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Volume 32 Issue 6
Dec.  2021
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Tom Andersen, O. Tapani Rämö. Dehydration Melting and Proterozoic Granite Petrogenesis in a Collisional Orogen—A Case from the Svecofennian of Southern Finland. Journal of Earth Science, 2021, 32(6): 1289-1299. doi: 10.1007/s12583-020-1385-8
Citation: Tom Andersen, O. Tapani Rämö. Dehydration Melting and Proterozoic Granite Petrogenesis in a Collisional Orogen—A Case from the Svecofennian of Southern Finland. Journal of Earth Science, 2021, 32(6): 1289-1299. doi: 10.1007/s12583-020-1385-8

Dehydration Melting and Proterozoic Granite Petrogenesis in a Collisional Orogen—A Case from the Svecofennian of Southern Finland

doi: 10.1007/s12583-020-1385-8
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  • Corresponding author: O. Tapani Rämö, tapani.ramo@helsinki.fi
  • Received Date: 2020-09-07
  • Accepted Date: 2020-12-01
  • Publish Date: 2021-12-30
  • Dehydration melting of metasupracrustal rocks at mid-to deep-crustal levels can generate water undersaturated granitic melt. In this study, we evaluate the potential of ~1.89-1.88 Ga metasupracrustal rocks of the Precambrian of southern Finland as source rocks for the 1.86-1.79 Ga late-orogenic leucogranites in the region, using the Rhyolite-MELTS approach. Melt close in composition to leucogranite is produced over a range of realistic pressures (5 to 8 kbar) and temperatures (800 to 850℃), at 20%-30% of partial melting, allowing separation of melt from unmelted residue. The solid residue is a dry, enderbitic to charnoenderbitic ganulite depleted in incompatible components, and will only yield further melt above 1 000-1 050℃, when rapidly increasing fractions of increasingly calcic (granodioritic to tonalitic) melts are formed. The solid residue after melt extraction is incapable of producing syenogranitic magmas similar to the Mid-Proterozoic, A-type rapakivi granites on further heating. The granitic fraction of the syenogranitic rapakivi complexes must thus have been formed by a different chain of processes, involving mantle-derived mafic melts and melts from crustal rock types not conditioned by the preceding late-orogenic Svecofennian anatexis.
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Dehydration Melting and Proterozoic Granite Petrogenesis in a Collisional Orogen—A Case from the Svecofennian of Southern Finland

doi: 10.1007/s12583-020-1385-8

Abstract: Dehydration melting of metasupracrustal rocks at mid-to deep-crustal levels can generate water undersaturated granitic melt. In this study, we evaluate the potential of ~1.89-1.88 Ga metasupracrustal rocks of the Precambrian of southern Finland as source rocks for the 1.86-1.79 Ga late-orogenic leucogranites in the region, using the Rhyolite-MELTS approach. Melt close in composition to leucogranite is produced over a range of realistic pressures (5 to 8 kbar) and temperatures (800 to 850℃), at 20%-30% of partial melting, allowing separation of melt from unmelted residue. The solid residue is a dry, enderbitic to charnoenderbitic ganulite depleted in incompatible components, and will only yield further melt above 1 000-1 050℃, when rapidly increasing fractions of increasingly calcic (granodioritic to tonalitic) melts are formed. The solid residue after melt extraction is incapable of producing syenogranitic magmas similar to the Mid-Proterozoic, A-type rapakivi granites on further heating. The granitic fraction of the syenogranitic rapakivi complexes must thus have been formed by a different chain of processes, involving mantle-derived mafic melts and melts from crustal rock types not conditioned by the preceding late-orogenic Svecofennian anatexis.

Tom Andersen, O. Tapani Rämö. Dehydration Melting and Proterozoic Granite Petrogenesis in a Collisional Orogen—A Case from the Svecofennian of Southern Finland. Journal of Earth Science, 2021, 32(6): 1289-1299. doi: 10.1007/s12583-020-1385-8
Citation: Tom Andersen, O. Tapani Rämö. Dehydration Melting and Proterozoic Granite Petrogenesis in a Collisional Orogen—A Case from the Svecofennian of Southern Finland. Journal of Earth Science, 2021, 32(6): 1289-1299. doi: 10.1007/s12583-020-1385-8
  • The origin of granite melts remains a controversial issue, not least because energy-conserved modeling schemes pertinent to granitic systems have not been fully developed. An important step forward in this regard was the release of the Rhyolite- MELTS software (Ghiorso and Gualda, 2015; Gualda et al., 2012) that delivers a much improved prediction of the quartz-feldspar saturation surface as a function of pressure and allows control on saturation conditions of H2O-CO2 mixed fluids in silicate liquids. Using this approach, we have embarked upon a project to evaluate possible modes of origin of two different granite suites from the Finnish Precambrian-Paleoproterozoic peraluminous leucogranites and Paleo- to Mesoproterozoic subaluminous (ferroan) rapakivi granites.

