Melting Of Metapelites Research Articles

High-pressure melting in metapelites of a 2 Ga old subducted oceanic crust (Usagaran belt, Tanzania): implications from melt inclusions, fluid inclusions and thermodynamic modelling

Investigation of polymineralic melt inclusions preserved in garnet of eclogite-facies metapelites of the Usagaran belt, Tanzania, is of particular importance as these metapelites, intercalated in oceanic metabasites, document the rare case of partial melting at high temperatures in a subducted oceanic crust. With an age of 2 Ga the rocks represent one of the oldest oceanic crusts and confirm a subduction process already at Paleoproterozoic times. Partial melting probably was initiated by dehydration melting under the presence of a CO2-rich fluid phase. The melt is preserved in siliceous polymineralic inclusions, while CO2 locally reacted with the garnet host to form dolomite-quartz-kyanite inclusions. During this reaction, the REE spectrum of garnet is adopted by the dolomite. Furthermore, graphite inclusions in garnet must have precipitated from the CO2 fluid by reduction. The highly ordered graphite structure indicates a formation temperature of at least 700 °C. Rehomogenization experiments of the siliceous polymineralic inclusions yield a homogeneous melt of rhyolitic, peraluminous composition. Thermodynamic modelling enables to deduce a P–T path in accordance with high P–T conditions (minimum 2.0 GPa, 900 °C) where a partial melt formed due to phengite breakdown leading to the preserved peak mineral assemblage garnet, alkali feldspar, kyanite, quartz and rutile. A very fast uplift of the oceanic crustal rocks can be deduced from the occurrence of very finely exsolved metastable ternary feldspar and from the preserved prograde zoning in garnet.

Prograde zircon growth in migmatites

AbstractEmpirical studies of zircon in migmatites document features compatible with growth during heating at suprasolidus conditions. However, numerical modelling of zircon behaviour suggests that suprasolidus zircon is expected to grow only during cooling and melt crystallization. Here, phase equilibrium modelling coupled with mineral–melt Zr partitioning is used in an attempt to reconcile the observations from migmatites with the predictions of previous numerical models of zircon behaviour in anatectic systems. In general, an equilibrium‐based model that includes Zr partitioning does not allow prograde suprasolidus zircon growth. However, melting of metapelites at temperatures just above the wet solidus may allow limited zircon growth because of the low solubility of zircon in melt coupled with a source of Zr from minor garnet and ilmenite breakdown. Preservation of this zircon requires entrapment in growing peritectic minerals during subsequent heating and further melting. Heating above muscovite exhaustion in metapelites is unlikely to grow zircon because of the progressive increase in zircon solubility as well as an increasing compatibility of Zr in the residual mineral assemblage. The modelled compatibility of Zr in the residue of a metabasite decreases during heating, but an increase in zircon solubility in melt counteracts this; prograde suprasolidus zircon growth in metabasites is unlikely. Infiltration of Zr‐rich melt into a migmatite during open‐system anatexis provides an additional potential mechanism for prograde suprasolidus zircon growth during high‐temperature metamorphism.

Sr-Nd-Hf Isotopic Disequilibrium During the Partial Melting of Metasediments: Insight From Himalayan Leucosome

Radiogenic isotopes of granitoids are widely applied to fingerprint the source of granitoids and study the magma mingling and assimilation processes, aiming to decipher the planetary differentiation. This weapon is based on the assumption that crustal melts inherit the radiogenic isotopes of protoliths. However, complicated melting processes in the crust would drive the radiogenic isotopes of melt away from the source, thus calling for a clear understanding of the behaviors of radiogenic isotopes during crustal melting. This study investigated the joint behavior of Rb-Sr, Sm-Nd, and Lu-Hf isotope systems during the melting of metasediments. Leucosome, schist, and leucogranite samples were collected from the Nyalam, South Tibet, where the leucosome was produced by muscovite dehydration melting of metapelites. Results show that the leucosome has a broad range of Sr-Nd-Hf isotopes (87Sr/86Sri: 0.763,48–0.875321, εNd(t): 14.6–11.6, εHf(t): 15.0–4.4) that deviate from the metasediments. We attributed it to the non-modal or disequilibrium melting of metapelites. The variation of Sr isotopes of leucosome is formed due to preferential entry into the melt of muscovite relative to plagioclase during melting. The changing 87Sr/86Sri and 87Rb/86Sr of leucosome define an errorchron at ∼ 500 Ma indicating an early Paleozoic tectono-thermal event supported by the U-Pb age of zircon cores (∼460 Ma) in the leucosome. The Nd isotopes of leucosome are mainly controlled by the preferential dissolution of apatite relative to monazite. The inadequate dissolution of zircon caused more depleted initial Hf isotopes compositions in the leucosome than the source. However, the leucosome with a higher crystallization temperature has radiogenic isotopic composition closer to the source. It indicates that the melting temperature greatly affects the isotope equilibrium between source and melt.

