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Precambrian Ophiolites and Related Rocks Edited by Martin J. van Kranendonk, R. Hugh Smithies and Vickie C. Bennett Developments in Precambrian Geology, Vol. 15 (K.C. Condie, Series Editor) © 2007 Elsevier B.V. All rights reserved. DOI: 10.1016/S0166-2635(07)15056-8 1
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Chapter 5.6
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TTG PLUTONS OF THE BARBERTON GRANITOID-GREENSTONE TERRAIN, SOUTH AFRICA JEAN-FRANÇOIS MOYEN, GARY STEVENS, ALEXANDER F.M. KISTERS AND RICHARD W. BELCHER* Department of Geology, Geography and Environmental Science, University of Stellenbosch, Private bag X 01, Matieland 7602, South Africa
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5.6-1. INTRODUCTION Plutonic rocks constitute a large part of Archean terranes and occur mostly in the form of variably deformed orthogneisses. The most common plutonic rocks are a suite of sodic and plagioclase-rich igneous rocks made of tonalites, trondhjemites and granodiorites, collectively referred to as the “TTG” suite. A large body of geochemical and experimental data exists for TTGs, and these studies have led to the general conclusion that TTGs are essentially melts generated by partial melting of mafic rocks, mostly amphibolites (as the dominant melting reaction involves hornblende breakdown) within the garnet stability field. However, the geodynamic setting for the origin of TTGs is still debated and contrasting interpretations are proposed, the most common being melting of the down-going slab in a ‘hot’ subduction zone setting (e.g., Arth and Hanson, 1975; Moorbath, 1975; Barker and Arth, 1976; Barker, 1979; Condie, 1981; Jahn et al., 1981; Condie, 1986; Martin, 1986, 1994, 1999; Rapp et al., 1991; Rapp and Watson, 1995; Foley et al., 2002; Martin et al., 2005), and melting of the lower part of a thick, mafic crust in an intra-plate settings (e.g., Maaløe, 1982; Kay and Kay, 1991; Collins et al., 1998; Zegers and Van Keken, 2001; Van Kranendonk et al., 2004; Bédard, 2006). In many cases, TTGs are the oldest component of Archean cratons. They generally appear as polyphase deformed gneissic complexes, commonly referred to as “grey gneisses”, which display variable degrees of migmatization. In such units, high finite strains and the tectonic transposition of different TTG phases obscuring original igneous contacts, renders the recognition of original protoliths difficult and detailed geochemical studies on individual magmatic intrusions are not possible. However, in the Barberton granitoidgreenstone terrain (BGGT)1 , many of the TTGs are characterized by weak fabrics and low * Present address: Council for Geoscience, Limpopo Unit, P.O. Box 620, Polokwane 0700, South Africa
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1 In this paper, we use “Barberton Granitoid-Greenstone Terrain” (BGGT) as an encompassing term to refer to
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the whole area of Archean outcrops (plutons and supracrustals), as opposed to the “Barberton belt” stricto sensu, that refers only to the supracrustal association.
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strain intensities, therefore allowing detailed study of their intrusive relationships, original compositions and comprehensive petrogenesis. TTGs from the BGGT range in age from ca. 3.55 to 3.21 Ga and the relationship between the greenstone belt and the surrounding TTG “plutons” is complex. The apparent domal pattern of TTG gneisses in tectonic contact with the overlying supracrustal greenstone belt is actually an oversimplification. In fact, each of the “plutons” has its own, distinct emplacement and deformational history (summarized in Table 5.6-1), with some of the “plutons” corresponding to relatively simple magmatic intrusive bodies, whereas others are composite units with complex and protracted emplacement and structural histories and are not really “plutons” in the classical sense. Likewise, the TTGs also have distinct petrological and geochemical natures, and while they all broadly belong to the “TTG” group, are actually petrologically and geochemically complex. Such a diversity points to different petrogenetic histories related to different geodynamic settings. The TTGs of the BGGT can be divided in to at least two “sub-series”: (i) a “low-Sr”, commonly tonalitic sub-series; and (ii) a “high-Sr”, commonly trondhjemitic sub-series. In most Archean cratons, tonalites and trondhjemites are typically associated in highly strained grey gneiss complexes, which are tectonically interleaved on a mm- to dm-scale, to such a degree that it gives the impression that both lithologies reflect only minor differences in terms of petrogenetic processes. In contrast, in the BGGT tonalites and trondhjemites occur as distinct intrusive bodies with well-defined margins and intrusive contact relationships. This allows their petrogenetic evolution to be studied independently from one another. In this paper, we demonstrate that the tonalitic and trondhjemitic bodies reflect two fundamentally different magma types, with different origins and evolutions. We propose that the two distinct TTG “sub-series” of the BGGT could reflect the results of two geodynamic environments important in the formation of Archean TTG’s, namely formation at the base of a thickened crust, and derivation from a subducting slab.