    Regarding possible crustal protoliths of granites, minerals such as muscovite and biotite, in some cases also amphibole, are important water-bearing constituents of low- to medium-grade metasupracrustal rocks. On heating at elevated pressure, these minerals will eventually break down and their water content will be liberated to a hydrous fluid phase through metamorphic devolatilization reactions or dissolve to form a water-undersaturated, aluminosilicate melt by processes known as dehydration melting (e.g., Bucher and Frey, 2002; Spear et al., 1999; Johannes and Holtz, 1996). Depending on the composition of the protolith, the melts produced will range in composition from tonalitic to granitic (Johannes and Holtz, 1996, and references therein). Dehydration melting of amphibole-bearing mafic protoliths has long been regarded as a process responsible for plagiogranite plutonism in orogenic belts (e.g., Wolf and Wyllie, 1994; Beard and Lofgren, 1991), but the process is in no way restricted to active convergent margin settings. Given a sufficient supply of thermal energy, any mica/amphibole-bearing protolith at deep to middle crustal levels will eventually reach temperature and pressure conditions where OH-bearing minerals will become unstable.

    Solidus curves for water-undersaturated rock systems have positive slope in the PT space (e.g., Holtz et al., 2001). It is therefore possible to induce partial melting in the middle to lower crust both by increasing the temperature at more or less constant pressure and by decompression. Sufficient temperature increase to induce partial melting may be caused by emplacement of hot, mantle derived magma into the crust (Frost and Frost, 1987). Decompression-induced melting is possible during crustal thinning and resulting rapid uplift and exhumation of deep crust in late-orogenic collapse of orogenic belts (e.g., Crawford et al., 2009; Vanderhaeghe et al., 1999). Furthermore, radioactive decay of 235U, 238U, 232Th and 40K will cause internal heating in compositionally evolved crust with localized concentrations of heat- producing elements (e.g., Clauser, 2011; Sandiford et al., 1998), which results in anatexis of the fertile lithologic units present (Chamberlain and Sonder, 1990). Such anatexis has been suggested to lead to voluminous melt fractions (e.g., Kukkonen and Lauri, 2009; Gerdes et al., 2000).

    In batch melting (e.g., Shaw, 1970), and even in a melting scenario in which melt is separated from residue quasi- continously (e.g., Bons et al., 2004) or episodically (e.g., Vigneresse, 2007), the solid residue formed in the process will be in at least local equilibrium with the melt that is extracted (e.g., Spear et al., 1999). The relationship between melting conditions (pressure, temperature), protolith composition, degree of melting and composition of the melt produced can therefore be evaluated from thermodynamical models incorporating data on relevant minerals and melt compositions. These have to be considered with the rheologic critical melt percentage (RCMP) that marks the rheology change of the anatectic system from brittle to ductile and the condition for separation of melt from residue (Arzi, 1978).

    In this study, we have applied the Rhyolite-MELTS software (Ghiorso and Gualda, 2015; Gualda et al., 2012) to simulate partial melting processes of metasupracrustal rock types in the Paleoproterozoic Svecofennia tectonic province of southern Finland, also studied e.g., by Kukkonen and Lauri(2016, 2009). The problems to be addressed are: (1) whether or not melting of realistic metasupracrustal protoliths will produce liquids that approach the composition of the widespread ca. 1.83 Ga leucogranites in the region, emplaced subsequent to the assembly of protocontinent Fennoscandia; (2) what was the amount of melt extracted; (3) what was the nature of the solid residue left behind after melt extraction; and (4) whether or not further heating of this residue may have contributed to the ca. 1 650- 1 540 Ma bimodal rapakivi granite magmatism in the region.

  • The Svecofennia tectonic province (Nironen, 2017) in southern and central Finland comprises an amalgamation of Paleoproterozoic crustal domains, assembled onto the Archean nucleus of the Fennoscandian shield by convergent processes by ~1.87 Ga (Fig. 1). Two major tectonic provinces, the western Finland subprovince and the southern Finland subprovince (Fig. 1) mark the bulk of the volume of the juvenile Svecofennian crust. In the southern Finland subprovince (or arc complex of southern Finland; cf. Korsman et al., 1997) calc-alkaline volcanic rocks, associated sedimentary sequences and synorogenic plutons are stitched together by two suites of granitoid rocks—the ~1.86-1.79 Ga late-orogenic leucogranites (Kurhila et al., 2011, 2010) and the ~1.65-1.54 Ga A-type rapakivi granites (Heinonen et al., 2017, and references therein; Rämö et al., 2014). The emplacement of the former followed the complex collisional events that had assembled protocontinent Fennoscandia by 1.87 Ga (Nironen, 2017) and was associated with transition from intraorogen extension at 1.84-1.83 Ga to late-Svecofennian collision at 1.82-1.80 Ga. The latter represented within-plate A-type magmatism with a characteristically bimodal (mafic-felsic) magmatic association (e.g., Rämö and Haapala, 2005).

