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Journal of Petrology Volume 41 Number 12 Pages 1653-1671 2000
© Oxford University Press 2000
The Archaean High-Mg Diorite Suite: Links to Tonalite–Trondhjemite–Granodiorite Magmatism and Implications for Early Archaean Crustal Growth
1GEOLOGICAL SURVEY OF WESTERN AUSTRALIA, 100 PLAIN STREET, EAST PERTH, WA 6004, AUSTRALIA
2AUSTRALIAN GEOLOGICAL SURVEY ORGANISATION, GPO BOX 378, CANBERRA, ACT 2601, AUSTRALIA
Received May 14, 1999; Revised typescript accepted April 17, 2000
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONThe 2·95 Ga Pilbara high-Mg diorite suite intrudes the central part of the Archaean granite–greenstone terrain of the Pilbara Craton, Western Australia, and shows many features typical of high-Mg diorite (sanukitoid) suites from other late Archaean terrains. Such suites form a minor component of Archaean felsic crust. They are typically emplaced in late- to post-kinematic settings, sometimes in association with felsic alkaline magmatism, and are either unaccompanied by, or post-date, tonalite–trondhjemite–granodiorite (TTG) magmatism, which comprises a much greater proportion of Archaean felsic crust. The TTG series comprises sodic, Sr-rich rocks with high La/Yb and Sr/Y ratios, thought to result from partial melting of eclogite facies basaltic crust. High-Mg diorite shares these characteristics but has significantly higher mg-number (
60), and Cr and Ni concentrations, suggesting a mantle source. Many compositional features of TTGs are also shared by Cenozoic felsic magmas called adakites. Adakites form by melting of a young, hot, subducting slab and provide an a priori reason to invoke a subduction origin for TTG. During ascent through the mantle wedge, adakite commonly assimilates, or is contaminated by, peridotite, and the resulting ‘wedge-modified adakite’ bears strong compositional similarity to Archaean high-Mg diorite. Nevertheless, the latter are not simply an Archaean analogue of ‘wedge-modified adakite’ (i.e. ‘wedge-modified TTG’) because their intrusion is post-tectonic and unaccompanied by TTG magmatism. The petrogenesis of the Pilbara high-Mg diorite suite requires remelting of a mantle source, extensively metasomatized by addition of about 40% TTG-like melt. However, although the generation of this metasomatized source appears to require a subduction environment, many Archaean TTG suites show no clear chemical evidence of having interacted with a mantle wedge, and on that basis are more likely to represent partial melts of basaltic lower crust rather than of subducted slab. High-Mg diorite suites appear to concentrate in the Late Archaean, suggesting that subduction may have become an important process only after 3·0 Ga. KEY WORDS: Archaean; high-Mg diorite; sanukitoid; TTG; adakite; crustal evolution; Pilbara Craton
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INTRODUCTION |
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The sodic and calcic granitoids that form a large proportion of many Archaean terrains can be subdivided into the tonalite–trondhjemite–granodiorite (TTG) series and the high-Mg diorite (or sanukitoid) suite. The TTG series represents, by far, the greater volume. These siliceous (SiO2
70 wt %) and aluminous (Al2O3 > 15%) rocks, with low Yb (<1 ppm), high La/Yb (generally >30), Na2O/K2O (>1) and Sr and Ba (both >500 ppm) (Barker & Arth, 1976
; Barker, 1979) (Table 1), are considered to be the result of partial melting of eclogite facies basaltic crust (e.g. Arth & Hanson, 1975; Barker & Arth, 1976; Barker, 1979; Tarney et al., 1979; Martin, 1986; Drummond & Defant, 1990; Rapp et al., 1991; Rapp, 1997). The major and trace element compositions of TTG are very similar to those of Cenozoic adakite (Table 1), which is thought to be restricted to zones where young, hot, oceanic crust is subducted and partially melted at pressures high enough to stabilize garnet ± amphibole (e.g. Defant & Drummond, 1990), and accordingly it has been suggested that TTG is the Archaean, subduction related, analogue of adakite (e.g. Drummond & Defant, 1990; Drummond et al., 1996; Martin, 1999).
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In contrast to TTG, rocks of the high-Mg diorite suite generally range in composition from diorite to granodiorite and are characterized by significantly higher mg-number [= Mg2+/(Mg2+ + FeTotal) x 100, with FeTotal as Fe2+], Cr, Ni and large-ion-lithophile element (LILE) contents (Table 1) (Shirey & Hanson, 1984; Stern et al., 1989). At about 60 wt % SiO2, high-Mg diorite has mg-number 60, Cr 200 ppm and Ni 100 ppm, requiring a source significantly more mafic than typical Archaean basaltic crust; that is, a mantle source. High-Mg diorite magmatism was first documented from the Superior Province of North America (Shirey & Hanson, 1984) and is generally regarded only as a minor, though widespread, component of that province (Beakhouse et al., 1999) and other Archaean terrains. Nevertheless, high-Mg diorite is the only Archaean felsic magma type that appears to be derived from the mantle, and an understanding of its petrogenesis, tectonic setting and possible relationship to the more voluminous TTG series may provide a better understanding of Archaean crustal evolution.
