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Supracrustal rocks of the Yilgarn orogen underwent deep burial, and later syn-shortening exhumation. Archean exhumation processes were likely less efficient that today, so that complete exhumation of high-pressure rocks was rare and accidental.

[Display omitted] • UHT peak conditions with counterclockwise P–T paths were constrained. • The UHT metamorphism might have persisted for more than 20 Myr (2.52–2.50 Ga). • Vertical sagduction process may be responsible for the Neoarchean UHT metamorphism. Archean supracrustal rocks that occur as rafts within tonalite–trondhjemite–granodiorite (TTG) gneisses can reach ultra-high temperature (UHT) conditions. Their metamorphic P–T paths and geodynamic regimes are debated. Herein, we present detailed analyses of UHT pelitic migmatites (JD1730 and JD1733) from the East Hebei, North China Craton, using petrology, phase equilibria modelling, and zircon dating, to recover the metamorphic history. Sample JD1730 is a sillimanite–orthopyroxene–cordierite migmatite that contains the sillimanite + orthopyroxene (+cordierite) symplectites, which are interpreted to be the pseudomorphs after sapphirine. JD1733 is a corundum-bearing garnet–cordierite–sillimanite migmatite that contains the corundum + spinel ± ilmenite/rutile aggregates, representing a stable mineral association of low pressures in local Si-unsaturated domains. The two rocks record consistent counterclockwise P–T paths involving pre-peak up- P processes to the UHT peak conditions followed by cooling. The pre-peak up- P processes occurred at 5–7 kbar and > 970 °C constrained from the textural relations involving (i) spinel enveloped by sillimanite and cordierite and mantled by the Sil + Opx symplectites in JD1730, and (ii) the inclusion variation in garnet of JD1733 with the Spl + Crn (±Ilm) aggregates embayed by sillimanite in the core and mantle, and worm-like quartz in the rim. The UHT peak conditions can be constrained to be 7–8 kbar, 980–1060 °C based on the stability of the inferred peak mineral assemblages in P–T pseudosections added with the plots of the re-integrated anorthite contents from perthite. The post-peak cooling processes to 5–8 kbar / 820–850 °C are marked by the growth of biotite in leucosomes of both samples. Zircon dating and geothermometry results suggest that the UHT metamorphism might have persisted for more than 20 Myr from ∼ 2.52 Ga to ∼ 2.50 Ga and the post-UHT cooling continued another 20 Myr or even 40 Myr. Considering the geological features in the terrane, a vertical sagduction regime was preferred for the Neoarchean tectonic evolution of the East Hebei terrane. [ABSTRACT FROM AUTHOR]

The oldest known evolved (felsic) rocks on Earth (c. 4.03 Ga) are found in the Acasta Gneiss Complex (AGC) in north‐western Canada and represent a fundamental keystone in unravelling the geological processes governing crustal growth and differentiation during the Hadean and early Archean. Although the timing of multiple episodes of magmatism, metamorphism and deformation in these tonalitic gneisses has been investigated extensively, the metamorphic pressure–temperature (P–T) conditions recorded by the rocks are poorly constrained. Here, we use phase equilibrium modelling coupled with in situ garnet Lu–Hf geochronology and trace element analysis for two garnet‐bearing tonalitic gneisses to decipher the metamorphic history of the AGC. The observed mineral assemblages are consistent with peak metamorphic conditions of T = 725–780°C and P = 4.5–6.2 kbar and the generation of a small amount of melt (<7 vol.%). Garnet geochronology constrains the age of metamorphism to 3.3–3.2 Ga, consistent with previous evidence for a late Paleoarchean tectono‐metamorphic event in the AGC. Subsequent isotopic disturbance of garnet at c. 1.9 Ga is interpreted to correspond to a modification of the primary Lu–Hf systematics in response to garnet resorption/recrystallization during the Paleoproterozoic Wopmay orogeny, resulting in significant scatter between these two age components. Our study adds to the small number of published P–T data for metamorphic rocks older than 2.8 Ga and shows that tonalitic gneisses in the AGC record a high apparent thermal gradient of ~140°C/kbar in the late Paleoarchean. This thermal gradient is the highest among the limited dataset, but is broadly similar to data from other Paleoarchean‐Mesoarchean crustal rocks in recording high T/P ratios (>77.5°C/kbar). [ABSTRACT FROM AUTHOR]

