Al-rich Metapelites Research Articles

Alumina Hosts in Fe- and Al-Rich Metapelites from Transangaria (Yenisey Ridge, East Siberia): Distribution, Composition, and Mining Potential

Fe- and Al-rich metapelite from the Transangarian segment of the Yenisey Ridge (East Siberia, Russia) is a potential new source of high-alumina refractories. The rocks have relatively high average contents of Al2O3 (20 wt%) and Fe2O3 (7.91 wt%), moderate K2O (3.44 wt%), and low CaO (0.74 wt%). Their dominant mineral assemblages are andalusite + muscovite + margarite + chlorite + biotite + quartz or staurolite + kyanite or/and andalusite + chlorite + muscovite + biotite + quartz with ±garnet and ±plagioclase. Al2SiO5 polymorphs occur as up to 1.5 cm andalusite porphyroblasts and partial or complete pseudomorphs after andalusite (kyanite and staurolite). Accessories include abundant Fe–Ti oxides and sporadic REE-, Y-, Ca-phosphates; sulfides are negligible. The composition of Al2SiO5 concentrates obtained in laboratory by heavy-media and magnetic separation from ≥0.06 mm fractions meet all requirements for raw material of this type: >56 wt% Al2O3, <42 wt% SiO2, <1 wt% Fe2O3, <1.2 wt% TiO2, and <0.2 wt% (CaO + MgO). The andalusite, kyanite, and mixed ores yield 0.7–4.1 wt%, 0.7–2.2 wt%, and 1.9–6.0 wt% of concentrate, respectively. The best-quality ores rich in Al2SiO5 polymorphs reside in zones of contact and/or dynamic metamorphism superimposed over regional metamorphism of Al-rich rocks.

Geochemistry, Formation Settings, Composition, and Age of the Protolith for the Fe- and Al-Rich Metapelites of the North Yenisei Ridge

Geochemistry, Formation Settings, Composition, and Age of the Protolith for the Fe- and Al-Rich Metapelites of the North Yenisei Ridge

Ferruginious-aluminous metapelites of the North Yenisei Ridge: Formation paleosettings, nature and age of protolith

Research subject. Fe- and Al-rich metapelites of the Teya, Garevka and Angara complexes of the North Yenisei Ridge (western margin of the Siberian craton) were studied. Research methods. Recently-obtained geochronologic (SHRIMP II U-Pb zircon dating) and geochemical data on the distribution of major and trace elements were used to reproduce the composition and age of the protolith, along with the paleofacies formation conditions. Results. The ferruginous-aluminous metapelites of the North Yenisei Ridge consitute redeposited and metamorphosed products of Precambrian weathering crusts predominantly of the kaolinite rather than lateritic type, as was previously thought. The chemical weathering of rocks in the Early Proterozoic on the Yenisei Ridge did not reach the deep stage of lateritization with the formation of zones of final decomposition of aluminosilicates; however, this process was limited to the formation of weathering products of predominantly kaolinite-illite-montmorillonite-quartz composition. Conclusions. The petro- and geochemical characteristics of the studied metapelites are mainly due to the features of sedimentation during the formation of the protolith, which could have been formed due to the erosion of the Lower Proterozoic microgneisses of the Siberian craton with ages in the range of 1962-2043 Ma with the involvement of granitoid and volcanic admixture of mafic rocks into the erosion area and subsequent accumulation in marginal continental shallow basins under the conditions of a humid climate and a calm tectonic regime. These results are consistent with the data of lithofacies analysis and geodynamic reconstructions of the Precambrian evolution of geological complexes in the North Yenisei Ridge.

