Garnet often traps droplets of anatectic melts (i.e. melt inclusions – MI), also called nanogran... more Garnet often traps droplets of anatectic melts (i.e. melt inclusions – MI), also called nanogranitoids once crystallized, whose characterization allows us to clarify deep crustal melting processes. the Adirondacks (US), is an ideal location to investigate mafic melting and crustal growth . As a portion of the Grenville Province, this massif mainly consists of intrusive bodies metamorphosed during the Ottawan Orogeny (1090-1050 Ma). Nanotrondhjemites were previously reported in in the giant garnets of Barton mine, Gore Mountain area (Ferrero et al., 2021) in the central-southern part of the Adirondacks. Melting is, however, not limited to such location: MI-bearing garnets are also found in the mafic granulites at Hooper mine, approximately 5 km NW of the Barton Mine. Such garnets have been divided in two types based on size, chemical zoning, habitus as well as the composition of the trapped melt.Type 1 garnets are large, euhedral porphyroblasts of diameter >5 cm, with a rather ho...
Detailed microstructural analysis of three basaltic sills of the Little Minch Sill Complex demons... more Detailed microstructural analysis of three basaltic sills of the Little Minch Sill Complex demonstrates that convection leaves a detectable signature in fully solidified bodies. The presence of dense clusters of equant grains of olivine and clinopyroxene in the central parts of sills can only be accounted for if they formed and were enlarged while suspended in convecting magma, with delayed settling to the sill floor. An associated stratigraphic invariance of plagioclase grain shape is consistent with growth while suspended in convecting magma. These microstructural indicators demonstrate that convection during solidification was vigorous and long-lived in the 135-m-thick picrodolerite-crinanite unit (PCU) of the composite Shiant Isles Main sill and vigorous and likely short-lived in the PCU of the composite Creagan Iar sill. In contrast, convection in the Meall Tuath sill was weak and short-lived: plagioclase grain shape in this sill varies with stratigraphic height, indicative of ...
Anatexis is the first step in granite genesis. Partial melting in the lower crust may produce leu... more Anatexis is the first step in granite genesis. Partial melting in the lower crust may produce leucoratic features of unusual chemical compositions, very different from the final products of crustal differentiation. Therefore, the links that exists between some migmatites and crustal-derived granites can be ambiguous. This study is an investigation of the anatectic history of a high-grade terrain: the Southern Marginal Zone of the Limpopo Belt (SMZ), north to the Kaapvaal Craton in South Africa. The work involved an integrated field, metamorphic, geochemical and geochronogical study of the metasedimentary granulites from two separate quarries in the northern zone of the Southern Marginal Zone, the Bandelierkop quarry and the Brakspruit quarry, where Neoarchean high-grade partial melting features can be observed. The project has aimed to address two main issues: (1) to accurately constrain the pressure temperature conditions and the age of the metamorphic episode in the SMZ, with impl...
<p>Inclusions of anatectic melt and fluids are probes used by geoscientist ... more <p>Inclusions of anatectic melt and fluids are probes used by geoscientist to investigate, understand and quantify devolatilization, melt production and chemical differentiation during orogenesis and crustal reworking. The large majority of case studies targeting these types of inclusions are in the Phanerozoic eon, whereas the Precambrian is still poorly represented. Here we present the first data on anatectic melt inclusions (<em>nanogranitoids</em>) and their associated primary fluid inclusions in garnets from the Mesoarchean basement in Southeast Greenland. Using optical microscope observations, MicroRaman spectroscopy and microprobe investigation we show that these metasedimentary migmatites contain two generations of garnets, a large xenoblastic garnet (Grt<sup>1</sup>) and a small idioblastic one (Grt<sup>2</sup>). Both garnet types contain clusters of primary nanogranitoids which are characterized by the presence of quartz and feldspar polymorphs + phyllosilicates, a phase assemblage typically observed in this type of inclusions. Nanogranitoids are associated in the clusters to primary CO<sub>2</sub>-CH<sub>4</sub> fluid inclusions, which appear to be