Bernardo Cesare

“Nanogranite” and glassy inclusions: The anatectic melt in migmatites and granulites

Using as a case study a granulite from the Kerala Khondalite Belt, India, we show that a former anatectic melt can be preserved as tiny (<25 μm) droplets within refractory minerals, in this case garnet. The melt is either fully crystallized as a Qtz-Ab-Kfs-Bt cryptocrystalline aggregate (“nanogranite”), or completely glassy in inclusions <15 μm. Both nanogranite and glassy inclusions have a peraluminous, ultrapotassic granitic composition that, in this case, does not correspond to a “minimum melt” and points to high melting temperatures, in agreement with the ultrahigh-temperature origin of the rock. This discovery indicates that peritectic minerals, growing during incongruent melting reactions, act as hosts for inclusions of anatectic melt, and that in the general case of slow cooling of the crust these inclusions will occur as nanogranite. Exceptionally, in the smallest inclusions, glass may be present due to inhibition of crystallization. Our results extend the frontiers of petrological and geochemical research in crustal melting, as the composition of natural anatectic melts can be directly analyzed rather than assumed.

Crustal anatexis and melt extraction during deformation in the restitic xenoliths at El Joyazo (SE Spain)

Abstract The dacite of El Joyazo contains abundant metapelitic xenoliths. These can be divided into two main types: garnet-biotite-sillimanite and spinel-cordierite xenoliths. In the xenoliths the widespread occurrence of rhyolitic glass as interstitial films, foliation-parallel layers and primary melt inclusions in all mineral phases indicates that these assemblages developed in the presence of a melt phase, i.e. during anatexis. The composition of the interstitial glass is comparable to that of the melt inclusions, suggesting that melt was locally produced. Phase equilibria indicate that anatexis occurred at P−T conditions of 5−7 kbar and 850±50°C . Several microstructural lines of evidence show that melt extraction was assisted by deformation during foliation development, and that on the scale of the xenoliths (up to 50 cm) melt escaped mainly by flow along foliation planes. The development of a syn-anatectic foliation also suggests that metapelitic rocks were involved in high-grade metamorphism and partial melting prior to fragmentation and dispersion in the host dacite. Mass balance calculations, based on the chemical composition of interstitial glass and melt inclusions in minerals, the bulk xenoliths and representative samples of potential pelitic sources support a model wherein the xenoliths represent restites after the extraction of 30 to 55 wt.% melt from graphitic metapelite protoliths similar to the rocks constituting the surrounding Alpujarride metamorphic complex.

Hydrogen deficiency in Ti-rich biotite from anatectic metapelites (El Joyazo, SE Spain): Crystal-chemical aspects and implications for high-temperature petrogenesis

Abstract Titanium-rich biotites from graphite-bearing metapelitic xenoliths, equilibrated at ca. 850 °C and 7 kbar in the presence of a granitic melt, have been studied through complete chemical analysis and single-crystal XRD refinements. The chemical study combines EMP analyses, hydrogen determination by both SIMS and C-H-N elemental analysis, and Mössbauer spectroscopy. Biotites in the analyzed xenoliths have TiO2 contents ranging from 4.5 to 4.9 wt% and an XFe of 0.67. Their F and Cl contents are negligible, and Fe3+/Fetot ranges from 10 to 16%. The H2O content of the biotites ranges from 2.4 to 2.8 wt%, and a small fraction of H is accommodated in the lattice as NH4. Based on these full chemical analyses, the calculated OH occupancy is 1.26 to 1.30 groups per formula unit, more than one third less than the stoichiometric value. The entrance of Ti in the octahedral site of biotite is consistent with the Ti-oxy exchange, whereas Ti-Tschermak or Ti-vacancy substitutions play a very minor role. The Fe3+-oxy exchange cannot account for the observed OH deficiency. From single-crystal XRD, biotites belong to the 1M polytype and contain variable amounts of stacking faults. The c cell parameter, K-O4 and outer distances provide an independent estimate of the OH content, which agrees with SIMS determinations. The linear relationship between VITi4+ and the bond-length distortion of the cis-M2 octahedron reveals the partitioning of Ti4+ in M2, and the Ti4+ off-center shift toward O4 supports the occurrence of the Ti-oxy exchange. The ordering of Ti4+ over two non-equivalent M2 sites, which would be favored energetically, is in agreement with the evidence for a third octahedral site suggested by Mössbauer spectroscopy. The biotite dehydrogenation combined with the partitioning of Ti4+ in M2 and the low thermal expansion of Ti4+- containing octahedra, are the keys to understanding the thermal stabilization of Ti substitution in biotites.

