Journal of Petrology | Volume 40 | Number 1 | Pages 61-77 | 1999
© Oxford University Press 1999
The Origin of Anorthosites and Related Rocks from the Lofoten Islands, Northern Norway: II. Calculation of Parental Liquid Compositions for Anorthosites
1 Institut Für Mineralogie, Petrologie Und Geochemie, Albert-Ludwigs-Universität Albertstrasse 23 B, D-79104 Freiburg, GERMANY
2 Department of Geology and Geophysics, University of Wyoming Laramie, Wy 82071, USA
Received February 10, 1997; Revised typescript accepted May 14, 1998
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ABSTRACT |
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 TOP ABSTRACT IntroductionWhole-Rock ChemistryCalculations With Partition...DiscussionGeneral ConclusionsReferences |
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Clinopyroxene, orthopyroxene and plagioclase in the Proterozoic Lofoten anorthosites are inferred to be in equilibrium with ferrodioritic melts, on the basis of literature partition coefficients for these phases in equilibrium with basaltic melts. Ferrodiorites and high-Al gabbros occur as fine-grained dykes in spatial association with the
1.8 Ga Lofoten anorthosites. They are linked to each other by an Fe-enrichment fractionation trend typical of tholeiites, and they are characterized by Al2O3 contents of 14–18.5 wt % and XFe [= FeOtot/(FeOtot + MgO)] between 0.5 and 0.8. The association of high-Al gabbros, ferrodiorites and anorthosites is common in many Proterozoic anorthosites. We infer that the high-Al gabbros to ferrodiorites represent the parental liquids from which these Proterozoic anorthosites formed. The Lofoten rocks show fractionation trends that are similar to those of high-Al gabbros and ferrodiorites from other Proterozoic anorthosite complexes. We interpret these trends in terms of incorporation of intermediate composition plagioclase into a typical tholeiitic magma with subsequent fractionation. The first process produces positive Eu anomalies and the high Al2O3 contents of the melts, and explains the lack of negative Eu anomalies in most of the ferrodiorites from Lofoten and Laramie. KEY WORDS: anorthosite; parental liquid; ferrodiorite; high-Al basalt; partition coefficients
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Introduction |
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TOPABSTRACTIntroductionWhole-Rock ChemistryCalculations With Partition...DiscussionGeneral ConclusionsReferences |
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Proterozoic massif-type anorthosites are huge intrusive bodies that contain hundreds to thousands of km2 of nearly pure (>90%) plagioclase cumulates of intermediate composition (An50±10; Ashwal, 1993
, and references therein). Most Proterozoic anorthosites are not associated with significant amounts of mafic or ultramafic cumulates. They contain plagioclase that is relatively sodic (usually around An50) and this makes them distinct from other anorthositic rocks (Archean anorthosites, lunar anorthosites, anorthosites in layered intrusions), which contain more calcic plagioclase. Proterozoic anorthosite complexes contain anorthosite sensu stricto (>90% plagioclase) as well as variable amounts of leuco-gabbroic, -troctolitic and -noritic rocks which respectively may contain up to 35% clinopyroxene, olivine or orthopyroxene.
The Proterozoic anorthosites of the Lofoten Islands in northern Norway (Heier, 1960; Green & Jorde, 1971; Romey, 1971; Griffin et al., 1978; Wade, 1985) are associated with large amounts of temporally related intrusive mangerites, charnockites and gabbros (Markl et al., 1998). There are two different anorthosite intrusions present. One is a relatively calcic complex, which occurs on the island of Flakstadøy (Flakstadøy Basic Complex, FBC), and the other is a relatively sodic complex, which is found at the Eidsfjord on the island of Langøy (Eidsfjord anorthosite). The FBC has the mineral assemblage cpx± ol ± opx + plag + ilm + mt, whereas the Eidsfjord anorthosite shows the assemblage opx ± cpx + plag + ilm + mt without olivine. The FBC crystallized at pressures from 9.5 to 4 kbar at temperatures between 1185 and 1140°C, whereas the Eidsfjord anorthosite equilibrated at temperatures of 1100–1135°C with a maximum pressure of 7.3 kbar. Both crystallized at oxygen fugacities above fayalite–magnetite–quartz (FMQ).
The anorthosites of Lofoten are part of a NE–SW trending belt of Proterozoic anorthosites that extends from Scandinavia across North America. These anorthosites range in age from 920 to 1789 Ma (Ashwal, 1993, and references therein; Schärer et al., 1996). The largest complexes occur within the Grenville and Nain–Churchill provinces of the Canadian Shield (Emslie, 1978, 1985; Wiebe, 1979, 1990, 1992; Morse, 1982; McLelland & Chiarenzelli, 1990; Owens et al., 1994). Well-studied Proterozoic anorthosites from this belt include the Laramie Anorthosite Complex in Wyoming (e.g. Scoates & Frost, 1996, and references therein), the Nain Plutonic Suite in coastal Labrador (e.g. Morse, 1988, and references therein; Emslie et al., 1994) and the Rogaland anorthosite complex, southwest Norway (e.g. Duchesne, 1984; Duchesne et al., 1989, 1993; Schärer et al., 1996).
