Metapelitic Assemblages Research Articles

Polymetamorphism and metastability in Paleozoic schists of the central Appalachian Baltimore Terrane, USA

Metapelitic assemblages are often considered highly reactive during prograde metamorphism, such that they effectively record a history of tectono-metamorphic processes. Across eastern North America, metamorphosed rift-to-drift stratigraphy has been central to disentangling the terminal history of Appalachian convergence. We report results from a single metapelitic outcrop in the Loch Raven Schist of Maryland's Baltimore Terrane, where regional metamorphism is historically interpreted to have derived only from the Ordovician Taconic orogeny. An aluminum-rich litho-horizon alludes to two phases of metamorphism: an early, low/medium-pressure (P) sillimanite-stabilizing phase largely overprinted by a subsequent medium/high-P kyanite-stabilizing event, terminating at c. 391–383 Ma. This provides evidence of substantial crustal disturbances in the central Appalachians during Avalonian collision of comparable timing to equivalent events in the north. In a subordinate and aluminum-poor litho-horizon, large (2–3.5 cm) garnet provides geochronological evidence only for a protracted phase of metamorphism at 440–424 Ma. We interpret this age as recording the early, low/medium-P phase of metamorphism that either: (a) extends the duration of the Taconic event; or (b) provides new evidence for metamorphism associated with Silurian tectonism and/or extension in the Central Appalachians. However, this assemblage appears to have been metamorphically unresponsive during Devonian overprinting, despite peak metamorphic conditions of ∼670 °C and ∼8 kbar. We suggest that a potential combination of the paucity of fluid, limited strain accumulation, and coarse refractive assemblages stabilized a kinetically sluggish and metastable composition.

Peak Alpine metamorphic conditions from staurolite‐bearing metapelites in the Monte Rosa nappe (Central European Alps) and geodynamic implications

AbstractThe tectono‐metamorphic evolution of the European Alps is still contentious. The Monte Rosa tectonic unit is a prominent nappe in the Central European Alps and estimates of its peak Alpine pressure (P) and temperature (T) conditions are essential for reconstructing its tectono‐metamorphic evolution. However, the reported peak Alpine pressure and temperature estimates vary considerably between 1.2 and 2.7 GPa and 490 and 640°C for a variety of lithologies. Here, we show petrology and pseudosection modelling of metapelitic assemblages from the western portions of the Monte Rosa nappe (upper Ayas valley, Italy). We present newly discovered staurolite–chloritoid‐bearing metapelitic assemblages. These assemblages exhibit an Alpine high‐P metamorphic overprint of a former contact‐metamorphic mineral assemblage generated by post‐Variscan granitic intrusions. Staurolite contains major amounts of Zn (up to 1.0 atoms per formula units), which is currently, in contrast to Fe‐ and Mg‐staurolite end‐members, not considered in any thermodynamic database. We employ two end‐member mixing models for Zn in staurolite, site mixing, and molecular mixing. Both models enlarge the pressure and temperature stability range for the observed assemblage, where site mixing has the largest influence of ±0.2 GPa and ±20°C. Our results for three metapelite assemblages, with and without staurolite, indicate peak Alpine pressure of 1.6 ± 0.2 GPa and peak temperature of 585 ± 20°C. These peak pressure estimates agree with previously published estimates for metagranites in the nappe, and are in stark contrast with peak pressure obtained from talc‐, chloritoid‐, phengite‐, and quartz‐bearing lithologies termed ‘whiteschists’ (>2.2 GPa). Our results confirm a variation of peak Alpine pressure of 0.6 ± 0.2 GPa between metagranite/metapelite lithologies and a nearby whiteschist lens (>2.2 GPa) within the metagranite. Field observations indicate that the studied region is structurally coherent and that the whiteschist is not a tectonic slice formed by tectonic mélange. We suggest that the consistent peak pressure for metapelite and metagranite assemblages represents the regional peak pressure and that the higher pressure recorded in the whiteschist lens is likely due to dynamic pressure, possibly resulting from tectonic and/or reaction‐induced stresses. If the calculated pressure of 1.6 ± 0.2 GPa represents regional peak Alpine conditions, then the Monte Rosa nappe was exhumed from a significantly shallower depth than previously assumed, based on peak pressure estimates > 2.2 GPa for whiteschist lithologies.

