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Genesis of Pyroxenite-rich Peridotite at Cabo Ortegal (NW Spain): Geochemical and Pb-Sr-Nd Isotope Data |
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| Authors: | SANTOS J F; SCHARER U; GIL IBARGUCHI J I; GIRARDEAU J |
| Institution: | 1DEPARTAMENTO DE MINERALOGÍA–PETROLOGÍA, UNIVERSIDAD DEL PAÍS VASCO, APTDO. 644, 48080 BILBAO, SPAIN 2LABORATOIRE DE GÉOCHRONOLOGIE, UNIVERSITÉ PARIS 7-IPGP, 2 PLACE JUSSIEU, 75251 PARIS CEDEX 05, FRANCE 3LABORATOIRE DE PLANÉTOLOGIE ET GÉODYNAMIQUE, UNIVERSITÉ DE NANTES, 2 RUE DE LA HOUSSINIÈRE, BP 92208, 44322 NANTES CEDEX 3, FRANCE |
| Abstract: | Petrographic and field data indicate the existence of four mainrock types within the allochthonous Cabo Ortegal ultramaficunits: (1) harzburgites; (2) dunites; (3) massive, occasionallygarnet-bearing, pyroxenites; (4) less abundant mafic rocks withvariable amounts of garnet-rich pyroxenite. The major and traceelement compositions of the analysed ultramafic rocks definewell-delimited fields in binary variation diagrams. Normalizedtrace element patterns, however, exhibit large ion lithophileelement (LILE) and light rare earth element (LREE) enrichmentthat do not correlate with the main rock types distinguished.NiO contents and fo-number of olivine in the harzburgites matchthose of the mantle olivine array, whereas a fractional crystallizationtrend is observed from dunites to pyroxenites. Spinel and olivinein the harzburgites have residual characteristics comparablewith those of abyssal peridotites or peridotites from arc settings,whereas in most of the dunites and pyroxenites the range offo-number and Cr/(Cr + Al) ratio suggests crystallization fromprimitive subduction-related magmas. Whole-rock major and traceelement and Pb–Sr–Nd isotope data suggest that regional-scalemassive pyroxenites from Cabo Ortegal originated from relativelyhomogeneous parental melts. Fractional crystallization processes,coeval with intense deformation, might result in the formationof cumulate layers (clinopyroxene, orthopyroxene, olivine, chromite,etc.). Some less abundant mafic rocks and associated pyroxenitesare also homogeneous but have different chemical and isotopicsignatures suggesting a different parental melt from that ofthe massive pyroxenites. Although some differences exist inthe major element and isotopic composition of the clinopyroxenes,their initial isotopic ratios (206Pb/204Pb = 17·845–18·305,207Pb/206Pb = 15·433–15·634; 87Sr/86Sr =0·70330–0·70476; 143Nd/144Nd = 0·512539–0·512916)suggest involvement of an enriched component in their mantlesource, which may be related to the subduction of terrigenoussediments (i.e. EMI). The new data obtained confirm that ultramaficunits of Cabo Ortegal experienced a complex tectonothermal historysimilar to that of other units of the same area and allow usto distinguish at least two different events. Sm–Nd whole-rock–clinopyroxeneages suggest formation of the ultramafic units at |
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