The major element geochemistry of high-Mg diorite resembles that of Miocene high-Mg andesite (sanukite) from the Setouchi volcanic belt of Japan (Shirey & Hanson, 1984) (Table 1). Tatsumi & Ishizaka (1982) suggested that sanukite results from the partial melting of hydrous mantle peridotite, and Shirey & Hanson (1984) argued for a similar origin for Archaean high-Mg diorite. However, high-Mg diorite has much higher La/Yb ratios and Sr and Ba concentrations than sanukite, and in these respects more closely resembles adakite that has been extensively contaminated by mantle peridotite (e.g. Kelemen, 1995; Yogodzinski et al., 1995; Rapp et al., 1999). Adakite melt rising off a subducting slab interacts with the mantle wedge, to produce rocks with significantly lower SiO2 and higher mg-number, Cr and Ni concentrations [e.g. Adak-type high-Mg andesite of Yogodzinski et al. (1995) or the high-Mg adakite of Rapp et al. (1999)] (Table 1). These Mg-rich rocks will be referred to here as ‘wedge-modified adakite’.
The close compositional similarities between high-Mg diorite and wedge-modified adakite suggest a similar origin (e.g. Yogodzinski et al., 1995; Drummond et al., 1996), through mantle contamination of a slab-derived melt. Intrusion of high-Mg diorite, however, is late- to post-kinematic (e.g. Shirey & Hanson, 1984; Stern et al., 1989; Evans & Hanson, 1997; Beakhouse et al., 1999), and is not associated with voluminous magmas commonly interpreted as slab derived, such as adakite or TTG. In this respect its tectonic association differs markedly from that of wedge-modified adakite.
Petrological and geochemical data are presented for a newly discovered, 2950 Ma, high-Mg diorite suite from the Archaean granite–greenstone terrain of the Pilbara Craton, Western Australia (Fig. 1). These rocks are post-kinematic, or anorogenic, intrusions into an ensialic setting and so their origin cannot be directly linked to contemporaneous subduction. They intrude the youngest supracrustal sequences of the central part of that terrain, and are associated with intrusion of alkaline magmas, but not with TTG.
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We develop a two-stage model similar to the one that Evans & Hanson (1997) proposed for the high-Mg diorites of the Superior Province. In our model, the Pilbara high-Mg diorite suite results from the remelting of a mantle source region, previously hybridized by the addition of significant amounts of a slab-derived melt. However, we also show that a subducted slab is not the most likely source of voluminous Archaean TTG suites. The compositional range for many TTG suites, and certainly most TTG from the Pilbara Craton, overlaps only the low mg-number, high SiO2 end of the spectrum of adakite and wedge-modified adakite, and as such, shows little, if any, evidence of having passed through a peridotitic mantle wedge. Melting of hydrated basaltic material at the base of thickened crust (e.g. Atherton & Petford, 1993; Petford & Atherton, 1996), rather than a subducting slab, may be a more appropriate model for most TTG. Because melt produced in this way has limited access to mantle peridotite, it cannot be the precursor to high-Mg diorite. The suggestion that subduction-derived TTG suites are rare may explain why high-Mg diorite suites are also rare, as modification of a mantle source by subduction-derived TTG appears to be a necessary precursor to high-Mg diorite magmatism. These apparent constraints are incorporated into two alternative tectonic scenarios for the evolution of the Archaean felsic crust.
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REGIONAL GEOLOGY |
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The Pilbara Craton lies in the northwest of Western Australia and contains well-exposed middle to late Archaean granite–greenstone terrains, overlain to the south by the little-deformed late Archaean to early Proterozoic volcano-sedimentary sequences of the Hamersley Supergroup (Fig. 1). The eastern part of the granite–greenstone terrain consists of large, ovoid, granitic complexes mantled by belts of tightly folded low-grade, volcano-sedimentary rocks (greenstones), that accumulated between
3600 and 2950 Ma. TTG magmatism appears to be predominantly confined to the older (>3440 Ma) portions of the eastern terrain (Champion & Smithies, 1998). In contrast, the western part of the granite–greenstone terrain is characterized by linear, northeast-trending structures and is not known to contain rocks older than 3270 Ma. In the western terrain, TTG magmatism occurred at 3260 Ma, 3100–3120 Ma and 2990 Ma.
The rocks of the Pilbara high-Mg diorite suite occur in the central part of the granite–greenstone terrain, where they were intruded into an extensive sequence of deformed turbidites that represent the remnants of the Mallina Basin (Fig. 1). The southeastern margin of this basin is an unconformity or is locally faulted, whereas the present-day margin to the northwest is a fault. The basin is a last stage in the geological evolution of the Pilbara granite–greenstone terrain. Recent geochronological studies (Nelson, 2000) confirm that basin formation occurred after 3000 Ma (e.g. Smithies et al., 1999). The bulk of the turbidites were folded by 2950 Ma, at which time they were intruded by the Pilbara high-Mg diorite suite, and by alkaline rocks of the Portree Granitoid Complex (Nelson, 1997, 1999). The rocks of both the high-Mg diorite suite and of the Portree Granitoid Complex are confined to the Mallina Basin, and the intrusions that form the high-Mg diorite suite define a belt that extends for 150 km, parallel to the long axis of that basin (Fig. 1). In the east of the basin, high-Mg diorite was emplaced into zones of active dilation related to extension along major basin-parallel faults (R. H. Smithies, unpublished data, 1999). Thus, emplacement of the suite appears to have been controlled by extensional reactivation of structures related to the earlier evolution of the Mallina Basin.