always affected by post-entrapment modifications with extensive formation of step-daughter minerals such as carbonates and pyrophyllite. Such occurrence proves for the first time that these rocks experienced partial melting with formation of garnet, and that this event took place in presence of COH-rich fluid. The re-evaluation of the metamorphic pressure and temperature conditions with up-to-date phase equilibria modelling, combined with the identification of nanogranitoids and fluid inclusions, suggests metamorphic peak equilibration and partial melting at T> 900 °C and P> 7 kbar. To date, this is the oldest verified occurrence of nanogranitoids as well as of fluid-melt immiscibility during garnet growth in a partially molten environment.</p>
<p>The Adirondack Mountains, New York State, USA belongs to the Canadian Gr... more <p>The Adirondack Mountains, New York State, USA belongs to the Canadian Grenville Province (Darling and Peck, 2016). The rocks exposed in the Adirondacks are interpreted to be the lower plate of a thrust-system at crustal levels during the Ottawan Orogeny (1090-1050 Ma) of the Grenvillian orogenic cycle. Garnet is abundant throughout the Adirondacks, with the greatest occurrence of megacrystic garnets within central Highlands. In the Gore Mountain area, the Hooper Mine is located 5 kilometers northwest of the Barton Mine and consists of partially melted mafic granulite. The mineral assemblage consists of medium grain size plagioclase, green hornblende and garnet in proportion 60:20:20. We separated the garnets of the Hooper Mine in two categories according to their size, chemical zoning and habitus: (1) Large, euhedral garnet porphyroblasts of diameter > 5 cm (LG), and (2) and small, xenoblastic grains (SG). Both types of garnets contain quartz, rutile and melt inclusions, similar to those observed in Barton Mine (Ferrero et al., 2021). In LG, chemical zoning is weak and inclusions are scattered randomly within the mineral. In SG, zoning coincides with the presence of quartz and melt inclusions in domain of low Ca and Y. Ti-in-quartz and Ti-in-amphibole thermometers in SG give equilibrium temperatures of 800-900 °C at 10 kbar.</p><p>Major and trace element analyses on rehomogenised melt inclusions in both types of garnet indicate two types of melts are present in the migmatite – granitic melt in SG and trondhjemitic melt in LG. Stable isotope ratios of oxygen and hydrogen in hornblende (δ<sup>2</sup>H: -62 to -73 <sup>0</sup>/<sub>00</sub> and δ<sup>18</sup>O: 4.7 to 6.7 <sup>0</sup>/<sub>00</sub>) indicate that partial melting occurs in a closed isotopic system and records the primary magmatic δ<sup>2</sup>H signature of the protolith. The range of melt chemistries, combined with the information previously collected in the Barton Mine defines a trend characteristic of primitive TTG melts or TTG embryos. These melts, combined with different proportion of peritectic phases (i.e. garnet, plagioclase and quartz), reproduces the full TTG chemistry range (Moyen, 2011). Therefore, the Mesoproterozoic mafic lower crust might be a perfect laboratory to test early granitoids genesis processes and better understand the link between melt inclusions, plate tectonics and the formation of the continental crust (Nicoli & Ferrero, 2021). </p><p> </p><p>References:</p><p>Ferrero, S. et al. (2021). Embryos of TTGs in Gore Mountain garnet megacrysts from water-fluxed melting of the lower crust. <em>Earth and Planetary Science Letters</em>, <em>569</em>, 117058.</p><p>Darling, R. S. and Peck W.H. (2016). Metamorphic conditions of Adirondack rocks. <em>Adirondack Journal of Environmental Studies</em>, <em>21</em>(1), 7.</p><p>Moyen, J. F. (2011). The composite Archaean grey gneisses: petrological significance, and evidence for a non-unique tectonic setting for Archaean crustal growth. <em>Lithos</em>, <em>123</em>(1-4), 21-36.</p><p>Nicoli, G., & Ferrero, S. (2021). Nanorocks, volatiles and plate tectonics. <em>Geoscience Frontiers</em>, <em>12</em>(5), 101188.</p>
By submitting this dissertation electronically, I declare that the entirety of the work con-taine... more By submitting this dissertation electronically, I declare that the entirety of the work con-tained therein is my own, original work, that I am the sole author thereof (save to the extent explicitly otherwise stated), that reproduction and publication thereof by Stellen-bosch University will not infringe any third party rights and that I have not previously in its entirety or in part submitted it for obtaining any qualification.
