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

Geochemistry of sapphirine-apatite-calcite-bearing gabbroic dykes from the Finero Phlogopite Peridotite (Ivrea-Verbano Zone): evidence for multistage interaction with the ambient peridotite

The Finero Phlogopite-Peridotite (FPP) is a mantle unit outcropping in the northernmost tip of the Ivrea-Verbano Zone (IVZ, Southern Alps). It shows a virtually complete recrystallization due to pervasive to channelled melt migration. The pervasive metasomatism formed a main lithologic association constituted by phlogopite harzburgites associated to phlogopite pyroxenites (mainly olivine-websterites, websterites and orthopyroxenites). These lithologies are also rich in amphibole and do not show significant chemical gradients among them (Zanetti et al., 1999). The channelled migration stages formed dunite bodies, which sometimes contain stratiform chromitites and, more rarely, pyroxenite layers similar to those associated to phlogopite harzburgite. The FPP also shows a discrete number of other, subordinate rock-types, which are characterised by the presence of apatite usually associated to carbonates (i.e. calcite or dolomite) and exhibit marked modal and chemical gradients with respect to the host phlogopite harzburgite. Examples of these lithologies are apatite-dolomitebearing wehrlites and harzburgites (e.g. Zanetti et al. 1999; Morishita et al., 2008), apatite-calcite zircon-syenites and hornblendites. Ar-Ar amphibole analysis and U-Pb zircon and apatite data return Triassic ages for these rocks, which have been considered to document the time of melt/fluid injection. Notwithstanding the apparent mineralogical and chemical differences with the main lithologic sequences, apatite-carbonates-bearing rocks have been frequently interpreted as cogenetic to phlogopite harzburgites. To debate the petrogenesis of these rocks, a detailed field, petrological and geochemical investigation has been carried out on a swarm of apatite-calcite-bearing gabbroic veins that randomly cut the main lithologic association. Preliminary investigation evidenced as these veins show complex metasomatic haloes and a symmetric internal layering, characterised by crystallisation of magmatic sapphirine (Giovanardi et al., 2013). The mineral assemblage of the veins is dominated by titanian pargasite towards the host peridotite and by plagioclase at the vein centre. The veins also present phlogopite and spinel. Field and petrographic evidence, major and trace element data and the O isotopic composition of such gabbroic veins indicate that they formed at shallow mantle conditions by multistage fractional crystallisation of a migrating melt unrelated to those forming phlogopite harzburgites. Besides, local strong enrichments in LILE, LREE and 18O in vein minerals confirm that such melt was deeply modified by interaction with the host phlogopite peridotite. The genetic relationships with other intrusive events recorded by the FPP and the associated crustal sequence will be addressed with the aim of placing new constraints on the petrologic and geodynamic evolution of the IVZ

Melt-Rock Interaction at Mantle Conditions: Evidences from Finero Gabbroic Dykes

The Finero Phlogopite-Peridotite (FPP) is a worldwide famous mantle massif recrystallized through several events of melt migrations. These events have enriched the FPP in hydrous phases and crustal components and have been commonly interpreted as related to a subduction/post orogenic geodynamic setting. The last of these metasomatic events has produced composite sapphirine-bearing gabbroic dykes, interpreted as the result of the interaction of channelized migrating melts with the host rock in a two-steps intrusion process. In the first step, the melt reacted with the FPP rocks and evolved by fractional crystallization of amphibole cumulates. In the second step, the evolved melt reacted with the first cumulates producing magmatic sapphirine and segregating plagioclase-rich bands containing abundant apatites at the nucleus of the dike. New data suggest, however, a more complex evolution. New O and in situ Sr isotopes on minerals suggest that the gabbroic dykes have evolved from melt(s) that progressively were contaminated by the interaction with the FPP rocks during its fractionation. The δ18O increases from 5.81‰ in orthopyroxenes at the dykes border to ~6.90‰ in cumulitic amphiboles and 8.60‰ in plagioclases. The 87Sr/86Sr values for plagioclase and coexisting apatite show isotopic disequilibrium between the two phases (plagioclases at 0.70474 ± 0.00033, n=23, and apatites at 0.70369 ± 0.00025, n=6). These isotopic variations could be explained with an AFC-like process between mantle-derived melt(s) and a crustal-enriched host (the FPP). In situ Sr isotope analyses were performed at the CIGS laboratory of the Università di Modena e Reggio Emilia using a Thermo Fisher Scientific Neptune™ coupled to a 213 nm Nd:YAG laser ablation system (New Wave Research™). During the analytical sessions a new in house plagioclase reference material for Rb-Sr systematic, named BC84, has been successfully tested and used

Mantle-Derived Corundum-Bearing Felsic Dykes May Survive Only within the Lower (Refractory/Inert) Crust: Evidence from Zircon Geochemistry and Geochronology (Ivrea–Verbano Zone, Southern Alps, Italy)

