Chromian spinel in the Ivrea-Verbano layered igneous complex, western Alps, Italy

TMPM Tschermaks TMPM Tschermaks Min. Petr. Mitt. 29, 33-53 (1981) Mineralogische

trod Petr0graphische /~tte i lungen

�9 by Springer-Verlag 1981

From the Istituto di Mineralogia del!'Universith di Ferrara, the Istituto di Mineralogia e Petrografia dell'Universit/t di Modena and the Istituto di Mineralogia e Petrografia dell'Universitg di Trieste, Italy

Chromian Spinel in the Ivrea-Verbano Layered Igneous Complex, Western Alps, Italy

G. Rivalenti, G. Garuti, A. Rossi, F. Siena, and S. Sirfigoi

With 7 Figures

Received February 5, 1981;

accepted June 9,1981

Summary

Spinel occurs in the pyroxenitic, peridotitic and gabbroic layers of the Ivrea-Verbano layered igneous complex. Its.composition varies between picotite and Mg-hercynite. Maximum Cr203 contents are found in the spinels of the dunitic cumulitic layers occurring at some height above the base of the complex. Chromium deposits are absent. The lack of chromite and of chromium deposits is attributed to the early fractionation of clinopyroxene (which depleted the residual liquid in chromium) instead of olivine, as a consequence of the relatively high pressure prevailing during crystallization (~ 8 kb). The relationships between)"O2-composition of spinel-composition of tile silicate phases indicate that fO2 exerted a major control on the internal stratigraphy of the single layers and on the pattern of fractionation. The variation offO2 are on their turn related to accidental variations of the pressure acting on the magma. It is finally suggested that chromium deposits are limited to low pressure layered intrusions, where the magma has been emplaced rapidly into a shallow magma chanaber. No chromium concentration is to be expected in those complexes that crystallized into deep-seated magma chambers and that fractionated at relatively high pressure, not markedly different from that at which the magma was produced. One of the major controls on the presence of chromium deposits results, therefore, to be the geotectonic environment of intrusion.

Zusammenfassung

Chrom-Spinell im geschichteten vulkanisehen Komplex yon fvrea-Verbano, Westalpen, ltalien

Fttr die Spinelie der Pyroxenit-, Peridotit- und Gabbrc-Lagen des Ivrea-Verbano-Intrusiv- komplexes wurde eine Zusammensetzung im Bereich Picotit und Mg-Herzynit bestimmt. Die Dunit-Lagen im basalen Anteil des Komplexes fuhren Spinelle mit Cr2 03-Gehalten bis zu 30 Gew.-%. Chromerz-Konzentrationen fehlen jedoch, was auf die fraktionierte Kristallisation yon Klinopyroxen (und Entstehung Cr-Restschmelzen) an Stelle yon Olivin, als Folge der bei relativ0hohem Druck (8 kbar) erfolgten Intrusion, zuriickzufNaren ist.

3 TMPM 29/1

0 0 4 1 - 3 7 6 3 / 8 1 / 0 0 2 9 / 0 0 3 3 / $ 04.20

34 G. Rivalenti et al.

Die Beziehungen zwischen Sauerstoffpartialdruck und Spinell- und Silikat-Zusammen- setzung zeigen den bedeutenden fO~-Einflug auf den stratigraphischen Aufbau der einzelnen Gesteinslagen und auf die fraktionierenden Kristallisationsvorg~inge. Die schwankenden fO2-Werte stehen in Zusammenhang mit den ebenfalls variierenden auf das Magma wirkenden Drticken. Es ist anzunehmen, d ~ Chromerzkonzentrationen nur in geschichteten Intrusiv- komplexen vorkommen, die sich unter niederem Druck bzw. in einer oberfliichennahen Magma-Kammer bildeten, und nicht in jenen, die tiefer intrudierten und bei hohem Druck kristallisierten. Von groger Bedeutung ftir die Bildung yon Chromerzlagerst/itten ist also das geotektonische Milieu der Intrusion.

