THE MOUNT PERKINS BLOCK, NORTHWESTERN ARIZONA - AN EXPOSED CROSS-SECTION OF AN EVOLVING, PREEXTENSIONAL TO SYNEXTENSIONAL MAGMATIC SYSTEM

The steeply tilted Mount Perkins block, northwestern Arizona, exposes a cross section of a magmatic system that evolved through the onset of regional extension. New Ar-40/Ar-39 ages of variably tilted (0-90 deg rees) volcanic strata bracket extension between 15.7 and 11.3 Ma. Pree xtensional intrusive activity included emplacement of a composite Mioc ene laccolith and stock, trachydacite dome complex, and east striking rhyolite dikes. Related volcanic activity produced an similar to 18-16 Ma stratovolcano, cored by trachydacite domes and flanked by trachyda cite-trachyandesite flows, and similar to 16 Ma rhyolite flows. Simila r compositions indicate a genetic link between the stratovolcano and g ranodioritic phase of the laccolith. Magmatic activity synchronous wit h early regional extension (15.7-14.5 Ma) generated a thick, felsic vo lcanic sequence, a swarm of northerly striking subvertical rhyolite di kes, and rhyolite domes. Field relations and compositions indicate tha t the dike swarm and felsic volcanic sequence are cogenetic. Modes of magma emplacement changed during the onset of extension from subhorizo ntal sheets, east striking dikes, and stocks to northerly striking, su bvertical dike swarms, as the regional stress field shifted from nearl y isotropic to decidedly anisotropic with an east-west trending, horiz ontal least principal stress. Preextensional trachydacitic and preexte nsional to synextensional rhyolitic magmas were part of an evolving sy stem, which involved the pending of mantle-derived basaltic magmas and ensuing crustal melting and assimilation at progressively shallower l evels. Major extension halted this system by generating abundant pathw ays to the surface (fractures), which flushed out preexisting crustal melts and hybrid magmas. Remaining silicic melts were quenched by rapi d, upper crustal cooling induced by tectonic denudation. These process es facilitated eruption of mafic magmas. Accordingly, silicic magmatis m at Mount Perkins ended abruptly during peak extension similar to 14. 5 Ma and gave way to mafic magmatism, which continued until extension ceased.