Corundum-rich (up to 55 vol.%) felsic dykes formed with albite, +/- K-feldspar, +/- hercynite and +/- biotite-siderophyllite cut the lower crustal rocks exposed in the Ivrea–Verbano Zone (NW Italy). Zircon is an abundant accessory mineral and its investigation through laser ablation-inductively coupled plasma (multi-collector)-mass spectrometer (LA-ICP-(MC)MS) has allowed results to directly constrain the timing of emplacement, as well as petrology and geochemistry of parental melts. Zircons are characterized by very large concentration in rare earth elements (REE), Th, U, Nb and Ta, and negative Eu anomaly. U–Pb analysis points to Norian emplacement ages (223 +/- 7 Ma and 224 +/- 6 Ma), whereas large positive EHf(t) values (+13 on average) indicate a derivation from depleted to mildly enriched mantle source. The mantle signature and the corundum oversaturation were preserved thanks to limited crustal contamination of the host, high-temperature refractory granulites and mafic intrusives. According to the geochemical data and to the evidence of the development of violent explosions into the conduits, it is proposed that dykes segregated from peraluminous melts produced by exsolution processes affecting volatile-rich differentiates during alkaline magmatism. This work provides robust evidence about the transition of the geochemical affinity of Southern Alps magmatism from orogenic-like to anorogenic during Norian time, linked to a regional uprising of the asthenosphere and change of tectonic regime

Brittle-ductile deformation effects on zircon crystal-chemistry and U-Pb ages: an example from the Finero Mafic Complex (Ivrea-Verbano Zone, western Alps)

A detailed structural, geochemical and geochronological survey was performed on zircon grains from a leucocratic dioritic dyke discordantly intruded within meta-diorites/gabbros forming the External Gabbro unit of the Finero Mafic Complex. This latter is nowadays exposed as part of a near complete crustal section spanning from mantle rocks to upper crustal metasediments (Val Cannobina, Ivrea-Verbano Zone, Italy). The leucocratic dyke consists mainly of plagioclase (An18-24Ab79-82Or0.3-0.7) with subordinate amounts of biotite, spinel, zircon and corundum. Both the leucocratic dyke and the surrounding meta-diorites show evidence of ductile deformation occurred under amphibolite-facies conditions. Zircon grains (up to 2 mm in length) occur mainly as euhedral grains surrounded by fine grained plagioclasedominated matrix and pressure shadows, typically filled by oxides. Fractures and cracks within zircon are common and can be associated with grain displacement or they can be filled by secondary minerals (oxides and chlorite). Cathodoluminescence (CL) images show that zircon grains have internal features typical of magmatic growth, but with local disturbances. However EBSD maps on two selected zircon grains revealed a profuse mosaic texture resulting in an internal misorientation of ca. 10o. The majority of the domains of the mosaic texture are related to parting and fractures, but some domains show no clear relation with brittle features. Rotation angles related to the mosaic texture are not crystallographically controlled. In addition, one of the analysed zircons shows clear evidence of plastic deformation at one of its corners due to indentation. Plastic deformation results in gradual misorientations of up to 12o, which are crystallographically controlled. Trace elements and U-Pb analyses were carried out by LA-ICP-MS directly on petrographic thin sections and designed to cover the entire exposed surface of selected grains. Such investigations revealed a strong correlation between internal zircon structures, chemistry, U-Pb isotope ratios and mylonitic fabric. U-Pb data return highly discordant and variable ages: in particular, the 206Pb/238U ages range from Carboniferous to Triassic within the same zircon grain. The youngest 206Pb/238U data derive from narrow axial stripes oriented parallel or at low angle with respect to the foliation planes. These stripes are characterized by an overall HREE, Y, U and Th enrichment possibly reflecting deformation of the grain in presence of interstitial fluid phases, likely related to a concomitant magmatic activity. Deformation related structures (cracks and fractures) within zircon grains acted as fast-diffusion pathways allowing fluids to modify the geochemistry and isotopic systems of zircon. Our results suggest that fluid-assisted brittle-ductile deformation can severely modify the trace elements and isotopic composition of zircon with unexpected patterns constrained by stress regime. In similar cases, our observations suggest that, for a more appropriate interpretation of the petrologic evolution and age variability, a direct characterization of the internal structures of zircons still placed in their microtextural site is highly recommended

Early exploitation of Neapolitan pozzolan (pulvis puteolana) in the Roman theatre of Aquileia, Northern Italy

