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Mineral behavior during anatectic metamorphism is a direct factor controlling the chemical and isotopic compositions of melts and residues. However, the mechanism that causes disequilibrium melting remains poorly understood. A combined study of petrological, geochronological and geochemical analyses was carried out on a stromatic migmatite outcrop, the Luoerling migmatite in the North Dabie zone. The in-source leucosomes and leucocratic veins are products of in situ partial melting and later magmatic intrusion, respectively. The finding of Neoproterozoic, Late Triassic and Early Cretaceous zircons suggests that the migmatites have Neoproterozoic protoliths, and experienced eclogite-facies metamorphism during collisional orogeny and anatectic metamorphism in the post-collisional stage. The occurrence of peritectic amphibole suggests that anatectic metamorphism mainly occurs through biotite hydration melting. The occurrence of orthopyroxene and clinopyroxene indicates local dehydration melting under granulite-facies conditions. Leucosomes show considerably higher 87Sr/86Sr i , but lower ε Nd (t) and ε Hf (t) values than the corresponding melanosomes. Such isotopic differences between leucosomes and melanosomes result mainly from the influx of external fluids derived from the surrounding eclogites and gneisses, indicating a coupled dehydration-hydration mechanism in the continental subduction zone. Peritectic and anatectic zircon, monazite, titanite and apatite that formed during anatectic metamorphism exhibit different petrological and geochemical characteristics. Peritectic and anatectic accessory minerals in the leucosomes and leucocratic veins exhibit smaller variations in Nd-Hf isotopes than those in the melanosomes, suggesting that the anatectic melts were homogenized during transport. Differential dissolution and growth of monazite, titanite and apatite affect the Sm-Nd isotopes of residues and melts during anatectic metamorphism. The dissolution of apatite plays a significant role in the hydration melting of metagranite. The continuous dissolution of relict zircon during cooling/transportation of melt results in decreased ε Hf (t) values in anatectic zircon, whereas the incomplete dissolution of relict zircon results in whole-rock Hf-Nd decoupling. The difference in Nd-Hf isotopes among whole-rock and different genetic minerals can distinguish between dehydration and hydration melting. Therefore, this study highlights that a combination of multiple isotope analyses on whole-rock and accessory minerals may be conducive to understanding not only the mechanism and process of disequilibrium melting but also the nature of anatectic metamorphism in collisional orogens. • Leucosomes have higher initial Sr, but lower ε Nd (t)-ε Hf (t) values than melanosomes. • Peritectic/anatectic minerals in leucosome have relatively homogeneous isotopes. • Peritectic/anatectic minerals in melanosome have widely variable isotopes. • Apatite dissolution controls the Sm-Nd isotopes during hydration melting. • Differential dissolution of relict zircon affects Hf isotope of anatectic melts. [ABSTRACT FROM AUTHOR]

The compositional diversity of Archean tonalite-trondhjemite-granodiorite (TTG) rocks is generally attributed to partial melting of mafic rocks at various depths or magmatic differentiation driven by the accumulation of plagioclase and/or amphibole. Our investigation of the approximately 130 km2 Paleoarchean Stolzburg pluton in the Barberton area of South Africa reveals a substantial compositional variability that cannot be explained by those processes. The variable Sr/Y and La/Yb ratios can correspond to high-, medium-, and low-pressure types based on pressure-dependent classification criteria, thus conflicting with partial melting simulations and ruling out the model of variable melting depths for such a small, single pluton. Instead, an integrated study incorporating petrographic analysis, mineral mapping, and thermodynamic simulation suggests that the heterogeneous accumulation of accessory minerals during crystal-melt separation in the magma chamber is the leading cause of the compositional variation of the ca. 3.45 Ga TTG pluton. This study highlights the critical role of accessory minerals in intrinsic magmatic differentiation within individual TTG plutons and advise caution when using bulk-rock compositions of TTGs to infer melt physochemical conditions and tectonic settings. [Display omitted] • Accessory minerals in magma mush can cause the compositional variation of TTGs. • Bulk composition of TTGs cannot be used as parent melt due to crystal separation. • Paleoarchean granites may be the product of magmatic differentiation. [ABSTRACT FROM AUTHOR]