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5.6-2. GEOLOGICAL SETTING
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Although supracrustal rocks (lavas The BGGT formed between ca. 3.51 and 3.11 and sediments) from the belt itself yield a relatively continuous spread of ages from 3559± 27 Ma (Byerly et al., 1996; Poujol et al., 2003) to 3164 ± 12 Ma (Armstrong et al., 1990; Poujol et al., 2003), the BGGT predominantly assembled during three or four discrete tectono-magmatic events (Poujol et al., 2003) at 3.55–3.49, 3.49–3.42, 3.255–3.225 and 3.105–3.07 Ga (see also Lowe and Byerly, this volume). The first two events (3.49–3.55 and 3.42–3.49 Ga) are well represented in the Ancient Gneiss Complex to the east (Kröner, this volume). However, in the BGGT proper, >3.42 Ga rocks are restricted to the high-grade “Stolzburg domain” (Fig. 5.6-1) (Kisters et 2 Ages indicated in millions of years (Ma) correspond to actual, measured ages with reference and error, while
dates given in billions of years (Ga) refer to generalized time intervals.
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Age (Ma)
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Leucocratic trondhjemite, medium to coarse grained. Intrusive (with intrusive breccias and dyke swarm) into the greenstone belt, contact deformed during the 3.2 Ga events The same
Vlaakplats granodiorite (intrusive in the Steynsdorp pluton) Elements of the Ngwane gneisses (in the ACG of Swaziland)
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The same
Tonalitic to trondhjemitic orthogneisses, interlayered with metasediments
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Table 5.6-1. Main field characteristic and ages of Barberton TTG plutons
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Tsawela gneisses (in the ACG)
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Tonalitic orthogneisses, forming a mappable, relatively homogeneous unit in the Ngwane gneisses
3) 3.23–3.21 Ga generation Kaap Valley pluton
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Dark, coarse-grained, amphibole bearing tonalite. Probably emplaced as a sub-concordant laccolith into the greenstone belt
Nelshoogte pluton
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Badplaas gneisses
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3290–3240 Ma (Kisters et al., 2006); Poujol (pers. comm.)
ACG = Ancient Gneiss Complex. See Kröner et al. (this volume). Surfaces are derived using GIS from the map of Anhaeusser (1981). a With inherited zircons dated at 3702 ± 1 Ma (Kröner et al., 1996). b Gneissic unit, intruded by younger plutons. Probably not only made of orthogneisses, contains some metasedimentary components (Hunter et al., 1978). See Kröner (this volume). c Interlayered with the Ngwane gneisses. Exact extension poorly known.
