REGIONAL DISTRIBUTION OF CR AND NI IN MOLDANUBIAN MORB-DERIVED AMPHIBOLITES OF THE BOHEMIAN-MORAVIAN HEIGHTS
Department of Mineralogy, Petrography and Geochemistry, Masaryk University, 611 37 Brno, Czech Republic
Předloženo 27.května 1997, do tisku doporučil Miloš Suk
Keywords: amphibolites, geochemistry, Cr, Ni, regional zonality, Moldanubian Complex, Moravian-Bohemian Heights.
Many amphibolite bodies occur within the Moldanubian Complexes of the Bohemian-Moravian Heights (Českomoravská vrchovina). They mostly represent N-type MORBs metamorphosed to higher zones of the almandine-amphibolite facies. A noticeble decrease of Cr and Ni, respectively, from 585 to 100 ppm and from 275 to 37 ppm, is observed in them from W to E. Concomitantly also the mg numbers and their bulk rock chemistry change. The Cr and Ni contents of the amphibolites show relations to the pre-metamorphic modes of the rocks (as it can be deduced from their CIPW norms), because they are controlled by the presence or absence of olivine and clinopyroxene, the main carriers of Ni and Cr, respectively, in the rocks. The probable cause of their regional zonation is magmatic differentiation of a primordial basalt magma generated by 15 - 20 % partial melting of a mantle source.
Amphibolites from the Moldanubian and Svratka Complexes which cover the area between the Boskovice and Blanice Furrows (Figs 1, 2) were studied. In addition to two large granitoid massifs the territory is largely built of various gneisses, which in the Moldanubian Complex were mostly metamorphosed to higher zones of the almandine-amphibolite facies, in the Svratka Complex to middle zones of the same facies. A detailed regional overwiev of the geology and petrography of the region was recently presented by Zoubek (1988) and Dalmeyer et al. (1995).
Fig. 1. Sketch map of the study area showing location of examined amphibolites. Cr contents, concentrational scale (ppm): squares - 500 - 600, triangles - 300 - 500, points 200 - 300, circles - 100 - 200. I - IV zones of different Cr concentrations. Abbreviations of towns: Vl - Vlašim, Ča - Čáslav, Ta - Tábor, Pe - Pelhřimov, JH - Jindřichův Hradec, HB - Havlíčkův Brod, Žď - Žďár, VM Velké Meziříčí, Je - Jemnice, MK - Moravský Krumlov. Crosses - granitoide massifs.
Most of the Moldanubian amphibolites of the Bohemian-Moravian Heights occur in the Varied group and, consequently, almost all the investigated samples were gained there. A few samples taken from the amphibolites which rim the Náměšť granulites are the only exception. The individual zones of the Varied group are unevenly distributed within the Bohemian-Moravian Heights and so also are the studied samples (Fig. 1, 2). The amphibolite bodies cover about one percent of the territory. The greatest accumulations of the amphibolite bodies occur in the Sázava area around the town of Rataje, and between the towns of Náměšť and Moravský Krumlov, and between Jemnice and Langau in Lower Austria. Large amphibolite bodies are rare. They mostly are approximately isodiametric. However, the most common shape are long strips concordant with the geological structures of the region. They are usually only some tens or hundreds meters thick but often many kilometers long. The amphibolites are usually only bimineral, consisting of predominant, mostly tschermakitic, hornblende and of plagioclase (basic andesine to acid labradorite). Clinopyroxene is an occasionally occurring minor constituent. Garnet appears only exceptionally in greater amounts.
About 200 amphibolite samples were taken in a, as far as possible, regular net and analysed for major components. On the whole 27 samples were analysed by XRF for minor and trace elements, partly in the Institute of Mineralogy and Petrology of the University of Parma, Italy (Dr. L. Toscani, analyst), partly in the Laboratory of the Czech Geological Institute in Prague. The data on amphibolites from the surroundings of Jemnice, published by Matějovská (1987), were used, too.
Fig. 2. Sketch map of the study area showing location of examined amphibolites. Ni contents, concentrational scale (ppm): squares 200 - 300, triangles 100 - 200, points 60 - 100, circles 35 - 60. Other symbols as in Fig. 1.