Slightly younger (2940–2930 Ma: Nelson, 1997, 1998, 1999) K-rich monzogranite and syenogranite intruded during the second major folding event to affect the central part of the granite–greenstone terrain. This magmatism represents recycling of earlier crust (Champion & Smithies, 1998). Intrusion of these K-rich granites was concentrated along the southeastern margin of the Mallina Basin, and systematically decreased in both age and abundance further to the southeast.
Tectonic setting of the high-Mg diorite suite
The Mallina Basin has been interpreted as an 3000–2950 Ma intracratonic basin (Smithies et al., 1999). The Pilbara high-Mg diorite suite, and the sodic (Na > K) alkali granites of the Portree Granitoid Complex (Smithies & Champion, 1998), are clearly late- to post-kinematic with respect to the first regional deformation of the Mallina Basin and around 10 my older than the second regional deformation (Smithies, 1998). They also post-date voluminous regional felsic magmatism by between 40 and 60 my, including extensive felsic volcanism in the Whim Creek area, along the faulted northwestern margin of the Mallina Basin (Fig. 1), and pre-date voluminous K-rich felsic magmatism by 10–20 my. In summary, intrusion of the Pilbara high-Mg diorite suite
- occurs in an intracontinental setting, unassociated with contemporaneous subduction;
- occurs along major faults, reactivated under regional extension;
- is directly associated with felsic alkaline magmatism;
- is late- to post-kinematic with respect to regional deformation;
- post-dates known significant felsic magmatism by 40 my;
- precedes widespread potassic magmatism.
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AGE, FIELD RELATIONSHIPS AND PETROGRAPHY OF THE PILBARA HIGH-Mg DIORITE SUITE |
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The Pilbara high-Mg diorite suite ranges from diorite and monzodiorite to tonalite and granodiorite. Most of the intrusions in the suite are stocks of <
20 km2 (Fig. 1). The Peawah Granodiorite is the largest intrusion (Fig. 1), covering an area of 180 km2, and is a composite body that can be geochemically subdivided into the Peawah Granodiorite (east) and the Peawah Granodiorite (west). The Peawah Granodiorite (west) has been dated at 2948 ± 5 Ma (U/Pb SHRIMP; Nelson, 1996). The Wallareenya granodiorite and the Geemas stock (Fig. 1) have been dated at 2954 ± 4 Ma and 2945 ± 6 Ma, respectively (U/Pb SHRIMP; Nelson, 2000). These intrusions are coeval with the alkaline rocks of the Portree Granitoid Complex, which have been dated at 2946 ± 6 Ma (U/Pb SHRIMP; Nelson, 1999). Many of the intrusions are partially surrounded by earlier intrusions of gabbro. Most intrusions also contain abundant rounded enclaves, up to 30 cm in diameter. Most of these are cognate inclusions of diorite and gabbro. Some intrusions preserve a chilled margin of fine-grained melanodiorite, which also occurs in dykes and sills of 1–2 m thickness in country rock. These rapidly cooled rocks are likely to be the least affected by cumulate processes and hence may preserve compositions close to that of a parental magma.
Mesocratic granodiorite is the most common rock type in the suite and ranges in texture from equigranular to seriate to porphyritic, with plagioclase phenocrysts up to 1 cm long. Plagioclase forms a connected framework of euhedral crystals, many of which show well-developed compositional zoning from inner zones of An35 to sodic rims of An18. Small sericite- and calcite-altered cores suggest compositions more calcic than An35.
Hornblende is the dominant mafic mineral. Euhedral hornblende forms part of the early crystallizing plagioclase framework. Subhedral to euhedral hornblende forms an intergranular phase or occurs in allotriomorphic-textured mafic clots. Hornblende shows little compositional variation throughout the suite, with mg-numbers ranging between 56 and 66. In most granodiorites, hornblende contains cores of diopside (Wo45–47En40–41Fs13–14; mg-number 75), variably altered to actinolite. The abundance of these cores indicates the rocks were initially clinopyroxene rich. Biotite mantles hornblende, or forms an anhedral intergranular phase. It is significantly more Fe rich (mg-number 53) than coexisting hornblende. Quartz and minor microcline are intergranular phases. Accessory minerals include magnetite, sphene, apatite and zircon, which are concentrated in hornblende and biotite. Mafic clots, up to 1 cm in diameter, are locally abundant, and contain hornblende with lesser diopside, biotite, plagioclase and magnetite.
Fine-grained melanodiorite forms a chilled margin to some intrusions. It contains 2–4 mm long phenocrysts of euhedral plagioclase (An43 cores to rims of An21), diopside (Wo41En42Fs17; mg-number 72) and hornblende (mg-number=65) ± orthopyroxene, in a flow-aligned groundmass of plagioclase and hornblende with lesser biotite (mg-number=52), quartz and magnetite. Orthopyroxene occurs as discrete subhedral crystals, as granoblastic clots containing minor diopside and plagioclase, or as anhedral cores in diopside. It shows significant compositional variation, between Wo3En59Fs38 (mg-number = 61) and Wo3En69Fs28 (mg-number = 71), with bronzite cores (Wo2En76Fs22, mg-number = 77) in larger crystals. Analysis of coexisting clinopyroxene and orthopyroxene give equilibration temperatures of 1020°C using the pyroxene thermometer of Wells (1977).