: The paper reports the results of the analyses on mortar-based materials from the Roman theatre of Aquileia (Friuli Venezia Giulia, Northern Italy), recently dated between the mid-1st Century BCE and the mid-1st Century CE. Samples were characterized by Polarized Light Microscopy on thin sections (PLM), Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) and Quantitative Phase Analysis by X-Ray Powder Diffraction (QPA-XRPD). Pyroclastic aggregates (mainly pumices and scattered tuffs), incompatible with the regional geology, were found in two samples from the preparation layers of the ground floor of the building. Their provenance was determined by means of QPA-XRPD, SEM-EDS, X-Ray Fluorescence (XRF) and Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS). Mineralogical and geochemical analyses demonstrated their provenance from the Bay of Naples, thus recognizing them as pulvis puteolana, a type of pozzolanic aggregate outcropping around the modern town of Pozzuoli and prescribed by Vitruvius (De Architectura, 2.6.1) in mortar-based materials to strengthen masonries and produce hydraulic concrete for harbor piers. This evidence represents the oldest analytically-established case of pulvis puteolana exploitation in Northern Italy up to now, and an early use of the material out of Campania adapted for civil constructions in a non-strictly maritime-related environment. Indeed, the theatre was built in the low-lying Aquileia's deltaic plain, prone to water infiltrations that are typical in lagoon-like environments. The data highlight the craftsmen's resilience in adapting and reinterpreting the traditional use of the Neapolitan volcanic materials to deal with the geomorphological challenges of Aquileia's lowland

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

The Cana Brava Complex is the northernmost and less-known layered intrusion of a discontinuous belt of mafic-ultramafic massifs within the Brasilia Belt, which also comprises the Niquelândia and Barro Alto complexes. Available geochronological determination by means of different systematics (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 statistically 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 Complex which suggest that the intrusion occurred during the Neoproterozoic, between 800 and 780 Ma, i.e. at the same age of Niquelândia. Discordant older 206Pb/238U ages are provided by inherited zircons, and match the age of the metamorphism of the encasing Palmeirópolis Sequence

MELT-PERIDOTITE MULTISTAGE INTERACTION AT MANTLE CONDITIONS: PETROLOGICAL ANO GEOCHEMICAL EVIDENCES FROM SAPPHIRINE-APATITE-CALCITE-BEARING GABBROIC DYKES FROM THE FINERO PHLOGOPITE PERIDOTITE (IVREA-VERBANO ZONE)

The Finero Phlogopite-Peridotite (FPP) is a mantle uni! outcropping in the northernmost pari of the Ivrea-Verbano Zone (IVZ, Southern Alps). Multistage pervasive lo channelled meli migrations had completely recrystallized the entire FPP. The main metasomatic event pervasively formed an association of amphibole-rich phlogopite harzburgite with subordinated phlogopite-pyroxenites which do not show geochemical gradients (Zanelli et al., 1999). Channelled migrations lately formed dunite bodies, sometimes containing stratiform chromitites and, more rarely, pyroxenite layers similar lo those associated lo phlogopite harzburgite. Several other lithologies, showing geochemical gradients with rocks of the main FPP association and characterized by the presence of apatite sometimes associated lo carbonates (i.e. dolomite and calcite), are subordinated in volumes and abundances. Commonly these lithologies occur as dykes or veins along deformation zones. Geochronological data from apatite-calcite zircon syenites and apatite-dolomite wehrlites provide Triassic ages assumed lo document the lime of the meltlfluid migrations. Notwithstanding the apparent mineralogica! and chemical differences with the main lithologic sequences, apatite-carbonates-bearing rocks have been frequently interpreted as cogenetic lo phlogopite harzburgites and related lo the main metasomatic event. Recently, apatite-calcite-bearing gabbroic dykes randomly crosscutting the FPP lithologic associations were recognized as possibly the las! (or one of the las!) melt migration event within the mantle unii (Giovanardi et al., 2013). The dykes show symmetrical internal layering formed by melanocratic bands towards the host peridotite dominated by titanian pargasite and a centrai leucocratic zone dominated by plagioclase. Magmatic sapphirine occurs in plagues al the contaci of the leucocratic zone within the melanocartic bands. New field, petrographic and geochemical studies were conducted lo constrain the gabbroic veins intrusion and their genetic relationships with other FPP metasomatic events. Petrographic evidences, major and trace element data and the O isotopic composition of such gabbroic veins indicate that they formed al shallow mantle conditions by multistage fractional crystallisation of a migrating meli unrelated lo those forming the harzburgite-pyroxenite association and the dunite bodies. However, local strong enrichments in LILE, LREE and 1i180 in vein minerals confirm that such melt was deeply modified by interaction with the host phlogopite peridotite. However, the amphiboles in textural equilibrium with sapphirine show a marked M-H REE and Y depletion associated lo a marked positive Eu anomaly, which suppor! meli evolution through plagioclase assimilation. The genetic relationships with other intrusive events recorded by the FPP and the associated crustal sequence will be addressed with the aim of placing piace new constraints on the petrologic and geodynamic evolution of the IVZ