Instability of Al2SiO5 “Triple Point” Assemblages as a Consequence of Polymetamorphism in Al-Rich Metapelites

Instability of Al2SiO5 “Triple Point” Assemblages as a Consequence of Polymetamorphism in Al-Rich Metapelites

Geochemistry, petrogenesis and age of metamorphic rocks of the Angara complex at the junction of South and North Yenisei Ridge

The mineralogical, petrological, geochemical and geochronological data were used to evaluate the age and petrogenesis of compositionally contrasting metamorphic rocks at the junction between Meso-Neoproterozoic Transangarian structures and Archean-Paleoproterozoic complexes of the Angara–Kan inlier of the Yenisei Ridge. The studied metabasites and metapelites provide clues for understanding the evolution of the region. The magmatic protoliths of low-Ti metabasites were derived by melting of depleted (N-MORB) upper mantle, and their high-Ti counterparts are interpreted to have originated from an enriched mantle source (E-MORB). The petrogeochemical characteristics of protoliths of the metabasite dikes resemble those of within-plate basalts and ocean island tholeiites. The Fe- and Al-rich metapelites are redeposited and metamorphosed products of Precambrian weathering crusts of kaolinite and montmorillonite-chlorite-hydromica compositions. The Р–Т conditions of metamorphism (4.9–5.5 kbar/570–650°С for metabasites; 4.1–7.1 kbar/500–630°С for metapelites) correspond to epidote–amphibolite to amphibolite facies transition. The evolution of the Angara complex occurred in two stages. The early stage (1.18–0.85 Ga) is associated with Grenville tectonics and the late stage is correlated with accretion/collision episodes of the Valhalla orogeny, with the peaks at 810–790 and 730–720 Ma, and the final stage of the Neoproterozoic evolution of the orogen on the southwestern margin of the Siberian craton. The correlation of regional crustal processes with globalscale geological events in the Precambrian evolution of the Earth supports recent paleomagnetic reconstructions that allow a direct, long-lived (1400–600 Ma) spatial and temporal connection between Siberia, Laurentia, and Baltica, which have been parts of ancient supercontinents.

Zn-rich spinel in association with quartz in the al-rich metapelites from the Mashan khondalite series, NE China

Al-rich metapelites from the Mashan khondalite series are characterized by the assemblage Spl+Grt+Sil+Crd+Bt+Pl (An72)+Kfs+Quartz+graphite. Large amounts of spinel+quartz assemblages occur as inclusions in garnet and prismatic sillimanite in the Al-rich metapelites of the Mashan complex, NE China. The chemical composition of spinel is characterized by Zn-rich (X Zn=0.33–0.40. X Zn=Zn/Zn+Mg+Fe*) (Fe*=Fe2++Fe3+) and Fe3+ rich (up to 0.31 p.f.u.). The characteristic chemical composition and the mineral association indicated that the formation of spinel and quartz assemblage may be due to the breakdown of Zn-rich staurolite. The geothermobarometers studies show that the peak temperature of the Mashan complex is around 820 °C and the peak pressures is 8.0 kbar. The Mashan complex shows a typical orogen style P-T path.

Ordovician and Cretaceous tectonothermal history of the Southern Gemericum Unit from microprobe monazite geochronology (Western Carpathians, Slovakia)

The Southern Gemericum basement in the Inner Western Carpathians experienced a polyphase regional deformation. Differences in the pre-Alpine and Alpine events have been constantly discussed. To address this, monazites from metapelites and acid metavolcanic rocks were dated using the Th–U–Pb electron microprobe method. Three monazite generations, such as Precambrian, Early Paleozoic, and Alpine, have been recognized in the greenschist facies pelites and acid metavolcanic rocks of the Southern Gemericum basement. Both inherited magmatic monazite grains in metavolcanites and rare relics of detrital monazites within the polyphase monazite grains in metapelites yielded the Precambrian age in the time span of 550–660 Ma. They prove the provenance and derivation from deeper crustal Cadomian fragments. High-Y magmatic monazites of Early Paleozoic age (444 ± 13 and 477 ± 7 Ma) have been recorded in the acid metavolcanites and their metavolcaniclastics. These ages roughly fit within the previously published magmatic zircon age determinations (at 494 ± 1.7 and 464 ± 1.7 Ma) that clearly indicate two-phase volcanic activity in the Early Paleozoic Southern Gemericum basin. The Early Paleozoic magmatic monazites were partly overprinted by the low-Y Alpine monazites (133 ± 5 and 184 ± 16 Ma) at their rims. In Al-rich metapelites, the newly formed low-Y monazites of Alpine age commonly occur, reflecting the polystage compression geodynamic evolution with three distinct peaks at 100 ± 8, 133 ± 5, and 190 ± 16 Ma, respectively. No data as the evidence of the pre-Alpine metamorphic events were observed in metapelites. Only some monazites yield the age indications for the Permian extensional thermal re-heating (260–290 Ma). The monazite age data from the Southern Gemericum basement indicate the strong overprinting due to the polyphase Alpine deformation at least in the greenschist facies conditions.