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GEOCHEMISTRY OF THE PILBARA HIGH-Mg DIORITE SUITE |
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Analytical procedures
Rock samples were prepared for chemical analysis by jaw crushing 2–5 kg of sample and then grinding a 50–70 g sub-sample in a tungsten carbide ring mill. Abundances of major and trace elements were determined either at the Australian Geological Survey Organisation (AGSO) in Canberra [X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS)], or at the Geology Department, Australian National University (ANU) in Canberra (XRF). Major and minor elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P and S) were determined, at both AGSO and ANU, by wavelength-dispersive XRF on fused discs using methods similar to those of Norrish & Hutton (1969)
. Precision for these elements is better than ±1% of the reported values. Loss on ignition (LOI) was determined by gravimetry after combustion at 1100°C. FeO abundances were determined at AGSO by digestion and electrochemical titration using a modified technique based on Shapiro & Brannock (1962). At AGSO, As, Ba, Cr, Cu, Ni, Sc, V, Zn and Zr were determined by wavelength-dispersive XRF on a pressed pellet using methods similar to those described by Norrish & Chappell (1977). A similar technique was utilized for samples analysed at ANU, using wavelength- and energy-dispersive XRF. Selected trace elements (Cs, Ga, Nb, Pb, Rb, Sb, Sn, Sr, Ta, Th, U and Y) and the rare earth elements (REE) were analysed at AGSO by ICP-MS (Perkin Elmer ELAN 6000) using methods similar to those of Eggins et al. (1997). Interlaboratory comparisons exhibit good agreement for major elements, typically within 1–5%, but to 20% and greater for low abundance minor elements (e.g. Ti, Mn and P—with a small absolute error of <0·05 wt %). Trace elements typically agree within 10% (Ba, Rb, Sr, La, Ce, Zr, Y, Ga and V) or up to 20% (Pb, Th, U, Nb, Sc, Cr, Ni, Cu and Zn).
Major and trace element compositions
The most mafic rocks of the Pilbara high-Mg diorite suite show many of the characteristics of high-Mg diorite (sanukitoid). According to Shirey & Hanson (1984), Stern et al. (1989) and Stern & Hanson (1991), these characteristics include MgO > 6 wt %, mg-number >60, Ni and Cr each >100 ppm, Sr and Ba each >500 ppm and high Na2O, K2O, light REE (LREE) and La/Yb at silica contents of 60 wt % or less (Fig. 2, Tables 1 and 2). Compared with high-Mg diorite suites from the Superior Province, the Pilbara high-Mg diorite suite has higher FeO, Sc and V and generally higher MgO, Cr and Ni, but lower Sr and generally slightly lower mg-number and K2O, at a given silica content.
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The rocks of the Pilbara high-Mg diorite suite have silica contents between 59 and 70 wt % (Fig. 2), with high mg-numbers(62–41) that decrease with increasing silica. Most intrusions plot separately on a SiO2 vs mg-number diagram, and the Wallareenya granodiorite has particularly high mg-numbers(Fig. 2). The concentrations of Al2O3 vary between 13·5 wt % and 16 wt %, whereas the concentrations of Na2O and K2O are moderately high (3·5–4·8 wt % and 1·7–3·3 wt %, respectively) and increase with increasing silica. High concentrations of Cr (20–210 ppm), Ni (15–100 ppm) and V (30–110 ppm) are positively correlated with mg-number.
Amongst the LILE, the concentrations of Sr and Ba are high (Sr 300–950 ppm; Ba 450–1600 ppm) but show no clear correlation with silica (Fig. 2), mg-number, or with each other. Rubidium varies from 70 to 120 ppm, and Rb/Sr ratios range between 0·14 and 0·25. REE patterns are strongly fractionated (Fig. 3) with La/Yb ratios increasing from 22 to 79 with decreasing mg-number, primarily through decreases in the concentrations of the heavy rare earth elements (HREE). No Eu anomalies are observed.
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For individual intrusions, trends to lower mg-number, Cr, Ni, Y and HREE with increasing silica must primarily reflect removal of hornblende ± clinopyroxene, which is consistent with the presence of these minerals as phenocrysts in fine-grained melanodiorite. Plagioclase is also a phenocryst phase in fine-grained melanodiorite, but the lack of Eu anomalies in the more silicic rocks and the poor correlations between the concentrations of Sr and SiO2 indicate that plagioclase fractionation was not extensive.
Compositional differences between the parental magmas probably relate more to variations in degrees of partial melting and/or, more particularly, in source composition. At similar mg-number, individual intrusions show significantly different concentrations of K2O, SiO2 and Th (Figs 2 and 4), suggesting that they did not form from a single parental magma.
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Comparisons with Archaean TTG series
Rocks of the TTG series are generally restricted to the older (>3440 Ma) parts of the Edgar and Shaw granitoid complexes in the eastern part of the Pilbara Craton (Fig. 1) (Bickle et al., 1983, 1989, 1993; Collins, 1983, 1993). Whereas high-Mg diorite and TTG have high LILE, rocks of the high-Mg diorite suite can generally be distinguished by their trends to lower silica and higher mg-number, MgO, Cr and Ni (Fig. 5). However, a single suite of TTG gneisses from the oldest (3·45 Ga) part of the Shaw Granitoid Complex overlaps the low-MgO, -Cr, -Ni end of the range for the high-Mg diorite suite. In terms of these elements and the LILE, the TTG gneisses appear to form a compositional link between the high-Mg diorite series and TTG.