The bulk rock composition of granitoids reflects the composition of their source and the conditions of partial melting, which are functions of the geodynamic setting in which they formed. Granitoids in active continental margins (continental arcs) are dominated by calc-alkaline rocks with subordinate alkaline compositions, although how these different magma compositions formed is not well understood. Neoproterozoic magmatic rocks are widely distributed along the western margin of the Yangtze Block in South China to form the >1000-km long Panxi continental arc system, which is dominated by granitoids with minor mafic–ultramafic and intermediate plutons. The granitoids are subdivided into sodic and potassic variants that occur as belts along the western and eastern sides of the continental arc, respectively. Sodic granitoids from the western part consist of tonalite, granodiorite, and monzogranite with crystallisation ages ranging from 870 Ma to 740 Ma. They have low K2O/Na2O ratios (0.1–1.0) and high Na2O contents (3.5–6.7 wt%), high but variable SiO2 (61–75 wt%) concentrations, and negative to positive whole-rock εNd(t) values (−1.7 to +2.9). Zircon grains from the sodic granitoids have εHf(t) values ranging from +0.3 to +9.6 and δ18O from 3.90‰ to 7.71‰. The potassic granitoids from the eastern side consist of monzogranite and syenogranite with crystallisation ages from 820 Ma to 790 Ma. They have high K2O/Na2O ratios (0.6–2.2), K2O (2.6–5.9 wt%) and SiO2 contents (69–78 wt%), but whole-rock εNd(t) (−0.9 to +2.9) and zircon εHf(t) (+1.8 to +12.9), and δ18O values (2.98‰ to 6.41‰) similar to those of the sodic granitoids. The isotopic compositions of both the sodic and potassic granitoids are similar to those of spatially- and temporally-related mantle-derived (mafic to ultramafic) rocks, and are considered to have been derived from juvenile mafic continental crust. Phase equilibrium modelling shows that the H2O content of the granitoid source rocks played a key role in their petrogenesis, both in lowering solidus temperatures and in controlling the compositions of the derived partial melts. Our results indicate that calc-alkaline sodic granitoids can be formed by water-fluxed melting of juvenile mafic crust at 750–900°C and 9–12 kbar in which the required H2O was derived from the dewatering of underplating mafic arc magmas. By contrast, the potassic granitoids were generated by fluid-absent (H2O-undersaturated) partial melting of a similar juvenile mafic source at 725–900°C and 6–9 kbar. We conclude that the sodic granitoids were derived from partial melting of the newly-formed mafic lower crust in the continental arc, whereas the potassic granitoids were likely generated in the back-arc setting induced by upwelling of asthenospheric mantle. [ABSTRACT FROM AUTHOR]

• In situ Lu–Hf garnet geochronology dates Eoarchean metamorphism to 3652 ± 69 Ma. • Phase equilibrium modelling indicates a thermobaric ratio (T/P ; bulk geotherm) of 800–1000 °C/GPa. • Metamorphic conditions are unexceptional when compared to Archean T/P globally. • No metamorphic evidence for Eoarchean subduction close to Isua supracrustal belt. Using petrography, in situ garnet Lu–Hf geochronology, garnet rare-earth element (REE) analysis, zircon U–Pb geochronology and phase equilibrium modelling, we provide unambiguous evidence for Eoarchean granulite-facies metamorphism in the northern Itsaq Gneiss Complex (IGC), southwest Greenland. In situ garnet Lu–Hf geochronology from two samples of variably migmatitic metabasic rocks least affected by subsequent (Neoarchean) reworking yield Lu–Hf isochron ages of 3641 ± 62 Ma (MSWD = 1.7, n = 45/67; all age uncertainties at 2σ level) and 3652 ± 69 Ma (MSWD = 1.8, n = 83/84) from garnet with REE patterns typical of single-stage prograde growth. From the same two samples, zircon grains with textures consistent with metamorphic growth give weighted-mean 207Pb/206Pb ages of 3620 ± 8 Ma (MSWD = 1.2, n = 45) and 3630 ± 8 Ma (MSWD = 0.6, n = 44), respectively. Phase equilibrium modelling constrains peak P–T conditions of Eoarchean (3640–3630 Ma) metamorphism to 8.3–9.0 kbar and 730–820 °C. The thermobaric ratios (T/P) of 800–1000 °C/GPa recorded by the investigated samples are considerably higher (warmer) than previously proposed for granulite-facies metamorphism in the northern IGC, and broadly similar to Archean metamorphic P–T data globally, with no evidence for the bimodality in T/P that characterizes younger metamorphism. Either subduction-driven metamorphism (and plate tectonics) did not operate in the Eoarchean, or the Eoarchean lithosphere had a rheology that prohibited exhumation of subducted rocks. [ABSTRACT FROM AUTHOR]

Author(s): Tim E. Johnson; Michael Brown; Nicholas J. Gardiner; Christopher L. Kirkland; R. Hugh Smithies Nature 543, 239242 (2017); doi:10.1038/nature21383 In this Letter we omitted to cite a paper [1] [...]