    Figure 1.  Overview map showing the main Paleoproterozoic lithodemic units of the Svecofennia tectonic province in southern and central Finland. Adopted and slightly modified from Luukas et al. (2017). CFGC. Central Finland granitoid complex; HämS. Häme migmatite suite; PimS. Pirkanmaa migmatite suite; LaS. Lapfors suite; VaC. Vaasa complex; TeS. Teuva suite; PiS. Pirttikylä suite. Blank areas south of the border (blue line) of the western Finland and southern Finland subprovinces mark Late Paleoproterozoic leucogranites, Late Paleoproterozoic to Mezoproterozoic rapakivi granites and Mesoproterozoic cratonic redbed sequences and associated dolerites. Inset shows map area relative to mainland Finland.

  • The late-orogenic leucogranites constitute the Svecofennian granite-migmatite zone (LSGM) of southern Finland (Ehlers et al., 1993). They are found as a discordant batholith-size intrusions (e.g., the Puruvesi pluton in the eastern part of the LSGM; Huhma, 1986), as small, relatively homogeneous bodies (a few km in diameter in general) with migmatic contacts with country rocks, and as pegmatitic segregations and flat-lying dike systems that cross-cut the earlier supracrustal and granitoid rocks (Fig. 2). In terms of volume, the leucogranite bodies are probably smaller than what may be suggested by the current exposure depicted in the geological map.

    Figure 2.  Lithological map of southern Finland, showing the distribution of the late-orogenic Svecofennian leucogranites and Mid-Proterozoic rapakivi granites amongst the synorogenic lithologic units of the Svecofennia tectonic province (cf. Fig. 1). Black lines denote major deformation zones, white blank areas denote lakes. Inset shows map area relative to mainland Finland. Modified from Fig. 1 in Kurhila et al. (2010).

    Most of the southern Finland leucogranites have SiO2 between 70 wt.% and 75 wt.%, clustering around the liquidus minimum in the Ab-Or-Qz system (Fig. 3a). They have K2O up to 6 wt.% and are moderately peraluminous, with aluminium saturation index (ASI) around 1.1 and normative corundum between 1.0% and 2.5%. Most of the leucogranite intrusions of the zone have crystallization ages (U-Pb on zircon and monazite) around 1.83 Ga, but the overall age range is surprisingly wide, from ~1.86 to ~1.79 Ga (Kurhila et al., 2011). Zircon in the leucogranites shows a large range of Hf isotopic variation, with εHf(t) from ca. -18 to +4, suggesting that a range of metaigneous and metasedimentary, Svecofennian rocks have been involved in their petrogenesis, the latter containing a component of Archaean detritus (Kurhila et al., 2010).

    Figure 3.  Compositions of leucogranites and rapakivi granites. Leucogranites are shown in blue, rapakivi granites in red. Source of data: Rock Geochemical Data of Finland, GTK 2020 (Geological Survey of Finland, 2020). (a) Normative Ab-Or-Qz; (b) wt.% SiO2 vs. ASI; (c) normative Ab-Or-An; (d) wt.% SiO2 vs. wt.% K2O. Qz. Quartz; Ab. albite; Or. orthoclase; An. anorthite.

  • The locus classicus rapakivi granites of southern Finland were emplaced at least 150-250 Ma later than the leucogranites, within the upper part of the denudated Paleoproterozoic orogenic crust. They are associated with contemporaneous mafic rocks (basaltic lavas and dikes, gabbroic rocks including anorthosite) and mark an anorogenic tectonic event with relatively extensive melting in the upper mantle and lower continental crust. The Wiborg batholith in southeastern Finland and adjacent Russia is the oldest center of rapakivi magmatism (1 644-1 627 Ma; Heinonen et al., 2017; see also Rämö and Mänttäri, 2015), rapakivi intrusions farther in the west (Fig. 2) and east (Lake Ladoga region, Russian Karelia) are younger (1 590- 1 530 Ma). Zircon in the rapakivi granites shows a restricted εHf(t) variation, from -4 to +4, suggesting an origin from relatively juvenile, Paleoproterozoic source rocks, whereas the associated mafic rocks approach a depleted mantle signature at εHf(t) of ca. +9 (Heinonen et al., 2010).

