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(a) Schlieren-rich granite in the transition zone between stromatic migmatite and diatexite, west side of the TAD (Walker Mountain, north of Roxbury, Maine; Station 95-215). The fabric is concordant with foliation and layers in the adjacent stromatic migmatite, and parallel to the length of the north-south trending transition zone (000° is left along the long dimension of the field). The schollen within the granite are calc-silicate-rich psammite that have internal structure similar to metasedimentary rocks of the CMB. Leucosome is concentrated at the interface with the granite. (b) Sub-planar granite sheets in stromatic migmatite that parallel the migmatite foliation and layers (Station 95-51; 012° is left). These sheets 'pinch-and-swell' to 'pinch' out the stromatic migmatite host along the fabric and length of the sheets. (c) Cylindrical granite bodies in diatexite that contains a block of biotite-garnet schist (Station 95-49), apparently residual after partial melting of diatexite. (d) Texture of the block of schist in (c) (plane-polarized light). Biotite defines a nearly decussate texture. Finer-grained groundmass is approximately equigranular biotite + plagioclase.
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In Maine, Siluro-Devonian turbidites were metamorphosed under high- T –low- P facies series conditions during deformation within a Devonian crustal-scale shear zone system, defined by kilometer-scale straight belts of apparent flattening strain that anastomose around lozenges of apparent constrictional strain. At upper amphibolite facies grade, met...
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... square meter (Fig. 3), in which relict facies mica schist (Smith & Barreiro, 1990). In com-primary structures are preserved, to rocks structurally parison, concordant U-Pb zircon and monazite ages disrupted by the migmatization process (diatexis; see from granite bodies in migmatite, as well as plutons, Brown, 1973; Fig. 4) and schlieric granite (Fig. 5a). range from c. 408 to c. 404 Ma (Solar et al., 1998); this Leucosome density and disruption of relict primary struc- suggests that granite emplacement was contemporaneous tures both increase across strike from the migmatite front with metamorphism (Brown & Solar, 1998a, 1998b, (Solar, 1999. Similarly, sheets and cylinders of granite ...
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... plunging lineation visible in the field, and elongate and between vein migmatite and diatexite within the quartz aggregates define a weak sub-horizontal lineation heterogeneous migmatite (Fig. 2). Commonly, bodies of seen only in cut hand specimens and suitably oriented schlieric granite are found within transition zones (see thin sections. Fig. 5a) that occur at AFZ-ACZ boundaries. In the In leucosomes, grains are equant and anhedral with northern part of the TAD and WAD, layers are pro- average sizes ranging from 0·1 mm in the thinner leuco- gressively eliminated across strike from the migmatite somes to 4 mm in the thicker leucosomes. Quartz is front by increasing volume of ...
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... by sillimanite the proportion of meter-scale composite granite sheet to (mostly fibrolite) and biotite (Fig. 4b). host stromatic migmatite such that the migmatite be- There are two types of heterogeneous migmatite, vein comes disrupted ultimately to occur only as isolated migmatite in the south and SW and diatexite in the schollen in granites (Fig. 5b) that make up a sheeted north and NE [Fig. 2;terminology after Menhert (1968) granite complex. andBrown (1973), as modified by Ashworth (1985)]. Contacts between the two types are gradational over tens of meters in transition zones. Vein migmatite ...
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... to biotite-sillimanite-dominated rock, and Quartz generally displays the largest grains (>5 mm in outcrop to outcrop from schlieren-rich migmatite to diameter), whereas plagioclase is usually >1 mm in schlieric granite with schollen of vein migmatite and length, and grains commonly show inhomogeneous nor-unmigmatized calc-silicate-rich psammite (Fig. 5a). Most mally compositionally zoned concentric shells of >50 m types are characterized by a discontinuous, weakly de- ...
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... are found in the defined by biotite and opaque minerals that we infer to be of magmatic origin, and formed where melt and interior of the granite cylinders. The granite cylinders lack a fabric, except proximal to the margin of these entrained crystals flowed around residue and/or un- melted protolith. Thus leucosomes tend to be rod-shaped, blocks (Fig. 5c). with a long dimension that plunges moderately to steeply ENE. This linear structure is sub-parallel to both the weakly defined mineral elongation lineation in the host Interpretation rock and the strongly defined mineral elongation ...
