Triassic plutonism in southern California: Southward younging of arc initiation along a truncated continental margin

Earliest Cordilleran magmatism in the southwestern United States is recorded by a belt of Triassic plutons that intrude Proterozoic basement of the Mojave crustal province and its cratonal/miogeoclinal cover. The belt extends from the western Mojave Desert through the Transverse Ranges to the Colorado River trough. Triassic plutons are predominantly alkali‐calcic, Fe‐ and Sr‐enriched quartz monzodiorites and monzonites. The northern part of the belt is composed of two older plutonic suites (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 younging continental margin arc 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 Cordilleran arc magmatism.