P-T modeling of margarite + anorthite-bearing Al-rich metapelites in the Taebaeksan basin, South Korea: implications for accretion-related metamorphism during the Late Permian-Triassic orogeny

Greenschist-facies Al-rich metapelites of the Pyeongan Supergroup in the eastern part of the Taebaeksan Basin, South Korea, contain unusual mineral assemblages, including the associations Mrg + Cld + Ms ± Chl ± Cal, An + Mrg + Ms, and An + Ms ± Grt ± St (all rocks contain Qtz). The XMrg [Ca/(Ca + Na + K)] and XPa [Na/(Ca + Na + K)] values of margarite range from 0.12 to 0.92 and from 0.10 to 0.28, respectively. High-Ca plagioclase porphyroblasts are slightly zoned, from An85 in the core to An98 in the rim. The Mrg + Cld-bearing assemblage yields P-T conditions of ca. 450–480 °C, 4.0–5.0 kbar based on the intersection of isopleths of Fe/(Fe + Mg) of chloritoid and XPa of margarite in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O (MnNCKFMASH) system. The anorthite-bearing mineral assemblages yield P-T conditions of ca. 480–520 °C, 3.8–4.4 kbar, and the An contents of plagioclase increased during decompression rather than during heating, based on isopleths of An content and the modal proportion of plagioclase. Phase relations in \(T - X_{CO_2 } \) pseudosections at 4 kbar in the CASH-CO2 system indicate that anorthite in the Al-rich metapelites grew during the consumption of margarite at \(X_{CO_2 } \) values of ca. 0.2–0.5 during sub-isothermal decompression at temperatures of 480–500 °C. Consequently, the overall P-T path for the formation of chloritoid, margarite and anorthite is characterized by relatively rapid compressional heating up to 480 °C at 5.0 kbar (geothermal gradient of 27–30 °C/km), followed by sub-isothermal retrograde decompression. This clockwise P-T path in the eastern part of the Taebaeksan Basin reflects the effects of the Late Permian-Triassic Songrim orogeny, which probably occurred at an accretionary forearc-related active continental margin.

Provenance of Upper Cretaceous quartz-rich sandstones from the North Sudetic Synclinorium, SW Poland: constraints from detrital tourmaline

Upper Cretaceous quartz arenites that fill the North Sudetic Synclinorium on the northern periphery of the Variscan Bohemian Massif show high compositional but low textural maturity. They have been interpreted for years as derived largely from nearby granite plutons, i.e. as first-cycle sediments. A different provenance was revealed on the basis of a combined light and heavy mineral analysis, and particularly a detrital tourmaline study, given that tourmaline is a sensitive indicator of granitic/metamorphic/sedimentary (multi-recycling) origin. Cenomanian, Turonian, Coniacian and Santonian sandstones contain abundant angular tourmaline grains, together with zircon and rutile, and subordinately staurolite, garnet, anatase, kyanite, sillimanite and monazite. The successive – Cenomanian to Santonian – tourmaline populations are similar with regard to shape, colour and chemical composition. The latter points unambiguously to various metamorphic rocks with a predominant group of Al-rich metapelites (Al-rich, F-poor dravite). It is concluded that, in the Late Cretaceous, large Sudetic granite plutons such as the Karkonosze and Strzegom–Sobotka massifs were not exposed but were buried under a thick siliciclastic cover. These results coupled with published apatite fission-track data from the granitic Karkonosze Pluton and the gneissic/migmatitic Gory Sowie Massif reveal that Late Cretaceous quartz arenites of the North Sudetic Synclinorium reflect gradual exhumation of the surrounding massifs, but do not record the final exposure of crystalline rocks. Since the latest Turonian, this exhumation corresponded to the Late Cretaceous inversion episode in Central Europe. The textural immaturity of Upper Cretaceous sandstones is misleading in terms of their recycled origin, and it is inherited from immature clastic source rocks.