    The rapakivi granites have a larger overall spread in SiO2 (65 wt.% to 80 wt.%) (Fig. 3) and K2O generally between 4 wt.% and 7 wt.% (Fig. 3d). They range from peraluminous (ASI ~1.1) to distinctly metaluminous (ASI < 1, Fig. 3b). The larger range of variation is also seen in normative Ab-Or-Qz and Ab-Or-An (Figs. 3a, 3c).

  • Rhyolite-MELTS (Ghiorso and Gualda, 2015; Gualda et al., 2012) is an adaption of melts (Ghiorso and Sack, 1995) with an improved calibration for quartz and K-feldspar that allows solid-melt equilibria in felsic systems at pressures up to 20 kbar to be modeled. Rhyolite-MELTS uses a Gibbs free energy minimization algorithm to estimate the amount and compositions of silicate melt and solid minerals in equilibrium in a given bulk composition as a function of temperature, pressure and oxygen fugacity. The underlying, internally consistent thermodynamical database incorporates data for minerals from Berman (1988) extended by solid solution models for minerals and data from experiments in natural silicate melts ranging from ultrabasic to silicic bulk compositions.

    Calculations have been performed using the pre-compiled Ubuntu Linux version of Rhyolite-MELTS release 1.0.2 (http://melts.ofm-research.org/unix.html) running in the Windows Subsystem for Linux on Windows 10 PC systems. Output ASCII files have been merged using the MELTS_Excel- Combine_tbl.xlsm template (Gualda and Ghiorso, 2015, file downloaded from https://magmasource.caltech.edu/alphamelts/links.php#combine_tbl). Normative mineralogy was calculated using GCDkit4.0 (Janoušek et al., 2006; http://www.gcdkit.org/) and results were plotted using graphical functions in Matlab.

    The compositions used as starting-points for the melting simulations span a comprehensive set of conceivable crustal lithologic units (Table 1): average felsic, intermediate and mafic metaigneous rocks (F, I and M, respectively), and metasedimentary rocks (pelite, P, and arkosite, A) from the Svecofennian belt of southern Finland, and combinations of these. The endmember compositions have been estimated from data in the rock chemistry database of the Geological Survey of Finland (Rock Geochemical Data of Finland, GTK 2020, Geological Survey of Finland, 2020). The compositions in Table 1 and models presented in subsequent figures are identified by the proportions (in fractions of ten) of the F, I, M, A and P starting composition.

    Initial compositions (wt.%)
    A10 P10 M10 I10 F10 F6I2M2 F8A2 F8P2 F6I2A1P1
    SiO2 73.1 63.0 51.6 62.5 70.4 65.1 70.9 68.9 68.4
    TiO2 0.4 0.8 1.2 0.8 0.4 0.6 0.4 0.5 0.5
    Al2O3 12.7 16.8 15.8 13.2 13.8 14.1 13.6 14.4 13.9
    Fe2O3 3.0 7.4 10.9 8.5 4.3 6.4 4.0 4.9 5.3
    MnO 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.0 0.1
    MgO 0.5 2.9 5.6 3.7 1.3 2.7 1.2 1.7 1.9
    CaO 1.2 1.5 9.8 4.2 2.6 4.3 2.4 2.4 2.7
    Na2O 2.7 2.5 2.9 3.4 4.3 3.8 4.0 3.9 3.7
    K2O 5.4 3.9 0.9 2.5 1.7 1.7 2.4 2.1 2.4
    P2O5 0.1 0.1 0.2 0.2 0.1 0.2 0.1 0.1 0.1
    H2O 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
    Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 99.9 100.0
    Residues (R) after melt-extraction at 6 kbar, 820 ℃ (wt.%)
    A10R P10R I10R F10R F6I2MR F8A2R F8P2R F6I2A1P1R
    SiO2 74.0 61.1 60.8 70.7 63.9 71.2 68.9 68.0
    TiO2 0.4 0.9 0.9 0.4 0.7 0.4 0.5 0.5
    Al2O3 12.5 16.5 13.2 13.9 14.2 13.7 14.1 13.9
    Fe2O3 3.5 9.9 11.0 5.1 7.9 4.8 6.1 6.7
    MnO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
    MgO 0.6 3.9 4.0 1.7 3.4 1.5 2.2 2.5
    CaO 1.0 1.7 4.8 2.8 4.9 2.6 2.7 2.9
    Na2O 2.7 2.9 3.8 4.6 4.2 4.4 4.5 4.2
    K2O 5.3 3.1 1.4 0.8 0.7 1.4 1.0 1.3
    P2O5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
    H2O 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
    Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0
    Key to the compositions: A10. average arkosite; P10. average pelite; M10. average mafic metavolcanic rock; I10. average intermediate volcanic rock; F10. average felsic metavolcanic rock; F6I2M2. 6 : 2 : 2, felsic : intermediate : mafic; F8A2. 8 : 2, felsic : arkosite; F8P2. 8 : 2, felsic : pelite; F6I2A1P1. 6 : 2 : 1 : 1, felsic : intermediate : arkosite : pelite.

