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Also, diffusion of H 2 O from the crystallizing leucosome melt back into the residue promotes crystallization of anhydrous products from the melt, accelerates solidification of the melt, and assists further development of hydrous products Fig. Thus, development of the leucosome patches in the Petronella metapelites seems to combine both mechanisms for the formation of anhydrous assemblages in the leucosomes, i. These anhydrous K-feldspar-poor domains triggered establishment of water gradients inside the patches, which could drain K-feldspar-rich domains to preserve garnet and orthopyroxene Fig.

The local transport of water from the K-feldspar-rich i. In fact, extensive chlorite formation is characteristic for the K-feldspar-poor domains Supplementary Data Fig. Biotite—quartz, biotite—sillimanite—quartz assemblages and later chlorite after orthopyroxene, cordierite, garnet and K-feldspar, both inside the patches and in the surrounding melanosome, imply that the evolution of the patches in the Petronella metapelites was governed by the action of essentially aqueous fluids issued during solidification of the melt inside the patches.

No aqueous-salt inclusions were found in the patches. However, primary carbonic inclusions in quartz in the patches suggest participation of small amounts of CO 2 in the patch evolution.

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There are several potential sources for CO 2 at the patch formation sites. CO 2 could be a product of cordierite decomposition during partial melting and also could be produced by oxidation of graphitic material within metapelites Hollister, ; Stevens, ; Cesare et al. Although we did not observe graphite in the studied Petronella rocks, this mineral is rather common in the Bandelierkop metapelites e.

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Stevens suggested that CO 2 in the metapelites of the SMZ was produced by interaction of aqueous fluids released from granitic magmas with this graphite. However, as well as water, CO 2 could also have been externally supplied, with leucocratic garnet trondhjemites similar to those at the Petronella locality, Fig. Clarke et al. MgO, FeO, MnO , as is reflected by similarities in compositions of peritectic phases within leucosomes and melanosomes. This is true for the studied patches, since the compositions of orthopyroxene and garnet in the patches are very similar to those present in the melanosome.

However, gradients could be still present with respect to trace elements. In this case, peritectic phases preserve trace element and REE patterns consistent with the dehydration of biotite in the melanosome Clarke et al. The newly-formed garnet and orthopyroxene in the melanosome became progressively enriched in Cr, Sc, Y and P during melting and segregation. Analyses of earlier generations of biotite in the melanosome rocks Fig.

Bea et al. Yang et al. In this case, at constant Cr content in the system, the distinct concave zoning of garnet cores Fig. Biotite was, probably, also the source of Sc for garnet e. This conclusion is supported by the concave Sc profiles of some garnet cores e. Xenotime is very rare, but monazite is a common accessory mineral in the leucosome patches Fig.

S2a , e, f. The wide variation of the Y content in monazite in the patches is consistent with continuous changes of its concentration from earlier to later garnet generations in the patches Fig. Simultaneously elevated concentrations of Y and Cr are considered as specific features of peritectic garnets e. This source could be apatite, which is a rather common accessory phase in the Bandelierkop metapelites, but is extremely rare in the garnet—orthopyroxene patches Supplementary Data Fig. Textures involving monazite and xenotime Supplementary Data Fig.

Along with plagioclase, apatite could also serve as an additional source for Ca. The discontinuous character of the distribution of Ca, Y, Sc, Cr and P between earlier and later garnet generations show that, because of low diffusion coefficients e. Carlson, and references therein , their concentrations within garnets were not severely modified during cooling in contrast to Fe, Mg, Mn. Carlson, Calcium and trace element-enriched garnet cores in the melanosome served as sites of extensive melt accumulation e.

Re-equilibration and partial redistribution of trace elements from the garnets to the melt resulted from the increase in the melt proportion around garnet cores. This process is manifested in garnet inclusions trapped by orthopyroxene crystals in the patches Fig. Rare garnets with oriented sillimanite inclusions in the patches also show elevated concentrations of Sc, Y, P and Cr Fig.

This additionally proves that garnets from the melanosome served as seeds for voluminous crystallization of Al-rich orthopyroxene. On further near-isobaric cooling, crystallization of orthopyroxene was accompanied or followed by formation of the next garnet generation from the melt.

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Although this garnet generation is poorer in Sc, Y, Cr and P than the garnet inclusions in orthopyroxene Fig. Thus, the latest garnet generation in the patches associated with myrmekitic intergrowths Fig. In the melanosome rocks, the cooling produced Sc, Y, Cr-free garnet intergrown with Cr-poor sillimanite after cordierite, which usually is strongly depleted in these elements e.