Amphibole-derived evidence of medium P/T metamorphic ratio in Alpujárride and Federico “HP” units (Western Betic-Northern Rif, Spain and Morocco): possible interpretations

The significance of zoned Ca-amphibole found in metapelites, quartzites, and synfolial veins of the Internal Zone of the Betic-Rif range (Federico units from Northern Rif and Alpujarride units from Western Betic) in the Alpine tectono-metamorphic evolution of these units is discussed for first time. Typical Al-rich metapelites from both areas show assemblages consisting of white mica and chlorite, with sporadic kyanite and chloritoid. Nevertheless, in the Rif zone, phyllites and synfolial veins of Permo-Triassic units show the assemblage pumpellyite + epidote + actinolite. In the Jubrique area (Betic zone), Ca-rich phyllites, fine-grained quartzites, and quartz veins show assemblages consisting of Ca-amphibole, plagioclase, epidote, titanite, chlorite, and quartz. The Al-in-amphibole thermobarometer defines clockwise pressure–temperature paths with a range of prograde temperatures and pressures between 272°C-1.2 kbar and 484°C-3.2 kbar for the Federico unit and between 274°C-1.1 kbar and 620°C-6.1 kbar in the Jubrique unit. Amphiboles from both areas define prograde pressure–temperature paths typical of Barrovian-type metamorphism. This finding contrasts with previous estimates, which deduced high-pressure conditions in both areas. The described amphiboles indicate metamorphic conditions similar to those found in the tectonically deepest complex (Veleta complex) of the Betic Internal Zone and suggest formation during a medium P/T Alpine event, which has not been previously identified in the Alpujarride complex.

The Teya polymetamorphic complex in the Transangarian Yenisei Ridge: An example of metamorphic superimposed zoning of low- and medium-pressure facies series

Two successive phases of metamorphism can be recognized based on mineralogical and petrological observations coupled with geothermobarometric estimates for chemical zoning in Fe- and Al-rich metapelites from the Teya crystalline rocks of the Transangarian Yenisei Ridge. The first phase is marked by the formation of low-pressure regional metamorphic complexes of the andalusite-sillimanite type (P = 3.9–5.1 kbar; T = 510–640°C), which were most likely related to the Middle Riphean Grenville events. In the second phase, metapelitic rocks underwent Late Riphean medium-pressure collisional metamorphism of the kyanite-sillimanite type (P = 5.7–7.2 kbar, T = 660–700°C), which resulted locally in an increase in pressure in the vicinity of thrusts. These results suggest that medium-pressure kyanite-bearing metapelitic rocks were formed as a result of collision-related metamorphism caused by thrusting of the Siberian cratonal blocks onto the Yenisei Ridge in the vicinity of the Tatarka deep fault.

Widespread relics of high-pressure metamorphism confirm major terrane accretion in Ecuador: a new example from the Northern Andes

Metamorphic rocks from two sections through the North Andean Cordillera Real in Ecuador contain phengites as relics of high-pressure (HP) metamorphism. Their maximum Si contents per formula unit are (1) 3.25 in Al-rich metapelites, (2) 3.35 in ordinary metapelites and garnet-rich metabasites, and (3) 3.45 in garnet-bearing orthogneisses. Detailed pressure–temperature paths were derived by calculating P–T pseudosections. Garnet-bearing metapelite and metabasite yielded a P–T path starting at 1.4 GPa and 525°C. Peak temperature conditions were 560°C at <1.4 GPa. The retrograde path passed through 0.7–0.8 GPa at 500°C. An adjacent metagranitoid experienced a similar P–T evolution at slightly lower temperatures of about 30–50°C. This is the first reported occurrence of HP metamorphism in the Cordillera Real; it is widespread and evidently was the result of continental collision. We hypothesize that a single microcontinent collided with the South American continental margin in Early Cretaceous time. In contrast to our findings, previous models have simply suggested the amalgamation of several terranes in the late Mesozoic or earlier in the region of Ecuador. Therefore, we propose that a widespread search for HP relics should be undertaken. Such relics provide a general criterion for defining subducted terranes and their respective boundaries.