The formation of accessory allanite, monazite and rutile in amphibolite-facies rocks across the Barrovian sequence of the Central Alps (Switzerland) was investigated with a combination of petrography and geochemistry and related to the known structural and metamorphic evolution of the Lepontine dome. For each of these minerals a specific approach was adopted for geochronology, taking into account internal zoning and U–Th–Pb systematics. In-situ U–Th–Pb dating of allanite and monazite by ion microprobe revealed systematic trends for the ages of main deformation and temperature in the Lepontine dome. Isotope dilution TIMS dating of rutile returns dates in line with this picture, but is complicated by inheritance of pre-Alpine rutile and possible Pb loss during Alpine metamorphism. Allanite is generally a prograde mineral that is aligned along the main foliation of the samples and found also as inclusions in garnet. Prograde allanite formation is further documented by rutile inclusions with formation temperatures significantly lower than the maximum T recorded by the rock mineral assemblage. Allanite ages vary from 31.3 ± 1.1 Ma in orthogneisses in the East to 31.7 ± 1.1 Ma for a Bündnerschiefer and 28.5 ± 1.3 Ma for a metaquartzite in the central area, to 26.8 ± 1.1 Ma in the western part of the Lepontine dome. These ages are interpreted to date the main deformation events (nappe stacking and isoclinal deformation of the nappe stack), close to peak pressure conditions. The timing of the thermal peak in the Lepontine dome is recorded in monazite that grew at the expense of allanite and after a main episode of garnet growth at temperatures of ~ 620 °C. Monazite in the central area yields an age of 22.0 ± 0.3 Ma, which is indistinguishable from the age of 21.7 ± 0.4 Ma from a metapelite in the western part of the Lepontine dome. In the central area some of the classical kyanite‐staurolite-garnet schists directly underlying the metamorphosed Mesozoic sediments contain monazite that records only a pre-Alpine, Variscan metamorphic event of upper greenschist to lower amphibolite-facies conditions dated at ~ 330 Ma. The new age data provide evidence that nappe stacking at prograde amphibolite-facies conditions and refolding of the nappe stack occurred between 32 and 27 Ma, only a few million years after eclogite-facies metamorphism in the Adula-Cima Lunga unit. Amphibolite-facies metamorphism lasted for about 10 My to ~ 22 Ma, allowing for multiple ductile deformation and recrystallization events. The long lasting amphibolite-facies metamorphism requires fast cooling between 20 and 15 Ma in the Central Alps. This fast cooling was not related to an increase in sedimentation rates in the foreland basins, suggesting that tectonic exhumation was responsible for termination of amphibolite-facies metamorphism in the Lepontine dome. [ABSTRACT FROM AUTHOR]

The suprasolidus behaviour of apatite and monazite is examined for an average metapelite composition using phase equilibria modelling coupled with solubility equations of these minerals. Both closed- and open-system scenarios are considered. Partial melting above the solidus requires apatite and monazite breakdown in order to saturate the anatectic melt in phosphorus and the light rare earth elements. In general, melt loss is predicted to increase the stability of apatite and monazite at high temperature. Most apatite is predicted to survive up to ultrahigh temperature conditions except for rocks with low bulk phosphorus concentrations. By contrast, most monazite is expected to be consumed by UHT conditions. Thorium substitution in monazite is expected to increase the stability of monazite to higher temperatures. The presence of LREE-rich apatite decreases the stability of monazite above the solidus, but the breakdown of this apatite during anatexis may generate prograde monazite at the apatite–melt interface in local pockets of melt oversaturation. However, prograde suprasolidus monazite along grain boundaries is expected to be consumed during further partial melting or during melt homogenization when an interconnected melt network develops. Anatectic melts are predicted to be saturated with respect to apatite except at UHT conditions and for rocks with low initial P 2 O 5 bulk concentrations. [ABSTRACT FROM AUTHOR]