There is still much debate as to how these immense bodies of nearly homogeneous mineralogy and plagioclase composition develop. The major problem is the lack of information on the compositions of the silicate melts from which the anorthosites crystallized. Two major theories have been proposed: the existence of hyperfeldspathic magmas (e.g. Wiebe, 1979, 1990; Morse, 1982) and two-stage polybaric crystallization of basaltic parent melts without involvement of hyperfeldspathic liquids (e.g. Emslie, 1978; Duchesne, 1984; Longhi & Ashwal, 1985; Wiebe, 1992; Ashwal, 1993; Emslie & Hegner, 1993; Longhi et al., 1993; Emslie et al., 1994; Scoates & Frost, 1996). The existence of a basaltic melt may be indicated by the presence of high-Al gabbros, ferrogabbros and ferrodiorites coeval with the anorthosites. These rocks have been suggested to represent parental or residual liquids of anorthosites (Olson & Morse, 1990; Mitchell et al., 1995, 1996).
In this paper we use published partition coefficients and mineral compositions from the Lofoten anorthosites to calculate the composition of the parental melt from which the anorthosites crystallized. We then compare these calculated melt compositions with the compositions of fine-grained rocks from the Lofoten and other well-studied anorthosites to better constrain the origin of the parent melts to massif-type anorthosites.
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Whole-Rock Chemistry |
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TOPABSTRACTIntroductionWhole-Rock ChemistryCalculations With Partition...DiscussionGeneral ConclusionsReferences |
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Analytical techniques
X-ray fluorescence (XRF) analyses of whole-rock samples and mineral separates of plagioclase, clinopyroxene and orthopyroxene were carried out at the Institut für Mineralogie, Petrologie und Geochemie at the University of Freiburg on a Philips PW 1450/20 machine using natural standards; accuracy and detection limits are of the order of 0.1 wt % for major elements and 1–10 ppm (depending on the specific element) for minor elements. Sample preparation was performed at the same institute. For whole-rock measurements, about 1–3 kg of material, depending on the grain size, was crushed and milled in agate mills, and powder and Li borate fusion discs were prepared to measure trace and major elements. Mineral separates were prepared from 63–125 µm fractions of the whole-rock samples; separations were made with heavy liquids, magnetic separators and a wave table. The resulting separates were again milled, and powder and fusion discs were prepared. Powders of six whole-rock samples were analysed for trace elements including La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Er, Yb and Lu by inductively coupled plasma mass spectrometry (ICP-MS) at the CRPG laboratory, Nancy, on a commercial basis.
Major and trace elements
Anorthositic rocks
Whole-rock analyses of anorthositic rocks typically reflect a cumulate mixture of various proportions of plagioclase, oxides and Fe–Mg silicates. Typically, it is difficult to derive genetic significance from whole-rock data, which in most cases only show that these rocks are cumulates (e.g. Haskin & Salpas, 1992). For example, a plot of wt % FeO vs wt % Al2O3 shows a linear relationship of decreasing Al2O3 with increasing FeO (Fig. 1). It is reasonable to assume that FeO in the rock is contributed by post-cumulate pyroxenes and oxides whereas the Al2O3 resides mostly in the plagioclase. Thus the linear pattern in Fig. 1 merely reflects different abundances of pyroxene and plagioclase. Similar trends are seen in other major oxides, indicating that the major element chemistry is controlled for the most part by the relative abundance of the minerals in the rock, rather than by the composition of the melt.
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Gabbro and ferrodiorite
In addition to the coarse-grained rocks, there are fine-grained mafic rocks, which we assume to represent chilled melts (Table 1). Of these samples, GM 405, 448, 406, 500, 508, 1120 and 1154 are dyke rocks, whereas GM 451, 443 and 420 are from bodies tens of metres in size. Eighteen additional analyses of evolved tholeiitic to ferrodioritic compositions from Lofoten dyke rocks have been given by Misra & Griffin (1972)
. The fine-grained samples listed here are interpreted to represent melt rather than cumulate compositions because of their fine grain size, the lack of Eu anomalies (where analysed, see Fig. 2 and Table 2) and the lack of correlations between chemistry and the compositions of possible accumulating phases.