The peak of regional metamorphism in western Maine was reached at ca. 404 Ma during the waning stage of Devonian Acadian deformation. Regional metamorphic mineral assemblages in metapelitic rocks range from greenschist to upper amphibolite facies. Subsolidus rocks are characterized by the association andalusite + staurolite; at the highest grades, anatectic migmatites are developed. Results of thermodynamic modeling in the MnNCKFMASH system are consistent with field data and imply a metamorphic field gradient that extends from 3.5-4.0 kbar at lower grades (500-520[degrees]C) to > 4.5 kbar at suprasolidus temperatures that exceeded 700[degrees]C. Regional isotherms that are inferred to have been shallowly inclined at lower grades are closely spaced around synmetamorphic granites and at the migmatite front, consistent with advection-controlled intracrustal redistribution of heat within the regionally extensive thermal high. Peak pressures vary both along and across the strike of the Central Maine belt, which is interpreted to record differential thickening during syntectonic metamorphism. Contact metamorphism associated with the Mooselookmeguntic igneous complex occurred ca. 35 million years after the regional metamorphic peak, and records higher pressure conditions than the regional event. We suggest that the final increment of late-Acadian thickening accounts for the pressure increase, consistent with regional cooling prior to the emplacement of the Mooselookmeguntic igneous complex. Pluton emplacement at deeper levels ca. 35 million years after the peak of Acadian metamorphism reflects lowering of the brittle-viscous transition zone, a level at which ascending magma is trapped, consequent on regional cooling and a steeper geotherm. An overall counter-clockwise P-T-t evolution is implied in the Central Maine belt, consistent with that proposed for Acadian metamorphism in western New Hampshire.

Subduction is a major process cycling material through Earth's geochemical reservoirs. Although trends in chemical composition of arc magmas imply assimilation of metasediment, the degree of such assimilation and the loci of that metasediment contamination (whether via subducted sediment or country rock assimilation) are poorly understood. To address these issues, we explore compositional data of oceanic and continental arc systems from circum-Pacific subduction zones. We find that high-silica continental arc rocks of the circum-Pacific are associated with higher aluminium saturation indices interpreted to reflect higher degrees of metasediment assimilation, with Sr/Y suggestive of shallow emplacement levels within the crust. In contrast, high-silica oceanic rocks of the circum-Pacific display lower aluminosity and equilibrated at deeper levels within the crust. Continental arc basalts are often assumed to be the source of high-silica continental arc rocks. However, phase equilibrium modelling of partial melting and crystal fractionation of continental arc basalts yield results that question this assumption. Furthermore, continental arc rock compositions show that the assimilated metasediments have protoliths that are most probably felsic greywacke and pelite rather than mafic greywacke. These findings are consistent with the hypothesis that high-silica rocks in continental arcs are directly influenced by anatexis of metasediment at shallow crustal levels (<20 km). Based upon a new method of discriminating the contribution of metasediment-derived melt, approximately one-third of felsic rocks in continental arcs have a demonstrable and unambiguous metasedimentary component. The degree of metasedimentary reworking in continental arc magmas plays an important role in the evolution of the continental crust and highlights the importance of using sediment-sensitive geochemical proxies and a magma's petrological history when deconvolving the histories of magmatic arcs. This study also underlines the caveats associated with the calculation of depleted mantle model ages, where traditional techniques may lead to discrepancies of the order of 0·5 billion years. [ABSTRACT FROM AUTHOR]

