WHERE DID THAT GRANITOID COME FROM? FOR THE SW USA TRY CHECKING ITS PB ISOTOPIC COMPOSITION

The pioneering Pb isotopic studies of B. Doe, J. Stacey and R. Zartman in the western U.S. during the 1960-70s established a framework for understanding the Pb isotopic evolution of the crust in this region. Additional Pb isotopic studies in the 1980-90s on Proterozoic rocks identified three Pb isotopic crustal provinces in the SW USA: Mojave, central and southern Arizona (AZ) which come together in a complex boundary zone in western AZ. The Proterozoic of the Mojave crustal province is particularly distinctive with highly variable and unusually high Th/U and the common occurrence of low long term U/Pb. The Pb isotopic compositions of the Mesozoic intrusive rocks of the Mojave, regardless of age, can be described as varying averages of those of the Proterozoic basement and define a coherent array at high ²⁰⁸Pb/²⁰⁴Pb within the field of the present day whole rock values. The Mesozoic data also define linear arrays on ²⁰⁶Pb/²⁰⁴Pb vs. ²⁰⁷Pb/²⁰⁴Pb plots with model ages ranging from 1.7-1.4 Ga and indicate sources with both low and normal long term U/Pb. Mesozoic and early Tertiary intrusives in AZ also reflect the long term average Pb isotopic compositions of those Proterozoic crustal provinces. These craton dominated Pb isotopic compositions are in strong contrast to those of the Mesozoic batholiths along the west coast of the U.S. Pb isotopic data from the continental margin batholiths define tight arrays on both ²⁰⁶Pb/²⁰⁴Pb vs. ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb plots with ²⁰⁶Pb/²⁰⁴Pb ranging from 18.7-19.6. These arrays indicate mixing lines between averaged oceanic and continental end members. Separation of the continental margin batholithic from cratonic data is particularly good for ²⁰⁶Pb/²⁰⁴Pb vs. ²⁰⁸Pb/²⁰⁴Pb with about 90% of the cratonic data having no overlap with the marginal batholithic data. Some specific sample groups, based on age, petrology and/or location, define distinctive areas on the Pb isotopic plots. The distinctive long term pre-Mesozoic U-Th-Pb isotopic evolution of the crust in the SW USA combined with the repeated subduction-related Mesozoic magmatism that swept back and forth across the craton and its hybridized margin created Mesozoic magmatic environments that are well defined by Pb isotopic compositions.