Contrasting fertility of metapelite vs diorite gneiss to produce leucogranite melts: Example from the Mahalapye Complex, eastern Botswana

High-grade terranes showing multiple episodes of leucogranite magmatism are ideal to assess the fertility of crustal protoliths. Two episodes of Paleoproterozoic leucogranite magmatism occur in the Mahalapye Complex high-grade terrane, at the northern edge of the Kaapvaal Craton. The older one (leucogranite 1) at ∼2.05 Ga represented by leucosomes, thin veins and dykes is related to the melting of ∼2.06 Ga biotite-bearing diorite gneiss. In comparison, the younger episode (leucogranite 2) at ∼2.03 Ga forms massive bodies. Leucogranite 1 is associated with the migmatitic diorite gneiss and crosscut by the ∼2.04 Ga granodiorite. Leucogranite 2 contains xenoliths of diorite gneiss and granodiorite, as well as inclusions of quartz-poor and quartz-rich metapelites (Bt + Crd + Pl + Kfs + Spl ± Sil ± Grt + Ilm + Qz). Phase equilibria modelling of a representative quartz-poor metapelite indicate that its partial melting occurred at ∼810 °C and ∼4.8 kbar in the presence of aqueous fluid. The peraluminous, ferroan and alkalic composition of the predicted melt is comparable to the chemistry of leucogranite 2. The larger volume of anatectic melt for the estimated P-T condition corresponds to the occurrence of leucogranite 2 as massive bodies. The lower temperature of in-situ anatexis (<750 °C) at a deeper level (∼7.3 kbar) and lack of additional fluid, likely did not generate significant melt fraction during the formation of leucogranite 1. The contrast in the fertility of the two protoliths with respect to production of leucogranite melts was also related to different setting of melting. Melting of diorite gneiss is related to the local influence of mafic dykes and sills, whereas melting of metapelite is related to regional collision. It follows that high pressures, water-deficient regimes and localized heat sources produce limited volumes of partial melt, whereas high temperatures, fluid-present regimes and a large-scale heat source produces higher volumes of melts.

The Effects of Source Mixing and Fractional Crystallization on the Composition of Eocene Granites in the Himalayan Orogen