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... low P atexite from more quartzo-feldspathic (leucocratic) to (Thompson & Tracy, 1976). At a depth of >15 km these more ferro-magnesian (melanocratic) types. In the ex-reactions indicate T of >700°C. Given the limited amount treme case, either schlieric granite is formed (leucocratic of water-rich metamorphic volatile phase that can be diatexite; Fig. 5a), or the mineral assemblage is dominated stored in rocks at upper amphibolite facies conditions, by biotite, sillimanite and garnet with <10 vol. % we expect that melting will be dominated by the muscovite plagioclase + quartz, to give the rock a melanocratic dehydration reaction. Patiño Douce & Harris (1998) appearance ( Fig. 5c and ...
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... be diatexite; Fig. 5a), or the mineral assemblage is dominated stored in rocks at upper amphibolite facies conditions, by biotite, sillimanite and garnet with <10 vol. % we expect that melting will be dominated by the muscovite plagioclase + quartz, to give the rock a melanocratic dehydration reaction. Patiño Douce & Harris (1998) appearance ( Fig. 5c and d). In all varieties of diatexite, investigated experimentally melting of metapelites similar sillimanite is found as clots within both plagioclase and to those in the CMB, using two schists with different retrograde muscovite. Although grain size varies, it is modes from the hanging wall of the Main Central Thrust generally >3 mm, with ...
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... two of the heterogeneous tulate in the light of the contemporaneous deformation migmatites (vein migmatite: 96-65, Fig. 4e; diatexite: 95- after describing the geochemistry of all rock types, but 35, Fig. 4c) and one of the schlieric granites (from the for the remainder of this paper we will refer to darker northern TAD transition zone: 95-215, Fig. 5a) were host rock that does not form a distinct melanosome like separated into the different structural components, and those in the stromatic migmatite as melt-depleted host analyzed separately. Major, minor and trace element ...
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... that were determined to have undergone diagenetic removal of the LREE ( Cullers et al., 1997) are excluded. highest in HREE concentrations consistent with the overall shape and steepness of these patterns are similar to those of the North American shale composite (Taylor higher garnet content in that part of the block (e.g. Hanson, 1980; see Fig. 5c for the centimeter -scale & McLennan, ...
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... Rb/Sr ratio of tematically higher than inferred from monazite the melt regardless of the aH 2 O during melting (Harris et al., 1993). Rb/Sr ratios of granite (excepting 95-121) solubilities, as a result of the more rapid dissolution of and leucosome specimens are in the range 0·2-1, which garnet schist in the cylindrical body of granite 95-49 ( Fig. 5) suggests it is an extreme example of residue from taken at face value are consistent with a low Rb-Sr fractionation during water-fluxed melting of the CMB partial melting of CMB metasedimentary ...
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... Among the many varieties of granites, strongly peraluminous (SP) granites or peraluminous leucogranites represent pure crustal melts invariably associated with collisional orogens (Patino Douce 1999). Although they are voluminously less abundant, SP granites are distinctive of modern and ancient collision orogens and have been reported from the Himalayas, Hercynides, Alps, and Caledonides (Miller 1985;Le Fort et al. 1987;Sylvester 1998;Nabelek and Bartlett 1998;Solar and Brown 2001). They have been identiBed as being formed by the partial melting of crustal rocks, particularly metasedimentary lithologies, in a thickened crustal pile either during collision (Pearce et al. 1984;Harris et al. 1986) or during the post-collision collapse of the orogen by lithospheric delamination/ tectonic thinning (Sylvester 1998). ...
... Located in the Oxford County pegmatite field in western Maine, northeastern U.S.A. (Fig. 1), the Emmons pegmatite is a dike intruded into Siluro-Devonian metasedimentary country rock (Bradley 1983;Bradley and O'Sullivan 2017;Solar and Brown 2001), belonging to the Sebago Migmatite Domain (SMD). The SMD is mainly composed of pelitic migmatites and diatexites with a few foliated granite intrusions (Solar and Tomascak 2009) and is part of the Central Maine Belt (CMB) in the northern Appalachian Mountains, which stretches from New Brunswick, Canada, to Connecticut, U.S.A. (Wise and Brown 2010). ...
