Rosaria Palmeri

Evidence of a close link between petrology and isotope records: constraints from SEM, EMP, TEM and in situ 40Ar–39Ar laser analyses on multiple generations of white micas (Lanterman Range, Antarctica)

K–Ar ages from white mica are commonly interpreted to record cooling below a certain temperature with the implicit assumption that all the requirements of the volume diffusion theory are fulfilled. Nevertheless, studies on metamorphic white micas have highlighted discrepancies with previously inferred closure temperatures and have evidenced a close link between petrology and argon isotope age records. This study uses the in situ 40Ar–39Ar laserprobe method in conjunction with scanning electron microscopy, electron microprobe and transmission electron microscopy (TEM) techniques to examine the relations between argon isotope records and microtextural, microchemical and microstructural variations in white mica. Gneisses and micaschists belonging to three different tectono-metamorphic complexes of the Lanterman Range (Antarctica) contain multiple generations of potassic white micas and are well-suited to investigate the relation between petrology and argon dating. Texturally resolvable white mica generations show microchemical and microstructural (TEM scale) variations that suggest development under different P–T deformation regimes, ranging from an eclogite facies stage down to low greenschist facies conditions. In situ 40Ar–39Ar laserprobe analyses on white mica samples from the three complexes reveal a complex intragrain and intergrain spatial distribution of argon ages which is closely linked to microtextural, microchemical and microstructural variations: texturally, compositionally and microstructurally older generations yield older ages whereas the younger ones yield younger ages. Results show that in the absence of re-crystallisation, white mica preserves argon isotope records pertaining to the high-pressure stage which survived amphibolite retrogression at temperatures of 550–650°C. The texture, petrology and isotope record of white micas in the studied samples preserve a nearly continuous record of P, T and deformation history within the same orogenic cycle.

Coesite in eclogites of the Lanterman Range (Antarctica): Evidence from textural and Raman studies

Quartz pseudomorphs after possible retrogression of coesite were recognized petrographically within garnets of mafic eclogites from the Lanterman Range (Antarctica). In one of the quartz pseudomorphs the presence of a pristine coesite is indicated by in situ Raman microprobe spectroscopy. The results of the Raman analyses show that the quartz inclusion in addition to the typical quartz vibrations has a weak band at 521 cm-1, which corresponds to the most intense fundamental vibration in coesite. This finding represents a first piece of evidence for ultrahigh-pressure metamorphism in Antarctica which was part of the Gondwana supercontinent affected by the Cambro-Ordovician orogenic cycle.

Ultra-high-pressure metamorphism in felsic rocks: the garnet-phengite gneisses and quarzites from the Lanterman Range, Antarctica

Mesostructural and microstructural relations between eclogitic boudins and country gneisses in the Ross Orogen of the Lanterman Range (Northern Victoria Land, Antarctica) are in some areas characterized by interlayering with sharp contacts on a cm scale, which indicate that the two rock-types underwent a common metamorphic evolution. Contrary to many other UHP felsic rocks that only preserve a poor record of the HP stage, the studied rocks have recorded a metamorphic history ranging from initial prograde amphibolite facies through the eclogite facies to the retrogressive amphibolite facies. The prograde amphibolite stage is documented by garnet relics preserving prograde zoning and bearing biotite, plagioclase, muscovite, phengite and rutile inclusions. The eclogite stage is characterized by the coexistence of phengite with pyrope-grossu-larite rich garnet, the latter containing phengite and paragonite inclusions, and by radial fractures within garnet around quartz pseudomorphs after coesite. Symplectites have formed during the amphibolite-facies retrogression. They consist mainly of biotite + plagioclase around phengite and garnet; muscovite, biotite and plagioclase grew along the main foliation. The reconstructed metamorphic evolution, involves a steep prograde and retrograde PT path as well as a HP-T peak. Along with the geochronological evidence of fast exhumation, this supports a model of arc-continent collision, with the HP rocks belonging to the over-riding plate. Their exhumation is mainly controlled by extension related to renewed “rollback” of subduction in front of the orogenic zone.