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These fine-grained rocks are chemically distinct from the gabbroic anorthosites. The most important differences are that the anorthositic rocks have higher Al2O3 and Na2O contents. Some of these fine-grained rocks (GM 448, 500, 420 and 443) have low XFe [= FeOtot/(FeOtot + MgO)] and have major element compositions that are similar to those of evolved tholeiites (e.g. Carmichael et al., 1974
), whereas others (GM GM 405, GM 406, GM 451, GM 1120) have higher XFe and have compositional affinities to ferrodiorites from other Proterozoic anorthosite complexes (e.g. high FeOtot, TiO2, P2O5; Emslie, 1980; Duchesne et al., 1989; Mitchell et al., 1995, 1996). In terms of their trace element chemistry, the latter group shows enrichment in Zr and Ba and depletion in Ni and Cr compared with the other fine-grained rocks and with the gabbroic anorthosites. GM 448, GM 1120 and GM 1154 were sampled from dykes within the Eidsfjord anorthosite. As this is of importance in the discussion below, the analysis of the gabbro sample BM-10 from the Laramie Anorthosite Complex in Wyoming (Mitchell et al., 1995) is listed for comparison in Table 1. |
Calculations With Partition Coefficients |
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TOPABSTRACTIntroductionWhole-Rock ChemistryCalculations With Partition...DiscussionGeneral ConclusionsReferences |
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Theory
The distribution of elements between a melt and coexisting crystals is given by the partition coefficient Di, which is defined as
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proposed a model that allows the calculation of the concentrations of all equally charged cations in the melt if their concentrations in the crystal, the partition coefficient for one element, the elastic moduli and the site sizes in the mineral are known. This model is based on the assumption that the structure of the crystal in equilibrium with a melt governs the partitioning between the melt and the crystal, and that the composition or structure of the melt is of much less importance. Because their experiments can account for the main network-modifying cations, this model system is also a good approximation of Fe-bearing basaltic compositions (see, e.g. Blundy, 1997). The D values and mineral analyses that were used in the calculations are given in Tables 3 and 4, and all other parameters are given in Table 5. Ionic radii are from Whittaker & Muntus (1970) and Blundy & Wood (1994); site radii were either adopted from Blundy & Wood (1994) or calculated as detailed below.
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The use of the Blundy & Wood (1994)
model is limited to 1+, 2+ and some 3+ elements because the partitioning behaviour of some 3+ and the 4+ elements is more complex, probably because of additional electron interactions. For this reason, and to better evaluate the validity of the results, we performed additional calculations using the partition coefficients from the works of Beattie et al. (1991) and Beattie (1993), who investigated partitioning of various major, minor and trace elements between orthopyroxene, clinopyroxene and basaltic melts. For comparison, we also calculated concentrations of K2O, Rb, Sr and Ba using the partition coefficients of Simmons & Hanson (1978) and of Morse (1988), and of Al2O3, TiO2 and FeO using the partition coefficients reported by Nolan & Morse (1986) that they derived for anorthosites or rocks related to anorthosites (Kiglapait troctolite). In the following, we will describe in detail which model and which values we used for the calculations with respect to the various minerals.
Calculation procedure
Plagioclase
The model of Blundy & Wood (1994) was used to calculate the concentration of Na2O, K2O, CaO and for some samples Rb, Ba, Sr, MgO and FeO, from the analyses of plagioclase separates and of electron microprobe (EMP) point analyses. In accordance with the data of Blundy & Wood (1994), we set DCa to unity for bivalent elements. The combination of the model of Blundy & Wood (1991) for DSr with the model of Blundy & Wood (1994) gave us another constraint on D values of 2+ elements and thus allowed us to calculate the size of the Ca site (roCa) in plagioclase according to the expression
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. The calculated sizes roCa were always within the range given by Blundy & Wood (1994) and hence support our choice of DCa. To determine D for univalent elements, the value of DNa between plagioclase and melt was fitted to the Na content of the melt as estimated from clinopyroxenes. From this we calculated DK from the expressions of Blundy & Wood (1994). For comparison, we also calculated Rb, K, Ba and Sr from the partition data of Morse (1988) and of Simmons & Hanson (1978). The results of these calculations are given in Table 3. The values for K2O and Rb from the Blundy & Wood model are significantly higher than those derived from the other data (by 0.5–2.5 wt %), whereas the Sr and Ba values are considerably lower (by 100–400 ppm).
