PROLONGED HISTORY OF SILICIC PERALKALINE VOLCANISM IN THE EASTERN PACIFIC-OCEAN

Socorro Island, Mexico, is an alkaline and peralkaline volcanic island located in the eastern Pacific Ocean on a mid-ocean ridge spreading c enter that was abandoned at similar to 3.5 Ma. Silicic peralkaline roc ks comprise up to 80% of the surface of the island, rendering Socorro virtually unique in the Pacific Ocean. Precise, replicate Ar-40/Ar-39 ages of 21 peralkaline trachytes and rhyolites reveal a history of epi sodic volcanic activity from similar to 540 to 370 ka that may have cu lminated with caldera formation; repose periods between these episodes may have had maximum duration of similar to 30 kyr. After up to 200 k yr of quiescence, Ar-40/(39)A. ages indicate that postcaldera silicic peralkaline activity commenced by 180 ka, forming the Cerro Evermann F ormation. Postcaldera mafic alkaline lavas of the Lomas Coloradas Form ation erupted dominantly between 70 and 150 ka based upon relative age relations. The dominant lithology of precaldera and syncaldera silici c peralkaline deposits on Socorro is nonfragmental and nonvesicular an d lacks lithic fragments and fiamme; despite this, numerous lines of e vidence including welding zonation, presence of a proximal ignimbrite or co-ignimbrite deposit, association with a caldera, and compositiona l heterogeneity within eruptive units suggest that they are dominantly ash flow tuffs. A change in eruptive style, from predominantly explos ive to predominantly effusive, followed caldera formation and suggests that a change in the efficacy of magma degassing may be linked to cal dera formation. On the basis of the presence of a caldera, the magma c hamber associated with Socorro Island is shallow and probably resides within the upper oceanic crust or the edifice. This together with a pr olonged history of silicic magmatism indicates that intrusion of mafic magma maintained thermal viability of the magmatic plumbing system. T he minimum calculated growth rate for the entire volcanic edifice (7x1 0(-4) km(3)/yr) exceeds those of nonhotspot off-axis volcanoes in the Pacific by almost an order of magnitude. Eruption rates for subaerial phases on Socorro may be several orders of magnitude smaller than this growth rate and are comparable to subaerial eruption rates of isolate d ocean islands related to mantle plumes.