Abstract Granites are generally the final products of crustal anatexis. The composition of the initial melts may be changed by fractional crystallization during magma evolution. Thus, it is crucial to retrieve the temperature and pressure conditions of crustal anatexis on the basis of the composition of the initial melts rather than the evolved melts. Here we use a suite of ∼46–41 Ma granites from the Himalayan orogen to address this issue. These rocks can be divided into two groups in terms of their petrological and geochemical features. One group has high maficity (MgO + FeOt = 2–4 wt%) and mainly consists of two-mica granites, and is characterized by apparent adakite geochemical signatures, including high Sr concentrations and Sr/Y and La/Yb ratios, and low concentrations of heavy rare earth elements and Y. The other group has low maficity (MgO + FeOt &amp;lt;1 wt%) and consists of subvolcanic porphyritic granites and garnet/tourmaline-bearing leucogranites. This group does not possess apparent adakite signatures. The low-maficity group (LMG) has lower MgO + FeOt contents and the high-maficity group (HMG) has higher Mg# compared with initial anatectic melts determined by experimental petrology and melt inclusions studies. Petrological observations indicate that the HMG and the LMG can be explained as a crystal-rich cumulate and its fractionated melt, respectively, such that the initial anatectic melt is best represented by an intermediate composition. Such a cogenetic relationship is supported by the comparable Sr–Nd isotopic compositions of the two coeval groups. However, these compositions are also highly variable, pointing to a mixed source that was composed of amphibolite and metapelite with contrasting isotope compositions. We model the major and trace element compositions of anatectic melts generated by partial melting of a mixed source at four apparent thermobarometric ratios of 600, 800, 1000 and 1200 °C GPa–1. Modeling results indicate that melt produced at 1000 °C GPa–1 best matches the major and trace element compositions of the inferred initial melt compositions. In particular, a binary mixture generated from 10 vol% partial melting of amphibolite and 30 vol% melting of metapelite at 850 ± 50 °C and 8·5 ± 0·5 kbar gives the best match. Therefore, this study highlights that high thermobarometric ratios and subsequent fractional crystallization are responsible for the generation of the apparent adakitic geochemical signatures, rather than melting at the base of the thickened crust as previously proposed. The thermal anomaly responsible for the Eocene magmatism in the Himalayan orogen was probably related to asthenosphere upwelling in response to rollback of the subducting Neo-Tethyan oceanic slab at the terminal stage of continental collision between India and Asia. As such, a transition in dynamic regime from compression to extension is necessary for the generation of high thermobarometric ratios in continental collision zone. Therefore, after correcting for potential effects of fractional crystallization and crystal accumulation on melt composition, granite geochemistry coupled with thermodynamic modeling can better elucidate the petrogenesis of granites and the geodynamic mechanisms associated with anatexis at convergent plate boundaries.

The Mencué Batholith: Permian episodic arc-related magmatism in the western North Patagonian Massif, Argentina

ABSTRACT The Mencué Batholith, western North Patagonian Massif, includes three major bodies. The Mencué Granodiorite, Cura Lauquén Granite and La Blancura Syenogranite. There are dikes of granitic and basaltic rocks cutting the above-cited rocks. The Mencué Batholith represents several episodes of magmatism, with ages varying between 294 and 239 Ma. The Mencué Granodiorite and the Cura Lauquén Granite are solid-state deformed and are S-type. They have high-K and normal calc-alkaline affinities. These rocks contain significant quantities of subduction-zone chemical components that decrease towards younger lithofacies. La Blancura Syenogranite lack subduction zone chemical components and represent A-type granite, typical of within-plate magmatism. The partial melting of metapelites could be the process of formation of these bodies. The older lithofacies of the Mencué Batholith, found westernmost, display a stronger deformation, but there is a progressive eastward change to younger and mildly deformed bodies and even non-deformed bodies. We conclude that the evolution of the Mencué Batholith start in the Sakmarian-Roadian (Early Permian) period. At this time, a subduction zone was active to the west and its thermal influence affected sedimentary or metamorphic rocks producing S-type granites During the Wordian and Capitanian, (Middle Permian) periods, the Mencué Batholith was mildly deformed, possibly in the process of the vanishing of the deformation and has a minor subduction chemical signature. Between the Wuchiapingian and Olenekia periods, the alkaline facies of the Mencué Batholith show an absence of deformation and the characteristics of within-plate magmatism. The Early Permian magmatic events in the western North Patagonian Massif are represented by the older bodies of the Mencué Batholith and were produced by subduction in the western margin of the Gondwanan continent. The Late Permian-Early Triassic magmatic events show a noticeable decreasing influence of subduction and an increasing influence of within-plate chemical components.

Plagioclase disequilibrium induced during fluid‐absent biotite‐breakdown melting in metapelites

