Garnet-bearing Assemblages Research Articles

Ultrahigh-pressure mantle metasomatism in continental collision zones recorded by post-collisional mafic rocks

Post-collisional mafic magmas are melts of mixed sources formed by the interaction between crust and mantle. We report the first identification of metasomatic sources formed at >200 km in continental collision zones that led to the Triassic post-collisional Shidao mafic magmas in the Sulu ultrahigh-pressure terrane, eastern China. The Shidao magmas are ultrapotassic with low Rb/K2O and Rb/Sr ratios, highlighting a major contribution of K-richterite to the melts, and with high Dy/Yb ratios indicating residual garnet in the source. Experiments have demonstrated that K-richterite coexists with garnet only at pressures higher than 6−7 GPa, which indicates that the Yangtze continental crust was subducted to >200 km, where it interacted with the overlying mantle to produce K-richterite + garnet-bearing assemblages. This ultra-deep source of the Shidao magmas spotlights an alternative mechanism for the genesis of post-collisional magmas, which are generally of shallow origin.

Two magma fractionation paths for continental crust growth: Insights from the adakite-like and normal-arc granites in the Ailaoshan fold belt (SW Yunnan, China)

Abstract Geochemical similarities between the continental crust and arc magmas have led to the inference that subduction zones may be the primary sites of crustal growth. Thus, it is necessary to unravel the petrogenetic mechanism(s) of granitoid generation in subduction-related settings to understand crustal growth through magmatic differentiation processes. In this study, we focused on granitoid generation in oceanic-continental subduction zones. We analyzed the whole-rock geochemistry and Sr-Nd isotopes, together with zircon U-Pb-Hf-O isotopes, of the newly identified Middle Triassic granitoids in the Ailaoshan high-grade metamorphic complex (Yunnan, SW China). All the studied granite samples were characterized by large ion lithophile element (e.g., Rb, Sr, and Ba) enrichments and high field strength element (e.g., Nb, Ta, and Ti) depletions, similar to arc-type rocks. They also showed a range of whole-rock Sr-Nd, (87Sr/86Sr)i = 0.7020–0.7048, εNd(t) = +0.6 to +4.2, and zircon Hf-O, εHf(t) = +10.3 to +18.1, δ18Ozircon = 5.09‰–6.65‰, isotope compositions, which overlap with those of previously reported coeval (ca. 237–235 Ma) hornblende diorite and granodiorite, the formation of which was interpreted to have originated from a mantle wedge metasomatized by a sediment-derived melt. Furthermore, the fractionation trends of some of the granitic samples and diorite-granodiorite suite overlap. They can be divided into two geochemical groups: Group 1 has intermediate to high SiO2 (66.9–73.8 wt%) and K2O (3.40–5.42 wt%) and low MgO (0.19–1.09 wt%) contents and shows depletion in heavy rare earth elements (HREEs; e.g., Yb and Y), resulting in adakite-like high Sr/Y (61–183) and La/Yb (47–90) ratios. Group 1 shows positive SiO2 versus Sr/Y and La/Yb correlations and negative SiO2 versus HREE and Y correlations, implying fractionation of a garnet-bearing assemblage. The negative correlations between SiO2 and εNd(t) and Nb/La reveal a crustal assimilation trend. Group 2 has relatively high SiO2 (72.6–76.5 wt%) and low K2O (1.93–3.82 wt%) and MgO (0.05–0.83 wt%) contents and shows depletion in middle REEs (MREEs; e.g., Gd and Dy) with low Sr/Y (1–10) and La/Yb (4–11) ratios. Group 2 granites show negative Gd/Yb versus SiO2 correlation, which indicates significant fractionation of an amphibole-bearing assemblage. Our results suggest that both group 1 and 2 granites were formed in a subduction setting from a common mantle-derived parental dioritic magma, but they experienced two distinct fractionation processes. While group 1 granites were likely formed by crustal assimilation and high-pressure (lower-crustal) garnet-dominated fractionation, group 2 granites were generated through low-pressure (middle-/upper-crustal) amphibole-/plagioclase-dominated fractionation. We suggest that these two fractionation trends are critical to crustal growth and the development of a more fractionated (felsic) upper crust.

