Seifert, Karl E.; Gibson, Ian L.; Weis, Dominique; Brunotte, Dale (1996): Geochemistry of metamorphosed cumulate gabbros from Hole 900A, Iberia abyssal plain. Texas A&M University, Ocean Drilling Program, College Station, TX, United States, In: Whitmarsh, Robert B., Sawyer, Dale S., Klaus, Adam, Beslier, Marie-Odile, Collins, Eric S., Comas, Maria Carmen, Cornen, Guy, de Kaenel, Eric, Pinheiro, Luis de Menezes, Gervais, Elisabeth, Gibson, Ian L., Harry, Dennis L., Hobart, Michael A., Kanamatsu, Toshiya, Krawcyzk, Charlotte M., Liu, Li, Lofts, Jeremy C., Marsaglia, Kathleen M., Meyers, Philip A., Milkert, Doris, Milliken, Kitty L., Morgan, Julia K., Ramirez, Pedro, Seifert, Karl E., Shaw, Timothy J., Wilson, Chris, Yin, Chuan, Zhao, Xixi, Proceedings of the Ocean Drilling Program; scientific results, Iberia abyssal plain; covering Leg 149 of the cruises of the Drilling Vessel JOIDES Resolution; Balboa Harbor, Panama, to Lisbon, Portugal; sites 897-901, 10 March-25 May 1993, 149, 471-488, georefid:2007-088108

Basement at ODP Hole 900A consists of metamorphosed cumulate gabbros with discontinuous foliation bands of recrystallized plagioclase and clinopyroxene containing a few large isolated porphyroclasts of strained plagioclase and clinopyroxene respectively. Locally the pyroxene has retrograded to fine-grained amphibole and other minerals and the retrograded cores are cut by multiple generations of veins. The alternating discontinuous felsic and mafic bands define a poorly developed foliation that is characteristic of high-grade metamorphism in ocean crust and quite distinct from high-grade continental metamorphism. The Hole 900A cumulate gabbros have very low concentrations of Zr (10-30 ppm) and other incompatible elements indicating little magma retention. Their geochemistry is similar to that of cumulate gabbros from the Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns are flat at 2X-4X chondrite with small positive europium anomalies. Spider diagrams reveal a secondary enrichment in the soluble large ion lithophile elements and U relative to less soluble incompatible elements. The association of K concentrations in alteration patches with amphibole indicates these elements were added by seawater during the hydrous retrograde metamorphism. Nd and Pb isotopic analyses indicate the gabbros were formed in an oceanic environment and the high positive initial epsilon Nd values (>6 at 136.4 Ma) indicate a MORB origin. High Sr isotopic ratios and high U/Pb ratios reveal seawater alteration. Based on a rare earth element pattern using literature partition coefficients for plagioclase and clinopyroxene, the average parental magma for the Hole 900A cumulate gabbros resembles transitional MORB. These geochemical data provide evidence for the presence of an active mid-ocean ridge magma chamber in the Iberia Abyssal Plain region 136.4 Ma.
West: -11.3616 East: -11.3616 North: 40.4059 South: 40.4059
Expedition: 149
Site: 149-900
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