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

Petrology and geochemistry of the back-arc lithospheric mantle beneath eastern Payunia (La Pampa, Argentina): evidence from Agua Poca peridotite xenoliths

This paper presents the results of new petrochemical studies carried out on mantle xenoliths hosted in Pleistocene basaltic rocks from the Agua Poca volcano in central-western Argentina. Mantle xenoliths studied are shown to be mainly anhydrous spinel lherzolites with minor amounts of harzburgite and banded pyroxenite, showing highly variable equilibrium temperatures ranging from 820°C to 1030°C at 1.0 to 2.0 GPa. This constitutes evidence that the mantle xenoliths are representative of a large portion of the lithospheric mantle column and that the geothermal gradient is not very elevated as reported in some other Patagonian provinces. Geochemical characteristics of clinopyroxene in the mantle xenoliths allow classification into two groups; Groups 1 and 2. Group 1 contains most of the lherzolites and has light-REE depletion, with slightly positive anomalies of Eu in some samples and extreme Nb and Ta depletion. Group 2 consists of two harzburgitic samples, has flat REE patterns with lower Sm to Lu concentrations, with enriched Sr and negative HFSE anomalies. Based on mineral and residua compositions estimated assuming equilibrium with clinopyroxenes, Group 1 can be considered to be refractory residua after up to 7%, non-modal, near-fractional melting of a spinel-facies Primitive Mantle. Group 2 can be considered to be after ca. 13% of partial melting. It is inferred that partial melting events in the lithospheric mantle beneath the Agua Poca occurred in different ages since the Proterozoic, but compared with Group 1, the metasomatic overprint is dominant in Group 2 mantle xenoliths. The calculated melt compositions from Group 2 are interpreted to be transient liquid compositions developed during melt-peridotite interaction, and are different from the host alkaline basalts. The HFSE-depleted composition estimated for the rising melt suggests the presence of a slab-derived component, although the possibility cannot be disregarded (on the basis of present data) that such a geochemical feature is due to segregation of HFSE-bearing minerals during the interaction with the peridotite. Thus, we attribute the metasomatic agent to a basaltic melt and to a minor amount of slab-derived fluids.Fil: Bertotto, Gustavo Walter. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Ciencias de la Tierra y Ambientales de La Pampa. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales. Instituto de Ciencias de la Tierra y Ambientales de La Pampa; ArgentinaFil: Mazzucchelli, Maurizio. Università di Modena e Reggio Emilia. Dipartimento di Scienze Chimiche e Geologiche; ItaliaFil: Zanetti, Alberto. Università di Pavia. Istituto di Geoscienze e Georisorse; Italia. Consiglio Nazionale delle Ricerche; ItaliaFil: Vannucci, Riccardo. Università di Pavia. Dipartamento di Scienze della Terra; Itali

Petrology and geochemistry of the back-arc lithospheric mantle beneath eastern Payunia (La Pampa, Argentina): evidence from Agua Poca peridotite xenoliths

This paper presents the results of new petrochemical studies carried out on mantle xenoliths hosted in Pleistocene basaltic rocks from the Agua Poca volcano in central-western Argentina. Mantle xenoliths studied are shown to be mainly anhydrous spinel lherzolites with minor amounts of harzburgite and banded pyroxenite, showing highly variable equilibrium temperatures ranging from 820°C to 1030°C at 1.0 to 2.0 GPa. This constitutes evidence that the mantle xenoliths are representative of a large portion of the lithospheric mantle column and that the geothermal gradient is not very elevated as reported in some other Patagonian provinces. Geochemical characteristics of clinopyroxene in the mantle xenoliths allow classification into two groups; Groups 1 and 2. Group 1 contains most of the lherzolites and has light-REE depletion, with slightly positive anomalies of Eu in some samples and extreme Nb and Ta depletion. Group 2 consists of two harzburgitic samples, has flat REE patterns with lower Sm to Lu concentrations, with enriched Sr and negative HFSE anomalies. Based on mineral and residua compositions estimated assuming equilibrium with clinopyroxenes, Group 1 can be considered to be refractory residua after up to 7%, non-modal, near-fractional melting of a spinel-facies Primitive Mantle. Group 2 can be considered to be after ca. 13% of partial melting. It is inferred that partial melting events in the lithospheric mantle beneath the Agua Poca occurred in different ages since the Proterozoic, but compared with Group 1, the metasomatic overprint is dominant in Group 2 mantle xenoliths. The calculated melt compositions from Group 2 are interpreted to be transient liquid compositions developed during melt-peridotite interaction, and are different from the host alkaline basalts. The HFSE-depleted composition estimated for the rising melt suggests the presence of a slab-derived component, although the possibility cannot be disregarded (on the basis of present data) that such a geochemical feature is due to segregation of HFSE-bearing minerals during the interaction with the peridotite. Thus, we attribute the metasomatic agent to a basaltic melt and to a minor amount of slab-derived fluids.Fil: Bertotto, Gustavo Walter. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Ciencias de la Tierra y Ambientales de La Pampa. Universidad Nacional de La Pampa. Facultad de Ciencias Exactas y Naturales. Instituto de Ciencias de la Tierra y Ambientales de La Pampa; ArgentinaFil: Mazzucchelli, Maurizio. Università di Modena e Reggio Emilia. Dipartimento di Scienze Chimiche e Geologiche; ItaliaFil: Zanetti, Alberto. Università di Pavia. Istituto di Geoscienze e Georisorse; Italia. Consiglio Nazionale delle Ricerche; ItaliaFil: Vannucci, Riccardo. Università di Pavia. Dipartamento di Scienze della Terra; Itali

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

Are the glasses in mantle xenoliths witness of the metasomatic agent composition?