    Table 1.  Source rock and unmelted residue compositions used in modeling (source rocks: Rock Geochemical Data of Finland, GTK 2020, Geological Survey of Finland, 2020)

    Modeling has been done over ranges of pressures (5-8 kbar) and temperatures (> 700 ℃) that are considered relevant for the tectonothermal evolution of the Svecofennian domain in southern Finland (Hölttä and Heilimo, 2017, and references therein), at oxygen fugacity corresponding to the NNO buffer. In order to avoid problems with algorithm instability at and below the solidus, models have been calculated "from top to bottom", i.e., cooling a total composition under equilibrium conditions from a starting temperature above liquidus (1 300 ℃) to the required temperatures within the partial melting range without removal of melt or solids.

    Files with the full set of results from the simulations can be found in ESMs 1 and 2.

  • The compositions of partial melts obtained from model calculations on all but one of the bulk compositions given in Table 1 approach the observed range of SiO2 in the leucogranites only at temperatures above 800 ℃, which corresponds to melt-yields of 20% or more (Fig. 4). The exception is the mafic protolith (M10), which does not yield melt with sufficient SiO2 under any of the conditions considered here (Fig. 4c). The yield depends on starting composition (Fig. 4a). The most melt-fertile source-rocks are the metasedimentary rocks (A10: pure arkosite, P10: pure pelite), followed by slightly less productive felsic metavolcanic rocks (F10) and mixtures of felsic metavolcanic rocks with intermediate metavolcanic and metasedimentary endmembers (F8A2, an 8 : 2 mixture of felsic and arkosite endmembers, F8P2, an 8 : 2 mixture of felsic and pelite endmembers and F6I2P1A1, which is a 6 : 2 : 1 : 1 mixture of felsic metavolcanites, intermediate metavolcanics, arkosite and pelite). Melt productivity decreases moderately with increasing pressure for all of these compositions, one of which is shown as an example in Fig. 4b.

    Figure 4.  Melt yields over the whole simulation range ~700 to 1 300 ℃. (a) Different compositions as function of temperature at a constant pressure of 6 kbar; (b) melt yield from one composition (F6I2A1P1) shows as a function of T at P=5, 6, 7 and 8 kbar; (c) SiO2 content of melts formed from the protolith compositions in Table 1 at 6 kbar as a function of temperature. Abbreviations as in Table 1.

    The melts produced at 800-850 ℃ (melt and residual compositions at 6 kbar, 820 ℃ are shown as examples in Fig. 5) are generally granitic, with 68 wt.%-72 wt.% SiO2 and 5 wt.% to 6 wt.% K2O (Figs. 5a, 5c) and with normative quartz and feldspar proportions overlapping with that of the late Svecofennian leucogranites (Figs. 5a, 5b). The K2O contents of the leucogranite samples are reasonably well reproduced by all of the source-rock compositions. All the partial melts produced are peraluminous, with ASI (aluminium saturation index) ≤1.2 for all but the pelite endmember (P10), which yields liquids with ASI≈1.8, and the F8P2 mixture at ASI=1.5, both of which are significantly more peraluminous than the leucogranites (Fig. 5c).

    Figure 5.  Model results compared to the range of leucogranite compositions (shaded background). In all of the diagrams, bulk compositions (source rock) are given by filled squares, melts at 6 kbar, 820 ℃ by circles and residues (in (c) and (d) only) by diamonds. Color coding is consistent as indicated in (d). For labels, see Table 1.

    The modal compositions of the residues coexisting with melt correspond to dry, granulite-facies rocks, all of which contain plagioclase and orthopyroxene as major minerals, and all but that of the mafic metavolcanic protolith contains quartz (illustrated at 6 kbar, 820 ℃ in Fig. 6). The modal quartz content amounts to 26% in the arkosite residual at 6 kbar, 820 ℃, and less in the others. Clinopyroxene is found in the mafic and intermediate metavolcanic systems and in the mixture involving the mafic component (F6I2M2). Garnet is present in the mafic and metasedimentary systems, and potassium feldspar in the metasedimentary (P10, A10) and intermediate (I10) systems, and in mixtures involving these (F6I2A1P1, F8P2, F8A2). None of the solid assemblages contain muscovite or biotite, and apatite is only present in small amounts in the residuals from melting of mafic and intermediate metavolcanic protoliths. With the exception of the residual in the arkosite system (A10), which has a charnockitic modal composition, the solid residuals shown in Fig. 6 are classified as enderbites or charnoenderbites.

    Figure 6.  Phase abundances in weight percent at 6 kbar, 820 ℃. Minor amounts of modal apatite are only present in bulk compositions M10 and I10.