Clinopyroxene and orthopyroxene
Experimentally determined partition coefficients of Beattie et al. (1991) and Beattie (1993) were used to estimate the contents of CaO, MgO, FeO, Na2O, Sr, Ni, Co, TiO2, Al2O3 and SiO2 in the melt from bulk and microprobe analyses of pyroxenes. We used the data of Beattie et al. (1991) and Beattie (1993) because the crystallization temperatures of the Lofoten rocks are similar to those used in his experimental work. For orthopyroxene, DCa was adjusted so that the results agreed with those from the plagioclase calculations. The D values of Mn, Mg, Ni and Co were then calculated from the expressions of Beattie et al. (1991). For the FeO calculations from orthopyroxene, we estimated the ferric iron from mineral formula calculations (normalization of the analyses to two cations and six oxygens) and used the Fe2+ content for the calculations. For clinopyroxene, we used data from Beattie’s (1993) experiments at 10 kbar as an approximation for our calculations. As a comparison, we calculated concentrations of Al2O3, FeO and TiO2 in the coexisting liquid from the partitioning data reported by Nolan & Morse (1986). For the FeO calculations, again the ferric/ferrous ratio was calculated as above; as Nolan & Morse (1986) gave a value for only DFeO/MgO, we chose the MgO content of the liquid as calculatedfrom the Beattie et al. (1991) expressions and then calculated FeO from the combination of these two values. These values are significantly higher (by 0.5 toseveral wt %) than those calculated from the Blundy & Wood model.
Problematic elements
Plagioclase mineral separatesconsistently have higher Fe than microprobe analyses. This is probably caused by the presence of small (micrometre-sized) inhomogeneously distributed grains of Fe oxides in the plagioclase. For FeOcontent of plagioclase, therefore, the microprobe analyses were used in the calculations because they are believed to give more accurate results. The estimate of Fe in the melt from clinopyroxene using the data of Beattie (1993) gives unreasonably high results. We discount these results because there is a strong dependence of DFe on the XFe of the clinopyroxene (Beattie, 1993), and our clinopyroxenes are more Fe rich than those used in Beattie’s (1993) experiments.
There are also discrepancies between estimations of K2O in the melt made from plagioclase separates and those made from EMP analyses, especially in the case of the FBC. Based upon thecomposition of plagioclase separates, the calculated K2O content of the FBC melt would be of the order of 2–4 wt %, whereas that of the Eidsfjord melt would be 1–2.5%. Incontrast, calculations based upon microprobe analyses give more reasonable values of 0.9–1.4 wt% K2O in both intrusions. We believe that these high K2O values and related highconcentrations of Ba and Rb are caused by the fact that the mineral separates included small amounts of late K-feldspar, which had formed from an interstitial liquid that wasconsiderably enriched in K, Ba and Rbcompared with the liquid from which the rest of the plagioclase and the mafic minerals crystallized. We therefore used theconcentrations of K2O in plagioclase as determined by the EMP analyses for the estimated liquidcomposition (Table 6). For the same reason, we do not report values for Ba and Rb in the calculated melt of the FBC.
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Comparison of the results with Lofoten rocks and with MELTS calculations
For several reasons, we areconfident that the calculated melts (Table 6)come close to thecomposition of the melt from which both the FBC and the Langoy anorthosites formed. First, the calculated liquids from the Lofoten anorthosites strongly resemble ferrodioritic rocks from Lofoten. Dykes of ferrodioritic and high-Al basalticcomposition are found throughout the Lofoten Islands (Table 1) and additional data have been reported by Misra & Griffin (1972)
. The calculated liquids of the FBC and the Eidsfjord anorthosite are both similar to GM 508 (although the FBC melt is more aluminous), a fine-grained basaltic dyke from the FBC. The Eidsfjord melt closely resembles the ferrodioritic rock GM 1154 from the margin of the Eidsfjord anorthosite (Table 1) in that they have both similarcontents of many major and minor elements and similar Na/Ca and Al/Si ratios. The calculated melts are also similar to some of the dykes reported by Misra & Griffin (1972) that are net-veined by anatectic material from thecountry rocks. This material has an apparent age (1795 ± 20 Ma, Rb–Sr whole rock, Misra & Griffin, 1972) similar to that of the mangerites and anorthosites. Pending more precise U–Pb geochronological work and because of the similarity between our calculated melts and some of these dykes, we propose that these dykes may represent basic meltscoeval with the anorthosites and mangerites and that observed differentiation trends in these rocks may reflect the differentiation of the basic magma. A plot of major element oxides against XFe (= FeOtot/(FeOtot + MgO) in the fine-grained rocks from Lofoten is shown in Fig. 3. XFe is chosen as a monitor of differentiation in this figure because SiO2 does not change much during crystallization of the ferrodiorites. There is acontinuous trend of increasing Fe2O3, P2O5, K2O and TiO2, and of decreasing Al2O3, CaO and MgO with increasing XFe. Also shown in Fig. 3 are the calculated melts for the FBC and Langoy anorthosites. They clearly lie within the trend defined by the fine-grained rocks and closely resemble some individual dyke rock samples.