... The SMD is mainly composed of pelitic migmatites and diatexites with a few foliated granite intrusions (Solar and Tomascak 2009) and is part of the Central Maine Belt (CMB) in the northern Appalachian Mountains, which stretches from New Brunswick, Canada, to Connecticut, U.S.A. (Wise and Brown 2010). It is composed of interlayered pelite and psammite rocks, metamorphosed to greenschist and amphibolite facies during the Acadian Orogeny (Solar and Brown 2001). Peak metamorphic conditions occurred at ca. 408-404 Ma with temperatures of around 500-520 °C for lower-grade metapelitic rocks (Solar et al. 1998;Johnson et al. 2003). ...
The anisotropic textures, including unidirectional solidification textures and graphic intergrowths, characteristic for pegmatites, are interpreted to result from disequilibrium crystallization at high degrees of undercooling. Experimental studies have revealed the existence of thin boundary layers surrounding the rapidly growing crystals. Here, tourmaline-bearing samples from the outer zones of the Emmons pegmatite (Maine, U.S.A.) are used to examine if a boundary layer can also occur in natural samples. Crystal morphology is linked with geochemistry to understand the evolution of pegmatite melts and to constrain disequilibrium conditions at large degrees of undercooling. Petrographic studies and semiquantitative micro-X-ray fluorescence element mapping were conducted to identify crystal morphology and zonation, complemented with electron microprobe analyses to determine major and minor element compositions and LA-ICP-MS analyses of selected trace elements. Three textural groups were identified: comb-like tourmaline, quartz-tourmaline intergrowths, and radiating tourmaline. The intergrowths are optically coherent and are split into three different morphologies: central, second tier, and skeletal tourmaline. Most tourmaline is schorl, but chemical variation occurs on three different scales: between textural groups, between different morphologies, and intracrystalline. The largest scale geochemical variation is caused by the progressive evolution of the melt as it crystallized from the borders inwards, while the intracrystalline variations are attributed to sector zoning. A model is suggested where the systematic variation of Mg, Mn, and Fe within individual intergrowths is proposed to be the result of crystallization from a boundary layer, rich in water and other fluxing elements (e.g., Li, P, B), formed around the rapidly growing central tourmaline. Here, we show the first examples of boundary layers in natural pegmatites. Furthermore, the results bring into question whether boundary layer tourmaline can be used as a bulk melt indicator in pegmatitic melts.
... Correlation between migmatite leucosome and granitic melts crystallized into plutons has been a subject of much interest, and ample evidence was obtained for close spatial, compositional, and genetic links between the products of partial melting and crustal granites (e.g., Bons et al., 2001;Solar and Brown, 2001;Brown, 2013). If such linkage exists, the question arises whether the events of partial melting are always synchronous with the formation of large granitic bodies. ...
Polymigmatites in the Early Proterozoic metamorphic complex of the Northern Ladoga area trace the evolution of thermodynamic conditions during anatexis. The P–T conditions inferred for anatectic leucosome correspond to the onset of partial melting under granulite facies conditions of 5.5–6.2 kbar and 720–810 °C. After the peak of metamorphism, pressure and temperature show a coupled decrease to 4 kbar and ~550 °C, respectively. The latest granitic veins were intruded into metamorphic rocks during an event of brittle deformation. The P–T trend correlates with changes in migmatitic mineral assemblages and in chemistry of minerals. Newly formed leucocratic material changed from plagiogranitic to granitic composition when melting involved plagiogneiss but melt derived from Al-rich metapelite remained granitic. The analyzed leucosomes and granitic veins originated by multistage melting, under P–T conditions changing from granulite to amphibolite facies, between 1875 and 1865 Ma. Judging by their ages, the leucosome and granitic vein bodies from the Lakhdenpokhia and Priozersky zones of the area differed in the total duration of crystallization. Therefore, a single thermal event involving different lithologies can produce intrusions of different ages.
... They represent the magma products which are derived from low-grade partial melting of continental crust followed by crustal thickening due to continental collision [1]. Although leucogranites can be observed in various terrains throughout the world, they are characteristic rock types of orogenic belts [2][3][4][5]. The most significant and widely distributed leucogranite outcrops are located in the Himalayan orogenic belt [6][7][8][9][10][11][12]. ...