SHRIMP dating of zircons in eclogite from the Variscan basement in north-eastern Sardinia (Italy)

SHRIMP (Sensitive High-Resolution Ion Microprobe) U-Pb ages of zircons from a single sample of mafic eclogite (Punta de li Tulchi, Sardinia, Italy) are reported. The study under cathodoluminescence (CL) reveals two groupsof metamorphic zircons and the SHRIMP analyses allow recognition of three ages: 1) 453 ′ 14 Ma; 2) 400 ′ 10 Ma, and 3) 327 ′ 7 Ma. The age of 453 ′ 14 Ma could be that of the magmatic protolith and an age of 327 ′ 7 Ma can reasonably be attributed to the main Variscan collisional event in Sardinia, which produced Barrovian-type metamorphism, and retrogression of eclogite under amphibolite-facies metamorphism. The intermediate age 400 ′ 10 Ma is difficult to interpret and it could represent either the age of the eclogite facies metamorphism or it is a result of Pb-loss during the main Variscan event at 327 ′ 7 Ma.

Eclogite at the Antarctic palaeo-Pacific active margin of Gondwana (Lanterman Range, northern Victoria Land, Antarctica)

Well-preserved eclogites were found for the first time in Antarctica, at the Lanterman Range, northern Victoria Land. They are part of a mafic-ultramafic belt that lies between the Wilson Terrane, representing part of the palaeo-Pacific margin of Gondwana, and the Bowers Terrane, a Cambro-Ordovician volcanic arc and related sediments, accreted to the margin during the Ross Orogeny. The eclogites formed at temperatures in the range 750-850°C and pressures above 15 kbar and subsequently experienced a decompressional path to low pressure amphibolite facies conditions. The formation and exhumation of eclogites and the attainment of the metamorphic peak in adjacent rock units is consistent with a plate convergent setting model at the palaeo-Pacific margin of Gondwana.

P-T paths and migmatite formation: An example from Deep Freeze Range, northern Victoria Land, Antarctica

Abstract Migmatites exposed in the Deep Freeze Range, northern Victoria Land, Antarctica contain the mineral assemblage biotite + quartz + plagioclase + K-feldspar ± garnet ± cordierite ± sillimanite in mesosomes, with concordant and discordant leucosomes. Partial melting is considered the main migmatite-forming process at approximately 700–750 °C and 4–5 kbar. In mesosomes, garnet displays contrasting microstructural and compositional features. In some samples, spessartine-rich almandines record growth zoning in core regions. These garnets, together with cordierite and K-feldspar, grew along the main foliation at the expense of biotite and sillimanite. This textural evidence suggests a PT heating path due to regional low P-high T metamorphism linked to the Ross Orogeny. In other samples, spessartine-poor almandines are resorbed and overgrown with coronas consisting mainly of cordierite aligned along the main foliation. In these rocks, the matrix is defined by the association biotite + cordierite; relics of sillimanite are present in cordierite. These textural features are indicative of an early higher pressure assemblage (garnet + sillimanite) that was overprinted by a lower pressure assemblage containing cordierite as a stable phase. These samples follow an isothermal decompression path in P-T space. Data suggest two contrasting P-T paths which are interpreted as P-T trajectories of different crustal levels in the same tectonic setting that developed during the Cambro-Ordovician Ross Orogeny.

First evidence of a “Barrovian”-type metamorphic regime in the Ross orogen of the Byrd Glacier area, central Transantarctic Mountains

Abstract The Selborne Group comprises two metamorphic rock units, the muscovite±dolomite bearing Madison Marble and the biotite-muscovite±quartz-calcite Contortion Schist, which contains thick lenses of variably deformed metabasalts and metaconglomerates. Petrological and structural data indicate a polyphase metamorphic evolution including: i) an early stage of upper greenschist regional metamorphism (P = ~0.15–0.3 GPa; T = ~380–450°C), ii) prograde metamorphism during D1 up to amphibolite facies peak conditions (P = 0.58–0.8 GPa, T = ~560–645°C), iii) syn-D2 unloading-cooling retrograde metamorphism, iv) a post-D2 contact metamorphic overprint at variable T between 450 and 550°C and ~0.2 GPa connected to the emplacement of granitic plutons and felsic dyke swarms. Geochronological data constrain the polyphase syn-D1/D2 evolution between ~ 510 and 492 Ma. A similar metamorphic path, including a medium P stage but at lower T conditions, is documented in greenschist facies metabasalts within the Byrd Group in the Mount Dick area. The metamorphic pattern and close lithostratigraphic matching between Selborne Group and Byrd Group sharply contrast with the high-grade Horney Formation that is exposed north of the Byrd Glacier and corroborate the hypothesis that the Byrd Glacier discontinuity marks a first-order crustal tectonic boundary crossing the Ross orogen.