Glass veins and pockets in mantle xenoliths are often considered as indicators of the composition of metasomatic agents affecting the mantle. Here we demonstrate that infiltration of, and reaction with, the host basalt may produce glassy veins and pockets whose composition encompasses that of different potential metasomatic agents. The xenoliths studied are 4-19 cm large, equigranular, spinel-facies harzburgites and lherzolites from the Patagonia lithospheric mantle. A reaction rim occurs at the contact with the basalt. Peridotite orthopyroxene is in reaction with the basalt and a glassy pocket (up to 600 μm in diameter) is thereby formed. New crystals of euhedral olivine (Fo = 84) and Ti-oxides crystallise in the glass pocket close to the basalt, while euhedral crystals of clinopyroxene (mg# = 85-89) and olivine crystallise close to orthopyroxene. The reaction-crystallisation processes induce dramatic compositional variations in theglass pocket from phonotephryte to trachyte. Trace element concentration and patterns and the element anomalies are controlled by the reaction-crystallisation process. Orthopyroxene dissolution has mainly a dilution effect, whereas clinopyroxene crystallisation and the crystallization of Ti oxides (and apatite) largely controls the trace element fractionation and element anomalies. The largest trace element variations are documented in the veins. The studied glasses obviously do not represent metasomatic agents that affected the lithospheric mantle, but the large compositional variations they document encompass those of glasses quoted in literature (Wulff-Pedersen et al., 1996; Coltorti et al., 2000) and believed to represent metasomatic agents, thus suggesting caution in drawing inferences on mantle processes and components from glass pockets and veins in xenoliths

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

DATING THE ONSET OF A LOWER CRUSTAL SHEAR ZONE: A (LUCKY) CASE FROM THE NORTHERN SECTOR OF THE IVREA-VERBANO ZONE (VAL CANNOBINA, ITALY)

A detailed textural and U-Pb geochronological investigation has been performed on zircons from a ductile shear zone established in lower crustal mafie intrusives. The investigated shear zone is hosted in the Finero mafic-ultramafic complex of the northeastern sector of the Ivrea-Verbano Zone, Southern Alps. Il developed al the base of the lower lo middle continental crust section, where mafie lo ultramafic rocks intruded into a sequence of metapelites and metabasites (i.e., Kinzigite Formation) constituting the polymetamorphic basement of the Adria plate. The shear zone forms a braided belt that can be followed for several km, from the Cannobino River lo the MI. Gridone, and developed entirely within gabbroic/dioritic rocks, partly intruded during Triassic lime (- 232 Ma; Zanetti et al., 2013). The shear zone is characterized by a well-developed foliation, a lineation, and a compositional banding, where amphibole, clinopyroxene and game! occur as large rounded lo elliptical porphyroclasts, resulting embedded in a fine grained matrix mainly consisting of plagioclase, amphibole, pyroxenes and ilmenite. The shear zone was active during uplift and cooling of the wall rock, and recorded retrograde metamorphic conditions ranging from 650 lo 500"C al 0.6-0.4 GPa (Kenkmann, 2000). The mylonitic deformation is generally placed no earlier !han 230 Ma, lasting until about 170 Ma al the latest (e.g., Handy & Zingg, 1991). Numerous zircon grains were obtained from the minerai separation of one (- 2 kg weight) sample collected in the Cannobino River (northem Ivrea-Verbano Zone, Southern Alps). Zircon grains were examined also directly on petrographic lhin sections of samples collected throughout the (ultra)mylonitic belt. According to petrographic and microstructural analyses zircon occurs as rounded grains within the mylonitic matrix, easily recognisable due lo their dimensions up lo 110 micron. Cathodoluminescence (CL) study al the SEM revealed a well developed systematic zoning of zircon grains with dark cores surrounded by brighter domains, locally asymmetric, and with a thickness up to 30 micron. The dark inner core can show zoning features suggesting magmatic growlh. Preliminary LA-ICP-MS U-Pb results from zircon separates yielded mainly late Triassic concordant ages of about 235 Ma for the dark CL cores and about 204 Ma for the brighter CL (mylonitic) overgrowths. The sizes, distributions, shapes and CL features of zircon grains provide strong evidence of a metamorphic response of zircon during ductile deformation. The observed features can be interpreted as evidence of dissolution/recrystallization in response lo fluid influx during high-temperature shearing. Further microstructural investigations, minerai chemistry and trace element characterization of different zircon domains are in progress and will provide more evidence for the petrochronological evolution of such a lower crustal shear zone

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

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