Leucogranites of Kalebalta in Central Anatolia are composed of plagioclase, quartz, orthoclase, and biotite and contains mafic microgranular enclaves (MME) in sizes ranging from few cm to 70 cm. In the total alkali-silica diagram, they fall typically in the granite field and show a calc-alkaline nature in the alkalis-iron-magnesium diagram whereas enclaves are Medium K series calc-alkaline, which represents the transition from tholeiitic to calc-alkaline. Leucogranites which have A/CNK(mol%) > 1 are strong peraluminous and seen as the products of magma derived from a metasedimentary source. Signs of magma mixing expressing the mantle inputs are also observed in many bivariation diagrams. Zircon and apatite saturation temperatures calculated on the basis of whole rock chemistry are 744–829°C for leucogranites and 761–832°C for their enclaves. According to the Raman spectra, biotite and plagioclase minerals in leucogranites and their enclaves show similar Raman spectrums. The biotite minerals have Mg–O and/or Fe–O translational (transformation) bonds between 182 and 552 cm⁻¹, Si–O–Si bending between 552 and 1,100 cm⁻¹ and Si–O–Si vibrational bonds between 1,100 and 1,200 cm⁻¹. The results of this study suggest that the leucogranites and enclaves are most probably derived from different magmas. In addition, according to geochemical and spectroscopic data, they may also have fractional crystallization, which is effective after the mixing process.
... The process during crustal anatexis are being understood in detail during the last few decades. The studies within the anatectic zone led to a better understanding of melt production reactions in the field as well as in the laboratory (Sawyer, 1994;Patino-Douce and Harris, 1998;Rushmer, 2001;Singh et al., 2022), melt distribution and nature of migration pathways (Solar and Brown, 2001;Imayama et al., 2010). The studies also throw light on deformation behavior, bulk permeability, melt pore pressure and melt emplacements (Hollister and Crawford, 1986;Dell'Angello et al., 1987;Wickham, 1987;Davidson et al., 1994). ...
Migmatites of Higher Himalayan Crystallines (HHC) along different river valleys are present above the Vaikrita Thrust in the upper package. They have experienced anatexis and protracted growth of in situ melt between ca. 46 Ma and ca. 20 Ma. Melt-filled conjugate set of extensional crenulation cleavage (ECC) of late-stage deformation indicating extension parallel to the preexisting foliation (Sm) and shortening normal to foliation plane. The field evidence indicates that ECC was formed during partial melting and later filled by the melt. During partial melting, the deformation got enhanced because of the accumulation of strain due to the prolonged presence of melt. That caused the increase in the melt pressure eventually leading to crossing the critical limit and forming extensional crenulation cleavage and making it the conduit for the emplacement of melt within the migmatite zone. The phenomenon of melt-filled ECC is due to melt-enhanced deformation occurring at the time of orogen parallel extension. The presence of episodic fluxing of melt as evident from U-Pb ages of zircon is very well be correlated with the building up of partial melt leading to an increase and drop of the melt pressure due to draining followed by rising of melt from the system along with the newly formed ECC until the next event. This episodic nature of fluxing appears to be a convenient mode to extract melt from partially melted terrains of migmatites leading to chaotic structures.
... Migmatite belts are most typically associated with collisional orogenies, but migmatites may also form due to heating of the lower crust by underplating of subductionrelated basaltic melts (Castro et al., 1999;Hegner et al., 2001;Stephens & Andersson, 2015;Stephens, 2020a). A genetic relationship between migmatites and granites has been documented from several orogens (Sawyer, 1998;Milord, 2001;Solar & Brown, 2001;Johannes et al., 2003), and spatial as well as temporal associations of granites with crustalscale shear zones are common (Brown andSolar, 1998a, 1998b). The generation of migmatites is closely associated with the tectonic evolution and development of meso and macroscopic geological structures of the bedrock. ...