Anatexis is the first step in granite genesis. Partial melting in the lower crust may produce leu... more Anatexis is the first step in granite genesis. Partial melting in the lower crust may produce leucoratic features of unusual chemical compositions, very different from the final products of crustal differentiation. Therefore, the links that exists between some migmatites and crustal-derived granites can be ambiguous. This study is an investigation of the anatectic history of a high-grade terrain: the Southern Marginal Zone of the Limpopo Belt (SMZ), north to the Kaapvaal Craton in South Africa. The work involved an integrated field, metamorphic, geochemical and geochronogical study of the metasedimentary granulites from two separate quarries in the northern zone of the Southern Marginal Zone, the Bandelierkop quarry and the Brakspruit quarry, where Neoarchean high-grade partial melting features can be observed. The project has aimed to address two main issues: (1) to accurately constrain the pressure temperature conditions and the age of the metamorphic episode in the SMZ, with impl...
Differentiation of the continental crust is the result of complex interactions between a large nu... more Differentiation of the continental crust is the result of complex interactions between a large number of processes, which govern partial melting of the deep crust, magma formation and segregation, and magma ascent to significantly higher crustal levels. The anatectic metasedimentary rocks exposed in the Southern Marginal Zone of the Limpopo Belt represent an unusually well-exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field. The formation of these migmatites occurred via absent incongruent melting reactions involving biotite, which produced cm- to m-scale, K2O-poor garnet-bearing stromatic leucosomes, with high Ca/Na ratios relative to their source rocks. Field investigation combined with geochemical analyses, and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting, show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution: (1) Melting of a portion of the source, with restricted plagioclase availability due to kinetic controls, to produce a magma (melt + entrained peritectic minerals in variable proportions relative to melt); (2) Segregation of the magma at near peak metamorphic conditions into melt accumulation sites (MAS), a.k.a. future leucosome; (3a) Reequilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion (H2O, K2O), which triggers the co-precipitation of quartz and plagioclase in the MAS; (3b) Extraction of melt dominated magma to higher crustal levels, leaving peritectic minerals entrained from the site of the melting reaction and the minerals precipitated in the MASs to form the leucosome in the source. The key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction. This results in elevated melt H2O and K2O and chemical potential gradient for these components across the leucosome/mafic residuum contact. The combination of all of these processes accurately explains the composition of the K2O-poor leucosomes. These findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which, according to the chemical modelling is < 5 years. We demonstrate that in some migmatitic granulites, the leucosomes constitute a type of felsic refractory residuum, rather than evidence of failed magma extraction. This provides a new insight into the ways that source heterogeneity may control anatexis. This article is protected by copyright. All rights reserved.
Studies on the Southern Marginal Zone (SMZ) of the Limpopo Belt have generally concluded that thi... more Studies on the Southern Marginal Zone (SMZ) of the Limpopo Belt have generally concluded that this terrain was affected by a single granulite-facies metamorphic event at 2.67–2.66 Ga (Kroner et al. 2000; Kreissig et al. 2000; Zeh et al. 2005; Elington and Armstrong 2004; Stevens and Van Reenen 1992a, b; Barton and van Reenen 1992; Barton et al. 1992; Van den Berg and Huizenga 2001; Rigby et al. 2008), with peak metamorphic conditions of 7.5–9.5 kbar and 800–850 C. In their recent paper, Belyanin et al. (2012) propose an ultrahigh-temperature (UHT) metamorphic event in the SMZ, with peak metamorphic conditions in excess of 1,000 C at approximately 12 kbar. Similar conclusions were proposed by earlier work on the same rock (Belyanin et al. 2010). The proposed UHT conditions are based on Al-rich orthopyroxene and ternary feldspar thermometry using reintegrated feldspar compositions. This evidence is derived from a single outcrop containing unusual aluminous layers within metasedimentary granulites, although Tsunogae et al. (2004) used Al-in-orthopyroxene and ternary feldspar thermometry to argue for a peak metamorphic temperature of [950 C from more representative metapelitic granulites of the Bandelierkop formation. Both these studies have proposed that UHT metamorphism affected the SMZ generally and thus that the relevant PT path of all rocks traversed UHT conditions. This interpretation is problematic because it is difficult to reconcile with the partial melting history of the rocks and with the peak metamorphic assemblage in most rocks. In essence, both the metasedimentary granulites of the Bandelierkop formation and the trondhjemitic to tonalitic Bavianskloof gray gneisses contain a significant proportion of biotite, which defines a syn-peak metamorphic fabric (Fig. 1). Within the metasediments of the Bandelierkop formation, compositional banding at the outcrop scale (Fig. 1) exists that is considered to represent primary differences in composition between different sedimentary units (van Reenen 1983). Anatexis of these rocks has occurred via three reactions (Stevens and van Reenen 1992a): Ms ? Qtz ? Pl = Sil ? Melt; Bt ? Sil ? Qtz ? Pl = Grt ? Melt; and Bt ? Qtz ? Pl = Opx ? Crd ? Melt. The first two reactions exhausted muscovite and sillimanite, respectively, while the third reaction had just begun to occur in the chemically most favorable rocks at the conditions of peak metamorphism. Pseudosection modeling of metapelitic granulites from the Bandelierkop quarry, which have undergone anatexis and that represent restitic, melt-depleted compositions, indicates that biotite in such rocks would not have survived temperatures significantly above 900 C and that the mineral compositions in such rocks are consistent with peak metamorphic temperatures of 820–860 C at 8–10 kbar (Fig. 2). At the peak metamorphic conditions proposed by Belyanin et al. (2012), these restitic granulites would have become Communicated by T. L. Grove.