ABSTRACT Petrochronological investigations of granulite‐facies metapelitic rocks from the eastern contact of the Closepet granite in the Central Dharwar Craton (CDC), southern India, provide new pressure–temperature–age (P–T–t) constraints on two seemingly discrete Archean metamorphic events during the Neoarchean and late Paleoarchean eras. Phase equilibrium modelling and conventional thermobarometry coupled with in situ monazite and garnet geochronology constrain Neoarchean (ultrahigh‐temperature; UHT) metamorphic peak conditions to ~930 °C and ~6.7 kbar at c. 2.63 Ga, then cooling and limited decompression to ~5.7 kbar at ≤810 °C. Monazite inclusions in garnet least affected by Neoarchean recrystallisation have distinct positive Eu anomalies and yield ages of c. 3.2 Ga, whereas garnet cores interpreted to have grown at the same time have slightly younger apparent ages of c. 3.1 Ga. We interpret this age mismatch to be the result of extensive resorption of garnet during Neoarchean UHT metamorphism, which led to widespread modification of the initial Lu–Hf systematics in garnet to produce younger apparent ages. The effect of retention and inward intracrystalline diffusion of Lu on the isotopic composition of garnet is most pronounced close to the resorbed grain margins and decreases towards the core, as reflected by progressively younger apparent single‐spot garnet dates from core to rim. Despite extensive overprinting of the sample at c. 2.63 Ga, the trace element composition of Paleoarchean monazite indicates growth in equilibrium with garnet but in the absence of feldspar, which is predicted to occur over a broad stability range at P ≥ 8 kbar and T ≤ 700 °C. Such P–T conditions are uncommon in the metamorphic rock record prior to the Neoarchean, but are typical of Barrovian‐type metamorphism, which is considered to be an expression of accretionary‐to‐collisional orogenesis. Rocks of similar age and metamorphic grade have been reported from the core of the Western Dharwar Craton (WDC) and may reflect regional subduction at the margins of the CDC and WDC since the late Paleoarchean. Neoarchean UHT metamorphism in the Dharwar Craton is coeval with a cluster of other UHT occurrences at 2.7–2.6 Ga, indicating the existence of globally elevated thermal gradients at that time. [ABSTRACT FROM AUTHOR]

Unlike many Archean diorites and granitoids that arguably formed in different geodynamic settings, their post-Archean counterparts are commonly regarded to have formed at convergent margins, although in detail their petrogenesis remains contentious. Here we present new whole-rock data and zircon Hf–O isotope analyses from dioritic (750–730 Ma), granitic (810–790 Ma) and tonalite–trondhjemite–granodiorite (TTG)-like intrusions (800–740 Ma) from the Panxi and Hannan regions, which form part of an extensive Neoproterozoic convergent margin exposed in South China. The dioritic rocks from the Panxi region exhibit high zircon εHf(t) (+10.1 to +13.1) and sub-mantle to mantle-like δ18O (3.1–6.3 ‰) values, whereas those from the Hannan region preserve low εHf(t) (+4.1 to +8.1) and high δ18O values (5.9–6.6 ‰), indicating that the dioritic melts were derived from subduction-modified lithospheric mantle sources and experienced variable degrees of lower crustal contamination. Zircons within granite and TTG from the Panxi region show a narrow range of Hf isotopic compositions generally spanning 2–4 εHf units (+3.1 to +7.9 for most felsic intrusions). By contrast, those from the Hannan region show a much wider range of zircon εHf(t) values spanning almost 10 εHf units (+1.1 to +10.9). Based on their O–Hf–Nd isotopic signatures, we propose that the granite and TTG from both areas were derived through partial melting of hydrated basaltic rocks in the arc root, and that the isotopic variability between the intrusions mirrors spatial and temporal chemical variations in these deep crustal source rocks. In both regions, the granites, along with mantle-derived mafic–ultramafic and intermediate rocks, show a coupled evolution associated with increasing εNd(t) and εHf(t) and decreasing δ18O with decreasing ages, whereas the TTGs formed during late-stage arc magmatism and preserve relatively homogeneous Nd–Hf isotopes and mantle-like δ18O values. Combined, these data record continuous crustal thickening through underplating of juvenile magmas and a progressive increase in the depth of melting, along with a decrease in the degree of interaction between the melts and basement rocks within the arc root. Our results suggest that slab melting was not required to produce post-Archean TTG signatures. Further, we suggest that the variability in the Hf–O–Nd isotopic compositions of metaluminous (I-type) granites mostly does not reflect a heterogeneity in upper mantle signatures, and that there is no conclusive evidence for the involvement of partial melts of subducted sediment based on Hf–O isotope signatures in zircon. [ABSTRACT FROM AUTHOR]

Phase equilibrium modelling is commonly employed to constrain the pressure–temperature (P–T) evolution of granulite facies rocks, from which their geodynamic setting may be inferred. However, defining a suitable equilibrium volume in such rocks is non‐trivial due to heterogeneities in protolith composition and open system behaviour, including melt loss or gain and deformation. Consequently, equilibrium volumes and the mineral assemblages they contain may vary both temporally and spatially within a single rock. Additionally, despite the extreme temperatures they attained, granulites commonly contain microtextures indicating gradients in chemical potential after the metamorphic peak. This study assesses the processes that control compositional heterogeneity between equilibrium volumes in a suprasolidus granulite and the consequences for the development of a range of microstructures. An approach combining phase equilibrium forward modelling and chemical potential diagrams is used to simulate the suprasolidus evolution of two adjacent equilibrium volumes in an Mg‐rich metapelite. Assuming equilibrium within the two compositional domains, evolving granulite facies assemblages vary along a high T/P (125°C/kbar) clockwise P–T path representative of high T/P metamorphic terranes. Many retrograde microtextures in metapelitic rocks can be reproduced by considering chemical potential (µ) gradients in µMgO, µFeO and µCaO between phases, assuming (a) the presence of melt on grain boundaries, (b) that Al2O3 is perfectly immobile and (c) that K2O and Na2O are perfectly mobile. Additionally, documented microtextures in granulites imply local SiO2‐undersaturation, despite the presence of matrix quartz in the rock. Preserving such chemical potential gradients requires that solid phases are chemically and physically isolated from melt. Efficient prograde melt drainage may lead to a loss in melt interconnectivity at high temperature (>900°C), meaning that equilibration of centimetre‐scale compositional domains is controlled by solid‐state diffusion. The presence of isolated pockets of viscous melt allows the formation of discontinuous intergranular reaction microtextures. [ABSTRACT FROM AUTHOR]