High-temperature metamorphic rocks are the result of numerous chemical and physical processes that occur during a potentially long-lived thermal evolution. These rocks chart the sequence of events during an orogenic episode including heating, cooling, exhumation and melt interaction, all of which may be interpreted through the elemental and isotopic characteristics of accessory minerals such as zircon, monazite and rutile. Developments in imaging and in situ chemical analysis have resulted in an increasing amount of information being extracted from these accessory phases. The refractory nature of these minerals, combined with both their use as geochronometers and tracers of metamorphic mineral reactions, has made them the focus of many studies of granulite-facies terrains. In such studies the primary aim is often to determine the timing and conditions of the peak of metamorphism, and high-temperature metasedimentary rocks may seem ideal for this purpose. For example pelites typically contain an abundance of accessory minerals in a variety of bulk compositions, are melt-bearing, and may have endured extreme conditions that facilitate diffusion and chemical equilibrium. However complexities arise due to the heterogeneous nature of these rocks on all scales, driven by both the composition of the protolith and metamorphic differentiation. In additional to lithological heterogeneity, the closure temperatures for both radiogenic isotopes and chemical thermometers vary between different accessory minerals. This apparent complexity can be useful as it permits a wide range of temperature and time ( T – t ) information to be recovered from a single rock sample. In this review we cover: 1) characteristic internal textures of accessory minerals in high temperature rocks; 2) the interpretation of zircon and monazite age data in relation to high temperature processes; 3) rare earth element partitioning; 4) trace element thermometry; 5) the incorporation of accessory mineral growth into thermodynamic modeling; and 6) Hf isotopic signature of zircon overgrowths and its use in linking zircon into major metamorphic phase development. [ABSTRACT FROM AUTHOR]

The relationships of Y/Nb, Th/Nb, La/Nb and Ce/Pb ratios in A-type felsic rocks from the Ediacaran Katerina Ring Complex, northernmost Arabian-Nubian Shield (ANS; S. Sinai, Egypt), are investigated in this work to understand their behavior during generation of highly evolved granitic magmas and to explore the nature of magma sources. Textural and compositional relationships of cognate Th-REE- and Nb-Ta-accessory minerals in Katerina felsic rocks show that chevkinite-group minerals (CGM), monazite, thorite, allanite and xenotime formed from residual liquids in quartz syenite porphyries, quartz monzonites and peralkaline granites, whereas in aluminous granites, allanite and monazite crystallized early, and thorite and columbite formed from residual liquids. Relationships of Y/Nb, Th/Nb, La/Nb and Ce/Pb ratios with Zr/Hf ratios in the aluminous granites and with Be abundances in the peralkaline granites suggest a decrease in La/Nb and Ce/Pb ratios in the former, and in Y/Nb and La/Nb ratios in the latter with crystallization progress. This contrasts with absence of systematic variations of Th/Nb and Ce/Pb ratios in the peralkaline compositions and of Y/Nb ratio in the aluminous ones. In this latter, Th/Nb ratio can present a significant decrease only in highly evolved compositions. An analysis of Y/Nb, Th/Nb, La/Nb and Ce/Pb relationships in worldwide OIB and subduction-related magmatic suites reveals that A-type felsic rocks with (Th/Nb) N < 1.3, (La/Nb) N < 1.3, and (Ce/Pb) N > 1 may have A 1 -type affinity, and those with (Th/Nb) N > 2, (La/Nb) N > 2, and (Ce/Pb) N < 1 tend to present A 2 -type affinity. The crystal fractionation of Th-LREE- and Nb-Ta-accessory minerals and mixing of components derived from the two granite groups may cause deviations from these compositional limits that can be evaluated using constraints imposed by Th/Nb-La/Nb, Ce/Pb-Th/Nb and Ce/Pb-La/Nb relationships in OIB and subduction-related magmatic suites. Three mantle sources might have been involved in the Ediacaran alkaline magmatism from northernmost ANS that are chemically similar to those of (1) OIB suites, (2) subduction-related magmatic suites, and (3) carbonatite-metasomatized peridotites. In addition, compositions characterized by (Y/Nb) N > 0.18, (Th/Nb) N > 2, (La/Nb) N > 2, (Ce/Pb) N < 1, and crustal-like Nb/Ta ratios may point to a significant contribution of crustal sources. [ABSTRACT FROM AUTHOR]