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A SOURCE FOR HIGH-Mg DIORITES |
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The low mg-number of most crustal rocks suggests that they cannot be a source for high-Mg diorite. This is true even for the Archaean basalts of the Pilbara Craton, which have mg-number values between 42 and 61 [430 analyses from Glikson et al. (1986)
] and commonly have concentrations of Cr and Ni that are only comparable with, or lower than, those of high-Mg diorite. A mantle source component is required to explain the high mg-number (>>62) and high Cr and Ni concentrations (Shirey & Hanson, 1984; Stern et al., 1989; Stern & Hanson, 1991). Nevertheless, even the most primitive high-Mg diorite also shows extreme enrichments in LILE. The high mg-number, Cr and Ni suggest that LILE enrichments are not primarily the result of crystal fractionation. The high-Mg diorite suite is LILE rich and high field strength element (HFSE) depleted (Fig. 6), with many trace element ratios (high La/Yb, Th/Nb, Ba/Nb, La/Nb) similar to either Phanerozoic subduction-related magmas (e.g. Kay, 1980; Pearce, 1982) or to magmas contaminated by continental lithosphere (e.g. Cox & Hawkesworth, 1985).
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Stern et al. (1989) used a range of compositions from basalt and komatiite to LILE-rich felsic crust to show that interaction between mafic to ultramafic melts and crustal material could not produce both the high Ni and Cr, and the high SiO2 and LILE compositions of Superior Province high-Mg diorite. For the Pilbara high-Mg diorite suite, no mixing model can account for the fact that some of the most silica-rich intrusions, such as the Jallagoonina stock and Wallareenya granodiorite (Fig. 2), also have particularly high mg-numbers: one end-member in such models would require the unlikely combination of extremely high SiO2 and mg-number. Negligible changes in Rb and Ba across the range of silica contents (or mg-number) also indicate that the high LILE content of the Pilbara high-Mg diorite cannot be a result of varying degrees of either magma mixing or assimilation of felsic crust. The most mafic rocks of the suite have Rb/Cs ratios around seven, similar to Archaean low Rb/Cs basalt and komatiite (<10; McDonough et al., 1992). In contrast, present-day felsic crust has an Rb/Cs ratio of 30 (Taylor & McLennan, 1985), and unless Archaean crust had a significantly lower ratio, it is unlikely that the high-Mg diorite incorporated a significant felsic component. In addition, rocks of the Pilbara high-Mg diorite suite show a virtual absence of inherited zircons (Nelson, 1996, 2000), further indicating very little crustal interaction. Consequently, the LILE-rich, HFSE-depleted nature of high-Mg diorite must reflect a mantle source component with similar characteristics.
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COMPARISONS BETWEEN HIGH-Mg DIORITE AND PHANEROZOIC ROCKS |
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The close compositional similarity between high-Mg diorite and Cenozoic high-Mg andesite has long been recognized (e.g. Shirey & Hanson, 1984
; Kelemen, 1995; Drummond et al., 1996; Rapp, 1997). Yogodzinski et al. (1995) identified two types of high-Mg andesite from the Miocene to late Pleistocene western Aleutian arc, and called these rocks Adak-type and Piip-type. Adak-type high-Mg andesite crystallizes early clinopyroxene (but no olivine), has high La/Yb ratios, very high Sr (and other LILE), low HFSE (Table 1) and a mid-ocean ridge basalt (MORB)-like isotopic composition. These characteristics closely match those of Cenozoic adakite, which is thought to be restricted to zones where young, hot, oceanic crust is subducted and partially melted at pressures high enough to stabilize garnet ± amphibole (eclogite to garnet amphibolite facies) (e.g. Defant & Drummond, 1990). However, the mg-number and concentrations of Cr, Ni and MgO are significantly higher in Adak-type high-Mg andesite than in most adakites (Table 1). Accordingly, Yogodzinski et al. (1995), proposed that Adak-type high-Mg andesite is the result of contamination of adakite by mantle peridotite (e.g. Kay, 1978; Kelemen et al., 1993; Sen & Dunn, 1994) (i.e. a ‘wedge-modified’ adakite). This process essentially involves assimilation of peridotite and crystallization of orthopyroxene (Yogodzinski et al., 1995), or garnet and orthopyroxene (Rapp et al., 1999). Rapp et al. (1999) showed experimentally that wedge-modified adakite would not only preserve typical adakite trace element ratios, but would show uniform increases in trace element concentrations, through consumption of melt during the contamination process.
The second type of high-Mg andesite identified by Yogodzinski et al. (1995), the Piip-type, crystallizes early olivine, and compared with wedge-modified adakite, generally has higher MgO, mg-number, Ni and Cr, lower K2O and Sr (LILE), and much lower La/Yb ratios (Table 1). These rocks are thought to be derived via direct melting of peridotitic mantle that was enriched in LILE through addition of either a slab-derived melt (i.e. adakite) or volatile phase. Alternatively, Yogodzinski & Kelemen (1998) suggested Piip-type melts may result when wedge-modified adakite mixes, in the mantle wedge, with arc basalt. Whichever is the case, wedge-modified adakite and Piip-type high-Mg andesite differ primarily in terms of the degree to which a slab-derived component contributes to magma genesis.