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Another reason why we haveconfidence in the veracity of our results is that we used the MELTS program of Ghiorso et al. (1994)
to calculate thecomposition of the phases that would be in equilibrium with our inferred melts at the temperatures and pressures that we estimated for the formation of these anorthosites (Markl et al., 1998). The calculated phasecompositions of plagioclase, olivine and clinopyroxene are very similar to those observed in the FBC and the Eidsfjord anorthosite (Table 7). Small discrepancies in the oxygen fugacities result from the fact that we have poorcontrol of the ferric ironcontent of the pyroxenes and hence poorcontrol of the ferric ironcontent of the calculated melts. This probably explains the predicted existence of olivine in the MELTS calculations at 4 kbar for the Eidsfjord melt, even though olivine seems to be lacking in these rocks.
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Discussion |
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TOPABSTRACTIntroductionWhole-Rock ChemistryCalculations With Partition...DiscussionGeneral ConclusionsReferences |
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Validity of the calculations
Choice of partition coefficients
Morse (1992)
cast doubts on the validity of the Blundy & Wood (1991) formulation for augite-poor, anorthositic liquids. He argued that in the Kiglapait intrusion the formulation for Sr of Blundy & Wood (1991) led to strong discrepancies between the calculated and his reconstructed liquid evolution trend. Blundy & Wood (1992) argued in a reply that Morse's dataset was dependent on strictly closed system behaviour and pointed out that his results were incontrast to the results of all other studies they used for the derivation of their model. Furthermore, recent experimental work by Blundy (1997) on the Kiglapait intrusion suggests that the calculated liquidcomposition of Nolan & Morse (1986) and hence the partitioncoefficients calculated therefrom may not becorrect. Hence, the discrepancy between the results calculated with the partitioncoefficients of Morse (1988) and Nolan & Morse (1986) and those calculated from Blundy & Wood (1994) (Tables 3 and 4) are likely to be caused by errors in Morse's partitioncoefficients. Blundy (1997) furthermore showed that the partitioncoefficients calculated after Blundy & Wood (1994) are suitable for rocks and liquids of the anorthosite kindred, and he suggested that inversion using appropriate D values is a better means of calculating parental meltcompositions rather than summation over exposed cumulates assuming closed system fractionation behaviour.
Validity and accuracy of the results
Because whole-rock analyses of cumulate rocks are sensitive more to variations in mineral abundance than to thecomposition of the melt from which they were derived, we have chosen to calculate thecomposition of thecoexisting melt from thecomposition of theconstituent minerals. This technique, though, is prone to some difficulties, which arise from the fact that anorthosites as cumulate rocks have had a long crystallization history and the melt from which the minerals crystallized certainly changedcomposition as crystallization proceeded. Calculations using cumulus and post-cumulus minerals will therefore give an average of the liquidcomposition that was in equilibrium with the anorthosites during its evolution. However, there are several arguments that support the validity of our results:
- the calculated melts are similar to gabbroic to ferrodioritic rocks observed in the Lofoten area;
- using the MELTS program and our calculated meltcomposition we can closely match thecomposition of the phases in the rocks;
- the calculated totalscome close to 100%, even though there is no reason why they should (without normalization!);
- where elementconcentrations in the liquid are calculated from various minerals and from various sets of partitioncoefficients (e.g. Ca from plagioclase and from clinopyroxene, or Sr from plagioclase and clinopyroxene), the results show only low variance (Tables 3 and 4).
Based on thecomparison between different samples and different sets of partitioncoefficients, we estimate that the calculated major elementconcentrations (SiO2, CaO, FeO, Al2O3) are precise to about 1–2 wt %. The Fecontent may be subject to larger uncertainties because of the effects of Fe3+/Fe2+ uncertainties. MgO, K2O and Na2O are probably precise to about 0.5–1 wt %. Sr gives a precision on the order of 30 ppm or 8–10 relative % when results from plagioclase and from clinopyroxene calculations arecompared, and this may be representative for the trace elements. The calculated CIPW norm of these liquids is slightly Ne-normative when low ferric/ferrous ratios are assumed. At higher ferric/ferrous ratios and if a P2O5 content of 0.5–1 wt % in the liquid is assumed, the calculated melts are only weakly Ne-normative or not at all, and hence resemble the observed ferrodioritecompositions from Lofoten which areHy- or slightly Ne-normative.
The reconstructed melts from the FBC and the Eidsfjord anorthosite (Table 6) have similar SiO2, TiO2 and K2O contents, and both are enriched in FeOtot and Al2O3 compared with tholeiitic basalts. The calculated Eidsfjord melt is much more evolved than the FBC melt as suggested by allcommoncompositional indicators (higher XFe, Na2O, K2O, lower CaO, lower An in plagioclase; see also Markl et al., 1998). Hence, we infer that the Eidsfjord anorthosite equilibrated with a more evolved melt than did the FBC.