AbstractIn the Bandelierkop Quarry (South Marginal Zone of the Limpopo belt), two generations of leucosome contain peritectic garnet produced by incongruent melting of biotite‐bearing assemblages, either in the presence or absence of sillimanite, respectively. This study focuses on mineral inclusions in large garnet crystals from both leucosome generations, which, in rare cases, host an abundance of euhedral plagioclase inclusions with variable compositions. Plagioclase in the residuum adjacent to the leucosomes ranges from An32 to An39, while the euhedral plagioclase inclusions are considerably more calcic with compositions that range from An50 to An83. These grains are also significantly zoned, with inclusions displaying both Na‐ and Ca‐enriched rims. Garnet zoning directly adjacent to the inclusions demonstrates that the Ca‐rich nature of the inclusions is not a consequence of Ca‐uptake from garnet. The euhedral plagioclase inclusions are interpreted to reflect disequilibrium during anatexis due to slow diffusion in plagioclase. Thus, the kinetic pathway followed by the incongruent melting reaction appears to be one where only the outer portions of the reactant plagioclase grains were involved, via dissolution, not diffusion, and with the precipitation of new Ca‐rich peritectic plagioclase as a consequence. This is predictable, as the coupled substitutions involved in plagioclase solid solution result in very slow intracrystalline diffusion of the major elements, while plagioclase dissolution in granitic melt is faster by several orders of magnitude. Phase equilibrium modelling demonstrates that the Ca‐content of the peritectic plagioclase is inversely proportional to the amount of plagioclase that participates in the fluid‐absent incongruent melting reaction in biotite‐bearing metapelites, with the specific compositions of the peritectic grains, reflecting local variations in effective bulk composition as the melting reaction progressed. The variation in composition and zoning patterns within plagioclase inclusions from individual garnet grains is interpreted to reflect melt segregation and flow simultaneously with garnet growth, allowing individual garnet grains to trap euhedral peritectic plagioclase grains that had formed within a range of effective bulk compositions. The preservation of this unusual garnet, which was most likely due to rapid and efficient melt loss from the residual source, as well as rapid cooling, provides a rare insight into the details of the anatectic process.

Source controlled 87Sr/ 86Sr isotope variability in granitic magmas: The inevitable consequence of mineral-scale isotopic disequilibrium in the protolith

The broad Sr isotopic variability exhibited by granitoid rocks is commonly interpreted to reflect the mixing of magmas from different sources. However, evidence from granites and migmatites indicates that melting and magma extraction from crustal sources can occur sufficiently rapidly that trace-element and isotopic equilibration between liquid and residual phases is commonly not achieved. Additionally, evidence from unmelted high-grade metamorphic rocks indicates that major reactant minerals in the fluid-absent melting process, principally biotite and plagioclase, do not always attain equilibrium during regional metamorphism. When these two circumstances occur in combination, the melt does not inherit its radiogenic isotopic signature from the bulk source in a simple way. In such situations, the isotopic composition of the melt will be dependent on the isotopic compositions of the reactant phases and the stoichiometry of the melting reaction. This study has used information from experimental studies of fluid absent partial melting in metapelites and metagreywackes to investigate the consequences of Sr isotopic disequilibrium between the reactant minerals for magma composition. The study demonstrates that a range of isotopically distinct magmas can arise from progressive melting of a single source that is able to undergo melting through different reactions as temperature increases. When translated to the typically layered sources constituted by sedimentary and volcano-sedimentary rocks, this process will produce magmas characterized by Sr isotope variability that reflects the differences in melting reaction stoichiometry within the different layers, even with no bulk-rock isotopic variability between layers. This study demonstrates that the Sr isotope variability commonly observed within granitic suites, as well as at the grain and sub-grain scale within individual magmatic bodies, can be primary, reflecting differences in composition between magma batches produced from the progressive melting of a single source.

Palaeoproterozoic high‐T, low‐P metamorphism and dehydration melting in metapelites from the Mopunga Range, Arunta Inlier, central Australia