Yogodzinski et al. (1995) noted that Miocene high-Mg andesite (sanukite) of Japan is typical of the Piip-type, consistent with experimental evidence suggesting that sanukite could be derived via melting of mantle peridotite under hydrous conditions (Tatsumi & Ishizaka, 1982). Shirey & Hanson (1984) recognized geochemical similarities between Miocene sanukite and Archaean high-Mg diorite (Table 1) and argued for a similar origin for high-Mg diorite. Apart from high mg-number, Cr and Ni, these rocks have similarly high concentrations of HREE, with Yb generally >1·0 ppm and commonly >1·6 ppm. Yogodzinski et al. (1995), however, pointed out that La/Yb ratios in high-Mg diorite (67–77) are much higher than in Miocene sanukite (5–11) or in other Piip-type rocks (4–6), and are more like those of adakite (19–65) and wedge-modified adakite (45) (Table 1 and Fig. 7). High-Mg diorite also has generally lower mg-number and K2O, and significantly higher Sr and LREE than sanukite and other Piip-type rocks, and in these regards is more like wedge-modified adakite (Table 1 and Fig. 6). The implication here is that melting of basaltic crust was significantly more important in the derivation of high-Mg diorite than is suggested by analogies to sanukite, and that the tectonic setting and petrogenetic models proposed for wedge-modified adakite may also be relevant to high-Mg diorite.
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PETROGENETIC LINKS BETWEEN ADAKITE, TTG AND HIGH-Mg DIORITE |
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Adakite is relatively rare in the Cenozoic, but rocks with similar major and trace element compositions form a major component of the voluminous Archaean TTG series (Table 1 and Figs 6 and 7). It is widely accepted that TTG also result from melting of basaltic material at pressures high enough to stabilize garnet ± amphibole, producing tonalitic melts with characteristically high La/Yb and Sr/Y ratios (e.g. Barker & Arth, 1976
; Barker, 1979; Tarney et al., 1979; Martin, 1986, 1999; Drummond & Defant, 1990; Beard & Lofgren, 1991; Rapp et al., 1991; Rapp, 1997). There is also wide acceptance that some form of plate tectonics was active in the Archaean (e.g. Hoffman, 1989; Card, 1990; De Wit et al., 1992; Kröner & Layer, 1992; Lowe & Ernst, 1992; Lowe, 1997; De Wit, 1998; but see Hamilton, 1995). It seems reasonable, therefore, to suggest that TTG may have formed in a subduction environment, and is an Archaean analogue of Cenozoic adakite (e.g. Drummond & Defant, 1990; Drummond et al., 1996; Rapp, 1997; Martin, 1999).
Wedge-modified adakites are subduction-related Cenozoic rocks with compositions very similar to Archaean high-Mg diorite. It is possible, therefore, that high-Mg diorite is an Archaean analogue of wedge-modified adakite, formed as TTG, derived via partial melting of subducted oceanic crust, interacted with the peridotitic mantle wedge (e.g. Yogodzinski et al., 1995; Drummond et al., 1996; Rapp, 1997).
However, none of the few reported occurrences of high-Mg diorite appears to directly relate to either subduction or TTG magmatism. Rather, intrusion appears to have been late-kinematic (e.g. Evans & Hanson, 1997) or post-kinematic (Davis et al., 1994) and there is a strong temporal and spatial link between at least some high-Mg diorite suites and generally sodic, felsic, alkali magmatism [e.g. Otto Stock Syenite of the Superior Province (Sutcliffe et al., 1990); Portree Granitoid Complex of the Pilbara Craton]. This fundamental difference in setting between high-Mg diorite and wedge-modified adakite shows that if a relationship exists between high-Mg diorite and subduction magmatism, including TTG, it is not via a single-stage process.
It is possible, however, that high-Mg diorite results from the remelting of a TTG–mantle hybrid source and this may also explain why high-Mg diorite has LILE concentrations that are at least as high as, and Yb concentrations that are higher than, those of TTG. Figure 8 shows that the LILE and REE concentrations of the Pilbara high-Mg diorites can be successfully modelled as a high-degree (25%) partial (batch) melt of a hybrid source produced by mixing 40% TTG–adakite into peridotite. Melting under garnet-free conditions, leaving an orthopyroxene-rich residue, reproduces the characteristically high HREE, whereas high La/Yb ratios are a combined result of remelting a source already LREE enriched through a significant TTG component.
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If high-Mg diorite is a result of any form (single or two stage) of TTG and melt–mantle interaction, a subduction origin for TTG is required. Although such an origin is commonly favoured for TTG (e.g. Drummond & Defant, 1990; Drummond et al., 1996; Rapp, 1997; Martin, 1999), Atherton & Petford (1993) and Petford & Atherton (1996) showed that hydrous basaltic material, underplating a thick crustal root, can also partially melt to produce adakite (TTG)-like rock. Martin (1986) also showed that steeper geotherms meant that the P–T requirements for TTG production may have been commonly attained in the Archaean. Consequently, the tectonic setting of TTG genesis needs to be carefully considered.