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Composite pluton, dominated by coarse grained, leucocratic trondhjemite Syn- to post tectonically emplaced as a laccolith into the greenstone belt Polyphased gneiss domain, emplaced during a ca. 50 Ma period, made of a variety of mutually intrusive, diversely deformed phases
Chapter 5.6: TTG Plutons of the Barberton Granitoid-Greenstone Terrain, South Africa
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Table 5.6-1. (Continued)
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Fig. 5.6-1. (On previous page.) Geological map of the southwestern part of the Barberton Greenstone Belt and surrounding TTG plutons (BGGT). Left: map modified after Anhaeusser et al. (1981). See text and Table 5.6-1 for comments and references. Top right: location map. Bottom right: Structural sketch indicating the position of the main terranes and structures. While the “Songimvelo block” of Lowe (1994) includes part of the Barberton Greenstone Belt, and the adjacent ca. 3.45 Ga plutons in the south, the latter are separated from the former by the Komatii fault, leading to the identification of a distinct “Stolzburg terrane” (Kisters et al., 2003; Kisters et al., 2004; Diener et al., 2005; Diener et al., 2006; Moyen et al., 2006) corresponding to the amphibolite-facies portion of the Songimvelo terrane. The main structure is the Inyoni–Inyoka fault system, separating the western (Kaap Valley block) from the eastern domain (Steynsdorp and Songimvelo blocks, including Stolzburg terrane). Note that the “Onverwacht Group” on both sides of the Inyoka fault actually corresponds to rocks with different stratigraphy and of contrasting ages: 3.3–3.25 Ga to the west, and 3.55–3.3 Ga in the east. Furthermore, the details of the stratigraphic sequences on both sides cannot be correlated, suggesting that the two parts of the belt evolved independently, prior to the accretion along the Inyoka fault (Viljoen and Viljoen, 1969a; Anhaeusser et al., 1981,1983; de Wit et al., 1992; de Ronde and de Wit, 1994; Lowe, 1994; Lowe and Byerly, 1999; Lowe et al., 1999; de Ronde and Kamo, 2000).
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al., 2003; Moyen et al., 2006; Stevens and Moyen, this volume), which corresponds to the high-grade, “lower” portions of both the “Steynsdorp and Songimvelo terranes3 ” (Lowe, 1994, 1999; Lowe and Byerly, 1999).
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The >3.5 Ga event is represented by the mafic and felsic volcanics of the Theespruit Formation (Lowe and Byerly, 1999, this volume, and references therein), which are coeval with the emplacement of the ca. 3.55–3.50 Ga Steynsdorp pluton (Kröner et al., 1996). Little information is available regarding the geological context of their formation. The 3.42–3.49 Ga event corresponds to the formation of the Komati, Hooggenoeg and Kromberg Formations of the Onverwacht Group (Lowe, 1999b; Lowe and Byerly, 1999, this volume, and references therein), which are mostly located in the lower-grade (upper plate of Kisters et al., 2003) portions of the Songimvelo and Steynsdorp terranes. These three formations are dominantly mafic to ultramafic lavas, with subordinate felsic volcanic rocks and cherts. At the contact between the Hooggenoeg and Kromberg Formations, the ca. 3.44–3.45 Ga “H6” unit (Kröner and Todt, 1988; Armstrong et al., 1990; Kröner et al., 1991a; Byerly et al., 1996) is nearly synchronous with the intrusion of TTG plutons in the Stolzburg domain (Theespruit, Stolzburg, and the minor plutons to the South defined by Anhaeusser et al., 1981). The H6 unit is a thin (few tens of meters) unit of dacitic lava flows and shallow intrusives (geochemically regarded as the extrusive equivalents of the 3 “Terrane” (or “block”) is used in this paper to describe a “fault-bounded geological entity with distinct
tectonostratigraphic, structural, geochronological and/or metamorphic characteristics from its neighbors (in the sense of Coney et al., 1980)” (Van Kranendonk et al., 1993), as opposed to “terrain”, which simply refers to a geographical region or area with no particular tectonic or genetic meaning.
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TTG plutons; de Wit et al., 1987) and clastic sediments and conglomerates. This suggests that some topography existed at that stage. The first, well constrained deformation event affecting the belt (D1 ) (Lowe et al., 1999) also occurred at about the same time and is interpreted to represent the development of an active margin (oceanic arc) (Lowe, 1999b; de Ronde and Kamo, 2000; Lowe and Byerly, this volume, and references therein) at ca. 3.45 Ga. Following the D1 event, the Mendon Formation was deposited in the Stolzburg domain (Songimvelo and Steynsdorp blocks) in the east (Lowe, 1999b), and the Weltvreden Formation in the western terranes, from ca. 3.42 to 3.25 Ga. Based on studies of the volcanic and sedimentary units, a period of quiescence (rift/intracontinental setting) is suggested (Lowe, 1999).
