TRIASSIC PLUTONISM IN SOUTHERN CALIFORNIA - SOUTHWARD YOUNGING OF ARCINITIATION ALONG A TRUNCATED CONTINENTAL-MARGIN

Earliest Cordilleran magmatism in the southwestern United States is re corded by a belt of Triassic plutons that intrude Proterozoic basement of the Mojave crustal province and its cratonal/miogeoclinal cover. T he belt extends from the western Mojave Desert through the Transverse Ranges to the Colorado River trough. Triassic plutons are predominantl y alkali-calcic, Fe and Sr-enriched quartz monzodiorites and monzonite s. The northern part of the belt is composed of two older plutonic sui tes (241-231 Ma) which are high K to shoshonitic; the southern part of the belt is a younger (218-213 Ma), sodic-alkalic suite. The plutonic record in southern California suggests a short-lived, southward young ing continental margin are setting for emplacement of Triassic plutons , superimposed on a continental margin modified by sinistral transform faulting Triassic plutonism in this region was followed by a magmatic lull prior to the onset of voluminous Middle to Late Jurassic Cordill eran arc magmatism.