Petrogenesis of isotopically enriched Quaternary magma with adakitic affinity associated with subduction of old lithosphere beneath central Myanmar

We present a model for the petrogenesis of magma with adakitic affinity in an old subduction zone, which does not involve slab melting and is constrained by new geochronological and geochemical data for Mt. Popa, the largest of three Quaternary volcanoes in central Myanmar (Popa, Monywa and Singu). The edifice is composed of Popa Plateau (0.8–0.6 Ma) with high-K rocks and a stratovolcano (< 0.33 Ma) predominantly composed of medium-K rocks with adakitic affinity (Mg# 45–63, Sr/Y > 40). The distinct K contents indicate that the adakitic magmas cannot be derived from Popa high-K rocks, but they share trace-element signatures and Sr–Nd isotope ratios with medium-K basalts from Monywa volcano. Our estimation of water contents in Popa magma reveals that primary magma for medium-K basalts was generated by partial melting of wedge mantle with normal potential temperature (TP 1330–1340 °C) under wet conditions (H2O 0.25–0.54 wt%). Its melting was probably induced by asthenospheric upwelling that is recognized by tomographic images. Mafic adakitic magma (Mg# ~ 63, Sr/Y ~ 64) was derived from the medium-K basaltic magma in fractional crystallization of a garnet-bearing assemblage at high pressure, and felsic adakitic rocks (Mg# ~ 45, Sr/Y ~ 50) were produced by assimilation-fractional crystallization processes at mid-crustal depths.

Monazite Microstructures and Their Interpretation in Petrochronology

The phosphate mineral monazite (LREE,Y,Th,Ca,Si)PO4 occurs as an accessory phase in peraluminous granites and Ca-poor meta-psammopelites. Due to negligible common Pb and very low Pb diffusion rates at high temperatures, monazite has received increasing attention in geochronology. As the monazite grain sizes are mostly below 100 μm in upper greenschist to amphibolite facies meta-psammopelites, and rarely exceed 250 μm in granulite facies gneisses and in migmatites, microstructural observation and mineral chemical analysis need the investigation by scanning electron microscope and electron probe microanalyzer, with related routines of automated mineralogy. Not only the microstructural positions, sizes and contours of the grains, but also their internal structures in backscattered electron imaging gray tones, mainly controlled by the Th contents, can be assessed by this approach. Monazite crystallizes mostly euhedral to anhedral with more or less rounded crystal corners. There are transitions from elliptical over amoeboid to strongly emarginated grain shapes. The internal structures of the grains range from single to complex concentric over systematic oszillatory zonations to turbulent and cloudy, all with low to high contrast in backscattered electron imaging gray tones. Fluid-mediated partial alteration and coupled dissolution-reprecipitation can lead to Th-poor and Th-rich rim zones with sharp concave boundaries extending to the interior. Of particular interest is the corona structure with monazite surrounded by apatite and allanite, which is interpreted to result from a replacement during retrogression. The satellite structure with an atoll-like arrangement of small monazites may indicate re-heating after retrogression. Cluster structures with numerous small monazite grains, various aggregation structures and coating suggest nucleation and growth along heating or/and enhanced fluid activity. Microstructures of monazite fluid-mediated alteration, decomposition and replacement are strongly sutured grain boundaries and sponge-like porosity and intergrowth with apatite. Garnet-bearing assemblages allow an independent reconstruction of the pressure-temperature evolution in monazite-bearing meta-psammopelites. This provides additional potential for evaluation of the monazite microstructures, mineral chemistry and Th-U-Pb ages in terms of clockwise and counterclockwise pressure-temperature-time-deformation paths of anatectic melting, metamorphism and polymetamorphism. That way, monazite microstructures serve as unique indicators of tectonic and geodynamic scenarios.

Trace elements in titanite: A potential tool to constrain polygenetic growth processes and timing

The petrological information preserved in the trace element signature of titanite is a valuable complement to in situ U/Pb geochronology, and can be used to refine interpretations on the growth of this mineral. We present trace element and U/Pb isotopic compositions of titanite grains in five amphibolite- to granulite-facies samples from the east Albany-Fraser Orogen of Western Australia. Chondrite-normalised rare earth element (REE) abundance patterns discriminate between titanite populations and correlate with backscatter electron (BSE) zonation. In two samples, titanite REE composition correlates with proximity to garnet, and in one sample titanite composition correlates with gneissic compositional banding.The Dy/Yb ratio can be used as a geochemical indicator to link titanite growth to garnet growth. Titanite that crystallised in a garnet-bearing assemblage is HREE-depleted with Dy/Yb > 2, whereas titanite that crystallised in a garnet-free assemblage typically has flat HREE patterns with Dy/Yb < 2. The titanite grains investigated in this study have a wide range of Eu and LREE signatures, with no obvious correlation to mineralogy, lithology, or growth environment. Furthermore, the Th/U ratio is not uniquely diagnostic of metamorphic or magmatic titanite. For the five samples reported here, Eu, LREE and Th/U are useful to discriminate between titanite populations, but not necessarily as growth process indicators.By integrating the REE signatures with U/Pb data, the five new titanite U/Pb ages can be linked to a range of processes: magmatic titanite crystallisation, metamorphic titanite growth, garnet growth, and/or cooling through the closure temperature for Pb diffusion in titanite.