Here, the petrological features of numerous primitive achondrites and highly equilibrated chondrites are evaluated to review and expand upon our knowledge of the chondrite–achondrite transition, and primitive achondrites in general. A thermodynamic model for the initial silicate melting temperature and progressive melting for nearly the entire known range of oxidation states is provided, which can be expressed as Tm = 0.035Fa2−3.51Fa + 1109 (in °C, where Fa is the proportion of fayalite in olivine). This model is then used to frame a discussion of textural and mineralogical evolution of stony meteorites with increasing temperature. We suggest that the metamorphic petrology of these meteorites should be based on diffusive equilibration among the silicate minerals, and as such, the chondrite–achondrite transition should be defined by the initial point of silicate melting, not by metal–troilite melting. Evidence of silicate melting is preserved by a distinctive texture of interconnected interstitial plagioclase ± pyroxene networks among rounded olivine and/or pyroxene (depending on ƒO2), which pseudomorph the former silicate melt network. Indirectly, the presence of exsolution lamellae in augite in slowly cooled achondrites also implies that silicate melting occurred because of the high temperatures required, and because silicate melt enhances diffusion. A metamorphic facies series is defined: the Plagioclase Facies is equivalent to petrologic types 5 and 6, the Sub‐calcic Augite Facies is bounded at lower temperatures by the initiation of silicate melting and at higher temperatures by the appearance of pigeonite, which marks the transition to the Pigeonite Facies. [ABSTRACT FROM AUTHOR]

Geochronological data from zircon in Archean tonalite-trondhjemite-granodiorite (TTG) gneisses are commonly difficult to interpret. A notable example is the TTG gneisses from the Lewisian Gneiss Complex, northwest Scotland, which have metamorphic zircon ages that define a more-or-less continuous spread through the Neoarchean, with no clear relationship to zircon textures. These data are generally interpreted to record discrete high-grade events at ca. 2.7 Ga and ca. 2.5 Ga, with intermediate ages reflecting variable Pb loss. Although ancient diffusion of Pb is commonly invoked to explain such protracted age spreads, trace-element data in zircon may permit identification of otherwise cryptic magmatic and metamorphic episodes. Although zircons from the TTG gneiss analyzed here show a characteristic spread of Neoarchean ages, they exhibit subtle but key step changes in trace-element compositions that are difficult to ascribe to diffusive resetting, but that are consistent with emplacement of regionally extensive bodies of mafic magma. These data suggest suprasolidus metamorphic temperatures persisted for 200 m.y. or more during the Neoarchean. Such long-lived highgrade metamorphism is supported by data from zircon grains from a nearby monzogranite sheet. These preserve distinctive trace-element compositions consistent with derivation from a mafic source, and they define a well-constrained U-Pb zircon age of ca. 2.6 Ga that is intermediate between the two previously proposed discrete metamorphic episodes. The persistence of melt-bearing lower crust for hundreds of millions of years was probably the norm during the Archean. [ABSTRACT FROM AUTHOR]

The article explores the life and work experience of the Michael Brown. Topics discussed include Brown's contribution in various articles on topics such as the high‐grade metamorphism and crustal melting, combining petrology and the structural geology; Brown's focused on identifying and interpreting secular change in the metamorphic record within the context of evolving global geodynamics; and the detailing about the Brown's research contributions towards metamorphism.

