Charnockite microstructures: From magmatic to metamorphic (original) (raw)
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Petrology
The paper presents data on the transformations in the mineralogy and mineral assemblages of quartz and feldspar free magnesian orthopyroxene-clinopyroxene-biotite metaultramafic granulites con verted into orthopyroxene-clinopyroxene-potassic feldspar-plagioclase-quartz charnockite. Within a 4 to 6 m aureole around the fluid conduit, the granulite has been affected by Na-K-Si-H 2 O-CO 2-Cl-F brine. Thereby the magnesian metaultramafite has undergone progressive debasification and leucocratization: potassic feldspar, oligoclase, and quartz have been metasomatically formed in the interstitial space between the mafic minerals and replaced ortho and clinopyroxene and biotite. The Fe mole fractions X Fe of all mafic minerals increase toward the fluid conduit as follows: from 0.25 to 0.54 for Opx, from 0.15 to 0.32 for Cpx, and from 0.16 to 0.56 for Bt. The whole rock compositions suggest that Na, K, and Si have been introduced and Mg, Fe, and Ca removed from the protolith, so that the metaultramafic rock has gradually been transformed first into a mesocratic rock and then, when partial melting started in its most significantly debasified domains, into leucocratic nebulitic migmatite with skialiths of the modified granulite, then into charnockite migma, and eventually into magma. The composition of the charnockite forming fluid was estimated as = 0.6, X (Na,K)F = 0.3, and = 0.1. The unusual F rich composition of the fluid is reflected in that both Bt and Hbl are enriched in F and contain almost no Cl. The P-T parameters of the process, which took place at the metamorphic peak, were T ~ 780°C, P = 8.5 kbar. Material balance plots of the rocks revealed three petro logical trends of the charnockite forming process controlled solely by the composition of the brine: (a) a trend that did not produce either Bt or Hbl during the metasomatic and anatectic stages, (b) that associated with intense amphibolization and biotitization during the metasomatic stage, and (c) a trend associated with the origin of melanocratic metasomatic Opx-Cpx-Bt-Hbl ± Pl selvages around newly formed charnocki toids, with the composition of the selvages close to the melanocratic veins produced in the peripheries of the charnockitization zone by the rapid redeposition of Mg, Fe, and Ca mobilized in the course of debasification. It follows that charnockitization proceeded according to the model of nonisochemical migmatization in an open system, a process driven first of all by the inflow of deep brines. This process differed strongly from sim ple closed system partial melting induced by an increase in temperature.
Geologica Carpathica
The Weinsberg granite, a coarse-grained biotite granite with abundant K-feldspar megacrystals, is the volumetrically dominant and most characteristic granite type of the late-Variscan Moldanubian Batholith in the Moldanubian zone of the Bohemian Massif. In the western batholith area, a local orthopyroxene-bearing variant (charnockite) of the Weinsberg granite has been identified and given the name of the Sarleinsbach quartz-monzodiorite in previous studies. Whole rock analysis of the charnockite and the relatively mafic Weinsberg granite in the immediate neighborhood show no significant geochemical differences with respect to either the major or trace elements. The mineralogy and petrology of the charnockite and surrounding granite are the same except for the presence of orthopyroxene ± clinopyroxene in the charnockite. In addition, the charnockite is characterized by the presence of dark grey, glassy orthoclase megacrysts with only some partial conversion to microcline, whereas in the granite the K-feldspar megacrysts consist of white microcline. The Fe-Mg silicates in the charnockite (orthopyroxene, clinopyroxene, amphibole, and biotite) are relatively Fe-rich (X Fe = 0.6-0.7) whereas the plagioclase is more albitic (X Ab = 0.6) than anorthitic. Fluid inclusions from the granite and associated charnockite are investigated and the results compared. The basic conclusion is that the magma responsible for the granite was dominated by an H 2 O-rich fluid with a CaCl 2 component. The magma responsible for the charnockite was dominated by a CO 2-rich fluid with a minor NaCl component, which lowered the H 2 O activity sufficiently below 1 such that orthopyroxene ± clinopyroxene was the stable Fe-Mg silicate phase during crystallization as opposed to the biotite in the granite. Taking into account that CO 2-rich and H 2 O-rich fluids are immiscible in the presence of NaCl and CaCl 2 over the P-T range of the overall crust, the implication is that in granitoid melts, if CO 2 is present, there will be regions dominated by CO 2 and regions dominated by H 2 O. The extent of either region will be determined by the overall CO 2 /H 2 O ratio in the melt. In the CO 2-dominated regions, the H 2 O activity could be sufficiently lowered such that orthopyroxene is the stable Fe-Mg silicate phase during crystallization, though this will also be dependent on the Fe/Mg ratio of these phases and the overall whole rock chemistry of the melt. In addition to incipient solid state charnockitization, commonly seen in the Archean terranes of southern India and elsewhere, this suggests that a certain subset of granites and granitoids worldwide should have patches and/or limited areas of charnockite if the amount of CO 2 present in the original magma goes above a certain fraction.
