METAMORPHIC AND STRUCTURAL CONSTRAINTS FOR PALEOPROTEROZOIC COLLISION BETWEEN THE MOJAVE AND YAVAPAI PROVINCES, BEAVER DAM MOUNTAINS, UTAH

The metamorphic and tectonic history of the Paleoproterozoic assembly of Laurentian continental crust is preserved in the Precambrian basement rocks of the southwestern US. Much of this tectonism is considered to have resulted from collision of juvenile Paleoproterozoic magmatic arcs with blocks of older Archean crust. However, the collisional history between these arcs and the Archean (?) Mojave Province remains enigmatic. Suturing of juvenile Yavapai Province with the Mojave Province is recorded in a broad swath straddling the Utah-Nevada-Arizona border region. Basement exposed in the Beaver Dam Mountains of southwestern Utah consists of variably deformed granitoids, amphibolites, and metasedimentary units. Current research in a 10 km ² area of the Paleoproterozoic basement exposed in the Beaver Dam Mountains has defined a structural boundary in which a less sheared block of lower metamorphic grade is juxtaposed against a block of intensely-sheared high-grade metamorphic rocks. There is clear-cut evidence for decompression in the western block, from significantly high pressures, that is not apparent in the eastern block. This evidence includes 1) Sil pseudomorphs after Ky in the migmatites, 2) development of Pl/Amp coronas around Grt in the amphibolite dikes, 3) later-stage development of amphibolite/granulite facies assemblages overprinting eclogite facies assemblages contained in the shear zone, 4) Grt cores which have inclusions of Rt, and 5) inclusion-rich garnet cores rimmed by an inclusion-free zones which suggests a 2nd stage of Grt growth over a higher-pressure assemblage. Decompression resulted in the formation of the migmatites. Leucosome rods preserved within the shear zone show that decompression and uplift were contemporaneous with deformation. Fabrics and preserved high-pressure assemblages indicate uplift of the western block from at least lower crustal depths.