    The solid residues are dry and depleted in the melt-forming components, most notably in K2O and to some extent also SiO2 (Fig. 5d). The only exception to this pattern is the arkosite protolith (A10), which is itself close to granitic in composition so that withdrawal of 20% of granitic melt does not lead to any major shift of residual composition away from the starting composition (Fig. 5). The degree of K2O depletion increases with temperature and thus with the degree of melting (Fig. 7). In terms of normative feldspar components, this amounts to a shift away from Or towards the Ab-An join (Fig. 5b) and away from the minimum liquidus temperature area in the Ab-Or-Qz diagram (Fig. 5a). The amount of K2O depletion is strongest in the felsic and mafic metavolcanic starting compositions (≥50%), less so for the intermediate felsic metavolcanite and F6I2A1P1 mixture (30% to 50%), moderate (10%-20%) for the metapelite, and negligible for the metaarkosite, which shows a minor increase in K2O in residues at < 800 ℃ (Fig. 7).

    Figure 7.  Potassium depletion (as wt.% K2O in residue/wt.% K2O in total system. All residues are progressively depleted in potassium, except for the metaarkosite (A10) which shows a small increase in K2O below ca. 800 ℃ (i.e., the melt is less potassic than the residue in this temperature interval). The grey bar at 820 ℃ is shown to connect to the panels in Fig. 5.

    Because the residues are anhydrous, they will only yield a second batch of melt when heated to the relevant dry solidus. Rhyolite-MELTS modeling suggests that extractable amounts of second-generation melt are produced at ca. 950 ℃ from the arkosite residue, and at ca. 1 000 ℃ from the pelite residue initially melted at 6 kbar and 820 ℃. The residues from the other starting compositions considered, temperatures of 1 050-1 100 ℃ and higher are required (Fig. 8). Initial low-fraction, second- generation partial melts will be roughly granitic in terms of major element compositions, but will move rapidly towards granodioritic to tonalitic compositions with increasing temperature and melt fraction (Fig. 9). All except the restites in the intermediate metavolcanic system (I10) and the F6I2M2 mixture will yield peraluminous second-generation melts (ASI > 1.2) that are also low in K2O (Fig. 9).

    Figure 8.  Melt yields on further heating of melt-depleted residues formed at 6 kbar, 820 ℃ (assuming full separation of melt). All systems are anhydrous, but 100 ppm water was added to the metapelitic system (P10) at T < 1 020 ℃ to prevent software from malfunctioning (broken part of the curve).

    Figure 9.  Second generation melts obtained by further heating of the residues of the first melt separation at 820 ℃, 6 kbar, compared to rapakivi granite compositions shown as grey background (panels and color coding as in Fig. 5). Diamonds represent bulk composition of the residues, the evolution of melts are shown as lines terminating (at point closest to bulk composition) at 1 100 ℃, calculated at 10° intervals (indicated by tick marks). The starting temperature on the curves indicated on the curves differ among the bulk compositions, and reflect the temperature where a non-zero melt production is first seen (cf. Fig. 8), i.e., 900 ℃ for A10R, 1 070 ℃ for F10R. Note that A10 is completely molten above 990 ℃. Color coding as in Fig. 5.

  • The numerical experiments done in this study show that it is possible to generate granitic magma with a major element composition appropriate for the late Svecofennian leucogranites of southern Finland by partial melting of metasupracrustal protoliths in the middle to lower crust at PT conditions that prevailed in the southern part of the Svecofennia tectonic province after the main collisional event (amalgamation of protocontinent Fennsocandia). Overall, the evolution of the Svecofennia tectonic province probably involved several orogenic stages, including microcontinent accretion, continental collision events with intervening extension and, finally, orogenic collapse before stabilization (Nironen, 2017; Lahtinen et al., 2009, 2005). The late Svecofennian period was initiated by 1.84 Ga in southern Finland by crustal extension and associated mafic to felsic magmatism (Nironen, 2017, 2005; Pajunen et al., 2008) and is characterized by high-T metamorphism and crustal anatexis and resultant leucogranite magmatism. Mantle-driven thermal perturbations and decompression associated with orgenic collapse presumably were the dominant driving forces for the late Svecofennian crustal anatexis.

    In a high-T, P anatectic scenario, the melting protolith seeks to minimize total interfacial energy between solid residue and melt by a process in which the generated melt penetrates melt-crystal boundaries forming a wetting angle, θ, the value of which is governed by the balance of interfacial energies (Jurewicz and Watson, 1985; Bulau et al., 1979). If 0° < θ < 60°, the forming melt eventually penetrates all grain edges and corners and forms an interconnected, stable network throughout the melting protolith (e.g., McKenzie, 1984). For granites, θ has been demonstrated to be on the order of ~45°-60° (e.g., Jurewicz and Watson, 1985). At this wetting angle range, the contiguity (extent of solid-solid contact) of a melting protolith has to decrease to 0.15-0.2 to make the system to reach the RCMP value. In an essentially static anatectic environment, this is reached at melt fractions of > 0.5 (Miller et al., 1988).