Calibration of the biotite-muscovite geobarometer for metapelitic assemblages devoid of garnet or plagioclase

Determination of metamorphic pressure (P) – temperature (T) conditions of metapelitic assemblages sometimes suffers from low-CaO content of garnet and/or plagioclase, thus the garnet-plagioclase-related geobarometers lose their usage under such circumstances. Therefore, calibration of garnet-plagioclase-absent geobarometers is very useful. In this paper, the biotite-muscovite geobarometer was empirically calibrated under P-T conditions of 1–12 kbar and 470–760 °C using 270 natural metapelite samples. About 86.5% of the total calibrant samples yielded similar biotite-muscovite pressure estimates to the well-calibrated garnet-Al2SiO5-plagioclase-quartz (GASP) geobarometer within error of ±1.5 kbar. The total error of the biotite-muscovite geobarometer is estimated to be less than ~±1.6 kbar. The applicability of the biotite-muscovite geobarometer has been certified through application to natural metapelite samples coupled with the Ti-in-biotite geothermometer. When garnet and/or plagioclase are absent in metapelitic assemblages such as two-mica schist / gneiss, this geobarometer may play its unique and irreplaceable role. An electronic spreadsheet (Table S3) is deposited at the journal's website for iterative application of the biotite-muscovite geobarometer in combination with the Ti-in-biotite geothermometer.

Electron microprobe petrochronology of monazite-bearing garnet micaschists in the Oetztal-Stubai Complex (Alpeiner Valley, Stubai)

Monazite dating in metapelites is an emerging method to investigate polymetamorphic areas. A protocol for Th–U–Pb dating of monazite by electron microprobe was adopted for a JEOL JXA-8530F. It was applied to the Variscan and Early-Alpine metamorphic Austroalpine Oetztal-Stubai Complex (OSC). In the Alpeiner Valley in the Stubai region, the Schrankogel complex is the eastern succession of the Central Metabasite Zone. In this part, metabasites are alternating with metapelites. In 4 samples from micaschist lenses, dominantly Carboniferous monazite isochrone ages at 335 ± 4 Ma, 320 ± 4 Ma; 319 ± 4 Ma and 319 ± 4 Ma were obtained. The micaschist samples with diverse modal compositions and variable bulk rock Ca contents of calculated assay, display distinct monazite microstructures, as quantified by automated SEM-MLA (mineral liberation analysis) routines. Clusters of small monazite could indicate new crystallization and yielded isochrones at 313 and 304 Ma. In contrast, corona structures of apatite and allanite around large monazites with isochrones between 350 and 315 Ma suggest a decomposition during decreasing temperature. Garnets in metapelitic assemblages display growth zonations with low pyrope contents in the cores and pyrope-rich rims. A prograde metamorphism with high pressure amphibolite-facies peak conditions at ~ 12 kbar and ~ 680 °C, and a post Pmax path with decompression to 4 kbar and 640–600 °C was estimated from the micaschists and from zoned Ca-amphiboles in retrogressed amphibolitized eclogites. The P–T path entered the monazite stability field during the decompression. This signals a Carboniferous age of the metamorphism. A minor population in one sample is composed of sporadic Permian single monazite ages. A Cretaceous monazite population is lacking. In the wide parts of the Austroalpine basement with Carboniferous-to-Cretaceous mica mixing ages, monazite age populations allow to discriminate a distinct Permian metamorphic event.