The magmatic evolution from basalts to ferrodiorites on the Lofoten Islands as recorded by olivine and feldspar
As noted above, the fine-grained rocks from the Lofoten Islands arecontinuous incomposition from high-Al gabbro to ferrodiorite. There is evidence for melts with even higher MgOcontents as indicated by thecomposition of magnesian olivines in the Moskenesøy cumulate (Fig. 4 ). The presence of an early Mg-rich basaltic magma is hence recorded by the mafic cumulates (Markl et al., 1998), whereas the more differentiated melts are recorded by the gabbros, anorthosites and ferrodiorites. The olivinecomposition in the Moskenesøy cumulate is Fo82–83. Progressive Fe enrichment is recorded by the Ramberg gabbro on Flakstadøy (olivine with Fo77), the anorthosites GM 263 and GM 265 with olivine of Fo72 and Fo68, respectively, and the anorthosite GM 139 with olivine of Fo66. In the Eidsfjord anorthosite, olivine is absent, but the calculated equilibriumcomposition from QUI1F (Andersen et al., 1993) would have been Fo58 (at lower aSiO2 than actually observed). The apparent general fractionation trend reflected by the whole-rock (Fig. 3 ) and olivine (Fig. 4 )compositions is also followed by the plagioclasecompositions. Plagioclase evolved from An69 in the Ramberg gabbro through normative An63 and An56 in the gabbros near Bjørnhaugen on Austvågøy (GM 420) and on Gimsøy (GM 440) to the FBC plagioclase (An57–47) and the Eidsfjord anorthosite plagioclase (An487–44). The ferrodiorite GM 451 has a normative ternary feldspar of An17Or23 (Markl et al., 1998).
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Comparison of the calculated meltcompositions with rocks from other Proterozoic anorthosite complexes
Figure 5 plots thecompositions of fine-grained rocks from Lofoten vs XFe together with a field for 50 analyses of gabbroic and ferrodioritic rocks from Harp Lake, Labrador (Emslie, 1980
) and with 63 analyses from the Laramie AnorthositeComplex in Wyoming (Mitchell et al., 1995, 1996). The fine-grained rocks from Lofoten havecompositions very similar to those of the gabbros and ferrodiorites from bothcomplexes (Fig. 5 ). The rocks from all threecomplexes clearly define similar evolution trends from high-Al gabbros to low-Al gabbros and to ferrodiorites in the majority of major and trace elements. The Lofoten rocks show the bend to high P2O5, K2O and Zr at lower XFe than the Laramie and Harp Lake rocks (Fig. 5 ). These discrepancies, though, do not negate the similar evolutionary trend from basaltic to ferrodioriticcompositions in both areas, as they are most probably due tocontamination of the evolving ferrodioritic melt with crustally derived melts at various stages of their evolution. Similarly (but not shown in Fig. 5 ), the Lofoten rocks closely resemble high-Al–Fe gabbros from the Adirondacks in New York (Olson & Morse, 1990), ferrodioritic rocks from the NainComplex in Labrador (Wiebe, 1990) and ferrodiorites from Rogaland, Norway (Duchesne et al., 1989). The Lofoten fine-grained rocks and the calculated melts evidently have equivalents in many large Proterozoic anorthositecomplexes. Emslie (1980), Olson & Morse (1990), Wiebe (1990) and Mitchell et al. (1996) have interpretedthese rocks as residual liquids of anorthosites, whereas Duchesne et al. (1989) have explicitly ruled out anycomagmatic relationship.
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Comparison of liquid evolution trends
Melt evolution as recorded by the fine-grained dyke rocks in the Lofoten Islands and the calculated melts is remarkably similar to evolution trends of tholeiitic andhigh-Al troctolitic liquids. Figure 6a compares the evolution of the liquids from Lofoten and the Laramie AnorthositeComplex with the trends reported from the Kiglapait intrusion in Labrador (Morse, 1981
) and from the Skaergaard intrusion (Wager & Brown, 1967) and the Basistoppen sill in Greenland (Naslund, 1989). Figure 6b compares the Lofoten trend with the Skaergaard trend as calculated by Hunter & Sparks (1987) and with the broad trend spanned by the Rogaland monzonorites of Duchesne (1990). With the exception of the Rogaland monzonorites, which were interpreted by Duchesne (1990) to be lower-crustal anatectic melts, all trends in Fig. 6a and b show initial, slightly variable Fe enrichment at nearlyconstant SiO2 followed by SiO2 enrichment and Fe depletion. The extensions of the Lofoten Islands and the LaramieComplex trends to higher SiO2 values cannot be reliably retrieved and are therefore shown dashed in Fig. 6a and b. Hunter & Sparks (1987) and Toplis & Carroll (1996) pointed out that the amount of Fe enrichment depends on thecomposition and on the fO2 of the parental magma. This argument further supports that the Lofoten rocks and calculated melts record an evolution trend typical of tholeiites.