In migmatitic environments the behaviour of the system is controlled by the generation
and amount of the anatectic melt. Accordingly, migmatites typically show a genetic
linkage between the tectonic deformation and melt migration. We investigated this
relationship in Olkiluoto (SW Finland) and identified four phases of ductile deformation,
which are distinguished by the multiple folding phases, ductile shear events and
cross-cutting features associated with pegmatitic leucosomes and/or a specific type
of diatexitic migmatite with feldspar megacrysts. U–Pb LA-MC-ICPMS data on zircon
cores and rims from migmatites and cross-cutting pegmatites indicate two distinct
metamorphic events associated with melt generation and migration at 1.87–1.84 Ga
and 1.82–1.78 Ga. These two migmatitic events suggest that the orogenic evolution of
the area was long-lasting and characterized by slow cooling. The structural data and the
age constraints presented in this paper support the idea of similar tectonic evolution
and metamorphic environment in SW Finland and central E Sweden.
... The genetic evolution of a magma begins when its protolith undergoes partial melting and ends when it completely solidifies to form rock. Migmatites are partially melted rocks, whereas granitic intrusions are structures that form during magma crystallization (Vigneresse et al., 1996). Despite the documented transitions of some migmatites to granites (Solar and Brown, 2001;Milord et al., 2001), the mechanisms by which anatectic magmas evolve from deeper-crustal partial melts to granitic plutons localized in the upper crust are still poorly understood. ...
The Velitkenay monzonite-granite-migmatite massif is a granite-core gneiss dome on the Arctic coast of Chukotka and a key outcrop for understanding mid-Cretaceous magmatism and metamorphism of the Arctic Alaska–Chukotka terrane. The petrology and geologic history of the complex were investigated using whole rock major and trace element and isotopic analyses (Sr, Nd and Pb), as well as electron microprobe thermobarometry, zircon U-Pb geochronology, trace element geochemistry, and O and Lu-Hf isotopic methods. The massif consists of two distinctive Albian plutonic suites. Deformed 106–103 Ma monzonitoids represent the early phase of plutonism; they were melted from a relatively mature crustal source with the bulk composition εNd(i) from –5.5 to –7.9; TNd (DM-2st) = 1.4–1.6 Ga, and zircon composition εHf(i) from –11 to –7, δ18 O from 10 to 8.4; and they evolved via assimilation and fractional crystallization processes. Late phase 102–101 Ma leucogranites (εNd(i) from –3.8 to –6.7, TNd (DM-2st) = 1.2–1.4 Ga) formed from melting of Neoproterozoic orthogneisses with mantlelike zircon (εHf(i) from +11 to +13, δ18 O ~ 5.8) and differ from early phase monzonitoids by the systematic presence of inherited Neoproterozoic (660–600 Ma) zircon xenocrysts. Migmatized Neoproterozoic orthogneisses are exposed in the central part of the dome, whereas the country rock on the flanks of the dome are paragenisses and schists with Devonian protolith ages. The intrusion of monzonitoid magmas was syntectonic with early stages of exhumation of the Velitkenay massif, whereas the leucogranite phase of magma intruded after peak metamorphism and does not exhibit ductile deformation. Based on subhorizontal mineral stretching lineations along the flanks of the dome and overall sigmoidal plan view of the massif, structural doming appears related to localized transtension in a more regional dextral strike-slip geodynamic environment. U-Pb isotope-geochronological data make it possible to distinguish seven episodes of granitoid magmatism, of which three major, in terms of volume (Aptian Bilibino, Albian Chaun, and Turonian-Coniacian Okhotsk-Chukotka granitoid magmatism subprovince), reflect the maximum rates of growth and modification of the Chukotka crust. Subordinate, in terms of volume, magmatic events of the Neoproterozoic, Devonian, Permian-Triassic, Late Jurassic, and Valanginian- Hauterivian have also been reconstructed.
... At the mesoscale, the recognition of leucosome in structurally-controlled sites in Ordovician migmatites such as dilatant shear surfaces, in between boudins, and strain shadows suggest that melt flow throughout the migmatitic domain proceeded simultaneously with ductile deformation (Solar and Brown, 2001). Moreover, SGC metasedimentary rocks may have behaved as potential contaminants of tonalitegranodiorite magmas which derived from the melting of the deeper crust. ...