Beard et al. (1993) pointed out that melt derived from a subducted slab should show clear evidence of having equilibrated, or reacted, with the mantle wedge through which it ascends. This appears to be the case for most adakite suites from subduction environments, which either comprise, or include, low-silica (<65 wt %) and high-mg-number (>47 and commonly >50) (Fig. 9), high-Cr and -Ni rocks (e.g. Defant et al., 1991; Mahlburg Kay et al., 1993; Sajona et al., 1994; Stern & Kilian, 1996). A small number of TTG suites also contain rocks with low silica (<65 wt %), reasonably high mg-number (between 47 and 50), and high Ni and Cr concentrations (Fig. 9). Such suites include the gneisses from the Shaw Granitoid Complex of the eastern Pilbara Craton (Bickle et al., 1983, 1989, 1993) and high-Al TTG from the southern Superior Province of Canada (Feng & Kerrich, 1992). However, average TTG has considerably lower mg-numbers, and lower Ni and Cr concentrations (Table 1), and the compositional range for most TTG suites is limited to the higher silica, lower mg-number end of the range for adakite and wedge-modified adakite (Fig. 9). If the trends to higher mg-number and lower SiO2 shown by Cenozoic adakite suites reflect the incorporation of a mantle component, then many (most?) TTG suites show no clear evidence of that component. Such suites, therefore, are either not subduction related or have somehow avoided contamination within the mantle wedge. The latter would seem thermodynamically improbable, as the mantle wedge should be hotter than either the subducting slab or the adakite melt. In addition, Rapp et al. (1999) showed that under experimental conditions, small amounts of contamination (10% peridotite) produce large (20%) changes in mg-number. Only TTG that interacted with mantle peridotite can have been the precursor to high-Mg magmatism, whether by a single- or two-stage process. If subduction-derived TTG suites are rare, then this would explain why high-Mg diorite magmatism is also rare.
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Regardless of how high-Mg diorites form, the scarcity of these rocks itself suggests that the required conditions were not met during all felsic crust-forming events, in all Archaean terrains. In the section below, we propose that Archaean high-Mg diorite results through the remelting of a mantle source region, extensively hybridized or metasomatized through the interaction of peridotite with a felsic (TTG-like) melt. We base this selection of source components on the close compositional analogies between high-Mg diorite and wedge-modified adakite, but we invoke a two-stage (remelting) process based mainly on the post-kinematic setting of the Pilbara high-Mg diorite suite.
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TECTONIC MODELS FOR HIGH-Mg DIORITE AND TTG |
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Some TTG suites, like those from southern Superior Province of North America and possibly the gneisses from the Shaw Granitoid Complex, show evidence of mantle interaction (high-mg-number, low-SiO2 TTG suites; Fig. 9). However, the compositions of many (most?) suites, including the majority from the Pilbara Craton, appear more consistent with melting of hydrous basaltic material at the base of the Archaean crust and under a typical Archaean geotherm (low-mg-number, high-SiO2 TTG suites; Fig. 9). One way to accommodate such compositional variation is if subduction during the Archaean was extremely flat, or approximated underthrusting, and locally excluded the mantle wedge (i.e. a tectonic underplate; Fig. 10, a1). Flat subduction of rapidly moving, young, thick and buoyant Archaean oceanic crust has been advocated as a significant mode of Archaean subduction by Abbott & Hoffman (1984)
and Abbott (1991). Flat subduction of oceanic crust, or possibly oceanic plateaux (e.g. Tarney & Jones, 1994), may have produced TTG melts, many of which may have ascended without encountering a mantle wedge. Other TTG melts (e.g. gneisses from the Shaw Granitoid Complex and TTG from the southern Superior Province; Fig. 9) passed through, and interacted with, peridotite in a thin mantle wedge.
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During reaction with mantle wedge peridotite, small-volume TTG melts would be totally consumed (e.g. Rapp et al., 1999), leaving a strongly hybridized or metasomatized mantle. According to Kepezhinskas et al. (1996), strongly metasomatized pyroxenite xenoliths in basalt from the northern section of the Kamchatka arc contain both a mineral assemblage matching products of experimental reactions between felsic melt and ultramafic rock, and dacitic veins broadly comparable in composition with adakite. This hybrid mantle may form the source for high-Mg diorite during a later melting event (Fig. 10, a2).
Alternatively, if melting of a lower, hydrous, basaltic portion of a thick Archaean crust is the primary mechanism of TTG genesis, then processes involving sequential stacking, or obduction, of buoyant oceanic crust (De Wit, 1998) and/or successive accretion of thick, buoyant, oceanic plateaux (e.g. Desrochers et al., 1993; Condie, 1997) might be more appropriate models of early Archaean crustal growth than subduction (Fig. 10b and c).
Sen & Dunn (1994) and Tarney & Jones (1994) noted that partial melts of oceanic basalt are depleted in Sr and Ba compared with either TTG or adakite. These deficiencies may be overcome if spilitized alkali- (Sr, Ba)-rich basalt is the source (e.g. Rapp et al., 1999), and it is also likely that the slab source for many adakites includes LILE-rich oceanic sediment (e.g. Stern & Kilian, 1996). Modern ocean plateau basalts commonly have trace element concentrations between those of mid-ocean ridge basalt and ocean island basalt (Weis et al., 1989; Richards et al., 1991; Lassiter et al., 1995; Kerr et al., 1997; Neal et al., 1997), with high Sr and Ba concentrations. Tarney & Jones (1994) noted that Archaean oceanic plateau basalts may also provide a suitable source for TTG.
An alternative to a flat-subduction and tectonic underplating model for TTG and high-Mg diorite magmatism begins with thick (>35 km; Martin, 1986) crust formed by stacking of locally spilitized oceanic crust and/or by of accretion of oceanic plateaux. If crustal thickening is rapid, the appropriate conditions (eclogite facies) for partial melting can be achieved before the crust dehydrates, and low-mg-number, high-SiO2 TTG suites should result (Fig. 10b and c). High-Mg diorite suites, and high-mg-number, low-SiO2 TTG suites, which require a mantle source component, may relate to possible (flat) subduction events during accretion of oceanic plateaux. An interesting feature of the documented high-Mg diorite suites is that they form late, not simply in terms of a given tectonic cycle, but in terms of the overall evolution of a given Archaean province, and all are younger than 3000 Ma. Therefore, it is possible that high-Mg diorite magmatism, or at least the subduction event that produced its hybridized mantle source, heralds a transition from early Archaean tectonic styles dominated by oceanic plate stacking and plateau accretion, to a period where subduction of the cooler, thinner and less buoyant oceanic crust became a more viable process (Fig. 10d1 and d2).