AbstractA sequence of psammitic and pelitic metasedimentary rocks from the Mopunga Range region of the Arunta Inlier, central Australia, preserves evidence for unusually low pressure (c. 3 kbar), regional‐scale, upper amphibolite and granulite facies metamorphism and partial melting. Upper amphibolite facies metapelites of the Cackleberry Metamorphics are characterised by cordierite‐andalusite‐K‐feldspar assemblages and cordierite‐bearing leucosomes with biotite‐andalusite selvages, reflecting P–T conditions of c. 3 kbar and c. 650–680 °C. Late development of a sillimanite fabric is interpreted to reflect either an anticlockwise P–T evolution, or a later independent higher‐P thermal event. Coexistence of andalusite with sillimanite in these rocks appears to reflect the sluggish kinematics of the Al2SiO5 polymorphic inversion. In the Deep Bore Metamorphics, 20 km to the east, dehydration melting reactions in granulite facies metapelites have produced migmatites with quartz‐absent sillimanite‐spinel‐cordierite melanosomes, whilst in semipelitic migmatites, discontinuous leucosomes enclose cordierite‐spinel intergrowths. Metapsammitic rocks are not migmatised, and contain garnet–orthopyroxene–cordierite–biotite–quartz assemblages. Reaction textures in the Deep Bore Metamorphics are consistent with a near‐isobaric heating‐cooling path, with peak metamorphism occurring at 2.6–4.0 kbar and c. 750–800 °C. SHRIMP U–Pb dating of metamorphic zircon rims in a cordierite‐orthopyroxene migmatite from the Deep Bore Metamorphics yielded an age of 1730 ± 7 Ma, whilst detrital zircon cores define a homogeneous population at 1805 ± 7 Ma. The 1730 Ma age is interpreted to reflect the timing of high‐T, low‐P metamorphism, synchronous with the regional Late Strangways Event, whereas the 1805 Ma age provides a maximum age of deposition for the sedimentary precursor. The Mopunga Range region forms part of a more extensive low‐pressure metamorphic terrane in which lateral temperature gradients are likely to have been induced by localised advection of heat by granitic and mafic intrusions. The near‐isobaric Palaeoproterozoic P–T–t evolution of the Mopunga Range region is consistent with a relatively transient thermal event, due to advective processes that occurred synchronous with the regional Late Strangways tectonothermal event.

Migmatite-granite relationships: origin of the Cooma Granodiorite magma, Lachlan Fold Belt, Australia

Most published interpretations infer that the Cooma Granodiorite (southeastern Australia) was formed by more or less in situ melting of metasedimentary rocks of the Cooma Complex. Detailed work has shown that melting of metapelites, which occurred by biotite breakdown during D3 (after muscovite had disappeared), produced relatively immobile, plagioclase-poor or plagioclase-free leucosomes that are compositionally unsuitable as the source magma for the granodiorite. However, melting of feldspathic metapsammitic rocks, which occurred during D5, as P-T conditions followed an anticlockwise path, produced mobile, plagioclase-rich leucosomes that are more appropriate for the granodiorite source magma. Though gradations from metapsammite-derived migmatite to Cooma Granodiorite are present, accumulation of magma derived locally from metapsammite melting appears to be unable to account for all of the exposed body of Cooma Granodiorite, implying some ascent of similar magma from deeper levels of the source rocks.

Water as a Petrologic Catalyst Driving 18O/16O Homogenization and Anatexis of the Middle Crust in the Metamorphic Core Complexes of British Columbia