Peritectic cores of garnets in granites are usually mantled by zones showing a rimward decrease in the trace element contents. Concordant and discordant relationships between undeformed orthopyroxene—garnet patches and the sheared metapelites Fig. Similar orthopyroxene-bearing patches termed nebulitic leucosomes within the Bandelierkop metapelites have also been described in other areas of the SMZ Taylor et al.

Patches described by Taylor et al. Therefore, at present, it is not possible to compare our data with the observations of Taylor et al. In contrast, temperatures obtained for the patches in the metapelites of the Petronella locality coincide with temperatures obtained for the garnet-bearing trondhjemites, which intruded these metapelites Safonov et al.

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It cannot be excluded that the temperature of the initial magmas were even higher. The close spatial relationship between patch-bearing outcrops and trondhjemite bodies at the Petronella locality Fig. These data can be grouped into three clusters. To discuss these ages is beyond the scope of our present study. Nevertheless, these ages are in a good agreement with the period of granite magmatism average at Ma in the Northern Kaapvaal Craton e.

Jaguin et al. This cluster fits to data recently interpreted as the age of peak metamorphism for the rocks of the Bandelierkop Formation Belyanin et al. The third cluster includes age data obtained from monazites only. We assume that monazite reflects the evolution of the patches. This conclusion is supported by the wide variation of Y 2 O 3 content in monazites from the patches Supplementary Data Table S8 , which is concomitant with continuous changes of Y 2 O 3 concentration from earlier to later garnet generations in the patches Fig.

The obtained P—T data and age span of — Ma imply a close relationship of the trondhjemite magmas with the commencement of interaction of the SMZ granulites with underthrusted greenstone rocks of the Kaapvaal Craton that occurred between — Ma e. The possible origin and source of the trondhjemites is beyond the topic of this study.

Whether underthrusted amphibolite similar to that from adjacent greenstone belts on the craton might be a possible source, or whether mafic granulites and amphibolites of the SMZ are a more viable source, is matter for future studies. Thus, the formation of the trondhjemite magmas is closely related to the timing of collision, orogeny and subsequent exhumation. The heat source for the production of these magmas was probably related to the upwelling of hot asthenospheric mantle during convergence and collision of continental lithospheric blocks as demonstrated in 2D numerical experiments by Perchuk et al.

The presence of abundant CO 2 in the fluid associated with the trondhjemites Safonov et al. A reasonable source for the CO 2 is carbonates that are often associated with the greenstone rocks. On crystallization of the trondhjemites, CO 2 could participate in metasomatic processes around the trondhjemite intrusions, including the localized partial melting recorded in the studied garnet—orthopyroxene patches.

Garnet—orthopyroxene patches developed in metapelites at the Petronella locality might thus be considered as an example of local partial melting related to the thermal impact of felsic intrusions derived from a deeper source, similar to that recorded in various metamorphic complexes worldwide e. The structural position of the leucosome patches within the Petronella Shear Zone closely resembles the undeformed coarse-grained orthopyroxene and garnet-bearing patches described by Morfin et al. These authors interpreted the patches as evidence for partial melting of host metagreywackes intruded by abundant leucogranite veins and dykes.

Taking these similarities into account, the Petronella locality can arguably be considered as an injection complex formed by pervasive intrusion of differentiated leucocratic granitic magma at P—T conditions which were close to the solidus of the host-rocks cf. Numerical modeling e. We have presented evidence for fluid-deficient, localized melting of cordierite—orthopyroxene—biotite metapelites of the Bandelierkop Formation of the SMZ associated with the Petronella Shear Zone.

The mechanism of partial melting was determined from common reactions involving biotite, plagioclase and quartz, which produced peritectic orthopyroxene and garnet along with a potassium-rich melt. Accessory minerals played an active role in these reactions. Monazite and apatite provided abundant Y and P for peritectic garnet, whereas the transformation of pyrrhotite to pyrite supplied additional FeO to the melt.

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The melt segregated into centimetre-scale patches that developed around peritectic minerals. On cooling, the segregated melts crystallized to produce abundant K-feldspar as well as Sc, Y, Cr and P-poorer garnet inside the patches. The evolution of the patches was accompanied by active partial loss of the melt. Close spatial and structural association with trondhjemite bodies that syn- to late-tectonically injected the SMZ at about Ma Belyanin et al. Supplementary data are available at Journal of Petrology online.

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