    The 20%-30% partial melting suggested by our modeling results for the generation of the Svecofennian leucogranites is well below the RCMP value of the static case. However, at high strain rates (a most likely scenario for the emplacement of the Svecofennian leucogranites; see Skyttä and Mänttäri, 2008), the RCMP may be reduced by 50% relative to the static case (see Fig. 4 in Miller et al., 1988). Hence the 20%-30% might be quite realistic. It is also well within the range inferred for other anatectic systems (e.g., Milord et al., 2001) and from experiments (Vielzeuf and Holloway, 1988). Segregation of such a fraction of melt from the source rock may require 1 Ma or more, which is sufficient to allow extensive equilibration with restite (Rabinowics and Vigneresse, 2004; Gerdes, 2001). A batch melting scenario is therefore more appropriate for formation of the leucogranite magmas than one involving extraction of small melt batches and disequilibrium between melts and solid residue.

    Spear et al. (1999) modeled partial melting of pelitic rocks in terms of a succession of low-variance melting reactions in a simplified system (NaKFMASH). The total melt productivity during the process could be described as a combination of discrete (i.e., essentially isothermal) and continuous melting reactions, resulting in a stepwise melt productivity curve at increasing temperature (Fig. 6 in Spear et al., 1999). This is similar to the approach to leucogranite petrogenesis of Kukkonen and Lauri(2016, 2009).

    When considering melting reactions independently of each other, the tendency of the whole-rock system to approach equilibrium is not taken into account. To illustrate this effect, equilibrium phase proportions in part of a Rhyolite-MELTS experiment on the pelitic composition (P10, with 3 wt.% water at 8 kbar) are shown in Fig. 10. Conditions and strarting compositon have been chosen to maximize stability of biotite and muscovite, for the sake of illustration. In this system, the abundance of muscovite decreases continuously with increasing temperature and melt fraction from 600 to 740 ℃, and shows an increasing rate of consumption above 700 ℃. The biotite abundance decreases gradually and reaches zero at 820 ℃. Although the temperatures at which muscovite and biotite finally disappear may be seen as the respective "melting" temperatures, the two minerals have contributed to the melt over the whole temperature range from solidus (< 600 ℃ in this example) to their final disappearance.

    Figure 10.  Phase proportion in the low-temperature part of a numeric melting experiment of the P10 metapelite composition (Table 1) with 3 wt.% water at 8 kbar. Heavy curves give phase abundance of muscovite (Mu), biotite (Bi) and liquid (Liq) as function of temperature. Thin curves are other solid phases present in the restite. Ilm. Ilmenite; Afs. alkali feldspar; Leu. leucite; Qtz. quartz; Plg. plagioclase; Grt. garnet; Opx. orthopyroxene.

    At pressures between 5 and 10 kbar, ca. 20% melting is achieved in the temperature range from 800 to 850 ℃. This is comparable, yet a bit on the low side, of the amount of melt produced by isobaric heating and resultant dehydration of a 1 : 1 muscovite-biotite assemblage in the NaKFMASH system assuming no added external water, presence of vapour phase at solidus and incorporation of all water released by dehydration reactions into the forming melt (Spear et al., 1999). Melt-yields in the order of 20 wt.%-25 wt.% suggest that the residue after melt extraction amounts to 3.8 to 4.7 times the volume of granite produced. Residues after melt extraction in the deep to middle crust are dry granulite-facies rocks depleted in water, K2O, and by inference also in incompatible trace elements, including the heath- producing elements U and Th. The results illustrate the fact that a dehydration melting process affects the whole phase assemblage of the rock undergoing melting, and cannot be seen as a selective breakdown process affecting only the water-bearing phases of the protolith (cf. Vielzeuf and Holloway, 1988).

    Kukkonen and Lauri(2016, 2009) proposed that a second episode of melting in the restitic crust of the Svecofennia tectonic province in southern Finland was responsible for the generation of the 1 650-1 540 Ma rapakivi granites of southern Finland. The melt productivity of the residual rocks left behind after leucogranite melt extraction is limited by their dehydrated and depleted mineralogy (Fig. 6), with heat productivity strongly reduced because of the reduced concentrations of the incompatible elements U and Th. Although a second generation of anatectic melt may be produced by continued heating after first melt extraction, temperatures above 950 ℃ are needed for a metaarkositic restite source, the other residues would require temperatures in the range above 1 000-1 050 ℃ and higher to yield extractable amounts of melt (Fig. 8). Small fractions of granitic magma of appropriate compositions can indeed be generated from some of the residual compositions studied at T≥1 050 ℃. On increasing degree of melting, however, the K2O of the melt would decrease and the normative feldspar components of the melt develop towards granodioritic, tonalitic and quartz dioritic compositions by the time 20%-25% of melt has formed. Residues after the first melting of the two metasedimentary protoliths (A10, P10) are able to yield larger quantities of anatectic magma overlapping with the range of rapakivi granites in terms of normative feldspar components, but these melts would be strongly peraluminous, with significantly higher ASI than observed in any of the rapakivi plutons (Fig. 9), i.e. magmas more akin to S-type than A-type granite.

