Tommaso Giovanardi

Igneous sapphirine as a product of melt-peridotite interactions in the Finero Phlogopite-Peridotite Massif, Western Italian Alps

Sapphirine is generally interpreted to be of metamorphic origin in high-MgO-Al2O3 rocks. Igneous sapphirine, i.e. sapphirine crystallised from melt, is very rare. We examined sapphirine-bearing magmatic veins in the Finero Phlogopite- Peridotite Massif, Western Italian Alps, to investigate a possible igneous origin for sapphirine by crystallisation from a melt, acquiring particularly high Al2O3 content via melt-rock reaction and fractional crystallisation. Sapphirine locally occurs in melano- cratic zones placed between leucogabbroic veins and the host peridotite. The leucogabbroic veins cut at high angle the mantle foliation and the lithological layering of the peridotite massif, which is defined by alternating phlogopite-rich harzburgites and pyroxenites. This observation, along with their peculiar major-element mineral chemistry, indicates that leucogabbroic veins were unrelatedtothe pervasivemetasomatic recrystallisation ofthe hostmantle sequence,recordingalater,distinctevent ofmelt injection. Melanocratic seams are observed on both sides of the leucogabbroic veins. They show a marked zoning of the mode: an orthopyroxene-rich zone overgrows upon the host peridotite (OPX zone), whereas an amphibole-rich zone occurs towards the leucogabbroic vein (AMPH zone). Sapphirine commonly mantles spinel or occurs as discrete grains, located either (1) within large light-brown pargasite crystals in the AMPH zone or (2) interstitially, between light-brown pargasite in both AMPH zone and OPX zone. Light-brown pargasite can also enclose spinel that does not have sapphirine envelope. To explain the petrochemical features of the sapphirine-bearingveinsandthe host peridotitea four-stageprocessinvolvingmelt-rockreactionsandfractional crystallisation is hereproposed.Duringthefirststage(StageA),theinteractionbetweentheuprisingSiO2-saturatedmeltandthehostperidotitecaused thereplacementofperidotiteolivine,amphiboleandphlogopitebynewlyformedorthopyroxeneclosetothecontactandAl2O3,TiO2, FeO enrichments in the host peridotite beyond the recrystallisation front. This mineralogical reaction resulted in high Al2O3/SiO2and MgO/FeO ratios in the migrating melt. The modified melt crystallised Al2O3-rich dark-brown pargasite (16.5 wt% Al2O3) and apatite in the open conduit (Stage B). Sapphirine/spinel saturation was actually achieved later, in the Stage C, in presence of a more differentiated melt, which reacted with the early dark-brown pargasite locally producing pseudo-symplectite textures made by light- brown pargasite and spinel/sapphirine. A peculiar Al2O3-enriched composition for the parent melt segregating sapphirine is indicated bythecompositionoftheassociatedlight-brownpargasite(17.5wt%Al2O3),phlogopiteandspinel.Locally,thismeltpercolatedalso the OPX zone, segregating sapphirine-bearing mineral assemblages. The sapphirine-free leucogabbroic vein was finally segregated during the Stage D, after splitting of the AMPH zone likely due to hydraulic fracturing.

Occurrence of phlogopite in the Finero Mafic layered complex

Phlogopite-bearing lithologies are the main constituent of the Phlogopite-Peridotite unit of the Finero sequence and the result of pervasive migration of metasomatizing melts/fluids. Conversely, the presence of phlogopite within the associated Finero Mafic Complex, a mafic-ultramafic pluton intruded into the metamorphic basement of the Adria plate, is mentioned in literature as rare. Recent detailed fieldwork has evidenced the presence of two distinct phlogopite-rich ultramafic lithologies within the Amphibole-Peridotite unit of the Finero Mafic Complex, where phlogopite is always associated with amphibole. Field and petrographic features of these occurrences, as well as major- and trace-element mineral chemistry, are here presented to i) place constraints on the nature of the parent melt from which they have been generated and ii) to address their relationship with the other lithologies of the Finero Complex. We find that these rocks were formed by late melt migrations along shear zones under high-T conditions. The geochemical affinity of these lithologies is different to the tholeiitic-transitional affinity reported in literature for the Finero Mafic Complex. The enrichment in LREE, Th, U and Sr of the associated amphibole possibly suggests that these phlogopite-bearing lithologies are genetically related to the metasomatic events that have affected the Finero mantle massif.