Metamorphic evolution of Pan-African Wadi El Miyah Metasediments, Central Eastern Desert, Egypt: a distinctive LP/HT metapelitic sequence from the northern Arabian–Nubian Shield

The meta-volcanosedimentary sequence from the SW sector of Wadi El Miyah area constitutes an example of the Late Proterozoic Pan African supracrustal rocks exposed in the Central Eastern Desert of Egypt. This folded meta-volcanosedimentary sequence is juxtaposed along about 8-km-long WNW–ESE-trending thrust contacts with a highly foliated ophiolitic melange comprising serpentinites, metagabbros and quarztofeldspathic blocks and intruded by a gneissose hornblende diorite. Chaistolite-bearing mica schists crop out in three compositionally different metamorphic zones, marked by garnet, andalusite, and staurolite porphyroblasts, respectively. Microstructural data combined with careful mapping indicate that the studied metapelites were subjected to three deformational phases, D1–D3, each having its own impact on the development of planar and linear structures. These rocks witness the coexistence of a less ubiquitous dextral sense of shear and a later sinistral one that correspond to the D2-thrusting and D3-related phases, respectively. Geothermobarometry of the garnet-bearing assemblages combined with Th–U–Pb dating of metamorphic monazite by an electron microprobe indicates a counter-clockwise P–T–t path with a thermal peak assemblage of garnet + staurolite + chiastolite ± fibrolitic sillimanite at 622 °C/4.1 kbars during the D2 phase, with an age around 640–658 Ma. This is concordant with the ages assigned for the arc accretion phase in the Nubian Shield. The determined counter-clockwise P–T–t path of the studied rocks suggests that such a Buchan-type LP/HT sequence may be considered as a typical example of a regional–contact metamorphism which formed in areas that suffer from crustal thickening accompanied by emplacement of granitoids that affected the geothermal gradient of the crust.

Shock conditions recorded in NWA 8159 martian augite basalt with implications for the impact cratering history on Mars

NWA 8159 is an augite-rich martian basalt, formed by cooling of a relatively evolved, Ca-rich, Ti-poor and LREE-depleted lava, under relatively oxidizing conditions, during the early Amazonian. In addition to its distinct igneous petrogenesis and high fO2, NWA 8159 is also set apart from most martian shergottites with respect to the low degree of shock metamorphism required to preserve crystalline igneous plagioclase (An50–65). In this study, mineral transformations within and adjacent to shock veins in NWA 8159 were investigated using scanning electron microscopy, Raman spectroscopy and transmission electron microscopy to better constrain the unusal shock history of this meteorite. The transformation of olivine to ahrensite (Fe-ringwoodite) along shock vein margins, and tissintite and coesite formed from igneous mineral (labradorite and silica) grains entrained as clasts within shock veins has been documented in this study. We report on a previously unidentified mineral assemblage of Ca-Na-majoritic garnet, sodic-clinopyroxene and stishovite crystallized from shock melt. This mineral assemblage indicates a crystallization pressure of approximately 16 GPa, which is within the range of previous shock pressure estimates for this meteorite (15–23 GPa). The presence of a majoritic garnet-bearing assemblage throughout veins up to 0.6 mm wide indicates that the sample remained at high-pressure throughout the melt vein quench. Based on thermal models, the sample must have remained at high pressure for ∼100 ms. This shock duration is an order of magnitude longer than those experienced by more highly shocked shergottites such as Tissint or Zagami (>30 GPa; 10–20 ms) and would seem to imply a relatively large impact event. Recent numerical models demonstrate that a range of shock pressures and durations are realized by rocks within the ejected spall zone of a hypervelocity impact. The shock conditions experienced by NWA 8159 therefore do not require an impact event distinct from other shergottites. Rather, our findings suggest that this meteorite originated from near the martian surface at the edge of the impact site. The shock history of NWA 8159 provides a picture of Mars consistent with that derived from remote observation; that of a random cratering process that samples a geologically long-lived and complex planet.