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Comparison with the calculations of Naslund (1989)
and Morse (1981) suggests that the fraction of melt crystallized until the point where the Lofoten high-Al basalts or ferrodiorites attain their highest FeOtot contents lies between 50 and 75 wt %. As the ferrodiorites are the liquids from which the anorthosites formed, there is a huge amount of these liquids to be crystallized after the formation of the anorthosites. During further fractionation and/or assimilation processes, these ferrodiorites may, for example, be used to produce the large amounts of mangerites and charnockitescommonly observed to accompany the anorthosites and ferrodiorites in Proterozoic anorthositecomplexes (e.g. Fuhrman et al., 1988; Emslie & Hunt, 1990; Emslie, 1991; Emslie et al., 1994). |
General Conclusions |
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TOPABSTRACTIntroductionWhole-Rock ChemistryCalculations With Partition...DiscussionGeneral ConclusionsReferences |
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Two types of high-Al basalts
The parental melts of Proterozoic anorthositecomplexes are high-Al basalts that fractionated to ferrodiorites (e.g. Emslie, 1980
; Olson & Morse, 1990; Mitchell et al., 1995, and references therein). It is important to note, however, that high-Al basalts associated with anorthositecomplexes are chemically distinct from those that are characteristic of subduction environments (e.g. Myers et al., 1986). The subduction-related high-Al basalts are higher in Al2O3, have relatively high XFe (e.g. Yoder & Tilley, 1962; Singer et al., 1992) and much lower TiO2 and Nicompared with the least evolved high-Al gabbros from Proterozoic anorthositecomplexes (Emslie, 1980; Mitchell et al., 1995). In addition, they do not exhibit the strong Fe enrichment that is characteristic of the anorthosite-related high-Al gabbros. Many of the high-Al gabbros from Proterozoic anorthosites show strong positive Eu anomalies, which are not characteristic of high-Al basalts from subduction environments (e.g. Myers et al., 1986; Myers, 1988). Because of these geochemical differences, it is evident that the high-Al gabbros related to Proterozoic anorthosites have an origin distinct from that of the subduction-related high-Al basalts.
The origin of the high-Al basalts associated with anorthositecomplexes can be evaluated using the analyses of high-Al gabbros from the LaramieComplex, which have a wide range ofcompositions (Mitchell et al., 1995). One way to show thiscompositional range is on a plot of MgO (which monitors the crystallization of ferromagnesian silicates) vs Al2O3 (which monitors the crystallization of plagioclase) (Fig. 7 ). There are two apparent trends on this diagram. The first shows variable Al2O3 at slightly decreasing, but high values of MgO, whereas the second starts at high Al2O3 and MgO values and shows decreasing MgO with decreasing Al2O3. We interpret these trends to indicate that two major processes were involved in the magmatic evolution of these rocks. Trend (1) shows a transition from a magma that is of normal tholeiitecomposition to high-Al basalt, and Trend (2) is the differentiation trend from high-Al basalt to ferrodiorite (Figs 3 and 5 ).
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The existence of the first trend implies that there is a mechanism by which tholeiitic basalts may evolve to the high-Al basalts of anorthositecomplexes. As suggested by Wiebe (1990)
, we propose that resorption of plagioclase andcontemporaneous fractionation of ortho- and/or clinopyroxene (which may be tentatively regarded as the megacrysts observed in many anorthositecomplexes) during rise of the magma or during influx of new, hot magma is responsible for the formation of this type of high-Al basalts. We observe two justifications for this process: first, resorption of plagioclasecombined with fractionation of, for example, Cr- or Ti-spinel will increase the melt in Al2O3 whereas MgO can stay relatively unchanged. Second, resorption of plagioclase also increases Eu in the melt, producing the positive Eu anomalies observed in many of the Proterozoic high-Al gabbros. The strongly positive Eu anomalies of the high-Al gabbros may also explain why ferrodioritic rocks often have low Eu anomalies or even lack them entirely (Mitchell et al., 1996; Scoates & Frost, 1996). It has long been argued that these rocks cannot be residual after extensive crystallization of plagioclase because they lack a negative Eu anomaly. Apart from the process of plagioclase resorption, the derivation of melts with strongly positive Eu anomalies from the mantle is difficult to reconcile with the known mantle mineralogy and geochemistry.