U-Pb zircon SHRIMP data from the Cana Brava layered complex: new constraints for the mafic-ultramafic intrusions of Northern Goias, Brazil

Abstract The Cana Brava Complex is the northernmost and least well known layered intrusion of a discontinuous belt of mafic-ultramafic massifs within the Brasilia Belt, which also comprises theNiquelândia and Barro Alto complexes. Available geochronological data from a range of techniques (K/Ar, Ar/Ar, Rb/Sr, Sm/Nd and U/Pb) provide a range of possible ages (time span from 3.9 Ga to 450 Ma), hence a precise and reliable age for the Cana Brava Complex is still lacking. Also, preliminary isotopic and geochemical data of the Cana Brava Complex suggest a significant crustal contamination, which could have affected bulk-rock Sr and Nd systematics resulting in meaningless age determinations. In this paper, we present new U-Pb SHRIMP zircon analyses from four samples of different units of the Cana Brava Complexwhich suggest that the intrusion occurred during the Neoproterozoic, between 800 and 780 Ma, i.e. at the same age ofNiquelândia. Discordant older 206Pb/238U ages are provided by inherited zircons, and match the age of the metamorphism of the embedding Palmeirópolis Sequence.

Short-scale variability of the SCLM beneath the extra-Andean back-arc (Paso de Indios, Argentina): Evidence from spinel-facies mantle xenoliths

Abstract Cenozoic basalts carrying ultramafic mantle xenoliths occur in the Matilde, León and Chenque hills in the Paso de Indios region, Argentina. The mantle xenoliths from the Chenque and León hills mainly present porphyroclastic textures, whereas the Matilde hill xenoliths have coarse-grained to porphyroclastic textures. The equilibrium temperatures are in the range of 780 to 940ºC, indicating a provenance from shallow sectors of the lithospheric mantle column that were subjected to a relatively low heat ffiux at Cenozoic Era. According to the modal compositions of xenoliths, the mantle beneath Matilde and León hills was affected by greater than 22% partial melting, while less depleted peridotites occur in the Chenque suite (starting from 10% partial melting). Such an observation is confirmed by the partial melting estimates based on Cr#Sp, which vary from 8 to 14% for the selected Chenque samples and from 14 to 18% for the Matilde ones. The common melting trend is overlapped by small-scale cross cutting local trends that may have been generated by open-system processes, such as open-system partial melting and/or post partial-melting metasomatic migration of exotic Na-Cr-rich melts. The two main mineralogical reaction schemes are: i) the dissolution of pyroxenes and the segregation of new olivine in olivine-rich peridotites, and ii) the replacement of primary olivine by orthopyroxene±clinopyroxene in orthopyroxene-rich peridotites. These were produced by channelled and/or pervasive melt extraction/ migration. Enhanced pyroxene dissolution is attributed to channelling of silica- undersaturated melts, whereas the replacement of primary olivine by orthopyroxene±clinopyroxene points to reaction with silica-saturated melts. Late disequilibrium reactions identified in the xenoliths comprise: the breakdown of orthopyroxene in contact with the host basalt, and (rarely) reaction coronae on orthopyroxene, clinopyroxene and spinel linked to glassy veins. Such features are apparently related to the injection of melt, likely during entrainment into the host basalts and ascent to the surface.

Filling the Gap in the Classification of Phlogopite-Bearing Ultramafic Rocks

In recent years, the many new occurrences reported in the literature of ultramafic rocks that have phlogopite as a major constituent and do not fall into the categories of kimberlites, lamproites, and lamprophyres have highlighted the need for a classification that includes this abundant mineral phase. Currently, a broadly accepted classification with phlogopite does not exist, and the only term used by scientists is “phlogopite-bearing” when this phase is above 5 vol% and up to 90 vol%. For this reason, we propose a new classification that integrates phlogopite into the current classification of ultramafic rocks without modifying the already accepted terminology or the classificative criteria (i.e., the mineral modal abundances). Phlogopite is added as an end member in the ultramafic-rocks classification diagrams, changing their shapes from triangular to tetrahedral. An Excel spreadsheet containing the new diagrams and a macro that automatically classifies the rocks is provided.