Garnet often traps droplets of anatectic melts (i.e. melt inclusions – MI), also called nanogran... more Garnet often traps droplets of anatectic melts (i.e. melt inclusions – MI), also called nanogranitoids once crystallized, whose characterization allows us to clarify deep crustal melting processes. the Adirondacks (US), is an ideal location to investigate mafic melting and crustal growth . As a portion of the Grenville Province, this massif mainly consists of intrusive bodies metamorphosed during the Ottawan Orogeny (1090-1050 Ma). Nanotrondhjemites were previously reported in in the giant garnets of Barton mine, Gore Mountain area (Ferrero et al., 2021) in the central-southern part of the Adirondacks. Melting is, however, not limited to such location: MI-bearing garnets are also found in the mafic granulites at Hooper mine, approximately 5 km NW of the Barton Mine. Such garnets have been divided in two types based on size, chemical zoning, habitus as well as the composition of the trapped melt.Type 1 garnets are large, euhedral porphyroblasts of diameter >5 cm, with a rather ho...
Detailed microstructural analysis of three basaltic sills of the Little Minch Sill Complex demons... more Detailed microstructural analysis of three basaltic sills of the Little Minch Sill Complex demonstrates that convection leaves a detectable signature in fully solidified bodies. The presence of dense clusters of equant grains of olivine and clinopyroxene in the central parts of sills can only be accounted for if they formed and were enlarged while suspended in convecting magma, with delayed settling to the sill floor. An associated stratigraphic invariance of plagioclase grain shape is consistent with growth while suspended in convecting magma. These microstructural indicators demonstrate that convection during solidification was vigorous and long-lived in the 135-m-thick picrodolerite-crinanite unit (PCU) of the composite Shiant Isles Main sill and vigorous and likely short-lived in the PCU of the composite Creagan Iar sill. In contrast, convection in the Meall Tuath sill was weak and short-lived: plagioclase grain shape in this sill varies with stratigraphic height, indicative of ...
Anatexis is the first step in granite genesis. Partial melting in the lower crust may produce leu... more Anatexis is the first step in granite genesis. Partial melting in the lower crust may produce leucoratic features of unusual chemical compositions, very different from the final products of crustal differentiation. Therefore, the links that exists between some migmatites and crustal-derived granites can be ambiguous. This study is an investigation of the anatectic history of a high-grade terrain: the Southern Marginal Zone of the Limpopo Belt (SMZ), north to the Kaapvaal Craton in South Africa. The work involved an integrated field, metamorphic, geochemical and geochronogical study of the metasedimentary granulites from two separate quarries in the northern zone of the Southern Marginal Zone, the Bandelierkop quarry and the Brakspruit quarry, where Neoarchean high-grade partial melting features can be observed. The project has aimed to address two main issues: (1) to accurately constrain the pressure temperature conditions and the age of the metamorphic episode in the SMZ, with impl...