Introduction

The Ivrea-Verbano Complex, belonging to the Hercynian basement of the Southern Alps, has been recently recognized as a deep-seated, possibly multiple, intrusion (Rivalenti et al., 1975; Rivalenti et al., 1980; Garuti et al., in press). According to Garuti et al. (1978-79) and Rivalenti et al. (1978-79) it crystallized at a pressure of 8 - 9 kb and was re-equilibrated during slow cooling at a temperature of 8 0 0 - 9 0 0 ~ at the same pressure. The complex displays a well defined stratigraphy consisting of: a) a lower layered series; b) a main basic body of massive gabbro; c) an upper diorite group (Rivalenti et al., 1975;Rivalenti et al., 1980; Garuti et al., in press). Slices of mantle peridotites, the best known of which are those of Baldissero, Balmuccia and Finero, occur in the lower part of the Complex (Lensch, 1971 ; Rivalenti et al., 1975 ; Capedri et al., 1977a; Shervais, 1979; Rivalen ti et al., 1980;Sinigoi et al., 1980). Stratigraphically controlled mineralizations of N i - F e sulphides and F e - T i oxides are found in the layered series (Bigioggero et al., 1978-79a) . The analogies with other well known layered intrusions suggest that chromium deposits might occur also at Ivrea-Verbano. However, they have never been found in this complex. This paper is aimed at checking why they do not occur and at discussing the processes controlling spinel crystallization and composition. Cursory petrographic examination reveals that spinel occurs mainly in the lithotypes of the layered series and, therefore, this study deals with this portion of the Complex. The chromian spinel in the mantle tectonites of Ivrea, related to partial melting phenomena, rather than fractionation (Medaris, 1975;Rivalenti et al., 1978-79) , is not re-considered here.

Stratigraphy of the Layered Series and Occurrence of Spinel

The best occurrences of the layered series are those of Sesia Valley and, therefore, the description is referred to the outcrops of this locality.

Chromian Spinel in the Ivrea-Verbano Layered Igneous Complex 35

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~ SULPHIOE MlllERALISArIO X

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Fig. 1. Schematic stratigraphy of the layered series in the Ivrea-Verbano Complex

The layered series is composed of cyclic repetitions of layers of pyroxenite, peridotite and gabbro in the lower portion and of pyroxenite, gabbro and anorthosite in the upper part (Rivalenti et al., 1975). Its schematic stratigraphy is shown in Fig. 1. The lithologicat succession begins with olivine- bearing websterites (~ 50 cm thick), followed by websterites with intercumulus plagioclase and grading upwards into pegmatoid clinopyroxenite. Peridotitic layers (the first peridotite unit) with intercumulus clinopyroxene appear near the base, but only after the crystallization of some pyroxenitic units. These peridotites grade upwards into olivine websterites and gabbros and are followed in turn by other units of websterites, gabbros and pegmatoid clinopyroxenites. Another peridotitic cycle (the second peridotite unit), where peridotites are interlayered with pyroxenites and leucogabbros, appears only much higher in the sequence, after the deposition of a sensible amount of pyroxenites and gabbros. As will be shown in a forthcoming paper, the internal stratigraphy of the single layers is complicated by igneous lamination, cryptic layering, small

3*


scale variation of phase composition. As in other complexes (Irvine, 1978; Jackson, 1970; Cameron, 1980), inverted fractionation pattern is common. It occurs at the stratigraphic height of the first and the second peridotite unit and is manifested by a higher Mg/Fe 2+ ratio of the phases in these units with respect to the same phases of the lower layers and by the appearance of Fo-rich olivine (cf. Fig. 2). Spinel is unevenly distributed in the cumulitic peridotites, pyroxenites and gabbros. In the peridotites, it occurs either as an interstitial phase, or, more rarely, as euhedral inclusions in olivine or orthopyroxene, or in rare, dis- continuous, sometimes boudinaged monomineralic layers, with the arrange- ment represented in the schematic stratigraphic sections of Fig. 2. The monomineralic spinel layers are generally very thin (1 or 2 cm), but occasionally some boudins may reach a thickness up to 20 cm. They are invariably followed immediately upwards by an orthopyroxenite layer. The highest spinel concentration are found when the peridotite is dunitic. In the pyroxenites, spinel occurs, together with plagioclase, as an interstitial intercumulus phase. Among the pyroxenites, only the olivine websterites have centimetric size spinel layering, while the pegmatoid clinopyroxenites have no spinel. In the gabbros, spinel, when present, occurs as a dispersed interstitital phase.

