Genesis of pyroxenite-rich peridotite at Cabo Ortegal (NW Spain): Geochemical and Pb-Sr-Nd isotope data

Abstract

Petrographic and field data indicate the existence of four main rock types within the allochthonous Cabo Ortegal ultramafic units: (1) harzburgites; (2) dunites; (3) massive, occasionally garnet-bearing, pyroxenites; (4) less abundant mafic rocks with variable amounts of garnet-rich pyroxenite. The major and trace element compositions of the analysed ultramafic rocks define well-delimited fields in binary variation diagrams. Normalized trace element patterns, however, exhibit large ion lithophile element (LILE) and light rare earth element (LREE) enrichment that do not correlate with the main rock types distinguised. NiO contents and fo-number of olivine in the harzburgites match those of the mantle olivine array, whereas a fractional crystallization trend is observed from dunites to pyroxenites. Spinel and olivine in the harzburgites have residual characteristics comparable with those of abyssal peridotites or peridotites from arc settings, whereas in most of the dunites and pyroxenites the range of fo-number and Cr/(Cr + Al) ratio suggests crystallization from primitive subduction-related magmas. Whole-rock major and trace element and Pb-Sr-Nd isotope data suggest that regional-scale massive pyroxenites from Cabo Ortegal originated from relatively homogeneous parental melts. Fractional crystallization processes, coeval with intense deformation, might result in the formation of cumulate layers (clinopyroxene, orthopyroxene, olivine, chronite, etc.). Some less abundant mafic rocks and associated pyroxenites are also homogeneous but have different chemical and isotopic signatures suggesting a different parental melt from that of the massive pyroxenites. Although some differences exist in the 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 mantle source, which may be related to the subduction of terrigenous sediments (i.e. EMI). The new data obtained confirm that ultramafic units of Cabo Ortegal experienced a complex tectonothermal history similar to that of other units of the same area and allow us to distinguish at least two different events. Sm-Nd whole-rock-clinopyroxene ages suggest formation of the ultramafic units at ∼ 500 Ma, an age similar to that of formation of the protoliths of associated HP/HT units. Internal Sm-Nd isochrons (Cpx-Grt-whole rock) from two pyroxenites indicate ages of ∼ 390 Ma for garnet crystallization in these rocks, which is consistent with previous U-Pb dating of HP/HT recrystallization in Cabo Ortegal. Pyroxenite-dunite-rich ultramafic massifs such as Cabo Ortegal might have originated within the lithospheric mantle above a subduction zone, variably modified by fluid or melt infiltration from the subducted oceanic crust.