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5.6-2.2. Main Orogenic Stage at 3.25—3.21 Ga
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The main, “collision” stage (D2–5 ), occurred between 3.25 and 3.21 Ga. Evidence for an accretionary orogen is presented elsewhere (Lowe and Byerly, this volume; Stevens and Moyen, this volume), and is thus only briefly summarized here. D2 corresponds to the amalgamation of the various sub-terranes that make up the belt, with the major suture zone corresponding to the Inyoni–Inyoka fault system (Fig. 5.6-1). Despite the apparently continuous stratigraphy across the fault, the sequences on both sides cannot be correlated (Lowe, 1994, 1999; Lowe et al., 1999; Stevens and Moyen, this volume). The D2 event is shortly followed by deposition (syn D3 ) and deformation (D4 and D5 ) of the 0.5). They plot in the medium to high-K fields of a SiO2 -K2 O diagram (Fig. 5.6-3(c)) at ca. 70% SiO2 , with no clear trends, and are mostly granites (in an O’Connor diagram) (Fig. 5.6-4). In the R1 -R2 diagram (Fig. 5.69(b)), they clearly plot below (lower R2 values) the other rock types. They have low Y (mostly 0.4), the validity of the approximation becomes doubtful, and we simply calculate the high-F melts as a weighted average of a F = 0.4 melt and the source. This approximation is still questionable, but not that important, as F = 0.4 corresponds to melts with 62% SiO2 , which is less than most of the rocks studied here. Trace elements are calculated using an equilibrium melting equation, Kd values from Bédard (2005, 2006), and mineral proportions interpolated from experimental data (Moyen
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and Stevens, 2006). According to the conclusions above (Section 5.6-6.1 and Fig. 5.6-12), a relatively enriched source composition is used (Sr = 240 ppm and Y = 20 ppm, within the range of the compositions of the non-komatiitic basalts of the Onverwacht Group in GEOROC database).
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5.6-6.2.2. Variations in P-T space The single most important parameter controlling the geochemistry of melts from metabasites is the degree of melting: higher degrees of melting (corresponding to higher temperatures) correspond to more mafic melts. Assuming both are primary melts of similar sources, trondhjemites corresponds to melt fractions lower than ca. 20% (Moyen and Stevens, 2006), whereas tonalites reflect melt fractions up to 40–50%. Experimentally, melt fractions sufficiently high to generate a ∼65% SiO2 liquid (equivalent to the tonalites) are attained at ca. 1000 ◦ C, below 15 kbar, but require higher temperatures as pressure goes up (to ca. 1200 ◦ C at 30 kbar) (Moyen and Stevens, 2006). Likewise, CaO/Na2 O values between 0.5 and 1, typical of the tonalitic rocks, correspond to the same P-T range. In contrast, the high silica, low CaO/Na2 O trondhjemites are generated at temperatures below 1000 ◦ C. The depth of melting controls the nature of the solid phases (residuum) in equilibrium with the TTG melts. There is a potentially major difference between low to medium pressure assemblages (amphibole and plagioclase stable, with garnet present but not abundant, and Ti mostly accommodated in ilmenite), and high pressure (eclogitic) assemblages dominated by clinopyroxene and garnet, with rutile as the main titaniferous phase. To complicate further, even at sub-eclogitic pressures, amphibole and plagioclase are consumed by the melting reactions, such that high melt fractions will coexist with amphibole- and plagioclase-free restites that are mineralogically rutile-free eclogites (Moyen and Stevens, 2006). Experimentally, both amphibolitic (Winther and Newton, 1991; Sen and Dunn, 1994; Patiño-Douce and Beard, 1995; Rapp and Watson, 1995) and eclogitic (Skjerlie and PatiñoDouce, 2002; Rapp et al., 2003) residuum have been demonstrated to be in equilibrium with TTG liquids. This is unsurprising, since both an eclogitic (clinopyroxene + garnet) and an amphibolitic (amphibole + plagioclase) residuum have similar major elements compositions, except for Na2 O. Sodium is indeed less abundant in eclogitic assemblages, resulting in high-pressure melts that are typically more sodic than their low-pressure counterparts for a given melt fraction (Moyen and Stevens, 2006). But a more important effect is associated with the melt fraction formed. In P-T space, the melt abundance curves are positively sloped, such that at high pressures the same melt fraction is approached only at higher temperatures, as mentioned above. Combining both parameters allows the identification of low-pressure liquids (relatively high-melt fraction, sodium poor liquids: granodiorites and tonalites) and the high-pressure liquids (lower melt fraction, more leucocratic and more sodic liquids: trondhjemites). A major “dividing line” thus exists, separating tonalites (and granodiorites) from trondhjemites (Fig. 5.6-13). The same division is observed in Barberton TTGs, where the
