AN EXAMPLE OF PRECAMBRIAN CRUSTAL GROWTH BY LATERAL ACCRETION: THE SOUTH PASS SUPRACRUSTAL BELT, WYOMING

Although much of Late Archean crustal growth in the Wyoming province was by magmatic addition, the South Pass supracrustal belt, southern Wind River Range, Wyoming, is one locality where the Wyoming province grew by lateral accretion.

The South Pass supracrustal belt (SPSB) is composed of mafic and ultramafic rocks of the Diamond Springs Formation, iron formation, pelitic schists, quartzites and amphibolites of the Goldman Meadows Formation, tholeiitic basalt of the Roundtop Mountain Greenstone, and graywackes of the Miners Delight Formation. The Miners Delight Formation is the most extensive unit, distinguishing the SPSB from typical greenstone belts that are dominated by mafic volcanic rocks. The SPSB was regionally metamorphosed to greenschist to lower amphibolite facies, and the northern portions were later contact metamorphosed during intrusion of the 2.63 Ga Louis Lake Batholith.

Zircon U-Pb dating of Miners Delight dacite and andesite yield ages of 2.65 Ga. Although always in fault contact with the other formations of the SPSB, the Miners Delight Formation is assumed to be the youngest portion of the SPSB. This age is similar to that for the Elmer’s Rock greenstone belt in the central Laramie Mountains, but younger than that of supracrustal belts in the Owl Creek, Granite, Seminoe and northern Laramie Mountains.

Initial e Nd are near 0 for the Diamond Springs and Roundtop Mountain Formations, and –5 for Goldman Meadows schist, suggesting that none of these rocks is derived solely from juvenile mantle sources. In contrast, graywackes and volcanic rocks of the Miners Delight Formation have positive initial e Nd values and Nd model ages of 2.66 to 2.95 Ga. We propose a continental margin or back arc setting for the older, less extensive units of the SPSB, but a juvenile, possibly island arc source for the Miners Delight Formation. The Miners Delight Formation represents a juvenile, Late Archean addition to the Wyoming province, accreted no more than 20 million years after deposition