Composition of Spinel

Representative microprobe analyses of spinels are reported in Table 1. The analyses have been recast to structural formulae on the basis of 32 oxygen atoms. Trivalent iron has been calculated assuming stoichiometry. The chemical characteristics of spinel are shown in the diagrams of Fig. 3 and 4. The composition of spinel in the layered series varies between picotite and Mg-hercynite. The only chromian spinels are found in the peridotite layers, where they show a wide range in composition, containing between 0 and 30% (by weight) Cr~ 03. In the pyroxenite and gabbro layers, spinel is always Cr-poor. Within the pyroxenites, spinels rich in chromium are found in the olivine websterites. The compositional variations of spinel from one layer to the other and within the same layer are summarized in the stratigraphic sections of Fig. 2. It is shown that, although the relationships between spinel composition and

lithology described above are generally respected, the variations within the same layer are very complicated. As the Fe-Mg silicates do, they do not follow a simple fractionation pattern. Spinels are frequently zoned. In the Cr-poor members zoning involves mainly


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XMg (= Mg/ER ~+) (b). Arrows indicate zoning from centre to rim. Dots = peridotites; squares = pyroxenites; triangles = gabbros. Filled symbols = first peridotite unit and basal units; open symbols = second peridotite unit

Fe3+-.AI and M g - F e 2+ substitutions, while in the Cr-rich spinels zoning results in variations o f Mg, A1 and Fe ~+. In bo th cases the variations have not a constant direction (Figs. 3a and b).

Relationship o f Spinel with the Silicate Phases

Spinel-olivine and spineforthopyroxene. Since the works o f Irvine (1965, 1967), the coexistence o f spinel and olivine has been widely used as a petrogenetic indicator (see e.g. Jackson, 1969; Medaris, 1975 ; Evans and Frost, 1975;Rivalenti et al., 1 9 7 8 - 7 9 ) . Fig. 4 is a plot o f the parti t ion coefficient for the M g - F e exchange equilibrium between olivine and spinel as a

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Fig. 4. Logarithm of the partition coefficient for Fe-Mg exchange equilibria between olivine and spinel (KD) as a function of YCr (= CRIER 3+) of spinel. Isotherms are taken fromMedaris (1975), Samples have been plotted on the plane Fe3+/Y,R 3+ = o. Symbols as in Fig. 3

function of YCr (= Cr/R 3§ of spinel for peridotite and olivine websterite layers. The trend of the samples cut across the theoretical isothermal planes. Owing to the constant distribution coefficient for Fe 2+ and Mg between olivine and orthopyroxene found in these rocks, it is expected that the relationships between spinel and orthopyroxene present the same features as those between spinel and olivine. Clinopyroxene-spinel. Fig. 5a shows the relationship between the chromium content of spinel and the modal amount of clinopyroxene: the two variables are negatively correlated. Fig. 5b shows the Cr distribution between the two phases. Generally a higher chromium content of spinel corresponds to a


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Fig. 5. a) Relationship between gCr (= Cr/ER ~+) in spinel and bulk CaO (wt.-%) of the rock, considered as indicative of modal clinopyroxene. Symbols as in Fig. 3. b) Relationship between Cr content (wt.-%) in spinel and clinopyroxene. Symbols as in Fig. 3. Asterisks are pyroxenite dikes in mantle peridotite of Baldissero and Balmuccia (Sinigoi et al., 1980; Capedri et al., 1977b; Rivalenti et al., 1978-79), The lines have been fitted by eye to the trends of the pyroxenites in the mantle peridotite, of the first and the second peridotite unit

higher chromium content in clinopyroxene. The trend, however, is different for the lower and upper peridotite layers: for a given Cr content in spinel the second unit shows a comparatively lower Cr content in clinopyroxene, which is also lower in AI with respect to the clinopyroxene of the first unit (Table 3).