&amp;lt;p&amp;gt;Inclusions of anatectic melt and fluids are probes used by geoscientist ... more &amp;lt;p&amp;gt;Inclusions of anatectic melt and fluids are probes used by geoscientist to investigate, understand and quantify devolatilization, melt production and chemical differentiation during orogenesis and crustal reworking. The large majority of case studies targeting these types of inclusions are in the Phanerozoic eon, whereas the Precambrian is still poorly represented. Here we present the first data on anatectic melt inclusions (&amp;lt;em&amp;gt;nanogranitoids&amp;lt;/em&amp;gt;) and their associated primary fluid inclusions in garnets from the Mesoarchean basement in Southeast Greenland. Using optical microscope observations, MicroRaman spectroscopy and microprobe investigation we show that these metasedimentary migmatites contain two generations of garnets, a large xenoblastic garnet (Grt&amp;lt;sup&amp;gt;1&amp;lt;/sup&amp;gt;) and a small idioblastic one (Grt&amp;lt;sup&amp;gt;2&amp;lt;/sup&amp;gt;). Both garnet types contain clusters of primary nanogranitoids which are characterized by the presence of quartz and feldspar polymorphs + phyllosilicates, a phase assemblage typically observed in this type of inclusions. Nanogranitoids are associated in the clusters to primary CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;-CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; fluid inclusions, which appear to be always affected by post-entrapment modifications with extensive formation of step-daughter minerals such as carbonates and pyrophyllite. Such occurrence proves for the first time that these rocks experienced partial melting with formation of garnet, and that this event took place in presence of COH-rich fluid. The re-evaluation of the metamorphic pressure and temperature conditions with up-to-date phase equilibria modelling, combined with the identification of nanogranitoids and fluid inclusions, suggests metamorphic peak equilibration and partial melting at T&amp;gt; 900 &amp;amp;#176;C and P&amp;gt; 7 kbar. To date, this is the oldest verified occurrence of nanogranitoids as well as of fluid-melt immiscibility during garnet growth in a partially molten environment.&amp;lt;/p&amp;gt;
&amp;lt;p&amp;gt;The Adirondack Mountains, New York State, USA belongs to the Canadian Gr... more &amp;lt;p&amp;gt;The Adirondack Mountains, New York State, USA belongs to the Canadian Grenville Province (Darling and Peck, 2016). The rocks exposed in the Adirondacks are interpreted to be the lower plate of a thrust-system at crustal levels during the Ottawan Orogeny (1090-1050 Ma) of the Grenvillian orogenic cycle. Garnet is abundant throughout the Adirondacks, with the greatest occurrence of megacrystic garnets within central Highlands. In the Gore Mountain area, the Hooper Mine is located 5 kilometers northwest of the Barton Mine and consists of partially melted mafic granulite. The mineral assemblage consists of medium grain size plagioclase, green hornblende and garnet in proportion 60:20:20. We separated the garnets of the Hooper Mine in two categories according to their size, chemical zoning and habitus: (1) Large, euhedral garnet porphyroblasts of diameter &amp;gt; 5 cm (LG), and (2) and small, xenoblastic grains (SG). Both types of garnets contain quartz, rutile and melt inclusions, similar to those observed in Barton Mine (Ferrero et al., 2021). In LG, chemical zoning is weak and inclusions are scattered randomly within the mineral. In SG, zoning coincides with the presence of quartz and melt inclusions in domain of low Ca and Y. Ti-in-quartz and Ti-in-amphibole thermometers in SG give equilibrium temperatures of 800-900 &amp;amp;#176;C at 10 kbar.