The Origin of Charnockite-Enderbite Complexes by Magmatic Replacement: Geochemical Evidence
Variations in the contents of trace and rare-earth elements in the charnockite--enderbite rocks of the Sharyzhalgai granulite complex cannot be accounted for by only metasomatic processes, which produce these rocks by the replacement of their metamorphic protolith. Using the magmatic replacement model for the fractionation of trace elements enabled us to calculate a series of model compositions that realistically approximate the REE patterns observed in natural enderbites and charnockites. As our data indicate, enderbite is a metasomatic rock, which is produced by the interaction between migmatizing fluid and metamorphic protolith. Charnockite is the product of enderbite melting in columns of magmatic replacement. As geochemical modeling indicates, rocks between enderbite and the melting front are produced by various stages of the partial melting. Throughout all of the magmatic replacement stages, the contents of trace elements in the phases were controlled by the chemistry of the percolating migmatizing fluid
Charnockites are Opx-bearing igneous rocks commonly found in high-grade metamorphic terranes. Despite being volumetrically minor, they show a wide range in both bulk geochemistry and intensive parameters. They form a characteristic component of the AMCG (anorthosite–mangerite–charnockite–granite) suite, but their association with typical S-type granites is less well-known. The Darongshan S-type granitic complex (DSGC) in Guangxi Province, southern China, contains granites varying in mafic silicate mineral assemblages from Bt + Crd (Darongshan suite) to Opx + Grt + Bt + Crd (Jiuzhou suite) and Opx + Crd ± Bt (Taima suite), corresponding to a geochemical transition from magnesian calc-alkalic to ferroan calc-alkalic. However, its genesis, even the accurate age of intrusion, remains highly contentious despite intensive research. In order to understand the genesis of charnockite and its genetic relationship with S-type granite; here, we first determined zircon U-Pb ages of each suite using a SIMS on the basis of a detailed petrological study. Zircon U-Pb ages show that all suites of the complex were emplaced contemporaneously at ca. 249 Ma. Monazite apparent U-Pb ages are indistinguishable from zircon U-Pb ages within analytical error. Further in situ zircon Hf-O isotope analyses reveal that the granitic complex was dominantly derived from reduced melting metasedimentary rocks (δ 18 O zircon = ca. 11‰; ε Hf (t) zircon = ca. −10; Δlog FMQ ≤ 0; Mn in apatite oxybarometer) with rare material input from the mantle. The variation in δ 18 O (7.8‰–12.9‰) is more likely a result of hybridization, whereas that in ε Hf (t) (−31.9 to −1.8) is a result of both hybridization and disequilibrium melting. The variation in mineralogy and geochemistry may be interpreted as a result of entrainment of peritectic garnets from biotite-dehydration melting. Nevertheless, heat input from mantle through basaltic intrusion/underplating is considered to play a major role in high-temperature (N 850 °C) melting at mid-crustal levels (i.e. the cordierite stable field) for generation of the granitic complex. We interpret that the granites were intruded in a back-arc setting and basaltic magmatism was directly associated with slab roll-back and tearing during the latest Permian and early Triassic times.