Long-lived metamorphic P–T–t evolution of the Highland Complex, Sri Lanka: Insights from mafic granulites

Sri Lanka is composed of four major tectonic units known as the Wanni, Highland, Kadugannawa and Vijayan complexes, and preserves important records of the history of the Gondwana supercontinent. The Highland Complex records Neoproterozoic–Cambrian metamorphism associated with the assembly of the supercontinent Gondwana. We combine phase equilibria modelling with isotopic and trace element geochemistry to provide a time-integrated P–T evolution using representative garnet-bearing mafic granulites from the Highland Complex. New zircon U–Pb geochronology and garnet trace element data from one locality in the EHC, together with a dataset compiled from published results, indicate garnet and zircon existed in equilibrium during long-lived high temperature metamorphism for possibly longer than 100 Ma (ca. 660–520 Ma). Calculated pseudosections imply high-temperature to potentially ultrahigh temperature (>850 °C) conditions for peak metamorphism. The Highland Complex is characterised by a small decrease in pressure and temperature following peak conditions. This was followed by an accelerated rate of exhumation, leading to the partial replacement of garnet-bearing assemblages by lower pressure parageneses. Although the post-peak cooling/modest pressure decrease is a comparatively minor part of the P–T evolution, compiled zircon U–Pb data suggest it was the temporally dominant part of the thermobarometric history, comprising >100 Myr. The long-lived high post-peak temperatures require a stable in-situ heat source, which based on regional litho-tectonic correlations, was almost certainly elevated rates of crustal heat production. The minor cooling interval from P–T space is in fact is the temporally dominant part of the P–T evolution highlights that without time analysis, the P–T evolution does not convey the long-lived baric stability of the high-temperature history of the Highland Complex.

Differentiation mechanisms of the early Hadean mantle: Insights from combined 176Hf-142,143Nd signatures of Archean rocks from the Saglek Block

Positive 142Nd anomalies in Eoarchean rocks provide evidence for early (4.40 ± 0.03 Ga; Morino et al., 2017) depletion of Earth's mantle. This model age lies within errors of the Pb-Pb “age of the Earth” (Connelly and Bizzarro, 2016), and is similar to model ages inferred for crystallization of the lunar mantle (McLeod et al., 2014), implying that this large-scale event may reflect crystallization of a magma ocean following the Moon-forming impact. However, differentiation mechanisms responsible for the formation of this early depleted mantle reservoir and the depth at which it formed cannot be constrained from the Sm-Nd isotope system alone, because the magnitude of Sm/Nd fractionation during partial melting or fractional crystallization shows little dependence on pressure-controlled changes in mantle mineralogy. In contrast, the Lu-Hf isotope system is highly dependent on mineralogy, notably the presence or absence of garnet, an upper mantle phase, and thus may be used to constrain the pressure of fractionation. This study provides the first 176Lu-176Hf isotopic results on mafic and ultramafic rocks belonging to the Eoarchean (Nulliak) and Mesoarchean suites of the Saglek Block (northern Labrador, 3.2–3.9 Ga). The 176Lu-176Hf dating confirms the distinction between these two groups of rocks and provides ages consistent with those obtained from 147Sm-143Nd dating. The whole rock 176Lu-176Hf errorchrons yield ages and initial epsilon values of 3766 ± 140 Ma, ε176Hfi = 6.0 ± 2.5 and 3023 ± 390 Ma, ε176Hfi = −0.3 ± 2.5 for the Nulliak suite and the Mesoarchean suite respectively. The time-integrated 176Lu/177Hf for the sources of the Nulliak and the Mesoarchean suites considering a time of differentiation at 4.40 ± 0.03 Ga are estimated to be 0.047 ± 0.005 and 0.033 ± 0.005, respectively. For the Mesoarchean samples, the combined 146,147Sm-142,143Nd and 176Lu-176Hf data are consistent with a near-chondritic mantle source. On the other hand, Nulliak ultramafic rocks were derived from a mantle reservoir with superchondritic Lu/Hf and Sm/Nd. The Nulliak parent reservoir, however, does not plot on the ε176Hf-ε143Nd mantle array defined by modern oceanic basalts. Instead, the Nulliak source more likely belongs to a distinct array defined by Eo- and Meso-Archean komatiites. These results are interpreted in the framework of a simple model of crystallization of a primordial magma ocean. It appears that the fractionation observed in the mantle source of Nulliak was most likely generated by crystallization of a garnet-bearing assemblage in the shallow mantle, above the transition zone rather than by perovskite fractionation in the lower mantle. To preserve this depleted reservoir from the rest of the hot and vigorously convecting mantle, the Nulliak mantle source may have been isolated either at the top of the mantle in a buoyant lithosphere or near the core-mantle boundary, with the latter setting being more consistent with the komatiitic nature of the erupted rocks. Given that the garnet signature argues for differentiation of the Nulliak source at relatively shallow depth (few hundred kilometers), its isolation in the deep mantle would require a cumulate overturn following crystallization of the magma ocean.