Geochemistry and Sr-Nd isotopes of amphibolite dykes of northern Victoria Land, Antarctica

Abstract Mafic dykes cutting the gneisses and migmatites in the Deep Freeze Range high-grade metamorphic complex of northern Victoria Land, Antarctica, have undergone strong recrystallization and deformation during amphibolite-facies metamorphism. Metamorphic mobility mostly affected the large-ion lithophile elements (LILE). Rare Earth (RE) and the high field-strength elements (HFSE) were essentially immobile during metamorphism. Together with the major-element geochemistry, this suggests primary characteristics of evolved tholeiitic magmas and mafic cumulates. No precise ages of intrusion are available for the dykes, but geological evidence suggest emplacement during the time interval 800 to 900 Ma. The Rb-Sr isotopic system in some of the dykes were also variably affected by a later thermal event, probably coincident with the time of amphibolite metamorphism, ca. 500–550 Ma ago. This event can be correlated with the Ross Orogeny in the Transantarctic Mountains. Nd isotopes and trace element abundances indicate that the dykes were derived by different degrees of partial melting and fractionation of heterogeneous sub-continental lithospheric mantle. The Nd isotopic compositions range from depleted to enriched signatures (ϵNd computed back to 850 Ma = +4.5 to −11.61), and are coupled to different trace element normalized patterns characterized by a slight positive Nb anomaly in the former case to a strong negative Mb anomaly for the latter samples. On isotopic and chemical ground the depleted signature of the mantle source resembles that reported for E-type MORB. The nature of the enriched components cannot be uniquely stated; nevertheless, on the basis of isotopic and geochemical data, it could be represented by sediments recycled into the sub-continental mantle or by crustal contamination during underplating of mafic magmas, or a combination of the two processes.

Partial melting of ultramafic granulites from Dronning Maud Land, Antarctica: Constraints from melt inclusions and thermodynamic modeling

Abstract In the Pan-African belt of the Dronning Maud Land, Antarctica, crystallized melt inclusions (nano-granitoids) occur in garnet from ultramafic granulites. The granulites contain the peak assemblage pargasite+garnet+clinopyroxene with rare relict orthopyroxene and biotite, and retrograde symplectites at contacts between garnet and amphibole. Garnet contains two generations of melt inclusions. Type 1 inclusions, interpreted as primary, are isolated, <10 μm in size, and generally have negative crystal shapes. They contain kokchetavite, kumdykolite, and phlogopite, with quartz and zoisite as minor phases, and undevitrified glass was identified in one inclusion. Type 2 inclusions are <30 μm in size, secondary, and contain amphibole, feldspars, and zoisite. Type 2 inclusions appear to be the crystallization products of a melt that coexisted with an immiscible CO2-rich fluid. The nanogranitoids were re-homogenized after heating in a piston-cylinder in a series of four experiments to investigate their composition. The conditions ranged between 900 and 950 °C at 1.5–2.4 GPa. Type 1 inclusions are trachytic and ultrapotassic, whereas type 2 melts are dacitic to rhyolitic. Thermodynamic modeling of the ultramafic composition in the MnNCKFMASHTO system shows that anatexis occurred at the end of the prograde P-T path, between the solidus (at ca. 860 °C–1.4 GPa) and the peak conditions (at ca. 960 °C–1.7 GPa). The model melt composition is felsic and similar to that of type 1 inclusions, particularly when the melting degree is low (<1 mol%), close to the solidus. However the modeling fails to reproduce the highly potassic signature of the melt and its low H2O content. The combination of petrology, melt inclusion study, and thermodynamic modeling supports the interpretation that melt was produced by anatexis of the ultramafic boudins near peak P-T conditions, and that type 1 inclusions contain the anatectic melt that was present during garnet growth. The felsic, ultrapotassic composition of the primary anatectic melts is compatible with low melting degrees in the presence of biotite and amphibole as reactants.