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LATE ARCHAEAN EVOLUTION OF THE PILBARA CRATON |
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In addition to the broader tectonic significance of high-Mg diorite magmatism, intrusion of the Pilbara high-Mg diorite suite also provides some of the most important clues to the Late Archaean evolution of the Pilbara granite–greenstone terrain. In the central part of the terrain, Pb isotopic ages were reset during a significant tectonothermal event at
2950 Ma (Oversby, 1976). Intrusion of the Pilbara high-Mg diorite suite and the alkaline Portree Granitoid Complex represents virtually the entire known extent of magmatism at that time, and both are restricted to the Mallina Basin (Fig. 1). The mantle-derived high-Mg diorite suite intruded along major extensional basin faults. Within less than 10 my, large volumes of monzogranite and syenogranite, of crustal origin (Champion & Smithies, 1998), swamped the central Pilbara region. These are most voluminous adjacent to the Mallina Basin, and from the available geochronology (Nelson, 1997, 1998, 1999), show a clear progressive decrease in age away from the basin towards the southeast.
De Wit (1998) and Beakhouse et al. (1999) speculated that high-Mg diorite magmatism occurs during late extensional collapse of an Archaean province. In the case of the Pilbara high-Mg diorite suite, we suggest that the restriction of 2950 Ma magmatism to the Mallina Basin and the concentration of 2940–2930 Ma monzogranite and syenogranite magmatism adjacent to the same region, with a seemingly systematic decrease in both age and volume away from the region, points to a process more localized than regional extensional collapse. It seems more likely that mantle upwelling beneath the central Pilbara Craton, possibly caused by either local lithospheric detachment–delamination or a mantle plume, remobilized a mantle source region, previously hybridized during a subduction event, to produce high-Mg diorite magmatism. At the same time, high-temperature melting of a metasomatized basal crust produced the sodic alkaline granite of the Portree Granitoid Complex, and inductive heating of the lower- to middle-crustal region caused partial melting and production of monzogranite and syenogranite magmas.
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CONCLUSIONS |
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The
2950 Ma Pilbara high-Mg diorite suite intrudes the youngest supracrustal sequences of the central part of the Pilbara granite–greenstone terrain, in a post-kinematic setting associated with alkaline magmatism but not with TTG magmatism. The rocks are broadly similar in composition to the 2700 Ma high-Mg diorite suites of the Superior Province and have distinctly higher mg-number and Cr and Ni concentrations than TTG, at equivalent silica contents. High LILE concentrations cannot be explained through crustal contamination of a mafic magma, and together with high mg-number and Cr and Ni concentrations indicate an LILE-enriched mantle source. TTG shows compositional similarities to felsic partial melts of subducted, young, hot, basaltic crust (adakite), and it is commonly suggested that TTG may likewise be subduction related. High-Mg diorite is compositionally very similar to Cenozoic high-mg-number adakite (wedge-modified adakite), thought to result when adakite is contaminated by peridotite during ascent through the mantle wedge. However, the late- to post-kinematic setting of high-Mg diorite excludes a similar origin. The close compositional similarities between wedge-modified adakite and high-Mg diorite can be explained by a two-stage model whereby high-Mg diorite results from the remelting of a mantle source region, previously hybridized by the addition of significant amounts of a subduction-derived TTG-like melt.
However, the lack of such evidence as rocks with high mg-number and low SiO2 contents for interaction between many TTG melts and mantle peridotite indicates that the amount of TTG derived via subduction may be limited. This would explain why high-Mg diorite magmatism is rare compared with TTG magmatism on a global Archaean scale. Melting of hydrated basaltic material at the base of thickened crust (e.g. Atherton & Petford, 1993; Petford & Atherton, 1996), may be a more appropriate model for most TTG.
Tectonic models that incorporate tectonic underplating of oceanic crust, sequential stacking, or obduction, of oceanic crust (De Wit, 1998) and/or successive accretion of oceanic plateaux (e.g. Desrochers et al., 1993; Condie, 1997) predict TTG generation mainly at the base of thickened crust and might be more appropriate models of early Archaean crustal growth than modern-style subduction and arc accretion. High-Mg diorite magmatism may herald a transition from early Archaean tectonic styles dominated by tectonic underplating, oceanic plate stacking and plateau accretion, to a period where cooler, thinner and less buoyant oceanic crust allowed processes more akin to modern-style subduction and arc accretion.
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ACKNOWLEDGEMENTS |
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This work has benefited greatly through numerous discussions with Arthur Hickman, Paul Morris, Steve Sheppard, Shen-su Sun and Martin Van Kranendonk, and through reviews by Michael Atherton, Kevin Cassidy, Suzanne Mahlburg Kay, Paul Morris, Steve Sheppard, Shen-su Sun and Marjorie Wilson. We thank S. Dowsett, L. Cosgrove, T. Edwards and P. Carroll for drafting of figures. This work is published with the permission of the Director, Geological Survey of Western Australia, and the Executive Director, Australian Geological Survey Organisation.
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FOOTNOTES |
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*Corresponding author. e-mail: r.smithies{at}dme.wa.gov.au
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