Two phases of granulite facies metamorphism during the Neoarchean and Paleoproterozoic in the East Hebei, North China Craton: Records from mafic granulites

Archean cratons are commonly reworked during the late Paleoproterozoic orogeny, which may result in multi-phase granulite facies metamorphism in many domains. The East Hebei terrane of the North China Craton is developed with two phases of granulite facies metamorphism in the Neoarchean and Paleoproterozoic. The Neoarchean granulite facies metamorphism is recognized mainly in the rafts or slivers of supracrustal rocks from the Taipingzhai ovate-structural domain, characteristic of anticlockwise P-T paths. The mafic granulites at this domain contain two-pyroxene granulite assemblages which are commonly overprinted by a late phase of granulite facies marked by tiny-grained vermicular orthopyroxene-bearing assemblages. They only record the Neoarchean metamorphic ages at 2.53–2.52Ga and 2.51–2.48Ga as indicated by zircon U-Pb data, para-simultaneously with the emplacement of TTG rocks. In contrast, the Paleoproterozoic granulite facies metamorphism can be well recognized in the Saheqiao linear-structural belt, which is characterized by high-pressure granulite assemblages in mafic rocks with clockwise P-T paths. In this belt, the Archean two-pyroxene granulite assemblages are locally preserved, showing “red-eye socket” overprinting textures consisting of garnet coronas or fine-grained garnet and clinopyroxene bearing assemblages surrounding the early-stage pyroxenes. Zircon U-Pb data indicate that mafic granulites from the Saheqiao belt record not only the Neoarchean metamorphic ages of ∼2.50Ga but also some Paleoproterozoic ages of 1.97–1.83Ga. The first application of Lu-Hf garnet-whole rock isochron method to garnet-bearing mafic granulites from the Saheqiao belt yields ages of 1766.6±7.0 and 1787±22Ma, confirming that the overprinting of garnet-bearing assemblages occurred in the late Paleoproterozoic. Thus, the Saheqiao linear-structural belt may represent a strongly reworked domain during the Paleoproterozoic orogeny, rather than an Archean high-grade greenstone belt as previously considered.

Discovery of MgTiSi2O7: a new high-pressure silicate with the weberite structure synthesized at transition-zone conditions

The crystal structure and chemical composition of a crystal of MgTiSi2O7 synthesized in the model system MgTiO3–MgSiO3 at 15 GPa and 1600 °C have been investigated. The compound was found to crystallize with the weberite-3T structure type, space group P3121, with lattice parameters a = 6.3351(7), c = 16.325(2) A, V = 567.4(1) A3, and Z = 6. The structure was refined to R1 = 0.059 using 2092 independent reflections, and can be described as a sequence of pairs of polyhedral layers (M and N) stacked along [001]. As far as the cation sites are concerned, M and N layers have general formula AB3 and A3B, respectively, where B are the octahedrally coordinated cations (B1, B2, and B3), which mainly accommodate Si. The octahedral framework gives rise to three types of larger cavities occupied by eight-coordinated Mg (A1), Ti (A2), and mixed (Mg,Ti) (A3) atoms. Electron microprobe analysis gave the Mg0.99Si1.62Ti1.39O7 stoichiometry for the studied phase. The successful synthesis of this phase demonstrates that titanium can stabilize heretofore unknown Mg–Si-oxides, the major Earth and rocky planet-forming materials, and can provide new constraints on thermobarometry of wadsleyite/ringwoodite, and garnet-bearing assemblages.

Garnet and Spinel Oxybarometers: New Internally Consistent Multi-equilibria Models with Applications to the Oxidation State of the Lithospheric Mantle

New thermodynamic data for skiagite garnet (Fe3Fe23+Si3O12) are derived from experimental phase-equilibrium data that extend to 10 GPa and are applied to oxybarometry of mantle peridotites using a revised six-component garnet mixing model. Skiagite is more stable by 12 kJ mol–1 than in a previous calibration of the equilibrium 2 skiagite = 4 fayalite + ferrosilite + O2, and this leads to calculated oxygen fugacities that are higher (more oxidized) by around 1–1·5 logfO2log⁡fO2 units. A new calculation method and computer program incorporates four independent oxybarometers (including 2 pyrope + 2 andradite + 2 ferrosilite = 2 grossular + 4 fayalite + 3 enstatite + O2) for use on natural peridotite samples to yield optimum logfO2log⁡fO2 estimates by the method of least squares. These estimates should be more robust than those based on any single barometer and allow assessment of possible disequilibrium in assemblages. A new set of independent oxybarometers for spinel-bearing peridotites is also presented here, including a new reaction 2 magnetite + 3 enstatite = 3 fayalite + 3 forsterite + O2. These recalibrations combined with internally consistent PT determinations for published analyses of mantle peridotites with analysed Fe2O3 data for garnets, from both cratonic (Kaapvaal, Siberia and Slave) and circumcratonic (Baikal Rift) regions, provide revised estimates of oxidation state in the lithospheric mantle. Estimates of logfO2log⁡fO2 for spinel assemblages are more reduced than those based on earlier calibrations, whereas garnet-bearing assemblages are more oxidized. Importantly, this lessens considerably the difference between garnet and spinel oxybarometry that was observed with previous published calibrations.