U-Pb ages of zircon from samples of tonalite and migmatite from the Central Iberian Zone (Spain), previously assigned to the Carboniferous were obtained using a sensitive high mass-resolution ion microprobe. These ages indicate a close temporal relationship between Ordovician tonalitic magmatism and high-grade (low-P) metamorphism in Iberia. The age obtained for Santa Cruz tonalite (476 ± 1 Ma) represents a constraint on the partial melting event experienced by Ediacaran Schist-Greywacke Complex siliciclastic rocks, while Risco Chico leucogranite that alternates with tonalites and migmatites yielded a younger age (468 ± 3 Ma). The age range of Neoproterozoic relict zircon grains preserved in Ordovician plutonic rocks coincides with that of host Ediacaran metasedimentary rocks (Puerto de Santa Cruz migmatites), which are the most plausible source of leucogranites. Altogether, the component of inherited zircon, peraluminous calc-alkaline composition, and an abundance of pelitic restites found in Ordovician tonalites provide evidence of a mixed contribution from old Ediacaran magmatic arc rocks and siliciclastic sequences. Our findings supply a missing piece in the Lower-Middle Ordovician tectonic puzzle of the northern Gondwana margin, revealing that tonalitic magmatism, ductile deformation, and high-grade metamorphism all occurred within a relatively restricted period simultaneously with siliciclastic deposition in active extensional basins.
Supplementary material: https://doi.org/10.6084/m9.figshare.c.5976757
... Commonly, these shears host melts, and the curving of the D3 fabrics near the D4 shears continued as long as the melt existed; beyond the melt-bearing zones, the shears peter out and the D3 fabrics are not warped [43]. This suggests the nucleation of the D4 shears occurred in the presence of melt through a feedback relationship [43,44]. Due to the melt-hosted nature of the conjugate NNE and ENE-striking shears, we consider the nucleation of the shears to be broadly contemporaneous with, albeit closely following, the D3 deformation at suprasolidus conditions in the granitoids. ...
The Great Indian Proterozoic Fold Belt (GIPFOB) is a curviplanar highly-tectonized zone of Precambrian crystalline rocks. In the GIPFOB, the N/NNE-striking western arm (the Aravalli Delhi Fold Belt, ADFB) and the E-striking southern arm consisting of the Chottanagpur Gneiss Complex (CGC) and the central/southern domains of the Satpura Mobile Belt (SMB) converge at the Godhra-Chhota Udepur sector. To investigate the tectonics of the sector, we combine the results of analyses of mesoscale and regional structures, U-Pb (zircon) geochronology, and monazite chemical dating to constrain the convergence. The sector is dominated by an ensemble of shallow-dipping granitoid mylonites (D2 deformation) and recumbently folded anatectic granulite-facies basement gneisses interleaved with allochthonous greenschist/epidote-amphibolite facies supracrustal rocks thrust top-to-the-south. The shallow-dipping carapace is traversed by a network of E-striking steep-dipping shear zones with sinistral and N-down kinematics (D3 deformation). The D3 shear zone hosted granitoids exhibit E-striking suprasolidus deformation fabrics and chessboard microstructures. In the shallow-dipping carapace, the partly overlapping stretching lineations associated with D2-D3 deformations share low-angle obliquities with the W/WNW plunging hinges of D2 recumbent folds and the upright/moderately-inclined D3 folds in the basement gneisses and the supracrustal rocks. The transition from thrust-dominated (D2) to wrench-dominated (D3) deformation involved flipping of Y and Z strain axes for similar orientations of orogen-parallel stretching caused by N-S shortening. U-Pb LA-ICP-MS (zircon) and monazite chemical dates suggest the D2-D3 deformation and felsic plutonism occurred at 0.95–0.90 Ga, the pre-D2 high-grade metamorphism in the anatectic gneisses at 1.7–1.6 Ga. The 0.95–0.90 Ga structures in the Godhra-Chhota Udepur are identical to those in CGC-SMB in the southern arm and terminate the N/NNE-striking structures in the ADFB. We suggest the GIPFOB comprises two Early Neoproterozoic accretion zones, e.g., the western arm (ADFB) and the younger (GC-SMB-CGC) southern arm.
... In fact, the low melt volume predicted by phase equilibria modelling is not enough for melt to leave the source. Another possibility to consider here is that a significant proportion of anatectic melt might have escaped from the melting site (Sawyer, 1987;Solar and Brown, 2001;Brown, 2002;Taylor et al., 2014;Nicoli et al., 2015). In other words, leucogranite formed by the melting of diorite gneiss might represent the residue remaining after extraction of anatectic liquid. ...
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.