The time‐scales and P–T conditions recorded by granulite facies metamorphic rocks permit inferences about the geodynamic regime in which they formed. Two compositionally heterogeneous cordierite–spinel‐bearing granulites from Vizianagaram, Eastern Ghats Province (EGP), India, were investigated to provide P–T–time constraints using petrography, phase equilibrium modelling, U–Pb geochronology, the rare earth element composition of zircon and monazite, and Ti‐in‐zircon thermometry. These ultrahigh temperature (UHT) granulites preserve discrete compositional layering in which different inferred peak assemblages are developed, including layers bearing garnet–sillimanite–spinel, and others bearing orthopyroxene–sillimanite–spinel. These mineral associations cannot be reproduced by phase equilibrium modelling of whole‐rock compositions, indicating that the samples became domainal on a scale less than that of a thin section, even at UHT conditions. Calculation of the P–T stability fields for six compositional domains within which the main rock‐forming minerals are considered to have attained equilibrium suggests peak metamorphic conditions of ~6.8–8.3 kbar at ~1,000°C. In most of these domains, the subsequent evolution resulted in the growth of cordierite and final crystallization of melt at an elevated (residual) H2O‐undersaturated solidus, consistent with <1 kbar of decompression. Concordant U–Pb ages obtained by SHRIMP from zircon (spread 1,050–800 Ma) and monazite (spread 950–800 Ma) demonstrate that crystallization of these minerals occurred during an interval of c. 250 Ma. By combining LA‐ICP‐MS U–Pb zircon ages with Ti‐in‐zircon temperatures from the same analysis sites, we show that the crust may have remained above 900°C for a minimum of c. 120 Ma between c. 1,000 and c. 880 Ma. Overall, the results suggest that, in the interval 1,050 to 800 Ma, the evolution of the Vizianagaram granulites culminated with UHT conditions from c. 1,000 Ma to c. 880 Ma, associated with minor decompression, before further zircon crystallization at c. 880–800 Ma during cooling to the solidus. However, these rocks are adjacent to the Paderu–Anantagiri–Salur crustal block to the NW that experienced counterclockwise P–T–t paths, and records similar UHT peak metamorphic conditions (7–8 kbar, ~950°C) followed by near‐isobaric cooling, and has a similar chronology during the Neoproterozoic. The limited decompression inferred at Vizianagaram may be explained by partial exhumation due to thrusting of this crustal block over the adjacent Paderu–Anantagiri–Salur crustal block. The residual granulites in both blocks have high concentrations of heat‐producing elements and likely remained hot at mid‐crustal depths throughout a period of relative tectonic quiescence in the interval 800–550 Ma. During the Cambrian Period, the EGP was located in the hinterland of the Denman–Pinjarra–Prydz orogen. A later concordant population of zircon dated at 511 ± 6 Ma records crystallization at temperatures of ~810°C. This age may record a low‐degree of melting due to limited influx of fluid into hot, weak crust in response to convergence of the Crohn craton with a composite orogenic hinterland comprising the Rayner terrane, EGP, and cratonic India. [ABSTRACT FROM AUTHOR]

A highly residual granulite facies rock (sample RG07‐21) from Lunnyj Island in the Rauer Group, East Antarctica, presents an opportunity to compare different approaches to constraining peak temperature in high‐grade metamorphic rocks. Sample RG07‐21 is a coarse‐grained pelitic migmatite composed of abundant garnet and orthopyroxene along with quartz, biotite, cordierite, and plagioclase with accessory rutile, ilmenite, zircon, and monazite. The inferred sequence of mineral growth is consistent with a clockwise pressure–temperature (P–T) evolution when compared with a forward model (P–T pseudosection) for the whole‐rock chemical composition. Peak metamorphic conditions are estimated at 9 ± 0.5 kbar and 910 ± 50°C based on conventional Al‐in‐orthopyroxene thermobarometry, Zr‐in‐rutile thermometry, and calculated compositional isopleths. U–Pb ages from zircon rims and neocrystallized monazite grains yield ages of c. 514 Ma, suggesting that crystallization of both minerals occurred towards the end of the youngest pervasive metamorphic episode in the region known as the Prydz Tectonic Event. The rare earth element compositions of zircon and garnet are consistent with equilibrium growth of these minerals in the presence of melt. When comparing the thermometry methods used in this study, it is apparent that the Al‐in‐orthopyroxene thermobarometer provides the most reliable estimate of peak conditions. There is a strong textural correlation between the temperatures obtained using the Zr‐in‐rutile thermometer––maximum temperatures are recorded by a single rutile grain included within orthopyroxene, whereas other grains included in garnet, orthopyroxene, quartz, and biotite yield a range of temperatures down to 820°C. Ti‐in‐zircon thermometry returns significantly lower temperature estimates of 678–841°C. Estimates at the upper end of this range are consistent with growth of zircon from crystallizing melt at temperatures close to the elevated (H2O undersaturated) solidus. Those estimates, significantly lower than the calculated temperature of this residual solidus, may reflect isolation of rutile from the effective equilibration volume leading to an activity of TiO2 that is lower than the assumed value of unity. [ABSTRACT FROM AUTHOR]