Primary melt inclusions in andalusite from anatectic graphitic metapelites: Implications for the position of the Al2SiO5 triple point

Anatectic crustal xenoliths in the Miocene volcanic rocks of Mazarron (southeast Spain) contain andalusite with melt inclu- sions, an unprecedented finding. Microstructures indicate that the melt inclusions were trapped during andalusite growth. The vola- tile content of the peraluminous inclusions is too low (Cl , 0.4 wt%, F , 0.3 wt%, P 1500 ppm, B , 850 ppm) to have caused a significant reduction of the wet solidus temperature. Moreover, the presence of graphite, as observed, during partial melting is expected to have raised the temperature of the wet solidus. Melting temperatures for the inclusions—obtained from quartz-albite-or- thoclase haplogranite system (680-790 8C), Zr (620-705 8C), and light rare earth element (615-725 8C) thermometry—indicate that the stability of andalusite 1 melt is incompatible with the position of the most commonly used andalusite 5 sillimanite equilibrium, and that the Al2SiO5 triple point must be placed at higher tem- peratures and pressures.

Recovering the composition of melt and the fluid regime at the onset of crustal anatexis and S-type granite formation

Using a metatexite from the Spanish Betic Cordillera as an example, we show that in situ and otherwise impossible to retrieve compositional information on natural anatectic melts can be reliably gained from experimentally rehomogenized melt inclusions in peritectic garnets. Experiments were conducted on single garnet crystals in a piston cylinder apparatus until the complete homogenization of crystal-bearing melt inclusions at the conditions inferred for the anatexis. The compositions of quenched glasses, representative of the early anatectic melts, are leucogranitic and peraluminous, and differ from those of leucosomes in the host rock. The H 2 O contents in the glasses suggest that melts formed at low temperature (∼700 °C) may not be as hydrous and mobile as thought. Providing for the first time the precise melt composition (including the volatile components) in the specific anatectic rock under study, our approach improves our understanding of crustal melting and generation of S-type granites.

Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (southeast India): Petrology and further insight into titanium substitutions

Abstract Precise chemical composition, including Fe3+ and H, of biotite from a pegmatite dike and its host granulite from the Kerala Khondalite Belt of SE India has been determined using a multi-technique approach involving EMP, SIMS, Mössbauer, and C-H-N elemental analysis. Biotite in these rocks formed at T > 800-850 °C and P = 5 ± 1 kbar. The full analyses were normalized on the basis of [O12-(x+y+z)(OH)xClyFz]. Biotite in the pegmatite is Ti-, F-, and Cl-rich (0.33, 0.46, and 0.16 apfu, respectively), H2O-poor (OH = 0.86 pfu), has XMg = 0.49 and Fe3+/Fetot ≤ 3%. The low octahedral vacancies (0.06 pfu) and the high oxygen content in the hydroxyl site (OH + F + Cl = 1.49 pfu) confirm the role of the Ti-oxy substitution as a major exchange vector in these high-T biotites. In the host granulite, fine-grained biotite is Fe3+-free, has low Cl (0.03 apfu), and more variable composition, with Ti, F, and XMg in the ranges 0.26-0.36, 0.52-0.67, and 0.67-0.77, respectively. The number of octahedral vacancies is relatively large (0.10-0.18 pfu) and the sum of volatiles (OH + F + Cl) varies from 1.71 to 2.06 pfu. Systematic variations of XMg are a function of the microstructural position and are in agreement with retrograde exchange reactions: biotite included in or in contact with garnet has the maximum values, whereas crystals in the matrix have the minima. Titanium has systematic negative correlations with F, XMg, and (OH + F + Cl), whereas Al and octahedral vacancies are virtually constant. These trends indicate that the Ti-vacancy, along with substitutions involving Al, cannot explain the observed short-scale variations. Conversely, the Ti-oxy exchange appears to be active, resulting from combination of two vectors: the more conventional hydroxylation Ti4+ + 2O2- = (Fe,Mg)2+ + 2OH- and the “fluorination” Ti4+ + 2O2- = (Fe,Mg)2+ + 2F-. The systematic retrograde redistribution involves not only Fe and Mg as commonly observed, but also Ti, F, and H, in a way such to eliminate the primary Ti-oxy component of biotite.

Eocene partial melting recorded in peritectic garnets from kyanite-gneiss, Greater Himalayan Sequence, central Nepal *

Abstract Anatectic melt inclusions (nanogranites and nanotonalites) have been found in garnet of kyanite-gneiss at the bottom of the Greater Himalayan Sequence (GHS) along the Kali Gandaki valley, central Nepal, c. 1 km structurally above the Main Central Thrust (MCT). In situ U–Th–Pb dating of monazite included in garnets, in the same structural positions as melt inclusions, allowed us to constrain partial melting starting at c. 41–36 Ma. Eocene partial melting occurred during prograde metamorphism in the kyanite stability field (Eo-Himalayan event). Sillimanite-bearing mylonitic foliation wraps around garnets showing a top-to-the-SW sense of shear linked to the MCT ductile activity and to the exhumation of the GHS. These findings highlight the occurrence of an older melting event in the GHS during prograde metamorphism in the kyanite stability field before the more diffuse Miocene melting event. The growth of prograde garnet and kyanite at 41–6 Ma in the MCT zone, affecting the bottom of the GHS, suggests that inverted metamorphism in the MCT zone and folded isograds in the GHS should be carefully proved with the aid of geochronology, because not all Barrovian minerals grew during the same time span and they grew in different tectonic settings.

