Abstract

A detailed survey was conducted of the Nd-isotope compositions of Precambrian metamorphic rocks both within and adjacent to the Cheyenne belt, a metamorphosed crustal segment in southeastern Wyoming that defines the Proterozoic suture zone between the Archean Wyoming craton and accreted Proterozoic island arc(s). The results of the study indicate that Proterozoic miogeoclinal sedimentary rocks exposed north of the Cheyenne belt and deposited at the southern margin of the Wyoming craton contain only Archean crust-derived material and no detritus from Proterozoic island arcs approaching from the south. Similarly, Proterozoic crust exposed south of the Cheyenne belt contains no crustal material derived from Archean sources. However, within the Cheyenne belt, paragneiss of the Barber Lake block has 2.0 to 2.4 Ga Nd model ages and is interpreted as metamorphosed intermixed sedimentary detritus from both Archean and Proterozoic sources. The paragneiss is thought to represent remnants of oceanic trench or foredeep sediments deposited in close proximity to both the Archean craton and the Proterozoic island arcs, similar to sedimentary rocks with 2.0 to 2.4 Ga Nd model ages exposed in the Penokean orogen in Wisconsin. The overall lack of 2.0 to 2.4 Ga modal age crustal material in the vicinity of the Cheyenne belt may reflect the fact that much of this crustal material was overthrust onto the Wyoming craton during accretion and then eroded during postorogenic rebound of the continental margin.

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