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Major and trace element analyses on rehomogenised melt inclusions in both types of garnet indicate two types of melts are present in the migmatite &amp;amp;#8211; granitic melt in SG and trondhjemitic melt in LG. Stable isotope ratios of oxygen and hydrogen in hornblende (&amp;amp;#948;&amp;lt;sup&amp;gt;2&amp;lt;/sup&amp;gt;H: -62 to -73 &amp;lt;sup&amp;gt;0&amp;lt;/sup&amp;gt;/&amp;lt;sub&amp;gt;00&amp;lt;/sub&amp;gt; and &amp;amp;#948;&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O: 4.7 to 6.7 &amp;lt;sup&amp;gt;0&amp;lt;/sup&amp;gt;/&amp;lt;sub&amp;gt;00&amp;lt;/sub&amp;gt;) indicate that partial melting occurs in a closed isotopic system and records the primary magmatic &amp;amp;#948;&amp;lt;sup&amp;gt;2&amp;lt;/sup&amp;gt;H signature of the protolith. The range of melt chemistries, combined with the information previously collected in the Barton Mine defines a trend characteristic of primitive TTG melts or TTG embryos. These melts, combined with different proportion of peritectic phases (i.e. garnet, plagioclase and quartz), reproduces the full TTG chemistry range (Moyen, 2011). Therefore, the Mesoproterozoic mafic lower crust might be a perfect laboratory to test early granitoids genesis processes and better understand the link between melt inclusions, plate tectonics and the formation of the continental crust (Nicoli &amp;amp; Ferrero, 2021).&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;References:&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Ferrero, S. et al. (2021). Embryos of TTGs in Gore Mountain garnet megacrysts from water-fluxed melting of the lower crust. &amp;lt;em&amp;gt;Earth and Planetary Science Letters&amp;lt;/em&amp;gt;, &amp;lt;em&amp;gt;569&amp;lt;/em&amp;gt;, 117058.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Darling, R. S. and Peck W.H. (2016). Metamorphic conditions of Adirondack rocks. &amp;lt;em&amp;gt;Adirondack Journal of Environmental Studies&amp;lt;/em&amp;gt;, &amp;lt;em&amp;gt;21&amp;lt;/em&amp;gt;(1), 7.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Moyen, J. F. (2011). The composite Archaean grey gneisses: petrological significance, and evidence for a non-unique tectonic setting for Archaean crustal growth. &amp;lt;em&amp;gt;Lithos&amp;lt;/em&amp;gt;, &amp;lt;em&amp;gt;123&amp;lt;/em&amp;gt;(1-4), 21-36.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Nicoli, G., &amp;amp; Ferrero, S. (2021). Nanorocks, volatiles and plate tectonics. &amp;lt;em&amp;gt;Geoscience Frontiers&amp;lt;/em&amp;gt;, &amp;lt;em&amp;gt;12&amp;lt;/em&amp;gt;(5), 101188.&amp;lt;/p&amp;gt;
By submitting this dissertation electronically, I declare that the entirety of the work con-taine... more By submitting this dissertation electronically, I declare that the entirety of the work con-tained therein is my own, original work, that I am the sole author thereof (save to the extent explicitly otherwise stated), that reproduction and publication thereof by Stellen-bosch University will not infringe any third party rights and that I have not previously in its entirety or in part submitted it for obtaining any qualification.
Anatexis is the first step in granite genesis. Partial melting in the lower crust may produce leu... more Anatexis is the first step in granite genesis. Partial melting in the lower crust may produce leucoratic features of unusual chemical compositions, very different from the final products of crustal differentiation. Therefore, the links that exists between some migmatites and crustal-derived granites can be ambiguous. This study is an investigation of the anatectic history of a high-grade terrain: the Southern Marginal Zone of the Limpopo Belt (SMZ), north to the Kaapvaal Craton in South Africa. The work involved an integrated field, metamorphic, geochemical and geochronogical study of the metasedimentary granulites from two separate quarries in the northern zone of the Southern Marginal Zone, the Bandelierkop quarry and the Brakspruit quarry, where Neoarchean high-grade partial melting features can be observed. The project has aimed to address two main issues: (1) to accurately constrain the pressure temperature conditions and the age of the metamorphic episode in the SMZ, with impl...
