LEON T. SILVER’S LEGACY OF U-PB-TH ZIRCON GEOCHRONOLOGY IN THE MAZATZAL PROVINCE OF SOUTHWEST ARIZONA

We present here for the first-time unpublished U-Pb-Th ID-TIMS zircon ages acquired by Lee Silver between 1966 and 1982 from Proterozoic basement of southwest Arizona. Multiple samples were collected via helicopter with USGS colleagues from remote localities of the Barry M. Goldwater bombing range and nearby areas. An extensive data archive at Caltech became available to Lee’s students following his passing on January 31, 2022. Paul Azimow graciously facilitated access to a collection of hand specimens, thin sections, zircon separates, and summary tables of U-Pb-Th analyses personally reduced by Lee. Most Proterozoic results reported below are upper intercept ages based on analyses of one or two large (5 to 50 mg) multi-grain zircon fractions.

Thirteen samples from the Mohawk Mountains, South Granite Mtns, Agua Dulce Mtns, John the Baptist Mtns, Ajo Mining district, and Cipriano Hills yielded ages between 1686 and 1582 Ma, expanding the coverage of the Mazatzal crustal province. Megacrystic granites from the Muggins Mtns and Yuma were found to have ²⁰⁷Pb/²⁰⁶Pb ages of 1426 and 1420 Ma, whereas granites from the Plomosa Mtns, Copper Mtns, Granite Mtns, and Sierra Pinta produced discordia lines with Late Cretaceous lower intercept ages and upper intercepts indicating inherited components of 1741, 1699, 1676, and 1632 Ma, respectively.

Single-crystal U-Pb-Th analyses of Silver’s leftover zircon separates performed on the SHRIMP-RG (Stanford) and the LA-ICP-MS (CSUN) show good agreement with the original age assessments. Rhyodacite porphyry from John the Baptiste Mtns and tonalite gneiss from South Mohawk Mtns are 1659±17 Ma and 1621±10 Ma, respectively. Two-mica granite from Sierra Pinta preserves 66.3±1.1 Ma zircon overgrowths on 1633±38 Ma cores. Eight additional samples never analyzed by Silver produced results as follows: John the Baptist Mtns diorite gneiss: 1652±14 Ma; Antelope Hills granite gneiss: 1652±8 Ma; South Mohawk Mtns granodiorite gneiss: 1643±15 Ma; South Mohawk Mtns quartzite: age maxima at 1625, 1670, 1740, and 1820 Ma; Bucks Peak granodiorite: 1659±17 Ma; Bucks Peak granodiorite gneiss: 1667±25 Ma; Window Mtn Well granite gneiss: 178±4 Ma; and Iglesita granite (Sonora): 68.3±0.8 Ma with 1689±8 Ma inheritance. Lee Silver’s zircon archive remains available for modern U-Pb-Th and Lu-Hf analyses.