Original Calibration of a Garnet Geobarometer in Metapelite

In many metapelitic assemblages, plagioclase is either CaO-deficient or even absent. In such cases, all the widely applied, well-calibrated plagioclase-related geobarometers lose their usage. Fortunately, it has been found that a net-transfer reaction including intracrystalline Fe2+–Ca2+ exchange in garnet is pressure-sensitive, therefore, a garnet geobarometer can be empirically calibrated under pressure–temperature (P–T) conditions of 430~895 °C and 1~15 kbar. The chemical composition range of the calibrant garnet is XCa = 0.02~0.29 and XFe = 0.42~0.91, and covers the majority of garnet in metapelite. The total error of this geobarometer was estimated to be within ±1.3 kbar. The application of this garnet geobarometer to metamorphic terranes certifies its applicability, and this geobarometer can play a unique role, especially when plagioclase is absent or CaO-deficient. Metamorphic P–T conditions can be simultaneously determined by the garnet–biotite pair through the application of the present garnet geobarometer in combination with a well-calibrated garnet-biotite geothermometer.

Conditions and timing of low-pressure – high-temperature metamorphism in the Montresor Belt, Rae Province, Nunavut

Pressure–temperature–time (P–T–t) estimates for the Montresor Belt, obtained using phase equilibria and geospeedometry modelling integrated with in situ U–Th–Pb monazite geochronology, shed new light on the tectonometamorphic effects of the Snowbird phase of the Trans-Hudson orogeny. Typical metapelitic assemblages of the lower Montresor group consist of white mica, biotite, plagioclase, quartz, and andalusite, which in some rocks is partly or completely pseudomorphed by white mica. The observed assemblages reflect peak P–T conditions centring at approximately 575 °C and 3 kbar. Rocks with high bulk Fe/Mg contents contain compositionally zoned garnet, permitting the addition of further constraints on the conditions of metamorphism in the Montresor Belt: Core compositions of earliest-grown garnets indicate initial garnet crystallization at approximately 535 °C and 2.3 kbar, suggesting a nearly isobaric P–T path of prograde metamorphism with a gradient of approximately 50 °C·kbar–1. Chemical age-dating of monazite inclusions in garnet yields ages of ca. 1870 ± 9 to 1837 ± 9 Ma. Retrograde, pseudomorphic andalusite replacement by white mica at approximately 540 °C is inferred to have been controlled by variations in bulk rock chemistry. Morphologically corroded and chemically heterogeneous monazite adjacent to white mica pseudomorphs suggests that andalusite replacement took place at ca. 1792 ± 10 Ma, possibly associated with extension and movement along the detachment fault separating the upper and lower Montresor groups. Simulations of diffusion across chlorite- and biotite-filled cracks in garnet assumed to be coeval with andalusite replacement suggest that the rocks have experienced the retrograde event for at least 20 My.

Dehydration of metapelites during high‐P metamorphism: The coupling between fluid sources and fluid sinks

AbstractPolymetamorphic metapelites and embedded eclogites share a complex, episodic interplay of dehydration and fluid infiltration at the eclogite type‐locality (Saualpe–Koralpe, Eastern Alps, Austria). The metapelites inherited a fluid content (i.e. mineral‐bound OH expressed in terms of mol.% H2O) of ∼6–7 mol.% H2O from high‐T–low‐P metamorphism experienced during the Permian. At or near Pmax of the subsequent Eoalpine event (∼20 kbar and 680°C), the breakdown of paragonite to Na‐rich clinopyroxene and kyanite in metapelites released a discrete pulse of hydrous fluid. Prior to the dehydration event, the rocks were largely fluid absent, allowing only limited re‐equilibration during the prograde Eoalpine evolution. Similarly, Permian‐aged gabbros have persisted metastably due to the absence of a catalyst prior to fluid‐induced re‐equilibration. The fluid triggered partial to complete eclogitization along a fluid infiltration front partially preserved in metagabbro. Near‐isothermal decompression to ∼7.5–10 kbar and 670–690°C took place under fluid‐absent conditions. After decompression, a second breakdown of phengitic white mica and garnet produced muscovite, biotite, plagioclase and ∼0.1–0.7 mol.% H2O that enhanced extensive fluid‐aided re‐equilibration of the metapelites. Potential relicts of high‐P assemblages were largely obliterated and replaced by the recurrent amphibolite facies assemblage garnet+biotite+staurolite+kyanite+muscovite+plagioclase+ilmenite+quartz. The hydrous fluid originating from the metapelites infiltrated the embedded eclogites at these P–T conditions and induced the local breakdown of the peak assemblage omphacite and garnet to fine‐grained symplectites of diopside and plagioclase. Further fluid infiltration led to the formation of hornblende–quartz poikiloblasts at the expense of the symplectites. The metapelites re‐equilibrated until the growth of retrograde staurolite consumed any remaining free fluid, thereby terminating the process. Further re‐equilibration is inhibited by both the lack of a catalytic fluid and H2O as a reactant essential for rehydration reactions. The interplay between fluid sources and fluid sinks describes a closed cycle for the rocks at the eclogite type‐locality. Final, near‐isobaric cooling is indicated by a slight increase of XFe in garnet rims. Post‐decompression dehydration and fluid‐aided re‐equilibration arrested by the introduction of staurolite might explain the apparently homogeneous retrogression conditions as well as the notorious absence of diagnostic high‐P assemblages in metapelites at the eclogite type‐locality.

