Nature and origin of Proterozoic A-type granitic magmatism in the southwestern United States of America

The mountain ranges of Arizona and adjacent California and Nevada contain large areas underlain by Proterozoic anorogenic granites comprising the southwesternmost portion of a transcontinental belt of 1.4–1.5-Ga-old anorogenic complexes that extends across North America northeast into Labrador. Of these, a two-mica, monazite-bearing granitic suite resides in central and southeastern Arizona as part of a peraluminous subprovince that is bordered on the south (southern Arizona to Sonora) and west (western Arizona and adjacent portions of California and Nevada) by marginally metaluminous granites bearing biotite-sphene ± hornblende and fluorite. All of these 1.4-Ga granites are distinctly more potassic, iron-enriched (relative to Mg), and depleted in Ca, Mg and Sr in contrast to typical orogenic granitoids. In general, the large-ion lithophile-element enriched composition is a consequence of limited melting of a water-deficient crustal source at depths greater than 25–37 km. For the peraluminous granites, this contrast is less extreme, perhaps resulting from a larger degree of melting as a consequence of a greater metasedimentary component and water in its crustal source. The anorogenic granitic magmas intruded into the upper crust at depths of 8–17 km or shallower at temperatures up to 790°C. The most dramatic variation in the crystallization-intensive parameters resides in the oxygen fugacity, which spans three orders of magnitude. Relative to other anorogenic suites, all of the magmas crystallized at elevated levels of ƒO2 as reflected in their assignment to the anorogenic magnetite series. Yet a regionally significant rise in primary ƒO2 levels, unmatched elsewhere in the transcontinental belt, occurs for plutons in western Arizona, including the Holy Moses and Hualapai granites. The most extreme case is the Hualapai granite whose biotite Fe(Fe + Mg) ratios drop (due to high ƒO2) to a low of 0.27, down from more typical levels of 0.54 to 0.75. Such extreme variations in primary levels of oxygen fugacity must be an indirect imprint of regional changes of the level of oxidation of the lower crust. The high-fO2 Holy Moses and Hualapai plutons have intruded near the regional boundary between the metaluminous and peraluminous granites and appear to be imaging a major change in the level of oxidation of the lower crust. This boundary is also approximately equivalent to significant changes in the Nd and Pb isotopic compositions of these granites and the metamorphic and magmatic character of the older orogenic terrane. On a global scale, the crust-forming orogenies ended by 1.6 Ga ago and the continents entered a long-lived era dominated by localized extension and transcontinental intrusion of anorogenic potassic rapakivi granite, mafic dike swarms, charnockite and anorthosite. The absence of orogenic deformation implies that plate consumption became intraoceanic during this time. The profuse and widespread nature of the igneous activity has no Phanerozoic analogue and is considered to be unique to the Proterozoic. A crustal overturn model ties the magmatism to heating within a largely undepleted subcontinental mantle, the eventual rise of mantle plumes, and the transfer of heat into the youthful, undifferentiated Proterozoic crust. Subsequent melting and rise of potassic granitic magmas from the lower crust leads to considerable crustal reorganization, a process that would continue until both the mantle and crust reached a stable configuration.