Isotopic and geochemical characteristics of Laramide igneous rocks in Arizona.

Abstract

Isotopic and trace element data on igneous rocks in nine multiphase magmatic complexes of Laramide age in Arizona place constraints on their petrogenesis and on the factors leading to the formation of porphyry copper deposits. The igneous rocks form a data array from ∊Nd(T) and Srₒ values of 0 and 0.704, to -14 and >0.710, respectively. Isotopic compositions indicate that early, intermediate volcanic rocks retained a mantle component whereas later intrusions were derived predominantly from Precambrian lower crust. The REE display temporally systematic behavior. Progressively younger igneous rocks in a district show a decreasing concentration of REE which is more pronounced for the HREE than for the LREE; they acquire greater upward concavity in their HREE profiles; and the Eu anomaly steadily becomes less negative. An increasing role for hornblende is indicated, either in the residuum of melting or as a fractionating phase. The evolving REE and isotopic behavior parallels the progression from barren, to subproductive, to productive intrusions. The geochemical behavior can be understood in the broader context of magmagenesis at the Laramide convergent margin. Early in the Laramide, the crust was cool and brittle, thereby allowing magmas formed in the mantle wedge as a consequence of volatile loss from the descending slab to ascend to high crustal levels. As the crust warmed the ascent of mantle-derived magmas was arrested in the lower crust where they induced anatexis in Precambrian crust. Three related models can account for the systematic REE behavior, crustal anatexis, and the timing of Laramide metallogenesis: (1) metasomatism of the lower crust, (2) progressively greater assimilation of hydrous crust by mantle-derived melts, and (3) migration of the anatectic zone into more hydrous rocks at higher crustal levels. Each process would allow melting to continue in confined columns of crust as well as provide increasingly volatile-rich magmas that were necessary for melts to evolve fluids capable of forming large porphyry copper deposits. The ultimate ability of a melt to form a porphyry copper deposit may, therefore, depend on characteristics obtained either in its crustal source region or during its passage through the crust.