Relationships o f fO2 with the Spinel Composition

As demonstrated by several authors (Speidel, 1967; Hill and Roeder, 1974; Hamilton et al., 1964; Yoder and Tilley, 1962; Cameron and Desborough, 1969) the oxygen fugacity of the magma exerts an important control on the amount and composition of the crystallizing spinel, fO2 may be estimated in presence of a suitable buffer, such as the coexistence of or thopyroxene, olivine and magnetite. In the present olivine-bearing rocks an estimate has been achieved by the magnetite s.s. content in spinel. The reliability of the results is affected by several uncertainties, such as the possible non-stoichio- metrs) o f the spinel (Ulmer, 1969), the uncertainties of the thermodynamic data, the methods for calculating the magnetite content of spinel and the


Table 2. Log f02 in the Rocks of the First and Second Peridotite Unit. Method for logfO2 calculations is given in the text

Log. (12oooc) Si/Cr Cr s 2

Sample Spinel Centre Spinel Rim Atomic Ratio ppm

Mo 935 -10.79 -11.77 31.75 3356

Mo 940 - 9.69 -!0.48 35.78 2500

Mo 941 B -!0.41 - 9.67 40.34 3050

Mo 943 -13.01 -!2.59 200.85 510

Mo 944 -10.72 -11.59 86.!2 2850

Mo 946 - 9,10 - 9.30 9.55 10506

Mo 1154 A - 9.28 10.75 9860

Mo 1 1 9 4 B - 9 . 2 7 - 9 . 2 0 0 . 4 3 56037

Mo 1 1 5 4 C - 9 . 6 4 - 1 0 . 5 1

Mo 1 1 5 6 Pd - 1 0 . 3 2 - 1 0 , 1 7 1895

Mo 947 - 8 , 2 8 - 8 , 4 8 1 1 . 2 0 9200

Fe 5 - 7 . 6 6 1 9 . 7 2 5080

Fe 10 - 8 . 2 9 1 8 . 3 4 5130

Fe 11 - 9 . 4 0 4 2 . 0 0 2760

Fe 12 - 8.51 8.62 11070

Fe 14 A - 1 0 . 4 4 4 3 . 6 4 2484

Fe 14 B - 9.65 4 3 . 4 2 2240

Fe 16 - 9.69 59.02 1980

Fe 22 -10,79 113.30 940

Fe 25 - 8.69 65.06 1625

Fe 26 - 9.64 43.34 2925

Fe 37 - 8.12 39.52 2780

Fe 39 - 8.84 39.84 2420

Table 3, Al203 Average Content (by Weighty of Clino- and Orthopyroxene in Pyroxenite and Peridotite Layers of the First and Second Perido tite Unit

A1203 (wt.-%)

First peridotite unit Second peridotite unit pyroxenite peridotite pyroxenite peridotite

clinopyroxene 7.94 6.09 4.49 3.50 orthopyroxene 5.39 3.44 3.88 2.68

Cbromian Spinel in the Ivrea-Verbano Layered Igneous Complex 43

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Fig. 6. Relationship between log fOx and YA1. (= A1/2R3§ XFe (= Fe/~R 2+) and YCR (= Cr/ZR 3+) of spinel, Symbols as in Fig. 3

relationships between concentration and activity of the components. There- fore, the calculated fO2 should be considered only as an approximation. The equation used, constructed on the basis of the thermodynamic data of Morse (1979) is the following:

log. fO2_ 25 470 - - T - - T ~- 8.992 + 0.0388 (P 1) +

+ 2 log a Fe 3 O.4--3 log a Fe 2 SiO 4 - - l o g a Mg2 SiO4 + log a Mg 2 Si2 06.

This equation can be resolved only if an independent determination of T and P is available. A constant P = 8 kb and a constant T = 1200 ~ have


been assumed here, as these values approximate the original conditions of crystallization (Garu t i e t al., 1978-79; Rivalen ti et al., 1978-79; Garu ti et al., in press;Rivalenti et al., 1980). In any case, an error in the assumed T and P should shift the calculated fO~ to higher or lower values, without affecting the reciprocal relationship of the samples. It has also been assumed that the T and P variations during the crystallization of the layered series are negligible. This is possibly true within the lower and the upper peridotite