Nd model ages are valuable tools for studying the structure and evolution of the continental crust. However, their calculation is severely affected by the 147Sm/144Nd lack of constancy during crustal evolution because it results in spurious Nd model ages that may induce wrong geodynamic and geochemical interpretations. An extensive database of granite analyses shows that 147Sm/144Nd may vary significantly in the high-SiO 2 range of granite rocks and that these variations are far more common in peraluminous than in metaluminous varieties. Only a tiny fraction of the latter has anomalously low ratios, likely resulting from apatite, perhaps titanite, fractionation. In contrast, many high-SiO 2 peraluminous granites also show abnormally high, even superchondritic 147Sm/144Nd ratios that likely resulted from monazite fractionation. A Sm and Nd residence study in peraluminous sources revealed that Sm/Nd ratios are lower in monazite and the feldspars than in the bulk rock but higher in apatite and garnet. Monazite carries 90% of Nd and 80% of Sm, apatite carries about 3–5% of Nd and up to 16% of Sm, and garnet may concentrate up to 27% of Sm. Accordingly, the interplay among monazite, apatite and garnet determines the 147Sm/144Nd of resulting magmas, obscuring the role of the other phases. Garnet is expected to be primarily a residual or early crystallized phase, although some can be entrained in melts. Apatite may also behave as a residual phase in high-SiO 2 metaluminous systems. However, it is highly soluble in peraluminous systems. Monazite may dissolve partially, remaining in the residua of peraluminous melts and crystallizing early from these, as seen in leucosome-melanosome pairs from the Ivrea-Verbano Zone, NW Italy, and Peña Negra, Central Iberia, and the upwards differentiated Pedrobernardo granite sill, also in Central Iberia. Garnet or apatite retention in residua, or segregated from a crystallizing magma, produces melts with low 147Sm/144Nd that may cause anomalously young Nd model ages, as exemplified by the Roccapietra granite of the Ivrea-Verbano zone, NW Italy. Monazite retention or crystallization, in contrast, produces high 147Sm/144Nd melts that yield anomalously old Nd model ages. The effects of intracrustal 147Sm/144Nd variations may be softened or eliminated by employing the two-stage Nd model age calculation method (DePaolo et al., 1991) with a Sm/Nd evolution model adapted to each specific crustal segment. • Realistic Nd model ages require the constancy of Sm/Nd during crustal evolution. • The constancy does not hold during the generation and evolution of peraluminous magmas. • Monazite fractionation causes abnormally high Nd model ages. • Garnet and, likely, apatite fractionation cause abnormally low Nd model ages. • These effects can be corrected using the two-step Nd model age calculation. [ABSTRACT FROM AUTHOR]

Abstract: A combined study of mineral compositions and zircon U–Pb ages was carried out for a zoisite―quartz vein and its host retrograded eclogite in the Dabie orogen. The results reveal two episodes of fluid action during exhumation of the deeply subducted continental crust. The vein has a mineral paragenesis of quartz+zoisite+epidote+white mica+garnet+rutile+zircon+apatite, which is almost the same as that of the host retrograded eclogite characterized by symplectites after omphacite and/or garnet in the eclogite. Residues of magmatic zircon in the vein are similar to those in the eclogite, suggesting local sources of the metamorphic fluid for veining and physical transport of the tiny accessory mineral from the host rock to veins. There are significant differences in major and trace elements between vein-forming minerals, which are attributed to two episodes of fluid action for veining. Metamorphic zircon from the vein yields concordant U–Pb ages of 215±4 to 218±4Ma. Trace element analysis indicates its precipitation from aqueous fluid during two distinct episodes under eclogite-facies and amphibolite-facies conditions, respectively. Ti-in-zircon and Zr-in-rutile thermometries yield variable temperatures from 537 to 683°C. Thus the veining postdates the UHP metamorphic event at 225–240Ma and took place during transition from HP eclogite-facies recrystallization to amphibolite-facies retrogression during the exhumation. The host eclogite was also retrograded in this stage to form metabasites such as amphibole eclogite to garnet amphibolite. The abundant occurrence of zoisite, epidote and phengite in the vein suggests that the metamorphic fluid is rich in Si, Al, Ca and K. The fluid composition is estimated in terms of zoisite composition and zoisite/fluid partition coefficients. The results suggest strong enrichment of LREE, Th, U, Pb, Sr, Rb and Ba, moderate enrichment of HREE, but significant depletion of HFSE. In this regard, the occurrence of metamorphic zircon and rutile in the vein suggests very local saturation of Zr and Ti in the Zo–Qtz vein-forming fluid. Therefore, the two episodes of fluid action would occur at the transition from eclogite to amphibolite facies during the exhumation and proceed with different fluid compositions and different flow behaviors. One was channelized under the eclogite-facies conditions, whereas the other is pervasive under the amphibolite-facies conditions. This provides robust constraints on the time, origin and property of metamorphic fluid during the exhumation of deeply subducted continental crust. [Copyright &y& Elsevier]