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Fig. 5.6-13. Melt composition in PT space, from parameterization of experimental data (Moyen and Stevens, 2006); a “ThB” source (tholeiitic basalt) has been used. (a) Nature of the liquid formed (in O’Connor (1965) systematics) as a function of the P-T conditions of melting. The thick grey line represents 10, 30 and 50% melt (F value). Fine lines correspond to the solidus and to the mineral stability limits (plag: plagioclase, amp: amphibole, gt: garnet). The two arrows labeled low and high pressure melting graphically display two possible geotherm leading to the formation of trondhjemites in one case, and granodiorites to tonalites in the second case. (b) Major element composition of the melts. The lines correspond to iso-values of SiO2 contents and CaO/Na2 O ratios of the melts. The thick dotted line is the “tonalite-trondhjemite divide” (panel (a), see text). (c) Sr contents of the melts in P-T space. (d) Sr/Y values of the melts in the P-T space.
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“low-Sr” group plots in the tonalite and granodiorite field in O’Connor (1965) diagrams, while the high-Sr rocks are almost exclusively trondhjemitic. Trace elements provide slightly different information and are far more sensitive to the pressure of melting. Indeed, trace elements will be partitioned in markedly different ways in eclogitic (garnet-clinopyroxene-rutile) and amphibolitic (amphibole-plagioclaseilmenite) assemblages. In addition, the mode of each mineral also changes with pressure (garnet becomes more abundant at higher pressure). Even within the realm of amphibolitic or eclogitic residues, melt composition vary significantly as a function of depth (Moyen and Stevens, 2006). For the elements used here (La, Yb, Sr, Y), the main control is exerted by the abundance of high Kd phases; i.e., garnet (for Y and Yb) and plagioclase for Sr. Therefore, trace elements in this case mostly record “pressure” information, with low pressure melts coexisting with plagioclase but not garnet, and having low Sr but high Y and Yb contents,
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whereas at high pressures, Sr is released because of plagioclase breakdown, but Y and Yb are locked in the garnet. Collectively, low Yb and high Sr/Y melts are produced only at relatively high pressures (>15–20 kbar); below this threshold, higher Yb and lower Sr/Y values are observed. Fig. 5.6-14 summarizes the geochemical trends predicted by both lowand high-pressure melting. Combining these observations allows the clear discrimination of the two sub-series. High-Sr melts are only trondhjemitic, and they form at high pressure (to the left of the dividing line), plotting in the P-T space from 1000 ◦ C at 15 kbar and below to 1200 ◦ C at 30 kbar. The low-Sr group contains tonalites and granodiorites (in O’Connor’s terminology, even if they are trondhjemites on the basis of their field appearance and mineralogy) and forms on the high-temperature side of this divide, at pressures below 15–20 kbar. It is worth noting that both types denote very contrasting geothermal gradients. High-Sr TTGs formed at relatively low temperatures (probably around 1000 ◦ C), but high pressures (>15 kbar), corresponding to a 15–20 ◦ C/km apparent geotherm. In contrast, the low-Sr group formed at lower pressures (10–15 kbar) and comparable or higher temperatures, corresponding to a distinct geotherm of 30–35 ◦ C/km. The model used here is dependent on the exact parameters used (position of the mineral stability lines, source composition, etc.). A more detailed treatment of the different cases is presented elsewhere (Moyen and Stevens, 2006). Importantly, however, even if the actual values are dependent on the model parameter, the same logic and the same opposition (low P, low Sr/Y, high F melts vs. high P, low F, high Sr/Y melts) remains. Interestingly, all of the Barberton TTGs are high-Al (Barker and Arth, 1976), and correspond to the Pilbara high-Al group of Champion and Smithies (this volume). These authors proposed that the difference between low Al (and low Sr) and high-Al groups reflects the depth of melting and stability of plagioclase in the residuum. In this model, both sub-series form at pressures above the plagioclase stability field (Fig. 5.6-13), yet the geochemistry of the melts evolves with depth, allowing a distinction between the two sub-series described here.