New evidence for Palaeoproterozoic high grade metamorphism in the Trivandrum Block, Southern India

In-situ zircon U–Pb isotopic and monazite Th–U–Pb chemical ages from leucosome in migmatitic paragneiss at Kanjampara in the Trivandrum Block, India, demonstrate the occurrence of high-T metamorphism and anatexis at 1.92Ga. This Palaeoproterozoic tectonothermal event was strongly overprinted by the main Neoproterozoic-Cambrian granulite facies tectonism which itself involved significant partial melting and formation of garnet-bearing assemblages at 0.65GPa and >820–860°C. Monazite grains, which enclose zircon grains and locally preserve Palaeoproterozoic chemical age domains, were extensively reset and recrystallized at 565±6Ma. These monazite grains were further modified to form high-Th cuspate rims at 517±15Ma, equivalent to the lower intercept age for extensive Pb-loss from the highly discordant 1.92Ga zircon, 528±18Ma. These results confirm that at least some of the metasedimentary paragneisses in the Trivandrum Block are polymetamorphic, initially metamorphosed in the Palaeoproterozoic in an event older than the 1.89–1.85Ga granitic orthogneisses recognised from the area. The nature of the relationship between the paragneisses and older (2.0–2.1Ga) granitic and charnockitic orthogneisses of the Trivandrum and Nagercoil Blocks, in particular whether they share the same Palaeoproterozoic history or were interleaved after 1.92Ga, requires further investigation focussed on the presence or absence of the isotopic imprint of this event in both rock suites throughout the area.The growth or recrystallisation of monazite at ca. 565Ma and its further modification at ca. 520Ma indicates that the high-T metamorphism in the Pan-African was long-lived, with a duration of at least 45Myr. This adds further weight to recent proposals, based on zircon-monazite isotopic studies from other localities in the Trivandrum Block, that the region formed the mid- to deep-crust of a long-lived collisional hot orogenic belt that welded Gondwana from ca. 580–510Ma.

Thermobarometry and electron-microprobe Th-U-Pb monazite dating in garnet metapelites from the Capelinha Formation, Araçuaí Orogen, Brazil

The Capelinha Formation (Macaubas Group) consists of a lower quartzitic unit with metamafic intercalations and an upper metapelitic sequence. It occurs in a complex tectono-metamorphic sector of the Aracuai orogen, where post-collisional collapse-related structures superimposed collisional structures. The garnet-bearing assemblages started crystallization in the collisional deformation stage that formed the main regional foliation around 570 Ma. Garnet porphyroblasts display a well-developed growth zonation of Fe-Mg-Ca-Mn and show, from core to rim, increasing almandine and pyrope contents in contrast with decreasing grossular and spessartine contents. Mineral relations and microstructures provide criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The P-T values calculated from cores to rims of garnets, aligned along clockwise trends, resulted in increasing temperatures (from 500 oC up to 620 oC) under decompression conditions (from 8.0 kbar to 4.5 kbar). The Th-U-Pb dating of homogeneous monazites by electron microprobe revealed a recrystallization period at around 490 - 480 Ma. These ages can be related to the tectono-thermal event associated with the gravitational collapse, constraining the youngest time limit for metamorphic processes in the Aracuai orogen.

Transition from orogen-perpendicular to orogen-parallel exhumation and cooling during crustal indentation — Key constraints from 147Sm/144Nd and 87Rb/87Sr geochronology (Tauern Window, Alps)