Abstract: In situ U–Pb age and Sr-, Nd-, Hf- and O- isotope analysis of accessory minerals (zircon, apatite and titanite) have been obtained for mafic microgranular enclaves and their host granites from the Early Cretaceous Fangshan pluton in the North China Craton, in order to constrain their sources and petrogenesis. The Fangshan pluton is composed of quartz monzodiorite at the margins, and granodiorite, with abundant mafic microgranular enclaves, in the core. Zircons from the marginal quartz monzodiorite record U–Pb ages of 130 to 133Ma, identical to those of granodiorite in the core. The enclaves have zircon U–Pb ages of 133 to 134Ma, identical within error to those of the host granodiorite, establishing that the mafic and felsic magmas were coeval. Furthermore, apatite and titanite from the enclaves and host granodiorite have the same U–Pb ages as the zircons, indicating rapid cooling of the Fangshan pluton. The granodiorites have relatively high SiO2 and low MgO concentrations. They have variable negative εHf(t) values of −25.1 to −18.2 and δ18O values for zircon of 6.2‰ to 8.3‰. The negative εNd(t) values of −19.7 to −14.0 for titanite and apatite, with initial 87Sr/86Sr ratios of 0.7052–0.7058 for apatite, indicate that they were derived from an ancient lower crustal source with addition of materials with higher εHf(t) and εNd(t) values and low δ18O values. The enclaves have relatively high MgO contents (up to 15.5wt.%) at intermediate silica concentrations, indicating that their parental magmas were derived from a mantle source. Their zircons have strongly negative Hf isotopic compositions (εHf(t)=−22.9 to −14.9). The Nd isotopic composition of titanite and apatite is also strongly negative (εNd(t)=−17.6 to −12.1). All these features, combined with the δ18O zircon values of 4.8‰–8.2‰ for zircons and relatively low initial Sr isotopic compositions of apatite (initial 87Sr/86Sr=0.7051–0.7058), indicate that they were derived from an enriched and/or depleted mantle source, but with involvement of crustal materials. The marginal quartz monzodiorites have transitional geochemical features between those of the enclaves and host granodiorites. In situ Sr, Nd, Hf, and O isotopic data of accessory minerals, coupled with field observations and whole rock geochemical data, rule-out simple crystal–liquid fractionation or wall-rock contamination as the major genetic link between the enclaves and host rocks. Instead, mixing of mantle-derived mafic magmas with crustal-derived felsic magmas, coupled with assimilation of wall rocks, is compatible with the data. Our study shows that in situ Hf, O, Nd and Sr isotopic analyses of accessory minerals are powerful geochemical tracers that provide unique petrogenetic information regarding the origin of granitoids. [Copyright &y& Elsevier]

