CRUST-MANTLE INTERACTION IN CONTINENTAL ARCS - INFERENCES FROM THE MEXOZOIC ARC IN THE SOUTHWESTERN UNITED-STATES

Arc magmas ranging in composition from basaltic andesites to rhyolites and intrusive equivalents were emplaced into the western margin of the North American craton starting in Late Triassic time giving way to rift-related sedimentation in the Late Jurassic. The region of this study cuts across Proterozoic basements of contrasting Nd model ages, 1.7-1.8 Ga (average E(Nd) approximately -11) in eastern Arizona and 2.0 to 2.3 Ga (average E(Nd) approximately -18) in western Arizona and eastern California (Bennett and DePaolo 1987). The Mesozoic rocks have initial E(Nd) of -3.4 to -6.4 in the eastern part of the study area and -7.1 to -9.2 in the western part. All of the rocks have elevated Sr-87/Sr-87 initial ratios (> 0.706). Trends in initial E(Nd) values of Mesozoic arc rocks are directly correlated with the Nd model ages of the basement through which they passed. Simple two-component mixing calculations indicate that recycled continental crust in the arc magmas represents on average about 65%. A minimum of 35% mantle input into continental arc magmas, as recent as the Mesozoic, represents a significant contribution to the growth of the continental crust, in the absence of a return flow of continental material into the mantle of similar magnitude. In a detailed study in the Santa Rita Mountains, Arizona, there is a pattern of increase of E(Nd) with time: early basaltic andesites have more negative E(Nd) than later felsic rocks. A correlated pattern of depletion with time is also observed with trace element and major element data. We attribute this either to progressive hybridization of the lower crust by repeated injection of mantle magmas, or the progressive thinning of the continental crust during prolonged arc magmatism. The present data do not allow distinction between the two models. Progressive decrease in crustal contribution to arc magmas with time may be an important feature of continental arc evolution. Hybridization of the lower crust due to repeated injection of mantle melts during arc magmatism may help contribute to small-scale heterogeneities in lower crust inferred from seismic and xenolith data. Similarly, whether there is a well defined MOHO or sharp crust-mantle boundary in any given segment of the continental crust may in part depend on the extent of crust modification as a result of continental arc magmatism.