Localized, solid-state dehydration associated with the Varberg charnockite intrusion, SW Sweden
The mineralogy, petrology, and fluid inclusion chemistry of two charnockite patches within a distance of 4–5 km of the Varberg magmatic charnockite intrusion, SW Sweden, are investigated and described utilizing SEM, EMPA, and fluid inclusion microthermometry. Garnet–clinopyroxene (890–930 • C), garnet–amphibole (600–800 • C), and garnet–biotite (670–860 • C) Fe–Mg exchange thermometry indicates high temperatures for charnockite Patch I compared to relatively lower garnet–orthopyroxene, garnet–amphibole, and garnet–biotite temperatures of 500 to 600 • C for charnockite Patch II. Plagioclase in the charnockitic patches tends to be more anorthitic and less albitic (X An = 0.20, X Ab = 0.76) than in the surrounding regional granitic gneiss (X An = 0.13, X Ab = 0.84). Replacement antiperthite is commonly found in unrelated plagioclase grains from either patch compared to the regional granitic gneiss where it is relatively rare. In either patch, K-feldspar is considerably less albitic (X Kfs = 0.90–0.92, X Ab = 0.05–0.10) compared to K-feldspar from the regional granitic gneiss. It can also be found as micro-veins along quartz grain rims. Both patches are dominated by clinopyroxene as opposed to orthopyroxene. Garnet, biotite, and amphibole and in both charnockite patches tend to have lower Fe and correspondingly higher Mg values compared with garnet, biotite, and amphibole from the surrounding regional granitic gneiss. Flu-orapatite tends to be relatively enriched in Cl and depleted in (Y + REE) compared with fluorapatite from the regional granitic gneiss. Fluid inclusions in charnockite Patches I and II are dominantly carbonic similar to what is seen for the Varberg charnockite. In addition to quartz, relatively high-density carbonic inclusions are also preserved in garnet and in fluorapatite. It is presumed that pure carbonic fluids must have once coexisted with relic magmatic H 2 O–CO 2 –NaCl fluids at peak metamorphic conditions. The most likely scenario suggests that charnockite Patches I and II were formed during the later stages of crystallization of the Varberg charnockite magmatic body during which copious amounts of CO 2-rich fluids with a brine (CaCl 2-dominated) component were expelled into the country rock via pegmatoid segregations both within and in the immediate surroundings of the charnockite body. Patch I appears to represent the extension of a pegmatoid segregation, whereas Patch II appears to represent fluid-induced lower temperature, solid-state dehydration. Transport was facilitated via a system of tectonic fissures and fractures generated in the regional migmatized granitic gneiss during its emplacement. Within the scope of what is known, these two charnockite patches fall into the generally observed parameters for localized dehydration zones in general.
The potential role of fluids during regional granulite-facies dehydration in the lower crust
Geoscience Frontiers, 2012
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Chemical Geology, 1995
The production of S-type granites has been related to the extraction of melts during water-undersaturated melting of metasediments in the lower continental crest. If this mechanism is of widespread applicability, a chemically distinctive granulitic restite must be left behind with a positive Eu anomaly and depleted incompatible element concentrations. However, little geochemical evidence has been found in support of this hypothesis. The rare-earth dement geochemistry of lower crustal granulites and associated melts will depend on the behaviour of the phases in which REE and trace elements are concentrated, i.e. zircon, monazite arid K-rich feldspar. New experimental data on synthetic metapelites show that K-feldspar can be a product or a reactant during water-undersaturated melting of biotite, depending on the H20/K20 of the melt relative to biotite. Past confusion over the role of K-fddspar during melting is due to the reaction being close to degeneracy and changing its stoichiometry in response to only sm~dl changes in bulk composition. Coherent interpretation of the role of K-feldspar in different experimental studies requires a better knowledge of phase water contents than has commonly been available. The leucosomes formed in some Antarctic granulite-facies migmatites are likely to have had low H20/K20 ratios, which suggests that K-feldspar would have been a reactant during raelting. This could cause the positive Eu anomalies observed in the leucosomes, through the disequilibrium melting of K-fddspar during water-undersaturated melting. The undersaturation of trace elements and REE in the leucosomes strongly suggests that disequilibrium melting took place, caused by the incomplete dissolution of accessory phases, monazite and zircon. These phenomena give the melts a distinctive geochemical character which has also been seen in the other granulitefacies leucosomes analysed to date. This geochemical character contrasts with that of S-type granites and therefore the production of granulites in the lower crust may not necessarily be associated with this type of magmatism. If lower-crustal granulite-facies migmatites are the source region for peraluminous S-type granites then the melts generated must undergo considerable modification during ascent.