Zoned Ca-amphibole as a new marker of the Alpine metamorphic evolution of phyllites from the Jubrique unit, Alpujárride Complex, Betic Cordillera, Spain

AbstractZoned Ca-amphibole from metaclastic rocks and synfolial quartz-rich veins of the Jubrique area (Alpujárride complex, Betic Cordillera, Spain), and their significance in the Alpine metamorphic evolution are described here for the first time. Typical Al-rich metapelites from this area show assemblages with white mica and chlorite, with sporadic kyanite and chloritoid. Nevertheless, some Ca-richer phyllites, fine-grained quartzites and quartz veins, show assemblages consisting of Ca-amphibole, plagioclase, epidote, titanite, chlorite and quartz. Ca-amphibole, appearing in large radial grains as well as in smaller subhedral crystals, displays a zonation with actinolite cores, overgrown by magnesiohomblende (and edenite) and minor tschermakite (and pargasite or hastingsite). The zoning suggests growth during prograde metamorphism. Retrogressive rims are variably developed in the several lithotypes. General zonation is well described in terms of the tschermakite, the edenite and the Fe3+Al-1, the MgFe2+-1, the NaK-1 and the OHCl-1 exchanges. Chemical parameters (IVAl, A(Na+K), BNa) indicate an evolution from low- to medium-grade temperature and pressure. The Al-in-amphibole thermobarometer provides a range of prograde temperatures between ∼300°C (for cores) and ∼600°C (for rims), for pressures from ∼1 to ∼6 kbar, respectively. Despite the uncertainty resulting from the fact that the thermobarometer used was calibrated for metabasites, amphibole defines a prograde pressure-temperature path typical of Barrovian-type metamorphism.

Geochemistry, protolith origin, and age of Lower Proterozoic Fe- and Al-rich metapelites in the Transangarian Yenisei Ridge

966 Argillaceous schists and their metamorphic prod� ucts are indicators of the continental crust composi� tion with respect to the behavior of some minor ele� ments, and their geochemical chronicling allows us to follow its evolution [1]. Pelitic rocks of specific iron– aluminous composition make up a significant portion in the structure of Precambrian metamorphic com� plexes in the folded frame of the southwestern margin of the Siberian craton; their metamorphism resulted in the formation of exotic mineral parageneses with extremely ferruginous minerals [2, 3]. Geochemical specifics of these rocks especially with respect to minor elements indicative for revealing geodynamic peculiarities of the protolith formation have been studied insufficiently, which causes controversial con� cepts on the peculiarities of their origin. According to the literature data, such a specific composition of rocks is interpreted as a result of the lateritic character of weathering processes [4], which is discordant with

Phase transition in the subducted oceanic lithosphere and generation of the subduction zone magma

Two metamorphic processes, i.e. subsolidus dehydration and partial melting occurring in MORB, metasediments and peridotite of subducted oceanic lithosphere are discussed on the basis of available experimental work and phase equilibrium modeling. Phase diagrams of hydrous MORB show that in most cold subduction P-T (pressure-temperature) regimes a large portion of water in the basic layer has released below the onset of blueschist facies (< 20 km), and at a depth (60–70 km) of transition from lawsonite blueschist to lawsonite eclogite facies through glaucophane dehydration; only a smaller portion of water will escape from the slab through dehydration of lawsonite and chloritoid in the depth range suitable for arc magma formation; and a very small portion of water stored in lawsonite and phengite will fade into the deeper mantle. The role of amphibole for arc magma formation is still arguable. In cold subduction P-T regimes, the dehydration of chlorite and talc in Al-poor metasediments, and chloritoid and carpholite in Al-rich metapelites at a depth around 80–100 km will make some contributions to the formation of arc magma. Comparatively, dehydration of serpentine in hydrated peridotite occurs at depths of 120–180 km, playing an important role in the arc magmatism. Subduction of oceanic crust along warm P-T regimes will cross the solidi at a depth over 80 km, resulting in partial melting under fluid-saturated and fluid-absent conditions in the metasediments involving biotite and phengite, and in the basic rocks involving epidote and amphibole. The melt compositions of the basic crust are adakitic at pressures < 3.0 GPa, but become peraluminous granitic at higher pressures.