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units, owing to their respective modest thickness, but it is possible that a variation, mainly of pressure, occurs between the two series. In spite of all these limitations, the results shown in Table 2 are in agreement with possible values of f O 2 of basaltic magmas, although here f O 2 remains constantly at a low level. The plots of Fig. 6 show that the variations o f fO2 are directly related to the Cr and Fe 2+ contents of spinel, while they are inversely related to its A1 content. These trends are in agreement with the experimental results of Muan (1957) and of Hill and Roeder (1974) on spinel crystallizing at constant T and varying fO2 from basaltic liquid. Oxygen fugacity is also related to the composition of silicate phases coexisting with spinel: the more Mg-rich olivines (and pyroxenes) are related to the highest fO2 (Fig. 7). The upper and lower peridotite units show, however, slightly different trend, as happens on the other hand in most plots. This may suggest either that the assumption that the two units have fractionated under the same T and P conditions is erroneus, or that from one unit to the other the magma had changed its composition, or both.


Nevertheless, a comparison of the trends of Fig. 7 with the experimental data of Mysen (1973) suggests that, at least within the same unit, the Fe/Fe+Mg ratio of the magma has remained constant. If this is the case, the compositional variations of spinel and silicate phases (within the same unit) are principally a function of fO 2 and cannot be attributed to the changing Fe/Mg ratio of the magma.

Chromium Content of the Parent Liquid

Mantle derived primitive liquids are theoretically rich in chromium (up and above 1000 ppm, see e.g. Irvine, 1979, p. 254; Kurat et al., 1980). The parent magma of most chromite-bearing complexes is not: provided that the chilled margins somehow represent the composition of the parent magma, the chromium content is 170 ppm at Skaergaard (Wager and Brown, 1967); 274 ppm at Stiliwater (Hess, 1960); 547 ppm at Muskox (Smith and Kapp, 1963). This suggests that primitive mantle-derived magmas undergo extensive Cr-depletion before crystallizing in the crust. On account of the deep seated environment of intrusion, chilled margins do not occur at Ivrea-Verbano. Capedri et al. (1977b) and RivaIenti et al. (1980), however, have postulated a genetic link between the parent magma of the complex and the episodes of partial melting that occurred in the closely associated mantle tectonites. Sinigoi et al. (I 980) indicate that melting in the spinel facies of these peridotites gave rise to a liquid of picritic composition. While flowing in the mantle (Irving, 1980), this melt underwent fractionation. It initiated in conditions very close to those of melting with the crystallization of Cr-diopside, orthopyroxene, tittle chromian spinel and traces of olivine. Under decreasing P and T, fractionation continued with the crystallization o f pyroxene and large amounts of AI-spinel (up to 30% by volume of the fractionate). These early fractionation episodes gave rise to pyroxenite dikes occurring in mantle tectonites. The residual liquid, represented by gabbroic pods occurring in the mantle peridotite close to the layered series of the complex, contains as much as 430 ppm Cr (Capedri et al., 1977b) and is considered to be the possible parent magma of the complex. The depletion of the Cr content of the primitive liquid down to the amount found in the residual magma, is thought to have been caused by the pyroxene fractionation that occurred dumag the flow through the mantle: the Cr203 content of the first pyroxene is higher than 1% (by weight) (Shervais, 1978-79 , 1979; Garuti et al., 1978-79; Sinigoi et al., 1980) so that a 10% fractionation would deplete a primitive liquid containing as much as 3000 ppm Cr20 a down to 300. The values of 430 ppm in the parent magma of the Ivrea-Verbano complex is, therefore easily explained by the fractionation predating the intrusion into the crust.


Discussion

It has been shown that the absence of chromite in the Ivrea-Verbano complex cannot be attributed to a particular depletion of chromium in the parent magma. It has also been shown that the only Cr-spinel is found in peridotite layers, corresponding to an inversion of the fractionation pattern, while spinel occurring in pyroxenites is invariably an aluminiferous phase. Fractionation and the causes controlling the inversion of fractionation are, therefore, the most probable controls of spinel composition. The fractionation pattern of the Ivrea-Verbano complex has been cursorely examined in a number of papers (Rivalenti et al., 1975, 1980; Capedri et al., 1977b) and is studied in detail in a forthcoming work. Here it is only briefly summarized.