The Chenaillet ophiolite, South of Montgenèvre (France), represents a preserved portion of the alpine ocean. The gabbros form lenticular bodies 50 to 200 m thick and a few hundreds of meters wide. They are intrusive in the serpentinites and often overlain by pillow-lavas. Plagiogranite/albitite veins are rare and volumetrically negligible. The petrology of the gabbro s.s. show great details of the progressive cooling of the massif, clinopyroxene are often rimmed by brown to green amphibole +/− ilmenite/titanite. Thermometry on these amphibole assemblages indicates a retrograde temperature evolution from late magmatic to subsolidus, between 950 and 900 °C, 800–750 °C, 600–500 °C. In a shear zone, a string of titanite associated with monazite and xenotime +/− ilmenite is located in a millimetric greenschist layer whose temperature is estimated at 600–550 °C. Accessories chemistry in this sample allowed in situ dating by LA-ICPMS on thin section in textural context. In the same sample, we performed U Pb zircon dating, constraining the magmatic age, as well as U Pb monazite, xenotime and titanite dating for the metamorphic evolution. In a Tera-Wasserburg diagram, monazite, xenotime and titanite yield 161.3 +/− 4.0 Ma, 161.5 +/− 2.4 Ma and 158.4 +/− 2.3 Ma lower intercept ages, respectively, while the zircon from the same sample indicates an age of 161.0 +/− 0.8 Ma. Finally, zircon dated in an albitite vein also yields a 161.8 ± 1.7 Ma age. These similar ages within the analytical uncertainties show that magmatism and metamorphism were concomitant. Thermal modelling predicts that a 150 m thick gabbroic massif crystallizes in a few hundred years and cools down in several thousand years. This is consistent with the ages reported in our study showing that the retrograde metamorphism in this massif records fast cooling. The development of this retrograde metamorphism required multiple fluid infiltrations during cooling. In the studied samples, 4 types of titanites could be distinguished based on their chemistries. 3 types variously depleted in REE are interpreted as related to water-driven fluids while another type has up to 10% REE, Y and Nb and suggest a possible felsic source. The two potential sources of water promoting this fast cooling are sea water-derived fluids and magmatic fluids exsolved from felsic veins. • New zircon crystallization ages of the Chenaillet gabbro yield 161 Ma. • Monazite, xenotime and titanite give a metamorphic age similar to the magmatic age. • We show very fast cooling rates for the Chenaillet gabbro. • Metagabbro thermal modelling show cooling to 600 °C lasted less than 7500 years. • Both sea water-derived and magmatic fluids are involved in this fast cooling. [ABSTRACT FROM AUTHOR]

Abstract: Fractional crystallization of peraluminous F- and H2O-rich granite magmas progressively enriches the remaining melt with volatiles. We show that, at saturation, the melt may separate into two immiscible conjugate melt fractions, one of the fractions shows increasing peraluminosity and the other increasing peralkalinity. These melt fractions also fractionate the incompatible elements to significantly different degrees. Coexisting melt fractions have differing chemical and physical properties and, due to their high density and viscosity contrasts, they will tend to separate readily from each other. Once separated, each melt fraction evolves independently in response to changing T/P/X conditions and further immiscibility events may occur, each generating its own conjugate pair of melt fractions. The strongly peralkaline melt fractions in particular are very reactive and commonly react until equilibrium is attained. Consequently, the peralkaline melt fraction is commonly preserved only in the isolated melt and mineral inclusions. We demonstrate that the differences between melt fractions that can be seen most clearly in differing melt inclusion compositions are also visible in the composition of the resulting ore-forming and accessory minerals, and are visible on scales from a few micrometers to hundreds of meters. [Copyright &y& Elsevier]

The study of accessory minerals in granitoid rocks can provide clues to the history of magmatic processes. In particular, the textural–chemical characteristics of accessories could represent effective markers of hybridization processes. The concomitant occurrence of contrasting reaction microtextures of REE–Y–Th–U accessory minerals in the Monte Capanne anatectic–hybrid pluton suggests the occurrence of transient chemical conditions (alumina saturation up and down in the same system) in the early stages of crystallization. Incongruent dissolution of apatite produced microcrystal clusters of huttonitic monazite, while monazite-(Ce) crystals were replaced by allanite-(Ce)±apatite assemblage at the same location. A magma mingling process, involving acidic peraluminous and mafic metaluminous end-members, can provide the expected initial strong differences in alumina saturation that are able to induce such contrasting reactions. The proposed double exchange of accessory minerals between the two magmas strongly suggests a dynamic setting (stirring and straining of crystal-rich melts) in which anatectic and hybridization processes evolved. [Copyright &y& Elsevier]

