OPEN-SYSTEM DEHYDRATION OF AMPHIBOLITE, MORTON PASS, WYOMING - ELEMENTAL AND ND AND SR ISOTOPIC EFFECTS

We have undertaken a geochemical and Sm-Nd and Rb-Sr isotopic study of banded metabasites in the contact aureole of the 1.43 Ga Laramie Anor thosite Complex, Wyoming, in order to determine the isotopic effects o f open-system, high-temperature metamorphism. Samples were collected f rom two locations along strike, one that experienced peak temperatures of 720 degrees C, and one that was subjected to temperatures up to 81 0 degrees C. At the higher temperature location, metabasites adjacent to marble have undergone extreme dehydration, presumably due to fluxin g of CO2-rich fluids. These hornblende-poor rocks also show textural e vidence of high-temperature shearing. Element mobility is most evident within this zone of dehydration: variations in chemical composition a re homogenized, and elements including Ti, Fe, K, Na, Rb, Sr, Y, Sm, a nd Nd have been removed. Metabasites at greater distances from the mar ble also provide evidence of open-system behavior, albeit less dramati c: Rb, K, and Sr contents are low throughout the high temperature loca lity compared to the lower temperature samples. Despite the more limit ed geochemical evidence of metasomatism outside the zone of shearing a nd extreme dehydration, the Nd and Sr isotopic systems have been distu rbed in all samples from both locations. Sm-Nd and Rb-Sr mineral isoch rons have been reset to the time of contact metamorphism indicating th at shearing and grain-size reduction is not prerequisite in dating met amorphism. Sm-Nd whole rock data show that Nd isotopes were largely ho mogenized on the outcrop scale during contact metamorphism, and that i n addition, the Sm/Nd ratio of some samples was changed. Rb-Sr whole r ock data also record changes in the Rb/Sr ratio, but in addition radio genic Sr-87 has been released preferentially during hornblende breakdo wn. These results indicate that Sm-Nd and Rb-Sr systems are much more sensitive to metasomatic processes than is commonly believed.