Fractionation pattern and causes of inversion. The parent liquid of the complex is normatively an olivine basalt. At the pressure of fractionation of the complex, it lay in the olivine primary field, close to the ternary peritectic point enstat i te-ol ivine-diopside, of the simplified system ol ivine-diopside-SiO 2 .This is shown by the modest amount of fractionated olivine (only 50 cm of olivine websterite) and by pyroxene-dominated composition of the first layers. The reversal o f the fractionation pattern, occurring/ 'or the first time at the stratigraphic height o f the first peridotite unit, implies a shifting of the magma in the olivine volume. Reversals o f this kind are common in most layered complexes and have been attributed to various causes: periodic injections of new magma and mixing with residual liquid (Irvine and Smith, 1969; Hughes, 1970; Coertze, 1970; Cameron, 1980), contamination of the magma by mixing with metasediments or granitic liquid (Irvine, 1975; 1977), temperature decrease due to heat loss against a cold wall or undercooling at the base of a new magma flow (Campbell, 1977), variations of pressure acting on the magma (Cameron, 1980; Osborn, 1980) and changes in the gas content (Ulrner, 1969; Hill

�9 and Roeder, 1974). For the following reasons the first three of this causes appear to be unaffective or umprobable at Ivrea-Verbano. It is doubtful that mixing of magma can be so perfect to give layers with the remarkable lateral continuity that they display in this complex, as well as in others. Contamination occurs at Ivrea-Verbano near metapelitic septs or inclusions, but the contaminated rocks, rather than showing reversals of fractionation, are the sites of deposits for Ni-sulphides (Ferrario and Garuti, unpublished data). The Ivrea-Verbano complex had intruded into deep crust, where the rocks were already in a fairly high thermal state (high amphibolite or granulite facies). Therefore, the temperature gradient between magma and country rock cannot have been high. Anyway, the reversal does not occur, as should be expected by cooling, at the contact with the country rock, but at some distance from it.


The last two causes (variation of pressure and gas phase) are, we believe the most reasonable candidates to cause the inversion at Ivrea-Verbano. Variations of pressure, due to tectonism affecting the magma chamber, are here suggested by the dynamic environment of intrusion (Rivalenti et al., 1980). As these variations acted on a liquid they would have an instantaneous effect over great masses and may therefore account for the lateral continuity of the layers (Cameron, 1977; 1980). The effect of pressure variations may be evaluated from the diagrams presented by Presnall et al. (1978) on the system Fors ter i te -Anor th i te -Diops ide-SiOz : a pressure decrease reduces the field of pyroxene, which coexist no more with spinel below 5 kb, expands the olivine and anorthite volume and deforms the spinel field. The composition of the basal layers indicates that the liquid, before the crystallization of the peridotite, was close to the ternary point diopside--orthopyroxene-spinel , in proximity of the olivine field. A pressure decrease may cause an excursion of the liquid into the olivine or spinel field or along a phase boundary, depending on the amount of pressure variation. A further support to pressure variations is given by the composition of pyroxenes: several experimental works indicate that pyroxene crystallizing at high pressure are richer in alumina with respect to those crystallizing at lower pressure from a liquid of the same composition. Table 3 shows that A1203 is higher in or thopyroxene and clinopyroxene of pyroxenite with respect to peridotite layers. This happens within both peridotite units, but the stratigraphically higher one has a lower A12 03 in pyroxenes. Variations of fO 2 are also able to cause the reversal in the fractionation trend and even the fractionation of monomineralic layers, as indicated by various experimental works (Ulmer, 1969; Mysen, 1973; Hill and Roeder, 1974) and supported by the diagram of Fig. 7. Mechanisms such as assimila- tion of ground water or breathing of a vent have been proposed by various authors as possible cause forfO2 variations (Ulmer, 1969; Cameron, 1971 ; Snethlage and Klemm, 1978), and criticized by others (Osborn, 1969; Cameron, 1977). It is possible that also the variations o f f O 2 are related to variations of the pressure acting on the liquid, as suggested by the experimental works of Kadik and Eggler (1974). These authors show that the H2 O/CO2 ratio of the magma increases at decreasing P and this conforms with a higher fO2 in the peridotite layers. The characteristics of the pegmatoid pyroxenites give a similar indication: their giant grain size points to the presence of a fluid, while the absence of spinel points to a minimum fO2. As they should correspond to moments of re!ative high pressure, the H20/CO2 ratio should have been small. While the variations of load pressure (and related fO2) easily explain the succession of the layers and the repetitive cycles, the fine variations within a single layer are probably the consequential path of the liquid towards equilibrium, after the shifting determined by a critical variation of P and


