists of depleted dunite and harzburgite. Rising arc magmas precipitate much additional olivine, and add the equivalent of 10 or 15% of basalt in clinopyroxene, orthopyroxene, garnet and spinel. The seismic M. discontinuity may be controlled primarily by the shallow limit of the depth zone in which crystallization is mostly of ultramafic components, and the fractionated magmas that reach the crust are mostly of gabbroic to granodioritic compositions. The lower crust is characterized by differentiated layered complexes, which often contain mafic or ultramafic basal cumulates, medial floated-plagioclase anorthosite, and upper quartz-poor pyroxene—mesoperthite granites, and by other magmatic rocks. Magmatic rocks are more voluminous than pre-existing rocks. Supracrustal rocks are in middle granulite facies, and much granitic material has been melted from them by magmatic heat. The magmatically modified middle crust consists primarily of migmatites in lower granulite facies in the deeper part, and upper amphibolite facies in the shallower part. Much dissociation of hydrous mineral assemblages is caused by the magmatic heat, producing water-rich, aluminous magmas, assimilation and anatexis. The high water contents restrict rise of the equilibrated magmas; voluminous pegmatites are expelled into the wall rocks, and crystallization is forced. Sheets of two-mica granites characterize the upper part of the middle crust. The comparatively dry magmas that rise into the upper crust are mostly tonalite to adamellite. These magmas spread out in steep-sided batholiths above the migmatites, erupt as ash flow sheets from calderas, and produce voluminous far-travelling volcanic ash. Inverted metamorphic gradients and outward-verging structures are produced beneath the spreading batholiths. Magmatic arcs are extensional at all crustal levels.