GEOCHEMICAL EVIDENCE FOR CRUSTAL MELTING IN THE ORIGIN OF THE IGNEOUS SUITE AT THE SIERRITA PORPHYRY COPPER DEPOSIT, SOUTHEASTERN ARIZONA (UNITED STATES, STRONTIUM, PIMA COUNTY, TRACE ELEMENTS, NEODYMIUM).

Abstract

Numerical values for parameters which characterize melting regimes and differentiation history have been determined for a suite of genetically-related calc-alkaline rocks. Isotopic ratios of Nd and Sr vary sympathetically, with the least differentiated and oldest rocks having ε(Nd) = -4.3 and ⁸⁷/Sr⁸⁶Srₒ = 0.7069 and the most differentiated and youngest characterized by ε(Nd) = -8.5 and ⁸⁷Sr/⁸⁷/Srₒ = 0.7092. These observations imply that a process which might relate the various units is that of invasion of the crust by mantle-derived magmas and progressive assimilation. The most negative Nd values may well represent the isotopic signature of the crustal rocks which melted. Melting of a 1.8 Ga source region (an age characteristic of the basement in southeastern Arizona) which had a ¹⁴⁷Sm/¹⁴⁴Nd ratio of .13 would yield Laramide melts with and ε(Nd) of -8.5. This Sm/Nd ratio is characteristic of a somewhat mafic (lower?) crust, a character consistent with petrological and chemical evidence which suggest that the source was intermediate to mafic in composition and of amphibolite grade. Solution of the isotopic data yields a value of r (that instantaneous ratio of assimilated material to crystallizing magma) equal to 0.6 to 0.9. These values are reasonable considering heat budgets of crystallization and fusion in the lower crust. Solution of the set of equations for changes in concentration of the trace elements yields numeric values for the f, fraction of remaining melt for each unit in the series. The values are: andesite, f-0.63; granodiorite, f-0.42 to 0.32; and the final granite stocks and dikes, f-0.34. The modeling provides insight into the way an igneous suite intimately associated with copper mineralization has evolved. The porphyry ore bodies are related to long-lived and large magma systems. At the level of mineralization and observation, we sample only a small portion of the system. The importance of subduction to metallogenesis may be that it provides a heat source, in the form of mantle-derived magma, which allows extensive melting of hydrous crust. Thus, as is becoming evident from other studies as well, assimilation and crustal anatexis are major processes in generating granitoid rocks at convergent plate boundaries.