Importance of Cr and Ni in the study of amphibolites
Fig. 3 is a cumulative multi-element diagram (spidergram) of minor and trace elements present in the examined amphibolites. The broadest variational range is shown by large ion lithophile elements. However, their abundances are not original, having been additionally markedly enhanced by an influx comming from the encasing gneisses during regional metamorphism which often was associated with migmatization (Němec in print b). The concentrations of Nb also show a considerable spread, but Nb is by some authors (Pearce and Parkinson 1993) also included into the number of the highly incompatible elements. Of all other examined elements, Cr and Ni display the broadest variational ranges of abundances. They both are highly compatible and are immobile during regional metamorphism (Beus and Scherbakova 1986, Schmidt 1990, Kataria et al. 1988 and others), yielding, therefore, the original pre-metamorphic concentrations. Of the common rock-forming minerals of basic igneous rocks, clinopyroxene is the best sink for Cr and olivine for Ni. Their partition coefficients are very high (compare Pearce and Parkinson 1993). For instance, even individual olivine megacrysts exhibit a drastic from core to rim Ni decrease (Mysen 1979, Leeman and Lindstrom 1978). This leads to large local variations of Cr and Ni in basic rocks and makes determination of the origin of the magmas and of paths and grades of their differentiation possible (Miyashiro and Shido 1975, Pearce 1980).
Fig. 3. Cumulative multi-element diagram of 16 MORB-derived amphibolite samples from the whole study area (with the exception of the surroundings of the town of Jemnice). The normalizing values from Pearce (1982).
Origin and geotectonic setting of the amphiboles
Abundances of major and trace elements, correlations between individual elements and trends of concentrational changes point unequivocally to igneous origin of the studied amphibolites. This is also endorsed by the Ni vs mg, Ni vs Co and Cr vs V diagrams as well as by the results of investigations in Moldanubian terranes adjacent to the Bohemian-Moravian Heights (Janda et al. 1965, Zaydan and Scharbert 1983, Steyrer and Finger 1994, Patočka 1991). The protoliths of the amphibolites encompass several types of metavolcanics and metagabbros (Němec 1994, 1996a, 1997, in print a). They can be mostly distiguished on the basis of their geochemical signatures (compare also Fig. 4). Among the basic metavolcanics
Fig. 4. Cr vs Y plot of the MORB-derived amphibolites showing the evolution path of magma (according to Pearce 1980). Field enclosed by broken line gives the loci of MORBs (according to Malpas et al. 1993). pm - partial melting line of C 3 chondrite. Metavolcanic amphibolites of the Bohemian-Moravian Heights (points) except for the surroundings of Jemnice (triangles) and the Svratka Complex (squares). Reversed triangles - Fe metagabbros, circle with asterisk - Mg metagabbro, diamonds - cummingtonite and hypersthene amphibolites (originated from within-plate basalts).
the MORBs are by far most numerous, while those of other types (the within-plate metabasalts and other types) are exceptional. The appurtenance of the rocks to MORBs is documented, for instance, by discrimination diagrams which use the following elements: Cr - Y (Fig. 4), V - Ti (Fig. 5), Zr - Zr/Y (Němec in print a), TiO2 - Zr (Němec 1994), V - FeO (Němec in print b) and Zr - Nb - Y (Němec in print b). From the Zr/Y-Zr/Nb-Y/Nb diagram (Fodor and Vetter 1984) which enables discrimination of individual MORB types, it follows that the metavolcanic amphibolites correspond to the N-MORB type. Only these rocks are object of the present study.The studied amphibolte samples originate from various zones of the Varied unit, whose lithological ages are unknown, but suggested (Zoubek 1988) to be approximately equal. Thus, all the investigated samples are treated together.
Fig. 5. V vs Ti plot, MORB-derived amphibolites of the Bohemian-Moravian Heights. IAT - island arc tholeiits, MARB - Mid-Atlantic ridge basalts, OIB - oceanic island basalts.
