Other
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994jgr....99..683s&link_type=abstract
Journal of Geophysical Research (ISSN 0148-0227), vol. 99, no. B1, p. 683-696
Other
4
Amphiboles, Boundary Layer Transition, Earth Crust, Earth Mantle, Garnets, Geology, Pyroxenes, Rocks, Lead Isotopes, Mineral Deposits, Neodymium Isotopes, Planetary Evolution, Strontium Isotopes, Tectonics, Temperature Measurement
Scientific paper
Garnet-pyroxene-amphibole xenoliths illustrate how P and T histories can be recorded in rocks from the crust-mantle transition and document the diversity of continental lithosphere below the Mohorovicic discontinuity. The xenoliths are from the Sullivan Buttes Latite in Chino Valley, Arizona, in the Transition Zone of the Colorado Plateau. The most definitive depth assignments depend upon garnet-pyroxene thermobarometry coupled with analysis of Ca and Al gradients in orthopyroxene. Websterites that record temperatures of 600-700C contain orthopyroxene zoned in Al but not Ca, and these rocks were carried up from depths of at least 43 km. Websterites that record temperatures of 800-900C contain more homogeneous orthopyroxene, and they were erupted from 70 to 80 km. Most eclogite and amphibole-rich xenoliths record temperatures in the range bracketed by websterites and so were probably erupted from similar depths. Element abundances and Sr, Nd, and Pb isotope ratios establish that protoliths of most xenoliths formed by crystal-melt fractionation from basaltic magmas. Diverse Sr and Nd isotopic compositions range from Nd-epsilon approximately equals +8 and Sr-87/Sr-86 approximately equals 0.7045 for two websterites to Nd-epsilon approximately equals -9 and Sr-87/Sr-86 approximately equals 0.7064 for both parts of a composite eclogite. Most xenoliths probably have Proterozoic protoliths, although many record more recent thermal and metasomatic events, and a few probably formed from Cenozoic magmas. Observations are consistent with a reconstruction of the lithosphere in which eclogite and amphibole-rich rock were volumetrically important to depths of at least 70-80 km at 25 Ma. Anhydrous peridotite may not dominate just below the Mohorovicic discountinuity beneath Chino Valley or beneath some other localities on the Colorado Plateau and elsewhere. No evidence was observed in the Chino Valley suite for replacement of continental lithosphere during Phanerozic tectonism or for significant underplating in Cenozoic time.
Arculus Richard J.
Manchester Janet E.
Smith Douglas
Tyner Nell G.
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