Petrogenesis of layered gabbros and ultramafic rocks from Val Sesia, the Ivrea Zone, NW Italy: trace element and isotope geochemistry

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Abstract
Summary: Layered gabbros and ultramafics, well exposed in Val Sesia, have been interpreted as a layered complex. The lower part (LLG; up to 1 km thick) comprises well layered pyroxenites and gabbros with minor peridotite and harzburgite. The upper part consists of fairly homogeneous gabbros (MG), which grade into diorite (up to 5.5 km thick). This gabbro body intrudes already highly deformed metapelites, but is itself relatively undeformed. These two groups (LLG and MG-diorite) are distinct on the basis of isotope, trace element and major element chemistry and cannot have been derived from the same liquid. The LLG has ɛ Nd ranging from +1.5 to +2.5 and ɛ Sr from +7 to −8 and was derived from a depleted source. The MG-diorite body has ɛ Sr ranging from +40 to +60 and a wide range of ɛ Nd from −2 to −6. The LLG pyroxenites and gabbros are dominantly cumulates of clinopyroxene with lesser amounts of orthopyroxene. Cumulus plagioclase appears towards the top of the LLG. The LLG is strongly depleted in LREE and LIL elements and was derived from a basaltic liquid with relatively depleted trace element characteristics. It probably fractionated at moderate pressures and was emplaced into the Ivrea Zone along with the adjacent Balmuccia peridotite. The MG comprises plagioclase ± olivine cumulates with minor cumulus apatite, ilmenite and pyroxene. The diorites additionally contain cumulus biotite. The whole body has strongly LREE-enriched patterns (Ce N 4.5–20) and large positive Eu anomalies (Eu/Eu* from 2–12). The MG has calc-alkaline features with high Ba and low Nb. The parental magma was also LREE-enriched, with low Ni, Cr and MgO and high Ba. It was possibly an andesite. The spread in ɛ Nd cannot be accounted for by contamination by any of the likely crustal contaminants. The spread in intial isotope ratios either results from a heterogeneous enriched mantle source or from a two stage process involving remelting of an intermediate mafic reservoir with moderate enrichment in Rb and LREE. The MG-diorite body was emplaced into sediments already undergoing amphibolite-facies metamorphism at pressures of 5–7 kbar. It appears as if the development of magma chambers in the crust was an important process of crustal growth in the Ivrea Zone.