EVIDENCE FOR PALEOPROTEROZOIC HIGH-PRESSURE METAMORPHISM AND DECOMPRESSION MELTING IN THE MOJAVE-YAVAPAI SUTURE ZONE, BEAVER DAM MOUNTAINS, UTAH

Precambrian basement in the southwestern US preserves a long and very complicated metamorphic and tectonic history. These rocks record Archean (?) and Paleoproterozoic assembly of Laurentian continental crust. Much of this tectonism resulted from collision of juvenile Paleoproterozoic arcs with older Archean crust. Suturing of Yavapai Province rocks 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 southwest Utah consists of variably deformed granitoids, amphibolites and metasedimentary units. Evidence exists for high-pressure eclogite facies metamorphism followed by decompression to amphibolite facies conditions. Relict eclogite occurs as diffuse greenish patches contained in northeast-trending amphibolite dikes. The eclogite consists of Omp+Grt+Czo+Ms+Qtz±Rt. The amphibolite contains Hbl+Qtz+Mt+Ep±Pl±Grt. When present, garnet is rimmed by plagioclase-hornblende intergrowths suggestive of retrograde decompression. Garnet-bearing leucosomes are contained in a Grt+Bt+Sil+Qtz+Kfs neosome. Rectangular sillimanite pseudomorphs after kyanite also suggest decompression. Stable mineral assemblages in amphibolite and migmatite indicate equilibrium P-T conditions in the range of 700 - 800°C and 0.7 - 0.8 GPa (25-28 km paleodepths). The relict eclogite suggests decompression from a minimum of 1.5 GPa, or 50-55 km depths. Exhumation of such deep-seated rocks is consistent with collision between the Mojave and Yavapai Provinces.