The process of plagioclase remelting may produce a variety of high-Al basaltic magmas. Depending on the amount andcomposition of the plagioclase and the fractionating pyroxenes and on the amount of fractionation in the tholeiitic basalt before resorption of plagioclase, the high-Al basalts can show various XFe, CaO or SiO2 values. Thus, then the samples with the lowest Al2O3 and highest MgO should represent the least evolved magma. Such rocks are found in the Laramie AnorthositeComplex (e.g. sample BM-10), where they havecompositions typical of tholeiites. Sample BM-10 has no Eu anomaly, high Ni and Crcontents, 11 wt % MgO and 14 wt % Al2O3 (Table 1). Extrusive rocks with nearly identicalcomposition are found in Hawaii (Macdonald & Katsura, 1964) and on the Hebrides (Carmichael et al., 1974), the former representing an oceanic, the latter acontinental tholeiitic province.
Plagioclasecompositions and density contrasts
Proterozoic anorthosites are characterized by plagioclase of thecomposition An45–An55 and rarelycontain plagioclase above An60. This observation may be explained by the experimentalconstraints of Fram & Longhi (1992) and Longhi et al. (1993), according to which more calcic plagioclase is not a liquidus phase of anorthositic parental melts at high pressures. However, we propose an additional process that maycontrol plagioclasecomposition in Proterozoic anorthosites. At low pressures the densitycontrast between more calcic plagioclase and basaltic melt is too small to permit efficient accumulation at the top of the deep-seated magma chamber. Figure 8 shows results of calculations with the MELTS program of Ghiorso et al. (1994), where the densities of the least fractionated high-Al gabbros, of tholeiites, of the calculated melts, of a ferrodiorite and of various plagioclasecompositions were calculated at 10 and 4 kbar. Figure 8 shows that the densitycontrast during early stages of fractional crystallization is around 0.1–0.15 g/cm3, until the plagioclase reaches acomposition of about An60 and, especially, until the melt reaches high values of Fe and of XFe. Thus, plagioclase of An>60–65 probably is not able to float in primitive tholeiitic magmas.
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The original magma to start with: a tholeiitic basalt
The arguments above imply that the parental melts to anorthositecomplexes are not of extraordinarycomposition; they are tholeiitic. Thecontrolling parameter in the formation of Proterozoic anorthosites is probably not the meltcomposition, but the specific crystallization history (resorption of plagioclase in melts), and this in turn may be strongly influenced by the tectonic setting. The most obvious way for resorption to happen is as a result of movement of a high-temperature tholeiitic magma through a deep-seated magma chambercontaining intermediate plagioclase. Because intermediate plagioclase will crystallize from relatively unfractionated tholeiitic melt only at high pressures (Fram & Longhi, 1992
; Longhi et al., 1993), this leads directly to the two-stage model of anorthosite formation (e.g. Emslie, 1978; Duchesne, 1984; Longhi & Ashwal, 1985; Wiebe, 1992; Ashwal, 1993; Emslie & Hegner, 1993; Longhi et al., 1993; Emslie et al., 1994; Scoates & Frost, 1996). Plagioclase stable at high pressurescould be remelted either because of a pressure decrease (Longhi et al., 1993) related toconvective lithospheric thinning (Corrigan & Hanmer, 1997) or because of injection of new hot magma into the magma chamber in a plume-related environment (e.g. Hoffman, 1989). Under high-pressureconditions at the crust-mantle boundary, the stable crystallizing plagioclase would be of intermediatecomposition and would float in the crystallizing magma;continuous fractionation of these high-Al melts can eventually lead to anorthositic cumulates buoyant enough to rise to high crustal levels as crystal-rich mushes (Fram & Longhi, 1992; Wiebe, 1992; Ashwal, 1993; Longhi et al., 1993; Emslie et al., 1994; Lafrance et al., 1996) whereas the melt entrained in this crystal mushcontinues to fractionate. During ascent and emplacement at mid-crustal levels, the successively more evolved liquids may be removed at various stages of fractionation because of filter-pressing. Our results strongly support that these liquids are the ferrodiorites observed in most anorthositecomplexes.
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Acknowledgements |
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This work has benefited from many stimulating discussions with B. Wood, Bristol, and from criticalcomments of K. Bucher and J. Schumacher, Freiburg, on an earlier draft of this manuscript. Extremely thorough reviews by J. Scoates, J.-C. Duchesne and D. H. Lindsley are very much appreciated. The analytical work of J.Eisele and S. Geiger with mineral separates used in this study is gratefully acknowledged. This work was funded by the Deutsche Forschungsgemeinschaft with Grants Bu 843/3-1, 3–2 and 3–3.
* Corresponding author. Telephone: xx 49 761 203 6414. Fax: 49 761 203 6407. e-mail: markl{at}ruf.uni-freiburg.de
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References |
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TOPABSTRACTIntroductionWhole-Rock ChemistryCalculations With Partition...DiscussionGeneral ConclusionsReferences |
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