Regional distribution of Cr and Ni in the amphibolites
The number of the examined samples is low. Nevertheless, general decrease of the Cr and Ni contents in the meta-MORB amphibolites from W to E is clearly conspicuous (Figs 1, 2). In the westernmost zone (No I), which, for the great part, covers the Sázava Varied area, the Cr and Ni abundances range between 500 and 585 ppm and between 245 and 275 ppm, respectively. In the No II zone the Cr and Ni concentrations vary from 300 to 475 and from 110 to 210 ppm, respectively. The eastern No III zone is characterized by lower concentrations (Cr mostly 100 - 300 ppm, Ni 35 - 100 ppm). There, in the surroundings of the town of Jemnice, the amphibolites are particularly depleted in these elements (Cr prevalently 100 - 200 ppm, Ni 35 - 75 ppm). Nevertheless, the zones are not homogeneous as to the chemistry of amphibolites, comprising probably temporarily different amphibolites with different Cr and Ni contents. So, in a marble quarry near the Menhartice village in the No III zone, two types of amphibolites occur: a garnet-rich variety having 114 ppm Cr and 43 ppm Ni, and a garnet-free varienty with 400 ppm Cr and 245 ppm Ni. Both varieties belong to MORBs, even though they have different geochemical signatures (Fig. 6, high K2O and Rb abundances of the garnet-free type are evidently due to an imput during regional metamorphism, Němec in print b). In similar cases, the best explanation is by two temporarily different intrusions (compare Němec in print a). In the area around the town of Chýnov presence of amphibolites widely differing by the time of their intrusion were proved recently by Janoušek et al. (1997) on the basis of geochronological study.
Fig. 6. Multi-element diagram of garnet-bearing amphibolites (points) and garnet-free amphibolite (triangles), Menhartice.
Amphibolites from the Svratka Complex (zone No IV) seem to be especially poor in Cr and Ni. Co is chemically similar to Ni.
However, its concentrations in the amphibolites vary only slightly (30 - 58 ppm) showing no noticeble regional differenciation. In contrast, the range of Ni concentrations is many times broader (37 - 270 ppm). Thus, the Ni/Co ratios (ppm) of the amphibolites drop strongly from W to E: 5.1-6.5 (area No I), 3.2-3.9 (area No II), 2.3-2.8 (area No III, except for the surroundings of Jemnice, where it mostly ranges 0.5 - 1.8).
Similarly to Cr and Ni, also the mg numbers of the amphibolites decrease continuously from W to E. However, their range is much narrower (0.4 - 0.7) and a considerable variability often exists even in the same localities (all the examples cited are from the No III area). The mg numbers of the amphibolites in the quarry of Mirošov range between 0.46 and 0.51 (4 samples), in the quarry of Vicenice between 0.51 and 0.64 (4 samples) and at Hostěradice between 0.48 and 0.63. (3 samples).The differences among individual areas are more apparent if the data are evaluted statistically (Fig. 7 which involves all the chemically analysed MORB-derived amphibolites).
Fig. 7. Histogram of mg numbers, MORB-derived amphibolites of the Bohemian-Moravian Heights. I - western zone of Fig. 1 (14 samples), II - middle zone (63 samples), III - eastern zone (91 samples).
As expected, also the bulk chemistry of the metavolcanics changes generally from W to E. Applying the currently used nomenclature of volcanics, which bases on the presence or absence of the normative hy, ol, ne and Q, we can see that in the areas No I and II solely alkaline olivine basalts and olivine tholeiites occur. In the No III zone, excluding the area around Jemnice, also tholeiites adjoin to them. The territory around Jemnice is characterized by olivine tholeiites and tholeiites, whereas in the Svratka Complex only tholeiites were encountered. In the No III area the petrochemical non-uniformity of the metavolcanics is particularly apparent in the territory of the Náměšť granulite. The granulite body is rimmed in the W by metamorphosed alkaline olivine tholeiites, and in the E by quartz tholeiites, tholeiites and olivine tholeiites (Němec 1996).
The Cr and Ni contents of the metavolcanics evidently were controlled by their pristine (pre-metamorphic) modes which can be inferred from the chemical composition of the rocks (see also Němec 1996 c). Fig 8 is a Cr-Ni correlation diagram. Within the range of low concentrations the Cr abundances increase more rapidly than those of Ni, so that the resulting relation can be approximated by a straight line which runs near the Cr axis. This relations holds up to a certain limit which lies between 200 and 300 ppm Cr. Further on, contents of both elements increase simultaneously. As Fig. 9 shows, the break coincides with the limit, below which the amphibolite samples possess only normative hy and di, but no ol (all these samples originate from the area around Jemnice and from the Svratka Complex). Clinopyroxene is the main carrier of Cr in basalts. Hence, in this section of Fig. 8 the correlation line is controlled preponderantly by clinopyroxene. Above the mentioned limit the samples display also normative olivine, which in the rocks is the main carrier of Ni. Therefore, beginning with this point both elements grow up simultaneously. (The data on the normative olivine need not reflect precisely the original situation, since the original degree of oxidation of iron could have changed during metamorphism and, furthermore, its analytical determination is difficult and therefore, often laden with faults). It can be also pointed out that the Cr/Ni ratios are approximately constant, displaying no marked excesses to higher values. Thus, presence of some special Cr and Ni minerals (chromite, pentlandite) in the pre-metmorphic rocks is improbable.
