The processes that control leucosome compositions in metasedimentary granulites: perspectives from the Southern Marginal Zone migmatites, Limpopo Belt, South Africa

Abstract

International audienceAnatexis of metapelitic rocks at the Bandelierkop Quarry (BQ) locality in the Southern Marginal Zone of the Limpopo Belt occurred via muscovite and biotite breakdown reactions which, in order of increasing temperature, can be modelled as: (1) Muscovite + quartz + plagioclase = sillimanite + melt; (2) Biotite + sillimanite + quartz + plagioclase = garnet + melt; (3) Biotite + quartz + plagioclase = orthopyroxene ± cordierite ± garnet + melt. Reactions 1 and 2 produced stromatic leucosomes, which underwent solid-state deformation before the formation of undeformed nebulitic leucosomes by reaction 3. The zircon U–Pb ages for both leucosomes are within error identical. Thus, the melt or magma formed by the first two reactions segregated and formed mechanically solid stromatic veins whilst temperature was increasing. As might be predicted from the deformational history and sequence of melting reactions, the compositions of the stromatic leucosomes depart markedly from those of melts from metapelitic sources. Despite having similar Si contents to melts, the leucosomes are strongly K-depleted, have Ca:Na ratios similar to the residua from which their magmas segregated and are characterized by a strong positive Eu anomaly, whilst the associated residua has no pronounced Eu anomaly. In addition, within the leucosomes and their wall rocks, peritectic garnet and orthopyroxene are very well preserved. This collective evidence suggests that melt loss from the stromatic leucosome structures whilst the rocks were still undergoing heating is the dominant process that shaped the chemistry of these leucosomes and produced solid leucosomes. Two alternative scenarios are evaluated as generalized petrogenetic models for producing Si-rich, yet markedly K-depleted and Ca-enriched leucosomes from metapelitic sources. The first process involves the mechanical concentration of entrained peritectic plagioclase and garnet in the leucosomes. In this scenario, the volume of quartz in the leucosome must reflect the remaining melt fraction with resultant positive correlation between Si and K in the leucosomes. No such correlation exists in the BQ leucosomes and in similar leucosomes from elsewhere. Consequently, we suggest disequilibrium congruent melting of plagioclase in the source and consequential crystallization of peritectic plagioclase in the melt transfer and accumulation structures rather than at the sites of biotite melting. This induces co-precipitation of quartz in the structures by increasing SiO2 content of the melt. This process is characterized by an absence of plagioclase-induced fractionation of Eu on melting, and the formation of Eu-enriched, quartz + plagioclase + garnet leucosomes. From these findings, we argue that melt leaves the source rapidly and that the leucosomes form incrementally as melt or magma leaving the source dumps its disequilibrium Ca load, as well as quartz and entrained ferromagnesian peritectic minerals, in sites of magma accumulation and escape. This is consistent with evidence fro...

Comment on “Ultrahigh temperature granulites and magnesian charnockites: Evidence for the Neoarchean accretion along the northern margin of the Kaapvaal craton” by Rajesh et al.

“…∼1000 • C and ∼12 kbar) are inconsistent with both field observations and experimental studies. The detailed metamorphic analysis provided by Taylor et al (2014) clearly demonstrates that the presence of peak metamorphic biotite in the Bandelierkop formation metapelites is inconsistent with peak metamorphic temperatures in excess of 900 • C. This is in agreement with earlier metamorphic studies (e.g. Stevens and van Reenen, 1992a,b), more recent conclusions from phase equilibrium modelling (Koizumi et al, 2014), as well as a very large body of experimental data on partial melting of biotite-bearing metasediments (e.g.…”

