TESTING TWO HYPOTHESES OF PROTEROZOIC CRUSTAL GROWTH USING GEOCHRONOLOGY AND THERMOBAROMETRY ANALYSES OF METASEDIMENTARY ROCKS IN NORTHWESTERN ARIZONA

The period between 1780-1600 Ma was one of extensive crustal growth in southern Laurentia. After this period, certain areas of southwestern Laurentia, such as western Arizona, are conventionally thought to have been tectonically stable and to have remained at mid-crustal depths until 1400 Ma or even later. However, a pre-1700 Ma period of isothermal decompression has been documented in the southern Hualapai Mountains suggesting that western Arizona underwent a tectonically active period of exhumation between 1740-1700 Ma. Furthermore, geologic mapping and petrographic analysis show that two sequences of metasedimentary rocks in the central Hualapai Mountains and the Cottonwood Cliffs have experienced different grades of metamorphism (granulite-facies vs. amphibolite/greenschist-facies). The observations imply that the first sequence was deposited, buried, deformed, and metamorphosed at granulite-facies conditions during a 1740-1720 Ma event. Then, the rocks were exhumed, and the second sequence was deposited. Subsequently both sequences were buried, deformed, and metamorphosed at amphibolite/greenschist-facies conditions during the ca. 1700 Ma Yavapai orogeny. To further test the preliminary results, U-Pb ages of detrital zircons in quartzites of both sequences have been obtained. In addition, pressure-temperature conditions of metamorphism of both sequences will be determined using two techniques: the garnet-biotite thermometer and GASP geobarometer and isochemical plots. The tectonic exhumation hypothesis is supported if the youngest zircons in the amphibolite/greenschist-facies (younger?) sequence are younger than the youngest zircons of the granulite-facies (older?) sequence and if the amphibolite/greenschist-facies sequence does yield lower pressure-temperature conditions than the granulite-facies sequence. If the results support the exhumation hypothesis, the history of southwestern Laurentia is more complex and dynamic than previously thought.