    Estimates of intrusive temperatures of the Fennoscandian rapakivi granites vary from < 700 ℃ to ca. 890 ℃ (Heinonen et al., 2017; Ehrlich et al., 2012; Eklund and Shebanov, 1999, and references therein). Similar A-type granitic magmas elsewhere have estimated temperatures from ca. 790 to 900 ℃, and up to 1 000 ℃ for the more mafic members of the association (Creaser and White, 1991; Anderson, 1983, 1980). A maximum temperature of 900 ℃ is thus a realistic estimate for the granitic magma compositions considered here. The ascent of an anatectic magma through the crust is a buoyancy-driven process that is near- adiabatic in nature. The dT/dP gradient of adiabatic decom- pression paths of a magma depends on the composition and phase proportion (Rumble, 1976). Nekvasil (1991) estimated that dT/dP < 10 ℃/kbar is required to preserve the typical feldspar textures of rapakivi granites during ascent of the magma. Eklund and Shebanov (1999) suggested a quasi-isothermal path for magmas of the rapakivi plutons in Finland. Assuming a maximum temperature of ~900 ℃ at the level of emplacement (~2 kbar pressure), the corresponding maximum temperature of the magma at 6-8 kbar would be < 960 ℃, i.e., short of what is required to generate extractible fractions of melts of realistic composition (i.e., mildly peraluminous to metaluminous syeno-monzogranite melts) from the residual compositions considered here (Fig. 8). The residue after the arkosite would generate melt at such temperature, but with impermissibly high ASI.

    Our findings thus support a model of rapakivi petrogenesis in which local radioactive heating provides a less significant contribution to the heat budget than do mafic, mantle-derived magmas emplaced in the middle to lower crust. Our preferred interpretation is a scenario of two separate melting events, first a late Svecofennian anatectic event related to orogenic collapse, during which melt-fertile components, heat and incompatible elements were removed from the middle to lower crust by lecuogranitic magmas. In a later, and not directly related process, lithospheric extension caused partial melting in the mantle, introduction of mafic magmas into the lower crust followed by partial melting of crustal rocks that were less affected by the leucogranite melting event.

    Our model is compatible with the well-established understanding of rapakivi granite petrogenesis in Fennoscandia (Rämö and Haapala, 2005, and references therein). The different, thought-provoking scenario proposed by Kukkonen and Lauri(2016, 2009) is far less realistic, at least as a major petrogenetic process for the voluminous primary melts of the locus classicus rapakivi granites.

  • Thermodynamic modeling using Rhyolite-MELTS software shows that anatectic melts approximating the composition of the 1 860-1 790 Ma leucogranites in the Svecofennia tectonic province in southern Finland can be produced by 20%-30% melting of different mixtures of Paleoproterozoic metasupracrustal rocks of the region at 800-850 ℃, 5-8 kbar. Metaigneous rocks are more important source components than metasedimentary rocks. Melt formation is best approximated by a batch melting process in which melt and minerals of the solid restite approach equilibrium. It is not a disequilibrium, fractional melting process in which water-bearing minerals in the protolith are selectively removed without equilibrating with the bulk solid residue.

    The residue is a dry, melt-depleted granulite-facies rock and will not be able to produce further anatectic melt of syenogranitic to monzogranitic, mildly peraluminous to metaluminous composition unless heated above 1 000-1 050 ℃. Moreover, in this case extractable melt fractions will be granodioritic or tonalitic, not granitic. Partial melting of such residues thus did not significantly contribute to the 1 650-1 540 Ma rapakivi granites of southern Finland. The latter is the results of mixing of mantle- derived magma and anatectic melts from source rocks that were unaffected by the earlier partial melting event in the region.

  • We thank Maria de Fatima Bitencourt and Valdecir de Assis Janasi for the invitation to submit this paper to this volume. Two anonymous reviewers gave helpful comments to the first draft of the manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1385-8.

    Electronic Supplementary Materials: Supplementary materials containing results of the individual model calculations (ESM1 and ESM2) are available in the online version of this article at https://doi.org/10.1007/s12583-020-1385-8.

Reference (59)
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