“…(1) This model is in conflict with the fact that the Matok pluton (suggested to be subduction related) intruded at ∼2.68 Ga (U-Pb age data of Barton et al, 1992;Laurent et al, 2013;Zeh et al, 2009), whereas terrane collision happened at 2.71-2.72 Ga, as indicated by U-Pb ages of metamorphic zircons obtained from granulite-facies rocks of the SMZ (Rajesh et al, 2014;Taylor et al, 2014). Furthermore, field observations and additional age data from the Hout River Shear Zone indicate that uplift and southward thrusting of the SMZ granulites over the Pietersburg block started prior to the intrusion of the Matok pluton, i.e.…”

Phase equilibria modelling of residual migmatites and granulites: An evaluation of the melt‐reintegration approach

“…A common approach consists in considering the probable melt productivity of the model protolith composition reconstructed by melt‐reintegration method (e.g. Cai et al., ; Diener et al., ; Guilmette et al., ; Hasalová et al., ; Indares et al., ; Nicoli et al., ; Taylor et al., ; White et al., ; Zhang et al., ). These amounts of melt are a maximum, because melt loss would reduce rock fertility at higher temperature (Korhonen et al., ; Yakymchuk & Brown, ).…”

“…This issue was circumvented by reintegrating a certain amount of melt to the residuum composition and by performing phase equilibria modelling of the new model protolith composition to reconstruct the probable prograde history (see White, Powell, & Halpin, ). The melt‐reintegration approach has become an increasingly routine method among metamorphic petrologists and various ways of calculating and reintegrating the extracted melt have been developed and applied (Anderson, Kelsey, Hand, & Collins, ; Boger, White, & Schulte, ; Cai et al., , ; Chen, Ye, Liu, & Sun, ; Diener, White, & Hudson, ; Diener, White, Link, Dreyer, & Moodley, ; Diener, White, & Powell, ; Dumond, Goncalves, Williams, & Jercinovic, ; Fitzherbert, ; Groppo, Rolfo, & Indares, ; Groppo, Rolfo, & Mosca, ; Groppo, Rubatto, Rolfo, & Lombardo, ; Guilmette, Indares, & Hébert, ; Hallett & Spear, ; Hasalová et al., ; Jiang et al., ; Kelsey & Hand, ; Kohn, ; Korhonen, Brown, Clark, & Bhattacharya, ; Indares, White, & Powell, ; Lasalle & Indares, ; McGee, Giles, Kelsey, & Collins, ; Morrissey, Hand, Kelsey, & Wade, ; Nahodilová, Faryad, Dolejš, Tropper, & Konzett, ; Nicoli, Stevens, Moyen, & Frei, ; Palin et al., ; Redler, White, & Johnson, ; Shrestha, Larson, Guilmette, & Smit, ; Skrzypek, Štípská, & Cocherie, ; Štípská, Schulmann, & Powell, ; Taylor, Nicoli, Stevens, Frei, & Moyen, ; Tian, Zhang, & Dong, ; Tucker, Hand, Kelsey, & Dutch, ; Wang & Guo, ; White et al., ; Yakymchuk et al., ; Yin et al., ; Zhang et al., ; Zou et al., ). Furthermore, the reconstruction of a plausible protolith composition is essential to assess the likely melt productivity of rocks (White et al., ) which, in turn, allows the potential role of loss and redistribution of melt in the evolution of the deeper crust to be explored (Diener & Fagereng, ; Diener et al., ; Korhonen, Saito, Brown, & Siddoway, ; Korhonen et al.,…”

Crystallization of Heterogeneous Pelitic Migmatites: Insights from Thermodynamic Modelling

“…The compositions of leucosome in migmatites have been compared with leucogranites to test possible relationships between granitic suites and source areas or protoliths (Obata et al, 1994;Castro et Johannes et al, 2003;Burda & Gawę da, 2009), and with experimentally produced and thermodynamically calculated melts to understand the formation of leucosomes (Solar & Brown, 2001;Cruciani et al, 2008;Taylor et al, 2014). Similarly, the compositions of leucogranites are compared with experimental and thermodynamically calculated melts to understand the source of the melt and the genesis of the granites (e.g.…”