ISOTOPIC AND GEOCHEMICAL CHARACTERISTICS OF LARAMIDE MAGMATIC SYSTEMSIN ARIZONA AND IMPLICATIONS FOR THE GENESIS OF PORPHYRY COPPER-DEPOSITS

Major element, trace element, and Nd-Sm and Rb-Sr isotope data are des cribed from igneous rocks in 11 multiphase magmatic complexes of Laram ide age in Arizona. Eight complexes contain barren stocks as well as p roductive plutons associated with the formation of porphyry base metal deposits, whereas in three complexes porphyry orebodies have not been recognized. Most of the complexes evolved through similar stages. Ear ly, dominantly intermediate volcanic rocks were intruded by multiphase complexes temporally characterized by increases in SiO2, changes in t he ferrormagnesian mineral assemblage, increasingly porphyritic textur es, and a progression from barren to subproductive to productive intru sions. Within complexes the sequence is independent of both the absolu te age of initiation of local magmatism and its duration. Analytical d ata and field relationships are combined to show elements of the petro genetic history of these complexes, potential relationships between pr oductive and barren magmas, and factors which may have influenced form ation of the characteristic magmatic hydrothermal sequence. The isotop ic composition of Laramide igneous rocks forms an array from epsilon(N dr) and (Sr-87/Sr-66)(initial) values of 0 and 0.704, to -14 and > 0.7 10, respectively. Early volcanic rocks have a more primitive signature consistent with retention of a significant mantle component. Subseque nt intrusions were derived predominantly from isotopically heterogeneo us, Precambrian lower crust but probably also contain an undefined amo unt of mantle component. Individual complexes do not exhibit any consi stent isotopic trends over lime and unique types of source rocks are n ot indicated for productive magmas. Ln contrast, rare earth elements ( REE) in several complexes display temporally systematic behavior. Prog ressively younger intrusions in a given complex follow a path of decre asing concentration of REE, steepening profiles, greater upward concav ity in heavy REE profiles, and changes in the Eu anomaly from negative to either markedly less negative or positive. The REE data from indiv idual complexes suggest an increasing petrogenetic involvement of amph ibole, either in the restite of tile source region or as a fractionati ng phase, and similar paths of petrogenetic evolution in spite of chro nologic and trace element evidence which argues against completely com agmatic development. Productive and barren intrusions cannot be clearl y distinguished by the isotopic data, but the systematic REE trends pa rallel increasing productivity of magmas in individual complexes. Mode ls which reconcile isotopic variation, systematic REE behavior, protra cted local but largely noncomagmatic magmatism and the increasing deve lopment of porphyry mineralization include: (1) progressive metasomati sm of the crustal component, (2) increasing assimilation of amphibolit ic crust by mantle basalts, and (3) migration of crustal anatexis from deep anhydrous rocks to shallower amphibolites. The relative importan ce of these processes cannot yet be clearly assessed from the data, bu t a plausible metallogenic impact of each would be upon the concentrat ion, and possibly the composition, of magmatic volatiles in the primar y melts and/or during subsequent fractionation. In this light, existin g models which propose that the principal control upon intrusion produ ctivity is a balance between crystal fractionation and the timing of s aturation and release of magmatic hydrothermal fluids may be strongly influenced by processes operative either in their source regions or du ring their interaction with the crust. Initiation of such processes ma y be a response to abrupt changes in tectonic regime in the volcanic-a re environment.