Contributions to Mineralogy and Petrology, 1993
The profound geochemical conseqences of accessory phase behaviour during partial melting of highgrade metapelites are demonstrated with reference to two geochemically distinct crustal melts produced by biotite dehydration melting reactions under granulite facies (kbar, 860 ~ C) conditions. These two leucogneiss suites, from the Brattstrand Bluffs coastline, eastern Antarctica, have similar field relations, transport distances (10-100 s of metres) and major element chemistry. Type 1 leucogneisses have low Zr, Th and LREE, positive Eu anomalies and Zr/Zr* and LREEt/LREEt* values less than 1.0 (i.e. less than required to saturate the melt). Mass balance constraints suggest that these melts have been extracted before equilibration with host melanosomes. The dry, peraluminous nature of vapourundersaturated melts inhibits monazite and zircon solubility and results in concentration of these phases in the residue. Melts are consequently depleted in LREE and HREE. Melanosomes show complementary enrichment in LREE, while HREE patterns are dominated by residual garnet. Type 2 leucogneisses, in contrast, have strongly enriched Zr, Th and LREE abundances, negative Eu anomalies and Zr/Zr* and LREEt/ LREEt* > 1 resulting from accessory phase entrainment. Vapour-absent partial melting under moderate (6-8 kbar) pressure granulite-facies conditions of a pelitic source containing monazite is likely to give disequilibrium melts depleted in LREE and HREE as monazite and garnet are concentrated in the residue. If temperatures are high enough (850-870 ~ C) to permit relatively large degrees of partial melting then the feldspar component of the source will be removed almost completely, giving melts with large positive Eu anomalies. Melts formed under vapour-present conditions are unlikely to show such extreme LREE and HREE depletion or positive Eu anomalies, even at high degrees of partial melting. Disequilibrium melting coupled with source entrainment could fortuitously produce REE and trace element signatures similar to those typical of S-type granites and usually ascribed to equilibrium melting conditions.
Lithos, 2019
The complementary roles of granites and rocks of the granulite facies have long been a key issue in models of the evolution of the continental crust. "Dehydration melting", or fluid-absent melting of a lower crust containing H 2 O only in the small amounts present in biotite and amphibole, has raised problems of excessively high temperatures and restricted amounts of granite production, factors seemingly incapable of explaining voluminous bodies of granite like the Archean Closepet Granite of South India. The existence of incipient granulite-facies metamorphism (charnockite formation) and closely associated migmatization (melting) in 2.5 Ga-old gneisses in a quarry exposure in southern India and elsewhere, with structural, chemical and mineral-inclusion evidence of fluid action, has encouraged a wetter approach, in consideration of aqueous fluids for rock melting which maintain sufficiently low H 2 O activity for granulite-facies metamorphism. Existing experimental data at elevated T and P are sufficient to demonstrate that, at mid-crust pressures of 0.5-0.6 GPa and metamorphic temperatures above 700°C, ascending immiscible CO 2-rich and concentrated alkali chloride aqueous fluids in equilibrium with charnockitic (orthopyroxene-bearing) gneiss will inevitably begin to melt granitic rocks. The experimental data show that H 2 O activity is much higher (0.5-0.6) than previously portrayed for beginning granulite facies metamorphism (0.15-0.3). Possibilities for metasomatism of the deep crust are greatly enhanced over the ultra-dry models traditionally espoused. Streaming of ultrasaline fluids through continental crust could be a mechanism for the generation of the discrete mid-crust layer of migmatites suggested to characterize younger tectonometamorphic regions. The action of CO 2-rich and hypersaline fluids in Late Archean metamorphism and magmatism could record the beginning of large-scale subduction of volatilerich surficial materials.