Here, we present results of the first 40Ar/39Ar dating of osumilite, a high‐T mineral that occurs in some volcanic and high‐grade metamorphic rocks. The metamorphic osumilite studied here is from a metapelitic rock within the Rogaland–Vest Agder Sector, Norway, an area that experienced regional granulite facies metamorphism and subsequent contact metamorphism between 1,100 Ma and 850 Ma. The large grain size (~1 cm) of osumilite in the studied rock, which preserves a nominally anhydrous assemblage, increases the potential for large portions of individual grains to have remained essentially unaffected by the effects of diffusive argon loss, potentially preserving prograde ages. Step‐heating diffusion experiments yielded a maximum activation energy of ~461 kJ/mol and a pre‐exponential factor of ~8.34 × 108 cm2/s for Ar diffusion in osumilite. These parameters correspond to a relatively high closure temperature of ~620°C for a cooling rate of 10°C/Ma in an osumilite crystal with a 175 µm radius. Fragments of osumilite separated from the sample preserve a range of ages between c. 1,070 and 860 Ma. The oldest ages are inferred to date the growth of coarse‐grained osumilite during prograde granulite facies regional metamorphism, which pre‐date contact metamorphism that has historically been ascribed to the growth of osumilite in the region. The majority of fragments record ages between c. 920 and 860 Ma, inferred to reflect the growth of osumilite and/or diffusive argon loss during contact metamorphism. The retention of old 40Ar/39Ar dates was facilitated by the low diffusivity of Ar in osumilite (i.e. a closed system), large grain sizes, and anhydrous metamorphic conditions. The ability to date osumilite with the 40Ar/39Ar method provides a valuable new thermochronometer that may constrain the timing and duration of high‐T magmatic and metamorphic events. [ABSTRACT FROM AUTHOR]

The processes of partial melting and the segregation and migration of melt underpin the differentiation of the lithosphere. The Sm–Nd and Lu–Hf isotopic systems, which are sensitive to these processes, behave similarly during mantle–crust differentiation, leading to isotopically coupled primary (basaltic) and continental (tonalite–trondhjemite–granodiorite, TTG) crustal compositions that define a linear terrestrial fractionation array in εNd vs εHf space. However, Eoarchaean basalts and TTGs from West Greenland do not sit on this trend and are isotopically decoupled, which may reflect their extraction from a mantle with a non-chondritic composition. We explore the effects of source composition vs fractionation on the production and evolution of early Archaean crust. We use phase equilibria and trace element modelling to characterize the Hf–Nd isotopic evolution of a chain of melting from anhydrous mantle through hydrated basalt to TTG. We show that ∼20% decompression melting of anhydrous mantle with a superchondritic Sm/Nd but chondritic Lu/Hf composition at a mantle potential temperature appropriate to the early Archaean produces basaltic melts with an isotopic composition similar to those measured in Eoarchaean tholeiitic basalts from Isua, West Greenland. In turn, 5–30% melting of hydrated basalt produces TTG melts with Hf–Nd isotopic compositions similar to those measured in Eoarchaean TTGs from the Itsaq Gneiss Complex, West Greenland. Thus, we chart a chain of melting from an isotopically decoupled Hf–Nd mantle composition to isotopically decoupled mafic and felsic crust. Our modelling defines an overall Hf–Nd isotopic fractionation trend that is parallel to, but offset from, that defined by modern rocks with coupled compositions. Primitive mantle contamination by 5% recycled continental crust (TTG) requires a higher degree of mantle melting (30%) to produce basaltic melt with a Hf–Nd composition similar to the Isua basalts. A mantle composition with greater than 5% crustal contamination is more enriched than the Isua basalts, placing an upper limit on the amount of crustal contaminant. A non-chondritic mantle source composition in the early Archaean likely imposed a first order control on the subsequent production of crust with decoupled Hf–Nd compositions. [ABSTRACT FROM AUTHOR]

