CHRONOLOGY AND ISOTOPE CONSTRAINTS ON TECTONOTHERMAL EVOLUTION OF THE MONTANA METASEDIMENTARY TERRANE, WYOMING PROVINCE

The Wyoming Province in the region of Wyoming and adjacent states is a well-exposed Archean craton that records the Precambrian continent assembly and cratonic lithosphere modification. On the basis of differences in chemical and isotopic evolution during late Archean, the Wyoming Province is recognized to be composed of three subdivisions. The Montana Metasedimentary Terrane (MMT) is a compositionally distinct subprovince in northwestern Wyoming Province, which preserves records of interaction between crustal accretion and tectonothermal reworking in the cratonic margin.

Even though the Precambrian basement has been intensively deformed by the complex processes associated with high temperature and pressure tectonism, the isotope systems of minerals that have survived can still potentially provide critical information about past magmatic and metamorphic history. Through a systematic chronological and isotopic investigation of geological samples, our new data help to understand the origin and development of Precambrian components as well as the nature and timing of tectonic events that occurred in the MMT. Three zircon U-Pb age populations of ~2.7, ~2.4, and ~1.8 Ga document major tectonothermal events in Neoarchean and Paleoproterozoic. The garnet Lu-Hf, Sm-Nd/titanite U-Pb/monazite U-Pb ages from the same samples are broadly consistent with the youngest zircon age group at ~1.8 Ga. Their apatite-titanite-whole rock Sm-Nd isochrons also yield ages of ~1.8 Ga, indicating a regional closure of the Sm-Nd system at that time. Meanwhile, the initial Hf composition of zircon calculated at ~1.8 Ga is generally subchondritic in most samples, which is in agreement with the initial Nd values of titanite and monazite. This data set shows evidence of the 1.8-Ga thermal event distributed throughout the MMT, which extensively overprinted previous records and demonstrates a reworking of early formed crust in Paleoproterozoic.