Differentiation of the continental crust is the result of complex interactions between a large nu... more Differentiation of the continental crust is the result of complex interactions between a large number of processes, which govern partial melting of the deep crust, magma formation and segregation, and magma ascent to significantly higher crustal levels. The anatectic metasedimentary rocks exposed in the Southern Marginal Zone of the Limpopo Belt represent an unusually well-exposed natural laboratory where the portion of these processes that operate in the deep crust can be directly investigated in the field. The formation of these migmatites occurred via absent incongruent melting reactions involving biotite, which produced cm- to m-scale, K2O-poor garnet-bearing stromatic leucosomes, with high Ca/Na ratios relative to their source rocks. Field investigation combined with geochemical analyses, and phase equilibrium modelling designed to investigate some aspects of disequilibrium partial melting, show that the outcrop features and compositions of the leucosomes suggest several steps in their evolution: (1) Melting of a portion of the source, with restricted plagioclase availability due to kinetic controls, to produce a magma (melt + entrained peritectic minerals in variable proportions relative to melt); (2) Segregation of the magma at near peak metamorphic conditions into melt accumulation sites (MAS), a.k.a. future leucosome; (3a) Reequilibration of the magma with a portion of the bounding mafic residuum via chemical diffusion (H2O, K2O), which triggers the co-precipitation of quartz and plagioclase in the MAS; (3b) Extraction of melt dominated magma to higher crustal levels, leaving peritectic minerals entrained from the site of the melting reaction and the minerals precipitated in the MASs to form the leucosome in the source. The key mechanism controlling this behaviour is the kinetically induced restriction of the amount of plagioclase available to the melting reaction. This results in elevated melt H2O and K2O and chemical potential gradient for these components across the leucosome/mafic residuum contact. The combination of all of these processes accurately explains the composition of the K2O-poor leucosomes. These findings have important implications for our understanding of melt segregation in the lower crust and minimum melt residency time which, according to the chemical modelling is < 5 years. We demonstrate that in some migmatitic granulites, the leucosomes constitute a type of felsic refractory residuum, rather than evidence of failed magma extraction. This provides a new insight into the ways that source heterogeneity may control anatexis. This article is protected by copyright. All rights reserved.
Studies on the Southern Marginal Zone (SMZ) of the Limpopo Belt have generally concluded that thi... more Studies on the Southern Marginal Zone (SMZ) of the Limpopo Belt have generally concluded that this terrain was affected by a single granulite-facies metamorphic event at 2.67–2.66 Ga (Kroner et al. 2000; Kreissig et al. 2000; Zeh et al. 2005; Elington and Armstrong 2004; Stevens and Van Reenen 1992a, b; Barton and van Reenen 1992; Barton et al. 1992; Van den Berg and Huizenga 2001; Rigby et al. 2008), with peak metamorphic conditions of 7.5–9.5 kbar and 800–850 C. In their recent paper, Belyanin et al. (2012) propose an ultrahigh-temperature (UHT) metamorphic event in the SMZ, with peak metamorphic conditions in excess of 1,000 C at approximately 12 kbar. Similar conclusions were proposed by earlier work on the same rock (Belyanin et al. 2010). The proposed UHT conditions are based on Al-rich orthopyroxene and ternary feldspar thermometry using reintegrated feldspar compositions. This evidence is derived from a single outcrop containing unusual aluminous layers within metasedimentary granulites, although Tsunogae et al. (2004) used Al-in-orthopyroxene and ternary feldspar thermometry to argue for a peak metamorphic temperature of [950 C from more representative metapelitic granulites of the Bandelierkop formation. Both these studies have proposed that UHT metamorphism affected the SMZ generally and thus that the relevant PT path of all rocks traversed UHT conditions. This interpretation is problematic because it is difficult to reconcile with the partial melting history of the rocks and with the peak metamorphic assemblage in most rocks. In essence, both the metasedimentary granulites of the Bandelierkop formation and the trondhjemitic to tonalitic Bavianskloof gray gneisses contain a significant proportion of biotite, which defines a syn-peak metamorphic fabric (Fig. 1). Within the metasediments of the Bandelierkop formation, compositional banding at the outcrop scale (Fig. 1) exists that is considered to represent primary differences in composition between different sedimentary units (van Reenen 1983). Anatexis of these rocks has occurred via three reactions (Stevens and van Reenen 1992a): Ms ? Qtz ? Pl = Sil ? Melt; Bt ? Sil ? Qtz ? Pl = Grt ? Melt; and Bt ? Qtz ? Pl = Opx ? Crd ? Melt. The first two reactions exhausted muscovite and sillimanite, respectively, while the third reaction had just begun to occur in the chemically most favorable rocks at the conditions of peak metamorphism. Pseudosection modeling of metapelitic granulites from the Bandelierkop quarry, which have undergone anatexis and that represent restitic, melt-depleted compositions, indicates that biotite in such rocks would not have survived temperatures significantly above 900 C and that the mineral compositions in such rocks are consistent with peak metamorphic temperatures of 820–860 C at 8–10 kbar (Fig. 2). At the peak metamorphic conditions proposed by Belyanin et al. (2012), these restitic granulites would have become Communicated by T. L. Grove.
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