The core of the Alpine orogen in the Eastern Alps (Tauern Window) experienced a change from orogen-normal shortening and post-nappe doming to predominantly orogen-parallel extension and tectonic unroofing during late Oligocene to Miocene northward indentation of the Adriatic Microplate. A new 147Sm/144Nd isochron age of 25.7±0.9Ma on a garnet-bearing assemblage from the lowest nappe complex forming a dome in the eastern part of the Tauern Window indicates that high temperatures related to post-nappe Barrovian-type metamorphism overlapped in time with the onset of doming. New 87Rb/87Sr biotite ages from this same complex combined with previously published 87Rb/87Sr white mica ages suggest that doming and exhumation began no later than 28Ma immediately in front of indenting Austroalpine crustal blocks, then migrated laterally to the ESE during rapid exhumation and orogen-parallel stretching beginning at about 23Ma. Rapid cooling ended at approximately 17Ma in the footwall of the Katschberg Normal Fault, which delimits the eastern margin of the Tauern Window. A similar pattern of migrating doming, orogen-parallel extension and cooling is recognized in the western part of the Tauern Window, where rapid exhumation began at about 20Ma, some 2–3Ma later than in the eastern part. This difference in the onset of rapid exhumation as well as the overall migration of doming from the center to the ends of the Tauern Window are attributed to fragmentation of the aforementioned Austroalpine blocks along the leading edge of the main Adriatic indenter (Southern Alps) as this indenter advanced to the north.

Multistage Svecofennian metamorphism: Evidence from the composition and U-Pb age of titanite from eclogites of the Belomorian Mobile Belt

The first geochemical study of titanite from eclogites and associated rocks of the Belomorian Mobile Belt (BMB) by secondary ion mass spectrometry made it possible to establish the compositional features of this mineral in the garnet-bearing and garnet-free assemblages. Titanite from garnet-bearing assemblages is characterized by upward convex REE pattern and lowered HREE content relative to LREE, as well as the average GdN/YbN ratio around 16.5. Titanite from metaultrabasic rock inherits the specific features of the host rock, which should be taken into account when comparing with titanite from metagabbro. Results of U-Pb (TIMS) dating of titanite confirms the identification of the early and late stages of the Svecofennian metamorphism in the studied areas of BMB: early metamorphism with the peak eclogite facies conditions at around 1900 Ma, retrograde amphibolite facies metamorphism at 1870–1880 Ma, and late allochemical metamorphism accompanied by the pegmatite formation with an age of 1840 Ma.

Genesis of garnet-bearing rocks at the Berezitovoe deposit, Upper Amur Region, Russia

The geological position, composition of mineral assemblages, and typomorphism of major minerals from garnet-bearing rocks at the Berezitovoe gold-base-metal deposit in the Upper Amur Region have been studied in detail. These are ore-bearing metasomatic rocks and metamorphosed porphyritic dikes. The garnet-bearing metasomatic rocks reveal zoning, which is caused by various degrees of metasomatic transformation of the Paleozoic porphyritic granodiorite that hosts the ore zone. The metasomatic replacement of granodiorite was accompanied by loss of Na, Ca, Ba, Sr and gain of K, Mn, and Rb. Garnet-bearing metamorphosed intermediate dikes occur within the metasomatic zone. The PT conditions of metamorphism and metasomatism are similar and estimated at 3.9 kbar and 500°C from various mineral equilibria. The results of physicochemical simulation of garnet-bearing mineral assemblages carried out by minimizing the Gibbs free energy and the geological data show that garnet-bearing mineral assemblages arose at the Berezitovoe deposit as a result of local high-temperature thermal metamorphism of previously formed low-temperature metasomatic rocks close in composition to classic beresite. In this connection, we propose considering garnet-bearing metasomatic rocks as high-temperature metamorphosed beresites.

Thermodynamic mixing properties and behavior of almandine–spessartine solid solutions

The heat capacity, Cp, of five solid-solution members of the almandine(Alm)–spessartine(Sps) binary, consisting of three synthetic polycrystalline and two natural single-crystal samples, was measured in the temperature range between 2 and 300K using relaxation calorimetry and between 282 and 764K using DSC methods. All garnets exhibit a λ-type heat-capacity anomaly at low temperatures resulting from a paramagnetic to antiferromagnetic phase transition. The temperature of the magnetic transition in Fe-rich garnets occurs between those of the two end-members (i.e. 9.2K for almandine and 6.2K for spessartine), but lies at lower values between 3.5 and 4.5K for more Sps-rich compositions with XMngrt>0.5.The calorimetric entropy at 298K shows mechanical-mixture behavior for Sps-rich garnets and a slight possible negative deviation from such behavior for Alm-rich compositions. At the 2σ level all data are, however, consistent with ideal mixing behavior and the Margules entropy interaction parameter, WS,FeMngrt, is zero for the Alm–Sps binary. Thermodynamic analysis of published high P and T phase-equilibrium Fe–Mn exchange experiments between garnet and ilmenite shows that the excess Gibbs free energy of mixing, ΔGex, for Fe–Mn in garnet is positive and asymmetric towards spessartine. Margules enthalpy interaction parameters of WH,FeMngrt=4170±518J/cation⋅mol and WH,MnFe=1221±588J/cation⋅mol are derived giving a maximum of ΔGex≈0.7kJ/cation⋅mol at XMngrt≈0.6. ΔHex obtained using autocorrelation analysis of published IR spectra of Alm–Sps solid solutions is in reasonable agreement with that derived from phase-equilibrium and calorimetry data. Previous diffraction and spectroscopic results on Alm–Sps garnets and quantum mechanical calculations made on almandine are used to interpret the macroscopic thermodynamic behavior from a microscopic basis.The relevance of the new garnet Fe–Mn mixing model for petrological calculations is demonstrated by incorporating it into the quaternary garnet mixing model of Berman (1990). Thus, better agreement for temperatures calculated using Fe–Mn garnet-ilmenite and Fe–Mg garnet-biotite geothermometry could be achieved. Temperatures calculated for Mn-poor and Mn-rich garnet-bearing assemblages, applying garnet-biotite thermometry, are in better agreement taking Fe–Mn mixing into account.