Fig. 8. Ni vs Cr plot of MORB-derived amphibolites of the Bohemian-Moravian Heights. Samples from the Svratka Complex (squares), from the surroundings of Jemnice (triangles) and all other samples of the region (points).
Factors controlling the regional distribution of Cr and Ni
Concentrations of Cr and Ni in basaltic magmas are controlled by many factors, particularly by T, fo (Murck and Campbell 1986, Barnes 1986, Roeder and Reynolds 1991) and the degree of differentiation of the magma. The degree of partial melting of the mantle source (Pearce and Parkinson 1993, compare also Fig. 4) and differences in mantle source compositions are only of a minor importance. Very broad range of Cr and Ni concentrations found in amphibolites and their relations to the bulk rock chemistry suggest magmatic differentiation as the main factor. In order to estimate the amount of the partial melting and the path of differentiation the same approach is used as outlined by Pearce (1980). The partial melting curve for the C 3 chondrite, which is believed to represent most closely the primordial mantle, is modelled on a Cr vs Y plot in Fig. 4, since these elements do not seem to be affected by variations in mantle composition (Pearce 1980). The points of the amphibolites cluster at the melting curve within the interval of about 10 - 15 % melt fraction suggesting thus that the amphibolites richest in Cr probably represent an almost unfractionated basaltic magma. (Let us also remember that the highest MgO contents found in the MORB-derived amphibolites range 8 - 9 wt %. Abyssal tholeiites having similar MgO contents are considered primary, Maaloe 1979.) The evolution path of the magma follows further, as expected, a near vertical trend. Similar pattern is also observed on the Y-Zr plot according to Pearce and Norry (1979) (Fig. 10), in which, again, the points of the amphibolites leave the partial melting curve and cluster around the fractionation curve.
Fig. 9. Normative ol, Q vs. Cr (ppm) plot, MORB-derived amphibolites of the Bohemian-Moravian Heights. Symbols as in Fig. 8.
Fig. 10. Y vs Zr (ppm) plot (according to Hoeck 1983). MORB-derived amphibolites of the Bohemian-Moravian Heights. pmc - partial melting curve, fc - fractional crystallization trend.
Differentiation of the basalt magma seems to have been only limited. Plagioclase participated only subordinately in it, as evidenced by the REE distributions in the amphibolites. The REE patterns of the amphibolites from the surroundings of Jemnice (Matějovská 1987) are completely devoid of negative Eu anomalies and those from the surroundings of Dolní Rožínka (Pešková 1973) exhibit only small Eu anomalies (Fig. 11).
Fig. 11. Chondrite-normalized REE pattern, amphibolites of the surroundings of Dolní Rožínka (data from Pešková 1973). Dotted line - garnet-bearing amphibolite, full lines - low-Ti amphibolites (0.76 - 1.15 wt % TiO2), broken lines - high-Ti amphibolites ( 2.6 and 4.2 wt % TiO2). The normalizing values from Nakamura (1974).
In the Moldanubian Complex of the Bohemian-Moravian Heights many bodies of amphibolites occur. Among them, various types of metagabbros were identified, but most of the amphibolites are metabasalts having geochemical signature of N-MORBs. Their characteristic feature is a drastic from W to E decrease of the Cr and Ni concentrations from 585 and 275 ppm to 100 and 37 ppm, respectively. It is accompanied by concomitant changes of mg numbers and the bulk rock chemistry of the original basalts which changed from alkaline olivine basalts and olivine tholeiites in the west to tholeiits in the east. The Cr and Ni contents of the amphibolites exhibit clear relations to the pristine pre-metamophic mode of the rocks, inferred from their CIPW norms (presence or absence and different amounts of ol, hy, di and Q, olivine and clinopyroxene are in basalts the main carriers of Ni and Cr, respectively). It can be suggested that the observed regional geochemical differences are due to magmatic differentiation of a primordial basaltic magma generated by 15 - 20 % melting of mantle.
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