fO2. This appears to be the case for the orthopyroxenites that follow the monomineralic spinel layers in the peridotites: spinel crystallization causes an increase in the activity of silica of the liquid, which must crystallize, for the given Mg/Fe ratio, orthopyroxene instead of olivine. The system might continue to evolve by auto-induced and reciprocally controlled variations of silica activity and fO2 according to the scheme proposed by Morse (1979). The mechanism presented above has been referred mainly to the crystallization of the basal unit, and it does not imply any sensible variation in the composition of the parent liquid. On account of a slight increase in the Fe/Mg ratio of the silicate phases in pyroxenites and gabbros, it is possible that the magma has suffered some iron enrichment before the crystallization of the second peridotite unit. However, the reappearance on the liquidus of olivine having the same average composition as that of the first peridotite unit (Fig. 2) suggests that the Fe/Mg ratio of the magma has not changed much and that pressure and related fO2 are the main controlling factor. Controls o f spinel composition. The model presented above permits to make some qualitative speculations on the chromium behaviour in the magma and on the controls on spinel composition. According to Burns et al. (1972) a high pressure favours the solubility of transition metals (such as Cr and Fe) in silicate liquids. It has been assumed that the parent magma contains 430 ppm Cr. The cumulus clinopyroxene of the basal pyroxenite layers contains up to 0.6% (by weight) Cr2 03. On account of the reaction relationship existing between clinopyroxene and chromite (lrvine, 1965, 1974; Dickey, 1975) it is therefore likely that the liquid was constantly under- satured with Cr during pyroxene fractionation. This gives the reason why the spinel coexisting with clinopyroxene is a high-A1 phase. The ignorance of what percent of fractionation the basal pyroxenites represen[, does not permit to estimate the degree of Cr-depletion caused by their crystallization on the residual magma. On account, however, of the modest thickness of these layers, it is likely that the Cr content of the magma did not change much. The pressure release, and related increase in fO2, which causes the disappearance of clinopyroxene and the appearance of olivine on the liquidus, causes concomitantly the disappearance of the c l inopyroxene- chromite reaction relationship. Chromium become therefore available to enter spinel. Only occasionally, however, the variations of P and fO z are sufficient to cause an oversaturation of the liquid with the spinel phase, as shown by the rarity and modest thickness of the spinel layers. It is likely that P and f O 2 variations, although have caused a shifting of the phase boundaries, have not been such to alter the generally high pressure and low fO2 regime of environment. According to Muan (1957) a high pressure favours the entry of A1 in spinel. This, together with the absence of plagio-


clase on the liquidus, has prevented the formation of chromite, and caused the crystallization of the chromian spinel found in the first peridotite unit. The continuous crystallization of pyroxenes, shown by the pyroxenite and gabbro layers occurring above the first peridotite unit, further depletes the chromium content of the residual liquid. The spinel of the second peridotite unit, however, have more chromium and a higher Fe3+/Fe 2+ ratio with respect to those of the first unit. Both these features find a possible explanation in a lower fractionation pressure (and related higher fO2 ) of the second unit with respect to the first: a lower pressure may saturate the liquid with less chromium, so, even if bulk Cr 2 03 of the magma has de- creased, spinel may contain a higher amount of this element. As for the Fe3+/Fe 2+ ratio, the experimental works of Osborn et al. (1979) in the range 1 - 1 0 kb, indicate that, for a large span of bulk composition of the liquid, this ratio increases in magnetites as the pressure decreases. By coincidence, also within the same unit, the spinels of the peridotite layers have, on average, a higher Fe3+/Fe 2+ ratio than those of the associated pyroxenites (Table 1). If pressure, and related f O 2 variations, control the composition of spinel, as a logical consequence also the spinel-silicate phases relationships are likely controlled by the same factors. This is the case of the relationship between the chromium content of spinel and that of clinopyroxene (Fig. 5b): the difference of variation trend at various stratigraphic levels is probably related to variations of the above parameters. Unfortunately, there is a surprising lack in experimental works on the dependence from pressure and f O 2 of the partition coefficient of chromium between clinopyroxene and spinel. However, the high Cr content of the clinopyroxenes of kimberlites (up to 12% Cr 2 03 by weight, Mayer, 1975) suggests that a high pressure favours more chromium enter this phase. In the scheme presented above the trend shown by Fig. 4 (relationship between the distribution coefficient for Fe 2+ and Mg between olivine and spinel and chromium content of spinel) has obviously no thermometric meaning: it is likely controlled mainly by fO2 of the system. The relationship between modal clinopyroxene and chromium of spinel (Fig. 5a) finds an explanation in the reaction relationship existing on the liquidus and in the subsolidus between clinopyroxene and chromite (Irvine, 1965, 1974;Dickey, 1975).