In this study, we investigate the metamorphic history of the Assynt and Gruinard blocks of the Archean Lewisian Complex, northwest Scotland, which are considered by some to represent discrete crustal terranes. For samples of mafic and intermediate rocks, phase diagrams were constructed in the Na2O--CaO--K2O-- FeO--MgO--Al2O3--SiO2--H2O--TiO2--O2 (NCKFMASHTO) system using wholerock compositions. Our results indicate that all samples equilibrated at similar peak metamorphic conditions of ~8-10 kbar and ~900-1,000°C, consistent with field evidence for in situ partial melting and the classic interpretation of the central region of the Lewisian Complex as representing a single crustal block. Meltreintegration modelling was employed in order to estimate probable protolith compositions. Phase equilibria calculated for these modelled undepleted precursors match well with those determined for a subsolidus amphibolite from Gairloch in the southern region of the Lewisian Complex. Both subsolidus lithologies exhibit similar phase relations and potential melt fertility, with both expected to produce orthopyroxene-bearing hornblende granulites, with or without garnet, at the conditions inferred for the Badcallian metamorphic peak. For fully hydrated protoliths, prograde melting is predicted to first occur at ~620°C and ~9.5 kbar, with up to 45% partial melt predicted to form at peak conditions in a closed-system environment. Partial melts calculated for both compositions between 610 and 1,050°C are mostly trondhjemitic. Although the melt-reintegrated granulite is predicted to produce more potassic (granitic) melts at ***700-900°C, the modelled melts are consistent with the measured compositions of felsic sheets from the central region Lewisian Complex. [ABSTRACT FROM AUTHOR]

• The influence of MgO variability on Archaean crustal fluid budgets is investigated using phase equilibrium models. • Ultramafic rocks store four times as much water as a typical Archaean basalt, which is released at temperatures >700 °C. • Hydrated ultramafic rocks provided a key source of H 2 O in the deep crust and upper mantle in the early Earth. Water is an essential ingredient in transforming primitive mantle-derived (mafic) rocks into buoyant (felsic) continental crust, thereby driving the irreversible differentiation of Earth's lithosphere. The occurrence in Archaean cratons of sodic granites of the tonalite–trondhjemite–granodiorite (TTG) series, high-MgO variolitic basalts, high-Mg diorites (sanukitoids) and diamonds with harzburgitic inclusion assemblages, all require the presence of hydrous fluids in Earth's deep crust and upper (lithospheric) mantle since at least the Paleoarchaean (3.6–3.2 billion years ago). However, despite its importance, where and how water was stored in Archaean crust, and how some water was transported into the upper mantle, are poorly understood. Here, we investigate Archaean crustal fluid budgets through calculated phase equilibria for three protolith compositions — a low-MgO mafic (basaltic) composition, a high-MgO (picritic) composition and an ultrahigh-MgO ultramafic (komatiitic) composition — that are representative of mafic to ultramafic magmatic rocks in Archaean greenstone belts. We show that the mode and stability of hydrous minerals, in particular chlorite, is positively correlated with protolith MgO content, such that high-MgO basalts can store up to twice the amount of crystal-bound H 2 O than low-MgO basalts. Importantly, ultrahigh-MgO rocks such as komatiite can store four times as much H 2 O, most of which is retained until temperatures exceeding 700 °C. Warmer geotherms in the early Archaean favoured dehydration of hydrated high-MgO and ultramafic rocks in the deep crust, leading to hydration and/or fluid-fluxed melting of overlying basaltic rocks to produce 'high-pressure' TTG magmas. Burial of Archaean mafic–ultramafic crust along cooler geotherms resulted in dehydration of ultramafic material within the lithospheric mantle, providing the source of enriched Archaean basalt that was parental to large volumes of ancient TTG-dominated continental crust. [ABSTRACT FROM AUTHOR]

Mantle temperatures during the Archaean eon were higher than today. As a consequence, the primary crust formed at the time is thought to have been extensive, thick and magnesium rich, and underlain by a highly residual mantle. However, the preserved volume of this crust today is low, implying that much of it was recycled back into the mantle. Furthermore, Archaean crust exposed today is composed mostly of tonalite-trondhjemite-granodiorite, indicative of a hydrated, low-magnesium basalt source, suggesting that they were not directly generated from a magnesium-rich primary crust. Here we present thermodynamic calculations that indicate that the stable mineral assemblages expected to form at the base of a 45-km-thick, fully hydrated and anhydrous magnesium-rich crust are denser than the underlying, complementary residual mantle. We use two-dimensional geodynamic models to show that the base of magmatically over-thickened magnesium-rich crust, whether fully hydrated or anhydrous, would have been gravitationally unstable at mantle temperatures greater than 1,500-1,550 °C. The dense crust would drip down into the mantle, generating a return flow of asthenospheric mantle that melts to create more primary crust. Continued melting of over-thickened and dripping magnesium-rich crust, combined with fractionation of primary magmas, may have produced the hydrated magnesium-poor basalts necessary to source tonalite-trondhjemite-granodiorite melts. The residues of these processes, with an ultramafic composition, must now reside in the mantle. [ABSTRACT FROM AUTHOR]