Fe- and Al-Rich metapelites of the Teiskaya Group, Yenisei Range: Geochemistry, protoliths, and the behavior of their material during metamorphism

The Fe- and Al-rich metapelite of the Teiskaya Group in the trans-Angara part of the Yenisei Range are characterized by variable P-T parameters of their metamorphism. Geochemical data on these rocks were used to reproduce the nature and composition of their protolith. The metapelites were determined to be redeposited and metamorphosed Precambrian weathering crusts of the kaolinite type, which were produced by the erosion of Archean rocks of predominantly acid (granitoid) composition in shallow-water continental-margin basins in a humid climate. These results are consistent with the results of the lithological-facies analysis and geodynamic reconstruction of the Precambrian geological evolution of complexes in the Yenisei range. Differences were revealed between the REE patterns in metapelites in various metamorphic zones caused by both the compositional heterogeneity of the protoliths and the prograde (in terms of pressure) mineral reactions of collision metamorphism with the predominant effects of various processes during different evolutionary stages.

On the application of in situ monazite chemical geochronology to constraining P – T – t histories in high-temperature ( &gt; 850 ° C) polymetamorphic granulites from Prydz Bay, East Antarctica

We present electron microprobe-based in situ (Th + U)–Pb monazite chemical age data from granulite-facies metapelites in Prydz Bay, East Antarctica. The monazite age data define two distinct age populations, Late Neoproterozoic to Early Palaeozoic ( c . 570–520 Ma) and Neoproterozoic ( c . 950–820 Ma), that confirm the polytectonic nature of Prydz Bay. Our data suggest that similarity in lineation orientation along the Prydz Bay coast is not sufficient to necessarily indicate time-equivalance. The minimum duration of Early Palaeozoic tectonism, spanning at least 60 Ma, is constrained from an Mg–Al-rich metapelite: monazite hosted by coarse-grained orthopyroxene defines the oldest Early Palaeozoic population, whereas cordierite + orthopyroxene symplectites define the youngest Early Palaeozoic population. The spatial distribution of monazites and their ages is correlatable with the inferred mineral assemblage evolution. We are able to characterize the evolution of Early Palaeozoic tectonism in quantitative P – T – t –mineral assemblage space, demonstrating that ultrahigh-temperature tectono-metamorphism in Prydz Bay is of Early Palaeozoic age. The survival of Neoproterozoic inheritance in Fe–Al-rich metapelites has implications for high Pb retentivity in monazite when chemical and kinetic conditions are favourable. The approach and logic applied herein are entirely and directly transferable to the interrogation of any other metamorphic terrane.

Aluminous sapphirine granulites from the Eastern Ghats Belt (India): Phase relations and relevance to counterclockwise P-T history

In the Eastern Ghats Belt, 7-9-3 sapphirine in biotite-free Crd + Crn ± Sil ± Spl restitic domains occurs interlayered with Opx + Crd + K-feldspar assemblages extensively replaced by biotite. Natural settings and experimental phase relations indicate the restitic assemblages were derived by low pressure ( 800 °C) anatexis of pelitic protoliths. In the restitic domain, sapphirine formed by the reaction Crn + Crd + Spl = Spr. Volumetrically minor amounts of sillimanite associated with sapphirine were produced by the ancillary reaction Crn + Crd = Spr + Sil. The decomposition of sapphirine + cordierite to En + Sil symplectites is attributed to the reaction Spr + Crd = En + Sil; occasional spinel in the symplectite may have formed by the reaction Spr = Spl + Sil + En. A quantitative activity-corrected M(F)AS grid constructed for Grt and Qtz-absent Mg, Al-rich metapelites suggests that sapphirine stabilization was induced by loading at ∼4.5 kbar, 900 °C. The decomposition of sapphirine occurred during cooling down to 700 °C at 8 Kbar in a thickened crust. The record of low-P prograde heating that led to high T (> 800 °C, P max attained broadly synchronous with P max at ∼9 kbar. The phase petrological relations in the sapphirine granulites strongly indicate that near-peak T was attained prior to the attainment of P max value. The initial high thermal anomaly is reconciled with asthenospheric heating below a thinned crust, and does not correspond with the thermal condition in the upper zone of a crust of normal thickness experiencing asthenospheric perturbation. Since loading induces temperature increase due to shallowing of geotherms, the cooling of the thickened crust can only be explained by conductive heat loss to “cooler” thrust slices above the hot crust, and/or asthenospheric depression. The post-T max cooling in the CCW orogen suggests the continuance of thermal relaxation in the thickened crust in its attempt to attain isostatic stability with its environs.