Conclusions

It has been shown that spinel occurs in the layered series of the Ivrea- Verbano Complex and that the more Cr-rich spinels are found in cumulitic peridotite layers. The absence of chromite and of chromium deposits has

4 TMPM 29/1


been related to the conditions of fractionation of the complex, and not to a low Cr2 03 content of the parent magma, which, in this respect, does not appears to be substantially different from that of other chromite- bearing complexes. Chromite has not formed at Ivrea-Verbano because of the high pressure of fractionation (~ 8 kb), which increased the solubility of chromium in the liquid and caused the early appearance of pyroxenes on the liquidus. The continuous crystallization of Cr-bearing clinopyroxene maintained the residual liquid constantly undersaturated with chromium. Saturation of the liquid with a spinel phase occurred only in concomitance with an inversion of the fractionation pattern corresponding to the peridotite layers and attributed to pressure releases caused by tectonism of the magma chamber. The composition of spinel formed during these episodes, never reaching chromite, is attributed to the generally high pressure of fractionation, and related fO2, besides to a high A12 03 activity of the liquid when plagioclase is not on the liquidus. An examination of other chromite-bearing layered complexes reveals that they are all of low pressure type, that they 'generally start with basal peridotitic zones and .that they are intruded into anorogenic continental zones (Carmichael et al., 1974; Hess, 1960; Wager and Brown, 1967). This implies that the magma was emplaced relatively rapidly, that there exists a great difference between the pressure of magma formation and that of its crystallization and that, therefore, the liquid starts its fractionation well in the field of olivine, where the accompanying liquidus phase is chromite. The Ivrea Complex differs from these chromite-bearing bodies for its orogenic environment of intrusion (Bigioggero et al., 1978-79 ; Garuti et al., 1978-79;Rivalenti et al., 1975;Rivalenti et al., 1980) and for the higher pressure of crystallization. In this situation the magma rose much slowlier with respect to that of the anorogenic complexes. So, it is possible that the geodynamic environment of intrusions exerts the major control over the presence of chromite deposits.

Acknowledgements

This work has been financially supported by the CNR grant No. 79.00796. The CNR is also acknowledged for financing the installation and maintenance, for use of all the scientists affiliated with the Committee 05 of CNR, of an electron microprobe laboratory at the "Istituto di Mineralogia e Petrografia dell'Universith di Modena", the facilities of which were used in the present work.

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Chxomian Spinel in the Ivrea-Verbano Layered Igneous Complex 53

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Authors' addresses: Prof. G. Rivalenti and Dr. F. Siena, Istituto di Mineralogia dell' Universit~ di Ferrara, C.so Ercole I. d'Este 32, 1-44100 Ferrara, Italy; Dr. G. Garuti and Dr. A. Rossi, Istituto di Mineralogia e Petrologia dell'Universith di Modena, Largo S. Eufemia 19, I-41100 Modena, Italy; Dr. S. Sinigoi, Istituto di Mineralogia e Petrografia dell'Universith di Trieste, Piazzale Europa 1, 1-34127 Trieste, Italy.

Chromian spinel in the Ivrea-Verbano layered igneous complex, western Alps, Italy

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