The Sklené garnet peridotite: petrology, geochemistry, and structure of a mantle-derived boudin in Moldanubian granulite

*Corresponding author The Moldanubian Sklene garnet peridotite, which preserves mineralogical, geochemical, and structural evidence for a protracted mantle evolution, is a small boudin (~1 m) enclosed by granulite of the Gfohl Assemblage. The peridotite has a porphyroclastic texture, in which large garnet and smaller olivine and pyroxene porphyroclasts (Stage 1) reside in a fine-grained, recrystallized matrix of olivine, pyroxene, and pargasitic amphibole (Stage 2). Garnet is surrounded by kelyphite, consisting of an outer zone of pargasite and an inner zone of spinel and pyroxene (Stage 3). Lattice-preferred orientations (LPO) for olivine, orthopyroxene, and clinopyroxene were measured by electron backscatter diffraction. The LPO of olivine (mostly Stage 2) has an [010] maximum parallel to the Z fabric direction and [100] and [001] girdles in the X–Y fabric plane. Both orthopyroxene and clinopyroxene have [010] maxima parallel to Z and [001] girdles in the X–Y plane. The Sklene peridotite is slightly depleted in incompatible major and minor elements relative to primitive mantle. Whole-rock REE contents are 0.3 to 0.8 times that of primitive mantle, with a convex upward pattern, and Rb, Th, U, Pb, and Sr show positive anomalies in an extended element plot. Among the highly siderophile elements, Os, Ir, Ru, and Pt are unfractionated, but Pd and Re are significantly depleted. A Sm-Nd garnet-whole rock-clinopyroxene isochron yields a cooling age of 338 ± 13 Ma, and e Nd and ( 87 Sr/ 86 Sr) i values for clinopyroxene (at 335 Ma) are +4.5 and 0.7035, respectively. By comparison with other Moldanubian garnet peridotites in the Czech Republic, the Sklene peridotite likely represents a fragment of subcontinental lithospheric mantle. It experienced early partial fusion and depletion of incompatible elements at moderate pressure in the spinel stability field, followed by transport to high-pressure conditions at 1230 oC and 50 kbar, where the Stage 1 garnet-bearing assemblage was stabilized. Subsequently, the depleted peridotite was metasomatized by transient, primitive basaltic melts, with the addition of Rb, Th, U, Pb, and Sr. Dynamic recrystallization, possibly with a progressive transition to grain size sensitive creep, occurred during Stage 2 at ~915 oC and 24 kbar in a flattening-dominated, transpressive deformation regime. This event occurred prior to incorporation of the peridotite in granulite and may reflect the early stages of convergence between Brunia and Moldanubia. The static recrystallization and local growth of spinel within kelyphite at 900 oC and 17–21 kbar (Stage 3) probably took place after entrainment in granulite and was likely isofacial with granulite peak metamorphism.

Origin and metamorphic evolution of magnesite-talc and adjacent rocks near Gemerská Poloma, Slovak Republic

*Corresponding author Magnesite and talc deposit Gemerska Poloma is located in Early Palaeozoic rocks of the Gemericum, Slovakia. It experienced both Variscan and Alpine greenschist-facies regional metamorphism and was overprinted by contact metamorphism connected with the intrusion of a Permian granite. As the origin of talc in Mg-carbonate rocks from this locality is interpreted as a result of either regional or contact metamorphism, relation of talc formation to these two processes is critically discussed. The estimated P–T conditions (6–7 kbar at 400 oC), reached during Alpine metamorphism, were obtained from the Permian metagranite that was not affected by the Variscan metamorphic event. The calculated P–T conditions were verified using various thermobarometers and pseudosections for biotite- and garnet-bearing assemblages in the metagranite.