The solubility of Ti- and P-rich accessory minerals has been examined as a function of pressure and K2O/Na2O ratio in two series of highly evolved silicate systems. These systems correspond to (a) alkaline, varying from alkaline to peralkaline with increasing K2O/Na2O ratio; and (b) strongly metaluminous (essentially trondhjemitic at the lowest K2O/Na2O ratio) and remaining metaluminous with increasing K2O/Na2O ratio (to 3). The experiments were conducted at a fixed temperature of 1000 °C, with water contents varying from 5 wt.% at low pressure (0.5 GPa), increasing through 5–10 wt.% at 1.5–2.5 GPa to 10 wt.% at 3.5 GPa. Pressure was extended outside the normal crustal range, so that the results may also be applied to derivation of hydrous silicic melts from subducted oceanic crust.For the alkaline composition series, the TiO2 content of the melt at Ti-rich mineral saturation decreases with increasing pressure but is unchanged with increasing K content (at fixed pressure). The P2O5 content of the alkaline melts at apatite saturation increases with increased pressure at 3.5 GPa only, but decreases with increasing K content (and peralkalinity). For the metaluminous composition series (termed as “trondhjemite-based series” (T series)), the TiO2 content of the melt at Ti-rich mineral saturation decreases with increasing pressure and with increasing K content (at fixed pressure). The P2O5 content of the T series melts at apatite saturation is unchanged with increasing pressure, but decreases with increasing K content. The contrasting results for P and Ti saturation levels, as a function of pressure in both compositions, point to contrasting behaviour of Ti and P in the structure of evolved silicate melts. Ti content at Ti-rich mineral saturation is lower in the alkaline compared with the T series at 0.5 GPa, but is similar at higher pressures, whereas P content at apatite saturation is lower in the T series at all pressures studied. The results have application to A-type granite suites that are alkaline to peralkaline, and to I-type metaluminous suites that frequently exhibit differing K2O/Na2O ratios from one suite to another. [Copyright &y& Elsevier]

Minor altered intrusions and drab retrogressed schists can easily be overlooked in geological studies but this contribution explores these rocks within the Archaean and Palaeoproterozoic southern São Francisco Craton (SSFC), Brazil using geological relationships and accessory mineral in situ analyses in the context of cratonic assembly. Three magmatic pulses are documented: i) Archaean and ii) Palaeoproterozoic felsic intrusions, both hosted by Archaean protoliths, and iii) Palaeoproterozoic felsic intrusions in Palaeoproterozoic supracrustal sequences. Archaean felsic intrusions confirm the Palaeoarchaean age of the mafic/ultramafic sequence of the Rio das Velhas Greenstone Belt and Rhyacian intrusions mark the collisional stage of the Mineiro Belt with the SFC at c. 2130 Ma. Greenstone belt schists show a wide distribution of rounded 'soccer ball' Archaean detrital and metamorphic zircon grains ranging in age from 3200 to 2750 Ma with an interpreted overprinting high-grade metamorphic event at c. 2700 to 2680 Ma. Most high-grade metamorphic rims have Th/U > 0.1, negative εHf (t) values and REE pattern consistent with eclogite/granulite metamorphic facies, reinforcing the hypothesis of a dehydrated-refractory crust formed during the stabilization of the SSFC, even though no such protoliths are preserved. This event links crustal thickening and partial melting of Archaean lower crust. Archaean rutile crystals from the greenstone belt schist were reset during the Palaeoproterozoic event but still preserve the early Archaean high-grade metamorphic signature. The presence of unstable ilmenite replaced by rutile in the schist, associated to felsic intrusions with the same age at c. 2130 Ma suggest high pressure, low temperature prograde metamorphism during the collisional stage of the Palaeoproterozoic orogen. Elongate and prismatic zircon grains from the Rhyacian intrusions have low εHf (t) signature and crystallised from partial melting of sedimentary protoliths. Accretionary events produced thicker and more differentiated crust by the end of Rhyacian time. Easily overlooked rocks in this study, when studied, have revealed a rich multi-event history of cratonic evolution. • Archaean felsic intrusions constrain the Rio das Velhas Greenstone Belt evolution in at least 200 Myr; • High-grade soccer ball metamorphic zircon grains depict conditions of Archaean crustal amalgamation; • First report of Rhyacian felsic intrusions in the Minas and Rio das Velhas supergroups of the Quadrilátero Ferrífero; • Rhyacian felsic intrusions in the supracrustal sequences derive from reworked crust; • Rhyacian felsic magmatism around the margins of the SSFC depicts the collisional stage of the Minas accretionary orogeny. [ABSTRACT FROM AUTHOR]