In the Thor-Odin and Valhalla metamorphic core complexes, we have documented a remarkable uniformity of mineral δ18O values in the middle continental crust beneath the detachment faults. For example, in the Thor-Odin Complex, throughout an 8 km thick section of metasedimentary rocks and early Tertiary leucogranites in the hanging wall of the Monashee decollement (MD), quartz δ18O = 12.3 ± 0.5% (lσ S.D.) for metapelite (N = 11), 12.0 ± 0.1% for quartzite (N = 2), 12.6 ± 0.6% (N = 4) for < 1 m thick amphibolite layers, and 12.1 ± 0.4% (N = 24) for the concordant leucogranites. No exceptions have been found to this remarkable 18O/16O homogeneity except locally in a couple of thick amphibolites and within a ductile, relatively impermeable, marble-rich section. Similar zones of 18O/16O homogeneity associated with leucogranite genesis are observed throughout the mid-crustal section of the Valhalla Complex and just beneath the MD in the Monashee Complex, the only difference being that those rocks are overall 0.5 to 1.5% lower in δ18O than in the middle crust at Thor-Odin. These zones of pervasive homogenization in 18O/l6O must be a result of exchange with magmatic or metamorphic H2O, and these same volatiles appear to have been responsible for the leucogranite anatexis. A wide range in quartz δ18O from +8 to +16 within and below the MD suggests that this major thrust fault was impermeable to aqueous fluid flow during the partial melting stage; at that time, the basement appears to have been isolated from the mid-crustal metamorphichydrothermal system. LITHOPROBE crustal seismic profiles establish the MD as a W-dipping, crustal-scale ramp with 20 km of vertical relief, and Carr (1992) proposed an anatectic origin for the leucogranites during decompression melting associated with tectonic shortening as the mid-crustal section moved up this thrust ramp. Partial melting of metapelites and feldspathic grits from the Late Precambrian Windermere Supergroup began in response to influx of metamorphic H2O, aided by internal muscovite dehydration at ≈8 kbar and ≈750°C at the base of the Monashee ramp. Metapelites are volatile rich, but feldspar poor, whereas the opposite is true for the grit lithologies. Thus, at the base of the Monashee ramp large-scale (≈30°) partial melting of the metapelites produced magmas near H2O saturation (10 tol4 wt°), whereas the intercalated arkosic grit-derived magmas were undersaturated (5 to 6 wt°). As these H2O-rich, pelite-derived leucogranite melts moved upward to shallower depths, they cooled adiabatically and underwent decompressive exsolution of H2O. The released H2O was then able to exchange oxygen with lithologies infertile to melting as it concurrently migrated through the section toward the feldspathic grit layers, where it could act as a catalyst and be re-used, promoting further hydrothermal melting of the arkosic grits. Continued decompression melting and exsolution occurred simultaneously in different parts of the section during uplift, tectonic shortening, and buoyant uprise of the magma bodies, until final crystallization of all of the leucogranites took place much higher in the crust, where almost all of the H2O was released and again re-used for a final episode of 18O/l6O exchange with the unmelted metamorphic lithologies. In addition to the direct l8O/16O exchange that takes place between the metamorphic rocks and the migrating leucogranite magmas, this use and re-use of the same H2O during repeated episodes of partial melting and exsolution in different parts of the section seems adequate to explain the pervasive oxygen isotopic homogenization of these metasedimentary rocks. It is estimated that 25 to 30° partial melting of a typical section of the Windermere Supergroup occurred as a result of these cumulative processes, and this probably played a pivotal role in determining the susceptibility of this orogen to subsequent extensional collapse along the detachment faults.

Dehydration melting of micas in the Chilka Lake Khondalites: The link between the metapelites and granitoids

Garnet-sillimanite gneisses, locally known as khondalites, occur abundantly in the Chilka Lake granulite terrane belonging to the Eastern Ghats Proterozoic belt of India. Though their chemistry has been modified by partial melting, it is evident that the majority of these rocks are metapelitic, with some tending to be metapsammitic. Five petrographically distinct groups are present within the khondalites of which the most abundant group is characteristically low in Mg:Fe ratios — the main chemical discriminant separating the five groups. The variations in Mg:Fe ratios of the garnets, biotites, cordierites, orthopyroxenes and spinels from the metapelites are compatible with those in the bulk rocks. A suite of granitoids containing garnet, K-feldspar, plagioclase and quartz, commonly referred to as leptynites in Indian granulite terranes, are interlayered with khondalites on the scale of exposures; in a few spots, the intercalated layers are thin. The peraluminous character of the leptynites and presence of sillimanite trails within garnets in some of them suggest derivation of leptynites by partial melting of khondalites. Here we examine this connection in the light of results derived from dehydration melting experiments of micas in pelitic and psammitic rocks. The plots of leptynites of different chemical compositions in a (MgO + FeO)-Na2O-K2O projection match the composition of liquids derived by biotite and muscovite dehydration melting, when corrected for co-products of melting reactions constrained by mass balance and modal considerations. The melt components of the leptynites describe four clusters in the M-N-K diagram. One of them matches melts produced dominantly by muscovite dehydration melting, while three clusters correspond to melting of biotite. The relative disposition of the clusters suggests two trends, which can be correlated with different paths that pelitic and psammitic protoliths are expected to generate during dehydration melting. Thus the leptynites evidently represent granitoids which were produced by dehydration melting in metapelites of different compositions. The contents of Ti, Y, Nb, Zr and Th in several leptynites indicate departures from equilibrium melt compositions, and entrainment of restites is considered to be the main causative factor. Disequilibrium in terms of major elements is illustrated by leucosomes within migmatites developed in a group of metapelites. But the discrete leptynites that have been compared with experimental melts approach equilibrium melt compositions closely.