Hercynite as the product of staurolite decomposition in the contact aureole of Vedrette di Ries, eastern Alps, Italy

Metapelitic hornfelses in the contact aureole of the Vedrette di Ries pluton exhibit the terminal decomposition of Zn-poor Fe-staurolite in a muscovite-quartz-free domain. The reaction takes place only within coarsegrained sillimanite that has replaced andalusite porphyroblasts during prograde metamorphism. The product is a gahnite-poor hercynitic spinel, which occurs as very small grains closely associated in space with resorbed staurolite. Microstructural observations indicate that bereynite growth postdates the pseudomorphs of sillimanite after andalusite. The textural evidence for a genetic relationship between hercynite and staurolite is confirmed by the identical Fe/Mg/Zn ratios of the two minerals, which causes the collinearity of hercynite, staurolite and Al2SiO5 in FeO−MgO−ZnO−Al2O3−SiO2−H2O composition space (FMZASH), and indicates hercynite formed by the reaction: Fe-staurolite = 3.85hercynite + 5.1sillimanite + 2.55quartz + 2H2O Staurolite inclusions within andalusite did not break down to form hercynite, indicating a kinetic control, as well as little overstepping of the equilibrium conditions, of the reaction forming hercynite. Assuming overstepping did not occur, modelling of the reaction with existing thermodynamic data in the simplified FASH system suggests that the terminal breakdown of staurolite to form hercynite occurred at 2.5–3.75kbar and 585–655°C.

Andalusite-sillimanite replacement (Mazarrón, SE Spain): A microstructural and TEM study

Abstract At Mazarrón, SE Spain, dacitic lavas of the Neogene Volcanic Province contain numerous xenocrysts and xenoliths with abundant andalusite that displays variable degrees of transformation to both fibrolite and coarse sillimanite. At the onset of replacement, andalusite dissolves along grain boundaries and (110) cleavage planes, probably assisted by fluids or melts. At the same time, fibrolite crystallizes together with plagioclase, cordierite, and graphite in newly formed embayments or in the adjacent matrix. With increasing reaction progress, fibrolite needles coalesce into coarser sillimanite prisms, and direct topotactic replacement of andalusite is observed. The mutual crystallographic orientation of andalusite and sillimanite obtained from TEM investigation deviates slightly from the topotactic relationship proposed in the literature (cAnd || cSil, aAnd || bSil, bAnd || aSil). The two lattices are rotated by ~2.5° around aAnd (= bSil). With this misorientation, the structurally equivalent {032}And and {302}Sil planes, which exhibit the smallest misfit between the two lattices, become parallel. Macroscopic interfaces with such orientations are rare. Microscopically, however, decomposition of faces into {032}And || {302}Sil and {110} facets are common. The mutual crystallographic orientation of the reactant and the product phases is, therefore, controlled by lattice misfit minimization. The prismatic shape of the final coarse sillimanite crystals, however, is controlled by kinetic factors. The reaction seems to proceed fastest parallel to the octahedral Al chains resulting in the development of crystals elongated along the c axis. The high activation energy and the large overstepping of the equilibrium temperature required for the transformation are probably responsible for the large differences in reaction progress observed in the samples from Mazarrón.

Armouring effect on Sr-Nd isotopes during disequilibrium crustal melting: the case study of frozen migmatites from El Hoyazo and Mazarrón, SE Spain

Crustal melting is responsible for the production of large volumes of rhyolitic melt and therefore is central to understand the rheology of the crust and the mechanisms of crustal differentiation. The attainment of isotopic equilibrium during melting of crustal rocks is implicitly assumed in most isotopic dating and tracing studies. This assumption considers the melting event as an instantaneous process and does not take into account the duration of anatexis. To assess the critical role of the timescale of crustal melting, we have studied the unique occurrence of erupted migmatites enclosed as xenoliths in the El Hoyazo and Mazarron dacites of the Neogene Volcanic Province of SE Spain. These xenoliths represent the residue after some 30-60 % rhyolitic melt extraction at P-T conditions of 5-7 kbar and ∼850 °C, and consist of biotite, plagioclase, sillimanite, garnet, cordierite, graphite and abundant glass inclusions (i.e., not extracted rhyolitic melt) within each mineral phase. The timescale of melt extraction was ∼3 Myr and <0.8 Myr at El Hoyazo and Mazarron, respectively, resembling the duration of melting events during rapid anatexis caused by basalt underplating and crustal assimilation processes. In both localities, the minerals and glass inclusions of erupted migmatites preserve a significant Sr and minor Nd isotope disequilibrium. At Mazarron the isotopic disequilibrium is most marked owing to the shorter residence time of the melt within the source. The isotopic disequilibrium is not caused by the major xenolith-forming minerals but rather by the accessory phosphate inclusions (apatite ± monazite ± xenotime) hosted in garnet and biotite. The preservation of isotopic disequilibrium in these accessory phases has been facilitated by both their intrinsically low Sr and Nd diffusion coefficients and the armouring effect caused by their occurrence within biotite and garnet crystals, which acted as chemical barriers to Sr and Nd diffusion. This result implies that modelling of radiogenic isotope equilibration in natural systems should consider elemental diffusion in a composite medium with a resistance at the interface, i.e. different partition coefficients between adjacent mineral phases.