The long-term high-temperature history of the central Namaqua Metamorphic Complex: Evidence for a Mesoproterozoic continental back-arc in southern Africa

Reaction textures, equilibrium assemblage modelling and in situ U–Th–total Pb monazite and zircon U–Pb LA-ICP-MS dating of metapelitic rocks provide new insights to the tectono-metamorphic evolution of the Kakamas Terrane in the central Namaqua Metamorphic Complex (NMC). Staurolite inclusions in garnet porphyroblasts and patchy sillimanite intergrowths probably after kyanite record an amphibolite facies prograde stage of the Mesoproterozoic Kibran/Namaquan orogeny. The peak assemblages in metapelites are (i) osumilite, replaced by alkali feldspar+cordierite+orthopyroxene+quartz±plagioclase, (ii) spinel+quartz and (iii) corundum+quartz in the southwest of the study area and (iv) garnet+cordierite+quartz+sillimanite±alkali feldspar±plagioclase±biotite parageneses in the northeast. The spinel+quartz bearing metapelites in the highest-grade domains in the southwest of the study area equilibrated at 800–900°C at ∼5.0kbar. Osumilite may have formed at somewhat higher temperatures in excess of 900°C. In shallower crustal levels, exposed in the northeast of the study area, the metamorphism reached 640–680°C at 4.0–4.5kbar, followed by minor decompression to 3.5kbar. Isobaric cooling during retrogression in the highest-grade rocks forms biotite+sillimanite+quartz intergrowths replacing Fe–Mg peak and post-peak phases at 650–700°C. Minor re-heating overprints these textures forming a new garnet generation. P–T conditions lower than the middle/upper amphibolite facies conditions are not recorded in the study area.Two generations of zircon showing oscillatoric zonation document an early Kibaran stage of anatexis in metapelites at 1351–1299Ma, followed by a more commonly seen younger stage of partial melting at ∼1200Ma. The UHT peak metamorphism is either associated with the first anatexis at ≥1300Ma or, more likely, is an early stage of the widespread partial melting in the NMC at ∼1200Ma. The monazite data set shows, in addition to ∼1200Ma ages, two further age populations. The first of these, dated as 1090±9–1071±6Ma, is associated with the second garnet growth. Younger monazite rims, formed at ∼1000Ma in lower grade rocks, are not related to other mineral reactions or textures and are interpreted as forming in relation to late-orogenic fluid influx.The long-lasting high-temperature history (>650°C) between ∼1350 and <1100Ma, locally reaching UHT-conditions, with slow cooling and heating episodes at near-constant mid-crustal pressures is not well compatible with crustal thickening and continent-collision models that have been proposed in the existing literature. We interpret the Mesoproterozoic evolution of the high-temperature crust in the central NMC as related to long-lasting, steady heat transfer from the mantle into a thinned lithosphere in a continental back-arc mobile belt setting, reaching higher granulite facies and in places UHT conditions during the Mesoproterozoic Kibaran/Namaquan orogeny.

