HR: 17:30h
AN: GP44A-07    [Abstracts]
TI: Mesozoic-Cenozoic reworking of the deep crust beneath the Bering Sea plate: Data from lower to middle crust xenoliths
AU: * Akinin, V V
EM: petrolog@neisri.magadan.ru
AF: North East Interdisciplinary Science Research Institute, Russian Academy of Science, 16 Portovaya St., Magadan, 685000 Russian Federation
AU: Miller, E L
EM: miller@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AU: Mukasa, S
EM: mukasa@umich.edu
AF: Department of Geological Sciences University of Michigan, 2534 C.C. Little Bldg., Ann Arbor, MI 48109-1063 United States
AU: Andronikov, A
EM: andron@umich.edu
AF: Department of Geological Sciences University of Michigan, 2534 C.C. Little Bldg., Ann Arbor, MI 48109-1063 United States
AB: Petrographic and geochemical investigation of lower to middle crustal xenoliths within Late Neogene alkaline basalts of the Bering Sea province (Enmelen volcanoes, Chukotka; Imuruk volcanic field, Alaska; Kookooligit volcanic field, Saint Lawrence island) were carried out to determine the composition, conditions of equilibration and age of the deep crust beneath the Bering Shelf. Mineral thermobarometry shows that xenoliths were derived from the middle to lower crust (~ characterized by pressures of 3 to 10 kb and elevated temperatures of 700 to 1050 C). Three groups of xenoliths can be distinguished: 1) charnockites and quartz- two pyroxene granulites enriched in REE and having Sr and Nd isotopic ratios close to BSE; 2) gabbroic restites and pyroxene-plagioclase cumulates which have relatively depleted and flat REE patterns, and positive Eu anomalies; 3) kelephytic (garnet) gabbro enriched in HREE and depleted in LREE. Radiogenic isotope in pyroxene and plagioclase separates from studied xenoliths have ratios fall into the major mantle array attributed to OIB-like mantle source (87Sr/86Sr =0.7040-0.70463; 143Nd/144Nd= 0.51252-0.51289; 206Pb/204Pb= 18.32-18.69). Isotopic data rule out a close genetic link between the host lava and xenoliths but shows possible relations of some xenoliths with Cretaceous calc-alkaline magmatic rocks associated with the Pacific Margin related Okhotsk-Chukotsk volcanic belt. Nunivak and Navarin xenoliths from the southern edge of the Bering shelf are different in their geochemistry and isotopic ratios (MORB-like signatures) from the rest of studied xenoliths and this might suggests diverse composition of the lower crust beneath the area. Zircons in charnockite and granulite xenoliths were dated by the U-Pb method using the SHRIMP RG and range from 60 to 107 Ma. The zonation patterns of the zircons, their geochemistry, U-Pb ages and how these characteristics vary with rock types and mineral association were used to reconstruct a general model for the petrologic evolution of the deep crust with added constraints provided by geophysical and geologic data. It can be concluded that the lower crust beneath the Bering Sea province has been modified significantly during Cretaceous (107 - 80 Ma) magmatic underplating and was later subjected to a thermal/metamorphic event at 75 - 60 Ma. These two events coincide with two major pulses of calc-alkaline magmatism represented by plutons and volcanic rocks at the surface. These two events occurred together with, and post-dated inferred extensional collapse of previously thickened crust beneath the Bering Shelf . Similar data from xenoliths across a broad area of the U.S., Russia and the Bering Shelf support the regional nature of these conclusions concerning the magmatic and tectonic evolution of the deep crust. Geotherms and metamorphic gradients derived from xenoliths study are too hot to be explained by relatively cold terrains involved in collision-type deformation and require the imput of heat into the crust by mantle derived magmas.
DE: 8109 Continental tectonics--extensional (0905)
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3670 Minor and trace element composition
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting