PUNCTUATED GRANITOID MAGMATISM AS CAUSE AND EFFECT OF PROTEROZOIC TECTONISM IN THE GRAND CANYON, ARIZONA

Granitoids comprise approximately 50% of the volume of middle Proterozoic rock exposed in the Grand Canyon. Three groups of granitoids were emplaced before, during, and after the Yavapai orogeny, during which a juvenile composite island arc terrane (the Yavapai province) was accreted to the stable North American continental margin (the Mojave province). The first wave of felsic plutons (1.74-1.71 Ga) preceded accretion, and consists of large volumes of monzogranitic to granitic magma that interacted with basaltic magma. Magma compositions are consistent with an origin by dehydration melting of amphibolitic lower arc crust or of older arc basement, represented by the 1.84 Ga Elves Chasm pluton, the oldest rock yet dated in Arizona. The second generation of felsic intrusions (1.71-1.66 Ga) are closer in composition to true granites, and occur as dike and pegmatite swarms in 6 kbar host rocks. They were emplaced synchronously with the peak of metamorphism and of NE-SW shortening possibly associated with the accretion of the Yavapai province to the Mojave province. Crustal thickening by thrusting and folding may have resulted in water-poor dehydration melting of fertile island arc greywacke at relatively low pressures (approximately 10 kbar). These crustally derived granites enhanced regional metamorphism and partitioned deformation. Finally, the ~1.4 Ga "anorogenic" granite event is represented by the (1.47-1.42) Ga Quartermaster pluton, which may have been generated as a result of extension of thickened crust, decompression melting of the mantle, and partial melting of the crust by basaltic magma, or by delamination of the base of the thickened crust, followed by mantle upwelling, melting, and crustal melting. The abundance of these rocks may be typical of the dominance of felsic to intermediate magmas at the middle crustal level during collisional orogeny on a global scale, resulting in heat transfer to the middle crust, modification of its rheologic characteristics, and initiation of explosive volcanism.