Calibration of a Ti-in-muscovite geothermometer for ilmenite- and Al2SiO5-bearing metapelites

Abstract The Ti-in-muscovite geothermometer was empirically calibrated as ln[T(oC)] = 7.258 + 0.289 ln(Ti) + 0.158[Mg/(Fe + Mg)] + 0.031 ln[P(kbar)] using ilmenite- and Al2SiO5-bearing assemblages in metapelites under P–T conditions of 450–800 °C and 0.1–1.4 GPa. The calibration was conducted for muscovites containing Ti = 0.01–0.07, Fe = 0.03–0.16, Mg = 0.01–0.32 and Mg/(Fe + Mg) = 0.05–0.73, respectively, on the basis of 11 oxygen per formula unit. Such compositional range covers more than 90% natural muscovites, and the random error of this thermometer is estimated to be of ± 65 °C. The geothermometer was validated against a set of independently determined temperature conditions between different degrees in samples from different prograde, inverted and contact metamorphic terranes. Application of this thermometer beyond the calibration conditions is not encouraged.

Calculated stabilities of sodic phases in the Sambagawa metapelites and their implications

Pressure (P)–temperature (T) pseudosection analyses were carried out on metapelites from Sambagawa belt by using Perple_X 07 so as to determine mineral equilibria and the stability of sodic phases, in the model system MnO–Na2O–K2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–(CO2) under high-pressure (HP) conditions (0.5–2.5 GPa/400–600 °C). A pressure–XNA [=Na/(Na+Al–2K–1.5Ca)] pseudosection at 500 °C is also calculated to examine the effect of Na/Al value of the bulk-rock composition on the stabilities of sodic minerals. The bulk-rock compositions of Sambagawa metapelite are variable in XNA values. The calculation results of stable assemblages in metapelites under the blueschist and eclogite facies conditions indicate that: (i) paragonite and glaucophane are stable throughout the wide XNA range of bulk-rock compositions of host rocks; (ii) stable P–T conditions of sodic pyroxene enlarge with increasing XNA value; and (iii) the stability field of paragonite enlarges with the presence of CO2 in the metamorphic fluid. The suggested wide stability of paragonite in metapelites and the relationships between the stability of sodic pyroxene and the bulk-rock compositions explain the reasons why (i) the occurrence of omphacite in metapelites from several subduction-related terranes is rare; and (ii) paragonite commonly occurs as inclusions in garnet of metapelites from the Besshi region of the Sambagawa belt. Paragonite is an important sodic phase of HP metapelites, and a combination of paragonite and quartz with high residual pressure included in garnet may be a useful indicator to verify the evidence for the eclogite facies metamorphism recorded in metapelites.

Garnet compositions as recorders of P– T– t history of metamorphic rocks

On the basis of tests made in recent studies, we conclude that the simultaneous solutions of the equilibrium conditions of garnet–biotite Fe–Mg exchange and Grs = Plag + Aluminosilicate + Qtz (GASP) reactions, as formulated on the basis selected experimental data and well constrained thermodynamic mixing properties of garnet and plagioclase, offer robust estimates of P– T conditions of metapelitic assemblages that consist of the above minerals. Additional calculations are presented to show the compatibility of the retrieved P– T conditions of natural assemblages with the aluminosilicate phase diagram. We also calculate the minimum grain sizes of garnets that should be used for the calculation of the peak metamorphic P– T ( P p, T p) conditions, using reasonable initial guessed values of minimum cooling rate and maximum T p. To retrieve the thermal history of metapelites, we have developed a finite difference scheme for modeling multicomponent diffusion profiles in garnet, incorporating provisions for continuous nucleation and growth. This has been interlinked with a genetic algorithm that permits retrieval of the temperature vs. time path of metapelites through modeling of the zoning profiles in garnets, keeping several model parameters as floating variables. The numerical code has been applied to retrieve the T– t history of the low pressure–high temperature Royke metamorphic belt, Japan, by modeling the concentration profiles of Fe, Mn, Mg and Ca of garnets of different sizes that have been inferred to have developed by a process of continuous nucleation and growth.

Trajetória P-T e condições do metamorfismo usadas como ferramenta para compartimentação tectônica da Faixa Brasília em Goiás

The observed mineral assemblages in metapelites of Araxá Group, in Goiás, are related to metamorphic peak, are typical of medium to upper amphibolite facies conditions and correlated to either an early stage of the main ductile deformation phase (Dn) observed in the region, or even to a previous deformation phase. The metamorphic peak is marked by the following mineral assemblage: quartz, biotite, garnet, plagioclase, kyanite ± staurolite; sillimanite is produced later during decompression. Intense retrometamorphism is observed in several samples, in which garnet is replaced by chlorite and later random grains of muscovite overgrew main foliation. Thermobarometry points to 610 ºC and 9.5 kbar as metamorphic peak conditions. The P-T path, inferred with textural and thermobarometric data, is clockwise, compatible with a collisional setting and similar with other P-T paths observed in other parts of Brasília Fold Belt.

Trajetória P-T e condições do metamorfismo usadas como ferramenta para compartimentação tectônica da Faixa Brasília em Goiás

The observed mineral assemblages in metapelites of Araxa Group, in Goias, are related to metamorphic peak, are typical of medium to upper amphibolite facies conditions and correlated to either an early stage of the main ductile deformation phase (D n ) observed in the region, or even to a previous deformation phase. The metamorphic peak is marked by the following mineral assemblage: quartz, biotite, garnet, plagioclase, kyanite ± staurolite; sillimanite is produced later during decompression. Intense retrometamorphism is observed in several samples, in which garnet is replaced by chlorite and later random grains of muscovite overgrew main foliation. Thermobarometry points to 610 oC and 9.5 kbar as metamorphic peak conditions. The P-T path, inferred with textural and thermobarometric data, is clockwise, compatible with a collisional setting and similar with other P-T paths observed in other parts of Brasilia Fold Belt.

Contrasting P–T–t paths for Neoproterozoic metamorphism in MacRobertson and Kemp Lands, east Antarctica

AbstractMineral equilibria modelling and electron microprobe chemical dating of monazite in granulite facies metapelitic assemblages from the MacRobertson Land coastline, Rayner Complex, east Antarctica, are consistent with an ‘anticlockwise’ Neoproterozoic P–T–t path. Metamorphism occurred at c. 990–970 Ma, achieving peak conditions of 850 °C and 5.6–6.2 kbar at Cape Bruce, and 900 °C and 5.4–6.2 kbar at the Forbes Glacier ∼50 km to the east. These peak metamorphic conditions preceded the emplacement of regionally extensive syntectonic charnockite. High temperature conditions are likely to have been sustained for 80 Myr by lithospheric thinning and repeated pluton emplacement; advection was accompanied by crustal thickening to maximum pressures of 6–7 kbar, followed by near‐isobaric cooling. This P–T–t path is distinct from that of rocks in adjacent Kemp Land, ∼50 km to the west, where a ‘clockwise’P–T–t path from higher‐P conditions at c. 940 Ma may reflect the response of a cratonic margin displaced from the main magma flux. In this scenario, crustal shortening was initially accommodated in younger, fertile crust (MacRobertson Land) involving metasediments and felsic plutons with the transfer of strain to adjacent older crust (Kemp Land) subsequent to charnockite emplacement.

The Proterozoic P–T–t Evolution of the Kemp Land Coast, East Antarctica; Constraints from Si-saturated and Si-undersaturated Metapelites

Integrated metamorphic and geochronological data place new constraints on the metamorphic evolution of a Neoproterozoic orogen in east Antarctica. Granulite-facies rocks from a 150 km stretch of the Kemp Land coast reflect peak conditions involving T � 870^9908 Ca t P� 7� 4^10 kbar, with pressure increasing westward towards an Archaean craton. Electron microprobe-derived (Th þ U)^Pb monazite ages from metapelitic assemblages indicate that the major mineral textures in these rocks developed during the c. 940 Ma Rayner Orogeny. Complex compositional zoning in monazite suggests high-T recrystallization over c. 25 Myr. Diversity in metapelitic reaction textures reflects silica and ferromagnesian content: Si-saturated Fe-rich metapelites contain garnet that is partially pseudomorphed by biotite and sillimanite, whereas Si-saturated Mg-rich metapelites and Siundersaturated metapelitic pods have reaction microstructures involving cordierite enclosing orthopyroxene, garnet and/or sapphirine, cordierite þ sapphirine symplectites around sillimanite and coarsegrained orthopyroxene þ corundum separated by sapphirine coronae. Interpretations based on P^T pseudosections provide integrated bulk-rock constraints and indicate a clockwise P^T^t path characterized by a post-peak P^T trajectory with dP/dT � 15^20 bar/8C. This moderately sloped decompressive-cooling P^T path is in contrast to near-isothermal decompression P^T paths commonly cited for this region of the Rayner Complex, with implications for the post-collisional tectonic response of the mid- to lower crust within this orogenic belt.

Lithium isotope fractionation between Li-bearing staurolite, Li-mica and aqueous fluids: An experimental study

We determined the lithium isotope fractionation between synthetic Li-staurolite and aqueous fluids containing excess LiCl or LiOH at 3.5 GPa between 670 and 880 °C and between Li-mica and similar fluids at 2.0 GPa and 300 to 500 °C. In all experiments for the system Li-staurolite-fluid, 7Li was weakly partitioned into Li-staurolite, whereas for Li-mica-fluid, 7Li preferentially fractionated into the fluid. The difference in the Li-isotope fractionation behaviour results from different Li-coordination in staurolite (Li [4]) and mica (Li [6]). For the system Li-mica-fluid, fractionation is about half as large along the same T-range as for spodumene-fluid [Wunder, B., Meixner, A., Romer, R.L., Heinrich, W., 2006. T-dependent isotopic fractionation of lithium between clinopyroxene and high-pressure hydrous fluids. Contrib. Mineral. Petrol. 151, 112-120]. This behaviour is consistent with differences in the bonding strengths of the Li-octahedra in Li-mica and spodumene. Subduction and concomitant dehydration of metabasic oceanic crust, containing chlorite and clinopyroxene with Li in six-fold coordination, releases fluids enriched in Li and 7Li into the fore-arc and arc mantle and thus introduces a light Li-component into the deeper mantle. Ongoing dehydration and Li-loss along the subduction path produces fluids with increasingly lower Li-concentrations and lighter lithium isotope compositions. However, as staurolite is the most important carrier of Li in metapelitic assemblages, the inverse fractionation of Li-isotopes relative to fluids enables staurolite to transport high Li-concentrations in combination with an isotopic composition enriched in 7Li into the mantle. Thus, depending on the Li-contents and Li-isotope signature of the protolite, breakdown of Li-bearing staurolite at high P and T can release fluids with high Li-contents and isotopically heavy Li into the deep mantle wedge.

Recalibration of the Garnet–Muscovite (GM) Geothermometer and the Garnet–Muscovite–Plagioclase–Quartz (GMPQ) Geobarometer for Metapelitic Assemblages

The garnet–muscovite (GM) geothermometer and the garnet– muscovite–plagioclase–quartz (GMPQ) geobarometer have been simultaneously calibrated under conditions of T 1⁄4 450–760 C and P 1⁄4 0.8–11.1 kbar, using a large number of metapelitic samples in the compositional ranges X grt Fe 1⁄4 0.53–0.81, X grt Mg 1⁄4 0.05–0.24, X grt Ca 1⁄4 0.03–0.23 in garnet, X pl Ca 1⁄4 0.17–0.74 in plagioclase, and Fe 1⁄4 0.04–0.16, Mg 1⁄4 0.04–0.13, Al 1⁄4 1.74–1.96 in muscovite on the basis of 11 oxygens. The resulting GM thermometer yielded similar temperature estimates (mostly within ±50 C) to that of the garnet–biotite thermometer, and successfully discerned the expected systematic temperature change of prograde sequences, thermal contact zones and an inverted metamorphic zone. The resulting GMPQ barometer yielded similar pressure estimates (mostly within ±1.0 kbar) to the garnet– aluminum silicate–plagioclase–quartz (GASP) barometer and placed the aluminosilicate-bearing samples in the appropriate aluminosilicate stability fields. Application of the GMPQ barometer to thermal contact aureoles or rocks within limited geographical areas confirmed the expected constant pressures that should exist in these settings. The random errors of the GM thermometer and the GMPQ barometer are estimated to be ±16 C and ±1.5 kbar, respectively. When biotite or aluminosilicate is absent in metapelites, metamorphic P–T conditions may be determined by simultaneously applying the GM thermometer and the GMPQ barometer.

The applicability of the GRIPS geobarometry in metapelitic assemblages

AbstractAlthough the garnet‐rutile‐ilmenite‐plagioclase‐silica (quartz) (GRIPS) geobarometer has been experimentally calibrated and widely applied, its applicability to metapelitic rocks has not yet been discussed carefully. In this paper, this barometer was recalibrated by fitting the available reversed‐phase equilibrium data incorporating different combinations of activity models of garnet, plagioclase and ilmenite. The resultant GRIPS barometer formalisms reproduce the experimental pressures well within ±0.2 kbar. The GRIPS and garnet‐aluminium silicate‐plagioclase‐quartz (GASP) barometer values are concordant within ±1 kbar for pressures above ∼6 kbar for natural metapelites, but the difference of pressure determinations between these two barometers becomes larger when pressure and/or the grossular content of garnet decrease. However, the pressure difference is independent of either temperature, or almandine in garnet, or anorthite in plagioclase, or iron content in ilmenite. After testing and application of the GRIPS barometer to aluminosilicate‐bearing metapelites and metapelitic assemblages within limited geographical areas as well as within contact thermal aureoles, it is concluded that this barometer may be applied to low‐ to high‐grade, medium‐ to high‐pressure metapelites. The application of the GRIPS barometer to metapelites is not advocated in situations where calcium is deficient in garnet ( &lt; 0.05) or plagioclase ( &lt; 0.17), or for pressures below ∼6 kbar.

Delamerian‐aged metamorphism in the southern Curnamona Province, Australia: implications for the evolution of the Mesoproterozoic Olarian Orogeny

AbstractMonazite electron microprobe U–Th–Pb and garnet Sm–Nd isotopic data from metapelitic assemblages in the Willyama Supergroup in the southern Curnamona Province, south‐central Australia, indicate that the terrain underwent regional greenschist to amphibolite‐grade metamorphism during the c. 500 Ma Delamerian Orogeny. The Delamerian‐aged mineral assemblages include prograde garnet–staurolite and kyanite‐bearing associations that overprint andalusite‐ and sillimanite‐bearing assemblages that are interpreted to have developed during the c. 1600 Ma Olarian Orogeny. Importantly, the development of secondary kyanite‐bearing assemblages in the southern Curnamona Province has been used previously to suggest that the Olarian Orogeny followed an anticlockwise P–T evolution. If such assemblages are the product of c. 500 Ma metamorphism, then the anticlockwise P–T path is an apparent path, due to the overprint of a distinct metamorphic cycle c. 1100 Ma later. Making such distinctions is therefore extremely important when using the textural and metamorphic evolution of polycyclic terrains to model the thermal behaviour of the crust during orogeny. This study highlights the utility of in situ geochronology, linking age data to petrologically important phases and assemblages.

Experimental data on Fe and Mg partitioning between coexisting sapphirine and spinel: an empirical geothermometer and its application

Specially designed experiments were carried out in the temperature range 950-1100 °C at 800 MPa to grow coexisting sapphirine and spinel-bearing metapelitic assemblages in the KFMASH system. Equilibrium phase assemblages contain these two phases with stoichiometric compositions. Compositional data obtained from the two coexisting phases indicate a strong dependence of the Mg-Fe distribution coefficient on temperature. An empirical thermometric expression has been formulated from the experimentally constrained relationship, lnKD = -6277/T(°K) + 6.4202, where KD = (XMg/XFe)Spr.(XFe/XMg)Spl. Application of this thermometer to natural mineral pairs indicate that earlier temperature estimates based on sapphirine-spinel need to be revised upward by 120-160 °C.