A petrogenetic grid for aluminous granulite facies metapelites in the KFMASH system

AbstractDue to the retrograde cation exchange problems experienced by conventional geothermobarometers above their closure temperatures, petrogenetic grids are a potentially powerful alternative to unravelling the P–T evolution of ultrahigh‐T granulite terranes. A new qualitative KFMASH (K2O–FeO–MgO–Al2O3–SiO2–H2O) petrogenetic grid for Mg–Al rich metapelites containing K‐feldspar, sillimanite and quartzofeldspathic melt that successfully accounts for the majority of assemblages composed of variations of sapphirine, spinel, garnet, orthopyroxene, cordierite, biotite and quartz is developed. Univariant reactions are predicted utilizing a newly derived ‘melt projection’ and these reactions are entirely consistent with algebraically calculated reaction coefficients obtained using a set of standard phase compositions. Based upon observations of commonly associated mineral assemblages in natural lithologies the [Spr, Spl], [Qtz, Spl], [Bt, Spl], [Opx, Spr], [Opx, Qtz] and [Bt, Opx] invariant points are assumed to be stable, whilst the [Grt, Spr], [Grt, Qtz], [Spr, Qtz] and [Crd, Qtz] are assumed to be metastable. Biotite‐bearing assemblages are confined to the lowest temperatures, and sapphirine + quartz to the highest temperatures. Orthopyroxene + sillimanite ± quartz assemblages are confined to the highest pressures, whilst spinel‐bearing assemblages are stabilized by lower pressures. The alternative choice of invariant point stability leads to significant differences between this grid and previously proposed topologies. Spinel cannot be stable along with the orthopyroxene and sillimanite assemblage as previously proposed. Further, more subtle differences in topology result from the treatment of H2O in the chemographic projection used to deduce univariant reactions, and projecting from a water‐bearing quartzofeldspathic melt does not yield the same reaction coefficients as projection from H2O. The new grid allows reinterpretation of previously proposed evolutionary P–T paths for Mg–Al rich granulites from the Napier Complex and Rauer Group, East Antarctica, and In Ouzzal, Algeria.

Metamorphic evolution of Verrucano metasediments in northern Apennines: new petrological constraints

The Triassic Verrucano metasediments largely crop out along the mid Tuscan ridge, in the northern Apennines. The metasediments mainly consist of ferriferous quartzites and Al-rich metapelites and contain a specific mineralogy which lead to the reconstruction of their P-T path. The newly-formed metamorphic minerals are syn-to post-kinematics and they consist of magnesiocarpholite, muscovite, chlorite, paragonite, pyrophyllite, sudoite, kaolinite, and chloritoid, Magnesiocarpholite locally occurs only in a few syn-metamorphic quartz veins. Microanalytical techniques (SEM-EDS, TEM) allowed to collect compositional and microstructural information at the micron scale, to recognize the actual mineral reactions and their relative chronology. For Verrucano quartzites the stable paragenesis is chloritoid-chlorite-pyrophyllite, with sudoite and kaolinite being incompatible AFM phases. Thermodynamic calculations for Verrucano quartzites indicates that these rocks experienced a) relative high P/low T metamorphism (peak conditions ranging from 350 degrees C at 8 kbar to 420 degrees C at 10 kbar); b) a retrograde path characterized by cooling during decompression. These data demonstrate the usefulness of metapelites in deciphering the conditions of very low grade metamorphism. Besides, they support the regional geodynamic evolution which outlines the subduction of the continental crust of the Adria microplate under the European plate (Oligocene).