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5.6-6.3. The Role of Fractional Crystallization Following Melting
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While fractionation has always been recognized as one possible process affecting TTG composition (e.g., Martin, 1987), it is generally regarded as a minor process that only marginally affects TTG composition. However, it has recently be suggested (Bédard, 2006) that it plays a far bigger role in shaping the trace element composition of Archean TTGs in general (and their high Sr/Y ratio in particular), and that equivalents of Barberton trondhjemites can be generated by fractional crystallization and differentiation of tonalites. The question is actually two-fold: (i) can fractional crystallization turn the low-Sr tonalites into low-Sr trondhjemites; and (ii) can fractional crystallization differentiate (low-Sr) tonalites into high-Sr tonalites? To investigate the potential effects of fractional crystallization, we modeled the differentiation of a ca. 65% SiO2 tonalite (Table 5.6-3), using three different mineral assemblages: amphibole + biotite (model 1; Bédard, 2006); plagioclase + amphibole (model 2;
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F:dpg15024.tex; VTEX/JOL p. 48
Chapter 5.6: TTG Plutons of the Barberton Granitoid-Greenstone Terrain, South Africa
dpg15 v.2007/05/23 Prn:1/06/2007; 14:54 aid: 15056 pii: S0166-2635(07)15056-8 docsubty: REV
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Fig. 5.6-14. Modelled melting and fractionation trends in binary or ternary diagrams. The field of low-Sr (tonalites and trondhjemites), and high-Sr (trondhjemites) are shown for comparison. Heavy arrows: melting trend from the solidus to ca. 1200 ◦ C. Grey: low pressure (13 kbar) melting; black: high-pressure (21 kbar) melting. Thin arrows: fractionation vectors; the length of the arrow corresponds to the biggest possible degree of fractionation (see text and Table 5.6-3). The dotted arrows correspond to models I and III, which do not fit the data. Note how the compositional spread of each individual rock unit is “shaped” by fractionation vectors (model II, Hornblende + plagioclase most likely), whereas their position in the diagrams is better explained by the melting trend.
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AI2 O3 13.5 8.7 20.9
FeO 15.5 9.1 25.1 50.0 0.2 16.7 100.0 0.1
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MgO 12.0 9.9 5.3
CaO 12.2 15.7 7.6
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KD Sr 0.389 0.032 0.019 0.0022 6.65 0.1 0.022 2.68 20 4.3 10.3 1.4
Na2 O 2.0 3.5 0.1 8.0 0.4
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Y 2.47 0.603 14.1 0.037 0.138 0.07 0.018 5.42 80
La
Yb 1.79 0.635 23.2 0.13 0.094 0.11 0.018 3.02 490 2.96 1.59 13
0.319 0.028 0.028 0.015 0.358 0.02 0.015 4.73 26.6 2.05 1005 12
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Source (undifferentiated liquid – low SiO2 tonalite at 64–66% SiO2 ) SiO2 65.14
Co
Al2 O3 15.57
FeO 2.9973
MgO 2.49
CaO 4.12
Na2 O 5.17
Sr 560.0
Y 13.5
Sr/Y 41.5
La 15.0
Yb 0.8
La/Yb 18.8
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F:dpg15024.tex; VTEX/JOL p. 49
Major elements composition SiO2 Amphibole 42.2 Clinopyroxene 52.2 Garnet 38.5 Ilmenite Plagioclase 60.7 Bioite 36.3 Magnetite Titanite 30.0 Zircon 32.0 Epidote 36.9 Allanite Apatite
dpg15 v.2007/05/23 Prn:1/06/2007; 14:54 aid: 15056 pii: S0166-2635(07)15056-8 docsubty: REV
Mineral compositions and KD ’s
5.6-6. Partial Melting of Amphibolites and Controls on the Melt Geochemistry
Table 5.6-3. Modelling fractional crystallization of a tonalite
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Model 1 (Bedárd, 2006 – Amphibole + Biotite) FeO
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Sr/Y 1.42
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2.32 1.58 0.74
